Re: aluminum primary coil AC resistance

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

Original poster: jimmy hynes <chunkyboy86-at-yahoo-dot-com> 

Hi,

I was afraid that the second conductor wouldn't carry any current because 
of proximity effect, but
I didn't think it would be that bad! Does proximity effect alone explain a 
25x increase in
resistance? I thought that since I did use a flat conductor, that it 
wouldn't be that bad.

In one spot, one of the inner turns arced to the second conductor of the 
same turn, but it didn't
happen in the outer turns. The voltage across the whole coil should have 
been only 2kv. Why would
the voltage between the two conductors be so much different at the inside 
turns but not at the
outside turns?

www.hot-streamer-dot-com/chunkyboy86/primaryarc.jpg

Do I just have to make a primary with a large inner diameter to keep the 
flux density down? What
do you think would be the best way to make a primary with very low Rac?

Thanks,
        Jimmy
--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
 >
 > Hi Jimmy,
 >
 > The same thing has been observed during extended runs on high power Spark
 > Gap coils as well. The current thinking is that the primary's own
 > electromagnetic field causes uneven current distribution within the winding
 > - similar to the current bunching due to proximity effect in multi-turn
 > coils. In a spiral TC primary, current tends to be constrained to flow
 > within smaller region of the inward-facing surface instead of being spread
 > out across the entire skin of a round conductor - and the stronger the EM
 > field, the greater this constraining effect.
 >
 > It can be shown that the power losses due to proximity effect scale as the
 > square of the local magnetic field. Current bunching is most pronounced at
 > the innermost turn of a spiral primary since the primary's EM field is
 > strongest there. A simulation done by Paul Nicholson last year showed that
 > the B field near the innermost turn of a typical spiral primary could be
 > the order of five times that of the outermost turn, implying 25X the degree
 > of joule heating as the outermost turn. Because of current bunching, a
 > spiral primary wound with a flat conductor may have significantly lower Rac
 > than a similar coil wound with round/tubular conductor.
 >
 > For more information, see Carter, G. W., "The Electromagnetic Field in its
 > Engineering Aspects", 2nd ed., Longmans, Green, 1967, pages 248 through 260.
 >
 > Best wishes and happy holidays,
 >
 > -- Bert --
 > --
 > --------------------------------------------------------------------
 > We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
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 >
 > Tesla list wrote:
 >
 > >Original poster: jimmy hynes <chunkyboy86-at-yahoo-dot-com>
 > >Hello everyone,
 > >The primary resistance was limiting the amount of power my DRSSTC could
 > >put out, so I built
 > >another primary that was supposed to have a much lower resistance. I built
 > >it out of 6" aluminum
 > >flashing, and used two layers spaced 1/8" apart. Each turn is separated by
 > >1/4" and it has 10
 > >turns. The ID is ~7"
 > >Here are some pictures of my new primary coil
 > >www.hot-streamer-dot-com/chunkyboy86/newprimarytop.jpg
 > >www.hot-streamer-dot-com/chunkyboy86/newprimaryside.jpg
 > >The DC resistance measures about twice as high as I calculated for pure
 > >aluminum, but at 4.9
 > >milliohms, it is small enough. The AC resistance of the primary is 0.25
 > >ohms! The skin depth at
 > >60khz is 0.013", so the 0.01" thick aluminum should have pretty uniform
 > >current distribution. I
 > >thought that it may be due to eddy currents in the two layers, so I tried
 > >using one layer. I got
 > >pretty much the same resistance.
 > >I ran the coil with no secondary, and was putting out 600watts. I then
 > >checked the temperature of
 > >the coil. The inside turn was the hottest, the next one out was not quite
 > >as hot, and so on. The
 > >outer turn was at room temperature.I was thinking that the inside turn
 > >could be shorted, but I
 > >didn't see any short. I can't figure out why the inside turns would be
 > >getting hot while the
 > >outside turns remain cold, can anyone else?
 > >=====
 > >Jimmy
 > >_
 > >
 > >