Power factor control of a three-phase induction motor

In an electronic controller for regulating the power applied by a three-phase AC supply to a three-phase AC induction motor of delta or wye winding configuration, the controller including a pair of power factor control circuits each requiring a positive DC supply voltage, a negative DC supply voltage and an isolated ground, a bipolar supply comprising:

• a step-down transformer having a single high impedance primary winding connected to sense a phase-to-phase voltage of the AC supply and a pair of voltage step-down secondary windings electrically isolated from each other, the secondary winding each providing an AC voltage substantially less than that impressed across the primary winding,
• first and second full wave bridge rectifiers for providing pulsating DC voltage output in response to an AC voltage input, each providing a DC output terminal and a DC return terminal, the full wave bridge rectifiers each including an AC input connected to a respective one of the secondary windings providing AC voltage inputs to the bridge rectifiers;
• first and second electrolytic capacitors each having a positive end and a negative end, the positive end of the first capacitor means being connected to the DC output terminal of the first bridge rectifier, the negative end of the first capacitor being connected to the DC return terminal of the first bridge rectifier, the positive end of the second capacitor means being connected to the DC output terminal of the second bridge rectifier, the negative end of the second capacitor being connected to the DC return terminal of the second bridge rectifier, the first and second capacitors filtering the pulsating DC voltage outputs provided by bridge rectifiers to provide generally steady state DC voltages;
• a first series-connected pair of Zener diodes having a cathode end and an anode end, the first pair being connected to sense at least a portion of the steady state DC voltage across the first capacitor means, the cathode end of the first series-connected Zener diode pair being connected to the positive end of the first capacitor, the anode end of the first series-connected Zener diode pair being connected to the negative end of the first capacitor, the cathode end of the first Zener diode pair providing a first positive DC supply voltage, the anode end of the first Zener diode pair providing a first negative DC supply voltage, the midpoint of the first Zener diode pair providing a first electrical ground relative to the positive and negative supply voltages provided by the first Zener diode pair, the first positive and negative DC supply voltages and their respective first electrical ground being connected to energize one of the power factor control circuits, and a second series-connected pair of Zener diodes having a cathode end and an anode end, the second pair being connected to sense at least a portion of the steady state DC voltage across the second capacitor means, the cathode end of the second series-connected Zener diode pair being connected to the positive end of the first capacitor, the anode end of the second series-connected Zener diode pair being connected to the negative end of the second Zener diode pair providing a second positive DC supply voltage, the anode end of the second Zener diode pair providing a second negative DC supply voltage, the midpoint of the second Zener diode pair providing a second electrical ground relative to the positive and negative supply voltages provided by the second Zener diode pair, the second positive and negative DC supply voltages and their respective second electrical ground being connected to energize the other of the power factor control circuits.