GB810119A - Improvements relating to the control of electric motors - Google Patents

Abstract

810,119. Control of A.C. motors. ELECTRIC APPARATUS CO. Ltd. March 10, 1955, No. 7134/55. Class 38 (3). Saturable reactors 13, 14, 15, connected in the primary circuit of a three-phase motor have a common D.C. control winding 16 the excitation of which varies in magnitude with, and in the same sense as, the secondary current of the motor. An increase of the load causes an increase of the output of a rectifier 21 in the circuit of the motor whereby the impedance of an auxiliary saturable reactor 25 is reduced and the primary voltage of transformer 27 increased. The control winding 16 is connected to the secondary winding of the transformer through a rectifier 31 so that an increase of motor load leads to a corresponding change in the motor primary voltage, so stabilizing the speed. The saturable reactors 13, 14, 15, have another common control winding 17 which opposes the winding 16 and is energized with rectified current from a transformer 33 having its primary winding either connected to supply terminals 7, 8, or in parallel with the A.C. winding of the reactor 13. In the latter case, the excitation of winding 17 decreases when that of the winding 16 increases. In another arrangement, the rectifier 21 is connected across a tapped rotor resistor 18, and a non- linear resistor 37 prevents appreciable excitation of the control winding of the reactor 25 until the rotor current exceeds a predetermined value. The reactor 25 may alternatively be connected on the secondary side of the transformer 27. A modified system comprises only two reactors in the primary circuit of the motor.