JPH0467780A - Output voltage control method of pwm inverter - Google Patents

Abstract

PURPOSE:To eliminate a change in voltage caused by changing a carrier pulse number from low speed to high speed by correcting the output voltage control signal of an inverter through a corrected value at the time of changing the pulse number and by correcting the change of the fundamental wave component of a PWM wave, which is caused by changing the pulse number, according to the corrected value. CONSTITUTION:In order that the output frequency is controlled from low frequency to high frequency, a carrier pulse number is changed according to a frequency domain and, at the same time, an output voltage control signal is corrected by a corrected value so that the change of an output voltage accompanying the change of the carrier pulse number is corrected. Therefore, the stepwise change of the output voltage at the time of changing the pulse number is eliminated so that it is possible to eliminate not only the uneven rotation of a motor 3 and the generation of an overcurrent in a power inverter part 2 but also the change in the ratio of frequency to voltage in each operating stage.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a PWM inverter, and more particularly to an output voltage control method.

B0 Summary of the Invention The present invention provides a PWM inverter that switches the number of pulses of a carrier wave according to the output frequency range, and corrects the output voltage control signal according to the number of pulses when switching the number of pulses, thereby improving the output voltage by switching the number of pulses. It is the absence of change.

C0 Prior Art A PWM inverter is configured, for example, in an open loop control system as shown in FIG. AC power with a PWM waveform is obtained from the DC power output of the rectifier 1 to the inverse converter 2, thereby enabling variable speed operation of the load such as the induction motor 3. The control device converts the programmed speed command from the speed setting section 4 into a frequency command with acceleration/deceleration adjustment through the cushion section 5, obtains an output frequency command from the frequency control section 6, and performs proportional and torque boost calculations, etc. The applied voltage command V is obtained from the voltage control section 7. Then, the PWM calculation unit 8 calculates the time to advance the vector and the zero vector time from the pulse pattern table such that the magnetic flux vector draws a circular orbit, the output voltage command V, and the output frequency command f, and calculates the time to advance the vector and the zero vector time, and calculate the time to advance the vector and the zero vector time. 10 and then the inverse conversion unit 2 using the PWM waveform.
Main switch control. The CPU II performs various controls such as various constant commands and operation status display control.

Here, in a PWM inverter, to increase the output frequency, P
In order to prevent the WM switching frequency from exceeding the limit switching frequency of the main switch itself of the inverse converter 2, switching is performed such that the number of pulses of the carrier wave is lowered as the frequency becomes higher. For example, the number of pulses is set to 27 pulses per cycle in low frequency operation with an output of 20 to 40 Hz, and for an output of 270 to 40 Hz.
At 360 Hz, there are three pulses, and the number of pulses is switched in multiple stages depending on each output frequency stage.

D9 Problems to be Solved by the Invention In conventional PWM inverters, the number of carrier wave pulses is switched depending on the output frequency step, so the voltage command V is applied to the fundamental wave component of the PWM waveform depending on the output frequency range.
A deviation occurs in the proportional relationship between This deviation is caused by a stepwise change in the fundamental wave voltage when switching the number of carrier wave pulses.
This may cause uneven rotation or overcurrent. In addition, during high-frequency operation, the rotational speed accuracy may decrease due to a change in the ratio of output frequency to voltage.

An object of the present invention is to provide an output voltage control method that eliminates voltage changes caused by switching the number of carrier wave pulses from low speed to high speed.

E9 Means and Effects for Solving Problems In order to achieve the above-mentioned object, the present invention provides a PWM inverter that switches the number of pulses of a carrier wave according to the range of output frequency to be controlled from low speed to high speed, and provides a method for switching the number of pulses. A correction value that eliminates the accompanying change in the output voltage of the inverter is obtained, and when the number of pulses is switched, the output voltage control signal of the inverter is corrected with the correction value, so that the change in the fundamental wave component of the PWM wave due to the change of the number of pulses is corrected. By correcting with the correction value, changes in the fundamental wave component are suppressed, that is, changes in the output voltage are eliminated.

F. Embodiment FIG. 1 is a main circuit diagram showing an embodiment of the present invention. C
The PUII takes in the frequency control signal that becomes the output of the frequency control unit 6, and determines the output frequency range from this control signal to determine the pulse pattern to be given to the PWM calculation unit 8.
One of the pulses to three pulses is selected, and a correction value of the voltage control signal corresponding to the number of pulses of the carrier wave is given to the voltage control section 7.

This correction value is a coefficient calculated in advance to eliminate changes in the output voltage due to changes in the number of pulses, and is multiplied by the output voltage control signal of the voltage control section 7 to correct the output voltage control signal. This correction value is as shown in the table below, for example.

(Left below) With the above configuration, the output frequency is controlled from low frequency to high frequency, but the number of carrier wave pulses is switched depending on the frequency range, and at the same time, the output voltage control signal is corrected by the correction value. The change in output voltage caused by switching the number of carrier wave pulses is corrected. Therefore, stepwise changes in the output voltage when switching the number of pulses are eliminated, uneven rotation of the motor 3 and occurrence of overcurrent in the inverse converter 2 can be eliminated, and changes in the ratio of frequency and voltage at each operation stage can be eliminated. It can be eliminated.

Note that in the embodiment, the voltage correction is not limited to being performed by software using the CPU II, but may also be a hardware configuration in which the correction value is switched in conjunction with switching of the pulse pattern.

G0 Effects of the Invention As described above, according to the present invention, the output voltage control signal is corrected according to the switching of the number of pulses of the carrier wave, so that the output voltage change due to the switching of the number of pulses can also be controlled from low to high speeds. This eliminates rotational unevenness and overcurrent during switching, and improves rotational speed accuracy for open-loop control.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a main circuit diagram showing an embodiment of the present invention, and FIG. 2 is a configuration diagram of a PWM inverter. 2... Inverse conversion section, 4... Speed setting section, 5... Cushion section, 6... Frequency control section, 7... Voltage control section, 8... PWM calculation section, 11...・CPU0 Figure 1 Gusomagnetic In, second part diarytic Figure 6-1: l Frequency? FIIIm part '7--a pressure-8-PWM friction part 1-CPU

Claims (1)

[Claims] (1) In a PWM inverter that switches the number of carrier wave pulses depending on the output frequency range to be controlled from low speed to high speed, a correction value that eliminates the change in the output voltage of the inverter due to the switching of the number of pulses is calculated, and the number of pulses is A method for controlling an output voltage of a PWM inverter, comprising correcting an output voltage control signal of the inverter using the correction value when switching.