Motor winding resistance 5

[mellomain]

I have a 480v 3 phase induction motor with 5 to 8% phase to phase resistive imbalance. Mangegemnf decided to start the motor without further troubleshooting. Current imbalance is low around 3%. What effects would I see with a resistive imbalance of say 5 to 10%? Should I push for more troubleshooting?

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[ashtree]

I would suggest that most motors would have a phase current imbalance of at least 3%. This can be caused by many things other than a phase resistive imbalance. I would also say that a 5% difference in resistance would not be that unusual either.

Most of the impedance in an electric motor is inductance so whilst some resistance imbalance might cause some phase imbalance , possibly a little more current, some extra heating, slight change in power factor and in theory a slight distortion in the rotating magnetic field i would not be concerned.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[zlatkodo]

Current imbalance is low around 3%. What effects would I see

It depends on FLA, SF, HP.
Generally, if the motor is driving the load and the highest amperage of the three legs is below the nameplate FLA, then it is safe to operate.

Motor Repair and Winding Design
Always attach a photo of nameplate.

 

[electricpete]

I have a 480v 3 phase induction motor with 5 to 8% phase to phase resistive imbalance. Mangegemnf decided to start the motor without further troubleshooting. Current imbalance is low around 3%. What effects would I see with a resistive imbalance of say 5 to 10%? Should I push for more troubleshooting?

I would suspect an error in winding resistance measurement associated with measurement conditions or equipment
1 - Where was resistance measured from? (directly at motor terminals... or from remote location thru cables)
2 - What type of measurement equipment was used? Was it a 4-wire measurement? What is the stated accuracy of the instruement? What is the winding resistance reading in milliohm (so we can compare accuracy to reading... want accuracy a small number compared to resistance)

I tend to feel good about the motor based on the satisfactory current unbalance. Likelihood of a motor problem (vs test equipment problem) seems very low. Nevertheless if it were a critical motor at our plant I would ask for reading to be investigated or repeated.


=====================================
(2B)+(2B)' ?

 

[mellomain]

The motor was 200HP, 1.15 SF, and 235 FLA. Some readings were taken at the motor leads and some readings were taken at the field leads in the breaker cabinet after the motor was reconnected. All readings were between 5 and 8 percent imbalance. I believe the readings were taken with a meter with too high of a range. They were using a calibrated fluke dvom that was supposed to be able to read milliohms but I think they should have been using a dlro or micro-ohmmeter. The current imbalance remained about 3% even after 24 hour run; i.e. did not get worse as the motor heated up. I was always told phase resistance imbalance (taken at the motor leads) should be less than 1%. And my question was what effects in other characteristics (voltage, current) would I likely see if there was a phase resistance imbalance. Thanks for all your responses.

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[electricpete]

. They were using a calibrated fluke dvom that was supposed to be able to read milliohms but I think they should have been using a dlro or micro-ohmmeter.

I agree. Fluke multimeter is not a 4-wire measurement. Minor deviations in connection resistance will disrupt the measurement. Try taking five readings in a row on the same phase, removing the clips and putting them back slightly differently in between measurements...I'll bet you'll see a pretty high variability.

And my question was what effects in other characteristics (voltage, current) would I likely see if there was a phase resistance imbalance. Thanks for all your responses.

My best practical advice is given above (if you want to rule out doubts for a critical motor, then repeat the measurement… if nothing else because you have not accomplished what you set out to do by checking resistances to begin with). But answering your question to my best ability: I think a condition of concern on the motor that showed up as resistive unbalance would most likely show up as current unbalance (but I don't have complete certainty on that).

What could cause resistive unbalance (other than measurement error): 1 - shorted turn, 2 - open parallel circuit, 3 - high resistance connection internal to the motor (assuming you have ruled out external high resistance by measuring directly at the motor).

1 - Would shorted turn lead to current unbalance? Shorted turn USUALLY leads to motor trip in very short order. But there are a few case studies reported here and there where a random wound motor operates with shorted turn for awhile. We ourselves had motor operate with shorted turn for awhile on ungrounded system (but it was also accompanied by ground). Would shorted turn lead to current unbalance? Most likely yes.

2 - Would open parallel path lead in one phase lead to current unbalance. Most likely yes.

3 - Would high resistance in one phase lead to current unbalance? (again we would be talking about potentially a faulty connection internal to the motor). Typically a dramatically high resistance in one phase (lug severely overheated on the verge of failure) will show up in a current unbalance. BUT we're not necessarily talking about a dramatically different resistance... only 8% resistive unbalance on a winding that might only be 0.1 ohm or so to begin with. Maybe this one is more of a coin-flip... the least likely of the three to show up in current unbalance by my guesstimation.


=====================================
(2B)+(2B)' ?

 

[mellomain]

Big Thanks! Management will not shut motor down now and let me repeat measurements. So all of this advice now will just make me smarter for next time.

Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber

 

[mikekilroy]

an idea for future use: since it is hard to measure .04 ohms with a simple ohmmeter, consider measuring inductance instead. You will find L is normally high enough (often double or triple digit mh) to see any variation between windings. This is what I do if I question a large motor for potential winding shorts. It has accurately identified good and bad each time over the years for me.

http://www.KilroyWasHere<dot>com

 

[flexoprinting]

Hello mikekilroy

"an idea for future use: since it is hard to measure .04 ohms with a simple ohmmeter, consider measuring inductance instead. You will find L is normally high enough (often double or triple digit mh) to see any variation between windings. This is what I do if I question a large motor for potential winding shorts. It has accurately identified good and bad each time over the years for me."

I did this once and remember the position of rotor affecting my readings, what I did then was carefully move rotor for that position to achieve the highest reading then change leads and find the highest reading there, I utilized the highest readings for comparison purposes. How do you recommend performing this procedure considering rotor position?

Thanks, Chuck

 

[edison123]

The best way to know if there are any winding turn shorts is to do a surge comparison test. A good winding shop will have one such tester. Resistance measurements, even with a micro-ohmmeter, won't cut it unless there is a serious turn short, in which case, the motor would have failed already.

Your 3% current unbalance is most likely due to voltage unbalance and not due to turn-shorts.

Muthu

http://www.edison.co.in

 

[electricpete]

...unless there is a serious turn short, in which case, the motor would have failed already.

That seems to be more a rule with form wound machines but not as much with random wound machines. I've seen many case studies of random wound motors that continued to operate after turn to turn short (perhaps with portion of winding blown open). We also had one case in that category although it caused a ground (which did not trip the motor...ungrounded system).

=====================================
(2B)+(2B)' ?

 

[LionelHutz]

The DVOM was the wrong tool for the job so it's results are meaningless.

In your case, since the running imbalance is <3% I would just let it keep running.

My experience says it's like Muthat posted. The good shops (both new manufactur and re-wind) I have dealt with surge comparison test their motor windings. The re-wind shop I deal with the most tests used motors to the same voltage level as a new motor would be tested to. They have found from experience that testing used or operating motors at a lower level like some standards say doesn't catch bad motor windings.

 

[edison123]

pete

It's not easy to detect turn shorts on-line. Ultimately, they will develop into a ground fault, at which point the protection kicks in. If the motor is not designed to trip on earth fault, it's a protection system lacuna (which is common and justified in small motors) and the motor will trip on over current/fuse blowing later on with possible severe core damage.

Muthu

http://www.edison.co.in

 

[electricpete]

I will just repeat, there are plenty of cases reported various places of random wound motors that experienced turn to turn shorts and did not trip.
I have seen them on maintenanceforums.com, I have seen them on our industry network, and we have one experience which perhaps is unique with our ungrounded system.

=====================================
(2B)+(2B)' ?

 

[mikekilroy]

I was going to second eletricpete's comment, but figured he did not need any seconding with his expertise.

Well, since it was brought up again, I can tell you I have experienced 3-4 cases of end turn shorts that let the motor continue to run in my 40+ yrs of fun in this field!!

I once made a trip to the old huge steel mill in Philly: our replacement motors kept "failing" (aka not failing but making horrendous audible noise) in 1-5 hours. After 15 or so motor failures and 4-5 service techs on site, I was called. I found a loose connection on one motor lead in the 150ft cable from drive to motor - it caused Dv/Dt spikes I measured at 2700V peak - on 230v insulated motor (good to approx. 2000v peak max). The result was ionization of air in air gaps in varnished end turns, eating the insulation, then turn to turn shorts. The result was a significant enough change in motor L that the current loop of the drive became unstable and oscillated at approx. 1500hz; hence the noise, yet the machine still performance was fine. Had this been on a v/hz drive, no one would have been the wiser, and each motor would have probably continued to run for a long time with just slightly different phase currents and maybe 5% more overall motor heat.

FYI, in addition to tightening the silly loose terminal strip screw I made them find under the machine bed, we began to vacuum impregnate the stator windings on this model motor to reduce/eliminate the air gaps.

http://www.KilroyWasHere<dot>com