JPH03139173A - Control method for multiplex current type inverter - Google Patents

Abstract

PURPOSE:To prevent simultaneous commutation of unit inverters and to suppress low order higher harmonic components by controlling each unit inverter such that the waveform is subjected to pulse width modulation, except the central part within a predetermined angle, at least in the low speed operation range of an AC motor. CONSTITUTION:PWM is carried out except the central 60 deg. section. iR2 lags by 30 deg. behind iR1. PWM is carried out for a combined waveform iR(iR1+iR2) except the central 30 deg. section. For the angles alpha1-alphaN, 30-60 deg. sections are (60-alphaN), (60-alphaN-1)...(60-alpha1) and there are N independent angle variables. In order to prevent simultaneous commutation of unit inverters, two angles beta, gamma are selected among a set of angles of iR1, iS1, iT1, iR2, iS2, iT2 so that they satisfy the formula shown on the drawing. thetamin is an angle corresponding to the commutation time defined by the turn OFF time of a thyristor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a control system for a multiple current source inverter suitable for precise control of an AC motor.

[Conventional technology]

FIG. 5 shows a configuration diagram when driving by a well-known current source inverter. Direct-coupled multiplexing is a method in which the current source inverter shown in FIG. 5 is used as a unit inverter and each phase of the output of each unit inverter is directly connected as shown in FIG. 2.

On the other hand, as shown in FIG. 7, a transformer-coupled multiple current inverter is a system in which each unit inverter is coupled via a transformer. Multiple current source inverters are used because they provide relatively better waveforms than unit current source inverters. For example, in the direct-coupled double type shown in FIG. 6, each unit inverter is operated with a phase difference of 30 degrees in electrical angle (hereinafter the same), so the waveform is improved as shown in FIG. 8.

In the unit inverter, as shown in FIGS. 8(a) and (b), the harmonic component of Ill+Ill is 5. 7.
it, 13th order of 20%, 14.3%, 9.1%,
Contains about 7.7%.

On the other hand, the harmonic component of tx=t*++t@2 is
5. As shown in FIG. 8(C). 7.11.13 The fifth-order and seventh-order components decrease in the order of 5.4%, 3.8%, 9.1%, and 7.7%. However, lower order 5, 7
Due to torque pulsation due to the following components, the speed may fluctuate at low speeds, making precise speed control impossible.

Therefore, a method is used in which the current of each unit inverter is pulse width modulated (hereinafter referred to as 'PWMJ').

Conventionally, there is a control method disclosed in Japanese Patent Publication No. 6O-52671). As shown in Figures 9(a) and 9(b), this is the output current of the unit inverter 'll+'1
2 is a 30° section BPWML, and the combined current tl is controlled so as to have a trapezoidal wave on average, as shown in FIG.

In this case, if the unit inverters commutate at the same time, commutation will not occur through the electric motor that is the load, but commutation will occur between the unit inverters. During commutation, a current source inverter uses the induced voltage of the load to perform commutation, so commutation may fail if the current is commutated between unit inverters without passing the load. In order to avoid this phenomenon, in the example shown in FIG. 9, when one unit inverter is commutating by PWM, the other unit inverters do not perform PWM.

[Problem to be solved by the invention]

However, since tll in FIG. 9(C) is an average trapezoidal wave, the harmonic component is 5. 7.11.13
Contains 4%, 2%, 0.8%, and 0.6% in the following order. However, since speed fluctuations at low speeds depend on torque ripple, the lower the frequency of torque ripple, the larger the speed fluctuations. In this case, due to the 5th and 7th harmonic components,
A torque ripple with a frequency six times the frequency f of the fundamental wave component of i occurs, and speed fluctuation becomes large. In addition, when the resonant frequency of the mechanical system is low, as seen in blowers, etc., resonance occurs due to this frequency and the torque ripple of the 6f component. It is required to raise the minimum frequency of
The control method shown in FIG. 9 has a problem in that low-order components such as the fifth and seventh orders cannot be suppressed.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an inverter control method using a PWM method that prevents simultaneous commutation between unit inverters and suppresses low-order harmonic components.

[Means to solve the problem]

In order to achieve this objective, the multiple current source inverter control method of the present invention provides a multiple current source inverter for driving an AC motor in which the outputs of a plurality of unit current source inverters are connected directly or via a transformer. In the control method, at least in the low speed operating range of the AC motor, the center of the waveform ±30
Under the conditions that each unit inverter is controlled so as to pulse width modulate the part other than °, and the commutation interval of each unit inverter and the commutation interval between each unit inverter is a certain value or more, the harmonic component of the current is The present invention is characterized in that each unit inverter is controlled so that the evaluation index configured by is minimized.

〔Example〕

Hereinafter, the present invention will be specifically explained based on Examples.

FIG. 1 shows a current waveform in an embodiment of the present invention.

tll is symmetrical with respect to the intersection with the θ axis, e.g. θ; 0° θ=180°, and the center point of the waveform, e.g. θ-90°,
It is assumed that there is line symmetry with respect to θ=270°.

PWM is performed on the sections excluding the central 60° section. t12 has the same waveform as CHI, but its phase lags tll by 30°. Composite current waveform εx (t@=61□
+L 112) is a section in which the central 30° section is not PWMed, and the other sections are PWMed.

Furthermore, Figure 2 (
Shown in a). For the angle α1~αi between 0~309, the 30~60° interval is (60−α,), (60−
αl+-1)... (60-α1), and the number of independent angle variables is N.

When h is expanded into a Fourier series, it can be expressed by equations (1) and (2).

cos n(-T-α2) +cos n(,-α*)-
cas n(-1 α,)...) (2) However, θ=(IJ t, n=1.5.7.1
1. 13. ...Generally, when PWM as shown in FIG. 2 is performed, the fundamental wave component in equation (1) becomes smaller than when no PWML is used.

Furthermore, since the characteristics of an AC motor are approximately determined by the fundamental wave component, the fundamental wave component must be set to a certain value or more. Therefore, in determining α, a condition is first established that the magnitude of the fundamental wave component is greater than or equal to Pl. Next, by selecting α, lower harmonic components are made zero. In this case, for the number N of independent variables, one is used to secure the fundamental wave component, and the remaining (
N-1) are used to suppress harmonic components, so (N
-1) harmonic components can be prevented from being generated. For example, when N=4, it is possible to prevent the generation of 5.7.11th order components. Therefore, evaluation index P
is defined as equation [3].

+　l　l　・　Φ (3) However, the number after a is (N-1>).

Next, in order to prevent commutation between unit inverters at the same time, '@I-tsI+tTI+'1
2+εSi+'? The angles are selected so that the following equation holds true for any two angles β and T in the set of two angles.

β−rl>θ,i, (4) Here, θ,7 is an angle corresponding to the commutation time defined by the turn-off time of the silisk.

Even if PWM is performed under the conditions of equation (4), there is no commutation failure and stable commutation is performed. The problem of determining the angle so as to minimize P and satisfy equation (4) is a constraint optimization problem, and it is well known that it can be determined by nonlinear programming or the like. For example, assuming a certain initial value, equation (3), (4)
Calculate α using the gradient method using a computer so that the formula is satisfied.

For example, when N=3, α, =8.4825°, α, =
12.293°, α, =24.05°. At this time, the fundamental wave component is 94.4%, and the fifth and seventh harmonics are zero. The frequency spectrum in this case is shown in FIG. N
Many harmonic components can be suppressed by increasing (4)
When the output frequency f of the inverter increases according to the conditions of the equation, it is necessary to decrease N. Therefore, for motor operation, the number of patterns is changed as shown in FIG.

In this case, N=0 means that PWM control is not performed.

It goes without saying that the evaluation index of formula (3) can be modified so that the angle is selected so that the effective value of the harmonic current is minimized in a case where the copper loss is to be minimized.

In direct connection type triple or higher inverters, the difference is that the phase difference between the unit inverters is generally 60/KC degree], where K is the number of other inverters, but the method for determining the angle is the same as in the case of connected duplexes.

In the transformer-coupled type shown in FIG. 7, each unit inverter is operated with a phase difference of 60/KC degrees, similar to the direct-coupled type. Further, in the transformer-coupled type, specific harmonics are not generated without PWM. For example, in a double type, if the transformer winding connected to INVI is Y-connected and the one connected to INV2 is Δ-connected, the fifth and seventh harmonics will not be generated. Therefore, for P in equation (3), by using harmonics other than harmonics that are canceled by the transformer, the commutation angle can be determined in the same manner as in the direct coupling type.

〔Effect of the invention〕

As described above, according to the present invention, harmonics can be suppressed while performing stable commutation, so torque pulsation is reduced, speed fluctuations based on this are also reduced, and the speed is lower than that of the conventional method. This enables precise AC motor operation.

[Brief explanation of the drawing]

FIGS. 1 and 2 are waveform diagrams according to an embodiment of the present invention, and FIG.
The figure is a frequency spectrum diagram of current in an embodiment of the present invention, FIG. 4 is an explanatory diagram of switching the PWM pattern in the embodiment, FIG. 5 is a circuit diagram for driving an AC motor by a unit inverter, and FIG. A circuit diagram of the direct-coupled multiplex type, Fig. 7 is a circuit diagram of the transformer-coupled type, Fig. 8 is a current waveform diagram of the direct-coupled duplex type without PWML, and Fig. 9 is a waveform diagram showing an example of the current waveform of the conventional method. It is. INV, ~INV, l: Unit inverter M: AC motor T: Transformer Vs, Vs+ ~Vsx: DC power supply Figure 1 Figure 3 1 3 7 9 3 Harmonic order 5 9 1 450- Figure 2 Figure 4 Figure 8 Figure 9

Claims (1)

[Claims] 1. A method for controlling a multiple current source inverter for driving an AC motor in which the outputs of a plurality of unit current source inverters are connected directly or via a transformer, at least in the low speed operating range of the AC motor. Then, the center of the waveform ±
Each unit inverter is controlled to pulse width modulate the part excluding 30 degrees, and the harmonics of the current are A method for controlling a multiple current inverter, comprising controlling each unit inverter so that an evaluation index composed of components is minimized.