GB1200865A - Speed control circuit for an electric motor - Google Patents

Abstract

1,200,865. Automatic speed control. JANOME SEWING MACHINE CO. Ltd. March 24, 1969 [March 22 1968; July 5, 1968], No.15385/69. Heading G3R. [Also in Division H2] A motor M is supplied from an A.C. source by a triggered conduction element SSS M controlled by an adjustable trigger circuit comprising a capacitor C1 (or inductor); a charging circuit therefor and a triggered switching device SSS T arranged to respond to a predetermined level of charge on the capacitor C1 to fire the triggered conduction element SSS M , the charging circuit comprising a full wave rectifier 4 with its input terminals A, B in series with C1 and its output connected to the emitter and collecter of a transistor Tr to the base of which is applied a control voltage which represents speed error. The speed error signal is derived by combination of a pre-set reference e s from a potentiometer R4 and a speed voltage eg derived from an A. C. tachometer generator PG driven by the motor M. The varying control voltage varies the impedance of the transistor and therefore the charging rate of C1; this causes SSS T to become conducting earlier or later in the A. C. supply cycle. When SSS T conducts, it discharges a capacitor C2 through a pulse transformer DT whose secondary is in series with the triggered conduction element SSS M . The triggered devices SSS T , SSS M are symmetrical devices supplying A. C. to the motor M. The capacitor C1 may be replaced by an inductor. In a modified circuit Fig.3 (not shown) the voltage e s is stabilized by using the forward voltage drop across several series connected silicon diodes. The tachometer generator may be a three phase type with a half wave rectifying arrangement.