VFD Block Diagram, Applications, Advantages, Circuit, Panel

Hey, today we are going to know about Variable Frequency Drive or VFD Block Diagram, Working principle, VFD Applications, Advantages, VFD Panel, and Circuit. Variable Frequency Drive (VFD) is an electrical device by which we can vary the frequency of an AC Power Supply. VFD is a solid-state converter or solid-state device.

VFD Block Diagram and Working

Here you can see the block diagram of VFD Drive system.

Generally, a VFD Drive system has three basic distinct subsystems or parts. These three parts are a Full Wave Bridge Rectifier, DC Link, and an Inverter.

Bridge Rectifier: The full-wave bridge rectifier circuit is used to convert the input AC power supply into DC power Supply. Single Phase low power VFDs have single-phase rectifier circuit using diodes but Three Phase VFDs have three-phase rectifier circuit using SCRs because SCR is suitable for high voltage and high power applications.

Four Diodes or SCRs is required for single-phase full-wave bridge rectifier circuit. Six diodes or SCRs is required for three-phase full-wave rectifier circuit.


DC Link or DC Bus: DC Link or DC Bus is nothing more than a DC Filter Circuit. This section consists of some Capacitors and inductors. This section removes the ripple components from the output DC supply of the rectifier.

Inverter Circuit: Actually, the main working of a VFD is depended upon this section. The Inverter circuit is used to convert the DC supply coming from DC link into AC supply. In a VFD, Full Controlled Inverter Circuit is used. Controlled inverter circuit means, the waveform of the output AC supply can be controlled. We know that, by controlling the waveform of an AC supply, we can vary the frequency.

Generally, Pulse Width Modulation Technique is used in those inverter circuits. Using Pulse Width Modulation Technique, the width of the pulses of the output AC supply is controlled, therefore the frequency also varies.


Control System: All operations are controlled by an embedded microprocessor. Microprocessor controls the whole control circuit. It gives control signals to the rectifier, inverter for their operation.

Variable Frequency Drive(VFD) Panel

The front side picture of a VFD panel is already given in the first paragraph. It is a 3.7 KW panel with DOL Starter. If you see the front side picture, then you will notice, there are two selective switches. One is used to start the motor in auto mode or manual mode. Another one is used to run the motor with VFD or Bypass.

When Auto Mode is selected, the motor will turn on automatically according to the set time of the timer.

When manual mode is selected, we need to turn on the motor manually by pressing the ON switch.

When VFD Mode is selected, then the motor will be run at the frequency which already set in VFD. Also, we can change the frequency at any time to change the speed of the motor.

You can see the VFD display shows 35.0 Hz which is the frequency at which the motor is running. It also shows 1.040 KW which is the power taking by the motor at running time.

When Bypass mode is selected, then the motor will be run at the main input supply frequency. In this mode, we cannot change the speed of the motor by changing the frequency.

Here you can see the inside picture of the VFD panel.

Here you can see the main VFD drive is placed at the right side inside the panel. Some electrical contactors are there. A timer also connected to turn on the motor automatically according to our requirement.

VFD Applications and Uses

1. VFD is mainly used for the speed control of AC motors(induction motors). The synchronous speed of the induction motor is depended upon the supply frequency that is why VFD is used to give the variable frequency to the induction motor to control the speed.

2. VFD is used with AHU in industries to control the speed of the AHU motors. AHU means Air Handling Unit. The cooling system in an industry is provided by AHU. If the frequency is set to high value, the colling will be more and vice-versa.

3. VFD is used in large Air Compressors. By changing the frequency value, air pressure, airflow can be changed.

4. VFD not only can vary the frequency, but it can also vary the voltage also. So VFD can provide variable frequency and variable voltage power supply.

5. VFD is used in Electrical Traction system for speed control of the Train Motors.

6. VFD is used in Lifts, Escalators, etc.

7. VFD is used in modern Air Conditioners, Refrigerators, etc.

8. VFD is used in electronic manufacturing companies for speed control of conveyors, various machines, large exhaust motors, etc.

Variable Frequency Drive(VFD) Advantages

1. The speed control of induction motor using VFD is very smooth and reliable.

2. Speed control of motor using VFD saves a huge amount of electrical energy.

3. VFD is a static device, so it can be controlled remotely, using an external computer, even wireless also.

4. When VFD is used, the induction motor takes very low current at the starting time and it starts rapidly and smoothly. But when normal starter is used, the motor takes comparatively large starting current.

5. There are no harmonics problem occurs when the speed control of motors is done with VFD.

6. There is no crawling effect occurs when speed control is done with VFDs.

7. As the VFD is astatic device and has no moving part, that is why it is more reliable and it can be placed anywhere and limited place.

8. VFD has an additional advantage that is Power factor Control. Electrical motors are the main reason for low power factor in an industry. VFD helps to improve the power factor also.



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