On 5/20/22 11:08 AM, pupman.com@xxxxxxxxxxxxxxxxxxx wrote:
> Hi Tedd,
>
> Cardboard tube works fine as a coilform by itself. As long as you bake
> the moisture out, then seal it you're all set. Clear polyurethane
> varnish from the hardware store works fine and requires no heat curing
> or special processing to use. The amount of water paper can retain is
> rather surprising. There was a capacitor factory here in Chicago that
> you could smell from down the block. It was an odor similar to cheap
> paper towels or wet paper bags and was just moisture being drawn from
> the capacitor elements in vacuum chambers. Nothing went in soggy- it
> was just moisture from the air they were removing from kraft paper and
> cardboard insulation. Paper dielectrics were dried for days and there
> was even one company that dried sections for something like 2 weeks
> (nobody knows why) before impregnation. The cans would be soldered
> shut under oil- that is they actually put large soldering irons into
> the oil tanks and soldered under oil.
I can guess why the two weeks - some "approved procedure" for a large
customer (i.e. US Government, Phone company, or similar) said "Capacitor
sections shall be baked out at X degrees and no more than Y inHg for no
less than Z hours".
We do that for spaceflight stuff, because gunk outgasses and then
deposits elsewhere contaminating either the vacuum chamber or your
optical equipment. Silicones are particularly troublesome - they're
really sticky. But we do it for almost everything - coax cables, for
instance, especially anything that might have "wicked" solvent or
something. Â A bakeout of several days at 60C at <1E-4 Torr would be
typical.
As you note, paper is remarkably absorbent. And water vapor is quite
"sticky" - if you're doing HV work, it doesn't take much water to
greatly reduce the insulating capacity of the oil - in a big
transformer, they'll have dessicant in the system, and it runs warm,
which helps boil out the water (because sometimes, you can't "seal" the
system - the changes in air pressure would make the enclosure buckle).
>
> Transformer manufacturers can bake parts at higher temperatures and
> bake stuff for less than a day, even if it contains paper. Final
> curing takes place in an oven again. These temperatues exceed what
> most plastic parts or any dielectric in a capacitor can withstand.
Exactly - baking out polyethylene at 70C would probably be a mistake.
>
> As for polyester resin as a choice, it sounds excellent. They even use
> it in reconstituted mica capacitors, which are still by far the most
> rugged, highest performing high voltage capacitors available.
>
You might look back in the archives - I believe Electrum, built by Greg
Leyh, used form created by wrapping a cardboard tube with
fiberglass/resin composite. Then pressure washed the cardboard out. I
can't remember if the windings were embedded in the composite, too.
https://en.wikipedia.org/wiki/Electrum_(sculpture)
http://www.lod.org/gallery/electrum/electrum.html - If you go to the
pictures page you can see the secondary being wound.
I think Greg posted some details on how he built it on the list.
>
>
> On Thu, 19 May 2022, Tedd Dillard wrote:
>
>> Is there any reason besides difficulty to use a cardboard tube or other
>> material as an arbor and use fiberglass fabric and epoxy Resin to make a
>> form?