AC line reactor vs DC choke - which is better?

The question is: what's better – AC line reactors or DC chokes? 

If you're looking at a variable frequency drive and you wanted to decide, should I put an AC line reactor or a DC choke, or should I specify one or the other, which one's better? Really, the answer is if you're buying it internally, you're going to have to go with whatever the vendor supplies and in some of our drives previously, we used to use internal AC line reactors. But, in all of our drives today, we're using the DC chokes and I'll show you what the difference is in how they're laid out. 

So, here's an AC line reactor. Here's a set of DC chokes that goes into the DG1 drives here behind me. Now, let me draw it out for you. So this is your converter from AC to DC, and then you have a capacitor on the DC bus and you have your inverter on the output. 

So, you go DC back to AC. But what we're talking about is inside that front part of the drive, we could put line reactors here on all three legs, three phase reactor and include that inside the case. What happens is that makes the case bigger. Historically what's happened is you could put up to about a 3% AC line reactor inside the drive. No, if we put on the DC bus inside here, if we put a DC choke here on each leg going to on each side of this capacitor here. Essentially you can put about a 5% DC choke on that side. If we look at the harmonic spectrum and we say, okay, here's my fundamental, my fifth harmonic is here. My seventh harmonic is here. If I have the AC line reactor five, the DC choke with a 5% I could be here and here. 

Honestly, the big difference is more about the percent that I can use in terms of the line reactor or DC choke. If I could put a 5% reactor here, theoretically, it'd be about the same as the 5% choke. Now, if I've already got my drive installed in my system, and I want to put in a reactor, I'm not going to put one inside the drive. So, I'm definitely going to put one on the outside of the drive, put my AC line reactor on the outside of the drive. And, even if I have an internal reactor, like let's say 3% or even 5% DC choke, I can still put up to a 5% AC line reactor on the outside of the drive and give me some really reduced harmonic current levels coming into the drive. Because remember, once those harmonics are produced, they're going to flow back out on the system and they have to flow back out and back, and we're going to have to deal with that harmonic current, once it leaves the drive. So, AC line reactor or DC choke really depends on if it's internal or external. And, I hope some of those reasons helped you to be able to make a decision about what might make sense for you.