Amperage not matching up

[meterchick]

We have a 100 HP motor on a 112.5 kVA transformer with a Benshaw VFD 277/480 Y, the consumer called in about the transformer being hot. We IRed the transformer and found it to be about 188 F which was 60 F hotter than other similarly sized and loaded transformers in the area. Amperage measured in the electric meter is running about 25 A which seemed low, but I thought maybe he doesn't have it running that hard. We checked amperage in the transformer, meter can, and his disconnect. All of these readings are running 70 to 90 amps, unevenly loaded, which matches the readings on the VFD. A phase is leading where B and C are lagging with no amperage whatsoever on the neutral. We then hooked up a three phase recorder and initial readings are showing Power Factor to be +14, -34, -96 ABC respectively. I don't understand how this is possible. Could this be a harmonics problem?? Any ideas would be welcome, I am totally stumped on this one!!

 

[charlie b]

There are several aspects of this situation that do not make sense to me. To start with, what are the primary and secondary voltages of the transformer? A current around 90 amps on a 100 HP motor seems to correlate with a 480 volt source, but it does not correlate with 25 amps on the primary side for any standard voltage rating of which I am aware. Also, if I understand correctly that the three phase conductors are not running at equal currents, then there should be some current on the neutral. If neutral current is zero, then the neutral wire is disconnected somewhere along the line.

Welcome to the forum.

 

[GoldDigger]

Have you considered that you are running with one phase line of a three phase supply open?
If the load is wired delta, there would still not be any neutral current.
Power factor is a number less than 1, so I guess what you listed was phase offsets in degrees between voltage and current on each line?

 

[meterchick]

Thanks! Sorry, I hoped I had got all the pertinent stuff in there, but knew I probably forgot something. Primary voltage on the transformer is 7960 and secondary voltage is 277 phase to ground, 480V phase to phase... We didn't see any obvious problems with the neutral, but need to check again. Our next step is to change out the meter and see if it is only a meter problem, but IMO that doesn't solve the hot transformer, or explain the weird phase angles and terrible power factor.

 

[meterchick]

Voltage is good and hot at 285ish on all phases to ground. Sorry about the power factor +0.14, -0.34, and -.96. Load dropped off suddenly about 25 days ago. Everything was fine before that. None of the equipment was changed and it has been running fine for several years.

 

[GoldDigger]

The transformer is a single core three phase?
Have you checked the phase angles of the line voltages as well as the currents?
Have you tested for a phase to ground (rather than neutral) fault?
If the VFD is drawing unbalanced current, it may be the result of bad phase voltages or else very uneven phase currents in the motor. The DC bus should isolate the input side current somewhat from phase imbalance in the output to the motor.
A scope would be the simplest way to look for harmonics, but an analyzer would be better if you have access to one.

 

[Smart $]

The transformer is a single core three phase?
Have you checked the phase angles of the line voltages as well as the currents?
Have you tested for a phase to ground (rather than neutral) fault?
If the VFD is drawing unbalanced current, it may be the result of bad phase voltages or else very uneven phase currents in the motor. The DC bus should isolate the input side current somewhat from phase imbalance in the output to the motor.
A scope would be the simplest way to look for harmonics, but an analyzer would be better if you have access to one.

Transformer is 3? by his info.

Checking angles of voltage may help diagnose, but I'm leaning towards no help, especially since he stated voltages are running 285-ish. With the transformer being SDS (not confirmed), L-G should be nearly identical to L-N if grounded wye and only load... but it doesn't hurt to verify. If an ungrounded secondary, the required GFP should indicate a GF. Current measurement with no load should be indicative of a resistive fault on the supply side of the VFD.

I agree the DC bus should isolate motor unbalance... so I'm leaning toward one or more bad rectifiers. This would account for the .96 pf on one phase, the plus pf on another, and poor pf on both other phases.

Curious how the motor is connected: wye or delta. Motor current measures, simple volts and amps would help in the diagnosis. If these are unbalanced, then testing motor circuit conductors and the motor would be indicated.

JMO