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RE: Induction heating in torroid / short circuit of secondary

HI Dale, all,

> Original Poster: Dale Hall <Dale.Hall-at-trw-dot-com> 
> 
> Hi Colin,
> I don't think I've seen a good explanation 
> re: why there is not much difference in performance
> between Shorted and Open topload toroids.
> 
> I keep a few AL dryer duct types around that can be
> easily separated at the junction (I keep the gap short ~1/4").
> 
> When I use the "Open" Toroid I connect it as an aiding turn.
> 
>   I am able to draw a small arc between the two "Open" edges,  
>   indicating there IS energy that may be lost using the shorted turn 
>   toroid; An optimized system could be theoretically create a bit 
>   longer maximum Spark using the Open turn version though likely to be
>   within a normal deviation distribution, so not readily apparent.
> from my experience:
> when Shorted, the power arc max distance difference is not discernable.

If the terminal conducts perfectly, there will be no power lost in it all 
as there will be no voltage drop across it. It is a long way from the 
primary (for typical small coils, k between the primary and toroid is 
likely to be less than 0.01) so doesn't feel the influence of the 
primary strongly. However, people have reported toroids getting 
warm in CW coils where the RMS current circulating the resonant 
circuits is fairly high.
     I once measured Fr of a secondary with a toroid mounted just 
below the windings, then cut a slit in the toroid and remeasured. The 
frequency difference was of the order of 1% for a four foot high coil 
and toroid diamter around 20" (twice that of the coil), again 
demonstrating a lack of overall coupling.
 
Regards,
Malcolm

> I wonder if there is a compensating alteration of Field difference.
> 
> *** Terry, have you ever modeled these two conditions ? 
>            Do the Field plots differ ?
>            This may be a case where you create the program,
>            so the results are pre-destined to agree with
>            that codes assumptions ?
> 
> I use galvanized and AL toroids interchangeably with indiscernible
>  max Spark difference.
> Any differences are more likely due to their surface texture
>  rather than the metal they are made from.
> 
> A note: higher resistivity is likely to result in greater inductive heating
> Stainless steel is more resistive than Brass
>  which is more resistive Galvanized ducting (zinc plated steel ?)
>  which is more resistive than AL
>  which is a bit more resistive than Gold
>  which is a bit more resistive than Copper
>  which is a bit more resistive than Silver.
> 
>  Has anyone measured discernable max arc differences using these metals ?
> (assuming similar dimension and surface texture)
> 
> Bottom Line:
> Heating is a low concern as its likely to be undetectable at most TCout avg
> powers
> unless it affects the max spark achievable.
> Spark length affect is real concern - and that appears to be zilch.
> 
> **** distantly related: I did some interesting experiments a while back,
> Stacking toroids dielectrically insulated from each other but  
> switched via spark gaps in an effort to provide electrical "delay"
> determining WHEN the next level Capacitance was "connected"
> The Sec output wire was likewise "gap switched" to the first toroid.
> 
> ...attempting to effect "dynamically stepped" tuning 
> as a funtion of having reached a minimal HV.
> 
> A thin little spark area appeared to be sufficient
>  to effectively connect the levels,
>  rather than an optimally large connecting surface area expected.
> 
> Regards, 
> Dale
<snip>