Re: Hot spot

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Resonance Research Corp. by way of Terry Fritz <teslalist-at-qwest-dot-net>" <resonance-at-jvlnet-dot-com>

A common problem in epoxy HV transformer design and construction.  There
probably was an air bubble in the drying epoxy.  The air bubble has a
different dielectric constant than the surrounding dielectric material.  As
the unit runs small sparks occur on the interior surface of the bubble.
This causes intense local heating which eventually can lead to hot spots or
final dielectric breakdown.

Commercial xmfr mfgrs overcome this by using vacuum epoxification
techniques.  The windings are shorted out and heated with a primary current
while the epoxy cures which drives out the air bubbles and the vacuum sucks
them away.

Dr. Resonance

Resonance Research Corp.
E11870 Shadylane Rd.
Baraboo,  WI  53913

 >
 > I ran my coil briefly yesterday and had an odd occurance.  After ten or
 > so seconds of run time at very reduced power a hot spot developed about
 > five or six turns up from the bottom of my secondary.  The run wasn't a
 > normal one in that the coil sounded "rough" and the safety gap firings
 > where more frequent, but the streamers seemed their usual length for
 > the power I was running.
 >
 > After shutting down and safing the coil, I examined the secondary and I
 > found an area where the epoxy resin appeared to have been burned off by
 > heat but with no charring.  The exposed turns seem intact save for some
 > of the coating being missing.  There was no indication of an arc-over
 > to the inner primary turn which is about an inch away, nor had I seen
 > such an arc when this hot spot appeared.  The hot spot looked like an
 > orangy-green ball about half an inch in diameter and was quite bright.