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Re: Toroid Eddy Current?

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Terry,
          I think of your three possibilities below, the current 
distribution is possibly the one that counts most unless the terminal 
is plonked right on the top of the windings. I can't buy the 
conductivity argument - toroids can and are made to be very 
conductive. It's an interesting one though - the presence of the 
terminal does modify the current distribution. As far as coupling 
goes - consider the difference between a flat spiral positioned a 
couple of inches below the bottom resonator turn (which can easily 
give k of 0.2 to the resonator) and a flat-bottomed "toroid" of the 
type I use positioned a couple of inches above the top of the coil. 



On 14 May 01, at 21:49, Tesla list wrote:

> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
> Hi Ray,
> I think there are three factors at work that diminish the shorted turn
> effect for toroids.
> 1.  The coupling is not real high.  I used MandKV31 (coupling
> calculation program*) to find the coupling for a 22 inch C-C toroid
> with an 8 inch cord that was 10 inches above a 30 inch long 10.25 Dia.
> secondary with 1000 turns.  The coupling was only 0.028.
> *
> http://hot-streamer-dot-com/TeslaCoils/Programs/Programs.htm
> 2.  Toroids are not real good conductors.  Corregated dryer pipe and
> other aluminum materials are faily resistive to RF currents.  Not sure
> how much difference that makes but...
> 3.  There is not much current near the top of the coil.  As Paul's
> TSSP project and my tests have shown.**  Thus, there is not a lot of
> nearby current to couple to the toroid.
> **
> http://hot-streamer-dot-com/TeslaCoils/MyPapers/NSVPI/NVSPI.htm
> http://www.abelian.demon.co.uk/tssp/pn1710/
> I think all three of these have some affect and add up to only a small
> Eddy current effect from the toroid.  The actual numbers and all are a
> bit fuzzy but it could probably all be figured out with a fair amount
> of precision.
> Cheers,
>  Terry
> At 09:58 PM 5/13/2001 -0700, you wrote:
> >Doesn't the toroid form a single shorted turn that would dampen the
> >secondary by lowering the Q? It seems that the toroid is close enough
> >to the secondary to have an effect. An interesting test could be
> >performed if someone had a working coil with an identical spare
> >toroid. That spare toroid could be cut with a band saw radially
> >toward the center stopping couple of inches from the middle. Probably
> >3 or 4 (maybe more) equally spaced cuts would be adequate and maybe
> >dabs of epoxy to stabilize the cuts. Then install the modified
> >toroid, retune the coil and compare the results with the original
> >toroid.
> >
> >Maybe I'm off base but I have never seen this discussed and if the Q
> >of the material for the secondary coil form matters, seems this just
> >might.
> >
> >Ray
> >