Re: Hot spot

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

Original poster: "Scott Hanson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

Weazle -

This is a bit odd, assuming that your coil has run previously in this
configuration without any similar problems. I don't put much stock in the
"bubble with internal breakdown" explanation. With the "hot spot" only a few
turns up from ground, there isn't much voltage present, ASSUMING that the
bottom of the coil is truly grounded. Also, since the coil has run before at
high power, it's not likely that this is a physically shorted turn or
turn-to-turn arcing.

Are you absolutely sure that the ground was connected, and electrically
solid? If you have a nice thick epoxy coating on the secondary, and you have
a discharge from the primary to the secondary, it will not look like the
bright, hot arc from bare metal to thinly insulated magnet wire. It will
appear somewhat diffuse until the epoxy gets hot enough to start to offgas,
at which time the arc will turn yellowish. There will not be any obvious
black charring of the epoxy, just some roughening of the texture and slight
discoloration, and possibly a really tiny pinhole.

How do I know? This is exactly what happened to my 6" coil long ago when I
foolishly forgot to reattach the earth ground wire to the system. As I
slowly ramped up the variac, the coil ran normally at low power, but was a
bit erratic and the safety gaps were firing abnormally at slightly higher
power. Of course I just kept cranking up the power, looking up at the
streamers, until my attention was drawn to arcing from the primary to the
secondary. That's when I saw that the ground wire was not attached to the
bottom of the secondary.

Is your "hot spot" at the closest point to any feature on your primary,
especially the start of the inner-most turn? Examine the "hot spot"
carefully under a magnifier and see if there is any evidence of heating of
the magnet wire, or if there is just a pinhole burned into the epoxy.
Discoloration of the enamel on two adjacent turns of the magnet wire for a
small distance under the epoxy will be evidence of a turn-to-turn short;
localized discoloration of the epoxy only and a pinhole are evidence of a
primary-to-secondary arc.

Regards,
Scott Hanson
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, January 26, 2003 11:36 AM
Subject: Hot spot


 > Original poster: "J. B. Weazle McCreath by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <weazle-at-hurontel.on.ca>
 >
 >
 > Hello Coilers,
 >
 > 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.
 >
 > Any ideas as to what caused this?  Any suggestions as to measures that
 > I could take to prevent a re-occurance?  The configuration of the coil
 > was exactly the same as I've been using for months with no problems.
 >
 > 73, Weazle, VE3EAR/VE3WZL
 >
 > "Don't take life so seriously, nobody has survived it yet!"
 >
 > Listening: 7,055, 147.030+, and 442.075+
 > http://www.hurontel.on.ca/~weazle
 >
 >