Re: Weird safety gap behaviour

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

Original poster: "Ed Phillips" <evp-at-pacbell-dot-net> 

I'm now measuring the total voltage across the hot terminals of the
safety
gap differentially (BTW getting the same answer as when measured single
ended between a hot terminal and ground and then doubling the
result).  Following are the measurements:

I slowly raise the variac voltage from 0 to 90V.  The peak differential
voltage across the two hot safety gap terminals increases to about
16KV.  With no further increase of the variac voltage, the 16KV starts
to
run away (exponentially it seems) and snaps to 30KV.  The safeties are
now
firing and healthy arcs are coming from the secondary top load.  The
safetys dont fire until after the runaway so don't seem to cause the
runaway.  It takes about one second to snap to 30KV.   In 30KV mode, the
variac output voltage is still 90V so the variac doesn't seem to be part
of
the runaway.

The next interesting thing is that I start to lower the variac
voltage.  The safety gap voltage stays locked on 30KV until the variac
voltage is reduced to 70V.  At this point, the safety gaps stop firing
and
the voltage returns to normal.

The safety gap spacings measured 0.21 and 0.26 inches.

Next, I set both safeties to about 0.20 inches.  Results were the same.

Next, I set both safeties to about 0.17 inches.  The runaway again
starts
at 16KV but the peak voltage after runaway is now about 26KV

If the safeties fired first and didn't regulate that well, then I could
understand that a transient response would be superimposed on top of the
steady state response.  But the runaway happens first and then the
safeties
fire.

Any ideas what is causing the runaway??  (maybe an engineering
explaination)

Also, could someone explain reverse voltage mode from an engineering
point
of view??

Many thanks for any responses,

Gerry R."

	Common behavior.  As you raise the primary voltage you reach a point
where the core permeability changes enough to cause the charging circuit
(leakage inductance and primary capacitor) to become resonant at the
power line frequency.  That causes the sudden jump in voltage.  Same
thing happens in reverse as you lower the primary voltage.  I once wiped
out the multiplier resistor in a multimeter doing the same sort of
thing.  In my case the "jump" occurred at only about   25 volts on the
primary.

Ed

	Do a search on "ferro resonance" and "parametric oscillations [in L/C
circuits]" and you'll probably find out some interesting things.