VFD work

 How a VFD Operates. A variable frequency drive takes in fixed AC voltage and frequency into a rectifier bridge to convert the voltage to DC, uses a capacitor bank and inductors to filter the DC, then inverts the DC voltage back to AC and sends it out to the motor at the frequency desired..

How is VFD used to control motor speed? 

With a VFD, voltage and current fed to the motor are controlled by turning on and off insulated-gate bipolar transistors (IGBTs). The amplitude of the voltage and current values is controlled by the frequency of the switching and on and off time of the IGBTs