GB815480A - Improvements in or relating to control systems for alternating current electric motors - Google Patents

Abstract

815,480. Automatic speed-control systems. GENERAL ELECTRIC CO. Ltd. Oct. 8, 1956 [Oct. 6, 1955], No. 28531/55. Class 38 (4). In a system wherein the speed of an A.C. motor 1 is controlled automatically by an adjustable liquid resistance 4 in the rotor circuit and in accordance with the difference between an adjustable reference voltage applied at terminals 9 and the output of a tachometer generator 7, 8, through a magnetic amplifier 11, a feedback or compensating signal dependent on the motor torque is obtained from a current transformer 21 and a rectifier 22 and applied through a delay network 24 to an auxiliary winding 20 on the magnetic amplifier to reduce variation of speed due to variation in load. The system provides for dynamic braking during overspeed periods by the application of a direct current from source 23 to the stator windings, through contacts 19 which are caused to be actuated by a polarized relay 14 in the event of overspeeding. During dynamic braking the output from the current transformer is shortcircuited by contacts 25 and the feedback voltage is taken from a series resistor in the stator circuit. The tachometer field 8 is arranged to be reversed when the motor 1 is reversed, and alternative arrangements for deriving the feedback proportional to torque are referred to. Specifications 717,253 and 738,310 are referred to.