Simplified power factor controller for induction motor

Abstract

A phase-triggered, gate-controlled AC semiconductor switch, in series with an induction motor and its AC supply, optimizes power flow to the motor under changing mechanical load conditions by varying the amount of supply voltage applied to the motor over each half-cycle of the AC supply. Such power flow optimization is effected by utilization of a control voltage generated by the freewheeling induction motor during nonconduction periods of the switch. The triggering point of the switch relative to the preceding zero crossing point of the supply voltage, that is, the switch firing angle or delay angle, is varied by the control voltage augmenting to a varying degree the charging rate of a capacitor that triggers the semiconductor switch into conduction. Under increasing mechanical load conditions, the control voltage decreases, the decreased control voltage accelerating the charging rate of the capacitor to trigger the switch into conduction at a reduced firing angle, wherein power flow to the motor is increased. Conversely, under decreasing mechanical load conditions, the control voltage increases, the increased control voltage decelerating the charging rate of the capacitor to trigger the switch into conduction at an increased firing angle, wherein losses caused by reactive current in the less-than-fully loaded induction motor are reduced with a resultant optimization of power factor.