Power factor controller for an induction motor using transistor switch

Abstract

A phase-triggered, gate-controlled AC semiconductor switch of the thyrister type 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. The triggering point of the semiconductor switch relative to the preceding zero crossing point of the supply voltage, that is, the switch firing angle or delay angle, is varied as a function of mechanical loading on the motor by means of a load current induced feedback voltage which varies the breakdown or triggering voltage of a two-transistor switch functioning as a variable trigger diode between the gate of the semiconductor switch and a turn-on capacitor charged by the AC supply voltage during non-conducting periods of the semiconductor switch. Under increasing mechanical load conditions, the feedback voltage increases in proportion to increasing load current, the increasing feedback voltage lowering the breakdown voltage of the two-transistor switch to trigger the semiconductor switch into conduction at a reduced firing angle, wherein power flow to the motor is increased. Conversely, under decreasing mechanical load conditions, the feedback voltage decreases in proportion to decreasing load current, the decreasing feedback voltage raising the breakdown voltage of the two-transistor switch to trigger the semiconductor 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.