Re: Toroid Design .

  D.C. -

  Thank you for your test data on the 40 inch dia VDG generator. Is the 9
inch wide belt running at 90 ft/sec or at 90 inches/sec? Also shouldn't the
50 ua/sq inch be 1 ua/50 sq inches as I show in my post?

  This gives -
  9 X 90 x 12 = 9720 sq inches/sec x 50 ua = 486000 ua/sec
  This is 486000 ucoulombs/sec - way too high
  This would indicate efficiencies of less than 0.5 percent??

  Should be? - 
  9 x 90 = 810 sq inches/sec divided by 50 = 16.2 ua/sec
  This is 16.2 ucoulombs/sec - possible - see my post

  Sphere capacity = 56.4 pf
  Voltage = 1 million volts

  Charge time  56.4/16.2 = 3.48 secs at 100% efficiency
  What was the rate of sparks per second?

  The max theoretical breakout voltage at 70 KV/inch rad
  V = 40/2 x 70 KV = 1400 KV = 1.4 million volts
  32 inch spark - what do you think the voltage was?

  My voltage equation
  KV = 65 x inch^0.7 = 65 x 32^0.7 = 735.4 KV 

  Note that the rate of sparks and the actual voltage is required to find
the efficiency of this VDG.

  There is much more to this type of problem. Nothing? in the VDG text books
is of any help when it comes to real world VDG work and efficiencies. Only
actual tests are of value. Ask Richard Hull. Fortunately we have plenty of
test data for Tesla coils thanks to the work of coilers. The graphs in my
books use much of this data. However, much more TC test data is required
from coilers.

  John Couture


At 09:38 PM 1/5/99 -0700, you wrote:
>Original Poster: "D.C. Cox" <DR.RESONANCE-at-next-wave-dot-net> 
>to: John
>Good post on the theory John.  One caution to watch is the leakage current
>from the relatively large area of very large balls.  Point in case, we
>built a 40 inch dia 1 MEV Van de Graaff several years ago and attempted to
>charge it with a 6 inch wide belt running at 75 ft/sec.  It barely worked. 
>Output spark was only 4 inches.  Terrible error.  With experimentation we
>discovered it required a 9 inch wide belt running at 90 ft/sec which now
>produces sparks around 32 inches (bright spark from side of terminal) in
>length.  It nows works properly but, in theory, should have worked ok with
>the first design.  The large surface area and per sq/inch leakage rate
>reduced the output to under 100 kV on our first tests.  Also, note at
>around 100 ft/sec or greater bearing noise becomes quite irritating.  We
>prefer 65-70 ft/sec as a better design value.  Charge is also "blown" off
>the belt as the charge collides with stationary air molecules on its upward
>Other info for designers is, in theory, 50 uA of terminal to ground current
>is produced with every sq. inch of belt passing over the pulley, however,
>in actual practice 28 uA per sq. inch is a much more reliable design value.
> Output potential is usually around 68% of theorectical max (30 kV/cm
>breakout value).