Confirmatory test for Defective rotor of SC Induction Motor 1

[Binnyjoseph]

Hi all,

We suspect that the inner cage of a 940 kW, 3.3 kV double cage induction motor is defective (broken bars and/ or end rings). A defective rotor is indicated with a motor checker. But with a growler, the outer cage appears to be healthy, and there is no access to the inner cage. So, no confirmation of the defect is possible. We have heard that there is an instrument available, by which the stator current spectrum can be analyzed for detection of rotor fault. Does anyone have specific experience with that instrument? How exactly it is used and the degree of accuracy? If someone can explain, it would be useful to practicing Engineers like us. Please help.

 

[gsimson]

U can contact either Predict or CSI for their product. This instrument is useful to detect rotor bar breakage when the motor is running.
Simple test is to do the hammer test - Keep a multimeter in resistance range across the suspected broken rotor bar. Hammer the bar with a little force. If there is any breakage the multimeter reading will fluctuate.

Generally the performance of the induction motor is affected only when the no. of broken rotor bars is more than 2. If not detected in time the failure rate of broken bars will accelerate.

 

[edison123]

How did the motor checker identify inner bar failure ? Normally, in double cage rotors, upper cage being of higher resistance (eg. brass) and seeing most of the starting in-rush current, fail more often (due to I^2R loss) than the lower bars which are of lower resistance (eg. copper) and the much lower running current. If lower bars are broken, one would expect hunting stator currents.

 

[electricpete]

We use current signature analysis.

CSI has a good article but their website is down.

We use our vib data collector (Entek) with a current probe. We look at the spectrum a few hz around 60hz. We compare the magnitude difference between the main peak at 60hz and pole pass sidebands around 60hz.

A difference of 52dB or more generally indicates good motor.
A difference 45dB or less generally indicates rotor problem.
In between is a grey area.

Load should be at least 75% of full load.
Some people suggest that limits should adjusted based on load, number of rotor bars and other motor parameters.

Also as you may know a single-phase test is a good test for rotor bar problems. Since slip is very high during the test it should be sensitive to the smaller cage used for starting. I'm not sure exactly which cage is inner and outer for double-cage rotor (Someone please remind me).

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[edison123]

pete,

The cage nearest to the air gap (called as upper or outer cage) is the high resistance winding mainly used for starting. The cage winding nearer to the shaft centre (called as inner or lower) cage is the low resistance winding mainly used for running.

 

[electricpete]

Thanks kumar/edison.

So that would seem to imply that inner cage problem is:
- more difficult to detect problem by single phase test
- more difficult to detect by visual inspection and tapping of bars for looseness (no access).
- easier to detect during running current signature test.
There is also Rotor influence test by pdma. I don't have any experience with it and don't know whether it would work in this situation.

It sounds like maybe loaded run is about the only way to confirm.
Check current as discussed.
Monitor vibration - look for pole pass frequency sidebands around running speed and slow-oscillating growling sound.
Possibly look for slightly higher slip than expected.

As you may also be aware some rotor problems are temperature sensitive and don't show up unless the rotor is at temperature and/or under load.

As kumar mentioned it would be interesting to know why you suspect a problem.

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[jbartos]

Suggestion: There are some threads in this Forum dealing with this topic, e.g.
Detecting Broken Rotor Bars using Vibration Analysis