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Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
Dr. Dark Current wrote ...
The heating is mostly due to eddy currents I believe. A stainless steel topload would heat more than aluminum one. The conductivity is the key parameter.
we don't care about heating, all we care that we have short circuit on top of secondary.
If some one have LCR with Q option, just measure primary Q, add toroid at hight it should be, and do Q measurement again. I believe Q will be lower, but by how much ?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'd say if the topload is highly conductive, the Q will not drop that much. On my 7kW CW SSTC the toroid made from aluminum ducting got warm, dissipating maybe 20W by guess. On my 1.5kW VTTC the topload made from paint can lid (so some generic steel) gets very hot, but its smaller, again dissipating maybe 20 watts. I believe if it was made from aluminum, it would heat less (and so drop the Q less).
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
Dr. Dark Current wrote ...
I'd say if the topload is highly conductive, the Q will not drop that much. On my 7kW CW SSTC the toroid made from aluminum ducting got warm, dissipating maybe 20W by guess. On my 1.5kW VTTC the topload made from paint can lid (so some generic steel) gets very hot, but its smaller, again dissipating maybe 20 watts. I believe if it was made from aluminum, it would heat less (and so drop the Q less).
As far as i understand, it doesn't matter with what you make short, Q will go down. Look, primary is made from 2-6 turns, bus voltage is 300Vdc, so you can expect like 50V, and you don't need low resistance to get it shorted And answer, use more conductive material to get low heat for me does not make scense, we care about efficiency of energy transfer, not energy transfer for circular currents inside toroid, what does nothing , only degrades performance
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Nope, you don't get it. The topload diverts the magnetic field, you can imagine it as a shorted secondary winding of a transformer with a large leakage inductance.
The current through the topload is given (at a given operating point of your coil). If you increase the resistivity, the dissipation in the topload goes up (P~IIR) and so your Q goes down. If the topload was superconductive, it would not dissipate anything and won't do anything to your Q.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
Dr. Dark Current wrote ...
Nope, you don't get it. The topload diverts the magnetic field, you can imagine it as a shorted secondary winding of a transformer with a large leakage inductance.
The current through the topload is given (at a given operating point of your coil). If you increase the resistivity, the dissipation in the topload goes up (P~IIR) and so your Q goes down. If the topload was superconductive, it would not dissipate anything and won't do anything to your Q.
Ok, now i get what you are saying, for me DRSSTC looks more like simple tranformer with load on very long rod. I have simple idea for test. if some one has DRSSTC without secondary, just hanging top-load where it should be with respect to primary. and simple single turn voltage should be measured as function of distance. Same measurement should be made without topload. If no one will do this, i will make it myself as soon as i get my bridge working
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes that could be an interesting measurement, but I guess it could be modelled as well by some finite element method (maybe the simple FEMM software can do this).
Registered Member #6541
Joined: Sun Sept 09 2012, 05:01AM
Location: Melbourne Australia
Posts: 16
What is the best shape for the top load? I"m new to this and am doing my research. I've seen round balls, donut shaped ones , ones made from curled copper pipe, slotted ones and some layered on top of others. What is the most efficient shape for the top load?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The donut (toroid) shape is best. It encourages the sparks to travel outwards away from the coil. With a sphere, they tend to go downwards and strike the primary more than you would like.
A large toroid stacked on top of a smaller one is a slight improvement.
From an efficiency point of view it doesn't really seem to matter whether you use a solid metal toroid, or a piece of flexible ducting bent round into a ring, or a skeleton frame made out of pipes, they all work much the same. However, if you are wanting to run the coil without a breakout point, you'll find that a smooth surface gives a single large streamer that wanders around the topload, whereas a rough one can generate several streamers.
I found running a DRSSTC without breakout point adventurous. Nothing happened until the variac was close to max and then suddenly the arc lashed out in an unpredictable direction.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, hardly anyone uses a DRSSTC without a breakout point. You have huge arcs flying around next to sensitive electronics so it is good to have as much control over the direction of them as you can.
When I tried it with Mjollnir, it would break out from the topload in a random direction about half of the time. The other half of the time, it flashed over down the inside of the secondary.
My AG2 coil works happily without a breakout and produces 2 or 3 streamers at once.
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Topload, effects on efficiency
