What is Variable Frequency Drive or VFD?

What is Variable Frequency Drive or VFD?

Today in this blog, we are going to learn about What is Variable Frequency Drive or VFD? How does a VFD work? Why do we use a VFD? What are the advantages of using VFD?

So the first question is What is the term of VFD? Well, a VFD stands for Variable Frequency Drive. In many industrial applications such as pumps, blowers, conveyors, crushers, etc. we have to control the speed of an electrical motor. A VFD is used mainly to control the speed of an electrical motor.

Before reading this blog, I prefer you to read my article about an electrical motor.

A variable frequency drive is a type of device that controls the speed of an electrical motor by varying the frequency and voltage of its power supply. The VFD also has ramp-up and ramp-down facilities to start and stop the electrical motor smoothly.

Why we need to control the speed of an electrical motor? Well, there are multiple reasons, such as

  • Save energy and improve system efficiency
  • To reach the desired torque or power to the process requirements
  • Lower the noise levels of pumps, blowers, fans, compressors, etc.
  • Reduce mechanical stress on the machines and improves their life cycle.
  • Improve the working environment.

The most common and general use of an AC drive is to control the speed of fans, pumps, blowers, compressors.

How does a VFD work?

The main two features that we are getting from VFD are Adjustable Speed and Soft Start/Stop Capabilities.

The VFD has mainly four sections:

  1. AC to DC Converter (Rectifier)
  2. DC Link
  3. DC to AC Converter (Inverter)
  4. Control Circuit

What is Variable Frequency Drive

Rectifier(AC to DC Conversion)

The rectifier is the first stage of an AC drive. It converts an AC supply from the mains to the DC power.

The rectifier can be a single direction or bi-directional rectifier. The simplest version of a rectifier is a diode rectifier, here each phase requires two diodes: one when the voltage is positive and the other when the voltage is negative. So the three-phase supply requires a six-diode. Thus it is also referred to as a six-pulse diode bridge.

The diode rectifier is simple and low in cost, but it has a limitation in that it only allows the energy to flow in one direction. So when the motor acts as a generator, the regenerated power can’t be fed into the main power supply.

What is Variable Frequency Drive

The other alternative for rectifiers is thyristor or IGBT. For a thyristor configuration, each thyristor bridge is used six thyristor and thyristor bridges are used in pairs. One pair of the thyristor is used when the motor is consuming energy and the other thyristor is used when the motor is regenerating energy.

For an IGBT rectifier, IGBTs are used with a combination of a diode. So six IGBTs are used with six diodes, this will allow the energy to flow in both directions.


DC Link(DC Voltage Smoothning)

This section of VFD contains capacitors and inductors. The DC power coming from the rectifier section contains voltage ripples which are smoothed using the capacitors.

The main function of the DC link is to store, smooth, and deliver the DC voltage.

What is Variable Frequency Drive


Inverter(DC to AC Conversion)

The DC voltage is converted back into AC voltage to feed to the motor. In this section of VFD, a DC voltage from the DC link is converted into AC voltage using transistors, IGBT, or thyristors.

What is Variable Frequency Drive

The modulation technique used to convert AC voltage and vary the output frequency is Pulse Width Modulation(PWM).

What is Variable Frequency Drive

Image Credit: ResearchGate.net


Control Circuit

Each drive consists of control circuitry, which is used to parameterize the drive. It consists of a microprocessor-based unit that performs various functions such as controlling the speed of the motor, monitoring the alarms and faults of the ac drive, interfacing the ac drive with different devices using a communication protocol, etc.

We can control the start/stop function, motor speed control, and receive feedback about current, speed, etc. using this unit.

Why should we use VFD?

⇒ Limits Starting Current

An induction motor draws 6 to 8 times the nominal current at starting in a conventional starter. While using a VFD, an induction motor will start with a lesser frequency and draws a lesser current even than the full load current.

⇒ Provides Smooth Operation

It provides a smooth start/stop operation which reduces the thermal and mechanical stress of the equipment.

⇒ Reduce Energy and Cost

VFD has an inbuilt power factor correction unit in the DC link circuit, it improves the overall power factor thus saves energy even the motor is running with full voltage and full frequency.

⇒ Controlling the Operating Speed and Acceleration

VFD facilitates the time to accelerate the desired speed and the time to deaccelerate the electrical motor. This time is known as acceleration time and de-acceleration time.

Operating speed we can adjust by using multiple protocols such as BOP/AOP, AI terminal on the drive, or by the communication protocol such as Profibus, Profinet, etc.

Where do we use VFD?

Variable Frequency Drive is used in almost all industries, but there are some common examples where VFD is most popular to use:

⇒ VFD is used to boost and control the pressure of air, gas, liquid, etc. in a booster pump motor.

⇒ VFD is used to control the pressure of air in the air compressor unit at a power plant.

⇒ VFD is used to control the flow of water at a centrifugal pump.

⇒ VFD is used at a closed-loop control to maintain the temperature, pressure, or flow.

⇒ VFD is used at an elevator system for a smooth start/stop operation.


I hope you get all the basic information about what is variable frequency drive from this article. Thank you for giving your valuable time to read this article.

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You can read more articles about Electrical and you can also find books that boost your knowledge in the field of Instrumentation

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