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bfranco21

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Reply with quote  #1 
 I was wondering if anyone has ever seen motor issues that have sub harmonics that aren't synchronous to running speed. I typically see this in direct coupled VFD motors to pumps. Its started in our plant about a year ago.
Walt Strong

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Reply with quote  #2 
That is a pretty vague question without a specific example identifying the machine and the frequencies of interest. I am not sure what you mean by "sub harmonics that aren't synchronous to running speed". Did you measure the actual shaft speed or simply rely on the VFD frequency? Do the frequencies shift as speed changes? Why are you calling them sub harmonics, if they are not related to shaft speed?

My general starting point would be to identify all possible forcing frequencies (mechanical, electrical, and fluid), and conduct one or more tests with variable speed/load, power on/off, and structural vibration tests. What events took place about a year ago there may or may not correlate with the sub harmonic frequencies? Some events might be major outage, power system changes, new VFD installations, and extreme weather (such as lightning strikes affecting the power system).

Walt
bfranco21

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Reply with quote  #3 
we have the operators put the machine in manual so the VFD stays at one constant speed until we are done taking the reading. the speed reference is at 1540 cpm and the sub harmonics are at 240, 480, etc... this is a new part of our plant that was commissioned 2 years ago now. this is the first reading on this motor that has shown this type of reading. and to my knowledge we have had no major power outages or extreme weather to cause this disturbance. they are below running speed and not a cage frequency or a belt frequency since it is direct couple with a Rexnord coupling. I attached a image to kind of give an idea of what im looking at. hopefully this helps out a little more.

Thanks,
Bryan
 

Attached Images
png motor_101.PNG (33.09 KB, 30 views)

Shurafa

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Reply with quote  #4 
Cage frequency first thing came to my mind but you are rolling this possibility out. How about background vibration?

Are the amplitudes significant?

Regards- Ali M. Al-Shurafa
bfranco21

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Reply with quote  #5 
Yes cage frequency was a though also, but I have the bearing frequencies put in the database and they aren't at these frequencies. there is no background vibration next to this piece of equipment. the amplitudes don't seem to be to significant around .005 IPS
electricpete

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Reply with quote  #6 
Yes, unusual pattern.

Apparently you've seen it in a few locations, all driven by vfd. 

My wild guess is that this is a result of the vfd control strategy.    Perhaps it creates an oscillation in torque at 240 cpm, which is being coupled to radial vibration through asymmetric stator support stiffness (assuming this is a horizontal motor).

Maybe check with a strobe to see if you can detect the shaft wiggling back and forth 4 times per second.  If you see that, would tend to support the theory. But if you don't see it, doesn't disprove the theory, because the inertia may filter out the torque oscillations so they don't result in much torsional movement.
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