Patent GB-485312-A (GB)
Title: Improvements in electric supply systems supplying current to d.c. motors
IPC: H02P7/34
Filed: 1936-08-17
Abstract: 485,312. Control of D.C. motors ; exciting dynamo electric machines. ENGLISH ELECTRIC CO., Ltd., BASTON, C. E., and BINNEY, E. A. Aug. 17, 1936, No. 22582. [Class 38 (iii)] [Also in Group XXXV] Varying E.M.F. ; regenerative braking.-A generator 6, the speed of which varies with the power transmitted, is so excited by a field winding 6a that any change in its armature current produces a corresponding change in excitation in a sense to oppose the change of current, and an exciter 5 for the generator 6 is coupled thereto to produce an excitation varying with the generator speed, so that upon any variation of armature current the excitation varies to oppose the variation, the power delivered, and with it the generator speed change, and the change in excitation continues until the armature current is restored to its original predetermined value. The generator 6 is driven by a compound-wound motor 1, and supplies a motor 7, while its field 6a is supplied from the line in series with the exciter 5 which has a field winding 5a carrying the generator armature current, and a field winding 5b supplied from the line in series with a regulating resistance 5c. To accelerate the motor 7 the motor-generator set is run up to speed, and the switch 9 closed. The switch 10 is then closed and current flows to the motor through the field winding 5a.. The action of the field windings 5a and 5b is such that for each value of excitation from the field 5b there is maintained a definite value of armature current. As the motor starts to rotate and build up a back electromotive force, the armature current tends to fall, but this disturbs the balance in the generator excitation circuit to increase the generator excitation and voltage. Equilibrium would be reached again at a slightly lower value of armature current, but as the result of an increased load on the generator its speed falls, and the exciter 5 exerts a corresponding control on the generator field so that the armature current is maintained at a predetermined value. When the motor 7 has been accelerated at constant current it may continue to receive its supply from the generator 5 or a switch 8 may be closed to connect the motor to the line and the regulator 5c adjusted to transfer the load to the line. A stabilizing resistance may be connected between the switch 9 and the field 5a. For regenerative braking the switch 8 is opened, the regulator 5c being previously readjusted if the motor is not to coast. The generator field 6a is then short-circuited by a switch 17 which also inserts an equivalent resistance 16. Then the generator voltage is destroyed. The regulator 5c is then adjusted to a setting corresponding to the desired braking current, and a variable resistance 14 connected across the motor field 7a. A reversing switch 11 for the motor 7 is then operated and the switch 17 operated to bring the generator field into action. The system operates to give constant current regenerative braking, switches 15 being operated in succession to strengthen the motor field. When the motor has been braked; the generator field is again short-circuited. In a modification, Fig. 2 (not shown), the exciter current also passes through the series field la of the motor 1 which motor may also have an additional field in the exciter circuit. In this modification also resistances in the armature circuit of the motor 7 replace its field diverting resistances, and the exciter current passes through the traction motor field. The application of the system to a compound-wound motor is described with reference to Fig. 3 (not shown). In this case the motor connections are not reversed for braking, but a reversing switch is provided for the exciter field 5a. The generator 6 may be additionally excited in dependence on the armature of the notor 1, and the generator exciter 5 may have a self-excited winding.