Failing 2 MW Siemens motor

[PowerfulStuff]

Hi,
I have a 28 year old Siemens motor that appears to be on the way out. Motor vital statistics are-
* 6.6 kV
* Wound rotor
* rod mill load

Over the last few weeks a new rod mill was put in and charged to an aim of 100% motor load. While it is a bit odd to adjust a process based on the capacity of a single motor, that argument has already over and done with. My problem is that the motor has developed a strong noise. The noise can be localised and is of a randomly varying nature. Vibration analysis shows rotor bar frequencies. The theory so far is that, due to the localised nature of the sound, some of the stator laminations are loose and the end result is likely to be a phase to earth fault. I am planning to change out the motor (4 day job due to equipment built over the top) but was wondering if there were any other thoughts or shared experiences out there.

Thanks,
PowerfulStuff

 

[electricpete]

It's a tough call from what little info is there. My two cents fwiw.

It would be interesting to see vibration spectra. What frequencies are dominant? Is the vibration directional? Is there significant axial vibration? What type of bearings (sleeve or anti-friction). What type coupling?

For completeness before you do anything drastic, it would also be worthwhile to do electrical tests on stator and rotor and an in-place inspection. Also a strobe check while running of the coupling and any accessible rotating parts might show something.

Striclty speaking, a wound rotor doesn't have rotor bars, but it does have rotor slots.

For squirrel cage motor (not wound rotor), RBPF pattern vibration (RBPF +/- 2*LF) is often present, often inherent in the design of the motor, often means nothing. Unless the thing you're calling RBPF is increasing, or changing in syncronism with the noise, I would say it is very likely that is means nothing.

So far the varying noise is the thing I hear most worth investigating. Often that is associated with looseness.

Loose stator core can show up as 2*LF, 4*LF, 6*LF, 8*LF etc in vibration spectra.

There can be many other sources of looseness. Many would normally show up as harmonics of running speed: 1x, 2x, 3x, 4x, 5x etc. Sources of looseness can be within bearing, bearing to housing, rotor core loose, rotor core loose on shaft, base looseness, and much more.

It's also possible but less likely that the noise is electromagnetic.

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