VFD? Working and Block Diagram, Parts of Variable Frequency Drive

VFD is a short form of a variable frequency drive also called Frequency converters, which has undergone extremely rapid changes, largely as a result of the development of microprocessor and semiconductor devices and their reduction in prices. However, the basic working principles of frequency converters remain the same. Other names of VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive. Here we are going to discuss working principle and block diagram of VFD.

Working principle of VFD 

The block diagram of the variable frequency drive is shown in the below diagram 

block diagram of VFD

A Variable Frequency Drive (VFD) is a type of motor speed controller that drives an Induction motor by varying the frequency and voltage supplied to the electric motor. 

Variable frequency drive(VFD) mainly has a Rectifier, Intermediate circuit, and Inverter to convert back dc voltage into ac as shown in the block diagram.

VFD Parts and Functions

  • Intermediate circuit
  • Inverter
  • Rectifier
  • Control circuit


The supply voltage applied to the rectifier is a three-phase alternating voltage or a single-phase AC voltage with a frequency of 50 Hz or 60 Hz. The rectifier is a circuit that converts the alternating voltage into the direct voltage. Rectifiers made up of diodes and thyristors, to convert AC supply voltage into the DC. 

Intermediate circuit:

The intermediate circuit also called DC-bus” or “DC-link can be seen as a storage facility from which the motor is able to draw its energy via the inverter.  It can be built according to three different principles depending on the rectifier and inverter

There are three types: 

  1. one, which converts the rectified voltage into a direct current.
  2. one, which stabilizes or smoothes the pulsating DC voltage and places it at the disposal of the inverter.
  3. one, which converts the constant DC voltage of the rectifier to a variable AC voltage.


The inverter is the last main part of the VFD drive. The inverter processes represent the final stage in terms of generating the output AC voltage and frequency. The inverter generates the alternating voltage which applied to the motor. 

Control Circuit:

The control circuit, or control card, is the fourth main component of the frequency converter and has four essential tasks:

  • control of the frequency converter semi-conductors.
  • data exchange between the frequency converter and peripherals.
  • gathering and reporting fault messages.
  • carrying out of protective functions for the frequency converter and motor.

Micro-processors have increased the speed of the control circuit, significantly increasing the number of applications suitable for drives and reducing the number of necessary calculations. With microprocessors, the processor is integrated into the frequency converter and is always able to determine the optimum pulse pattern for each operating state.

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Reference:Facts Worth Knowing about AC Drives Danfoss

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