Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
Hi Jim,
I have Terman documents, so I'll go and investigate what is said on
the subject. Agree with your statement about proximity losses. That
is actually my point here. Depth penetration at the kHz ranges we
run our coils at is in no way going to require a 5 awg wire size.
The 8 kHz coil was outside our norm, so it stood out and showed me
there is a problem with the sD recommendation. In design
programming, more emphasis should probably be put on proximity,
power, and dielectric losses.
Those are my main points with this discussion. Yes, problems with
skin depth are real, however, those losses are not being put into
perspective and the sD recommendation is probably doing more to
minimize basic power losses than actual sD losses. Q would still go
up, but the reason may not have actually been sD losses.
Regarding proximity losses in round conductors, this paper may be of interest:
http://www.classictesla.com/download/Proximity_Effect_Loss_Calculation.pdf
"An Improved Calculation of Proximity-Effect Loss
in High-Frequency Windings of Round Conductors"
Xi Nan and Charles R. Sullivan
Take care,
Bart
Tesla list wrote:
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 09:56 AM 9/23/2005, Tesla list wrote:
Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
Hi Jim, All,
In every reference I've been reading regarding skin depth, I can
find nothing stating round conductors and sheet conductors have a
difference in depth penetration due to frequency, and it just
doesn't make sense that they would (at least, I'm not getting it).
The only difference I can find is that for round conductors, the
math gets messy to define exactly when the abrupt change occurs
and tails off toward zero.
I think Terman has a discussion of this. I don't have a copy here,
so I'll have to check with someone else who does.
In any event, there is never an "abrupt" change. It's always a
gradual decrease (exponential in the infinite flat plate case)
Skin depth is defined as the distance from the surface of a
conductor where the current density is 1/e times the surface
current density. This is nothing more than a density ratio used to
describe the effective conducting area.
I'll agree with this, because it happens that the integral of
exp(-x) from 0 to infinity is = exp(-1).
Skin depth occurs because a changing flux induces a voltage loop
or eddy current which is coincident with the voltage. This eddy
reinforces the main current at the surface and opposes the current
in the center of the conductor. The result is that as frequency
rises, current density increases at the surface and tails off
exponentially toward zero at the center because of these frequency
dependent eddy currents.
In a conductor, the eddy current at some depth is affected by not
only the current directly above it, but also by the current on
either side. Imagine a bunch of filaments with equal current all
laid next to each other. In the flat plate case, this winds up
giving you the exp(-x) characteristic. In the round conductor case,
the filaments next to the one directly above are closer than they
are in the flat plate case, so the current decays faster.
It should be noted that the current is not uniform around the
wire. The current density will occur adjacent to magnetic fields.
That's an entirely different (proximity) effect. Even for single
straight wires, round conductors have an AC resistance greater
than you'd get from circumference*flat plate skin depth.