Re: Off-axis tuning coil? (was: I Think I Messed up!)

["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: "Patrick Leonard by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <transactoid-at-rogers-dot-com>
> 
> Someone suggested I use an additional coil in series with my primary to
> increase the inductance. I've only ever seen one or two other coil like this.
> It had maybe only 4 or 5 turns, and used an additional coil of maybe 7 or 8
> turns from which a wire was used to tap the secondary. I think the builder
> called it an off-axis "tuning coil".  One of the coil's on
> <http://www.hvguy-dot-com>www.hvguy-dot-com also has such a "tuning coil".
> 
> How would I calculate the dimensions, or is this something I can just kind of
> guess at? Also, is it wound flat spiral like the primary or cylindrically?
> (that's how the above tuning coil was made).

The effect of the extra inductor (Lx) is to increase (add to) the 
inductance of the primary (L1), without changing the mutual inductance 
(M) with the secondary (L2).

The coupling coefficient (k) is reduced, from:
k=M/sqrt(L1*L2) 
to
k'=M/sqrt((L1+Lx)*L2)

The shape of the extra inductor is not important. Just keep in mind that
it has to carry all the primary current, and sustain a good fraction of
the primary voltage. Just make something easily adjustable, with thick
wire, and as insulated as the primary coil.
 
> Lastly, how does such an addition work? >From what I understand, the flat
turns
> of the primary generate a magnetic field which induces a current flow in the
> secondary. I don't see how a seperate coil, located somewhere else, not
in the
> "plane" of the secondary transfers any energy over?? Am I mis-understanding
> inductance?

The inductance that continues to be part of the transformer transfers
the
energy. The extra inductor, as said above, increases the primary 
inductance and reduces the effective coupling coefficient of the system. 
Energy continues to be transferred through the magnetic coupling, but 
at a slower rate, in more cycles.

Antonio Carlos M. de Queiroz