Re: more on air core chokes (fwd)

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

Original poster: Tesla List Moderator <mod1-at-poodle.pupman-dot-com>



---------- Forwarded message ----------
Date: Wed, 07 Nov 2001 17:11:54 -0800
From: Ed Phillips <evp-at-pacbell-dot-net>
Reply-To: ed-at-alumni.caltech.edu
To: Tesla list <tesla-at-pupman-dot-com>
Subject: Re: more on air core chokes

Tesla list wrote:
> 
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>
> 
> Whipping out NBS Circular 74, I found the following equation (p256) for
> long multiple layer coils.
> 
> L = L0 - 0.01257 * N^2 * a * c/ B * (0.693 +Ba)
> where L0 calculated as below
> n = number of turns
> a =radius of coil center of winding cross section
> b = length of coil
> c = radial depth of winding (i.e. distance from center of first layer to
> center of outer layer)
> Ba is a correction factor depending on b/c and ranges from 0-at-b/c=1, to
> 0.1200-at- b/c=2, to .2292 at b/c=5, to .2844 -at-10, to .3099 -at- 20
> 
> L0 (the single layer coil formula) is calculated by equation 153, (p 252)
> 
> L0 = 0.03948 * a^2 * n^2 / b * K
> 
> where a = radius of coil, b = length of coil, K is a function of 2a/b, and
> tends to 1 for long coils,
> For 2a/b = 1, K=.6884
> 
> dimensions are cm, L is in microhenries
> 
> The example given is of some interest, since it is for an inductor of 12.9
> mH, a convenient value.
> 
> 400 turns, 20 cm diameter form, 10 turns per cm (i.e. 40 cm long coil).
> 
> I note that bare AWG #10 wire is 0.10 inches in diameter, so you couldn't
> wind a single layer coil this tightly.  However, 400 turns * roughly 2 ft
> per turn is 800 ft.
> 
> Iron cores would greatly reduce the size and number of turns required for a
> given inductance.
> 
> I suspect you probably want something like 0.1 H maximum inductance... This
> would take 1200 turns or so, which is quite a pile of wire (2400 ft),
> especially for AWG10.

	If anyone's interested I can provide programs for calculating
inductance, based in the methods of C74.  They are in general good to
better than 0.1%, the usual accuracy with which you know coil
dimensions.  I also have an approximation to K above (Nagaoka's
constant) good to one part in 100,000 or better.

Ed