JPS5812588A - Drive device for ac motor - Google Patents

Abstract

PURPOSE:To suppress a rotor vibration with a simple structure by altering at least one of the output voltage and frequency of a power converter in a stepwise manner in a specific range of a motor speed. CONSTITUTION:An AC motor 12 is driven by the output of a power converter 11. The output voltage of the converter 11 is controlled by the output of a voltage controller 13, and is controlled by the butting of the outputs of a voltage reference circuit 14 and a voltage feedback circuit 15. The rotating frequency of the motor 12 is detected by a speed detector 16, and the output of the detector is inputted to a frequency discriminator 17. The discriminator 17 generates a signal when the rotating frequency of the motor 12 is in the specific frequency range, a voltage throttling circuit 18 is operated by the signal, thereby decreasing the voltage reference of the circuit 14.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a drive device for an AC motor, and more particularly to a drive device designed to safely perform direct start-up of an AC motor.

Conventionally, the most commonly used method for starting AC motors is the so-called full-voltage direct start method.This method is used not only for relatively small capacity motors used in commercial systems, but also for power conversion of inverters, etc. Currently, it is also applied to devices driven by devices.

This direct start method is advantageous in that it is easy to control and is economical, and is easily adopted when there are no problems with the heat generation conditions of the motor and the torque matching conditions with the load.

However, if the structure of the motor is special, for example, if the rotor has a thin disk structure, the rotor will vibrate due to resonance at a certain rotation speed when normal direct startup is performed, and the strength will increase. This can be a problem.

This resonance phenomenon is due to the structure of the rotor. This occurs when the full resonant frequency becomes twice the slip frequency during direct startup. This phenomenon will be explained using drawings.

FIG. 1 is a structural model diagram of an AC motor in which the rotor has a thin disk structure, the stator has a flat structure, and there is an air gap in the axial direction. This type of motor has the characteristic that its axial dimension can be shortened, and is usually an induction motor or a hysteresis motor.

In Fig. 1, 1 is a rotor, 2 is a stator core, 3 is a stator coil, 4 is a shaft, 5 is a bearing, and 6 is a frame.
In the figure, the number of poles of the motor is 2, and the stator coil is f
A power supply frequency of , is applied, and the four-day rotation frequency at this time is f.

Since the frequency ftVC is therefore excited, if we consider f3 as the reference coordinate, the rotational frequency f of the magnetic flux in the air gap is fs=f.

-f. Since this magnetic flux has two magnetic poles, if we focus on a certain point on the gap surface of the rotor as shown in FIG. 2, it will receive a magnetic attraction force F with a frequency of 2f.

When this 2f coincides with the axial resonance frequency of the rotor, the rotor vibrates greatly, and as shown in FIG. 3, the vibration stress limit may be exceeded at a certain frequency.

In other words, when the stator coil is excited by a power supply with a constant voltage and a constant frequency, fr(f
r is the resonant frequency), that is, f, = f1−−L−fr
When the stress G due to vibration reaches its peak, the stress limit Gc of the rotor is exceeded around that frequency.

The present invention has been made in view of the above-mentioned drawbacks, and provides an AC motor drive that can moderate the rotor vibration mentioned above by adding appropriate control to the drive device side, and that does not impair economic efficiency. K said they would provide the equipment.

The present invention will be described in detail below using the drawings.

FIG. 4 is a block diagram of an AC motor drive device showing one embodiment of the present invention.

In FIG. 4, AC motor 12 is driven by the output of power converter 11. In FIG. The output voltage of the power conversion device 11 is controlled by the output of the voltage control circuit 13, and this value is controlled by matching the output of the voltage reference circuit 14 and the output of the voltage feedback circuit 15. The circuit described above is a conventional feedback constant voltage control system.

The structural features of the present invention are as follows. The rotational frequency f of the AC motor 12 is detected by a speed detector 16, and its output is input to a frequency discrimination circuit 17. The frequency discrimination circuit 17 issues a signal when the rotational frequency f3 of the AC motor 120 falls within a specific frequency range fA<fs<1mK, and the voltage throttling circuit 18 is activated by this signal to lower the voltage reference of the voltage reference circuit from vN to vL.

For these series of operations, the rotational frequency ft is plotted on the horizontal axis,
FIG. 5 shows the output voltage of the power conversion device along the vertical axis. As shown in FIG. 5, fl=fI−−4−
In the vicinity of fr, the output voltage is reduced to vL. Of course, unless the value of vL is below a certain level, the AC motor will not be able to accelerate, so the value of vL should be set near the lowest voltage that allows acceleration.

The effects of the present invention in the above configuration will be described below. Since the magnitude of the magnetic attractive force F shown in FIG. 2 is proportional to the acid gap magnetic flux, it is approximately proportional to the square of the voltage V applied to the stator coil. Moreover, F in FIG. 2 and rotor stress 0 in FIG. 3 are surprisingly proportional to #1, so when the present invention is applied, FIG. 6 is obtained from K as a diagram corresponding to FIG. 3. That is, in the region of fA(f, (fB), the voltage is set to vL, so the stress peak at the resonance frequency is greatly reduced and can be suppressed to below the stress limit Gc of the rotor.

Next, FIG. 7 is a block diagram of an AC motor drive device showing another embodiment of the present invention. In FIG. 7, the turtles having the same numbers as those in FIG. 4 are the same elements as in FIG. 4, so a description thereof will be omitted.

In FIG. 4, the output frequency of the company electric converter f11 is assumed to be constant and the explanation thereof is omitted, but in FIG. The frequency reference circuit 20 normally outputs the rated output frequency fN, but when the frequency change circuit 19 operates with the output of the frequency discrimination circuit 17, it outputs the output frequency f. FIG. 8 illustrates these operations.

FIG. 8 shows a situation in which the output frequency is jumped to fH in the vicinity of fB = f N 2 f r, and this control prevents the resonance of the rotor from occurring.

The resonance speed corresponding to the output frequency fHK is fH-"-fr, and in this case, if f, = f, is returned, resonance will not occur.

Therefore, as shown in FIG. 9, the rotor resonance phenomenon no longer occurs, and there is no stress peak at any rotational speed of the rotor.

In FIG. 5, the output frequency is controlled to be increased, but it is clear that the same effect can be expected even if the output frequency is controlled to be decreased. Furthermore, when controlling the frequency, if the V/f is controlled to remain constant, it is possible to maintain the acceleration time of the AC motor as before.

As described above, according to the present invention, it is possible to provide a drive device that can suppress the resonance phenomenon of the rotor when directly starting an AC motor by simply adding a circuit.
The advantage of the direct startup method, which is cheaper than the p-drive method, is not lost, so the effect is great.

In the above explanation, we only explained ``one electric motor and [control by speed detection]'', but this can be changed to ``many electric motors (in this case, sampling speed detection can be applied)''.
``It is clear that it is possible to extend the control to control based on vibration detection or control 1 based on a preset tie i, etc., without relying on speed detection.

[Brief explanation of drawings]

Fig. 1 is a structural model diagram of an AC motor, Fig. 2 is an explanatory diagram of the magnetic attraction force applied to the rotor, Fig. 3 is an explanatory diagram of rotor stress, and Fig. 4 is an AC motor drive showing an embodiment of the present invention. A block diagram of the device, FIG. 5 is a diagram explaining its operation, FIG. 6 is a diagram explaining its effect, FIG. 7 is a block diagram of an AC motor drive device showing another embodiment of the present invention, and FIG. 8 is a diagram explaining its operation. The explanatory diagram, FIG. 9, is an explanatory diagram of the effect. 1... Rotor 2... Stator core 3... Stator; coil 4... Shaft 5...
... Bearing 6 ... Frame 11 ...
・Power converter 12...AC motor 13...
・Voltage control circuit 14...Voltage reference circuit 15...
...Voltage feedback circuit 16 Four-speed detector 17...
Frequency discrimination circuit 18...Voltage throttle circuit 19...
・Frequency change circuit 20... Frequency reference circuit (7
317) Agent Patent attorney Noriyuki Chika (1 person) Figure 1 Ka4 ft'-+fr

Claims (1)

[Claims] In an AC motor drive device that accelerates or decelerates an AC or current motor using a power conversion device that can vary at least one of the output voltage or output frequency, the output voltage or output frequency of the power conversion device decreases within a specific range of motor speed. A drive device for an AC motor, characterized in that one of the two is changed into a stepped shape.