Re: Overcoupling?

To: tesla-at-pupman-dot-com
Subject: Re: Overcoupling?
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Thu, 06 May 1999 08:13:33 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Hello Antonio,

>Original Poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
>> >       The corona area you mention is probably a natural high field
>stress area
>> >that linear wound Tesla coils have about 2/3 up the secondary.
>
>Why?
>
>Frequently I see the term "overcoupling" in this list, but can't see
>what it means. I don't see a reason for increased voltage gradients
>along the secondary just because the coupling coefficient to the
>primary is increased. A primary closer to the secondary would induce
>sparking between the two coils, certainly, but why sparks along the
>secondary? Would not this effect be due to tuning to a higher order
>resonance mode instead?

Here is my idea why:

The primary coil´s magnetic field induces a current into the secondary,
right? Changing the coupling factor also changes the amount of magnetic
field that is effectivly "being seen" by the secondary. As you increase the
coupling, you also increase the "amount of secondary" (couldn´t find any
better words for this) being encased by the magnetic field. This increases
the output voltage of the secondary coil. At one time or the other, you will
hit the maximum voltage the secondary wire can take (meaning from one
winding to the next). If you surpass this coupling factor and go even higher
the wire´s insulation breaks down and you get racing sparks. You can
compensate for this by increasing the secondary loading. This will then
decrease (!) the secondary voltage, yet you will get longer sparks,
because the topload stores more Joules (voltage is within "safe" values,
but C is bigger, hence more Joules). This would also explain to me, why
magnifiers allow a higher coupling coefficient. In a magnifier setup the
secondary coil is much more loaded with Ctopload, Ctertiary (and
topload) and Ltertiary. The current flow in such a setup is increased,
but due to the loading, the voltage does not climb to destructive levels.

The voltage levels allowed in the secondary are dependant on the total
insulation. My testbed coils were all just wound with wire and not
coated. I experienced the racing spark phenomenon many times. My
8" coil was coated (before winding) with several layers of PU varnish,
wound, coated with 2 layers of resin (with sanding in between) and two
final layers of PU varnish. In other words the coating is very thick. With
this coil I can couple pretty high. The primary to secondary distance is
around 1.9". I can raise the primary(!) approximately 4.5" above the
lowest secondary turn and get absolutely no interturn arc overs. At
~5" above, I start getting the phenomenon of racing sparks. Of course
this depends on input power, but for equal input VA, a coated coil will
allow higher coupling than a non coated coil. It has happend to me
twice that I got fierce arcing from primary to secondary (one time the
primary coil slipped and tilted during a run, the other time the
secondary wasn´t centered properly within the primary). I examined
the secondary with a high powered magnifying lens. There was
absolutly NO damage. Although I wouldn´t recommend "testing" a
coil this way, it seems as if proper coil preparation is vital for a coil
to survive such accidents (and it increases "looks").


Coiler greets from germany,
Reinhard