In this article, Variable Frequency Drive (also, named: Variable Speed Drive) basics are presented in a simple and easy way with explanation graphs. To understand how the Frequency Speed Controller operates, it is necessary to understand how the VSD supplies variable voltage and frequency for speed control.
The below block diagram illustration depicts a typical three-phase AC variable speed drive system. It has three main components: an Operator Control, a Drive Controller, and an AC Motor.
An Operator Control device provides a means to start and stop the motor and adjust the operating speed. The Drive Controller consists of a variety of components that work together to convert an AC input into a frequency and voltage output necessary to change the speed of an AC motor.
Main sections of a Variable Frequency Drive:
The converter section:
This section converts the incoming 3-phase AC voltage to DC voltage. The converter is essentially a 3-phase, full wave rectifier with Silicon Control Rectifiers, a specialized type of control diode, in the bridge.
The following video explains what SCR is and how it works: