Electricity: motive power systems – Induction motor systems – Power-factor control
Patent
1980-09-12
1983-01-18
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Power-factor control
318345D, 318812, H02P 540
Patent
active
043694039
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to electronic controllers for motors adapted to drive varying or less than full mechanical loads, and more particularly, to control circuits for automatically reducing the power applied to a less than fully loaded AC induction motor, such reduced power application reducing losses caused by reactive current to improve the power factor of the induction motor.
U.S. Pat. No. 4,052,648 to Nola discloses an AC induction motor control circuit of the subject type which utilizes a phase-triggered thyristor means such as a Triac switch (Triac is a trademark of the General Electric Company of Syracuse, N.Y.) in series with an induction motor to vary the time of supply voltage application to the motor, on a half-cycle basis, the time of supply voltage application changing in direct proportion to the sensed motor power factor (a sensed decrease in motor power factor results in a decrease in the time of supply voltage application to inherently increase and optimize the motor power factor). Nola continuously senses the phase angle between the motor supply voltage and load current, and then uses a phase angle related signal to continuously adjust the firing point of the Triac switch relative to the zero crossing point of the supply voltage. For a sensed increasing phase angle (decreasing power factor) between motor supply voltage and load current, Nola shifts the Triac firing point away from the line voltage zero crossing point to apply a smaller portion of each half-cycle of the supply voltage, which inherently decreases the phase angle (increasing power factor) and reduces the heat loss (I.sup.2 R) caused by the reactive current.
While Nola recognizes the energy-saving advantages of duty cycle controlling an induction motor as a function of mechanical load with a series-inserted, phase-triggered, Triac switch, his phase angle measuring requirement and the resultant circuitry are undesirably complex and costly as compared to the relative simplicity and low cost of a small, single phase induction motor which exhibits the greatest need for reliable power factor regulation. Also, the Nola circuitry has been found to be unstable under certain operating conditions.
SUMMARY OF THE INVENTION
In accordance with the present invention, a solid state semiconductor AC switch, inserted in series with an AC induction motor and its AC supply, regulates the AC power applied to the motor. In one embodiment of the invention, current detecting means samples the alternating load current pulses through the motor for effective power factor control thereof without sensing the related motor voltage or supply voltage. In another embodiment of the invention, a voltage detecting means samples alternating supply voltage pulses applied to the motor, in addition to the sampling of the alternating load current pulses by the current detecting means. A control means responsive to the detecting means triggers the AC switch into a conducting state a predetermined time period (either fixed or variable as a function of load current) after the trailing edge of each detected load current pulse or of each detected line voltage pulse, at least when the peak value of the sensed load current pulses is less than a predetermined load current value indicative of a near full load condition experienced by the motor.
Preferably, the AC switch is a gate-controlled thyristor, such as a Triac switch, triggered relative to sampled trailing edges of load current pulses or of applied line voltage pulses. The sensed or detected trailing edge of each load current pulse or of each applied line voltage pulse is, in effect, delayed for a fixed time period, or, in the alternative, a load variable time period, and then applied to trigger the Triac switch, which results in the generation of the next load current pulse. The load variable time period in accordance with the invention is determined by the magnitude of load current through the motor as a function of time. Unlike Nola, the firing angle of the Triac, when varied in accordance with the inven
REFERENCES:
patent: 3525916 (1970-08-01), Chodash
patent: 3539892 (1970-11-01), Lindberg
patent: 3646439 (1972-02-01), Broski
patent: 3875485 (1975-04-01), Hornung
patent: 4243926 (1981-01-01), Phillips
patent: 4266177 (1981-05-01), Nola
"Power Factor Controller", Tech. Support Package, NASA, George C. Marshall Space Flight Center, Summer 1977.
Smith Jr. David
The Scott & Fetzer Company
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