Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1987-01-29
1988-01-12
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Primary circuit control
363 56, H02P 540
Patent
active
047194006
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a motor control apparatus and, more particularly, to a motor control apparatus capable of preventing the dead zone of a pulse-width modulating circuit from causing a short circuit in an inverter circuit.
A control circuit for an AC or DC motor generally makes use of an arrangement, namely a pulse-width modulation drive circuit, for pulse-width modulating an AC or DC current command to drive an inverter circuit (a transistorized amplifier circuit) and for applying a pulse-width modulated sinusoidal or direct current to a motor. It is sufficient if the arrangement is capable of withstanding the high voltage required solely by the final stage of the inverter circuit, the circuit construction is thus simple and control is performed comparatively well.
In the case of direct current the inverter circuit has four transistors. For three-phase AC drive the inverter circuit has two transistors in each phase, for a total of six transistors, with control being such that when one of the transistors in each phase turns on, the other turns off. A transistor drive signal (a pulse-width modulated signal) is provided with dead time in such a manner that the two transistors in each phase will not be turned on simultaneously at the time of the changeover, that is, in such a manner that the DC power supply will not develop a short circuit. This dead time is set by providing a pulse-width modulating circuit with a dead zone. However, since the pulse-width modulated signal is provided with the dead time in this manner, the pulse width of the modulated signal is reduced and distortion is produced in the pulse-width modulated sinusoidal current or in the direct current. This results in certain disadvantages, namely greater excitation noise and torque irregularity, with the commanded torque originally not being obtained. In particular, in the case of operation at low speed, the sinusoidal current command takes on a low frequency, and pulse width is small with a direct current command of a low voltage. Accordingly, the influence of the dead zone is of such proportions as cannot be ignored.
As means for compensating for the dead zone there has been proposed a system in which the output voltage of the inverter circuit is fed back to a pre-stage of the pulse-width modulating circuit to raise the gain using this feedback loop. However, with an arrangement in which an arithmetic circuit such as a microprocessor is used as the motor control circuit, it is necessary to provide separate analog circuits for these feedback loops. The processing of these analog circuits cannot be performed by the arithmetic circuit, thereby resulting in a complicated construction and high cost.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a motor control circuit which is capable of compensating for the influence of a dead zone in a pulse-width modulating circuit by using an arithmetic circuit.
Where a PWM circuit having a dead zone and a transistorized amplifier is used for controlling a motor, the present invention is arranged such that an arithmetic circuit for generating a current command also generates a compensating signal for offsetting the dead zone of the PWM circuit. This compensation signal is added to the current command and the result applied to a holding circuit.
According to the invention, loss of motor torque due to the influence of the PWM circuit dead zone can be prevented, and the elimination of loss ascribable to the dead zone can be carried out by processing performed by the arithmetic circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a first embodiment of the present invention;
FIG. 2 is a block diagram of a principal portion of the arrangement of FIG. 1;
FIG. 3 illustrates waveforms associated with various portions of the arrangement of FIG. 2;
FIG. 4 illustrates an input/output characteristic of a PWM circuit;
FIG. 5 is a block diagram of a system for eliminating dead zone;
FIG. 6 is a block diagram of dead zone el
REFERENCES:
patent: 3944907 (1976-03-01), Weit
patent: 4051418 (1977-09-01), O'Berto et al.
patent: 4451771 (1984-05-01), Nagase et al.
patent: 4456868 (1984-06-01), Yamamura et al.
patent: 4458192 (1984-07-01), Sakamoto et al.
patent: 4459534 (1984-07-01), Nagase et al.
patent: 4477762 (1984-10-01), Karakake et al.
patent: 4481457 (1984-11-01), Zach et al.
Kurakake Mitsuo
Sakamoto Keiji
Fanuc Ltd.
Smith Jr. David
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