a three - phase power factor control system for an ac induction motor monitors the power factor of a first - phase motor winding and produces a rectangular - wave pulse power - factor signal which is used to directly control the duration of &# 34 ; on &# 34 ; time of each cycle of input power to the first - phase winding and to indirectly control the &# 34 ; on &# 34 ; time of each cycle of input power to second - and third - phase windings . to provide a signal for controlling the power to second - and third - phase windings the power - factor pulse signal is synchronized with a square - wave oscillator whose frequency is three times that the power - factor pulse signal . each second and third positive pulse of the square - wave oscillator is used to control the &# 34 ; on &# 34 ; time of each cycle of input power to the second - and third - phase windings . a current sampler for monitoring current flowing through the first - phase winding samples voltage across a triac and thereby avoids unduly loading the first - phase winding with a resistor as was done in the prior art . the current sampler is coupled to following circuits through an optical isolator for producing pulses corresponding to current phase .