Dave,
Another thing to consider is the spacing of the electrodes (at closest
approach), and the maximum available voltage. I've seen various cases
where the spark could barely jump the gap, and even missed some
firings. In these cases I had to set the electrodes so close that they
almost touched as they whirred by the fixed electrodes. In general,
when viewing the sparking gap with a video camera, The brightest glow
at the gap is seen while the electrodes are aligned. I was not able
to visually see evidence of firing before the gaps aligned. I would
imagine that in these cases, the
spark is not able to jump very much before the electrodes align,
perhaps not at all. In almost all cases, the gaps quench (stop
firing) while the electrodes are still aligned (before they pull apart
from each other). Under certain conditions this can cause a bad
problem of "re-firing" of the gaps while they are still aligned. Factors
that contribute to this are; slow rotary rpm, small diameter disc,
wide diameter electrodes, and small main capacitor. This problem
is unlikely to occur with 120bps sync rotaries however. It's mostly
likely to occur in higher break-rate systems (whether sync or async).
In any case rotary gaps do not quench the spark by stretching it.
The sparks quench on their own while the gaps are still aligned.
The only case where the sparks could possibly be quenched by
stretching is in a huge coil running at super low resonant frequency
of maybe 10kHz or whatever. The math shows that the mechanical
dwell time of the rotary gap tends to be large compared to the
typical quench times for a typical Tesla coil. This is what makes
the bad problem of rotary "re-firing" possible. But again it's usually
not a problem as explained above.
In any case when setting up a sync rotary 120bps system, you just
adjust the phase controller for maximum output spark length at full power.
Adjustment has to be done at full power because the best adjustment
variac position varies as the power level is varied (at least in NST powered
setups). I forget offhand if this occurs in PT/Pig powered setups (it probably
does since it has to do with how fast the capacitor charges and how much
voltage is available to jump the gap.
In some cases with NST powered coils, best spark length was obtained
when the spark gap fired below the maximum voltage that was reached.
The gap fired as the voltage was decending from maximum.
It was not possible to make the gap fire at maximum voltage. I think
the energy storage within the NST was upset when this was attempted,
and the gaps refused to fire and kicked the resonant charging system
into a different mode. This seems to have a lot to do with the
setting of the ballast (in ballasted setups), and the manner and rate of
voltage rise on the capacitor which depends on the relative charging
current of the transformer and the capacitance value. This may also
be a part of the reason that certain sized LTR cap values are best
for certain NST's.
Regards,
John Freau
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