evaluating stator wedge/winding tightness 2

[electricpete]

I am looking for general information on evaluating stator coil and wedge tightness. Particularly information on the wedge tap test.

(By the way I am not interested in links to advertisements for wedge tightness testing devices unless they contain technical info.)

For a 4kv form-wound individual-coil vpi machine (not global vpi), is there any standard that addresses max side clearance of coil within slot?

If we we see red dust from apparent fretting of wedge within the groove, does that imply the coil is loose/moving or just that the wedge is loose. (I heard someone say the wedges don't fret unless they are pushed by loose coils, but I don't believe it).

If we see red dust at the boundary between coil-side and core (visible within air-duct and at coil slot exits), but no sign of damage of the nomex liner, should that be a concern?

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[DougMSOE]

There may be other sites but the only site that I know of is one buried in EPRI.
The tap test is a simple one to do and interpret. I have used either a small hammer, like a tinners hammer or a small center punch, pin punch, etc. Using the "hammer end" of the punch i.e. the pointy end up, hold the punch adout 1 inch (a.k.a. 2.54 cm) above the wedge and drop it. With a little practice you will be able to tell the difference between a good wedge and a loose wedge. Both usually bounce but the sound is very different. If you hold the punch with your fingers and tap lightly on the wedge you can also do this test up-side-down so you don't have to roll the stator over.
Coils should always be tight in the slots regardless of the rating.
Wedges can fret even with the coils tight.
The red dust from the wedge can and will go everywhere.
A test that you may want to consider on machines 4kv and up would be a partial discharge test.
Iris Power Engineering does this very well and I have worked with them.

 

[aolalde]

The comments by DougMSOE are great. I agree with him. I have seen magnetic wedges disappear pulverized due to vibration.

 

[jbartos]

Suggestion: Visit

http://www.adwel.com/PDF/TWP_6.pdf

etc. for more info