RE: More MOT supply questions

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dave,

On 18 Oct 2002, at 7:18, Tesla list wrote:

> 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!

Gaps firing with an unexpectedly low setting of the variac is an 
indication that resonant charging of the cap/s is going on. That in 
turn shows that leakage inductance is present in the circuit. The cap 
is ringing up.

Regards,
Malcolm



 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.
> 
> 
> 
> 
> 
> 
> 
>