no load current

[coppens]

three phase motor, 4kv, 39amps FLA, 200hp?, running at FLA unloaded (uncoupled) from coal pulverizer (mill). repair shop ran unloaded at 31amps. what's my problem?

 

[Skogsgurra]

It might be too high a motor voltage. Cos(phi) goes down drastically when voltage is increased above name-plate voltage. Is whole factory lightly loaded when you do the uncoupled test?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[vibetech]

coppens,
You mentioned the motor repair shop; what repairs were performed at the shop? No load currents should be significantly less than FLA. (1/2 to 1/3) If the rated FLA is 39, I would expect the shop to see about 1/2 that during their test run.

Thanks,
Jeff

 

[oftenlost]

This sounds like a slow speed motor with high torque characteristics. I am not particularly alarmed at the no load current. What is the loaded value?

 

[aolalde]

The reported current at full load (39 amps)is too high for average 200 HP motors at 4 kV. Apparently your motor has very low speed (You did not mention it but is very important information).

If your motor has 20 or more poles, nothing is wrong, the magnetizing current is so large that almost matches the full load current figure. Test the motor under full load to verify that the final current under load stays into the nameplate 39 amperes. Remember that the magnetizing current is 90° out of phase to the voltage and the load current is in phase to the voltage. The final total current is the vectorial addition of both currents.

 

[vibetech]

Excellent point---as I reviewed the post---the same thought came to mind; what is the rpm???? Aplologies for that, it has been a long week. Coppens, what is the rated rpm? 6pole? 8 pole?
Thanks, Jeff

 

[JBinCA]

It seems to me that everyone but Gunnar is missing the point. The OP seems to be asking why he's getting different results when attempting to perform the same test that was conducted prior to his motor leaving the shop.

I think Gunnar is probably correct that a difference in voltage is the most likely culprit.

Of course, it's also possible that your indications are wrong or the shop's indication were. Your indications probably rely on installed CTs that were sized for your motor. Who knows what the shop used? Were the meters in cal, what is their accuracy in this range? Etc.

You may be able to find data from the MFG on no load current of the motor. At least then you'd have a third data point to work with.

Regards,

JB