JPS5999994A - Power feeding device for 3-phase ac motor - Google Patents

Abstract

PURPOSE:To smoothly shift a phase current to a trapezoidal shape in a low speed range and to a sinusoidal wave shape in middle and high speed range by inserting a reactor during a neutral unblanking period between a power converter and the motor. CONSTITUTION:A reactor 6 is inserted and connected during neutral unblanking period for connecting the neutral point MU of cycloconverters 1-3 and the neutral point MM of a motor winding. Since the impedance given by the reactor 6 is low in a motor low speed range, near a shortcircuit state between the neutral points MU and MM, and a zero phase current is flowed without limit, a phase current flowed to the motor becomes trapezoidal shape. On the other hand, as the speed rises, the impedance of the reactor 6 increases, the zero phase current is gradually limited, the phase current is decreased in the trapezoidal shape to approach to the sinusoidal wave, and becomes apparently not connected between the neutral points in the high speed range, and the phase current becomes sinusoidal waveform.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply device for a three-phase AC motor using a power conversion device comprising a semiconductor conversion valve such as a thyristor.

[Prior art and its problems]

It goes without saying that the phase current waveform of the AC motor is preferably a sine waveform. When supplying sinusoidal waveform phase currents in this way, the conversion valve of one phase flows the peak value of the phase current at each half-wave maximum point, while the conversion valves of the remaining two phases supply half of that value. Just bear the current of amnesty. The period during which one phase σ) conversion valve continuously flows a current near the peak value is short in the high speed (high frequency) region, but long in the low speed (low frequency) region. In other words, in the low speed region, a large current near the peak value flows intensively into the conversion valve of one phase for a long time.

Therefore, capacity selection in the power conversion device must be performed based on the W current and the peak value of the voltage. In such a case, the power converter has a considerable extra capacity in the speed range corresponding to the rated speed where the operating time is the longest, and the utilization rate of the capacity of the power converter is poor.

In other words, it is disadvantageous for the power conversion device to make the phase current waveform a sine waveform.

It has already been proposed to make the phase current waveform a trapezoidal waveform for the purpose of improving the utilization rate of the power converter without sacrificing the output and torque of the motor. That is, the phase current J) has a trapezoidal waveform consisting of a sinusoidal fundamental wave and a third harmonic. The motive is to regulate the flow.

Of course, between a sine waveform and a trapezoidal waveform having the same effective value, the wave height value of the latter is smaller, so the conversion valve capacity can be selected to be smaller accordingly. however,
In this way, the third frequency component included in the trapezoidal phase N flow cannot flow without a neutral return line between the motor and the 7-power converter, so it is necessary to provide a neutral point return line. There is. What should be noted here is that the magnetization and torque characteristics of the motor are hardly adversely affected by the mold cost of such third harmonic components. Because the magnetomotive force components generated by the zero-sequence current flowing from the third harmonic component 11IIK are canceled out thanks to the symmetrical structure of the motor, the composite magnetomotive force vector is determined only by the sine wave fundamental wave component. It is.

However, the above-mentioned conventional solutions are not completely without negative effects on the electric motor. From the perspective of the heat generation characteristics of the AC motor, the zero-phase W flow produced by the harmonics VC is undesirable.

Therefore, as a way to improve this, if we focus on the fact that the current capacity of the power converter has a margin in the medium to high speed range, we can consider making the phase current waveform closer to a sine waveform in the medium to high speed range. 4).

[Purpose of the invention]

In view of the above, it is an object of the present invention to provide a power supply device equipped with extremely simple means for making the phase current have a trapezoidal waveform in a low speed range and smoothly transition to a sinusoidal waveform in a medium/high speed range.

[Key points of the invention]

The present invention achieves the above object by inserting and connecting a reactor in the neutral return line connecting the neutral point of a power converter that outputs a phase voltage having a trapezoidal voltage waveform and the neutral point of one motor. It is something. In other words, as a result of the zero-phase N current being automatically adjusted due to the frequency dependence of the impedance of the reactor, a phase current with a trapezoidal waveform corresponding to the trapezoidal phase voltage of the power converter flows in the low speed range, and as the speed increases. The zero-sequence current is gradually suppressed and the phase current approaches a sine wave.

Embodiment The figure shows an embodiment of a power supply device according to the present invention. The power conversion device provided for feeding power to the three-phase AC motor 4 is composed of, for example, three cycloconverters 1 to 3, and each cycloconverter 1 to 3 is connected to a pair of 3-phase AC power supplies connected to the three-phase AC power supply 5. It consists of a phase thyristor bridge, and both thyristor bridges are connected in antiparallel on the output side. Ten thousand output terminals of each of the cycloconverters 1 to 3 are connected to each phase terminal of the motor 1, and the other output terminals are connected together to form a neutral point Mu. During the neutral point retrace line connecting this neutral A Mu and the neutral point Mm of the motor winding, the accelerator 6
is inserted and connected. The phase voltage applied to the speaker from each cycloconverter is controlled to have a trapezoidal waveform. The inductance of the reactor 6 is selected so that the 3rd harmonic reactance value is much larger than the fundamental wave reactance value due to the leakage inductance of the motor armature circuit.

In this way, in the motor low speed range, the impedance at the neutral return line given by the reactor 5 is low, and the neutral point Mu-Mm is close to a short-circuit state, and the zero-sequence current flows without being suppressed. (5) The phase current flowing through the motor VC has a trapezoidal shape corresponding to the trapezoidal phase voltage output by the power converter. In contrast to this Vr, as the speed increases, the impedance at the neutral point 1tjlK given by the accelerator 5 increases, the zero-sequence current is gradually suppressed, and the phase current becomes less trapezoidal and becomes a sine wave. Coming closer. In the altitude range, the neutral voices are apparently not connected, and the phase tltf& has a sine waveform.

〔Effect of the invention〕

According to the present invention, by using an extremely simple circuit means of inserting a reactor in the neutral return line between the power converter that outputs a trapezoidal phase voltage and the motor, the frequency dependence of the impedance of the reactor can be adjusted. Utilizing this, it is possible to flow a trapezoidal phase current in the 1-low speed range without zero-sequence, and since the zero-phase circuit impedance changes continuously until high-speed completion, the phase current can be smoothly sine-shaped. It is possible to get closer to the waves, and the negative effect on the electric motor can be minimized while utilizing the capacity of the power converter to utilize effective pressure.

(6)

[Brief explanation of drawings]

The figure is a circuit connection diagram showing an embodiment of the power supply device according to the present invention. 1-3...Power conversion device, 4...AC motor, 5.
...AC power supply, 6...reactor. (7) 507-

Claims (1)

[Claims] 1) A reactor is inserted and connected in the neutral point return line connecting the neutral point of the power converter that supplies a phase voltage having a trapezoidal waveform to the three-phase AC motor and the neutral point of the three-phase AC motor. A power supply device for a three-phase AC motor.