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Re: Glass cap with polyethylene
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Sunday, September 14, 1997 6:09 PM
To: Tesla List
Subject: Re: Glass cap with polyethylene
>
> From: Alfred A. Skrocki[SMTP:alfred.skrocki-at-cybernetworking-dot-com]
> Sent: Saturday, September 13, 1997 9:02 PM
> To: Tesla List
> Subject: Re: Glass cap with polyethylene
>
> On Saturday, September 13, 1997 10:21 AM DR.RESONANCE
> [SMTP:DR.RESONANCE-at-next-wave-dot-net] wrote;
>
> > To: Rich
> >
> > Using two different dielectrics in a cap is very bad. The
electrostatic
> > fields will not be uniform which could lead to local corona and
hence a
> > breakdown. Suggest using single sheets of plastic and then
series
> > connecting each subsection.
>
> I find one major flaw in your logic, namely the fact that almost
all
> polyethylene caps used for TCs are in fact dual dialectric caps.
The
> polyethylene being one dielectric and the oil being the other. If
> your logic was true then, all oil immersedimmersed polyethylene
caps would
> breakdown . In fact they rarely break down when built to tolerate
the
> voltages used.
>
>
> Sincerely
> Alfred A. Skrocki
>
Alfred,
Perhaps the reason why oil and poly work so well together is that
they have almost the same dielectric constant. This being the case,
the electrostatic fields in this type of capacitor would actually be
quite uniform in practice.
So maybe what Dr. Resonance meant to say was that using two
dielectrics which have very different electrostatic properties can
sometimes lead to local corona and hence breakdown. Then again, maybe
that's NOT what he meant.
I wonder if using oil with a higher dielectric constant (such as the
4 rating in Ralph Down's SUNISON compressor oil capacitor) might
exhibit the effect Dr. Resonance mentions.
Glass has a dielectric constant between 4 and 7, depending on the
type of glass. The really bad thing about the glass is its
dissipation factor, which causes it to get VERY hot in TC use. The
use of poly with glass does not address this dissipation problem, so
the main problem would remain, and if anything, the poly would then
REDUCE the effective dielectric constant. So poly and glass do not
really seem to have any advantage when used together.
But, some OTHER combinations (such as OIL and POLY) are definitely
very useful.
Hope this helps.
Fr. Tom McGahee