RE: More MOT supply questions

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

Original poster: "Dave Hartwick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ddhartwick-at-earthlink-dot-net>

Ted,
Regarding which gaps are firing:  Assume MOT designation 1,2,3,4,5,6. 3 and
4 are the inners, 1 and 6 are the outers carrying max potentials, and 2 and
5 are in the middle of each group of 3.

3 - 2 - 1
|
4 - 5 - 6

None of these MOTs are matched, though they are the large ones producing
from 2200 to 2400 volts. I think I've seen 1, 6, 2, and 4 fire occasionally
at a main gap setting of 5/8" to almost 3/4". The individual gap distances
are not precisely set to exactly the same distance. I just eyeballed it. On
a couple of rare occasions when I get a system strike--one time I had a tank
wire incorrectly positioned--I got power arcs across all 6 gaps and shut
down immediately.

But that 5/8 - 3/4" main gap distance is too much, not necessary. I don't
want to turn my CP caps into explosive devices.  It behaves well at about
1/2" in which case the MOT safety gaps never fire. The main safety gap only
fires rarely.

Your conjecture, "high di/dt in the xfmrs due nature of load  generating
high voltage spikes" is interesting, but I'm not sure. I'm not the lists
greatest theoretician, as everybody must know at this point. Clearly
something is going on, because the main gap fires (1/2" setting)  when the
variac reaches about the 50% position! Keep in mind, however, that inner
MOTs 3 and 4 are connected directly to 240 VAC service. 1,2 and 5,6 are
connected series/parallel and connected to the Powerstat 1256D 240 VAC
variac. This being the case, the voltage developed by the supply at 50%
variac setting should be roughly <9 kV. That's calculate, I've not measured
it.

This supply, with shunts in, pulls an arc about 18" long when just at the
threshold of tripping the 30 amp breakers. Most of us have seen longer, but
I don't think I'd call that tame!

Dave




Original poster: "tesla by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<tesla-at-paradise-dot-net.nz>

Hi Team (Ted Linney in NZ with comment)
I also see these huge voltages and do intend to find out more as to how this
tame liittle 13kvrms supply is doing this. Those safety gaps sound like a
good idea, Can you comment on which gaps are firing is it the inner ones or
the outer ones or are they all going?
The interesting point I note is I'm getting big volts with 6MOT/88nF and you
are seeing the same with 6MOT/25nF hmmmmm I wonder if we are seeing high
di/dt in the xfrmrs due nature of load  generating high voltage spikes ?
Ted L

>SNIPPED BELOW
Let me say, however, that I have safety gaps on each MOT in my 6 pack and
I've not regretted that inclusion. Set to about 3/8-1/2", I've seen them
fire when perating at what might be considered running at the ragged
edge--leaning, large main gap distance settings, and >5 kVA power levels.