VFD Damage

Dont buy cheap Christmas Crackers...

What I neglected to mention that may also have a bearing and may contribute to a better understanding of the problem is that the damage was caused not only to the VFDs that were energized (motors running) but also to those whose motors were not running.

When a short supply interruption occurs the VFD "soft-start" pre-charge system may not get switched out. The result would be a massive current surge when the supply comes back on (as the DC Bus capacitors are re-charged from the supply without any current limitation). The only way I can see to stop this effect would be to ensure that, when the supply is interrupted, it is enforced for a MINIMUM time - enough time for the pre-charge system in the VFDs to kick out.

Just a thought...

VFD manufactures usually indicate the maximum supply current that should be available:

EG

- Suitable for use on a circuit capable of delivering not more than 5000 rms symmetrical amperes, 480 V maximum, when protected by fuses.

- Suitable for use on a circuit capable of delivering not more than 50000 rms symmetrical amperes, 480 V maximum, when protected by CC, J, T or R class fuses.

Please use the telephone to discuss the problem with the equipment manufacturer(s), then report back.

<subscribes>

Could the HV reclosers create harmonics not filters by your RLC filters?

If possible, measure the odd harmonics.

It might be a problem caused by the power coming back, while the VFD is in the process of "shutting down" as a name to use.

The electronics are in some areas still getting power from capacitors that have not lost their charge, but the overall control is gone. Inductors are shooting high voltages around as the power is dropped....

This can cause damage as parts are simply going "wild" as a word to use.

What could possibly prevent this to a degree, is some sort of breaker (I don't know the right name!), that drops off when no voltage is available...so the device cannot power up while still powering down! If power gos, it remains gone!! You need to go and reset them before powering up again.

This will probably prevent most problems, but possibly not all.....powering down such units while still operating, is bound to cause possible problems still from time to time.

Maybe the manufacturer needs to address such possible problems with a better design, stopping transients from entering sensitive electronics....

I would contact various manufacturers and ask how they handle uncontrolled power downs, or power "Hiccups".

Firstly, you need to fix the problem at the source (the power drop) and eventually possibly buy better quality VFD units that can handle that better as well......

Me again. Thank you all for your responses. We are getting more info from the field all the time and no doubt we will be zeroing in on the solution as soon as we have identified the source of the problem. Another piece to the puzzle is that the motors controlled by VFDs but have not been fitted with passive filters that are connected to the low voltage busses have not suffered any damage. Also the damage that has occurred to the damaged VFDs has been to the IGBTs (at the output end) as well as the diodes of the uncontrolled rectifier (yes, the VFDs are the PWM type) - in other words, the damage has been pervasive.

There is a long story as to why damage like this can happen, the end of which comes down to this anyway. You must have more impedance on the INPUT side of the VFDs. People often focus on the output filters to help protect the motors, which is fine, but neglect the input side simply because they may not be concerned about harmonics. Input reactors do little for harmonic mitigation, so people tend to leave them out, missing out on the OTHER more important benefit they bring, input impedance. It comes down to a combination of what was said earlier in this thread, but for different reasons. I'm on my iPad right now and this forum doesn't like Safari so it's too hard to do the long version, but if you want this to stop happening, go get line reactors for your drives.

AndyGermany said "What could possibly prevent this to a degree, is some sort of breaker (I don't know the right name!), that drops off when no voltage is available...so the device cannot power up while still powering down! If power gos, it remains gone!! You need to go and reset them before powering up again."

Can i suggest a breaker controlled by a low-v or differential-v relay, or both.

I "feel" that such a breaker as you mentioned, would reduce the problems significantly.

As I see it, you feel possibly the same way?

Possibly not fix all the problems, but close to at a guess.....certainly a relatively cheap alternative, easily installed.

If the problems are (hopefully) reduced by at least 80% (or more as I believe they will be!) it would be very positive...

But till such a device is installed, we will not know!!

Guessing is simply not good enough!!