Commercial magnet wire is 100% tested for insulation pinholes, and
usually for other defects like blisters, lumps, and minimum
insulation thickness. You will find "reject" magnet wire sold on
eBay as "hobby wire".
The amount of damage sustained by a secondary during a "racing arc"
incident can be very severe. A few years ago at the NorCal
Teslathon, someone had a unique coil (with around an 8" X ~50"
secondary, I think) that had a truly monstrous toroidal topload. It
also had an unusual motor-driven rotating primary, with a fixed
"wheel" as the tap point, to adjust tuning.
During his initial attempt to run the coil, the operator slowly
cranked up the power and simultaneously "adjusted" the tuning. The
spark gap was firing continuously, but the toroid was so large that
there was nothing really happening as far as a discharge, excepr
corona starting to develop everywhere. My perception that the whole
system was on the verge of a massive breakdown. Suddenly, there was
a blinding arc from the top of the secondary down to a point near
the bottom of the secondary. There was a fireworks-like spray of
sparks and glowing molten copper, and then all was silent. If I
remember, there was about a 3/4" diameter spot on the secondary
where multiple turns of the heavy magnet wire were completely
vaporized ..... simply "gone". There was a large smudge of
carbonized insulation and vaporized copper deposited in a zone at
least 6" in diameter around the arc spot. This coil would have
required a lot of work to salvage.
This also points out the need for EVERYONE around a large Tesla coil
to wear eye protection, or to stay back at least 20-30 feet from the
operating coil. We always talk about the danger from HV
transformers, charged capacitor banks and disintegrating rotary
spark gaps, but rarely is a spray of molten metal discussed.
As for the ability of insulation applied over the secondary windings
to prevent or reduce surface arcing or "racing sparkss", I'm
convinced that it can, at least on small secondaries. I overcoat all
my secondaries with a very heavy single layer of clear epoxy resin.
This provides mechanical protection to the windings, and definitely
increases the resistance to insulation breakdown. For instance on a
small desktop coil with a 2" dia X 11" tall secondary, I use a very
tightly coupled helical primary. During initial testing at night,
there was a very obvious "bloom" of corona forming at a single tiny
point about 1/3 of the up from the coil base. Increasing the input
power induced surface arcs from the top of the coil to this point
where the corona was seen. The secondary was then examined under a
stereo microscope, and a tiny bubble (literally microscopic) was
found in the epoxy overcoating. There was no defect in the magnet
wire insulation, just a thin spot in the overcoating. This spot was
repaired, and on retest the corona formation was eliminated, and no
corona or surface arcing at this spot could be induced at any power level.
I epoxy overcoat all of my secondaries, and have never had a
secondary damaged by racing sparks or insulation damage. Early on I
did have a secondary that suffered arcing and breakdown on the
inside of the coil form, but since then I always use one or two
internal baffles and this has completely eliminated this source of failure.
Regards,
Scott Hanson
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Saturday, April 22, 2006 8:37 AM
Subject: Re: Primary coil configuration
Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
Hi Gary,
Interesting. I also think that one can buy "virgin" wire and find
that somewhere there will be pin holes in the insulation. It is a
matter of probability. Seems like if one coats the coil with
polyurythene, these "pin holes" will be covered. If a racing arc
causes a "permanent" problem, seems like one only needs to remove
any carbon tracking and re coat. I have heard of people repairing
their secondary coils this way.
Gerry R.
Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>
(SNIP)
I don't know that it's ever been conclusively demonstrated that any
secondary sealing technique will render it invulnerable to racing
sparks.
Regards, Gary Lau
(SNIP)