JPS607919B2 - Power supply method for polyphase AC motor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for powering a polyphase AC motor having windings provided with a neutral return wire and supplied with phase current by a power converter.

It goes without saying that it is preferable that the phase current waveform supplied to the AC motor be a sine waveform when considering the motor, but for the power converter that supplies current to the motor, the waveform of the current and voltage is the peak value of the current and voltage. Since the capacity of the converter valve is selected based on , it is disadvantageous to make the current sinusoidal.

This is because, as is well known, when supplying a sinusoidal waveform phase current, a converter valve for one phase must guide the peak value of the phase current at each half-wave maximum point.
Since the converter valves of the remaining two phases only carry half the current, and all converter valves must be designed based on the peak value of the phase current that occurs only for a short time in each case, This is because the overall utilization rate of the conversion valve decreases. In order to obtain good converter valve utilization without sacrificing motor output and torque in AC motors fed by power converters, one phase current in each case has a predetermined maximum limit near the center of the half-wave. Tsubaki Kaisho 50-53 introduced a power supply method in which harmonics are superimposed on the sinusoidal fundamental wave of the phase current so that the remaining phase current is strengthened by the deviation.
It is known from the publication No. 818.

According to this known power supply method, since the phase current is controlled to a predetermined maximum value, it goes without saying that the capacity of the power converter can be reduced compared to the case where a sinusoidal waveform phase current is supplied. It should be noted here that the magnetization and torque characteristics of the motor are not substantially adversely affected by such limitations. This is because the magnetomotive force components caused by the superimposed harmonic components are canceled out due to the symmetrical structure of the motor, so that the resultant magnetomotive force vector is determined only by the sinusoidal fundamental wave component. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved power supply method that allows phase currents supplied to a motor to approximate a sine wave without increasing the capacity of a power converter.

The purpose of this is to set the maximum value for limiting the magnitude of the phase current corresponding to the phase current fundamental wave frequency or motor speed in the above-mentioned known power supply method in which harmonics are superimposed on the sinusoidal fundamental wave of the phase current. This is achieved by variably setting the value as follows depending on the signal, that is, variably setting the maximum value to be small in the low frequency or low speed region.

The present invention provides that when an AC motor is operated at very low speed or zero speed, a large current may flow for a long period of time only in the conversion valve of one phase, and that the power conversion device should be designed to withstand even under such conditions. It is recognized that if designed in this way, there will be a margin in the current capacity of the power converter in medium and high speed castles, and from the viewpoint of the heat generation characteristics of AC motors, zero-sequence current generated by superimposition of harmonic components is preferable. This is based on the recognition that it is preferable that the phase current supplied to the AC motor has a waveform close to a sine wave.

According to the present invention, since the phase current is limited to a small maximum value in the low frequency or low speed region, it is possible to prevent a large current from concentrating on the converter valve of one phase for a long time, and the current burden on each phase is suppressed. On the other hand, in medium and high speed castles, the maximum value for limiting the phase current is increased, so the phase current approaches a sinusoidal waveform that is preferable for the motor.

Further, calculation results have confirmed that when the phase current approaches a sine waveform, harmonics generated in the grid power supply to which a power converter (for example, a cycloconverter) is connected are reduced. Therefore, according to the present invention, while fully utilizing the effects of known power supply methods, further improved results can be obtained for the entire system including the grid power source/power converter and the AC motor. Hereinafter, the present invention will be described in more detail with reference to an apparatus for implementing the power supply method according to the present invention shown in the drawings.

FIG. 1 is a block diagram showing an example of the overall schematic configuration of an apparatus for implementing the power supply method according to the present invention. The cycloconverters 1, 2, and 3 as power converters are connected in a star shape, and each phase output terminal is connected to each phase terminal R, S, and the AC motor 4, respectively.
T, motor neutral point N and cycloconverter 1
A retrace line is provided between the neutral point of ~3.

The input side of the cycloconverter is connected to the grid power supply 5. The cycloconverters 1, 2, 3 of each phase obtain control pulses by pulse generators 6, 7, 8 which receive control inputs from current regulators 9, 10, 11, respectively. Current setpoint value generators 12, 13, 14 are used to control the phase currents iR, ·s, iT supplied to the motor 4 from the cycloconverters 1, 2, 3 according to the method according to the invention.
First, current target values jR*, is*', iT*' having desired amplitude, frequency and phase are given by
R*, is*, and iT are converted. Current regulator 9,1
0 and 11 are the corresponding phase current target values iR* and iS, respectively.
*, iT* so that the actual phase current values iR, isiT detected by the current detectors 16, 17, 18 match the respective target values iR*, is*, iT*,
The cycloconverters 1, 2, 3 are controlled via pulse generators 6, 7, 8, respectively. FIG. 2 is a block diagram showing a specific configuration of the conversion circuit 15 in FIG. 1.

According to this, the sinusoidal electromagnetic target values iR*', is*', iT*' from the target value generators 12 to 14 are guided to the summing amplifiers 21, 22, and 23, respectively, and are sent to the output side of each summing amplifier. are provided with restriction stages 31, 32, and 33, respectively. Each limit stage 31, 32, 33 is given a maximum positive and negative limit value 10B, 1B from a limit value transmitter 40. Further, difference amplifiers 51, 52, 53 are provided which form a difference between the input and output voltages of each limiting stage 31, 32, 33, and the output voltage of each of these difference amplifiers is equal to or less than the remaining phase except the one to which it belongs. The signals are guided to summing amplifiers 21 to 23 belonging to two phases. For example, when the absolute values of the sinusoidal current target values iR*', is*', and iT* are all below B, the output voltages of the difference amplifiers 51 to 53 are all zero, and the current regulators 9 and 10 .

For example, if the sine wave current target value iB*' is in phase near the half center and becomes larger than the positive limit value +B,
Due to the limiting stage 31, iR*=B, and the output voltage of the difference amplifier 51 becomes B-iR*', which is the output voltage of the summing amplifier 22,
23. As a result, the remaining two phase current target values are: is*=is*'+B-jR*'iT*=iT*'+B-iR*'.

The fact that iR*' is near the center of the positive half wave means that
*', iT*' have negative polarity, and the deviation B-i
Considering that R*' has negative polarity, the phase current target values is* and iT* are respectively adjusted in magnitude by the deviation magnitude iB-
This means that it has been strengthened by the amount of iR*'. In other words, the phase current target values iR*, is* taken out via the conversion circuit 15
, iT*'Therefore, the phase current actual value iR, which is adjusted,
In each case, one of ls and iT is reduced to a predetermined maximum limit value near the center of the half wave, and at the same time, the harmonic component is superimposed on the fundamental wave component of the sine wave so that the remaining two are strengthened by the deviation. That will happen. This harmonic component flows to the neutral return wire as a zero-sequence current. What has been explained above is described in more detail in Tokukan Sho 50-15 Patent Publication No. 18.

Where the present invention differs from that is that the limit value transmitter 40
This is how to set the limit value for one column. In other words, while the limit value is fixed in the above publication,
In the case of the present invention, the difference is that the limit values are variably set in a predetermined relationship, and this difference results in an improved power supply system as described above. FIG. 3 shows an embodiment of the limit value transmitter 40 in FIG. 2 according to the invention.

According to this, a setting device 41, a multiplier 42 which multiplies a preset value by a variable coefficient k, receives an input signal corresponding to the motor rotational speed n or the power supply frequency f', and calculates the input signal and a predetermined value. A function generator 43 that generates a coefficient output signal k having a functional relationship and an inverting amplifier 44 connected to the output terminal of the multiplier 42 are provided, and the multiplier 42 outputs, for example, a positive limit value +B=+kB. , the negative limit value 1B=1k& is output from the inverting amplifier 44. FIG. 4 shows the function characteristics of the function generator 43 provided to change the coefficient input k of the multimeter 42, which affects the magnitude B of the positive and negative limit values, according to the speed n or the frequency f. . As can be seen, the coefficient k has a value of 1 in a range above a predetermined speed or frequency, and becomes smaller as the speed or frequency decreases below that range. According to the present invention, since the current is controlled to have a trapezoidal wave in the low speed or low frequency region, it is possible to suppress the current load from shifting to the conversion valve of any one phase, thereby reducing the required capacity of the power converter. Moreover, this can be done without affecting the torque generated by the motor, and furthermore, in medium and high speed ranges, the capacity of the power converter is fully utilized to transfer the current to the thermal characteristics of the motor and other systems. It is possible to approach a sine wave current which is preferable for the whole.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a schematic configuration of a device for implementing the power feeding system according to the present invention, FIG. 2 is a block diagram showing an example of the configuration of the main parts of the device in FIG. 1, and FIG. A block diagram showing a specific configuration example of a limit value transmitter, which is a further part of the main configuration example shown in FIG. 4, and FIG. 4 shows the function characteristics of the function generator used in the limit value transmitter shown in FIG. It is a diagram. 1 to 3... Power converter (cycloconverter), 4... AC motor, 5... System power supply, 6 to 8... Pulse generator, 9 〜１１・・・・・・
・Current regulator, 12-14... Current target value generator, 15... Conversion circuit, 16-18...
・Current detector, 21-23...Summing amplifier, 3
1-33...Limiting stage, 40...Limit value transmitter, 51-53...Difference amplifier, 41...
...Setter, 42...Multiplier, 43...Function generator, 44...Inverting amplifier. Figure 'Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. In a power supply method for a polyphase AC motor that has a winding that is equipped with a neutral return wire and is supplied with phase current by a power converter, a sinusoidal target value is commanded for each phase current, and those sinusoidal When the instantaneous absolute value of the target value exceeds a predetermined maximum limit value, the instantaneous absolute value of the sinusoidal target value of the corresponding phase is reduced to its maximum limit value, and at the same time, the resulting deviation from the sinusoidal target value is reduced. is calculated, and the sine wave target value for the remaining phase currents is corrected so as to strengthen the remaining phase currents by an amount corresponding to the deviation, and the maximum limit value is determined by a signal corresponding to the phase current fundamental wave frequency or motor speed. A power supply method for a polyphase AC motor characterized by variable setting so that the power becomes smaller in a low frequency or low speed region.