# High axial vibration at permanent magnetic generator

1. Feb 6, 2010

### floyds

hi guyz,

I have Turbiine(5500RPM)-Gearbox(output 1500RM)-Generator-Exciter-Permanent Magnetiic Generator(PMG).

I have problem with high axial vibration at ball bearing permanent magnetic generator.
Axial vibration at 5500RPM, but it rotate at 1500RPM. This overall is 3-5 times radial direction.

I have confirmed that there is no bearing cocked on shaft.

Is there anyone can help me..?

Thanks

floyds

2. Feb 6, 2010

### FredGarvin

Have you examined the bearings themselves? PMGs also can introduce vibrations under loads when there are issues with the windings and drastic differences between phases. I would have to do some research on that end since it has been a long time since I have troubleshot a generator's electrical side.

3. Feb 6, 2010

### floyds

dear FredGarvin,

we did.
we have change the bearing both side, and nothing change in amplitude. we always keep it(PMG) runs at 50Hz, so I guess electrical problem due to line frequency(difference air gap between stator and rotor) should come at 6000 RPM (is not the real problem). Because our dominant frequency is 1XRPM highspeed gearbox about 5500RPM read at PMG ball bearing. permanent magnetic generator(PMG) itself rotate at 1500RPM.

and I dont think it could be bent shaft.

can it be a resonance..?or any probable that I miss in thought?

Thank You

Last edited: Feb 6, 2010
4. Feb 6, 2010

### FredGarvin

A 1E vibration is surely an inbalance. The tell tale is if the dominant frequency increases with increase in rotational speed. If it rises and then goes away with increasing frequency (speed) then it is most likely either passing through a mode in the rotating group or a structural resonance. If it were a bearing you would see sidebands around the the ball pass frequency and, most likely, the 1E as well.

5. Feb 7, 2010

### brewnog

Axial vibration at running frequency is typically angular misalignment between the driving and driven equipment. Measure the phase difference between the gearbox and generator, if they're out of phase you should definitely look for angular misalignment.

6. Feb 7, 2010

### floyds

Dear fredGarvin,

thank you for the opinion,
I dont know what the 1E Vibration mean..? when unbalance happen, 1XRPM(1500RPM) at running speed would present at spectrum. Resonance itself hard to understand for me. can resonance happen due to our activities in changing the bearing(part of bearing or any other equipment) which have natural frequency around running speed..? how can i confirm it. this amplitude show up just about several days ago. or is there any field activities to confirm the part's natural frequency without bump test..?

from increasing speed, how can I justify that one part or more part(structural) have natural frequency around running speed?

dear brewnog,

thank you for the opinion.
first I thought it was a misalignment or bent shaft due to high amplitude at axial direction. but clearly inspect at its spectrum, the dominant frequency is about 5500RPM. at the time condition our PMG surely running at 1500RPM. that's a little weird for me.
for misalignment and bent shaft the dominant frequency should be present at 1500RPM or its harmonic.

thanks

7. Feb 7, 2010

### floyds

Dear fredGarvin,

thank you for the opinion,
I dont know what the 1E Vibration mean..? when unbalance happen, 1XRPM(1500RPM) at running speed would present at spectrum. Resonance itself hard to understand for me. can resonance happen due to our activities in changing the bearing(part of bearing or any other equipment) which have natural frequency around running speed..? how can i confirm it. this amplitude show up just about several days ago. or is there any field activities to confirm the part's natural frequency without bump test..?

from increasing speed, how can I justify that one part or more part(structural) have natural frequency around running speed?

dear brewnog,

thank you for the opinion.
first I thought it was a misalignment or bent shaft due to high amplitude at axial direction. but clearly inspect at its spectrum, the dominant frequency is about 5500RPM. at the time condition our PMG surely running at 1500RPM. that's a little weird for me.
for misalignment and bent shaft the dominant frequency should be present at 1500RPM or its harmonic.

thanks

8. Feb 7, 2010

### brewnog

You said the turbine was running at 5,500rpm. Seeing vibration at this frequency on the output shaft (although geared down to 1,500rpm) wouldn't be unusual, particularly for a torsional or misalignment issue. Are there helical gears in the gearbox?

9. Feb 7, 2010

### floyds

dear brewnog,
it is. we used helical gear at gearbox.

thank you

10. Feb 7, 2010

### FredGarvin

By the way, are you measuring radial and tangential vibes as well? If so, what are they like?

11. Feb 7, 2010

### brewnog

Will await an answer to Fred's question, but this could easily be (natural) torsional or linear (vertical/horizontal) vibration from the turbine at running speed (5,500rpm) being translated to axial vibration on the driven equipment by thrust loading through the helical gearbox, and/or by angular misalignment between the turbine and gearbox.

Your relative measurements between the turbine, gearbox and generator will diagnose these theories.

12. Feb 7, 2010

### floyds

Dear FredGarvin,

I did.
Vibration spectrum at radial(vertical & horizontal), have the same dominant frequencies at 5,500RPM. Both frequencies(5,500RPM and 1,500RPM) have harmonics. Please have a look at 2 files attached.

Dear Brewnog,
Thank You for the opinion. I'll make a new measurement today, and look more clearly at possibility of angular misalignment and the gearbox condition.

Thank You & Regards

floyds[/QUOTE]

#### Attached Files:

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• ###### Horizontal IB PMG.JPG
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