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ksteensma (Electrical) (OP)
30 Jan 08 18:22
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.
dpc (Electrical)
30 Jan 08 19:04
It would be helpful to know the voltages at the motor.  

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.  
davidbeach (Electrical)
30 Jan 08 22:42
What is the phase relationship between the three voltages?
7anoter4 (Electrical)
31 Jan 08 8:40
hi Ksteensma
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
gepman (Electrical)
31 Jan 08 11:44
Since you have the problem with all of the motors I doubt that it is in the wiring to individual motors.  You should check the voltage relationship at the MCC which serves all of these motors.  If there is no problem there then I would check it at the motors (which I realize is usually more difficult).  I have read that a 1% voltage unbalance can cause up to a 10% current imbalance.  You have a 0.35% voltage imbalance.  You need to calculate the current imbalance with the method that dpc states.  If it is around 3.5% then this would be within normal.

I don't think that it is as 7anoter4 states unless the parallel cable load imbalance is causing a voltage imbalance to the motors.
ksteensma (Electrical) (OP)
31 Jan 08 12:54
Thank you for all your interest. To answer the questions:

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
dpc (Electrical)
31 Jan 08 15:03
If all the motors show the same current imbalance, it is reasonable to assume the voltage to the motors is not balanced.  But I think you need to get voltage readings as close to the motors as you can.  

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.  
ksteensma (Electrical) (OP)
31 Jan 08 16:44
I will check voltages at the load side of the starters, as this is as close as I can get it. I will let you all  know.
7anoter4 (Electrical)
3 Feb 08 11:56
Hi
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
SpaceAce69 (Electrical)
6 Feb 08 12:02
Since the voltage is nearly equal, and the PF readings coorespond to current imbalance, then you are left with efficiency and resistance.  This seems to be pointing to resistance (including feed wires).  My suggestion is to measure resistance leg to leg (with power off of course) from the bottom side of the contactor.  If nothing sticks out (all less than 5%: 100 x [max. deviation from average]/average), then turn the motors on one at a time and compare current deviation (same way).
jraef (Electrical)
7 Feb 08 12:01
I'm with gepman and SpaceAce69 on this. The first thing I would suggest doing is to roll your conductors from the service and see if the problem follows. If it does, it is in the conductors.
LionelHutz (Electrical)
12 Feb 08 12:52
FYI, A 6.5% current imbalance requires about 2% motor derating according to NEMA MG-1.

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.

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