Generator Vibration

[ele.engineer]

Hello,
I first want to say thank you in advance for those who help.
I need your advise for the following problem
Generator (10 MW and 6600 volt)coupled to gas turbine via gear box.
we make re winding for the generator stator winding and after generator operation it have high vibration reading (after vibration analysis unbalance is found on rotor shaft)
BUT
after 4 months of continuous operation the vibration reading backs to its normal !!
also, after stopping the generator for two hours and start it again the vibration readings become more high again !!
Why the vibration decreased and increased again ???

 

[electricpete]

The possibilities are wide open. Ideally you'd like to compare everything you can about the conditions under which vibration occurred and when it didn't occur (loading, excitation, temperature).

You mentioin unbalance so I gather the vibration is 1x. Some sources of 1x that can change with conditions:

1 - thermal bow of rotor can cause vibration to get worse as temperature increases. If this is trim balanced based on balance conducted at high temperature, the vibration may be higher at low temp and lower at high temp. Or there may be a compromise balance attempted.

2 - Shorted rotor turn can cause something resembling a thermal unbalance that may be sensitive to rotor excitation level (and time since excitation was applied).

3 - Thermal changes can affect supporting structures causing misalignment which causes 1x vibration.

4 - Bearing oil and seal oil temperature changes can dramatically change characteristics (stiffness) of bearings and seals which can tune resonances toward/away from running speed.

That's all that comes to mind at the moment but I'm sure there are more possibilities. But again the best approach is probably to start with the known facts about what conditions changed and work from there. Also more details about the vibration might provide some clues (did it affect all bearings, was it directional, purely 1x, separate measurements of bearing housing and shaft relative vib, etc).


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(2B)+(2B)' ?

 

[NCTHAI]

In addition to the suggestions from electricpete - has anything changed for shaft earthing? It is Gas Turbine Generator - so i believe one bearing is insulated and shaft earthing is provided. Is there any change in shaft grounding brushes or currents?

Have experienced similar problem for steam turbines of quite bigger size. We determined root case was grounding.

NC