JPH0446080B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a cycloconverter control method, and more particularly, to a method for controlling a cycloconverter, and more specifically, for controlling a cycloconverter from a first AC voltage to a variable voltage/variable frequency second AC voltage for each phase of an AC motor. A conversion means for obtaining a voltage is provided, and an AC bias voltage having a frequency three times that of the second AC voltage is superimposed on the second AC voltage.
A cycloconverter control method that aims to improve the input power factor by controlling the AC voltage in a trapezoidal waveform,
It is related to.

[Technical background of the invention]

So-called bias control is known in which the input power factor is improved by controlling the output voltage of a cycloconverter in a trapezoidal waveform by superimposing an odd harmonic bias voltage on a sine wave motor voltage. In this control method, a bias voltage is superimposed on a motor voltage that changes in a sinusoidal manner, and the cycloconverter is operated at a high output voltage. Generally, a third harmonic of the load voltage is given in consideration of the load voltage phase, and by reducing the peak value of the output voltage of the cycloconverter, an approximately trapezoidal waveform output voltage is obtained. By doing so, the secondary voltage of the input transformer can be reduced, and the range in which the cycloconverter can be operated at a high output voltage can be expanded, thereby improving the input power factor. .

FIG. 1 shows the main circuit configuration of a device implementing such a control method. In particular, a configuration example is shown in which the number of series-connected converters for each phase is two in order to obtain a trapezoidal waveform output voltage. The number of connections may be 1 or 3 or more depending on the case. In Fig. 1, input transformer 1,
The second and third primary windings 11, 21, and 31 are each connected to a three-phase AC power source (not shown), and the two sets of secondary windings 12, 13, 22, 23, 32, and 33 are connected to a converter 411, respectively. ,412,421,42
It is connected to the AC input side of 2,431,432. These converters each consist of a pair of reversible thyristor converters 4P, 4N for positive output and negative output connected in antiparallel as shown in FIG. Each converter itself is configured as a three-phase bridge rectifier circuit. Converter 411, 41
2 are connected in series on the DC output side to constitute a U-phase converter 41 that can flow current in both forward and reverse directions to the U-phase winding of the AC motor 5. Similarly, converters 421, 422
is the V phase converter 42, and the converters 431 and 432 are W
A phase converter 43 is configured respectively. The U, V, and W phase converters 41, 42, and 43 are controlled by a control device (not shown) so that their output currents are three-phase alternating currents having a phase difference of 120 degrees. Two sets of converters connected in series in each phase are asymmetrically phase-controlled so that their respective operating voltages are high.

For example, now the output voltage of the U-phase converter 41 is
V _U , and the output voltages of the converters 411 and 412 are respectively V _1U and V _2U when a combination of well-known asymmetric control and bias control is performed, each output voltage V _U ,
V _1U , V _2U and converter output line voltage V _UV (U, V
The waveform of the phase line voltage is as shown in FIG. The output voltage V _U has a third harmonic voltage superimposed on the sinusoidal motor phase voltage V _P with the peak value V _M shown by the broken line, resulting in an almost trapezoidal output waveform, and the output voltage peak value is reduced. I know that there is. Since the secondary voltages of input transformers 1, 2, and 3 are selected according to the maximum output voltage value of each converter, the transformer secondary voltage, that is, the cycloconverter input voltage, can be designed to be lower by the above bias control. Can be done. In addition, in FIG. 3, V _D is the converter 411, 41
2 maximum output voltage.

As shown in FIG. 3, by asymmetrically controlling the output voltages V _1U and V _2U of the converters 411 and 412 as shown in the figure, a symmetrical trapezoidal waveform output voltage of a predetermined frequency is obtained.
In obtaining V _U , if the converter 411 is operated at the maximum output voltage V _D , the output voltage V _2U of the converter 412 is given by V _U - V _D , and conversely, the converter 412
When operated at maximum output voltage V _D , the output voltage V _1U of converter 411 is given by V _U −V _D.
In this way, one of both converters 411 and 412 will always be operated at the maximum output voltage V _D ,
It is possible to improve the input power factor.

The reactive power generated by the cycloconverter when the above control is performed is compared to the case with bias control.
FIG. 4 shows the results calculated as a function of the output voltage ratio γ when the input transformer secondary voltage value is set to the same value in each case, with P _IB and no case being P _IN . Here, the reactive power P _I is standardized so that the motor active power P _R is 1 (=100%) when the output voltage ratio γ=1. The output voltage γ is defined as input transformer secondary voltage effective value V _I and cycloconverter output line voltage effective value V _O , γ=√2・(V _O /√3)/(3/π)・√ 2.V _I ... is defined by (1), and its maximum value is 2/√3 in the case with bias control, assuming γ=1 in the case without bias control. This means that with bias control, the peak value of the cycloconverter output voltage V _U is the sine wave phase voltage V _P of the broken line in Figure 3.
This is because it can be reduced to √3/2 with respect to the peak value V _M of .

[Problems with background technology]

As is clear from Fig. 4, once the input transformer secondary voltage is selected to be reduced by applying bias control, the operation in the region where the output voltage ratio γ is small is lower than that without bias control. It can be seen that the generated reactive power is smaller than in the case with. In addition, in Fig. 4, when the output voltage ratio γ is 1 or less, the output voltage V _U of the cycloconverter
This shows that it is possible to operate with the selected transformer secondary voltage even if it exhibits a sine wave. In this case, when the bias voltage is superimposed, the peak value of the output voltage of the cycloconverter is suppressed to a low value, and on the contrary, the reactive power increases.

In general, the input transformer secondary voltage must be selected to allow stable operation under the maximum voltage of the AC motor during overload operation at maximum frequency. In particular, when there is automatic weakening operation of a large-capacity motor, such as when applied to a main rolling machine, the motor terminal voltage at the base speed and top speed often changes significantly, and therefore the output voltage decreases in the weakening region and below the base speed. It will be operated at a value where the ratio γ is smaller than the maximum value. Therefore, in the conventional control method of superimposing a bias voltage at a fixed ratio on the sinusoidal phase voltage of the motor, the effect of reducing reactive power is reduced at a value other than the maximum output voltage value of the cycloconverter. Additionally, the maximum reactive power generated by the cycloconverter is during overload operation below the base speed, and the fact that the power factor improvement effect is small under this condition is
This also results in the loss of the economic effect of reducing the maximum reactive power and lowering the filter capacity of the grid.

[Purpose of the invention]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a cycloconverter control method that eliminates the drawbacks of the prior art described above and can improve the input power factor even when the motor voltage takes a value lower than the maximum value. It is in.

[Summary of the invention]

In order to achieve the above object, the present invention reduces the output voltage peak value of the cycloconverter to the maximum extent with respect to the maximum phase voltage value of the motor, and as a result, lowers the input voltage of the cycloconverter, that is, the secondary voltage of the input transformer. In order to be able to select the
The amount of alternating current bias of harmonics is superimposed, and if the motor phase voltage is less than the maximum value, the amount of bias superimposed on the sine wave phase voltage is adjusted so that the motor can operate at an output voltage as close to the maximum value as possible. It is characterized by:

[Embodiments of the invention]

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

Figure 5 shows that the motor phase voltage V _P (U phase) is V _M・sinωt
This figure shows the output voltage V _U of the U-phase converter 41 and the bias voltage V _B superimposed on the motor phase voltage V _P when it takes the maximum value in the form of . This bias voltage is given as V _B =1/6V _M・sin(3ωt), and the motor voltage V _P =V _M・
For sinωt, the amplitude component 1/6·V _M is given from the condition of minimizing the peak value of the converter output voltage V _U.
Although this point is the same as the conventional system shown in FIG. 3, the present invention is characterized in that the bias voltage is set to this value only when the motor phase voltage gives the maximum value. Converter output voltage V _U
is expressed as the sum of the motor phase voltage V _P and the bias voltage V _B , and it becomes V _U = V _M · sinωt + 1/6 V _M · sin (3ωt) ... (2).

As already mentioned, in the conventional control method shown in FIG. 3, 1/6 of the phase voltage output peak value V _M is always uniquely given as the bias amount over the entire output voltage range of the motor. Therefore, when the motor output voltage is low, the peak value of the converter output voltage V _U is suppressed to a low value, and the effect of reducing reactive power due to the trapezoidal waveform output voltage is reduced.

On the other hand, in the embodiment of the present invention described here, 1/6 of the motor voltage output peak value V _M is added as the bias amount in order to select the input transformer secondary voltage as low as possible. Limited only to the case of the motor maximum output voltage value, and when the motor phase voltage is less than the maximum value, as shown in FIG. 6, the bias voltage V _B is changed depending on the magnitude of the motor phase voltage V _M The size shall be adjusted according to the following formula:
That is, V ₁₀₀ ≦V _M ≦V ₁₁₅

Claims (1)

[Scope of Claims] 1. Conversion means for obtaining a second alternating current voltage of variable voltage and variable frequency from the first alternating voltage for each phase of the alternating current motor;
In the cycloconverter control method of controlling the second AC voltage into a trapezoidal waveform by superimposing an AC bias voltage of twice the frequency, if the maximum output voltage of the conversion means is equal to or higher than the peak voltage of the AC motor, the AC A cycloconverter control method, characterized in that the bias voltage is set to zero, and when the maximum output voltage does not reach the peak value voltage, the AC bias voltage is controlled to a value such that the output voltage of the conversion means is equal to or lower than the maximum voltage. .