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OLi

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Reply with quote  #1 
So am I the only one that seen 1xLF on a generator? It was a out of the ordinary naturally. I was working on a site that swapped a stator in nuke size 3000RPM apprx 800T weight and in 2 weeks operation it was full of sawdust from isolation material. I did a heroic shaker test at full load and OEM spent 10 months detuning the endwinding assy resonance. It was like 49.9 Hz and it came and went with excitation and load. You can argue it wasn't electrical but mechanical resonance so ok then maybe so. It has been on my "electrical" list since maybe wrongly so triggering a inspection of the generator if these things pop up or the machine with current problem is a rare case. Adjustments was done on 6 machines and maybe 10-11 similar designs was just lucky. None was so bad as the one we got as replacement that triggered the findings also a pilot machine that was apprx. 50% in size had the problems so it scaled nicely. It has since been running 40 years with only a few replaced winding's due to isolation fail... It will be decommissioned next couple of years. Any other ways to generate that feature? I know, normal is 2xLF but this was not about normal. Bad balancing does not count..... It must at least be electrical related.
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John from PA

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Reply with quote  #2 
Quote:
Originally Posted by OLi
So am I the only one that seen 1xLF on a generator?


I'm not sure that I've ever seen a line frequency or rotational but according to the paper at https://irispower.com/wp-content/uploads/2017/02/Optimize-Stator-Endwinding-Vibration-Monitoring-with-Impact-Testing.pdf it can occur.  Note in particular the paragraph on page 2, the section titled "Background" which reads

Quote:
The dynamic response of stator endwinding bars resulting in vibration can be attributed to two primary forces. The main force is the electromagnetic frequency at twice power frequency (100Hz/120Hz). This is generated from the magnetic fields that are produced between two parallel current-carrying conductors. The electromagnetic forces between two adjacent bars are proportional to the square of the current [2]. Harmonics of this force can occur from the power system currents that excite the stator endwindings resulting in vibration at exact multiples of this fundamental frequency. Another force on a stator endwinding during normal operation is at turning speed: 50/60 Hz for 2-pole machines and 25/30 Hz for 4-pole machines.


I've changed some of the text to bold to indicate that the authors of the paper cite a force on the stator that can occur at "turning speed" which would be the same as line frequency since you cite a 2-pole machine.

OLi

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Reply with quote  #3 
So I am not alone, good to know! Thank you!
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electricpete

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Reply with quote  #4 
I'm pretty sure there are no 1x or 1LF forces acting directly on the endwindings.

I can see two possible sources of 1x vibration of the stator core, which supports the stator coils / endwindings and may transmit vibration to them:
1 - Unbalance / misalignment forces might make their way from bearings to to core via support structures
2 - If rotor is orbitting at 1x within the air gap, there is a rotating unbalanced magnetic pull between the rotor core and stator core.  This is sometimes called dynamic eccentricity.  On induction machines the frequency would show as 1x with pole pass sidebands. On sync machines  the frequency is simply 1x.   I presume Oli's generator is a sync machine. 

How do those two fit into the mechanical / electrical classification? 
#1 is mechanical (*)
#2 is mechanical in origin (*), but certainly a part of the transmission path is magnetic.
* there is a little bit of a grey area in calling rotor movement mechanical.  The motion is a result of all the forces present, some are mechanical and some are electrical. But traditionally we call it mechanical. 

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