Re: 2 layer primary puzzle

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

Original poster: "Peter Lawrence by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>

James,


>Original poster: "James T by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
<jamest2000-at-att-dot-net>
>
>Hi Everyone,
> I will keep this brief. I am working on an 8" coil with a 2 layer primary.
>I have constructed the primary and
>it is not behaving as I expected. I measured the inductance of the primary
>by itself.
>  What happens is, I measure from the outside of the top coil to the inside
>and the inductance goes up as
>expected. Then I drop down to the bottom layer and measure from the
>top/outside to the bottom/turn, oops, the
>inductance goes down as I go out. The inductance of the bottom coil seems
>to cancel the inductance of the top
>coil. Please set me straight on this.
>1.understanding/myth - the winding direction between the 2 coils is
irrelevant.


the misunderstanding here is that you have _one_ coil in 2 parts rather than
2 coils - from the point of view of inductance etc.

fact: all the turns of _a_ coil should be in the same clockwise or counter-c-w
direction. any turns in the opposite direction cancel that many turns in
the original direction (approx).

myth: with _two_ coils (eg primary and secondary of a TC) they should be
wound in the same direction. reality: it does not matter, and in fact many
of us have proved this by direct experiment.


>2. understanding/myth - the "mutual inductance" of the 2 coils increases
>inductance, not cancels out the
>others inductance.

the formula for inductance is related to N**2, ie a function of the square
of the number of turns, intuitively this means that the total inductance
of a coil goes up not by a constant increment for each new turn, but by
an amount that increases as the number of turns that already exist increases.
So yes, not only does "two coils" (in your terminology, I'ld call this 2-parts
to a single coil) end up with more inductance, in fact it ends up with four
times the inductance rather than twice!



>3. 2 layer primary's are a practical approach. I seem to recall some posts
>on this, and it was considered
>reasonable. Brilliant to ask now that it is done!

sounds like a possible recipe for arc-over between layers and resultant
loss of energy???  But if the layers are far enough apart to prevent this
I wonder if there would be any advantage???  If the inner turns are tied
together (electrically) and the outer turns are connected to the power
supply then the inner turns will be at about ground voltage and will have
less tendency to arc to the secondary coil.... all in all it might be more
compact... any others???


>Any comments will help. Thanks,
> James Cart
>


Let us know how it works once you get the winding directions all consistent, 
we are always interested in the results of experiments!

Peter Lawrence.