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Re: Overcoupling?

Hi Terry, all,

My comments are interspersed.

Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
At 02:21 AM 5/6/99 +0200, you wrote:
snip...
>
>The primary coil´s magnetic field induces a current into the secondary,
>right? Changing the coupling factor also changes the amount of magnetic
>field that is effectivly "being seen" by the secondary. As you increase the
>coupling, you also increase the "amount of secondary" (couldn´t find any
>better words for this) being encased by the magnetic field. This increases
>the output voltage of the secondary coil. At one time or the other, you will
>hit the maximum voltage the secondary wire can take (meaning from one
>winding to the next). If you surpass this coupling factor and go even higher
>the wire´s insulation breaks down and you get racing sparks. You can
>compensate for this by increasing the secondary loading. This will then
>decrease (!) the secondary voltage, yet you will get longer sparks,
>because the topload stores more Joules (voltage is within "safe" values,
>but C is bigger, hence more Joules). This would also explain to me, why
>magnifiers allow a higher coupling coefficient. In a magnifier setup the
>secondary coil is much more loaded with Ctopload, Ctertiary (and
>topload) and Ltertiary. The current flow in such a setup is increased,
>but due to the loading, the voltage does not climb to destructive levels.
>
>The voltage levels allowed in the secondary are dependant on the total
>insulation. My testbed coils were all just wound with wire and not
>coated. I experienced the racing spark phenomenon many times. My
>8" coil was coated (before winding) with several layers of PU varnish,
>wound, coated with 2 layers of resin (with sanding in between) and two
>final layers of PU varnish. In other words the coating is very thick. With
>this coil I can couple pretty high. The primary to secondary distance is
>around 1.9". I can raise the primary(!) approximately 4.5" above the
>lowest secondary turn and get absolutely no interturn arc overs. At
>~5" above, I start getting the phenomenon of racing sparks. Of course
>this depends on input power, but for equal input VA, a coated coil will
>allow higher coupling than a non coated coil. It has happend to me
>twice that I got fierce arcing from primary to secondary (one time the
>primary coil slipped and tilted during a run, the other time the
>secondary wasn´t centered properly within the primary). I examined
>the secondary with a high powered magnifying lens. There was
>absolutly NO damage. Although I wouldn´t recommend "testing" a
>coil this way, it seems as if proper coil preparation is vital for a coil
>to survive such accidents (and it increases "looks").

Start of Terry´s mail:
>You post here and Malcolm's on this subject are VERY interesting!
>Perhaps what you are saying could be looked at in the following way which
>may be more clear and easier to quantify.  In the loosely coupled case, a
>Tesla coil develops voltage across it's secondary length "basically" as a
>function of SQRT(Cp/Cs).  However, as the coupling is increased, the coil
>begins to act more like transformer where the voltage rise is governed by
>Ns/Np (N is the number of turns).  In the first case, for my coil, the
>SQRT(Cp/Cs) is equal to 25 and Ns/Np is equal to 67.  So, if the coil were
>just barely able to handle the voltage in the first case, increasing the
>coupling could substantially increase the voltage across the secondary
>length and cause the arcing to occur.  With a firing voltage of 20000
>volts, the first case would produce 500KV across the secondary while the
>second case would produce 1300KV!

Thanks for putting it into more correct terms and making it easier to
read and understand.

>If this is indeed the basic mechanism, Reinhard's observations about using
>heavy insulation could be optimized to produce Tesla coils capable of
>handling very high coupling.  I was a bit confused where you mention that
>increasing the distance from 1.9 to 4.5 to 5 inches produced racing arcs at
>the further distances?  That seems backwards?

Oh, sorry about the confusion. The 1.9" is the spacing (in)between my (flat)
primary and secondary. The secondary is 7.8" in diameter and the inside
diameter of the primary is 11.6" (11.6-7.8/2 = 1.9" spacing). Normally, the
secondary is raised or level with the primary (turns of both being at equal
heighth). I can raise my primary (most coiler raise their secondary to
decrease coupling) up to 4.5" above the lowest secondary winding. Meaning
the primary is now 4.5" higher, than the bottom of the secondary winding (not
coilform). This increases my coupling. At ~5" I start getting the racing spark
phenomenon.

>Of course, increasing coupling has a number of effects on the system that
>need to be balanced into the whole equation as well.  But racing arcs can
>be very destructive and a greater understanding of why they occur and ways
>to avoid them would be very helpful!!  It is possible to vary the winding
>pitch, add insulation, change the way the primary is constructed, etc. to
>produce all kinds of wonderful effects.  In fact, since the currents are in
>phase across the secondary, one could add primary windings anywhere along
>the secondary as long as arcing could be avoided.  Perhaps a very large
>diameter secondary that would couple loosely to the whole coil instead of
>just the base would have some beneficial effect?  Tesla's big Colorado
>Springs coil was much like this in some configurations.  This stuff is not
>easy to do by trial and error.  If we know what we are trying to accomplish
>and what the governing mechanisms are, we can do wonderful things to the
>design of coils...

I tried exactly what you described a while ago with a 3.34"x21" secondary. The
bottom part of the coil was wound with magnet wire (7.5" length), whilst the
top part was wound with the same gauge wire, but with cotton & plastic
covered wire. I used a helical primary, which was 12" in diameter and 10"
high. I don´t remember the capacitance I used for the tank circuit, as this
coil was just a testbed for various things. Best spark length was around
62cm (~25") using a pretty big (size, not capacitance wise) toroid. Input
power was ~560VA (not compensated, tho). The toroid was 2" in minor
diameter and about 32" in major diameter. This coil wasn´t coated,
however. I tried an even larger primary (in heigth), but started getting
more sparks to the top of the primary, then to a grounded object. Sorry,
I never calculated the coupling factor, but at that heigth and using a helical
form, I guess it must have been pretty high.


Coiler greets from germany,
Reinhard