TK 9... rsi Darnell 5Ilnlneratta ffiihtatg 3tifaca, Mew ^atk V. Cornell University Library TK 2851.J28 Controllers for electric motorsja treati 3 1924 021 408 822 ^Utyl^ ^r^^^^^^^-^^^^ Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924021408822 CONTROLLERS FOR ELECTRIC MOTORS A TREATISE ON THE MODERN INDUSTRIAL CONTROLLER, TOGETHER WITH TYPICAL APPLICATIONS TO THE INDUSTRIES BY HENRY DUVALL JAMES, B.S., M.E. Fellow, American Institute of Electrical Engineers, Member Engineers' Society of Western Pennsylvania 259 ILLUSTRATIONS NEW YORK D. VAN NOSTRAND COMPANY 2; PARK PLACE 1919 Copyright 1919. by D. Van Nostrand Company PRESS OF THE NEW ERA PRINTING COMPANY LANCASTER, PA. PREFACE ' This volume consists of the series of articles originally published in the Electric Journal during 191 7 and 1918, to which I have now added some new text and illustrations. The object of this book is to bring the Industrial Con- troller Art to the attention of technical students, operating engineers, purchasers and users of electrical apparatus, in the hope that it will give them a good general idea of In- dustrial Controllers and explain their principles of opera- tion. Illustrations have been used for the purpose of making the text clear. No attempt has been made to have these illustrations represent the various forms of com- mercial apparatus. This information can be obtained from trade publications. Some of the chapters are elementary for the benefit of those not experienced in the art. No attempt has been made to cover every method of control, those selected being for the purpose of showing principles of operation. Most of the diagrams have been very much simplified for the purpose of clearness. Anyone attempting to repair or con- nect up a controller, with which he is not familiar, should obtain a diagram and instructions from the maker. It is not possible in a book of this kind to give complete diagrams of commercial control. It is the hope, however, that this book will help in the understanding of such information. In the application sections, the processes and methods of operation are described in some detail, as this information is always essential to a proper understanding of the control. Many controller troubles arise from the lack of complete information of this nature. I wish to express my appreciation for the assistance I re- ceived from the engineers associated with me, and particu- IV PREFACE larly to the editorial staff of the Electrical Journal, who checked my articles and made many value suggestions. I was assisted in the preparation of Chapter VI and VII by Mr. A. A. Gazda; Chapter X by Mr. H. C. Nagel; Chapter XII by Mr. A. H. Candee; Chapter XVI and XVII by Mr. A. L. Harvey; Chapter XVIII by Air. R. T. Kintzing; Chapter XIX by Mr. E. S. Lammers; Chapter XXVI by Mr. W. L. Hartzell; Chapter XXVI I by Mr. H. H. Johnston. H. D. James. Pittsburgh, July, igig. TABLE OF CONTENTS Chapter Page Preface iii I. Introduction 1 II. Historical 13 III. Design Details 41 IV. How to Read Controller Diagrams 48 V. Methods of Accelerating Motors 65 VI. Starting Characteristics of Motors with Dif- ferent Methods of Control 75 VII. Methods of Speed Control and Dynamic Brak- ing 89 VIII. Direct Current Magnetic Contactor Con- trollers 102 IX. Alternating Current Controllers 114 X. Resistors 125 XI. Protective Devices 135 XII. Series-parallel Control and the Electro- pneumatic Contactor 147 XIII. Voltage Control for Direct Current Motors. 158 XIV. Mine Hoists 167 XV. Hydraulic Pumps 184 XVI. Machine Tool Controllers 198 XVII. Machine Tool Controllers (Continued) 210 XVIII. Control for Machinery Requiring Low Initial Speed, such as Printing Presses and Rubber Calenders 226 VI CONTENTS Chapter Page XIX. Steel Mill Floor Controllers for Auxiliary Drive 240 XX. Cranes 254 XXI. Car Dumpers 266 XXII. Ore and Coal Bridges 275 XXIII. Coke 290 XXIV. Elevators 301 XXV. Electrical Equipment for Oil Wells 322 XXVI. Locomotives for Mines and General Indus- trial Purposes 338 LIST OF ILLUSTRATIONS Figure Page 1. D.C. Commutator Controller 15 2. A.C. Commutator Controller 18 3. A.C. Face Plate Controller 21 4. D.C. Face Plate Controller of a More Modern Type 22 5. A Commutator Controller Showing Modifications from Fig. I . . . . 23 6. Commutator or Grindstone Controller 24 7. Cam Contactor Control Built Prior to igoo 26 8. Automatic Cam Controller 27 9. A.C. Automatic Cam Controller 29 10. Automatic Cam Type Elevator Controller 30 11. Lever Type-Controller 31 12. Automatic Elevator Controller 32 13. Drum Controller Shovping the Use of a Wooden Drum and Wooden Finger Base Support 33 14. Drum Controller for 2,200-volt A.C. Motor 34 15. Automatic A.C. Pump-Controller Autotransformer Type 35 16. A.C. Oil Immersed Contactor 36 17. D.C. Elevator Control Magnetic Contactor Type 37 18. D.C. Automatic Controller for Mine Hoist 38 19. D.C. Automatic Controller 39 20. Modern Contactors 40 21. Elementary Controller Diagram, with Face-plate Rheostat 48 22. Starting Rheostat 49 23. Diagram of Starting Rheostat 5° 24. Diagram of Starting and Regulating Rheostat 51 25. Typical Reversing Drum Controller 53 26. Industrial Drum Controller 55 27. Diagrammatical Representation 56 28. Diagram of Non-reversing Controller 57 29. Diagram of Reversing Controller 58 30. Magnetic Contactor Control Panel 61 31. Contactor 62 32. Diagram of Connections 63 33. Simplified Diagram of Connections 66 34. Part of Diagram of Connections 68 35. Diagram of Connections 69 36. Diagram of Series Lock-out Magnetic Contactor 71 vii via LIST OF ILLUSTRATIONS FipURE Page 37. Automatic Starter 73 38. Diagram of Connections of Automatic Control Equipment 77 39. Starting Tests of a 20-HP., 750-R.P.M. Motor Belted to 50-KW. Generation with no Load on the Generator 78 40. Starting Tests of a 20-HP., 7S0-R.P.M. Motor Belted to a 50-KW. Generator with no Load on the Generator 79 41. Starting Tests of a 20-HP., 7S0-R.P.M. Motor Loaded with a Prony Brake Set for Full-load Torque at Full Speed 80 42. Starting Tests of a 20-HP., 750-R.P.M. Motor Belted to a 50-KW. Generator with no Load on the Generator 81 43. Starting Tests of a 20-HP., 500-1, 500-R.P.M. Motor Belted to Two 50-KW. Generators to give 20-HP. Torque at 1,500-R.P.M. 83 44. Same as Fig. 43, Except a Prony Brake was Used Instead of the Generator to Give Full-load 84 45. Effect of Field Variation on Dynamic Braking 85 46. The Same Arrangement as Fig. 45 86 47. Starting Tests of a 15-HP., 825-R.P.M. Motor Driving a Prony Brake Set for Full-load Torque 87 48. Same Starting Test as Fig. 47 87 49. Starting Tests of a 20-HP., 250-1,000-R.P.M., 230-Volt Shunt-motor Operating a Planer 88 50. Diagram of Connections for Varying the Speed of Direct-current Motors 89 51. Characteristic Regulation Curves of Direct-current Motors 90 52. Diagram of Connections for Speed Regulation 91 53. Characteristic Curves of Direct-current Motors 92 54. Typical Speed-torque Curves of a Wound-rotor Induction Motor. 95 55. Typical Speed-torque Curves on a Squirrel-cage Induction Motor. 97 56. Cascade Connection of Induction Motors 98 57. Typical Speed-torque Curves of a Polyphase Motor 99 58. Typical Non-reversing, Automatic Controller 102 59. Diagram of Panel Shown in Fig. 58 103 60. Double-pole, Reversing, Automatic Controller 105 61. Double-pole, Reversing, Automatic Controller, Arranged for Dy- namic Braking from Either Direction of Rotation 106 62. Push Button Station 108 63. Six-point Drum-type Master Controller 108 64. Enclosed-type Float Switch log 65. Diaphragm Pressure Regulator 109 66. Diagram of Automatic Controller for a Three-phase Induction Motor ilj LIST OF ILLUSTRATIONS IX FiGUBE Page 67. Double-pole Contactor and Series Accelerating Relay 116 68. Liquid Controller 116 70. Connections for Starting a Squirrel-cage Motor by an Autotrans- former Starter 118 71. Autotransformer Starter with Cover and Oil Tank Removed 120 72. Connections of Autotransformer Starter 121 73. Connections for Multipoint Starting with an Autotransformer .... 122 74. Modified Method of Producing Multipoint Starting with an Auto- transformer 123 75. Cast-iron Grid Resistor Unit 125 76. Grid Resistor 125 77. Embedded Type Tubular Resistor 126 78. Rheostat made up of Grid Resistors 127 7g. Relation of Temperature Rise and Energy Loss per Resistor Unit. 128 80. Energy Which can be Dissipated in a Grid for Different Lengths of Service 130 81. Dash-pot Type of Inverse Time Element Over-load Relay 138 82. Induction Type, Time Element Over-load Relay 138 83. Combined Phase Failure and Phase Reversal Relay for a Three- phase Circuit 141 84. Combined Phase Failure and Phase Reversal Relay for a Two- phase Circuit 142 85. Combined Phase Failure and Phase Reversal Relay of the Watt- meter or Motor Type 143 86. Steps in the Open Circuit Transition Method of Series-parallel Control 149 87. Speed-torque Curves with Open Circuit Transition 150 88. Steps in the Shunt Transition Method of Series-parallel Control.. 151 89. Speed-torque Curves with Shunt Transition 151 90. Steps in Shunt Tarnsition Using an Electropneumatic Controller. 152 91. Steps in the Bridging Transition Method of Series-parallel Control. 153 92. Speed-torque Curves with Bridging Transition 155 93. Electropneumatic Contactor 156 94. Connections for Reversing a Steel Mill Motor 159 95. Connections of Slip Regulator to Induction Motor 160 96. Diagram of Connections of Equalizer Flywheel Hoisting Set .... 161 97. Equalizer Flywheel Hoisting Set 162 98. Power-demand Curves of a Typical Hoisting Set 164 99. Load-time Curve of n Typical Hoisting Set 165 no. Hoist Operated by Voltage Control Method 177 loi. Diagram of Connections for Contactor Controller . . .- 169 X LIST OF ILLUSTRATIONS FiGUEE Page 102. Contactor for an Alternating-current 2,200-VoIt ilotor i/O 103. Contactor Panel for a Direct-current Automatic Hoist i/l 104. Coned Drum 1 72 105. Liquid Rheostat with H-slot Device 1/4 106. Liquid Rheostat, Cross Section 174 107. Single Lever H-slot Device 175 108. View of Hoisting Room 175 109. Hoist Operated by Slip-ring Motor 176 no. Hoist Operated by Voltage Control Method 177 111. Liquid Controller Installation 177 112. Cam Limit Switch with Worm Reduction Gearing 179 113. Geared Limit Cam-type Switch with Traveling Xut 180 114. Hatchway Limit Switch 182 115. Connections of a Two-point Direct-current Automatic Controller, Operated by a Float Switch 186 116. Connections of a Two-point Direct-current Automatic Controller, Operated by a Pressure Cage 1S7 117. Diagram of Connections of an Automatic Autostarter 1S8 118. Gage Type Pressure Regulator 188 119. Open Type Float Switch 189 120. Typical Capacity and Horse-power Curves of a Centrifugal Pump. 191 121. Connections of a Wound-secondary Induction Motor for a Large Elevator Installation 192 122. Diaphragm Pressure Regulator 193 123. Typical Motor-operated Centrifugal Pumps 195 12^, Drum Reverse Switch for Direct-current Dynamic Braking Service. 199 125. Xon-reversing Controller 203 126. Connections for Xon-reversing Controller Panel of Fig. 125 204 127. Connections of Xon-reversing Panel with Dynamic Brake 204 127a. Same as Fig. 127 with Addition of Drift Point Contactor 204 128. Connections of Reversing Control Panel 205 129. Same as Fig. 128 with D>Tiamic Brake 205 130. Same as Fig. 129 with Addition of Drift Point Contactor 205 131. Separately Mounted Field Rheostat 207 132. Combined Master Switch and Field Rheostat 207 133. Reversing Controller with Field Rheostat 208 134. Universal Wood Milling Machine 210 13;. Electrically-operated Radial Drill 211 136. Electrically-operated Turret Lathe 212 137. Small Coil Winding Table 21^ 138. Turret Lathe with Drum Controller 214 LIST OF ILLUSTRATIONS XI FiGTJRK Page 139. Motor-driven Engine Lathe 214 140. Wheel-lathe Controller 215 141. Push Button Pendant Switch 216 142. Planer Master Switch 216 143. Reversing Planer Controller 217 144. Reversible Planer Controller 218 145. Connections of Reversing Planer Motor Control Panel Shown in Fig. 144 218 146. Group of Wood Turning Lathes 219 147. Arrangement of Controller for a Hydraulic Accumulator 220 148. Electrically-operated Slotter 221 149. Method of Grouping Machine Tool Controllers 222 150. Motor-driven Shaper with Drum Controller 223 151. Planer with Autostarter and Continuously Operating Alternating- current Motor 224 152. Single Motor and Single Voltage Control 226 153. Single Motor and Double Voltage Control 227 154. Single Motor and Double Voltage Control 228 155- Controller for a Single Motor with Adjustable Armature Series and Armature Shunt Resistance 229 156. Control Scheme for Operation over a Wide Range of Speeds .... 230 157. Control Scheme for Alternating-current Motor 231 158. Modification of the Scheme Shown in Fig. 156 232 159. Modification of the Scheme Shown in Fig. 156 232 160. Control Scheme for Use with a Mechanical Gear Changer 233 161. Control Scheme for Operation from Two Voltages 234 162. Two Motor, Full Automatic Direct-current Controller 235 163. Control Scheme for Automatic Acceleration with Face Plate Rheostat 236 164. Double Two Motor, Direct-current Controller 237 165. Two Motor, Full Automatic Controllers 238 166. Two Motor, Full Automatic Controllers 238 167. Push Button Control Station 239 168. Charging Machine 241 169. Scheme of Main Connections for Form AA Controller 242 170. Scheme of Main Connections for Form BB Controller 242 171. Scheme of Main Connections for Form CC Controller 243 172. Scheme of Main Connections for Form DD Controller 243 173. Form CC Controller , 244 174. Form DD Controller ; 244 xii LIST OF ILLUSTRATIONS Figure Page 175- Typical Application of Electric Motors to Reversing Roll Tables of Blooming Mill 246 176. Typical Application of Motors and Gears to Tilting Roll Table of Plate Mill 248 177. Main Connections of Controller with One Point Slow-Down 249 178. Cam Limit Switch for Gear or Sprocket Connection 250 1 79. Two-point Master Switch 251 180. Load Curve of loo-HP., 230-volt, Compound-wound Motor Opera- ting Main Roll Table of a 40-inch Blooming Mill 252 181. Six-ton, Hammer-head Type Ship-building Crane 255 182. Connections for Hoist Controller 256 183. Connections for Bridge or Trolley Controller 256 184. Connections for a Hoist Controller 257 185. Connections for a Hoist Controller 257 186. Crane Protective Panel 258 187. 1.5-ton Semi-Portal Crane 259 188. Operator's Cab of a Traveling Crane 260 189. Cam Contactor z6l 190. Cam Contactor Type of Drum Controller 262 191. 75-ton Full-portal Crane 263 192. 150-ton Electrically-operated Revohang Pontoon Crane 264 193. Car Dumper 267 194. Back View of Car Dumper of Fig. 193 268 195. Diagram of Connections for Barney Haul 269 196. Car Dumper Discharging the Coal from a Car 270 197. Scheme of Connections for Cradle Hoist 270 198. Car Dumper Loading Coal into a Boat 271 199. Car Dumper Arranged for Dumping Coal or Ore into a Yard ... 272 200. Motors and Controllers for Operating Car Dumper 273 201. Ore Bridge 276 202. Connections of Direct-current Motors and Controllers for Hoist Installations 278 203. Connections for Alternating-current Hoisting 279 204. Connections of Motors and Controllers for Trolley Applications . . 280 205. Direct-current Control Panel 281 206. Ore Bridge 282 207. Ore Bridge Unloading Coal from Boats 283 208. Bridge Operated by Alternating Current Motors 284 209. Coal Handling Bridge at the Panama Canal 286 210. 15-ton Ore Rehandling Bridge 287 211. Car Dumper and Coal Handling Equipment 290 LIST OF ILLUSTRATIONS Xlll Figure Page 212. Contactor Panel for Pusher Ram Controller 291 213. Protective Panel for Pusher Ram 292 214. Interlocking Control Panels for the Coal Handling Machinery ... 293 215. Pusher, Leveler and Door Machine Control facing 294 216. Combined Pusher, Leveler and Door Machine 294 217. End of Oven Battery Showing Larry Car, Quenching Car and Pusher Machine. 296 218. Coal Handling Control facing 295 219. Coke Quenching Station of the Lehigh Coke Company, South Bethlehem, Pa 297 220. Coke Wharf and Coke Quenching Car 299 221. Complete Elevator Equipment 302 222. A Worm-gear Drum-type Elevator Machine 304 223. Direct-current Motor Driving an Elevator Equipment 306 224. Geared Traction-type Elevator Machine 309 225. A Worm-gear Drum-type Machine 310 226. D.C. Elevator Control Using a Dash-pot for Controlling Accel- eration 313 227. Automatic Elevator Controller 315 228. Diagram for Automatic Elevator Controller 316 229. Electric Elevator Showing the Automatic Switch for Push Button Control 317 230. Door Lock and Contact 318 231. Full Magnetic Controller for a Two-speed Alternating-current Motor 319 232. Four Tandem Worm-gear Elevator Machines 320 233. A Transformer Sub-station in an Oil Field 322 234. An Oil Well Equipment 324 235. Combined Line Switch and Oil Circuit Breaker with Maximum Torque Relay 326 236. Controller for Motor Primary and Secondary 327 237. Two-speed Wound-secondary Induction Motor 328 238. Maximum Torque Switch 328 239. Diagram of Connections for a Wound-secondary Induction Motor. 329 240 and 241. An Installation of a Two-speed, Wound-secondary Induc- tion Motor Used for Pumping and Pulling 330 242. Diagram of Connections for Wound-secondary Two-Speed Induc- tion Motors 331 243. Diagram of Connections for a Wound-secondary Induction Motor. 332 244. Speed-torque Curve of a Two-pole, Wound-secondary Induction Motor, Having a Rating of 15 and 30 Horse-power 334 XIV LIST OF ILLUSTRATIONS Figure Page 245. Steel Pumping Power 33^ 246. 30-ton Tandem Mine Locomotive Made up of Two 15-ton Units . . 339 247. Drum Controller 34° 248. Diagram of Drum Controller Shown in Fig. 247 '340 249. Traction Reel Gathering Locomotive 34i 250. Motor- Operated Conductor Cable Reel 342 25 1. Conductor Cable Reel Locomotive 343 252. Diagram of Motor Operated Conductor Cable Reel 344 253. Diagram of Contactor Control 345 254. Pusher Type Locomotive with Third Rail 346 255. Arrangement of Bus Line Receptacles for Tandem Locomotive . . 347 256. Storage Battery Locomotives in an Industrial Railway 347 257. Diagram of a Two-motor Storage Battery Control 348 ■ 258. Combined Trolley and Storage Battery Locomotive 349 259. Diagram of Controller for Combined Trolley and Storage Battery Locomotive 350 CHAPTER I INTRODUCTION Proper understanding of an electric controller requires that it be considered as a part of the motor; the controller should be designed to take care of the functions not incor- porated in the motor design, in order to enable the latter to operate under the specified conditions of load. Every motor has certain inherent characteristics which enable it to adapt itself to some of the conditions encountered in prac- tice. In many cases, ho\ve\er, the motor would be very expensive and also ver\' inefficient if it were given the neces- sary characteristics to prevent its being injured or to pre- vent injury to the load during the cycle of operation. The functions usually supplied b_\- the controller are as follows : TO LIMIT THE CURRENT DURING THE ACCELERATION OF THE MOTOR The ohmic resistance of a motor is \-ery low, so that when it is at rest a very large current would be drawn from the line if external resistance were not used to limit this current. As the motor accelerates it deAelojas a counter e.m.f., which reduces the voltage available for causing a current to flow. The current at any instant can be calculated by subtracting the counter e.m.f. from the line voltage, and dividing the result by the ohmic resistance in circuit. It is evident from the above that as the motor increases in speed the starting resistance should be reduced, until the motor is finally con- nected to the line without any external resistance. The short-circuiting of this starting resistance can be done in several different ways. 2 CONTROLLERS FOR ELECTRIC MOTORS Some direct-current motors are designed to be accelerated from rest, without the use of external starting resistance; these motors are, however, for use in particular applications. The squirrel-cage induction motor, in small sizes, can also be started by connecting it directly to the line. TO LIMIT THE TORQUE DtTRING ACCELERATION The torque of a motor is proportional to the current multiplied by the field strength. It is often desirable to start a motor with a gradually increasing torque; this can easily be done with a shunt motor by starting it with zero field strength. The shunt field of the motor is connected to the line at the same time as the armature. Since it takes an appreciable time for the field to build up to lull strength, the torque will increase gradually and give an easy start. In this way a shunt motor can be started with twice full- load current, or even greater current, and not cause a shock, or jar, to the motor or apparatus to which the motor is connected. A series or compound motor, started in the same way, will build up its torque much faster. The induc- tion motor also builds up its torque rapidly. No motor, however, will build up its field strength instantly, so that it is not likely that any type of motor will give a hard shock to the machinery if it is started with zero field strength. TO CHANGE THE DIRECTION OP ROTATION OF THE MOTOR In many classes of service the motor is required to re- verse its direction of rotation repeatedly. It is a well- known fact that the direct-current motor can be reversed in rotation, by reversing the current through the armature, and keeping the field in the same direction. The induction motor can be reversed by reversing one of the phases, and this is usually done by interchanging any two leads on a three-phase motor, or interchanging the two leads in one INTRODUCTION 3 phase of a two-phase motor. Where the motor operates continuously in one direction, these connections can be ad- justed at the time of installing the motor, but where the reversal of rotation occurs frequently, some substantial form of reversing switch should be included in the controller. TO LIMIT THE LOAD OF THE MOTOB This is usually done by means of fuses, or a circuit breaker. Where the main line switches are operated by magnets, an overland relay is used to de-energize the magnet and allow the switch to open. All overload de- vices should be provided with some form of time element attachment, usually a dash pot. This will allow the motor to take a short peak load of a few seconds duration with- out disconnecting itself from the line. This is very desir- able, as such short-time peaks occur during acceleration, and often during the normal operation of the motor. Sometimes an overload relay is provided for inserting the starting resistance in the motor circuit in case of overload. This arrangement is required in special cases where it is undesirable to have the motor torque entirely cease. Such relays are sometimes called "jamming relays." TO DISCONMTECT THE MOTOR UPON FAILURE OF VOLTAGE The voltage supply sometimes fails, and a serious injury might result upon the re-establishment of voltage if the motor were left connected to the line without the starting resistance. Manually-operated starters and controllers are provided with a latch, held in place by a shunt magnet. This latch retains the controller in the running position. Upon failure of line voltage, the catch is released and the controller is mechanically returned to the starting or off position. 4 CONTROLLERS FOR ELECTRIC MOTORS Magnetic contactor control automatically returns to the off position upon failure of line voltage, as the magnets are de-energized under such conditions. The controller can be connected so that it will automatically start the motor again upon return of voltage to the line, in which case the device is known as " low voltage release." Where it is necessary for the operator to perform some function, such as pushing a button after failure of voltage, the advice is known as "low voltage protection." This latter device is usually required, as an operator may be working on the machinery, and there is danger of its being started auto- matically and injuring him upon return of voltage. TO EEGULATE THE SPEED OF ROTATION Frequently a motor is connected to a load requiring dif- ferent operating speeds. This speed change can be effected in several differeht ways, the common ways being: Armature Control. — This consists in putting resistance in series with the motor. Direct-current motors have this re- sistance in the armature circuit, and the voltage across the motor brushes is less than the line voltage, due to the drop through this external resistance. Wound secondary induc- tion motors are controlled in a similar way, by connecting the resistance in the secondary circuit. The drop in speed is in this case a little more complicated to calculate; the reduction in speed is proportional, however, to the voltage drop through resistance. Motors controlled in this way are called "variable speed motors." The speed at which the motor operates depends directly upon the torque required by the driven load. A change in torque causes a corre- sponding change in current, and the drop through the ex- ternal resistance is equal to the current multiplied by the ohms. Changing the Field Strength of direct-current shunt motors varies their speed. This is usually done by connect- INTRODUCTION 5 ing a rheostat in series with the shunt field winding of the motor. Such motors are called " adjustable speed motors," since the speed remains practically constant under all con- ditions of loading. Commercial motors of this type are built with speed ranges as high as four to one. At present no alternating-current motors of this class are in com- mercial use. Changing the Voltage of the Supply Circuit also varies the speed of a motor. This is usually done by supplying each motor from a separate generator. This system of control is used only for large motors, and at present is ap- plied principally to mine hoists and reversing steel mill motors. TO START AND STOP THE MOTOR AT FIXED POINTS IN THE CYCLE OF OPERATION, OR AT THE LIMIT OF TRAVEL ON THE LOAD This feature can be obtained by the use of limit switches, which are operated by the machinery to which the motor is connected. They usually interrupt only a small circuit, thereby opening magnetic contactors which, in turn, dis- connect the main motor circuit. These limit switches may be connected by gearing to the driven machinery, in which case they are called " geared limit switches." Where the limit switch is mounted along the runway, and operated by the machinery striking the switch, they are called "track" or "hatchway limit switches." TO STOP THE MOTOR The motor can be brought to rest by either mechanical or dynamic braking. Mechanical Braking is accomplished by a friction brake, which is usually applied by a heavy spring and released by a magnet in series with the main circuit of the motor. In D CONTROLLERS FOR ELECTRIC MOTORS this case the brake is set whenever there is no current in the motor, and consequently no special arrangement is neces- sary on the controller to apply the brakes. Dynamic Braking requires at least one additional switch on the controller. It is accomplished by disconnecting the armature from the line and short-circuiting it on itself through a resistance with full field strength, the energy stored in the rotating parts being dissipated in heating the resistance. TO PROTECT THE OPERATOR PROM INJURY It is very important to insure the operator who uses the machinery from injury either during the starting of the motor or during the subsequent operation of the machinery. This requires the control apparatus to be properly pro- tected, so that there is little danger of the operator's receiv- ing a shock, or being burnt by an arc, in starting or during the operation of the machinery. Accidents may occur which require quick stopping of the machinery. To effect this result, safety stop devices are frequently placed around the machinery, which are either operated automatically or manually, depending upon conditions. These devices must be adapted to each particular application, but are very im- portant, and should be carefully considered by engineers in specifying the electric drive. Electric controllers can be roughly divided into two general classes: (i) Manual Acceleration, (2) Automatic Acceleration. 1. MANUAL ACCELERATION These comprise control apparatus in which the accelera- tion of the motor is entirely under the control of the oper- ator. Illustrations of this are the face plate and drum type controllers. INTRODUCTION 7 2. AUTOMATIC ACCELERATION" These comprise the control apparatus in which the ac- celeration of the motor is performed automatically. These terms are usually applied to the method of acceler- ation. Controllers may have a combination of these two methods; for example, a magnetic contactor type of crane controller may have a master switch with five or six notches. The acceleration between notches is automatic, but the oper- ator can determine the direction of rotation and the speed of the motor. The rate of change in speed, however, is automatic. Controllers including starters may be divided into the several groups given below. These groups do not include every type built, but give a good idea of present practice. The advantages and disadvantages listed must be inter- preted in a very general manner as they may not apply in many special cases. The magnetic contactor control is usually automatic. The other types are generally manual; but automatic acceleration may be obtained by using an electric motor or an air cylinder to operate them. 1. FACE PLATE CONTROLLERS A dvantages : 1. Low price. 2. Compact, usually with self-contained resistor. 3. Easy to mount on wall or switchboard. 4. Flexible design, can be readily altered. 5. Renewals of contacts and repairs inexpensive. 6. Low voltage release feature easily applied. Disadvantages : 1. Design not usually well adapted to take care of arcing. For this reason it is not good for heavy or frequent service. 2. Design usually not rugged mechanically. 3. This type presents difficulties where the connections are compli- cated, such as a reversing control for wound secondary motors. CONTROLLERS FOR ELECTRIC MOTORS 2. LEVER CONTROL (THIS TYPE CONSISTS OF INDIVI- DUAL CONTACTORS, EACH OPERATED BY ITS OWN LEVER OR HANDLE) Advantages: 1. Medium price. 2. Panel-mounted — may have self-contained resistor. 3. Can be readily combined with a circuit breaker. 4. Flexible design — can readily make up special combination. 5. Can use rolling contact. 6. Closing and opening of contact positive. 7. Low voltage release feature easily applied. D isadvan tages : 1. Requires the manipulation of a number of handles. 2. Not suitable for quick operation. 3. Cannot be readily enclosed to protect operator. 4. Not as compact as the drum type. 3. DRUM CONTROLLERS (CYLINDER TPYE) Advantages: 1. Low price in small and medium sizes. 2. Compact, but separately mounted resistor. 3. Entirely enclosed. Can be made dust-proof, spray-proof, or gas- proof. 4. Strong mechanically and simple to operate. 5. Can have various mechanical retarding devices attached to pre- vent too rapid acceleration. 6. Complicated connections can be made; i.e., forward and reverse, or power and brake, on the same drum. Disadvantages : 1. Difficult and expensive to modify design. 2. Frequent adjustment of contacts necessary. 3. Rapid deterioration of contacts under severe conditions. 4. Sometimes difficult to take care of the energy of the arc. 5. Limitation as to size. 6. Large sizes difficult to operate. INTRODUCTION 9 4. CAM CONTBOLLERS DRUM-MOTJNTED (MANUALLY OPERATED) Advantages: 1. Low price. 2. Compact design but resistor separately mounted. 3. Entirely enclosed and can be made dust-proof, spray-proof or gas- proof. 4. Strong mechanically and simple to operate. Can be provided with reciprocating as well as rotating handle. 5. Can have various mechanical retarding devices attached to the handle to prevent too rapid acceleration. 6. Complicated connections can be made readily. 7. New combinations can be made easily by changing the number of units and the shape of individual cams. 8. Rolling contacts are used which are usually free from welding. 9. Contacts are easily and cheaply renewed. 10. Entire unit can be replaced if necessary. 11. Easy to inspect. 12. Quick closing and opening not found in drum controllers. Disadvantages: 1. The larger sizes require more power to operate than does a master switch. 2. On account of the enclosure the continuous capacity may be reduced and the energy of the arc must be limited. 3. Limitations in size. 5. CAM CONTROLLERS PANEL-MOUNTED (MOTOR- OPERATED) Advantages: 1. Compact design. 2. The necessary complicated sequences of operation easily obtained. 3. Positive time element obtained through pilot motor and gearing. 4. The motor may be replaced by an air cylinder when air pressure is available. 5. A rolling contact is obtained which is free from welding and gives long life. 6. Contacts are easily and cheaply renewed. 7. Entire unit can be replaced if necessary. 8. Easy to inspect. 9. A saving in copper connections, as the controller can be located close to the motor, and the master switch wherever convenient. lO CONTROLLERS FOR ELECTRIC MOTORS Disadvantages : 1. Expensive in small sizes. 2. The cycle of operation must be completed in order to restore the controller to its normal position. This may require too long a time element for fast service. 3. Contactors are usually mounted in a row which limits the size of the panel. 6. MAGNETIC CONTACTOR CONTBOL Advantages: 1. Long life of contacts due to the rolling action and a quick opening and closing. 2. Positive opening and closing of contacts. 3. Flexibility in adapting controller to various designs. 4. Can be arranged with various safety attachments. 5. More "fool-proof" than other types. 6. Strong and rugged mechanically. 7. Automatic control can be obtained. 8. Contacts are easily and cheaply renewed. 9. Entire unit can be replaced if necessary. 10. Easy to inspect. 11. A saving in copper connections, as the controller can be located close to the motor, and the master switch wherever convenient. 12. A time element in closing and opening not found in drum con- trollers. Disadvantages : 1. More expensive than manual controllers. 2. The larger sizes occupy considerable space. 3. The wiring diagram is complicated. 7. LIQUID CONTROLLERS Advantages: 1. Large thermal capacity for starting. 2. Very gradual change of resistance. 3. Absence of arcing or other wear, as no contacts are used (except line switch). 4. The resistance can be easily adjusted by varying the amount of soda in the solution. 5. It can be readily adapted for automatic operation. INTRODUCTION II Disadvantages : 1. Considerable floor space is required. 2. The electrodes are subject to corrosion. 3. Cooling water is required for the larger sizes. 4. The use of the liquid limits its application. SAFETY BITLES Two sets of safety rules now in force apply to control apparatus. One set has to do with fire protection and is known as the National Electric Code. It comprises the regulations of the National Board of Fire Underwriters for electric wiring and apparatus. The other rules have to do with personal hazard and are known as the National Elec- tric Safety Code, issued by the Bureau of Standards, Wash- ington, D. C. The fire protective rules have been in force for a great many years and are pretty thoroughly worked out and understood by the public in general. Copies of these rules can be obtained by anyone interested in them. They pro- vide for the proper method of installing electrical apparatus and place certain restrictions upon the design of this appa- ratus. Usually standard apparatus for sale to the general trade is submitted to the Underwriters' Laboratories for in- spection and test. If satisfactory, this apparatus is listed by them among their approved fittings and is not questioned by local inspectors. The rules for protection againt personal hazard have re- cently been compiled by the Bureau of Standards in the form of the National Electric Safety Code. These rules have been worked out in connection with the engineers of commercial companies and are well thought out and logic- ally arranged. The recent war has interfered considerably with their general consideration and a number of changes will be made as the requirements develop. It is probable that these rules will be followed by many industrial estab- 12 CONTROLLERS FOR ELECTRIC MOTORS lishments, whether required by law or not, as the protection of workmen is of growing importance and is receiving care- ful consideration. In specifying control apparatus, the following informa- tion should always be given : 1. The characteristics of the power circuit, such as voltage, fre- quency, etc. 2. A brief description of the control apparatus required. 3. If the motor speed is to be adjusted, the speed range should be given and the load at the maximum and minimum speed. 4. The cycle of operation should be stated in detail, particularly the number of starts made by the motor per hour. 5. A description should be given of the machine which the motor drives, particularly the torque required to accelerate to full speed. 6. A description of any unusual features of the installation, such as moisture, dirt, acid fumes, limited source of power, etc., should be noted. All of this information is necessary for the intelligent selection of both the motor and control. In the absence of any of this information, the engineer furnishing the elec- trical apparatus must guess at the requirements on the basis of the best average practice. This will often take care of the situation, but exact information is very much better. CHAPTER II HISTORICAL In order to obtain the proper perspective on controller practice, it is necessary to know something of the events which led up to the present state of the art. This chapter will not give a complete history of industrial development and it must, to a considerable extent, be limited to the per- sonal experience of the author. A sufficient number of im- portant stages in the development will be outlined to enable the reader to understand how present practice has been developed. The earlier forms of control were manually operated. They were usually for D.C. motors and consisted of a line switch and a resistor for limiting the starting current, to- gether with means for short-circuiting this resistor. Some- times the line switch contained means for reversing the motor. The most common form of starting device became familiarly known as a "starting rheostat." The other de- vices were known as " reversing controllers,'' whether they controlled the speed of the motor or merely started it. The automatic controller followed the manual controller and in many cases was quite complicated, most of the controllers involving the use of a dash-pot, which was combined with weights or magnets to control the cutting out of the resistor during acceleration. Most of the early controller development took place in the railway and elevator fields. The former was carried on in connection with the series motor, the latter with the shunt motor, which was usually compounded during the starting period. A study of the early patent art shows a marked 13 14 CONTROLLERS FOR ELECTRIC MOTORS difference in the development of these two lines of control apparatus. This distinction exists today to a considerable extent; railway control concerning itself chiefly with the series motor; elevator control has broadened into industrial control and deals with shunt, series and compound motors for a veriety of applications. 1. CONTROL FOR SERIES MOTORS, PRINCIPALLY IN THE ELECTRIC RAILWAY FIELD The use of motors in connection with trolley cars made it convenient to locate the controller on the platform of the car. This led to the compact enclosed type of controller, its prin- cipal representatives today being the drum type shown in Fig. 247, Chapter XXVI. This usually consisted of a cyl- inder rotated by the handle with contact segments which en- gaged stationary flexible fingers. Some of the earlier designs consisted of a rotating cam shaft actuating individual con- tactors and various modifications of these two arrangements. The cylinder type of control proved the cheaper and better commercially. The weak point of this controller is in the contact fingers which require frequent adjustment and are apt to stub or bend when the drum contact approaches the finger in the direction of the finger support. Many types of fingers were de^yised to minimize this diflficulty ; and quite a variety of designs are in use today. At first, the cylinder holding the contacts was made of wood and the fingers themselves supported on wooden bases. The contacts and finger supports were attached to the wood by means of ordinary wooden screws. If moisture was present in the wood, a small leakage current would be set up between the sdrews, which charred the wood around the screw and ulti- mately loosened the contact or finger support. The most important improvements consisted in the use of the "magnetic blow-out" for extinguishing the arc and substituting a metal cylinder for the wooden cylinder, the HISTORICAL 15 contact strips being attached with metal screws. The metal segments of the cylinder are clamped to a square or hexa- gon shaft insulated by tubing made of various materials. The finger supports which were first attached by wooden screws were afterwards clamped to the wooden bases by L^ Fig. I. D.C. Commutator Controller. This is an early type of reversing controller built prior to igoo. It was used for crane and hoist applications. The fundamental idea comprises a controller with a set of commutating segments similar to the commutator on a D.C. generator. 1 6 CONTROLLERS FOR ELECTRIC MOTORS through bolts. Later the wooden bases were discarded and insulated steel bars substituted, the finger supports being clamped to these bars. Another early form of controller consisted of the com- mutator type placed under the front platform and operated by a system of rods and levers. The system of control first consisted of an ordinary rheo- static reversing controller, the motor being connected to the line through resistance, which was afterwards short-cir- cuited. The construction of a trolley car required the use of two and sometimes four motors. Advantage was taken of this plurality of motors to place the two motors in series to obtain a slow-operating speed and to give the necessary starting torque with a minimum line current. After the motors had been accelerated to half speed, they were changed to full speed by connecting them in parallel with resistors in series and afterwards short-circuiting the resistors. ' This system of control presented a new problem in. chang- ing from the series to the parallel connections. The three methods of accomplishing this are explained in the chapter on Series Parallel Control. When it became desirable to connect several cars together to form a train, some form of remote control was necessary. One of the early types consisted in operating the control for each car by means of an electric motor, known as a pilot motor. A later modification in this system consists in substituting air cylinders with magnet valves for the pilot motors. These air devices were developed quite recently and are giving successful operation. Later railway developments consist in using contactors in groups together with remote control reverses. These con- tactors and reverses are operated either by electro magnets or by air cylinders, the air cylinders being controlled by small magnet valves. The presence of compressed air on HISTORICAL 1 7 railway equipment in general makes its use for actuating the control equipment both convenient and economical. Some air-operated industrial controllers have been installed, but their application is limited, as compressed air is not usually available. 2. CONTROL OF SHUNT AND COMPOUND MOTORS Most of the early development was in connection with elevators. At first a manual controller was used, mounted close to the motor and actuated from the elevator car by means of a rope or some form of lever device. This fol- lowed closely the hydraulic practice for actuating valves and needed no new development as far as the actuating means was concerned. A good many slow speed elevators today use a hand rope control, but the hand rope operates only the reverse switch, the acceleration being performed by separate means, usually magnetic contactors. When the operating rope or lever was moved, it first closed the direction switch (which was often separate from the line switch) and afterwards the line switch, which in turn actuated automatic accelerating means. It consisted of a weight operating against a dash-pot and retarded by a coil in series with the armature. The weight moved an arm across a set of contacts which gradually short-circuited the starting resistance and afterwards the series coils, causing the motor to operate as a shunt motor. These controllers gave only one elevator car speed and were satisfactory for slow-moving cars. Another form of accelerating means was a magnet connected across the armature of the motor for lifting an arm which short-circuited the starting resistance. Still another variation consisted in connecting the magnet across the starting resistor, using the drop across this re- sistor to retard the cutting out of the resistance during ac- celeration. Many combinations of these devices were used, but the electrical and mechanical difficulties led up to the i8 CONTROLLERS FOR ELECTRIC MOTORS development of a contactor control. It might be interesting to note that the pilot motor control was used quite suc- cessfully on a number of elevator installations approxi- FiG. 2. A.C. Commutator Controller. This controller was made for use with a polyphase A.C. motor in which a segment of a circle was used instead of the complete circle as shown in Fig. I. mately twenty years ago. This pilot motor actuated a face plate controller, which not only short-circuited the starting HISTORICAL 19 resistance, but changed the motor speed by changing its field strength and also provided for dynamic braking. One of the difficulties with this control, apart from its expense, was the slowness with which the pilot motor returned the arm to the starting position. The contactor type of control developed later consists of individual magnetic contactors. Sometimes these are two- pole for use as combination direction and line switches. The contactors used for short-circuiting the starting resistor are usually actuated in one of two ways : 1. The shunt coils are connected across the armature of the motor and closed in succession as the counter e.m.f. of the motor increased. This is known as a counter e.m.f. method. 2. The closure of the accelerating contactors is controlled by the main motor current and is known as current limit ac- celeration. This usually consists of a relay or relays for governing the closure of the accelerating contactors. The details of this system will be given later. About twenty-five years ago, a prominent engineer ex- perimented with elevator motors, controlling their speed by changing the field strength of the motor. These experi- ments were among the first in which the speed of a shunt motor was varied by changing the resistance in the field windings. Very little was done with this method of con- trol until recently. Its commercial development first took place in connection with machine tool drive and was later used with high-speed elevators. Another modification in the control system for elevators consisted in obtaining a slow speed by using a resistance in shunt with the armature, as well as in series with the armature. The combination of shunt and series resistance, together with an adjustable speed motor, will give three operating speeds, a slow speed for making the landings, a speed representing the full field strength in the motor, and 3 20 CONTROLLERS FOR ELECTRIC MOTORS a third speed representing the weakened field of the motor. Usually this is sufficient, but additional speeds may be obtained. MACHINE TOOL CONTEOLLERS There are two other applications of electric drive ; namely, the machine tool and the steel mill applications, which have exerted a very marked influence on controller development. The machine tool application has been principally respon- sible for the development of the adjustable speed D.C. motor and the controllers which are used with this type of motor. The study of machine tool requirements has pro- duced several special motor and control combinations, the best known of which is the reversing planer equipment. STEEL MILL CONTBOLLEKS The steel mill application can be briefly divided into main drive and floor auxiliaries. The main drive used motors of several thousand horse-power, the first of which • was installed in this country in 1906. The motor is con- nected to a motor generator set and controlled by changing the field of the generator. This method will be described in Chapter XIII. The floor auxiliary devices in steel mills use series and compound motors that are of very rugged design. The motors are made special for this class of service and are known as the steel mill type. The controllers are also very heavy and rugged and must be liberally rated. The con- trol problems worked out in this application differ from the elevator and the machine tool fields because of their dealing with series motors. It was in this field that the lockout switch found one of its principal applications. The application of electrical apparatus to cranes, printing presses, and a number of other industries, has had its effect on controller development. It is for the purpose of bring- HISTORICAL 21 ing out the effect of these industries on controller types and systems that the applications are given in the latter part of this book. MANUAL CONTKOLLERS The drum type controller has been briefly described above. This controller is used extensively today and is a Fig. 3. A.C. Face Plate Controller. This is a development from the commutator type of control in which the segments are mounted on the surface of the insulating plate, rather than around the periphery of the insulating material. 22 CONTROLLERS FOR ELECTRIC MOTORS Fig. 4. D.C. Face Plate Controller of a More Modern Type. very reliable device when applied within its capacity. Re- cently, the cam contactor type of control has been further developed. Fig. 190, Chapter XX, shows a contactor type controller, using the drum mounting. This development HISTORICAL 23 has been too recent to make definite statements of its range of application, but it is believed that it will extend the use Fig. 5. A Commutator Controller Showing Modifications From Fig. i. of manually operated controllers and provide the desired flexibility to enable special controllers to be built readily and for a reasonable cost. 24 CONTROLLERS FOR ELECTRIC MOTORS The commutator or grindstone controller, illustrated in Figs. 5 and 6, is not being built at present, as the cylinder and cam type of drum controllers perform the same func- FiG. 6. Commutator or Grindstoxe Controller. A more recent development of the commutator type control. This shows an earlier form of resistor in which the units were wound up in a spiral with mica insulation and formed individual cells. This type has been super- seded by the cast-iron grid. tions and are more compact. It illustrates a type of design representating a great many controllers which have been in HISTORICAL 25 commercial use. There is a possibility that it may be used again, as the art advances. The face plate controller, illustrated in Figs. 3 and 4, is quite similar to the commutator type, except that the con- tact segments are placed on a flat surface, rather than around the arc of a cylinder. Some of these controllers are still being manufactured commercially. A great many of them were used in the past and proved very satisfactory. Their size and exposure at present is somewhat against them, particularly since the safety requirements make it necessary to enclose them for many applications. Under this head- ing should be included field rheostats, which may be either hand operated or motor driven, depending upon their size and application. MAGNETIC CONTACTORS This type of control is the preferable one at the present time for many applications. It is practically the only type used for automatic acceleration. A description of the de- tailed development of the magnetic contactor is very interest- ing and brings out many ingenious features. We shall have space only to mention a few of the more important changes. Practically all the early contactors were actuated by magnets of the coil and plunger type. Theoretically, this is the most economical form of magnet, but its adoption to practice developed a fundamental difficulty. The plunger passes through the magnetic frame, the magnetic circuit being completed between the upper end of the plunger and the stationary core. Where the plunger passes through the frame, it is necessary to provide a permanent air gap, usually consisting of a brass bushing. Although this introduces an appreciable air gap in the magnetic circuit when the plunger is brought in contact with the core, it is often necessary to use an additional brass washer to prevent the residual magnetism from holding the plunger 26 CONTROLLERS FOR ELECTRIC MOTORS against the core. These air gaps reduce the efficiency of the magnet. If the air gap, where the plunger passed through the core, was made small, it was impossible to keep the plunger from being pulled strongly to one side, which caused wear on the brass bushing and the further the bush- ing wore down, the greater was the side pull, due to unequal magnetic distribution. This not only introduced consider- able friction in the movement of the plunger, but it caused Fig. 7. Cam Contactor Control Built Prior to 1900. This form of control was the forerunner of the lever type controllers now in use. It consists of a series of contractors closed by cams actuated by a common handle. rapid wear which increased the maintenance charge. In dirty places, dust and grit accumulate inside the coil and sometimes cause sticking of the plunger. In the last few years, designing engineers have changed from the coil and plunger type to the clapper type of magnet. This has avoided the former difficulties and is proving very durable and satisfactory in practice. The earlier forms of contactors employed a variety of contacts and these contacts were located sometimes at the top and sometimes at the bottom. In general, the design HISTORICAL 27 has settled down, so that the rolling type of contact is usually employed and the contacts themselves are located at the top of the contactor above the operating coil. The arc Fig. 8. Automatic Cam Controller. This illustrates an automatic controller of the cam type for use in connec- tion with a pump. A pressure regulator controls a hydraulic cylinder which actuates the cam shaft starting the motor when the pressure decreases to a predetermined value. When the pump has increased the pressure to the proper amount, the pilot valve makes the necessary connecctions for return- ing the controller to the off position. shields are arranged so that they can be revolved around their core in order to expose the contacts for inspection and repair. All contacts, operating coils, arc shields and other parts of the svs^itch which may be injured, can be removed and replaced from the front of the board by one individual, unless the part is too heavy to be handled. 28 CONTROLLERS FOR ELECTRIC MOTORS Trouble with the earlier designs of contactors led to the practice of mounting each contactor on an individual base. If trouble developed, which could not be readily repaired by the electrician, the complete contactor, including its base, was removed and a new contactor with base mounted in its place. This practice is being followed at present by some manufacturers. It results in a larger and more ex- pensive structure than necessary and does not present as good an appearance. The modern contactor is so well de- signed that very little trouble is experienced with the magnet frame. All of the parts which might be affected can be readily removed from the front of the board, so that there is no further necessity of using individual bases for each contactor; in fact, it is easier to dismantle the contactor from the front of the board than it is to disconnect all of the wiring and substitute a new contactor and base complete. This change in construction has a very important bearing in places where it is necessary to provide cabinets or guards around the control apparatus, as the space requirements be- came an important item and a compact control panel is desirable. SYSTEM OF CONTROL When the automatic control was first introduced, it was found to be comparatively easy to provide a great many different safety connections. For instance, the line con- tactors were interlocked electrically with the accelerating contactors, so that all of the latter must be opened before the line contactors were closed. With the old coil and plunger type of magnet, the sticking of a contactor might occur quite frequently and such an arrangement was neces- sary. With the modern design of clapper contactor, these interlock connections have been abandoned. The older de- sign of controllers were characterized by many electrical interlocks and much complication of the detail wiring. The HISTORICAL 29 recent tendency has been very strongly towards a simplifica- tion of the control wiring. Electrical interlocks are apt to give trouble and should be avoided wherever possible. By comparing the modern diagram for a reversing control with Fio. 9. A.C. Automatic Cam Controller. , This shows a series of cam contactors actuated by an A.C. magnet, the time element being a dash-pot. 30 CONTROLLERS FOR ELECTRIC MOTORS one in use ten or fifteen years ago, a marked improvement in this respect will be noted. An interlock connection should not be used unless the damage from its omission will be Fig. 10. Automatic Cam Type Elevator Controller. This illustrates a series of cam contactors actuated by a weight and con- trolled by a dash-pot. This controller was introduced over ten years ago. greater than the trouble which might be experienced from its failure to operate. One of the points of merit in a con- trol panel is the small number of electrical interlocks. With HISTORICAL 31 these improvements, the magnetic contactor control has be- come an exceedingly reliable piece of apparatus and com- paratively easy to understand. Fig. II. Lever Type-Controller. This form of controller was introduced about 1904 and consists of a series of lever switches actuated by starter handles. This controller shows the rolling contact which has since been universally adopted for magnet contactors. CURRENT LIMIT ACCELERATIOIT Very little change has taken place in the counter e.m.f. method of acceleration or in the dash-pot method, except that dash-pots have been materially improved recently. The principal developments in accelerating systems have had to do with the current limit method. This can be broadly divided into the relay type and the lockout switch type. The relay type was used much earlier than the lockout switch (the latter coming into use about ten years ago). 32 CONTROLLERS FOR ELECTRIC MOTORS Originally only one relay was used for each controller. The contactors had two coils, one of which was known as the closing coil, controlled by the relay, the other a holding coil, which was sufficient to retain the switch in the closed position. The relay alternately opened and closed at each current peak, thus retarding the acceleration. It was after- " — ■ fkj K ^ _ ■ _ ' ~ dS ml %A ^ f - ? m ' ^ mM \ K 1 D E s ■ J ^L r - 1 i i 1 S -, B S I'mJ If i H 5 Fig. 12. AuTOJiATic Elevator Controller. This controller is of the drum type having the resistor units forming part of the drum. Note the use of wood as insulating material. This form of controller was used about 1902. wards found that a single coil on the contactor was sufficient if a four-point interlock were used. This interlock trans- ferred the contactor circuits to the relay in succession. This arrangement was economical in the use of relays, but the extra interlocks were both expensive and sources of trouble. At this time, a limited use was made of a single relay having a series of connections, one to each contactor. It was found \'ery difficult to maintain all of these connections through the relay, but the advantage of eliminating the interlock was sufficient to warrant the use of an individual relay for HISTORICAL 33 each contactor. This reduced the auxiliary contacts to the present practice which is used in some cases without extra precautions. This arrangement causes the relay to open and close its contacts each time the motor is accelerated and Fig. 13. Drum Controller Showing the Use of a Wooden Drum and Wooden Finger Base Support. This construction was in common use fifteen years ago, but has been superseded by a more substantial design. 34 CONTROLLERS FOR ELKCTRIC MOTORS Fig. 14. Drum Controller for 2,200-VoLT A.C. Motor. This shows a modern construction of drum made up of metal segments clamped to an insulated steel shaft. The contacts for the primary of the motor, which is connected to a 2,200-volt supply line, are mounted in an oil tank at the bottom of the controller. HISTORICAL 35 a further improvement consisted in mechanically holding the relay open by the preceding contactor, so that the func- tion of the relay was only to make contact. Even this improvement was not ideal, as the magnetism in the relay takes an appreciable time to build up after the current has decreased to the low value. Sometimes this would permit Fig. 15. Automatic A.C. Pump-Controller Autotransformer Type. This illustrates one of the earlier forms of pump controllers for A.C. motors. It consists of a standard starter having an automatic device at- tached in place of the handle. The sheave wheel is connected by ropes to a float in the tank to which the pump delivers water. The raising and lower- ing of the water level throws the weight over center and actuates the con- troller to either start or stop the motor. The time element is controlled by a dash-pot. the relay to drop and touch its contacts before the magnetism had reached its maximum value. To overcome this, some engineers wound the relay with compound coils, which over- magnetized the relay until it was time for it to operate. For large motors, the extra copper connections added a great deal of expense. By arranging the mechanical inter- lock of the relay so that it does not release until the preced- ing contactor has closed its arcing tip has overcome this difficulty to a considerable extent. By winding the relay 4 36 CONTROLLERS FOR ELECTRIC MOTORS with a shunt coil and connecting it across a section of the starting resistance, the same results are obtained as using the straight series coil and the relay may be readily over- magnetized without any additional contacts or connections. Fig. i6. A.C. Oil Immersed Contactor. This shows an early form of oil immersed contactors actuated re- spectively by a D.C. or an A.C. magnet. This form of contactor was also actuated by air cylinders. The contacts are on the lower part of the struc- ture and are immersed in the oil tank. The lockout switch now in use is of several different de- signs, the one most commonly used being known as the single coil type. Considerably more than ten years ago a contactor was placed on the market having two coils. In HISTORICAL 37 the off position the contacts were open. When the circuit was energized one of the coils lifted a plunger or latch and released the contact, which however was prevented from Fig. 17. D.C. Elevator Contkol Magnetic Contactor Type. This shows an earlier form of magnetic contactor. The acceleration is controlled by a series relay. The first contactor is provided with a dash-pot to give a fixed time element in order to take care of the initial rush of cur- rent on the elevator. Note the complicated set of interlocks. The progress in the art has resulted in eliminating most of these interlocks. closing by a second series magnet coil. This arrangement of lockout switch is used successfully today. 38 CONTROLLERS FOR ELECTRIC MOTORS- The more common type of single coil switch depends upon the saturation principle. There are twD parallel paths for the magnetic flux, one being the closing path and the other the lockout path. A part of the magnetic path in the Fig. i8. D.C. Automatic Controller for Mine Hoist. This controller was built about 1906 and provided for the automatic opera- tion of a 500-h.p. D.C. motor connected to a mine hoist. closing circuit is of small sections so that it becomes satu- rated at high current values. This permits the parallel or lockout path to predominate when the switch is held open. When the magnetism decreases to a fixed value, the closing magnetism predominates and the switch contacts are brought HISTORICAL 39 Fig. 19. D.C. Automatic Controller. This photograph shows a construction which eliminated the use of slate panels. It made a lighter controller and one that was more accessible. It was in use about 1908. 40 CONTROLLERS FOR ELECTRIC MOTORS together. If the current drops too rapidly the ampere turns in the closing circuit are not sufficient and the switch fails to close. This is one of the chief drawbacks to this Fig. 20. Modern Contactors. This shows one form of the modern contactor, one of these being for D.C. and the other for A.C. circuits. Note that the form of magnet has changed from the coil and plunger type to the clapper type. Illustrations in other chapters will show the use of these contactors in different controllers. type of switch. Chapter V. A further description will be found under CHAPTER III DESIGN DETAILS The detail design of controllers should be left to the manufacturer who guarantees their operation and satis- factory service. The application engineer must select the proper type and size of controller for each particular drive and he is therefore interested in knowing how variation in design may affect the performance of a controller. The capacity of a controller to carry current with a given rise in temperature may often be of much less importance than the durability of the contacts under repeated arc rup- turing, and the desigft of bearings to withstand wear in dirty places. A knife switch will carry current but will not last long if used for rupturing current. Current-carry- ing ability may- be affected by long periods of operation without the scouring action on the contact surface due to opening and closing. When the controller is used for short periods at consider- able intervals of time, a much smaller controller can be selected than normal for the motor. An application of this kind would be a crane in a power house used only for repair work. For rapid operation, such as occurs with motors driving reversing tables in steel mills, the controller should be more than usually liberal. The continuous current capa- city has little to do with either of these applications. If the motor drives a condenser pump in a power house, continu- ous current capacity is the most important consideration. Contacts. — The current-carrying capacity of contacts de- pends upon the following: 41 42 CONTROLLERS FOR ELECTRIC MOTORS 1. The Material from which it is Made. — Hard drawn or forged copper has given the best results. 2. The Pressure between Contacts. — Other things being equal, the heavier the pressure, the more current the contact will carry. There, of course, are limits to this pressure, but the carrying capacity can be materially increased with an increase of pressure. 3. The Mass of the Contact. — The greater the mass, the more heat carried away from the contact surfaces, and dis- tributed through the adjacent material. 4. Radiation. — This factor determines the amount of energy which can be dissipated with a given temperature rise. With considerable mass in the contact, there is a greater radiating surface available for dissipating the heat. 5. The Surface of the Contact. — This surface should be clean and free from the oxide scale which forms when the arcing takes place in the air. This oxide is a non-conductor and interferes with the passage of current. Arcing under oil results in more or less of a carbon deposit which is a conductor, although it may not be as good a conductor as the original material. Usually the design of contact will give a small amount of wiping action which cleans the contact surface when they are being closed. This sliding or wiping action wears away the contact, and should be limited to a very small motion. Contacts are usually of three general types : 1. The "Butt" Contact. — An example of this is the lami- nated copper brush. 2. Sliding Contact. — An example of this is the drum controller using fingers sliding on a cylindrical surface. 3. Rolling Contact. — This is the form of contact that has given such good service on magnet contactors. Contact is made at the tip and rolls down the surface of the contact to the heel. (See Fig. 20.) During this rolling action there is always a slight amount of sliding action which causes DESIGN DETAILS 43 enough abrasion to keep the surface clean. Only a very small amount of sliding action is desirable. It is very diffi- cult to weld this form of contact when properly designed, as the rolling action creates a very powerful leverage for sepa- rating any spots which may become fused during closing. This form of contact has been used for years on railway equipment and for steel mill controllers. Its advantages can be summarized as follows : ( 1 ) The current is carried at the heel of the contactor. This is kept clean by a slight sliding motion during the closing period. The contact at this point is under the maximum spring pressure. (2) The arcing takes place at the tip of the contact, as this is the last part of the contact to separate. (3) The rolling action minimizes the bounce upon closing. (4) Little chance for the contacts to become welded. The slight bounce which is always present when contacts are brought together with any speed causes a small arc to be drawn during the closing of the contact. With the rolling action the surface is quickly shifted from the tip to the heel under a very powerful leverage action, so that there is little opportunity for this arc to weld the contacts together. (5) The absence of any considerable sliding action pre- vents the contacts from sticking if the surfaces become roughened. (6) Heavier pressures can be maintained between the contact surfaces than where sliding contacts are used. The pressure is limited to about lO lbs. per lineal inch for slid- ing contacts, on account of the cutting action. The design of contacts to rupture current depends upon the following : 44 CONTROLLERS FOR ELECTRIC MOTORS 1. Shape of contact. 2. Size of contact. 3. Material from which contacts are made. 4. Direction of the arc. 5. Separation of contacts when opened. 6. Speed of opening. 7. Arrangement of arcing box and shape of magnetic field when blow- out is used. All of these items have a bearing on the maximum rup- turing capacity of the contact and its durability under severe service. Exact information is not available to enable the engineer to predetermine the proper design of magnetic blowout without some experimenting. At present, this knowledge is largely a matter of classified experience and some considerable test data. It is hoped that in the near future, some exact information will be available on this subject. AKCING When two surfaces which are carrying current are sepa- rated, the intervening space is ionized, which makes it a conducting gas. The resistance which the gas offers to the passage of the electric current depends upon the dimensions of the arc, the current being carried between the surfaces by the flow of ions. The usual method of extinguishing this arc is to establish a magnetic field at right angles to the flow of ions. The magnetic field then forces the ions outward away from the contact and extinguishes the arc. This ac- tion is known as a magnetic blowout. In addition to the flow of ions, there are stray ions which are set in motion by the static field due to the difference in polarity between the two separate contacts. If the space between the contacts is small relative to the voltage, this static field may reestablish the arc by causing a flow of these stray ions. Where two sets of contacts are used in series for rupturing an arc, one contact member of each set is dead after the arc has been DESIGN DETAILS 45 extinguished, which materially decreases the static field between the contacts, and an arc is much less likely to be reformed. This is the reason that two separate breaks in series are less likely to be reestablished than when only a single break is used. The arc has a tendency to move vertically as the hot gasses rise. The most natural way to extinguish an arc is for the magnetic blowout to force it in a vertical direction. If, however, the magnetic blowout is properly designed and of sufficient strength, the arc can be blown out with success, in any desired direction. Difficulty is sometimes experienced with large contactors in rupturing small currents, when the arc is not blown in the vertical direction, as a small current value gives a very weak magnetic field. MAGNETIC BLOWOUT A magnetic blowout is used for rupturing the arc which is established when contacts are separated under load. The proper design of this blowout is very important. The mag- netic field must have the correct strength and distribution to reduce the wear on the contacts. The arcing box must be of the correct design to prevent the arc from flashing over to other parts of the apparatus. Investigations have been made using arc splitters in the arc box. The function of these splitters is to increase the length of the arc without having it project much beyond the edge of the arc box. They also cool the arc, which materially assists in rupturing it. The shape and size of the arc when using these splitters is very different from similar designs without the use of these splitters, and it is probable that many contactors, par- ticularly the larger ones, will be provided with arc splitters in the future. The material of which the box is made has an important effect on the arc. Unless the material is very refractory, it 46 CONTROLLERS FOR ELECTRIC MOTORS will be rapidly destroyed. If it contains a fusible flux, this flux will appear on the surface next to the arc and may assist in maintaining the arc. In some of the larger con- tractors a molded composition is used for the arc box and carborundum is molded into the box opposite the contacts to give increased refractory substance. RATING The method of rating contacts depends entirely upon their service requirements. Two general methods are followed : 1. Rating on a basis of a continuous ampere capacity. 2. Rating on a basis of durability. Where the load on the contacts is continuous, the temper- ature rise under load conditions is an important item. In many cases, however, the wear on the contacts is severe and a much larger contact is used than would be necessary for continuous carrying capacity. OPEBATING COILS The temperature of operating coils on magnetic con- tactors is affected by 1. Variation of line voltage, and by variation in frequency on A.C. circuits. 2. Variation of the air temperature, ventilation, moisture in the air, etc. 3. Variations in the load on the current parts of the contactor. Heat is readily transmitted from one part of the contactor to another, so that local heating in some other part of the contactor may materially affect the temperature of the operating coil. Sometimes the use of a small lead wire to a contactor will make considerable difference in the temperature of the contacts and operating coil. The rating of contactors or other control apparatus is affected by an enclosing case or cover. Any restriction in ventilation increases the temperature and this is particularly true for control apparatus where the speed regulation of DESIGN DETAILS 47 the motor is obtained through the sue of resistors. Care should always be exercised in locating heat-producing appa- ratus, so that proper ventilation will be obtained. REGULATION A variation in line voltage has a bad affect on control apparatus. A decrease in voltage may cause difficulty in the closing of contactors or the operation of other magnetic devices. An increase in voltage will cause overheating in shunt coils. If a motor is operating at a given voltage and this voltage is suddenly decreased below the counter e.m.f. voltage of the motor, the current through the motor will be reversed and generator action take place. A small difference of voltage is sufficient to cause a heavy flow of current if the motor is connected directly to the line with- out external resistance. This not only causes a jolt in the motor, but it is very bad on the drive. If gearing is used, the back lash in the gearing may cause a serious blow to the teeth, and in many cases, gears have been stripped in this way. If a chain drive is used, the results are even worse. Successful operation of electric drive depends upon a reason- able steady voltage; rapid fluctuations of the voltage are particularly detrimental. CHAPTER IV HOW TO READ CONTROLLER DIAGRAMS The present discussion is intended to deal, in an elemen- 'tary manner, with a few simple forms of controllers, in an endeavor to explain some of the fundamental principles of operation to those who are not well versed in the art. A thorough understanding of this section will be of material Regiilating Resistor^ L- L+ Shunt Field Series Field oenea rieia j^— ^ WV^- (^ Fig. 21. Elementary Controller Dlagram of face-plate rheostat. assistance in following subsequent discussions of more com- plicated commercial forms of controllers as used in various industries. 48 HOW TO READ CONTROLLER DIAGRAMS 49 FACE PLATE CONTROLLERS The face plate controller is the simplest type used for starting or regulating the speed of an electric motor. Fig. 21 illustrates the elements of this controller. While this arrangement is operative, commercial apparatus usually has additional features, which in this instance are omitted for the purpose of clearness. L -\- and L — • represent the two power wires leading to the controller and a compound- wound motor. If the rheostat arm be moved from the off position, shown in the diagram, to the contact R-^, current will flow from Z, -|- to the arm, from this to contact i?i, through the regulating resistance to R-^^^, thence through the armature and series field of the motor to L — . The shunt field is connected from R^ to L — , and is energized as soon as the rheostat arm makes contact with R-^. The voltage across the armature will be equal to the line voltage minus the voltage drop through the regulating resistor. The torque of the motor will be proportional to the arma- ture current and the field strength. When the contact is first made at T?-, the field strength is zero, and it takes a short interval for the field to reach its full value, so that under ordinary conditions the torque will increase gradually from zero to a value which will start the mo- tor. The rotation of the armature in the mo- tor field generates a voltage known as the counter e.m.f. which op- poses the line voltage. As the motor increases in speed, the diiference between the line and counter e.m.f. becomes less and the motor current Fig. 22. Starting Rheostat, Of face-plate type showing grid resistors. so CONTROLLERS FOR ELECTRIC MOTORS decreases until a balanced condition is reached. When this balancing condition is reached the speed of the motor may be further increased by moving the rheostat arm to contact R.^. Additional increments of speed are obtained by additional movements of the arm to other contacts until all of the regu- lating resistance is eliminated from the circuit and the arm Resistor ivyvvvvvvvT--, Low Voltage Release Magnet Shunt Fidd rW\AA- Series Field Fig. 23. Diagram of Starting Rheostat, Of the type shown in Fig. 32. rests on contact iSjQ. The arm should be allowed to remain on each contact until the motor reaches its balancing speed for that step of the resistance, so that the minimum amount of current will be taken by the motor. In bringing the motor to rest, the reverse operation of the arm is made. In passing from contact R^ to the ojf HOW TO READ CONTROLLER DIAGRAMS SI position, the connection between the motor and L -{- is in- terrupted, causing the motor to come to rest. The shunt field, however, will still be connected across the armature of the motor, including the regulating resistor. This con- nection should be used wherever possible, as it allows the Armature Regulating Resistor F egi lat immil] V\AA^ — (a^ Fig. 24. Diagram of Starting and Regulating Rheostat, Having both armature and field resistors. field current to die down gradually as the speed of the motor decreases. The drop in voltage through the starting re- sistor, with only the field current flowing, is so small that it may be neglected and the field can be considered as having a voltage equal to the counter e.m.f. of the motor. The shunt field winding consists of a large number of turns of fine wire. Any change in the value of the field current 52 CONTROLLERS FOR ELECTRIC MOTORS is opposed by the self-induction of this winding, so that a change in the current should be made gradually. If an attempt is made to open the field circuit abruptly, the self- induction will cause a high voltage to build up between the terminals of the field coils, which may result in the break- ing down of the insulation. If the rheostat is to be used for starting purposes only, the resistor is made of less current-carrying capacity than for regulating purposes. It is called a " starting rheostat," or a " regulating rheostat," depending upon the purpose for which it is used. The connections, however, are the same, the difference being only in the capacity of the resistor. A commercial design of starting rheostat is shown in Figs. 22 and 23. This rheostat difi"ers from the one previously de- scribed, in the addition of the low-voltage release magnet. The rheostat arm is provided with a spring, which returns it to the ojf position if the handle is released during the starting of the motor. After the motor has been brought up to speed, and the rheostat arm rests upon contact R^-^, the low-voltage release magnet holds the arm in this posi- tion. Brush B bridges between the terminals M and N, so that in the running position the current passes from Z, -|- to terminal M, through the brush B to terminal A'^, thence to the armature of the motor and through the series field to L — . This provides a parallel circuit to the one through the rheostat arm to contact iJ^j, so that the continuous flow of current will not overheat the rheostat arm and its con- tacts. In the running position the rheostat arm is held firmly by the low-voltage release magnet, so that current flows from L -j- through the rheostat arm to point P on the magnet. One circuit then passes through the magnet wind- ing to L — . The other circuit is connected to the shunt field. If, for any reason, the line wires are disconnected or the voltage on the circuit fails, the low voltage release magnet will be de-energized and the spring will return the rheostat arm to the starting position. HOW TO READ CONTROLLER DIAGRAMS 53 A controller provided with both armature and field regu- lating resistance is shown in Fig. 24. The motor is known as an adjustable-speed motor, and can have its speed changed by adjusting its field strength. The rheostat arm is made in two parts, the under part making contact with the segments marked R^ to 7?i2 and with the contact ring E, while the top arm engages the upper row of round con- tacts. When starting the two arms are held together by a Fig. 25. Typical Reserving Drum Controller. latch. The bottom arm is provided with a notched segment engaging a plunger forming part of the low-voltage release magnet. The notched segment and pawl hold the arm in any operating position after the low-voltage magnet is energized. To start the motor, the contact arms are moved from the off position to contact 7?^. The current flows from L -{- 54 CONTROLLERS FOR ELECTRIC MOTORS through the arm to contact R-^, thence through the arma- ture regulating resistor to contact i?i2, and then through the armature and series field to L — . The shunt field current flows from L -\- through the arm to the segment E, to the field windings and thence to L — . Connected with i?i is a shunt circuit passing from the positive side of the line through the low-voltage release magnet to the negative side of the line. The arms are gradually moved to the right, eliminating successively each section of the armature resistor until the bottom arm makes contact with R-^^. In this posi- tion the armature is connected directly across the line and the segment E disconnected from the rheostat arm. The shunt field circuit now is from the positive side of the line through the upper rheostat arm to the right-hand field con- tact F-y2, thence to the field winding. This gives a motor speed due to full field strength. If it is desired to increase the speed of the motor, the upper arm can be moved to the left across the field contacts to insert resistance gradually in the shunt field circuit and thus within its range give the increased speed required, while the low-voltage release magnet holds the lower arm on contact R-^2- If the circuit is interrupted, the low-voltage release magnet will allow the lower arm to be carried to the ojf position by means of its spring. It, in turn, picks up the upper arm and the two are moved quickly to the ojf position. DEXIM: COMrTROLLERS A drum type of controller is shown in Fig. 25. Such a controller consists of two rows of contact fingers attached to the frame work of the controller, but insulated from it so as to be electrically separated from each other. Between these rows of fingers is mounted an insulated cylinder or drum, which is revolved by the handle. On this drum are mounted copper segments of different lengths which engage the contact fingers. The length and location of these seg- HOW TO READ CONTROLLER DIAGRAMS 55 ments are such as to make different connections for each "notch" of the controller. A drum, the controller box with contact fingers in position and cover are shown in detail In Fig. 26. Attached to the drum shaft at the top is a wheel Fig. 26. Industrial Dkum Controller. Showing drum, position of fingers in controller box and cover. having notches corresponding with each of the operating positions of the controller handle. A roller is forced into one of these notches by a spring whenever a set of contacts is properly engaged, thus indicating to the operator the correct running positions of the controller and preventing motion from any of these positions, due to vibration or other accidental means. Fig. 27 shows the segments of such a drum as they would appear if rolled out flat. The two vertical rows of circles represent the stationary contact fingers. The horizontal strips represent the segments of the rotating drum, and the vertical dotted lines show the 56 CONTROLLERS FOR ELECTRIC • MOTORS position of the segments with respect to the controller fingers at each successive position of the drum. A slip-ring motor control arrangement with the controller connected only to the secondary circuit of the motor is shown in Fig. 28 with the drum rolled out or " developed" as in Fig. 27. When the primary of the motor is connected to the power line, current passes through the secondary wires, and thence through the resistor, completing the Fig. 27. Diagrammatical Representation Of drum and contact fingers rolled out flat or developed. circuit. When the motor is at zero speed the controller drum should be in position /. If the cylinder of the drum is now moved from right to left, the dotted line 2 travels over to the center line of the contact fingers and the resistor section E to E-^ is short-circuited, decreasing the resistance in part of the secondary circuit of the motor. As the speed of the motors increases, a further movement of the drum will cause the vertical line j to intersect the contact fingers. This will short-circuit the resistor section from D to D-^. At each increase of the motor speed a further movement of the drum may be made until the vertical line zj intersects the controller fingers. In this position all of the resistor is short-circuited and the motor is operating at full speed. HOW TO READ CONTROLLER DIAGRAMS 57 rJ > O 'O « - ■Z. T3 o c U S Pi M o s so bo C A drum controller diagram similar to that shown in Fig. 28, except that it provides for reversing the direction of rotation of the motor, is gievn in Fig. 29. One motor terminal marked C is connected directly to the line. The other two terminals of the motor, marked A and B, are con- nected to correspondingly marked terminals of the con- troller. In the forward direction, the drum segments on 58 CONTROLLERS FOR ELECTRIC MOTORS t-S J J J #41 n .^ 23 no a ^ the right-hand side of the diagram move toward the left- hand row of fingers, and the segments on the left-hand side of the diagram move toward the middle row of fingers. This will be understood if the developed diagram showing the drum contacts is replaced so as to fit on the surface of a cylinder or drum and the contact fingers marked on two vertical sticks of wood mounted on each side of the cylinder HOW TO READ CONTROLLER DIAGRAMS $9 1 80 degrees apart. When the drum segments are moved from left to right for forward operation, the terminal A of the motor is connected through finger A-^, and the second and third segments from the top, which are connected together as indicated, to Zj power wire. Likewise, the terminal B of the motor is connected to Z-j power wire. The arrangement of the drum contacts for short-circuit- ing the secondary resistors differs somewhat from that shown in Fig. 28. The first notch in the forward direction closes the contacts to the primary of the motor A to Z-j and B to Z,2. A drum segment is brought in contact with the finger marked D^ on this notch, but as no other connection is made to the resistors this contact causes no change in the secondary resistance. The motor, therefore, starts to rotate at its minimum speed with all resistance in the secondary. When the drum is moved over until dotted line 2 intersects the central row of fingers, the one marked E-^ is connected to the drum, short-circuiting the section of resistor between Z?i and El. Next the dotted line marked j intersects the central row of fingers and E^ is connected to the drum, short- circuiting another section of the resistor. This sequence is continued until the dotted line 8 intersects the central line of fingers connecting Eg to the drum. A further movement of the drum causes the dotted line p in the right-hand part to intersect the left-hand row of fingers connecting resistor E^ to the drum. The dotted lines 10 to 75 successively in- tersect the left-hand row of fingers, gradually short-circuit- ing all of the resistor, which brings the motor up to full speed. The reverse direction of operation causes the drum seg- ments to move from right to left. In this case the left- hand dotted lines are brought into contact with the left- hand row of fingers and the right-hand row of dotted lines into contact with the middle row of fingers. The primary 6o CONTROLLERS FOR ELECTRIC MOTORS terminal A of the motor is connected to line Lj and the terminal B of the motor to line L-^, when the dotted line i intersects the middle row of fingers. A further movement of the drum from right to left causes the dotted lines 2 to 8 to intersect successively the central row of fingers. This short-circuits a part of the starting resistors. Further movement of the drum from right to left causes the contact shown on the dotted line p to intersect the finger F^. A further movement brings the dotted lines 10 to 75, inclusive, so that they successively intersect the left-hand row of fingers. This short-circuits all of the resistor and brings the motor up to speed in the reverse direction. MAGNETIC CONTACTOR CONTROL A simple starter of this type is illustrated in Fig. 30. The controller consists of a slate panel, at the top of which is mounted a knife switch and two fuses for overload pro- tection, with four contactors underneath. A contactor is a switch which is held in the open position by gravity and closed by a magnet. Fig. 31. Contactor i. Fig. 30, is pro- vided with a blow-out coil, as it opens and closes the main motor circuit. Contactors 2^ j and 4 are used for short- circuiting the starting resistor sections, which are mounted at the rear of the panel. This arrangement is shown dia- grammatically in Fig. 32. At the left is a scheme of main connections. Each set of parallel lines represents a con- tactor. The numbers opposite these parallel lines are the same as shown in Fig. 30. The single loops represent the series coils for contactors 2 and j. An arrow is drawn between the parallel lines representing these switches and the loop representing the coil for each particular con- tactor. Underneath this scheme of main connections is shown a table called " Sequence of Switches." This table has four vertical rows in which circles are drawn. The first HOW TO READ CONTROLLER DIAGRAMS 6l row represents the first start- ing position of the con- troller, and the last row the running position of the con- troller. Where a circle is shown opposite a switch number it indicates that that contactor is closed. This table is used as follows : In the first vertical col- umn, opposite I, is shown a circle which indicates that the contactor i is closed. This operation connects the shunt field of the motor from the point B to the nega- tive side of the line. This arrangement of shunt field connections is the same as previously explained. Now refer to the schematic dia- gram and trace the main cur- rent from the positive side of the line through contactor / and the coil of 2, to R-^ of the resistor, through this resistor to the A^ terminal of the motor, through the motor armature and series field, to the negative side of the line. Referring to the table, in the second column contactor 2 is also closed, the path of the current being from the positive side of the line through i and 2 contactors, the coil of contactor j, to R^ on the resistor, and from there it follows the same path as for the first column. The third column of the table shows that contactor j is closed. The current now Fig. 30. Magnetic Contactor Control Panel. 62 CONTROLLERS FOR ELECTRIC MOTORS passes to R^, as previously described, through one section of the resistor to R^, from here to ^i on the motor arma- ture, and through the armature and series fields to the nega- tive line. The fourth column of the table shovi^s that con- tactors I and 4. only are closed. The path of the current then is from the positive line through switch / to B, then through switch 4 io A-^, through the armature and series field to the negative line. This connects the motor directly to the line, without any external resistance, and is the full- sped position of the controller. Referring now to the main diagram, Fig. 32, the mag- netic contactors are represented by a circle and the operat- ing coils are shown inside of the circles. The moving con- tact is represented by the bottom of the two parallel lines. The upper parallel line is the sta- tionary contact connected to the upper terminal of the switch. When the coil is energized sufficiently to attract the armature, the two par- allel lines are brought together and current can flow between the top and bottom terminals of the switch. Underneath contactor i is a small switch, indicated by square dots, and a pivoted arm. When i is open, this auxiliary switch, known as an " interlock," is also open. The two contacts connected to the arm of this interlock are joined together elec- trically and to the hinge joint, so that they complete the circuit be- tween the stationary contacts 10 and 700 and the pivot, when i closes. To the right of the diagram is a rectangle enclosing two push-buttons. The button marked start is held in the Fig. 31. Contactor Used on control panel of Fig. 30. HOW TO READ CONTROLLER DIAGRAMS 63 open position by a spring and the stop button is held in the closed position by a spring. If the start button is depressed for a moment, current flows from the positive line through the knife switch and fuses to the coil of switch I, through this coil to the terminal i of the stop Control Panel (Rear View) Scheme of Main Connections starting resistance E*' JWUWL-A R2I R3 .^4 Push Button Station Series Compensating Field VWSlAjV-(a^ Fa Fi Fig. 32. Diagram of Connections Of controller shown in Fig. 30. button, to terminal 10 and through the start button to the negative side of the line. This energizes the coil of con- tactor I and closes the main contact. This coil is now connected from terminal / through the stop button to ter- minal 10 on the interlock underneath this switch to the negative side of the line, which makes the circuit to the coil I independent of the start button, so that this button may 64 CONTROLLERS FOR ELECTRIC MOTORS now be released. The current now passes from the positive side of the line through the contact i and across to terminal B on 2, through the coil of this contactor to R-^, through the starting resistor to R^, thence to the bottom terminal of ^ to ^1 on the motor, through the armature and series fields of the motor to the negative side of the line. As the motor increases in speed the current through the coil of 2 de- creases until this contactor closes. The contactor operates on what is known as the "lockout" i^rincinle. i. e.. it closes when its current is below a set v much lower value. When 2 closes tion of the starting resistor from t the contact on 2 to the terminal C, i?2 on the resistor. This again inc motor and prevents j from closing creased to a fixed value. When j c» section of the resistor between i?3 A resistor section between Rn and R^ \ The coil of contactor /^. is in shunt! the motor. The circuit is from \^ terminal 100 on the interlock underneath 7, through this interlock to the hinge joint which is connected to the nega- tive line. Since the A^ terminal of the motor armature is connected to the negative side of the line, the voltage across this coil is equal to the counter e.m.f. of the motor. When the speed of the motor reaches the proper value, contactor /j. closes, connecting the A^ terminal of the motor directly to L -\- through the contact on i. To stop the motor, push on the button marked stop, thus opening the circuit between terminals / and 10, and discon- necting the operating coil of contactor / from L — . This opens contactor / and disconnects the motor from the posi- tive line. The opening of switch i opens the contacts 10 and 100 on the interlock underneath, thus disconnecting the coil of contactor 4. from its motor armature circuit and leav- ing the equipment ready for operation again. CHAPTER V METHODS OF ACCELERATING MOTORS In starting a motor from rest and bringing it up to full speed, resistance is inserted in the armature circuit of a direct-current motor, or the rotor circuit of an induction motor, to limit the current. This resistance may be short- circuited gradually by a manually-operated controller, as explained in Chapter IV, or the resistance may be short- circuited automatically as the speed of the motor increases. There are several methods of short-circuiting this resistance automatically, as follows : 1. Counter e.m.f. method. 2. Series relay method. 3. Series lock-out switch method. 4. Time element method. Sometimes a combination of several of these methods is used in one controller. The present article explains the fundamental principles involved in these different methods of automatic acceleration. COUNTER E.M.F. METHOD This method has been developed for use with direct- current motors only, and is commonly used with shunt, or standard compound-wound motors. When a motor is started from rest and accelerated to full speed, the voltage across the rotor terminals increases as the speed of the motor increases. If the coil of a magnetic contactor is connected across the motor brushes, the current in this coil will in- crease as the speed of the motor increases. By adjusting the air-gap in the magnet, the contactor may be made to 65 66 CONTROLLERS FOR ELECTRIC MOTORS close at a fixed voltage across the motor brushes. The clos- ing of the contactor can be made to short-circuit a section of the armature resistance. By adjusting several contactors to close at different armature voltages, the series steps of the starting resistance can be short-circuited and the motor brought up to full speed. A simple diagram with one step of armature resistance, and one magnetic contactor for short-circuiting this resist- Push Button Start Stop Sequence of Switches Sw. Run 1 1 O O 2 O Fig. 33. Simplified Diagram of Connections, Using the counter e.m.f. method of accelerating motors. ance, is shown in Fig. 33. Several steps of starting resist- ance could, of course, be used with a contactor for short- circuiting each section. The closing of switch i, which is operated by a push- button, connects the motor to the line in series with the starting resistor. One end of the operating coil of this switch is connected to the negative side of the line, and the other end is connected through the pushbutton to the posi- tive side of the line. The coil of switch 2 is connected across the brushes of the motor armature, and will close the switch when the counter e.m.f. of the motor reaches a pre- determined value. The closing of this short-circuits the starting resistor R-^R^, and places the motor directly across the line in the regular operating position. With the elementary arrangement shown, if switch i is opened by pushing the stop button, switch 2 will not open METHODS OF ACCELERATING MOTORS 67 immediately, as it will be held in by the counter e.m.f of the motor. With most commercial switches a counter e.m.f. of 25 per cent, of the full-speed value will hold the con- tactor closed. Under this condition, when the motor speed has been reduced to one-fourth full speed, and with con- tactor 2 still in the closed position, the start button can be pushed, thus closing line switch i, with the result that the motor will be connected directly across the line, without any starting resistance, and may cause a severe jar to the motor and machinery which the motor drives. In order to avoid such a possibility, in commercial controllers an interlock is usually provided on switch i, which opens the current of the coil on switch 2 whenever switch i is opened. The advantages of this method of acceleration consist in its simplicity, since the switch does not need any additional parts, and no auxiliary relay or other devices are required. Disadvantages arise where there is a considerable varia- tion in the line voltage. An increase in line voltage will cause the contactor to close sooner than it should, and a drop in line voltage sometimes prevents the contactor from closing. These, however, are extreme cases. With a rea- sonable system of power distribution, especially if the power circuit is used for lights the variation of voltage will be small, and no trouble should be experienced. Another dis- advantage may be caused by a change of adjustment, due to a change in temperature in the operating coil of the con- tactor. With a properly designed contactor, however, changes in the coil temperature will not cause trouble. Where several contactors are to be installed it is often necessary to furnish different coils, in order that adjust- ments can be made over the wide range of voltage necessary for the operation during acceleration. Interlocks are used for dropping out all but the last switch, in order to protect the low-voltage coils from overheating. A modification of the connection shown in Fig. 33 is often 6 68 CONTROLLERS FOR ELECTRIC MOTORS' used, in order to keep all of the coils alike, and eliminate the interlock on the last switch. This arrangement is shown in Fig. 34. The operating coils of all contactors have one side connected to the motor brush farthest away from the starting resistor. The other sides of the operating coils Fig. 34. Part of Diagram of Connections, Showing three contactors for regulating acceleration by the counter e.m.f. method. are connected to the taps on the starting resistor, the coil on switch i being connected to R^ on the resistor. The voltage on this coil is equal to the line voltage, less the drop in voltage through the first section of the resistor. As the speed of the motor increases, the counter e.m.f. causes a de- crease in the armature current. This reduces the drop through the first section of the starting resistance. The voltage on the operating coil of switch / is gradually in- creased, until this switch closes. Switch 2 has its operating coil connected to R^ on the starting resistor. The voltage on this coil is increased by the closure of switch i. The increase in current, however, at this instant, causes a con- siderable drop in the second section of the starting resist- ance. As this current gradually decreases with the in- creased speed of the motor, switch 2 closes. Switch j is connected across the motor armature, and closes when the counter e.m.f. of the motor is nearly equal to the line voltage. METHODS OF ACCELERATING MOTORS 69 ACCELERATION BY SEBIES RELAY METHOD There are a number of different schemes for using a series relay to control the acceleration of a motor. The principle involved in all of these schemes is a relay having a series winding which holds the relay contacts in the open position when the current exceeds a predetermined value. When the current is reduced sufficiently, the relay arma- ture completes the circuit to the shunt coil of a magnetic contactor. This method of acceleration can be used for either alternating or direct-current motors. The arrange- ment most common in industrial applications consists of a series relay for each magnetic contactor. The relay con- tacts are held open mechanically until the electric circuit is Sequence of Switches Sw RunJ 1 2 Using Fig. 35. Diagram of Connections, a series relay to regulate acceleration of the motor. closed with the maximum resistance in series. The relay armature is then released and allowed to drop when the current is reduced to the value for which the relay is set. The dropping of the armature completes the circuit for the operating coil of a magnetic contactor, which short-circuits a section of the starting resistance. A simple form of this type of control is shown in Fig. 35. Switch I is controlled by a pushbutton in the same way as in Fig. 33. This contactor is provided with a series 70 CONTROLLERS FOR ELECTRIC MOTORS relay mounted directly beneath the switch, whose contacts are connected to the positive line and through the operating coil of switch 2 to the negative line. When the relay arma- ture is released, these contacts are connected together, thus causing switch 2 to close. When switch / is open, the con- tacts of the relay are held in the open position by a spring. When switch i closes, it releases this spring by mechanical means, so that the contacts may close. The current, how- ever, in the series coil holds the armature in the upper or open position until the current has been reduced to a pre- determined value. The armature then drops and its con- tacts are closed. This will not occur until after the motor has approached full speed, so that when switch 2 closes and short-circuits the starting resistor the increase in current will be limited. Several sections of armature resistance may be used with switches for short-circuiting each section, each switch being controlled by a series relay mounted on the preceding switch in the manner described. The advantages of this method of acceleration are : 1. The sections of the starting resistor are short-circuited in direct pro- portion to the motor current. 2. This method is not affected by variation in line voltage, providing there is sufficient voltage to close the magnetic contactors. 3. The adjustments for closing are not affected by the heating of the coil. 4. This method limits the load under which the motor will start. If the load is too great to allow the motor to accelerate sufficiently to reduce the current to the predetermined value the relay will not drop and close its contacts, and therefore the starting resistance will not be short-circuited. The disadvantages of this method are : 1. This method may result in too rapid an acceleration of the motor under light loads. 2. Additional apparatus is required, viz., a relay for each resistance contactor. 3. The motor may fail to start under overload. This was given as an advantage, but in some cases it may be a disadvantage, depending upon the application. METHODS OF ACCELERATING MOTORS 71 Magnet Yoke Magnet Core Operating Coil Brass Stop This method of acceleration is the most reliable for heavy service and frequent operation. SERIES LOCK-OUT SWITCH METHOD The series lock-out method of acceleration also depends upon the value of the armature current. It differs, how- ever, from the preceding method, in that the magnetic con- tactor is provided with a series coil, and does not require a separate relay for controlling it. The closing of the mag- netic contactor depends upon the saturation of the iron in one portion of the magnetic circuit. This can be under- stood from the diagram of a contactor of this design, shown in Fig. 36. The flux or mag- netism in the iron is caused by current flowing through the operating coil. This flux passes through the air-gap to the armature of the contactor. Part of this flux passes from the armature through the arma- ture bracket to the magnet yoke, and thence to the magnet core. Another part of the flux passes from the armature through the tail piece to the magnet yoke. The flux through this last circuit exerts a pull which prevents the contactor from closing. The magnetic path through the armature bracket has a small cross-section, so that when the current flowing through the operating coil exceeds a certain value it becomes saturated and forces the balance of the flux through the tailpiece, holding the contactor open. As the current decreases, the flux in the saturated armature bracket remains constant, but the flux through the tailpiece de- creases until it is not sufficient to hold the contactor open. Tailpiece tock-Out Air-Gap Iron Calibrating Screw Fig. 36. Diagram of Series Lock- out Magnetic Contactor. 72 CONTROLLERS FOR ELECTRIC MOTORS The switch can be adjusted to close at a predetermined cur- rent value by changing the hold-out air-gap between the tailpiece and the magnet yoke. This air-gap is adjusted by means of a calibrating screw. The greater the air-gap at this point, the higher the current value at which the switch will close. When the circuit is first completed through the operating coil there is danger of the switch closing before the flux in the tailpiece is sufficient to lock it open. This tendency is overcome by placing a heavy copper damping coil around a portion of the armature bracket. When the operating coil is energized, this coil forces the flux to build up in the tailpiece ahead of the armature bracket. The advantages of this method are the same as for the preceding method, with the additional ad- vantage that less apparatus is required. The disadvantages are the same as the preceding method, with the additional disadvantage that the switch may not close if the current decreases too rapidly during accelera- tion. This sometimes happens when the motor starts with a light load. If the current should drop to a low value after the motor has reached full speed, the switch may drop open. To correct this latter difficulty a shunt hold coil is sometimes used on the last contactor. This requires a con- nection which will short-circuit the series coil, otherwise the switch would drop open if the motor current should be reversed, due to regeneration. TIME ELEMENT METHOD The apparatus used for short-cifcuiting the starting re- sistance is controlled by a dashpot, or other timing device. Each contactor may be provided with an individual dash- pot, or a small master switch can be used, controlled by a dashpot. Such a master switch completes the circuit to each contactor in turn, with a time interval between, so that the METHODS OF ACCELERATING MOTORS 73 resistance is cut out in steps. Sometimes a face plate con- troller is used, similar to the one shown in Fig. 23, in which the arm is moved by a magnet and retarded by a dashpot. A very successful type of time element device consists of a drum controller driven by a pilot motor through worm Fig. 37. Automatic Starter, Showing a time element device consisting of a motor-operated drum controller. gearing. The time of acceleration is adjusted by changing the speed of the pilot motor. The segments on the drum short-circuit sections of the starting resistance. A com- mercial controller of this type is shown in Fig. 37. The circuit to the motor is opened or closed by a magnet con- tactor. The motor-operated drum short-circuits the arma- 74 CONTROLLERS FOR ELECTRIC MOTORS ture resistor during acceleration. The advantages of such devices consist in their simplicity. The acceleration is smooth under all conditions of load, and the motor will start with an overload, as the time element device gradually reduces the starting resistance until the torque of the motor is sufficient to start the load. Excessive torques can be guarded against by a proper setting of the circuit breaker. The disadvantages of the time element system are chiefly due to troubles with dashpots, which have more or less fric- tion, and are hard to keep tight. CONCLUSION The opinion of controller engineers differs somewhat on the proper devices to use for controlling the acceleration of motors. The four schemes, described above, are the ones usually employed. Various combinations of these de- vices are used and other means have also been employed. A good deal depends upon the practical application for which the controller is designed. In general, the counter e.m.f. method is used for small controllers, although it can successfully be employed for quite large units. The series relay method is the most expensive, but very satisfactory, and should be used for heavy service, where the load varies throughout wide limits. The series lock-out switch is satisfactory for starting service where the acceleration is always under load. The last resistance switch is often provided with a holding coil to keep it from dropping out on light loads. Sometimes the last switch is operated by the counter e.m.f. of the motor and the preceding switches on the lockout principles. The time element device is the most satisfactory to use where the voltage varies over wide limits, particularly where rapid starting is not necessary. This device in its various forms, has been employed for a good many years, and at one time was the only means used for controlling the acceleration of a motor. CHAPTER VI STARTING CHARACTERISTICS OF MOTORS WITH DIFFERENT METHODS OF CONTROL The usual method of testing motors and controllers has been to use an ammeter and a voltmeter. These instru- ments gave good average readings, but due to the inertia of their indicating member, only average values could be ob- tained. However, the improvement in the oscillograph and its general adaption to commercial work has made possible the determination of many factors, which are not shown by an ammeter. In some cases even the oscillograph was not rapid enough to indicate excessive conditions of voltage and a spark-gap has been used. Considerable practice is re- quired to obtain good results with an oscillograph and experience is required in reading these results. Investiga- tions of this kind have proven valuable and in many cases, mechanical analyses have been made, explaining in detail the phenomena observed with the oscillograph. DIRECT-CXTRBENT MOTOR The simplest form of motor and control, as well as the oldest, is the direct-current shunt motor with a controller which short-circuits the armature resistance during accelera- tion. When this combination was first used, considerable care was necessary in accelerating the motor, to prevent excessive sparking or flashing. The starter was operated by hand and had a considerable number of steps. ^ This was necessary to cut down the burning on the difi'erent steps 1 Such a starter is shown in Figs. 22 and 23. 75 76 CONTROLLERS FOR ELECTRIC MOTORS and to introduce a considerable time element, so that the operator, if he were careless, would not short-circuit the starting resistance too rapidly. Due to this practice, engi- neers have become accustomed to quite a number of start- ing steps in accelerating these motors. They have based their calculations of the accelerating current peaks on Ohm's law and have neglected a number of factors which enter into this problem. The use of circuit breakers or overload relays without a time element attachment has also tended toward the use of a considerable number of starting steps, in order to prevent their tripping on the overload peaks which occur. The introduction of the magnetic contactor has provided a method of switching electric currents of considerable energy without the rapid destruction of the contact. The contactor can be used in connection with automatic devices for short-circuiting the resistance during acceleration, which eliminate the personal element and prevent careless manipu- lation. The use of contactors for automatic acceleration immediately reduced the number of starting steps, as com- pared with the manually operated starter. A feeling still exists that several steps are needed when starting even small motors. In order to obtain some actual information, a series of tests was made with an oscillograph on a direct- current motor accelerated automatically and belted to a generator of about double the motor size. A record was made of the armature current, the armature voltage, and the field current. In a few cases a prony break was used for loading the motor instead of belting it to a generator.^ The diagram of connections is given in Fig. 38, and Figs. 39 to 46 show results of some of the tests. The internal resistance given in the captions includes the complete resist- ance of the controller and motor armature circuit, also the 2 The results of these tests appeared in the Proc. A.I.E.E., Feb., 1917. p. 233- STARTING CHARACTERISTICS OF MOTORS 77 leads between the motor and controller. It was measured from the -|- to the — terminals of the controller with the starting resistance short circuited. In addition, there was some resistance in the lead wires between the controller and the source of power which will occur in any installation. fWt^ 1 K°^ SEQUENCE OF SWITCHES >w RUN 1 1 O o o 2 o o 3 o < o SCHEME OF MAIN CONNECTIONS Fig. 38. Diagram of Connections of Automatic Control Equipment, Used in tests to analyze the starting characteristics of motors. The 50-kw. direct-current generator belted to the motor represents more inertia than usually occurs in practice. Figs. 39 to 44 cover a period of about two seconds. Figs. 45 and 46 cover a period of about four seconds. SUMMARY or TESTS3 1. It seems unnecessary, with automatic acceleration, to use more than one switch to short-circuit the armature resistor used with small motors except where special requirements are to be met. It is practicable to use one switch with motors as large as 15-hp. for general purposes and operate this switch by counter e.m.f. setting the switch to close at 75 per cent, of normal voltage. 2. If the motor field is zero, or has a small value when the line switch is closed, the starting torque is also zero or has only a small value and it will increase gradually so that the motor, or its load, will not be subjected to a heavy shock or jar when the lost motion in the drive is taken up. ' From the author's A.I.E.E. paper, loc. cit. 78 CONTROLLERS FOR ELECTRIC MOTORS 3. The shunt field of small adjustable speed motors can be reduced in one step under normal load conditions without fear of undue torque or current. This practice can be safely followed with SO-hp. motors and perhaps larger. This covers the usual range of sizes for this type of motor. Most machine tool motors are started light. Under this condition, the motor can be started successfully with minimum field strength and the field relay omitted. This will enable the use of the same controller for constant speed and adjustable speed motors supplying a separately mounted field rheostat for the latter. TIME IN SECONDS Fig. 39. Starting Tests of a 20-hp., 750-R.p.M. Motor Belted to 50-KW. Generation with no Load on the Generator. Resistance 1.35 ohms starting -j- 0.25 ohm internal. The starting resistance was short-circuited in two steps at 120 and 160 volts counter e.m.f. 4. Adjustable speed motors can use one step of resistance for dynamic braking as the change in field strength tends to maintain the braking current constant over a considerable range of speed. 5- The time required to accelerate to 95 per cent, of speed is very short. In these tests the time did not exceed three seconds. During the discussion of these tests, it was pointed out that the results obtained may have been materially affected by a drop in line voltage. It was admitted that some drop in line voltage would probably occur in most installations. The effect that the use of a small number of starting steps would have if the motor and controller were located close to the power house was, however, questioned. In order to determine this point, the writer had a number of tests made STARTING CHARACTERISTICS OF MOTORS 79 with the motor connected to the bus-bars in the power house ; an oscillograph record was taken of the armature amperes, armature volts, and line voltage. The tests showed that 1- 1 0- 200 1 ^LL= — — " — 5 100 f "• — — —— _^_ -T .^ p J Sioo . — — — — — " "^ 1 , — — — " ""^ ■"— — _^_ ^ TIME MSEC ONOS Fig. 40. Starting Tests of a 2o-hp., 750-R.p.M. Motor Belted to a 50-KM. Generator, with no Load on the Generator. Resistance one ohm starting -|- 0.25 ohm internal. The starting resistance was short-circuited in one step at 190 volts counter e.m.f. there was practically no change in the line voltage during acceleration and that the current peaks obtained were about two-thirds of the value, as usually calculated, based upon Ohm's law. The results of these tests are shown in Figs. 47 and 48. In calculating the starting resistance for a shunt motor, the steps are usually arranged in geometric progression. This method is based on the assumption that each step of resistance is short circuited when the motor current has reached a uniform minimum value. This value is that necessary to overcome the torque which the motor is re- quired to develop during the accelerating period. A stan- dard 15 hp., 230 volt, 825 r.p.m. shunt motor during these tests, was accelerated under full load obtained by means of a prony brake. The minimum accelerating current to over- come this torque was 50 amperes. The line voltage was 8o CONTROLLERS FOR ELECTRIC MOTORS 258 volts and the resistance was short-circuited in one step. Following the usual method of calculation, and assuming two equal current peaks, a calculated external resistance of 0.905 ohms would be required in series with the mot9r armature. This would give two equal peaks of 220 amperes each. The oscillograph record of this test, shown in Fig. 47, was obtained with 1. 282 ohms in series with the arma- ture and shows maximum current peaks of 168 amperes at start and 163 amperes when the resistance was short-cir- d 11 — 1 1 — 1 1 l_ 100 J- — -*— 100 ^: ^^ T1H€ IN SECONDS Fig. 41. Starting Tests of a 2o-hp., 750-R.p.M. Motor Loaded with a Prony Brake Set for Full-Load Torque at Full Speed. Resistance 0.725 ohm starting -|- 0.25 ohm intemaL The starting resistance was short-circuited in one step at 125 volts counter e.m.f. cuited. This short-circuit occurred when the counter e.m.f. across the motor brushes was 61.5 volts. This voltage di- vided by 0.268 ohm, which is the internal resistance, would give a peak of 230 amperes. By extending the current peak to the instant when the resistance switch closed, the curve shown by dotted lines in Fig. 47 gives a close check upon the calculated value of current The effect of the armature self induction is shown by the difference between the dotted line and the heavy line. The starting peaks are thus shown to be about two-thirds of the calculated value STARTING CHARACTERISTICS OF MOTORS 8 I using the geometric progression method and neglecting armature reaction. In Fig. 48 is shown the results of a similar test, accelerat- ing with one-half full-load torque applied by prony brake. TIME IN SECONDS U LLU^4-++^ r=— == TIME IN SECONDS Fig. 42. Starting Tests of a 20-hp., 7So-r.p.m. Motor Belted to a jo-kw. Generator, with no Load on the Generator. Resistance 1.3S ohms starting -[- 0.25 ohm internal. The starting resist- ance was short-circuited in one step at 150 volts counter e.ra.f. for the upper set of curves and at 120 volts counter e.m.f. for the lower set. The adjust- ment for the upper curves gives equal current peaks and represents a prac- tical controller. The peak calculated in the usual way would require 138.6 amperes based on a minimum accelerating current of 20 82 CONTROLLERS FOR ELECTRIC MOTORS amperes. The actual peaks obtained were no and 102 amperes, showing the calculated peak to be about 3 1 per cent, in excess of the actual value. The following mathematical analysis has been worked out for calculating the true current peak shown in Figs. 47 and 48.* By taking into account inductance and inertia, it is possible to calculate the true peak current' when a portion of the external series resistance is short circuited. The differential equation for the transient current is di di r , Lj:+ Mj; + Ri + Ki j idt = E} ' dt^ dt The effect of mutual inductance M is very small and can be neglected. On the other hand, the counter torque Te will be considered and assumed constant; hence, Lj-^ + Ri + Ki jidt-K2 f T,dt = E. Since we are considering the case in which the motor has come up to some percentage of full-load speed and a part of the series armature resistance is short circuited, the initial counter voltage of the motor must be added to the left hand side of the above equation giving Ei+Lj-^ + Ri+ R^j idt -K2 f Tcdt = E. The general solution of this equation takes the form -B+VRf-ALKit -R-YBS-iLKit i=Ai+A2e " +A,e in which A-^, A2 and A^ are constants of integration. The value of these in terms of known quantities when the motor * By Mr. A. A. Gazda, who made these tests. ^ Starting characteristiccs of Direct-Current Motors, K. L. Hansen, Proc. AJ.EM., Feb., 1917, p. 272, Eq. 22. STARTING CHARACTERISTICS OF MOTORS 83 is accelerating under a constant resisting torque is Ai = I ^ the initial current. A2 = I(Rs - R)_ ^ Vi?2 _ ^LKi ' ^3 = IjRs - R) Vi?2 - LKi ' Rs = series resistance that is short circuited. R = final resistance. When 4LK1 is greater than R^, the current oscillates around the value required for the constant torque load, and the above general solution becomes 2liR,-R) ( . <^LKi-RH^ ^^LKi - i?2 sm 2L -Bt The values of current represented by this equation were calculated for the period immediately following the closing of the accelerating switch, as shown in Figs. 47 and 48. These values check the oscillographic curve very closely and are shown in Table I. This brings out the feasibility of TIME IN SECONDS Fig. 43. Starting Tests of a 20-hp., 500-1 500-R.p.M. Motor Belted to Two so-Kw. Generators Connected to Give 20-hp. Torque AT 1 500-R.p.M. Resistance 0.76 olim starting -|- 0.34 ohm internal. The starting resistance was short-circuited in one step when the counter e.m.f. was 100 volts. The field resistancce was inserted when the starting resistance was short-circuited, the gradual rise of current being due to weakening the field. 7 84 CONTROLLERS FOR ELECTRIC MOTORS calculating the actual peak current during the acceleration of the motor by means of series resistance. TABLE I Accelerating Current Values Time in Amperes at Half Load Amperes at Full Lo Seconds. Torque (Fig. 48). Torque (Fig. 47)- 0.00 20 SO O.OI 76 120 0.02 104.4 153-5 0.03 1 12.8 166 0.04 I07-S iSi 0.06 83.2 129 0.08 54-2 93 O.IO 330 S6.3 0.123 20 50 0.14 16.5 45-5 0.16 iS-7 45 0.18 16.9 46 0.24 20 50 Time is calculated from the instant that the resistance short-circuiting switch makes contact. I- 2 „ t = 5 "■' ^ - -.00 3 ■^ P^s^ —■ 'N i " 1 ^ ' 'X. . TIME IN SECONDS Fig. 44. Same data as Fig. 43, except a prony brake was used instead of the gener- ator to give full-load at 1,500 r.p.m. This gives a heavier starting torque and less inertia. The starting resistance was short-circuited at 120 volts counter e.m.f. These last tests confirm the writer's opinion that only a small number of starting steps are required in accelerating STARTING CHARACTERISTICS OF MOTORS 55 a modern direct-current shunt motor under ordinary condi- tions. Where a compound or series motor is used, the start- ing torque will build up more rapidly, particularly with the series motor, so that this may prove a limiting condition during acceleration. A number of tests were made upon a reversing planer equipment to obtain a detailed analysis of the different parts of the operation. These curves, one set of which is shown in Fig. 49, proved of considerable interest and value in designing these controllers. ALTERNATING CURRENT SQUIRREL-CAGE MOTORS Under certain conditions an excessive current may be ob- tained in starting alternating-current squirrel-cage motors. ° r r- .^ — C ^ _ _ C 1 . r -^ 5 J , .~_ — '■^ 1 ^ -J _ UJ «' V- L^ ^' cz ■^ ^ _ TIME m SECONDS Fig. 45. Effect of Field Variation on Dynamic Braking, Made with a 15-hp., 400-1600-r.p.m. motor belted to a 50-kw. generator with no load on the generator. When the motor was operating at 1,600 r.p.m. the armature was disconnected from the line and connected to a resistance to give dynamic braking. At the same time the motor field rheostat was short-circuited, strengthening the field to the 400-r.p.m. value. The curves show that the field built up faster than the speed decreased so that the arma- ture voltage at first increased and then remained practically constant for a considerable period. A strong dynamic brake was thus maintained until the motor speed was quite low so that it could be easily stopped by friction or a mechanical brake. ° See paper on " Transient Conditions in Asynchronous Induction Ma- chines," by Mr. R. E. Hellmund, Proc. A.I.E.E., Feb., 1917, p. 205. 86 CONTROLLERS FOR ELECTRIC MOTORS These conditions are not apt to occur in the smaller size motors commonly used. The increasing use, however, of large size motors of this type, particularly with two and four poles, has made it necessary to consider these phe- nomena and shown the importance of analyzing the motor and controller as a unit. Several years ago, a series of breakdowns in the insula- tion of a wound-secondary motor occurred, due to the in- ductive effect between the windings when the secondary circuit was opened before the primary winding was dis- connected from the line. Oscillograph tests at first did not :: 200 ,^ * .. ' ^^ £ 100 * A. ' '~~ 5 ° 9 i^'; ^ e 0- TIME MSEC ONDS Fig. 46. The Same Arrangement as Fig. 45, Except that the motor field was not increased but remained at the 1,600- r.p.m. value, showing that a decreasing torque with decreasing speed will cause considerable drift before the motor comes to rest. disclose this dfficulty but the use of the spark gap showed that approximately five times normal voltage might be ob- tained in the secondary circuit under these conditions. Prior to that time, it was the general belief that an alternat- ing-current motor had very little inductive effect of this kind. The writer believes that there is an opportunity for con- siderable valuable work to be done along this line by uni- STARTING CHARACTERISTICS OF MOTORS ^7 versities and technical schools. Most of their laboratories are equipped with oscillographs and other means for this kind of an investigation. The work is very interesting and I A 280 1 1 „0—-^'^ Armalure Voliage 160 ■L20 60 40 { Tune in Seconds Annamre Curreni \: 240 ■220 •200 IBO ■160 ■140 120 •100 ■80 ■60 , . . J°. U.B UO U (r li ft- Timq in Seconds Fig. 47. Starting Tests of a is-hp., 825-R.p.M. Motor Driving a Prony Brake Set for Full-Load Torque. Starting resistance 1.282 ohms + 0.268 ohms internal resistance. The starting resistance was short-circuited in one step when the counter e.m.f. was 161.S volts. Tune in Seconds Fig. 48. Same Starting Test as Fig. 47, E.xcept that the prony brake was set for one-half full-load torque. The start- ing resistance was 2.062 plus an internal resistance of 0.268 ohm. instructive; in the foregoing description only the more im- portant phenomena have been discussed. An analysis of the curves show that the armature voltage is approximately equal to the line voltage at the instant of closing the circuit. 88 CONTROLLERS FOR ELECTRIC MOTORS The shunt field amperes start at zero and at first have a negative value, probably due to the reactive effect of the armature current. The peak values of the armature current show a round-off due to the reactive effect of the circuit. R m p5 » I ( \ i% ,, Arm ture It \ . k "^ n y- A & li \y If Time m Seconds Fig. 49. Starting Tests of a zo-hp., 250- to i.ooo-r.p.m., 230-voLT Shunt- Motor Operating a Planer. 24-inch tool travel, 500-r-p.iii. cut and i,ooo-r.p.ni. return stroke. Begin- ning at the left, the motor is accelerated for the cutting stroke, then dynamic braking occurs, followed by acceleration for the return stroke. The last loop is dynamic braking from the return stroke. This can be varied by changing the mechanical inertia of the parts or by changing the inductance of the complete circuit. The shunt field is assumed to follow the field cur- rent quite closely, although it undoubtedly does not reach the instantaneous values shown by the curve of field amperes. Further investigation in this line would be interesting. CHAPTER VII METHODS OF SPEED CONTROL AND DYNAMIC BRAKING DIBECT-CTJRKENT VABYING SPEED MOTOKS Variations in the speed of a direct-current motor can be obtained by changing the voltage across the motor brushes. Usually this is accomplished by placing a resistor in series with the armature, as shown in Fig. 50. The drop in voltage through the resistor is equal to the resistance ±,^yirinn_0^;2vJ^,, Fig. 50. Diagram of Connections for Varying the Speed of Direct- Current Motors, Using a resistor in series with the armature. multiplied by the current. If the line voltage remains con- stant, the speed of the motor can be varied by changing either the ohmic resistance or the load. The characteristic curves for direct-current shunt, compound and series motors are shown in Fig. 51. Curve A is for a shunt motor. The difference in speed between full load and no load is caused by the drop in voltage due to the internal resistance of the motor. The amount of change in speed is called the regula- tion of the motor, which may be expressed by giving the speeds at no load and full load or as a percentage change in speed. The shunt motor is assumed to have a constant field strength, and therefore the change in speed is small. If, however, a resistor is placer in series with the armature, 89 90 CONTROLLERS FOR ELECTRIC MOTORS a curve such as A A is secured, which shows considerable reduction in speed at full load, the amount of speed reduc- tion depending upon the resistance in series with the arma- ture. In this case, it can readily be seen that the speed of the motor depends upon the load and will vary with different values of torque. It is for this reason that these motors are called varying speed motors. Torque Fig. 51. Characteristic Regulation Curves of Direct-Current Motors, Using the series resistor scheme of Fig. 50. If part of the field winding is made up of series turns, the field strength will increase with increased torque, so that the difference between no-load and full-load speed is quite marked, as shown by curve B. If external resistance is used, a curve such as BB is secured. This curve can be changed by varying the amount of resistance in series with the armature. Curve C shows the regulation curve of a series motor. Theoretically, at no load, there is zero field strength and therefore infinite speed. It is therefore neces- METHODS OF SPEED CONTROL 9 1 sary to have a definite load on a series motor to prevent its running away and series motors should not be used for any applications where the load is reduced to a very small value. Usually 25 per cent, of full load is required to keep the speed within safe limits. If a resistor is used in series with the armature, curve CC results, which can be varied by changing the resistor. It will be seen from these curves that the series motor is best adapted for speed regulation by the use of series resist- ance, the slope of its curve being much steeper than those for the shunt or compound motor. Where the load may be- come very light at times, it is necessary to either make a special arrangement for energizing the series field, as will be explained later, or use a compound motor. The curve for the compound motor will vary between that of the series and shunt motor, depending upon the percentage of compounding used. With this method of speed control, it is necessary to change the amount of armature resistance to obtain the correct speed with a changing load. This at first sight may Shunt Resistor Series Resistor wmmh-^ Fig. 52. Diagram of Connections for Speed Regulation, Using the combined series and shunt resistance control method. seem to be rather complicated, but in practice the operator can move his controller lever forward or back until the desired speed is obtained. Where it is desirable to regulate the speed of the motor within closer limits, the arrangement of resistors shown in Fig. 52 is used, where RA is a resistor in series with the 92 CONTROLLERS FOR ELECTRIC MOTORS armature and RS is a resistor in shunt with the armature. This is known as a combined armature series and shunt resistance control. If RS has a very low ohmic resistance, the speed of the armature can be held at a low value throughout its range of load. Fig. 53 shows the various curves for this arrangement. Curve / is the armature current. Curve 2 is the line current, and curve j is the shunt current passing through the resistor RS. Curve 4 is the speed-current curve for the motor connected directly degenerative CiuTent Motoring Current Fig. S3. Characteristic Curves of Direct-Current Motors, Using the control scheme of Fig. 52. to the line. It can be seen from these curves that the motor prformance is somewhat similar to a badly regulated shunt motor. At no load, the series field obtains current through the resistors RA and RS so that the speed of the motor has a fixed value. The speed can be changed by varying either RA or RS. Sometimes both are changed simultaneously. In figuring the speeds obtained with this arrangement, the circuit it rather complicated and the problem can be simpli- fied by drawing curves similar to the ones shown in Fig. 53. Two points in these curves can readily be determined. The armature and line current can be found for zero load and METHODS OF SPEED CONTROL 93 for zero speed. The shunt current in resistor RS can be figured from these two values. BIRECT-CURBENT ADJUSTABLE SPEED MOTOBS Neglecting the internal resistance of the armature and assuming that the number of conductors on the armature is fixed, the speed of the motor will depend upon the strength of the field, providing a constant voltage is maintained across the armature brushes. If the motor is shunt wound and a rheostat is placed in series with the field winding the current can be changed and the motor caused to operate at different speeds. The speed of the motor will be practically constant for whatever value the rheostat is adjusted and will not vary under changing loads. For this reason, the motor is called an adjustable speed motor. There will be a small change in speed between no load and full load, due to' the drop through the armature resistance. This change in speed, however, is usually negligible for most applications. The amount of speed range, which can be obtained on a given motor, depends upon the iron in the magnetic circuit. The slowest speed is obtained with the maximum field strength. This field strength, of course, is limited when the iron in the magnetic circuit becomes saturated. The maxi- mum speed is obtained with the weakest permissible field strength. This can be reduced only a limited amount, as sufficient field must remain to give stable operation of the motor. To obtain a wide speed range by field control in- volves considerable expense in the motor and usually a maximum of four to one is all that is attempted com- mercially. For many applications, the combination of armature re- sistance and field control is used. The slow speeds are often required only for short intervals of time or at reduced loads, so that the loss in armature resistance is small. 94 CONTROLLERS FOR ELECTRIC MOTORS DIRECT-CtTRRENT MOTOR VOLTAGE CONTROL SYSTEM If a separate generator is provided for each motor, the speed of the motor may be altered by changing the field strength of the generator. This alters the voltage of the generator and the motor has a speed corresponding to the generator voltage. The generator field may be reversed to reverse the rotation of the motor. Very slow speeds can be obtained and the speed remains fairly constant between no load and full load. This method of control is used prin- cipally for large motors ^ several hundred horse-power, and is particularly adapted for reversing service, such as for large mine hoists and reversing steel mills. The con- troller is small and inexpensive, as only the field current of the generator is manipulated. It is the practice to connect the generator and motor armatures in a closed circuit pro- vided with a single overload circuit-breaker, which is opened only in cases of emergency. Sometimes this arrangement of control is combined with a motor having a two to one speed range, so that higher speeds can be obtained for certain operations. The initial cost of this combination is less than for the constant speed motor, as it does not add a great deal to the cost of the motor to obtain the additional increase in speed, and a smaller generator can be used than if the entire speed range were obtained by varying the generator voltage. ALTERNATING-CURRENT VARYING SPEED MOTORS The relation between the speed and torque of an induc- tion motor having a wound secondary with collector rings is shown in Fig. 54. When the collector rings are short- circuited, the speed of the motor decreases very little from no torque to the maximum torque. When this point is reached, the speed of the motor drops abruptly and con- tinues to decrease until zero speed is reached. If, how- METHODS OF SPEED CONTROL 95 ever, the torque is sufficiently reduced, the motor will again increase in speed until it is very close to full speed. The curve shows that there is a maximum torque, which the motor is capable of exerting and that, if this torque is ex- ceeded, the motor will stop. This maximum torque is usually called the " puUout torque " of the motor. The drop in speed from no load to maximum load is small and compares with that of the direct-current shunt motor. This drop in speed is due to the internal resistance of the motor. A| Bl 1 "N 1 /I J X 1 /I \ ^v^ • "^1 ^ 1 \ ^*^ ^ ^>< \ ^\ ^ /T V. ' \ ^s^ /^j ^ "^ I \ ' ^s/ 1 \^ \ s/ /\ \ / / ^ v 1 \ ' / X. 1 \ / / Nj / V / 1 ^ /! * \/ \ / / X I < / /\ 1 / \ / / \4 1 / \ 1 / / \ 1 . / \i ' / \ i/ \i / / \ / \ / / \ A 1 .Torque Fig. 54. Typical Speed-Torque Curves of a Wound-Rotor Induction Motor, Obtained by varying the resistance of the secondary circuit. If resistance be introduced in the secondary circuit of the motor, the slope of the speed curve is increased, so that the difference in speed between no load and maximum load is considerable, as shown in curve 2. Sufficient resistance can be used to bring the speed to zero at the maximum torque, as shown in curve j. With a still further increase in resistance of the secondary, curve 4 results. These 96 CONTROLLERS FOR ELECTRIC MOTORS curves are typical, and any particular curve desired can be obtained by adjusting the resistance in the secondary cir- cuit of the motor. If a controller is arranged for varying the resistance in the secondary of the motor, the speed of the motor can be adjusted to any required value at a given torque. The speed, however, will change with the torque, and for that reason the motor is called a " varying speed motor." ' When the motor is started from rest it may not commence to rotate on the first notch of the controller, and the operator will therefore move the controller handle to reduce the resistance in the secondary ; this reduction of resistance will increase the starting torque by changing the shape of the curve up to the point of maximum, as shown in curve j. If the resistance is reduced still further, the current will in- crease, but the torque will decrease, as shown in curves 2 and I. It will therefore be seen that care must be exercised in starting these motors under heavy load to prevent reduc- ing the resistance in the secondary beyond the value for maximum torque. The speed of the motor at no load is called the synchro- nous speed. When the motor is loaded the actual speed of the motor is less than the synchronous speed. This differ- ence in speed is called the " slip " of the motor. The curves show that this slip in speed is dependent upon the resistance in the secondary circuit. The torque of the motor is proportional to the square of the voltage. A reduction of 10 per cent, in voltage reduces the torque to 81 per cent, of its maximum value, or the pull- out torque of the motor is reduced from the dotted line B in Fig. 54 to the dotted line A. The speed torque curve of the motor with short-circuited secondary and 90 per cent, normal voltage is shown by the dotted curve 5. Thus the output of a crane or hoist is often seriously affected by poor voltage regulation. It may happen that when the voltage METHODS OF SPEED CONTROL 97 regulation is poor the motor will fail to start its maximum load from rest. If the motor is provided with a high-resistance second- ary, as shown in Fig. 55, curve /, which is a speed torque curve similar to curve j in Fig. 54, the speed of the motor may be regulated by reducing the primary voltage, as shown in Fig. 55, curves 2, j and /j.. This method has been Torque Fig. 55. Typical Speed-Torque Curves on a Squirrel-Cage Induction Motor, Obtained by primary voltage control. employed, but has the disadvantage of giving reduced torques at decreased voltages. Usually the motor is re- quired to exert as much torque at the slow speed as at the high speed, and therefore this method is seldom used. The method in universal use at present for varying the speed of induction motors is to maintain the primary voltage con- stant and change the resistance in the secondary of the motor. There are several other methods of controlling the speed 98 CONTROLLERS FOR ELECTRIC MOTORS of the slip-ring induction motor, some of which are ap- plicable only to large motors. They consist in connecting the slip rings of the motor to some source of voltage supply, the voltage and frequency of which can be varied. These methods of control are not yet in common use. ALTERNATING-CTTBBENT ADJUSTABLE SPEED MOTOB The best-known means at present for adjusting the speed of alternating-current motors is to change the number of poles in the primary. Quite frequently motors are built having two sets of poles, one set giving high speed and one set slow speed. Usually these motors are provided with squirrel-cage secondaries, but slip-ring motors also have been built of this type, particularly for elevator and hoist work. The controller consists of a double-throw switch or its equivalent for changing the connections of the motor primary, so as to give the desired number of poles. Another method, which is equivalent to changing the number of poles, consists in connecting two motors in cas- cade, the secondary of Motor No. 1 Motor No. 2 „ „ ^^ - , . t- ^^ — . ^ ^ — ^ , n nn, the first motor bemg connected to the pri- mary of the second, as shown in Fig. 56. Let Fig. 56. Cascade Connection of Indhc- ^g consider that the TioN Motors For speed regulation. nrst motor IS WOUnd with six poles and the second motor with four poles. These motors can be con- nected together, so that they have a combined speed equiva- lent to the sum or difference of the poles, which would be a ten-pole or a two-pole speed. Either motor can be oper- ated separately as a six-pole or a four-pole motor. With this combination it is possible to get a speed equivalent to two, four, six and ten poles. It is necessary that both METHODS OF SPEED CONTROL 99 motors be mounted upon a common shaft or rigidly coupled together. Arrangements of this kind have been used in a number of cases, particularly with large motors. 1 (' }//\ Torque Fig. 57. Typical Speed-Torque Curves of a Polyphase Motor, Having a single-phase secondary winding. A special condition of operation exists in the slip-ring type of induction motor when one phase of the secondary circuit is open. This gives a single-phase circuit only in the secondary, and the motor operates at approximately half speed over quite a range of load. The explanation of this is shown in Fig. 5 7. Curve / represents the standard speed- troque curve set up by the primary, and curve 2 the speed- .torque curve of the single-phase secondary. Curve j is the resultant of i and 2. As curve 2 passes through zero speed it approaches a horizontal position, and therefore a slight change in the speed of the motor results in a considerable change in torque, which tends to maintain the speed con- stant at this value. If the motor is started from rest and the load is not too heavy it will accelerate to half speed, and 8 lOO CONTROLLERS FOR ELECTRIC MOTORS remain approximately at this speed unless the external torque is less than the value shown by the dotted line A. If the torque is less than this value, the motor will continue to accelerate on the upper part of curve j and approach full speed. After the motor has reached the upper part of curve J it will not again be brought to half speed unless the primary circuit is first opened, thus allowing the motor to drop down below half speed. However, the motor will stop if the load exceeds the dotted line B. The value shown by the dotted line A depends upon the secondary resistance of the motor and a number of other factors. This arrange- ment of speed control might be applicable for fans where the torque depends largely upon the speed. DYNAMIC BRAKING Direct-current motors are sometimes operated as gen- erators in order to bring the motor to rest. This method of braking is usually referred to a dynamic braking. The field of the motor is excited -in a positive manner and the armature is connected in a closed circuit through external resistance. The stored energy of the rotating part of the motor and the load is dissipated as heat in this external re- sistance. The torque exerted by the motor is equal to the field strength multiplied by the armature current. If a load is being lowered by means of dynamic braking, the speed of the motor can be adjusted by changing either the field strength or the resistance in series with the armature. The latter method is usually employed. Sometimes, as in the case of crane controllers using series motors, a combination of both armature and field control is used, as will be ex- plained later. Where shunt and compound-wound motors are used no special provision needs to be made for energiz- ing the field. A series motor usually has its field connected across the line through a fixed resistance, in order to insure a positive field when using the dynamic brake. METHODS OF SPEED CONTROL lOI As the braking torque depends upon the voltage gen- erated by the armature, the value of the torque decreases quite rapidly as the motor approaches slow speed and, unless there is considerable friction in the load, it is neces- sary to provide a mechanical brake for bringing the load to a positive stop. The advantage of the dynamic brake is in reducing the wear and tear on the mechanical brake, as most of the energy of the moving load can be absorbed through dynamic braking, allowing only a small amount of energy to be absorbed by the mechanical brake through fric- tion. The extra work done by the motor during dynamic braking must always be taken into consideration when de- termining the size of motor that is required for applica- tions where dynamic braking is frequent. Controllers are often provided with a dynamic brake for emergency pur- poses. Where the motor is stopped infrequently, the heat- ing effect of the dynamic brake need not be considered. The only practical method of obtaining dynamic brak- ing with induction motors is to supply the primary of the motor with direct current. This gives a stationary field, which produces a voltage in the rotating element of the motor. This voltage will cause a current to flow through a resistance connected to the slip rings of the motor and thus produce a dynamic braking effect similar to that ob- tained with a direct-current motor. CHAPTER VIII DIRECT-CURRENT MAGNETIC CONTACTOR CONTROLLERS Magnetic contactor controllers may be divided broadly into non-reversing and reversing controllers. Where sev- eral contactors are used for short- circuiting sections of the resistor in the armature circuit, the con- nections to the operating coils of these contactors can be made in succession by means of a drum-type master switch. The motor can in this case be oper- ated with more or less armature resistance, depending upon the number of contactors whose circuits are closed through the master switch. The number of resistance steps can be altered to suit the application. The writer believes that one step of resistance is sufficient for start- ing direct-current motors up to 15 horse-power. 230 volts, where the starting duty is light, and that two steps may be used for heavy starting. If it is de- sirable to regulate the speed Fig. 58. Typical Non-Revees- ^f ^^e motor, additional steps iNG, Automatic Controller, ' ^ Arranged for dynamic braking, should be provided. The ac- DIRECT-CURRENT MAGNETIC CONTROLLERS 103 celeration of the motor quite frequently is automatic, even when several running points are provided. BrON-REVERSING CONTROLLERS Non-reversing controllers are usually connected on one side of the motor circuit only and consist of a line contactor, a resistor and one or more contactors for short-circuiting Sw Off Run 1 1 3 3 lA ^unt Fi eld | Field Dnchargfi RctuUnce Fig. 59. Diagram of Panel Shown in Fig. 58. Pushing the start button results in closing line switch i, which closes the mechanical interlock contacts and opens the bottom contacts at lA. The motor then starts, with the starting resistance in series. Switch 5 simul- taneously short-circuits the field rheostat, ensuring full field as long as the starting resistance is in series with the armature. When the counter e.m.f. of the accelerating armature reaches a predetermined value, switch z closes and shunts the starting resistance out of the circuit. Short-circuiting this starting resistance causes switch 3 to open, so that field control is possible by the use of the field rheostat, after the motor comes up to speed. The dynamic braking resistance is connected across the armature terminals whenever switch / is open. Series operating coils on lA prevent the motor from being started while the heavy dynamic braking circuit is flowing, and also ensure good contact pressure. I04 CONTROLLERS FOR ELECTRIC MOTORS the resistor. A two-pole knife switch is usually required with this type of controller, the knife switch being connected so that it will disconnect both sides of the motor and con- troller from the line. The line contactor may be provided with a bottom contact for short-circuiting the armature of the motor through a fixed resistance to give dynamic brak- ing when the -line switch is opened. A diagram and an illustration of a controller arranged for dynamic braking are shown in Figs. 58 and 59- In some cases it is desirable to open both sides of the motor circuit when the controller is in the ojf position. This can be done by providing two line contactors. This arrangement, however, is not usually employed, as it adds to the expense, and opening the knife switch by hand ac- complishes the same result. The knife switch is needed so that the contactors can be disconnected from the line to renew the contacts or make adjustments. BEVERSING CONTBOLLERS In order to reverse the armature of a direct-current motor it is necessary to disconnect both sides of the armature from the line. This necessitates the use of four single-pole con- tactors or two double-pole contactors, two contacts being closed for either direction of operation. It is the usual practice to arrange either a mechanical or an electrical in- terlock between these contactors, so that the forward and reverse contactors cannot both be closed at the same time, as this would result in a short-circuit and might in- jure the apparatus. The resistor in series with the arma- ture is short-circuited in steps by magnetic contactors in the same manner as for non-reversing controllers. Fre- quently the resistor is connected directly to one side of the motor, so that the line contactors in their o-ff posi- tion disconnect the armature entirely from the line, as DIRECT-CURRENT MAGNETIC CONTROLLERS IDS SCHEME OF MAIN CO^fNECT10NS 2 1 Slartuig Ri Fig. 6o. Doubi.e-Pole, Reversing, Automatic Controller. The master switch makes contact between o and x in the off position ; between u and 4 in the forward position ; and between o and 3 in the reverse position. Switch 8 provides overload and low-voltage protection. Its operating coil is connected directly across xy through the master con- troller in the off position, and across xy through the overload relay in the running positions. This means that in case the motor is stopped for any reason, it is impossible for it to be restarted until the master controller has been turned to the off position. Switches b and 7 are of the magnetic lock- out type, and cannot close until after the current flowing on their control coils drops below a predetremined value. shown in Fig. 60. Since both sides of the motor circuit are opened by contactors, the shunt field cannot be con- nected so it will discharge through the armature of the motor. -^ It is therefore good practice to provide the shunt field with a parallel resistance to take up the inductive dis- 1 As explained in Chapter IV. io6 CONTROLLERS FOR ELECTRIC MOTORS charge when the field circuit is opened. If this resist- ance is omitted, a high voltage is generated in the field windings, which may ultimately result in puncturing the insulation. IContro] Panel (RtM View) «:HEME of main C0NNECTI0N5 ^ ITo Line Circuit '-UUUUUUU-' Field DiicbBTKe Reeiitsnce Fig. 6i. Double-Pole, Reversing, Automatic Controllek, Arranged for Dynamic Braking from Either Direction of Rotation. Switches / and s, as well as 2 and 4, are mechanically connected and con- trolled by the same operating coil, thus forming a double-pole contactor. Dynamic braking is accomplished, as in Fig. 59, by bottom contacts lA and zA. The reversing controller can be arranged for dynamic braking by providing a bottom contact on one of the for- ward and one of the reverse direction switches, so con- nected that when both of these switches are open the arma- ture will be short-circuited through a resistor, as shown in Fig. 61. DIRECT-CURRENT MAGNETIC CONTROLLERS IO7 FIELD RHEOSTATS Where a controller is used with an adjustable speed motor, a field rheostat is used for changing the resistance of the field circuit to adjust the speed of the motor. This field rheostat may be mounted on the controller panel or separate from it. It is considered better practice to mount this rheostat separate from the control panels so that the operator will not be required to place his hand close to the magnetic contactors when they are in operation. The sepa- rately mounted field rheostat can be covered to prevent the operator from coming in contact with any live parts. Usually the contactor panel is quite large and must be mounted in a more or less inaccessible place. The field rheostat, however, is small and can be located close to the master switch. Sometimes the master switch and field rheostat are combined in one unit. To sum up : A controller is made up of a magnetic con- tactor panel, either reversing or non- reversing, with or without dynamic brake. Some form of master switch is provided to operate the controller, and a field rheostat may be added. This gives the essential elements for the control of an electric motor. MASTER SWITCHES The master switch is an auxiliary switch which serves to govern the operation of contactors and auxiliary devices of electric controllers. It usually takes the form of a push- button, float switch, pressure switch, thermostat or drum switch. Other forms, of course, are in use, and any device which opens and closes the circuit may be used as a master switch. The push button switch is the most common and is perhaps used more than all of the other types combined. An ordi- io8 CONTROLLERS FOR ELECTRIC MOTORS Fig. 62. Pushbutton Station. nary start and stop pushbutton is shown In Fig. 62. More elaborate combinations of pushbuttons are used for printing presses, electric elevators, and other ap- plications where a complicated sequence of operation is required. A float switch is shown in Fig. 64 and a pressure switch in Fig. 65. These switches are used in connection with pumps and similar installations. The float switch, as its name implies, consists of a hollow metal box used as a float, which opens and closes the master switch for diff'erent levels of the liquid in which it is placed. The pressure switch has a diaphragm which opens the contacts at the maximum pres- sure and closes them for the minimum pres- sure for which it is ad- justed. Various forms of thermostats are used in connection with re- frigerating machinery for starting and stop- ping the motor-driven machinery at diff'erent temperatures. A drum-type master switch is shown in Fig. 6;^. This is usually in the form of a small drum controller and may be operated by the rotation of a handle or with a forward and back motion of a lever. Fig. 63. Six-Point Drum-Type Master Controller. Under this class should be in- DIRECT-CURRENT MAGNETIC CONTROLLERS 109 eluded also switches in which the contacts are arranged on a faceplate instead of a drum, and where the contacts are operated by cams. The drum type of master switch is usually applied where the sequence of operation is more or less complicated; also where frequent operation is required. Most master switches may be connected to either reversing or non- reversing controllers. There are two general methods employed, one of which is known as low-voltage release and the other as low-volt- age protection. The Electric Power Club defines these two meth- ods of voltage protec- tion, as follows : Low- Voltage Release. — The effect of a de- vice operative on the reduction or failure of voltage, to cause the interruption of power to the main circuit, but not preventing the reestablishment of the main circuit on return of voltage. Low- Voltage Protec- tion. — The effect of a device operative on the reduction or failure of voltage to cause and maintain the interruption of power to the main circuit. Fig. 65. Fig. 64. Fig. 64. Enclosed-Type Float Switch. Fig. 65. Diaphragm Pressure, Regulator. no CONTROLLERS FOR ELECTRIC MOTORS The reason for making the above distinction is largely a matter of safety. If the motor drives a woodworking ma- chine, for instance, the machine may be at rest due to the absence of line voltage. Under these circumstances, the operator might be engaged in adjusting the machinery, or for some other reason have his hands close to the cutting tools. If the line voltage is restored at such a time, the operator might easily be injured. This applies to a variety of machinery. Gears are a source of danger from this cause. These gears are usually protected under operating conditions, but this protection might be removed when the machine is at rest. In order to guard against accident from this cause, the master switch is connected to the controller so as to afford low-voltage protection. This requires the operator to perform a manual operation on the master switch in order to start the motor after it has once come to rest, due to a failure of voltage. Where the motor operates a fan or pump, it is very de- sirable to have the apparatus start automatically after a failure of voltage. In such cases there is little or no danger due to the automatic starting of the motor, and a great deal of inconvenience and possible danger might result from the failure of the motor to start when the voltage is restored to the line. The master switch under such conditions is con- nected to give low-voltage release. This arrangement auto- matically starts the motor again when the line voltage is restored.^ OVEBLOAD PROTECTION" It is usually necessary to provide some form of overload protection in connection with the controller. The most 2 Fig. 32, in Chapter IV, shows a pushbutton master switch connected to a controller to afford low-voltage protection. If a float-type master switch were connected between the terminals i and — , this same controller would give low-voltage release. By providing three terminals for the master switch of such a controller, it can be connected for either low-voltage release or protection. Many standard diagrams show both methods of connection. DIRECT-CURRENT MAGNETIC CONTROLLERS I I I common form of protection is the fuse, usually of the car- tridge type. These fuses have a short time element and therefore are well adapted for control apparatus. In start- ing a motor or during its operation, heavy currents may be taken by the motor for a few seconds. These do not injure the motor or controller, and therefore it is desirable to have a time element in the overhead device. This occurs to a limited extent when a standard enclosed fuse is used. Another common form of overload protection is a series relay which opens a contact by means of a magnet when the current exceeds a certain value. These relays are often furnished without a dashpot, and therefore their action is instantaneous. It is much better practice to use a dashpot to obtain a time element, so that the relay can be given a setting more nearly equal to the full-load current of the motor. Where a magnetic contactor is used on only one side of the line, a single-pole relay may be used with this contactor, but it is necessary to use a fuse on the other side of the line. The advantage of a relay is that it can be restored to its normal position very easily. The fuse, however, must be replaced by a new one. If a combined relay and fuse is used, it is usual to select a fuse with a higher rating than the setting of the relay, so that under ordinary conditions of overload the relay will operate and save the fuse. It takes some time to raise the temperature of a motor to a dangerous point when it is overloaded. If this over- load is within the commutating limits of a motor, the addi- tional load will not injure the motor until the temperature has been raised to the danger point. This usualy takes from five minutes up to a half-hour or more, depending upon the size of the motor and the amount of overload. If the overload is continued indefinitely it will injure the motor, but short periods of operation are permissible. The result has been that operators set the overload protection high enough to take care of the short-time peak loads. I 1 2 CONTROLLERS FOR ELECTRIC MOTORS I"herefore, the motor is without any real protection against continuous overloads. Very few fuses, overload relays, circuit breakers or similar apparatus give complete protec- tion to industrial motors. They operate in case of short- circuits or abnormal overloads, but usually they are set too high to open on a small overload, which may be sufficient to injure the motor if continued for a long time. /There is, therefore, opportunity for developing tiise=«iement over- load devices which will have time elements of five to thirty minutes instead of five to thirty seconds, as is the case with most of the existing apparatus. The longer the time ele- ment can be made, the more desirable it is for operation of motors. It is often stated that an overload relay having a long- time element will not operate quickly enough in case of a short-circuit or a ground to protect the motor. Standard practice, however, requires the use of fuses or a circuit breaker at the point where the motor circuit leaves the main power wires. The fuses or circuit breaker at this point must be set to protect the wires leading to the motor. If these wires are made a little larger than for the full-load capacity of the motor, the circuit breaker at this branch point may be set so that it will not operate on normal over- loads of the motor, but will protect the motor under abnor- mal conditions, and also afford protection to the wires lead- ing to the controller. The ideal protection, therefore, is feeder protection against short-circuit and a time-element overload device on the controller to protect the motor against continuous overloads, but which will permit over- loads for short periods of time. Overload relays are commonly made up in three forms : I. To allow the armature to return to the open position immediately after the overload has been removed. The function of such a relay is to open the line contactor. Con- DIRECT-CURRENT MAGNETIC CONTROLLERS I I 3 nections are made so that the contactor will be retained in the open position until the- master switch is manipulated. 2. The overload relay is provided with a catch which holds the armature in the closed position and requires the energizing of a separate magnet to release the catch and restore the relay to its normal condition. J. This is the same as 2, except that the catch is released by hand instead of by a magnet. This is objectionable be- cause the operator must place his hand on the device, which is in the neighborhood of live parts. Arrangement i is the more common and is usually pre- ferred, owing to its simplicity and cheapness. Arrangement 2 is usually connected to the master switch, so that the relay is reset by moving the master switch to the center or off position. APPLICATIONS The magnetic contactor controllers described above are those having the most general application. A few of the typical applications are for motors driving line shafting, pumps, machine tools, woodworking machinery and, in fact, any apparatus that is motor driven and does not require a special arrangement of circuits. CHAPTER IX ALTERNATING-CURRENT CONTROLLERS Many types of alternating-current motors have been built and a variety of methods of control have been devised. However, very few alternating-current motors, other than induction motors, are used in industrial work, and the methods of control in common use are quite simple. WOUND SECONDABY MOTOBS When resistors are used for accelerating or controlling the speed of induction motors they are usually placed in series with the three-phase wound secondary of the motor. The ends of this winding are brought out to three slip rings on the motor shaft, so that resistance may be inserted between each of the three rings.-' The method of control corresponds very closely to that of a compound-wound motor with only a small amount of series field. In study- ing this type of control and applying it, the problem is simplified if the operating conditions are considered to be those of the corresponding direct-current motor and no attempt is made to analyze the complicated reactions which are taking place in the wound secondary motor. This method of control requires the use of a primary switch, which should be separated electrically from the part of the control which is used for changing the re- sistance of the secondary circuit. Usually the second- ary is wound for less than 6oo volts, and in standard motors the secondary voltage and current do not change 1 The operation of a motor controlled in this way is explained in Chapter VII, Fig. 54- 114 ALTERNATING-CURRENT CONTROLLERS IIS PUSHBUTTON STATION STpP S TAR T CONTROL PANEL (REAR VIEW) From Three-Phase Lino X'Xl Jjuui. F. F. F, F SEQUENCE OF CONTACTORS :o, Run 1 O O O o O o o 4 o J|Ul^|llllL Fig. 66. Diagram of Automatic Controller for a Three-Phase Induc- tion Motor. Contacts i — lo are held open mechanically when switch i is open, and closed when switch i is closed. Contacts x — 2 are held open by a spring when switch i is open, and the spring pressure is released when switch / closes. These contacts are still held open magnetically, however, until the current in the coil DE drops below a predetermined value. Contacts x — j and X — 4 are similarly interlocked to switches 2 and 3, respectively. The interlock at the bottom of switch 4 is thrown to the right when the switch is open, and to the left when the switch is closed. Pushing the start button causes line switch / to close, and this switch is held closed through contacts i — 10 after the start button is released. Switch 2 is closed through circuit x-2-20-y as soon as the current in coil DE drops below a predetermined value. Switches 3 and 4 are similarly closed auto- matically when the current in D\ Ei and D^ Ei drops below the predeter- mined value. The interlock below switch 4 interrupts the control circuits to switches 2 and 3, allowing them to drop open, and holds switch 4 closed independently of the interlocks of switches 2 and 3. Pushing the stop button interrupts the control circuit of switch i, causing it to open; and this action opens contacts x — 2 allowing switch 4 to open. ii6 CONTROLLERS FOR ELECTRIC MOTORS Fig. 67. Double-Pole Con- tactor AND Series Ac- celerating Relay. This contactor illustrates switches 2 and 3 in Fig. 66, with their respective relays. materially for different primary voltages. This fact makes possible the use of the same secondary con- troller with a number of different primary switches. For a primary potential of 600 volts or less, air- break switches are used. If it is 2,200 volts or more, oil switches are frequently employed, although for certain classes of service air- break magnetic contactors have come into use. \\'here the motor operates in one direction only, the primary switch often consists of a circuit breaker or a knife switch and fuses, and either a face plate or drum-type secondary controller is used. If automatic acceleration is required, both the Rod Fconi Operating Levee 'Reservoir - Counterweight To Motor Secondary —Electrodes , —Electrode Tank —Cooling Coils --Regulating Va1v:e --Pump Pump Motor Master Switch Fig. 68. Liquid Controller. For large wound secondary induction motors. primary and secondary control consist of magnetic con- tactors, as shown in Fig. 66. If the motor is to be reversed at frequent intervals, a ALTERNATING-CURRENT CONTROLLERS I 1 7 drum controller having both primary and secondary switches is generally used for small motors.^ Large motors usually require magnetic contactors, in which case the same master switches and the same method of overload protection may be employed as for direct-current control.^ Low-voltage release or low-voltage protection can be obtained in the same way. For motors of 200 horse-power and larger, the second- ary control often consists of a liquid controller. Fig. 68. Each phase of the secondary of the motor is connected to a series of iron plates. These iron plates extend down into a tank, which may be filled with an electrolyte, usu- ally carbonate of soda and water. These plates are of different depths and so arranged that the number of plates, as well as the immersed area, increases as the water level rises. By properly proportioning these plates the desired acceleration can be obtained. In Fig. 68, the iron plates or electrolytes extend into the upper tank, while under- neath is a larger tank used for storing and cooling the electrolyte. A centrifugal purhp, driven by a small motor, lifts the electrolyte from the lower into the upper tank. A valve or weir in the upper tank can be adjusted to give any desired level, and is operated by the same lever that operates the master switch. A master switch con- trols contactors in the primary of the motor. A move- ment of the operating lever in either direction first op- erates the master switch to close the primary circuit of the motor with a small amount of electrolyte in the upper tank. A further movement of the lever increases the height of this electrolyte until full speed of the motor is obtained. The pump has only a limited capacity so that an appreciable time elapses between the movement of the weir and the increase in the height of the electrolyte; 2 This is shown in Fig. 29, Chapter IV. 3 As described in Chapter VIII. ii8 CONTROLLERS FOR ELECTRIC MOTORS this time element can be adjusted so that the minimum period of acceleration is fixed at a safe value. The weir is large enough to permit the electrolyte to discharge very rap- idly, so that when the SEQUENCEOF l^ver is thrown quickly from the forward to the reverse direction the electrolyte will be at approximately its min- imum level when the Fig. 69. Connections foe Starting a primary switches are Squirrel-(^ge^Motor ey Series reversed. The contin- ual pumping of the electrolyte from the lower tank to the upper tank and the dis- Sw Run 1 1 2 , From Three-Phase Line Overload Trip Actuated by either Overload CoU Fig. 70. Connections for Starting a Squirrel-Cage Motor by an Ahtotransformer Starter. In the starting position contacts / are closed. In the running position contacts i are open and contacts 2 are closed. The amperes given are for the starting position only and are merely relative. The no-voltage coil serves to hold the handle in the running position against the action of a spring which returns it to the oflF position if the voltage is interrupted or reduced below a certain value. Either one of the overload coils will inter- rupt the circuit of this no-voltage coil. ALTERNATING-CURRENT CONTROLLERS I 1 9 charge through the weir causes a rapid circulation and dissi- pates the heat energy with a minimum amount of steaming. The advantages of the liquid controller are : ( i ) Its sim- plicity; (2) its large thermal capacity, which enables it to sustain heavy overloads for short intervals of time, and (3) the absence of definite notches or steps, so that abso- lutely smooth acceleration is obtained. In this country there has been little demand for this form of controller in small sizes, partly due to a prejudice against the use of a liquid and partly because the other forms of controllers are usually cheaper for small motors. SQXriB,BEL-CAGE MOTOBS The current taken by an induction motor in starting can be limited by placing resistance in series with the pri- mary and using a short-circuited secondary. This form of secondary is commonly known as a squirrel-cage second- ary and the motor is spoken of as a squirrel-cage motor. If resistance is used in series with the primary, the cur- rent will be reduced in proportion to the resistance in- serted. The torque of the motor, however, will decrease as the square of the voltage across its terminals. This method of starting. Fig. 69, is very simple and inex- pensive and is sometimes used with small motors. There are a few applications for large motors which will re- quire 90 per cent, of normal voltage to start where this form of starter would be more satisfactory than the transformer type. The maximum current is taken at the time the motor starts from rest, and this current grad- ually decreases as the motor increases in speed, so that the voltage on the terminals of the motor will gradually increase, as the drop through the resistance is proportional to the current. I20 CONTROLLERS FOR ELECTRIC MOTORS Autotransj ormer Starter s- Nearly all squirrel-cage in- duction motors are started by using a trans- former to apply a re- duced voltage to the primary of the motor. The advantage of a transformer over a re- sistor is the fact that the reduced voltage is ob- tained with little or no loss in power, and there- fore the current drawn from the line is less than the current taken by the motor in the ratio of the starting voltage of the line voltage. The con- nections commonly used for this type of starter are shown in Fig. 70. The trans- former has only one winding and is usually called an " autotrans- former." The complete device is often called an "autostarter" or a "com- pensator." The relative values of current in the different circuits during starting are shown on the diagram, not including losses in the transformer. These values of current are given merely to bring out the Fig. 71. AuTOTRAXSFORMER Starter with Cover and Oil Tank Removed. ALTERNATING-CURRENT CONTROLLERS 121 Fmm Thrcc'Phaw'L'ini Sw Run I o l o l o _i_o 25 J.O o" . oo Fig. 72. Connections of autotransformer Starter With resistor to obviate opening the circuit when changing from starting to running positions. saving in power which results from this method of start- ing. The starting voltage is assumed to be 65 per cent, of normal, which gives approximately 42 per cent, of the torque which the motor would exert at standstill, if it were ™— ™-.. connected to full voltage. This starting torque of the motor at ^^'m^M^^HW^ full voltage is usually from 150 ^ ^ to 200 per cent, of full-load torque, so at 65 per cent, of line voltage, the starting torque would be from 65 to 85 per cent, of full-load torque, which is sufficient to start most loads. Other taps are provided on the transformer so that the starting voltage may be adjusted. If more than 80 per cent, of the line voltage is required to start the motor there is very little use of employing a transformer for starting pur- poses. Such a condition usually indicates that the motor is not suitable for the particular application ; either a wound secondary motor should be used or a larger squirrel-cage motor. The starting torque of a squirrel-cage motor can be increased by increasing the secondary resistance. This, however, decreases the efficiency of the motor. Usually the secondary resistance of a squirrel-cage motor i? adjusted by using different materials in the rings short-circuiting the secondary windings and is not adjustable after the motor is built. One method of increasing the starting torque of an existing motor is to saw slots in the short-circuiting rings between the connections to the winding bars. Where the motor is used for continuous operation and requires heavy starting torque it is undesirable to use a high-resistance secondary, and therefore the wound secondary motor is preferable. 122 CONTROLLERS FOR ELECTRIC MOTORS A commercial starter for small and medium-sized motors is shown in Fig. 71. The switch in the lower part of the case is immersed in oil. Above the switch is the trans- former, and in front of the tran^ormer is located the over- load protection. In the of position the handle stands central. To start the motor the handle is moved in the di- rection which closes the contacts marked i, Fig. 70. After the motor has accelerated to approximately full speed, the handle is moved in the opposite direction and the contacts, which are marked 2, are closed. The handle is held in this position by a small magnet called the low-voltage coil, which releases the handle on the failure of voltage and TraniTonnei T ' TraMforma- A rv^i,. (-~ii % 3t Choke Ctiit JEiSOEWCE OP aWlTCHEa 4CholKCoiI73 ■■■■i 5t s. R.. ° Fig. 73. Connections for Multipoint Starting with an autotransformer. allows it to return to the ojf position. A latch is usually provided to prevent the operator from accidentally throw- ing the handle into the running position first. In passing from the starting to the running position with this arrangement, it is necessary to open the connections, so that the motor will not be connected to the starting tap on the transformer and at the same time connected to the line, as this would short-circuit a section of the transformer and probably destroy it. In starting large motors it is often desirable to pass from the starting to the running position without opening the motor circuit. This may be done by using a resistance connected as in Fig. 72. This resistance is inserted between the starting tap and the line ALTERNATING-CURRENT CONTROLLERS 123 From Thrce-PSasc Line VL. 4 t3 Svy Bun I 01010 _a_ o o _ 3-S.— 00 Fig. 74. Modified Method of Producing Multipoint Start- ing WITH AN AuTOTRANSFORMER. to prevent short-circuiting the transformer at the time the connections are changed. In this arrangement the contacts marked 2 and j represent the starting connections. They are not opened until after contacts 4 are closed. Where several starting steps O A fiEnlJENCE OF are used, the arrangement shown in Fig. 73 may be made. This makes use of a small autotransformer or coil, the center of which is connected to the motor load. After the autotransformer is energized, contact J of this auxiliary coil is connected to the starting tap. One-half of the coil then acts as a choke coil and gives the minimum starting voltage. When contact 4 is closed an intermediate voltage is obtained. Contact J is then opened and 5 cl&sed, which connects the motor directly to the line, giving the final step in starting. This method makes a very simple set of connections for a magnetic con- tactor starter where a multipoint starter is required. A modification of this arrangement is shown in Fig. 74. This arrangement is similar to Fig. 70 but the switches are operated in a different manner. The contacts marked /, 2 and J are first closed. This gives the same connection as closing contacts i in Fig. 70. If contacts 2 are now opened, one end of each autotransformer is disconnected from the line and the motor is left across the line with a part of the transformer winding in series, which acts as a choke coil. Switch 4 is immediately closed, connecting the motor to the line. This short-circuits a section of the transformer but, since the winding is not energized, no harm results. Switch J is now opened, disconnecting the motor from the trans- former. 124 CONTROLLERS FOR ELECTRIC MOTORS Disconnecting the primary of an induction motor from the line when passing from the starting point to the running position makes very little difference with small motors. With large motors, however, in special cases, it may cause surges of current and voltage.* These surges are more apt to occur in high-speed than in slow-speed motors. They depend, however, upon the characteristics of the motor and can readily be taken care of by the manufacturer when he furnishes the motor and the starter as a combined unit. Automatic acceleration for alternating-current controllers at present is limited to the series relay method or a time- element method. The time-element method is very satis- factory where the timing device can be relied upon. For rapid acceleration and reversing service, the series relay method,^ shown in Fig. 66 is the best method to use. * This is brought out in Mr. Hellmund's paper on " Transient Conditions in Asynchronous Induction Machines and Their Relation to Control Prob- lems," in the Proc. A.I£.E., Feb., 1917. ^ Described in Chapter V. CHAPTER X RESISTORS^ The most common form of resistor is the cast-iron grid, shown in Figs. 75 and ']6. Cast iron is admirably adapted for this purpose on account of its cheapness, high electrical resistance, freedom from corrosion, and small temperature Fig. 75. Cast -Iron Grid Resistor Unit. Fig. 76. Grid Resistor. coefficeint. The resistance of cast iron increases about 15 per cent, with a change of temperature of about 250 degrees. Its principal limitation is for small apparatus where a large ohmic value is required with a small capacity, requiring high resistance units of small size. Where a large ohmic value is required in a small space 1 The Electric Power Club defines a " resistor " as : " An aggregation of one or more units possessing the property of resistance, used in an electric circuit for the purpose of operation, protection or control of that circuit." This term was coined to express properly the part of a controller often referred to as the " resistance." The word " resistance " expresses the prop- erty of a substance and should not be used to denote the material itself. 125 126 CONTROLLERS FOR ELECTRIC MOTORS 77 Embedded Type Tubu- lar Resistor. the embedded type resistor is used. It is made in various forms, the resistance material usually consisting of a wire or ribbon embedded in enamel or some similar compound, as in Fig. 77. It is also common to make up units in the form of plates. Embed- ding the resistance material gives increased thermal ca- pacity and mechanical pro- tection. It also prevents con- ducting material, such as metal dust, from collecting on the unit and reducing the re- sistance. Most of these em- bedded units can be heated to destruction without any external flash or drippings. Cast iron grids are placed on insulating rods and mounted between end frames, thus forming convenient units. Where it is desirable to combine the cast iron girds with an em- bedded type of resistor, either the embedded resistor should be in the form of a plate or the tubes should be mounted so that they will fit in the same frame with the grids. A " rheostat " is defined by The Electric Power Club as a resistor provided with means for varying its resistance. This usually takes the form of a series of contacts mounted on the surface of an insulating material having an arm arranged for making connection between a central post, which forms the pivot of the arm, and the various contacts which are arranged in a circle. Mounted back of this face plate is a series of resistor units, as shown in Fig. 78. For small sizes, the contacts and resistance material are both embedded in a compound forming a complete unit, known as a plate-type rheostat. In applying resistors, two points should be considered: RESISTORS 127 ( I ) The ability to radiate or conduct the heat from the unit to the surrounding atmosphere, (2) the ability to absorb heat. Usually the mass of the resistor is small compared with that of a motor or controller for dis- sipating the same amount of heat energy, so that the energfy absorbed by the resistor will raise its tempera- ture more rapidly. Where the resistor is used for start- FiG. 78. Rheostat Made up of Grid Resistors, Showing series of contacts on face plate. ing purposes only, the absorption forms a very impor- tant item in the design. For intermittent duty, radia- tion is the controlling feature. The relation between tem- perature rise in degrees C. and the watts per unit for five different conditions of operation is shown in Fig. 79. The 128 CONTROLLERS FOR ELECTRIC MOTORS heavy line is the average value and the dotted lines on either side represent the variations due to the different cross-sections of the grids used. These curves, of course, apply only to one particular size of grid and are shown for the purpose of illustrating the effect which different classes Fig. 79. Relation of Temperature Rise and Energy Loss Per Resistor Unit. For five different classes of service. of service have upon the capacity of a resistor of a given size. Curve / is for continuous service; Curve 2 for a cycle of duty, in which the resistor is in circuit for two minutes out of every four, with two minutes rest between. Curve 2 represents a duty cycle of an ordinary shop crane of one minute service and three of rest. Curve 4. repre- sents a duty of 30 seconds with 3.5 minutes of rest. Curve 5 is a duty of 15 seconds in each four minutes. Curve 5 corresponds to ordinary starting duty. In making these RESISTORS 1 29 tests, it was found that the spacing between the grids was an important factor and preliminary tests were made to de- termine the most economical spacing. Similar data should be obtained for any form of resistor used, so that an in- telligent application can be made. In applying resistors, it is usual to design to a temperature of from 200 to 250 degrees, although the exact temperature of the material and also of the air issuing from the enclosing case is rather difficult to determine. The air issuing from the case should not be hot enough to cause a fire hazard or injure objects in the vicinity of the resistor. Destruction of the resist- ance material itself is usually by fusing, which occurs at a very high temperature, so that a difference of 25 or 50 degrees in the actual temperature of this material represents only a small error, as compared with the fusing tempera- ture. Unless the grids are mounted properly, they have a tendency to sag at a dull red heat, at about 600 degrees C. This sometimes results in destruction of the resistor unit, although fusing does not take place. The maximum tem- perature of a resistor depends largely upon its mounting. There should be a free circulation of air, and the frames should not be mounted one above the other to any consider- able extent. A maximum of three high usually represents the limit of good practice, and space should also be provided between the tiers of frames. In most applications, the maximum heating occurs in different resistor frames during different parts of the cycle. If a little judgment is used in arranging these frames, the heating may be distributed in different tiers, so that it will not all be concentrated in one place at any one time. It should be remembered that a resistor is used for the purpose of dissipating energy in the form of heat. For large motors, a considerable amount of energy is often dissipated in this way. The space surrounding the resistor should be arranged for carrying off this heat. Sometimes I30 CONTROLLERS FOR ELECTRIC MOTORS trouble is caused by mounting the resistor in the corner of a room or some other restricted space, which has not suffi- cient ventilation to carry off the heat dissipated. For small resistors, this is a less important item. Care should be taken to mount the resistor so that it will not directly heat the motor or controller. — n 1 100— 1 ^ -^ \ ■^ X 10 F .3 \ t^ S-*^ ^ -D I 1 . \ ■arSf^. S^ ^ fi 1 " a . \ ^ >- ■» 3 5 X ! / K„ 10 5 1 1 / / 1 *^ F^ »C / •5 10 1 / "1 ' 5 E 1 -.5 V 1 7 f 90 ,05 , inds Service m Each a) 1 to Set 5 1 nds(* 1 Mimil 1 1 Is 2 i y"ao Fig. 8o. Ehergy which can be Dissipated in a Grid for Different Lengths of Service. Assuming a maximum temperature rise of 250 degrees C, Fig. 80 gives the relation between the watts dissipated and the time a grid is in service. It also shows the per- centage of current which at continuous rating would give the same temperature rise. This curve is useful in illustrat- ing the rapidity with which the capacity of the resistor changes with time, and the importance of deteermining ac- curately the service requirements Avhen designing the re- sistor. These curves represent the average values for a particular size of grid. In order to vary the ohmic value of a grid, the cross-section is changed. This makes a small difference in the curve for each individual grid, but usually this difference may be neglected and an average value used. RESISTORS I 3 I In designing resistors for a particular service, the cal- culation becomes quite complicated, if all of the variables are taken into consideration. Furthermore, it is often im- possible to predetermine exactly how the controller will be manipulated, as this depends upon the judgment of indi- dividual operators. It is evident from Fig. 79 that the difference of a few seconds in the cutting out of the resistor during acceleration will make a considerable difference in the temperature of this resistor. The variations which occur in well-known service applications have been classified on an empirical basis, which is the result of experience ex- tending over a number of years. New applications must be approximated and actual operating data obtained. Where the operating conditions are not well known, the error should be made on the safe side and a larger resistor ordered than necessary, rather than one which is too small. In order to arrive at a commercial classification of service, The Electric Power Club has recently adopted the follow- ing five standards for service conditions: Light starting duty (15 seconds out of each four minutes) ; heavy starting duty (30 seconds out of each four minutes) ; light inter- mittent duty (one minute out of each four minutes) ; heavy intermittent duty (two minutes out of each four minutes), and continuous duty. The service conditions are also di- vided into the following current standards, the figures repre- senting per cent, of full-load current on the first step : 25, ^Oj yOj loOj ijOj 200 or over. While this classification is empirical, a careful check made from existing data shows that it closely represents present practice. These designations do not constitute complete data for testing a resistor, as it is a common practice for many ap- plications, such as cranes, hoists, etc.\ to taper the resistor, so that the capacity on the first step is less than the capacity on the last step. The percentage of resistance short-cir- cuited on each notch of the controller is also a variable. It 132 CONTROLLERS FOR ELECTRIC MOTORS is the aim to so proportion the resistor that equal peaks occur during the acceleration at the specified load. This classification of resistors, it is believed, is sufficiently ac- curate for practical purposes. As explained previously, it is very difficult to calculate a resistor closely, and therefore the limit of error in design is considerable. In view of this fact, it would seem unnecessary to adopt classifications for intermediate service values, such as a service represent- ing three minutes on and i minute off. At the same time, it allows sufficient latitude in tapering resistors to enable engineers to adapt their particular designs to different classes of service. The tapering of a resistor depends to some extent upon the type of controller. Where automatic acceleration is provided a less number of steps is used and the current value at which the resistor is short-circuited can be adjusted with considerable accuracy, so that full advantage can be taken of the refinements of design in pro- portioning such resistors. Where manual control is used, so much depends upon the operator that refinements in design are of less value. Data for proportioning resistors is given in Table II., which is partly based on theory and partly on experience. This table illustrates the propor- tions that are sometimes used in the various steps of a resistor. The method of calculating the ohms per step in a given resistance takes the form of a geometric series, — R -\- RX-y -\- RX2 -j- RX2, etc., where R is the internal resistance of the motor and controller and X is the ratio of maximum and minimum accelerating current. The derivation of this formula can be obtained from text-books on the subject or electrical hand books. The values obtained from such a formula are based entirely upon Ohm's law and do not take into account the effect of inductance in the circuit. It is sufficiently accurate, however, for most calculations. Where automatic acceleration is used and the resistor is short-cir- RESISTORS 133 cuited in a small number of steps, it is desirable to take into account the inductance in the circuit, as this has a marked effect upon the current value of the intermediate steps. ^ In order to check still further the effect of induct- TABLE II Light Intermittent Duty (i Min. Out of Each 4), 100 Per Cent. Full- Load Amperes on the First Step Total Num- Percentages to Each Step. ber of Steps. I 2 3 4 5 6 7 8 9 I < R A 100 2 < R A 80 37 20 75 aj R A 65 37 25 60 10 70 4| R A 49 37 27 53 15 59 9 64 s| R A 43 37 26 48 16 52 9 57 6 60 ej R A 38 37 25 47 16 SO 10 53 7 57 4 59 7| R A 34 37 23 45 16 48 II 50 7 53 5 55 4 58 sj R A 31 37 22 44 16 46 II 48 8 50 6 53 4 55 3 57 pj R A 28 37 21 42 16 43 II , 45 8 48 6 50 4-5 52 5-5 54 2.S 56 R ^ Per cent, of total ohms. A = Per cent, of fuU-Ioad amperes at continuous rating. ance, a 15 -horse-power, 230-volt, direct-current shunt motor was connected to the power-house bus-bars and loaded with a prony brake to give half full-load torque. The motor 2 This was brought out in several papers before the A.I.E.E., Feb., 1917. 134 CONTROLLERS FOR ELECTRIC MOTORS was then accelerated and the value of current and armature voltage measured by an oscillograph. These tests show that the actual current value obtained is about two-thirds of the calculated value. The ratio between the value calculated by the above formula and the actual value depends upon the inertia of the load. With a heavy inertia load, the actual value will approach more closely to the calculated value. In most cases, there will be some line drop, as very few motors are connected to the busbars in a power plant. This drop of voltage, together with the resistance of the feeders leading to the controller, will have an appreciable effect on the maximum current peak. It is probable that further tests on various installations will show that it is safe to assume a value for this peak in the neighborhood of 6o per cent, of the calculated value. The speed of a motor using a resistor in series with the armature depends upon the load. Where a resistor is re- quired to give speed control, it is very necessary to know the exact load on the motor, so that the resistor can be cal- culated accurately. Commercial ratings have been so standardized that a larger motor than necessary is often specified, so as to use a standard rating. This often mis- lead the engineer who designs the resistor; the purchaser may specify a 50 per cent, speed reduction at full load giving only the horse-power rating of the motor and not the actual load conditions. Sometimes an error is made in estimating the horse power required ; in such cases, it may be necessary to readjust the resistor or to furnish addi- tional sections. CHAPTER XI PROTECTIVE DEVICES Industrial controllers are commonly provided with one or more protective devices, such as overload, low voltage release, etc. Some of these devices are designed to protect the motor against abuse; others are for the protection of the operator or the machinery driven by the motor. The more common devices are for protection against : I. Overload. ■z. Low voltage. 3. Phase reversal. 4. Phase failure. 5. Shunt field failure. OVERLOAD PROTECTION Fuses. — The oldest form of overload protection is the fuse, consisting of a strip of metal in the main circuit which is melted or fused when the current exceeds a predetermined value. The earlier forms of fuse consisted of an open link. A better and more accurate fuse was obtained by enclosing the fusible link so as to give it a more definite time element and prevent the particles of molten metal from dropping on surrounding objects. Fuses are easy to obtain in the ordinary sizes, as they are carried by most supply houses. Small fuses are inexpensive where only occasional overloads are experienced. Where the motor is worked hard result- ing in repeated blowing of the fuse, the cost of fuse re- newals, even for small motors becomes excessive and it is cheaper to use some form of overload device which does not require renewal. A knife switch should be provided for 135 136 CONTROLLERS FOR ELECTRIC MOTORS disconnecting the fuses from the line before they are re- newed. Even the best designs of fuse are not very ac- curate, so that it is necessary to overfuse a motor somewhat to be sure of having a fuse of sufficient capacity. The in- herent time element in a fuse is a distinct advantage on a motor load, as the fuse will not respond to momentary variations in load, although it will act promptly on excessive overloads. Circuit Breakers. — The circuit breaker is a switch pro- vided with an overload trip which usually consists of a magnet with a movable core. The attraction of the core of the magnet trips the circuit breaker and opens the circuit. Usually the current at which the circuit breaker trips is ad- justed by changing the air-gap between the core and its pole face. Most circuit breakers are reset or closed by hand, although magnetic reset can be provided. No new parts are required for reestablishing the circuit after the circuit breaker has opened. The continual rupturing of the circuit gradually wears away the arcing trips of the circuit breaker so that these must be renewed occasionally. The overload trip can be provided with a dashpot for giving it a time element which should always be done for motor loads. Overload Relay. — The overload relay is a small circuit breaker which is actuated by a magnet and opens the circuit to the operating coil of a magnetic contactor or to the low voltage coil of a circuit breaker. The relay closely re- sembles the overload mechanism of a circuit breaker, with the addition of the small contacts referred to above. These relays should be provided with dashpots to give an inverse time element when used with motors. When the overload relay is used in connection with magnetic contactors arrangements can be made for reestablishing the electric circuit from a push button or master switch. When the relay trips its pilot circuit, this circuit may be maintained open by a mechanical latch on the relay or it may be opened PROTECTIVE DEVICES 137 through an electrical interlock on the magnet contactor. If a mechanical latch is used on the relay, this latch may be released either by hand or by another small magnet. The two methods are known respectively as " hand reset " and " magnet reset." Where the circuit through the relay con- tacts is opened on an interlock attached to the main con- tactor or through another relay, the device is known as " electrically reset." The hand reset on the relay is not recommended for most applications, as it is not desirable for the operator to place his hand near the live parts on the control panel. Time Element Overload Relays. — Engineers have recog- nized for years the desirability of having an overload device which would have a time element proportional to the time required to heat up the motor to a definite temperature with various loads. Such a device would give overload pro- tection to the motor but would not protect the motor from a short-circuit or severe overload. This, however, could be readily taken care of by a fuse, as it would not be called upon to operate except in cases of emergency. Many motors are operated on intermittent loads, such as cranes, hoists, machine tools, elevators, etc. The motor is capable of carrying a heavy overload for a short period of time. This period of time is much longer than given by the time element in commercial forms of relays. It is necessary, therefore, to set the relay so as to prevent its operating on these short time overloads. This results in the motor being without adequate overload protection in case of prolonged operation at the maximum load. Large units, such as turbogenerators, have thermocouples incorporated in their windings. These thermocouples are connected to suitable switchboard instruments to indicate the temperature of the windings so that the load can be regulated properly. The same device could be used for opening the circuit breaker, if desirable. These thermocouples, however, are expensive 138 CONTROLLERS FOR ELECTRIC MOTORS and the apparatus required too elaborate for ordinary motor applications. There is therefore a field still open for the Fig. 8i. Dash-Pot Type of Inverse Time Element Ovek-Load Relay. development of a long time overload device which will permit the motor to op- erate at a heavy load for relatively short pe- riods of time and still protect the motor against continuous op- eration at this load. Fig. 8 1 shows a com- mercial form of over- load relay having a dashpot to give it an inverse time element feature. Another form ^ „ ^ of relay, shown in Fig. Induction Type, Time Element o -l Over-Load Relay. o2, has a copper disc Fig. 82. PROTECTIVE DEVICES 139 rotating between the poles of permanent magnets to provide a definite time element. The dashpot type of relay is usually designed to give an inverse time element on increasing loads. Sometimes it is operated from series transformers with saturated fcores to limit the pull on abnormal overloads. This latter form of relay is known as a fixed time element relay. It is the preferable form to use in connection with a controller which is connected to a large power supply line and is provided with a separate feeder circuit breaker for taking care of short-circuits. Some operators are under the impression that it is desir- able to adjust the time element of overload relays. This adjustment would be desirable if a long time element were obtained. Commercial forms of relays do not afford a time element which compares in length to the time required to heat up even small motors. It is desirable, therefore, to obtain as long a time element as possible with the dashpot relay and any adjustment provided should be set to give the maximum time element. If too long a time element is at- tempted with the dash-pot relay, there is a tendency for it to stick under adverse conditions. It is necessary, there- fore, to adjust this time element so that the maximum time given will insure satisfactory operation. All motor circuits should be protected by feeder circuit breakers or fuses. If the feeder circuit is connected to a very large transformer or to a power circuit having large capacity back of it, the feeder circuit breaker should be of ample capacity to take care of the power ahead of it in case of a short-circuit to the feeder or the apparatus con- nected to this feeder. This circuit breaker must have a less time element on its trip than that obtained with the over- load relay on the controller panel. This latter will require a relay with a fixed time element to insure the opening of the circuit breaker before the contactor on the control panel. The circuit breaker can be set for a high enough value so I40 CONTROLLERS FOR ELECTRIC MOTORS that it will not be affected by ordinary overloads. The overload device on the control panel should be set low enough to protect the motor against abuse. Where over- load relays are used in connection with feeder circuit breakers, it is desirable to have an adjustable time element. In many cases the same overload relay is used for both classes of service. It is therefore provided with an ad- justable time elenient, although the short time element is not desirable when used on the controller. LOW VOLTAGE PBOTECTION OK RELEASE Devices of this kind are arranged for disconnecting the motor from the line on failure of voltage. The Electric Power Club has recognized two forms of this protection : (a) Low Voltage Release. — This provides for discon- necting the motor from the line on failure of voltage, but permits the motor to start automatically when line voltage is reestablished. Such a device is the magnetic contactor control with automatic acceleration. It is used for pumps, fans, and similar applications which should restart auto- matically when the voltage is restored to the line. {b) Low Voltage Protection. — This device disconnects the motor from the line on failure of voltage and prevents the motor from being started again on reestablishment . of line voltage. In order to start the motor, the operator must push a button or operate a lever. This latter is a very necessary precaution where the motor is used for driv- ing machine tools or woodworking machinery, printing presses or in fact, any device which might cause injury to a person working on the machine. These devices are sometimes known as " under voltage " instead of " low voltage," both terms having the same significance. Usually they do not respond to a small drop in voltage. PROTECTIVE DEVICES 141 PHASE REVERSAL PROTECTION This device operates to disconnect the motor from the line in case one of the phases of the polyphase circuit has been reversed. Such rever- sals sometimes occur I'^l- when repair men are m- " - stalling service trans- formers or making other repairs. The ef- fect of such a reversal is to cause the motor to operate in the opposite direction. For some ap- plications such a rever- sal will not cause any damage, but where the motor drives an eleva- tor or hoist a serious accident may result. Some public service corporations supplying electric power to users require the installation of a reverse phase re- lay device on all eleva- tor motors to protect them from liability re- sulting from such an accident. There are a number of these devices now in the market. Some of them consist of a small relay having two parts corresponding to the stationary and movable element of a motor or wattmeter. Power sup- plied to these two parts causes rotation in a definite direc- Rclay 2 Fig. 83. Combined Phase Failure and Phase Reversal Relay for a Three- Phase Circuit. This consists of two small relays, the contacts of which are closed by electro- magnets. Relay 2 is connected across one phase of the circuit and remains closed as long as that phase is energized. The coil of relay i is connected between the three phases of the circuit, one end of the coil having an inductor in one branch and a resistor in the other branch. This com- bination brings the current in the two branches of the circuit so that its effect upon the coil of relay / is added when the phase relation is correct. On a reversal of phase relation, the current in the two legs of the circuit through the coil or relay / oppose each other and the relay drops open. On failure of voltage in any one of the three phases, relay / is opened either directly or through the opening of relay 2. The contactor coil for the control is in circuit with the contacts of relay t so that the opening of this relay disconnects the motor from the line. 142 CONTROLLERS FOR ELECTRIC MOTORS tion. The torque thus established maintains a contact in the closed position and represents normal operation. If either phase is reversed, the torque of the relay is also reversed which opens the contact and disconnects the con- trol and motor from the line. Another form of relay is shown in Fig. 83. This relay is made up of standard contactors. The operating coil is supplied through two circuits, as shown. One of these cir- cuits has a resistance in series with it, the other an induct- ance. The resistance and inductance cause a displacement To Motor ,-,\ ■ To Power and Controller .^-^ ■ Supply Inductor Fig. 84. Combined Phase Failure and Phase Reversal Relay for a Two-Phase Circuit. This device consists of a single relay having two coils, the coil across one phase having a resistor in circuit with it. The coil across the other phase has an inductor in circuit with it. The use of the resistor and in- ductor in the two coil circuits results in the current in each coil being ap- proximately in the same phase relation and their action is added. If, how- ever, the phase relations of the supply circuit are reversed, the magnetic action in the two coils is opposite and the relay opens. On failure of volt- age in either circuit, one coil is de-energized and the relay opens. The coil for the magnetic contactor for the main circuit is connected through the contacts on this relay so that when the relay opens the magnet contactor coil is disconnected, opening the contactor and disconnecting the motor from the line. between the two phases so that their affect is added and the relay maintained closed. If either phase is reversed, the phase angle is changed, causing the two circuits to oppose each other, reducing the magnetic action on the coil and opening the relay. In the case of the two-phase arrange- ment shown in Fig. 84, the contactor is provided with two separate coils, one in each phase. PROTECTIVE DEVICES 143 Some devices of this character have been designed to close a cir- cuit on reversal of phase rather than open it. Such devices have special applications in connec- tion with power circuits, but are undesirable for industrial control as the failure of the contacts to make a good electrical con- nection or the breaking of one of the wires would prevent such a device from operating. Where the contact is closed for nor- mal operation, the breaking of a wire or the failure of the con- tact would disconnect the con- troller from the line and auto- matically stop the motor, which is a safer arrangement. To Motor and Controller 1: -p _ Contactor Coil "^00^ OOP / Reverse I Phase Retay To Power Supply Fig. 85. Combined Phase Failure and Phase Rever- sal Relay of the Watt- meter OR Motor Type. The coil for the contactor in the motor circuit is con- nected through the contact in the relay. This contact is held open by a spring and is closed due to the torque ex- erted by two coils connected to two of the three phases. These coils set up a motor action which forces the con- tacts together against the spring pressure when the phase relation is correct. If either of the three phases is reversed the torque on the watt meter movement is re- versed and the contact opens. If the voltage fails in either phase, the torque is reduced to zero and the spring opens the contacts. PHASE FAILTTBE PKOTECTIOIT Sometimes one line of a poly- phase circuit may be opened acci- dentally. If the motor has not started, it will fail to do so and may be injured by leaving it connected to the line. This can easily happen in a mechanically-operated elevator con- trol where the failure of the motor to start might cause the operator to leave the controller in the running posi- tion. - By using a low voltage device across two phases of the three-phase circuit, or one relay for each phase of the two-phase circuit, the motor can be protected from such an accident. The relays are connected in such a way that the main switch will not close until both relays are 144 CONTROLLERS FOR ELECTRIC MOTORS closed. This arrangement is often combined with a phase reversal relay device to give protection to the elevator both from phase reversal and phase failure. If a motor is rotating and one phase is opened, the motor will continue to operate single phase if the torque does not exceed the single-phase torque of the motor. Such opera- tion, however, causes all of the load to be carried by one phase of the motor and may seriously overheat these wind- ings. If the overload protection is set at a low enough value, it will protect the active phase from an excessive overload. Unfortunately, such overload devices are fre- quently set too high to afford proper protection. While the motor is operating, voltage is maintained across all three terminals of a three-phase motor or across both phases of a two-phase motor, due to the active phase generating voltage in the inactive circuit. The voltage generated in the inactive circuit is very little less than the normal volt- age, so that any phase failure device depending upon a drop in voltage for operating it will not respond, when connected to a rotating motor. Fortunately many installa- tions, such as elevators, hoists, etc., operate for only a short time without coming to rest, so that a phase failure device will operate the first time the motor is brought to rest and prevent restarting it. SHTTNT FIELD rAILTTBE Shunt-wound direct-current motors may operate at an abnormal speed and destroy themselves by centrifugal ac- tion if the shunt field becomes disconnected from the line. While this kind of an accident is of very rare occurrence, it is thought advisable to guard against it in some particular cases. The usual method of guarding against this form of accident is to provide a relay and place its magnet wind- ing in series with the shunt field circuit of the motor. When this relay is energized, it closes the pilot circuit to PROTECTIVE DEVICES 145 the controller. If the shunt field circuit should open, this relay will open the pilot circuit to the controller, which in turn disconnects the motor from the line. One serious objection in the use of this relay is the trans- former action which takes place in the motor due to sudden changes of load. This action is particularly noticeable when the motor is compound wound. A rapid change in load causes a change in the field flux. This exerts a trans- former action on the shunt field windings and may be suffi- cient to momentarily reverse the current in these windings. This does not mean that the flux in the field circuit of the motor is reduced to zero. It simply means that the rate of change of the flux is sufficient to generate a counter voltage in the shunt field windings large enough to cause a momentary pause in the current through these windings. This is not very hard to do as the shunt field usually has a very large number of turns, which, multiplied by a small change in flux, will cause a considerable voltage. A reac- tion of this kind in the shunt field circuit of the motor may cause the relay to drop out and disconnect the motor from the line. The connections to the controller are such that when the relay does open the circuit, the motor will not start again automatically. It requires action on the part of the operator to reset this relay. A number of devices have been used to delay the action of this relay to prevent an interruption of service. One method consists in adding considerable inertia to the mov- ing parts of the relay by means of a pivoted weight or similar device. Another method is to use a heavy tube of copper around the magnet core. This copper tube acts as the short-circuited secondary of a transformer and de- lays any change in magnetism in the relay. Usually one or the other of these devices will prove satisfactory, although in aggravated cases, additional precautions must be taken. 146 CONTROLLERS FOR ELECTRIC MOTORS Engineers, as a rule, do not consider it necessary to use a shunt field protective relay except with large motors which may run light under certain conditions of load. Safety devices of any character should be avoided where unnecessary, as they add complication to a control equip- ment and require additional inspection and care to maintain in an operative condition. It is seldom that any safety devices are used other than overload and low voltage. Wherever a safety device is used, it should be tested at frequent intervals to insure its proper operation in case of accident. CHAPTER XII SERIES-PARALLEL CONTROL AND THE ELEC- TRO-PNEUMATIC CONTACTOR The series-parallel system of control is applied to two motors, or groups of motors, so arranged thait they are con- nected in series across the line for acceleration to half speed and operation at this speed. The motors may then be connected in parallel and accelerated to full speed. The motors must be mechanically connected together. Other- wise, one motor may accelerate faster than the other, which results in unequal distribution of load. This control is usually applied to cars moving along a relatively horizontal track, such as street railway, interurban and main line cars and locomotives. It is also used for cars in steel mills, coke plants, etc., for the purpose of conveying material from one point to another. These latter cars may be controlled automatically by a push button, or by a standard street car controller. This system of control is sometimes used for slope hoists, where the angle of the slope is small, and for the bridge travel of large ore bridges. Some controllers are arranged for operation in either series or parallel, the motors being connected permanently in series or in parallel by means of a change-over switch. This switch is interlocked so that the controller must be turned to the off position before the connections are changed. An arrangement of this kind is known as " series and parallel " control. It has only a very limited application. If the resistor is designed for accelerating with one com- bination of motors, it gives poor acceleration on the other II 147 148 CONTROLLERS FOR ELECTRIC MOTORS combination, unless the connections to the resistor are changed, which further complicates the control. The only occasion for using such a control would be where there are considerable periods of time when the apparatus is re- quired to operate at half speed, which can be obtained by- series connection. The series-parallel control is usually associated with street railway and steam railway electrifications; the system, however, is applicable to many industrial railways, to mining locomotives, automobile trucks, and similar appli- cations. The advantages of the series-parallel control are obtained when the period of acceleration extends over a considerable period of time and represents an appreciable part of the complete duty cycle. This is obtained in horizontal trac- tion and many of the applications already enumerated. A saving is accomplished when the motors are connected in series, since the current drawn from the line is one-half of the value which would be taken if the motors were connected in parallel. This is particularly desirable for starting a car or train of cars where the static friction requires a considerable torque in excess of the running torque. Where the period of acceleration is short, the sav- ing is often counter-balanced by the loss during the transi- tion period. The series-parallel control gives operation at one-half normal speed. This is desirable where trolley cars are operating through a congested portion of the city. If the industrial car is operated automatically, the series combina- tion would give a slow speed, from which the stop could be made more gradually than from the parallel combina- tion. The reduction in starting current may sometimes permit the use of smaller feeders for a trolley system or other power distribution. The acceleration of the motors SERIES -PARALLEL CONTROL 149 in series to half speed, and then in parallel to full speed, results in a saving in the weight of the resistors, which is considerably more than the additional weight to the control equipment, so that a net saving in the total equipment is obtained. The disadvantage of the series-parallel control is the added complication in additional parts to the controller. Where rapid acceleration is required, the transition period from series to parallel introduces a time element, which is objectionable. For instance, many industrial motors are accelerated in approximately three seconds. If one second were taken for the transition period, this would add 33 per cent, to the total time of acceleration. If the complete cycle were completed in six seconds or ten times a minute, the introduction of this extra second would eliminate one cycle per minute, which might be very undesirable. Even where the bridging system is used and no loss is expereinced in the progressive acceleration, the ad- d. . , . . , <• , Resistor No.] Motor Na2 Motor itional time required tor the opera- ^jmSL-(T)—JM'—r2)—wi^ tion of the extra switches would still F,rst&rin ^'■ add a considerable time element to ^Miif -^yyj^fji.^(Jyu^/^f^ the cycle. Where the motor oper- fuji sp«d s«ri=s °' ates a vertical hoist, or has a similar ,— ""Ml — (7)— W- -(7)— w/4l load, the reduction in torque, which -r.„....„„ usually occurs at the transition luumiL period, would cause a slowing down of the motors which would be very undesirable and would more than compensate for any saving which ^- ,, ^ might be effected during the period fuh sp^od paraiiei of acceleration of the motors in „ „, „ . Fig. 86. Steps in the Open series. Circuit Transition Where the series-parallel system Method op Series- ^ ' Parallel Control. is considered for a new application, careful analysis should be made of the accelerating condi- ' — ^%Z^- ISO CONTROLLERS FOR ELECTRIC MOTORS tions to determine whether this system of control is the most desirable. There are three common methods of changing motors from series to parallel. They are known as : 1. Open circuit transition. 2. Shunt transition. 3. Bridging transition. Open Cricuit Transition was the first method introduced. It is illustrated in Fig. 86. Fig. 87 shows the relation between speed and torque during acceleration. In passing from the full series to the first parallel notch, the circuits of both motors are opened. This method is practical for small motors and ordinary service. It is objectionable because the motor circuits are opened, causing arcing at the contacts of the controller and a loss of torque in the mo- tors. The method of control is simple and easily understood. The motors are per- manently connected in series and started by inserting resistance in series with them. This Speed resistance is gradually Fig. 87. Speed-Torque Curves with Open short-circuited as in Circuit Transition. ''^°" Circuitea, as m rheostatic control. This gives half speed with both motors across the line in series. The motor circuits are then opened and the motors are connected in parallel and accelerated from half speed to full speed by introducing resistance in series with each motor, and gradually short-circuiting it. SERIES-PARALLEL CONTROL 151 Shunt Transition is an improvement over open circuit transition. It is based on the principle that a short-circuit can be placed around the armature and fields of a series motor without injuring the motor. The short-circuiting of the armature through the field reverses the field current, which in turn, reduces the fields and counter e.m.f. to zero. The method of control is shown in Figs. 88 and 89. This Firsr Scnea *^' Full Sptcd Sencs '^' —Torque of ihe Motor in Circuit at Transition ^UUUUl^ FuJlSptcd Parallel Fig. 88. Steps in the Shunt Fig. 8g. Speed-Torque Curves Transition Method of Series- with Shunt Transition. Parallel Control. system allows one motor to remain active while the other motor is being short-circuited, and in this way, an active torque is maintained on the apparatus during the transition period. In passing from full speed series to the first notch in parallel, the proper amount of series resistance is first inserted and then motor No. 2 is short-circuited. This resistance limits the current to compensate for the counter c.m.f. of motor No. 2. No. 2 motor is then connected in parallel with No. i motor, and the series resistance gradu- ally is short-circuited until the motors are connected across 152 CONTROLLERS FOR ELECTRIC MOTORS First Series Gr Second Series Gr Third Scries Gr. Fourth Series Or. i- yiTJiiffliy -^ — vw — — vw-i Full Speed Scries Transition ^- *Gr the line, giving full speed. This method of control has been used in type K drum con- trollers, which are still in ex- tensive use on trolley cars. One of the type HL elec- tropneumatic controllers uses this principle of control. Fig. 90 shows the connections in detail. An improvement has been made in this controller by using some of the sections of resistors several times. This is permissible, as the use of re- sistors in series requires the short-circuiting of these re- sistors in sections. Ordinar- ily, the first section of the resistor has the maximum re- sistance and is in circuit the least amount of time. In order to use cast iron grids for these resistors, this first section usu- ally has more capacity, and requires less capacity in pro- portion, than the balance of resistors, as it is impractical to obtain high ohmic value with a small number of grids. If this section of resistor is now used in another part of the acceleration by connecting it in parallel with other resistors, the weight of metal is used to better advantage, and reduces the total weight of the resistor. The advantages of this method of transition are: 1. Ab active torque is maintained upon one motor during the transition period. 2. It is the simplest method in general use. Full Speed Parallel Fig. go. Steps in Shunt Transi- tion Using an Electro- Pneumatic Controller. SERIES -PARALLEL CONTROL 153 The disadvantages of this method of transition are : 1. A reduction in torque is obtained during transition, as only one motor is active. 2. This active motor is subject momentarily to a very heavy overload. 3. The change in torque on the short-circuited motor during transition usually results in the motor being momentarily driven by the other motor which takes up the lost motion in the driving gears in the reverse direction. When the motor becomes active again, this lost motion is again taken up in the positive direction. This double action results in two shocks in the driving machinery and has a tendency to cause excessive wear and loosening of parts. Bridging Transition. — The transition from series to parallel by this system consists in placing a shunt or bridge between the motors so that all the motors are active during the transition period. This is illustrated in Figs. 91 and 92. Its operation is similar to a Wheatstone bridge. The two sides of the bridge consist of the motor plus a resistor. If the drop in voltage through ^ ^ the resistor is equal to the .;lnmra£0^A«--J«"H£>^ drop through the motor, the two parts of the circuit con- nected by the bridge will be at the same potential, and no current will flow. It is difficult to obtain this exact balance with manual operation; it can be closely approximated, how- ever, where automatic acceler- ation is used. The arrange- ment of circuits for this method consists in a portion of the series resistance being inserted between the two motors. Pass- ing from the full speed series position to the first parallel notch, the two sections of resistance are inserted in parallel with the motors, so that each motor has a circuit from trol- First Scries Jl'ilUUilL, r-^M™^^ Full Speed Scncs >^nnnnnn>^' Juiniui ' 1 (7yfn- Full Speed Parallel Fig. 91. Steps in the Bridging Transition Method of Series-Parallel Control. 154 CONTROLLERS FOR ELECTRIC MOTORS ley to ground through the motor and a section of resistance, the final series connection forming the bridge of the cir- cuit. This bridge circuit is then opened and the resistance gradually short-circuited until the motor is brought up to full speed. If the resistors are so adjusted that more current passes through the two resistors than through the two motors dur- ing the bridging period, the opening of the bridging switch will interrupt this excess current and give an increased torque on the motors, equivalent to an additional notch of the controller. This notch can be so adjusted as to be equal to the other accelerating notches, so that the acceleration through the transition period compares favorably with that during other periods. The advantages of this method of control are: 1. An active torque is maintained on the two motors during the transi- tion period. 2. By, properly adjusting the resistors at the time of transition, an active accelerating notch is obtained at this time, which gives a smooth acceleration. Since both motors are active, no jerks are obtained. These advantages make this method of transition the best for heavy traction applications. The disadvantages of this method of control are : 1. Added complication. 2. Additional switches. 3. Increased arcing. This latter is objectionable only where drum con- trollers are used. Contactor switches are well adapted for this service and the arc can be properly distributed so as not to cause excessive wear. The use of series-parallel control in railway work has identified it with the electropneumatic controllers. This controller is in general use for railway work but is also used for some industrial applications where compressed air is available, or where the installation is of suflScient size to warrant the use of compressed air. Often compressed air proves a valuable means for operating mechanical brakes. SERIES-PARALLEL CONTROL 155 clutches, and similar apparatus, so that on some large ore bridges and similar industrial applications, the electropneu- matic controller has been used instead of the magnetic con- tactor controller. An electropneumatic contactor is shown in detail in Fig. 93. The valve is held in the open position by a small spring immediately under the valve magnet. When the magnet is energized, the armature presses the valve down Fig. 92. Speed-Tokque Curves with Bridging Transition. connecting the cylinder to the air pressure and closing the exhaust. The air pressure then closes the main contacts, against the action of a heavy spring. The valve stem ex- tends through the top of the valve magnet, so that the valve can be operated by hand. The design of the con- tactor is such that a failure of the air pressure or a failure of the electric circuit to the valve magnet causes the contact to open, placing it in a safe position. Some of the advantages of the electropneumatic system are as follows : I. A large operating force is available for closing the switches. This permits the use of a. strong spring for opening the sviritches so 156 CONTROLLERS FOR ELECTRIC MOTORS that a large surplus of power is available for both opening and closing the contactor. The spring for opening the contactor is placed between the piston and the top of the cylinder. The amount of air required is very small, as the cylinder seldom ex- ceeds three inches in diameter and the stroke is usually less than one inch. Fig. 93. Electropneumatic Contactor. 2. Only a small amount of power is required for operating the valve. This permits the use of a low-voltage control circuit, which may be as low as ten volts. With this small wattage on the control circuit, practically no wear from arcing occurs on the contacts of the master switch or interlocks, and there is little or no insulation trouble in the connecting wires. A low voltage permits the inter- lock and master switch contacts to be placed close together, en- abling a compact design to be used. Dust or dirt has little effect on these parts, as the voltage is too low to cause trouble from creep age. 3. The design of these switch groups is such that the live parts con- nected to the line voltage are well protected from contact by an SERIES-PARALLEL CONTROL I 57 operator, so that any trouble with the control circuit can be in- vestigated with little personal hazard. If the power circuit is dis- connected, the operator has only a low voltage control circuit to work with, which renders the testing out of this control circuit free from hazard. 4. The use of air for closing the switches gives a slower motion than is usual with a magnetic contactor. This reduces the bouncing of the contacts at the instant of closing, which is very desirable. The time of opening and closing can be adjusted within certain limits and the time element taken advantage of to protect the apparatus during acceleration. In many applications, it is more convenient not to use automatic acceleration. For instance, a trolley car may be accelerated on a level track, on an up grade or a down grade. It is obvious that the current for accelerating this car will be diiferent under the three conditions. Automatic ac- celeration for this application would not give as good results as an intelligent operator can obtain without the automatic feature. The introduction of a slight time element in the closing of each switch gives the motors a chance to adjust themselves to new conditions and protects the apparatus to a considerable extent. The speed with which the switch opens is also adjustable so that quick open- ing can be obtained where it is desirable. 5. The operation of the contactor is independent of the line voltage. This is particularly desirable in railway work where the range of voltage is often 2:1. Where the low-voltage control circuit is supplied by a shunt on the main line, the valve magnets can be adjusted for operation over a wide range of voltage. This is par- ticularly desirable on alternating-current circuits, the operating circuit being supplied from a low-voltage transformer. The force required for closing an alternating-current magnet is directly proportional to the square of the voltage, so that magnets which are designed to operate on a low voltage will slam badly on a high voltage. The armature of the valve magnet is so small that it has very little inertia, and the slamming effect is minimized. If, how- ever, the contactor were closed by a magnet, the moving parts would be heavy and the excessive slamming detrimental to the life of the switch, as a whole. Where the control circuit is sup- plied from a battery, this battery is automatically charged in con- junction with the air compressor motor, so that its maintenance is reduced to a minimum. 7. The weight of an electropneumatic contactor is usually less than that for a correspinding magnetic contactor. The space re- quirement is also less. These are important considerations for railway applications and some industrial control. CHAPTER XIII VOLTAGE CONTROL OF DIRECT-CURRENT MOTORS If a direct-current motor has its field excited at a con- stand voltage, its speed will be proportional to the voltage impressed on its armature. The methods of obtaining a reduced voltage by means of resistance in series with the armature have already been described. Another method that is little used at present is to provide a source of power, using four or more power wires, the voltage between the different wires being proportioned so that a considerable number of operating voltages can be obtained by connect- ing the motor armature to different pairs of wires. This method is known as the " multivoltage system." It is ob- jectionable because it requires a number of power wires and also special generator equipment. There are also power circuits provided with two outside wires and a cen- tral or neutral wire, the voltage between the outside wire being double that between either wire and the neutral. These systems usually have voltages of 115 and 230 respec- tively, and are used for a limited number of industrial applications. The increasing size of direct-current motors for hoisting, and the application of motor drive to reversing steel mills during the last ten years has brought into general use a system of voltage control, in which a separate generator is provided for each motor. This generator may be driven from any source of power but is usually driven by a con- stant speed, alternating-current motor. The armature of the generator is connected directly to the direct-current 158 VOLTAGE CONTROL OF DIRECT-CURRENT MOTORS 1 59 motor, as shown in Fig. 94, both machines having their fields supplied from a constant voltage exciter. The slow speeds of the motor are obtained by reducing the strength of the generator field. If the generator field is reduced to zero and energized in the reverse direc- tion, the rotation of the motor will be re- versed. The controller in Fig. 94 shows one means of doing this. The rheostat consists of a closed circuit in the form of a circle. Points A and B are connected to the -(- and — side of the ex- citer, and C and D are connected to the generator field. When the rheostat is in the position shown, the generator field is zero, and consequently the motor speed is zero. If contact i is moved to coincide with point Aj contact 2 with Cj contact j with Bj and contact 4 with D, current will flow from the exciter to A through I and 2 to C, thence through the generator field to D and from contact ^ to j to B, and thence to the exciter. This will give the maximum field strength to the generator and cause the motor to operate in a forward direction at full speed. Any position between the one shown in the diagram and the one just described, will give intermediate values of field strength and cause the motor to operate at reduced speeds. If the controller is moved so that contact I coincides with point Cj contact 4 with point A, etc., cur- rent will flow from the exciter to A through contacts 4 and J to D, through the generator field to C, through contacts. Motor Field Rheostat Fig. 94. Connections for Reversing a Steei. Mill Motor, Showing the field reversing switch of the alternating-current generator. i6o CONTROLLERS FOR ELECTRIC MOTORS I and 2 \.o B and then to the exciter. This will cause the motor to operate at full speed in the reverse direction. In- termediate positions of the control will give intermediate speeds. The advantage of such a method of control is obvious. The speed and direction of rotation of the large motor M in Fig. 94 is controlled by switching the small field current of generator G. This current may be in the neighborhood of lOO amperes, while the armature current flowing from G to M may be several thousand amperes. This method gives a large number of fixed running speeds and the only losses which occur are the usual losses in the generator and motor in addition to the rh ecstatic losses in the field control. The speed of motor M may be retarded by reducing the generator voltage to a lower value than the counter e.m.f. of the motor. This causes the motor to regenerate and ijnt produces dynamic braking. If the generator G is driven by a suitable motor, this method of slowing down will return power to the line. If it is driven by an engine, the generator cannot ab- sorb the power, and the regen- erated current must be wasted in a resistance. The field of motor M can also be varied to increase the speed It is found in practice that the motor M can be arranged for speed control of i : 1.5 or i : 2 by varying its shunt field without much additional expense. If less than the full range of operating speed is obtained from the generator, a smaller generator can be used. Therefore, the combined field control of the generator and field control of the motor gives cheaper commercial appa- ratus than to obtain the entire range of speed control from Fig. 95. Connections of Slip Regulator to Induction Motor. range of the combination. VOLTAGE CONTROL OF DIRECT-CURRENT MOTORS l6l the generator. This double method of control may seem complicated, but as it is usually combined in one master switch, very little additional apparatus is required. When the generator G is driven by a motor, this method of controlling the operating motor is usually combined with a system for reducing the maximum power demand from the generating station. The power supplied for large in- FiG. g6. Diagram of Connections of Equalizer Flywheel Hoisting Set, A.C.M., wound-secondary induction motor ; F, flywheel ; D.C.G., separately- excited direct-current generator ; E, exciter ; D.C.M., separately-excited direct-current motor; S.R., automatic liquid slip regulator; T, torque motor for slip regulator ; O.C.B., oil circuit-breaker ; F.C., reversing field controller for generator ; R, rheostat for motor field ; V .R., voltage regulator for ex- citer. A, ammeter; V, voltmeter; W, watthour meter; I.W., integrating wattmeter. stallations is usually alternating current, so the generator G in Fig. 94 is driven by an induction motor, shown in Pis'- 95- O'^ the motor-generator shaft is placed a large flywheel, which is used for storing energy during the low demand period of the cycle and for supplying energy during the maximum demand periods. This flywheel performs l62 CONTROLLERS FOR ELECTRIC MOTORS a similar function to a storage battery floating on the direct- current system. The flywheel gives out energy or absorbs energy, depending upon the speed of the motor generator set. The driving motor for this set is provided with a wound secondary and a slip regulator is introduced in its second- ary circuit. If the resistance in this secondary circuit is varied automatically with the load, the motor will take an Fig. 97. Equalizer Flywheel Hoisting Set, Showing automatic liquid slip regulator. approximately constant amount of power from the line. This is only desirable above, say, full load on the motor. If the demand for power is in excess of this load, the slip regu- labor introduces more resistance in the motor secondary and allows the motor-generator set to slow down, so that the flywheel can supply this excess of power. When the de- mand on the generator is less than normal, the resistance in the secondary of the induction motor is decreased and the excess power input is used in accelerating the flywheel, thus storing up mechanical energy to be given out later when an excess demand occurs. VOLTAGE CONTROL OF DIRECT-CURRENT MOTORS 1 63 A diagram combining the control of the motor-generator set with the control of the operating motor is shown in Fig. 96. This diagram is merely a scheme of connections in- tended to illuMrate the principle, and does not show any of the control apparatus in detail. The slip regulator in the secondary of the induction motor consists of three fixed electrodes marked B in Fig. 95. Each of these electrodes is insulated and connected to one of the slip rings of the induction motor. Above them in the liquid are suspended three electrodes marked A attached to a common support C and electrically connected through this support. The solu- tion in the tank, known as the " electrolyte" is a solution of washing soda and water. Referring to Fig. 96, it will be seen that the movable electrodes are raised or lowered by a small torque motor T which is energized from three series transformers in the primary circuit of the induction motor. The weight of the moving element is partly counterbalanced, but is still sufficient to move the plates together. The torque motor tends to separate the plates. This motor operates in the same manner as an ammeter ; the plates move up or down until the torque of the motor just balances the weight of the moving element, which occurs at substan- tially the same current values for all positions of the plates. The friction in a commercial regulator does not require more than five per cent, diiference between the torque for raising or lowering the plates. The upper curve in Fig. 98 shows the regulation obtained with one of these slip regulators in commercial work, as compared with the regulation obtained with a magnetic contactor control for cutting resistance in and out of the secondary of the motor, as shown in the lower curve. These curves show the advantages of the liquid regulator, and that the maximum power input is quite uniform. The minimum power input depends largely upon the way in which the load comes on and off the operating motor, so that these low 164 CONTROLLERS FOR ELECTRIC MOTORS peaks decrease to a very small value if the demand for power is small over a considerable period of time. An exciter is shown mounted on the shaft of the motor- generator set in Fig. 96. As the speed of this exciter varies with the speed of the motor-generator set, it is necessary to provide a voltage regulator, in order that the voltage of this exciter will remain constant over the speed range which is obtained in practice. This is a much simpler arrange- ment than to use a separately-driven exciter. 3000 IjIIU -_^^ - ^ -%-,^ ^-^ ^w— ^ IJLU - - ~V ' V. ^^\y "■■ 1 2 02 P M. 3:01 P.M. !;00 p. M. k "^ 1 -s in ^\ y v^ ] /\ ^/»-^ ■V / "' \ J ''\ V»/^ \, / \. -y ■v .--V^-^ \ V ^ L. ^ n Fig. 98. Power-Demand Curves of a Typical Hoisting Set. Upper curve shovps the regulation, using a slip regulator; lower curve the regulation using magnetic contactor control. It is becoming the practice of power companies to make a charge based upon the maximum demand required from the power system. This is a just method of charging for power, as the size of the generating station must be deter- mined by the maximum demand of the customers. Where a large motor is applied to a hoist, considerable power is taken from the line to accelerate the hoist, if the motor is connected directly to the supply system. The charge for power may be large, on account of this maximum demand. If the motor is operated from a flywheel motor-generator set, as previously described, the maximum demand can be kept quite low, as shown in Fig. 98, particularly if a liquid regulator is used, so that a reduction is made in the power bills by using this system. Another saving results from the regeneration of power when a load is lowered or when the hoist is brought to rest. By means of this voltage system VOLTAGE CONTROL OF DIRECT-CURRENT MOTORS 165 of controlling, there is very little rheostatic loss, so that less power is taken and a large percentage of the energy given out by the descending hoist is returned to the line by regeneration. It is not difficult to determine the size of a flywheel to absorb the peak loads when used in connection with a motor- generator set. A curve should be drawn showing the rela- FiG. 99. Load-Time Curve of a Typical Hoisting Set. tion between the horse-power required at any particular in- stant and the time. It is usual to plot the horse-power as ordinates and the time as abscissae. From this curve, the average input can be obtained and the maximum demand over any given period of time. To illustrate, in Fig. 99 is given a load-time curve of a hoisting set, to lift 5,000 lbs. at 1,200 ft. per min. The accelerating and retarding periods will be equal to 1,760 horse-power seconds and the constant speed periods will be 3,540 horse-power seconds, making a total of 5,300 horse- power seconds in excess of the average requirements. This represents the total energy which must be given out by the flywheel over the maximum demand period. This energy must be returned to the flywheel during the periods where the power demand is less than the average. The weight of the flywheel depends upon the type of construction and the maximum peripheral speed. Let Fj 1 66 CONTROLLERS FOR ELECTRIC MOTORS equal the velocity in feet per second at the radius of gyra- tion for maximum speed, and Fj for the minimum speed. The simplest form of flywheel and one of the best is made up of solid circular plates. For this type of wheel, the radius of gyration is equal to 0.707 of the wheel radius. If 20,000 ft. per min. is selected as the maximum peripheral speed, which corresponds to good practice, the maximum velocity at the radius of gyration will be : 20,00 X 0.707 , (^ Vi = =236 ft. per sec. If a minimum speed is assumed equal to 85 per cent, of the maximum speed, which is good practice, the minimum peripheral speed will be V2 = 236 X 0.85 = 200 ft. per sec. The weight of the flywheel can now be calculated as follows : _ Hp. sec, to be supplied X 550 X 2g _ 5.300 X 35400 _ ~ (236)2 - (200)2 ~ ^^'°5°- The rotating element of the motor and the generator fur- nish some flywheel eff'ect, so that the horse-power seconds of the flywheel effect obtained from these two units can be subtracted from the total in calculating the size of the fly- wheel if it is desirable to figure very closely. ^ With a slip regulator it is possible to keep the motor load almost con- stant at the average value. Without a slip regulator the motor load increases with the decreasing speed, and the calculation is more complicated. See article on " Relation of Flywheel and Motor Capacity for Industrial Loads," by S. A. Fletcher and Chas. R. Riker, Electric Journal, March, 1912, p. 270. CHAPTER XIV MINE HOISTS Mine hoists may be divided into two general classes ; those for coal mines and for metal mines, the essential dif- ference being that coal mines are shallow and the metal mines deep. Coal mining practice differs in the anthracite and bituminous fields, and the practice varies in different Fig. 100. Hoisting Room for Voltage Control Equipment. Showing the flywheel motor-generator set to the right, the liquid regu- lator in the center and the control panel at the left. Magnetic contactors are used for varying the strength of the generator field. states. In general, hoists for coal mines run from 250 to 1,000 horse-power. In metal mines, the motor may reach 167 1 68 CONTROLLERS FOR ELECTRIC MOTORS double the size of that for coal mines. In addition to the main hoists, small hoists are frequently put in for handling men and supplies. These hoists may be so small that the ordinary form of drum controller may be used. The large controllers may be divided into three classes : 1. Contactor Controllers. 2. Liquid Controllers. 3. Voltage Controllers. Most mine hoists are operated from alternating-current supply lines and therefore use alternating-current slip ring motors, either to drive the hoist directly or through a motor- generator flywheel set to supply direct-current to the hoist motor. In some cases where direct-current power is avail- able, the direct-current motor which operates the hoist is controlled directly from the supply lines. CONTACTOB CONTROL This form of control has been used almost entirely for the smaller motors and also for some of the larger motors up to approximately 1,000 horse-power. It is much more durable than the drum controller for small motors and is often preferred, although the first cost is greater. The method of control is illustrated in Fig. lOi. The drum type master switch causes the motor to operate in either the hoisting or lowering direction and by changing the posi- tion of the handle, the speed of the motor may be changed.-* Ordinarily, a mine hoist is used for the purpose of hoisting material out of the mine. It is therefore operated normally under load conditions and a rheostatic control of this kind gives satisfactory operation. In the off position of the con- troller, a mechanical brake is applied for stopping the hoist 1 The speed-torque curves for this method of control are shown in Chap- ter VII. These curves show that the speed of the motor depends upon the load. MINE HOISTS 169 and holding it securely at the landing. This brake is often released by an electro-magnet, which is deenergized in the off position of the controller and applies the brake. Maslcr Switch Fig. lor. Diagram of Connections for Contactor Controller. This controller consists of two primary contactors, / and 2, for the pur- pose of connecting the primary of the motor to the line to give the proper direction of rotation. Four secondary contactors are controlled by current limit relays. These secondary contactors automatically short-circuit the resistor in the secondary circuit of the motor during acceleration. The primary and secondary contactors are controlled by a drum-type master switch. Two track limit switches automatically stop the hoist at either limit of travel. The motor is protected by a three-pole circuit breaker in the supply line. 170 CONTROLLERS FOR ELECTRIC MOTORS One of these controllers built for a 2,200-volt primary and a low voltage secondary is shown in Fig. 102. The primary contactors are provided with large magnetic blow- outs to take care of the high voltage. The secondary rh%^ J Alternating-Curkent >R. Tli%^ . rol the primary of the motor. One contuwi^^ jggSKI. ,:vi»s:.„jss: s _ bj} o o >-. en n « > ■s — el p. C3 01 a _ -*^ -^ d C u u 1h 1-1 ^ U o » >> ■^ >. >. -t-) tj >» i^. >. >x 5 ■" ■u 3 Jj S; -«-> -«-> M " ■Q _ •^ t^ i "^ ^ J3 - C3 •si s a ^ii J3 ^i B B 3 3 £5 Q ^ Im IH 3 z^ >- ^ Z> ;z; ZZ ■■=s m -. 0) CD 03 03 CO CO >^E-2 o o o o qS 2 Z2 > z> >< >< >> > ZZ > o o s o o OJ o o CO o o o c2 ZZ >• Z2 > s ■ZZ' > ZZ Ih V _o _o ,CJ _o _o ^o u o *i u o 3 +J '+J '■H '+J *J '+J *^ *5 *^ +3 '-3 (U (U rt ea o U SIS S isa ^ s ss 1 H ^^!^!^'!^!^^r!!^ !^. !^. M M N W CO ■=!■ M ^; CO « fo ro "^I" •* ro ^^ "" -^'— ' 1 - — --^ ! • o lO >o o o o O ^ lo' So \n o C4 o o o ■" O M 2 o O O ■t-i X rr) ro ^ " i> ro "i o s (Q 03 CO o . o Slcn o > E j ffi S o M 1" M g o = ■■ .5 -*- 'S 3 U M «., ^i d) s es a 3 C7* 3 "o o 3 3* 3-0 n S " M O T3 J3 " pi A> -■ at o o ^ 6 MACHINE TOOL CONTROLLERS 20I d Is B e u u dJ dJ CO to 11 Separate Separate On panel as E E 2 5 QQ E E 2 5 QQ u u 22 m to Mh o2 o o o o 2Z II o o o Z2Z o o o o Z2 (U o to > (2 O O o o 22 Z o >'2 o o S zz>; 1 c o o o bO bO ^ a o o 5i ^^1 o _y ^y OJ OJ OJ hJl U ojO nJ rt rt 1-s M M IH W H M IH tH M M M N IN (N '^ n N Tj- ^ « fO ro bo c a. 22 li-) Tt ^2 t~ o 2^ o2 °2 a III C Xh .S 1 n 22 £2 1 CO 'as S D > 2 V. ^1 3 *^ ii!;>Q ■s ^23 m c5 c 2 3 t 1 if) 1 fl ^ 202 CONTROLLERS FOR ELECTRIC MOTORS quired to give the desired flexibility of control and at the same time avoid unnecessary complication. In providing control apparatus for machine tools, it is very desirable to have as many parts as possible interchangeable, so as to minimize the repair parts required, to make it easy for the electrician to understand the control apparatus and make replacements where necessary. The speed of a direct-current motor may be changed in two ways : 1. By changing the resistance in series with the armature. This is known as varying speed control. The speed of the motor on any notch of the controller depends upon the load on the motor, the light loads giving higher speeds than the heavy loads. 2. By changing the field strength of the motor, known as adjustable speed control. This gives practically a uniform speed for each notch of the controller. Alternating-current motors are furnished for varying speed only. These motors are of the slip-ring type and have external resistance in the secondary circuit which is changed to vary the speed. The characteristics of control are practically the same as for a direct-current motor with armature control. The use of a varying speed motor is uneconomical if the motor is operated much of the time at reduced speeds, as considerable loss occurs in the series resistance. A wide range of speed can, however, be obtained with a less ex- pensive motor than if adjustable speed were used, and for some applications having intermittent service, such as bend- ing rolls, this form of motor is satisfactory. The adjust- able speed direct-current motor is the one usually employed where a change in motor speed is required. MACHINE TOOL CONTROLLERS 203 A list of the more common magnetic control applications is given in Table III. While the form of control given in this table is not always used with the application indicated, it represents a practical form of control and one that is suitable for most installation. The question of dynamic brake and drift points is governed to a considerable extent by the particular work performed by the tool; in some cases, it is a matter of personal preference. Where the motor is small, the controller shown in Fig. 124 can be used and both dynamic braking and drift obtained at small addi- tional expense. If the control of machine tools be analyzed, the various arrangements may be classified as follows : I. Nonreversing Control Panel. — The wiring dia- gram of this panel is shown in Fig. 126. It con- sists of a line contactor and one or more acceler- ating contactors, together with the starting resistor. The panel should also con- tain a knife switch and overload protection by either fuses or an over- load relay. It is prefer- able to mount this panel in a cabinet and arrange the handle of the knife switch so that the switch can be opened from the outside of the cabinet and can be locked in the open position to prevent the accidental starting of the machine tool when the attendant is adjusting or repairing it. This knife switch should be used only for disconnecting, and the motor should be started and stopped with the line contactor. The con- nections provide for low voltage protection. One of these controllers is shown mounted in a cabinet in Fig. 125. Fig. 125. Non-Reversing Controller. Mounted in cabinet. 204 CONTROLLERS FOR ELECTRIC MOTORS 2. Nonreversing Control Panel with Dynamic Brake. — The diagram of connections for this control panel is shown in Fig. 127. It is the same as for the control panel shown in Figs. 125 and 126, with the addition of a back contact, to provide for dynamic braking. The diagram also shows the connections to a field rheostat. Suimng Rts-istof ««uijilpniiiY- +-II ^1 1 II i Series ">»■ Ru„| 1 2 3 Fig. 126. Connections of Non-Reversing Controller Panel of Fig. 125. Starting Braking Fcsisri-,r B ^lsi_stGi J^ Field Rheo stat ■^ iijjA/\ferV- II i -Shun( Fu;ld Con Off Si. ni?un 1 2 lA Field Disctiarge ResLStor Fig. 127. Connections of Non-Reversing Panel with Dynamic Brake. Starting _^ BraKing KesistuT rZ Resistor 3 A F»id_Rheostat — Tiuwlauiii^*- Sories 2 Fields, SJiuniFif-rrf Field Dtcharge Resistor ^on Off Dfi SUrt) s D :5 f, Fig. 128. Connections of Reversing Control Panel. Slarting Rcsisior s. Rpv "in -or ^ J. BL ^ s f ^- c s r. \^ 4A Wffi^ SliumField Fig. 129. Same as Fig. 128 with Dynamic Brake, •'^'ni, SurimsRra iSWl}. Rh T,.,...rd! su„ : ¥( 5? Sui. 1 3C r ■ c f> i ;i ( : i - lA : ■ 1/ 553 fi cc D D :c Fig. 130. Same as Fig. 129 with Addition of Drift Point Contactor. knife switch and overload protection and should be mounted in a cabinet, as described for the controller under item /, Figs. 125 and 126. 5 Reversing Control Panel with Dynamic Brake. — This panel. Fig. 129, is the same as Fig. 128, with the addition 206 CONTROLLERS FOR ELECTRIC MOTORS of a back contact on two of the line contactors. When both the forward and reverse contactors are open, the back con- tacts establish the dynamic brake circuit. 6. Reversing Control Panel with Dynamic Brake and Drift. — This panel is shown by diagram in Fig. 130, which is the same as Fig. 129 with the addition of another line contactor, 8. This additional contactor is necessary only where the forward and reverse switches are double pole. Where single pole switches are used for reversing the motor, the drift position can be obtained by opening the reverse contactors, 2 or j, which are not equipped with back contacts. The opening of one of the reversing contactors disconnects the motor from the line on one side and allows it to drift. The opening of the other line contactor completes the dyna- mic brake circuit and stops the motor. Since these con- trollers with reversing contactors are usually applied to large motors, the addition of the extra contactor on the other side of the line is frequently desirable, as it entirely disconnects the controller from the line. This disconnect- ing is not always necessary, as the opening of the knife switch will effect the same results. The knife switch, how- ever, should never be opened under load, and the use of the extra contactor is sometimes desirable to clear a short-cir- cuit, due to a ground. Items 3 and 6 provide for this extra contact, which may or may not be opened in advance of the dynamic brake contactor, depending upon whether or not a drift point is required. 7. Field Rheostat Separately Mounted. — This rheostat. Fig. 131, can be used in connection with any control panel to provide adjustable speed control for the motor. It is desirable to have this field rheostat mounted separately from the panel, so that the operator does not have to place his hand near the main control circuit for operating it. It is also desirable to have the rheostat separately mounted so MACHINE TOOL CONTROLLERS 207 it can be located in a convenient place and the control panel located on the machine tool or the wall where it is not readily accessible to unauthorized persons. Where the con- trol panel is enclosed in a cabinet, the rheostat may be mounted in the cabinet with the handle extending to the Fig. 131. Separately Mounted Field Rheostat. Fig. 132. Combined Master Switch AND Field Rheostat. outside. This requires the control cabinet to be located in an accessible place and is not convenient for many machine tool applications. 8. Push Buttons. — Push button stations may consist of one or more buttons arranged for manipulating a controller panel. They are usually applied to nonreversing panels and consist of a start and stop button. They may be located close to the field rheostat making a very compact and neat arrangement where the motor is operated for considerable periods of time. g. Drum Reverse Switch. — A small switch is illustrated in Fig. 124. It may be used in cennection with nonrevers- ing panels to give reversing control. The same arrange- ment can be extended to larger switches when desirable. 70. Master Switch and Field Rheostat Combined. — This arrangement, Fig. 132 is very convenient for lathes and 208 CONTROLLERS FOR ELECTRIC MOTORS similar tools, particularly where large motors are used. It is used in connection with one of the panels described in items / to 6. 11. Reversing Switch and Field Rheostat. — This consists of a controller for reversing the direction of rotation of the motor, combined with a field rheostat and provided with contacts for operating one of the control panels, items I to 3. 12. Reversing Controller with Field Rheostat. — Fig. 133 shows a small controller which is selfcontained and can be used for adjustable speed mo- tors on reversing service. A great many controllers of this type are used for motors up to ten horse-power capacity. It is easy to operate and proves quite durable for these motors. It does not require a controller panel. It is usu- ally connected directly to the line through the knife switch and fuses. Where adjustable speed mo- tors having a considerable range of speed adjustment are used, it may be desirable to provide for starting with full field strength. This can be taken care of in several ways. Where the field rheostat is not mechanically connected to the master switch a conven- ient method is to use a small accelerator for short-circuiting the field rheostat during acceleration. This contactor can be made to operate automatically on a variation of current Fig. 133. Reversing Controller WITH Field Rheostat. Armature starting and field control. MACHINE TOOL CONTROLLERS 209 strength when desirable, but precautions should be taken to reverse the functions of this contactor or relay during re- generation. The relay, however, should short-circuit the field rheostat during dynamic braking. The amount of comjDlication involved in the use of this relay will depend upon the size of the motor. For small motors, a small con- tactor held closed until the last accelerating switch is closed will be found very satisfactory and will eliminate most of the complication. The twelve items described above, make a very convenient combination for a manufacturing standpoint, as well as from the standpoint of the user. The control panels, par- ticularly item I, can be used for pumps, fans, and many forms of drive other than machine tools. Items 7 to 12 will be found useful for a variety of other applications. A manufacturing establishment requiring machine tool con- trollers has other apparatus operated by motors requiring control. It is therefore desirable to adopt a few standard controllers which have a wide application. CHAPTER XVII MACHINE TOOL CONTROLLERS (CONTINUED) One of the most significant movements of the past few- years has been the effort to guard employees from physical injury. The principal danger from electrical apparatus is a shock or burn, due to contact with live parts. Control apparatus should be so guarded that the operator will not come in contact with live parts when handling any of the control necessary for his work. It should also be protected Fig. 134. Universal Wood Milling Machixe, Driven by a reversible direct-current motor with speed adjustment by field control. 210 MACHINE TOOL CONTROLLERS 211 SO that tools, pieces of iron, chips, or other material cannot come into accidental contact with live parts. This may be accomplished by enclosing all current-carrying parts and providing projecting handles for the operation of the switches; or the control panel may be protected by grill work; or placed eight feet above the floor. The master con- ^H^^^^^^^f^ Fig. 135. Electrically-Operated Radial Drill, Driven by non-reversing shunt motor with speed adjustment by field controL troller should be arranged in a convenient manner so that the operator is not required to reach across his machine or in any way expose himself to injury during operation. This convenience will also increase production. In some cases, it is desirable to provide several stations, from which a machine can be stopped in case of accident. These stations usually consist of push buttons wired in series 15 212 CONTROLLERS FOR ELECTRIC MOTORS with the operating coil of the contactor or a low-voltage coil, so arranged that the pushing of any button opens the circuit and disconnects the motor from the line. A uni- versal wood-milling machine which is motor-driven and provided with three control elements is shown in Fig. 134. At the base of the pedestal is mounted a cabinet containing Fig. 136. Electrically-Operated Turret Lathe, Equipped with old style control panel which has the field rheostat mounted with the contactors. the line switch and fuses together with a line and an ac- celerating contactor. On the inside of the cover which can be locked in the closed position is attached the wiring dia- gram and instructions. The knife switch is operated by a handle extending through the right-hand side of the cabinet. This knife switch is used only for disconnecting purposes and can be locked in the open position. Near the center of the table and located on either side of the operating levers MACHINE TOOL CONTROLLERS 21 3 of the machine is a field rheostat and a drum reversing switch. Both of these are covered to prevent contact with live parts. A radial drill is shown in Fig. 135. The same cabinet is used as in Fig. 134. The motor is nonreversing and the master switch is combined with the field rheostat. Fig. 137 illustrates a coil winding table. A number of these Fig. 137. Small Coil Winding Table, Operated by a reversing direct-current motor, with speed adjustment by field control and low-voltage protection with reset button. tables are located together and in the background can be seen the control cabinets for six tables. There is a handle on the outside of the box for opening the knife switch. A con- tactor with a blowout is in series with the motor ; two other contactors short-circuit the starting resistor during accelera- tion. Each table is provided with a reversing drum con- troller and a field rheostat. The drum controller also serves as a master switch and is operated by a treadle. The push button, shown underneath the reverse switch in Fig. 137, is a reset button for low- voltage protection. In case of failure of voltage, the motor cannot be started again without push- ing this button. 214 CONTROLLERS FOR ELECTRIC MOTORS A turret lathe with an older form of control panel, in which the field rheostat is mounted on the panel with the cantactors is shown in Fig. 136. While many of these Fig. 138. Turret L.ithe with Drum Controller^ For reversing service. A field rheostat in the bottom of the controller is used to adjust the speed of the motor. Fig. 139. MoTOR-DRn-EX Engine Lathe. The controller is operated from the spline shaft of the lathe. panels are still in use, they may not comply with many safety requirements now enforced, as the operator can MACHINE TOOL CONTROLLERS 215 readily obtain a shock by carelessly placing his hands on a live part of the control. Fig. 140. Wheel-Lathe Controller. When this panel is equipped with a cover, the field rheostat is mounted on the inside and operated from a handle on the outside. Fig. 138 represents the use of a drum controller arranged for armature starting and speed regulation by field con- trol on a turret lathe. The controller is located close to the motor on the head of the machine, making a compact in- stallation. The operator manipulating the machine stands within easy reach of this controller. This same form of controller is shown in Fig. 139 operated from the spline 2l6 CONTROLLERS FOR ELECTRIC MOTORS shaft of the lathe. This same arrangement can be used with the drum reversing switch and a separate control panel. The methods of control illustrated in Figs. 134 to 139 are very simple, consisting of a line switch which may or may not reverse the motor, together with suitable means for Fig. 141. Pushbut- ton Pendant Switch. Fig. 142. Planer Master Switch. short circuiting the starting resistance. A field rheostat may be added where adjustable speed motors are used. Ap- plications of this kind do not present any unusual control features. While they can readily be made up from the units described in Chapter XVI, the arrangement must be made to suit the particular design of machine. Some other applications, however, require considerably more study. WHEEL LATHE The application to the wheel lathe requires special con- sideration in order to obtain the maximum convenience in the operation of a machine designed for this particular pur- pose. The controller is illustrated in Fig. 140 and is oper- ated by a push-button station having buttons marked start, stop and slow. This push-button station can be arranged MACHINE TOOL CONTROLLERS 217 for suspension by a flexible cord, and used as a pendant switch as shown in Fig. 141. The controller is non-revers- ing and is provided with current limit acceleration. If it is necessary to reverse the lathe for any purpose, it can be done by means of the knife switch shown at the bottom of the panel in Fig. 140. In the cover of the panel is located the field rheostat with the handle projecting to the outside. In turning up a large wheel, hard spots are often encoun- tered requiring a slow cut over a part of the circumference. This can be obtained by depressing the button marked slow. "Inching" of the motor can be obtained by manipulating the start and stop buttons. This is very desirable in setting up work. PLANER CONTBOL When a reversible motor is used for driving a planer. Fig. 143, the motor must be stopped and started quickly in Fic. 143. Reversing Planer Controller. Operating 34:1 speed direct-current motor. The handles for the field rheostats are shown in the cover of the controller to the right of the motor. One handle is used for adjusting the speed of the cutting stroke and the other for the return stroke. On the side of the planer is shown the master switch connected to the reversing gear operated by the platen of the planer. In front of the planer head is shown the pendant switch. Close to the main motor is a small drum reversing switch for controlling the motor which operates the tool carriage. 2l8 CONTROLLERS FOR ELECTRIC MOTORS the reverse direction. This requires a special motor, as well as a special controller. It is desirable to have a motor which gives a large torque with a small diameter of arma- ture. The work done in re- versing the platen of the planer consists in dissipating the stored energy in the mov- ing parts until the plateau comes to rest, and then stor- ing energy in the moving parts during acceleration in the reverse direction. As the planer platen moves slowly, it has very little stored en- ergy. Most of the energy stored is in the motor arma- ture; hence the larger the di- ameter of the armature, the more work must be done in Fig. 144. Reversible Planer Con- • „ tuv -„„.,;,^.>ionf TROLLER reversmg. Ihis requirement For use with adjustable speed di- has resulted in the production rect-current motors. The field rheo- r , j •„ j f„« t^Uli, stats are mounted inside the cover of motorS designed for thlS and operated from the handles on particular service and known the face of the cover as shown in , . -n. .„„ Fig. 143. One rheostat is for the as planer motors. These mo- cutting stroke and the other for the tors usually have a speed ad- return stroke. . . ,. i_ 1. 4. justment of 4:1 by shunt field control. Two field rheostats are used, one of which JYfinriftiiiiiuY'^ SEQUENCE OF CONTACTORS ^ Field I Field Resistor^ n Zni Cu. fc t m R=>| " c c 4 c lA c c !A : c 5A c Fig. 145. Cox.vECTioNS of Reversing Planer Motor Control Panel. Shown in Fig. 144. MACHINE TOOL CONTROLLERS 219 controls the speed during the cutting stroke and the other rheostat during the return stroke. An arrangement of this kind is necessary so that the adjustment of the cutting stroke to suit the work will not interfere with the speed of return. During the stopping and acceleration period, both field rheostats are short-circuited automatically to give the motor the maximum torque during this part of the cycle. A master switch, as shown in Fig. 142, is located on one side of the planer and operated by a shifting mechanism controlled by projections from the platen of the planer. These projections or "dogs," can be adjusted to limit the travel of the platen in each direction. The master switch is operated like the belt shifting device on the old planers. Sometimes this master switch has been combined with a switch for reversing the direction of the motor. This caused considerable arcing in this switch and therefore much Fig. 146. Group of Wood Turning Lathes. Equipped with four-speed, squirrel-cage motors which are totally enclosed and operated by drum-type controllers. These controllers are located back of the leg of the lathe with their operating handles near the head of the lathe. better service can be obtained by using magnet contactors for switching the motor circuit and using the master switch only for the purpose of controlling the small wire circuits to the contactors. 220 CONTROLLERS FOR ELECTRIC MOTORS Chain and Pulleys Highest Position Lowest Position-^ % Floating Weight jOr '^ Rheostat '^^^^^s^^^^^te^s^ss;?;^ ReduehiK .SprocketOpeiatedt FieU RlieosBt Controllers for planer service require rapid acceleration and therefore the starting resistance is short-circuited in one or two steps. Even with a 150 horse-power motor, starting in one step has been found to give the best results when the motor and control are adapted for such operation. When it is realized that the platen of the planer may pass through the cutting and return stroke, making a complete cycle in six or seven seconds, the speed of stopping and acceler- ating is very important. It is desirable to pro- vide for an emergency Fig. 147. Arrangement of Controller stop in case of failure FOR A Hydraulic Accumulator. , , . For use in forge shops. 01 voltage. A common method of obtainingthis is to short-circuit the motor armature through a resistance and at the same time short-circuit the field rheostat. This causes the motor to operate as a self-excited shunt generator. Usually there is sufficient residual magnetism in the motor field to make this operation satisfactory. Where it is neces- sary to take extra precaution, a mechanical brake with a magnetic release can be mounted on an extension of the motor shaft. The magnet windings are energized by line voltage to release the brake, therefore on failure of line voltage the brake sets. The brake wheel adds to the stored energy of the armature of the motor; the magnet winding consumes energy while the planer is operating; the brake itself takes up extra room and requires a special exten- sion of the motor shaft. For these reasons, the brake is not used except where it is important to make a positive emergency stop. Fig. 144 shows a standard control panel, and Fig. 14S MACHINE TOOL CONTROLLERS 221 a Wiring diagram of a controller which has had a wide application. This panel provides for dynamic braking by connecting the motor as a self-excited shunt generator in case of the failure of line voltage. The direction of rota- tion of the motor is con- trolled by double-pole magnetic contactors, shown at the top of the panel in Fig. 144. The contactors are interlock- ed by a steel rod, which prevents both directional switches being closed at the same time. Each di- rectional switch is pro- vided with a back con- tact shown as lA and 2A on the diagram in Fig. 145, which com- plete the dynamic brake circuit. When either di- rectional switch is closed, this back contact is opened, disconnecting the brake circuit. The shunt field remains con- nected across the line when the planer is being operated. It is provided with two field rheostats, that shown in Fig. 144. only one of which is shown on the diagram, one rheostat being for forward operation and the other for reverse operation. The par- ticular rheostat in use is selected by the master switch. Connections are arranged so that these field rheostats are Fig. 148. Electrically-Operated Slotter. Driven by a 20-hp., 4 : i reversing planer motor. The controller used is similar to 222 CONTROLLERS FOR ELECTRIC MOTORS short-circuited during acceleration by means of a contact attached to the accelerating contact 5 in Fig. 145. The small contactor in the lower right-hand side of the panel '■~-^4 »2r_._^ T--^ ;^-"">- .; •^'i k^ -~u^ A— i^:i^^«- >^-'?^^- — 'i^li-f: ySSA sajgi? ' -i^^' i£^3^ ^ 'i'-' ^ !■ • j r ■ly ^^^. W V. I a' m I w Ad.--- V n^«i ■ \ m K- "■.!.• 1 i Fig. 149. Method of Grouping Machine Tool Controllers On either side of the distributing cabinet. If this row of controllers is protected by a screen, it will meet safety requirements, as the operation of the motor is by a push-button or master switch and the operator is not re- quired to handle any apparatus on the control panel. is used for no voltage protection. On failure of voltage, the planer is stopped and cannot start again until the reset button is operated. In addition to a master switch operated by the platen of the planer, a pendant switch, Fig. 141, may be provided having push buttons marked master, pendant, cut and re- turn. When the master button is pressed, the master switch controls the operation; when the button marked pendant is pressed, the operation is controlled by depressing either the cut or return button. It is necessary to hold these buttons down, as the motor will come to rest automatically if the MACHINE TOOL CONTROLLERS 223 operator releases the pendant switch. In addition to these buttons, the reset button may be included in the pendant switch. The use of this pendant switch adds greatly to the convenience of the operator in setting up his work. As an added precaution, the master switch is wired so that both sides of the operating coils are disconnected when Fig. 150. Motor-Driven Shaper with Drum Controller Having armature starting and field regulation, showing a neat and compact arrangement for electric drive. the switch is in the " ojf " position. In designing con- trollers, it is desirable to eliminate as many interlocks and relays as possible. The planer type of control has been used with slotters, Fig. 148, planers, shapers, key seaters, and gear-cutting machinery. Each of these applications differs to some ex- tent from the standard planer, and it is not desirable to apply a standard controller without an investigation. This is particularly true in the application to gear cutting ma- chinery. Some of these machines oscillate the gear, and at the same time move the tool forward and back. If the gear is very heavy, the rapid oscillation of this gear im- 224 CONTROLLERS FOR ELECTRIC MOTORS poses a severe duty upon the whole equipment and care must be taken to prevent seriously racking the apparatus. A modification of the reversing planer equipment is the non-reversing equipment. The direction in which the platen travels is changed by mechanical means and the motor permitted to run continuously in one direction. Provision, however, is made for changing the speed of this Fig. 151. Planer with Autostaeter and Continuously Operating Alternating- Current Motor. The reversing of the platen is done by shifting belts. This makes a very good arrangement for alternating-current drive. The guard on the side of the planer serves to prevent injury to the operator. motor by adjusting the field so that a different speed of platen can be used for the cutting and return strokes. This equipment permits the use of a standard motor, but in many respects is not as economical as the reversing equipment and sometimes difficulty is experienced with the mechanical reversing drive, due to wear. Such an arrangement, how- ever, lends itself readily for attachment to standard planers, formerly designed for line shaft drive. MACHINE TOOL CONTROLLERS 22 5 The development of the wheel lathe and the planer con- trollers shows the advantage of electric drive for machine tools where a proper equipment is designed and the ma- chine tool built for such an equipment. Many special machine tools are being designed for motor drive; if a proper study of the conditions is made, present experience in this art enables the electrical enginer to provide a suit- able equipment to meet the most exacting conditions. CHAPTER XVIII CONTROL FOR MACHINERY REQUIRING LOW INITIAL SPEED, SUCH AS PRINTING PRESSES AND RUBBER CALENDERS Control equipment providing a slow operating speed of about one-twentieth of the normal speed is used in setting up a machine preparatory to the commercial run; for instance, in a paper or rubber calender, the paper or rubber must be threaded between the cal- ender rolls by hand. Un- less these rolls turn at a very slow speed, the op- erator is in danger of in- jury. Another application is the cloth printing machine in textile mills. The cloth is threaded through the machine in a manner similar to a calender and in addi- tion, it is necessary to see that the printing regis- ters properly before operating at normal speed. The different applications require minor changes in de- tails but the underlying principles are the same. Recent 226 m^ 1' f ."^■, 1 * J ■^J^^^^bH ■MH^' i '•*: Vtt ■: ij-w^-^ Fig. 152. Single >[otor and Single Voltage Control. CONTROL FOR LOW INITIAL SPEED MACHINERY 2 2; legislation, has been enacted in several states requiring the live parts of the control equipment to be inaccessible to un- authorized persons. This is tending very strongly to in- crease the use of remote con- trol from push button stations or master switches. The push button controller is the most desirable but costs more. In the past, it has been selected in the main on account of its convenience. In the future, the question of personal safety to the operator will be an important additional factor in the selection of this type of control. In equipments of this kind as a rule, the torque required from the motor is constant. The load is mostly friction and therefore steady. In paper cal- enders the quality of the stock and the pressure between the rolls also affect the load, but this load remains mately uniform for any par ticular combination. The load on rubber calenders is more variable on account of the numerous operations performed by the same machine. It is affected considerably by the amount of rubber in the rolls. The variation in the torque of the motor is a very important factor in designing a suit- able control for operation at slow initial speeds. After the machine has been made ready to operate, it is accelerated smoothly to the proper operating speed. Most of these applications require a large number of economical l6 |^;| l»««.-i.,v •1 1 HH m Fig. 153. Single Motor and Double approxi- Voltage Control. 228 CONTROLLERS FOR ELECTRIC MOTORS operating speeds, which are obtained by changing the field strength of the direct-current motor, or if an alternating- current motor is used, the resistance of the secondary circuit is changed. Fig. 154. Single Motor and Double Voltage Control. The field rheostat is operated by a push button controlled motor. In some applications, a two voltage direct-current circuit is provided, usually 115 and 230 volts, shown by diagram. Fig. 161. The double voltage system reduces the speed range required from a motor by field adjustment and, there- fore, decreases the first cost of the motor. The disadvan- tage of this system is the extra wire required and the addi- tional complication in the generating equipment. In some applications, a rotary converter is used to change the power CONTROL FOR LOW INITIAL SPEED MACHINERY 229 from alternating to direct-current. This converter can readily be arranged to give both 115- and 2 30- volt, three- wire power circuits. In most cases, this converter is located close to the ma- chinery and the extra ex- pense of wire is small. The cost of a 4 : I adjust- able speed direct-current motor, as compared with a 2 : I motor, depends to a considerable extent upon the commercial demand for this 4 : i motor. These motors are built in consid- erable quantities up to 50 or 75 horse-power and, for such sizes it is usually more economical to use a 4:1 motor than a 2 : i and a double voltage system of control. For larger sizes, the reverse may be true. This is, of course, under- going continual commer- cial change and a decision should only be reached after a careful investiga- tion of the apparatus avail- able. The speed control over the normal operating range presents no problems different from those previously described. The problem which makes this class of control different from others, is the method of obtain- ing a very slow operating speed for making up the Fig. 155. Controller for a Single Motor with Adjustable Armature Series and Armature Shunt Resistance. This controller is represented dia- grammatically in Fig. 163. The contacts in the lower left corner are those con- trolled by switch 3 and the contacts in the lower right corner are those con- trolled by switch 4. 230 CONTROLLERS FOR ELECTRIC MOTORS machine. The starting or threading speed for direct- current equipments may be obtained by using resistors in series and in shunt with the motor armature.-^ Where the size of the motor is small or the minimum operating speed comparatively high, this method is economical and frequently used. For large motors or where a wide difference exists be- tween the slow speeds for threading purposes and the maxi- mum operating speeds, four methods of control have been applied : I. This arrangement can be used for either alternat- ing or direct-current. It consists in using a small auxiliary motor geared to the main drive to give the proper slow SCHEME OF MAIN CONNECTIONS Larfc Motor , «J Sh. Fli SEQUENCE OF CONTACTORS Contactor 3t Run Bl ^ 1 2 1 Q 2 o o 3 o o 4 o o i o o o P.R. o o o o lA O y e Q O o 7 O ^ 8 o Fig. 156. Control Scheme for Operation Over a Wide Range of Speeds. A large motor and small motor are used. The small motor operates the machine at minimum speed and is cut out of circuit when the large motor is used for driving the machine at normal speed. After all the contactors have closed, during acceleration, the motor is gradually brought up to the speed corresponding to the rheostat setting, by the field relay FR. speed with the small motor operating near its normal speed. This motor is started up and controlled in the usual manner. After the machine has been made up, the main operating motor is connected to the line and accelerates the machine to its normal operating speed. At the same time, the small motor is disconnected from the line aad automatically un- coupled from the machine. The schematic diagram for a 1 This method of speed control was described in Chapter VII. CONTROL FOR LOW INITIAL SPEED MACHINERY 23 1 direct-current motor is shown in Fig. 156 and for an alter- nating-current motor in Fig. 157. SCHEME OF MAIN CONNECTIONS Fig. 157. Control Scheme for Alternating-Current Motors. The small motor is connected to the line by contactor 2 and the large motor by contactor /. The resistors in the small motor circuit are adjusted to give the proper minimum speed. The large motor may or may not be connected to the line, while the small motor is operating the machine. The speed of the large motor is adjusted by changing the resistors in the sec- ondary with a drum controller. The small motor is disconnected when the large motor begins to accelerate the machine. 2. This scheme is applicable to either alternating or direct-current motors. It is similar to Ne. i, except that both the main motor and the auxiliary motor are started together, the motors being geared together. The resistor in series with the main motor is adjusted to give less than the required speed of the machine. The additional speed is obtained by loading the auxiliary motor. Since this auxiliary motor operates close to its normal speed, its opera- tion is quite stable and it readily adapts itself to any change in the load without a material change in its speed. If the equipment is operating at the required speed and the load increases, the machine tends to slow down. A small change in the speed of the machine makes a large change in the speed of the auxiliary motor and materially increases the load on this motor so that it supplies the additional torque required with only a slight decrease in speed. If the load is reduced instead of increased, a slight increase in the ma- 232 CONTROLLERS FOR ELECTRIC MOTORS chine speed will cause a considerable increase in the speed of the auxiliary motor, which materially reduces the load on this motor and compensates for the difference in torque required by the machine. Since the auxiliary motor acts' ""TMni B^ Braking Res. Fid. Res. SCHEME OF MAIN CONNECTIONS Arm, Reg. Res Large Motor Ser. Fid. Rs /r4 R3 Kz K, Flfl™; F, r V r V T y"i^yTj^Jv-ii/vvv\A ■O. L, Relay . Small 8* Motor , Ser. Fid. Starting Res. Brake Res. I - 1 n nn 1 "^ ' 8A Bi2"iJlrLruu*B77 Sh. Fid. Fid. Dis. Res, ,nnnnn;i^ SEQUENCE OF CONTACTORS CO as S c Increase^ u ^ ^01 1 do ok> 2 060 3 4 30 5 c 6 CO 7 0| 8 9 lA 8A OB cpl Fig. 158. Modification of the Scheme Shown in Fig. 156. Both the small motor and the large motor are connected to the line during the slow-speed operation. The diagrams are essentially the same, except for the sequence of the contactors. f\y \ I Large SCHEME OF MAIN CONNECTIONS i 4 Motor Ser. Fid. B2 Braking Res. fO, L. '■, "6 R5 R4 R3 Rj Rj Relay, rfotor Ser Fid Starting Rm. B12 p Sh. Flip Fid. Dis. Res, .nnnnnn , SEQUENCE OF CONTACTORS 11 Off Inch Is Eo C Decrease ' 1 P P 00 JO 3 n 4 n 5 fi 7 30 fl M Q 8A e Q 03 Fig. 159. Modification of the Scheme Shown in Fig. 156. The main motor and small motor have their armatures connected in series for the minimum speed. The small motor is disconnected when the large motor accelerates the machine to normal operating speed. CONTROL FOR LOW INITIAL SPEED MACHINERY 233 only as a stablizer, it can be smaller than where it alone is used to drive the machine at the slow speed. The schematic diagram of this control, Fig. 158, differs from the former diagrams in that the main motor is in circuit during the start. After the machine is made up, the main motor ac- celerates it to the normal operating speed and the small motor is disconnected from the line and disengaged from the gearing so that it remains at rest during normal operation. 3. The third scheme is applicable only to direct-current motors. Fig. 159. The armatures of these two motors are connected in series, and they are connected together through gearing for the slow speed. Both motors are started at the same time, the small motor running close to its normal speed. Its armature sets up a counter e.m.f. which absorbs most of the line voltage available for 3 SCHEME OF MAIN CONNECTIONS ArmJRegulaling Res. Ser. Fld._ _ ^.L. Relay SEQUENCE OF CONTACTORS 11 Incr Recr 1 2 D 3 3 4 1 A Fig. 160. Control Scheme for use with a Mechanical Gear Changer To give the slow speed operation. The control is similar to those described in previous issues. the main motor. A small change in speed of the machine causes this auxiliary motor to alter its counter e.m.f. which makes a considerable change in the voltage across the main motor and adjusts the total torque for the two motors with only a very small change in speed. After the ma- chine is made up, it is accelerated to the normal operating speed and the small motor is disconnected, as previously described. 234 CONTROLLERS FOR ELECTRIC MOTORS 4. The fourth scheme, Fig. 160, consists in a double set of gearing connected by some form of friction or magnetic clutch. The machine is operated by one motor, which starts by driving the machine through the reduction gear. After the machine has been made up, the large motor is gradually transferred from the low gearing to the high gearing by means of the clutch. A considerable number of mechanical devices have been placed on the market for effecting this change in speed gradually without shock or jar to the drive. No attempt will be made to describe these devices in detail, as they differ widely for different classes of service. Ser. Fid. SCHEME OF MAIN CONNECTIONS S2 nntuinnnnjiniuii"- ' SEQUENCE OF CONTACTORS P^^- V V p r If icccl. Relay -vmnKnnAnn-l — ^wyvVW" Ficfd Accel. Relay §1 n steps Steps o> 1 2 3-11 ~ 12 1 n 3 3 > CO 4 a 3 ' ? 5 E-l fi 7 F.R 3 lA 3 Fid. pi3._Res. Fig. 161. Control Scheme for Operation from Two Voltages. The lower voltage is between positive and neutral and the higher between positive and negative. Resistors are provided both in series and shunt with the armature to give the slow-speed operation at the minimum voltage. A two-pole, double-throw knife switch is shown for reversing the armature if occasion should require. This switch is operated only when the motor is disconnected from the line. Steps 3 to 11 are field control at the low voltage, and 13 to 19 are field control at the high voltage. The selection of the best scheme for any particular drive is determined largely by the first cost and maintenance, which is in turn influenced by the commercial apparatus available. An analysis of each particular problem should be made. A description of a push button controller of this general type will, in the main, cover the manual and semi- automatic controller. Panels for several different auto- matic controllers are shown. These panels consist of a power-driven master switch having sliding contacts, the CONTROL FOR LOW INITIAL SPEED MACHINERY 235 contact arm being moved either by a small motor or a magnet. This motor or magnet is controlled by a set of push buttons. The movement of this master switch controls the circuit to the main and auxiliary motors, partly through magnetic contactors and partly by direct contact. The power- operated master switch may be replaced by a drum type or face plate master switch operated manually. The motor-operated switches shown are provided with a handle for operating the face plate by hand, when occasion requires it, and move in a clockwise direction for an increase in speed. The solenoid operated master switch moves downward for increasing the speed of the machine and up- ward for decreasing the speed. The acceleration of the main motor may be controlled by any of the usual methods of auto- matic acceleration.^ Fig. 167 shows a typical push button station consisting of five buttons marked safe, run, fast, slow, and inch. Several of these stations may be used for one controller. If we assume that this push button station is connected to one of the motor- operated panels controlling a main and auxiliary motor supplied with direct-current power, the operation of the system will be as follows : 2 Described in Chapter V. Fig. 162. Two Motor, Full Automatic Direct-Curkent Controller. For remote control by push buttons. 236 CONTROLLERS FOR ELECTRIC MOTORS The push button marked run at all the stations must first be depressed, as the control is inoperative if any of the buttons marked safe are depressed. After all of the run buttons are depressed, the machine may be started by press- ing the fast button. As long as this button is held down SCHEME OF MAIN CONNECTIONS 1 R2 R3 R, Rs Kb Fid. Res. Motort' ^Cool. Fid. A2 Ser. Fid. . . ±1 O. L. SEQUENCE OF CONTACTORS L Fi Sh. Fid.. ^2 Fid. Dis. Res. Relay ^1 Off [ncfa Increase , t3 bI , Decrease ■l&\ 1 3 D D 3 2 .T 3D ♦ 5 5 i 3biOl lA Fig. 163. Control Scheme for Automatic Acceleration with Face Plate Rheostat. This is a controller having both armature, series and shunt resistors for the minimum speed, together with field control for giving the various operating speeds. This control is usually applied to smaller motors or where the speed reduction is medium. A photograph of this controller is shown in Fig. 155. The series resistor is short-circuited in steps by the motor operated handle designated as B on the diagram. At the start, B is at the extreme left. This gives so much resistance in series that the motor might not be able to overcome the starting friction. To obviate this, con- tractor 3 is closed on the first position, and is automatically opened when the motor starts to rotate, leaving the full resistance in series. Contactor 4 closes when 3 opens, giving the lowest operating speed. the controller gradually increases the speed of the machine. When the desired speed is reached, the putton is released. This stops the pilot motor or solenoid and permits the ma- chine to operate at that particular speed. If this speed should prove too fast, the button marked slow is depressed, which causes the speed of the machine to decrease. In starting up, the fast and slow buttons may be manipulated until the desired "make ready" speed of the machine is obtained. After the machine has been made ready, the fast button is again depressed and the motor accelerated to the desired operating speed. Automatic means may be pro- vided for stopping the pilot motor when the desired speed is reached, or this may be left to the discretion of the operator. CONTROL FOR LOW INITIAL SPEED MACHINERY 237 If it is desirable to move the machine only a short dis- tance in order to "spot" a particular part of the appa- ratus, the button marked inch may be pressed. This moves the machine at a very slow speed and stops it as soon as the button is released. This method of operation is Fig. 164. Double Two Motor, Direct-Current Controller. Arranged to operate as a single unit. more expeditious than using the fast and slow buttons, where it is desired to operate the machine only mo- mentarily. In order to stop the machine, the push button marked safe is depressed. This opens the main circuit to the motors and, where direct-current is used, it applies a dy- 238 CONTROLLERS FOR ELECTRIC MOTORS namic brake. Where alternating-current power is used, it releases a magnetic brake. These brakes are set to bring the machine to rest as quickly as safety will permit, in order to decrease the effect of any accident which might occur Figs. i6; and i66. Two Motor, Full Auto.matic Controllers. Fig. l6s is for direct current and Fig. i66 for alternating current. Both are for remote control from push button stations. while operating. The quick stop is not required under normal conditions, but it is always a convenience to bring the machine to rest with as little delay as possible, even if no accident should occur. When the safe button is de- pressed, the pilot motor is connected so as to move the master switch back to the starting position. CONTROL FOR LOW INITIAL SPEED MACHINERY 239 A number of safety features are employed in these controllers, as fol- lows : 1. If any safe button is pushed, the machine cannot be started again until th^ run button at that particular push button station has been pushed. Provision can be made for locking the safe button in the depressed position so as to prevent the machine from being started while an attendant is working on the interior of the machine. 2. When the safe button is depressed, the machine cannot be started up again until the master switch has returned to the off position. 3. A current limit relay can be provided to stop the acceleration temporarily when the current exceeds a fixed value. This provides current limit acceleration and guards against the pilot motor operat- ing the arm faster than the main motor can accelerate the machine. 4. An overload relay is provided for disconnecting the motor in case of overload. 5. Low-voltage protection is provided so that, on failure of power, the machine will not start again without the action of an attendant. 6. Mechanical means are provided for disconnecting the operating lever from the pilot motor and moving it by hand, if such an emergency should arise. This converts the controller from full automatic to manual control. Fig. 167. Push Button Control St.^tion. It can readily be seen from the description of the auto- matic controller that either all or a part of the automatic features may be replaced by manual control to reduce the cost. CHAPTER XIX STEEL MILL FLOOR CONTROLLERS FOR AUXILIARY DRIVES The application of electric motors and controllers to steel mills naturally divides itself into two classes; namely, the main drive and auxiliary drives. The main drive consists of large motors which may be reversing or non-reversing, depending upon the character of the mill. A great deal has been written on the subject of this main drive, the articles appearing in the proceed- ings of the A. I. E. E., Association of Iron and Steel Elec- trical Engineers and the Electric Journal. The reversing drive presented more difficulties than the continuous mill. A very interesting description of the reversing drive is given in an article on the subject of " Electric Drive for Reversing Rolling Mills," by Messrs. Sykes and Hall in the Proceedings of the A. I. E. E. for June, 1916. The floor controllers for auxiliary drives comprise a large number of applications on which there is very little pub- lished data. It is hoped that the Association of Iron and Steel Electrical Engineers, together with the Steel Mill Committee of the A. I. E. E., will make a study of this problem and standardize this equipment as far as it is prac- tical to do so. So far, the practice is uniform only in a general way, individual engineers trying out their respec- tive ideas and profiting by their own experience and the data obtained from other mills. The D.C. series motor is still extensively used, but recently there has been strong tendency towards using a compound motor which has a limited maximum speed and seems to reduce the commu- tator trouble. 240 STEEL MILL FLOOR CONTROLLERS 241 Few applications of electric motors and controllers involve more severe service than floor controllers use in steel mills. The motors are started and stopped very frequently, usually under heavy loads, and large starting currents are used to obtain quick acceleration. The control apparatus must be rugged and should have a minimum number of interlocks Fig. 168. Charging Machine. The illustration shows one end of a modern charging machine with control equipment of the magnetic contactor type. and other auxiliary contacts. Provision should be made for repairs and renewals in the minimum length of time. It is very desirable to have the parts subject to wear or accident removable from the front of the control board, and one man should be able to handle this work without assist- ance, except where the parts are very heavy. 242 CONTROLLERS FOR ELECTRIC MOTORS Floor controllers group themselves into four general classes for reversible service. Where a non-reversing con- troller is used, it follows the same grouping except that one set of reverse switches is omitted : AA. Plugging on (he reverse, without speed control; BB. Plugging on the reverse with speed control; acceleration by shunt switches with current limit relays. CC. Dynamic brake without speed control. DD. Dynamic brake with speed control ; the acceleration is by shunt switches with current limit relays. Schematic diagrams showing the reversible controllers are illustrated in Figs. 169 and 1 72. All of these controllers consist essentially of two double pole line switches, one for forward and the other for the reverse direction of operation. SCHEME OF MAIN CONNECTIONS 1 , 2 Starting Resistor SCHEME OF MAIN CONNECTIONS Starting Resistor Field Discharge Resistor SEQUENCE OF CONTACTORS ^ 6 '7 "fc Shuni Field load fWWVV Field Discharge Resistoi Con ^. Off ^.\ : - t> / ■: o 3 : A ■; ■■ '■> (J ■> 6 ;;: ;; SEQUENCE OF CONTACTORS Con •te. Off ^..l 1 : - - ■1 3 D n : : r. - -: ■■ f> : n n - 7 3 u Fig. 169. Fig. 170. Fig. i6g. Scheme of Main Connections for Form AA Controller. Acceleration is by means of series lockout magnet switches 6 and 7, which are operated by series coils B and C respectively. The motor is connected for forward or reverse operation by closing contactors / and 3 ox 2 and 4 respectively. With this form of controller, acceleration is entirely automatic. Fig. 170. Scheme of Main Connections for Form BB Controller. The operation of this form of controller is the same as the AA, except that the automatic acceleration is by means of series relays, the switches being closed by shunt coils which are in series both with the master con- troller and with the series relay contacts. With this scheme, acceleration is automatic up to the position at which the master controller is set. Coils D and E are the series coils of the relays which control the operation of switches 6 and 7 respectively. STEEL MILL FLOOR CONTROLLERS 243 The controllers illustrated use a mechanical interlock so that both forward and reverse switches cannot be closed at the same time. These switches for controllers CC and DD are provided with back contacts; when both direction switches are opened, the two back contacts complete the SCHEME OF MAIN CONNECTIONS' Stniting Resistor -R SCHEME OF MAIN CONNECTIONS - Field Discharge Resislgr Oven- SEQUENCE OF CONTACTORS Con For, Off Rev.| 1 D z D ^ ") ■:> 3 3 4 ■) 1 S n 1 fi c n 1 iO -) lA ■1 ;: 4A => ? ? SEQUENCE OF CONTACTORS Con For. Off ^.l 1 5 5 ; 1 ■; : r> C 3 c ■: : 3 : : : 3 6 5 : 7 T ?t lA 3 : 5 4A ) ;: : Fig. 171. Fig. 172. Fig. 171. Scheme of Main Connections for Form CC Controller. This controller is the same as that shown in Fig. 169, except that it is arranged for dynamic braking in the off position of the contactors. The brake resistor Bi B2 is connected across the motor armature through con- tacts I A and 4A. These contacts are on the bottom of switches / and 4 so that both sets of reverse switches must be open before dynamic braking is obtained. The current in passing through contacts lA and 4A energizes magnets which press these contacts firmly together as long as the dynamic brake current is flowing, and effectually prevent the reversal of the motor as long as its speed is sufficient to send current through these coils. Fig. 172. Scheme of Main Connections for Form DD Controller. This controller is the same as that shown in Fig. 170, except that dynamic braking is provided in the off position in the same manner as shown in Fig. 171. dynamic brake circuit. The coils maintaining pressure on these back contacts are in series with the contacts and hold them closed as long as current is flowing through this cir- cuit. This arrangement also prevents the motor from being operated in the reverse direction as long as the dynamic brake current lasts. The single-pole contactor with blow- 17 244 CONTROLLERS FOR ELECTRIC MOTORS out, which is used for opening the negative side of the line is shown in the top row, Fig. 173 and 174. At the bottom of the panels are two contactors for short-circuiting the starting resistor. In Fig. 173, these contactors are wound Fig. 173. Fig. 174. Fig. 173. Form CC Controller. This controller corresponds to the diagram shown in Fig. 171. The re- verse switches have two top contacts, / and 3 for forward operation with back contact lA, and 2 and 4 for reverse operation with bottom contact 4A. These switches are mechanically interlocked by the rod at the left, to pre- vent both from closing at the same time. Contact 5 is shown in the upper right hand corner, and lockout accelerating switches 6 and 7 at the bottom of the panel. Both the reverse switches and the line contactor 5 are actuated by shunt coils not shown in the diagram. An over-load relay is located directly under switch 5 and two-pole knife switches are provided for dis- connecting the main and control circuits from the line. Fig. 174. Form DD Controller. This controller corresponds to the diagram shown in Fig. 172. It is the same as Fig. 173, except that accelerating switches 6 and 7 are actuated by shunt coils which are controlled by the series relays, one of which is shown mounted underneath switch 6, with series coils and operated on the lockout principle. In Fig. I 74, the contactors have shunt coils and are controlled by current limit relays. One of these relays is shown STEEL MILL FLOOR CONTROLLERS 245 mounted underneath the resistor switch. The other relay is mounted under the direction switch. These panels have a two-pole overload relay and a low- voltage protective relay. The overload relay is provided with two coils, each having a dashpot time-limit device. The plungers actuated by either of these coils engage a single switch member, which opens the potential relay cir- cuit in case of overload. The use of the two-pole overload relay and the negative line switch gives maximum pro- tection against grounds, which are apt to occur in applica- tion of this kind. In the off position, the controller dis- connects the armature from the line. If no negative line switch is used, one end of the series field is connected to the line, so that it is not safe to work on the motor without ■ opening the knife switch. In the upper right hand corner of the panel is a two- pole, single-throw knife switch for disconnecting the feed wires from the panel. A similar two-pole single-throw knife switch and fuses are located underneath the overload relay for the control wiring. The main line knife switch, as well as the smaller knife switch and fuses, can be pro- vided with sheet metal covers to protect the operator from accidental flashes if the' switch is opened under load. The large knife switch is provided with an attachment for a pad- lock so that the switch can be locked in the open position when repair work is going on. By opening the main knife switch and closing the control knife switch, the control cir- cuits can be tested out and the operation of the contactors observed, to see that the equipment is in working order, before power is applied to the main contacts. Usually these control panels are protected by grill work or in some other manner, or they are mounted in a gallery, which is accessible only to authorized persons. In some states these precautions are required by law. In the main, however, the steel mill companies recognize the importance 246 CONTROLLERS FOR ELECTRIC MOTORS of such safety provisions and use them whether required by law or not. The reason for dividing the controllers into four classes can best be understood by giving some of the applications for each class of control. Class A A controllers are applied to all of the main and auxiliary tables requiring one speed only. These tables usually consist of a series of rollers; the steel rests on the rollers and is moved forward or back by revolving the rollers. Class BB controllers are used for similar applications where two or more speeds of operation are required. Class CC controllers are applied to screw-downs, lifting and tilting tables, manipulator fingers, and side guards. Class DD controllers are used for metal mixers and Bessemer converters, and are also used for the same ap- plications as class CC where speed control is desired. The use of four controller combinations is not desirable where one or two combinations will do the work. In addi- tion, a number of steel mill engineers object to the present lockout switch, as its operation is not reliable on light loads. It is hoped that development in the controller art will enable Fig. 175. Typical Application of Electric Motors to Reversing, Roll Tables of Blooming Mill. a single controller to be used for all applications. The dynamic brake feature need not be connected in where a series motor is used. For compound motors, the dynamic brake is preferable to plugging, as it limits the commutating requirements in the motor and the controller can be so de- STEEL MILL FLOOR CONTROLLERS 247 signed that the reversing will be as quick or even quicker than where plugging is used. The more extensive use of compound wound motors for steel mills will make the use of dynamic brake more general. A two high reversing mill is outlined in Fig. 175. The circles A and B represent the main rolls for fabricating the steel. The horizontal row of circles represents the table rolls which are driven by electric motors through suitable gearing. The steel billet is moved up to the main rolls in the direction shown, by revolving the table rolls. After the billet has entered the main rolls, it is carried through by the action of these rolls and is delivered to the table rolls on the left hand side. By reversing the direction of the main rolls and table rolls on both sides, the billet is fed back through the main rolls, the process being repeated until the desired reduction is secured. Each of these opera- tions is known as a " pass." After the billet has been rolled to a given size by one set of main rolls, it is often passed to other rolls, or the same set of rolls may have several different shaped grooves, so that the billet can be moved sideways and made to enter these grooves during successive passes. The movement of the billet sideways is controlled by the side guards, which consist of horizontal bars, which are moved across the table rolls for placing the billet in the proper position. When it is desired to turn the billet over, manipulator fingers are used. These fingers are attached to the side guards and extend underneath the billet between the table rolls. These manipulator fingers are connected so that they can be raised and lowered. This movement, when properly directed, serves to turn the billet over. The distance between the main rolls A and B is ad- justed by means of an electric motor, geared to screws which raise the top roll. If the screws are raised, the hydraulic pressure will cause the top roll to follow. If it is desired to 248 CONTROLLERS FOR ELECTRIC MOTORS lower the top roll, it is necessary for the motor to drive the screws down with sufficient force to overcome the hydraulic pressure. This is known as the " screw-down " motion. It can be readily seen that the adjustment between these rolls must be very exact, and therefore it is important that the control of the screwdown motor provide for stopping this motor with practically no drift. Fig. 176. Typical Application of Motors and Gears to Tilting Roll Table of Plate Mill. A three-high mill with tilting tables is shown in Fig. i "jd. The main rolls are illustrated by circles A, B and C. Roll B is an idler. The steel to be rolled in this form of mill is usually a plate and the mills used are usually called " plate mills." The red hot steel plate is passed alternately between rolls A and B from right to left and back from left to right between rolls B and C. The screw-down motion adjusts the distance between rolls A and C, roll B being free so that when the pass is made between rolls A and B, roll B is forced against roll C. When they pass in the oppo- site direction between B and C, roll B is pressed against A. In order to pass the steel plate alternately between A and B and B and C, it is necessary to tilt the tables ; hence, the term " tilting table." The end of the table away from the rolls is hinged and the opposite end of the table is raised or lowered by means of an electric motor. These tables have two fixed positions, which are previously adjusted so STEEL MILL FLOOR CONTROLLERS 249 that the motors are automatically stopped when the tables reach these positions. The tables are counter-weighted, as shown, in order to equalize the work done between raising and lowering. The masses moved, however, are great and as an accurate stop is required, it is necessary to provide SCHEME OF MAIN CONNECTIONS 1 2 Slow starting Resistor ^ ^Vr p ''Senes Brake Bo C Over* Field Discharge Resistor r^UUVUUUUUUIS -MAAAAA-L ■^WW\A- t Field SEQUENCE OF CONTACTORS Con T o.?„ Rev. 1 i 1 i J s ^ r 4 : s : ; : - jR e 1 M f n lA ■ ]<> ; 1 S 5a i Fig. 177. Main Connections of Controller with One Point Slow-Down. This controller is the same as Fig. 171 with the addition of switch S. When it is desired to slow down the motor, preparatory to 'stopping, the shunt coil of switch 1? is disconnected from the line by means of the limit- switch shown in Fig. 178. This, in turn, causes coil B to open 6 and there- fore inserts all the starting resistance and closes a shunt around the armature through contact SA, leaving the motor connected to the line with the starting resistor in series with, and the slow-down resistor shunted across the arma- ture, providing a slow running motion. The bottom contact of switch S, marked SA, has a series coil to retain the switch in the open position as long as current is flowing through contact SA. If the line switches were open and the dynamic resistance omitted, some dynamic brake action would occur due to the resistor across the armature through contact SA. This resistor, however, is of too large an ohmic value to give a quick stop, and it is, there- fore, necessary to use the additional dynamic braking resistor to bring the motor quickly to a standstill. a slowdown before the final stop. Fig. 177 illustrates the controller used for this purpose. When the table ap- proaches either limit of travel, switches 7 and 8 are opened and switch 8-A is closed. This inserts the starting resist- ance in series with the motor and also provides a shunt 250 CONTROLLERS FOR ELECTRIC MOTORS around. the motor armature. These connections can be ad- justed to give a positive slow speed, from which an accu- rate stop can be made. The problem is very similar to that of an elevator or skip hoist, which is moved up and down between fixed limits. In bringing tables to rest, switches i-A and 4-A are closed at the same time that the line switches are opened, which provides a dynamic brak- ing path of low resistance around the armature. In addi- tion, the circuit through the mechanical braking magnet is opened so that the brake shoes set and assist in stopping the load, as well as hold it securely. The accurate placing of these tables at either limit of travel is facilitated by the use of a crank motion. The stop is made when the crank is at either the top or bottom of the travel, so that a slight variation in the point of stopping makes little differ- ence in the location of the table. The controller switches are operated automatically by a stop motion switch, shown in Fig. 178, whose actuating shaft is connected to the mechanism. On the shaft is mounted a set of cams which open or close the limit switches at the proper time. The cams are adjustable, so that each switch can be set to open and close at the proper point in the travel. The use of the cams gives a quick motion to these switches and enables an accurate setting to be obtained. Fig. 178. Cam Limit Switch for Gear OR Sprocket Connection. STEEL MILL FLOOR CONTROLLERS 25 I The motor is started by means of the master switch shown in Fig. 179. This master switch is also used in connec- tion with the other controllers, previously described. The central position of the handle disconnects the motor from the line and resets the no-voltage switch in case the over- load has opened it. The movement of the handle either side of the center operates the controller forward or re- verse. One slow-speed point and one full-speed point are provided in each direction. Where the application re- FiG. 179. Two-Point Master Switch. quires more running notches, a different form of master switch is used. A lifting table is very similar to a tilting table, except that the table is moved vertically up and down instead of having one end hinged and the other end raised and lowered. The results accomplished are the same, but a somewhat different mechanism is used. Usually the lift- ing table is confined to rolling operations which do not re- quire a very long table. Tests indicate that, on many applications, the best results are obtained with very few accelerating points. There are two ways of approaching this problem. Those who argue for a large number of accelerating points call attention to the fact that the current can be maintained at a higher level 252 CONTROLLERS FOR ELECTRIC MOTORS and thus furnish a larger average accelerating torque. The advocates of a small number of steps show that each switch requires a certain time in which to operate. Where the acceleration must take place in two or three seconds, too much time is consumed in the closing of these switches ; also high current peaks must be used in any event. X IT - - - - i^^ , X 600 ir? i-i^ i \\i \[ Ml 400 t I II Jjt Im I - ~ ' J" --5 _ a ^ _ I200 t^ '^ "• - ''■'v. •< " ^ , • 1 " * 2 : : a J : 3 3 (53 : 4 5 z : c i S n ■- c : - 1 u J, 'J, 5 11 n n 1 c J, [_ : r (- -) c c c :; i'A - J L J 14 ■ c D iti 1 LJ Fig. 183. Connections for Bridge or Trolley Controller. the direct-current motors and in shunt with alternating- current motors. In some cases an additional brake of this type is placed on the secondary shaft, the double brake equipment giving added insurance against dropping the load. The motors operating the trolley and bridge are usually stopped by plugging or reversing the motor. The movement of the trolley is slow and no difficulty is experi- enced in handling it. Usually the bridge is equipped with CRANES 257 a foot brake which may be used instead of plugging the motor or as an added safety feature should anything happen to the electrical equipment. Small cranes are frequently operated from the floor. The controller is provided with a spring for returning it to the central position and ropes are attached to an operating wheel or sheave on the controller shaft so arranged that the operator can control the motors by pulling one or the other SCHEME OF MAIN CONNECTIONS S2 B2^'"*'"S Resistor . SEQUENCE OF COWT ACTORS No 7 Contactor Closing Coil. ,^ Hr.,s. s Lower 1 fi ■; 4 \ 2 1 1 2 :i 4 ■s B 1 n n n~> n ? D c?; c s d -> U 4 r [0 g A ODbb ooBlo 5 ^ s 11 ce do 17 r r f qu 1.1 c H , J_ _ _ Fig. i^ Limit Switch' Connections for a Hoist Controller. With an electric limit switch. SCHEME OF MAIN CONNECTIONS! SEQUENCE OF CONTACTORS Hnis. a: Lower _| ■; 4 3 2 1 T 2 .1 4 .S6 1 :> :> 2 c t; CO s '1 4 c c c :) :> :i ~lO i- C) c :: ^ c 3 Q 11 r : 1? c c : 1.1 c H <5 Fig. 185. Connections for a Hoist Controu.er. With a mechanical limit switch. Switches Nos. 7 and 8 are normally open and Nos. 9 and 10 are normally closed. of these ropes from the floor. Larger cranes have a cab for the operator. This cab is attached to the bridge and con- tains the control equipment. Some very large cranes have the cab attached to the trolley. This is known as a " man trolley." The scheme of control is shown diagramatically in Fig. 182 for the hoist and Fig. 183 for the bridge or trolley. Various schemes of control have been devised for the hoist, 258 CONTROLLERS FOR ELECTRIC MOTORS that shown being in most general use at present. The dia- grams indicate a contactor form of control but the same con- nections may be made with a drum controller. Drum con- trollers are in common use with the smaller cranes and magnetic contactor controllers on the larger cranes. The dividing line is roughly about 50 hp., but the class of service and frequency of operation are the determining factors. The magnetic contactor control is more durable but also more expensive and takes up additional room. When a manual controller is used, either of the drum type or some other design, it is desir- able to provide a protective panel for each crane. One of these panels is shown in Fig. 186. It consists of a slate panel having mounted on it a knife switch, two single-pole con- tactors, one overload relay for each motor circuit, and one re- lay in the common return wire. The latter is called a "totaliz- ing relay" and sometimes two of them are furnished, one for each side of the line. These relays provide automatic overload protection for each of the motors, as well as for the complete system. The knife switch is provided with means for locking it in the open position so that an attendant may lock the switch open when he is working on the crane. Each controller may be pro- vided with a contact for resetting the relays when the handle is in the off position or a reset push button may be used. Fig. 186. Crane Protective Panel. CRANES 259 It is desirable to provide a means for limiting the upward travel of the crane hook. This is usually done by a limit switch. The diagram in Fig. 184 shows the connections for an electrical limit switch using a contactor and Fig. 185 for the mechanical limit switch. These limit switches may Fig. 187. 1.5-T0N Semi-Portal Crane. This crane is equipped with a 35-hp. hoist motor; ij-hp. crane-travel motor and 7.5-hp. rotating motor. either be geared to the hoisting drum or operated by the hoisting block through mechanical means. The limit switch corresponding to the diagram in Fig. 185 consists of a rotating shaft having a quick make and break attachment. When the shaft is rotated through a given angle, the mov- able element of the switch is snapped from the normal posi- tion to the emergency position, which disconnects the motor from the line and applies dynamic braking. When the hook is lowered a short distance, a weight or spring moves the shaft back to its normal position and the switch is snapped back, establishing the regular connections. The magnetic contactor control consists of a slate panel having mounted on it the magnetic contactors, knife switches and overload relays. No protective panel is used 26o CONTROLLERS FOR ELECTRIC MOTORS in connection with this form of control, as the safety fea- tures are embodied in the controller itself. The master switch usually consists of a small drum type controller mounted in front of the operator. The contactor panel it- self is usually mounted in the upper part of the cab above the operator's head. This requires a longer cab than is necessary with the manual controller. Fig. i88. Operator's Cab of a Traveling Crane, Showing cam controller and crane protective panel. The most common form of manual controller is of the drum type. This gives a very convenient and compact form of controller but one which is subject to considerable wear for the larger sized motors. In order to provide a more durable form of manual controller, various forms of CRANES 261 face plate and grindstone types have been devised. These controllers are usually quite heavy and have a great many exposed parts which endanger the operator. Recently some attempts have been made to cover up these controllers, but this has added to their bulk so that ip-aT^- -^ •'^ ^ than formerly. A n&w form of controller embodies many of the good and the contactor type of cc form of a drum controller tactors operated by cams, th the drum cylinder. One f trated in Fig. 190. The c shown in Fig. 189. The manner as with a magnetic Fig. 189. Cam Contactuk. Showing contacts open, beginning to close and closed, illustrating the rolling contact. are used, which makes the parts interchangeable. The same blowout is used so that the rupturing of the arc takes place in as efficient a manner as the ordinary magnetic contactor. The cam gives a quick motion for opening and closing so that it is difficult to just touch the contactor. On account of the contactors moving in a horizontal plane and the air space being restricted by the drum mounting, these 262 CONTROLLERS FOR ELECTRIC MOTORS controllers are not recommended for as large motors as the magnetic cantactor type. They form, however, a very valuable intermediate step and may be used even with the smallest motors, as they are available in small sizes. The controller itself is lighter than a drum controller and the construction is such that various combi- nations can easily be ob- tained ; for instance, the plan reversing controller for the bridge service dif- fers from the hoist con- troller only in the cam shaft, the balance of the controller being the same. The bridge motion con- troller can be used for either alternating or di- rect-current motors by simply changing the con- nections. Where alter- nating-current motors are used, dynamic braking OP. „„.,x,voi,L£R. cannot easily be obtained so that the load brake is still used requiring the hoisting motor to be operated under load when lowering. This makes the alternating- current controller the same for the bridge and trolley motions. The cam controller can be connected to the alter- nating-current motor to start up with single-phase second- ary, which gives a small starting torque and corresponding slow speed for light loads. For the bridge service, the same controller is used with both phases closed on the first CRANES 263 notch. The adaptability of this design of controller and the use of interchangeable parts with the magnetic con- tactor control will reduce the spare parts carried in stock. The rolling contact is acknowledged to be the best by all engineers and it should be used wherever possible. Fig. 191. 75-ToN Full-Portal Crane. This crane has a 54-ft. radius and is 54 ft. from track to pivot pin. Hoist motor, 80-hp. ; crane travel motor, 25-hp. ; rotating motor, 33-hp. The present demand for cranes is very great, which has focused the attention of engineers on this apparatus, and many improvements are being made in the design of cranes and the electrical equipment for them. A good example of a special design of heavy cranes is illustrated in Fig. 192. This shows a 150-ton electrically- operated revolving pontoon crane, which is perhaps the largest ever constructed in the United States. The pontoon contains a complete boiler plant and an engine-driven gen- , 264 CONTROLLERS FOR ELECTRIC MOTORS CRANES * 265 erator, which supplies the electric current for operating the various motions of the crane. The crane is controlled from a small house mounted above the pontoon deck by means of master controllers; one operator is able to control all of the motions of the crane. When the load is lowered, the motors operate as generators and, in case of accidental interruption of electric current, the crane motions are automatically locked by means of friction brakes to prevent the possibility of dropping the load. The crane motions consist of a main hoist of 150 tons divided into two parts of 75 tons each. These hoists are fixed on the boom. In addition, there is an auxiliary hoist of 25 tons capacity, which is movable up and down the boom. A rotating motion is obtained by two 60-hp. motors and the boom hoist is operated from the vertical to an angle of 30 degrees by two screws operated from 60-hp. motors. In addition to the crane proper, the pontoon is equipped with four electrically-operated cap- stans, one in each corner. CHAPTER XXI CAR DUMPERS The car dumper is a special machine designed for un- loading ore or coal from open type railway cars. The ap- paratus consists of two essential parts — the barney haul and the cradle hoist. A train of loaded cars is placed on a slight incline ap- proaching the car dumper. An individual car is detached and passes down the incline to the barney haul, which con- sists of a small car with a pusher arm projecting above the track between the rails, attached to a cable driven by an electric motor. The arm engages the rear of the car and pushes it up an incline to the cradle hoist. The car is fastened securely in the cradle by clamps, shown in Figs. 194 and 196, which engage the top of the car and hold it firmly against the rails. These clamps consist of vertical members which are curved into hooks at the top. There are several of them on each side of the car, set high enough to clear the largest car and held down by counterweights, such as the small rectangular counterweights shown in Fig. 194. When the cradle is hoisted they engage the top of the car, exerting sufficient force to retain the car in place when the cradle turns it over. By the use of counterweights a very flexible form of clamping is obtained, which' is entirely automatic in its operation. Clamping bars may be used instead of hooks as shown in Fig. 196. The cradle hoist lifts the car to a fixed elevation and turns it over, emptying the contents of the car onto an apron provided with a chute for directing the contents into the proper place. This method of unloading is used extensively in connection with 266 CAR DUMPERS 267 coal and ore. The material may be loaded onto a conveyor, into a boat or a hopper car. The empty car is returned to the track level and the clamps removed. The next loaded Fig. 193. Car Dumper. Showing the incline on the approach side. This dumper is used for load- ing coal into boats and is provided with a special form of pan and chute for this purpose. car pushes the empty car onto an incline located on the other side of the cradle hoist. The empty car descends by gravity to an assembling track or switch, where it is taken care of in the usual way. BARNEY HAUL The barney haul is usually operated by a direct-current compound-wound motor, although a series motor may be used where there is sufficient friction to eliminate any 268 CONTROLLERS FOR ELECTRIC MOTORS danger of over-speeding. The controller is usually of the rheostatic reversing type, provided with one armature shunt point to give a slow speed when the barney engages the car. A series or compound motor is able to exert the heavy torque required during the period of moving the car up the Fig. 194. Back View of Car Dumper of Fig. 193. incline. On the reverse motion the weight of the barney is insufficient to overhaul the cable; it is therefore necessary to operate the motor drive in the reverse direction. A series or a compound motor gives a high speed return under this light load. In calculating the size of motor and control the heating effect is based upon the period of heavy load during hoisting and the light load on the return stroke. CAR DUMPERS 269 The motors run from 150 to 300 hp., at either 230 or 500 volts. The controller is of the magnetic contactor type Fig. 195. Limit switches are provided to stop the motor SCHEME OF MAIN CONNECTIONS SEQUEN CE OF CONTACT ORS 1h Dis.Rcsislor juiiuiiuuiniL, juuiMJUiJuu, , Zon i Off Un 1 1 z 1 I 2 3 4 5 6 03 7 11 01 12 0K>|O| 13 op 14 2p c Df5 od 21 op o|oM Fig. 195. Diagram of Connections for Barney Haul. automatically at each limit of travel. Both the gear type and the track type limit switches have been used. CRADLE HOIST The motors for the cradle hoist are of the direct-current series type. Sometimes a single motor is used, for other applications — two motors. Where the barney haul requires only half the horse-power of the cradle hoist, the use of three motors of the same rating makes a good arrangement. The armatures and other spare parts of the motors remain the same, the only difference being the field windings. While the loaded car is being hoisted, the maximum amount of torque is required. After the proper height has been reached and the car begins to turn over, the load de- creases but still remains positive due to the arrangement of counterweights. In returning the empty car the motor first operates under a friction load while swinging the cradle over to the upright position of the car. The cradle and car are then lowered under dynamic braking to the track level. The cradle is counter-balanced to make the work done during the total cycle as small as possible. 270 CONTROLLERS FOR ELECTRIC MOTORS Fig. 196. Car Dumper Discharging the Coal from a Car. The car is held against the track by means of bars instead of hooks. The controller connections, Fig. 197, provide for full reverse with dynamic braking in the lowering. The con- troller consists of magnetic contactors operated from a 20 nnn scheme of mah " " "-tsenes CONNECTIONS >1 3 [ ' i 1 1 r r 1 JULUJULI^lA ShuntBrake 21 -N-j > % rUlTL, Snnnnnnnnr 1 Series Dis. Resistor 1 • ' 'T T T T J " ■it- 3 uuiAjuu'*, SEQUENCE OF CONTACTORS ::on Hoisl DH J^wer 1 1 n c :>c 2 C ; t •3 J 3C J f^ ~i 7C D 8 1A 6 3A 20 00 : 6 IllJ eI a 00 _ Fig. 197. Scheme of Connections for Cradle Hoist. Using two motors in parallel. Each motor has a separate set of resistors short-circuited simultaneously by double pole contactors. The acceleration is by current limit relays and the relay coils are shown in the circuit of No. 2 motor. The motors are geared together rigidly and must be accelerated as a single unit. CAR DUMPERS 271 master switch. The limit of travel in both directions is con- trolled by limit switches, either of the gear or track type. The motors range in size from a total of 250 to 400 hp., sometimes divided between two motors. Usually the single motors do not exceed 350 hp. Fig. iq Car Dumper Loading Coal Into a Boat. The distance of the vertical hoist before the car is turned over depends upon the applications. In some cases this hoist may be 40 to 50 ft., in other cases only a short dis- tance. Where the vertical travel is considerable, it is usually necessary to slow down the cradle where the motion changes from vertical to rotating. In coming back, the rotating motion is slowed down where it changes to vertical. This slowing down is to avoid shocks when the cradle enters and leaves the hooks or trunions. 272 CONTROLLERS FOR ELECTRIC MOTORS - The above description is general. It applies in the main to various commercial types of car dumpers, each particular design embodying ingenious features for taking care of the details. The arrangement of the barney differs with vari- FiG. 199. Car Dumper Arranged for Dumping Coal or Ore Into a Yard. Separated from the track by «- wall. The coal or ore is taken from this point by a bridge with a grab bucket. This form of car dumper is arranged for moving along the wall and is carried on a special track. ous companies. It is necessary to return the barney, so that it will pass underneath the next car. This may be done by a system of two track levels or by an arrangement for rotating the arm into a horizontal position during the return part of the travel. The details of changing the motion of the cradle from the vertical hoist to the rotating motion differ in various designs. The car dumpers illustrated in Figs. 193 and 198 employ an ingenious arrangement to permit the operator on the outer end of the pan to get back CAR DUMPERS 273 and forth to the main cab. It consists in a duplicate set of controllers for hoisting the pan so that the operator may bring the hinged end of the pan back on a level with the stationary cab and then raise the pan to a horizontal posi- tion to permit him to walk from his cab to the main cab. Various designs of pans or chutes are employed for direct- ing the contents of the car in the desired manner. PAN OR APRON Where the coal or ore is loaded into the vessel, an ad- justable pan or apron provided with a chute is furnished, the nozzle of which can be turned in different directions. This arrangement is illustrated in Fig. 193. The pan is Fig. 200. Motors and Controllers for Operating Car Dumper. raised or lowered by a pair of screws, one of which is at- tached to each upright. These screws are driven by revers- ible motors with the ordinary rheostatic control. The pan is hinged to the two uprights of the car dumper and the outer end is raised or lowered by means of an electric motor. The control for this motor is similar to the control for the cradle hoist, only much smaller. It provides for rheostatic 2/4 CONTROLLERS FOR ELECTRIC MOTORS control In the hoisting direction and dynamic braking in lowering. The chute is provided with a small motor for rotating it. The operator is located in the cab immediately above the chute and controls the height and location for the opening in the chute by means of the controllers just described. CHAPTER XXII ORE AND COAL BRIDGES Ore bridges are used for the handling of ore and coal, principally on the Great Lakes. During the navigation period, ore is shipped from the Lake Superior region to the lake ports in the East, and coal is shipped back. The ore boat is a long vessel made up almost entirely of cargo space. The propelling machinery is usually at the rear, which leaves the body of the boat free for cargo. The ore is loaded into the boats at the Lake Superior ports from bins by means of chutes. When the boat reaches its east- ern destination in the neighborhood of Cleveland or Buf- falo, the ore is taken out of the boat and, either loaded in cars for transportation to the blast furnaces, or placed in stock piles. The coal is then loaded into the boat from cars by means of a car dumper and is taken out of the boat at the Lake Superior ports by means of an unloading bridge. An ore or coal bridge. Fig. 201, consists of a struc- tural steel span supported on two piers. These supports are mounted on tracks and moved by electric motors. On the bridge is a run-way supporting a trolley, which is moved forward and back on the bridge by electric motors and carries the hoisting mechanism for raising or lower- ing the clam-shell bucket. Usually the operator rides in this trolley familiarly known as a ''man trolley." The bridge is moved along the tracks until the clam-shell bucket is in position above the hatchway. The bucket is lowered into the boat and lifts the ore or coal up through the hatch- 19 275 c « O ORE AND COAL BRIDGES 277 way, carrying it back to bins or a stock pile. The bins are used for loading cars in the usual manner. During the summer, a surplus of ore is accumulated at the Great Lake stations in large piles known as " stock piles." At the time of unloading, the ore is graded, so that the burden in the clam-shell bucket may be dropped close to the dock or a considerable distance back, depending upon the location of that particular grade of ore. Special machines are frequently used for the purpose of unloading the ore from the boat. Where such machines are employed, the ore bridge is used for transferring the ore from the unloader proper to the stock pile or bins. Where the ore bridge is used for unloading the boat, provision is sometimes made for twisting the bucket through 90 degrees to facilitate picking up the ore in the hold of the boat. The application of motors and control to an ore or coal bridge divides itself into three distinct classes; viz., hoist, trolley and bridge. HOIST The hoist is equipped with two drums, one drum handling the shell line and the other the closing line. The shell line is attached directly to the stirrup of the bucket, while the closing line is attached by levers in such a manner that pulling on the closing line causes the jaws of the clam-shell to come together and retain the load. In picking up a load, the bucket spades are in a vertical position, with the closing line slack. The bucket is then lowered into the ore or coal and the closing line pulled up. The tension on the closing line moves the two shells together, after which the hoisting may be done on both the closing and the shell lines. To discharge the burden, the load is held by the shell line and the closing line is slackened. When only one hoisting motor is used, the two drums are 278 CONTROLLERS FOR ELECTRIC MOTORS (b) 3._ controlled by clutches. Where two motors are used, one is connected to each drum, each motor having a separate con- troller and usually a separate master switch. By using two motors and adjusting the resistance, the proper tension can be kept on the closing and shell lines so that the bucket will not open until the operator changes the adjustment. Direct-Current Mo- tors are series wound and operated as stand- ard series motors in the hoisting direction. In lowering, a kick-oflF notch is provided which connects the armature and field in parallel across the line. This causes the motor to op- erate as a shunt motor. After the bucket has started down, there is always sufficient weight to overhaul the hoist- ing mechanism so that the armature generates through the series field and provides dynamic braking. After this braking cir- cuit has been once established, the speed of lowering can be controlled by changing the resistance in the dynamic brake circuit. The motor ma}- or may not be disconnected from the line, depending upon which condition gives the most economical operation. The schematic arrangement of such a control for a single motor is shown in Fig. 202. If two motors are used, two controllers are supplied, one for each motor. The switch Notches 1 2 ■3 4 5 6 Hoist — ClObe Switches IJ 4 S 6 7 8 Lower Kiclt Off Close Switches 1A-2-; Lower Brake - Close Switches lA-3 4 5 6 7 8 Lower — Close Switches lA-2-3 4 5 6 7 8 ,Otf Position - Close Switches 3A _ _ _ (-Method I Mcthod^J Fig. 202. Connections of Direct-Curre.vt Motors and Controllers for Hoist Installations. ORE AND COAL BRIDGES 279 marked jA in Fig. 202 (a) is closed only in the central or off position. This dynamic brake circuit does not in- clude the magnet brake^ coil; therefore, the friction shoes set and the motor is brought to rest. Secondary Hoist- Close Switches Lower-Close Switches Brake- Close Switches 2-4-5-6 10-1611-17 12-18 in-lrill-17ll2-l>lil3-14-19-2d 11-1712-1813-14-19-20 13-14-19-20 Fig. 203. Connections for Alternating-Current Hoisting. Used in installation shown in Fig. 207. The connections for hoisting are given in Fig. 202. (b) This is a straight rheostatic controller, the speed of hoisting being controlled by a cutting out of resistance in series with the armature and field. The connections for lowering are given in Fig. 202. (c) On the first notch, the current comes in through switch 2 to a point between the armature and field. It passes through the field and brake magnet to the starting resist- ance and thence through switch j to the line. Another branch of current passes through the armature in the reverse direction and through a resistance to contact i-A and thence to contact j and the line. This keeps the cur- rent flowing through the field in the same direction while 1 By " magnet brake " is meant a friction brake which is set by a heavy spring and released by a magnet. 28o CONTROLLERS FOR ELECTRIC MOTORS it reverses the current through the armature, causing the motor to rotate in the opposite direction. After the motor A _ r, r» B D o Overload ^ . ^ 5 R, R2 R3 R^ Rs Series Field o Relay "•tRj^Timi^ (a) 1 7 rOn 5 Resislorl ll4nltlUln^^nJl^(l^ ' ^ BrakeRelay SEQUENCE OF CONTACTORS Coa "orward Brake Off , Brake Reverse | 1 2 3 4 5 6 7 8 00 9 10 U IZ 13 20 21 ° ° Fig. 204. Connections of Motors and Controllers for Trolley Applications. starts, the load drives the motor as a generator, the circuit being through the field and resistance i to resistance 2 and ORE AND COAL BRIDGES 281 thence through the armature, completing the circuit. When this condition is established, switch 2 may be opened. The motor then runs as a generator at its maximum speed. By successively closing switches 4. to 8 inclusive, the resistance of this circuit is decreased and the speed of lowering de- FiG. 205. Direct-Current Contkol Panel. Designed for two motors and used in trolley applications. Double-pole switches are used throughout to insure that the direction and acceleration of the motors is maintained uniform. creased. In order to stop, the controller is moved to the central position, closing switch J-A, in Fig. 202 (a), which sets the magnet brake and brings the hoist to rest. Other methods of connecting the motor armature and field for lowering have been proposed, but they all follow the same essential arrangement, as shown in Fig. 202 ; namely, the motor is operated as a shunt generator for the kick-off or starting point and as a series generator for 282 CONTROLLERS FOR ELECTRIC MOTORS lowering. Sometimes a reverse current relay is placed in the armature circuit to disconnect the motor from the line automatically as soon as regeneration begins. If a heavy load is lowered frequently, it may be more economical to keep the motor connected to the line, so that power will be returned to the system. Usually, however, the bucket is Fig. 206. Ore Bridge. Equipped with direct-current motors and controllers, and having a lO-ton bucket. Two motors are used on the hoist and one on the trolley. A large pile of iron ore is shown in the foreground and the blast furnaces in the background. lowered empty and hoisted with a load. It is trolleyed over to the proper point and the load dropped without lowering the bucket, as this saves time. Alternating-Current Motors. — Wound-secondary induc- tion motors with external resistance have been in successful use for about ten years on bridges of this type. The control is shown diagrammatically in Fig. 203. For opera- tion under power, the control does not differ in any way from the standard reversing controller. The motor is con- nected to the line with all of the resistance in the secondary and this resistance is gradually short-circuited to bring the ORE AND COAL BRIDGES 283 motor up to full speed. Dynamic braking is obtained by connecting the primaries of the motors in series to direct- current power, which is usually furnished by a motor-gen- erator set. The resistance is inserted in the secondary. With the motor at full speed, the direct-current power pro- vides a stationary field, which produces the same effect as when the motor is at rest and the alternating-current rotat- ing field is provided. As the resistance is reduced in the Fig. 207. Ore Bridge Unloading Coal from Boats. Showing pier leg of bridge, bucket and man trolley. This bridge was the first one which used alternating-current motors for coal handling and is equipped for dynamic braking. The bucket has a five ton capacity and this installation has been in successful operation for about ten years. secondary of the motor, it is gradually slowed down until it operates at a very slow speed with the secondary short- circuited. The controller illustated in Fig. 203 can be arranged for the same number of power and brake notches on either side of the off position when used for trolley Missing Page Missing Page 286 CONTROLLERS FOR ELECTRIC MOTORS ORE AND COAL BRIDGES 287 O o H W 288 CONTROLLERS FOR ELECTRIC MOTORS the current passing through the armatures in the reverse direction. The brake connections are shown in Fig. 204 (c) and (e) , the only difference being the direction of current through the armatures. The shunt around the field coils is disconnected and current is applied through switch 8, re- sistance J, the field coils and switch 2 to the line. The armatures are connected in the closed loop through the starting resistance and switches j, 4, and 6. This causes the motor to operate as a shunt generator, the amount of dynamic braking being controlled by closing switches 10 to Tj inclusive, which decreases the resistance in the arma- ture circuit. When the mechanical brakes are released by an electromagnet, the magnet is provided with a shunt winding, which is disconnected in the off position. Where a single motor is used, no shunt connection is required around the series field, as provided for in switch p and resistance 2 ; otherwise, the connections are the same. Sometimes two motors are provided, each with a separate controller. When this is done, it is desirable to use double pole switches and common accelerating relays. Fig. 205, so that the rate of acceleration of both motors is maintained constant and there is no danger of connecting the motors to the line in reverse directions. BRIDGE One or two motors are provided on each pier of the bridge for moving it along the track. These motors are provided with an ordinary reversing controller having a slow-down point by using a resistance in shunt with the armature. The friction brake is released by a shunt magnet, which is disconnected, thereby setting the brake, only in the off or central position of the master switch. Usually, two separate master switches are used, as the two ORE AND COAL BRIDGES 289 piers of the bridge may not always move at a uniform rate, due to slippage and other causes. If one pier moves faster than the other, the proper manipulation of one or the other controller will straighten the bridge. Usually, limit switches are provided for automatically disconnecting the leading motor in case the skewing of the bridge exceeds the safe limit. CHAPTER XXIII COKE Every American should be interested in the conservation of our national resources and the making in America of dyes and other chemicals, which were formerly purchased from Germany. For years this country wasted perhaps the Fig. 211. Car Du.mper and Coal Handling Equipment. most valuable part of the coal in the manufacture of coke. Coke is produced from coal by the action of heat. The process drives off the tar and volatile matter, leaving a hard gray substance known as coke. Until recently the process was carried on in this country in what are known as "bee hive coke ovens." This process produced only coke, and wasted the byproducts. 290 COKE 291 During the past ten years a large number of plants have been built which conserve the byproducts and are often referred to as byproduct coke and gas oven plants. These plants eliminate a great deal of the dust and fumes which originally ruined the country for miles around and, while Fig. 212. Contactor Panel for Pusher Ram Controller. Diagram of connections shown in Fig. 215. there is still dust in some of the operations, there is no nuisance or damage to the surrounding country. This dust consists largely of coke breeze which is a very fine coke dust. There is also coal dust in and about the crushers and conveying apparatus. It is therefore necessary to protect all control apparatus from this dust to reduce the main- tenance and insure reliable operation. Many of the con- troller panels are enclosed in cabinets provided with felt packing around the door. Examples of these cabinets are shown in Figs. 212 and 213. The control applications to a byproduct coke plant can best be described by taking up the process of manufacturing 292 CONTROLLERS FOR ELECTRIC MOTORS coke from the time the coal is received at the plant until the coke is ready for shipment. The method of hand- ling the coal from the cars or barges to the re- ceiving bins depends upon the locality and method of receiving the coal. Where the coal is shipped by rail, the car is often elevated to a height of 40 or 50 ft. and emptied di- rectly into the receiv- ing bin. In other cases the coal is emptied into a track hopper and conveyed by belts to the bins. This latter process is shown in Fig. 211. When the coal is received by barges, a regular unloader may be used or some form of hoist with clam-shell bucket. Where the coal is stored in the open, a coal bridge is required. Coal is conveyed from the receiving bins to the breakers and crushers. These crushers consist of hammer mills or rolls driven by constant speed motors, the controller being of a plain rheostatic t\'pe for direct-current motors or slip Fig. 213. Protective Panel for Pusher Ram. Doors open and closed. Diagram of con- nections shown in Fig. 215. o c u ^ 5 u z 294 CONTROLLERS FOR ELECTRIC MOTORS ring motors. Where squirrel-cage motors are used standard autostarters are applied. The starting requirements for these rolls are sometimes quite severe and care should be taken in applying squirrel-cage induction motors to see that sufficient starting torque is available. Fig. 2i6. Combined Pusher, Leveler and Door Machine. The control equipment for coal handling machinery is often combined into one large board located in a dust-proof building. The master controllers, which are usually push buttons, are located close to the individual motors, so that the attendant must be close to the motor when it starts. This arrangement makes it necessary for the operator in charge of starting the motors, to walk the entire length of different conveyors and inspect them together with the driv- ing machinery before starting the motors. Safety stop buttons are located at convenient intervals along the entire length of the conveyor galleries so that the machinery can CONTROLLER B^ CONTROLLER B, CONTROLLER Bj CONT ROLLER. A: CONTROLLER Al CONTROLLER As CONTROLLER Bl j^^ SEQUENCE OF CONTACTORS CONTROLLER B s 0-,n ^un 1 1 too o o n o i o o c c ?i o n c 4 ^ n 4 o SEQUENCE OF CONTACTORS CONTROLLER Bj SEQUENCE OF CONTACTORS CONTROLLER Aj SCHEME OF MAIN CONNECTIONS CONTROLLERS Ai and A5 From 3 Phase Line Con. ft 1 ■-; u ^ C-^n Run 1 1 n n -) n ■y ? ;i D 5 i Lj tL2- Bs 4Pf To Motor Primary Fig. 218. Coal Handling Control. The entire motor and control equipment is alternating current. Part of the motors are squirrel-cage and part of the slip-ring type. Each controller is of the contactor type and self-contained, provided with a push button for starting and another button for stopping. The controllers are interlocked so that motor A must be started first, then motors B, C, etc. If motor C is stopped, motors D, E, F, G, H and / will also stop automatically. It will then be necessary to start motor C before the others can be started. The operating of any one of the stop buttons stops all of the motors ahead of that control. This diagram has been simplified by omitting all duplicate equipment. The actual control system provides for additional motors. COKE 29 S be stopped if anything goes wrong or an accident should occur. The coal handling control is so interlocked that the machine at the delivery end of the system must be started first and the other machines in their order up to the receiv- ing end of the system. Should a machine in any part of the system stop, all of the machines back of it are automatic- ally stopped to prevent the loading up or accumulation of material in any part of the system. The remaining ma- chinery on the delivery end continues to operate, thus un- loading all of the material in that part of the conveyor system. In some plants a relay is used for stopping the con- veyor if the load goes off, due to the breakage of a belt, gear, etc. This relay operates to open the contact if the load on the motor falls below a fixed value. In some instances as many as twenty motors have been interlocked on a system of this kind. As a rule, however, they are divided into two or more groups, each group con- veying the material to a storage or mixer bin. Lights or other signal devices are used so that when one group of con- \'eyor motors is started up the operator on the second group is signaled automatically to start up his group. The ovens are arranged in batteries and are opened at both ends. Each oven is about 8 inches wide by 10 feet high and 40 feet long and has a capacity of from 11 to 16 net tons of pulverized coal. Coal is charged into the ovens by a larry car having four hoppers. This car conveys the coal from the storage above the ovens to the ovens them- selves. These cars are generally operated by direct-current motors with the ordinary rheostatic reversing drum con- trollers. The current is collected and returned to the line from two collector shoes operating on collector bars at the side of the track. A door machine is provided on each side of the oven. On one side this door machine is combined with a leveling bar and pushing machine, as shown in Fig. 216. On the 296 CONTROLLERS FOR ELECTRIC MOTORS < O" ^-1 o o X COKE 297 Other side of the oven the door machine is combined with a coke guide through which the coke is forced from the oven into the quenching car. This arrangement is shown in Figs. 217 and 220. Fig. 219. Coke Quenching Station of the Lehigh Coke Company, South Bethlehem, Pa. After an oven has been emptied, the doors at each end of the oven are placed in position by the door machines and then sealed with clay to make them air-tight. A small opening is provided in the top of one of the doors for the leveling bar which levels off the coal discharged into the oven by the larry car. After the oven has been completely 298 CONTROLLERS FOR ELECTRIC MOTORS charged this small opening is closed and sealed. The cok- ing process requires from 14 to 22 hours. When the coke is ready for removal from the oven the door on the one side is removed, the coke guide is placed opposite the opening and the quenching car is placed opposite to coke guide. The pushing bar is inserted from the opposite side and the coke forced out through the chute into the quenching car. This is shown in Figs. 216, 217 and 220. The control for the combined pusher, leveler, and door machine is shown in Fig. 215. The various operations are often interlocked to insure safety in handling the coke and in the movement of the machine. The controllers are of the usual rheostatic reversing type. Both drum controllers and magnetic contactor panels are used, depending upon the size of the motor and the work to be done. Limit switches are provided to prevent over-travel for the machines which have positive limits of travel. The quenching car is moved by a 20-ton electric locomotive which takes it to the quenching station where water is discharged on the coke in the car to reduce its temperature so that it can be handled readily. Fig. 219 shows a load of coke being quenched at the quench- ing station. The locomotive then conveys the coke to the coke wharf Fig. 220 where the coke is discharged and allowed to cool still further. From here it is fed onto belt conveyors by a rotary feeder and conveyed to a screen which separates the coke from the coke breeze or dust. The electrical apparatus which operates the feeding conveyor and screen is subject to a great deal of coke dust and must be carefully protected. The control apparatus is usually the ordinary rheostatic type; either alternating-current or direct-current motors may be used, the latter being preferable on account of having fewer moving parts. The gas and fumes which are distilled from the coal during the formation of coke are conveyed to various tanks COKE 299 and stills where the byproducts are separated and refined. The electrical apparatus for this part of the equipment has no unusual features. During the heating process when the coal is changed to coke, in a well known type of oven, the direction of the gas through the ovens and flues is changed about every thirty minutes. This change is usually taken care of by clock-work which operates the motors that change the dampers. Fia. 220. Coke Wharf and Coke Quenching Car. Alternating-current motors are preferable for all con- veyors, crushei's, damper regulators and screens. These motors have fewer moving parts than a direct-current motor and are less likely to be affected by the dust. Direct-cur- rent motors are preferable for traction purposes, also for the pusher, leveler and door machine, and for the unloading machines, either a car dumper or a hoist. All of the motors 300 CONTROLLERS FOR ELECTRIC MOTORS and controllers in a coke plant are subject to a great deal of dust and fumes. Special treatment should be given the exposed windings of motors and in some cases artificial ventilation is provided so that clean air can be taken from the outside and passed through the electrical apparatus. Little or no speed variation is required on most of the apparatus. Where speed regulation is required, direct- current motors are recommended. An important feature in the control equipment is the proper interlocking of groups of controllers to prevent accident. CHAPTER XXIV ELEVATORS Fifteen years ago, the electric elevator was used for small installations. The large office buildings and hotels used hydraulic power to operate their elevators. The hydraulic elevator has the disadvantage that it is inherently a constant power machine, as it takes the same amount of water whether the car is operated empty or loaded; and it is not feasible to overbalance the car on account of the hydraulic elevator being a single-acting machine and part of the car weight must be left unbalanced in order to circu- late the water when the car is descending. There is another disadvantage; every hydraulic installation- requires a power plant consisting of pumps, tanks, regulating aparatus, and a considerable amount of expensive piping. Many large elevator installations still use hydraulic elevators, but the improvement in electric elevators has extended their use so that today they are the preferred type of elevator for all installations. In fact, some of the highest buildings could not have been successfully equipped with hydraulic ele- vators, on account of the length of car travel. The electric elevator has less investment and maintenance cost than the hydraulic, due to the elimination of the power plant. It is also more economical to operate because the power used is proportional to the load. Electric elevators are used for a wide range of service conditions. The simplest form of elevator mechanism con- sists of a driving sheave operated from a counter shaft by suitable belting. The countershaft is run continuously by an electric motor and the elevator is operated up or down 301 by shifting the belts from the loose pulley to either the forward or reverse operat- ing pulley. The elevator is brought to rest by means of a mechanical brake con- nected to the belt shifting mechanism. These elevators usually lift from l,ooo to 2,000 pounds at 50 feet per minute. The next advance step is made by belting the motor to the drum with a single belt and securing up or down operation of the elevator car by running the motor in either the forward or reverse direction. These two types of machines are used for slow-speed freight service. Fig. 221. Complete Elevator Equipment. 'The rope passes from the top of the car around the traction drum on the hoisting engine and thence to a counterweight running on a pair of guides at the rear of the hatchway. Another set of cables runs from the bottom of the counterweight to the bottom of the car. These cables are used for counterbalancing purposes only and do not carry any of the load. The controller is shown at the right of the motor, both motor and control being located above the hatchway. Along the side of the runway for the car at both the top and bottom are shown three limit switches for bringing the car to rest automatically at either limit of travel. On the bottom of the hatch- way is shown an oil buffer to stop the car in case it should creep be- yond the usual limits of travel. ELEVATORS 303 A more substantial machine is made by mounting the motor on a common bed plate with the drum and gearing, thus forming a self-contained unit. For passenger service, a worm gear is used. The gearing consists of either a single wheel with ball bearing end thrusts on the worm, or a double wheel with right and left worms. The latter arrangement is usual for heavy loads. This type of ma- chine has been successfully operated with car speeds of 300 feet per minute and even higher. For large office buildings, hotels, and similar applica- tions, car speeds from 450 to 600 feet per minute are usual. In place of the winding drum, a grooved wheel is used, the drive depending upon the friction between the rope and the wheel. This type is known as a " traction elevator ma- chine." The rope connection between the car and the machine may be arranged for i :i gearing, in which case, the car tra\els at the same speed as the circumference of the driving sheave. Another form has a rope connection, which gives a car speed equal to one half the peripheral speed of the driving sheave. This is known as "2:1 traction." In some cases, the driving sheave is mounted directly on the motor shaft and a slow-speed motor is used, operating from 30 to 60 revolutions per minute. Another form has her- ring-bone or worm gears between the motor shaft and the driving sheave. This is known as a " gear type traction machine." For freight service, where the loads are heavy and high speds are not necessary, a drum machine having a combina- tion of worm and spur gearing is used. A pinion is mounted on the worm wheel shaft and meshes with an internal gear bolted to the end of the drum. This gives extra lifting power at a slower speed, but the spur gearing causes the machine to run less smoothly than a straight worm gear drive. Fig. 222. A Wokm-Gear Drum-Type Elev.\tor M.a.chixe. Located in the basement and set back from the hatchway so that the car and counterweight ropes pass around the two idler sheaves shown. They are technically known as vibrating sheaves because they travel to the right or left along their shaft as the rope winds on or off the drum. To the right of the machine is shown a rope wheel, which is operated from the ear by means of a lever or wheel. This rope wheel operates the reverse switch back of the large gear, which connects the motor to the line through the magnetic contactor shown next to the drum. The motor is of the squirrel- cage induction-type, which can be connected directly to the line. The fric- tion br^ke is located between the motor and the worm gearing, and consists of a wheel with two shoes, which are pressed against the wheel by coil springs and released by the polyphase brake magnet shown above the brake. ELEVATORS S^S The smooth running of the car is affected by a number of conditions. The car travels on two guide rails mounted vertically in the shaft. Any irregularities in these rails will be felt in the car. The guide shoes mounted on the car must have flexibility, in order to take up the lost motion and at the same time, not cause undue friction. A poor adjust- ment of these guide shoes will often cause a swaying or rattling of the car. Improperly adjusted shoes or gritty rails may cause a disagreeable scraping sound. Gearing when used should be very smooth in operation and have little or no back lash. The effect of a rough worm wheel or bad end thrust bearings can readily be felt by a passenger in the car. Defects of this kind cause tremors or vibra- tions, which are very disagreeable. A friction brake is used to effect the final stop and to hold the car securely at the landing. Considerable skill is re- quired in designing this brake to make its action soft and gradual. The acceleration and retardation or deceleration of the car depend largely upon the controller, although it is essen- tial that the electric motors have proper characteristics. The passengers notice the rate of change of acceleration more than the acceleration itself. Therefore a controller should be designed so that the rate of acceleration will not be changed abruptly, either in starting or stopping the car. Usually the motor is started with resistance in the arma- ture circuit. This resistance is short-circuited in steps, finally bringing the motor to full speed. The steps of re- sistance have a finite value and therefore each step repre- sents a change in the rate of acceleration, which has a tendency to be abrupt. The design of the motor and con- trol should be so adjusted that these abrupt changes in the torque of the motor will be minimized. This adjustment is improved by having a motor that does not respond too quickly to a change in impressed voltage, and by providing 3o6 CONTROLLERS FOR ELECTRIC MOTORS a considerable number of steps in the starting resistance. The change in the amount of this resistance is usually con- trolled automatically, in which case the switch in the car Fig. 223. Direct-Current Motor Driving an Elevator Equipment. The controller is mounted directly above the motor and is used in connec- tion with a reverse switch which is mounted above the brake and operated by the rack and gear shown on the left. The device projecting from the machine on an e.xtension of the drum shaft is the stop-motion gearing, for returning the controller to the off position and applying the friction brake automatically at either limit of travel. determines the amount of resistance which may be cut out, but the automatic arrangement in the controller fixes the rate at which the resistance is short-circuited. ELEVATORS 3°? Where an adjustable speed motor is used, the changes in field strength must be effected gradually. This is to a cer- tain extent an inherent characteristic of the motor. In some cases, however, it must be assisted by automatic means embodied in the controller. Counterbalances are used having sufficient weight to com- pensate for the weight of the car and a part of the load. The object of this counterbalance is to reduce the work done by the motor in moving the car from one landing to another. The counterweight is connected to the winding drum in the reverse direction from the hoisting ropes, so that when the hoisting rope is wound up, the counterbalance rope is un- wound and vice versa. If the load in the car is such as to just equal the counterbalance, the motor overcomes fric- tion only. If the car is at the top of the runway and a load in excess of the counterbalance is placed in the car, the car drives the winding drum, due to its excess weight, and causes the motor to operate as a generator. The same con- dition is obtained if the car is at the bottom of the runway, and starts up with a very light load. The counterbalance in this case being heavier than the car, drives the motor as a generator. The elevator is "overbalanced" to limit the maximum current drawn from the power line. If the overbalance is equal to half of the maximum load the car is required to lift, it is obvious that the motor need be only half the size that would be required if no overbalance were used. A some- what larger motor is generally selected and the overbalance reduced to equal half of the average load, which is from 30 to 40 per cent, of the maximum load. The over-counterbalance of the car weight causes the motor to operate part of the time as a motor and part of the time as a generator. This generator action may occur either with the car ascending or descending, depending upon the loading. It complicates the controller problem ma- 308 CONTROLLERS FOR ELECTRIC MOTORS terially. If the controller is adjusted to accelerate with the maximum positive load on the motor, it will have a tendency to jerk the car when accelerating with the maximum nega- tive load. In stopping a car, the same conditions hold, so that a great deal of care must be exercised where high- speed cars are used. If the speed of a car is I GO to 150 feet per minute, the change in rate of acceleration between posi- tive and negative values of loading is not very pronounced. If, however, the car speed is 600 feet per minute a marked difference may be notified. In order to obtain slow speed for making landings, resist- ance is used both in series with the armature and in shunt with the armature. This gives a positive slow speed, even with negative loading, the speed of course, being faster with a negative load than with a positive load. For high-speed service, several speed points are provided on the master switch so that the speed in making the landing can be ad- justed to suit the loading of a car. In stopping, this resist- ance in shunt with the armature is used for dynamic braking. In addition to this armature shunt resistance, a second or emergency resistance is frequently used, so that should an open circuit occur in either resistor, at least one resistor would be available for dynamic braking. Some- times both resistors are used each time the master switch is placed in the off position. If a car is descending with the maximum load at full speed and the operator throws the master switch quickly to the off position, the stopping of the car can usually be felt by the passengers. Often a per- ceptible bump, or sometimes a series of bumps occur due to sudden changes in the rate of deceleration. A good operator, however, moves his switch lever gradually so as to give a smooth control. To a certain extent, this same thing is noticeable in the handling of railroad trains, street cars, etc., where the engineer or motorman may jerk his train or car very badly if he is careless in starting or ELEVATORS 309 stopping. The skilled operator will bring his car flush with the landing almost every time. It is the abrupt jerking of the car back and forth to make a landing that is disagreeable to the passengers and indicates an unskilled operator. f Li 4-4. %1 \ ) ^mi ^^H^Um J ^^S' *J ^BI^bI Mi " ""S ^^p^^ Fig. 224. Geared Traction-Type Elevator Machine. The reverse switches, as well as the remainder of the control, are operated electrically. There is no stop-motion gearing on this machine, as this form of limit stop is not used on traction-type machines. An elevator car has a limited travel in either direction. Downward, it is the bottom of the pit and upward, is the sheave beams. It is therefore very important that the car be brought to a positive stop at the top and bottom landings. As a matter of safety, both stops must be independent of the operator. There are various ways of doing this. Where the ropes are wound on a drum, a mechanical attach- ment known as the " stop motion gearing," is provided, usually on an extension on the drum shaft. This gearing operates the controllers and sets the brake at either limit of travel. For high speed cars, several slow-down steps are provided before the final stop is made. As the speed of the car decreases, fewer slow-down steps are required, and for slow-speed passenger and freight cars, only one step is 3IO CONTROLLERS FOR ELECTRIC MOTORS used. This method is known as a " machine limit stop " because it is driven by the winding machine. Another device, known as the "hatchway limit" consists of one or more small switches placed in the hatchway and arranged to be operated by the car in passing. These switches are arranged at either limit of travel and can give both slow-down and stop when required. For traction ma- chines, where the rope is not positively attached to the Fig. 225. A Worm-Gear Drum-Type Machine. Operated by a two-speed wound-secondary induction motor. The friction brake is released by an alternating-current magnet and the controller, shown in Fig. 231, is actuated entirely by magnets. machine, as in a winding drum, the machine limits stops are not safe or satisfactory, so that hatchway limit stops are used ; a limit switch attached to the car is also used. A traction drive for an elevator car is inherently safer than a drum, because the car or counterweight, on reaching the bottom of its travel takes most of the tension off the ropes and allows them to slip on the driving sheaves, pre- venting a further movement of either car or counterweight. ELEVATORS 3 I I Oil buffers are provided for cushioning the impact at the limits of travel. The successful operation of an elevator depends largely upon the control equipment including the magnet brake. Every direct current controller should have the following features: 1. A line switch to disconnect the motor from the electric circuit. This switch may be operated every time the car is operated, or it may remain closed normally and be opened only by some of the safety devices or when the line voltage fails. 2. A reversing switch to change the direction of rotation of the motor. (Sometimes several switches are used combining i and 2.) 3. An accelerating device for gradually short-circuiting the starting resistance. Where a variable speed direct-current motor is used, this device must also gradually weaken the motor field to attain the maximum speed of the elevator car. 4. A dynamic brake for slowing down the car when the elevator is brought to rest. This device is sometimes omitted in slow-speed freight service. 5. A mechanical brake for stopping the elevator car and holding it securely at the landing. 6. A top and bottom limit stop for bringing the car gradually to rest at either limit of travel, independently of the operator. 7. A controlling device in the car for operating the elevator. This may consist of a master switch or a mechanical device such as a rope or lever. 8. A slack cable device for stopping the motor in case the car or counter-weight should become obstructed in its travel, causing the ropes to lose their tension. This does not apply to the traction type of elevators. 9. An over-speed switch for disconnecting the motor from the line at the time that the car safety device operates to grip the guide rails. In some forms of machine a slack cable switch is depended upon to perform this operation. 10. Overload protection to the motor. This frequently consists of fuses, although sometimes overload and no-v'oltage circuit breakers are used, or a circuit breaker relay with magnet control. In addition to the above, high-speed passenger elevators must have the following control features : 11. One or more positive slow-speed notches. It is necessary to have a slow-speed notch to enable the operator to stop his car accurately 312 CONTROLLERS FOR ELECTRIC MOTORS at the landings. This slow speed is also necessary to bring the car to rest properly at either limit of travel. Where the speed of the car exceeds 400 feet per minute, several slow speeds are de- sirable in order to obtain efficient operation of the elevator. 12. An emergency switch on the car to enable the operator to stop the car in case his regular controlling device becomes disarranged. For high-speed elevators, it is customary to provide oil- buffers under the car and counter-weights ; also a mechan- ical brake on the car, by which the guide rails are gripped to retard the car in an emergency. This brake usually is an attachment to the regular safety stop. In addition to the controller proper, it is desirable to provide contacts on the hatchway doors or gates which will prevent car operation until the gate is closed. Sometimes the car itself is equipped with a gate having one of these contacts. The above functions of the electric control are accom- plished in a variety of ways, the devices used depending upon the characteristics of the electric motor. DIEECT CUBBENT ELEVATOES The first successful electric elevators were operated by compound-wound direct-current motors. These motors had a fixed shunt field ; the compound winding was cut out after starting so that the motor operated as a shunt motor at full speed. This design gave a good starting torque, together with good speed regulation after full speed was reached. The reversing drum was operated first, being followed im- mediately by the closing of the line switch ; next the resist- ance arm was released, moving the brush over a series of contacts which short-circuited the starting resistance and afterwards the series turns on the motor field. The resist- ance arm was raised by a series wound solenoid and retarded by a dash-pot. This form of controller with various me- chanical modifications, is used successfully today for slow- speed passenger and freight service. ELEVATORS 313 The first developments were improvements in the method of cutting out the starting resistance. Some controllers used a shunt coil connected across the motor terminals and actuated by the counter e.m.f. of the motor for moving the resistance arm. This same arrangement is now used, except that the resistance is short-circuited by individual contactors actuated by the counter e.m.f. of the motor and the dash-pot Fig. 226. D.C. Elevator Control Using a Dash-Pot for Controlling Acceleration. is omitted. Other -arrangements were developed for closing these individual resistance switches, depending upon the amount of current taken by the motor. The dash-pot method of acceleration has one distinct advantage; namely, the resistance can be proportioned to start the motor under light load, and the accelerating device will continue to re- 314 CONTROLLERS FOR ELECTRIC MOTORS duce this resistance until the car starts even under heavy load, so that it is very seldom that an elevator having this method of acceleration will fail to start. This advantage has caused many manufacturers to retain the use of the dash-pot, although there are some troubles inherent in this type of apparatus. Another improvement has been in the line switch and reversing switch. The line switch has been replaced in most controllers by an electrically operated contactor. Sometimes several of these contactors are used for reversing the motor, while in other cases a mechanical reverse switch in combination with an electrically-operated line contactor is found to be cheaper and just as satisfactory for slow- speed service. A number of other devices for accelerating the motor have been tried, but are seldom used at present. One such device was a rheostat driven by a pilot motor. In addition to the expense of this device, it was too slow in returning to the starting position. With the development of the shunt motor with a two to one speed adjustment, a new feature was introduced in ele- vator control. This motor gave two fixed running speeds and made the control of high-speed elevators more positive. Trouble was experienced at first in changing from maximum to minimum speed and vice versa. When the field was reduced, the motor had a tendency to jerk the car and take a heavy current; when the field was strengthened, the motor acted as a generator and reversed the direction of current, tending also to jerk the car. An improved method for con- trolling the field of the shunt motor is the so-called " flutter- ing " relay, which strengthens the shunt field on an excess of current, and reduces the field by inserting resistance when the current drops to the proper value. A further increase in car speeds made it necessary to pro- , vide a slower running speed than could be economically obtained by shunt field control. This slow speed is secured ELEVATORS 315 by shunting the armature with a resistance at the same time that resistance is inserted in series with the armature, as shown diagramatically in Fig. 228. This diagram is ar- FiG. 227. Automatic Elevator Controller. ranged for a complete magnetic controller having several speeds in each direction of operation. A view of the con- troller is given in Fig. 227. The method of operating the car at first consisted of a mechanical device, either a rope, a wheel or a lever, con- nected through the proper gearing to the controller. De- 3i6 CONTROLLERS FOR ELECTRIC MOTORS |^ji(og^=^i 47 ^ 1 iiS: Sit sl" ■ "HS'^S^ |i 1 gq 3 J^^r — riTI ' I^J '■' o = ■ pSf ' II " -t-§^fe — :-^|gg : 1 ,|S 4 '— ff-^°- ■ •S'-Q-d-^^-SaiM ■ - ELEVATORS 3 I 7 vices of this kind are still In use on freight and slow-speed passenger elevators. The modern elevator, particularly for high speeds, is provided with a master switch in the car, so arranged that it is returned to the central or off posi- FiG. 22g. Electric Elevator Showing the Automatic Switch for Push Button Control. This switch is arranged for calling or sending the elevator to some specific floor by the pushing of the button at the landing. tion if the operator releases the handle. In addition, a safety switch is usually provided to stop the car in case of accident to the master switch. In private residences, hospitals and other places where the elevator is not used enough to warrant the expense of a regular operator, the push button system of control is used 3l8 CONTROLLERS FOR ELECTRIC MOTORS (Fig. 229). This is an elaborated form of master switch. Each landing is provided with a push button to bring the car to that landing, and a set of push buttons is located in the car for dispatching it to a predetermined landing. These push buttons operate a selecting circuit which is opened by mechanical means when the car reaches that particular Fig. 230. Door Lock and Contact. This shows a device used for locking the hatchway door when not in use. A cam on the car automatically operates this lock so that the door can be opened when the elevator car is opposite the landing. When the door is opened the contactor is also opened, which prevents the operation of the elevator while the door is in the open position. landing. A cam on the car unlocks the door when the landing is reahced (Fig. 230), and the opening of the door breaks the control circuit so the car cannot be started until ELEVATORS 319 the door is closed again. A safety button is provided in the car for stopping the car at the will of the passenger. Many of these devices are in successful operation. They first began to meet with favor about twenty years ago. As far back as 1887, devices of this kind were applied to hy- draulic elevators, the valves of which were controlled by electro- magnets. ALTEKNATING-CTTR- BENT ELEVATORS About 1897, the alternating -current motor commenced to be a factor in the op- eration of elevators. These motors are of two types — squirrel- cage and wound sec- ondary. Thesquirrel- cage motor is started and controlled mere- ly by closing the pri- mary switch, no ex- ternal resistance be- ing used. In small sizes, this motor pre- sented no difficulties, but in sizes over five much current at starti Fig. 231. Full AFagnetic Controller for a Two-Speed Alternating-Current Motor. Four two-pole contactors are connected to the primary of the motor. The two contactors at the left, connected by mechanical interlocks, determine the direction of rotation. The re- maining two contactors in the bottom row connect the line wires to either the high-speed or low-speed primary winding. The other contactors are used for controHing the resistors in the secondary circuit of the motor. horse-power the early motors took so ng that they caused serious disturbance 320 CONTROLLERS FOR ELECTRIC MOTORS to the power supply. At that time, very few central stations had sufficient line capacity for starting these large size motors. The wound secondary motors were more common in the larger sizes. They have external resistance in the secondary circuit. This resistance was short-circujted dur- ing acceleration by means of a mechanical switch controlled by a dash-pot. They were fairly successful for car speeds of 150 to 200 feet, but could not be used at higher speeds. They also gave trouble due to the large amount of stored energy in their rotating elements. As the problem of in- duction motors for elevators became better understood bv Fig. 232. Four Tandem Worm-Gear Elevator Machines. Driven by direct-current motors. These machines are located above the hatchway, which is the preferable arrangement for an electrical machine. The tandem gearing is used for heavy loads. The two gear wheels mesh together and are actuated by right and left worms. This arrangement takes up all end thrust upon the worm shaft and is a very substantial construction. motor designers, a better type of induction motor was de- veloped for elevator service. This motor had a long rotat- ing element of small diameter, giving a minimum stored energy and having an electric design well adapted for heavy starting torque. The squirrel-cage motor of this type is designed to develop its maximum torque at starting, and takes a comparatively small current when it is first started. ELEVATORS 3 2 I It is generally used in preference to the wound secondary type up to the 20-horse-power size, although a number of wound secondary motors are used in small sizes where the power supply is limited. Larger motors of the same gen- eral design arranged with slip rings for external resistance at starting are also used. The latest development in alternating-current elevator l^ractice is a two-speed motor, the slow speed being usually one-third of the full speed. Such a motor is adapted for high-speed elevator work as the slow speed of the motor will enable the operator to control the car easily when making landings. It also permits a satisfactorily top and bottom limit stop being used. The car is made to approach either limit of travel on the slow speed, so that the mechan- ical brake can easily stop the car when the limit of travel is reached. The control for this motor (Fig. 231) consists of a reversing switch, a pole-changing switch, and an ac- celerating device for short-circuiting the secondary winding used in connection with the high-speed combination. CHAPTER XXV ELECTRICAL EQUIPMENT FOR OIL WELLS Oil is used extensively today in industrial establishments and has entered into many phases of every day life. Next Fig. 233. A Tkansformek Sub-Station in an Oil Field, With derricks in the background. to coal, It is our principal source of power. Many of our battleships and merchant vessels, and railway locomotives. ELECTRICAL EQUIPMENT FOR OIL WELLS 323 use oil for fuel instead of coal. During 191 7, the produc- tion of crude oil in the United States exceeded 340,000,000 barrels, which was worth nearly one billion dollars. After the oil was refined, it represented a much higher value. This production in the United States amounted to 65 per cent, of the world's total production of crude oil. The distribution of oil wells in the United States may be divided broadly into five fields as follows : /, Eastern or Appalachian Field. — These oils are generally of the paraffin base and are large producers of gasoline and lubricating oil. The wells are gaseous and gas engines are extensively used for pumping. 2. The Mid-Continental Field, including Kansas, Oklahoma and Northern Texas. — This field produces oils having both paraffin and asphaltum bases. In this field, there are about 400 wells operated by electric motors and in the neighborhood of 1,000 new wells are being equipped with electric drive. 3. Louisiana and South Texas. — These oils are principally of the as- phaltum base. The electric drive is just being introduced; and 100 new wells will be equipped in the near future. 4. Colorado and Wyoming Field. — This field produces oil with -- paraffin base. It is a new field and the development has been retarded by lack of material during the war. 5. California Field. — The oil in this field has an asphaltum base and the pumping has been done by burning it under boilers to produce steam. This was very uneconomical and the electric drive has found its widest applica- tion here, there being now something over 2,000 wells equipped with electric motors obtaining power from hydroelectric plants. The introduction of electrical equipment for oil wells has been gradual, and there was much skepticism expressed at first as to the feasibility of this drive. It is dependent upon central station power for its economic operation and it is not probable that any considerable development will take place in fields which are not adjacent to large central station lines. In southern California, due to the far-sighted policy of two of the large power companies, this development has been carried forward rapidly. The power company supplies the high tension service to a centrally located substation where outdoor-type transformers step down the power to the low- voltage distributing system. The electric meters are 324 CONTROLLERS FOR ELECTRIC MOTORS located at this substation and the distribution is taken care of by the owners of the lease. In addition to pumping and pulling the wells, electric power is available for operating Fig. 234. A.v Oil Well Equipment, Showing stands of tubing resting against the side of a derrick. the machine shops, pumping stations, dehydrating plants, and for welding the pipe joints for pipe lines. It also furnishes light where required and can be made of general service throughout the whole lease. The use of welded ELECTRICAL EQUIPMENT FOR OIL WELLS 325 joints on pipe lines is a comparatively recent development and is rapidly superseding the screw joints, where electric power is available for welding purposes. Welded joints remain tight and materially reduce the loss from leakage. The pipe sections can readily be cut apart with the electric arc, where renewals or changes are required. When an electric motor is applied to a well, it must per- form two functions ; namely, pumping and pulling. The pumping service is used about 98 per cent, of the time. With the pulling service is included the lifting and lower- ing of tubing and tools, the cleaning of the well, which is known as "swabbing," "agitating," baling sand and other general work. PUMPING This application does not differ in any essentials from an ordinary pumping installation, as far as the electrical equipment is concerned. For the shallower wells, a squirrel-cage induction motor is commonly used, operating on what is known in the trade as " steel pumping power." This rig may also be used to pump water from gas wells. The motor is started by connecting it directly to the line, as only a small motor is required. For pumping service, the primary windings of the motor are connected in star and are protected by fuses. Where the oil wells are shallow, another form of pump- ing power is sometimes used, known as a " band wheel power," which is connected to a number of wells, usu- ally from fifteen to twenty. These wells are grouped together and pumped by means of pull rods running from the well to the "power." By balancing one well against another, the pull can be distributed throughout the rev- olution of the band wheel and a remarkably small amount of torque is needed for operating the pumps. The band wheel is driven by a wound-secondary induction motor, 326 CONTROLLERS FOR ELECTRIC MOTORS SO that the speed may be adjusted. When properly installed and connected up the op- eration is very smooth and makes an ideal ap- plication for an electric motor. Deep wells are pumped by individual motors of the wound- secondary type. These motors are started by inserting resistance in the secondary circuit. The primary is pro- vided with low voltage protection and is con- nected to the line through a time element overload relay. PULLING SERVICE Under pulling serv- ice are included nu- merous miscellaneous operations. The prin- cipal function, how- ever, is to pull the tubing out of the well and replace it. It is necessary to pull the tubing in order to clean out the well, when it becomes clogged with sand, or a new working barrel installed. During this time the well is not producing oil and there is a direct loss of produc- tion. Furthermore, since oil is of a migratory nature, Fig. 235. Combined Line SwrrrH and Oil Circuit Bre.aker with Maximuji Torque Relay. ELECTRICAL EQUIPMENT FOR OIL WELLS 327 the operators feel that they are actually losing the oil which they do not pump. It is therefore desirable to reduce this inactive period as much as possible. The time required for pulling the tubing and clean- ing is proportional to the depth of the well. For shal- low wells, which are pumped by squirrel-cage motors, the pulling is ef- fected by erecting an A- shaped derrick above the well and pulling the pipe out in short sections. The pumping motor can be used and the extra torque for pulling purposes ob- tained by connecting the primary windings in delta. The motor is coupled to a hoisting drum by means of a clutch. Pulling serv- ice requires a reversal of the motor, which is ob- tained by the use of a small drum reverse switch. The motor is connected to the line by moving the drum switch to either the forward or reverse di- rections. Deep wells, having a standard rig, use wound secondary motors, with the primary windings connected in star for pumping and in delta to get the extra torque for pulling. Usually the pulling torque is from three to Fig. 236. Controller for Motor Pkijiary and Secondary. This controller is made up of cam contactors operated by a sheave wheel. The contactors are mounted in a self- contained case similar to a drum con- troller. 328 CONTROLLERS FOR ELECTRIC MOTORS Fig. 237. Two-Speed Wound-Secondary Induction Motor. four times the torque required for normal pumping. A double- throw switch is mounted on the frame of the motor which changes the connections from star to delta, the star side being marked "pumping" and the delta side "pulling." Where a two speed motor is used, the slow speed is for pumping and the high speed for pulling. The pump may- operate for short intervals of time from the high speed con- nections for "shaking the well" or agitating the sand at the bottom of the well. Often this agitation will make it unnecessary to pull the tubing. It takes only a few minutes and may effect a considerable reduction in the idle period of the well. The control of the squirrel-cage motor presents no par- FiG. 238. Maximum Torque Switch. ELECTRICAL EQUIPMENT FOR OIL WELLS 329 ticular features. The windings of the motor are connected to a knife switch mounted on the motor frame, or other convenient location, for giving either tlie star or delta con- nection. The motor is started and stopped or reversed by a small drum switch which connects the primary windings Secondary Resistor Seconilaiy Resistor Jiniymiyinn. SEQUENCE OF CONTACTORS Forward Re e se | Pumping To Threc-Phase Line Pulling Fig. 239. Diagram of Connections for a Wound-Secondary Induction Motor, With primary connections in star for pumping and in delta for pulling. directly to the line. The overload protection usually con- sists of fuses, the fuses being eliminated during the pulling period. The control for the wound secondary motor has several novel features. It consists of a combination line switch and circuit breaker, a controller with resistors to vary the speed of the motor, and a maximum torque switch. The line switch consists of a three-pole double-throw contact member mounted in a case with a two-coil overload relay, a low- voltage magnet and a maximum torque relay. The handle 330 CONTROLLERS FOR ELECTRIC MOTORS Can be locked in the central or off position to prevent acci- dental starting of the motor in case work is being done on the machinery. The two running positions of the handle are marked "pumping" and "pulling." When the handle is thrown into the pumping position, it is held in this posi- tion by the low-voltage coil and the overload relays are Figs. 240 and 241. Ax Installation of a Two-Speed, Wound-Secondary Induction Motor Used for Pumping and Pulling. connected in series with two legs of the motor circuit. In case of overload or low voltage, the handle is returned to the central position and the circuit opened at the contacts. When the handle is thrown to the "pulling" position, the primary of the motor is connected directly to the line with the maximum torque relay in series with one of the motor leads. The function of this relay will be described later. The secondary of the motor is connected to the controller, ELECTRICAL EQUIPMENT FOR OIL WELLS 331 which is used for short-circuiting the secondary resistor. Only a part of this resistor is connected to the drum con- troller, one section in each phase being connected to a mag- netic contactor, known as the "maximum torque switch." The coil of this switch is energized by the second point of the controller and remains closed during normal operation. Secondary Re^stor SEQUENCE OF CONTACTORS » Forward "H R„e™> 1 _ ^W^ 1 1 uj r rr — SS ibWoi^ ;J! - m -1--- E n 1* ^1 No Vollage Relay CoJ Pulling PumpiDg Ti> Thrrc-Fhaae Line Fig. 242. Diagram of Connections for Wound-Secondary Two-Speed Induction Motor. The slow speed is for pumping and the high speed for pulling. For pumping, the operation of the motor follows normal practice. The primary of the motor is connected to the line through the two overload relay coils and is provided with low voltage protection. The secondary resistance is used for starting and for regulation of motor speed when required. For pulling service, the double-throw motor switch con- nects to the side marked "pulling." This connects the ELECTRICAL EQUIPMENT FOR OIL WELLS 333 windings or the motor to give the high horse-power rating, either by changing them from the star connection to delta connection where a single-speed motor is used, or by con- necting the windings to give half the number of poles where the two-speed type of motor is used. At the same time, the line switch is closed in the pulling position. With this switch in the pulling position, a maximum torque relay replaces the overload circuit breaker. This change is necessary to guard against an accident during the pulling process. If the motor becomes overloaded while pulling the tubing, instead of opening the circuit breaker as in ordinary service, the maximum torque relay will lift, operating the maximum torque switch which inserts the proper resistance in the motor secondary to give the maxi- mum torque. The load applied may be sufficient to stall the motor when pulling, but the motor remains connected to the line exerting its maximum torque so there is no danger of dropping the tubing. If the motor primary were dis- connected from the line on overload, the tubing might drop down into the well at a high velocity. If the brake is ap- plied and is successful in stopping the hoisting drum, the strain set up may be severe enough to strip the tubing apart at one of the couplings and drop a section to the bottom of the well. This results in a long and tedious "fishing" process during which time the well is non-productive. The star delta arrangement of primary winding is shown in Fig. 239. The combined line switch and circuit breaker is connected in the same way, but only one set of slip rings is required for the motor secondary. DRILLIK"G SERVICE Where electric power is available, new wells are drilled with an electrical equipment. The drilling rig is substan- tially the same as that used with an engine, except that a motor is substituted for the engine. Drilling is effected by attaching the string of tools to the end of a rope or cable. 334 CONTROLLERS FOR ELECTRIC MOTORS The lower end of this string of tools is provided with a cutting edge and the drilling is done by alternately rais- ing and dropping this string of tools in such a manner as to impinge upon the rock surface. The weight of this -1200 "^ ^ ■d _ 01 lut- i^Pgv ;> ^ k° 4^ \ "", =?^ k ^ ■3800 c 2 NA 200a- \ / -^00 3 "^ ^ \ // 150" 'N / / / \^ < > L ^400 1 N /. / \ lOO— \ I'.o / y \ 200 C^ / 1 ^ t^ fif- / ^ liu- ,/ / ' 1 30 Torq 2 Poun 4 Is at One 4 'OOt 1 adiu. SI Fig. 244. Speed-Torque Curve of a Two-Pole, Wound-Secondary Induc- tion Motor, Having a Rating of 15 and 30 Horse-Power. These curves show very clearly the operation of the maximum torque relay. From the speed torque curve for 30 hp. marked A, is obtained the performance of the motor with a short-circuited secondary. This shows that the maximum torque is obtained at 850 r.p.m. If the motor is loaded be- yond this point, the torque will decrease with the speed so that at zero speed, the torque has changed from 500 to 300 foot pounds. At this point, the current shown by the curve has increased from 200 to nearly 300 amperes, the full load current being in the neighborhood of 50 amperes. If the torque relay is set to operate at 200 amperes, when the current reaches this value the relay will open the maximum torque switch, inserting sufficient resistance to give the motor its maximum torque of 500 foot pounds at zero speed (curve 5). When the relay operates, the current will drop from 200 to less than 100 amperes. As the motor decreases in speed, the current will gradu- ally increase to the 200-ampere value. This not only reduces the demand on the line, but insures maximum torque being maintained during these abnormal conditions. In practice, the maximum torque relay would be set to operate at a lower value than above, depending upon circumstances. ELECTRICAL EQUIPMENT FOR OIL WELLS 335 string of tools varies from 500 to 2,000 pounds, depending upon the size of the rig. The force, with which it strikes the rock depends upon its stored energy at the time the blow is delivered. This energy is proportional to the weight of the tools, multiplied by the square of the velocity, or speed. The speed is imparted to the tools by allowing them to fall through a short distance. The rope is attached to a crank motion on the drilling rig, which is rotated by the motor. The speed of this rotation has an important bearing upon the strength of blow delivered by the tools. The alternate up and down motion of the string of tools and cable has a definite time period. It is therefore necessary to adjust the speed of the motor so that the alternate pull and release of the cable synchronizes with this natural time element. An adjustment of this kind increases the upward movement of the string of tools and therefore stores up the maximum of energy with which to strike the blow. When the tool strikes, it rebounds and this adjustment continues the up- ward motion from the rebound without a pause. The driller can tell by the feel of his string of tools when he has adjusted for the proper speed. The correct speed changes with the depth of hole so that the speed adjust- ment given by the controller must be in small increments. This is obtained by furnishing an auxiliary controller which changes a short section of the secondary resistor in small steps. The total range of this small conductor need not exceed the range of one step on the large controller. The driller can adjust his main controller for approximately the correct speed and then make the fine adjustments on the auxiliary controller. The diagram of connection is shown in Fig. 243 and in every other respect is similar to the pumping and pulling controller shown in Fig. 239, the connections for the drilling side corresponding to the pump- ing side of the other equipment. The motor used for a drilling rig is usually larger than for pumping, on account of the extra powSr required for 336 CONTROLLERS FOR ELECTRIC MOTORS handling the casing. In deep wells, the driller starts in with a large size casing and decreases the size as the depth of the hole increases. Sometimes the hole is not entirely- straight, due to a boulder or other hard substance being located at one side of the hole. This. deflects the drilling tool slightly. In putting the casing in past a crooked spot Fig. 245. Steel Pumping Power. The electric motor is located in the wooden enclosure at the right. The wells shown are in the bed of the Arkansas River in Oklahoma, the plat- form being raised above high water level. Pumping is continued during high water, the control being located on the river bank. ELECTRICAL EQUIPMENT FOR OIL WELLS 337 of this kind, it is necessary first to drop the casing and raise it again, repeating this operation a number of times until the casing passes down free of the obstruction. To do this with large casings requires motors which will develop from I GO to 150 horse-power for short intervals. Usually a motor connected in star for drilling and in delta for hand- ling the casing is satisfactory. The delta connection would overheat the motor if it were operated in this way for long periods of time, but the pulling operation is short and the motor can be designed for continous operation on the star connection only. The motors for both pumping and drilling should have a high efficiency and power-factor, as these operations are continuous and the performance of the motor materially affects the power consumption. The introduction of elec- tric motors and controllers has increased the production and earnings of oil wells on account of the ease with which the exact speed can be adjusted. The aim of the operator is to get maximum production at all times. There must be a minimum of delays due to breakdowns or repairs. Every shut-down means less production and a loss to the producer. The electric equipment has demonstrated its ability to furnish this service with a minimum of repairs and atten- tion. In many cases the cost of the electrical power is more than made up by reduction in the repairs which were form- erly necessary with the older methods of drive. In addi- tion, the increase in production has been a material factor in the success of the electrical equipment. Electric drive is now being adopted in the oil fields as rapidly as central station power is made available. CHAPTER XXVI LOCOMOTIVES FOR MINES AND GENERAL INDUSTRIAL PURPOSES The electric locomotive is a very important means of conserving the man power in oux mines and industrial establishments. One man with a mule can handle a very small tonnage per day, compared with that which the same man can haul using an electric locomotive. Although the extra cost of the electrical equipment is considerable, it can usually be recovered in a short time where the tonnage is sufficient. The electric locomotive not only reduces the expense of hauling, but increases the efficiency of a mine or industrial establishment by the speed with which coal or finished material can be moved out of the way of workmen and new material brought to them. When coal mining operations extended only a few hun- dred feet, the coal cars were hauled out to the shaft or entry, by mules. As a mine was extended, gasoline engines and afterwards compressed air locomotives were tried out. The most recent development is the use of electric locomo- tives, both for the main haulage and for gathering pur- poses. During the last few years rapid strides have been made in substituting electric locomotives for mules where any considerable tonnage is taken out. This same development has taken place in industHal establishments. Formerly, the material was trucked from one place to another by man power. This is being rapidly superseded by the automobile truck and storage battery locomotives. 3.^8 340 CONTROLLERS FOR ELECTRIC MOTORS Fig. 247. Dkum Controller, Showing separate drum for reversing and series or parallel connections. FoTvrard Reverse 1 2 J 4 s s" 6 tj a TroUey To Headlights Reverse* Grou""* Drum Fig. 248. Diagram of Drum Controller Shown in Fig. 247. LOCOMOTIVES FOR MINES 341 At first, the mining operations were taken care of by one type of locomotive, which was used for hauling the cars through the main tunnel. As the mines became de- veloped, and larger areas were worked, a new class of loco- motive was developed for gathering the cars from the side rooms. With this development, larger and more powerful haulage locomotives were built which increased the size of the control apparatus and in many cases, the manual con- troller has been superseded by remote control. Fig. 249. Traction Reel Gathering Locomotive. Electric locomotives for mine and industrial service may be divided as follows : 1. Trolley tyjje locomotives for main haulage. 2. Trolley type locomotives for gathering service. 3. Storage battery locomotives. 4. Combined storage battery and trolley locomotives. The trolley locomotive is particularly well adapted for high speed, long hauls and heavy grades. It may employ a single trolley wire or third rail and use a bonded track 342 CONTROLLERS FOR ELECTRIC MOTORS for the return circuit. The single trolley is the usual prac- tice in mines. In some industrial establishments, a double trolley or double third rail is used, in which case the track is not Used to form part of the elec- tric circuit and need not be bounded. The trolley locomo- tive for gathering serv- ice is employed only in mining operations. In addition to the usual locomotive equipment it may have a traction reel or a cable reel. The traction reel gath- ering locomotive is equipped with a motor driven reel on which is wound a steel cable. The locomotive remains on the main haulage track and the cable extends into the room or entry and is con- nected to the cars. The motor operating the reel draws the cars out to the main track so that the locomoti\-e can couple to them. The reel motor is controlled by a simple rheostatic reversing drum controller. \\ni.ere the gathering reel is of the conductor cable type. Fig. 250, it may be mechanically dri\'en from the axle or equipped with an individual motor. The conductor cable enables the locomoti\'e to run on a room track for hauling Fig. 250. Motor Operated Conductor Cable Reel. LOCOMOTIVES FOR MINES 343 the cars to and from the room working face. Where the electric cable is single conductor, the room track must be bonded. In many cases double conductor cable is used to avoid bonding these tracks. Where the reel is mo- tor driven, an electric motor is built inside of the reel Fig. 251. Co.xDi'CTOR Cable Reel LoroMumE. drum and is self contained with the reel. The motor is connected across the power lines with a resistance in se- ries. As the cable is wound off the reel, the motor is driven against its torque and keeps the cable taut. As the locomotive returns, the motor winds the cable on the reel. The action of the motor is thus equivalent to a spring. The torque, however, remains constant and does 344 CONTROLLERS FOR ELECTRIC MOTORS not vary as in the case of a mechanical spring. The cable passes through a guide driven by the reel which moves back and forth and causes it to wind uniformly across the face of the reel. Fig. 252 shows a diagram of electrical connections for the motor driven reel and its control circuit. Hooh Motvr Driven Peel Circuit Brsiker *" or Fuse *-7o Controller Fig. 252. Diagram of Motor Operated Conductor Cable Reel. The controllers used on all but the very large locomotives are manually operated, usually the drum type. Each con- troller is provided with two handles — the operating handle controls the speed of the locomotive; the other handle makes the proper connections for forward or reverse opera- tion, and where two or more motors are used, it provides for either series or parallel connections. The handles are interlocked so that the direction of rotation and series or parallel connections cannot be changed unless the operating handle is in the off position. Controllers for mine locomo- tives must be made short in the vertical direction and as flat as possible, as the overall dimensions of the locomotive are very limited in mining work. Controllers for industrial locomotives need not meet these requirements. LOCOMOTIVES FOR MINES 345 Each locomotive usually has two or more motors which can be connected in series or in parallel. Fig 248 shows the diagram of connections for a drum controller arranged for two motors. The drum connected to the operating handle is shown at the left of the main diagram and at the top of the schematic diagram. It connects the motors to the trolley circuit through a resistance which is short-cir- cuited in five steps. To the right is the forward, reverse s eoOe n ce of contacto rs Sg gRjR^^Ej §i§ Blowopt Coil No. 2^ , Motor Cut out Fig. 253. Diagram of Contactor Control. and series parallel drum. - These connections can be changed only when the drum shown on the left is in the off position, the interlock consisting of a mechanical connection between the handles on the inside of the controller. Larger locomotives are equipped with air brakes and con- tactor control. The operating handle provides for both series and parallel connections. A separate handle sets the circuit for either forward or reverse operation. Fig. 253 illustrates a typical controller for this type arranged for two motors. These locomotives are sometimes quite large and are often used for handling several freight care at one time. A pusher locomotive differs from the regular locomotive by the addition of a bar which projects from the side of the locomotive and enables it to move the cars on an ad- jacent track, as illustrated in Fig. 254. 346 CONTROLLERS FOR ELECTRIC MOTORS It is often difficult for a locomotive operator to "spot" cars properly where the train is of any considerable length. It is possible to improve these conditions, by erecting a tower adjacent to the track and arranging the control so that the locomotive operator can leave the cab and operate his locomotive from the tower. Contactor control with Fig. 254. PusHEK Type Locomotive with Third Rail. On the Cleveland Ore Dock of the Pennsylvania Lines. This loco- motive runs on tracks parallel to those of the cars, and pushes the cars with a pneumatically operated arm. master switches are used and a control wire is run from the locomoti\'e to the tower by means of an additional trolley or third rail. Where a considerable tracti\-e effort is required and the size of rail or operating conditions will not permit the use of a single locomotive, the locomotive is divided into two parts and is known as a tandem locomoti\-e, Fig. 246. One of the units is a leading or primary unit and the other a LOCOMOTIVES FOR MINES 347 secondary unit. The primary unit is equipped with a four motor controller to control two motors on each unit. The electrical connections between the two units are made with jumpers, each locomotive being equipped with its own resistor. The two units are operated electrically in parallel, Primary Locomotive Secondary Locomotive Srid ( , ,, J Tl / Grid eaistor | W^ JX^A-l - Resistor j r JlMotoisJik ^^"^'^^^^^ SjiiMotors^ [jj I .._J Fig. 255. Arrangement of Bus Line Receptacles for Tandem Locomotive. the motors on each unit being connected in series or in. parallel in the customary way, and the resistors on the two units being paralleled step to step by the controller contacts on both the series and parallel connections of the motors. Fig. 256. Storage Battery Locomotives in an Industrial Plant. When operated separately the connections on the primary unit which are provided for the motors and resistor of the second unit remain idle. Where the locomotive is equipped with air brakes and remote control, arrangement can be made for connecting 348 CONTROLLERS FOR ELECTRIC MOTORS ^f.'^ "^Cst^^' ,-i|i|-©_a5a-^wvjr 2-iiii-(T)-iaww\jgi (D_aB7vvu!m_ 3-iii[-(jMoavwviioii fgy-flja-Avyga- Hllh- -@m^^ — lll^■• -(D-fiKLvVIA-flll 1 1| ^.. _(g>-ga-Aw oao -flu- several of these units together and operating as a single locomotive. A good illustration of this is three 2 5 -ton locomotives recently installed by a large copper company. Each of these locomotives can be operated as a single unit or the three units can be connected together, giv- ing the equivalent of a 75- ton locomotive with a dis- tributed weight, so that no heavier track is required than for the single 25 -ton unit. Storage battery locomo- tives. Figs. 256 and 257, are usually designed for slow speeds of about 3.5 miles an hour. They are particularly adapted for short hauls and intermittent service. Heavy grades and high speeds cause a very high discharge rate on the battery, which makes the size of the battery out of proportion to the service rendered. These locomo- tives are operated in shifts to allow time for changing the battery. Sometimes ex- tra batteries are provided to keep the locomotives in continuous service, the idle battery being charged while the other battery is in service on the locomotive. Stor- age battery locomotives are particularly well adapted for ,:b-i|i| T(j>JmvwLfflia^ Hllh -(^yUBl /Jfm— <\; ^ ", " "^ «, ^ 1 2 6 3a 3b e. c c. 4n c 6 i 4b Q c n c q r> 5 c 5b c q J>, L- p c c c C Fig. 257. Diagram of a Two-Motor Storage Battery Control. LOCOMOTIVES FOR MINES 349 industrial purposes where a trolley wire or third rail would be objectionable. The loading and unloading of cars consumes time and a great deal of the work consists in shifting cars from one place to another, and in most in- dustrial plants few if any grades are encountered. These locomotives are also well adapted for gathering service in mines. The volts per cell for commercial storage batteries is quite low so that motors are wound for considerably less voltage than is common for power service. This increases Fig. 258. Combined Trolley and Storage Battery Locomotive. With cover removed showing location of the motors and storage battery. the current per horse-power considerably and makes it necessary to use larger contacts for a given horse-power. For a single motor, the controller consists of an ordinary drum with rheostatic control. The forward and reverse connections are made on a separate drum, the change in connections being made when the operating handle is in the off position. Where two or more motors are used, the motor arma- tures and fields are combined in series and parallel rela- 3 so CONTROLLERS FOR ELECTRIC MOTORS tions which gives a wide range of speed with very little rheo- static loss and conserves the energy in the storage battery. Fig. 257 is a schematic diagram showing such an arrange- ment. The numerals / and 2 represent the two motors. Each motor is provided with a double field winding. The motors are first connected with their armatures and all of Forward Reverse Fig. 259. Diagram of Controller for Combined Trolley and Storage Battery Locomotive. their field windings in series. This gives a maxirnum field strength and maximum torque with a relatively small cur- rent, y^fter the starting resistance has been short-cir- cuited, the field windings are connected in parallel, giving a higher speed by field control. Higher speeds are ob- tained by changing the armatures from series to parallel connections. Six operating speeds are shown on this dia- gram. The steps marked with sub letters, such as ja^ ^a, etc., are transition steps and not intended for running positions. Sometimes, the locomotive is designed for operation both from a storage battery and a trolley, the battery being used for short hauls where it is not convenient to provide the LOCOMOTIVES FOR MINES 35' trolley wire, such as for gathering purposes in a mine. Fig. 258 shows such a locomotive and Fig. 259 a diagram of connections. The operating handle provides ordinary rheostatic control. The separate handle which can be moved only when the operating drum is in the off position provides for connections for either forward or reverse operation on either the trolley or battery. A locomotive of this kind has considerable flexibility, but the added weight of the battery is a disadvantage in operating on the trolley wire. Some quite large industrial locomotives of this kind are employed by a large plant where it is necessary to haul cars across property where a trolley cannot be installed. In such cases, the battery can be charged, if desired, while the locomotive is operating from the trolley. INDEX. A Acceleration, Counter E.M.F. .. 65 Acceleration, Methods 65 Acceleration, Series Lockout Switch 71 Acceleration, Series Relay .... 6g Acceleration, Time Element.. 7^-74 Accumulator, Hydraulic 220 Adjustable Speed, A,C. Motors, 98, 231 Adjustable Speed, D.C. Motors, 93. 97. 199, 230 Alternating Current Elevators. 319 Apron Hoist 273 Arcing 44 Automatic Mine Hoist 171, 182 Autotransformer Starters ..118-124 B Barney Haul 267 Brake 182 Brake, Dynamic .... 85, 100, 106, 204-206, 270 Bridge Controller for Cranes, 256, 288 Bridging Transition 153 C Calculating Starting Current. 81-84 Calender Control 226 Cam Controller .... 261, 262, 327 Cam Limit Switch 250 Car Dumpers 266 Cascade Connections for A.C. Motors g8 Centrifugal Pumps 184, igi Characteristics of A.C. Motors, 95, 99. 334 Characteristics of D.C. Motors, 90, 92 Circuit Breakers 136 Coal Bridges 275 Coke 290 Commutator Controllers 24 Constant Speed Control for Ma- chine Tools 198 Contactor, A.C 40, 116 Contactor, D.C 40 Contactor Control 25-28 Contacts 41-44 Counterbalance of Elevators... 307 Counter E.M.F. Acceleration... 65 Cradle Hoist 269 Crane Protective Panel 258 Cranes 254 Current Limit Acceleration. 31, 69 D Dash Pot Type Overhead Relay. 138 Direct Current Elevators 312 Door Lock and Contact 318 Door Machine 294 Double Voltage Control . . 228, 234 Drilling Oil Wells 333 Drum Controller .... 33, 34, 54-60 Drum Reverse Switch 207 Dynamic Brake .... 85, 100, 106, 204-206, 270 E Electropneumatic System of Control 155 Elevator, Alternating Current. 319 352 INDEX. 353 Elevator, Direct Current 312 Elevator, Complete Equipment. 302 Elevator Control 13, 17, 301 Elevator Pumps ..^ 190 Equalizer Flywheel Hoist Con- trol 161 F Face Plate Controllers 25, 4Q Field Rheostat 107, 206 Float Master Switch 109, 189 Freight Elevator 303 G Gathering Locomotives for Mines 344 Grid Resistors 125 Grid Resistor, Energy Dissi- pated 80 Grindstone Controllers 24 H Heavy Intermittent Duty Re- sistor 131 Heavy Starting Duty Resistor. 131 Hoist, Apron 273 Hoist, Automatic for Mines... 182 Hoist Controller for Cranes, 2.';6, 257 Hoist Controller for Bridges.. 277 Hoisting Calculations 165 Hydraulic Accumulator Control. 220 I Irrigation Pumping 196 K "K" Type Drum Controllers.. 151 Leveler Bar Control 294 Light Intermittent Duty Re- sistor 131, :33 Light Starting Duty Resistor.. 131 Limit Switches .. 180, 250, 257, 310 Liquid Controller .... 116, 173-178 Load Curve of lOO-H.P. Motor. 252 Locomotives 338 Locomotives, Types 341 Locomotives, Gathering . . 342-344 Locomotives Storage Battery, 248-351 Low Voltage Protection. . 109, 140 Low Voltage Release .... 109, 140 M Machine Tool Application Table 201 Machine Tool Controllers .... 198 Magnetic Blowout I4i 45 Magnetic Contactor Control . 25-28, 60-64, 76, 102, 168 Master Switch 107, 207, 251 Maximum Torque Switch. 328, 333 Methods of Series-parallel Con- trol 150 Mine Hoists 167 Municipal Pumping Plants . . . 194 N Non-reversing Controllers. 103, 203 O Oil Wells 322 Oil Well Drilling 333 Oil Well Fields 323 Open Circuit Transition 150 Operating Coil 46 Operation of Elevators 311 Ore Bridges 275 Overbalance of Elevators 307 Overload Protection .... 110-113, 135-140 354 INDEX. P Pan Hoist 273 Phase Failure Protection 143 Phase Reversal Protection . . . 141 Planer Control 88, 217 Plugging Control 242-245 Pressure, Regulator or Master Switch 109, 187 Printing Press Control 226 Protective Devices 135 Pulling Service for Oil Wells.. 326 Pumping Oil Wells 325 Pumps, Centrifugal 184, igi Pumps, Elevator 190 Pumps, Positive Acting 184 Pusher Locomotive 345 Pusher Ram Control 294 Q Quenching Coke 297 R Railway Control 14 Rating 46 Regulating Speed of D.C. Motors 91 Regulation 47 Resistors 125 Resistor, Calculation 132 Reversing Mill, Too High .... 247 Resistor Temperature 128 Reversing Controllers . . . 104, 205, 242-245 Rheostat 50, 51, 126 Rubber Calender Control 226 S Safety Devices 180 Series Lockout Switch 71. 72 Series-parallel Control 147 Series Relay Method of Accel- eration 69-71 Shunt Field Failure 144 Shunt Transition 151 Slip Regulator 160 Speed Control 89 Squirrel Cage Induction Motor Starters 119-124 Starting Characteristics of Motors 75 Starting Tests 77 Steel Mill Controllers 240 Steel Pumping Power 336 Storage Battery Locomotives, 348-351 Systems of Control 29, 186 T Temperature of Resistors .... 128 Tests on A.C. Motors 86 Tilting Roll Table 248 Time Element Acceleration . . 72-74 Trolley Controller 137 Tubular Resistors 126 Too High Reversing Mill .... 247 U Unit Switch Controllers, see Magnetic Contactor Control. V Varying Speed A.C. Motors . . 94 Varying Speed D.C. Motors . . 199 Voltage Control, Double Volt- age 228, 234 Voltage Control of A.C. Motors 97, 118 Voltage Control System 94, 158, 178 W Warf, Coke 299 Wheel Lathe Control 216 Wound Secondary A.C. Motor Control .. 114, 169, 192, 282, 329 ^ D. VAN NOSTRAND COMPANY 25 PARK PLACE NEW YORK SHORT-TITLE CATALOG OF Publications i Tinportations OF SCIENTIFIC AND ENGINEERING BOOKS This list includes the technical publications of the following English publishers: SCOTT, GREENWOOD & CO. JAMES MONRO & CO., Ltd. CONSTABLE & COMPANY, Ltd. TECHNICAL PUBLISHING CO. BENN BROTHERS, Ltd. for whom D. Van Nostrand Company are American agent« Descriptive Ciroulars sent on request. September, 1919 SHORT=TITLE CATALOG OP THE Publications and Importations OF D. VAN N05TRAND COMPANY 25 PARK PLACE Prices marked with an asterisk (*) are NET All bindings are in cloth unless otherwise noted Abbott, A. V. The Electrical Transmission of Energy 8vo, *Ss oo A Treatise on Fuel r6mo, 075 Testing Machines i6mo, o 75 Abraham, H. Asphalt and Allied Substances 8vo, 500 Adam, P. Practical Bookbinding i2mo, *2 50 Adams, H. Theory and Practice in Designing 8vo, *2 So Adams, H. C. Sewage of Seacoast Towns 8vo, "2 50 Adams, J. W. Sewers and Drains for Populous Districts... .8vo, 2 50 Addyman, F. T. Practical X-Ray Work Svo, 5 00 AdJer, A. A. Theory of Engineering Drawing Svo, *2 50 Principles of Parallel Projecting-Line Drawing Svo, 'i 00 .4i.: nan, C. M. Manures and the Principles of Manuring. .. Svo, 2 50 Mtken, W. Manual of the Telephone Svo, *8 00 D. VAN NOSTRAND COMPANY'S SHORT-TITLE CATALOG 3 d'Albe, E. E. F. Contemporary Chemistry i2mo, *i 25 Alexander, J. Colloid Chemistry lamo, i oe Alexander, J. H. Elementary Electrical Engineering. .. .izmo, 2 50 Allan, W. Strength of Beams under Transverse Loads. i6mo, o 75 Allan, W. Theory of Arches i6mo, Allen, H. Modern PoTsrer Gas Producer Practice and Applica- tions i2mo, Anderson, J. W. Prospector's Handbook i2mo, Andes, L. Vegetable Fats and Oils 8vo, Animal Fats and Oils 8vo, Drying Oils, Boiled Oil, and Solid and Liquid Driers. .8vo, Iron Corrosion, Anti-fouling and Anti-corrosive Paints. 8vo, Oil Colors and Printers' Ink 8vo, Andrews, E. S. Reinforced Concrete Construction i2mo, Treatment of Paper for Special Purposes i2mo, Theory and Design of Structures 8vo, Further Problems in the Theory and Design of Struc- tures 8vo, The Strength of Materials 8vo, Elastic Stresses in Structures 8vo, Andrews, E. S., and Heyevood, H. B. The Calculus for Engineers i2mo, 2 00 Annual Reports on the Progress of Chemistry. Fifteen Vol- umes now ready. Vol. I, 1904, to Vol. XV, igig, 8vo, each *2 00 Argand, M. Imaginary Quantities i6mo, o 75 Armstrong, R., and Idell, F. E. Chimneys for Furnaces and Steam Boilers i6mo, o 75 Arnold, E. Armature Windings of Direct Current Dynamos. 8vo, *2 00 Asch, W., and Asch, D. The Silicates in Chemistry and Commerce 8vo, *d 00 Ashe, S. W., and Keiley, J. D. Electric Railways. Theoreti- cally and Practically Treated. Vol. I. Rolling Stock r2mo, *2 50 '2 50 I 75 *6 00 *, 00 *6 00 *e 00 *3 00 "'1 00 *3 00 3 50 2 50 *4 00 9 00 4 D. VAN NOSTRAND COMPANY'S SHORT-TXTLB, VATALOG Ashe, S. W. Electric Railways. Vol. II. Engine*-' ? Pre- liminarie and Direct Current Sub-Stations . - i2mo, "250 Electricity: Experimentally and Practical. ()plied. i2nio, *2 00 Ashley, R. H. Chemical Calculations lamo, *2 00 Atkins, W. Common Battery Telephony Simplified. ...i2mo, *i 25 Atkinson, A. A. Electrical and Magnetic Calculations. .8vo, *i 50 Atkinson, J. J. Friction of Air in Mines i6mo, o 75 Atkinson, T. J., and Williams, E. H., Jr. Gases Met with in Coal Mines i6mo, o 75 Atkinson, P. The Elements of Electric Lighting i2mo, i 50 The Elements of Dynamic Electricity and Magnetism. 1 2ni o, 2 00 Atkinson, P. Power Transmitted by Electricity i2mo, 200 Auchincloss, W. S. Link and Valve Motions Simplified. . . .8vo, *r 50 Atdley, J. A. Silica and the Silicates 8vo (hi Press.) Austin, E. Single Phase Electric Railways 4to, *5 00 Austin and Cohn. Pocketbook of Radiotelegraphy (In Press.) Ayrton, H. The Electric Arc 8vo, *5 50 Bacon, F. W. Treatise on the Richards Steam-Engine Indica- tor i2mo, I 00 Baff, W. E. Sale of Inventions i2mo (In Press.) Bailey, R. D. The Brewers' Analyst 8vo, *s 00 Baker, A. L. Quaternions 8vo, *i 25 Thick-Lens Optics lamo, 'i 50 Baker, Benj. Pressure of Earthwork i6mo. Baker, G. S. Ship Form, Resistance and Screw Propulsion, 8vo, "4 50 Baker, I. 0. Levelling i6mo, o 75 Baker, M. W. Potable Water i6mo, o 75 Sewerage and Sewage Purification.. i6mo, 075 Baker, T. T. Telegraphic Transmission of Photographs. i2mo, *i 25 Bale, G. R. Modern Iron Foundry Practice. Two Volumes. r2mo. Vol. I. Foundry Equipment, Material Used *3 oo Ball, J. W. Concrete Structures in Railways 8vo, *2 50 D. VATi NOSTRAND COMPANY'S SHORT-TITLE CATALOG 5 Ball R. S. Popular Guide to the Heavens 8vo, 5 00 Natural Sources of Power 8vo, "2 50 Ball, W. V. Law Affecting Engineers 8vo, *3 50 Bankson, Lloyd. Slide Valve Diagrams i6mo, 075 Barham, G. B. Development of the Incandescent Electric Lamp 8vo, "2 50 Barker, A. F. Textiles and Their Manufacture 8vo, 2 00 Barker, A. F., and Midgley, E. Analysis of Woven Fabrics, 8vo, 3 50 Barker, A. H. Graphic Methods of Engine Design. .. .lamo, *2 00 — — Heating and Ventilation 4to, *8 00 Barnard, J. H. The Naval Militiaman's Guide. .i6mo, leather, i 00 Barnard, Major J. G. Rotary Motion i6mo, 075 Barnes, J. B. Elements of Military Sketching i6mo, *o 75 Barnett, E. de B. Coal-Tar Dyes and Intermediates .... 8vo, 3 50 Barrowclif!, M.. and Carr, F. H. Organic Medicinal Chemicals, 8vo (/n Press.) Barrus, G. 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Dyeing of Cotton Fabrics 8vo, *5 00 Dyeing of Woolen Fabrics 8vo, *3 50 Beggs, G. E. Stresses in Railway Girders and Bridges. . . .(In Press.) Begtrup, J. The Slide Valve 8vo, *2 00 Bender, C. E. Continuous Bridges i6mo, o 75 Proportions of Pins Used in Bridges i6mo, 075 Bengough, G. D. Brass (/" Press. ) Bennett, H. G. The Manufacture of Leather 8vo, *6 00 Animal Proteids 8vo (In Press.) Bernthsen, A. A Text-book of Organic Chemistry. .. .i2mo, *3) 50 Bersch, J. Manufacture of Mineral and Lake Pigments. .8vo, *6 00 Beveridge, J. Papermaker's Pocket Book izmo, *4 00 Binnie, Sir A. Rainfall Reservoirs and Water Supply. .8vo, *3 00 Binns, C. F. Manual of Practical Potting 8vo, *8 00 The Potter's Craft rzmo, *2 00 Birchmore, W. H. Interpretation of Gas Analysis i2mo, *i 25 Blaine, R. G. The Calculus and Its Applications i2mo, *i 75 Blake, W. H. Brewer's Vade Mecum 8vo, *4 00 Blanchard, W. M. Laboratory Exercises in General Chem- istry i2mo, I 00 Blasdale, W. C. Quantitative Chemical Analysis. .i2mo, *2 50 Bligh, W. G. The Practical Design of Irrigation Works. .8vo, Bloch, L. Science of Illumination 8vo, "2 50 Blok, A. Illumination and Artificial Lighting i2mo, *2 00 Blucher, H. Modern Industrial Chemistry 8vo, *7 50 Blyth, A, W. Foods: Their Composition and Analysis. ..8vo, 7 50 Poisons: Their Effects and Detection 8vo, 8 50 Bockmann, F. Celluloid i2mo, *2 50 Bodmer, G. R. Hydraulic Motors and Turbines i2mo, 5 00 Boileau, J. T. Traverse Tables 8vo, 5 00 Bonney, G. E. The Electro-plater's Handbook i2mo, i 50 Booth, W. H. Water Softening and Treatment 8vo, *2 00 Superheaters and Superheating and their Control. . .8vo, *i 50 Bottcher, A. Cranes: Their Construction, Mechanical Equip- ment and Working , , , 4to, *ro 00 D. VAN NOSTRANb COMPANY^S SHORT-TITLE CATALOG 'j Bottler, M. Modem Bleaching Agents i2mo, *2 50 Bottone, S. R. Magnetos for Automobilists lamo, *i 00 — ^ — ^ Electio-Motors, How Made and How Used i2mo, i 00 Boulton, S. B. Preservation of Timber i6mo, 075 Bourcart, E. Insecticides, Fungicides and Weedkillers. .8vo, *6 00 Bourgougnon, A. Physical Problems i6mo, o 75 Bourry, E. Treatise on Ceramic Industries 8vo (/« Press.) Bowie, A. J., Jr. A Practical Treatise on Hydraulic Mining . 8vo, 5 00 Bowles, 0. Tables of Common Rocks i6mo, 75 Bowser, E. A. Elementary Treatise on Analytic Geometry. 1 2mo, i 75 • -Elementary Treatise on the Differential and Integral Calculus i2mo, 2 25 Bowser, E. A. Elementary Treatise on Analytic Mechanics, i2mo, Elementary Treatise on Hydro-mechanics i2mo, ■ A Treatise on Roofs and Bridges i2mo, Boycott, G. W. M. Compressed Air Work and Diving .... 8vo, Bradford, G. Whys and Wherefores of Navigation. .i2mo, Sea Terms and Phrases lamo, fabrikoid {In Press.) Bragg, E. M. Design of Marine Engines and Auxiliaries. ... Brainard, F. R. The Sextant i6mo, Brassey's Naval Annual for 1919 8vo, Briggs, R., and Wolff, A. R. Steam-Heating i6mo, Bright, C. The Life Story of Sir Charles Tilson Bright. .8vo, Telegraphy, Aeronautics and War 8vo, Brislee, T. J. Introduction to the Study of Fuel 8vo, Broadfoot, S. K. Motors Secondary Batteries i2mo, Broughton, H. H. Electric Cranes and Hoists Brown, G. Healthy Foundations i5mo, Brown, H. Irrigation 8vo, Reprinting) Brown, H. Rubber 8vo, Brown, W. A. Portland Cement Industry 8vo, Brown, Wm. N. The Art of Enamelling on Metal. .. .i2mo, Handbook on Japanning i2mo, House Decorating and Painting i2mo, History of Decorative Art i2mo, Dipping, Burnishing, Lacquering and Bronzing Brass Ware i2mo, Workshop Wrinkles 8vo, 3 00 2 50 "2 *4 25 25 2 00 res *3 s.) 00 10 00 75 *4 SO 6 00 *3 00 *o 75 75 (t1 *2 50 3 *2 00 00 *2 00 "2 00 *0 50 *I *I 50 00 8 D. VAN NOSTRAND COMPANY^S SHOkT-TITLE CATALOG Browne, C. L. Fitting and Erecting of Engines 8vo, - *i 50 Browne, R. E. Water Meters i6nio, o 75 Bruce, E. M. Detection of the Common Food Adulterants, i2mo, I 25 Brunner, R. Manufacture of Lubricants, Shoe Polishes and Leather Dressings 8vo, *3 50 Buel, R. H. Safety Valves i6mo, o 75 Bunkley, J. W. Military and Naval Recognition Book, .izmo, i 00 Burley, G. w/ Lathes, Their Construction and Operation, i2mo, 2 00 • Machine and Fitting Shop Practice 2 vols..i2mo, each, ^ 00 Testing of Machine Tools i2mo, 2 00 Burnside, W. Bridge Foundations i2mo, *2 00 Burstall, F. W. Energy Diagram for Gas. With t«xt. ..8vo, i 50 Diagram sold separately *i 00 Burt, W. A. Key to the Solar Compass i6me, leather, 2 50 Buskett, E. W. Fire Assaying i2mo, *i 25 Butler, H. J. Motor Bodies and Chasis 8vo, *3 00 Byers, H. G., and Knight, H. G. Notes on Qualitative Analysis 8vo, *i 50 Cain, W. Brief Course in the Calculus i2mo, *r 75 Elastic Arches i6mo, o 75 ■ Maximum Stresses i6mo, o 75 ■ -Practical Designing Retaining of Walls i6mo, 075 Theory of Steel-concrete Arches and of Vaulted Struc- tures i6mo, o 75 Theory of Voussoir Arches i6mo, o 75 ■ Symbolic Algebra i6mo, o 75 Calvert, G. T. The Manufacture of Sulphate of Ammonia and Crude Ammonia i2mo, 4 00 Camm, S. Aeroplane Construction i2mo, 3 00 Carey, A. E., and Oliver, F. W. Tidal Lands Cvo, 5 00 Carhart, H. S. Thermo-Electromotive Force in Electric Cells i2mo (In Press.) Carpenter, F. D. Geographical Surveying i6mo. Carpenter, R. C, and Diederichs, H. Internal-Combustion Engines 8vo, *s 00 Carter, H. A. Ramie (Rhea), China Grass r2mo, *3 00 D. VAN NOSTRAND COMPANY'S SHORT-TITLE CATALOG 9 Carter, H. R. Modern Flax, Hemp, and Jute Spinning. .8vo, *3 50 • Bleaching, Dyeing and Finishing of Fahrics 8vo, *i 25 Cary, E. E. Solution of Railroad Problems With the Use of the Slide Rule i6mo, *i 00 easier, M. D. Simplified Reinforced Concrete Mathematics, i2mo, *i 00 Cathcart, W. L. Machine Design. Part I. Fastenings. . .8vo, *3 00 Cathcart, W. L., and Chaffee, J. I. Elements of Graphic Statics Svo, *3 00 Short Course in Graphics i2mo, i 50 Caven, R. M., and Lander, G. D. Systematic Inorganic Chem- istry i2mo, *2 00 Chalkley, A. . P. Diesel Engines Svo, *4 00 Chalmers, T. W. The Production and Treatment of Veg- etable Oils 4to, 7 50 Chamber's Mathematical Tables Svo, 2 00 Chambers, G. F. Astronomy i6mo, *i 50 Chappel, E. Five Figure Mathematical Tables Svo, *2 00 Charnock. Mechanical Technology Svo, *3 50 Charpentier, P. Timber Svo, *6 00 Chatley, H. Principles and Designs of Aeroplanes i6mo, o 75 ■ How to Use Water Power i2mo, "150 — — Gyrostatic Balancing Svo, *i 2s Child, C. D. Electric Arc Svo, *2 00 Christian, M. Disinfection and Disinfectants lamo, 2 50 Christie, W. W. Boiler-waters, Scale, Corrosion, Foaming, Svo, ■'3 00 ■ Chimney Design and Theory Svo, *3 00 Furnace Draft i6mo, o 75 Water Its Purification and Use in Industries .... Svo, *2 00 Church's Laboratory Guide Svo, *r 50 Clapham, J. H. Woolen and Worsted Industries Svo, 2 00 ClappertOB, G. Practical Papermaking Svo, 2 50 Clark, A. G. Motor Car Engineering. Vol. I. Construction *4 00 Vol. II. Design 3 50 Clark, C. H. Marine Gas Engines. New Edition 2 00 Clark, J. M. New System of Laying Out Railway Turnouts, i2mo, I 00 10 D. VAN NOSTRAND COMPANY'S SHORT-TITLE CATALOG Clark, J. W., and Scott, W. Plumbing Practice. Vol. I. Lead Working and Plumbers' Materials. .8vo, *4 oo Vol. II. Sanitary Plumbing and Fittings (/n Press.) Vol. III. Practical Lead Working on Hoofs {In Press.) Clarkson, R. P. Elementary Electrical Engineering {In Press.) Clausen-Thue, W. ABC Universal Commercial Telegraphic Code. Sixth Edition (/» Press.) Clerk, D., and Idell, F. E. Theory of the Gas Engine. i6mo, o 75 Clevenger, S. R. Treatise on the Method of Government Surveying i6mo, mor., 2 50 Clouth, F. Rubber, Gutta-Percha, and Balata 8vo, *6 00 Cochran, J. Treatise on Cement Specifications 8vo, *i 00 Concrete and Reinforced Concrete Specifications. .. .8vo, *2 50 Cocking, W.C. Calculationsi of Steel-Frame Structures. i2mo, 2 50 CofSn, J. H. C. Navigation and Nautical Astronomy. .r2mo, '3 00 Colburn, Z., and Thurston, R. H. Steam Boiler Explosions. i6mo, o 75 Cole, R. S. Treatise on Photographic Optics izmo, i 50 Coles-Finch, W. Water, Its Origin and Use 8vo, *5 00 Collins, C. D. Drafting Room Methods, Standards and Forms 8vo, 200 Collins, J. E. Useful Alloys and Memoranda for Goldsmiths, Jewelers i6mo, o 50 Collins, S. Hoare. Plant Products and Chemical Fertilizers, 8vo, 3 00 CoUis, A. G. High and Low Tension Switch-Gear Design. 8vo, *3 50 ■ Switchgear i2mo, o 50 Colver, E. D. S. High Explosives 8vo, 12 50 Comstock, D. F.. and Troland, L. T. The Nature of Matter and Electricity i2mo, 2 00 Coombs, H. A. Gear Teeth o 75 Cooper, W. R. Primary Batteries 8vo, *6 00 Copperthwaite, W. C. Tunnel Shields 4to, *9 00 Corfield, W. H. Dwelling Houses i6mo, 075 Water and Water-Supply i6mo, o 75 Cornwall, H. B. Manual of Blow-pipe Analysis 8vo, *2 50 Cowee, G. A. Practical Safety Methods and Devices. . .8vo, *3 00 D. VAN NOSTRAND COMPANY'S SHORT-TITLE CATALOG 1 1 Cowell, W. B. Pure Air, Ozone, and Water i2mo, *2 50 Craig, J. W., and Woodward, W. P. Questions andl Answers About Electrical Apparatus i2nio, leather, i 50 Craig, T. Motion of a Solid in a Puel i6mo, o 75 Wave and Vortex Motion i6mo, o 75 Cramp, W. Continuous Current Machine Design 8vo, *2 50 Creedy. F. Single-Phase Commutator Motors 8vo, *2 00 Crehore, A. C. Mystery of Matter and Energy i2mo, i 00 New Theory of the Atom (/» Press.") Crocker, F. B. Electric Lighting. Two Volumes. 8vo. Vol. I. The Generating Plant 300 Vol. II. Distributing Systems and Lamps Crocker, F. B., and Arendt, M. Electric Motors Svo, *2 50 and Wheeler, S. S. The Management of Electrical Machinery i2mo, *i 00 Crosby, E. U., Fiske, H. A., and Forster, H. W. Handbook of Fire Protection i2mo, 4 00 Cross, C. F., Bevan, E. J., and Sindall, R. W. Wood Pulp and Its Uses Svo (Reprinting) Crosskey, L. R. Elementary Perspective Svo, i 50 Crosskey, L. R., and Thaw, J. Advanced Perspective .... Svo, 2 00 Culley, J. L. Theory of Arches i6mo, o 75 Gushing, H. C, Jr., and Harrison, N. Central Station Man- agement , *2 00 Dadourian, H. M. Analytical Mechanics. Svo, *3 00 Danby, A. Natural Rock Asphalts and Bitumens Svo, *2 50 Darling, E. R. Inorganic Chemical Synonyms i2mo, i 00 Davenport, C. The Book Svo, *2 00 Davey, N. The Gas Turbine Svo, *4 00 Davies, F. H. Electric Power and Traction Svo, *2 00 Foundations and Machinery Fixing i6mo *i 00 Deerr, N. Sugar Cane Svo, 8 00 Deite, C. Manual of Soapmaking 4to, De la Coux, H. The Industrial Uses of Water Svo, *s 00 Del Mar, W. A. Electric Power Conductors Svo, *2 00 Denny, G. A. Deep-Level Mines of the Rand 4to, *io 00 Diamond Drilling for Gold *5 00 De Roos, J. D. C. Linkages i6mo, o 75 12 ft, VAN NOSTRA Nb COMPANY^S SHORT-flTLE CAfAL6Q Derr, W. L, Block Signal Operations Oblong izmo, *i 50 Maintenance of Way Engineering (In Preparation.) Desaint, A. Three Hundred Shades and How to Mix Them. 8vo, 9 00 De Varona, A. Sewer Gases i6mo, o 75 Devey, R. G. Mill and Factory Wiring i2mo, *i 00 Dihdin, W. J. Purification of Sewage and Water 8vo, 6 50 Dichman, C. Basic Open-Hearth Steel Process 8vo, *3 50 Dieterich, E. Analysis of Sesins, Balsams, and Gum Resins. 8vo, *3 50 Dilworth, E. C. Steel Railway Bridges 4to, *4 00 Dinger, Lieut. H. C. Care and Operation of Naval Ma- chinery i2mo, 3 00 Dixon, D. B. Machinist's and Steam Engineer's Practical Calculator i6mo, mor., i 25 Dommett, W. E. Motor Car Mechanism i2mo, *2 00 Dorr, B. F. The Surveyor's Guide and Pocket Table-book. i6mo., mor., 2 00 Draper, C. H. Elementary Text-book of Light, Heat and Sound i2m.o, i 00 Draper, C. H. Heat and the Principles of Thermo-dynamics, i2mo, 2 00 Draper, E. 6. Navigating the Ship i2mo, 2 00 Dron, R. W. Mining Formulas i2mo, i 00 Dubbel, H. High Power Gas Engines Svo, *5 00 Dumesny, P., and Noyer, J. Wood Products, Distillates, and Extracts Svo, *5 00 Duncan, W. G., and Penman, D. The Electrical Equipment of Collieries Svo, *5 00 Dunkley, W. G. Design of Machine Elements. 2 vols. i2mo, each, *2 00 Dunstan, A. E., and Thole, F. B. T. Textbook of Practical Chemistry i2mo, *i 40 Durham, H. W. Saws Svo, 250 Duthie, A. L/ Decorative Glass Processes Svo, *2 00 Dwight, H. B. Transmission Line Formulas Svo, ''s 00 Dyke, A. L. Dyke's Automobile and Gasoline Engine Encyclopedia Svo, 5 00 Dyson, S. S. Practical Testing of Raw Materials Svo, *5 00 and Clarkson, S. S. Chemical Works Svo, 9 00 D. VA^f NOSTRAND COAIPANy's SHnRT-TTTLE CATaLoG I3 Eccles, W. H. Wireless Telegraphy and Telephony. .121110, *8 8-^ Eck, J. Light, Eadiation and Illumination 8vo, "2 5c Eddy, H. T. Maximum Stresses Under Concentrated Loads, 8vo, I 50 Eddy, L. C. Laboratory Manual of Alternating Currents, I'iiTlO, O 5c Edelman, P. Inventions and Patents lamo, ^i 50 Edgecumbe, K. Industrial Electrical Measuring Instruments, 8vO, 5 01 Edler, R. Switches and Switchgear 8vo, *4 '^t Elssler, M. The Metallurgy of Gold Svo, o -^i ■ ^The Metallurgy of Silver Svo, ^ m The Metallurgy of Argentiferous Lead Svo, 625 ■ A Handbook of Modern Explosives Svo, 5 02 Ekin, T. C. Water Pipe and Sewage Discharge Diagrams, folio, *3 00 Electric Light Carbons, Manufacture of Svo, i 00 Eliot, C. W., and Storer, F. H. Compendious Manual of Qualita- tive Chemical Analysis i2mo, '1 2$ Ellis, C. Hydrogenation of Oils Svo, 7 50 Ultraviolet Light, its Application in Chemical Arts, i2mo (In Press.) Ellis, G. Modern Technical Drawing Svo, *2 00 Ennis, Wm. D. 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The Treasures of Coal Tar i2mo, 2 00 Firth, J. B. Practical Physical Chemistry i2mo, *i 25 Fischer, E. The Preparation of Organic Compounds. .i2mo, *i 50 Fish, J. C. L. Lettering of Working Drawings. . . .Oblong 8vo, i 00 Fisher, H. K. C, and Darby, W. C. Submarine Cable Testing. 8vo, *4 00 Fleischmann, W. The Book of the Dairy 8vo, 450 Fleming, J. A. The Alternate-current Transformer. Two Volumes 8vo, Vol. L The Induction of Electric Currents *6 so Vol. II. The Utilization of Induced Currents *6 50 Fleming, J. A. Propagation of Electric Currents 8vo, *3 50 A Handbook for the Electrical Laboratory and Testing Room. Two Volumes 8vo, each, *6 50 Fleury, P. Preparation and Uses of White Zinc Paints. .8vo, *2 75 Flynn, P. J. Flow of Water i2mo, o 75 — — Hydraulic Tables i6mo, o 75 Forgie, J. Shield Tunneling 8vo. {In Press.) Foster, H. A. Electrical Engineers' Pocket-book. 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