Motor stator failure

[migueldiazch]

Hello,
In May 2008 trying to diagnose a motor for noise, we did a vibration analysis an saw in the spectrums some signs of internal rub. We decided to remove the stator and found the squirrel cage broken (se attachment "May2008 failure") the motor was a GE 591 HP, Model: 5K82094062501, Poles: 2, Voltage: 4000V, Frequency: 60 Hz, Phase: 3, Frame 8209S, Enclosure: TEAAC. We started the procurement of the same motor and GE told us that they declined to manufacture this motor and gave us a GE 591 HP, Model: 5K82104062501, Poles: 2, Voltage: 4000V, Frequency: 60 Hz, Phase: 3, Frame 8210S, Enclosure: TEAAC as a sustitude, and after one year of operation we found some craks on the rotor (se attachment "Sep2010 failure"). Any advise?. Posible failure cause?. Repair technique if it possible?.
Regards,
M. D.

 

[migueldiazch]

Sorry, here is the other file..

M. D.

 

[electricpete]

Starting imposes a wide variety of stresses as the copper expands more than the steel (higher temperature and higher coefficient of expansion). My guess would be severe starting duty. For example frequent dol starting, high inertia load, possibly low voltage. Also high speed is a severity factor and you have 2-pole motor. Maybe you can enlighten us on the other starting duty facotrs

My suggestion is to repost in the motor forum (or at least post a link there to this thread):
forum237



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(2B)+(2B)' ?

 

[dicer]

What is the material that broke? What is the speed of rotation? That looks like overspeed damage. What is the application? It looks like centrifugal force pulled at that end ring and distorted the bars. If you feel the failure occured first, then either some bad material or machining or assembly, caused stress risers.

 

[electricpete]

What is the material that broke?

That would be copper.

What is the speed of rotation?

The machine is 2-pole powered by 60hz. That would make the speed 3600 rpm, minus a small amount of slip.

Another factor related to thermal expansion is called "thermal ratcheting". Each time you start, the copper bars expand axially within the slot in the iron. If bars are captured at the center and expand equally/oppositely in each direction, then return to their original position upon cool-down, then life is good. But the slot consists of the ends of laminations which you can imagine is not a smooth surface, and so bars sometimes do not slide easily within the slot and the points where they get stuck can be unpredictable. And if the laminations are not flipped after die cutting, then they can have edges which tend to allow movement only in one direction but not in the other . The net result after many cycles of expansion/contraction can be that a bar moves further and further in one direction after each start. You can imagine that if this happens to only one bar, you create some streses on the end rings.

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(2B)+(2B)' ?

 

[electricpete]

Sorry, I should have let the OP state the bar material. I guess there is small possibility that this is fabricated aluminum bar rotor, but I don't think so.

Is this a Siemens motor by any chance?

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(2B)+(2B)' ?

 

[powerjunx]

pete, had a good guess relates to thermal expansion of the material.
Aside from severe starting duty cycle, one factor that may contribute would be the intermittent short cycle " rotor stalling " where the protective relay unable to pick the fault amplitude duration as if the relay sees through the amplitude as the starting current. kindly check continuously the load current upon operation, what is the connected load of your motor, by the way?


"..the more, the merrier" Genghis Khan

 

[unclesyd]

As a matter of curiosity what type of environment are these motors working in?

The reason I asked is that we had two rotor failures of 1500 hp blower motor due to their working environment. The failure were like your second failure.