Re: eddy current with secondary coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> 

Tesla list wrote:

 > Original poster: Dave Lewis <hvdave-at-earthlink-dot-net>

 > One way to visualize how much space you need is to imagine the magnetic
 > flux density within the center of the secodary as more or less evenly
 > distributed along the cross section.   Given that the area is pi*D where
 > D is the secondary diameter, you'd like at least that much area for the
 > flux lines to exit the top of your secondary and bypass the torroid
 > without compressing.  That would be a vertical spacing of at lease D/4.
 > Thats derived by equating the area of the secondary pi/4*D^2 to the area
 > below the torroid and last turn pi*D*X.

I have just observed that I can use my program for inductance
calculations to evaluate what is the effect of a "toroid" above the
secondary coil.

I place a flat coil with all the turns in parallel above the secondary
and calculate the inductances of the system. I have then a transformer
model with a secondary with modified inductance, have the inductance
of the terminal, and have the coupling between them. From this I can
calculate the currents, and even the losses if I estimate the resistance
of the toroid.

It would not be difficult to adapt the calculations for a true toroid.
A single turn doesn't model the toroid correctly, because the
calculations assume uniform current and turn radius much larger
then wire radius. The change in the secondary inductance is not
observed.

Example:

A flat coil with 50 turns in parallel, maximum radius=10 cm, minimum
radius=5 cm, 1 mm wire, 1 cm above my usual secondary coil with
1152 turns, 0.2 mm wire, 32 cm of length and 4.4 cm of radius:

Original secondary inductance: L2:   28.2669505945 mH
Primary inductance L1:      0.0001557802 mH
Secondary inductance L2:   28.2587159298 mH
Mutual inductance:    0.0106327465 mH
Coupling coefficient k:    0.16025554187763

Unless for possible losses, the effect on the secondary inductance
is negligible. The program can list the complete inductance matrix of
the system (it doesn't list it if so many turns are used), and a
simulator could calculate the currents in the toroid from it.
I don't have time now to try this, but it's not so difficult.

By the way, I have recently updated the program, with the
"instantaneous"
calculations by explicit formulas as default and allowing inputs in
inches and AWG gauges:
Inca program: http://www.coe.ufrj.br/~acmq/programs

Antonio Carlos M. de Queiroz