What is the difference between EC motors and motors with VFDs?

A question we've been getting recently is “what's an EC motor or electronically commutated motor versus putting a variable frequency drive in front of an induction motor”.

Basically an electronically commutated – or EC motor – contains similar components to what a drive has on the front end, but it's almost like it converts it to DC and drives a DC motor where a variable frequency drive goes from AC to DC converts from DC back to AC and drives an AC induction motor with a different frequency to change the speed and change the process. For example, with an AC motor, you change the speed by almost changing the voltage on a DC motor, and you can control the speed that way. Let me just draw that out for you and show you what I mean. 

Let's say we have an AC motor here and if we want to change the speed of this motor, we can put a VFD in front of it. And what the VFD does is change the frequency. And by changing the frequency, for example, let's say we have 60 Hertz in and we run the motor at 30 Hertz out. We're gonna run the motor at half speed. We can change the speed variable frequency and also change the speed of the motor for an AC induction motor. 

Now, with a EC motor, basically think of the EC motor as sort of this motor here with an attachment to it. That's a converter. It's almost like you have a VFD built into the motor itself, but think of the motor more like a DC motor. It's almost like we have the front end of a VFD. So, what's in a VFD is AC to DC or rectifier. And then we have a DC bus with a cap and we have DC to AC on the output. By doing that with IGBTs here, we can change the speed of the motor on this end.

We just convert from AC almost directly to DC. So essentially what we do is drive the motor by changing the magnitude that goes to the motor in terms of voltage. Now, if we have an EC motor again versus VFD, what are my concerns with harmonics? Well, with a drive, we know that we're going to have harmonic spectrum that looks like this, and it's going to have six pulse. So this is six pulse. It typically has fifth and seventh. So, you have your fundamental, then you have fifth and seventh and 11th and 13th and so forth. My concern with EC motors is that it has a similar front end wave form, but it doesn't usually have any kind of inductance or a line reactor or a DC choke in there. So the current wave form often looks very steep.

You still have fifth and seventh, 11th, 13th, but these magnitudes for an EC motor might be double what they are for a VFD. And so, as you consider all that, an EC motor has less components to pull everything together. You can drive the fan or the motor that you're trying to drive at a different speed. But, from a harmonic standpoint, you might have to add in additional harmonic control after the fact. So, that's the difference between a variable frequency drive and an EC motor as it relates to harmonics.