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marksl

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Posts: 153
Reply with quote  #1 
Called out Yesterday to quote on replacing noisy bearings in an electric motor driving a Mycom Screw Compressor. The noise sounded more electrical in nature than a dry bearing so I had the data collector in the car and collected some data. The motor is a 2 pole 50Hz but running at 60Hz due to refrigeration requirements. The dominate frequency is 2xLF at 5.3 mm/sec RMS in the vertical direction. There is also a strong tone at 47xTS (4.3g's) surrounded by very low amplitude sidebands of 60Hz and 120Hz respectively which I'm assuming is MB in the horizontal direction. I'm thinking of checking the soft foot as there is no history on this compressor only that it is 2 years old and has been noisy for some time now. Do you think the 2xLF and motor bar frequencies are related, maybe the soft foot creating an eccentric rotor has loosen a rotor bar?

Appreciate your thoughts.


Dave Reynolds

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Reply with quote  #2 
IF the vibration is directional, like higher in horz than vert, you are 100% sure it is 2XLF and not 2XRPM by taking high resolution low frequency data and there are no pole pass sidebands around 12345XRPM, then yes it could be a soft foot

You state 50hz but running 60hz, I assume there is a VFD, does the machine exhibit 5.3mm/sec/rms at 50hz also? The vibes are almost .3 in/sec/pk which is quite high. The speed change could be exciting a resonance, again vibration would be directional if a resonance, also a phase shift from 50-60hz
  Capture.jpgMonitoring live time, using high resolution, you should see the 2XLF decrease as you loosen one motor foot at a time.
Dave

Walt Strong

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Reply with quote  #3 
"Do you think the 2xLF and motor bar frequencies are related, maybe the soft foot creating an eccentric rotor has loosen a rotor bar?"

2xLF is a common modulator for rotor bar and stator slot frequencies. Any soft foot that distorts the shape of the motor frame/stator would not change the shape of the rotor or loosen a rotor bar (my opinion).

How was the "noisy bearing" characterized, by sound or vibrations? Was the sound considered to be low, medium, or high frequency? Do your vibration and possibly sound measurements cover the frequency range? Is the sound actually coming from the bearing(s)?

Is the motor run from 50 Hz power with a VFD to get to 60 Hz speed or is the power 60 Hz running the motor with a 50 Hz nameplate at 60 Hz speed?

No harm is doing a soft foot check to confirm/eliminate it as a source of the noise.

Walt
marksl

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Posts: 153
Reply with quote  #4 
Here is a plot showing the 2xLF with is dominate in the vertical direction. The resolution is 1600 LOR just enough to see that it's not 2xTS. Note I have a marker on the turning speed and harmonics.

2xLF.png

T
he acceleration plot below shows what I believe to be the motor bar frequency in the verticals direction 

Motor Bars.png 

ivibr8

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Posts: 163
Reply with quote  #5 
FWIW-
I had a relatively high 2LF (NOT 2X) on a condensate pump, motor end.
Suspected soft-foot.

Got permission from operators and Plant Mgmt to loosen one foot at a time on the motor.
Clear as day - 2LF dropped dramatically as I loosened one corner of the motor....
took 5 minutes of my time

Fixed confirmed soft-foot next time it was down. Case closed.

Jim P
marksl

Sr. Member
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Posts: 153
Reply with quote  #6 
I just received an email from the electric motor OEM and the 48xTS is motor slots NOT rotor bar frequency. I can't say I've even seen an elevated MS frequency in the past. Anyone like to speculate if the 2XLF and MS frequencies are some how related
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