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Re: [TSSP] Measured vs. Operating coil coupling?

Original poster: "Terrell W. Fritz by way of Terry Fritz <twftesla-at-uswest-dot-net>" <terrellf-at-qwest-dot-net>

Hi Paul,

At 11:46 AM 5/15/2001 +0100, you wrote:
>I agree with your comments to Ray, but I'll add a few of my own...
>4. With a large toroid, the top current can be a large proportion of
>the base current, eg 70%, so that there can be a sizeable current
>available for induction at the top.

So we could see say 10 amps in the toroid.  Interesting since we do not
often think of the toroid as a current loop.  However, perhaps we should!!

>5. The induced currents in a shorted turn, toroid, or ground sheet
>are not in themselves a bad thing - in fact they can be quite useful
>if carefully managed. Its the I^2R loss in the shorted loop that
>really counts, and whether or not the shorted loop destroys the Q of
>the coil depends on the impedance match between the loop resistance
>and the rest of the system.  For any shorted loop there will be a
>worst possible loop resistance that will have,  for the particular k
>factor involved, the max impact on system Q, and that worst resistance
>will be the matched resistance.  If the loop R is below or above this,
>the impact on Q will be reduced accordingly. Thus a shorted loop which
>has a very low R will only have a tiny effect on Q,  even though the
>induced current can be quite high,  as seen by the apparent reduction
>of the secondary inductance. To demonstrate this qualitatively I've
>generated a set of response curves for a resonating solenoid coupled
>to a shorted turn:
> http://www.abelian.demon.co.uk/tssp/tmp/eddy-coupling.gif
>The green curve is the worst-case scenario - the resonator is well
>matched to the shorted loop resistance of 1K ohms and the Q is very
>low. The blue and red traces show the response with a loop resistance
>of 100K ohms and 10 ohms respectively, and you can see that, when we
>are well away from the 'matched' resistance, the effect of the
>shorted turn on the Q is much reduced. 
>So the moral is - if you're going to have a shorted turn, make it a
>good one, eg the nice wide conductor of the toroid. There is an
>advantage to this ...

What a fascinating set of curves!  It is not a big deal to add an outer
aluminium tape ring to a toroid made from dryer duct to vastly decrease the
resistance of the loop.  Dryer duct toroid tend to have strangely poorer
performance than smooth toroid.  Perhaps this is a truly new concern for
toroid builders!  I also note that 5% frequency shift!!  E-Tesla does not
consider such things affecting resonant frequencies...  I feel some
experiments coming on :-))

>Yes, I've prepared two graphs showing the V/I profiles of Marco's
>Thor system when driven by a CW voltage applied to the (untuned)
>primary terminals, as modeled by tssp:
> http://www.abelian.demon.co.uk/tssp/tmp/tviplot.thor-p.60.gif
> http://www.abelian.demon.co.uk/tssp/tmp/tviplot.thor-p.66030.gif
>for drive at 60Hz and at 66.03kHz respectively. Clearly we can expect
>the k factor at the operating frequency to differ from that measured
>with low frequency currents, although of course the mutual inductance
>is the same in both cases since that is frequency-independent and is
>fixed by the geometry of the coils.
>As you know, tssp's time domain simulator is currently under test, so
>hopefully over the next few months we can turn up some reliable
>statements about how the effective operating k relates to the mutual
>inductance M. Note that the familiar formula k=M/sqrt(LpLs) only
>applies to coupled resonant circuits where the two separate coils are
>tuned to the same frequency, and when the system is driven near to
>that frequency. Away from resonance, or with mis-matched resonators,
>the k is a more complicated expression, so we are on shaky ground if
>we try to talk about k at 60Hz - better to stick with M.
>On the whole, I'd expect a modest Fres shift - say of order 1% down,
>on making a radial cut through the toroid, but no noticeable
>difference in performance. I believe this has been reported by those
>who have taken the trouble to test this.

E-Tesla tends to have a little unaccounted for error in the sub 1% area.
Perhaps this is the key!

>Just to add one final piece of speculation, there is some possibility
>that the shorted turn of the toroid may be of some slight benefit, in
>that it's resistance may dampen the higher resonant modes rather more
>than the two desired operational modes either side of Fres, thus
>perhaps usefully suppressing the unpleasant consequences of higher
>mode excitation, eg racing arcs and difficult quenching.  Plenty of
>research to be done on this topic.


>By all means forward these comments to the pupman list if you wish.

I did!

>Paul Nicholson,
>Manchester, UK.