Apparatus for variable speed drive of an induction motor from a fixed

Abstract

A control system for driving a variable speed AC motor employing a fixed frequency AC power source, combines pulses representative of a desired slip frequency with pulses of a frequency proportional to the speed of the motor. The combined pulses are employed with a speed command signal to set a threshold which determines the fraction of each cycle of the AC primary power which is fed to the motor. In addition, the speed and slip pulses are employed to direct the AC power to specific ones of the windings of the motor to thereby create a rotating magnetic field in the induction motor. An array of silicon-controlled rectifiers does the actual gating and an SCR protect circuit ensures that transfer of control from one SCR to another is inhibited until all current creases to flow in an SCR being extinguished. A forced commutation circuit forces the extinction of all current remaining in the last conducting SCR feeding a motor winding at the transfer time from that motor winding to the next.