GB637129A - Improvements relating to the dynamic braking of induction motors - Google Patents

Abstract

637,129. Control of A.C. motors. EVANS, O. T., GHALIB, S. A., and METROPOLITANVICKERS ELECTRICAL CO., Ltd. Sept. 19, 1947, No. 25621. [Class 38 (iii)] In a dynamic braking system for induction motors a load resistor is automatically adjusted in accordance with motor speed, the adjustment being responsive to two signals, one proportional to motor speed and the other proportional to the displacement of the load resistor adjusting means. An induction type tachometer generator G is driven by a winder motor M and is of the type having two stator.coils and a copper tube rotor, one coil being fed from an A.C. source, thus inducing a voltage in the other coil which is of the supply frequency and of magnitude proportional to the motor speed and to the magnitude of the excitation. A device I, such as a variable inductor of the induction regulator type, potential divider or the like, is adjusted by a servomotor SM which operates the adjusting means for the load resistor R. The output voltage of the device I is of the same frequency as the output of the member G and is proportional to the setting of the load resistor R. The two signals are combined in a phasediscriminating amplifier A, the output of which, after amplification in a power amplifier P, is arranged to operate the servomotor. The load resistor is thus set to the correct value at any speed and for a constant D.C. excitation, during braking, torque will be maintained constant down to a speed of, say, 5 per cent full speed. A manual control may vary the braking torque by alteration of the D.C. excitation of the motor or, by controlling the magnitude of the signal proportional to motor speed. The load resistor may also be used for acceleration control during winding, the control being either manual or automatic. Specifications 632,831, 634,815 and 637,128 are referred to.