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Steve Conner

Sun Sept 28 2014, 09:02AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

FWIW, the "frame covered in rounded edge discs" actually works, it is used in some ultra high voltage test rigs in the power industry.

There's probably a mathematically optimal shape for the disc, but if you imagine the structure shrink wrapped and try to avoid sharp edges poking through the wrap, you won't go far wrong.

Uspring

Sun Sept 28 2014, 10:06AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Spacing between ring was about 2cm. I later replaced the toroid by a solid one of the same overall size. Didn't even have to retune the coil and the arc problems went away. I think thin tubing is much more an issue regarding unwanted breakout rather than capacitance.

Kizmo

Sun Sept 28 2014, 05:40PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

My stacked ring toroid has space between tubes that is roughly 1.5x the tube diameter.

Works well enough, i do get stray spark now and then but that happens at low power. Crank it up and the main streamer from breakout rod does the energy drawing all alone :D

Graham Armitage

Mon Sept 29 2014, 11:35AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Steve Conner wrote ...

FWIW, the "frame covered in rounded edge discs" actually works, it is used in some ultra high voltage test rigs in the power industry.

Yup, that's what I was thinking about, but on a smaller scale.

From what I am hearing, keeping the rings closer together definitely helps. The fact that the ring toroid can be swapped out with the solid one and no re-tuning is required answers the question about capacitance difference (or lack thereof).

Graham Armitage

Mon Sept 29 2014, 06:41PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

So I combed through photos and websites, pulling anecdotal data from working ring toroids. Coupling that with the feedback from this thread, I came up with some tables with rough numbers. To be used as a guideline for ring-toroid construction. Would like to know if this matches what others have done successfully through experimentation?

Dr. Slack

Mon Sept 29 2014, 07:16PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

My interest was piqued by the discussion on partial surface toroids, as it's always been a puzzle to me why so many folks use hard to make continuous surfaces rather than bent pipe or mesh.

I especially like Steve's 'shrink wrap' analogy for what happens to the electric potential around a bundled conductor (as the power line companies call those multiple conductors you see strung across the country to reduce the electric field and hence corona losses on their cables).

So I thought I'd put some numbers to the hand-waving, to see just how effective mesh is compared to solid.

Rather than do the sums for a toroid, I set up the equations for a cylinder, which will produce the same form of results, and the same results as the toroid becomes skinnier.

Rather than set up the conductive surfaces in a Finite Element solver like atlc or something, or do any serious analytical stuff, I instead chose the method of using linear superposition to sum the potential from each conductor, of either a central charge of N units, or N wires each carrying unit charge. The voltage of any equipotential gives the capacitance that a conductor with that profile would have. Although in general the equipotential contours are not exact circles, the approximation to a circle is remarkably good, especially for small radius conductors.

Results for a few selected geometries

The bundle's nominal diameter is that of the wire centres. The equivalent solid cyclinder is given with respect to this dimension. The bundle overall diameter will be a little larger, to the tune of one wire diameter.

The wires are dimensioned as a fraction of their spacing. So a 0.2 diameter conductor fills 20% of the surface, regardless of how many there are spaced round the bundle. This may not be the most consistent way to present it, but it was how I coded it, and you only need to mess about with pi to get to and from absolute dimensions.

For 6 conductors of diameter 0.02 spacing,
the bundle is equivalent to a solid cylinder of 63% nominal diameter
for 0.2, 92%
for 0.4, 99.2%

For 12 conductors of diameter 0.02 spacing,
the bundle is equivalent to a solid curface of 79% of nominal diameter
for 0.2, 95.7%
for 0.4, 99.6%

For 24 conductors of diameter 0.02 spacing,
the bundle is equivalent to 89.1% of nominal diameter
for 0.2, 97.8%

For 100 conductors of diameter 0.02 spacing,
the bundle is equivalent to 97.3% of nominal diameter

Dr. Dark Current

Mon Sept 29 2014, 07:21PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The effective capacitance also depends on the secondary coil dimensions and relative topload positioning. I believe JavaTC assumes this.

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