# AC motor testing.



## MDShunk (Jan 7, 2007)

Other than megging each winding to the motor frame and ohming out each winding to see that it's the same as it's companion winding, what other electrical tests are to be done to pass or fail a suspect motor? I know that a motor can "pass" my two proposed tests and still be bad.


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## 10492 (Jan 4, 2010)

Spin the motor and measure voltage between windings?


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## Big John (May 23, 2010)

Our go-to test is a surge comparison test. It will show interturn ungrounded shorts that a low-voltage resistance test or a megger can't find. It works like a high voltage TDR and thumps each winding with a series of pulses and looks at the reflections. With no rotor, or the rotor in the proper position on each test the error-ratio should be low enough that each waveform overlaps. 

The shape of the wave generated also depends on the inductance in the winding, so you can also find failing core laminations and shorted rotor bars.

While Baker Instruments makes the machine we use, there is supposedly a way to do it with a simple HV capacitor and a scope. It might be something to play with, I've never attempted it or seen it done.

EDIT: If you're interested, check out page 658 of The Testing Bible. It gives a rundown of a surge test and a basic example of the test circuit as well as just being a good read on motor-testing.


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## MWayne (Nov 8, 2010)

We had a motor short between loops in the same winding and the surge test was how the testing company verified the problem.


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## Lone Crapshooter (Nov 8, 2008)

With almost 40 years of experience I can safely say that most 3 phase motor problems can be found with a DMM and a Megger. The DMM for winding resistance and winding balance and the Megger for insulation resistance to ground. At work we have a Ductor ( High current low resistance ohmmeter that puts out 10 amps DC). I use that when checking a high voltage high horse power motors.
If I am not satisfied with that I will hook the motor in question to a Variac and apply voltage phase to phase (monitoring both voltage and current) in steps then I will calculate the 60 cycle impedance and look at the balance of the phases.

LC


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## RICHGONZO1 (Mar 5, 2012)

PDMA, I do it on all motors 100 HP and above and test them yearly. The cost is significant, but if you're running high HP motors, it's nice to trend. BTW a PDMA runs roughly $3k a month to rent or 60k plus to own. We rent here and we have up to 3000 HP motors, I've tried to talk them into buying, but they won't pay for preventative maintenance, go figure. To me, it has a nice trend from new motor along its life and will keep motor rewind shops honest, it should pay for itself, try explaining that to the big wigs.


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## RICHGONZO1 (Mar 5, 2012)

BTW, I worked at places that own the PDMA and it makes to be a valuable troubleshooting tool when calling high dollar motors bad when actually you have other intermittent issues. I'm sure it has stumped many a good electrician in calling a motor bad when actually it was an intermittent issue other than the motor. We have all been there with bosses breathing down your neck and they have a motor on standby just in case and you look a fool with when the breaker trips again with a new motor in its place!


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