aluminum primary coil AC resistance

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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

_