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Re: Corums New Tesla Coil Theory Paper
Terry wrote:
> "Terry's Thoughts on the Corum's New Paper"
Commenting a few parts:
> Much of the paper's statements are based on the "coherence theory" or
> resonant rise effects that the Corums have explained in previous papers.
This
> theory states that when the primary arc is quenched, the secondary system
will
> be free to ring independently and the voltage rise will only be limited
by the
> VSWR (a function of Q) of the secondary system. I would point to the
following
> flaws with that theory.
What is said on -that- paper appears to be just the known fact that a
long
coil is a kind of low-loss transmission line, with multiple resonance
frequencies. This is well known since Tesla's time.
> 1. The secondary system has a relatively fixed secondary capacitance
that
> must be charged to increase voltage. The moment the gap quenches, the energy
> in the system is fixed and any additional voltage rise will require
additional
> energy to the secondary system which is not possible. If one simply takes a
> sine generator and excites the secondary system, very high voltage increases
> can be created. However, the sine wave generator is supplying a continuous
> source of energy to the system that is not available a real operating
coil with
> a fixed amount of energy per discharge cycle.
The paper does not distinguish between a forced system and a capacitor-
discharge system, and appears to mix relations that apply to these two
very different cases...
> 2. The VSWR voltage rise theory does not consider the enormous system
> losses in a Tesla coil. Once streamers and other losses are added into the
> system. The system Q drops dramatically. This large system loss effectively
> damps any resonant rise effects.
The same happens in the lumped model.
> 4. They state that the secondary voltage is proportional to the equation
> "Vmax = S x Vmin" where S is the standing wave ratio. This has not been
> independently demonstrated in any way.
And the papers doesn't make clear if this relation applies to the forced
mode or to the transient mode.
> Much of the paper's dispute with lumped element Tesla coil models is based on
> the fact that a lumped element inductor has equal current entering and
leaving
> the coil were a real Tesla coil secondary does not. However, NO lumped
element
> model ever assumed that. The work of Medhurst demonstrated that the current
> decreases along the secondary, due to capacitive charging of the space around
> the coil, in his landmark paper of 1947. Every lumped element representation
> of a secondary coil has an inductor in parallel with a capacitor representing
> the self-capacitance of the secondary. This allows two lumped elements, the
> inductance and self-capacitance, to represent the very complex secondary
> behavior. Papers, like my "Modeled and Actual Voltage and Current Waveforms
> within a Tesla coil", use this method to represent the secondary and
> demonstrate, through actual measurement, it's validity.
Exactly. The paper doesn't recognize that the lumped model is just a
low-frequency approximation of the more exact and complex transmission
line model, and doesn't mention that everybody models the coil with
an empirically determinated "self-capacitance" in parallel.
I repeated the calculations about the coil mentioned in the paper:
Length: 97.8" (2.48 m)
Radius: 1' (0.30 m)
Turns: 342
Terminal capacitance: 48 pF
My Teslasim program, using nothing more than Wheeler and Medhurst
formulas, finds the resonance frequency as 137 kHz. The paper mentions
a measured frequency of 140 kHz, and a predicted resonance of 133 kHz.
Conclusion: The lumped model works better in this case.
Note that a finite coil is not an uniform transmission line, having
important end effects that change its behavior measurably.
> 1. Tesla coils only operate acceptably at the fundamental frequency.
But they can be operated, less efficiently of course, in other modes.
Antother comment, related to the FAQ being built: That paper can be
used as an example of what happens with html papers with equations and
pictures. Gif line drawings are ok, but photos must be in jpg,
otherwise they create enormous files, with low quality due to the
256 colors only. Equations in image files in the text may look
well, but take long time to load, and some browsers (as mine...)
are not intelligent enough to save the pictures along with the html
text (as I found after saving separately the text and what I could
see that were pictures...).
Better to concentrate the equations in a single file, and never put
image files mixed with the text.
Antonio Carlos M. de Queiroz