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Does the top load present a shorted turn?

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LAN_403

2Spoons

Tue Dec 14 2010, 02:38AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Disclaimer- I've never built a TC, so this is all purely theory and conjecture (ie food for thought). A TC is on my 'to do' list - but its such a long list ...

Looking at the construction of most TCs it seems to me that putting a large conducting toroid or sphere on the end of the coil effectively presents a shorted turn to the coil, albeit a badly coupled one. This implies power loss and lowered Q, therefore lower output.
Has anyone tried to build a 'segmented' top load? I would envisage something akin to a dandelion head, with an array of radiating spokes with small plates on the end, forming the approximation of your favourite topload shape without the continuous conducting surface.

Arcstarter

Tue Dec 14 2010, 04:14AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Some people with small drsstcs and sstc's have reported topload heating due to what they presumed to be eddy currents. But, sstc's and drsstc's have much tighter coupling than sgtcs. I think that you would see no difference with a segmented one and conducting one in a sgtc scenario.

In fact, at least in the case of a sgtc, the bottom part of the secondary acts more like a transformer sourcing current for the upper part, which acts as a resonator. This would mean Tesla coils already act somewhat like a 'magnifier' setup. Im not sure about these theories, and i have done no testing on this whatsoever, but i have just absorbed this from the forum over the past 3 or so years.

Steve Conner

Tue Dec 14 2010, 09:59AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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The toroid only lowers the Q if it's made out of poorly conducting material. A good conductor like aluminium or copper will reflect the EM field instead of absorbing it, and that just lowers the inductance of the coil, shifting the resonant frequency a little.

Same goes for a metal baseplate under your primary. I've built a DRSSTC with a thick sheet of aluminium checker plate under the primary for mechanical and Faraday cage reasons. It worked fine, but I think if I'd used thin sheet steel, it would have got red hot and started a fire.

On my old OLTC2 coil, I once accidentally shorted the ends of the strike rail together. It threw the coil out of tune, and it wouldn't draw current or make any sparks.

Pinky's Brain

Tue Dec 14 2010, 02:46PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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I always thought it was just a capacitor.

U4R1A

Tue Dec 14 2010, 05:30PM

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Location: Albuquerque NM
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@ pinky Your right it does act like a capacitor but it also helps shield the top windings from corona. The position of it also goes into tuning and eliminating racing sparks up the coil.

2Spoons

Tue Dec 14 2010, 09:39PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Steve McConner wrote ...

On my old OLTC2 coil, I once accidentally shorted the ends of the strike rail together. It threw the coil out of tune, and it wouldn't draw current or make any sparks.

Which is exactly my point. Copper may be a good conductor at DC, but at TC frequencies the skin depth is going to be under 0.05mm, so there will be some resistance. And as it would be essentially a single turn the circulating current could be quite high.
I concur regarding steel - skin depth in ferrous materials will be very small. And you have to add hysteretic losses too.

It would make an interesting experiment to compare two toploads: solid vs segmented construction
My apologies for not being in a position to do the comparison myself.

ubuntupokemoninc

Tue Dec 14 2010, 10:01PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
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If you getting eddy current in a toroid wouldn't common sense tell you that you secondaries too short?

Even anyways in a sgtc the pulse is so crude, i don't know how it would heat the toroid, and sgtc, use high voltage witch would mean it would take more curren't to make a bigger eddy current, were as in a sstc, you use low voltage witch will make a large eddy current.

some one please correct me if i'm wrong.

Marko

Wed Dec 15 2010, 07:30AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys -

Yup, the topload does present a shorted turn, and in CW SSTC's I have had it heat up so much that it became a fire hazard. In those cases I highly recommend using a topload that is not a completely closed ring - also, a topload that is made of multiple tube rings is better than one thick ring.

In SGTC's and DRSSTC's, rep rate is usually low enough for this heating to be negligible, and losses negligible as well considering the loses in other parts of the coil, especially of the sgtc.

Marko

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