The variable frequency power supply is an AC – DC – AC - filter frequency conversion device which is composed of the whole circuit. It has been widely used. Variable frequency power supply can not only simulate and output power grid index of different countries, but also provide pure and reliable sine wave power output of low harmonic distortion and high stable voltage and frequency for the export appliance manufacturers in the design and development, production, detection and other applications. The variable frequency power supply is very close to the ideal AC power supply, which can output power grid voltage and frequency of any country. The frequency converter is composed of AC – DC – AC (modulating wave) and other circuits. The standard name of frequency converter should be variable frequency governor. The waveform of the output voltage is pulse square wave, and the harmonic component is much, and the voltage and frequency are changed proportionally at the same time, and cannot be adjusted separately, which does not meet the requirements of AC power supply.