Motor winding resistance
Motor winding resistance
(OP)
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





RE: Motor winding resistance
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"
RE: Motor winding resistance
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.
RE: Motor winding resistance
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.
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(2B)+(2B)' ?
RE: Motor winding resistance
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
RE: Motor winding resistance
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.
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(2B)+(2B)' ?
RE: Motor winding resistance
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
RE: Motor winding resistance
www.KilroyWasHere<dot>com
RE: Motor winding resistance
"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
RE: Motor winding resistance
Your 3% current unbalance is most likely due to voltage unbalance and not due to turn-shorts.
Muthu
www.edison.co.in
RE: Motor winding resistance
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(2B)+(2B)' ?
RE: Motor winding resistance
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.
RE: Motor winding resistance
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
www.edison.co.in
RE: Motor winding resistance
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.
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(2B)+(2B)' ?
RE: Motor winding resistance
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.
www.KilroyWasHere<dot>com