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Mniroshan

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Reply with quote  #1 
We have multistage pump with VFD drive & currently we have increased the frequency from 27Hz to 39Hz. Then overall vibration remained same and motor bearing condition values (g value) increased sharply. This happened only for Motor & for the pump all values are good. Motor rated frequency is 50 Hz. We put some grease to Motor bearings and re checked the vibrations. There was not any change.

Any idea for this? Maximum g value was 6. Machine is medium power unit.
electricpete

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Reply with quote  #2 
1 - "motor bearing condition values (g value) increased sharply"
In general I would suggest check the spectra and time waveform to understand what that g value is trying to tell you.

2 - "we have increased the frequency from 27Hz to 39Hz. Then....motor bearing condition values (g value) increased sharply"
I'd say you can't rule out coincidence (bearing degraded around the time the speed changed) unless / until the machine goes back to 27hz and bearing-related vib goes back down.  Maybe there could be some reasons for bearing related noise to change with speed, but I'm reluctant to go there without more info. 


Mniroshan

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Reply with quote  #3 
Quote:
Originally Posted by electricpete
1 - "motor bearing condition values (g value) increased sharply"
In general I would suggest check the spectra and time waveform to understand what that g value is trying to tell you.

2 - "we have increased the frequency from 27Hz to 39Hz. Then....motor bearing condition values (g value) increased sharply"
I'd say you can't rule out coincidence (bearing degraded around the time the speed changed) unless / until the machine goes back to 27hz and bearing-related vib goes back down.  Maybe there could be some reasons for bearing related noise to change with speed, but I'm reluctant to go there without more info. 



Thanks for the reply
But what do you think about the g value. It is around 4-6
Noknroll

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Reply with quote  #4 
what is the frequency of the g value? is it an overall or specific frequency that is 6 g? 
Mniroshan

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Reply with quote  #5 
Quote:
Originally Posted by Noknroll
what is the frequency of the g value? is it an overall or specific frequency that is 6 g? 


That g value is for 2624.44Hz & shaft speed is 39Hz


Noknroll

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Reply with quote  #6 
Ok, approx 67 X, up around the rotor bar/slot pass frequency range. 
What is the g value at 27 Hz

do you have any spectrum's or waveforms to post
Mniroshan

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Reply with quote  #7 
g value at speed 27 Hz is 0.0077
Mniroshan

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Reply with quote  #8 
Quote:
Originally Posted by Noknroll
Ok, approx 67 X, up around the rotor bar/slot pass frequency range. 
What is the g value at 27 Hz

do you have any spectrum's or waveforms to post



Mniroshan

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Reply with quote  #9 
Please find the files

 
Attached Files
docx g RMS - 27 Hz.docx (30.23 KB, 13 views)
docx g RMS - 39 Hz.docx (29.75 KB, 11 views)

MarkL

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Reply with quote  #10 
First thought would be electrical noise from vfd and that hump is around the electrical fluting 'sweet spot'. Are the side bands at the hump at motors running speed?
Is there a noticeable loud 'whirr' (Vfd noise' from the motor, grease isn't working as I reckon it's electrical.
Can we see the time wave form? Is it very 'tight' closely spaced cycles?

Not sure did you mention the size of the motor? That would also help. What is the history of the machine how long have you been monitoring the machine any waterfall charts of the acceleration fft?


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