Electromagnetic vibration such as RBPF with sidebands is strongly affected by the space-harmonic fluxes created by the combination of rotor slotting and stator slotting. If I read between the lines, it may be that the combination of of rotor bars and stator slots is improper and causes the vibration. (In that case it applies to all the sister motors).

In that case, the shop might be looking to change the number of stator slots (requires a a new core). Other options might include stiffening the stator against the modeshapes of interest, changing the fractional pitch used on the winding of existing core, or changing the rotor. Also checking for eccentricity, since that can shift the mode number of the space harmonics.

There was some discussion of a few "thumbrules" about rotor and stator slots here:
http://maintenanceforums.com/eve/forums/a/tpc/f/7161085912/m/8621030173

You have 180 rotor bars and 216 stator slots, 24 Poles
N1 = 216
N2 = 180
N1 - N2 = 36
P = 24

It looks to me like you meet all the "thumbrules", but that is not a guarantee that the design will be free of unusual electromagnetic noise/vibration.

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

In real life (#rotor_bars * RPM + 2xLF) has the highest magnitude as oppose to what is always shown in the text books ( for instance, Vibration Analysis 1, by TA p.2-41, 1993) where the highest peak is at RBPF=(# rotor_bars * RPM ) is surrounded by multiple 2xLF SBs.

(#rotor_bars * RPM + 2xLF), as oppose to RBPF=(# rotor_bars * RPM ), has also standing out harmonics magnitude wise. And this is puzzling, as Pete has mentioned.

Jason,
I think it will be interesting to take a look at the TWF. It will allow to see modulation pattern. Vibration in the middle of the stator has the highest amplitudes, which is expected, and therefore will be the best candidate. If possible please measure the frequency from the TWF.

Thanks,
David

Jason,

The data indicates that the rotor is not at fault, and the fault (if any) is associated with the stator. I suggest that you find (vibration or sound measurements and impact test) any component on or attached to the motor case that may be resonant. Verify air gap dimension and uniformity before rotor is removed. When rotor is removed inpsect for any high spot on the stator (along length and around perimeter). Test stator for natural frequency. Simply changing components without having a good reason could be time consuming, expensive, and not alter the vibrations.

Walt

Walt,

The motor shop had performed all the test that you are requesting; sound and impact, air gap (corrected to industry specs), no high spots on stator, high spots on rotor were removed, impact testing all around the stator core and iron.
Replacing the core is about the only thing that hasn't been done. Even though the core test all look really good with no evidence of degradation.

I wanted to point out one thing of interest (not much practical significance, just interesting). The fact that RBPF + 2LF is the highest (compared to RBPF and RBPF - 2LF) is expected for this particular slot combination, because the corresponding mode shape has the lowest order

Revisiting my previous post 10 April 2008 10:01 AM
1: cos((R-S-2P)*theta+2*Pi*<RBPF-2LF>*t)
2: cos((R-S)*theta-2*Pi*<RBPF>*t)
3: cos((R-S+2P)*theta+2*Pi*<RBPF+2LF>*t)
where R is number of rotor slots and S is number of stator slots and P is number of pole pairs and theta is the mechanical angle coordinate and 2LF is twice line frequency and RBPF is rotor bar pass frequency.

Plug is R=180, S=216, P = 12 (pole pairs) and look at the "spatial mode number" (not sure if that's the right term... I'm talking about the coefficient of theta)
1 has mode number |180 - 216 - 2*12| = 60
2 has mode number |180 - 216| = 36
3 has mode number |180 - 216 + 2*12| = 12

So the third one (which has frequency RBPF + 2LF) has the lowest mode number. The stator core in general has the lowest stiffness for bending at the lowest mode number and highest vibration is expected at this frequency.