Primary AC resistance measurements

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Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>

Hey All:

The thread several weeks ago about the observed heating of inner primary
turns got me to wondering about the actual AC resistance of primary coils
wound with different conductors and what would be the best choice.  I have
access at work to a very nice instrument that can measure an inductor's AC
resistance over any range of frequencies.  So I constructed four
identically dimensioned test primary coils, using these conductors:
1. 1/4" copper refrigeration tubing, the standard for Tesla Coil primaries 
2. #7AWG-equivalent Litz wire, 259 strands of #38AWG enamel-insulated wire,
about 1/4" diameter overall 
3. #10AWG stranded wire, 105 strands of #30 tinned copper 
4. 1/2" wide by .008" thick copper ribbon 
And I plotted the AC resistance from 40 KHz to 800KHz.  

The performance of the Litz wire was marginally better than copper tubing
to poor, depending upon frequency.  And yes, I'm sure that it truly is Litz
wire and that all strands were stripped and tinned together at the ends.
The AC resistance was only slightly lower than 1/4" tubing below 120KHz,
and rose to more than 3 times the resistance of tubing at 800KHz.

The performance of the copper ribbon was quite good considering that the
thickness was only 8 mils, probably less than the skin depth.

And not surprisingly, the performance of the stranded wire was terrible.

The conclusion is that the 1/4" copper tubing that we use is a very good
choice.

Graphs, data, and photos are on my web site at
http://www.laushaus-dot-com/tesla/primary_resistance.htm

Regards, Gary Lau
MA, USA