What is the use of Variable Frequincy Drive?

Question

AC & Dc Drive

Answer 1

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. 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 or inverter drives.

Answer 2

The purpose of an electric motor (regardless of whether it is an Alternating Current - AC - or Direct Current - DC - machine) is ultimately to produce usable mechanical energy - also referred to as torque - from an electrical energy input.

In some processes, torque is variable with speed. In other cases, torque is required to remain constant regardless of speed.

Torque is related to speed and power by a simple formula:
(Torque) = (Constant) * (Power) / (Speed) ... the additional constant is to translate the power/speed result into some recognized units. For example, the constant is 5252 when using HP as the power units and RPM as the speed units to obtain torque units of lb.ft.

The speed of a DC machine is independent of frequency, so Variable Frequency Drives (VFDs) have no impact on their operation. The DC drive rectifies an incoming AC signal to DC ... an output signal with effectively zero frequency.

An AC machine operate with a fixed relationship to frequency. This can be expressed as:
(Speed in rpm) = (120) * (frequency in hertz) / (poles)

Therefore to achieve different speeds, either the number of poles has to change - not really a practical method, since the machine is already built! - or the frequency has to change. VFDs use rectification to change the incoming AC signal to one of a different frequency by chopping the signal into a number of shorter pulses and recombining them.