JP2000184729A - Method for modulating pwm inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To change the electromagnetic sound of PWM from noisy monochromatic sound quality to white noise by changing the time ratio of the half cycle of a plurality of PWM modulations in the sample period for controlling current. SOLUTION: When the cycle and the current control cycle of PWM modulation are to be synchronized by a PWM inverter, the sample period of current control is set to two times larger or more than the half cycle of a PWM carrier. Then, the sample cycle period itself is made constant, and a plurality of half cycle periods of PWM included in it are distributed with a certain ratio instead of being equally distributed, thus changing time width for each half cycle and hence modulating a carrier frequency and changing the electromagnetic sound of PWM from noisy monochromatic sound quality to white noise for reducing noise in terms of auditory sensation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PWM inverter, and more particularly to a modulation method for reducing noise caused by a carrier frequency generated from a motor.

[0002]

2. Description of the Related Art FIG. 2 shows an example of a speed control device using a PWM inverter. The speed control of the induction motor 1 is driven by the inverter body 2 having a current control system in a minor loop of the speed control system.

[0003] Speed controller 3, a proportional integral deviation between the pulse pick-up and the speed detection signal of the motor 1 by the speed detector 4 by the speed detecting circuit omega _r and the speed command ω _r * (PI)
Calculate to obtain torque command T *. The vector control unit 5 obtains a current command i * from the torque command T * by a vector operation.

The current control unit 6 performs a proportional integral operation on a deviation between the current command i * and the output current i of the inverter body 2 to obtain a voltage control signal v *. PWM modulation unit 7, to obtain a phase PWM pattern signal PWM modulated by comparison of the voltage control signal v * and the carrier signal omega _c, on-off control of each arm of the inverter 2 by the PWM pattern.

In such a PWM inverter, the current control unit 6, the speed control unit 3, the vector control unit 5, and the PWM
In many cases, the M modulator 7 performs digital arithmetic processing using a microcomputer. In particular, in the case of the vector control system, digital arithmetic processing is often used. Further, in the case of using the speed sensorless method, a microcomputer may be provided with a magnetic flux observer function for estimating magnetic flux and speed from a current detection signal of the electric motor 1 and electric motor constants.

Here, since the PWM modulation section 7 modulates the voltage control signal with a triangular carrier frequency, an electromagnetic sound caused by the carrier frequency is generated from the electric motor. In particular, when the carrier frequency is 10 kHz or less, the sound quality is offensive to humans, and its frequency spectrum distribution is a single frequency.

Originally, the carrier frequency itself should be increased. However, from the viewpoint of increasing the switching loss and the amount of noise generated in the main circuit of the inverter, it is desirable to lower the carrier frequency as much as possible.

However, when the carrier frequency is low, the PWM
, The level of the electromagnetic noise of the motor increases.

Therefore, even if the carrier frequency is low,
A method has been proposed to disperse the frequency spectrum of electromagnetic sound over a wide frequency band by randomly modulating this frequency, and to improve sound quality by converting electromagnetic sound, which was conventionally a single spectrum, to white noise. I have.

[0010] This is called a random modulation method.
As shown in FIG. 2, the frequency or cycle of the carrier signal fc generated by the carrier signal generator 8 is randomly modulated by the random signal generator 9.

In this random modulation method, the carrier is FM
This is realized by performing modulation (frequency modulation) or switching a plurality of carrier frequencies at random.

[0012]

However, in the method of synchronizing the PWM cycle and the current control cycle, if the carrier is randomly modulated while the current control gain for each sample cycle is fixed, the sample cycle changes. As a result, the current control gain is also modulated.

In order to prevent the current gain from changing, there is a method in which the coefficient of the current control operation is also corrected in accordance with the carrier modulation and the gain is equivalently kept constant.

However, this has the disadvantage that the operation time of the microcomputer increases. In particular, when applying not only current control but also calculations such as a magnetic flux observer, there are many coefficients that are affected by the sampling period, and a large amount of calculation is required to correct carrier modulation.

An object of the present invention is to provide a PWM inverter modulation method in which the carrier frequency is modulated while the current control sample period is fixed to reduce motor noise.

[0016]

SUMMARY OF THE INVENTION According to the present invention, the amplitude and phase of a PWM current ripple are changed by changing the time ratio of a plurality of PWM modulation half periods within a current control sample period. The frequency component of the generated electromagnetic sound is changed from a single spectrum to a dispersed spectrum having a bandwidth, and is characterized by the following method.

In a PWM inverter that modulates a voltage control signal with a carrier signal synchronized with a current control cycle, a sample cycle for current control is set to a half cycle of the PWM modulation.
It is characterized in that the time ratio of a half cycle of a plurality of PWM modulations within the sample period is changed while the sampling period for current control is fixed at a constant value while synchronizing to the above integer multiple.

Further, the ratio of the half period of the PWM modulation is
It is characterized in that it is changed into a sine wave or a triangular wave synchronized with an integer multiple of the output frequency of the inverter.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present embodiment, in a PWM inverter in which the PWM modulation cycle and the current control cycle are synchronized, the half cycle of the PWM carrier is set to twice or more in the sample period of the current control. Then, the sample cycle period itself is fixed, and the time width of each half cycle is changed by distributing at a certain ratio instead of equally dividing a plurality of PWM half cycle periods included therein.

Here, PWM is one cycle (half cycle is twice)
When the random modulation is applied to each time, this is equivalent to changing the time of the middle point of the PWM carrier halfway back and forth.

As a result, the output voltage of the PWM changes every PWM half cycle by carrier modulation. Due to its influence,
As shown in FIG. 1, the time width and the temporal position of the PWM of the line voltage such as the PWM signal of V _U -V _V are changed. However, since the three-phase modulation signal itself does not change,
The average voltage during the current sampling period can be output as instructed, and the fundamental component excluding harmonics can be output normally. In other words, only the harmonic components are modulated.

However, when the ratio or phase of the PWM voltage is shifted, the current ripple waveform is affected by the integrated value of the voltage, so that the amplitude and phase position of the ripple component change as in the U-phase current waveform in FIG. . As a result, the electromagnetic sound can be whitened, and the tone like "Keen"
Can be changed.

Therefore, it can be seen that by modulating the ratio of the plurality of PWM half cycle periods, an effect similar to the effect of randomly modulating the carrier frequency can be obtained.

This method is characterized in that the current control operation has a constant cycle, so that the conventional fixed gain can be used, and the operation is simple.

In particular, when a magnetic flux observer or the like is also calculated in the current control calculation cycle, there are many gains such as motor constants, so that an enormous amount of calculation is required to correct the effect of random modulation.

On the other hand, in the modulation method of the present embodiment, such a problem does not occur because the operation cycle itself is fixed.

In the above embodiment, the current control cycle is fixed and the time width of a half cycle of the PWM modulation is randomly modulated. In this modulation method, the amount by which the ratio of the half cycle of the PWM modulation is changed may be other than a random component, or may be changed into a sine wave or a triangular wave synchronized with an integral multiple of the output frequency.

[0028]

As described above, according to the present invention, the carrier frequency can be modulated in order to change the time ratio of a half cycle of a plurality of PWM modulations within the current control sample cycle, and to modulate the PWM frequency. The electromagnetic sound can be changed from harsh monochromatic sound quality to white noise, and the noise can be reduced audibly.

In addition, since the sampling period of the current control system can be fixed, the influence of random modulation does not appear on the current control gain.

[Brief description of the drawings]

FIG. 1 is an example of a modulation waveform in which a current control sample is synchronized with each period of PWM in a PWM modulation method according to the present invention.

FIG. 2 is an example of a speed control device using a PWM inverter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Induction motor 2 ... Inverter main body 3 ... Speed control part 4 ... Speed detection part 5 ... Vector control part 6 ... Current control part 7 ... PWM modulation part 8 ... Carrier signal generator 9 ... Random signal generator

Claims (2)

[Claims] 1. A PWM inverter for modulating a voltage control signal with a carrier signal synchronized with a current control cycle, wherein a current control sample cycle is synchronized with an integral multiple of two or more half of the PWM modulation cycle, and A modulation method for a PWM inverter, characterized in that a time ratio of a half period of a plurality of PWM modulations during a sample period is changed while a current control sample period is fixed. 2. The PWM inverter modulation method according to claim 1, wherein the ratio of the half cycle of the PWM modulation is changed in a sine wave or a triangular wave synchronized with an integral multiple of the output frequency of the inverter.