I had a resonance suspect at running speed in a feed pump which cannot be turned off to do a bump test (Equipment off), so I had to go with the bump test (Equipment running), I am using CSI 2130 analyzer, I have a confusion about how the negative averaging works, I read about it, although I am not sure about the results.
I have done the test first with the following settings
Expert analysis: (Bump test – equipment running)
Fmax = 400 HZ
LOR = 1600
Averages = 12 (one hit per average)
Pre trigger = 10 %
Trigger level = 1 G’s
Window = Hanning
The resulting spectrum was very similar to the normal spectrum before negative averaging but lower in amplitude.
I thought maybe I didn't leave it the enough time to settle in the second step (live spectrum) before I press (Stop) button,
So I repeated the test again twice; one with the default setting in the analyzer, I left it for long time before I pressed stop, the result was a flat spectrum (Just some noise - see attached)
Second, I used a high resolution (Fmax = 200 HZ, LOR = 3200), I tried this high resolution to seperate between natural frequency (If exist close to 1X), the result was a similar spectrum as before but 1X almost disappear, leaving little peaks @ 2X, 3X.
Could anyone please tell me what wrong I am doing and the validity of the tests done.
- How long should I wait in the second step (Live spectrum) before I press stop?
- What if the natural frequency was exactly the same as 1X, and I waited for a long time taking too many averages, wouldn't it cancel each other leaving a flat line? the "valley" shape in the peak as described in CSI manual, should it always appear if I have resonance @ running speed?
- In the last trial, If I don't have a natural @ running speed so that 1X completely gone, why the spectrum still showing 2X & 3X after negative averaging?
Please refer to the attached file, spectrum photos.
Thanks in advance, appreciate your help.