RE: sync, phase, LTR-ness

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi John:

Your report about the "normal" vs. "atypical" modes of sync gap operation
got me thinking.  I never actually scop'ed the "atypical" waveform before
and your report that the voltage always reverses before firing in this mode
makes sense, and is consistent with the observed high current draw!  At the
peak cap voltage before the reversal, there is substantial energy in the
cap, but the gap is not yet aligned to fire.  So the cap voltage reverses
and passes through zero.  At the zero crossing, the energy that was in the
cap, plus a bit more from the ride down, is now stored in the magnetic
field of the NST.  But NST cores weren't designed to store energy like
this, so the core (shunts?) saturate, and this results in the observed high
current draw.  Funny how this seems to be coming up a lot lately?

Let's think up some more descriptive names for these modes.  I'm at a loss
for something short that describes this without resorting to some new
acronyms.  "Normal" vs. "Firing After Voltage Reversal with Core
Saturation" or "FAVRCS"?  Yuk!

Regards, Gary Lau
MA, USA

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

All,

Some follow up findings....

I finally took the trouble to add more caps to the MMC array,
to bring the C to 0.0189uF, up from the original 0.0142uF.
This brings it to more than 2X reso, instead of about 1.6X reso.
The NST is a 9/30.  With the added C, the system demanded
to be run in the normal phase mode, it no longer worked well
in the atypical mode which worked so well before with the
smaller C value.  As expected, now that the coil is running in
the normal mode, with a more suitable C value,  it works fine
with the 1800 rpm sync rotary, and gives about 32" sparks.
The results show that a 3.25" rotary disc works well provided the
coil is run in the normal phase mode.  The atypical mode
requires either a larger rotor, or a 3600 rpm motor.  

I looked back in my notes from a few years ago, and I see
evidence from other work I did which suggested that certain
LTR values demand the atypical phase setting.

Somehow the sparks seem a little weaker though.  Maybe
it's because I had to tune inward on the primary by about two
turns, so the primary surge Z is now lower.  I'm only using about
9.4 turns now. This may increase the gap losses to a noticeable
degree.  

I thought that maybe the caps would run cooler in this mode, 
both because there are more caps and because the voltage 
does not reverse direction before firing.  However, as far as
I can tell the caps get just about as warm as before.  I simply
added a forth string to raise the C value.

None of this explains why the system did not work well in the
normal phase mode before, despite the firing voltage being
the same for both the normal and atypical phase modes.
Maybe I measured incorrectly.

Now that the coil is running in the normal mode, the gap stops
firing if I move the phase position too far.  This matches the
behavior of the TT-42 TC, and that of other folk's coils who are 
using LTR systems.  There is no doubt that different input power 
levels require a different phase position.  If I set the phase for best
spark at 120 volts, then I need to reset the phase for 140 volts.
Otherwise, the gap stops firing.  In the atypical mode, the gap
does not stop firing as the phase is moved in either direction,
so this could be considered to be a good thing about the
atypical phase position.  Another good thing about the atypical
phase mode is it permits a smaller C value to be used, which
may result in lower gap losses.

To summarize:  

1)  Too small an LTR C value may demand the use of the 
     atypical phase mode.  In this mode, the cap voltage rises,
     then reverses to the opposite polarity before firing.  This
     mode (from previous work), tends to give a poor power factor.
     I'm not sure if there are certain C value nodes which demand the
     atypical phase position, or if it covers a wide range of C
     values around 1.6X reso.

2)  The atypical phase mode does not seem to work well with a
     low rpm motor and a small rotor disc.  This may be because
     the gap arc jumps ahead to the approaching electrodes.  This
     happens in all rotaries, but may be more important in the atypical
     phase mode.  I explained this in the previous posting.  The 1800 
     rpm motor with a small rotor works fine in the normal phase mode.

3)  In the normal mode, the gap stops firing if the power is increased.
     In the atypical mode, this doesn't occur.

4)  The larger C value demanded by the normal phase mode
     may indirectly result in larger gap losses.

Cheers,
John