Patent GB-594272-A (GB)
Title: Improvements in d.c. electric motor control systems
IPC: B60L9/04
Filed: 1945-06-27
Abstract: 594,272. Control of D.C. motors. ENGLISH ELECTRIC CO., Ltd., and BINNEY, E. A. June 27, 1945, No. 16390. [Class 38 (iii)] To regulate automatically the acceleration of a D.C. motor a bucking machine 6 in series with the motor, for example a traction motor 7, has its excitation controlled automatically so as to maintain a predetermined current through the motor until the E.M.F. of the bucking machine is substantially zero, i.e. until the motor has accelerated to a speed where its E.M.F. is substantially equal to the supply voltage, whereupon voltage responsive switching means automatically connect the motor 7 directly across the supply lines 4, 5. The bucking machine 6 is coupled to a shunt wound machine 1 and exciter 10, the machine 1 being connected across the supply lines 4, 5 and functioning initially as a motor, while the exciter serves to energise the field 6a of the machine 6 when contacts 9b are closed. The exciter has three field windings, viz. a field 10a connected initially in series with the motor armature 7 and bucking machine 6 over closed contacts 9a, the contacts 21 then being open; a field 10b connected across a resistance 8 in series with the motor armature 7 and acting in the same sense as field 10a; and an opposing field 10c in a battery circuit provided with a bank 13 of resistances shunted by contactors 15, 16, and 17 controlled by the operator. The exciter field 10a requires only a small current to produce in the exciter a voltage such that the E.M.F. of the bucking machine becomes quickly equal to line voltage when the line switch 3 of machine 1 is closed, the current through the motor armature 7 being consequently small. When the bucking machine voltage is substantially equal to that of the supply a relay coil 20 connected across its armature is energised to open contacts 20a in the circuit of a relay coil 9, and also closes contacts 20b to energise a relay coil 21a, the latter then operating to close a line switch 21 and short circuiting the exciter field 10a. The operator's controller is next operated to close contacts 14 in the battery circuit and energise exciter field 10c in opposition to 10b, thereby reducing the output voltage of the bucking machine and allowing the accelerating current through the traction motor to increase, until a balance is reached. As the traction motor accelerates, its back E.M.F. increases, and the E.M.F. of the bucking machine, which is motoring, decreases correspondingly until eventually the traction motor back E.M.F. is equal to the supply voltage and the E.M.F. of the bucking machine is zero. This causes de-energisation of relay 20 and the opening of contacts 20b and closing of contacts 20a, and since the controller contacts 14a have been closed in the meantime, relay 9 is energised to close contacts 9c and open contacts 9a and 9b, thereby by-passing the bucking machine to connect motor 7 directly across the supply, and also disconnecting machine 6 from the supply and disconnecting its field 6a from the exciter 10. Rheostatic braking is effected in two stages, the first being the return of the driver's accelerating controller to the " off " position to open contactors 14-17 to deenergise the exciter field 10c, regeneration being prevented meantime by the exciter providing a gradually reducing motoring current. The second stage is the operation of the pedal - actuated brake controller which closes contactors 14-17 to energise exciter field 10c according to the current desired through the armature 7 during braking, the current through motor field 7b being determined by the opposing exciter fields 10b, 10c, of which 10c is governed by the pedal - controlled resistance 13. Operation of the brake pedal simultaneously opens contacts 25 to de-energise the line switch 21, and contacts 11 to open exciter field circuit 10a, and closes contacts 26 in series with the rheostatic braking resistance 27. In a modification the bucking machine may be provided with a shunt field winding and the exciter armature connected in series therewith but acting in opposition to the E.M.F. of the bucking machine, in the manner described in Specifications 509,884 and 518,329. The bucking machine may be coupled to a machine acting as a motor to start the bucking machine and to return power to the line during acceleration of the main motor. Specifications 514,433, [Group XXXV], 560,531, and 579,570, [Group XXXV], also are referred to.