Large VFD on ungrounded delta

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Randy S.

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One of the large chillers I am responsible for is VFD driven and powered by an ungrounded 480 delta, This is in the 500 HP range. We have seen some unusual dark spots on the motor windings, and unusual wear on the motor shaft thrust ring and bearing surface. The motor megs fine, and was additionally tested by a very reputable motor shop.

I'd like to hear from those knowledgeable about VFDs on an ungrounded system.
I know that more than one manufacturer advises to not use their VFD on an ungrounded system, since ther is no real ground reference in the system, and fault current would be phase to phase, not necessarilly phase to ground unless there were ground faults on more than one phase. We have found no signs of unintentional grounding, but the anaomalies I wrote of remain.

Thanks to all for your comments.
 
I have done it on a similar system but the VFD manufacturer had to come out and take a screw out because we were ungrounded. Have you consulted the manufactuerer?
 
The screw removal is because a lot of VFDs designed for European installations come automatically set up with a ground reference as a Y configuration on the front-end filter (because all power systems are Y in Europe), so you disconnect that ground reference if using it on a Delta system. It does affect the Ground Fault protection capabilities of the VFD though, hopefully they told you that. Without that ground reference, the VFD can get a lot of nuisance GF trips, so most of the time you have to disable it. Sometimes reactors can help alleviate that nuisance tripping.

I'm not so sure that has anything to do with the output though. It's far more likely to be the bearing currents issue, that's a very common problem regardless of y or Delta input power. More good reading on it here.
 
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