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#170821 11/12/07 09:17 PM
Joined: Sep 2004
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I received a phone call today from an HVAC contractor who was in the process of changing out the compressor in a RTU which was fed with a grounded B phase system. He wanted to know if he could install a 3phase monitor that would prevent his compressor from running on single phase if the unit lost a phase.

I have never had the "pleasure" of working on a grounded B phase system so I didnt' have an answer for him. He said he had changed this compressor out 1.5 years ago and it had already died.

So I started thinking.....would the compressor for this type of electrical system be different from one used on a modern 3phase system ? Could the grounded B phase be the cause of the compressor failing so soon if the compressor wasn't designed for use on a grounded B phase system ?


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There is nothing special about corner delta that you wouldn't have in red leg delta. Anything that works on centertapped delta should work on corner delta (except 120v loads)
He does know this is 240, not 208?


Greg Fretwell
Joined: Jul 2001
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Originally Posted by sparky 134
I received a phone call today from an HVAC contractor who was in the process of changing out the compressor in a RTU which was fed with a grounded B phase system. He wanted to know if he could install a 3phase monitor that would prevent his compressor from running on single phase if the unit lost a phase.


He doesn't need a different motor (which does not exist), he needs a Phase Loss Relay added to his starter control circuit instead.


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