Weird power supply

[thewellguy]

I was working in northern Indiana and ran across a power supply that I had never seen before. Its a 3 phase 480 system across the legs, but leg to ground it measures 390, 360, and 140. I thought they had a power problem but a local electrician said thats a normal supply for the area. My question is 2 part. 1) how is this system created and 2) can I safely put a vfd on it. Thanks for you help.

 

[jraef]

Sounds like you have a grounded open delta system, not terribly uncommon. Do a keyword search here on "open delta" and you will get previous discussions, but this one is likely most applicable to your situation. thread238-157326

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[jraef]

Sorry, forgot to say something about the VFD supply. There are a number of VFDs on the market which cannot be connected to unbalanced supplies. We discussed this at length in the Motor Control forum earlier this year, but unfortunately that thread was deleted for some reason. I believe Baldor was one manufacturer, AB was maybe another. Anyway, you can check in your VFD's manual to be sure. The ones that have a problem usually state it.

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[stevenal]

I'd guess it's ungrounded. Ground is closer to the potential of one phase due to more capacitive coupling or leakage on that phase.

 

[jraef]

I was actually thinking it is grounded but they have a bad connection.

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[wbd]

This sounds like an ungrounded, delta 480V system that has developed a partial ground through wire insulation going bad. If possible, I'd start isolating loads to see if the phase to ground voltages change.

 

[thewellguy]

Thanks for the informantion. I talked to the local guy again and he says that this is not just at the location that I was working at, it is a common supply from the power company for the area.

 

[waross]

If this is an open delta, the common leg may have more leakage to ground than the single legs. That would skew the voltages to ground with a well insulated system and a high impedance voltmeter.
A grounded delta typically gives two low voltages to ground and one high. Hear we have two high voltages and one low.
respectfully