Re: Primary Coil INSIDE Secondary Coil

Tesla List wrote:
> Original Poster: Cabbott Sanders <cabbott-at-cyberis-dot-net>
> Tesla List wrote:
> > Original Poster: "Jeff Corr" <corr-at-enid-dot-com>
> >
> > Ok, what about doing it with the primary INSIDE the secondary?
>   Jeff, this is suicide for your coil. think about the magnetic field donut
> coming off your primary.  you will have opposite flowing edges of flux lines
> contradicting eachother in the lowermost turns of your secondary and you
> get inversed potentials all over the place !  this will result in the
> secondary
> coil arcing over to itself, cancelling itself out, and the most fantastic
> you have ever seen!  the inside of the secondary should be treated like a
> sterile laboratory. there is a high density magnetic field in there, so dont
> let anything get inside it....dry it out, seal it up, and keep that primary
> coil down low! ;-)
> my 3.14159 cents,
> Cabbott Sanders


Forget about visualizing those flux lines (unless you've got the special
Flux Line Visualizer Glasses) and think more in terms of two coils,
doesn't matter which one is the outer and which one is the inner.  They
have magnetic coupling through their mutual inductance.  This is an
air-core transformer.  It doesn't matter which coil is employed as the
input coil and which is the output, it works either way through a
reciprocity law.  In fact when a tesla coil is operating there is power
transfer both to and from primary and secondary coils depending on the
moment in time that you look. 

Greg Leyh employed an inside primary with success on his Electrum
Project, but was forced to go this route by a cosmetic requirement. I
would employ one myself *only* if the design did not allow room for a
flat spiral outer primary which I would use first out of preference. 

To obtain the same L as a large flat outer spiral any internally wound
primary will have to be quite long if wound as a solenoid on the inside
of the HV resonator.  This makes achieving proper K values more
difficult to control in the design and can cause additional HV
insulation problems from the upper regions of the primary solenoid to
the adjacent inside of the secondary.  To prevent flashovers here the
primary solenoid must be made with healthy clearance to the secondary
coil.  The the primary gets smaller in diameter to achieve this
clearance it now gets even longer to achieve required L. Employing an
oil barrier here can be used to advantage to deal with this clearance
problem, but the required primary length causes another problem. The top
of an inside primary will also be a good target for inside secondary
coil flashovers from the entire inside length of the secondary coil on
up to the top terminal.  Such flashovers can be very destructive in high
powered coils and may not be easily visible to the operator by virtue of
their being hidden.

Internal primaries can be made to work but I see them only as a very
special application headache.  As for seeing those flux lines, I wanna
buy a pair of those special glasses as soon as they come down in price.
: )

Robert W. Stephens