JPH05168244A - Pwm control device - Google Patents

Abstract

PURPOSE:To enable a magnetic noise of an AC electric motor which is connected to a three-phase PWM inverter to be reduced without increasing a switching frequency. CONSTITUTION:A modulation rate corresponding to a three-phase voltage command is input and then an offset value which minimizes an evaluation function expressing a constituent of a high frequency of voltage between output lines of a three-phase PWM inverter 1 is obtained. A value which is obtained by subtracting the offset value from the modulation rate is compared with a triangular wave at a comparator 5 as a new modulation rate and then a switching signal controlling three switches of the PWM inverter 1 is output. A frequency of the triangular wave is controlled according to a value of the evaluation function.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width control device for a voltage type PWM inverter, and more particularly to a technique for reducing magnetic noise of an electric motor connected thereto.

[0002]

2. Description of the Related Art A conventional technique will be described with reference to FIG. Three-phase P
In the system in which the AC motor 2 is driven by the WM inverter 1, the ON / OFF signals Su, Sv, Sw of the three switching elements of the three-phase PWM inverter are compared with the modulation factor proportional to the three-phase voltage command and the triangular wave carrier. Obtained by That is, the modulation rates Vu, Vv, Vw of the modulation rate generator 3 and the triangular wave C of the output of the carrier generator 4
a and are input to the comparator 5, and Vu> C in the comparator 5.
If a, then Su = 1 and if Vu <Ca, output a switching signal Su such that Su = 0. Similar switching signals Sv and Sw are output in the case of Vv and Vw. 3-phase PW
The M inverter inputs them, and if Su = 1 means connecting the U-phase switch to the positive side of the DC power supply 11 of the inverter 1, and if Su = 0, switches the U-phase switch to the DC power supply of the inverter 1. Means to connect to the negative side of 11.
The same applies to the V-phase and W-phase switches.

[0003]

The voltage waveform between the output lines of the three-phase PWM inverter is a rectangular wave and has harmonics.
Therefore, when an electric motor is connected to the output, magnetic noise caused by its harmonics is generated from the electric motor. The prior art f _c ± 2f mainly of harmonics is sideband of 1 times the frequency of the triangular wave by, f _c ± 4f (or f _c component) and, 2
Double sidebands 2fc ± f, 2fc ± 5f (more than 2f
c component). Where fc is the frequency of the triangular wave,
f is the frequency of the voltage command. In order to reduce the magnetic noise of the above problem, the frequency of the noise should be higher than the human audible frequency. Therefore, the frequency fc of the triangular wave may be set higher than the audible frequency, but since the switching frequency of the switching element and the frequency of the triangular wave are equal, there remains a problem that switching loss increases. The present invention solves this problem and is also effective when it is desired to reduce the noise of either the fc component or the 2fc component, or when resonance of the mechanical system due to one of those components is avoided. Can adapt to.

[0004]

In order to solve the above problems, in the invention according to claim 1, the modulation factor generating means for outputting the modulation factors Vu, Vv, Vw corresponding to the voltage command and the output thereof are provided. Offset generating means for inputting and outputting an offset value Vo which minimizes an evaluation function representing one or a plurality of harmonic components contained in the output line voltage waveform of the three-phase PWM inverter, and the modulation factors Vu, Vv, Subtract offset value Vo from Vw Vu '= Vu-Vo ... [1] Vv' = Vv-Vo ... [2] Vw '= Vw-Vo ... [3] Subtracting means and the subtraction Vu 'of the output of the means,
Comparing means for inputting Vv ', Vw' and the triangular wave of the carrier generating means and outputting switching signals Su, Sv, Sw of the three-phase PWM inverter according to their magnitude relation is provided.

Here, the evaluation function of the offset generating means is
Determine as follows. FIG. 3 shows the waveforms of the voltage commands Vu ', Vv', Vw 'of the output of the subtraction means, which are compared with one cycle of the triangular wave, and the switching signals Su, Sv, Sw of the comparison result which are the outputs of the comparison means, and the PWM inverter. Su-Sv, Sv-Sw, Sw- equivalent to the output line voltage waveform of
The waveform of Su is shown. As shown in the figure,
Corresponding from π radians to π radians, and assuming that the frequency of the triangular wave is very large due to the change in the modulation rate, the switching times P _u1 and P _u2 of the U phase are P _u1 = −0.5 (Vu ′ + 1) π, P _u2 = − P _u2 ... [4] Switching times P _v1 , P _v2 , P _w1 , of the other phases,
The same applies to P _w2 . For example, the evaluation function of the fc component is obtained. Among the harmonics of the Su-Sv waveform, the magnitude of the component of the same frequency fc as the triangular wave is

[Equation 1] = (2 / π) {sin (P _v1 ) −sin (P _u1 )} ... [5]. Similarly, the fc components of the Sv-Sw and Sw-Su waveforms are also Av = (2 / π) {sin (P _w1 ) -sin (P _v1 )} ... [6] Aw = (2 / π) {sin ( P _u1 ) -sin (P _w1 )} ... [7]. The square root of the sum of these squared values is used as the magnitude V of the modulation factor.
Divided by

[Equation 2] Be the evaluation function of the fc component. Where V is

[Equation 3] Is.

The evaluation function of the 2fc component is also the same.

[Equation 4] Becomes Here, A _2u = (1 / π) {sin (2P _v1 ) −sin (2P _u1 )} ... [11] A _2v = (1 / π) {sin (2P _w1 ) −sin (2P _v1 )} ... [12] A _2w = (1 / π) {sin (2P _u1 ) -sin (2P _w1 )} ... [13]. In the offset generating means, the modulation factors Vu, Vv,
Vo which minimizes the above evaluation function is obtained from Vw and output. However, Vo is set to a range in which the new modulation factors Vu ', Vv', and Vw 'do not exceed the maximum peak value of the triangular wave.

The invention according to claim 2 further comprises carrier generating means for inputting the evaluation function value output from the offset generating means and increasing the frequency of the triangular wave to be output as the magnitude thereof increases.

[0008]

According to the first aspect of the invention, in the prior art, even if the same value is subtracted (or added) for all three phases from the modulation rate of three phases, P
This is based on the fact that the output line voltage of the WM inverter does not change. That is, Vu'-Vv '= Vu-Vv, and the output line voltage does not change. The offset value Vo to be subtracted is obtained by the offset generating means so as to minimize the value of the evaluation function representing the fc component or the 2fc component of the harmonic components of the output line voltage waveform of the PWM inverter, and the subtracting means is used for the three-phase. The offset value is subtracted from the voltage command of, the triangular wave is compared with the offset value, and the switching signal is output. In this way, the voltage between the output lines of the PWM inverter does not change, and only the harmonic components represented by the evaluation function contained therein can be minimized, so that the frequency of the triangular wave is not increased, that is, the switching element The magnetic noise due to the above harmonics can be reduced without increasing the loss.

According to the second aspect of the present invention, since the evaluation function represents the magnitude of the harmonic component, the frequency of the triangular wave is increased when the value of the evaluation function is large, and the frequency of the triangular wave is decreased when the value of the evaluation function is small. Lower. That is, when the value of the evaluation function is large, the magnetic noise becomes large. Therefore, the frequency of the triangular wave is increased so as to increase the frequency of the magnetic noise so that it does not become too large. By doing so, the magnetic noise can be further reduced as compared with the invention of claim 1 without raising the average frequency of the triangular wave so much.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG.
The modulation factors Vu, Vv, and Vw corresponding to the three-phase voltage commands output from the modulation factor generator 3 are input to the offset generator 6. There, an offset value Vo that minimizes the evaluation function represented by the equation [8] and an evaluation function value Ifc at that time are obtained. The subtracters 71, 72, 73 perform the operations [1], [2], and [3] to output new modulation rates Vu ', Vv', and Vw '. The carrier generator 4 inputs the evaluation function value Ifc of the output of the offset generator 6, and outputs a triangular wave Ca having a higher frequency as the value is larger. In the comparator 5, subtractors 71, 72, 73
Depending on the magnitude relationship between the output of the carrier generator 4 and the output of
The switching signals Su, Sv, Sw are output as in the conventional method.

[0011]

According to the present invention, since the three-phase modulation rate is modified by the optimum offset value using the evaluation function and is modulated by the triangular wave, it is included in the output line voltage waveform of the three-phase PWM inverter. The fc component of the harmonic can be minimized, and the magnetic noise from the AC motor connected to the three-phase PWM inverter can be reduced without increasing the frequency of the triangular wave. Further, by increasing the frequency of the triangular wave only when the value of the evaluation function is large, it is possible to reduce the magnetic noise of the electric motor without increasing the average frequency of the triangular wave too much.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a PWM control device of the present invention.

FIG. 2 is a block diagram of a conventional PWM control device.

FIG. 3 is a relationship diagram of a triangular wave, a modulation rate, and a switching signal.

[Explanation of symbols]

 1 3-phase PWM inverter 2 AC motor 3 Modulation rate generator 4 Carrier generator 5 Comparator 6 Offset generator 11 DC power supply 71, 72, 73 Subtractor

Claims (2)

[Claims] 1. A modulation rate generation means for generating a modulation rate proportional to a three-phase voltage command, a carrier generation means for generating a one-phase triangular wave, an output of the modulation rate generation means and an output of the carrier generation means. In the PWM control device comprising a comparison means for inputting and, and outputting a signal for controlling ON / OFF of three sets of switching elements connected in series in the three-phase PWM inverter, the output of the modulation factor generating means is input. Offset generating means for generating an offset value that minimizes an evaluation function representing a harmonic component of one or more frequencies included in an output line voltage waveform of the three-phase PWM inverter; and an output of the modulation factor generating means. And a subtraction unit for subtracting the offset value of the output of the offset generation unit from the three-phase modulation factor. 2. The PWM control device according to claim 1, further comprising carrier generation means for inputting the evaluation function value output from the offset generation means and increasing the frequency of the triangular wave according to the magnitude thereof. ..