What is a Variable Frequency Drive (Video)?

By Yin on 19th Jun 2014

Video Transcript

Today we are going to talk about what is a VFD.

In the 1880s, we saw the introduction of three-phase AC power. This was a marvelous invention by the great scientist Nikola Tesla. And running on that three-phase power was Tesla’s three-phase AC motor. This was a fabulous invention. It’s the greatest labor saving device of mankind, and changed our lives forever. But it had one significant drawback – it could only run at one speed. So we have come up with many devices and inventions in order to compensate for that basic deficiency, things such as belts and pulleys, sheaves, dampers, inlet guide vanes, eddy current couplings, fluid couplings and so on. All to compensate the motor that could run in one speed.

It took us 100 years to come up with a device that could reliably and cost-effectively run the motor at different speeds. Finally, at the 1980s, we saw the introduction of the variable frequency drive, or VFD.

Now I’d like to look at the schematic of a VFD. If you look at the schematic in front of you, you will see three arrow looking devices on the left. These devices in the electrical world are called dials; in the mechanical world and plumbing world, we might think of these as check valves. They basically let the current flow in one direction that is the direction of the arrows. So we have an AC to DC converter or just converter for short, this converts the 3-phase AC power to DC. The DC that comes out of this is rather choppy and not very smooth. So we put in a capacitor in order to clean that DC power up. A capacitor is like a reservoir in a plumbing circuit, and that reservoir soothes everything out and gives us a nice, clean DC.

Then going to the right, we see 6 switches. These 6 switches compose the DC to AC inverter or just inverter for short. So by switching these switches on and off, we can create any frequency that we like. And that frequency will regulate the speed the motor goes. So this entire circuit is called a variable frequency drive. If you have heard terms such as variable speed drive, adjustable frequency drive, adjustable frequency AC drive, variater; all of these are just synonyms for the variable frequency drive.

Now let’s take a look at an actual variable frequency drive. This VFD was manufactured in the 1980s. You’ll see that there is a lot of spare space in this drive, so we weren’t packaging it in the small packages we see now. However, the basic topology of a VFD has not changed since 1980, over the last 30 years. Now if you look at a modern drive, you will have the exact same parts but in a smaller enclosure with less space, packed together, and many times, stacked on top of one another.

We come down here with 3-phase AC, and this block you see right there is the AC to DC converter, or the dial block. There are 6 dials in there to convert the AC to DC. Then we come up here to these two blue cylinders are called capacitors. These are electrolytic capacitors that smooth out that DC. Going from these capacitors over here, we have the transistors. So there are 3 transistor blocks here. In each transistor block, we have 2 transistors. And they switch on and off, and thus create a frequency that allows us to regulate the speed of motors. This has tremendous potential for energy savings and allows us to adjust the speeds of our fans, conveyers, or process control systems.