High 2X vibration at NDE axial on 4-pole generator

Dear All,

I have been seeing similar pattern in vibration spectra too often at test bed condition at manufacturer.

Sometimes, 1X axial vibration is too-high, sometimes 2X axial vibration is too high, and sometimes only NDEA 2X vibration is too high, which result in high overall vibration for axial direction.

This pattern I have observed in 4-pole generators irrespective of rating, exciter rotor is overhang type at NDE.

Please put some light on this problem as I want to understand the source of the problem.

Thanking you.
 
B T V Jagadeesh
One of my assignments was to witness testing of a 21,000 Hp compressor drive motor. Mfg personnel said they had checked everything... many times. At quitting time I asked permission to remain after everyone left to do some paper work. They said yes, but left an employee to keep an eye on me !
After everyone left I went to the test stand, and kicked the shims at each pedestal, like kicking tires on a new car. Sure enough, the shims of one pedestal moved ! Problem solved !
Regards, Phil Corso.
 
Hi Jagadeesh,

When you say 1X and 2X, I am assuming you must have some sort of software from where you are able to see the spectrum data..Right? Does it have other 3x and so on synchronous harmonics..?? A snapshot of the spectrum data would be great to do an analysis.
1x and 2x can be a lot of stuffs like unbalance, shaft clearance, eccentricity, bent shaft,misalignment,looseness.
Also Overhung machineries would generally be associated with some imbalance. If you are taking phase reading, could you provide the phase data..? that would be really helpful.Also can you provide some idea about your test bed setup???
 
Hi udshred,

Here is the spectral data I got. Regarding test bed set-up, people normally does the straight edge alignment from drive motor to generator.

Hi Jagadeesh,

Check whether the 2x is not a part of the line frequency component. If it is not then it might be a overhung imbalance or misalignment or bent shaft or looseness but requires additional analysis to pinpoint the problem. Phase readings/ Orbit plot would have given a much clear picture. What is the ISO balance grade level??
It would have been much better to give a solution by actually being present at the site but what you can do is eliminate each of the individual components and pinpoint the problem.
 
Hello Jagadeesh...
1) Do you have a vibration study of the period between the time the generator is on-line, and the time it stops rotating?
2) Is it a Salient-pole or a Non-Salient Pole machine?
3) Has it been tested without the exciter?
4) When you said "too often... at manufacturer." are you referring to one mfg?
5) I'm curious, what's the Generator's capacity?
Regards, Phil Corso
 
Hi PhilCorso,

Thank you for the response.


1) Do you have a vibration study of the period between the time the generator is on-line, and the time it stops rotating? The generator at test bed.
2) Is it a Salient-pole or a Non-Salient Pole machine? non-salient pole
3) Has it been tested without the exciter? without excitation
4) When you said "too often... at manufacturer." are you referring to one mfg? different capacity generators.
5) I'm curious, what's the Generator's capacity? 13.75 MVA

Please refer to the previous reply with 2X Vibrations pdf.

Regards
Jagadeesh
 
Hi Jagadeesh,

Check whether the 2x is not a part of the line frequency component. If it is not then it might be a overhung imbalance or misalignment or bent shaft or looseness but requires additional analysis to pinpoint the problem. Phase readings/ Orbit plot would have given a much clear picture. What is the ISO balance grade level??
It would have been much better to give a solution by actually being present at the site but what you can do is eliminate each of the individual components and pinpoint the problem.
Hi udshred,

Does overhung configuration excites 2X axial and radial vibrations? This is a sleeve bearing generator
 
Absolutely..
Hi udshred,

Does overhung configuration excites 2X axial and radial vibrations? This is a sleeve bearing generator
Hi Jagadeesh,
Overhung imbalance would generally have high 1X axial..generally recommended range is from 0.8X to 1.2X and misalignment can have 1X,2X,3X and so on because of rocking movement of the overhung confguration. Also looseness would have the same but with a higher floor noise. Bent shaft would again have higher 2X vibrations if bent near a coupling or joint point (would look like misalignment) and 1X if towards the center (would look like imbalance). And so I said it requires a checking a lot of other stuffs to pinpoint the problem. 1X and 2X are the hardest part of vibration analysis as there can be a lot of effects which might be into play.

But generally I would start with a shaft centerline plot, probe voltage plot and orbit plot to look for patterns and get a feel/idea from those plots about how the shaft is moving during coast-up, running and coast down conditions. Personally I feel those are much more effective for fluid film bearings when the fault has already been generated.
 
Absolutely..


Hi Jagadeesh,
Overhung imbalance would generally have high 1X axial..generally recommended range is from 0.8X to 1.2X and misalignment can have 1X,2X,3X and so on because of rocking movement of the overhung confguration. Also looseness would have the same but with a higher floor noise. Bent shaft would again have higher 2X vibrations if bent near a coupling or joint point (would look like misalignment) and 1X if towards the center (would look like imbalance). And so I said it requires a checking a lot of other stuffs to pinpoint the problem. 1X and 2X are the hardest part of vibration analysis as there can be a lot of effects which might be into play.

But generally I would start with a shaft centerline plot, probe voltage plot and orbit plot to look for patterns and get a feel/idea from those plots about how the shaft is moving during coast-up, running and coast down conditions. Personally I feel those are much more effective for fluid film bearings when the fault has already been generated.
Mr. Udshred,

Thank you for your response, I will try to gather SCL and orbit plots. Since the generators are at manufacture, I do not think there will be potential failure apart from the alignment (cardan shaft without any laser alignment, do you strongly suggest to align with laser alignment the cardan shaft?) and test-bed mounting etc,. But as said, 2X is dominant sometimes at NDE side (which has overhung exciter rotor), I want to investigate 2X along axial at both DE&NDE.

If you could show some light on possible sources, I am grateful to you.
 
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