Could use some help with a small motor I am trying to troubleshoot.
I have attached a PDF with an overall amplitude trend, a spectrum, and a zoomed in view of the same spectrum that shows the side bands I mention below.
This is a 250 HP motor (ball bearings) driving a small overhung pump (ball bearings) in reduced crude service (600+ F). This is a constant speed machine (3594 RPM) , and the load on the pump/motor never changes as the process condition does not vary.
In the vibration data I have collected, at the motor inboard horizontal location there is a dominant 2x peak (119.8 Hz) that varies in amplitude considerably through out the time I recorded vibration data. It is anywhere from .35 IPS peak all the way up to .65 IPS peak.
When I look at the spectrum, there is a dominant 2x peak with an amplitude of approximately .23 IPS peak... this peak amplitude stays fairly constant throughout the data that I have recorded. There are also side bands spaced at 4.6 Hz around this peak which is confusing to me.
I have checked for bearing frequencies in the motor and also for rotor bar related issues.... neither seem to be the source.
If you look at the time wave form, there are random and non-repeating impacts that occur... they are there and then gone... I may see them in one 4 second time block and then not again for another 8 to 12 seconds. Again.... not repeating and random frequencies... this appears to be where the amplitude variance is coming from but I am unsure of the cause.
I have looked at the cross channel phase data at 2x run speed, but there is nothing to suggest there is misalignment between the pump and motor.
The current balance for this motor was even across the 3 phases and the current draw was fairly consistent.
I am skeptical to call this a potential electrical issue, as the dominant peak is not at 2x FL rather true 2x TS.
Does anyone have any ideas on what may be the cause of this?
Please let me know if you need any more vibe data.