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Re: Gap Dwell Times (formerly: Beating Solved)
Tesla List wrote:
>
> >From bert.hickman-at-aquila-dot-comWed Oct 9 22:50:21 1996
> Date: Wed, 09 Oct 1996 20:58:31 -0700
> From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Gap Dwell Times (formerly: Beating Solved)
>
Bert,
Thank you a very definitive answer. I stll have mixed feeling about
dwell timed and energy transfer. As you and numerous others have
commented in the past there are so many unanswered questions about what
we do, it sometimes seems that every new answer brings on 5 new
questions.
> Skip,
>
> There's no easy way to explain this! We want the primary to ring for 1/2
> a BEAT [Fupper - Flower], not for 1/2 a primary cycle. Coupled LC
> circuits work BEST when their uncoupled resonant frequencies are close
> or the same.
I hear what you say but I don't see why.
>From my radar days we had to drive the pulse forming lines from the
matched driver impedances to get maximum energy transfer. We took a
block of energy and dumped it all at once into the line and took it out
of the other end all at one time. In the case of the TC, I have always
assumed that when we match up the frequencies we are essentially
matching up the impedances of the primary and secondary. The pulse
former in our case is the primary cap and the combination of the gap and
cap make up our delay line. Our problem is to get complete energy
transfer from the cap to the secondary without reflecting any back.
Loosely inductively-coupled resonant circuits exchange
> energy back and forth between the primary and secondary at the "beat"
> frequency. Because of the loose coupling, it takes a number of cycles
> for all of the primary energy to transfer to the secondary or
> vice-versa.
Putting aside for the moment the problem of arcover, etc., could we very
closely couple the primary/secondary and with one of Richard Hull's very
fast quenching gaps apply a huge pulse to the primary, immediately
quench the gap, and let the secondary ring, except that hopefully the
pulse running up the secondary will create a very large spark and start
generation of the ion cloud getting ready for the next energy pulse.
If we had a lossless system, this exchange and re-exchange
> of energy would continue indefinately at the beat frequency. Steve Roy's
> earlier example using coupled-pendulums was a fairly accurate
> representation using a mechanical analog.
>
> The greater the coupling coefficient, the fewer primary oscillations
> required to complete an energy transfer "beat". Since coupling only
> takes place while the gap is firing, we would like to turn off the gap
> at the _first_ point where the primary has minimal energy (NOT the first
> zero current rossing). In this case, most of the energy that was
> originally in the primary LC pair now resides in the secondary/toroid
> pair (less losses).
>
Are you saying that the energy transfer continues after the first 1/2
cycle of the first beat if the gap is still conducting? If so do you
think that the energy is somehow being stored in the secondary, perhaps
in the toroid and Cself? If this were true, how is the energy
transferred to the secondary during the first beat, prevented from
transferring back to the primary during subsequent beats (assuming the
gap continues in conduction). How can there be an accumulation of energy
in the secondary for the making of the big discharge?
Shouldn't we strive to get a single pulse of energy to the primary
secondary system and thereby reduce gap losses and oscillatory losses?
> With the gap quenched, the secondary will ring down at its uncoupled
> resonant frequency. The rate at which the secondary's energy is
> dissipated now depends on its Q, and whether energy is _also_ being lost
> via a corona discharge (the case we usually want). If the gap fails to
> quench, much of the secondary's energy couples back to the primary and
> is subsequently dissipated in the gap. In this case, any further
> increases in primary power merely heat up the gap and may result in
> decreasing output from the secondary.
>
> You CAN increase the coupling coefficient and jam more energy into the
> secondary in a shorter amount of time. The "gotcha" is that the
> secondary may be stressed beyond its dielectric strength during the
> primary to secondary energy-transfer.
But don't we eventually achieve the same amount of total amount of
energy transferred to the secondary and therefore the same maximum
stresses just a few beats later.
This transfer usually occurs at a
> _higher_ frequency than the secondary's 1/4 wave ring-down frequency,
> causing voltage peaks at points lower than the top of the secondary
> winding. Increasing the coupling coefficient only worsens this effect:
> 2-coil systems typically "max-out" at k=0.28 or less due to secondary
> winding flashovers.
>
> Going to an oil-immersed coil might allow you to go to higher coupling
> coefficients IF you can also prevent the gap from re-igniting when the
> secondary tries to dump its energy back into the primary. 3-coil systems
> avoid the secondary breakdown problem (but not the quenching problem) by
> base-driving the third coil/toroid _at_ its natural ring-down frequency
> from a tightly coupled primary:secondary pair. Proper high speed
> quenching is still somewhat of an art.
>
> Some other backup information (ignore if you bore easily...):
>
BIG SNIP
> If you quench too early (say at the first zero crossing of primary
> current), only a _small_ portion of primary energy will have been
> transferred to the secondary. The rest of the energy gets expended in
> trying to heroically extinguish the gap! This case is illustrated in the
> graph in the appendix of the Corums' booklet "Vacuum Tube Tesla Coils",
> page IV-10. The predicted "best case" quenchtime was calculated to be
> about 10 uS based upon the upper and lower coupled resonant peaks.
> When quench times were forced to be less than 10 uS, there was a
> _dramatic_ falloff of output, MUCH more severe than having too long a
> quench time. For example, at 5 uSec, secondary output is only about
> 10-15% of the optimal value. Dave could probably punch-in a slightly
> different set of gap opening times on his model to simulate this effect
> graphically as well (hint, hint...).
>
> Whew!! This is _heavy_ stuff! Think I'll go play with some sparks now...
>
> As usual, flames, semantic corrections, and insults are heartily
> welcomed! :^)
>
> -- Bert --
>
> <The mother of all snips...>
>
> > Richard and all
> >
> > This brings up a question which has been on my mind for a long, long
> > time.
Thanks again or a great discussion
Skip
> >
> > If we only want the primary to ring up for 1/2 cycle .... why is it
> > necessary that the primary and secondary be correctly tuned? I know this
> > to be true and you know that I continue to try to build a 1/4 wave
> > secondary but I still can't put it together. Why can't we just closely
> > couple the primary to the secondary and jam the power in? I guess I need
> > something physical to put picture what is going on. I will certainly
> > appreciate any light that the group can shed on this.
> >
> > Skip Greiner