Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1986-12-15
1989-01-24
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Primary circuit control
318806, 318811, H02P 540
Patent
active
048003271
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
BACKGROUND OF THE INVENTION
This invention relates to a three-phase induction motor control method in which the output is controlled so as to be constant by sensing fluctuations in power supply voltage.
Recent control of three-phase induction motors is in many cases performed through either of the following methods: supply by an inverter, or stator current and generating a torque equivalent to that of a shunt DC machine.
Since the output of an induction motor generally varies in proportion to the square of the voltage impressed upon the motor, the output will fluctuate when the AC input voltage undergoes a large variation, irrespective of whether the inverter method or vector control method is employed. Countermeasures devised in order to prevent such a fluctuation in output include varying inverter control pulse widths in dependence upon the AC input voltage in the inverter method and varying the amount of maximum slip in dependence upon the AC input voltage in the vector control method. Nevertheless, satisfactory results are not obtained in terms of holding the output of the induction motor constant.
SUMMARY OF THE INVENTION
The present invention has been devised to solve the aforementioned problem of the prior art and an object is to provide a three-phase induction motor control method in which the output is controlled so as to be constant, even if there is a fluctuation in power supply voltage, by sensing a DC voltage, which is converted from an AC input, as the voltage of a DC link section, and by varying a flux command to deal with this voltage.
The three-phase induction motor control method of the present invention includes a step for sensing voltage at a DC link section which converts an AC input voltage into a direct current; a step, implemented when the input voltage is low, for increasing a flux command above a base velocity and raising the amplitude of a signal, output by a pulse width modulation (PWM) circuit, with respect to a triangular wave, thereby raising motor terminal voltage, and a step, implemented when the input voltage is high, for decreasing the flux command from a region below the base velocity and reducing the amplitude of the PWM circuit signal with respect to the triangular wave, thereby lowering motor terminal voltage.
Thus, in accordance with the present invention, the output of the induction motor is controlled so as to be constant in an ordinary vector control-type control circuit, even when there is a variation in the AC input voltage. Further, according to the invention, the output of the induction motor can be held constant even if the input voltage fluctuates by .+-.10% of the rated input voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a control block diagram illustrating an embodiment of the present invention as applied to vector control of a three-phase induction motor,
FIG. 2 is an equivalent circuit of one phase of a three-phase induction motor,
FIG. 3 is a vector diagram showing the relationship of elements of a three-phase induction motor; and
FIGS. 4 through 9 are views for explaining various characteristics of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will now be described in detail with reference to the drawings.
FIG. 1 is a control block diagram in which an embodiment of the present invention is applied to vector control of a three-phase induction motor, where the pulse-width is controlled by a PWM circuit.
Before describing the arrangement shown in FIG. 1, let us refer to FIGS. 2 through 6 to describe the principles on which the present invention is premised.
FIG. 2 is an equivalent circuit of one phase of a three-phase induction motor including primary resistance R.sub.1, primary reactance X.sub.1, terminal voltage V.sub.1 and secondary resistance divided by slip R.sub.2 /S., FIG. 3 is a vector diagram showing the relationship of various elements of a three-phase induction motor, and FIGS. 4 and 5 are views for explaining characteristics of the present inventio
REFERENCES:
patent: 4047083 (1977-09-01), Plunkett
patent: 4480299 (1984-10-01), Muto et al.
patent: 4587605 (1986-05-01), Kouyama et al.
patent: 4672287 (1987-06-01), Fujioka
patent: 4680525 (1987-07-01), Kobari et al.
Fujioka Yoshiki
Kouno Shin-ichi
Fanuc Ltd
Smith Jr. David
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