Stator core repair problems

[dule00]

Hello!

We have to overhaul an 250 kW, 6 kV induction motor which has suffered rotor/stator rub. Motor didnt have any working protection device so the occurrences in motor (when it failed) were pretty dramatical.

Rotor/stator rub appeared from bearing failure. On about 1/3 of stator and on the rotor there is some smearing visible. Affected stator teeth are about 0,3 mm shorter. The bigger problem is that rotor rods are made of brass and that they have melted and leaked onto the stator core (brass melting point at 900 deg C). Leaking wasnt large.

We have made coil loop test of stator core and found that 1/3 of stator core was heating fast and very bad (cca 80 deg C) while the rest of the core didnt change its temperature (cca 23 deg C). The heating was evident both on teeth as in the yoke.

We are planning to take of smeared material from rotor in lathe. But how to find out about the condition of rotor, especially interlamination insulation? Should I even be worried about it? Losses in operation will be slightly higher, I know, but the motor should work normally I think.

The real question is how to fix stator core? I think that we will all agree that present condition of stator core is unsatisfactory. I dont know if the problem is only in the interlamination insulation. Could it be that the material has reached its Curie temperature? I dont know if it gets back its ferromagnetic properties when it cools down, or its properties stay paramagnetic?

What are your opinions and how would you personally try to fix the stator core?

Thank you! I really appreciate all the help!

Dule00

 

[electricpete]

Some thoughts fwiw:

Lately, whenver a restack is required on our large motors, our shops says it is more cost-effective to replace all laminations with new rather than try to re-coat the existing ones. If the cost of both options is not much different and there is concern about those laminations, then why take a chance with trying to recoat existing laminations… replace them. (I guess schedule would be the only thing that might drive you the other way).

The rotor… you are going to rebar it I assume to replace some bars. The concern about laminatinos on the rotor is that even thought they don’t contribute to losses significantly, they might possibly create some kind of thermal bow. As a customer I would feel better if at least you did a balance and TIR while cold, then heat up the rotor in an oven to 200F or 250F, then repeat to make sure it doesn’t change in either magnitude or phase. That’s not a guarantee (because there is no differential heating of bars hotter than core), but I’ve heard that it does reveal a lot of thermal bows I’ve heard.

Perhaps an even better option to check for thermal bow would be load test before it leaves the shop. Even that is still not a guarantee if you don’t do a DOL start (assuming plant does a DOL start).


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(2B)+(2B)' ?

 

[electricpete]

Also maybe there are some tricks that can be done to improve rotor core performance after a rub. Spreading,cracking, etc

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(2B)+(2B)' ?