Re: for Inductance Calculation

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<jimlux-at-earthlink-dot-net>
> 
> >     The formula L=Nr*2/9r+10h........for a single layer air coil.
> >
> > Matt, the 9 and 10 in the denominator equation, are those a constant of
> > some sort??? and of what sort???
> 
> That's the Wheeler formula, and is an approximation for single layer coils
> that accounts for things like end effects, etc.

	It gives reasonably accurate results for solenoids whose length to
diameter ratio is greater than around 3.  That applies to all TC
secondaries  of which I am aware.  For short large-diameter coils the
results can be quite inaccurate.

> > This is the formula I have;  L=N(squared) x permeability x A / l
> >
> > L is inductance in henries
> >
> > N is # of turns
> >
> > Permeability is a constant of 1  (for air cores)
> >
> > A = area of core
> >
> > l = length of coil
> >
> 
> That's the formula for an idealized solenoid, ignoring end effects and
> "inter turn" interactions.

	That formula is NG without multiplication by a constant dependent on
the length to diameter ratio and on the system of units being used
(English? metric?). The constant is often called "Nagaoka's constant";
Nagaoka worked out and published the values early in the 20th century
and his name has stuck to them.  There are analytic expressions
available which give inductance values to within a few parts per million
for fanatics who worship accuracy and precision as I do.  They have no
practical use as the geometry of the coil is never known that well.  In
order to get really precise inductance values it is necessary to include
corrections for the wire thickness and spacing, as well as the
frequency.

	A convenient source of inductance formulae is any edition of the
"Radiotron Designer's Handbook", although they can be found in almost
any good electrical or physics handbook.

	Since we all have to tune our coils to account for extra capacitance or
streamer loading, the main value of doing the inductance calculations is
to make sure there are "enough turns on the primary" and for that you
need to know the resonant frequency of the secondary, which depends on
the capacitance (hard to estimate) as well as the inductance (much
easier to estimate).

Ed