6.6 KV Motor overheating after repair and during solo run

[petroelecric]

Dear Friends,
I am new to this forum and finds it very informative and all the members very much helpful in solving the issues.
We have problem with a 6.6 KV Motor and the case history is written below. Can anyone give any clue to this issue of overheating?

Rating : 355kw, 6600V, 35.5A, 50Hz
RPM : 2987.Make Toshiba F&G
Degree of Protection : IP56
DOR : CW viewed from DE side
DE Bearing : 6318C3
NDE Bearing : 6318MC3VL024.1
Insulation Class : F
Driven Equipment : Centrifugal Pump (Service : Lean Solvent)


1ST Failure : During Commissioning observed Rotor fan bolt looseness and touching stator winding which resulted into slight damage to the winding.
Action : Winding damage repaired by wrapping.
Motor has averaged running period of almost 6 months without any abnormality till 2nd failure on 18th Aug 2011.

2nd Failure : On 18th Aug 2011, Motor DE side Bearing seized and smoke was observed.
Action : Motor shaft, Bearing Housing repaired. Bearings replaced.

3rd Failure : On 25th Sep 2011, After repair of 2nd Failure immediately during load trial Motor DE side Bearing seized and smoke was observed.
Action : Motor shaft repaired and Bearings were replaced with the procedure guided by Vendor.

Following the Repair after 3rd Failure, Motor No-Load Trial conducted at site on 12.01.2012. Following were the observations.
Motor winding temperature was found to be increasing abnormally. After almost 4 hours of running, winding temperature reached 130C and was still on increasing trend. Hence we decided to stop the motor.
Motor Bearing Vibration and Temperature were found normal during the run.
Motor No-Load Amps were also found normal.
Motor IR value and Winding R is healthy.
Motor core-loss test was done and found OK.
Motor solo run at reduced voltage was done and observed no temperature rise. Motor was running normal.

Experts, kindly give your feedback.

Regards,
george

 

[MiketheEngineer]

Why is this so hard - CALL the MFG. They owe you a repair or new motor!!!

 

[electricpete]

Most likely sounds like core degradation to me. That can happen if something scrapes the bore of the stator.

When core test was done, did you look for hot-spots using IR camera? Was test sufficient duration and flux density to identify hotspots?

Much less likely, some kind of winding problem. Have electrical tests of winding been done (winding resistance, insulation resistance).



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(2B)+(2B)' ?

 

[DTR2011]

Pete-

"Motor IR value and Winding R is healthy."

I believe that this was done and stated above.

 

[burnt2x]

I have had a similar incident like this one a long time ago. It took us a long time to decide what to do since everytime we test the motor (after it was rewound), it always gets hot. Ours was a problem on a bolt loosened inside the motor cavity and end-part of the core bore got some areas chamferred by ensuing arc as well as the windings nipped at the root of the end-winding.

We were not in a tight situation then as there were actually three identical units serving the same duty. I decided to have motor re-worked, advising the motor rewinder to include among other things a re-stacking of the motor iron. Made sure all core cooling ducts cleared.

BTW, the motor I was talking about was a 3.45 kV, 355kW, 60Hz, 1800rpm. 4-poles.

 

[petronila]

Hello

If the stator core test was done accurately then could be a design problem, check the cooling system could be inneficient

is the motor new or surplus? or was rewinded?

Then the problem could come due to the workshop use a wire with less mm2 and the overheating come from watt looses.

Regards

Carlos

 

[iop995]

I supose that stator winding became in some short-circuit at nominal voltage and after some temperature increasing (after under load working). Recheck winding repair.

 

[flandrax]

Could there be a possibility that when the motor was repaired the third time that maybe the air baffels (if there are any) were left out? Or as stated above the cooling system ineficient.

 

[petroelecric]

Thanks everyone for your support. The core Test was not done properly in the workshop. We did another test and found the Hotspot. The core was damaged.

Once again Thanks to all,

Kind Regards,
George

 

[DTR2011]

Thanks for the follow up.