Alternator Vibration During Excitation

[ashrafec]

Hi guys,

I'm new to this forum and this would be my first post :-)

Hope you guys can guide me on what to do on this alternator that's having problem with vibration. I'm running out of ideas now on what to do with this alternator.

The alternator was sent to our workshop by the client for repair. They noticed alarming vibration. Upon opening the alternator, we found the rubbing mark at the bottom side of the non-drive-end (NDE) stator core. At the same time, there are rubbing mark on all 4 poles of the rotor, also at the NDE section meaning all 4 poles have rubbed with the stator core at the NDE side. This is a salient type rotor. we have done repair work on the stator core lamination at the rubbing area, also we have rewound the main stator and all 4 poles. We have replaced both bearings DE and NDE. Also, we have fixed the NDE bearing housing to get it to within tolerance.

After repair, we tested the alternator in the workshop. However, the vibration is still there. When we disconnected the AVR from the excitation i.e. running without excitation (no output voltage), the vibration dissappear i.e. reduces to below 1mm/s. When we introduce the excitation, then the vibration will appear around 4mm/s.

We have looked at the vibration spectrum. It seems that there might be possible cause of eccentricity.

It seems that there is a problem with eccentricity based on 2xline frequency but how to check the actual air gap when the machine is totally enclosed ? Is there any other factor that can contribute to this problem ? Or perhaps it's not actually symptom of 2xline frequency, but symptom of 4xrpm instead ?

Any idea what should I do now ?

Brief details of the alternator :

- Leroy Somer LS F44M2ATR16
- 4 poles
- power rating : 30kVA, 24kW
- voltage : 415V
- ampere : 42A
- speed : 1500rpm/50Hz

Please find attached vibration spectrum for both DE and NDE when the alternator is excited and when it's not excited. There is a big different between magnitude for these 2 conditions.

Thank you for your guide !

 

[Bystander]

Missing spacers? Can you see endwise motion/shifting of the shaft? You might be able to spot it just exciting without rotation. Spins where it's happy no load, turn on excitation and shift it endwise to where it starts dragging/striking something.

 

[Baluncore]

An electrical fault can give asymmetric physical vibration.
Because it results in load imbalance it can cause resonant vibration and physical contact at some RPMs.
My first guess would be an open circuit phase or a shorted turn in one phase on one pole.
Depending on how the excitation is wound there could be a shorted turn in one pole of the rotor.