# VFD on ungrounded system



## telsa (May 22, 2015)

Lone Crapshooter said:


> Found this out today
> Have a AB 150 HP VFD don't know what the model is but They tell me it is about 15 years old.It is on a 480 volt ungrounded system.
> 
> The complaint is when the drive has power applied to it's line side it brings in *a* ground alarm at the switchgear. The drive has a ground fault monitor*;* it shows no grounds. With all of the wiring disconnected the wiring and motor check fine.
> ...


You want to stay away from autotransformers in such ungrounded schemes.

Total isolation is best.

Ungrounded schemes and autotransformers just don't mix well.

Indeed, EUSERC was founded ENTIRELY because of wholly unexpected nightmares triggered by faulting autotransformers in an ungrounded Service -- a war plant, no less. (1945)


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## Lone Crapshooter (Nov 8, 2008)

It's just what AB used in the drive to power the cooling fans. probably put out amp and a half at most.
Don't know the number on the drive but there was a fairly new 1336 on the other pump.

LC


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## Big John (May 23, 2010)

Off-the-cuff guess: The drive ground-fault-detection is only looking at the circuit after the DC bus and isn't capable of detecting a fault behind the front end?


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## Lone Crapshooter (Nov 8, 2008)

That is correct.

LC


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## JRaef (Mar 23, 2009)

99% chance that was a PowerFlex 700H drive, a temporary solution Rockwell used a long time ago to extend the range of the PowerFlex 700 line by brand-labeling a drive from Vacon and putting the PowerFlex 700 control boards on it. Rockwell ceased doing that when they released their own versions that extended the range, mostly because there were so many problems with the Vacon power structure design, ESPECIALLY the fan circuit. There were several recalls and service advisories on that issue, warning people to change the fans out. Someone before you likely ignored them because they were not having issues, or someone bought those fans from a surplus house after someone else yanked them out of service.

Also, on all drives that have the MOVs protecting the line side (which is most drives), there will be a recommendation somewhere in the manual to disconnect the ground reference on the MOVs. They are always connected in Wye and the Wye point id grounded. that's what you WANT if you have a Wye power source. But if you have Delta source, the MOVs attempt to become the Wye point for your entire facility if there is a ground fault anywhere, albeit only for a very brief moment before they vaporize and spew their guts all over everything around them. Rockwell makes it easy by providing a simple jumper that you must remove. the same is true if the VFD has Common Mode Chokes to reduce common mode noise getting onto the ground plane, and all PowerFlex drives have that too, so there is another jumper for that as well. RTFM, it's all in there.


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## Lone Crapshooter (Nov 8, 2008)

Thanks for the advice. I cannot remember I think the transformer is a delta but I cannot be sure I will call the engineer tomorrow. They are using a large blower to blow air down through the drive because the fans failed some time ago.
Thanks again LC


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