What is a variable frequency drive and how does it work in the a system?

Answer 1

A variable frequency drive (VFD) is an oscillator that is used to pulse the electric for a motor on and off. If the pulses occur infrequently the motor will run slow. If they occur as fast as (or a speck faster than) the line frequency the motor goes at full or higher speed..

Incoming AC power is rectified to make DC. That drives an oscillator whose frequency can be varied depending upon a reference input (either an analog voltage or a digital command number). The pulses from that oscillator give a voltage that runs the motor. To the motor it is an AC voltage (not a sine wave) and the motor responds by turning. Higher frequency means faster turning.

If you have a system in which a pump needs to have a speed that depends on how much water needs to be pumped into a tank a VFD is a natural solution. If the water level is high the VFD sends out pulses infrequently and the pump runs slow (but not so slow that no water moves). If the water level is too low the pump would run at a high speed because the VFD sends out pulses about at the line frequency. That makes the pump run at its highest speed.