Delta/Wye Motor 1

[bgriii96]

I was dealing with a Delta/WYE wired motor for start and run. The issue was that the terminal blocks feeding main 480 volts to the equipment shorted between A and B phase which is also feeding the motor starters for the motor. After repairing the terminal blocks, I OHMed the motor. It is a 6 lead 480 volt 33.7 amps (high voltage) rated 30 HP motor. I ohm across all leads and all showed open. All leads did not Meg to ground or between phases. After several hours the motor would ohm between phases. The ohm readings where .99 to 1 ohm. Now according to ohms law the motor should have a higher ohm rating. This motor was later wired back up and it runs, and all voltage and amp draws are correct. I am unsure what I am missing, to me that motor showed bad at the time of the issue. Any thought on what I have missed would be appreciated.

 

[electricpete]

Now according to ohms law the motor should have a higher ohm rating.

I think perhaps you were thinking you could calculate the motor winding resistance from voltage rating and current rating? That would be incorrect. The input voltage is not dropped accross the the stator resistance... there is induced emf. Perhaps start with motor equivalent ciruict. Sorry if I misundertsood your meaning.

I ohm across all leads and all showed open. ... After several hours the motor would ohm between phases. ...This motor was later wired back up and it runs, and all voltage and amp draws are correct

The first reading sounds bad. Either you read wrong (maybe didn't make good contact) or something is changed within the motor.

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(2B)+(2B)' ?

 

[waross]

You can't use the simple form of Ohm's law for motors. The current in a motor is limited by a combination of resistance, inductive reactance and back EMF.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

You won't get an ohm reading unless you ohm between the start and end of a coil.

 

[bgriii96]

Thank you all that responded to my post. From the responces and research I have come to the conclusion that it does not matter what a motor ohms as long as all windings ohm the same. It will help to use a more suitable meter such as a Fluke 87 and not a T5 that will not show below 1 ohm.

 

[jraef]

Actually you can take it a bit further. Using any kind of ohmmeter on a motor winding is really only useful for one thing; telling you if the motor winding is completely open or not, which you can usually tell looking anyway. Beyond that, all it is telling you is that the winding insulation can withstand the rigors of all the horrendous abuse that a 9VDC battery can inflict upon it. It's like saying that if this elephant cannot feel a fly landing on it's back, it won't have a problem carrying another elephant.

"Will work for (the memory of) salami"

 

[electricpete]

Just to recap, there are two kinds of resistance measurement we can make:

Winding resistance - follows the same path as normal current from terninal to terminal. I agree with you bgriii96 that it's important to check for balance among the three sets of readings. Short among turns may show up as imbalance among phases. Sometimes something like one parallel in a phase winding will be open, giving imbalance reading among phases. Accuracy of measurement method is important for sensitivity. We use a bridge for this measurement. For the most part, motor with faults detectable by difference in winding resistance typically would not run, so some may question the value of this test.

Insulation resistance - test between winding and ground. Test voltage is important for sensitivity. As jraef mentions, 500vdc megger is better than multimeter.

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(2B)+(2B)' ?

 

[mikekilroy]

there is another reading you can take that will tell more. Inductance. If your meter has it, once you verify no open winding, you can measure L between legs: rather than trying to measure .2 ohms, you will often has 10-20-30mh - easy to see. I have found partially shorted windings this way when the ohmeter of course just shows ok.

 

[electricpete]

Inductance is proportional to turns^2 while resistance is proportional to turns. So a 1% change in series turns (due to shorted turns) might cause a 2% change in inductance (more sensitive) and a 1% change in resistance.

Magnetic properties are highly non-linear (and not just in the saturation range as many thing), so value depends on voltage, but this does not affect phase comparison approaches at a single voltage.

There is one factor to be aware of. When you deenergize an induction motor stator, the rotor current is exponentially decaying dc in the rotor reference frame. That leaves a residual magnetism in the rotor. We can see the effects of rotor residual magnetism in a number of ways:

1 – rotate the rotor by hand with the motor deenergized and you will see a few hundred millivolts on an oscilloscope. Handy for rotation test of large motors where you can’t accelerate them from rest fast enough to get a distinct first deflection direction with standard rotation meters.

2 – surge comparison test of stator sometimes shows differences among phases when tested with rotor in. These differences go away with rotor removed.

3 – PDMA uses “rotor influence check” to look for rotor bar problems and can get false alarms due to motor residual magnetism – a whole article here

http://www.pdma.com/pdfs/Articles/Influence_of_Residual_Flux_on_the_Measurement_of_Inductance.pdf

It will also affect inductance test of stator conducted for purposes of finding winding shorts as you suggest. Could perhaps be discrimiated by rotating rotor over a series of tests similar to pdma (or else testing with rotor out). Inductance testing of stator is not a standard test described in IEEE standards (unless you want to include 1410… actually under debate by the working committee at the moment whether should be include). It probably can have some use but you would need to be aware of effects of residual magnetism and consider that in your test method and/or interpretation.


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(2B)+(2B)' ?

 

[electricpete]

I just noticed that the article I linked above suggests that rotor residual magnetism is associated with a faulty rotor, not a healthy one. That is pure sales mumbo jumbo imo. They are trying to take a known weakness in their method (sensitivity to rotor residual magnetism) and pretend that it is useful. Sad.

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(2B)+(2B)' ?

 

[petronila]

Hello Bq

My conclusion is maybe you don´t remember exactly how you was test the initial and after several hours test and in the initial test you have tested 1 and 2, 2 and 3, 4 and 5, 5 and 6 leads (Please see attached file) and several hours later you could tested 1-4, 2-5, 3-6 having a ohm value of each phase winding like Lionel Sayd.

Regards

Carlos