Re: aluminum primary coil AC resistance

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

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 --
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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
>_
>
>.