JPH0669290B2 - Cycloconverter control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a cyclone system in which an AC bias voltage is superposed so that an output voltage supplied to an AC motor as a load can be higher than an input voltage. The present invention relates to a converter, and more particularly, to a control method of a cycloconverter suitable for an AC variable speed drive system in which its output frequency range is required to be operated close to an operable upper limit value.

[Conventional technology]

Cycloconverters have been widely put into practical use because they have various features as a power supply device for variable-speed driving of AC motors of relatively large capacity, such as induction motors.
On the other hand, some restrictions on the contents of control and characteristic methods are found, and examples thereof include JP-A-60-194794 and JP-A-60-28772. it can.

That is, of these publications, the former (JP-A-60-194794)
No.) superimposes an AC bias voltage with a triple frequency on the terminal voltage of an AC motor to be driven by a cycloconverter,
This discloses a control method characterized in that the output voltage of the cycloconverter is approximately trapezoidal, and as a result, the cycloconverter can operate under an output voltage larger than the input voltage of the cycloconverter, while the latter ( JP 60-
No. 28772) discloses that the output voltage should be limited due to the relationship between the input frequency and the output frequency of the cycloconverter.

[Problems to be solved by the invention]

By the way, as can be seen from the above-mentioned prior art, in a variable speed drive system for an AC motor using a cycloconverter, a so-called AC variable speed drive system, it is sufficient to increase the input voltage of the cycloconverter It is desirable that a high electric machine voltage can be obtained and that the output frequency can be controlled sufficiently higher than the input frequency.

However, in the above-mentioned conventional technology, no consideration is given to increasing both the output voltage and the frequency in comparison with the cycloconverter input voltage and the frequency. Therefore, it is necessary to increase the cycloconverter input voltage. However, there is a problem that the cost is increased due to the increase of the withstand voltage of the semiconductor switch element to be used and the increase of the capacity of the transformer on the power source side.

An object of the present invention is to provide a control method of a cycloconverter which can be applied to a high-speed and large-capacity AC variable speed drive system and can be operated up to a sufficiently high frequency without increasing the capacity of the power supply side device.

[Means for solving problems]

The above object is to control the bias voltage superposition ratio in accordance with the output frequency in a bias control type cycloconverter that superimposes an AC bias voltage having a tripled frequency on the output voltage of the cycloconverter to cope with an increase in the output frequency. Is achieved by reducing the bias voltage.

[Work]

In the cycloconverter, normal operation can be obtained only in a range in which the rate of change of the output voltage does not exceed the rate of change of the input voltage.

On the other hand, in the bias control method, the rate of change of the voltage is increased by superimposing the bias voltage.

Therefore, by controlling the bias voltage superimposition rate and decreasing the superimposition rate as the output frequency increases, an increase in the output voltage change rate can be suppressed, so that operation can be continued up to a high frequency.

〔Example〕

The control method of the cycloconverter according to the present invention will be described in detail below with reference to the illustrated embodiment.

FIG. 2 is a schematic block diagram of an AC variable speed system to which an embodiment of the present invention is applied. In the figure, AC power having a voltage v _i and a frequency f _i is converted into a cycloconverter 3 from a power source 1 through an input transformer 2. , The cycloconverter 3 performs the conversion operation, and the voltage v ₀ and the frequency f
The AC power of ₆ is output and supplied to the AC motor 4. Then, the terminal voltage of the electric motor 4 becomes v _m.

Therefore, a control device (not shown) in the cycloconverter 3 generates an AC power having an arbitrary variable voltage v ₀ and a variable frequency f ₀ according to operating conditions such as the rotation speed required for the electric motor 4, A variable speed drive of the electric motor 4 is realized.

Next, the waveform of each part in the embodiment of FIG. 2 is shown in FIG. 3 for a motor voltage v _m that changes sinusoidally
By superimposing the bias voltage v _B of the double frequency, the output of the cycloconverter becomes a substantially trapezoidal output voltage v ₀ ,
The peak value V ₀ is reduced. By doing so, the output voltage E _i (E _i = effective value of v _i ) of the input transformer 2 can be reduced. Conversely, for the same E _i ,
The electric motor terminal voltage E _m (E _m = effective value of v _m ) can be increased.

Therefore, the ratio of increase in the input voltage and the terminal voltage at this time _Em /
Consider a 72-arm cycloconverter with E _i as an evaluation value for bias control.

First, looking at the maximum voltage that can be output, the peak value V ₀ (FIG. 3) of the output voltage v ₀ is the input voltage v _i and the minimum conduction angle of the thyristor which is the main circuit switching element of the cycloconverter 3. If the effective value of the input voltage v _i is limited to E _i by E _i , this peak value V ₀
Is determined by the following equation (1).

Next, let E _{m be} the effective value of the motor voltage v _m , and K = V _m /
If you say V ₀ , So Becomes

Next, let us consider the condition where the rate of change of the output voltage does not exceed that of the input voltage, that is, the condition of the following expression (3).

Here, when the input voltage v _i is 72 arms, Can be expressed as

On the other hand, the output voltage v ₀ is the fundamental wave and the third harmonic (bias)
Is the sum of Can be expressed as Here, a is the bias voltage superimposition ratio, and ω _i and ω ₀ are 2πfi and 2πf0, respectively. (3) ，
From (4) and (5) To get

Fig. 4 shows the equations (2) and (6) in Fig. 4, which is the reciprocal of the superposition ratio of the third harmonic bias voltage, that is, 1 / a,
The output voltage ratio, that is, the magnitude of E _m / E _i is shown.In this figure, the curve 10 shows the limiting equation (2) from the maximum output voltage, and the voltage ratio E _m / E _i can be The range is the area below curve 10.

If the influence of frequency is not taken into consideration, E _m / E _i has a maximum point 17 at a = 1/6, as is conventionally said.

On the other hand, the curve groups 11 to 16 show the expression (6) for limiting the rate of change of the output voltage with the output frequency f ₀ / f _i standardized by the input frequency f _i as a parameter. The range of output voltage E _m / E _i is the area below each curve.

Therefore, the optimum value of the superposition rate a determined by the two conditional expressions (2) and (6) is in the common area of the conditional expressions (2) and (6).
It is expressed as a point that maximizes the output voltage ratio E _m / E _i . That is, in f ₀ / f _i ≦ 0.65, the maximum point of a = 1/6, and f ₀
As / f _i increases from 0.65, the maximum shifts to the right on curve 10. On the other hand, when the bias control is not performed, K = 1 in the equation (2), the maximum value of E _m / E _i is 1.73, and the straight line 18 shows this. Therefore, the amount of increase in output due to the bias control is above this straight line.

As described above, the optimum value of the superposition rate a determined by the conditional expressions (2) and (6) is the output voltage ratio _Em / E in the common region of these expressions (2) and (6). _It is expressed as a point that maximizes _{i. In} other words, this is to control the bias voltage superposition rate a to be high to the limit where the rate of change of the output voltage v ₀ does not exceed the rate of change of the input voltage v _i . Therefore, in the above embodiment, the bias is controlled so that this condition is satisfied.

FIG. 1 shows the value of the superimposition ratio a necessary to give the maximum value of the voltage ratio E _m / E _{i to} the output frequency f ₀ / f _i as the reciprocal 1 / a of a and Voltage ratio E _m / corresponding to 1 / a at
It shows the value of E _i , and therefore represents the characteristics obtained by the above-mentioned embodiment.

As is apparent from FIG. 1, if the bias voltage superposition rate is changed according to the output frequency, that is, as in the above-described embodiment, the output voltage ratio is increased to a sufficiently high frequency. I know what I can do.

FIG. 5 shows a comparison with the conventional bias control, that is, a case where a constant control is performed with a = 1/6. In FIG. 5, a curve 20 indicates a change in E _m / E _i according to the above-described embodiment. Curve 21 shows the case where constant bias control is performed at a = 1/6, and the straight line 22 does not perform bias control, that is, the case where a = 0.

Accordance connexion, according to the FIG. 5, see that to obtain a high output up to a frequency higher that of the above embodiment, after all, according to this embodiment, without increasing the input voltage v _i, sufficient The variable range can be widened to a very high frequency, and the AC motor 4 can be controlled at a variable speed over a wide range.

〔The invention's effect〕

According to the present invention, while fully utilizing the advantage of the bias control method of increasing the output voltage of the cycloconverter, the output frequency can be controlled as close to the input frequency as possible to the limit, so that the capacitance of the input transformer can be controlled. There is an effect that it is possible to provide a cycloconverter that can be applied to an AC variable speed drive system and can sufficiently improve its performance without increasing the cost due to the increase in the withstand voltage of the switch element.

[Brief description of drawings]

FIG. 1 is a characteristic diagram of an embodiment of a cycloconverter control method according to the present invention, FIG. 2 is a configuration diagram showing an example of an AC variable speed drive system to which the embodiment of the present invention is applied, and FIG. Waveform diagrams for explaining the operation, and FIGS. 4 and 5 are characteristic diagrams for explaining the operation, respectively. 1 ... AC power supply, 2 ... Input transformer, 3 ... Cyclo converter, 4 ... AC motor.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor, Mitsugu Matsutake 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi factory Hitachi Ltd. (72) Inventor, Junichi Takahashi 5-chome, Omika-cho, Hitachi-shi, Ibaraki No. 1 Incorporated company Hitachi Ltd. Omika factory (56) Reference JP-A-60-194794 (JP, A) JP-A-63-181665 (JP, A)

Claims (2)

[Claims] 1. A cycloconverter of the type in which an output voltage waveform is distorted into a trapezoidal shape by superimposing a bias voltage to increase the output voltage so that the rate of change of the output voltage does not exceed the rate of change of the input voltage. A control method for a cycloconverter, characterized in that the superposition rate of the bias voltage is controlled according to the output frequency. 2. The superposition rate of the bias voltage is a constant value until the output frequency of the cycloconverter reaches a predetermined value, and the frequency rises above this predetermined value. A control method for a cycloconverter, which is characterized by controlling so as to decrease in proportion to.