JPS605786A - Inverter for electric motor drive - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an inverter for driving an electric motor,
More specifically, the present invention relates to an inverter for driving a motor suitable for performing regenerative braking.

[Background of the invention]

A square wave inverter is used as a conventional motor drive inverter. When the electric motor is suddenly decelerated using such a conventional electric motor drive inverter, the ratio of the voltage applied to the electric motor during regeneration and the frequency changes greatly in a low frequency region, resulting in an excessive voltage.

At this time, the windings of the motor were short-circuited by the transistor and diode pair, and the transistor was sometimes destroyed. In order to avoid such accidents, conventional technology requires measures such as lengthening the deceleration time, detecting the regenerative current at several tens of points below the current and temporarily stopping the deceleration, which places restrictions on customer use. The drawback was that it was necessary.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an inverter for driving a motor in which power elements such as transistors are not destroyed even when the motor is suddenly decelerated, by eliminating the drawbacks of the prior art described above.

[Summary of the invention]

The inverter for driving a motor of the present invention includes a rectifier, a smoothing capacitor, a plurality of switching means, a regenerative current detection means, and a control circuit for driving the plurality of switching means, and the control circuit outputs an output from the regenerative current detector. The present invention is characterized by comprising means for receiving a regenerative current detection signal and switching control of the plurality of switching means from square wave mode to PWM mode.

This invention was born from the following idea.

In other words, at low frequencies, because the terminal voltage of the smoothing capacitor is low, it was initially thought that a large amount of energy would be returned, but in reality, almost no energy was returned, and the energy was dissipated through the transistors in the inverter. . The reason for this is that the voltage increase of the smoothing capacitor becomes relatively large at low frequencies, making it difficult for energy to return from the motor side. For this reason, if variable voltage square wave inverter control is stopped during this period and PWM operation is used, the voltage of the smoothing capacitor will always remain constant and high, so even if the voltage rise is the same, the relative Because only the voltage does not become abnormally high, even at low frequencies,
There is less chance of shorting the windings in the transistor.

To give an example, the voltage of the smoothing capacitor Vdc=300
V, when output frequency f=60HzO, regenerative voltage V=60
V, the ratio vac/f of the smoothing capacitor voltage V(10 and the output frequency f is 300/60 to (300+(
SO)/(So, which is an increase of +20% at most. However, when the output frequency f = 50Hz, the capacitor voltage Vdc = 5 oo/2 = 15ov, and vac/f is 150/ 30 to (150+60)/3
0, resulting in an increase of +40%. However, in reality, the motor does not generate electricity until such an overvoltage occurs, and the motor's windings and transistors consume energy.
It is operated while maintaining the regular ■/f+20%.

At low frequencies, the voltage Vac of the smoothing capacitor is originally low, so during sudden deceleration, a relatively large voltage is applied to a(v)'
be accepted as such.

[Embodiments of the invention]

The present invention will be described in more detail below with reference to embodiments shown in the accompanying drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

In the figure, 1 is a main circuit of a conventionally used inverter for driving a motor, 2 is a control circuit improved from the conventional technology according to the present invention, and 3 is an electric motor. The main circuit 1 of the inverter for driving a motor includes an AC power supply 10
1, forward converter 102, smoothing capacitor 103, transistors 104 to 109, and diodes 110 to 11
5 and a junction 116. The control circuit 2 includes a comparator 201, a reference voltage 202, a NAND circuit 203, an oscillator 204, an error amplifier 205, an adder/subtractor 206, a speed setter 207, a hexadecimal ring counter 208, and a modulator 209. It is composed of.

The operation of the motor drive inverter having the above configuration will be described next. That is, when a regenerative current exceeding a predetermined value flows into the junk 116, the comparator 20
1 operates and its output becomes H. Therefore, the oscillator 20
4 output pulses pass through a NAND circuit 203 and are input to a modulator 209 composed of an AND circuit. The modulator 209 connects the output of the hexadecimal ring counter 208 and the oscillator 2.
04 output pulses are mixed to form a so-called PWM pulse. This PWM signal is applied to the bases of transistors 104-109 to control conduction of transistors 104-109. That is, the moment the regenerative current is detected, the square wave inverter is automatically switched to the PWM inverter.

During normal operation (powering and stopping), the output of the comparator 16 is L, so the output of the NAND circuit 203 is always H, and the modulator 2
09 outputs the output of the hexadecimal ring counter 208 as it is, and is applied to the transistors 104 to 1090. The main circuit 1 therefore operates as a square wave inverter.

〔Effect of the invention〕

According to the present invention, the following effects can be obtained.

(1) Since a large regenerative voltage can be returned, rapid deceleration operation of the motor is facilitated.

(2) At low frequencies, the voltage across the smoothing capacitor is low and the mode is switched to PWM mode, so it is possible to receive more regenerative energy than in normal PWM operation, and the motor can be decelerated more rapidly than a PWM inverter.

(3) Since the time for short-circuiting the motor windings with the transistor is shortened, the transistor will not be destroyed even if the motor suddenly decelerates.

(4) Since the smoothing capacitor is a double-ended type, the voltage can be kept high regardless of the frequency, so when using a motor regenerative energy consumption device that consumes the voltage across the smoothing capacitor with a resistor, it is recommended to Since it is sufficient to detect and operate the absolute voltage regardless of the relative judgment of the proportional capacitor voltage, the system can be assembled at a low cost.

(5) As shown in Figure 1, by amplifying the error difference between the terminal voltage of the smoothing capacitor and the speed command and constraining the duty of the oscillator, the excitation current of the motor during regeneration can be controlled, simply by temporarily changing the frequency. It can be controlled more reliably than the method of stopping and adjusting the amount of regeneration (the power from the negative slip region to the return to the positive region is unrestricted in the frequency pause method), and constitutes a highly reliable device. can.

Furthermore, when excessive deceleration occurs, the configuration of the control circuit is simpler than the method of slightly increasing the frequency, and deceleration can be reliably achieved under any load. Therefore, unlike the method of slightly increasing the frequency, the rotational speed does not become locked in a certain frequency range depending on the magnitude of the inertia of the load and become uncontrollable.

[Brief explanation of the drawing]

1 is a circuit diagram showing an embodiment of the present invention.

Claims (1)

[Claims] In an inverter for driving a motor comprising a rectifier, a smoothing capacitor, a plurality of switching means, a regenerative current detector, and a control circuit for driving the plurality of switching means, the control circuit is outputted from the regenerative current detector. An electric motor drive inverter (
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