Re: Primary coil idea

["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: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Hollmike-at-aol-dot-com>
> 
> Hi Pete,
>     What you are saying would be workable, but you must keep a couple of
> things in mind.  First, the coupling between the coils needs to be
> loose(around 0.2 for the coupling coefficient).  What you are suggesting
> could easily lead to overcoupling which will cause two frequencies to be
> generated on the secondary(which are equally displaced on either side of the
> actual resonant frequency of the coil).  The problem with this is that
> constructive/destructive intereference will occur and cause spots of high
> voltage to appear at locations below the top of the coil.  I think the hot
> spots will actually travel up and down the secondary at a rate equal to the
> difference between the two frequencies assuming it doesn't burn up the
> windings on the first pass.  This is why most people simply use a flat spiral
> primary.

Again this story of "overcoupling"... A Tesla transformer -always-
operate
with two resonance frequencies, really one above and another below the
resonance frequency of the separate primary and secondary circuits.
When the primary and secondary circuits are coupled, originally tuned
to the same frequency, two resonances at different frequencies appear,
for any degree of coupling. This is what causes the beating waveforms
seen
at the primary and secondary circuits, and the transfer of energy from
one circuit to the other. Voltage peaks at other points than the top of
the secondary can appear if the primary circuit is tuned to one of the
other resonances of the secondary (odd multiples, approximately) instead 
of to its main resonance. Transmission line effects can also produce
small voltage irregularities along the secondary, but it's not clear if
they are important in the generation of stresses in the secondary coil.
Tesla coils can theoretically be operated with coupling coefficients as 
high as 0.6 (or even higher), but this really results in serious 
insulation problems. Lower coupling coefficients are a compromise that
allows easier construction.

>     Another problem to consider is that if you use a small clearance between
> the primary and secondary, is that you will have to provide adequate
> insulation between the coils to prevent flashovers.  This can pose quite a
> challenge.  Again,  this is a good reason to stick with a flat spiral
primary.

This is the real problem with high coupling coefficients.

Antonio Carlos M. de Queiroz