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

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

On 9 Oct 2005, at 9:17, Tesla list wrote:

> 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).

An additional note: Adding a core reduces the amount of copper
required in achieving the design goal for a transformer and makes the
finished item more compact for a given I^2.R loss. Note that adding
the core increases the inductance of all coils coupled to it which
also proportionately increases the mutual inductance M leaving the
coupling constant largely unaffected! (except when conductive objects
are located within close proximity to the windings in which case it
increases k somewhat).

Malcolm

> >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
>
>
>
>