Patent GB-784517-A (GB)
Title: Improvements in or relating to electric motor control systems
Inventor: TURRALL JAMES CHARLES
IPC: B60L7/04
Filed: 1954-07-07
Abstract: 784,517. Control of several D.C. motors. GENERAL ELECTRIC CO., Ltd. Sept. 6, 1955 [July 7, 1954], No. 19899/54. Class 38 (3). A traction system comprises four compoundwound direct-current motors having armatures 1, 2, and 3, 4, connected in two parallel groups, each group consisting of two motors in series. Separately-excited field windings 1F-4F of the motors are connected in series, and field control is obtained by means of adjustable resistance FR. The motors have series field windings 1SF-4SF, and starting resistance SR is provided for control of the armature circuits which comprise reversers R<SP>¢</SP> and R<SP>¥</SP>. When a power master controller, Figs. 2, 3 (not shown), is moved to a first motoring step, switches LP1, LP2, LN2 and LN1 are closed together with a contactor P, the current of armatures 1 and 2 passing through the coils CLM, CLB, respectively of motoring and regenerating currentlimit relays and coil NCR of a no-current relay. A contactor F is closed to energize the separately-excited field windings, the rheostat FR being in the zero-position A. The starting resistance is controlled by contacts M1-M7 of an electromagnetic pawl-operated cam-shaft controller, whilst the controller for the field resistance is of the multi-notch type consisting of a commutator, the segments of which are connected to tappings in the field resistance and the rotating brushes being driven by an air engine. When the power master controller is moved to the second notch, the cam-shaft controller is arranged to remove the resistance SR under the control of relay CLM until contact M7 is closed. This is followed by movement of the field controller from position A to B, again under the control of the relay CLM. The equipment stops in this position until the master controller is moved to the third notch wherein the field controller progresses from position B to C, and selection of weak field by movement of the master controller to the fourth notch causes the field controller to be adjusted to position D whereupon contactor F opens. During progression of the master controller from its first position to the closing of contact M7, switching- off of the master controller results in the opening of line switches LP1, LP2, LN1 and LN2. However, after the field controller has started to move, the switching-off results in opening of the switch LP2 only for coasting operation, the cam-shaft controller being returned to its first position and the contactor F closed, if it is not already closed. If, under these circumstances, a motoring current passes through the coil CO1 of a polarized relay, the field controller is arranged to increase the field strength until a small regenerative current is produced, and the field controller is halted until a motoring current is again taken, whereupon the field strength is further increased. The no-current relay NCR is effective only during motoring, loss of supply causing the relay to open line switch LP2 and return the cam-shaft controller to its first position. When the supply is restored, the armature resistance is again reduced, the field strength remaining the same as when the interruption took place, so avoiding a braking effect which would be produced if the field were strengthened before reapplication of power at high speed. The setting of the relay CLM is adjusted to compensate for the reduction of torque which would occur when the fields were weakened unless accompanied by an increase in armature current. A polarizing coil PCO1 of the relay CO1 is connected in series with a similar coil PCO2 of another relay having main coil CO2 and the coil OV of an over-voltage relay. The contactor P breaks the parallel circuits to avoid damage in the case of a faulty reverser or when cars are towed. The separately-excited field windings are protected by discharge resistor DR and rectifier DRT. To obtain regenerative braking, a brake master controller is moved to close contactor G, short-circuiting all but a small part of the resistor SR, and to cause the field controller to increase the motor excitation until the braking effort is checked by the relay CLB. The setting of the current-limit relay is varied by the value of field current passing through its second coil CLB1, so that the allowable braking current increases with increase of field strength. After full field has been reached, the braking effort decreases as the speed is reduced, and, at a suitable current value, the polarized relay having coils CO2, PCO2, responds to open the line switches. If the field controller has been adjusted towards the full-field position and insufficient regenerative current to lift a relay having coil RP results, all the overload relays are tripped and regeneration is replaced by air braking. The same occurs with the lifting of the over-voltage relay. In a modification, Figs. 4, 5, 6 (not shown), a system comprising two motors with their armatures connected in parallel is provided, the cam-shaft controller for regulating the resistor SR being replaced by a number of unit switches adapted to operate in sequence. An additional contactor is provided to give a higher maximum separately-excited field strength during regenerative braking than during motoring, this contactor being closed just prior to the closing of the contactor G to assure an increase in field strength when entering regenerative braking operation. The function of the polarized relay having the coil CO1, for increasing the field strength until a small regenerative current is produced when the power controller is switched off under reducedfield conditions, is now undertaken by a relay which is polarized by a metal rectifier. Another relay, which is oppositely polarized, is used for decreasing the field excitation when the regenerative current reaches a predetermined value during coasting operation.