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# Measuring secondary voltage (fwd)

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From:  Robert W. Stephens [SMTP:rwstephens-at-headwaters-dot-com]
Sent:  Thursday, January 22, 1998 8:58 PM
To:  Tesla List
Subject:  Measuring secondary voltage (fwd)

<major snippola>

John Couture wrote:
> >   Jim -
> >
> >   Wouldn't "extrapolating to infinite impedance" give you infinite voltage?
> >
> >   However, I believe your idea could be used by extrapolating to a very high
> > impedance where the reduction in voltage could be negligible.
> >
> >   Maybe Robert Stephens would want to try this. It certainly could be used
> > for small coils.
> >
> >   John Couture

Malcolm Watts replies:
> I think the last system that one should attempt to extract an
> accurate figure from is a small one. IMHO, a large system (one with a
> large Cself) running at much reduced Ep to bring it into the measuring
> range of test equipment would be the best option. For one thing,
> large Cself minimizes the influence of divider probes and the like.
> Is there any reason why the result could not be extrapolated to
> high values of Ep if an accurate measurement at low Ep could be made?
>
> ?
> Malcolm

Malcolm,  All,

Precisely!  Why would anyone with anything at all to do bother putting a
model airplane engine on a dynamometer?  On the larger coils, where you might
actually have a real need to know what the voltage actually is it makes much
more sense.  Also as you point out, the divider C becomes less and less of
the total distributed and topload C as the coil begins to reach Greg
Leyh dimensions.  As for linear extrapolation, I agree entirely with
you.............right up until the point where corona is formed around the topload
terminal or the HV bussbar or hot end of the divider column.  At this point
the linearity of system  voltage rise as a function of increasing input power hits
a sharp knee and essentially tops out.

I in-fact plan to calibrate my large divider by using a high powered
CW Tesla coil (my Coronatron) and calibrating the large divider at say
100 kV RMS with a NIST traceable calibrated probe operated in
parallel.  You then remove the 100 kV probe, retune your test setup
to resonance and crank up the power.  At some point when a flare
finally breaks out of the top end somewhere the measured voltage dips a little
and will successfully clamp any further voltage rise no matter how much power
you press into it.  I've seen the US NAVY scientists employ this
calibration procedure on a 100kW, 30 kHz (magnifier Tesla coil) high voltage
test generator and large divider that they used to operate at Forest
Port, NY, while invited there to observe some commercial testing.

Although it's certainly true that more top C reduces the voltage
developed, this is really only a pain-in-the-butt engineering wise in
CW systems of very high voltage where any topload C increses the
circulating currents in your resonator and a several hundreds of
picofarads from the combined Resonator,  Divider-Column, Bussbar and
Load-Under-Test  conspire quickly to demand heroic conductor dimensions or
the use of large diameter and expensive Litz wire for the resonator.
Since such setups are built solely to develop voltage, any C up there
which electrically shortens the resonator to less than 90 degrees and
conspires against it is a necessary evil.

On the other hand, with your typical disruptive coil system where the average
resonator currents are low enough to be handled by modest wire sizes even in
quite large systems producing  very impressive peak powers it seems to be that
a low ESL storage capacitance which is represented by a large ROC topload
actually helps make the streamers longer as it can source high
instantaneous currents as needed to the base of the outwardbound
streamer.  Although its capacitance reduces theoretical voltage gain,
its large ROC actually holds off the formation of voltage clamping
corona, allowing it to rise higher in reality than it could without
the large topload.

I look forward to the day when I have time to put a model airplane
engine on a dynamometer.

Robert W. Stephens
Director
Lindsay Scientific Co.
RR1 Shelburne, ON Canada L0N-1S5
Tel: 1-519-925-1771   Fax:
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