How does a VFD work?

We’ve explained Electric Motors in principal, but we all know it’s not that simple. In large part, the guts are pretty similar in most electric motors. It’s windings and magnets, which can be arranged in difference sizes and positions, to get a specifically desired performance. In the examples we gave, it was assumed that the electrical power going in was constant. What if it wasn’t?


Less Than 100% On-Time

What if we didn’t keep dumping power into the motor? Think of it like a manual transmission. We know we need power to push ourselves up a hill, but we can push in the clutch (put it in neutral) and coast down the other side without using engine power. We can do this with an electrical motor, gaining a massive amount of control and performance out of it. If we attach some sensors and measure the RPM, we can determine when it needs power and when it doesn’t to maintain a given speed or torque output.

This difference in driving the motor means we may power the motor for only 1/10th of a second, every other 1/10th of a second. Thus, we end up with something that might look like:


Power on, power off… Power on… Power off. The motor only does a tiny bit of work when it needs to. This cuts down on waste heat and allows us to cheaply regulate the power going into the motor. In other systems, this type of control would require changing the amps going in or performing some really absurd transformations to the electrical power. Here? We can just make a computer and teach it to be a half sugar-crazed toddler playing with a light switch.


This Sounds Familiar…

If this idea sounds familiar to you, it should. We talked about Transformers a while back. At the time, I mentioned what we call a Switched Mode Power Supply. That is, a device which rapidly turns on and off to change the voltage. The voltage is more or less averaged over time. If something is only receiving power for half of a given time, the voltage is essentially halved.

We can do the same thing to a motor. If it’s on for half the time, it’s at about half the voltage. If it’s on for 90% of the time, it’s 90% of the voltage. The on/off time is entirely arbitrary with modern technology. We have things like mosfets, relays, and transistors which can be toggled billions of times on and off per second.


Affordable Control

All said and done, this means that we can cheaply control every aspect of a motor. We can have computerized controls that instantly speed up or slow down. We can have systems which detect the need for more power to increase output torque. We can cut down on the wear and tear of the motor by literally only running it exactly as much as necessary rather than burning it with current all day and night.



This whole concept is called a Variable Frequency Drive. We’re varying the voltage and electrical frequency. We’re varrying how often the motor runs. These drives are used in a massive range of fields. It’s possible to use a VFD to run a compressor, to run carwashes, to power trains, and in short, they’re everywhere. They can be used on fans in HVAC systems, in pumps, in process controls, and everywhere you need to make a motor spin. The reductions in energy usage and wear in the hardware offer immense savings, while the better drive control allows for precise regulation in any application.

The Wrap Up

What did you think? Did we get something wrong? Got something for us to cover next time? Let us know in the comments below.

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