[Prev][Next][Index][Thread]

Re: Overcoupling?

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").
>
>
>Coiler greets from germany,
>Reinhard
>
 

Hi Reinhard,

	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!

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?

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...

Cheers,

	Terry