Re: [TCML] tank circuit power

[Steve Ward <steve.ward@xxxxxxxxx>]

Jim,

I've used some FEMM software in the past to study the RF losses of my
primary coil designs.  You are right, the inner turn gets it the
worst.  My FEMM plots show differing density patterns on each turn for
a spiral primary.  The inner turns have more current on the innermost
surface of the copper instead of the current being evenly distributed
around the whole tube.

You can get your own free FEMM software at:

http://www.femm.info/wiki/HomePage

I've found it a very useful tool for HV design too, especially when
*trying* to guarantee corona free surfaces.

Back to primaries... I found that using smaller copper tubing, and
water cooling, improved the efficiency of my primary coil (simply
measured power usage without making any sparks).  The smaller tubing
actually had about the same AC resistance because this current
bunching "proximity effect" was less wasteful of the copper even going
from 5/8" to 3/8" and achieving the same required inductance.  To make
up for the limited heat dissipation ability of the 3/8" stuff, i used
a cheap pond pump to circulate some water through it.  The leakage
current to ground is pretty small... at least my IGBTs dont care.  You
may have to watch out on higher voltage systems (Vpk is about 9kV on
my primary).  Keeping the smaller copper at a lower temperature
actually makes this primary more efficient than my 5/8" copper design
which had no active cooling.

Another comment i'd offer up is using spice or something to do your
energy integrals.  You just gotta set up a circuit with the right R,
L, and C and let it ring down, and spice will figure out the numbers
for ya...  Well thats the lazy way i guess.  Talk to Antonio about
solving the real integral, though im sure i did it at some point for
homework ;-).

Steve


> Hi Jim,
>
> I don't believe for a second that air flow is the culprit. What I'm
> referring to regarding proximity loss are where the coils are mutually
> coupled and physically close to one another. My choice of terms is probably
> not the best. I'm not necessarily referring to turn to turn losses in the
> primary, but rather the magnetic field density where the two coils are
> physically nearest. That of course is the inner turn on a planar type
> primary. Would be nice to run something like JMAG on the situation (wish I
> had that software).
>
> Take care,
> Bart
>
>
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