Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1999-12-30
2001-12-11
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S774000, C318S781000
Reexamination Certificate
active
06329783
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electric motors. The present invention relates more specifically to induction motors utilized in applications demanding a range of variable speeds.
2. Discussion of the Related Art
Many applications for electric motors demand variable speeds with a known load on the motor. For example a blower motor in a household heating, ventilation and air-conditioning (HVAC) system will typically be a fractional horsepower motor driving a blower unit or fan blade which represents a known load varying regularly by speed in revolutions per minute.
Inexpensive induction motors are desirably utilized in many applications. These motors are not particularly well adapted for variable speed usage. Rather they are designed to operate efficiently only at one best speed and inefficiencies result when trying to run the motor at other than the designed speed. However, many systems, such as the above HVAC applications, would benefit greatly from having a wider range of motor speeds available.
In the past art, a variable range of speeds from one induction motor was obtained through the use of expensive controllers changing the frequency and voltage of the input to the motor windings or by using a multi-tap motor to attain a number of fixed selectable speeds by mechanical switching between the taps.
Expensive controllers such as these were necessary because, as the input to the motor windings strays farther from sinusoidal, motor efficiency and power factor drop while total harmonic distortion rises, resulting in unacceptable noise, heat, efficiency loss, and motor life.
Thus, known motor controllers utilizing inexpensive switching mechanisms, such as triacs, to control power to the motor windings by “chopping” the sinusoidal waveform input were thought to be of limited use in applications of continuously variable motor speed control.
In an article entitled “A Single Phase Induction Motor Voltage Controller with Improved Performance”, J. D. Law, T. A. Lipo,
IEEE Transactions on Power Electronics
, Vol. PE-1, No. 4, October 1986, pp 240-247; triac control of paired main and auxiliary windings is suggested to run the pairs first in series then in parallel to maintain constant motor speed as the load varies from a low to a high, or fully rated, load. A constant firing delay angle based on empirical study is input to the triac controller using DIP switches. The phase delay is measured with a voltage zero crossing detector and zero current detector. The current hold off angle is then computed and adjusted to make the phase delay and current hold off equal to the predetermined firing delay to maintain constant rated or near-rated speed under the varying load conditions to maintain as closely as possible the full speed the motor was designed for.
The present invention is rather concerned with the opposite effect of obtaining reasonably efficient variable speed for a load of known characteristics with a low cost induction motor and controller system.
SUMMARY OF THE INVENTION
In a variable speed motor application a particular motor speed is called for according to an environmental demand placed on the motor function e.g. moving air or other compressible fluids. For example, a thermostat may determine that more conditioned air needs to be moved in a ventilation system, thus requiring an increase in blower unit rotation and concurrent motor speed.
The controller decodes the speed demand signal and determines if the main windings should operate in series or in parallel configuration. It also determines the firing rate or delay angle, of the triacs to achieve the desired motor speed and greatest motor efficiency at the expected load. The auxiliary windings are preferably left unswitched to provide a constant sinusoidal component to the input power in order to increase power factor, and lower total harmonic distortion in the motor and thereby increase efficiency and reduce noise and heat.
The present invention provides an inexpensive system for obtaining variable speed electric motor operation over known load ranges. The stator main windings of the motor are switch-controlled, preferably by triacs, in an exclusive OR function, to run in series at lower speeds and in parallel at higher speeds. The switch point between parallel and series operation is determined empirically according to the motor usage, or load, and designed into the motor controller in the form of memory such as a look-up-table or by calculable result of an algorithm. Because the load of a blower varies in known relation to the speed of the motor, the slip can be determined and controlled by adjustment of the firing delay angle of the triacs with use of only a zero crossing voltage detector for feedback.
Where fine adjustments are necessary, a tachometer may be added as a motor speed feedback to the controller to ensure continuously variable speed adjustments. Where available, the tachometer may also be used to determine the switch point between series and parallel main winding operations.
REFERENCES:
patent: 3582737 (1971-06-01), Brown et al.
patent: 4443749 (1984-04-01), Douthart et al.
patent: 4467257 (1984-08-01), Douthart et al.
patent: 4477760 (1984-10-01), Kuznetsov
patent: 4779034 (1988-10-01), Shepard, Jr.
patent: 5614799 (1997-03-01), Anderson et al.
patent: 5650707 (1997-07-01), Lipo et al.
patent: 5675230 (1997-10-01), Dunfield
patent: 5680021 (1997-10-01), Hollenbeck
patent: 5867005 (1999-02-01), Brown
patent: 3607162-A1 (1987-09-01), None
patent: 4222431-A1 (1994-01-01), None
Joseph D. Law and Thomas A. Lipo:A Single Phase Induction Motor Voltage Controller with improved Performance, IEEE Transactions on Power Electronics, vol. PE-1, No. 4, 240-247, Oct. 1986.
M.M. Marcos et al.:A Solid-State Speed Controller for Capacitor Motors Driving Ventilation Fans, IEEE Transactions on Industry Applications, vol. 30, No. 3, 656-663, May 1994.
Eichorn Ronald L.
Vrionis Nickolas G.
Gas Research Institute
Pauley Petersen Kinne & Fejer
Ro Bentsu
LandOfFree
Apparatus for continuously variable speed electric motor... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus for continuously variable speed electric motor..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus for continuously variable speed electric motor... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2590687