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Results of Paper & Poly & Oil
---------- Forwarded message ----------
Date: Sun, 28 Sep 1997 11:57:22 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Results of Paper & Poly & Oil
I have interspersed some comments within the following post
Fr. Tom McGahee
>
> From: teslaman-at-juno-dot-com[SMTP:teslaman-at-juno-dot-com]
> Sent: Wednesday, September 24, 1997 9:30 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: mixed dielectric cap
>
>
> On Wed, 24 Sep 1997 18:06:45 -0500 Tesla List <tesla-at-pupman-dot-com>
writes:
> >
> >From: Gwyn Zucca[SMTP:gwyn-at-tcbod.demon.co.uk]
> >Sent: Wednesday, September 24, 1997 12:29 PM
> >To: tesla-at-pupman-dot-com
> >Subject: mixed dielectric cap
> >
> >Hello all,
> >
> >I am considering building a couple of high power discharge
capacitors,
> >
> >to a design that I have done some previous experiments with. The
> >design
> >uses 1mm (40 mil) polypropylene sheet as the dielectric with
grease
> >proof cooking paper between the foil and the poly as a wicking
agent.
> >This mixed dielectric construction is of similar form to that of
> >professional build, but the grease proof paper is a substitute for
> >high density capacitor paper.
Personally, I would be somewhat wary of using *grease proof* paper,
as it will most likely NOT give you the oil wicking action you
desire. The paper is normally grease-proofed by adding a waxy
material to the paper. Something like kraft paper is going to be much
more oil absorbant.
> >The experimental caps thus far have been of the
> >flat plate type, and of a sealed, oil immersed form. The proposed
new
> >ones will be 65nF in capacitance, 40Kvdc working, to be used in
series
> >
> >in a symmetrical equi-drive type tank circuit. This covers the
basic
> >detail.
> >
> >What I would like to know is, has any one else constructed caps of
a
> >mixed dielectric type, and what experiences have they had. I think
> >that
> >home made mixed types offer the advantage of better oil
penetration
> >and
> >so help expel trapped air, in which corona tends to form. I also
agree
> >with the view voiced by others on the list that certain oils can
> >enhance
> >dielectric properties.
> >
> >any comments welcome ( useful ones anyway!!! )
> >
> >See ya...
> >
> >Gwyn
> >
> >
> >Gwyn,
>
> It's a small world. I am on vacation this week and my project is
the
> same as yours. I am experimenting with 4 mil building supply type
> polyethylene with masking paper soaked in mineral oil. This paper
can be
> found at building centers also. It's approx 3 mil thick and comes
in
> different size rolls. I am using 12" x 60 yards cut into sections
and
> immersed in oil before assembly with the poly and foil. I believe
the
> paper will more or less assume the dielectric constant of the oil
and be
> helpful keeping an oil supply between poly sections. This is just
what
> the big boys do in there pulse caps. referring to Aerovox
technical
> data/design literature I have.
>
> I agree on the entrapped air as it must be removed. Watching a
test
> capacitor under oil as voltage was brought up to corona onset,
revealed
> the ominous blue glow in areas of small air bubbles and eventual
rupture
> in that same area. The oil I plan on using will be transformer
grade
> mineral oil as it is on hand. Hopefully we will both have success
with
> our venture. More input from others out there on this subject
would be
> appreciated.
>
> Kevin E.
> >
>
>
I recently disassembled a flat plate capacitor that I had built using
both poly and paper under oil. The reason for disassembly was to cut
the poly into smaller sheets to make the capacitor less cumbersome.
The original used poly plates 20" x 16" and was housed in a homemade
plexiglass container with outside dimensions of 24" x 18" x 2.5"
This capacitor had seen extremely heavy duty for a period of several
months, and upon disassembly I was particularly interested in how the
paper had fared. I found that there was a slight discoloration where
the edge of the plates were located. Very slight. In fact, on many of
the papers it was barely discernible. But it was most evident at the
ENDS of the plates, as opposed to the other sides. I attribute this
to the fact that it is here that there is the shortest total distance
between one plate and the next. The lesson here is that I need to
make this distance even greater in the future. The effects of edge
corona appear to be the greatest danger in homemade caps. On one
sheet of paper I noticed a little darker line at the edge where the
plate corona had actually cut through the paper in one spot. Closer
examination revealed that there was a weak spot in the poly at that
exact location due to a slight cut in the poly material's surface.
The paper was very useful in making visible the effects of corona at
the edges of the plates and at poly weak points. In no way did the
paper appear to be degrading the operation of the capacitor, and it
was particularly revealing that there was absolutely no discoloration
of the paper that was directly under the plates. The paper had done
an admirable job of wicking in the oil, and I highly recommend its
use.
I really want to point everyone's attention to the fact that the
greatest corona problem appears to be at the ENDS. I have begun using
a distance here of double that used along the edges. I know we all
want to squeeze the most use out of our poly, but the dominant danger
zones are not so much the short poly separation thickness between
plates, but the END and edge distances around the plates. But
especially at the ENDS.
By the way, I cut the poly down to 10" x 16" and re-assembled the
unit into a Rubbermaid container. Instead of flashing I used Heavy
Duty Reynolds aluminum foil. I folded the 12" width over to 6" and
then folded the end section over again in an attempt to cut down a
little on the sharp edge effect at the end. Foil strips were cut to a
length of 24". I allowed for a distance of 4" from the end of the
foil and the end of the poly, and 2" along either side of the foil.
Because the number of sheets increased, and each sheet has the
"wasted" edge matierial that contributes to HV holdback, but not to
capacitance, the capacitance of the new capacitor is much less than
the original. The original was .01 mfd, and the new one is now .007
mfd. Total poly thickness between plates is 120 mils. Not my ideal
configuration, since I am using two 60 mil sheets of polypropylene
plus paper between plates. I would have preferred splitting this into
two .014 mfd caps and then put them both in series. This would have
been much better, but would have required two containers. Sometimes I
do things that I know are not optimal, simply because I happen to
already have this or that item on hand. That's life, folks!
Hope this helps.
Fr. Tom McGahee