GB632571A - Improvements in or relating to load relay circuits for induction motors - Google Patents

Abstract

632,571. Control of A.C. motors. IGRANIC ELECTRIC CO., Ltd. Aug. 27, 1945, No. 21961. Convention date, Aug. 31, 1944. [Class 38 (iii)] A relay associated with the primary circuit of a three-phase induction motor is rendered responsive to the load component only of the line current. The winding a of the relay LR, Fig. 1, is supplied with the vector difference between the current from the current transformer CT-proportional to the total load current, and that from the potential transformer PT-adjustable to a value proportional to the magnetizing current or the no-load current of the motor. Thus the current in winding a is proportional to the actual load component of the motor current. The primary winding of transformer PT is connected to line L2 and through the variable resistor R to the variable tapping on the potentiometer resistance R1 connected across lines L1 and L3. In one modification, transformer CT supplies a second primary winding on the potential transformer, of which the secondary winding is connected in a closed circuit with winding a. In place of winding a the relay may have two windings, Fig. 4, viz : winding i which takes the place of the current transformer CT, and winding j which takes the place of the primary winding of the potential transformer PT, the relay again being energized in accordance with the vector difference between the two currents.