Why do most UPSs have active front ends but VFDs have diode rectifiers?

A question we get a lot is – “why do most UPSs today have active front ends, but most drives still have diode front ends.”

I'm going to explain this and draw it out for you so that you can see the difference, but it really comes down to economics. Let's take a look at a previous example. If I have a UPS, it is likely going to have a transformer on the front end and it is going to have some sort of a harmonic filter. And then it has either a 6-pulse or 12-pulse rectifier that converted from AC to DC. It has a capacitor on DC bus and battery, and an inverter. And when the inverter is typically going from DC to AC, it is I G B T based. And that would feed the loads. Let's compare that to what a variable frequency drive looked like. A variable frequency drive would have a diode front end that converted from AC to DC. And a converter on the UPS. So this is the UPS. This usually had SCRs.

This has a capacitor on the DC bus for smoothing, and this is our BFD. Then going from DC to AC to AC, it is obviously feeding a motor. We had I GBTs. So, when you look at it, you say, okay, the back end of the UPS and the back end of the driver, the same, the front of the drive, this has diodes. And, so diodes commutate naturally SCRs. You can force to turn on, but you have to wait till they turn off. So, these are less expensive, more expensive, most expensive when you look at the components. This is what happened in UPS space – they took these components and they removed all of that and they replaced all of that with this essentially. So now you have a converter on the front end that uses IGBTs with pulse, with modulation, and they put a small filter here that filters out this PWM frequency.

Now on the front end of a UPS, instead of having a 6-pulse or 12-pulse wave form like that, it relatively has a pretty smooth sign. Your soil, maybe a little bit of high frequency on there, but that's what the PWM filters and has a nice clean sinusoid away form. Not only that. It corrects the harmonics, but it also can correct power factor. So, you have good power factor. You have low harmonics, and what you've done is removed a large transformer and a filter and spent more money on the front end converter. And that's why today, UPS typically has that kind of configuration. If I were to do that on a VFD, what's my benefit? Well, I would have to convert this to this version. And, so I don't have my less expensive diode front end.

And, what it does is just adds more cost. So you see the difference is that I took away a lot of cost in these parts, combined it with this. And, the net benefit is that the cost is about the same, or maybe even less for a much cleaner solution, as opposed to taking and adding more components and therefore more cost. So, hopefully that explains why with UPS's today, we use the active front ends in almost every case. And, with drives, we still use diode front ends for maybe 80 or 90% of the drives out there because it's less expensive and we're not replacing all those components that we had to do with UPS.