JPH0898540A - Pwm inverter - Google Patents

Abstract

PURPOSE: To simplify an output filter which reduces harmonic components. CONSTITUTION: In a PWM waveform generation circuit for a neutral-point clamp PWM inverter, carriers generated from two carrier-wave generators 4A, 4B and sine waves generated from a sine-wave generator 5 are level- compared by respective comparators 6A, 6B, outputs of both comparators are added by an addition circuit 6C, their phase is returned by a sign-inverting circuit 6D, and a PWM waveform which comprises a high level H, a low level L and a zero level 0 is generated. By this constitution, the carriers by both carrier-wave generators are changed into sawtooth waves. Thereby, a constitution in which the spectrum distribution of harmonic components contained in an inverter output concentrate near a frequency which is integral multiples of a carrier frequency and in which a specific frequency acts intensively on a specific frequency in an output filter to reduce the harmonic component, i.e., a filter constitution, is made simple, and the harmonic component is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a neutral point clamp type P
The present invention relates to a WM inverter, and more particularly to a PWM waveform generation method for reducing harmonic components.

[0002]

2. Description of the Related Art Inverters convert DC power from a DC power supply 1 into AC power whose frequency / voltage (or current) is controlled by controlling a switching mode of an inverter main circuit 2, as shown in FIG. The load 3 such as an induction motor is supplied.

In the PWM inverter, each phase switch of the main circuit 2 is controlled according to the PWM waveform. The PWM waveform for this is the carrier signal from the carrier wave generator 4,
The sine wave generator 5 controls the frequency f and the voltage V to generate a sine wave, and the PWM waveform generator 6 performs a level comparison. This PWM waveform is distributed by the gate circuit 7 as a gate signal required for each switch of the inverter main circuit 2. Reference numeral 8 is a filter for removing harmonics from the output of the inverter.

Here, in order to generate a PWM waveform, generally,
As shown in FIG. 3, a PWM waveform is generated by level-comparing a carrier of an isosceles triangle and a sine wave.

In the neutral point clamp type PWM inverter,
As shown in FIG. 4, the levels of two isosceles triangle carriers with a zero level as a reference are compared with a sine wave, and a high level H is obtained when the sine wave is higher than both carriers, and both sine waves are obtained. The low level L is obtained in the period lower than the carrier and the 0 level is obtained in the period in which the sine wave is between both carriers.

[0006] In addition to PWM, there is single edge modulation in which a carrier is a sawtooth wave, in addition to double edge modulation in which a carrier is an isosceles triangle. This single-edge modulation can simplify the circuit configuration and the like as compared with the double-edge modulation, but has a higher harmonic content rate. For example, when the modulation rate is 1, the distortion rate of double edge modulation is 33.8% and the distortion rate of single edge modulation is 43%.

[0007]

In the conventional neutral point clamp type PWM inverter by double edge modulation,
As shown in FIG. 5, the output voltage spectrum is distributed substantially evenly on the high-frequency side and the low-frequency side, centering on an integral multiple frequency of the carrier frequency.

For this reason, the frequency range in which harmonics of a non-negligible magnitude exist is considerably widened, and in order to remove all of them, a filter 8 having a complicated circuit configuration with a wide band is required, or a large number of them are required. A filter 8 provided with filters in parallel is required.

It is an object of the present invention to provide a neutral point clamp type PWM inverter which simplifies an output filter for reducing harmonic components.

[0010]

In order to solve the above-mentioned problems, the present invention provides a neutral point clamp type PWM provided with a PWM waveform generating means for generating a PWM waveform by comparing the levels of two carriers and a sine wave. In the inverter, the carrier has a sawtooth waveform.

[0011]

With respect to the neutral point clamp type PWM inverter in which the carrier waveform is a sawtooth wave, the spectrum of the output voltage can be obtained as shown in FIG. 6 in the simulation by the inventor of the present application.

This spectrum is similar to the case of the double edge modulation in that it is centered on a frequency that is an integral multiple of the carrier frequency, but it was found that the sawtooth wave is more concentrated in that distribution.

In the simulation in which the rising and falling slopes of the triangular wave are set to 1: 2 as the carrier waveform, the spectrum is close to the isosceles triangular carrier spectrum distribution as shown in the spectrum of FIG.
It was confirmed that the spread of the spectral distribution was not concentrated.

Further, in a normal PWM inverter, even if a carrier is a sawtooth wave, the spectrum distribution of the harmonic component does not change so much.

Therefore, the present invention provides a neutral point clamp type PW.
By making the carrier of the M inverter a sawtooth wave, the spectral distribution of the harmonic components contained in the output of the inverter is concentrated near the frequency that is an integral multiple of the carrier frequency.

As a result, the inverter output filter for reducing the harmonic component is concentrated on a specific frequency, that is, the filter structure is simplified to suppress the harmonic component.

[0017]

1 is a PWM waveform generation circuit diagram showing an embodiment of the present invention, showing elements 4 to 6 in FIG.

The sine wave generator 5 generates a sine wave whose frequency f and voltage V are controlled. First carrier generator 4A
Generates a carrier signal having a sawtooth waveform on the positive side with reference to the zero level.

The second carrier generator 4B generates a carrier signal having a sawtooth waveform on the negative side with reference to the zero level.

The PWM waveform generation circuit includes a comparator 6A
And 6B, an adding circuit 6C and a sign inverting circuit 6D. The first comparator 6A includes a first carrier generator 4
The carrier from A and the sine wave from the sine wave generator 5 are compared in level, and a high level H is obtained when the level of the sine wave is higher than the level of the carrier.

Similarly, the second comparator 6B has a second
The carrier from the carrier wave generator 4B and the sine wave from the sine wave generator 5 are compared in level, and a low level L is obtained when the sine wave level is lower than the carrier level.

The adder circuit 6C is composed of an operational amplifier OA and an operational resistance as an adder, and diodes 6D ₁ and 6 that slice the outputs from both comparators 6A and 6B at zero level.
A PWM waveform having a high level H, a low level L, and a zero level is obtained by adding both signals as an input signal via D ₂ .

The sign inverting circuit 6D is composed of an operational amplifier OA and an operational resistor as a polarity inverting circuit, and obtains an output obtained by inverting the phase of the output from the adding circuit 6C.

In this embodiment, carrier wave generators 4A, 4A
The carrier frequency of B is switched and controlled according to the output frequency of the sine wave generator 5 in the case where the inverter is controlled to a variable frequency and a variable voltage.

In the embodiment, an analog circuit configuration is shown. The carrier wave generators 4A, 4B and the sine wave generator 5 are generated by digital processing, and the comparator 6 is also used.
A similar operation and effect can be obtained by using A, 6B, the adder circuit 6C, and the sign inversion circuit 6D as a digital operation circuit.

[0026]

As described above, according to the present invention, the neutral point clamp type P having the PWM waveform generating means for generating the PWM waveform by comparing the levels of the two carriers and the sine wave is provided.
In the WM inverter, since the carrier has a sawtooth waveform, it is possible to concentrate the spectral distribution of the harmonic component included in the inverter output near the frequency that is an integral multiple of the carrier frequency, simplifying the filter configuration and reducing the harmonic component. Can be suppressed.

[Brief description of drawings]

FIG. 1 is a PWM waveform generation circuit diagram showing an embodiment of the present invention.

FIG. 2 is an example of a PWM inverter.

FIG. 3 is a general triangular wave comparison method for generating a PWM waveform.

FIG. 4 is a triangular wave comparison method of a neutral point clamp type PWM inverter.

FIG. 5 is a harmonic distribution in the case of isosceles triangular wave comparison PWM.

FIG. 6 is a harmonic distribution in the case of sawtooth wave comparison PWM.

FIG. 7 is a harmonic distribution in the case of triangular wave comparison PWM having a slope of 1: 2.

[Explanation of symbols]

 2 ... Inverter main circuit 4, 4A, 4B ... Carrier wave generator 5 ... Sine wave generator 6 ... PWM waveform generation circuit 7 ... Gate circuit 8 ... Output filter 6A, 6B ... Comparator 6C ... Addition circuit 6D ... Sign inversion circuit

Front page continuation (72) Inventor Yasuo Kataoka 2-1-1-17 Osaki, Shinagawa-ku, Tokyo Stock Company Shameidensha

Claims (1)

[Claims] 1. A neutral point clamp type PWM inverter having PWM waveform generating means for generating a PWM waveform by comparing the levels of two carriers and a sine wave, wherein the carrier is a sawtooth waveform. PW
M inverter.