Wild leg power

[nya1]

I have encountered problems with a 5 HP 230/3/60 volt pump service. The motor is a 230/460 (dual wound) motor.

The problem does not ocur when using real 3 phase power. The amp draw is normal and balanced. FLA is 14.7A, max reading is 12.8A

On a wild leg service the pump draws almost 25% more amps on one leg and about 15% on the second and the 1st is about FLA. The voltage seems balanced, phase to phase is within 5 volts AC.

Phase to Phase - 245V, 243V & 247V
Phase to ground - 120V, 120V & 208V

Any ideas?

Thx,
nya1

 

[dpc]

Are you measuring those voltage with the motor running? If so, voltage balance is pretty good.

If these were measured with the motor off, the voltages may look a lot different when the motor is running.

As far as "real 3 phase power" goes, a wild-leg system actually provides real three phase power - the problems generally start when the single-phase loads become too big a portion of the total load.

Are you maybe dealing with an open-delta system that only has two transformers and not three?

 

[nya1]

Yes, motor voltages were taken while running. Amps and volts were taken with a recently calibrated power meter.

Yes, we are using the open delta two transformer system. This is the only place we have encountered trouble. When using three transformers, true 3 phase, the problem goes away. This has occured at more than one location.

thx, nya1

 

[raisinbran]

Nya1
Have you checked the motor with a meggar for an internal ground? One possibility -You could be seeing current flow between an internal motor ground and the grounded center tap ( I assume) of one of the legs of the open delta transformer connection. This would now become a solidly grounded supply - sort of. If this motor were connected to a truly ungrounded 240 volt three phase system, a ground in the motor would not cause any additional current flow.
Regards,
Raisinbran

 

[dpc]

The open-delta connection will always produce unbalanced voltages under load. All you can do is keep it within acceptable limits. If transformers are mismatched, that will make the problem worse.

You might try rotating the motor connections so you can see if the current imbalance is a function of the system phase or the motor phase. If the imbalance follows the motor leads, then the problem is likely in the motor.

 

[electricpete]

I think you ruled out problems with the motor when you saw balanced currents on another power supply.

It seems two possibilities:
1 - the balanced voltages of your supply become unbalanced when you add a load as mentioned above (did you check voltage at the motor under load?)
2 - the voltage magnitudes are equal but the phase angles are not 120 degrees apart.

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[SteveKW]

Possible open primary on the transformer?

 

[ccjersey]

An open primary on an open delta transformer bank would leave only single phase wouldn't it?

Best thing to do is roll the phases and find the line/lead combinations with the least imbalance. As mentioned above this is also a way to diagnose where the problem is. Also look for any high resistance connections on whichever side (of the point you where you roll the phases) is indicated. I have also seen this on some motors on open delta service and our voltages are closer than the ones in this post.

Jim

 

[electricpete]

Once again, I don't think the high current would do anything other than follow the lead when rolled. He has already established balanced current on the motor with another power supply which rules out the motor in my book.

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[rbulsara]

I agree with electricpete.

Are the currents measured in the condcutors of the motor branch circuit or at the mains? If at the mains it may be due to other loads (most likely single phase) connected to the system.