Variable-frequency Drive (VFD) is an electrical control device that controls an ac motor by changing the frequency of the motor's working power supply.

Frequency converter is mainly composed of rectifier (current alternating direct current variable dc), filter, inverter (direct current variable alternating current), brake unit, drive unit, and detection unit micro-processing unit. The inverter adjusts the voltage and frequency of the output power supply by internal IGBT breaking, and provides the required power supply voltage according to the actual needs of the motor, so as to achieve the purpose of energy saving and speed regulation. In addition, the inverter also has many protection functions, such as overcurrent, overvoltage, overload protection and so on. With the continuous improvement of industrial automation, frequency converter has been widely used.

The main circuit of VFD is the power conversion part that provides the voltage and frequency modulation power supply to the induction motor. The main circuit of the frequency converter can be roughly divided into two types: the voltage type is the conversion of the dc of the voltage source to the ac converter, and the filtering of the dc circuit is the capacitance. The current mode is a frequency converter which transforms the dc current source into ac, and the dc loop filter is inductance. It consists of three parts, which transforms the power supply into a dc power "rectifier", absorbing the "flat wave loop" in the voltage pulsation generated by the converter and the inverter.