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Re:



Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Tesla list wrote:

Original poster: "Gav D" <gdingley@xxxxxxxxx>

Hi all,
I've been writing some MATLAB code to explore the use of current
filament analysis to calculate the inductance of the standard coil
geometries: flat-disk, helical and conical. The results I've been
getting have been interesting when compared to the traditional
formulae used in calculating inductance. Assuming my code works, this
is what I have found:-

Helical: Wheeler works fine as long as the diameter/radius is greater
than about 5, beyond this the error is small but still present. Below
this it can get quite way-off.

Flat-Disk: The code generates values of the right magnitude to the
standard "Wheeler-variation" formula for this coil geometry, but the
figures are quite different.

Conical: Have not looked at this one yet.

I was wondering if anyone else has looked into this, perhaps have a
program by which I can compare my own code? Has anyone else noticed
how inaccurate the standard formulae can be? Perhaps my code is just
wrong.

Thanks,

Gavin

I have used an Excel spread sheet to compare Wheeler's formulae for helical coils (the two in the Radiotron Designer's Handbook - there are more) with the "classical" method using Nagaoka's constant and also with Lundgren's approximation to the same. I just found the sheet comparing Wheeler to Lundgren and will send it to you directly. Bottom line is that by using both the "long" and "short" approximations in RDH you get results good enough for most purposes (surely for TC's) for all coils with length greater than 0.1 diameter.

Ed