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Re: Oil Cap + Paper
Subject: Re: Oil Cap + Paper
Date: Sun, 04 May 1997 16:31:10 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
To: Tesla List <tesla-at-pupman-dot-com>
References:
1
Tesla List wrote:
>
> Subject: Oil Cap + Paper
> Date: Fri, 2 May 1997 19:49:12 -0400
> From: "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
> To: <tesla-at-pupman-dot-com>
> CC: "Tesla-2" <tesla-2-at-emachine-dot-com>
>
> Fellow Coilers,
>
> A little while ago there was some discussion as to whether or not adding
> paper to a Tesla coil capacitor would be useful. I have just finished a
> few
> experiments along this line, and would like to share with all of you my
> observations.
>
> I just finished building a flat plate capacitor using TWO sheets of .06
> poypropylene between each metal plate. By the way, the poly rectangles
> were 24 inches by 18 inches, and the metal plates 20 inches by 16
> inches.
> Border was 1" along each long side, and 2 inches at one end, the other
> end
> being the end the metal plate stuck out.
>
> To reduce the problem of air bubbles I placed a sheet of paper between
> each
> surface. The resulting sandwich of materials then looked like this:
>
> metal flashing
> paper
> poly
> paper
> poly
> paper
> metal flashing
> paper
> poly
> paper
> poly
> paper
> metal flashing
> ... repeated for entire capacitor.
<SNIP>
> I had several heavy blankets covering the
> little
> capacitor, and I put my head under the blankets to observe what was
> happening INSIDE the capacitor. The only light was that produced by High
> Voltage activity within the capacitor. With NO paper I observed corona
> activity where the bubbles existed. The bubbles would GROW as time
> progressed, and it was evident that a fair amount of heat was generated.
<SNIP>
>
> I hope these experiments are useful to some of you. If I find any
> deterioration of the paper or any other items of note in the future, I
> will
> send a post about it to the Tesla list.
>
> Fr. Tom McGahee
Fr. Tom,
Excellent experiment, Tom!! Development of corona in the air bubbbles is
truly the kiss of death for caps running at HF and HV. Commercial
vendors either design their caps so as to keep stress levels below this
point (inception voltage), or use various vacuum/impregnation techniques
to make sure that the air is removed. Looks like the use of kraft paper
allows more "forgiveness" in the design and is very appropriate for
home-brewed caps. It's a trick the professionals use as well, and your
experiments help shed light on the practical reasons why (which the pros
may not be willing to share). :^)
Especially interesting is the apparent disruption and absorbtion of the
small bubbles into the oil under HF use when using the kraft paper.
While it was known that air (or water) bubbles tended to elongate under
high E-field stress, it wasn't obvious that they can also dissapear(!)
under high frequency excitation. Do you suppose they may they re-appear
later when the "excess" absorbed gas comes out of solution once the HF
energy is removed? VERY interesting stuff! It's also convenient that
kraft paper is very inexpensive and readily available in office supply
stores.
You and Dave Flinn have made me a convert!
The two-ply 120 mil dielectric should let you run up to 15 -18 KV RMS
input. More layers of thinner material would allow you to take it higher
for the same total dielectric thickness. Once you've eliminated the air
bubbles, the cap may be usable at even higher levels. It's now more a
function of other defects which may lurk - sharp edges, deep scratches,
or imbedded defects/impurities in the dielectric material. Going to the
two dielectric layers alone can triple the dielectric strength versus
that of a single equally-thick layer, and more thinner layers are even
better as long as you remove the air. 15 layers of 6 mil poly, for
example, should be able to handle 20+ KV RMS in Tesla Coil service....
as long as you can remove the air.
Safe cappin' to you!
-- Bert --