Has anyone ever seen 2xLF modulated by 1xRPM? If so, how can that physically happen?

I've got a spectrum showing 2xLF with pole pass (PP) sidebands. However, there are also 4 sets of frequencies surrounding 2xLF spaced at 1xRPM. I'm considering two possibilities for this pattern:

First, the peaks spaced at 1xRPM could be 1xRPM sidebands surrounding 2xLF. Again, how can this happen? Will a rotating air gap cause this?

The second explanation is that they are the upper sidebands of 1xRPM harmonics. If they are the sidebands of 1xRPM harmonics, they are higher in amplitude than the adjacent harmonic. The spacing between the sideband and the 1xRPM harmonic is PP.

I've also got PP showing up in the spectrum by itself. 1xLF is 0.34 ips. By comparison, 1xRPM is roughly 0.005 ips - almost non-existent.

I haven't seen 1xRPM sidebands surrounding 2xLF anywhere in the literature I have so I'm hesitant to believe that's what I've got.

I believe the spectral content suggests a motor rotor eccentricity issue and possibly a cracked bar, but I'd also like to understand if/how 2xLF can be modulated by 1xRPM.

Data was collected in the horizontal position at the inboard bearing of the motor. 12000 CPM / 3200 FFT lines / 2 averages.

Thanks -

I believe a math analysis of forces will show that in the case of dynamic eccentricity (rotor moving at 1x due to excess clearance or bow) you will see:

1 – If we calculate the magnetic force applied at a SINGLE point on the stator, the frequency is 2*LF +/-1X

2 – If we calculate the TOTAL vector sum of all the magnetic forces all the way around the stator, the frequency is 1X +/- Fp. (you can sort of see derivation of this in my thread “motor math”).

If you were measuring near the bearing and seeing forces transmitted from rotor to bearing, the vibration you see is related to the total force on rotor, and expect 1X +/- FP.

If you were measuring at the stator frame, the vibration you see may be related to only the local force at that point in the stator and you would expect to see 2*LF +/- 1X.

Therefore I believe you have dynamic eccentricity, and I believe you are measuring on the stator frame, not directly on bearing housing (can you confirm?)

All of the above are my thoughts... never heard anyone else say that before and I never thought of it until I read your question and thought about it for awhile.

I have seen this pattern once. It was on one of our large vertical slow-speed circulating water motors. We measure far out on the stator frame since bearing housing is not accessible. The vibration pattern went away after we refurbished the motor (lower bearing housing as-found fit was loose during refurbishment).

This message has been edited. Last edited by: electricpete,

Attached was the example I mentioned.
Running speed = 325.6 rpm.
Log scale. Magnitudes are fairly low.
Almost every peak in there is identified as either a harmonic of running speed (7x) or a running-speed-harmonic-sideband around 2*LF (2LF-7X).
The closest sidebands were about 15% of the 2*LF magnitude.
I believe these specific motors in our case are particularly sensitive to magnetic pull effects due to 1 - flexible rotor; 2 - series winding group configuraion 3- skewed rotor bars.

This message has been edited. Last edited by: electricpete,

cw14_vib_7_24_00.ppt (76 Kb, 54 downloads)

Yes, the data was collected on the stator frame and not the bearing housing.

We obtained the same results vertically and horizontally. Both collections were done on the stator frame.

I have thought it through some more. It still makes sense that the force at a given location on the stator with dynamic eccentricity present will be 2LF +/-1x. It also still makes sense that the total force in presecne of dynamic eccentricity will be 1X +/- FP.

What doesn't make sense is putting these two facts together. Total force should be a linear combination of the individual forces and shouldn't change the frequency. I have to think about that contradiction for awhile. (any suggestions?)

What about the second explanation asserting that they are the upper PP sidebands of 1xRPM and its harmonics?

They appear to be sidebands of 2xLF, but that could simply be do to the math coincidence. And, I have been using linear scale. If I go to log scale (recommended for this type of problem), it is obvious that 1xRPM and its harmonics are surrounded by PP sidebands. It's just that the upper sidebands (the first upper sideband in particular) are higher in amplitude.

We loosened both feet on one side of the motor and dropped the peak at 2xLF from 0.23 ips to 0.03 ips. The spectral pattern remained the same.

I attached a screen grab of the pattern that has been discussed for those interested. The first one is linear scale. The second is log scale. I limited the frequency range from 4 - 10 kCPM.

The collection was 12 kCPM/3200 lines/2 avgs/50% overlap.

And here's log scale.

Yet another possibility for the pattern:

RPM harmonics

- AND -

RPM sidebands surrounding 2xLF and its harmonics

You can not have sidebands without modulation. Modulation typically comes from loose mechanical fits, such as loose coupling hub or components, bearing fit to shaft, bearing fit to housing. It could also be bad rotor bar, BUT you would have pole pass sidebands, not 1x sidebands. Motor would over heat, draw more current than normal, make a funny noise then go BOOM.
Can you perform a quick lift of the shaft with dial indicator?
It sounds like you have a soft foot and mechanical looseness. The 2xLF is merely responding to some other defect, like the soft foot.