JPS62217860A - Control system for voltage-type pwm inverter - Google Patents

Abstract

PURPOSE:To easily eliminate higher harmonic, by combining frequency component nfc+ or -mf0 (m, n are natural numbers) generated by switching action, with the signal of opposite phase, when each fundamental frequency of the modulation signal and the reference signal of a PWM inverter is fc, f0. CONSTITUTION:The phase theta0 of desired fundamental wave component and the phase thetac of modulation signal are directed to ROMs 11, 12 for input, and are multiplied 41 by m1 and multiplied 42 by n1 to be added 51 to each other. The phase is directed to a ROM 13 for input to obtain sine wave output. In the meantime, by a ROM 14, a non-linear function with the input of control factor lambda is produced, and higher harmonic DELTAe0 with the added output of the signal and of the ROM 13 multiplied 21 by each other is found out. To the reference signal e0 of a sine wave found out with the output of the ROM 11 and the control factor lambda multiplied 21 by each other, the higher harmonic signal DELTAe0 is added 52, and new reference signal e0 is produced and is compared with the modulation signal ec by a comparator 3. According to e0'>ec or e0'<ec, a switch on the positive side or on the negative side of a PWM inverter is turned ON. As a result, higher harmonic component is easily eliminated or reduced.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention reduces harmonics of a specific order contained in the output voltage in a voltage source PWM inverter that obtains single-phase or multi-phase AC voltage from a DC power supply. Regarding the control method for

[Conventional technology]

It is generally desirable that the output voltage waveform of a voltage source inverter be a sine wave with few harmonic components.

As a method for improving the output voltage waveform, for example, a pulse width modulation (hereinafter also abbreviated as PWM) method is generally known.

FIG. 5 is a circuit diagram showing a three-phase output voltage type inverter main circuit composed of transistor switches (hereinafter simply referred to as switches). To perform PWM control, the switches T and TI3 connected to the positive potential and the switches T'z+ to T0 connected to the negative potential are operated to output an alternating current voltage. For example, regarding the terminal voltage of the U phase, the positive side switch'
When l”11 is on and the negative side switch TZI is off, V
Switch T on d/2! When I is on and T is off, it becomes -Vd/2, and an alternating current voltage is obtained by changing the ratio of the period when the positive side switch is on and the period when the negative side switch is on. Traditionally commonly used sine wave PWM
In this case, by changing this ratio into a sine wave shape, it is attempted to reduce particularly low-order harmonic components. 6th
The figure shows an example of the terminal voltage waveforms of the U phase and ■ phase in FIG. 5, and the line voltage waveforms of both phases. In addition, in the same figure (a) is the U-phase output voltage, (b) is the V-phase output voltage, and (c) is U-V.
Each phase-line voltage waveform is shown.

Figure 7 shows the conventional sine wave pwMlil! An example of the 11B method is shown. In the same figure, 11 and 12 are respectively sine waves,
A memory in which triangular wave data is stored; 21 is a multiplier;
3 is a comparator. Also, θ. is the phase of the fundamental component of the desired output voltage, which is stored in read-only memory (
Hereinafter, it will be referred to as ROM. ) 11, and its output becomes a sine wave. This signal is multiplied by λ (hereinafter referred to as the control rate), which is the target value of the output voltage magnitude, in a multiplier 21 to obtain the target value eo of the output voltage (hereinafter referred to as the reference signal). . θゎ is the phase of a signal that modulates eo, and this is input to the ROM 12 to obtain, for example, a triangular wave signal ec (hereinafter referred to as a modulation signal). eo and e
The magnitude of c is compared by comparator 3, and when eo > ec, the positive side switch of the PWM inverter is turned on, and e6<e
When c, a signal S is obtained that turns on the negative side switch. 8th
The figure shows an example of the relationship between the reference signal e0, the modulation signal ec, and the output voltage waveform.

As is clear from the figures, the output voltages in Figures 6 and 8 contain quite a few harmonic components, and these harmonic components can have an unfavorable effect on the load or DC power supply connected to the output terminal, for example. It is known that certain frequency components of each harmonic component can be particularly harmful. For example, when driving an AC motor, the load is an inductance load, so low frequency The harmonic current due to the harmonic voltage component becomes a relatively large value, which significantly increases the winding temperature or causes large rotational irregularities.

The harmonic components included in the line voltage of the output terminal will be described in detail below, taking the case of a three-phase PWM inverter as an example.

That is, the output voltage can be classified into each frequency component shown below.

(a) Fundamental wave of reference signal e0 and harmonic components excluding multiples of 3 (b) nfc+mf0 component (hereinafter referred to as sideband). However, fc is the fundamental wave frequency of modulation signal ec, and fo
is the fundamental frequency of the reference signal e0. Further, n and m are natural numbers.

From the above, if the reference signal is a sine wave, there are no harmonic components belonging to the above item (a). Therefore, for example, if fc is set to a high frequency in an attempt to reduce the harmonics generated at low frequencies, the harmonic components belonging to the (b) term become harmonics, and the harmonic components at low frequencies become small.

[Problem that the invention seeks to solve]

However, when fc is increased, the number of times the switch is switched per time increases, the loss generated by the switch element increases, or the operation delay of the switch element increases.
Problems arise, such as not being able to perform desired switching. Therefore, as a method to reduce disturbances caused by output voltage harmonics of a PWM inverter, harmonics can be compensated for with a device that generates voltage or current that is in the opposite phase to the generated harmonics, or multiplexed inverters can be used to reduce harmonics. Although methods have been employed in the past, they generally have the problem of being expensive.

Therefore, the present invention determines the switching operation of the switch by an intermediate and inexpensive method without increasing the switching frequency of the special main circuit equipment or the switch, and reduces harmonic components that are harmful to the load in particular. The purpose is to provide a control method.

[Means for solving problems]

The fundamental frequencies of the modulation signal and the reference signal are respectively fc,
fo, PWM control is performed by superimposing frequency components nfc±mfo, which are generated based on the switching operation, on the reference signal, and frequency components that have the same frequency and magnitude but opposite phases. Let's do it.

[Effect]

In PWM, the switching operation of a switch is determined by comparing the magnitude of a modulation signal and a reference signal corresponding to the desired output voltage.When trying to obtain an output voltage close to a sine wave, the reference signal is generally set to a sine wave. Often called waves. However, based on the switching operation NFC:TM
A sideband wave of the fo component is generated. By the way, the frequency f in the reference signal.

If the signal obtained by adding the harmonic Δe0 with
Kinaru. The present invention reduces the frequency f to n1f, which is particularly harmful.
(:!=mtfo (n+ + mz are n, respectively)
The harmonics to be added to the reference signal are determined so as to match the frequency of the specific value of m), and so that both harmonics have equal magnitudes and opposite phases. As a result, both harmonics cancel each other out and no longer appear at the output terminal of the inverter. In terms of the ratio of the period during which the positive side switch and the negative side switch are on, the method of the present invention controls this ratio so that it becomes a sine wave containing the harmonics that you want to eliminate. It can be said that this prevents specific harmonics from being generated.

[Embodiments of the invention]

According to various studies so far, if the control rate λ is constant, the magnitude of the sideband wave depends on the modulation frequency rc and the reference frequency f0.
is constant regardless of Also, the phase of the sideband wave is the phase θ of the modulation signal. and the phase θ of the reference signal. given by only. Therefore, the magnitude of the harmonic Δe0 that should be added to eo in order to suppress the generation of a specific harmonic is a function only of the control rate λ, and its phase is θゎ and θ. It can be found by

FIG. 1 is a block diagram showing an embodiment of the invention.

The parts indicated by the symbols If, 12, 21, and 3 in the figure are the same as in the case of FIG. 7, so the explanation will be omitted, and the differences will be mainly explained. 41°42 are m respectively
It is a multiplier of l and n3 times, and m and n are respectively selected to values corresponding to the harmonic order to be reduced. 4
The outputs of 1.42 are added by an adder 51, and a sine wave output is obtained by the ROM 13 using this phase as an input. on the other hand,
The control rate λ is manually set as a nonlinear function by the ROM 14, and this signal and the output of the ROM 13 are multiplied by the multiplier 22 to obtain Δe0. Δe0 is added to the sine wave reference signal e0 in the adder 52, and this signal is signal e
By using 0/ as a new reference signal and comparing the magnitudes of e0' and modulation signal e with comparator 3, on/off of the switch is determined as in the case of FIG. e,' is a signal in which harmonics of n, fc + m, f components are added to a sine wave, and by determining the operating state of the switch according to this signal, an output that does not have harmonic components of n Hfe + m + fo components is obtained. A voltage waveform can be obtained.

Further, according to the principle of the present invention, it goes without saying that by adding a plurality of harmonics to eo, it is also possible to reduce a plurality of harmonics included in the output voltage. Second
The figure shows another embodiment of the present invention, in which the modulation frequency fc is expressed as an integral multiple of the reference frequency. In this case, both the modulation signal and the reference signal are θ. can be obtained from θ.

are manually input to ROM11 and 12 to obtain sine wave and triangular wave outputs, respectively. The signal Δe0 added to the reference signal e0 is also θ. and hence θ. is also input to the ROM 13. The function of the ROM 13 is a sine wave having a frequency that is an integral multiple of the fundamental wave frequency. Other parts 14.
21. 22. The functions of 3 and 52 are the same as in the case of FIG. 1, so a description thereof will be omitted.

In the above explanation, eo was explained as a sine wave, but in the case of three-phase output, even if eo contains harmonics that are multiples of 3, these harmonics will cancel each other out in the line voltage of the output terminal. Doesn't appear. For example, IEEJ Study Group Material EPA-7
9-3R Cycloconverter Power Supply AC Variable Speed Drive System and Converter Capacity", a method is proposed in which the target value of the -phase output voltage, that is, eo, includes harmonics that are multiples of 3 to make eo into a trapezoidal waveform. If adopted, the fundamental wave component of the output voltage for the same power supply voltage can be increased, which is even more preferable.

In addition, when multiplexing multiple PWM inverters to reduce the output voltage, for example, when duplicating
The present invention can eliminate harmonic components that cannot be eliminated by duplication, and as a result, the present invention is also effective for multiple PWM inverters.

In Figure 3, the modulation frequency fc is set to nine times the reference frequency f0,
An example will be shown in which the waveforms of each part of the one-phase output voltage, current, and torque generated by the induction motor are obtained by digital simulation when the induction motor is driven by a conventional general three-phase output pressure sinusoidal PWM inverter. in this case,
The output voltage has an fc-2f0 component as a low-order harmonic component, that is, re"9fo, so 9fo 2fo
A large amount of −7fo component (seventh harmonic) is included, and therefore the current also contains a large amount of the seventh harmonic. Furthermore, it is well known that the seventh harmonic current generates six times as much torque pulsation, and this also appears in the simulation results. On the other hand, FIG. 4 shows a case in which the seventh harmonic voltage is particularly reduced according to the present invention, and the reference signal shown by the dotted line in FIG. 7th harmonic components with various magnitudes and phases are included. At this time, the output voltage and current shown in Figures (B) and (C) hardly contain the 7th harmonic component, and the torque pulsation, which is six times the generated torque, is reduced as shown in Figure (D). , low-order torque pulsations are almost eliminated.

High-order torque pulsations are less likely to cause uneven rotation due to the effect of the moment of inertia of the electric motor, so stable rotation can be obtained.

〔Effect of the invention〕

According to the present invention, in a voltage-type PWM inverter in which the operation of a switch is determined by comparing the magnitude of a triangular wave modulation signal and a reference signal that is a desired value of output voltage, the reference signal has at least one or more The advantage of superimposing the harmonic components of the output voltage is that it is possible to eliminate or reduce the harmonic components of the output voltage having the same frequency.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing another embodiment of the present invention, and Fig. 3 is a conventional general 3
A waveform diagram showing the -phase output voltage, current, and motor generated torque when an induction motor is driven by a phase output pressure sinusoidal inverter. Figure 4 shows each component similar to Figure 3 when the present invention is applied. The waveform diagram shown in Figure 5 is a three-phase output voltage type PWM.
A circuit diagram showing a general example of an inverter main circuit, Fig. 6 is a waveform diagram showing the terminal voltage waveforms of the U phase and V phase in Fig. 5, and line voltage waveforms of both phases, and Fig. 7 is a pwM control A block diagram showing a general example of the system, and FIG. 8 is a waveform diagram of each part for explaining the operation of FIG. 7. Code explanation 11 to 14... Read only memory (ROM), 2
1.22... Multiplier, 3... Comparator, 41.42.
... Multiplier, 51.52 ... Adder, Tll to T11
... Positive side switch, T□~T2. ...Negative side switch, D, ~DI31D2. ~I) zt...diode. Agent Patent Attorney Akio Namiki Agent Patent Attorney Kiyoshi Matsuzaki Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7N λ Figure 8

Claims (1)

[Claims] The output voltage waveform is generated by switching and igniting a switching element connected to the positive potential or negative potential side based on the magnitude relationship between the modulation signal and a reference signal representing the output voltage command value. In this method, when the fundamental frequencies of the modulation signal and the reference signal are f_c and f_o, respectively, the nf_ that is generated based on the switching operation of the reference signal is
c±mf_o (n, m are natural numbers) A voltage source PWM inverter control method characterized by performing PWM control by superimposing a frequency component having the same frequency and magnitude as the frequency component and having an opposite phase relationship. .