Electricity: motive power systems – Braking – Automatic and/or with time-delay means
Patent
1986-10-17
1988-05-31
Shoop, Jr., William M.
Electricity: motive power systems
Braking
Automatic and/or with time-delay means
318331, 318434, 318341, 318334, H02P 500
Patent
active
047483890
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a speed control apparatus for a motor which stabilizes the speed of the motor under a varying torque when the motor operates at a low speed.
BACKGROUND ART
The motor speed control apparatus of this type is shown in FIG. 7. As shown, a DC motor 3 and a thyristor 4 are connected to the DC output of a full-wave rectifier 2 which is connected to the terminals of an AC mains supply 1. Across the terminals of the thyristor 4 is connected a resistor 5 and a capacitor 6 in a series circuit so that when the thyristor 4 is in a turned-off state, the capacitor 5 is charged through the resistor 5. Capacitor 6 is discharged through a bidirectional breakdown diode, or diac 7 which is coupled between the gate of thyristor 4 and one end of the capacitor 6 when the voltage across the capacitor 6 reaches a predetermined level.
More specifically, if the anode of thyristor 4, shown at A in FIG. 7, is impressed with a voltage having a substantially sinusoidal waveform as shown in FIG. 8, the voltage of the capacitor 6 developed at point B increases gradually and the bidirectional breakdown diode 7 is turned on when it reaches its breakdown voltage V.sub.B1 at time T.sub.1, turning the thyristor 4 on to control the phase of the motor 3.
In this way, the capacitor 6 is charged with a voltage which is equal to the difference between the mains supply voltage and the counter EMF (electromotive force) generated in the motor 3 so that it operates at a low speed. Under this low speed condition, the prior art apparatus operates to maintain the speed of the motor 3 constant under varying torque.
However, the prior art apparatus suffers from speed instability when the motor is operated at a still lower speed. Assume that the breakdown voltage of the diac 7 is raised V.sub.B1 to to V.sub.B2 as shown in FIG. 8 in an attempt to reduce the motor speed, the rate of voltage increase at point B decreases gradually and the breakdown occurs at time T.sub.2. Thus, the increase in breakdown voltage results in a disproportionally large shift in breakdown time. This implies that the motor would suffer speed instability if an attempt is made to lower the motor operating speed by merely increasing the breakdown voltage.
Furthermore, if the mains supply voltage has increased, the voltage at point A increases, causing the charging time to reduce with a resultant decrease in trigger intervals. Thus, the motor speed increases with an increase in mains supply voltage.
SUMMARY OF THE INVENTION
The motor speed control apparatus of the present invention includes a thyristor connected in a motor current circuit to phase control the motor. When the motor speed is to be reduced by delaying the thyristor trigger timing, the trigger timing occurs at a point in time which is advanced by an amount equal to the interval between the time prior to the occurrence of a speed change and the time following the occurrence of the speed change if the thyristor trigger timing would otherwise be delayed significantly from the zero voltage point of the mains supply. As a result, the motor speed stabilization is ensured even though the speed is low.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a motor speed control apparatus according to an embodiment of the present invention,
FIG. 2 is a waveform diagram showing various waveforms appearing in the apparatus,
FIG. 3 is a circuit diagram of a motor speed control apparatus according to another embodiment of the present invention,
FIG. 4 is a waveform diagram associated with the embodiment of FIG. 3,
FIG. 5 is a circuit diagram of a motor speed control apparaus according to a still further embodiment of the present invention,
FIG. 6 is a waveform diagram associated with the embodiment of FIG. 5,
FIG. 7 is a circuit diagram of a prior art motor speed control apparatus and
FIG. 8 is a waveform diagram of the prior art apparatus.
BEST MODE FOR CARRYING OUT THE INVENTION
The following is a description of an embodiment of the present invention with ref
REFERENCES:
patent: 3629615 (1971-12-01), Gurwicz
patent: 3868554 (1975-02-01), Konrad
patent: 4171532 (1979-10-01), Sloan et al.
patent: 4195254 (1980-03-01), Gurwicz et al.
patent: 4303874 (1981-12-01), Iwai
patent: 4358730 (1982-11-01), Beifus
patent: 4369403 (1983-01-01), Lee
patent: 4417187 (1983-11-01), Bateika
patent: 4503370 (1985-03-01), Cuneo
patent: 4504769 (1985-03-01), Fushiya et al.
patent: 4517502 (1985-05-01), Aschoff et al.
patent: 4524309 (1985-06-01), Hisatake et al.
patent: 4574226 (1986-03-01), Binder
patent: 4658692 (1987-04-01), Bagus
Ip Paul
Matsushita Electric - Industrial Co., Ltd.
Shoop Jr. William M.
LandOfFree
Motor speed control apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Motor speed control apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Motor speed control apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1876493