GB636083A - Improvements relating to alternating current electric motor control systems - Google Patents

Abstract

636,083. Control of A.C. motors. MULLIGAN, J. W., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Nov. 21, 1947, No. 30959. [Class 38 (iii)] In a control system for an alternating-current synchronous motor the direct-current excitation of the motor is controlled in accordance with the load on the motor by means which is responsive to changes in the load current and in the exciting current and operates to control the exciting current such that the ratio of the exciting current to the load current is maintained substantially constant at a predetermined value. The said means is preferably so arranged that the excitation of the motor cannot be reduced below a predetermined minimum value. An exciter E is coupled to a synchronous motor M and supplies directcurrent to the rotor winding of the motor through slip-rings SR. The field winding F of the exciter is connected across its armature through a variable resistance RS which is adjusted by means of a servomotor SM. A relay BR, having a coil LC which is energized in accordance with the load current through a current transformer CT and a rectifier R, and an opposing coil EC which is energized in accordance with the exciting current through a shunt SH in the armature circuit of the exciter, controls the operation of the servomotor. If the ratio of exciting current to load current has a desired pre-determined value, the contacts of the relay will be open and the servomotor de-energized. If the load increases, for example, the servomotor is energized to reduce the resistance RS. The exciting-current is increased and the ratio between it and the load current is brought back to the pre-determined value, whereupon the motor SM is de-energized. Alternatively, the balanced relay may operate through contactor switches or a carbon pile regulator. A voltage regulator, transductor, or electronic apparatus may replace the balanced relay.