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Antonio Nobel candidate
Tesla List wrote:
>
> Original Poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
>
> John Freau wrote:
>
> > I've seen racing sparks occur just about anywhere along my secondary
> > coils. Often too-tight coupling causes racing sparks near the
> > bottom of the secondary at around the height of the shallow inverted
> > cone primary. Other times, the racing sparks occur at 20 % from
> > the bottom, 30 %, 50 %, 70 %, 90 %, or anywhere in between. I've
> > also noticed that sometimes, the sparks jump just a short distance
> > of an inch or two, other times, the sparks seem to jump 4" or so.
> > They may be traveling along the surface in a sense. Sometimes the
> > sparks loop out from the secondary, and look like the looping
> > promenenses that loop out from the Sun's surface.
>
> The other discussion about static charges in the surface of the
> secondary gave me another idea: These racing sparks can be charges
> deposited in the surface of the secondary, moving due to the
> coil varying electric field. A varnished secondary coil can store
> significant charge at the surface, capturing charges from the
> ionized cloud that surround an operating coil.
> The small distance to the primary would result in charge spraying
> over the secondary coil by corona, enhancing the effect.
> The combination of a grounded conductive layer (the coil wire) and
> a thin dielectric(the varnish) create a large grounded capacitance.
> These accumulated charges can create areas of relatively conductive
> material at the surface of the varnish, that serve as terminals and
> charge supply for the racing sparks.
> (A similar phenomenon is what makes MOS transistors conduct.
> Charges at the surface of a charged vertical capacitor move when
> excited by a lateral electric field.)
> If this is true, coils that show racing sparks would also
> retain significant charges after turned off. The idea of gluing
> plastic rings spaced along the coil, or blocking corona from the
> primary with well rounded surfaces and added insulation could then
> be effective against "racing sparks". Another, more strange,
> idea would be to make the surface of the varnish slightly
> conductive to impede the slow accumulation of static charges.
>
> Antonio Carlos M. de Queiroz
Antonio,
Congratulations! I think you have perhaps, before anyone else IMHO,
suggested the correct hypothesis for an enhancement mechanism that would
contribute to the 'racing spark' problem on classical TC secondary HV
resonators.
I've personally seen that any area of the HV resonator that is exposed
to fringe discharge or heavy corona seems to develop extremely high
surface potentials that remain as a high electrostatic field stored in
the dielectric coating. This effect gets pretty bazaar, and
unexpectedly high in intensity and perseverance (longevity) sometimes,
perhaps in the form as would be expected from an electret. I've gone
back and re-discharged the same damned local area of a secondary with a
ground wand after a good run several times, each time extracting spark
action from the same area!
While I think most of us have been looking for a simple 'dynamic'
explanation of the 'racing surface sparks' which are a problem to some
of us at one time or another, the idea you pose of the resultant
'static' surface charge condition which occurs simultaneously to any
'dynamic' coil activity sure sounds to me like it could be a _major_
part of the explanation of the real contributor to our common problem!
Because of the rather extremely high instantaneous voltages involved
along the surface of the secondary in any Tesla coil, and more-so in one
with higher K values, the infintesimally small capacitance developed by
the surface coating to the outside world certainly has proven itself
capable of giving an unsuspecting coiler a damned good wallop, myself
included! I say there is REAL and unignorable amounts of energy stored
by this mechanism! This shock hazard which reveals this storage
mechanism can continue to be dangerous even days after a coil run
depending on coil materials and local humidity. The instantaneous energy
storage capability of this dielectric effect must be quite intense!
It seems reasonable to me that while areas of the HV resonator have
developed such dielectric charges as a static DC potential along the
outside of the dielectric coating, the dynamics of coil operation causes
an extreme opposite voltage condition to occur in the conductive
windings below these previously established charges. The result is
simple voltage addition and something has to break down when the
insulation system is overly stressed.
A sudden voltage reversal peak occuring in an area of winding that had
previously charged its outwardsided dielectric coating to some high
local value would certainly and 'suddenly' enhance its surface charge.
The violent surface creepage discharges I have witnessed in several
coils up to a 10 kVA coil system certainly initially start out looking
like a non-breech of the insulation system, and merely the
redistribution/cancellation of surface electrostatic charges.
When first observed there is no puncture damage to the insulation. If
this mode of operation is continued beyond a few to tens of seconds, it
would appear those surface electrostatic charges, working against
dynamically varying voltage risefronts underneath in the copper windings
can subsequently quickly lead to coil destruction through dielectric
puncture and carbonisation.
Comments?
Robert W. Stephens