Re: Why tesla coil is air-cored to operate efficiently at high freq?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:
> Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
>
> It's all about time.  It requires a finite period of time to 
> magnetize an iron core, and at high frequencies this time period is 
> too long, so RF coils work better without a core.  This effect will 
> be covered in detail in my forthcoming book, "Tesla's Marvelous 
> Transformer:  The Tesla Coil, which I am presently hard at work 
> on.  It looks like approx 650 pages when I finish it with a lot of 
> drawings, figures, and photographs.

This is not necessarily true... Assuming reasonable linearity, all
that a core does in an inductor is to increase its inductance for
a given shape and number of turns. The current grows exactly as in
an air-core coil, more slowly due to the higher inductance. But
in a transformer, if the magnetizing inductance (the inductance of
the primary coil) is increased, the transformer works more close
to an ideal transformer, and so a core is a good thing. In a
Tesla coil, we don't want an ideal transformer, but just two loosely
coupled coils. Actually, not even a transformer is required. A
directly coupled system works too, in the same way (although with
less degrees of freedom in the design).

> I believe the "swinging" action you are referring to is the rapid 
> transfer of charge from the capacitor to inductor in a tank circuit.
> Another type of "swinging" transfer occurs between the primary and 
> secondary circuits as energy is swapped back and forth.  This is 
> indesireable and that's why efficient quenching (turn off) of the 
> spark gap is necessary --- to prevent this type of action.  Ideally, 
> all energy is transferred to the sec coil in the first burst.  In 
> most classic TC oscillators it usually requires 2 to 2.5 pri-sec 
> swaps to get all the energy in the sec coil.  Energy is wasted. 
> That's why the solid state coils are so much more efficient --- the 
> IGBTs can turn off rapidly and trap the energy in the sec coil.

This is not so simple to do. If the IGBTs are really turned off,
the voltage over them grows immediately (if not for more complex
reasons, just because the square wave over them is replaced by a
sinusoid (almost) 4/pi times larger. In practice, the reverse
diodes in the IGBTs conduct and the system operates as if the
driver continued to operate for some cycles, returning energy
to the DC bus.

Antonio Carlos M. de Queiroz