JPS63181668A - Controller for pulse-width modulation control inverter - Google Patents

Abstract

PURPOSE:To suppress a magnetic noise by varying the frequency of a modulation carrier signal of a PWM inverter proportionally to the amplitude of its output current, and preventing upper and lower limit values from exceeding predetermined ranges. CONSTITUTION:A DC power from a DC power source 2 is converted by a PWM inverter 3 into an AC power of desired voltage and frequency to drive an induction motor 4 at a variable speed. The inverter 3 is pulse-width modulated by a command value from a frequency setter 11 through a calculator 12, etc., to control ON, OFF transistors. In this case, in order to vary a carrier signal input to a comparator 13 corresponding to the output current of the inverter 3, carrier frequency varying means which has an upper/lower limiter 23, a voltage/frequency converter 24, an integrator 25, etc., is provided. Thus, the switching losses of the transistors can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a control device for a pulse width modulation control inverter that drives an AC motor at variable speed.

[Conventional technology]

FIG. 3 is a circuit diagram showing a conventional example of driving an induction motor at variable speed using a pulse width modulation control inverter.

In FIG. 3, a DC power supply 2 is converted into AC power with a desired voltage and frequency by a DC chamber cover, and a pulse width modulation control inverter (hereinafter abbreviated as an inverter) 3 made up of transistors. Therefore, the induction motor 4 can be driven at variable speed with this output AC. Current detectors 5A and 5B are provided between the PWM inverter 3 and the induction motor 4.

5C is provided, and if it is detected that the output current of the PWM inverter 3 has reached an overcurrent level, a protective operation such as stopping the operation of the PWM inverter 3 is normally performed.

Control for outputting alternating current of desired voltage and frequency from the PWM inverter 3 is performed as follows. That is, the frequency setter 11 outputs a command value of the frequency (and thus the speed of the induction motor 4) to be output by the PWM inverter 3 to the voltage command value calculation circuit 12. This voltage command value calculation circuit 1
2 is comprised of, for example, an acceleration/deceleration calculation circuit, a voltage frequency conversion circuit, a function generation circuit, etc., and outputs a sine wave signal with an amplitude and frequency corresponding to the input frequency command value as a voltage command value.

The carrier signal generation circuit, which is the encoder 5, outputs a carrier signal of a predetermined frequency, so the comparators 13A and 1
3B and 13C input the above-mentioned voltage command value and carrier signal, and by comparing the magnitude relationship between the two, pulse width modulated pulse train signals are sent to the pace drive circuits 14A, 14B, 1.
Appear to 4C. The output from each of these base drive circuits is such that each transistor constituting the 2wM inverter 3 is sequentially controlled on and off.
The inverter 3 outputs an alternating current of voltage and frequency corresponding to the frequency command value set by the frequency setter 11 to drive the induction motor 4.

[Problem that the invention seeks to solve]

By the way, the frequency of the carrier signal for pulse width modulation used in the above-mentioned PWM inverter 3 is often on the order of several kilohertz, and when driving the induction motor 4, magnetic noise at a frequency corresponding to the carrier frequency command is induced. It occurs in the electric motor 4. Since this magnetic noise is in a frequency band that is jarring to humans, it is desired to suppress this noise. To achieve this, (a) it is possible to raise the carrier frequency to a higher value than the human audible frequency, but as the carrier frequency increases, the semiconductor switching elements (transistors in FIG. As the frequency of on/off operations increases, switching loss increases (generally, switching loss increases in proportion to the carrier frequency). Therefore, in this type of system, it is necessary to increase the capacity of transistors, increase the number of cooling bodies, etc.
This method has drawbacks such as increasing the cost and external dimensions of the device and reducing efficiency.

(b) Contrary to the above, it is also possible to lower the carrier frequency to a frequency so low that it is not audible to humans, but in this case, the output current ripple of the PWM inverter increases, and the transistor as a switching element This current ripple causes disadvantages such as large torque ripple of the induction motor 4 and an adverse effect on the load.

Therefore, an object of the present invention is to suppress magnetic noise that is unpleasant to humans without increasing the loss generated by the switching elements of the PWM inverter that drives the AC motor at variable speed, and without increasing the output current ripple. It is in.

[Means for solving problems]

In order to achieve the above object, the control device of the present invention has the following features:
Pulse width modulation control that drives an AC motor at variable speed using AC output based on a pulse width modulation signal obtained by comparing a voltage command signal obtained from the motor speed setting signal and a carrier signal that determines the modulation frequency. The inverter includes means for changing the frequency of the modulating carrier signal in accordance with the magnitude of the current output by the pulse width modulation control inverter, and the carrier frequency changing means is for controlling the pulse width modulation control inverter. Assume that the carrier frequency increases as the output current value increases.

[For production]

This invention lowers the frequency of a modulating carrier signal of a PWM inverter that drives an AC motor at variable speed when the output current of the PWM inverter is small, and gradually increases the carrier frequency as the output current increases. and suppress the magnetic noise generated by the AC motor.

〔Example〕 FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In FIG. 1, DC power from a DC power supply 2 is input to a PWM inverter 3 that uses transistors as switching elements, and the P''wM inverter 3 outputs AC power of a desired voltage and frequency to a Nyopa induction motor. 4 is driven at a variable speed in the same manner as in the conventional circuit described in FIG. voltage command value calculation circuit 12) comparators 13A and 13B.

13C and base drive circuits 14A, 14B, 14C
This is also the same as in the conventional circuit described in FIG. 3.

In this invention, the above-mentioned comparator 13A.

The carrier signal input to 13B and 130 is
In order to change the output current value of the M inverter 3 in accordance with the output current value, the output current detected by the current detectors 5A, 5B, and 5C is controlled by a full-wave rectifier circuit 21, a smoothing circuit 22), an upper and lower limiter 23, a voltage/frequency A carrier frequency changing means composed of a converting circuit (hereinafter abbreviated as a /F converting circuit) 24 and an integrating circuit 25 changes the frequency of the carrier signal outputted by the converting circuit in accordance with the output current. .

That is, a voltage signal proportional to the output current of the concave inverter 3 is created by the full-wave rectifier circuit 21 and the smoothing circuit 22, and the V7. By inputting it to the conversion circuit 24, a pulse signal with a frequency proportional to the output current is obtained, and this pulse signal is converted into a triangular wave carrier signal by the integrating circuit 25.

Here, the upper and lower limiter 23 determines the lower limit value so that the frequency of the carrier signal is not too low and the current ripple is not too strong, and also sets the lower limit value so that the frequency of the carrier signal is not too high so that the transistor does not perform switching operation. ◇Figure 2 is a graph showing the characteristics of the carrier frequency changing means explained in Figure 1, and the horizontal axis is the Taki inverter 3.
The absolute value of the output current is shown on the vertical axis, and the carrier frequency is shown on the vertical axis, but due to the action of the upper and lower limiters,
The upper and lower limits of the carrier frequency are set so as not to fall outside a predetermined range.

〔Effect of the invention〕

According to this invention, by pulse width modulation control, the frequency of the modulating carrier signal of the PWM inverter that outputs AC power of a desired voltage and frequency to drive the AC motor at variable speed is controlled by the PWM inverter. The output current is changed in proportion to the size of the output current, and the upper and lower limits are kept from exceeding a predetermined range. Therefore, when the load current of the AC motor is small, the carrier frequency is lowered and the output current is The magnetic noise generated by the electric motor can avoid frequency bands that are uncomfortable for humans, thereby exhibiting the effect of suppressing magnetic noise during driving.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a graph showing the characteristics of the carrier frequency changing means explained in FIG. FIG. 3 is a circuit diagram showing a conventional example of driving an induction motor at variable speed using a pulse width modulation control inverter. 2... DC power supply, 3... Inverter, 4... Induction motor, 5A, 5B, 50...
...Current detector, 11...Frequency setter,
12... Voltage command value calculation circuit, 13A, 13B
, 13C...Comparator, 14A. 14B, 14C---Base drive circuit, 15...
... Carrier signal generation circuit, 21 ... Full wave rectifier circuit, 22 ... Q- smoothing circuit, 23 ... Upper and lower limit limiter, 24 ...
....eta conversion circuit, 25...integration circuit.

Claims (1)

[Claims] 1) An AC motor is driven by an AC output based on a pulse width modulation signal obtained by comparing a voltage command signal obtained from a speed setting signal of the motor with a carrier signal that determines a modulation frequency. A pulse width modulation control inverter capable of being driven at high speed, comprising means for changing the frequency of the modulation carrier signal in accordance with the magnitude of the current output by the pulse width modulation control inverter. Control device for modulation control inverter. 2) In the control device according to claim 1, the carrier frequency changing means is a means for increasing the carrier frequency as the output current value of the pulse width modulation control inverter increases. A control device for a pulse width modulation controlled inverter.