Hi
Actually, I don't have any experience about this subject. But I like to tell you my opinion. According to the symptom,
I don't expect an electrical problem. I think the lubrication system and bearings must be checked first.
Regards.

Azmi Demirel
azzmi@elk.itu.edu.tr
Welcome
I am handsome but married :-)

you do not give much background but some things to consider:

1.cold and/or hot balance of the driven equipment is bad but the balance correction has been done on the motor such that it is "unbalanced" when not coupled. (if this is the case then why worry about uncoupled vibration.)
2.make sure there is no soft foot conditions anywhere along the train.
3. make sure the motor is dowelled to the foundation! (motor hold down bolts are NOT designed to over come sliding forces on the feet; that's what dowells are for)
(note problem areas 1 & 2 can cause changes in vibration levels as a function of changes at the installation such as cold and hot ambients).
4.oil viscosity in sleeve bearing type motors can greatly influence COLD vibration, coupled or uncoupled. check with your friendly oil supplier on the temperature range of the oil you are using. many outdoor motors are supplied with oil sump heaters for this reason.

Hi,
Motorman, you are right all about an unbalanced system. Maybe I'm wrong but if there is a balance problem, I think the vibration must be repeatable.
Regards,

Azmi Demirel
azzmi@elk.itu.edu.tr
Welcome
I am handsome but married :-)

It should be repeatable unless thermal forces are changing things. The above is not much to go on; we do not even know what the dominating frequency of the vibration is. I suspect the problem is due to oil film instability.

more to this thread at:http://www.eng-tips.com/viewthread.cfm?SQID=1844&SPID=384&page=1&CFID=3302598&CFToken=35781439

woops- I meant to direct for additional interest to "motor vibration non-repeatability" under Mechanical Engineers/Mechanical Acoustics/Vibration Engineering

Latest data showed that non-repeatability was worse with higher amplitudes with cold oil, compared to normal DTE light turbine oil at 115 degree F. design temperature. Motor base vibration was over ½ amplitude at bearings in horizontal direction. As rotor vibration was much worse near the coupling end bearing, with a non-symmetrical orbit, bearing was taken apart and re-assembled. As impact tests on casing during coastdown at slow roll speed showed frequency near 1950 cpm, giving only about 10% margin, motor supports were also stiffened in the horizontal direction - insuring that the rotor/bearing first critical speed remained above operating speed. After re-installing bearing, and checking for soft foot after welding support braces, motor then ran with much lower vibration levels.