GB773135A - Saturable reactor control for a polyphase motor - Google Patents

Abstract

773,135. Control of A.C. motors. SQUARE, D., CO. Oct. 20, 1955 [Nov. 22, 1954], No. 29964/55. Class 38(3) [Also in Group XXXIX] A squirrel-cage induction motor 10 is started in the forward direction by actuating push-button 21 to close contacts 21A and energize relay coil 30. The motor is placed across the power supply lines L1, L2, L3, with the parallel-connected output windings A-A, B-B, of a saturable reactor in series with the line L2 and motor terminal T2 so that the motor is supplied with unbalanced voltage to limit the starting torque. Further depression of the pushbutton closes contacts 23A which energize the control winding C-C of the saturable reactor through bridge rectifier 20 and series resistor 50, causing partial. saturation of the reactor and an increase of torque. The push-button, is then actuated to close contacts. 25A, 27A, which shortcircuit successive parts of, the resistor 50 until the reactor is fully saturated and the motor voltage approaches a balanced condition. Finally, closure of contacts 29A energizes relay coil 60 whereby contacts 62 close to short-circuit the windings A-A, B-B, and contacts 61 open the circuit of the winding C-C. After release of the pushbutton 21, the motor may be operated in the reverse direction by closure of an interlocked push-button 22 to energize relay coil 40. Subsequent operation is similar to that described for forward running. A timedelay effect during starting is obtained by transient current circulating in the loop circuit formed by windings A-A, B-B, and induced by changes in the control current flowing through windings C-C. This retards change in the saturating flux and reduces the value of peak currents to afford a cushioning effect. The saturable reactor comprises a core having three legs, the alternating-current output windings being wound on the outer legs, and the directcurrent control winding on the centre-leg (Fig. 2, not shown).