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New phenomenon in Vibrations of Electric Generator
2

New phenomenon in Vibrations of Electric Generator

New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!
A new phenomenon has been seen in a Run Down of an electric generator. It is shown in the attached graphic.

The machine has ball bearings with grease in two sides. One side is fixed and another one is non fixed in axial direction. The horizontal line is appeared only in 15% of manufactured machines. Why?

The colour-map generated is obtained in a Run Down test. So there is no electric supply. The machine goes to 2000rpm and then the electric supply is switched off. Now, the rotor speed goes down until 0rpm. The horizontal line appears near 1600rpm.

Do you need more data? Wich the reason is that generates this horizontal line?

Thanks in advance.
 

RE: New phenomenon in Vibrations of Electric Generator

Looks like 1/2 order knocking (impacts) to me.  Can you hear anything unusual at that speed?  I can't begin to guess why it's 1/2 order though.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi Steve,

At that speed the noise is normal. There is no impact noises. The orders are the diagonal lines that you can see in the colour map. The horizontal line isn't a order. I don't know what it's...

Personally, I think some non linearity (but wich?? maybe the ball bearings??) is producing that horizontal line. I don't know...

The next fb-waterfall shows the same phenomenom but in this case to 850rpm.

Thanks in advance,
Best Regards

RE: New phenomenon in Vibrations of Electric Generator

I can read colormaps (I write software that creates them!).  My point was that at your problem speed, 1/2 order excitation is evident, which isn't something I'd expect to see for a simple rotational machine like this, hence my suspicion that this is connected to the problem.  Horizontal lines in colormaps ususlly point to something misbehaving (knocking, instability, non-linearity, etc) so that all harmonics are excited.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

Do you know what the rotor naturial Freq?

Also how big is this unit (kw), operating speed?

I have seen things like this before, and it was related to a naturial freq.  I would do some more diging to see if this is repeatable.  If this is a naturial freq, it can be anything in the system.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

RE: New phenomenon in Vibrations of Electric Generator

I agree with Somptingguy in terms of the frequency content of that yellow line. Looks like 1/2x (12hz) with harmonics of 1/2x (12hz) occuring at machine speed ~ 24hz.  The fundamental frequency of motion appears 12hz, but it is not sinusoidal (presence of harmonics)

And I agree with Motorvib - has the characateristics of a resonance.  If you look when the machine speed gets down to 12x (the lower part of the graph), you see a slight intensification of the yellow again - which supports the idea that 12hz is a resonant frequency.

Putting it together, as 1/2 decreases through resonance, the magnitude gets so high that it excites non-linear effects, giving harmonics.

Another interesting thing - there is a faint vertical line at this frequency and it's harmonics.  Then as we move down the chart, the left-most vertical line (12hz) tends to curve left toward slightly lower frequency and next right veritcal line  (24-hz) curves slightly right.

It is a little bit of strange behavior. Thinking out loud - two things come to mind that may or may not be of interest (see for example Den Hartog or Harris' Shock and Vib Handbook)

1 - Self-excited vibrations of rotors.  Typically they occur at rotor resonant frequency.   Also typically during increasing of machine speed (a different scenario than your test), we would not any of these instability effects until rotor speed gets near/above twice the natural frequency.  That is generally the threshhold speed.  As rotor speed continues above that, the vibration would continue at the natural frequency (not the rotor speed).  Reasons for this type of behavior include: oil whip... internal hysteresis of the rotor due to loose parts such as loose laminations... fluid trapped in rotor... etc.  Although typically we don't think of this type of vibration continuing as rotor speed decreases below twice the natural frequency).
2 - If I'm remembering correctly (and I'm not sure I am) I  think he talks about criticals at half speed due to the effect of gravity.  

 

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RE: New phenomenon in Vibrations of Electric Generator

" criticals at half speed due to the effect of gravity" .

One Hartog's example (might have been others) was for a rotor of non-uniform bending stiffness.
2X4 wood beam, 2 pole motor rotor, gets twice per rev vertical bounce on earth with shaft ~ hrizontal

RE: New phenomenon in Vibrations of Electric Generator

Thanks Tmoose.  Twice per rev would of course be different.  Maybe I was mistaken.  I'll see if I can find it.

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RE: New phenomenon in Vibrations of Electric Generator

Am I right in reading it as rpm up the y axis and Hz across the bottom?

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: New phenomenon in Vibrations of Electric Generator

That's the way I read it. (reading points off the 1x line, the vertical axis of rms is around 60* higher than the horiziontal axis of hz).

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RE: New phenomenon in Vibrations of Electric Generator

Let's try that again (correction in bold)
That's the way I read it. (reading points off the 1x line, the vertical axis of rpm is around 60* higher than the horiziontal axis of hz).

=====================================
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RE: New phenomenon in Vibrations of Electric Generator

If you squint you can just about read Hz on the X-axis and rpm on the Y-axis.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!
Sorry about the bad quality of colour map. In this one:
x-> Hz
y-> rpm
colour scale-> vibration level

In the 2D graphic:
x-> rpm
y->Vibration Level

In some cases, the vibration level is important and the machines should be rejected.

The natural frequency is 30% far from nominal speed (1200rpm) by design.

One question: When you say 'criticals at half speed due to the effect of gravity'... This is only for flexible rotors or rigid too?

Finally, I have found a paper of Dra. Muszynska that says that the horizontal line is due to Nonlinear Stiffness. ponder. But it doesn't says what causes that phenomenon.sadeyes. It says:"Horizontal line is not an indicator of crack. This is due to Nonlinear Stiffness".

See soon!
Best Regards,


 

RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!

I am thinking about this problem ponder ponder. I would like to know your opinion about the next one:

Is it possible that a difference among diameters of inner balls of ball bearings produces this nonlinear stiffness showed as horizontal line?

In negative case, how could you see this another problem?surprise

Best Regards,
 

RE: New phenomenon in Vibrations of Electric Generator

"Is it possible that a difference among diameters of inner balls of ball bearings produces this nonlinear stiffness showed as horizontal line?"

Have you gotten familiar with the rotational frequencies of components and events in a ball bearing?  If one or a group of undersized or oversized ball significantly changed stiffness, that influence would be rotating at about 0.4X shaft speed.

RE: New phenomenon in Vibrations of Electric Generator

Pete said "Twice per rev would of course be different."

I think you had it right.
The frequency of excitation for the [2 pole motor - 2X4 lumber] effect is 2 vertical bounces per shaft rotation.

If INDAR's calculated rotor "natural frequency is 30% far from nominal speed (1200rpm) by design", AND that natural frequency is first bending mode, AND the natural frequency is above rotating speed, THEN discounting damping and what not, if he has a two pole motor then a "critical speed" will be apparent as the rotor coasts thru about 65% nominal speed.  The frequency of the vibration during the critical however will 2X the rotor rotating frequency.

(The fact a single calculated/designed rotor natural frequency is mentioned suggests bearing and support stiffness may not have been included in the cyphering process)    

RE: New phenomenon in Vibrations of Electric Generator

Wow. I think you may be on to something there.

Best guesstimate of the frequenies where the horizontal line appears from attached is 1800 rpm (30hz) machine speed and 12 hz vibration frequency.  (note I positioned the vertical red dashed line based on the underneath vertical yellow line... that vertical yellow line easy to see in the original version but not so much in this revised version).

13hz/30hz = 0.43

Which as was mentioned is a very typical value for Fundamental Train Frequency - the frequency that the cage and rotating elements orbit around the bearing.  (Note in my simple mind I think about the motion of earth about the sun... FTF equates to one year and BSF equates to one day).  If you have one larger rolling element, it would show up at the same position (relative to a stationary reference frame) at frequency FTF.

Do you have a bearing part number?  Manufacturers and vib programs provide fault frequencies based on bearing part number.

Putting together the story line - we have a resonance at 13hz, and a forcing function (FTF) which gets to 13hz when machine speed is 1200 rpm.  And perhaps the vibration magnitude is so high that it excites non-linear effects giving those harmonics of 13hz.

But here's a part that doesn't fit that particular story line - why don't see see the 13hz vibration again due to  1x excitation (unbalance etc) when the machine speed passes through 780 rpm?   

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RE: New phenomenon in Vibrations of Electric Generator

Note INDAR's second post:

"The next fb-waterfall shows the same phenomenom but in this case to 850rpm"

That's close to 780 rpm.  It has to be bearing-based.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

INDAR,

Thanks for this thread.  It's making me realise how much I miss the detective work that follows some relatively simple vibration measurements.  It's all a big puzzle and the measurements have the answer.
 

- Steve

RE: New phenomenon in Vibrations of Electric Generator

Thanks, I didn't see that attachment

Quote:

The next fb-waterfall...
What does fb stand for? How does the 2nd attachment relate to the first? Different machine? Different bearing on same machine? Different run on same machine?

 

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RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!
The 2nd attachment is not related to the first one. This is of another machine of the same type. The testing was the same: Run Down.

I am sure that the problem comes from the bearing zone. I don't know if there is some looseness, the size of ball bearings, maybe the Non Drive End that has mechanical play in axial direction... By other side, the design of this area and the ball bearings was recommended by SKF, so I think this should be OK.

Besides, as this is a production line, I can't do a lot of testing.

I would like to find some information about this phenomenon.

Best Regards,

RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi Guys,

I have get the full information in pdf with very good graphics of this testing. There is no more information.

This one is from another machine. The horizontal line appears in vertical and axial direction.

For its application, the vibration level is high.

Best Regards,

RE: New phenomenon in Vibrations of Electric Generator

Well, there's no .5, .4, .43 or similar order in any of those plots.  All I see is the expected 1st order dominance cutting through resonances, exciting harmonics.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

Ah, no at 850 rpm there is a lot of activity at all the integer order lines, that looks either like a signal acquisition problem (distortion), or else something is going off in a non linear fashion.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: New phenomenon in Vibrations of Electric Generator

Agreed, it does go a bit AWOL in some directions at that speed.  Hadn't thought of distortion (or simple overloading), always a possibility.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!

What is AWOL? I don't understand this word.

In the beginning I think so. But after some measurements (15% of measured machines)and after check it with a hand analizer, I think the measurements are fine. The channels (accelerometers) weren't overload.

The measurements are made with the same equipment and the same projects always. So I think the problem comes from machine, exactly from bearings.

Besides, although bearings are well lubricated, there is a screeching noise from the bearings. (This screeching noise dissapears with the running of machine as well as the temperature of machine raises - maybe the grase of ball bearing is more liquid and the lubrication is better-). But in some machines that appears this noise its behaviour is fine. Not horizontal line appears. mad

Somebody has see this horizontal line before in other machines? ponder      

Best Regards,
 

RE: New phenomenon in Vibrations of Electric Generator

For the bearings take a look at NTN or SKF Web page, they have an on line informasion about bearing freq's.

"In this house, we obey the laws of thermodynamics." Homer Simpson

RE: New phenomenon in Vibrations of Electric Generator

All I can really say is that it looks like a 'clunk' to me if the signal is OK.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: New phenomenon in Vibrations of Electric Generator

It looks as if everything is confirming some kind of resonance around 870 or so.  The excitation is sometimes 1x and sometimes this 0.43x which we have postulated by be bearing fault frequency FTF.

Try googling: "bearing fault frequency" FTF

Meanwhile, if you have a bearing handy, you can easily convince yourself of the physical significance of this fraction around 0.43 (exact fraction can be calculated from bearing geometry or from bearing part number).

I have done it in the attachment, although I think I need to explain what is shown in the attachment since the visual cues are not well displayed.

The bearing starts out with blue dots at the 12:00 position on all three components:  inner ring, cage, and outer ring.

Then I rotate the inner ring one full rotation so that it comes back to the 12:00 position.  Where does the blue dot on the cage end up?.... around the 5:00 position.

During one full revolution of the shaft, we have seen 5/12 ~ 0.4 revolutions of the cage.  And if we had one large rolling element, we have seen 0.3 revolutions of that element.

If shaft is traveling at X rpm, then that rolling element will pass by a given location on the outer race at a rate of 0.43 * X rpm.





 

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RE: New phenomenon in Vibrations of Electric Generator

Ok, just being thick here...

Is that 0.43 based on the contact radii ratios of the solid parts of the bearing?  If so, is 0.43 some kind of optimal ratio?

- Steve

RE: New phenomenon in Vibrations of Electric Generator

It is a slightly different number for each bearing depending on geomtry as you mentioned.  Could be maybe 0.38 to 0.45.  I don't know the number for this bearing.

0.43 = 13hz/30hz is the ratio that I calculated between the resonant frequency and the running speed frequency from the first attachment.  I have ASSUMED that this is the value of the FTF for Indar's bearing.

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RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!
Some doubts: ponder

1. What's AWOL?
2. What's FTF?

I do not understand what it gives you the information ratio between the resonant frequency and the running speed frequency.

Best Regards,

RE: New phenomenon in Vibrations of Electric Generator

AWOL means Absent Without Leave, a military term.  It's often used to describe something that's out of control, off the scale, etc.  I shouldn't have used it really, sorry.

- Steve

RE: New phenomenon in Vibrations of Electric Generator

FTF stands for Fundamental Train Frequency.  It is the frequency that the cage travels around the bearing.  It is also the frequency that a given ball travels around the bearing.  It is one of the 4 common rolling bearing "fault frequencies" (FTF, BSF, BPFO, BPFI).  If we compare a ball going around bearing to earth going around sun, then FTF is analogous to one orbit per year, BSF is analogous  to one rotation per day. Please google bearing fault frequency and you should find a lot more info.

Quote:

I do not understand what it gives you the information ratio between the resonant frequency and the running speed frequency.
My discussion was in the context of the first attachment.  We see evidence of a resonance at 13hz which is excited when machine frequency is 30hz.

If the source of excitation is one whose frequency is proportional to  machine speed, then the proportionality constant in this case is Fexcitation / Fmachinespeed = 13/30 ~ 0.43

Bearing fault frequencies are proportional to machine speed.  For the case of an 8-element bearing, we might have something similar to:
BPFO = 3.1 *  Machine Speed
BPFI = 4.9 *  Machine Speed
FTF = 0.4 * Machine speed
(where the exact multiple can be determined from the bearing part number or geometry).

Looking at these, we see the FTF proportionality constant (~0.4) is very close to the proportionality constant suggested by the data (0.43).  So we make a guess that the fault may be one which varies at frequency FTF.   

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RE: New phenomenon in Vibrations of Electric Generator

(OP)
Hi guys!

Through an exchange of mails, roller bearings manufacturers have confirmed that the "nonlinearity" could come from the bearings. This rigidity depends on the type of bearing, size and clearance / preload. thumbsup2
I have to talk with them by telephone. I'll let you know.

Best Regards

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