Three phase power-factor control system for A.C. induction motors

Claim 1

A multi-phase control system for an AC induction motor comprising: a current sampling means including means adapted to be placed in circuit with a first-phase winding of said multi-phase motor for providing an output signal indicative of the phase of current drawn by said first phase winding; a voltage sampling means adapted to sense a voltage signal of an electrical input to said motor and for providing an output signal indicative of the phase of said voltage signal; a phase-difference detection means responsive to the output signals of said current sampling means and said voltage sampling means for providing an output signal which varies in accordance with the phase difference between said current and said voltage signals; a first control means adapted to be electrically connected in series with said first phase winding of said motor, and responsive to the output signal of said phase difference detection means for varying the duration of "on" time of each cycle of input power to said first-phase winding inversely to the difference in phase between said current and said voltage signals; and, at least a second control device respectively adapted to be electrically connected in series with a second-phase winding of said multi-phase motor, and responsive to the output of said phase detection means for varying the duration of "on" time of each cycle of input power to said second-phase winding inversely to the difference in phase between said current and said voltage signals, wherein said phase-difference detection means produces first-phase control pulses to gate said first control means, each pulse determining the on time of a half-cycle of input power to said first-phase winding, and wherein is further included logic circuitry coupling said phase-difference detection means to said second control device, said logic circuitry including a frequency-multiplying means for frequency multiplying the first-phase control pulses to create further pulses, a phase-lock means for phase locking said further pulses to said first-phase control pulses and a distributing means for distributing some of these further pulses to said second control device to be thereby used as control pulses for said second control device.

Claim 6

A method of controlling power applied to a multi-phase AC induction motor comprising the following steps: sampling current flowing through a first winding of the AC induction motor and thereby determining its phase; sampling a voltage signal applied to said AC induction motor and thereby determining its phase; comparing the phases of said current flowing through said first phase winding with the phase of said voltage applied to said motor and producing a first pulse signal representative of the difference between said compared phases; utilizing the first pulse signal to vary the duration of "on" time of each cycle of input power to said first-phase winding inversely to the difference in phase between said current and said voltage; multiplying said first pulse signal to create a second pulse signal having a frequency that is a multiple of the frequency of the first pulse signal by a factor which is a multiple of the number of phase windings; and, phase locking said second pulse signal to said first pulse signal; and, using pulses from said second pulse signal for varying the duration of "on" times of each cycle of input power to each additional different-phase winding.

Claim 7

A method of controlling power applied to a multi-phase AC induction motor comprising the following steps: sampling current flowing through a first winding of the AC induction motor and thereby determining its phase; sampling a voltage signal applied to said AC induction motor and thereby determining its phase; comparing the phases of said current flowing through said first phase winding with the phase of said voltage applied to said motor and producing a first pulse signal representative of the difference between said compared phases; utilizing the first pulse signal to vary the duration of "on" time of each cycle of input power to said first-phase winding inversely to the difference in phase between said current and said voltage; multiplying said first pulse signal to create a second pulse signal having a frequency that is a multiple of the frequency of the first pulse signal by a factor which is a multiple of the number of phase windings; and, phase locking said second pulse signal to said first pulse signal; and, using pulses from said second pulse signal for varying the duration of "on" times of each cycle of input power to each additional different-phase winding.