Can a bad motor sound be caused by a VFD?

Discussion in 'General Electronics Chat' started by strantor, Nov 5, 2011.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    I have a 15hp 3phase motor that was making wierd groaning/vibrating noises. The oscillating noise was consistent, but not consistent with the rotation of the motor, i.e. it wasn't every revolution that the noise happened, but rather maybe every 10th revolution and lasted maybe 10 revolutions (pure guess as to the revs, I didn't put a tach on it, it's only an example). I am not the one who troubleshot it; a guy on another shift troubleshot it. He disconnected the motor from the load and ran it, and said it still made the same noise. He therefore diagnosed that the motor was bad and a new one is coming. I was told to take the motor out, which I did, and afterwards I wrapped a cord around the shaft and pulled it like a pull-start lawnmower. That got it up to maybe 100-200rpm and I could not hear any noise coming from it at all, and it was smooth as butter. My supervisor (a mechanic) said that one time he saw the same thing, and it turned out to be the drive and not the motor. I was wondering, has anybody seen a drive do this before? I have seen some BLDC controllers that can send specific frequencies to motors an make them play a little tune, and you can hear the frequency of an industrial drive while it's running, so I guess its plausible that if something went wrong in the VFD, it could produce an audible noise in the motor. thoughts? experiences?
     
  2. praondevou

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    If you had asked me this like ten years ago I may have had an answer. Our company was manufacturing VFDs for 3-phase asynchronous AC motors.
    The only I remember of noise coming from from the motor was when the switching frequency was decreased on inverter overtemperature. It was still high frequency though (8kHz).

    I think it's not impossible that something goes wrong changing PWM signals inside an VFD and therefore causing weird noises in a connected motor. I personnally can't remember a particular case. You would need to do some measurements on the inverter output.
     
  3. Lundwall_Paul

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    Oct 18, 2011
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    I sure would try a VFD first.
     
  4. jimkeith

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    VFD's can excite mechanical resonances inside the motor, but not a common issue. A fellow engineer once designed an anti-resonance option to add to a VFD to make it skip past a specific resonant frequency--I do not think any actually sold. The VFD may also be defective--e.g. intermittent half-bridge IGBT--with a DC component, things go crazy. If a standard motor, throw it across the line and see how it runs.
     
  5. shortbus

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  6. GetDeviceInfo

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    personally, I'd suspect mechanical first. When the motor is running, one would do the long screwdriver to the ear thing to listen for bearings. One could also watch the output shaft for any movement correlating with the sounds. Grease extrusion is a sure sign of problems ahead.

    If your operation allowed, one could run the drive through it's speed range. Low speed is particularly telling, as drive problems of the shake/rattle type, typically escalate when rolling inertia is reduced, whereas mechanical symptons abate.

    I'd fire that puppy across the line and take a listen.
     
  7. PackratKing

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    Jul 13, 2008
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    a rolling vibration or knock, is bearings..............likely sleeve.
    These are replaceable, though it is almost a lost art in a good motor shop to ream them in for proper fit, and the end-bells must be repacked with wicking for lubricant.
    Ballbearings are far easier to replace.
     
  8. strantor

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    UPDATE:
    I installed the new motor (brand new) and I am experiencing the exact same problems. While the load is coupled to the shaft, it growls intermittently. While the load is uncoupled from the shaft, it growls constantly. While the load is coupled or uncoupled from the shaft, it draws FLA. Here are some videos:

    loadedhttp://s273.photobucket.com/albums/jj238/strantor/?action=view&current=2011-11-07_18-24-16_505.mp4
    unloadedhttp://s273.photobucket.com/albums/jj238/strantor/?action=view&current=2011-11-07_18-32-23_171.mp4


    Here is a picture of the output of the VFD. It looks normal enough to me. The sine wave is 60hz mains; I had to use mains as a trigger because I couldn't get the scope to show a good VFD output otherwise. all the phases looked the same.
    [​IMG]
     
    Last edited: Nov 9, 2011
  9. GetDeviceInfo

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    unless you have documented runtime parameters, a reset to factory parameters should run the load without problems. Otherwise, she's busted. The vids not clear, but there appears to be a brake or extended encoder housing on the motor. I'd override a brake, and remove encoder/vector operation for base testing.
     
    Last edited: Nov 9, 2011
  10. jimkeith

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    Looks like you now have a spare motor...
     
  11. gerty

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    I have a VFD on a small (3/4 hp) motor trainer that I use. When it's run at a very slow speed it makes a racket that sounds mechanical in nature but when the speed is ramped up the noise dissapears. The motor and the VFD were bought at the same time, meaning they're new.
    Also when the motor is run at 120 hz and twice the normal speed it's sounds like it's trying to come apart.
     
  12. strantor

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    You're talking about the drive being busted? I sure hope you're not talking about the motor!

    There's no brake; the housing covers a fan and an encoder. I'll try removing the encoder and switch to sensorless.
     
  13. strantor

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    My theory is that on your drive, That might be due to different PWM frequency used at slower speeds. At the high end of the 120Hz it might switch PWM so fast you can't hear it, or so high pitched that it doesn't sound like death, but at the low end of the 120 Hz, it may switch the PWM frequency to something lower, which gives you a lower pitched sound.

    Mine makes this noise all the way up to 60Hz, and it didn't do that last week.
     
  14. strantor

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    UPDATE #2:
    Status: fixed, Cause: unknown

    The tech on the other shift called Rockwell Tech Support and they walked him through some parameter changes, most of which he didn't remember. What he did remember is that he did an autotune and changed to sensorless vector mode. I suspect that what fixed it was the autotune and that changing to sensorless vector wasn't necessary or desired. This application requires fine tuned speed control, and by changing to sensorless vector, I fear it will now be subject to speed change when the load changes, which is not good. However, I have been instructed to "not fix it it if it aint broke" so I guess we'll have to wait and find out the hard way if I'm wrong.
     
  15. GetDeviceInfo

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    one of the first things I did in my current project, besides backing up all the PLC listings, was to save all the drive parameters (DriveExecutive over networked PowerFlex). I also implemented a document procedure whereby anyone who makes any changes, documents those changes.

    I would look further into your encoder wiring, paying attention to terminations, intact shielding, and the possibility that is paralleling a noisy powerline. I had an electrician one time who inadvertantly pulled one phase of a 3 phase line through a seperate panduit, creating a ton of grief until I asked him to show me exactly how he had did that job.
     
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