Re: Primary Heating

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

Original poster: "Mark Fergerson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <mfergerson1-at-cox-dot-net>

Tesla list wrote:

> Hi All,
> 
> I integrated the Biot Savart equation in the space around a flat
> spiral primary to produce some plots of relative B field strength,
> 
>  http://www.abelian.demon.co.uk/tmp/primary-field1.gif
> 
> for a uniform current in the primary.  The arrows show the direction
> of the field and their length is proportional to the flux density B.
> 
> Notice that the lines are very much longer across the inner few
> turns, compared with the rest of the primary.  A closer look is in
> 
>  http://www.abelian.demon.co.uk/tmp/primary-field2.gif

  Hello Paul:

  Both these images show some asymmetry in the vertical
direction, particularly some kinks (apparent discontinuities) in
the portion of the field below the primary. Is this a program
artifact, or is it real, and due to the solenoidal secondary
influencing the final field shape? If the latter, what will
happen with a flat spiral secondary? I would think it should be
perfectly symmetrical, top and bottom.

  Also of interest is that the field takes one direction at the
outer edge of the coil, goes through a minimum not quite
one-fourth of the width in, and then shows four increasingly
stronger maxima (and increasingly stronger "sub-minima") as you
go inward. Is that (also) an artifact of the program, or does it
indicate actual small-scale field variations? If the latter, what
is the cause? It _looks_ like a transmission-line effect, nodes
and antinodes, but that can't be correct.

> suggesting that the inner-turn field is significantly stronger than
> elsewhere on the primary.  A factor of 5 might be a suitable figure
> for the back of an envelope.  As Bert says, those inner-most few turns
> are getting 25 times the proximity loss of the other turns.  So
> proximity loss could be the culprit.

  I'm finally assembling my magnifier system, and if your
calculations are correct, I'll have to tear it apart and rebuild.
The driver was a pair of flat spirals (with different #'s of
turns and different C's) in the same plane, but as Maggie drivers
are said to do better with tight coupling, I'd like to get as
much of the primary flux into the secondary as possible. That
means the coils will have to be in adjacent planes, with the
secondary _smaller_ than the primary. Since I don't like waste,
I'm considering splitting the secondary into identical windings
above and below the primary to grab both "sides" of the field.
Fortunately, I archived on my HD the previous posts about winding
sense re: connecting multilayer flat primaries.

> But is this high proximity loss sufficient in size to cause the
> observed heating effect?  I don't know how to calculate the proximity
> loss for conductors with radius greater than the skin depth.  The best
> I can do is apply
> 
>  http://thayer.dartmouth.edu/inductor/sfdj.pdf
> 
> which is quite easy and I might put into acmi, but only applies to
> thin wires, not primary tubes.

  I'm not even a newbie when it comes to this kind of
programming, so all I can say is thanks for what you've done so
far, and good luck with any further attempts.

  Mark L. Fergerson