Re: A new idea? ...different geometries in coil construction more efficient?

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Mr Gregory Peters by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>

Some incorrect ideas...

> The ultimate source of voltage rise in a tesla coil is resonant rise.
> However, it is unavoidable that some parasitic conventional turns ratio
> transformation will take place. This is "all bad" as it interferes with
> the free resonating of the coil and therefore reduces output and can aid
> in coil failure.

There is no resonant rise in a capacitor-discharge Tesla coil. The
energy transfer is just by two oscillations at different frequencies
producing beats. The maximum voltage gain can be expressed as:
Voutmax/Vinmax=sqrt(Lsecondary/Lprimary)
That, not by coincidence, would be the turns ratio if the two coils
had the same shape.
 
> Primary shape determines the maximum coupling possible. The coupling for
> a "Conventional" two coil tesla coils needs to be fairly low (0.05-0.3,
> depending on size and construction of coil). Any higher coupling will
> result in the point of critical coupling being exceeded,

There is no such a thing as "critical coupling" in a Tesla coil. There 
is a maximum practical value of k=0.6, that results in the fastest
possible energy transfer. Higher values are possible, but result in 
more oscillation cycles to the complete transfer and, more serious
problem, insulation difficulties.

> whereby the
> source of voltage rise in the coil will become dominated by transformer
> action rather than resonant rise (much less effective), and power input
> will be greatly restricted as the coil "burns up".

Take a look at the waveforms for high k in a simulator, and what you
see are still two oscillations, but they become widely different for
high k. The output voltage starts to look as a high-frequency
oscillation
added over a low-frequency oscillation, and the beats that
appear when k is low don't appear.
The coil will really quite probably "burn up" due to racing sparks if 
k is too high (although it's still not clear why).

> The flatter coils
> such as pancake and cone primaries will generally easily achieve these
> low coupling levels if properly constructed. It therefore makes sense to
> use these forms, as they are less prone to strikes, and although they
> can't couple as well as some other shapes, they provide enough coupling
> for most purposes.

Correct.
 
> It is only in magnifier systems, where the secondary is intentionally
> used mostly as a conventional transformer, that tighter coupling is
> required for maximum power transfer. In these systems, the highest
> possible coupling is desired and a traditional cylindrical shape primary
> is used. Magnifiers really begin to perform at coupling levels over 0.4.

Magnifiers are a trick to avoid insulation problems when k is high.
Their operation is very similar to the operation of a two-coils
system, but with the high-voltage third coil moved away from the
primary coil the coupling can be made higher. The driver transformer
is not used as a conventional transformer. The coupling of the
primary-secondary transformer is made high because the presence of
the third coil reduces the effective k. A magnifier can be exactly
designed for maximum power transfer (assuming low losses) by adding
a capacitance across the secondary:
http://www.coe.ufrj.br/~acmq/tesla/magnifier.html
 
> In summary, I wouldn't bother too much trying to increase the coupling
> of your coil.

The losses decrease with high k due to the faster energy transfer. 
What is frequently mentioned is that too high coupling causes racing 
sparks, and that best performance is obtained just below this condition. 
So, there is experimental evidence that increasing k is a good thing.

Antonio Carlos M. de Queiroz