Phase detector for three-phase power factor controller

Claim 1

In a three-phase power factor controller for a three-phase induction motor of the type comprising a plurality of electronic switching means individually connected between the respective phase terminals of a three-phase supply line and the corresponding phase windings of the motor; phase detector means for producing, for each phase, an output proportional to the phase difference between the motor voltage and current; summing means for summing the outputs of said phase detector means; power factor command signal generating means for generating a power factor command signal; and control means, responsive to the output of said summing means and to said power factor command signal, for controlling switching of said switching means, each said phase detector means including means, comprising an operational amplifier whose inputs are connected across the corresponding electronic switching means for that phase, for sensing the phase angle between the current for that phase and the line voltage by sensing the voltage across said corresponding electronic switching means, the improvement comprising means providing positive feedback between the output and input of said operational amplifier such that switching of the output of the operational amplifier is synchronized with switching of the voltage across the electronic switching means.