motor ampere meter readings

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1madison

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Given: A 25hp, 208 volts,60 hz,3 phase continuous duty compressor motor. rated fla 74.
When running, steady readings are;
a-phase 72 amps, b-phase 69 amps, c-phase 60 amps.Supply voltage is stable with little fluxuation.
Any input, anyone regarding the wide range in phase loads ampere readings.
Thanks in advance.
 
Re: motor ampere meter readings

Check connections from starter to the motor connection box. Seen many loose connections cause these problems. Even just a little annealing on a wire in the lug, so don't just check torque. A thermal camera can show a loose connection real quick if you have one, but a quick inspection on one motor shouldn't be to much time either. Other than that, make sure you have a spare motor ready to go. Usually watch for about a 10% difference and you are exceeding that.
 
Re: motor ampere meter readings

I'm watching realtime readings on some fairly large motors driving compressors right now. One 800hp (480V) for example is loaded thus:
A - 770A
B - 813A
C - 792A

Another 900hp
A - 788A
B - 830A
C - 816A

I don't recall seeing too many motors that approached being really balanced. Probably has to do with the way they're wound, line voltage balance and maybe gremlins in the motor. :confused:
 
Re: motor ampere meter readings

After checking all contactor connections, which all tested ok (approx. 95 deg.f.,about 10 deg. higher than controller ambient surface temp).Other than meggering the motor coils I will keep an extra eye on this unit, being that it is still under warrantee.
 
Re: motor ampere meter readings

kentirwin, your motor's only have about a 6% difference between the highest and lowest reading. Bigger motor allow larger variance. Look at % not the actual difference. The impedance imbalance between the windings is what causes some kind of difference. No mater what the coils may even have the same turns but core variances and just the way the wire's are laid in the slots can change a coils impedance. So yes there is always some kind of imbalance, but that imbalance shouldn't exceed about 10%. Once it's over that in the motor itself you could be developing turn to turn shorts, or just have bad connections even on the inside of the motor.

If you have a shop that knows what there doing you can put a surge tester on the motor. Unfortunately if you don't know what you are looking at while a motor is still assembled it could read bad. A large motor you can do this with. A small 25HP, have a spare ready to go, and just be ready. Keep monitoring.
 
Re: motor ampere meter readings

Also 1madison, where are you taking your current readings from. Right from the starter contactor feeding the motor or the incoming feed to the machine. You may have various single phase control devices that could change the current on one leg.
 
Re: motor ampere meter readings

1madison, check the connections inside the motor connection box also.

edited to remove offensive slang term

[ February 23, 2005, 04:02 PM: Message edited by: don_resqcapt19 ]
 
Re: motor ampere meter readings

Gee Don, it doesn't bother me but I remember in the Navy quite a ruckus about said offensive term. It got attention from some rather high level brass. It came down that it was an accepted trade name and was not being used in an offensive manner. Some of the motor people I have worked with, when I said connection box haven't even understood what I was talking about until I used the offensive term. Like I said it doesn't bother me but It just gets funny how language can get messed (almost used an offensive term for that) up. :D
 
Re: motor ampere meter readings

I would suspect that variations in the air gap between the rotor and stator iron might tend to create these variations. This effect might be aggravated by worn bearings especially if the motor is driving a belt which pulls the rotor to one side.
 
Re: motor ampere meter readings

All readings were taken at the "T" terminals, the control circuit load was not part of the load totals that I had previously mentioned in initial post.I will laser heat scan the temperature on the connections made at the "pecker-head" (a New York City term sometimes used to indicate the motor housing connection box)
Thanks All
Contents should not be misconstrued as taking on any sexual connotation.
 
Re: motor ampere meter readings

The times that I have seen this problem it was caused by poor incoming power. I always see it when supplied by an open Delta transformer bank that have large motor loads. I have also seen a standard delta arrangement that had a bad transformer which in effect is an open delta.
 
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