On 7/13/11 12:03 AM, Carl Noggle wrote:
Hey-- A toroid or any top electrode will light up with corona when the
electric field at its surface is about 30 kV per cm. For a sphere, the E
field at the surface is voltage/radius. For example a sphere with a 10
cm radius will light up at 300 kV or above. A cylinder will require
twice the voltage to get the same E field, so a 10 cm radius cylinder
will light up at 600kV. The field at the surface of a toroid is a little
lower than that of a cylinder, but not much. So we would expect a 7x30
toroid (8cm radius x 37.5cm radius)to turn on at 30kv/cm*8cm*2 = 480kV,
or about 0.5 MV. When this voltage is reached, many dancing streamers
will suddenly appear. Using toroids or spheres like this is a good way
to measure the voltage.
I don't know that corona inception voltage is a good way to measure
voltage. It's affected by things like surface imperfections, dust, etc.
Even in a well controlled laboratory environment, looking for
breakdown in polished sphere gaps with a radioactive source to make
sure there's ions floating around is probably only good to a few percent.
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