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Walt Strong

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Reply with quote  #16 
Dave,

You can't look for resonance near 120-Hz with a piece of wood on a heavy machine structure and a F-max of 80,000 cpm! You would need a 5-lb or much heavier hammer with a medium-soft impact tip (like a hockey puck). An F-max of about 200-Hz (12,000 cpm). I can't comment on your test locations or directions without photos or drawings. I have several impulse hammers with load cells with head weight ranging from 1-lb. to over 10-lb.. I would not conclude there is no resonance there without using a better test method.

Walt
RRS_Dave

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Reply with quote  #17 
Best I had Walt. I don't have test hammers, and probably won't be getting any with just a couple years to go.

I'll take a sledge hammer and wrap in in tape next time I go. Poor men have poor ways.

Dave
RRS_Dave

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Reply with quote  #18 
Not sure what happened to the "quote" when replying to posts.....

EPete, I'm now going to have to have the tech at the station take a photo or two for me (station is 4 hours from home) of the stator just to be sure I'm remembering right, but I recall wondering what the small coils were between the big ones.
Here is the nameplate that was on the side (Third page of attachment in first post):

IMG_0437.jpg 

They don't run this generator much because of the vibration unless absolutely necessary, so the run time will be a small percentage of calendar time.
I blew off the suggestion of shorted coil from experience in motors with it having a cascading effect. The hotter it got the worse it got and the worse it got the hotter it got. As you said, they don't last long. However, I am intrigued by the thought of a DC rotor coil short which could last a long time.

I will do more research on synchronous generators and try to learn more so I can communicate somewhat intelligently 😉 

I will also try and get back up there to beat some more on the structure.

Dave




electricpete

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Reply with quote  #19 

Thanks for the nameplate. Sorry, I only read the first page… didn't realize there was more.  I should have looked at all that to begin with.

The second page depicts the rotor in red, stator in blue.  The little wings at top and bottom of the stator bore depict the ends of diamond coils.  So it is showing a the stator as a typical lap-wound stator with diamond coils and a relatively smooth bore (similar to a large induction motor stator).  There shouldn't be any pole pieces on the stator.  The pole pieces (54) are shown on the rotor as expected - salient pole rotor for slow speed machine.   I'll assume that's the case unless there's a photo of something different.   

So in summary, we have the standard construction for a slow-speed ac synchronous generator, the same as everyone else probably assumed from the beginning. Sorry for creating an unnecessary sidetrack on this.

RRS_Dave

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Reply with quote  #20 
I appreciate your responses Epete , no apologies necessary. I learn from all comments. I expecting a call here shortly from a gentlemen who may have had to deal with same type problem at Duke Energy’s Keowee hydro plant. A fraternity brother who just retired from the Oconee nuke station there said this finding sounded awfully familiar to him, but it’s been a while. We’ll see what if anything he can add.

Thanks again for replies
D
Walt Strong

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Reply with quote  #21 
Dave,

Seriously, for low frequency impacts get a Hockey Puck for an impact pad. I attached an "handle" to it to avoid smashing my fingers!

Note that my previous comments for testing were very general. Some large generators have end windings that can be resonance at 2xLF or at least have damping that changes with restraint tightness and insulation deterioration. Here are a few links for a quick search:

Search: synchronous generator stator vibrations

https://www.bing.com/search?q=synchronous+generator+stator+vibrations&qs=PF&cvid=9d7a73c53cdd44638ee67e1a39492b55&cc=US&setlang=en-US&elv=AY3!uAY7tbNNZGZ2yiGNjfPUWXb6m1cunO9iLgyZSX!*fHIKx62cJBpvumal95ErQt6UJSM1pABf2*Fm*t9aAn1HspzsS6bjuZ9SdR1obG6S&first=10&FORM=PERE

 

Electromagnetic Vibrations of Hydropower Generator Stator Core and Transformer Core at 100 Hz Frequency

From <https://www.scientific.net/SSP.260.278

file:///C:/Users/w_f_s/Downloads/Synchronous%20Generators%20Power%20Generation%20Handbook-Part4%20Chap-31.pdf

http://www.tf.llu.lv/conference/proceedings2015/Papers/063_Griscenko.pdf


Walt

RRS_Dave

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Reply with quote  #22 

Thank you for the links Walt. The Griscenko paper was interesting. It kinda fits in with what a couple gentlemen discussed separately today about the stator. Both agreed it was a stator problem, and after seeing the data they both agreed that the 120Hz should not be seen in that magnitude and in those many places. 

I spoke with a retired gentleman from Canada that three others with Duke and Synergy thought was the smartest man they knew on this type subject, and he gave me some steps to take and narrow down the area.

Even though this is a relatively low head high flow unit, there has been some concern with hydraulic resonance in the runners. This same company had that problem in one of the higher head units at another facility that they were cycling for demand type power. I have not seen any of that type signature (can be # of buckets X RPM and can be 1X RPM)  on this unit, but we're going to test and see whether the water wheel has any play in this particular problem.

Evidently the stator core is made of lamination's (much like a motor) but they are made in segments (probably 6 according to this gentleman) that are varnished, baked, and then assembled so that each layer "overlaps" the segment joints. Westinghouse uses threaded rods spaced around the core (in notches that are cut into the lamination's) that are torqued down, then the nuts welded (hopefully he meant tacked) and the rods welded to the stator frame. If these lamination's are loose, then the 120 Hz. signal will appear, and according to him can cause a rather hard vibration. This is a big problem if left alone just due to the fretting that occurs between the segments which wears the varnish insulation off, and you know what happens next.

His first recommendation was to check all these rods to ensure they were tight. How tight? Look at the book for the rebuild, that should be in there. (Hope they have that)
To see if the vibration that they can feel is indeed electrical or mechanical, run the unit up to speed with no excitation (for a motor we just kill power right?), then add excitation, then add load. Measure vibes at each stage and see when it appears. No excitation with full speed, it vibes then it's in the runners or mechanical in nature, if excitation and load increase brings vibe increase, then you'll find the stator loose.

So I have a plan going forward. They are down for a week starting next week. I'm going to try and get there before then to see what we can determine running wise. If I can't get it to happen, I'll see if they can get someone to start checking clamping rods.

Something he said made me think of a comment the tech made to me. If the stator is loose, the vibration will be the highest in that area, and the tech told me when we were down at the lower bearing that the brake piping vibrated worse between 6 and 9 o'clock (from where we were standing looking toward the operator station).

Its a place to start.

Thanks for the response.
I appreciate all ideas and suggestions.

Dave

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