Our 150 MW Generator is runnig as Synchronous Condenser. We notice
that vibration spectrum at the turbine end bearing indicates higher
vibration at the 4th harmonics than at the 3rd and 2nd harmonics.
According to literature on vibration, a higher 4th harmonics vibration
means electrical causes. Quote "If some magnetically affected part is
slightly loose(such as iron encapsuled by wires or slightly loose
armature laminations) the loose part will be pushed in one direction
at the instant of the positive portion of the electrical cycle and
then pushed in the negative direction at the instant of the negative
cycle" unquote. I wish to hear from members who have such experience.

Here is the self-proclaimed Turbine Doctor:

http://www.turbinedoctor.com/services/vibration.html

How many poles?

If 4-poles 1,800 rpm in US), then 4 times running speed would be twice line frequency (frequency of magnetic force variation), and the claim would be reasonable IF we also saw slightly incrased 4*LF, 6*LF etc. I have seen that pattern on 6 sister motors which all had loose stator slot wedges (as determined by physical inspection). The pattern disappeared on 5 when they were rewound and on the 6th when it was rewedged.

Chris, is the 4th harmonic real or a function of the FFT process? An examination of the waveform should reveal if there is truly a 4th harmonic or its the result of a square wave. I would agree with Pete, that the 4th harmonic problem as you describe should apply to a 4 pole and not a 2 pole machine. If you look at the waveform and there is truncation in the sine wave (e.g. square waveform), you may be looking at looseness in the bearing support system.

Thanks for the messages. I attach herewith the vibration spectrum for reference. You will notice the vibration at the 4th harmonics higher that those at 2nd and 3rd. The generator now running as Synch Condenser is a 2 pole machine.