We have a 2-pole 800 hp horizontal sleeve bearing motor. What can account for the fact that after repair and initial run of about half hour uncoupled vibration levels were circa 0.1 ips ,however, after running uncoupled for about 2 hours the readings all went down to about 0.03 ips.Remember it is being run uncoupled.

I would guess that you had some rotor distortion that took about 2 hours to straighten out. If you shut it down after the vibration drops to .03ips and start it right back up, does the high vibration return? What happens if you let the motor get cold again and start it up?
Is the motor any hotter after 2 hours than after half an hour?
What repairs were made to the motor?

Do you have any spectral data to show the forum with this question? Was it mounted on a test stand or on the permanent base?

Thanks Guys for the quick response,
Clav: rotor distortion? would you give some examples.
We did not start up the motor after shut off.. sorry
Yes the temperature went up to 100 deg F on the OB and 132 deg F on the IB bearing fro circa 80 deg F in the first half hour.
Motor bearings were changed, some rotor and stator checks were made.
This motor was test run in the shop with 440 V, but on the plant it is run on 2300 V, will this have any significant impact on the vibration readings?

Don
The motor was on its permanent base.

I was thinking the rotor may have gotten a little sag while it was sitting in the shop. That would have taken longer than a half hour to straighten out. If it were a lot of sag it might not straighten out.
There is also the possiblity that a bearing was missaligned though I would have expected the temperature to have been greater than what you found.
Like Don, I'd need to see the vibration data before I tried to offer more than general ideas.

Sorry about the format guys, but here are some signatures

MOTOR_UNCOUPLED.doc (143 Kb, 61 downloads)

Could not open document. Maybe because of the format?

I couldn't open the file either.

Sorry guys , I am resending hope it opens.

MOTOR_UNCOUPLED.doc (2 Kb, 32 downloads)

Sorry but it would not open. Comment was cannot open this web page. Before it said basically it had a path or file issue.

I can't read the files either. What application was used to generate the file? MS Word?

I have usually seen thermally-related problems increase over time after start, even in the case of unloaded motor.

But, in theory, I believe a thermal bow can depend not only by the temperature, but by uneven temperature distribution. And the gradient in temperature distribution can be highest just after start. During the first second of the start a tremendous amount of heat is put in... then it takes awhile for the system to come to thermal equilibrium. I would have thought 30 minutes would be enough, but who knows. An analogous scenario that comes to mind is a typical ceramic dinner plate taken out of the freezer and put directly into the microwave can shatter (I speak from personal experience). The reason is that the change in temperature is so rapid that the temperatures don't have a chance to equilibriate... and the thermal gradients within the plate create uneven thermal expanasion and resulting stresses.

Another factor that may be at work is bar movement within the slot. The moment of start again is likely to create thermal stresses resulting in a bow. If this process moves the bar axially to a new position within the slot where it get locked in place, then the bar may act to keep the rotor bow even after the thermal stresses disappear. But over time when the thermal stresses are gone, the rotor wants to return to it's normal position and the bars are preventing it. The bars may slip in the slots over time back to their original position, allowing the rotor to unbow.

Trying to think what else might change over time:
* Voltage? I have seen voltage affect 1x on a motor before. Got change in the shop while varying voltage. It was a slot speed motor... doesn't seem as likely for normal speed motors.
* Oil temperature? that seems like a longshot for 1x, might affect subsync vibration.

This message has been edited. Last edited by: electricpete,

Vibehigh,
Is the change subsynchronous, 1x, 2X line frequency, harmonics etc?

I am sorry everyone for the word doc not opening,I have a tremendously slow internet connection, maybe that's the problem.
We took a 6400LOR plot and yes we are seeing the dominant peak to be at 120 HZ together with 1x,3x, and 4x.

I am sorry everyone for the for the word document not opening, I have a tremendously slow internet connection, I am not sure if that's the problem, as the document opens fine on my PC.
Yes we are certainly seeing 2x line frequency, we used 6400 LOR and there is one distinct peak at 120 Hz, together with 1x 3x 4x etc, but the predominant peak is at 2x,
I will try once more to send the file, if it doesn't work let me apologize in advance.

MOTOR_UNCOUPLED.doc (159 Kb, 21 downloads)

I am sorry everyone for the for the word document not opening, I have a tremendously slow internet connection, I am not sure if that's the problem, as the document opens fine on my PC.
Yes we are certainly seeing 2x line frequency, we used 6400 LOR and there is one distinct peak at 120 Hz, together with 1x 3x 4x etc, but the predominant peak is at 2x,
I will try once more to send the file, if it doesn't work let me apologize in advance.

MOTOR_UNCOUPLED.doc (168 Kb, 27 downloads)