Re: Secondary resonant frequency vs. Skin Effect

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi Ck,

Yes, skin affect does apply at 278kHz, but has nothing to do with the human 
body. You were probably standing on the ground when you were took the 
discharge and took the full brundt of a "then" grounded arc. I'm certainly 
glad it wasn't fatal and happy your here.

The skin affect is a term which describes the density of flux linkages in 
the conductor as they are related to conductance "within" the conductor. 
Alternating current expands and callapses and as this occurs at higher 
rates, the flux lines become displaced. It becomes less dense in the center 
and more dense toward the edge or skin of the conductor. This causes a 
reduction in the useful cross-sectional area of the conductor and 
effectively decreases the conductance. In effect, it is as if a smaller 
wire is now being used to carry the same current. The result is I^2R losses 
in the form heat (as with everything). This will cause Q to drop.

The only reason to bother with the "skin effect" is to keep our eyes open 
for AC losses. Should the losses be higher than we want, we increase the 
wire size to ensure we have enough conducting area to carry the current 
(without great losses).

Proximity effect is identical as far as causing the flux linkage 
displacement and associated conductance loss, except that proximity sources 
are external (i.e., typically adjacent wires).

Oh, question:

20"x48" at 278kHz? No way, it can't be done (at the stated 560 turns. Now 
at 260 turns, yes).
Is it possible you meant 78 kHz? This would make sense and quite probable 
with a 560 turn coil which has the geometry you described (with a topload, 
not the self Fres).

Also, as this coil was built 28 years ago, did it use a helical primary or 
a ribbon primary?

Just curious.

Take care,
Bart

Tesla list wrote:

>Original poster: "C. Kollett" <ckollett-at-falconaerosystems-dot-com>
>Hi All,
>
>I am new to the list, but jus  twanted to warn of the low frequency which 
>can be typical of a large coil. I built my first coil 28 years ago. It was 
>20" diameter x 48" and easily produced a 50-60" discharge. The resonant 
>frequency was 278Khz. This is actually a very low frequency and "skin 
>effect" does not apply at these low freqencies. In my opinion it is the Q 
>of the secondary that produces the high voltage rather than a turns ratio. 
>At the time I should have focused on producing the highest Q possible 
>rather than having a large number of turns (apprx 560 per memory). Anyway, 
>I was accidentally hit by the main streamer with only one or two snaps, 
>but it nearly killed me since the discharge went through my body rather 
>than over the surface (as it would if the frequency were much higher).
>I know many of you out there are very experienced.. but for anyone coming 
>into this with limited experience that one must be very careful. These 
>machines can toast you. I'm passing this on for  what its worth .
>
>It's very nice to see there is an enthusiastic group on this subject. I 
>want to build another coill some day. When I built mine back in 75-76 
>there was no internet and I was totally on my own.Fortunately I had a lot 
>of electronic background. Im also happy to share any information on this 
>subject to anyone.
>
>CK
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