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The frequency converter is a high-tech product, and its main components are power electronic devices and microelectronic devices. The three-phase alternating current of the power grid is connected to the input terminal of the frequency converter. The input side of the frequency converter is a rectifier circuit, and the output side is an inverter circuit. Therefore, the voltage and current at the input and output sides contain rich harmonics, which will cause grid waveform distortion. Distortion; at the same time, whether the grid voltage is symmetrical and balanced, the size of the transformer capacity and whether there are nonlinear devices connected to the power distribution bus will also affect the normal operation of the inverter. There are computer chips or DSP chips inside the current inverters, which are used to realize the function control of the inverter and the driving control of the inverter of the main circuit. Because the voltage and current of the computer chip are small and the working speed is high, it is very easy to be affected by some external electrical interference. Therefore, in order to realize the safe and reliable operation of both the power grid and the inverter, it is necessary to take suppression measures for the mutual interference between the two.

The rectification circuit and inverter circuit of the frequency converter are composed of nonlinear devices, and their circuit structure will cause the voltage and current waveforms of the power grid to be distorted. The three-phase AC voltage UR, US, UT converts the AC power into The direct current is filtered by the electrolytic capacitor, so that the direct current voltage is basically constant. The diode used in the rectification circuit is a non-linear device. The rectified output voltage charges the filter capacitor, and the waveform of the charging current depends on the difference between the rectified voltage and the capacitor voltage. Figure 10-5 shows the input voltage u and current i of the inverter. Measured waveform. It can be seen that when the input voltage of each phase line is a sine wave, the input current of each phase line is not a sine wave.
When the inverter is in different frequency and different current working conditions, the input current waveform is also different. Figure 10-6 shows the input current waveforms when the 55kW inverter drives the cage-type asynchronous motor load at 10Hz, 20Hz and 50Hz. It can be seen that as the frequency and current of the motor increase, the input current changes from intermittent to continuous, and the waveform distortion of the current becomes smaller and smaller. Decomposing these waveforms with mathematical Fourier series will result in many harmonic current components. These harmonic current components vary depending on the type of inverter circuit and its operating state, system, and conditions.