Overheated sign on stator coil 3

[Leendabests]

Hi,

I would like to get an advise and opinion regarding to the overheated coil in the induction motor. Recently I have dissemble 1 unit 315 KW Induction motor . We found out that the rotor has marked such as the color of metal at the rotor turned to rainbow color. It is sign of the motor is heating while running. The insulation resistance test has been made and the reading is 2-3 Gohm for all phases. I want to know from the mark at the rotor, is there any posibility the copper also heated and can cause to the failure? Attached is the photos for your reference.

http://files.engineering.com/getfil...84a4c5e748df&file=20160115_091028_resized.jpg

http://files.engineering.com/getfil...94385f403348&file=20160115_091053_resized.jpg

http://files.engineering.com/getfil...6c8b8a406042&file=20160115_091148_resized.jpg

Thank you

 

[jraef]

The apparent concentration at one end makes me think of a bad end ring joint in the rotor cage.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ozsalty]

Looks like it has been "poling".Could even be something like a broken shaft inside the rotor core,or loose fitting of rotor to shaft..

 

[electricpete]

"overheated coil in the induction motor….is there any posibility the copper also heated and can cause to the failure"

You didn't exactly say it but I'm assuming there was a stator failure resulting in motor trip? Are there any more details about the failure scenario and visual observation of stator failure location. How about bridge test? Surge test? Stator core test (thermography and watts loss)?

Some possible links between apparent overheating of rotor and damaged stator coil:

1 - Rotor rubbed stator bore. This caused friction heating of both. Also smeared the stator laminations creating eddy heating of the stator core. Localized heat contributed to stator coil failure.

2 - Damaged stator coil continued to operate for some period but creating severe unbalance in stator field, creating reverse rotating field which is 120hz, cause deep bar heating of rotor (don't know exactly how big this motor is).

3 - Attempts to restart motor with damaged stator might also have similar effect.

4 - Rotor bows in response to heating from stator fault (and resulting heating from effect 2) and causes rotor bow. But in that case I'd normally expect rub at one arc of circumference of rotor and all around stator...doesn't seem to be what you've got.

5 - Stator failure unrelated to visual observations shown. Example maybe there was a rotor/stator that occurred years ago and you only notice it now that you tear everything down after the stator failure. It doesn't seem particularly like that they are unrelated, but I don't think you'd be able to rule it out with complete 100% certainty unless / until you can examine the area of the stator failure closely.

Since you included photos the ID of stator bore I gather maybe you're focusing on scenario 1. I tend to agree for what little it's worth from a distance, but you have much better info available to you there. End ring distress may be rub marks. On stator there appear to be rub marks primarily at the near end at bottom of photo. If worst stator bore marks are on the same end as worst rotor marks, it might tend to explain why the rotor heating is concentrated at one end and corroborate the scenario 1. In that case there is some more thinking to do about how it happened (bearing holding rotor off-center within stator at one end seems most likely but there may be other possibilities)


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(2B)+(2B)' ?

 

[electricpete]

deleted

 

[Leendabests]

The motor is failed due to damage bearing. Thus, it has melted the rotor around bearing and broke it down. Then we check the insulation resistance and we got 2-3 GigaOhm for all 3 phases. Hipot test has been done using Baker Winding tester. Result as attached. We found out there is overheated mark at the rotor as photo 1. We afraid that the coil also will be slightly overheated even though the test result shows that the coil is in "good condition". Please advice.

http://files.engineering.com/getfil...157-d0b13176402c&file=IMG-20160116-WA0001.jpg

 

[edison123]

Bearing failures result invariably in stator-rotor rubbing and in most cases, the winding will fail either due to direct mechanical damage to the coil(s) or due to core hot spots. The first thing you do on such rubbed core scenarios is to do a core flux test or ELCID test to check if there are core hot spots. If hot spots are found, it is imperative they be corrected before the motor is put back in service. In many cases, this may involve removing the stator winding (essentially a rewind) & repairing the hot spots and in a few cases, even dismantling of stator core. In some cases, it is possible to repair the core without damaging the stator winding. After such repairs, the above test must be repeated and presence of hot spots must be ruled out.

Surge comparison testers normally do only DC hipot. An AC hipot at 1.2 to 1.5 times the rated voltage most likely would have resulted in the winding failure showing the winding is not healthy. Also, what you have done is surge comparison test (which mostly tests for only turn shorts) and not a real hipot test, which tests the ground insulation.

This is why bearing temperature monitoring and protection is a must for all critical motors since the collateral damages could be much more severe and much more expensive to fix than just fitting such a bearing protection system.

Muthu

http://www.edison.co.in

 

[dArsonval]

Giving this one, the classic bearing fit tolerance, [OUT of Tolerance] Stamp.

The top sticks clearly show rub/smear marks. The rotor has the typical over heating signature appearance found in a rotor rub/drag scenario.
The rotor (it-self) is likely Okay.
An excessive amount of gray metal dust would indicate a shaft/rotor fit issue. That’s not visible in the photos, so it is likely ruled out, as a cause… for rotor rubbing.
The rotor shaft to the right... in one of the photos appears larger in diameter, and matching the dirtier end of the motor which would correspond with the stator end showing marked rubbing.
I’ll guess, that’s the drive end.

If the bearing JOURNAL fit has failed, again there would be a more metal dust appearance.
So, it looks like the [end bracket bearing fit] at the drive end of the motor is likely worn out.
(It could just be a bearing issue due to loss of lubricant, yet kind of doubt it.)
It would be helpful to know if the motor is direct-coupled, or otherwise.
It also appears to be a form-wound motor indicating Medium Voltage operation.
The stator should be completely cleaned up, dried out, and then electrically re-tested.
Electrically testing a dirty stator does not provide productive information.

Nice Rotor Stands!

John

 

[electricpete]

...Then we check the insulation resistance and we got 2-3 GigaOhm for all 3 phases. Hipot test has been done using Baker Winding tester. Result as attached. We found out there is overheated mark at the rotor as photo 1. We afraid that the coil also will be slightly overheated even though the test result shows that the coil is in "good condition". Please advice.

As edison123 points out, core loss test is next logical step. If core is in bad enough condition that it requires restack, your winding will have to be replaced even if it's in pristine condition.

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(2B)+(2B)' ?