Autotransformer wiring

[Zero999]

my autotransformer
here is a 1000w microwave oven transformer the primary winding taps at 120v 220v 240v  . the secondary windings  LT 18v winding. and the deadly HT 1KV  winding - I will cut this off.  my plan is to use it as a step down transformer. add ground tabs and input fuse.   I plan to use a old computer ups power supply case for a box.   tapping the  primary 0v and 240v  as input & also tapping the same primary 0v and 220v as output.  this transformer also can have a US 120v outlet too if I need one. does anyone think the re-use of microwave oven transformers is a good idea? if the earth leakage test is ok.  may just add a 200amp 5v winding for spot welding maybe as a multifunctional unit. ?

Start your own thread, rather than hijacking someone else's.

[SeanB]

Now it is open the wiring is easy. The innermost connection is neutral for 115VAC and 230VAC, the middle one is the 115VAC output and the outer is the 230VAC input.

Smell and well warmed are kind of typical for these, they tend to be left on for decades and almost always have a mild overload on them, thus they run hot and the paper slowly cooks. A coat of transformer varnish ( plain polyurethane varnish and dunk it for an hour, then let air drip dry for 24 hours followed by a bake in an oven at 70C overnight - note do not do this in an oven you ever will want to cook in again, you never will get the smell out, get a cheap small oven on sale and use it) and it will work well again.

[belzrebuth]

Isn't that varnish too flammable for this purpose?
Also ,what about its thermal properties?
I've seen whole power supplies (some old commodore 64 ones) and transformers dipped in epoxy but it said that epoxy (once cured) has thermal dissipation qualities but I don't know about plain varnish..

[SeanB]

The wire insulation is pretty much the same polyurethane, and all transformers are coated in it. The solvent is the flammable part, and once you evaporate it all off ( that is the 24 hour air dry) you can bake it to a hard finish in the oven.

[belzrebuth]

Okay!
Thanks for the clarification.I will try that once I find an oven cheap enough for the job.
 

[Brumby]

Now it is open the wiring is easy. The innermost connection is neutral for 115VAC and 230VAC, the middle one is the 115VAC output and the outer is the 230VAC input.

That's how I would suggest it gets connected - but, please, CHECK the thing.

I seriously feel there is a short.  If there is one, an oven isn't going to help.

[belzrebuth]

I already connected the transformer as suggested above and it works as expected so no shorts.

The short described in my earlier posts and the associated confusion caused by a molten wire knot that acted as a strain relief for the cables leaving the tranformers' windings ends inside the bell of the transformer.

The transformer probably heated to an extreme and the caused the insulation of the big wires to melt so they shorted.

Once I cut the wires leaving the transformers' leads, the windings' resistances read fine.

[Brumby]

AH - Good news.