JPS6052670B2 - Control method for multiplexed current source inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling a multiplexed current source inverter suitable for powering an AC motor to be driven at variable speed.

As shown in FIG. 1, unit inverters constituting a multiplexed current source inverter include, for example, bridge-connected thyristors U--Z, diodes inserted and connected in series with each thyristor, and a combination of thyristors and diodes. The DC input current Id, which is smoothed from the variable DC power supply Vs through the DC reactor DcL, is converted into a three-phase output current having a rectangular waveform with a width of 1200. 1 supplied to phase AC motor M
A 200 energization type inverter is used.

In the case of this output current waveform, the fundamental wave current is 100%
Then, the 5th harmonic current is 20%, and the 7th harmonic current is 14%.
．． 3%, 11th harmonic current 9.1%, and 13th harmonic current 7.7%. Therefore, with only a single inverter, the motor will generate relatively large torque pulsations, especially at low speeds. There is an inconvenience that sometimes it appears as rotation irregularity. In order to prevent this, multiple unit current source inverters are connected in parallel on the output side and the control pulse phases of the corresponding thyristors of each unit inverter are shifted from each other to generate a composite output current of multiple staircase waveforms. A multiplexed current source inverter is known that supplies .

FIG. 2 shows a connection diagram of a duplex current source inverter, which is one type of this type of inverter. A unit current type inverter In which receives smoothed DC input currents 1d1 and Id2 from DC currents FSl and VS2 through DC reactors DcLl and DCL2, respectively.
vl, Inv2 are connected in parallel to the AC motor M on the output side. Although each unit inverter is indicated by only bridge-connected thyristors Ul, 4, U2-4, the detailed connection configuration is the same as that in FIG. 1. The control pulse phase difference between corresponding thyristors between both inverters is selected to be 30 degrees, so that the combined current supplied to the AC motor M has a step waveform. This staircase waveform is 12k±1 (k=1, 2, 3...
・The harmonic content of the harmonics of the order of ri is the same, but the harmonic content of the other orders is significantly smaller. For example, in the case of a 120 large rectangular waveform, the 5th harmonic content is 20% and the 7th harmonic content is 14.3%, whereas in the case of a duplex staircase waveform, the 5th harmonic content is 20% and the 7th harmonic content is 14.3%. The wave content rate becomes 5.4% and the seventh harmonic content rate becomes 3.8%, which are greatly improved. By further increasing the number of unit inverters, a step waveform with an even larger number of steps can be obtained, and the harmonic content rate can be further reduced. Furthermore, as a method for reducing harmonic current components, the output current waveform of one phase (R) shown in Figure 3 is shown. A current cutting method or electric current that pulses the half cycle of the output current so that
The WM method is known.

According to this method, it is possible to reduce the low-order harmonic current to a few percent or less, so that the pulsating torque generated by the low-order motor becomes a practical problem. It can be reduced to no extent. However, in this normal current cutting method, the high-order harmonic current is around the current cutting frequency.
It is larger than the 200 width rectangular wave, and 2
It will be around 0%. This harmonic current causes noise in the motor.
This not only causes an increase in iron loss, but also increases primary and secondary copper loss in the motor. Especially in the secondary, the increase in secondary resistance due to the skin effect can cause the motor to overheat. Furthermore, this current cutting method is only possible in the low frequency range, and in high frequency ranges (at high speeds) it is necessary to switch to the normal 120 rectangular waveform, which increases harmonic current and pulsating torque at high speeds. There are drawbacks.
An object of the present invention is to provide a current source inverter that can sufficiently reduce harmonic current components over a wide range of output fundamental wave frequencies. Specifically, by combining current step waveforming and current cutting through multiplexing, we can take advantage of the advantages of both while negating the disadvantages of the current cutting method, resulting in better multiplexing with a smaller number of inverters. This is an attempt to construct a current source inverter. This object is achieved according to the invention by the features described in the claims. Embodiments of the present invention will be described with reference to the drawings. First, the main circuit configuration of a multiplexed current source inverter to which the control method according to the present invention is applied is, for example, a second
It may be a general diagram.

FIG. 4 shows the R-phase output current 1r1,
A waveform diagram of ir2 and R phase combined output current 1r is shown.

As can be seen from FIG. 4, by controlling the firing of the thyristors U19l9Wl9Xl9Yl9Zl of the unit inverter Invl with a delay of the corresponding thyristors U2, V2, W2, X2, Y2, and Z23O of the unit inverter In2, the unit The output current 1r2 of the unit inverter Inv2 has a phase delay of 30 with respect to the output current 1r1 of the inverter Invl.

15 whose firing phase difference is 30 wider than 1200.
By cutting both sides of the rectangular wave having a width of 0.01 in a pulse shape by 30 corresponding sections, each unit inverter output current 1r1, ir2 becomes a trapezoidal wave current as shown by the broken line in the figure when viewed in terms of average value.

When viewed from an average value, the combined output current 1r also becomes a trapezoidal wave current as shown by the broken line in the figure. Partial current cutting control as shown in FIG. 4 is performed in the low speed operating range of the AC motor.

In the high-speed operation range where current cutting control is difficult, the output current of each unit inverter 1r1,
jr2 is a normal rectangular wave current with a width of 120 bar, so that the combined current becomes a normal staircase wave current. As can be seen from FIG. 5, the current cutting control section indicated by the broken line in FIG. 5, which corresponds to the broken line in FIG. By doing so, sudden changes in the output current phase immediately after starting or stopping cutting control can be avoided. The control pulses of the negative side thyristors Xl, Yl, Zl of the first unit inverter and the negative side thyristor X2 of the second unit inverter,
If the control pulses of Y2 and Z2 are swapped with each other, the output current fundamental waves of both unit inverters will simply be in phase, and no change will occur in the waveform of the combined output current. A waveform diagram corresponding to this is shown in FIG.

According to the present invention, in a low speed operation range, it is possible to obtain a desired sufficiently low harmonic content with a smaller number of unit inverters than in the case of a conventional stepped waveform multiplexed current source inverter, and It is also possible to overcome the disadvantages of using a simple current cutting method, and to take advantage of the advantages of both the multiplexing method and the current cutting method, it is possible to provide an advantageous current supply to an AC motor.

The control method of the present invention is not merely a combination of a known multiplexing method and a known current cutting method.As can be seen from the waveform diagram in FIG. Another major feature is that the width of the sections was determined so that they did not overlap in time.

That is, by doing this, as can be seen from the waveform of the composite output current 1r in FIG.
It becomes 2. On the other hand, for example, if two unit inverters have the output current of the current cutting waveform shown in Fig. 3 and multiplex these, the step size will be twice the value 1.
d occurs. That is, the two unit inverters each output output currents Jrl and ir2 having waveforms as shown in FIG. 7, and their combined output current 1 is obtained.
When obtaining r, as can be seen from the enlarged time axis diagram in FIG. 8, there is a step in the magnitude of Id. For this reason, when multiplexed current cutting control as shown in FIGS. 7 and 8 is performed, not only is it impossible to eliminate the drawback of single current cutting control that a large amount of high-order harmonic current is included, but also The effect of reducing low-order harmonic currents is also smaller than in the case of the present invention. The control method according to the present invention is based on n(
≧2) It is applicable to a multiplexed current source inverter consisting of unit inverters.

In this case, the firing phase of the corresponding controllable valve between each inverter is shifted by 600/n, but the width of the cutting section is
By controlling to 00/n, it is possible to prevent the cutting sections of each inverter from overlapping each other in time.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram showing a configuration example of a current source unit inverter, Fig. 2 is a connection diagram showing a configuration example of a duplex current source inverter, Fig. 3 is a waveform diagram showing a conventional current cutting waveform, 4 and 5 are waveform diagrams for explaining an embodiment of the control method according to the present invention, FIG. 6 is a waveform diagram for explaining a modification of the control method according to the present invention, and FIGS. 7 and 8 are waveform diagrams for explaining a modification of the control method according to the present invention. The figure is a waveform diagram for a control method not according to the present invention, which is exemplified to explain the advantages of the control method according to the present invention.

Claims (1)

[Claims] 1. By connecting a plurality of current source unit inverters in parallel to an AC motor, and controlling the firing of controllable valves corresponding to each other between the unit inverters by shifting their phases from each other. , in a multiplexed current source inverter in which the combined output current supplied from these unit inverters to the AC motor has a stepped waveform, at least in the low speed operating range of the AC motor, the width is wider by a predetermined angle than the 120° width. Each unit inverter is controlled so that each inverter has an output current of a waveform obtained by cutting both sides of a rectangular wave in a pulse shape by a section corresponding to a predetermined angle, and the output current of each inverter is A method for controlling a multiplexed current source inverter, characterized in that the predetermined angle is selected so that the sections cut into pulses do not overlap in time. 2. The control method according to claim 1, wherein the pulse-like cutting control is performed so that the output current waveform of each unit inverter has a trapezoidal shape when viewed from an average value. 3. The control method according to claim 1 or 2, wherein the predetermined angle is selected to correspond to an ignition phase difference of the controllable valve due to the correspondence between each unit inverter. 4 In the high-speed operation range of the AC motor, the pulse-shaped cutting control is stopped, and the output current of each unit inverter is reduced to the normal 120
Claims 1 to 3 are characterized in that each unit inverter is controlled so as to produce a rectangular wave current with a width of
The control method described in any of paragraphs. 5. The pulse-like cutting control is performed so that the output current phase during the cutting control and the non-cutting time are the same in order to prevent the output current phase of each unit inverter from suddenly changing due to its stop. A control method according to claim 4.