Axial movement of large synch motor

[eugen55]

I have a problem. I have a large motor, horizontal,(syncroneous)driven by an LCI. After one or two hours at high speed the rotor starts to slowly move back and forth. If I lower the speed the oscilations increases up to aprox 1HZ. What would be the axial forces doing that. I do not think that the drive is at fault. After cooling down, all is allright, for 1-2 hrs. There are no thrust bearings.
The motor just came from the shop rebuild.
There are to stator windings (two motors) 2000HP

Thank you,
Eugen

 

[electricpete]

Is this in the coupled or uncoupled condition?
What kind of coupling?

There are to stator windings (two motors) 2000HP

I don’t know what that means. What is the relevance of a 2nd motor... it has the same problem or doens’t have the problem or not running or... what are we supposed to do with the 2nd motor information.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

And what does this motor drive?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

As long as the shaft shoulders do not touch the bearing faces and the coupling allows the axial float, it should not be a problem.

Muthu

http://www.edison.co.in

 

[eugen55]

Thank you very much for the replay.
@electripete
There are two motors on the same shaft, 30 degrees shifted. The LCI is driving both of them. Only one exitation
It drives a gear box.

@edison123
I do not hear any noise or banging. The motor runs smooth.
Could be a gap, pole, alignment problem?

 

[edison123]

More likely leveling / axial magnetic centering issue or even an excessive axial thrust by one side centrifugal fan. I have seen one case where the centrifugal fan forced the rotor axially away from its mag center but since the bearings had a longer axial play, it was not an issue.

Muthu

http://www.edison.co.in

 

[electricpete]

We have some large horizontal sleeve bearing motors that drive centrifugal pumps through a shim pack coupling. We see 1 hz axial oscillation up to 0.125 hz which occurs ONLY when the pump is at low flow. The pump has something like 0.010” clearance within it’s dual thrust bearing. Obviously the coupling is flexing. We believe there is broadband excitation from the pump at low flow that excites the natural frequency of the motor mass moving on the springy coupling.

If you have thrust bearings in your gearbox and the motor shaft movement is more than than the small clearance of those bearings, , then you also must be experiencing flexing of your coupling (regardless of the origin of the force). If it’s a shim pack coupling, and the oscillation occurs continously, you might think about fatigue (our oscillation only occurs during low flow condition that we don’t operate in except during plant startup/shutdown). That’s not a big deal, just make sure the shim packs get inspected once in awhile. If it is a gear coupling and you are seeing motor movement, I would be very concerned about the sliding wear that would be occuring.

By the way... what kind of coupling do you have?


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

By the way if it's a NEMA motor, then you know your endplay... 0.5" for a motor this big. How well centered you are within that is unknown without further checks.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[eugen55]

Thank you sirs for all the usefull inputs.

The problem was the coupling, bit rough and pulling the rotor, in combinaition with a slight missalignment.
Ran the motor not coupled and all was OK.



Thank you very much for all the answers.