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electricpete

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Reply with quote  #16 

The first photo shows some lengthwise blemishes.  Those look like they originated as a rub between rotor and stator (rather than to overheating) since the seams between the laminations look blended.  Based on the rub alone, it was a good call to pull the rotor.

Beyond that, I'm not sure. Everything from here on is sort of rambling.   I don't fully understand the construction of this rotor.  It is semi-enclosed slot (so bars are inserted from the end). I'm not sure why a jagged piece of the bar appears to be coming up through the slot opening everywhere but since it's doing that everywhere I don't think it's a problem.  Then there is the stairstep pattern as if the laminations are shifted from each other - could be putting stress on the bars.  I can't really see what's going on at the endjoint other than some discoloration which might indicate arcing or overheating.   There may be some good evidence here to a trained eye but (not being familiar with this rotor construction and what I'm looking at), the only thing I can say for sure is there was a rub (the other stuff is strange but I'm not sure what it means).

So, based on photos alone I personally couldn't  say whether there is an electrical defect in the rotor. BUT considering the important fact that your slip went so high, I'd say there definitely was an electrical defect affecting the torque speed characteristic as discussed (I would rule out the scenario that a continuous rub caused increased mechanical loading which increased slip... ruled out because in that scenario the motor should trip on overload and there should be a helluva lot more heating evident at location of the rub).  There is some testing you can do on rotor to localize an electrical defect on the rotor.  Apply current at each end with clamps and inspect using thermography or green paper.  Or since it looks like you have some thru-holes, you can do a core loop test and inspect with thermography.   Or you can do growler test or attempt bar to bar DLRO resistance measurements and more sophisticated electrical tests.

Relationship between the known rub and everything else? Hmmm.  If rub was an initiator, then it would  smear the laminations which creates the potential for uneven rotor iron heating which causes a bow which causes dynamic eccentricity which can cause pole pass sidebands even in absence of rotor electrical defect.  But again because of the high slip we already know  there is a rotor electrical defect. So the more likely scenario would be that electrical defect caused uneven copper I^2*R current heating which led to a bow which cuased the rub. 

As far as explanation from the vib pattern, you might be right. On the other hand, there are a lot of complicated things that go on with spatial flux harmonics that create the slot related vibration patterns and I personally have given up on trying to understand them a long time ago. I look more for the pole pass sidebands around 1x in vib and then try to confirm them with pole pass sidebands in current.  Have seen that on three different large motors. None of those was bad enough to have noticeable increase in slip.  

That's just my take and unfortunately not very much decisive to offer. Maybe others can give better insight what's really going on with the rotor or the vib pattern. It's an interesting case study though.

Walt Strong

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Posts: 889
Reply with quote  #17 
"spatial flux harmonics that create the slot related vibration patterns"
Definitely needs a Flux Capacitor installed to control the flux harmonics![wink]
electricpete

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Posts: 647
Reply with quote  #18 
Hah. Don't say I didn't warn you:
Quote:
Everything from here on is sort of rambling.
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