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Re: Top Toroid
<< Hi Malcolm and all
> 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?
> 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 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. 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.
> Flames and comments will be appreciated
> Skip
>>
Skip,
In the normal situation where the gap fires at full capacitor charge, the
input voltage has already passed its peak due to the shift in phase that
occurs when a cap is charging. The result is that the firing gap "shorts"
the power supply when it's at a low power point, and less transformer energy
is dissipated (and wasted) in the gap.
In contrast, when the gap fires "early", before the cap is fully charged,
the intantaneous transformer voltage may actually at its peak AC voltage
point. Shorting the transformer at the AC peak will burn up a lot of power.
This happens normally and occasionally in a NON-synchronous gap and is a
disadvantage. Perhaps this effect is what is heating your gaps, when your
sync-gap fires "early".
In a related matter, the longer the dwell time, the more power will be wasted
in shorting the transformer. In one non-synchronous experiment, I reduced my
dwell "distance" from 3/4" (considering the "overlap" of electrodes) to zero
dwell, by "offsetting" the gaps. My input power was reduced from 16 amps, to
13 amps and spark length stayed the same. This is probably one of the
easiest ways to improve TC efficiency, BTW (in a non-sync system).
Ballasting, and resonant charging effects, and external series gaps, will
also determine how much power is wasted during this "power arcing". I
suspect that many of the ballasting difficulties in non-sync systems can be
traced to this "shorting at the AC peak" effect. This effect may create
the need for using a combination of inductive and resistive ballasting in
some high powered TCs to reduce tranformer saturation and possible
"thumping".
Taking this further, it would seem that a non-sync gap with a lower break
rate should be more efficient than one with a high break rate, since more
"full power shortings" will occur during each half cycle when using the high
break rate.
Regarding the gap resistance, it seems to me that a gap with low current that
fails to vaporize much metal would have a higher resistance.
Coiling for today and tomorrow, comments welcome.
John Freau