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Re: Weird safety gap behaviour



Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi Malcolm,

 > 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.

Cp
O---------------------- 1K -----------------------------------||-------->
pri tap
     hot        |                                 |                 |
                |   |   filter                    O               O
                  |                           safety
O---------------------------------------O            SRSG (removed for this
experiment)
    gnd        |                           safety
                |   |   filter                    O               O
                  |                                 |                 |
O----------------------- 1K -------------------------------------------->
pri non tap
      hot

 > Knowing how long the leads between
 > the safety and main gap are would also be helpful as would knowing
 > the filter component values.

There is no wiring.  They are next and parallel to each other and  connected
by 1/8" thick brass sheeting 2 inch wide.  Of course the main gap is removed
for this phenomenum.  The safety gap has three 12 awg wires (hot+, GND,
hot-) going down (thru the shelf) to the terry filter.  Length 6 inches.
Ground of the terry filter is connected to NST chassis via another 6 inches
of 12awg.  Terry filter (each side) has 8 1800V MOVs in series and 8
0.0047uf in series.  The 1K wire wound resisters are of course in series
with the two hot wires coming down thru the shelf.  The hot sides of the
filter are connected to each NST via 2 inches of 12awg (physical arrangement
is NST filter NST, so 2 inches to one NST and 2 inches to the other NST).

NST's and filter are on the bottom shelve.  The Cp, safety, and main gap
(removed for this purpose) are on the second shelf.  The primary and base of
the secondary are on the third shelf.  Cp is 26.6nf (2.5*Cres) [4 strings of
14 0.068uf + 1 string of 14 0.1uf].  All high current paths are kept as
short as possible in the second shelf using 0.5x0.5" brass bar to connect
the strings together and 1/8" thick 2 inch wide brass sheet to connect the
brass bar to the safety and rotor gap.  Two connections up to the primary.
Each is done with a pair of 12 awg speaker wires acting as one conductor.
The non tap is 6 inches long and the tap is 2 feet.  Primary is 17 turns to
the tap point of 1/4" tubing

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.

Yes.  I'm thinking the NST shunts are saturating.


 >
 >       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.

The variac output voltage does not change when this happens so I too dont
think it is part of the problem.


 >
 > 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.
 >  >
 >  >
 >  >
 >  >
 >
 >