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Chief Flunky
Field Service Engineer
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3,775 Posts
Only the manufacturer can really answer that question properly.

On thing to have it repaired. Another thing to have it tested. And another thing to put in to service….

Like did it get dented and the varnish is scraped?

Or did it fail.?
BS from someone that doesn’t know how to do it/

It depends on the extent of the damage: if it passes standard transformer tests a little epoxy to clean up the damage is all you need.

Rewinding is usually only done on very large or unusual transformers where lead time is an issue. Removing the varnish or epoxy means heating to 550-650 at for several hours to burn it off. You can’t realistically do a small area.
 

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Chief Flunky
Field Service Engineer
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3,775 Posts
Standard tests are Megger, TTR (turns ratio), ohms/inductance or tan delta, There are other more advanced tests but these tests look specifically for insulation and wiring damage. These are the same tests specified in NFPA 70B and NETA MTS. Nothing special except the TTR and ohms/tan delta test equipment costs several thousand and not something most electricians have. The TTR for instance gives you a very accurate turns ratio. Works great on 3 phase. On single phase it would be hard to tell anything except compared to name plate. Or just do a smoke test (run it…pass/fail)!and check temperature. This is one of those situations where it’s often hard to tell.

We have repaired a few when it’s really just 1 or 2 turns and easily accessible even if we had to crimp or TiG the wiring then reinsulate the repairs for moisture protection. If it gets to the point where it’s more than that though you are looking at rewinding. If you feel confident just get a couple of the year off packs of epoxy that comes with the popsicle stick. Mix it up on cardboard and smear a THIN coating on the wiring once the epoxy gets a little thick.
 

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Chief Flunky
Field Service Engineer
Joined
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3,775 Posts
That doesn't look bad at all. The tape is just glass tape. Slice it open with a utility knife and check what the damage is, if anything. It looks like it shouldn't be a problem. Then rewrap tightly with glass tape and coat everything in epoxy. Getting glass tape started can be tricky...if it doesn't stick you can use a piece of vinyl tape (88/33+) or even just tie a knot in it back along where you have good insulation.

You can buy glass tape and epoxy yourself from EIS Sales or even McMaster-Carr. Or if you know of a local transformer shop or even a local motor shop they can probably sell you/give you the stuff very inexpensively since they buy it in bulk. Those bundles are groups of parallel flat wires...it will look like "ribbons". Once you verify that nothing has been seriously damaged then just reinsulate. Even if the insulation is damaged you can take strips of glass tape and lay between them then wrap the whole bundle. I'm probably giving away the shop secrets but once in a while things like this happen and every shop will make minor repairs as long as it doesn't compromise the product.
 
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