VFD - DC Link Capacitors Useful Life

[Drivesrock]

Hi all.
When to replace Voltage Source Inverter drives DC link capacitors?
I have some customers asking me about this (in large power drives) and how do we measure them to know if they need replacing? A capacitor has a design lifetime in number of hours at a certain ambient temperature. I have seen data of 100 000 hours at 40 degrees C. That's over 11 years continuous use. If the drive is not in continuous use or in a lower ambient temperature we could see a much longer lifetime than that but there is the other side to the coin!
I don't think it is a sensible option to simply want to replace drives - low voltage (high powers) or high voltage because of the DC link capacitors at say, 6 years on a whim! That's a big investment and if these drives have been working well and there are still spares and support available there are plenty of reasons to keep them - except that the drives might become unreliable?

I found this information on capacitors:
Manufacturers of capacitors usually define “end of life” as that point when the ESR (Effective Series Resistance) of the capacitor doubles from its original ESR value. This does not mean that the capacitor stops working, blows up or dies. In fact, the capacitor will continue to behave as a capacitor but with degraded performance. The effect of this on the drive will generally be an increase in the peak to peak ripple voltage on the DC link. This may or may not be significant depending on the application and other factors. The increased ripple will also lead to increased ripple current which in turn will create more heating effect in the capacitor. This increased heating will further degrade the performance.

I'm not sure what 'significant' above means - I hope trips - but on what? I suppose it depends on the manufacturer. Trips and not catastrophic failures!

What experience do you guys n gals have on this subject? Any suggestions on what to measure and compare to?

 

[buzzp]

I once spent a lot of time diagnosing a drive problem on a PAM drive used on a 480V, 100HP submersible pump and motor. The drive would start to ramp up and trip on OL. After many hours of trying to figure out the issue, I discovered a cap breaking down at about 300Volts so we replaced the whole bank.

On the same type of drives, I have had to clean up after a cap bank let loose and it was no fun at all. The electrolyte is incredibly sticky and gets everywhere when one really lets go.

So I have seen catastrophic failure and failures that don't show up until the cap is energized, or at least started to energize, and does not cause any external physical damage to the cap (sometimes the vent at the top will swell or open up indicating a bad cap and other times you won't see anything wrong). I don't see scheduling an equipment replacement based solely on the life of the caps in the drive but then again it depends on the application. Of course, nowadays, an electronic device that lasts 10 years is not unheard of but has technology made it obsolete?

Electrolytic capacitors are usually one of weak links in any electronic design, whether big or small, so it is something to be concerned with.

I think this is a case where one has to determine the application and the cost of the down time vs. the cost of stocking a spare. We repaired these drives on a regular basis and kept 50 or so on hand in the event we had to replace a whole capacitor bank.

 

[roydm]

If the VFD has been sitting unpowered for a long period the capacitors may need reconditioning, the manual can tell you how to do that.