Variable frequency drive is an electric device to change AC power frequency to control AC motor speed, In addition, it also can change the AC power voltage.

In the past, variable frequency drive was included in motor generators, rotating converters and other electrical equipment. With the emergence of semiconductor electronic devices, VFD can be completely manufactured independent.

Variable Frequency Drive allows the electric motor smooth start up, control startup current growing from zero to motor rated current, reduce impact to the power grid and avoid the motor being burned out, also provide protect in  motor running process. Besides these functions, the main function of variable frequency drive is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

Generally, variable frequency drive contains two components: rectifier and inverter. The rectifier converts incoming AC power to DC power, then the inverter converts DC power to the desired frequency AC power. In addition to these two parts, variable frequency drive may also contain transformer and battery. Wherein the transformer changes the voltage and isolates input/output circuit, the battery compensates energy loss inside the VFD drive circuit.

The variable frequency drive not only changes the AC power frequency, but also can change electric AC motor rotation speed and torque. In such conditions, the most typical VFD structure is a three-phase two level source variable frequency drive. The VFD controls each phase voltage by the semiconductor switch and pulse width modulation (PWM).

In addition, variable frequency drive also can be used in aerospace industry. For example, the electrical equipment inside aircraft needs 400Hz AC power, but generally the power on ground is 50Hz or 60Hz. Therefore, when the aircraft is parked on ground, the variable frequency drive will convert 50Hz/60Hz to 400Hz AC power to suitable for the aircraft.