2.5X Freq. on 600hp Induction Motor

[NRaaum]

I don't know if this is the approriate forum, but I'm having a vibration problem on an older Fairbanks Morse Mill Motor. It is an 8 pole 600hp motor that turns at 888rpm. The dominant vibration on the spectrum is present only in the axial direction and is around .3in/s at 2200cpm. This problem has been gradually increasing over the last year. There is really no other vibration in the spectrum, 2x line freq., RBPF, and PPF do not show up at all so it doesn't appear to be a typical electrical problem. What I'm suspecting is that the motor is being pulled or pushed out of its magnetic center by either the axial thrust bearing or the coupling. My question is could this cause a 2.5x freq. any ideas? Thanks

-Nick

 

[cbarn24050]

A bearing is my first guess.

 

[electricpete]

2.47x? Hmmm. Don't have any ideas. Just some random thoughts (brainstorming).

Might possible be bpfo on a six-ball bearing (are these ball or sleeve bearings?).

Here is one thing I would suggest. Put your spectrum on a log scale (so you can see very tiny peaks) and see if this peak fits into a harmonic series as either a harmonic or a fundamental.

For example you might see 0.5x, 1x, 1.5x, 2x, 2.5x (large), 3x etc. That might indicate some looseness with the 2.5x magnified by resonance.

You might see 2200cpm, 4400cpm, 6600cpm, 8800cpm, which would seem to indicate fundamental frequency of 2200cpm, perhaps consistent with ball bearing problem. Or if this is sleeve bearing, perhaps worse might indicate impacting on the shaft shoulder (should not occur if coupling gap is set to maintain motor near mechanical center).

If no evidence of impacting (no harmonics above or below), then I would go more with your idea possibly magnetic center problem hunting and interacting with a flex coupling.

Or the load may induce some axial movement which through the flex coupling pushes the motor off-center slightly and some kind of interaction grows to large shaft movement. We have such a case one horizontal sleeve bearing motor drives pump through Thomas Shim Pack coupling. At high pump flows everything is fine. At low pump flows you can see the motor shaft move up to 1/8" at fairly low frequency perhaps 2-3hz. Interesting that the motor can move so much when pump thrust bearing only allows approx 10 mils movement. Clearly the movement at the pump is being magnified by an axial resonance of the coupling/system.

In your case with higher frequency perhaps put a strobe on shaft and check for axial movement. While you're at it put the strobe on the coupling.

Perhaps there is some resonance on the end bells. Might be checked with a bump test.

By the way, is there any load attached to this machine? What type? Any belts or gearbox attached to this machine by changce (those can generate strange frequencies).

It might be interesting to check if the vib exists on both ends of the motor. Also is it in phase across the motor. And does the frequency show on the pump or whatever load?

 

[jbartos]

Suggestion: If it is paper mill application, it is possible that there is some axial dynamics caused by the motor load rollers and their load.
Bearings may be worn out
Motor stator winding ends may be less structurally sound.
The electromagnetic field inside the motor housing may be asymmetric.
Etc.

 

[jOmega]

NRaaum:

As we are analysing the case you presented, it would be helpful to eliminate voltage imbalance as a cause of vibration. So, if you have the data ... please advise the phase-to-phase voltage (hopefully at the motor terminations) ....

Also, are the phase currents well balanced.... a clamp-on ammeter could be used to measure the phase currents... and
to post the numbers along with the voltage....

Is this a Horizontal ? or Vertical ? motor

And is it driving a pump ?

Has the motor ever been out for repair... ? if so, what ?

and how is the mechanical load connected to the motor shaft....
[ul][li]direct coupled ?[/li]
[li] belted ?[/li][/ul]

Thanks ..

j

&##937

 

[NRaaum]

Thanks for the replies, The motor is a horizontal motor that is coupled to a coal Mill which drives a large worm gear. The worm gear in turn drives a bull gear, there really is not a possiblity for a 2.47X freq. within this gear drive. Also vibration data is taken throughout the worm gear and on the axial end which suggests that this is not something transmitted from the loading.
The motor also has sleeve bearings so it is not a rolling element fault freq. The peak does not fit in a harmonic, the only other vibration that shows up are some low (.01in/s) peaks at multiples of running speed 5x,6x,7x ect. The motor is directly coupled to the worm gear, and it allows for axial movement. I do not have phase data on the axial vibration, but it is present on both ends of the motor, but more so in the outboard direction. I also do not have phase to phase current or voltage data. The only work done on the motor in the last 2years was lead connection work. There was an EMI test performed last year, but I am very green in that area as I am a Co-op student with a backgorund in ME. The EMI test indicated excessive static charges on the outboard axial end, but I do not know what if any the correlation would be to this problem. I will try to get phase data on the problem for both axial vibration and electrical connections. Thanks for the help.

 

[jbartos]

Suggestion: In more complex motor-load sets, it is more probable to have some questionable impact from the load side rather than from the electrical side.