Question for Those Who Rewind Transformers
 

I am attempting my first transformer rewinding project. Actually I am hoping it will need not be an entire rewinding task. It is a transformer from a EICO 720 transmitter I built from a kit many years ago. It developed a low resistance (200 ohms) between the high-voltage winding and a tap off of the primary winding. (The tap is used to provide 120 VAC to an optional external modulator that I never use because I am an all-CW operator). But the shorted condition needs to be cleared up and I am hoping I can locate a simple carbon track or something to clear the primary-to-HV path.

I have disassembled the "E" and "I" laminations and am about to begin unwinding the first
winding. Every lamination has discoloration and a deposit of (what looks and feels like) varnish and some carbon that must have allowed to get quite hot.
My first question: Should I wire brush the laminations or would I be scouring away something that the manufacturer put on them to protect them?

My next question: Once I have found the problem and reassembled the transformer should I re-varnish the frame composed of all those laminations?
If so, is there any special type of varnish I should be using?

As always, thanks to this forum for its members' wisdom and help.

Re: Question for Those Who Rewind Transformers
 

Hi, quite a few possible problems there.

Lams, they may need to be cleaned in a solvent to remove the varnish. They often have an oxide coating to provide a very light insulation. This should not be scrubbed off, but a solvent will be OK on them.
Without doing this you will not get all the lams back in !

The windings were "stick" wound, you may find it easier to add cheeks for your rewind.

Use double insulated wire (Grade 2) and there is then no need to interleave the layers with tissue. You still need insulation between windings !

You might be lucky and see the short at the end of a winding section and you may be able to dribble some varnish or Araldite in there to cure the problem, but be sure to check with a megger after the insulation has cured.


I suspect you will have to remove all windings to be sure you have fixed the problem, so take careful note of the construction.

You can re-varnish if you wish, you could also impregnate with molten candle (paraffin) wax as an easier option.

When re-stacking the lamination's ensure the E & I,s butt closely together or the no load current of the transformer will be higher.

Note that through bolts in the core should be insulated at each end with nylon or fibre washers, the bolts are sometimes also covered in transformer tape. This is to reduce circulating currents.



Feel free to ask as other issues arise, good luck, Ed

Re: Question for Those Who Rewind Transformers
 

Hi Ed, Thank you so much for the most helpful information! Indeed I will follow your recommendations. May I ask what kind of solvent would you recommend for the laminations? Would you say I should just soak them in a "bath" of the solvent or should I lightly brush or use a cloth? I'd hate to disturb the coating on those laminations done by the manufacturer. Thanks again.

Re: Question for Those Who Rewind Transformers
 

Turps should remove the varnish or white spirits. Useful info here - http://ludens.cl/Electron/trafos/trafos.html on rewinding tfmr's.

Andy.

Re: Question for Those Who Rewind Transformers
 

Hi, petrol and other solvents will also work, brushing or rubbing should not be a problem as the coating on the lams is usually from a thermal oxidation process.

Ed

Re: Question for Those Who Rewind Transformers
 

I'd use petrol (keep in a closed container while it does its stuff, such as a plastic lunch box) or paint stripper. Don't use a wire brush! But a mild scouring pad should be OK.