Do I need to worry about input harmonics, output harmonics or both with a VFD?

Question: When we talk about harmonics on a VFD, do I need to worry about the input harmonics, the output harmonics, or both? 

The real answer to the question of input or output harmonics depends on what you're worried about. If you're worried about the motor, then you have to worry about the output harmonics, and that's really more of a pulse width modulation. I don't really consider that too much from a harmonic standpoint, but when we're talking about harmonics, mostly we're talking about the input harmonics going into the drive and that's on the rectifier part of the drive. So, let me show you what I mean on the import of the drive, we have a rec. So, if this is a VFD, we have an input that comes in three phase and we have a rectifier and that has changes from AC to DC. 

Then we have a DC bus with a capacitor, and an inverter and then an inverter as DC back to AC. Then we feed our motor with the three phase pulse width modulation. That again has high frequencies. The types of things that we're talking about is the 39th harmonic or something very high, so harmonics of high orders. If you look at a frequency spectrum, you see the fundamental and then you see way out here some high frequencies, but on the input of the drive, we have typical six pulse drives that are going to have fifth, seventh, 11th, 13th, 17th, 19th, et cetera. That's for a typical six pulse drive. And, what that means is we have six diodes converting from AC to DC. Now, when we have that, our harmonics spectrum looks like this fundamental fifth, seventh, 11th, 13th, 17th, 19th, and so forth. And so again, the real answer to the question is - do we worry about the input or output harmonics? If I look at my power system, what I'm worried about is the input harmonics, because those are the harmonics that are going to come up from my drive and effect the rest of the power system that current either flows through this transformer, maybe causes resonance on another load, but this harmonic current that's coming out of the drive. If this is my VFD here, and it's a constant current source. 

I'm not really worried about going through the driving on the other end, feeding through the inverter motor, because all of those, again, PWM output harmonics are going to be only affecting that particular motor. The harmonics on the import are going to affect other loads and the power system itself. So importer harmonics are what we're talking about.