Overheated sign on stator coil
Overheated sign on stator coil
(OP)
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/getfile.aspx?folder=3...
http://files.engineering.com/getfile.aspx?folder=1...
http://files.engineering.com/getfile.aspx?folder=a...
Thank you
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/getfile.aspx?folder=3...
http://files.engineering.com/getfile.aspx?folder=1...
http://files.engineering.com/getfile.aspx?folder=a...
Thank you





RE: Overheated sign on stator coil
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Overheated sign on stator coil
RE: Overheated sign on stator coil
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)
=====================================
(2B)+(2B)' ?
RE: Overheated sign on stator coil
RE: Overheated sign on stator coil
http://files.engineering.com/getfile.aspx?folder=7...
RE: Overheated sign on stator coil
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
www.edison.co.in
RE: Overheated sign on stator coil
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
RE: Overheated sign on stator coil
=====================================
(2B)+(2B)' ?