Re: Toroid Modeling

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi John, All,

See below:

Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<FutureT-at-aol-dot-com>
>
> In a message dated 8/27/01 2:18:06 AM Eastern Daylight Time, tesla-at-pupman-dot-com
> writes:
>
> > Using a 3" x 15" (5:1 ratio secondary coil), I ran sims on different
> >  toroid sizes starting with a minor diameter of 1" and
> >  increasing up from there. As you know, when the toroid is placed upon
> >  the secondary, the top load capacitance varies
> >  with proximity to the secondary or walls and objects. It appears,
> >  regardless of minor diameter, as the major diameter is
> >  increased (increased proximity to the secondary) the top load
> >  capacitance increases until the top load capacitance equals
> >  the "free-space" capacitance of the toroid, then the top load
> >  capacitance begins to decrease.
>
> Bart,
>
> Normally, as a toroid is made much larger, the secondary will
> be made somewhat taller to match, the power input will be
> increased, and the room also has to be larger to accommodate
> the longer sparks.  Does your simulation take these factors into
> consideration?  Also, why does the capacitance decrease after
> a certain size in your simulations?  Is it because the toroid is
> close to the floor or walls, etc.?  If so, it seems your results
> would change if the room was a different size?  I'd appreciate
> it if you could go into a little more detail about your simulations
> to address these issues.
>
> John Freau

This is a model of a 3 x 15 inch coil using 28awg wire (Ls = 14.53mH). The
secondary is modeled by itself (1" off the ground). Ceiling height is 108"
and wall
distance is 72". There is no doubt that with increased toroid size, the
wall and
ceiling distance do affect the output parameters. However, I don't think
this is
what is causing the capacitance decrease beyond the free-spaced toroid value. I
don't know what's causing it. It may be an E-Tesla6 error (but I doubt it). The
pattern repeats itself for any given secondary and toroid configuration. I
suspect
the removed proximity to the secondary and increased proximity with the ground
plane may be the cause.

What I showed in the graphs are just examples for a specific coil with
increased
toroid parameters. I'm currently working on a 4 x 20" model and the same
affect is
occuring. The method used is to model the coil in E-Telsa6 for each toroid
diameter
increase (changing only the toroid diameter). On each sim, the frequency is
logged
into a speadsheet against the expected frequency. The expected frequency is the
frequency using the same toroid size and free-space value for the toroid. I
then
remove a percent of the toroid capacitance until the expected frequency matches
E-Tesla6. This is just a method to determine how much of the top capacitance is
being seen. If this sounds like a long process, your right. It takes a lot
of time
to log the data.

In reality, the toroid is probably not changing as much as is shown and the
secondary itself is affected by the toroid change to some degree probably
both in
capacitance and inductance. However, for all practical purposes, lumping
this into
a change in top capacitance is simply a method to show the change in frequency
regardless of where the actual capacitance and inductance distributions are
located
(something we've been doing for a long time probably just because it's
easier to
deal with). Finding the actual distribution of these changes is best left
to TSSP.

When I modeled my coil in E-Tesla6 against measured frequency's with changes in
toroid sizes, I was just amazed at the accuracy. Wouldn't it be nice to
model each
and every possible coil and toroid size? Nice, but impossible. If I change the
above coil simply in wire size or turns, I change Ls and Cs. The same toroid
modeling would show different numbers, however, if I continued increasing
toroid
diameter, I would see the same pattern.

At some undetermined diameter, the coil will see the full free-space value
of the
toroid as indicated "when E-Tesla6 frequency matches that of what you would
expect
using the full capacitance value of the toroid". What I didn't expect was
the turn
or decreased capacitance as the toroid diameter grew beyond that point.
Again, I
believe this has more to do with the ground plane than with the walls.

Tell you what, tonight I'll double the wall distance on one of the previously
determined graphs and see what affect this has.

Take care,
Bart