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Re: Top Toroid
Tesla List wrote:
>
> Subscriber: FutureT-at-aol-dot-com Sat Feb 1 22:15:26 1997
> Date: Sat, 1 Feb 1997 17:49:36 -0500 (EST)
> From: FutureT-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: 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
John and Skip,
The other problem with excessive dwell times is that the energy that's
transferred to the secondary/toroid couples back into the primary
circuit, to be dissipated as heat. This problem becomes even worse if we
don't have heavy secondary streamers "helping" to reduce the secondary
energy coming back into the primary circuit.
Robert Stephens showed me a very interesting videotape of his large
disruptive system where the toroid was intermittently breaking out.
During the times he had no breakout, the light intensity coming from the
rotary was MUCH brighter than when he was getting good streamers. Lots
more energy was clearly being dissipated in the gap, and I have no doubt
a lot more heat production and electrode erosion were occurring as well.
John, your thoughts regarding slower versus faster break rates may be
true. However, I would also propose that its more important to have the
minimum amount of dwell time (through offset electrodes for example) so
that we will complete the first full primary-to-secondary energy
transfer, but quench before transferring any back. This should also be
the most efficient operation as referenced from the input power mains.
Assuming we have proper dwell, we can increase the break-rate (and
assuming that we can recharge the tank cap quickly enough), get more
overall power transferred to the secondary. [This implies that we may
want to use a larger tank cap and fire the gap at a lower voltage so as
to get the same energy per bang, and more bangs per AC half-cycle]. Once
we start breaking out, more power delivered to the secondary should
result in more energy going into the longer, and hotter streamers, and
less proportionately being dissipated in the gaps.
I'd even contend that an "ideal" asynchronous system (other than a
DC/charging choke system) might employ a very rapid rotary (assuming
adequate dwelltimes). The ballast characteristics and voltage-point of
the incoming AC waveform will govern how quickly the tank cap will be
recharged. If we size these properly, they, and not the rotary, will
govern how many times per half cycle we actually fire, and these could
be adjusted to limit the actual breakrate to below 600 BPS. By using a
high rotary speed, we'd always have another electrode presentation
within a relatively short period of time, so that if we "just miss" one,
we'll hit the next very quickly, minimizing jitter. A static gap, set at
a slightly higher breakover voltage, shunted across the rotary could
also be used to handle "misses".
Flames, brickbats, and cat-calls are heartily welcomed. :^
-- Bert H. --