Re: skin depth in round conductors Re: 8 kHz Tesla Coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Robert,

Tesla list wrote:

> Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>
>
> >
> > It should be noted that the current is not uniform around the wire.
> > The current density will occur adjacent to magnetic fields. In an
> > iron core transformer, the current flow through a conductor is drawn
> > to the high permeability core side of the winding. In an air core
> > transformer, it's toward adjacent wires. In a helical single layer
> > coil, toward the 2 adjacent wires.
>
> My understanding is:
> The core flux has the effect of pushing the current to the outside of the
> winding.
> The flux from adjacent wire has the effect of push the current away from the
> adjacent areas.
> The net effect is the current tends to flow more on the outside of the
> solenoid.
I should clarify the core as I did use the term "transformer". With 
the secondary next to the core and primary wound around the secondary 
on a typical E-core, the greatest field strength is between the two 
windings and is where the current density, i.e, current flow, will occur.

In a single winding (typical E-core with small center air gap), the 
field outside the winding is completely shorted out and the field 
intensity is greater in the gap than in the winding area within the 
core because of the energy stored in the gap. Current flow is 
concentrated on the inside surface of the winding and no current flow 
on the outside of the winding.

The flux from adjacent windings can attract or repel and that depends 
on the current flow direction in each winding. Typically the current 
flow is in the same direction on a solenoid winding and current flow 
is attracted to the outside near each winding and on the inside where 
the field strengths are greatest in a solenoid (which is where the 
dielectric just happens to be on Tesla transformers).

That's my understanding anyway. The cause of greatest losses I 
believe has far more to do with proximity and dielectric losses than 
with skin depth in our coils. We are on the low frequency end of the 
spectrum. We aren't running lot's of power at MHz ranges, we are 
running moderate power at low KHz. I think there is too much emphasis 
on skin depth and not enough on proximity and power losses. I am 
guilty of this.

Take care,
Bart