An overview of a VFD

A VFD is a device used to vary the speed of Asynchronous or Synchronous (AC) electric motors by varying the frequency and voltage supplied. A VFD can also be referred to as an Adjustable Speed Drive (ASD) or inverter. It is the most effective way of controlling the speed of an induction motor. The principle of operation of a VFD is based on the fact that the speed of an induction motor is directly proportional to the frequency of the voltage supplied to the motor. When frequency increases, speed also increases, and when frequency reduces, the speed will also reduce. A synchronous motor runs at synchronous speed (the speed of the rotating magnetic field of the stator) while an asynchronous (induction) motor’s speed is slightly less than synchronous speed. In the previous chapter, it was mentioned that synchronous speed is determined by the frequency of the voltage source and the number of poles, as shown by the following formula:

Synchronous speed = 120f...