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High motor cyclic vibration

  • 1.  High motor cyclic vibration

    Posted 09-30-2023 02:21

    Dear gents,

    Kindly I need your expert advise in a case in my site.

    The case for a 2-pole, 2980 RPM induction motor, sleeve bearings, driving a multi stage horizontal pump  

    trend shows intervals of stable readings and times of fluctuating readings (cyclic vibration) on both motor sides, more observable in the NDE. The fluctuation

    appeared to occur during the night, however some events were recorded during the day morning.

    - Vibration vs temperature trend showing correlation during steady and cyclic periods with

    temperature.

    - During the fluctuation period, cyclic vibration magnitude varies from below 20 microns to more

    than 50 microns pk-pk within 2 minutes.

    - The frequency of cyclic event is around 4.5 minutes

    - Full spectrum waterfall showing 1X forward component responsible for the direct vibration

    variation.

    - The polar plot showing repeating loops - while the machine is under steady conditions without variations in load - 

    What do you think the cause of this fluctuation? Is it something we can live with? Any additional plots can reveal more clear vision 

    is this a typical symptoms of "Morton Effect"?

    Thanks in advance 



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    Ahmed Gomaa
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    Attachment(s)

    pdf
    DOC-20230927-WA0000..pdf   633 KB 1 version


  • 2.  RE: High motor cyclic vibration

    Posted 10-01-2023 14:01

    Any similar equipment running near?



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    Vibanalyst
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  • 3.  RE: High motor cyclic vibration

    Posted 10-01-2023 14:17

    This is a new installed machine, the sister machine has another problem, couldn't start for enough time to monitor the same phenomenon occurrence.



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    Ahmed Gomaa
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  • 4.  RE: High motor cyclic vibration

    Posted 10-02-2023 01:39

    Was the ˝sister machine˝ ON while measuring this one? 



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    Vibanalyst
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  • 5.  RE: High motor cyclic vibration

    Posted 10-02-2023 04:19

    No sir, no running machines nearby 



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    Ahmed Gomaa
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  • 6.  RE: High motor cyclic vibration

    Posted 10-02-2023 02:04

    How about waterfall and SCL during the vibration change?

    Could you show the correlation between Vibration and temperature? How about the winding temperature of motor?

    Could you share bearing clearance and shaft diameter?



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    Dinh Ngo
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  • 7.  RE: High motor cyclic vibration

    Posted 10-02-2023 04:39
      |   view attached

    The waterfall shows 1x forward, responsible for the vibration fluctuation.

    Scl for the motor during the change is almost stable, however the pump scl in the NDE is abnormal and strange for me, but would it related to motor fluctuation? And if not is it normal, what would the cause of?

    Attached waterfall, bearing temperature variations with vibration, the winding temperature is not displayed in MMS unfortunately. Also the motor bearing clearance is not provided, only for the pump is available. 

    Thanks again for your consideration 



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    Ahmed Gomaa
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    Attachment(s)

    pdf
    AMP.pdf   763 KB 1 version


  • 8.  RE: High motor cyclic vibration

    Posted 10-02-2023 09:59
    Edited by Dinh Ngo 10-02-2023 10:13

    It is an interesting case. However, I do not think that this is a morton effect in this case by its period fluctuation and bearing temp not high (64oC).

    This motor uses circulation lubrication or Oil Ring. Could you share with us:

    • Direct orbit (two time: 1. normal and 1. at high vib).
    • All vibration trend from pump + motor in 1 plot.
    • Could you zoom to see how it fluctuate in detail.
    • Have you checked any abnormal issue related to the machine or nearby.
    • A spectrum with high resolution.

    In my opinion, I think it could be a modulation case which need to see closely side band of 1X in high resolution.



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    Dinh Ngo
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  • 9.  RE: High motor cyclic vibration

    Posted 10-02-2023 11:35
    Edited by Ahmed Gomaa 10-02-2023 11:37
      |   view attached

    The motor uses circulation lubrication

    Attached direct orbit for the high vs low values during the fluctuating period

    The spectrum doesn't show sidebands

    the motor trends shows the fluctuation in both ends, more in the NDE, the pump seems very less in the DE and nothing in the NDE (I don't have right now - will collect later on)

    Thanks for your interest



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    Ahmed Gomaa
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    Attachment(s)

    pdf
    AMP - 2.pdf   543 KB 1 version


  • 10.  RE: High motor cyclic vibration

    Posted 10-02-2023 20:16

    Good morning!
    I think this is modulation issue with problem from motor. The beating could happen but to see it you need to setting sampling rate and resolution. The sideband  is closed to 1X so with existing setting, you couldn't see any thing.

    Hope that help.



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    Dinh Ngo
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  • 11.  RE: High motor cyclic vibration

    Posted 10-03-2023 01:38

    Morton Effect" is very rare case, but seems u have very good observation. it might be the case. 

    we need data file to analysis it. 



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    Basit Khan
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  • 12.  RE: High motor cyclic vibration

    Posted 10-05-2023 10:51

    If I were there, I definitely take high-resolution data to verify the existence of sidebands. Typically, online vibration monitoring systems do not use a high number of lines. In this case, I wouldn't be surprised if the step in the spectrum > 1Hz which is a lot to examine sidebands.

    Based on the data, the Martin effect and rubbing are at the top of my possible causes.



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    Ali M Shurafa
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  • 13.  RE: High motor cyclic vibration

    Posted 10-06-2023 03:56

    Thanks all for your valued comments

    I agree the resolution may not be sufficient to capture the nearby sidebands, the set resolution is 0.75 Hz step, if this is a beating which is repeated once every 4.5 minutes, so the beat frequency is 0.22 Hz, so I need 12800 lines in a 1000 Hz span, am I correct? 

    I can see the filtered orbit dot is moving with time (during the cyclic event), could be indicative of subsynchronous peak just below the 1X? Or could it be higher like the line frequency, I can think also of electrical source (PPF), I have collected high resolution data with the portable analyser CSI 2140 but couldn't capture sidebands.

    If not electrical, what could be the potential causes of very close peak to 1X?

    How can I confirm if it is a rubbing or not, other symptoms of rubbing like reverse precession, temp, scl change, doesn't exist, values also are not high, I understand that doesn't eliminate rubbing occurrence, but should I look for other symptoms? And what would be the cause of rubbing, as you all well know it is a secondary effect?

    I know these are too many questions in my head 😔, a strange case for me with a new machine 

    I will try to collect higher resolution from proximity probes, and I have also requested to increase the lube oil temperature a few degrees to check if it has any effect, still waiting for approval to implement. I will update with the outcomes. 

    Thanks all for support



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    Ahmed Gomaa
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  • 14.  RE: High motor cyclic vibration

    Posted 10-06-2023 05:20

    I think this is interesting case and it is first time to see it on motor issue. I just share what I think (it could be wrong):

    1. I see that this is period change of vibration with circular pattern on polar plot while the rubbing is more randomly (as I know).  Besides, the rubbing in lubrication could be at 1X but during rubbing it will increase stiffness and make vib value decrease, the temp could be increase at the time. So I think the rubbing is not a case.
    2. I have not had experience with Morton effect issue but from some paper of the phenomenon, it was known that it is the phenomenon happen with temp issue at the bearings (normally, at overhung compressor type, high speed). Under the consideration that this is motor between bearing and circulation oil and period characteristics, I think it could not Morton effect issue. 
    3. From vibration you zoom in, I see that the amplitude has a pattern like modulation issue. And this is coincident that the modulation issue has period characteristics. So, I think it would high possible.
    4. From SCL, it looks that the misalignment in vertical could be a case, but the value is not much. So, it could be not a cause of the problem.

    Just my point of view. If you find the cause, please share. 

    Best Regards,

    NVDINH



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    Dinh Ngo
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  • 15.  RE: High motor cyclic vibration

    Posted 11-07-2023 15:57

    I do not think this has anything to do with Morton effect.  This is a 2-pole motor, which are notorious for having vibration problems.  What you are seeing is called slip frequency modulation.  Look in the appendix written by Robert Wall in the book "Vibration Spectrum Analysis" by Goldman (ISBN 0-8311-3088-1).  Basically, vibration at twice line frequency (2LF) is being modulated by two times the slip frequency.  One easy way to determine if this is the problem is to change the load on the motor, as the load is increased, the modulating period should decrease.  At low loads, the modulating period can be several minutes, and at no load, it can be several hours.

    We tested some 2-pole motors driving some blowers a few years ago.  All had this modulating vibration.  The motors also had elevated stator slot vibration, which is a very high frequency since the number of stator slots is usually quite large.  On the motors we tested, the number of stator slots was 54, so you will need to take high resolution frequency data out to at least one times the stator slot frequency and preferably twice this frequency.  The slot frequency was side-banded by 2LF.

    The question, of course, is what to do about this modulation.  The first thing I would recommend is to perform a motor electrical signature analysis with the motor running and at load.  The results from this test, in terms of the voltage, current, and impedance should give you an idea if something is wrong electrically.  I'd look primarily at voltage imbalance between phases and harmonic distortion.



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    Lyn Greenhill
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