Re: Weird safety gap behaviour

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Gerry,
           I think I have a picture of how your system is wired but a
circuit diagram would be helpful. Knowing how long the leads between
the safety and main gap are would also be helpful as would knowing
the filter component values. You might have inadvertently created an
undesirable resonant circuit with the filter caps and the
transformer's leakage inductance. Your description of "snapping" into
resonance suggests the non-linearities inherent in the transformers
are coming into play when the secondary current exceeds a certain
value.

      The variac should play little if any part since it is supposed
to be close to an ideal transformer. In any scenario like this,
unintended resonances are the cause and either stray or unsuitable
component choices causing such resonances are the problem. I'm
surprised that your safety gap is planted on the coil side of the
filter. It would be effectively in parallel with the main gap if the
leads between it and the main gap are short. Your description of the
problem suggests this is not where the problem lies however.

Malcolm

On 2 Sep 2004, at 7:54, Tesla list wrote:

 > Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
 >
 > This is a repost, the first one never came out of the pipe :-((
 >
 > Hi All,
 >
 > This is a follow on to the weird SRSG behavior and I thank everyone
 > that responded to that post.
 >
 > I'm getting a safety gap behavior that I don't understand.  I got the
 > SRSG removed from the system so that added complexity is gone.  I have
 > two 15/30 NST's (magnetek) in parallel connected to the terry filter
 > that in turn is connected to a 3 terminal safety gap (center terminal
 > grounded).  One side of the safety gap is connected to Cp (2.5* Cres).
 >   The other side of the safety gap is connected to the primary.  Cp is
 > in series with the primary (standard TC topology for a two bushing
 > power source).  The safety gap consist of brass heat sinks that are
 > threaded for 3/8" carrage bolts.  The carrage bolts are adjusted to
 > just not fire when the unloaded NST is driven with a variac at 140Vac.
 >
 > 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.
 >
 >
 >
 >