GB706455A - Improvements in or relating to control systems for direct current electric motors - Google Patents

Abstract

706,455. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. March 26, 1952 [April 7, 1951], No. 7729/52. Class 38(3) Relates to means for preventing too rapid reapplication of power to a traction motor when the supply is interrupted during reduced field operation. The motor has, in addition to the armature A, field windings 6, 7, which are wound on the same poles, the series field winding 6 being used for motoring operation only and the shunt winding only for dynamic braking. An auxiliary motor AX, which must be started before the main motor, is provided, together with pedal-operated master and brake controllers M, B. A cam controller C and a change-over switch P' are operated by air engines. Depression of pedal 13 to move the controller M first operates the switch P to open the braking connections and connects the traction motor across the supply-lines. Further movement of the master controller causes the acceleration of the traction motor to proceed under the regulation of limit relay LR which has a contact 22 biased to the closed position by means of spring 23 coupled to pedal 13. The relay LR also has a main series coil 20 and its sensitivity is enhanced by a coil 25 responsive to sudden increases of traction motor current since it is connected across the shunt field winding 7 which in transformer relation with the series field winding 6. Improper response of the coil 25 during reverse motor operation is prevented by rectifier 26 on reverser contacts 28. The relay LR controls, through the valve V3, the rate of advance of the controller C which has contacts regulating the armature resistance R1 - R6. When the master controller M is returned to its offposition the switch LS is opened and the cam controller C moved towards its first notch, the switch P effecting the connection of braking resistors RA, RB, across the armature A. Means are provided to prevent reclosure of the line switch until the cam controller has returned to a low speed position. If a brake pedal 14 is depressed when the traction motor is coasting and disconnected from the line, contactors SF1, SF4, are energized to excite the shunt field winding 7. The voltage-drop across the brake resistor RA is applied to the holding-coil 33 of relay SFR, thus maintaining the energization of contractors SF1, SF4. The voltage-drop across resistor RB opposes the line voltage applied to the shunt field winding 7, the field excitation thus being increased as the braking current decreases so that the braking effort is held substantially constant. The amount of braking effort is determined by the pedal 14 which selects, by means of controller B, the portion of field resistance R8-R10 connected in circuit. When the braking current falls, the relay SFR drops and de-energizes the shunt field winding. An auxiliary relay X prevents advancement of the controller C in the event of power failure during motoring operation A coil 39 of the relay is connected in series with motor AX, an additional relay winding 43 producing a de-energizing effect in response to a sudden decrease of traction motor current when there is power interruption, so that the relay responds quickly to such a condition despite regenerative feed-back from the main motor to the auxiliary motor. The relay X has contact 42 adapted to open the control circuit of the traction motor, and contact 41 which prevents reclosure of the relay until the controller C has moved to a suitable low-speed position. As shown, the winding 43 is connected during weak-field operation across the series field winding 6 which, due to its reactance, develops a voltage responsive to the rate of charge of motor current. In a modification; Fig. 2 (not shown), the coil 43 is connected across the shunt field winding 7. Specification 706,456 is referred to.