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Power-factor-correction-for-variable-speed-drive

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Power factor correction for variable speed drive

Details

Inventors: Leuthen, John M.;
Assignee: Hughes Tool Company (Houston, TX)
Primary Examiner: Rubinson; G. Z.
Assistant Examiner: Evans; Arthur G.
Attorney, Agent or Firm: Felsman; Robert A., Bradley; James E.

A variable speed drive for an AC motor has features to improve the power factor. The system uses a rectifier for converting AC supply voltage into DC voltage on a positive rail and a negative rail. Frequency switches are controlled to alternately connect the rails to the power conductors at a variable rate to define a selected voltage frequency. An amplitude switch is located on one of the rails and controlled to vary the potential between the rails in proportion to the frequency. A current sensor senses the current waveform in one of the conductors and applies it to a phase detector which detects the difference in phase between the current and the voltage. Phase difference pulses are produced which are averaged into an average DC value. The DC value is applied to a demand voltage that controls the amplitude switch to reduce the amplitude to improve power factor.

DETAILED DESCRIPTION The system employs means to vary the amplitude of the voltage in the positive and negative rails to improve the power factor.
An amplitude switch is located in one of the rails for switching on and off the DC voltage.
A current sensor senses the current waveform in one of the conductors.
Phase detector means detects the phase difference between the current and the voltage frequency.
The difference is converted into an average DC value and applied to a control circuit which controls the amplitude switch to reduce the potential between the rails proportionately.

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