It can be seen from the above formula that when the slip rate does not change much, the rotation speed n of the asynchronous motor is basically proportional to the power frequency f. The frequency of the power supply can be continuously adjusted through the frequency converter to smoothly change the speed of the motor.Inverter is a type of AC electric drive system. It is based on control technology, electronic power technology, microelectronic technology and computer technology. It converts AC power frequency power into a variable voltage and frequency suitable for AC motor speed regulation. The power electronic conversion device is called VVVF (Variable Voltage Variable Frequency).The AC low-voltage AC/DC traffic frequency converter system is mainly composed of rectifier and inverter (the block diagram is shown in Figure 2). Rectifier: A power electronic device that converts AC power to DC. Its input voltage is sine wave, and the input current is non-sinusoidal, with rich harmonics. Inverter: A power electronic device that converts DC power to AC power. Its output voltage is non-sinusoidal. Sine wave, the output current is approximately sine.