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Re: IMPORTANT: RESEARCH ON WHAT?
Hi Marco,
The very best of luck with your PhD. I wish you well.
Below are some suggestions for your research. These are things I
have pondered and long wanted definitive answers to:
> Original Poster: "Marco Denicolai"<Marco.Denicolai-at-tellabs.fi>
>
>
>
> From: Marco Denicolai-at-MARTIS on 23.11.98 09:10
>
>
> To: Tesla List <tesla-at-pupman-dot-com>
> cc:
> Subject: IMPORTANT: RESEARCH ON WHAT?
>
> To make a long story brief, I'll just tell you that the High Voltage
> Institute of Helsinki (part of the Helsinki University of Technology) is
> interested in Tesla Coils.
>
> They are going to support me financially in order to build one (or more)
> Tesla Coils and to make extensive research on it. That will constitute my
> PhD thesis.
>
> They have got facilities and measurement instruments we have always been
> dreaming about and they are researching only about high voltage and related
> topics.
>
> Having such a great opportunity I have to lay down a proposal about WHAT I
> am going to research about and here I need YOUR help, because, after all, I
> am just a beginner who has built one TC only.
>
> My questions are:
>
> 1. What is still not understood about Tesla Coils and what would be worth
> investigating about? In that laboratory I am confident I could, for
> instance, measure all the parameters of a TC you have always been dreaming
> about and try to make a model of it.
First and foremost, I would like to know exactly what the truth is
about resonator behaviour with no topload present and no corona or
spark issuing. I would like to know the relative phases of base
current vs current (say), several turns short of the top with
(a) the gap firing
(b) no gap firing
Further to that, I would also like the current distribution mapped
for the entire coil under both conditions to know whether it changes
from condition (a) to condition (b) or not. I would suggest a
resonator with an h/d around 5 or so so that it approaches a that of
a slow wave structure more closely (than that of a squat coil).
I would also like an accurate mapping of the voltage
distribution under both conditions.
Second thing:
We have to date been able to design a resonator with no topload
with an accurately predicted frequency using the lumped inductance
and something said by Medhurst to be the coil's self-capacitance.
However, I believe that while this is a most useful recipe, I do not
believe that it represents the truth when it comes to quantifying
Cself. The reason for this is that unless the current distribution
test shows that current in the very top turn = base current, the
inductance of this part of the coil is not contributing substantially
to the lumped figure when the coil is used like this and hence Cself
cannot be a true figure either. Moreover, I don't think it conforms
to that of an isolated cylinder either. I think this itself needs to
be checked. It is worth bearing in mind that whatever math Medhurst
applied to his results, the formula he came up with had a caveat (one
end grounded) and the math followed the measurements. It would have
been more convincing had the measurements confirmed the maths. With
regard to the "cylinder" capacitance, I have tried hooking one end of
the LCR analyser I use to the base of resonators only to find that it
gives readings far higher than one calculates if one uses L and Fr
and derives a value for Cself. Maybe the meter is lying - maybe not.
Third thing: I would like to see a formula produced that
accurately allows a frequency to be predicted for such and such a
resonator with such and such a topload fitted. Bearing in mind the
points made above and also bearing in mind that the apparent
capacitance of a coil+topload <> the sum of the individual components,
I would like to know what the nature of the modification is. I
suspect that shorted-turn coupling into the resonator by the topload
modifies the L/C distribution over the top portion in a fashion
commensurate with the degree of k between various portions of the
winding and topload. However, tests also show that cutting a slit in
the topload to remove the short also have minimal influence on Fr so
it would be nice to see this checked out as well.
I have constructed a number of toploads and resonators with a
view to conducting such experiments myself should a suitable venue
become available. Great isolation is called for and this includes any
probing of the coil to minimize distorting the elctric field due to
the high shunt impedance of the resonator. So far I have been unlucky
in finding a suitable place to conduct the tests. I feel sure a
formula of some sort (whether true-picture or recipe) can be derived
and feel that it would involve terminal shape and size, winding shape
and size and probably winding pitch and height the terminal is
mounted above the winding. We know that raising the terminal has the
effect of apparently raising overall structure capacitance as
witnessed by a drop in Fr when this is done (this could be a
consequence of increasing overall structure size). It also raises
the Q.
Fourth thing: I and I'm sure everyone here would dearly love to
know the exact mechanisms that cause voltage hikes back at the
transformer terminals that cause safety gaps to go beserk under some
conditions, most notably overcoupling. At what point in the sequence
do these occur? When the gap first conducts? When it has just gone
out? During zero current crossings in the primary? Answers to these
questions should point to definitive solutions to the problem IMO.
> 2. Would it be more challenging insteas of a 2-coil TC to build and measure
> a 3-coil TC (Magnifier)?
I feel there are still plenty of points to be investigated more
rigorously in 2-coil systems. With regard to magnifiers, it would be
nice to see a formula that predicts the resonant frequency of an
arbitrary combination of secondary and free resonator. We know that
for tight driver coupling the inductance of the driver secondary
plays a large part but of course such coupling occurs only while the
gap is firing. I have found that a relatively modest secondary
inductance/Cself allows one to roughly tune the primary to the extra
coil system and have a working system. What about when the secondary
has an inductance/capacitance that is commensurate or even
considerably greater than the extra coil system?
I would also like to see the voltage distribution between the
driver secondary and extra coil (with particular attention given to
systems such as Richard Hull's Mag #11-E). I have long held the view
that a solenoidal winding is limited in the voltage it can ultimately
withstand by its length. I would be interested to know what the final
voltage of Mag #11-E is and what proportion of the output voltage is
generated by the resonator.
> 3. What you would start researchin about, if you were me? >
All of the above and probably other things I haven't thought of. If
something else comes to mind I'll post.
> P.S. I promise I'll keep the list all the time constantly informed about
> results and findings, but please help me to find a good subject. I don't
> want to discover something that has already been known by the majority of
> you. Instead I think some of you could have some technical questions
> bothering but no possibility to find answers for missing of instrumentation
> or facilities.
The challenge is there. I think answers to the questions I've posed
are well suited to PhD research. I've sometimes thought about doing
it myself.
Best Regards,
Malcolm