Re: The Next Coil
From: Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Wednesday, July 30, 1997 3:12 PM
Subject: Re: The Next Coil
Thought I should address one point in your excellent
> Reply To: rwstephens-at-headwaters-dot-com
> Sent: Tuesday, July 29, 1997 8:52 PM
> To: Tesla List
> Subject: Re: The Next Coil
> John Freau comments and asks a question of Greg Leigh after Greg's
> announcement of a magnificent new large TC project:
> > Greg,
> > This is really great news!!! Do you still feel that capacitance in the
> > secondary can be substituted for top load capacitance--that they have
> > an equivalent effect for the same C value?
> > John Freau
> John, Greg,All,
> When I was starting my rapid learning curve about how a Tesla coil
> works about 3 years ago, external information embossed me with the
> notion that secondary coil self-C is a bad property which cannot be avoided in
> the real world, and which robs us of power because it has to be charged at the
> same time as we are trying to get a peak pulse to occur at the top terminal.
> I agree with your recent comment Greg of how stepped leader formation can be
> enhanced through support of a large topload C where the energy is available
> without series inductance right there at the base of the ionization
> channel. I've written a comment about exactly that somewhere in the
> recent past. I also made a post a while ago rambling about how the local
> air along the streamer which stays ionized after the streamer
> extinguishes, acts more than like a momentarily persistent pathway
> upon which the next TC output pulse will instantly fill and then
> extend through stepped leader action (although I used the term dart
> leader because I am old and confused), but I also went a step further
> and postulated that this locally ionized pathway surrounding the
> extinguished plasma channel might actually be storing energy which
> becomes added to following Tesla coil streamers that go this route
> instants in time later. The streamers which follow this channel as
> we know go well beyond the distance records made when the streamer
> completely changes direction and starts new pathways. I received
> absolutely no comments on this idea.
I thought there was general agreement with this idea so didn't
comment at the time.
> The energy which is stored amongst the turns of the secondary in the
> self-C is not available to the streamer the same way nor as
> immediately. The energy stored nearest the topload will be available
> first, but not as soon as that from the topload itself. Like a
> stretched out pulse, the self-C energy will be communicated from
> secondary coil regions progressively further down this helical delay
> line to the top terminal when a low impedance ionization channel
> calls for it.
> The way this slower current helps in streamer growth
> and maintenance is another question. Perhaps it does not aid in
> streamer growth at all but in fact is responsible solely for channel
> maintenance after it is laid out by the fast currents available from
> the topload.
That sounds remarkably like a lightning scenario where charged
regions progressively join in after the channel is established.
> Also responsible for channel maintenance is the wave
> which propagates up from the bottom of the secondary upon each
> of the break, and successive beat envelopes if any. This energy from
> the source would be mingled and become one with the discharge pulse of the
> self-C of the secondary coil (delay line), I would think, i.e., you would
> not be able to distinguish the two as separate entities in the scoped
> output waveform.
> With the hands-on experience I now have I no
> longer accept the statement handed me initially that self-C robs us of
> output power. The picture is much more complicated than that and I am still
> trying to piece it all together. I have not yet had the advantage of
> any form of coiling or CAD program from which to plug in variables
> and watch the theoretical results. So far, if I wanted to see what
> increased secondary L, or a larger top C would do, I would wind a new
> secondary or build a larger topload. The nice part about this expensive
> tedium is that I now have a good assortment of equipment which is
> darned handy to have in a Tesla coil laboratory.
> Malcolm recently commented that the self-C energy is entirely
> reactive so there is no power loss. I hope he's 100% right.
I was making the point that capacitance per se is not a lossy element.
However, the distributed C can couple power to surrounding lossy
elements. But from your next paragraph I would think inductive
coupling is a more likely mechanism.
> I went to great effort about a year and a half ago to put a 15 inch
> diameter, AWG #8, polythermalese magnet wire wound coil 48 inches long on a
> fiberglass form inside a large 200 gallon steel tank filled with
> transformer oil. The performance of this coil has yet to be gleaned.
> It just doesn't seem to light up like much smaller coils in air that
> I have built using the same input power. In 'Fundamentals of Radio' by Terman
> he mentions how coils may be installed inside metallic housings and that eddy
> current losses in the surrounding container will drop to an
> acceptable level once the radial distance to the containment vessel from the
> coil matches the radius of the enclosed air core coil. My enclosed coil follows
> these suggestions exactly. I haven't had the opportunity to pump
> enough input power into this resonator to overcome thermal inertia and
> feel heat on the steel walls or anything, but I have driven it up to
> about 4 kW CW and 900 VA disruptive style. In disruptive mode , by
> observing the discharge to a ground wand it is apparent that there is
> *a lot* of C being discharged in the resonator's output. Streamers,
> although shortened as compared to a similarly powered normal air
> cored Tesla coil are very much louder and visibly brighter indicating
> discharge current from a proportionately large self-C. I get a
> similar effect when I place an external air-dielectric 100 pF cap
> built from an aluminum spinning suspended within an empty, grounded 55 gallon
> steel drum on the output of a regular disruptive Tesla coil and retune to
> compensate for the mucho extra topload C, except in this case I don't appear to
> be losing nearly as much voltage at the same time. We understand well that
> topload C foreshortens the 1/4 wave resonator in electrical degrees so that a
> lower Vmax is produced at resonance.
The conductivity of the container is going to affect losses even with
those guidelines. Perfect conductivity = no loss. Iron is not a very
good conductor though. From memory, it is one of the worst of the
metals. Also, you can make helical primaries considerably larger than
2 x Dsec and still couple power from one circuit to the other most
effectively. The proof of the pudding would be to place the resonator
in a similarly sized aluminium container such as is normally used for
housing radio coils. The size and mass of the container would be
contributing to heatsinking too. As well as all that, heat
conductivity from the inside to outside is rather poor for iron.
A few thoughts.