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Re: spark length vs toroid diameter
Subject: Re: spark length vs toroid diameter
Date: Fri, 9 May 1997 22:04:17 -0500
From: "Robert W. Stephens" <rwstephens-at-headwaters-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
> >
> > dw pierson wrote:
> >
> > > Assuming the rest of a given coil setup can provide some
> > > arbitrarily high voltage (eg: low enuf losses, high enuf
> > > power), the spark over distance is set, largely, by the
> > > terminal (toroid, sphere, whatever) diameter. A 'zero
> radius'
> > > terminal (point) will spark out at a relatively low
> voltage.
> > > If LOTS of power is available, there may be long sparks.
> A
> > > larger toroid will 'allow' higher voltages before
> sparking over.
> > > (Of course changing toroid size also changes tune, all
> the
> > > way back thru the system, so simply dropping on a new
> toroid
> > > to see the effect can be complicated to interpret...)
> >
> >
> > There are several points to consider here:
> >
> > 1) Why then wouldn't a hemisphere on the top of the coil be the
> best?
> >
>
> A FULL Sphere would be best as far as CAPACITANCE goes. It would also
> have the edge as far as BREAKOUT of the discharges goes. But a sphere
> is lousy at shielding the top windings of a Tesla coil, compared to a
> toroid. The sphere has a greater radius of curvature for a given
> diameter size, but it also takes up more room, uses more metal (or
> whatever you make your top loads out of...), and provides less
> shielding (for a given diameter size, of course).
>
> By "hemisphere" you might mean the configuration where the top half
> is perfectly hemispherical, and the bottom "half" is more like the
> BOTTOM of a toroid. This would have more useful capacitance than a
> toroid of same diameter, but less than a full sphere. It would have
> shielding equal to the toroid, which would be an improvement over the
> full sphere. They use this shape a lot with commercial Van de Graaf
> generators, as it reduces the tendency for discharges to shoot down
> the outside of the support column. Note that such a shape is not as
> easy to build out of the kind of glorified junk that we coilers tend
> to use as our materials of choice.
>
> > 2) Some coilers report getting longer arcs by putting breakout
> points
> > on their toroids, which actually compromise the hold-off
> voltage.
> >
>
> The breakout points focus the breakouts to a smaller area of the
> toroid. That will cause the ionization near that area to be somewhat
> increased. Ideally the breakout point should be a BUMP rather than a
> point. A point will start corona discharge early and drain off energy
> that COULD have gone into actual discharge production. Note that
> sometimes a point's ionization may actually scare up a discharge that
> is longer than normal... depends on many conditions. But in general a
> bump is better than an actual point.
>
> > 3) The streamer itself, once formed, acts as a sharp wire hanging
> off
> > the toroid, further compromising its holdoff voltage.
> >
>
> Yes, the discharge compromises holdoff voltage, but the same
> phenomenon also allows the discharge to GROW on itself, so to speak,
> over multiple oscillations.
>
> > It would seem that a larger radius of curvature isn't necessarily
> > the reason for longer arcs. What else could it be?
> >
> > -GL
>
> A big old humongous sphere is still one of the absolute BEST toploads
> you can put on a Tesla coil. But toroids have so many things going
> for them, that they are the topload of choice for coilers. I believe
> that the shielding action of the toroid is one of its best features.
> You want max capacitance, go with a sphere. You want close to max
> capacitance with less size and better shielding, then go with the
> toroid.
>
> I LOVE to steal the spheres off of Van De Graaf generators and use
> them as toploads. But the physics department always comes along and
> demands that I give them back! Maybe if I built them a Van de Graaf
> generator using a toroid they would be happy? Nope, it has to look
> like the thing in the textbook or they get totally confused. Poor
> souls!
>
> Fr. Tom McGahee
> (I don't actually "steal" the spheres... I just borrow with
> permission and keep for as long as I can)
Fr.Tom,
What about employing a rather small radius of curvature toroid just
above the top secondary winding for electrostatic field control (I do
this on all my larger coil systems successfully) and then having
taken care of corona breakout from the topmost windings of the coil,
let's get down to business up here making a top terminal that cooks!
As I see it, once you have taken care of shielding of the top of the
coil, above that you should be able to employ a sphere, so long as
the diameter/spacing of the topload protection toroid is adequate.
Although what I am postulating here is probably a good idea, I will
not use this as I have come up with a better shape to throw streamers
up and away, and miraculously, I came up with it by experiment using
REAL HARDWARE.
Please pitty me and understand that I am handycapped. Unlike many
others, I have no spreadsheets, no Teslac or similar programs,
absolutely no CAD programs, etc. I have to simulate everything I do
in reality. I admit, this costs real money as opposed to simulated
money (wrong forum to discuss the fact that these are now the same), and
real
time as opposed to time compaction simulation, but
somehow I manage........and oh yeah, somehow I also get REAL results.
MMMMMMMMM smell that ozone! : )
rwstephens