Most of the frequency converters are frequency converters in the form of PWM modulation. That is to say, the voltage output by the inverter is actually a series of pulses, and the width and interval of the pulses are not equal. Its size depends on the intersection of the modulating wave and the carrier wave, that is, the switching frequency. The higher the switching frequency, the more the number of pulses in a cycle, the better the smoothness of the current waveform, but the greater the interference to other equipment. The lower the carrier frequency or the bad setting, the motor will make unpleasant noise.
By adjusting the switching frequency, the noise of the system can be minimized, the smoothness of the waveform is the best, and the interference is also the smallest.
Influence of switching frequency on #inverter output current
The inverter (DC/AC conversion) part of the inverter is formed by the IGBT through the sine pulse width modulation SPWM, and then through the motor winding to form a sine wave current waveform. Then the size of the switching frequency directly affects the quality of the current waveform, as well as the size of the interference, and the size of the switching frequency is more sensitive and direct. Consider adding various harmonic suppression devices, such as AC reactors, DC reactors, filters, additional sequence reactors, as well as installation wiring, grounding and other measures.
This approach is more reasonable and effective, and must not be reversed. To deal with the problem, this is a very important principle. When the switching frequency is high, the current waveform is sinusoidal and smooth. In this way, the harmonics are small, the interference is small, and vice versa, when the switching frequency is too low, the effective torque of the motor decreases, the loss increases, and the temperature increases. On the contrary, when the switching frequency is too high, the loss of the inverter itself increases. , IGBT temperature rises, and the rate of change of output voltage dv/dt increases, which has a greater impact on motor insulation.
1) The higher the operating frequency, the larger the duty cycle of the voltage wave and the smaller the current high-order harmonic component, that is, the higher the switching frequency;
2) The smoothness of the current waveform is better;
3) The higher the switching frequency, the less current the inverter allows to output;
4) The higher the switching frequency, the smaller the capacitive reactance of the wiring capacitor (because Xc=1/2πfC), and the greater the leakage current caused by the high frequency pulse.
The switching frequency is generally better at the default value of the system. It is not recommended to adjust it casually. Generally, it will only be adjusted when problems are encountered. This is because:
The higher the switching frequency of the inverter, the smoother the output waveform; the lower the switching frequency of the inverter, the worse the output waveform. The switching frequency, to put it bluntly, is chopping. Suppose we chop the sine wave into 100 segments and 1000 segments. It must be that the combined waveform of 1000 segments is more like a sine wave, which should be understandable.
The chopper device of the inverter also has a switching life. The higher the switching frequency, the shorter the life of the inverter, and the output current and power of the inverter will be reduced. The smaller the number of chopping, the higher the harmonic content and the more serious the harmonic pollution.
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