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Axial 2xLF problem in vertical induction motor

Axial 2xLF problem in vertical induction motor

Axial 2xLF problem in vertical induction motor

(OP)
I’m having a problem with what appears to be a 2xLF axial vibration of the motor shaft in a vertical, 1 hp, 1800 rpm (single phase, 4 pole) induction motor with upper and lower sleeve bearings.  The same motor can be used at either 120v, 60 Hz or 100 v, 50 Hz.   In application, there is a rotating plate attached to the motor shaft above the top bearing and a container body attached to the motor and mounted to a kitchen sink (garbage disposer).  Either mounted to the sink or placed on resilient pads on the floor, the units are noticeably louder running at 50 Hz than at 60 Hz.  If I suspend the units by bungee cords so there is no axial restraint, the noise difference between the units seems much less.

I've made acoustic and vibration measurements (radially with accelerometers, axially with a laser vibrometer) for both 50 and 60 Hz, suspended and on the floor, for both the motor only and the motor with the rotating plate attached.

For both 50 and 60 Hz, the motor alone is noisier than when the rotating plate is added.  For the motor only, on the floor, the 100 Hz component is 10-15 dB higher than anything else in the 50 Hz case but in the 60 Hz case, the 60, 120, 240, and 360 are all very similar in level and are the highest components.  They are about 25 dB less than the peak value in the 50 Hz case.  

I do know that this motor is constructed with 2 more lams in the stator than in the rotor and the endplay is variable due to tolerance stackup.  Any suggestions as to what could cause the differences between the 100v 50 Hz and the 120v 60 Hz cases, what measurements might be useful,  or what I might look at to reduce the noise at 50 Hz would be appreciated.  
 

RE: Axial 2xLF problem in vertical induction motor

Flux-related forces should be roughly the same since volts/hz is the same

I think maybe there are some mechanical resonances at work.... particularly near 100hz.

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