514,433. Control of D.C. motors. ENGLISH ELECTRIC CO. Ltd., BINNEY, E. A., CORNER, H. S. POUND-, and SEDGFIELD, H. B. May 4, 1938, Nos. 13312/38, 26224/38 and 5787/39. [Class 38 (iii)] Varying electromotive force. -In a motor-control system comprising a Ward-Leonard or booster set which draws energy from a power supply and supplies or bucks and boosts the supply to a motor, switches are provided which operate under the control of means responsive to conditions in the motor armature circuit to connect the motor directly to the power supply. In the form shown motors 7 and 7b are connected in series with a booster 6 to a supply 2, 3 and switches 12a, 12b and 16a are provided respectively to parallel the motors to the supply and isolate the booster. These switches are controlled by means of a relay 31 which is connected across the open contacts of either switch 16a or 12a according to which is closed and responds when the combined voltages of the machines 6, 7b falls to zero. The relay 31 is supplied from a battery 8 and a rectifier 32 is included in circuit to prevent reverse energization. The booster set is started by the closure of a switch 18 and is run up to speed by a relay 21 which cuts out resistance 20 and finally closes the switch 19, a switch 19a in the controller circuit also being closed. Movement of the controller 29 to the motoring position energizes relays 14 and 30. Relay 30 closes switches 30a and 30b and causes energization of the relay 16 which closes the switch 16a connecting the motors in series with the booster across the supply. The field 6a of the booster 6 is in series with an exciter 5 having fields 5a-5d, the field 5a being in series with the motor 7, the arrangement being such that the booster voltage assumes such a value that the current through the winding 5a is proportional to the current through the field 5b. Since the windings 5b and 5c are short circuited the booster builds up to a voltage equal to the supply so that the voltage across the motors is zero. As the motor speeds build-up the generator voltage reduces to zero, reverses and builds up to a value equal and opposite to that of the supply. At this point the relay 31 responds and reverses the contact 31a to de-energize relay 16 and energize relays 12 and 33. Relay 12 causes switches 12a, 12b to close and relay 16 causes switch 16a to open thus parallelling the motors to the supply and disconnecting the generator. The switch 31a is of the type in which both circuits are momentarily energized so that switches 12a, 12b close before 16a opens. The relay 33 operates with a time delay and reverses the connections of a polarizing winding 31b for the relay 31 to prepare it for reverse operation, connects the field windings 7d, 7e by means of the switches 13, 13a to the generator, and reverses the connection of the exciter 5. This system then operates with the motors in parallel across the supply and their fields supplied from the generator. Regenerative brakings is obtained by moving the controller 29 to the reverse position 6 to de-energize relay 14 and energize relay 15. This reverses the connections of the field windings 5b and causes reverse current to flow in the motor circuit. This current may be under the control of the axle generator 10 down to the point at which the speed control cuts out the generator or, alternatively, the controller may be arranged to operate the switch 11 to cut out the generator 10 when it is moved to the braking position. Coasting may be brought about either by reducing the voltage applied to the fields 5b to zero or by moving the controller to its neutral position, in which case the field 5b is shortcircuited and the switches 16a, 12a, 12b open. Overload relays 26, 27 are provided in the motor circuits and a differential relay is provided havings windings 28, 28a in the supply circuit and responsive to earth faults. Resistance is reinserted in the circuit of the motor 1 by means of a no-current relay 25 if the current is interrupted for an appreciate time, by means of an overload relay 22 if the current become excessive, by means of an over-voltage relay 23 if the voltage rises excessively during regeneration and by means of an under-voltage relay 24 should the supply voltage to the set fall to a low value or the motor become disconnected. Specifications 485,312, 509,041, 509,138, 509,239 and 509,884 are referred to.