Motor load current imbalance topic again
Motor load current imbalance topic again
(OP)
I know this topic can get old at times, but hear me out once.
We are expierencing a large current imbalance on our 2000 amp 480 volt Y service which serves motor only load. We have 10 refrigeration compressors on this service of varying horsepowers between 75 hp and 300 hp. The service and all motors have been checked and all exhibit the same facts:
A phase current middle value
B phase current lowest value
C phase current highest value
Voltage at the service entrance 481,484,482
Neutral current is zero
There is a 13% difference between the low and high readings on the current. The voltages at the incoming service entrance all look good. Entire system including motor starters have all been IR scanned showing nothing.
Every motor has its own microprocessor and displays current readings. All 10 show the current imbalances as described above. The service equipment has it's own metering and shows the same as the motors. All have been checked with a hand held True RMS clamp-on meter
I have checked for harmonic currents riding on the 60hz using a Fluke 41B Power Analyzer. It showed all current to be 60hz, and no harmonic component. The one thing that stood out was the following:
A phase power factor .72pf
B phase power factor .61pf
C pahse power factor .68pf
These measurements were at the service entrance.
I am stumped where to go next. Any ideas are greatly appreciated.
We are expierencing a large current imbalance on our 2000 amp 480 volt Y service which serves motor only load. We have 10 refrigeration compressors on this service of varying horsepowers between 75 hp and 300 hp. The service and all motors have been checked and all exhibit the same facts:
A phase current middle value
B phase current lowest value
C phase current highest value
Voltage at the service entrance 481,484,482
Neutral current is zero
There is a 13% difference between the low and high readings on the current. The voltages at the incoming service entrance all look good. Entire system including motor starters have all been IR scanned showing nothing.
Every motor has its own microprocessor and displays current readings. All 10 show the current imbalances as described above. The service equipment has it's own metering and shows the same as the motors. All have been checked with a hand held True RMS clamp-on meter
I have checked for harmonic currents riding on the 60hz using a Fluke 41B Power Analyzer. It showed all current to be 60hz, and no harmonic component. The one thing that stood out was the following:
A phase power factor .72pf
B phase power factor .61pf
C pahse power factor .68pf
These measurements were at the service entrance.
I am stumped where to go next. Any ideas are greatly appreciated.





RE: Motor load current imbalance topic again
Do the motors have power factor correction capacitors and if so, have the fuses been checked?
Voltage and current imbalance per NEMA MG-1 is defined as the maximum percentage deviation from the AVERAGE of the three values. NOT the variation from min to max. On this basis, what is the measured current imbalance?
A relatively small voltage imbalance can cause a current imbalance many times larger.
That's a pretty lousy power factor measured at a service entrance.
RE: Motor load current imbalance topic again
RE: Motor load current imbalance topic again
I think is a problem with your 2000A feeder.It is possible the feeder consists of 2 or more parallel single phase cables per phase.In this case the unbalance may be produced by unbalanced position in the magnetic field produced in assamble of all phases.
The single cables have to be layed in certain order to avoid unbalance.
In october was a similar thead:
Parallel Cable Load Imbalance
thread237-201037: Parallel Cable Load Imbalance
Best Regards
RE: Motor load current imbalance topic again
I don't think that it is as 7anoter4 states unless the parallel cable load imbalance is causing a voltage imbalance to the motors.
RE: Motor load current imbalance topic again
Voltage at the motors? I don't have now.
PF capacitors? none used
Voltage phase relationship? Don't have vector meter yet
Parellel conductors? Yes, incoming conductors are 5 parellel sets in steel underground conduits. Matched lengths? I am sure the conductor lengths vary even due to conduit routing. Conductors are 5 sets of 500kcmil 40 feet long to transformer.
Current imbalance calculated as the deviation from the mean is 6.5%.
Hope to hear more opinions soon. Thank you
RE: Motor load current imbalance topic again
The voltage readings taken at the service entrance do not explain a 6.5% current imbalance, but the voltages at the motors may be different.
RE: Motor load current imbalance topic again
RE: Motor load current imbalance topic again
You are right gepman, I am afraid .According to my late calculation it is an insignificant voltage difference even on a wrongest possible cable run. So it is not this direction.
Best Regards
RE: Motor load current imbalance topic again
RE: Motor load current imbalance topic again
RE: Motor load current imbalance topic again
I do find it hard to believe every motor has the same 6.5% current imbalance. That is interesting since I've always noticed no 2 motors are the same and that the current imbalance of a motor will change with the load. I've seen motors that were 40% when unloaded and drop to 3-4% when loaded.
All you can do is provide good 3-phase power. After that, the current imbalance of the motor will be whatever it is.