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Re: [TCML] measure VDG voltage
Hi Bart,
Sorry for the delay in getting back with you. Jim and Antonio have
already responded with excellent answers. Unlike most home experimenter
setups, the voltage breakdown tables reflect measurements made using
rigorous setup, scrupulously clean spheres, gaps that are a small
fraction of sphere diameter, and precision DC voltage sources or impulse
voltages with well defined leading and trailing parameters. Typical home
experimenter setups use smaller spheres, dirty or unpolished electrode
surfaces, low impedance circuits and oscillatory waveforms, and minimal
E-field control. It's not too surprising that the results may differ
from the "ideal". I would agree that, under these conditions, a more
typical breakdown threshold may indeed be in the range of 26 - 27 kV/cm.
Best wishes to all for a very Merry Christmas and only good things in 2008!
Bert
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Barton B. Anderson wrote:
Hi Bert,
I measured a 1" ball gap back when doing TSSP measurements for Paul. I'm
at a very low altitude of about 100 ft above sea level here in a CA. I
measured 26kv/cm at that time. I think the 30kv/cm is a nice roundabout
number, but I think 26kv/cm is a more accurate number and not just from
my own measurements. I know where the 30kv/cm is from, but from my own
measurements and others who have measured (including papers on the
task), I've seen values between 26 and 27 and "never" higher. That
4kv/cm does make the terminal voltage little lower.
When corona inception voltage is reached, I have to assume breakout at
that point in this application. Although the corona itself "grows" the
ROC a tad bit, it's not enough to change the voltage significantly.
The ROC is the best method of predicting top volts. Far lower than
energy calcs and certainly more real.
Take care,
Bart
You can estimate the corona-limited maximum voltage for the VDG based
on the radius of curvature of the top terminal. However, this only
provides the maximum that the VDG generator could achieve, not
necessarily what your generator is achieving. The following
relationship assumes an air breakdown voltage of 30 kV/cm at sea level
and a polished sphere. The maximum voltage will be reduced for
locations with higher elevations or if the sphere's surface was not
polished. The Radius of Curvature (ROC) is in centimeters. So, a
spherical topload 20 cm in diameter would have a ROC of 10 cm, and an
estimated maximum VDG voltage of 300,000 volts.
Vmax = 30*ROC
You can also approximate the actual maximum voltage by using a spark
gap and a suitable table that converts gap distance to voltage. This
has the advantage of not loading down the VDG until actual spark-over
occurs. You can use the VDG top terminal as one gap electrode and a
similarly curved grounded spherical terminal (a gazing globe or a
metal "float" ball or even a suitably round metal bowl) as the other
gap electrode. Jim Lux's HV Handbook contains sphere gap construction
hints and a gap-voltage table:
http://home.earthlink.net/~jimlux/hv/spherev.htm
Bert
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