Hellow everybody:
We replaced one motor because this motor had big vibration at 12 order running speed about half year ago. The vibration decreased clearly after replaced this motor. I think it should come from Electrical fault. So I did cascade test at workshop with our automation people. We try to record the vibration data when we cutoff the motor's power suddenly. Please see attached file for more details.
We have no enough experience at motor electrical fault, We don't know what's the next step , So any comments are welcome.
Thanks a lot.

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

Motor_cascade_test_.zip (1,138 Kb, 38 downloads)

Dear Weng:
I am a little confused. The first motor had high vibration at 12X while running the motor (on VFD) at approximately 58Hz. You are comparing that to the new motor which shows a much smaller vibration at 8X while running the motor at 50Hz. I'm not sure the two vibration patterns are comparable so I don't know what was happening at 12X with the first motor. However, the second motor has something (small) going on at 8X (which is possibly near some sort of rotor-related resonance when 8X coincides with 200Hz). It doesn't appear to be a problem but you will notice that there is a peak vibration at 8X as the rotor slows and the peak does not remain at 200Hz. I do not think this is electrical since (obviously) the power is off but the 8X still exists. Also, 8X (or 12X) is not a usual electrically-related frequency. Sorry, not much help for determining the problem with the first motor but I'll think about it some more.

Regards, H

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

Noticed something further on the second motor (and, again, I don't know if this is comparable to the first motor). Although the amplitudes don't appear to be objectionable, notice that the 2X and 4X behave very similarly to the 8X - particularly 4X. The double peaks at/near 4X are similar (except for magnitude) to the double peaks at/near 8X. I suppose that one peak could be at 4X and the other at twice line frequency (2xf) which is close to but not exactly the same as 4X. This pattern is repeated at 8X and 4xf but at the higher amplitudes - that's fairly odd unless, in this case, 200Hz is near a resonance and being excited by the 8X and 4xf frequencies when they all nearly coincide (8 x 1500 RPM and 4 x 50Hz line frequency). Without other information on the first motor, it would be too much speculation to say that the behaviour of the second motor is similar to the first.

--H

Hi Hokeith:
Thank you for your comments.
I try to compair these two motors at similar speed (plesse see attached file),maybe you can get more clear information for this case.

Motor.zip (497 Kb, 14 downloads)

IMO, the old motor was zero loading when we did the cascade test, and the vibration level should be more high if this motor at full loading.
Can we say the the 4X line frequency and 6X line frequency was disappear after the motor's power was off?
Thank you.

Dear Weng:
I'm sorry. I'll have to retract my earlier suggestions about resonances (for now). This new file raises more questions. See the attached file on which I have included some comments and questions. Moreover, I am confused by the neighboring motor and think that there may be something about the spreader roll that may be significant. The neighboring motor shows much higher (of its own) 1X vibration as measured on the old/new motor than the old/new motor itself! What is the 1X vibration on the neighboring motor itself? Can the 345Hz frequency be detected on the neighboring motor? What is the relationship between the neighboring and the old/new motor? Was the spreader roll cleaned or have any maintenance when the old motor was replaced? Is '12' a significant number with respect to the spreader roll (spokes, gears, belt ratio, etc)? What bearings (bearing numbers may be on the motor nameplates) are in the old/new motor, the neighboring motor and on the spreader roll?

These questions all focus on some mechanical significance at or near 345Hz. I am not expecting that there was/is electrical problem with the old motor.

Sorry for all the questions but these and the ones on the attached file should clear things up enough for a better diagnosis.

Regards, Hokeith

WengMotor.pdf (159 Kb, 19 downloads)

Hi Hokeith:
Thank you for you reply.
The neighboring motor vibration is low, but its coupling is too closeing the motor's basement of we talk about.(maybe i need take a picture next week).
The spreader roll is OK.
Hi Hokeith,I must say that I didn't analyse the plot so carefully like you.I will analyse this case again follow your comments.
Have a good day.