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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
>>
After I posted, I thought about this some more, and wonder also if you may be
seeing what Robert Jameson spoke about at an ITS Symposium: If your dwell
time is relatively long, and your gaps are close, with no external series
gaps, the gap may "refire" while the gap is still "presenting". The gap fires
BEFORE the electrodes line up (due to close spacing), the cap discharges, but
starts recharging, The gaps are becoming even closer as they whiz past each
other, allowing the gap to "refire" at a lower voltage than normal. Since in
the "early firing set up" of the sync gap, the voltage is rapidly rising,
this could make the scenario more likely to occur. Robert found that when
this undesireable "multiple firing" occurs, it causes heavy current and rapid
gap heating. They heat due to this scenario: The cap can charge up, and fire
the gap, charge and fire the gap, many times during one presentation. Each
of these firings is at such a low energy level that very little energy is
transfered to the secondary. Since the cap never charges very much during
each "refire", the transformer is basically constantly forced to charge a
mostly uncharged cap. The mostly uncharged cap draws terrific current during
each multiple firing and rapidly heats the gaps.
John Freau