How does Variable Frequency Drives work?

A variable frequency drive is a power electronics-based device that converts a basic fixed frequency, fixed voltage sine wave power to a variable frequency. It regulates the speed of a three-phase induction motor by controlling the frequency and voltage of the power supplied to the motor. Variable frequency drives are also known as adjustable frequency drives(AFD), variable speed drives(VSD), and AC drives.


(Source of images are Google: they are used only for knowledge purpose) 

VFD consists of mainly four parts- rectifier, intermediate DC link, inverter, and controlling system:

1.  Rectifier

It is the first stage of a variable frequency drive. It converts AC power from mains to DC power. This section can be unidirectional or bidirectional based on the application used like the four-quadrant operation of the motor. It utilizes diodes, SCR’s, transistors, and other electronic switching devices.

2.  DC bus

DC power from the rectifier is fed to the DC link. This section consists of capacitors and inductors to smooth against ripples and store the DC power. The main function of the DC link is to receive, store, and deliver DC power.

3.  Inverter

This part comprises electronic switches like transistors, thyristors, IGBT, etc. It receives DC power from the DC link and converts it into AC which is delivered to the motor. It uses modulation techniques like pulse width modulation to vary output frequency for controlling the speed of the induction motor.

4.   Control circuit

It consists of a microprocessor/microcontroller unit and performs various functions like controlling, configuring drive settings, fault conditions, and interfacing communication protocols. It receives a feedback signal from the motor as current speed reference and accordingly regulates the ratio of voltage to frequency to control motor speed.

Important features of Variable frequency Drive

     Variable speed

     Improve process efficiency and control

     Acceleration and deceleration control

     Saves energy

     Reduce mechanical stress on machine to extend their lifetime

     Improve the working environment

     High power factor


Application of VFD

     They are mostly used in industries for large induction motors whose power rating ranges from kW to MW.

     They are used in traction system(a component used in the train)

     They are also used in modern lifts, escalators, and pumping systems.

     In some applications such as speed control for a conveyer, the variable frequency drive is installed with a remote potentiometer that personnel can adjust manually to set the speed for the conveyer.

     They are widely used on machine tool drives, compressors, and in ventilation systems for large buildings.



     The main advantage of VFD is that it can vary the power supplied to match the energy requirement of the driven equipment, and saves energy.

     Increased motor longevity

     Reduced power line disturbances

     Energy saving

     Reduced risk of motor damage during start-up and stop

     High efficiency


    Initial drive cost

    Motor heating at low speed

    High maintenance

    Induced power line harmonics 

To see how does it programmed and used in Automation systems go through this article - Getting started with AB Powerflex VFD 









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