A variable-frequency drive (VFD) is a system for controlling the speed of a rotational or linear alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.[1][2][3] A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives.

A variable frequency drive (VFD) refers to AC drives only and a variable speed drive (VSD) refers to either AC Drives or DC Drives.

VFD's vary the speed of an AC motor by varying the frequency to the motor. VSD's referring to DC motors vary the speed by varying the voltage to the motor.

Variable frequency Drive is not compatible with a DC Motor. VFD can be used only with AC motors, where frequency plays a key role in defining the speed of the motor.

Main function of Variable Frequency Drive is to be able to run the AC Induction motors at different speeds (by changing the frequency). The speed is a function of frequency.

N - Speed.

F - Frequency

P - Pole

N= ( 120 x F)/ P

Single phase variable frequency drives are commercially available. Building it by an individual is complicated.