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Re: Results of Paper & Poly & Oil (fwd)



---------- Forwarded message ----------
Date: Tue, 30 Sep 1997 15:08:17 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Results of Paper & Poly & Oil 



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Results of Paper & Poly & Oil 
> Date: Monday, September 29, 1997 11:36 PM
> 
> ---------- Forwarded message ----------
> Date: Mon, 29 Sep 1997 07:08:58 -0700
> From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Results of Paper & Poly & Oil
> 
> <SNIP>
> > "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
> 
> Fr. Tom,
> 
> Great post! I fully agree with your comments on using an absorbent
paper
> - Kraft paper is very inexpensive and its what the professionals
use.
> Especially interesting is the discoloration where the paper has
been
> physically altered by the action of the corona. This effect is also
> observed in high voltage oil insulated cables used in power
> distribution. Are your sheets of kraft paper the same size as your
poly
> sheets, and did you see any signs of tracking on the ends? What
voltage
> levels were you running on this unit?

Used it with transformers ranging from 6KV to 15KV, but it was doing
daily duty at 12KV.
> 
> Your results certainly add support to the idea of making the caps
from
> identical series sections to reduce the voltage stresses seen by
each
> section. Another benefit, however, is that since each cap sees less
> voltage, more dielectric area could be used on each, making for
> significantly higher volumetric efficiency. 

Yes, there are actually TWO really good reasons to go that route.
Firstly, you get increase the size of the plate and thus the
capacitance, because you don't have to witstand as high a voltage per
section. Secondly, the voltage breakdown PER MIL thickness is
actually HIGHER when using thinner dielectric sections. It's a
win-win situation.
 
> However, I'm not sure I
> understand why reassembling these as two 0.014 uF caps connected in
> series would require 2 containers. Shouldn't you be able to fit the
> multiple-sections in a single container? The flat caps I'm using
> actually consist of four identical sections, connected in series,
in one
> container. Unfortunately, I didn't use kraft paper when they were
> built.. :^(.   
> 
> Thanks for the info, and safe coilin' to you!
> 
> -- Bert H --

Bert,
You are right, of course. I COULD have crammed the whole assembly
into a single container. In fact, I just might disassemble and
rebuild this cap that way at some future date. The actual reasons
that go into making any given piece of equipment any given way on any
given day are complex and weird. It is not always just reason that
enters in. It can be the time of the day. Or night in my case. 

After I opened the original cap and disassembled it and cut down the
oily poly, it was past midnight. My brain cells do strange things
after midnight following four preceding days in which sleep was
relegated to 4 hours per day. Want to know why I used aluminum foil?
It was the only thing I had enough of at that ungodly hour. I
"borrowed" it from the school cafeteria, because my own personal
supply had run out. Want to know why I didn't immediately make two
series-connected .014 mfd units instead of the single .007 mfd unit
that I actually made? So do I. My brain was not 100% fully
functional. I messed up. I blew it. I started slapping together
double poly sets with paper touching each poly surface, and I was
really trying to make sure that I got the foil sheets aligned just
right.

I became aware that I was building a sub-optimal capacitor somewhere
past 1 am. Desiring greatly to get at least three hours of sleep in
before my alarm clock was set to go off at 5 am, hands dripping with
transformer oil, I stared at my sub-optimal cap assembly for a
minute, shook my head, and then continued the assembly. I got my
desired three hours of sleep. I also got a sub-optimal cap. At least
I have a cap. I am reasonably happy.  And humbled.

Hope this helps.
Fr. Tom McGahee