Patent GB-726274-A (GB)
Title: Improvements in or relating to control systems for electric motors
IPC: H02P3/12
Filed: 1953-06-19
Abstract: 726,274. Control of D.C. motors; controllers. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. June 19, 1953 [June 26, 1952], No. 16968/53. Class 38 (3). Traction motors are operated as series generators for dynamic braking, and means are provided for reducing an excessive rate of increase of motor current and voltage when the braking circuits are established while the motors are operated at high speeds. The motors have armatures A1, A2, and field windings SF1, SF2, and are accelerated by series-parallel control and field-shunting. During coasting operation, the spotting-current adjustment is made by a field controller FC under the regulation of a spotting relay SR, resistors R2, R3 and R5, being included in the braking circuit. A service braking-application is made by the closure of a brake-lever 202 which transfers the control of the braking progression from the spotting relay to a current-limit relay CR. Closure of a brake-relay BR effects movement of the field-controller FC towards its full-field position under the regulations of the relay CR. When the field-controller reaches its full field position, a relay B2 is closed to short-circuit part of resistance RS in the common dynamic-braking circuit of the traction motors wherein each of the armatures of the motors is loaded by the field winding of the other motor. The relays B5 and B6 are closed in succession to remove further portions of the braking- resistance, all these braking-progression operations being under the control of the limit relay CR which interrupts the progression whenever an excessive motor-current causes an opening of the backcontact 199, thereby preventing the progression until the motor-current falls to a desirable value. After removal of the resistor R5, switches 1-2, 3-4, and 9-10 are closed in succession to remove accelerating resistors R2, R3, which are in the individual portions of the respective dynamicbraking circuits, this operation being under the same limit-relay control. The calibration of the relay CR, to cause it to drop out in response to various motor-current values during both acceleration and dynamic braking operation, is effected by a rate-coil RC which is connected in series with a weight-responsive rheostat 200 during motoring operation. A second rate-coil circuit involves a braking-responsive resistor 201 which is automatically changed in accordance with the position of brake-handle 202, so that the rate-coil has maximum excitation when a low braking-rate is called for. Means for responding to an excessive rate of increase of motor-current comprise a stepup transformer 301 having its primary winding in the motor circuit and having its core separate from that of the motor-field. The secondary voltage of the transformer is applied to the rate coil RC. An auxiliary switching-segment 306 on the field-controller is used to prevent undesirable operating conditions which may arise when the master controller MC is left in its No. 1 position while the vehicle is operating at high speed. When the field-controller has advanced during the accelerating-period to any of the positions F3-SF, before the controller MC is moved back to the No. 1 position the contact 306 connects control wires 4 and 6 so rapidly removing all motor resistance and increasing the field strength of the motors. This prevents the driver from moving directly from the No. 1 position of the controller MC to establish dynamic braking operation under favourable conditions of motor armature resistance and field excitation. Specification 722,671 is referred to.