JPS5895983A - Protecting device for pwm inverter - Google Patents

Abstract

PURPOSE:To protect an inverter by detecting the frequency of the output signal of a comparator which compares the current command with an actual value and stopping the inverter when the frequency exceeds the prescribed value. CONSTITUTION:When the switching frequency increases at the abnormal time, the output F0 of an F/V converter 212 becomes large. When the output F0 becomes larger than the output of a setter 213, and hence when the switching frequency of the transistor exceeds the upper limit of the switching frequency of the transistor, the output SP of the inverter 212 becomes H level, the signal SP is outputted as the abnormality, thereby performing protecting operation. The inverter is protected by detecting the increase in the switching frequency at the abnormal time as described above.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection device for a current comparison type PWM inverter.

FIG. 1 shows an example of a conventionally used control device for a current comparison type PWM inverter.

In FIG. 1, a forward converter 1 performs full-wave rectification of an AC power source,
This is smoothed by capacitor 7 to create a DC voltage with less pulsation. This DC voltage causes the transistors 11 to 16 and the diode 21 connected in parallel to the transistors 11 to 16 to
-26 PvJM inverters IN drive the AC motor 9 by controlling the output voltage and frequency by controlling the conduction of the transistors. Control of the inverter IN is performed by a trigger circuit 40.

Details of the trigger circuit 40 are shown in FIG. Inverter I
The current command i/ which commands the output current (sine wave) of N and the output of the current detector 8 are output by the current detection circuit 44U. is added to the comparator 210 having hysteresis characteristics, and depending on the magnitude relationship, a transistor 110 on/off signal U as shown in FIG. 2 is obtained. The on/off signal UN of the transistor 12 is obtained by taking the inverted signal of Up at the front stage of the inverting circuit 230. Note that FIG. 1L-3 is for one phase, and on/off signals for the transistors 13 to 16 are similarly obtained for other phases.

The operation for one phase will be explained below. In comparator 210, inverter current command i and inverter output current 1
Compare °1, current command! 1, the inverter output type current (detection signal) i is the hysteresis width Δ of the comparator 210.
If the upper limit of H is exceeded, the output signal of the comparator 210 turns off the inversion 1 transistor 11, thereby controlling the current l in a decreasing direction. The slope of the current 10 is determined by the circuit impedance. Conversely, when the decreased inverter output current 10 exceeds the lower limit of the hysteresis width ΔH, the output signal of the comparator 210 is inverted again, turning on the transistor 11, and causing the current 1. increases. As is clear from the above description, the current deviation with respect to the current command 11 is determined by the hysteresis width ΔH of the comparator 210, and the switching frequency is determined by the circuit impedance and the hysteresis width ΔH. In order to reduce the deviation,
Although it is necessary to reduce the hysteresis width ΔH, when the hysteresis width ΔH is reduced, the switching frequency of the main circuit transistors 11 to 16 of the PWM inverter IN increases. Since the transistor has an upper limit on the switching frequency, it is necessary to determine the hysteresis width ΔH of the comparator 210 in consideration of the deviation and the switching frequency.

However, even if the hysteresis width ΔH is determined as described above, when an abnormality occurs, such as a layer short circuit in the motor or a short circuit between windings at ground, or when Cake.

If a short-circuit accident occurs in the circuit, the circuit impedance decreases and the slope of the current 10 increases, so the switching frequency of the transistor increases.

If winding burnout or temperature rise does not occur, it is difficult to detect these abnormal phenomena, and the switching frequency of the transistor may exceed its upper limit, causing the transistor to be destroyed.

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, it is an object of the present invention to provide a protection device for a PWM inverter that detects an increase in switching frequency during an abnormality and protects the switching frequency.

The feature of the present invention is that the frequency of the output signal of the comparator that compares the current command and the actual value is detected, and when the frequency exceeds a predetermined value (upper limit), the output is stopped. It is in.

The present invention will be explained below using a specific example. FIG. 3 shows an example of a control circuit implementing the present invention, and FIG. 4 is a diagram illustrating its operating principle. P shown in Figure 1
As described above, each of the transistors 11 to 16 of the WM inverter IN is controlled by applying the trigger signal shown in FIG. 4. The output of the inverting circuit 230 can be applied to a frequency/voltage (F/V) converter 212 to determine the switching frequency of the transistor.

When the switching frequency increases during an abnormality, the output Fo of the F/V converter 212 increases. By comparing the output Fo of this F/V converter 212 with the output FM of the setting device 213 which sets the upper limit value of the switching frequency, it is possible to know whether or not the transistors in the inverter main circuit are operating at the allowable switching frequency. be able to.

Suppose that something goes wrong as shown in Figure 4! +, when the output Fo of the F/V converter 212 becomes larger than the setting device 213, that is, when the switching frequency of the transistor exceeds the upper limit of the switching frequency of the transistor, the output S PI of the inverting circuit 212 /iH level, outputs signal SP indicating abnormality and performs protection operation.

That is, the trigger signal amplifier 43 in FIG. 1 has a suppressing function, such as blocking the control signal by the output signal SP of the ratio converter 211. Alternatively, protection can be achieved by opening the AC side circuit breaker 31 -0 Note that a delay may be provided as necessary to prevent errors in the F/v converter 212.

Similarly, a function of determining an abnormality by outputting an abnormal signal to the comparator 212 for a predetermined period of time or longer is also provided.

As explained above, according to the present invention, the switching frequency becomes high during an abnormality, thereby preventing burnout of the control element constituting the inverter.

Although transistors are used in the embodiments described above, it is clear that a GTO thyristor or a general thyristor can be used as the semiconductor conduction control element.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of a control device for a PWM inverter, FIG. 2 is a waveform diagram explaining the operating principle of a conventional control method, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIG. The figure is a waveform diagram for explaining the operation of FIG. 3.

Claims (1)

[Claims] 1. PWM inverter that drives an AC motor, the PWM
A current detector that detects the output current of the inverter, the PW
Comparison means for comparing the command signal of the output current of the M inverter with the output signal of the current detector and outputting an output signal for conducting conduction control of the PWM inverter, and frequency detection for detecting the frequency of the output signal of the comparison means. and a circuit;
When the output frequency of the comparison means exceeds a predetermined value, the P
A protection device for a PWM inverter, characterized in that the WM inverter is stopped.