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Re: Top Toroid

Hi Skip,

<snip>
> I have been following with great interest the various posts on gap
> losses but not always accepting the conclusions put forth. I assume from
> your answer above that as the surge impedance goes down then the surge
> current increases. Are you then saying that gap losses go up due to this
> increased current? Doesn't this assume that the "resistance of the gap
> while it fires remains constant?

Not at all. If the gap has a more-or-less fixed conduction voltage,
then losses scale as I. With a normal resistance they scale as I^2.
Case 1 is V*I, Case 2 is I^2*R
 
> My experience with synchronous gaps leads me to a different conclusion.
> I feel quite sure that the current in a firing gap is a function of the
> voltage in the cap as well as other factors.

I agree completely. Vcap/Zsurge = Igap in rough terms.

> I note that the contacts in
> my gaps run much cooler when the gap is firing near the peak of the
> mains voltage. When firing ahead of the peak (by adjusting the gap
> position with respect to the motor shaft) I notice that the gap pins get
> much hotter. I attribute this increase in gap heating to increased
> resistance in the gap due to a lower current/voltage. I believe that the
> gap resistance is dynasmic and definitely increases at lower currents
> which can definitely be controlled when using a synchronous gap.

Also true. If you take Vgap as being fixed, then Rgap is inversely 
proportional to Igap. I can't explain the extra heating by my scenario
but it does stand to reason that the transformer will dissipate more 
of its power in the gap if the gap is fired earlier? I can say that 
the lower the initial cap voltage, the higher the proportion of cap 
energy will be wasted in the gap.

 My
> primaries usually are 3 to 7 turns and I think on the low impedance side
> and therefore have low surge impedance. Still at 1800va input I have no
> trouble using .25" brass contacts in the rotary and they give very long
> life.

That will be due to self cooling I would think. A static gap will not 
be so happy in my experience.

Malcolm