What is a VFD?

2023-09-29 | By Kyra Meadows

License: None Capacitors Current Diodes Drivers / Controllers Transistors Voltage

As an electrical engineer, you should associate the acronym “VFD” with the term “variable frequency drive”. A variable frequency drive can be most easily defined as a type of motor controller that drives an electric motor by varying the frequency and voltage supplied. There are many applications of the VFD, especially in industrial facilities.

So how does a VFD work?

Simply, a VFD is comprised of three stages: a rectifier, a filter, and an inverter. Each of these stages can be made from rudimentary components in any electronics lab. This setup will produce a “six-pulse VFD”, which is the current standard.

The rectifier will convert AC power to DC. Then, the filter is used to remove the AC ripple from the DC bus. Finally, the inverter does the opposite of the rectifier! Thus, making the DC power AC.

The rectifier is made up of six diodes which allow current to flow in one direction. These diodes will take each phase of the AC voltage and direct the current of the most positive phase voltage through that phase's diode, thus making the voltage DC. The filter, which can be made from a capacitor or can also include an inductor, will remove the ripples from the pulsating DC output of the rectifier. Lastly, the inverter is made up of six transistors which switch rapidly to provide alternating voltage at the output. 

What is the Application of a VFD?

Now that we understand what makes up a variable frequency drive, how can we use it? As I mentioned before, VFDs are used widely in industrial settings. Mainly, VFDs help to control the speed of a motor, which can be useful in many ways. For example: how fast a conveyor belt will move a piece of material or even how fast a fan may blow. VFDs can also be used to ramp up a motor for a smooth startup or to prevent a heavy load from straining the motor on startup.

To conclude, variable frequency drives are a useful piece of equipment in today's world. While they may seem complicated, they are simpler than you think! When you break it down, at its core, a VFD is just a handful of components we have all heard of before.

See a VFD schematic in the below Scheme-It project!

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