Rotary Phase Convertor

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laketime

laketime

Senior Member
2 years ago we installed a customer sized and supplied rotary phase convertor to feed a 3 phase compressor. The customer let us know today that the compressor service company took ampergae readings on the compressor. Phase A=79 amps
Phase B=79 amps
Phase C=45 amps

What gives?
 
laketime said:
2 years ago we installed a customer sized and supplied rotary phase convertor to feed a 3 phase compressor. The customer let us know today that the compressor service company took ampergae readings on the compressor. Phase A=79 amps
Phase B=79 amps
Phase C=45 amps

What gives?
Click to expand...

It does not take much of a voltage imbalance to cause a much larger percentage current imbalance in a motor. If the capacitors which form part of the rotary phase converter are not tuned to the size of the motor load the voltage on the derived leg can differ from the voltage on the supplied phase conductors.

It seems odd, but if the C lead is the derived phase, the AC and BC currents could be different from the AB current, leading to the measured currents seen.

The very first piece of additional information I would want to see is the full set of line voltages.
Presumably the compressor is wired in delta, as is the rotary converter. Does the source single phase have a center tap which is grounded? That would make the line to ground voltages on the converter output look like a high leg delta.
 
You can never balance an air compressor either - tried it. Problem is it draws more current when system is near cut off pressure then when it is near or even below cut in pressure.

Most you can hope for is that the motor isn't smallest motor that can drive the load and still be within rating when unbalanced like that.
 
Thanks for the information. It was really helpful. I will let you know what we find out. :thumbsup:
 
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