GB835924A - Improvements in or relating to electric motor control arrangements - Google Patents

Abstract

835,924. Control of A.C. motors. GENERAL ELECTRIC CO. Ltd. Feb. 25, 1958 [March 1, 1957; Sept. 30, 1957], Nos. 6972/57 and 30543/57. Addition to 774,691. Drawings to Specification. Class 38 (3). [Also in Group XXXV] In a control system as in the parent Specification in which a resistance in the rotor circuit of an induction motor is automatically varied to maintain a value of rotor current corresponding to a desired torque, the desired value of torque is set by varying the effective ratio of the current transformer, responsive to rotor current, which energizes an electromagnet controlling a fluid-operated servo-mechanism varying the rotor resistance. A three-phase liquid resistance is employed in the arrangements described. In one arrangement current transformers energize the electromagnet through auto-transformers which are adjustable by a manually operated lever to set the desired value of torque. In a modification in which motor speed is automatically maintained at a desired value in either direction, the current transformers are of the saturable type and their effective ratios are controlled by energizing control windings in accordance with the difference between the voltage of a tachometer generator coupled to the motor and the voltage from a potentiometer adjusted by a manually operated lever which thus serves to set the motor speed. In this arrangement the difference voltage also controls the electromagnet during motoring. In both arrangements, as in the parent Specification, the rotor current is automatically controlled during direct-current dynamic braking.