evaluating coil and wedge tightness 1

[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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[jbartos]

Suggestion: Visit

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

etc. for more info

 

[rhatcher]

Pete,

I have never seen the situation you have described in a motor that has been in service. Are you certain that the material that you are seeing is from the wedges? How long has the motor been in service since the coils were installed?

My experience is that loose wedges come out...either the coil lifts them up until the rotor hits and they are dragged out or vibration makes them "slide" out of the slot axially (less common).

With respect to the clearance between a coil and the slot walls for individual VPI coils, it should be on the order of thousandths of an inch.

 

[jbartos]

Suggestion: Visit

http://www.lincolnmotors.com

http://www.lincolnmotors.com/whatsnew/VVVF Hoist Motors.pdf

for Lincoln Motors practices covering high quality motor winding production. Namely, they place the motor winding turns side by side, which fills the slot properly and tightly. Then, there is very little wedging required.

 

[electricpete]

Hi Ray
We had the red powder analysed. Largest components are iron (presumably from rust) and Silicon (presumably from wedges... wedges are know to be polyester glass laminate).

Also we noted the pattern of the red powder on wedges matches the pattern of loosest wedges by tap test. It seems there must be some kind of cause/effect relationship at work.

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[edison123]

pete,

I use a simple tap test (wooden or nylon half pound mallet) to check for wedge looseness.If a wedge is loose (hollow sound) in between two adjacent tight wedges, I will ignore it. If two consequtive wedges are loose, then I would retighten them by adding liners. Normally, wedge looseness occurs in the first 1-3 years of winding (original or rewind). After that, all the settling and shrinkage of insulation is almost complete and I would be surprised if any looseness occurred again.

Now, we use ripple springs (on top under the wedges and on side with conductive porperties) which help in keeping the wedges tight even after coil settling and insulation shrinkage.

However, I have seen some magnetic wedges (used in diesel generators)totally missing when they reach my shop for an overhaul. (May be someone here explain the mystery of vanishing wedges). Since these machines are invariably impregnated (VPI), I do not risk rewdeging them to avoid any coil damages and they are all working fine for the past 7 to 2 years.


jb,

The question relates to form wound coils and not random wound ones.

 

[jplinn]

I have seen red dust on wedges before. In my experience the dust is iron oxide from the wedge abraiding the iron laminations. The only force in the slot that can result in wedge movement is coil force.

Two test I use.

First I perform a tap test. I use a tac hammer. This hammer has a split striking surface that rings when hitting a solid surface. I cut off the back of the hammer and shorten the handle. After shorting the handle install a circular eyelet screw. When using the hammer in a location where droping it would be a diaster, tie it off through the eyelet to your body.

Tap the wedges in continuous motion while moving about 1 inch distance between taps. Listen for tight (ping), hollow (weak ping), and loose (thud). Mark wedges T,H & L. Contstruct an wedge tightness map for trending.

If there is concern that looseness may be damaging the windings. (Wedge damage is less of an imeadite concern) Use a boroscope to examine the coils. Insert the boroscope through the radial cooling vents (either front iron or back iron) on both wedges that tap out tight and loose. Examine the coils at the point where they just enter the the radial vent. If the coil is moving in the slot there should be abrasian on the coil surface and dusting.

Take corrective actions as necessary.