3-Phase Voltage imbalance?

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guilbertj2006

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Pompano Beach, FL, USA
A 3-phase 480V HVAC compressor, with a nameplate RLA of 50A, tripped an interlocked 68A 3-pole CB. A megohm reading showed some deterioration on the stator insulation. (The compressor had been replaced in January 2012.) I took these readings: A to B: 477V. A to C: 477V. B to C: 461V. Amperage: A: 52A. B: 50A. C: 41A. There was no FOP (fall in potential) on the line-side of the 3-pole starter when the motor went across-the-line; and there was no voltage drop across any of the contacts to the load side. I calculated the voltage imbalance as approximately 2.2%. I'm thinking either a step-down transformer within the building, or possibly the utility voltage is off. But I don't want to lose this expensive-oh-so compressor. I'm installing a phase monitor and a new CB. Any help and pointers would be greatly appreciated, and thank you in advance.
 
Is the service 3 separate transformers or a true 3-phase transformer? An open delta (2 transformers) will have crappy voltage across the "missing" transformer.
 
guilbertj2006 said:
A 3-phase 480V HVAC compressor, with a nameplate RLA of 50A, tripped an interlocked 68A 3-pole CB. A megohm reading showed some deterioration on the stator insulation. (The compressor had been replaced in January 2012.) I took these readings: A to B: 477V. A to C: 477V. B to C: 461V. Amperage: A: 52A. B: 50A. C: 41A. There was no FOP (fall in potential) on the line-side of the 3-pole starter when the motor went across-the-line; and there was no voltage drop across any of the contacts to the load side. I calculated the voltage imbalance as approximately 2.2%. I'm thinking either a step-down transformer within the building, or possibly the utility voltage is off. But I don't want to lose this expensive-oh-so compressor. I'm installing a phase monitor and a new CB. Any help and pointers would be greatly appreciated, and thank you in advance.
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having voltage imbalance means a loss in motor efficiency. NEMA graphs show an almost 10% drop on a 3% voltage unbalance. running the motor at full load with high unbalance would likely result to slowly deteriorating insulation.
 
Swap input leads, but still maintain rotation, and see if the supply or the load changes voltage and current values. If the unbalance is still the same on the load (EI T1 lead is still same voltage and current but is now connected to L3 instead of L1 of the supply)then the problem is likely not in the supply.

Make sure voltage is balanced under no load conditions also.
 
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