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Re: Large Transformer Question (fwd)





---------- Forwarded message ----------
Date: Sat, 14 Feb 1998 09:09:50 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Large Transformer Question (fwd)


> 
> 
> ---------- Forwarded message ----------
> Date: Fri, 13 Feb 1998 08:09:37 -0800
> From: Joshua Resnick <seraphim-at-WPI.EDU>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Large Transformer Question (fwd)
> 
> >
> > NEVER NEVER NEVER fire your poly caps "dry". Surface conduction
> > can cause a 15KV spark to snake along several inches of surface
> > even though it might only be capable of 'jumping' a fraction of
> > an inch. You NEED the oil!!!
> 
> Believe me it is a capacitor, a very big capacitor. I know that I need
> the oil but I figured that I should at least test the system briefly to
> make sure I wasn't wasting my time and money potting the cap.  My
> makeshift gap for the cap test was only 3 mm larger than the dielectric
> thickness so I was sure that no severe breakdown would occur. I also did
> not feel that powering up slowly with a variac would cause any sudden ,
> violent startup conditions. I learned alot from this test including the
> method in which I should bus the cap. in order to distribute current
> more evenly.
Oddly enough, two electrodes on an insulating surface (i.e. the insulator
isn't directly interposed in the gap, but is next to it) have a much lower
breakdown voltage than the gap alone (typically 3 times lower). Fr. Tom's
comment about not firing it up without oil is very well taken. I've made
arcs many inches long across a sheet of plexiglas with only 10-15 kV. The
inevitable surface imperfections help the propagation of the spark, as
well.