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Re: Insulation of HBT s
>> I sort of have to agree/disagree with this. First of all PE only melts
at
>> temperatures above 100=B0C. If your xformer gets hotter than this, you
wi=
>> ll
>> start cooking the Formvar on the wire.
Well, usual practice is to make the *normal* operating temperature at the
hottest
spot of the transformer something like 75 or even 100 C. That will allow
less
copper to be used than for example if you wanted the transformer to be
much
cooler. (Hottest spot is not at the surface, beware!)
Then there's the need for errors, overload and tolerances. Insulators are
therefore
much overrated. Usual practice might be to use insulators than can take
something
like 160-200 deg. C temperature continuously.
>primary and the core. Look at what real manufaturers use. :)
Usual choises in all kinds of transformers are polyamide foils (like
mylar) and
polyimide foils (like kapton), Mylar can handle about 250 deg. C before
it gets
fractile, (mylar A, specs from DuPont) and some kapton grades can take
much
more.
If the transformer is in oil the good choise is always paper if you can
get good
transformer quality paper. (Definately a hard-to-find product!) There are
also
many special plastic products which have roughened surface or a
sponge-like
surface layer over a mylar or capton film. The idea of this sponge-like
layer is
to get a lot of oil in there. Paper/plastic laminated double films are not
uncommon
either. Multilayer products do exist too.
For hv transformer I'd go for submerging it in oil or vacuum molding (for
smaller
transformers). Insulators like mylar or kapton might be good for
layer-to-layer
insulation (if they just are resistant enough to oil). For
primary-secundary
insulation I'd probably get some heavier products like kapton with a
sponge-like
layer. A few layers of that should do the trick for a tiny pig.
Oil layers are desirable because they are self-healing to some extent.
When
plastic has failed it is done forever. Oil might take a tiny arc and still
be usable
after that. Lot's of those and it will definately fail too due to excess
contamination.
Oh, you might also want to consider low-sulfur transformer oils if using
copper.
Copper tends to react with those sulphur compounds and contaminate the oil
to some extent. With aluminum there is no risk of this.
--
Harri Suomalainen mailto:haba-at-cc.hut.fi
We have phone numbers, why'd we need IP-numbers? - a person in a bus