Hi

Could anyone tell me what effects could an AVR or an exciter would have on axial vibration on the generator end?

After a major shutdown, the machine was running smoothly at 5mm/s axial vibration for two weeks. In approx. 4 hours, the axial vibration increased to 10.5mm/s. The stator was re-wedged during the shutdown and the rotor was not touched. Electrical tests did not show any issues with the rotor or the stator.

I would really appreciate some help on this subject.

NaHaS

On the surface, it has the appearance this could be a thermal cause, where the behavior changes over the first few hours of operation as things heat up. . Possibly thermal bow due to shorted rotor turns. In that case you may expect a gradual increase in 1x as the temperature increases. Why it would be axial, I'm not sure.

Sounds like electripete is on the right track. If re-wedging was not done right it prevents the rotor bars free expansion and creates rotor bow. This would be my bet.

Thank you for your responses.

But would that take 2 weeks to show?
The generator heats up in hours.

Re-wedging a stator is in no way related to rotor winding expansion. Rotor wedges are purposely kept loose to allow for coil movement & expansion.

An inter-turn short in the rotor should have been picked up by a RSO/surge test. If you have a Generatortech flux probe mounted, that can do the job too.

At what frequency is the vibration occurring, what were the previous patterns like? What is the radial vibration like? Do you have any prox. probe data?

Regards,

Aditya

Was anything else done on this major shutdown? Coupling inspection? Oil change? Any repiping?

To verifiy if it is thermal monitor the VAR load and trend it with vibration. Holding power constant have them vary the VARs and see what happens.

A seal rub could cause local heating and vibration. Is it equeal at both ends. You could check your set screws. 2 weeks? Any system changes heating up your seal oil?

Nahas, In some applications exciter can cause high axial vibration. I came across this situation for three Gas Turbine generators (33 MW) where vibrations started increasing after few months of operation. In that case exciter rotors were mounted overhung on generator rotors. Two bearing in total for generator/exciter/AVR. In that case vibrations were reaching up to 20 mm/sec and rotor change along with field balancing resolved the issue. Just sharing my experience...