Good Day
Just a quick question with regards to looking at electrical issues such as slot pass pole pass line frequency 2Xline frequency and other faults. What is the best way to veiw these in the spectrums. Velocity Accelerration or P-P Displacement or a combination? Any help on this would be greatly appreciated.

Lee

My company makes machinery diagnostic software. For motor and generator issues, we use acceleration spectra. If you have spectra in either acceleration, velocity, or displacement, it's easy to convert to another parameter.

Duncan
Thanks Duncan

My preference is to use velocity for low frequency faults e.g. 2LF and use acceleration for high frequency faults e.g. rotor bar.

With respect to the amplitude, it is a good idea to use the log scale.

I'm not sure if there is any advantage of using displacement for normal speed motors (1200-3600 rpm), if we are measuring the casing vibration.

And I could be totally wrong.

quote:

Originally posted by lee:
What is the best way to veiw these in the spectrums. Velocity Accelerration or P-P Displacement or a combination?

Does measurement unit type really matter since all these measures are solidly related ( if no error introduced during integration or differentiation process)?

To enhance viewing ability in cases of wide dynamic range a log scale can be used. I don't think an alarm value can be applied in these cases, with a possible exception of using statistically processed data, but for trending purposes untits don't matter, IMO.

Any other opinions?

Dave

I see slot/bar pass most often at the outboard end of the motor in the vertical direction on the stator frame (not the bearing endbell). On TEFC motors this is the "default" location for measuring the motor outboard bearing since the fan cover (and the fan, of course) keeps you from getting directly at the bearing.

On some motors, this will be a very significant vibration but often it never changes and usually nothing can be done about it. It shows up just as well in acceleration or velocity, but the acceleration level is directly related to how well your sensor is "coupled" to the housing, so it can vary considerably. This can be confusing. Also, if you use acceleration waveforms and peak acceleration level trending to monitor bearing condition, it can throw things out of wack if data is not taken in a very consistent manner.

On motors where bar/slot pass is not a normal vibration, it can be a significant indicator of changes in the rotor/stator air gap. This is especially useful on large motors with plain bearings as excessive wear will often result in changes in the air gap.

Just some thoughts.

Thanks guys
I agree rusty that this is normal in some motors and does not change over time, that being said if you where to call a motor based on a bearing fault lets say. When this motor was pulled and it did show a lot of slot pass and rotor bar and 2XLF would you have the motor shop look into this or just let them do the bearings. The reason I am asking is I think this is what should be done to make sure this is running at peak efficiency when it comes back into service. But I have seen lots of motors go out and come back with these issues. This was at the begining of my time in the predictive field. It seem that the eletrical Dept. may not be aware these issues even exist if PDM technologies are not used. Does the fact that these faults exist in the first place regardless if they change or not mean this motor is not performing as good as one without these type of faults?

Whenever I call out a motor for a bad bearing, I will request that the motor is checked thoroughly for electrical defects if I see evidence in the vibration data. I do not think that it is done very often though.