Control for three phase A.C. motor

Claim 1

A solid state trigger circuit to provide variable period output signals within a timed sequence of signals for controlling conduction angle firing of thyratron type switching devices in an A.C. phase line comprising: a bi-stable multivibrator with a first and a second input and a first and a second output which are coupled to form a third output; timing means providing successive timing signals derived from the alternating current in said phase line to be 180° out of phase with each other, said timing signals respectively being alternately applied to a corresponding one of said first and said second inputs of said multivibrator to oscillate said multivibrator at a desired timing rate, said first and said second outputs of said multivibrator providing a voltage transition spike at said third output at the beginning of each positive half cycle of said timing signals; means responsive to said voltage transition spike to provide a saw tooth wave form with periods corresponding to said timing rate; means providing a linear voltage input control signal indicative of a desired time period pulse width; and means comparing said control signal with said saw tooth wave form and providing corresponding square wave trigger output signals with signal pulse periods equal to said desired pulse width.