The speed of an induction motor depends on the frequency of the supply. A VFD (Variable Frequency Drive) controls the motor speed by changing the frequency but how does it actually change the frequency of electricity?
let us understand in simple language. The synchronous speed of the induction motor is directly proportional to the supply frequency.
Synchronous speed = 120*f /N
where, f is operating frequency
N = number of poles
If frequency increases the motor runs faster. If frequency decreases the motor runs slower. A VFD first converts the incoming AC supply into DC using a rectifier. The DC is stored in capacitor inside the VFD. After that the inverter circuit inside the VFD converts this DC back into AC but a new frequency is chosen by the chosen by the operator. For this reason, the motor receives AC at any frequency between 0 and 50 Hz or even higher. Due to this the motor speed can be controlled smoothly from zero to full speed. In such condition VFDs save a large amount of energy in applications like pumps, fans, and conveyer belts where full speed is not always needed.