JPS6028798A - Operating method of inverter device - Google Patents

Abstract

PURPOSE:To perform stable operation even at the low speed drive time of an AC motor by reducing a carrier frequency in a low frequency drive range of a modulated wave frequency with the carrier frequency variable. CONSTITUTION:When the frequency of a modulated wave (a) becomes low frequency drive range, a signal is delivered from a modulation wave generator 5 to reduce the output frequency of a carrier wave generator 6 by the signal, thereby reducing the frequency of the carrier (b) to suppress the variation in the output of an inverter. A comparator 7 outputs while the carrier (b) generated from the generator 6 is larger than the modulated wave (a) outputted from the generator 5, and delivers a pulse train as a firing signal.

Description

[Detailed description of the invention] The present invention relates to a method of operating an inverter device.

FIG. 1 shows a conventional example of an inverter device that drives an AC motor at variable speed. In the figure, 1 is a DC power supply, and 2 is an inverter main body of a variable pulse width type, which is made up of six transistors Tl-T6 connected in a bridge, as shown in FIG. D1 to D6 are diodes. 3 is an induction motor, and 4 is a control device for the inverter main body 2. This control device 4 includes a modulated wave generation circuit 5, a carrier wave generation circuit 6, and a comparison circuit 7 that compares the outputs of both and sends a firing signal to each transistor of the inverter main body 2. This comparison circuit 7 compares the constant frequency t generated by the carrier wave generation circuit 6.
While the U transmission wave is larger than the variable frequency modulated wave a output by the modulated wave generation circuit 5, the pulse train C shown in FIG. 2 is output.
is sent as the above-mentioned ignition signal. The pulse train d in the same figure is
When the pulse train C is the first phase pulse train, the pulse train of the other phase shifted by 120 degrees with respect to the first phase is shown, and the waveform e in the figure is the interphase voltage waveform, and f is the fundamental wave component of the interphase voltage waveform e. It shows.

The inverter main body 2 receives the pulse-modulated firing signal from the comparator circuit 7 as described above, and receives the firing signal from each of the transistors T1 to T1.
T6 repeats on and off in a predetermined order, converts DC power into frequency-controlled AC power, and supplies power to the induction motor 3. However, in order to prevent a DC short circuit in the inverter main body 2, it is usually shown in Fig. 4. As shown, the interlock time t is set. In the figure, g indicates the pulse train supplied to the base of the transistor T1 of the first phase, and h indicates the pulse train supplied to the base of the transistor T2. In this case, even if the load power factor (motor power factor) is 00%, For example, the output potential of the first phase of the inverter (Fig. 3 (potential 11) is the output potential of the transistor T1
It becomes a positive potential during the on period of the transistor T2, and becomes a negative potential during the on period of the transistor T2. However, when the load power factor is 0%, the positive potential period of the output potential changes as shown in i in the figure, and the negative potential period changes as shown in j in the figure. As is clear from the figure, this is because the phase voltage of the first phase can be seen as a modulated wave.
On the other hand, when the load power factor is 0%, the diode D1 is conductive during the interlock time t in the range of -90 degrees to +90 degrees, and the output potential becomes positive, and in the range of +90 degrees to 270 degrees. In this region, the diode D2 becomes conductive and the output potential becomes a negative potential.

In this way, in conventional devices, the carrier wave frequency is fixed, so when the modulating wave frequency becomes low, the inverter output that is fed to the induction motor is greatly affected by the above interlock time due to changes in the load power factor. This has the disadvantage that stable operation of the induction motor becomes difficult.

This invention was made to eliminate the above-mentioned conventional drawbacks, and by making the carrier wave frequency variable and lowering the carrier wave frequency in the low frequency drive region where the modulation wave frequency is low, compared to the conventional one, It is an object of the present invention to provide a method of operating an inverter device that suppresses voltage fluctuations in the inverter output caused by the interlock time and facilitates stable operation even when an AC motor is driven at low speed.

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 5, the frequency of the modulated wave a is the same as that in FIG. 4, but the frequency of the carrier wave A is 1
2 shows the output potential when the voltage is set to /2. It can be seen from a comparison with FIG. 4 that when the frequency of the carrier wave is lowered, the influence of the interlock time t on the inverter output becomes smaller. Therefore, when the frequency of the modulated wave a falls into a low frequency driving region, the modulated wave generating circuit 5 is configured to send out a signal, and the output frequency of the carrier wave generating circuit 6 is lowered by this signal, as shown in FIG. This lowers the frequency of the carrier wave and suppresses fluctuations in the inverter output.

When carrying out the present invention, the conventional modulated wave generation circuit is added with a function of generating the above signal when the modulated wave frequency falls to the above low frequency region, and the carrier wave generation circuit 6 is added with the function of generating the above signal at the above low frequency range. Since it is only necessary to add a function to switch and reduce the size according to a signal, the cost is low.

Although the above embodiment uses transistors to configure the inverter main body, the present invention can also be implemented in an inverter device configured using other switching elements such as thyristors and gate turn-off thyristors to obtain similar effects. can be obtained.

As described above, according to the present invention, when the modulation wave frequency becomes low frequency, the carrier wave frequency is also made small to reduce the influence of the interlock time for preventing DC short circuit on the inverter output. , voltage fluctuations in the inverter output caused by changes in the load power factor and the interlock time can be reduced compared to conventional methods.
The AC motor can be stably driven even when the AC motor is driven at low speed.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional pulse modulation type inverter device, Fig. 2 is a waveform diagram of each part in the above inverter device, Fig. 3 is a circuit diagram of the inverter main body of the above inverter device, and Fig. 4 is the above impark FIG. 5 is a waveform diagram for explaining the problems of the device, FIG. 5 is a waveform diagram for explaining the operating method of the inverter device according to the embodiment of the present invention, and FIG. 6 is a block diagram of the inverter device implementing the present invention. be. In the figure, 2-inverter main body, 4-control device, 5-
Modulated wave generation circuit, 6 - = (general transmission wave generation circuit, 7 - comparison circuit. In the figures, the same symbols indicate the same or corresponding parts. Agent Masuo Oiwa Figure 3 Figure 4

Claims (1)

[Claims] An inverter device using a pulse modulation method for driving an AC motor at a variable speed, characterized in that the frequency of a carrier wave that is compared with a modulated wave is made variable, and the frequency is made smaller when the modulated wave is in a low frequency region. How to drive.