Hi Dave,
even if I come in a few days late:
> I understand shellac was used for the
> insulation of individual plates in the past- is this still done?
No. Nowadays the laminations most often come from the factory with an
insulating coating. Several types of coatings are available. One of them
is what they call "organic coating". This is simply a varnish, with much
better thermal characteristics than good old shellac. Another is an
oxyde coating, which isn't a perfect insulator, but is extremely thin,
and cheap to create. There are also some other chemical coatings.
So when one winds transformers using new laminations, it's not necessary
to apply additional insulation to the laminations.
And when rewinding an old transformer, most commonly it isn't necessary
either, because those laminations will have some of their insulation
still on it, and enough rust to fill the voids! Basically all the areas
that have lost the coating, will be rusty. That's fine!
The insulation between the laminations doesn't need to be perfect. The
idea is to have a resistance high enough to make the eddy currents
negligible. Since the induced voltage is low, and there are many layers
of insulation, the only requirement is that the coating has a resistance
significantly higher than the metal.
If I had to insulate laminations that for some reason are completely
bare, I would use electrical varnish of the proper thermal grade. These
varnishes exist in air-dry and oven-dry versions. The former dry out in
several hours at room temnperature, while the latter require heating to
fully, but then they are harder, and they dry well even in confined
spaces, such as between the laminations. That allows you to stack the
core together with soft varnish, and let it bake together into one very
solid, hum-free block. But if you ever want to take it apart again,
you'll have to use dynamite.
> It was suggested to me to
> just use AC-43 spray on each plate but, given the plates are all
> "used", I have to wonder if that will be create too much build-up
> between plates & hence a larger core en toto.
That's a problem indeed. Used plates inevitably have some dirt and some
rust on them. If you shellac or otherwise paint them, these
irregularities will become hard, and the laminations will no longer
stack well enough. IF you really want to apply a new insulation, then
FIRST you must get them very smooth. And that's a lot of work, and not
worth the improvement, if any.
> Any information, or direction to a reference on plate
> preparation/assembly would be appreciated.
My suggestion: First assemble the core with the plates as they are.
Check how the transformer performs. If under zero load it gets hot, and
you are sure the turns per volt is enough, then you need to take it
apart again and insulate the laminations. But if it stays only lightly
warm when running under no load, then the insulation is good, and you
don't need any more.
If it hums, you can let some varnish wick in, preferably with the
transformer quite warm.
Manfred.
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|