Aditya...

Is the coupling the same coupling in each instance or has it been replaced somewhere along the line? A 3x can also be indicative of a defective coupling depending on type [not balance but an actual defect of some sort, even as simple as a slight misbore of one hub]. If this is the same coupling it would appear to be a potential constant throughout this [unless of course the 3x shows up with the motor running solo].

Dana

Hi guys,

The rotor problem described by you would be if the rotor bars are broken. However, loose bars show at rotor bar pass frequency & its multiples.

The 3X dominates the spectrum during coastdown. The bearings & housings have been checked physically, clearances are normal.

The 3X continues to appear even without the coupling, so I guess we'll have to look elsewhere.

The new rotor bars have not yet arrived. I should be able to get the motor for testing next week. Lets see what shows up with the new fans & rotor bars.

Aditya

Any more news Aditya?

Did you get to the bottom of it?

A 3xSS frequency can certainly indicate looseness on the rotor. I worked on a large motor that had a loose cooling fan (aluminum with shrink-fit hub).
Keep your eyes, ears and mind open so the solution can present itself!

Hi Every body
Mr Aditya: Finally what was the problem with motor and how it was corrected?
Any one who is aware may answer please as I am also facing a similar problem & yet to get to the root cause.
Thanks

The problem was found & corrected, two things were detected:

1. The internal cooling fans which were supposed to be shrunk fit on the shaft were found to be loose.

2. We actually de-brazed the short-circuiting rings from both ends to inspect the rotor bars. The bars are supposed to be tight in the slots, we could slide them out easily with our hands.

Both problems were rectified & the motor went back to site. A small component remained at 3X, however it did not increase with coupling & the motor is running fine to date.

Till date, I have not been able to corelate some of the data with what we found, but anyway the motor works.

BTW, are you working at Eleme?

Regards,

Aditya

Hi Aditya
Yes I am at Eleme now, but how u know that? Better if you write more detail. Also pl send me your email Id.
Rgds
Amit Kumar

quote:

Originally posted by Aditya:

Till date, I have not been able to corelate some of the data with what we found...

Aditya,


My guess is that loose internal fan was resonant. It is hard to prove it now with data on hand though...

When resonance was first detected and then confirmed with a coastdown test the offending component should have been searched. Motor stator housing, stator core, rotor shaft, fan, etc. - all of them should have been bumped. Instead, just the motor housing was propped up.

Also, it appears that correcting off-centered stator bore did not produce any effect. How bad was it?

David

Amit,

That was just a guess, as I know Sunil Laulkar over there in the Electrical Dept., used to work with him at Reliance Hazira, PP Plant. My mail ID is adityakorde@yahoo.com.

David,

You've made a good point about bump testing all components. I had assumed that the natural frequency would be identical for the entire assembly. Will keep this in mind for the future.

I do remember machining that stator bore but can't recollect by how much. That particular motor manufacturer's plant is shut down now, so no chance of getting any records. Sorry about that.

Regards,

Aditya

have you perofrmed phase analysis to look for cocked bearings?

collect phase data at 4 points around the bearing in the axial direction. if the bearing is cocked, the phase shift will be about 180 from top to bottom or side to side.

Balancing and aligment attempts will not make any change to the phase