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Re: That secondary behaviour, E-Tesla5, and Corum's thing...
Hi all,
Thanks for the responses. My posting was just a vote. I have already written
perhaps four pages or more on the subject to the list and Terry. I was gob
smacked to find one of my posting pop up in a search for Corums papers. How
does that happen Terry?
However I do feel obligated to provide responses to the major points
raised even if my responses are only cut and paste from previous posts or
e-mails . However I will not be responding to the alleged bathing habits of
the masses nor do I have a golden pen or golden keyboard and I have no
desire to be a tutor on transmission line theory (read a book or do a web
search). If you don't think its applicable to your coiling or think its
rubbish then forget it. For me and I suspect others, who are fascinated by
science and Tesla coils in particular, its not enough to know that a
formula produces approximately the right answer we want more. We want to
know how it really works and if it has a practical application that will be
a bonus.
First I should restate my views. The theory is not mine even Tesla reported
it.
In its usually operation a Tesla coil secondary resonates due to the
reflections of signals from what is normally the high voltage end in a
similar way to a 1/4 wave antenna or any quarter wave stub (as it is
referred to when not used as an antenna). A good analogy is how an organ
pipe resonates. The coils behaviours as a classical transmission line due to
its distributed capacitance and inductance which result in the comparatively
slow propagation of ALL SIGNALS along the axis of the coil. Typically a
thousands of time slower than the velocity along a wire or coax cable. The
resonance occurs if the reflected signal returns to the driving end with the
opposite polarity to that of the driving signal. For an open circuit
transmission line the minim length at which this can occur is 1/4 of a
wavelength. If the line is terminated with capacitance, inductance or a
short circuit the phase of the reflected signal is modified and hence the
line will resonate at a different length. It can also resonate at odd
multiplies of a 1/4 wave length again due to the phase of the reflection.
The fundamental requirement for this to occur in the average Tesla coil at
say 100KHz is the slow propagation of signal along the axis of the coil.
A Tesla coil has all the theoretical requirement of distributed capacitance
and inductance to support slow propagation. I have personally measured the
slow propagation in a coil that I also used in a Tesla coil including the
90deg phase shift (terminated coil), more of this later. There are also
some devices that use the effect such as the luminance delay line in a
television and the delay line used in some scopes to allow observation of
the trigger pulse (the display channels are delayed).
One could assume two modes of signal probation for the coil. One mode
would have a slow propagation time and be used to explain standing waves,
signal delays, transmission line effects etc.. and the above effect. The
second mode would have a fast propagation time where the input signal is
able to influence the whole coil relatively instantly (fast compared to the
period of the operating frequency of a coil). This mode is used to explain
how the inductance and capacitance of the coil behave collectively as
lumped components at resonance.
I far as I am aware the only direct evidence to support the view that a fast
propagation mode exists at the fundamental resonance frequency of a coil is
the approximately zero phase shift measurements between the top and bottom
of
a resonating coil. As Terry can testify, initially this appeared to me as
strong evidence to
support a fast propagation mode. However this was due to my lack of
understanding of the properties of a standing wave. The reason that it is
called a standing wave is just that it does not move only its amplitude
varies i.e. no phase shift. This is because it is the sum of a forward
travelling wave and the reflected or backward travelling wave. I have
already posted this point so I am surprised that it is still believed there
would be any measurable phase shift at resonance. I guess this point may not
be understood or believed. It can be verified from theory and modelling.
If you want to measure the phase shift on a real system you must use an
edge signal or terminate the coil so the reflection does not confuse the
measurement.
Possible my greatest objection to the fast mode was trying to understand how
the
signal could propagate rapidly from one end of the coil to the other. The
classical lumped model of a transmission line is a large number serially
connected inductors(non coupled) with a capacitor at each junction to
ground. Signals propagates relatively slowly due to the delay caused by
charging the parallel capacitors via the series inductors.
There is no particular requirement for the capacitors and inductors to all
have the same value but it does make the maths simpler. Now this appears to
be a good representation of a Teals coil except that there is no
representation of the interturn capacitance and interturn inductance or
coupling. However the interturn capacitance is thousands of times too small
for any significant signal to reach the end rapidly. You could assume that
the signal is inductively coupled to the end from turn to turn. However
coupling between the turns is in opposition to the inducing signal and so
this increases the propagation impedance which slows the signal even more.
This is verified both by direct experiment, modelling and theory.
For those individual that are not into transmission line theory and are
convinced that a signal can propagate instantly down the coil. I suggest
that if they have access to a signal generator, scope and a coil check it
out. I have and it just doesn't. You can readily observe the slow
propagation, reflections and resonance up to several times the fundamental.
With the small coil I used (12in by 1.25in) it was even possible at
resonance to light a small florescent tube and observe the multiple voltage
peaks at the higher resonance modes. Although I have not tried it I am
certain the same would be confirmed with a Spice model and again no instant
signal.
If I remember correctly one respondent said he has. What are the details of
the model? If the model was a series of Ls and Cs with a bit of c at one
end for the top load, it was a model of a transmission line resonating at
1/4 wave length. If it was one L and one C, what was correct , the
propagation time, the output voltage?
Around resonance the input impedance of an open ended transmission line
looks like a tuned circuit i.e. a lumped circuit. Even Corum uses such
representations for transmission lines but it is only a representation not
how it resonates.
I agree the lumped models can predict the majority of voltages a round the
secondary and even the secondary top voltage. I had believed that we
discussing how the secondary works not how it may be modelled.
I have read all of the responses and except for the phase issue there are
few technical issues I can respond to.
It appears that several people accept that multiply resonances or harmonics
occur but are unable to give a explanation with a lumped model. Terry and
his artist are on the verge of grasping the transmission line theory.
They recognise the problem with the harmonics and have given a technical
justification for how such harmonics may be supported and they still cling
to the erroneous phase issue. I believe the problem at least for Terry is
that it is difficult to accept that signals can not instantly propagated
down the coil. This is difficult to accept. For me it was helped by seeing
it on the scope so check it out Terry you have all the equipment. The
artist and or Terry have suggested that coil is a series of parallel tuned
circuit that can resonate at different frequencies I don't see how it can do
that. But you are getting close just add the capacitance to ground and a
bit of coupling between the coils and you will have a good model . Plug it
in to Microsim or spice and see how it works, instant transmission no,
standing waves yes. If you want I will try and convert one of your coils in
to a 10 element model. It should be good for the fundamental and third
harmonic. May also be able to prove that the turn to turn capacitance is
insignificant which will allow you to use a standard L and C model Then
hopefully I want have to right an other long post on this subject.