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Re: Primary field size
Subject:
Re: Primary field size
Date:
Mon, 24 Mar 1997 09:08:57 -0500 (EST)
From:
msr7-at-po.cwru.edu (Mark S. Rzeszotarski, Ph.D.)
To:
Tesla List <tesla-at-pupman-dot-com>
Hello All,
Kevin M. Conkey said:
>How can you determine the useful field size that a given primary puts
>out?
You can determine it by calculating the mutual inductance
between
the primary and secondary, using Neumann's formula for the line
integration.
Having said that (and having calculated quite a few), you really don't
need
to. Whether you use a flat spiral or an inverted cone geometry, most of
the
coupling will be close to turns near the base of the coil. If you have
a
specific geometry you want to evaluate, send me private E-mail and I can
do
the calcs for you.
>How did someone come up with the aspect ratios of pri/sec? Was it
>strictly through trial and error, or can you figure it out?
The general rule of thumb of using a primary height/diameter
ratio
of between 3:1 and 4:1 for diameters larger than 3 inches or so was
developed empirically by a number of experimenters. Smaller coil
diameters
may necessitate higher ratios, in order to increase the inductance.
I have investigated the secondary using a helical resonator
model
similar to that employed by the Corum brothers, with additional more
realistic component values, and have found similar results. There is
little
to be gained by using H/D ratios in excess of 4:1. I build all of my
conventional coils using a 3.5:1 ratio now.
Most of the primary energy couples to the secondary near the
base of
the coil. ( I have posted graphs of this, available in the archives.)
As a
result, coil height is unimportant from a mutual inductance standpoint.
Resonant rise is what you want to optimize, and the tradeoff between
resonant rise and coil losses (due to A.C. coil resistance and
distributed
capacitance) limits the optimal ratio to around 4:1 or so.
Flames, comments welcomed,
Mark S. Rzeszotarski, Ph.D.