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Modeling a magnifier
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From: Robert W. Stephens [SMTP:rwstephens-at-headwaters-dot-com]
Sent: Tuesday, February 24, 1998 3:26 PM
To: Tesla List
Subject: Re: Modeling a magnifier
> From: FutureT-at-aol-dot-com [SMTP:FutureT-at-aol-dot-com]
> Sent: Sunday, February 22, 1998 11:08 AM
> To: tesla-at-pupman-dot-com
> Subject: Re: Modeling a magnifier
John Freau,
You wrote:
> This has been suggested by Lou Balint, Duane Bylund, and myself
> (the tuning part), (and probably others) for some time. As far as
> tuning is concerned, it is clear from experiment that the primary
> must be tuned to the combined secondary and resonator L and C.
This has been the mental model I have always used as well.
> Regarding the use of special gaps, I have used both special rotary
> gaps and normal rotary gaps both on magnifiers and on classic coils
> and have seen little difference in performance. Scope tests show
> the performance of series quenching rotaries to be only marginally
> better than regular rotaries. More on K below.
You have had more experience of experiment here than I.
>
> Then Robert replies:
<snippage>
> > Just a related point. If you take a classical Tesla coil and start
> > unwinding the secondary from the top down, stopping at intervals to
> > make K measurements, you will find your K factor increasing steadily as you
> > remove wire, until most of the secondary has been removed, then the
> > trend will reverse itself as you continue to remove the last of the
> > secondary. That point where the K has maxed out represents the right turns
> > ratio and coil geometry IMO for a good maggy driver coil, if you are
> > going to use that partially unwound coil.
>
> This is very interesting, can you give an approximate proportion of
> winding length that you found to be best for the secondary? Did you
> wind up with 30%, 40%, etc., of the original length for example. This
> may vary maybe from system to system. But I'd be interested to
> hear of an example of what proportions you found to be good in a
> particular system.
It depends a lot on the aspect ratio of the secondary you start with.
The more the coil approaches a *candlestick* design, the more turns
you will have to remove to get to this sweet spot. On a coil with an
aspect ratio of 4:1 I'm gonna guess you'd be around the 25% mark, but
this also has to do with the type of primary you are using, and
EVERYTHING to do with the resultant field shape. Your goal here is
to maximize the common volume of field shape developed by both coils
with little or no part of either coil creating a field in empty space
where the other's field is weak.
> > Maybe the unwashed masses representing the classical croud among us should
> > try rotary gaps that quench in tens of microseconds rather than the hundreds
> of
> > microseconds which is normal on most of our classical coils.
> > Certainly the multi-swept gaps of the maggys perform the
> > function of putting out the fire quickly by virtue of increasing the
> > approach and departure velocities of the electrodes greatly over
> > simpler 2 or 4 gap rotary designs. Could the extra trouble
> > involved in the manufacture of such superior quenching, shorter dwell gaps
> lead
> > to better operation of classical coils if employed?
>
> I've posted numerous times that in all my tests, I've seen only a
> slight difference in quenching capability and coil performance
> between multi-swept gaps, and regular rotary gaps, when used
> in either magnifiers or classic TC's up to 5kW.
> Also, I find that rotaries usually do not quench by stretching the arc.
> The gap generally quenches while the electrodes are still lined up in
> a good system. For instance the dwell may be 200uS, but the arc may
> quench in 40uS because the energy is all gone by that time in many
> closely coupled systems. In most systems that I have personally
> scoped, the actual quench occured well in advance of the mechanical
> dwell time.
I agree with you. The effect that a high speed gap with multi series
contacts being superior in quenching has to do a lot with airflow and
how this arrangement keeps the electrode surfaces cool. Electrodes
that operate in an incandescent mode throw a lot of metal ions into
the plasma in the gap. This makes it harder for the natural ability
of the system to self-quench as you describe to work. After zero
crossing where a good gap would have quenched, an incandescent gap
will often reignite, or simply stay lit as a result of 60Hz supply
power arcing if the supply is a powerful low impedance model like a
pole pig.
<snip>
When I say gap dwell I am speaking of the electrical activity, unless
I specifically refer to *mechanical* dwell which you aptly point out
are two completely different animals. Perhaps in the future I should
specify the type of dwell being considered in all accounts.
>
> Robert, BTW I've obtained 22" sparks from a classic tube coil using
> (1) 833A, in steady running, at 15A, on a 120V line, using an MOT
> with voltage doubler. Thanks for your inspiration and info in that area.
I figured you would! : ) Oh well, there goes another record. : (
>
> Comments welcomed,
> John Freau
Cheers,
Robert W. Stephens
Director
Lindsay Scientific Co.
RR1 Shelburne, ON Canada L0N-1S5
Tel: 1-519-925-1771 Fax:
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