Field Method for Verifying Stator Short Circuits

[Laplacian]

What is a good quick and dirty field method of determining if the stator of a 3-phase induction motor has shorted phase-phase (preferably at the motor "T" leads)? The motor trips on instantaneous overcurrent during start, but DC resistance tests to ground show very good results. The feeder cables test good also. I do not want to close back in on a suspected fault to capture current magnitude during the overcurrent protection operation.

 

[Skogsgurra]

A simple way of testing the stator winding is to check its inductance with a low-frequency meter. That is the standard field measurement. If all phases have roughly (say within 5 percent) inductance, then the windings are probably OK.

No inductance bridge or meter available? Then check the inductive voltage drop across each winding. Connect a suitable wattage lamp in series with the winding and feed the combination with AC. Measure the ac voltage drop across the winding. Repeat for all windings. The voltage drop shall be the same (+/-5% again) if the windings are healthy.

Please note that the motor windings shall be isolated from ground (they usually are) and that you are using lethal voltages. This test shall not be performed by non-electric people and the motor shall not be left without supervision during the test.

 

[aolalde]

Rent a Three Phase Surge Comparison Tester.
A surge ringing pulse is applied to each phase and the trace of each phase circuit is compared. Turn to turn, phase to phase or wrong connections are detected. The test voltage level must be high enough to bridge weak or defective insulation.

 

[edison123]

As far as I know, surge comparison testing is the best way to detect turn to turn shorts and phase to phase shorts (without isolating phases).

 

[Skogsgurra]

I agree with both aolalde and edison. But I wouldn't call a Baker "quick and dirty". But if you drop that prerequisite, a Baker or similar is of course the best way to go.

 

[electricpete]

Another "quick and dirty" check would be resistive balance check among phases using DLRO.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[peebee]

DLRO?

 

[busbar]

Digital low-resistance ohmmeter... up to 10 amperes of test current {and still reasonably portable} with Kelvin test leads.

 

[TomcSR]

Has anyone ever heard of testing a stator by removing the rotor,applying a low votage to the coils and inserting a bearing in the field a watch it spin?

 

[SteveKW]

Look at these...

http://www.eng-tips.com/viewthread.cfm?qid=17776

http://www.eng-tips.com/viewthread.cfm?qid=97804

 

[mc5w]

Another classic way to test is to take the motor off line and then connect a capacitor across 2 leads. Then connect a storage oscilloscope or high speed strip chart recorder across the capacitor and apply a DC pulse from a battery about 5% to 10% or motor rated AC voltage. If you get an oscillation waveform that indicates low damping your inductance or motor is OK. If the oscillation in response to the pulse indicates heavy damping you have found your short. Essentially you are checking the resonance and Q of the capacitor and inductance combination.

Repeat the test twice using the other possible phase to phase connections treating the motor as 3 single phase inductances.

Also works for transformers and just about ANY inductive device that does NOT have shading poles such as an AC only relay coil or a shaded pole motor but those are throw away items anyways.

Mike Cole, mc5w@earthlink.net