The variable frequency power supply changes the alternating current in the mains supply through AC→DC→AC, and outputs pure sine wave, and the output frequency and voltage are adjustable within a certain range. It is different from variable frequency speed regulating controller used for motor speed regulation, and it is also different from ordinary AC stabilized voltage power supply. The ideal AC power supply is characterized by stable frequency, stable voltage, zero internal resistance and pure sine wave voltage waveform (without distortion). Variable frequency power supply is very close to ideal AC power. Therefore, more and more advanced developed countries use variable frequency power supply as standard power supply, so as to provide the best power supply environment for electric appliances, and facilitate objective assessment of technical performance of electric appliances. There are two major types of variable frequency power supply: Linear amplification type and SPWM switching type.