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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, I wanted to design a secondary for a tube coil, which could possibly run up to 10 kilowatts input and operate at 350 kHz. Probably the best secondary design I could get had winding dimensions of 12.5 x 75cm wound with two parallel 0.50mm wires, as I quickly realised one 1mm wire would get smoking hot from the eddy current losses induced from the primary magnetic field. But still, based on the formula in this article , with the input of Bp calculated from primary parameters and recalculated for dissipation per cm2, I am still getting 5 times higher losses per cm2 than a previous coil running on a similar frequency on which I know the secondary got very hot after a while. When I multiply the results by the DC to AC resistance ratio of the coils to get a possibly more accurate answer, the dissipation per cm2 is still 2-3 times higher. Do you think this formula could give answers accurate enough for this problem?
So, how would you go about designing Tesla secondaries, running at high powers and high frequencies? Thick single conductor wires and high frequency magnetic fields really don't seem to mix very well...
Some ideas: -Use a thinner wire with interturn spacing, but no easy way to do this. -Make the winding with more parallel running wires, but this also gets pretty hard to do with higher wire count... -Use a stranded conductor made from individual enameled strands, but I have no idea where to get such a wire. -Base feed the secondary to eliminate primary magnetic field. But this opens a whole new design chapter...
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Based on experience with coils for amateur radio more than TCs;
'skin' effect is a problem, at 350 kHz the easiest solution is Litz wire, which is quite expensive (e.g. ) some good info is here a cheaper solution is just thicker single-strand copper wire. At 350 kHz silver plating etc makes negligible difference.
more significant is 'proximity' effect for which there are no simple analytical formulas, in practice winding with about one wire diameter space between turns dramatically increases the 'Q' of the winding.
You will have to live with the windings getting hot, so plastic formers are useless, I've tried glass (flower vase) which is ok, more rugged and better heat capability is ceramic, e.g ceramic (pottery) flower vase, sewer pipe etc. But since you can't use varnish etc the windings slip when hot if space-wound, I haven't solved this problem yet.
I have made a few high power pseudo-single-layer solenoid inductors using 1mm2 tinned multi-strand pvc coated wire that was originally for underground trains. I wound two wires flat in parallel then added a third turn as a second layer on top between them, if interested I can photo' an example.
Google for "H.F. Resistance and self capacitance of single-layer solenoids' by R.G. Medhurst, 1947, for some good information.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Sulaiman, thanks for the info. I am more worried about the eddy current losses, which are because of the magnetic field from the primary winding. Often the secondary coil gets very hot right under the primary but stays cold on the upper part. I think winding the coil with more parallel running thinner wires is probably the easiest solution, but will probably be a PITA to wind.
I was also thinking about a higher powered ~3 MHz coil, which would probably be impossible to do the traditional primary-secondary method as the coils would probably overheat very quickly, so I am thinking about going the "base feed" method on this one. Still not sure what's the best way to do this...
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High power high frequency secondary coil design
