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Re: Gap Dwell Times (formerly: Beating Solved)
Tesla List wrote:
>
> >From MALCOLM-at-directorate.wnp.ac.nzThu Sep 26 22:18:09 1996
> Date: Fri, 27 Sep 1996 08:13:59 +1200
> From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> To: tesla-at-pupman-dot-com
> Subject: Re: Gap Dwell Times (formerly: Beating Solved)
>
> In reply...
>
> > I did some research this weekend, pouring through a Corum brothers
> > monograph and some old posts to this newsgroup and found out that the
> > optimal dwell is 1/(k*Fr), *not* 1/(2*k*Fr). In other words, the spark
> > gap should conduct for only 1/2 of the superimposed beat-frequency
> > envelope. Primary current peaks at that point, and maximum energy is
> > transferred into the secondary.
>
> I started from k approx dF/F, and ringup time = 1/2dF so you can see
> how I derived that. The problem when cutting the gap off (if you
> could) when Ip is maximum is that with k<1, most of the primary
> energy is coupled to the primary. I tried doing exactly this with the
> MOSFET gap and the spikes hit the roof. Virtually none of that energy
> was coupled to the secondary. The spikes in a real gap would have re-
> ignited it anyway. I wonder if they have actually tried doing this?
<SNIP>
Malcolm,
It looks like they did - in the appendix of their booklet "Vacuum Tube
Tesla Coils", page IV-10, they present a chart which shows experimental
data points connected with a curve. Their system had Fu=123.33kHz and
Fl=74.18 kHz, and k=.46(!). The predicted "best case" quenchtime was
1/(2*(Fu-Fl))=10.17 uS. They tested dwell times from slightly less than
5 uS through 55 uSec, finding that the peak secondary output occurred at
the expected value of 10 uSec.
Alas, they did not indicate just how they controlled the dwell times,
nor how they were able to quench the arc during primary current peaks.
Interestingly, if they quenched at the worst time (the 2nd primary
current peak after the first energy transfer - about 20 uSec), they
still got over half of the maximum voltage output from the secondary.
There is a dramatic falloff of output with quenchtimes under the
optimum, since the secondary does not have a chance to fully ring up. At
5 uSec, output is only about 10-15% of the best case.
> I found the ideal dwell to be when the secondary was fully rung-up
> and the primary had virtually nothing left. Even then, quenching at a
> primary zero-crossing is a no-no because of spike generation. I sent
> some photos I took of this process to several people.
>
> Malcolm
I concur with your theory and analysis, having made similar storage
scope measurements on my system. The spiking may be markedly less if the
secondary is "loaded" with a corona or a dummy load of some type, since
the energy in the secondary will have somewhere to go other than trying
couple back to a non-existent primary circuit (MOSFET open).
Any possibility of scanning some of these photos in for the rest of us?
Safe coilin' to ya!
-- Bert --