re: sync, jitter, cap charge time

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

All,

I mentioned previously how my spark was much shorter using
an 1800 rpm sync motor than when using a 3600 rpm motor,
both with a 3.25" rotor for the spinning electrodes.  I think I found
the answer.

I had thought previously that it had to do with jitter.  Now I've
concluded that jitter is not much of a factor.  Instead I think the
problem was caused by a too short capacitor charge time.  The
key to understanding this is that the distance the arc can jump
ahead as the electrodes approach each other does not vary as 
the rotor speed is changed, but the distance between electrodes 
does change.  This is because an 1800 rpm motor needs 4 spinning
electrodes and the 3600 rpm motor needs 2 spinning electrodes.
So the 1800 rpm electrodes are positioned closer together on
the rotor.  This would make no difference at all if the arc did not
jump ahead as it fires.  But the arc does jump ahead as the 
electrodes approach each other.  Consider the distance the arc 
jumps ahead as a percentage of the total electrode travel of the 
electrodes between firings.  It will be seen that the jump ahead 
distance may be about 6% of the total electrode travel for the 
1800 rpm motor, but only about 3% for the 3600 rpm motor.  As
a result of this, the 1800 rpm motor gives the cap perhaps 3% 
less time to charge compared to the 3600 rpm motor.  I'm figuring 
the jump-ahead distance is about 1/8" or so combined for both
firing gaps on the rotor.  This all has nothing to do with mechanical
dwell time in the sense of "time of electrode presentation", or 
electrode overlap time.

Now, in the normal LTR phase mode, this all wouldn't matter very
much, because the voltage level is flattening out or falling just before
firing.  But in the atypical phase mode which my TT-32 demands,
it makes a difference.  In this atypical mode, the voltage is rising
very rapidly in a straight line.  It rises from 0 to full firing voltage in
about 1mS, just before firing.  Doing a few quick calcs shows that
I may be losing about 20% of my firing voltage because the gap 
fires too soon at the low rpm.  I lose about 235uS of charge time
using the low rpm motor.  A 20% loss of charging voltage may
result in a 20% spark length reduction.

Now taking this further, since the spark is so weak using the
1800 rpm motor, the spark loading is poor.  Poor spark loading
results in poor quenching.  When the quenching is slow, capacitor
charging time is lost.  Again, this wouldn't matter for a coil that
runs in the normal phase mode, but for the atypical phase mode
of the TT-32, with its rapidly rising voltage, a slow quench may
result in another 5% of lost charge time, which may equate to
another 5% spark length loss.  This gives a total of 25% spark
length loss, which is perhaps what I'm seeing.  Also, the firing
voltage, as seen on the scope, does seem to be about 20% to
25% lower.  

So that may be the answer to that mystery.  My apologies if anyone
was implying the above explanation when they mentioned a possible
change in capacitor charge time.  This explanation may also explain
why folks sometimes have problems when they use a rotor with many
closely spaced spinning electrodes, and they slow their rotor to
lower the bps.  Usually I assigned blame to refiring of the gaps, but
the percentage of jump ahead time may be an issue also.  All of
this argues for fast motors, and large rotors.

A mystery that remains unsolved is why does the normal phase
mode give a shorter spark length than the atypical phase mode
despite equal firing voltages, even using a large high speed 120
bps sync gap, in the TT-32 TC?  I think the only suggestion I've
heard so far is that the NST may feeding power to the system
during the gap firing in the atypical mode.  Another possibility
that comes to mind is that some firings may be skipped in the
normal phase mode, but I'm not seeing that on the scope.  
I'm measuring my voltage using a HV probe across half the NST
(ground to one end of NST).  I figure this should give an 
acceptable relative voltage measurement on the scope, between
phase modes.

Cheers,
John