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Re: NST Saturation during TC use?



Gary, Terry, Ed, and others,

I re-designed my safety gap yesterday and today.  My old safety gap was
slightly
flexible, and the gap tended to expand when I re-hooked it up (decent design,
poor materials).  My new one is now inflexible and works well, though I set it
too close (.156") and was firing far too much (even when the RSG was set "just
right").  I re-set it to about .170", but haven't tested again.  I also set the
RSG gap spacing to about .025", as they used to be about .032".

My primary cap is a LTR-style (23nF) MMC.  My PFC caps (~180uF) are
connected in
parallel with the NSTs (2x 15/30).  I had intended to measure the NST current
this morning before, during, and after the RSG is firing, but was unable to get
the RSG to fire consistently (safety gap set too close).  I can very easily
adjust the NST's PFC (in increments of 60uF...), and this may be something that
I'll try before I hook up more trannies.

Would I end up messing anything up if I just set the variac to 120V and threw
the switch?  (This Question REALLY shows my experience level ;-)

Thanks!

Mark

Tesla List wrote:

> Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
>
> Hi Mark:
>
> I wouldn't recommend running your NST into your cap without a properly set
> spark gap, even for a tiny brief time.  Depending upon how close your cap is
> to being mains-resonant with the NST secondary, the cap/NST voltage will
> climb to very hazardous (to the NST and cap) levels, kind of like flooring
> the gas with the clutch down.  That growling power must be going someplace,
> right?
>
> I have to differ with Terry's answer suggesting that the NST primary current
> will settle to more reasonable levels once your spark gap is in the circuit.
> My coil uses a single unmodified 15kV/60mA NST with 233uF PFC cap, Larger
> Than Resonant (LTR) .02uF tank cap, and static gap.  Even with the PFC, the
> NST pulls 20 Amps from my 15A variac.  When I attempted to use a sync RSG, I
> up-ed the tank cap to .03uF, and the 20A-rated PTC current surge protector
> in my variac melted down, so I don't know the actual current, but WAY more
> than 20A.
>
> While running with no gap does represents a worst case current-shunt
> saturation and current draw case, running with LTR caps and a normal spark
> gap may cause MUCH higher than expected current draw (and performance
> too!!!).
>
> Regards, Gary Lau
> Waltham, MA USA
>
> >Original Poster: "Mark Broker" <broker-at-uwplatt.edu>
> >
> >While testing current loads to determine whether I needed to run my RSG
> >motor and my NSTs on seperate circuits, I noticed that the NST current is
> >non-linear.  My testing was conducted on the full primary with the spark
> >gap open (so that it wouldn't arc) without a secondary, and with 100% PFC.
> >
> >I measured a fairly linear current-voltage relationship up to about 85 V on
> >the variac.  Then the current jumped from about 5A to about 14A (roughly)
> >between 90 and 115V!  The NSTs also started to hum very loudly around 90V.
> >
> >This is rather important, as my gap (distance ~.030" each side) won't fire
> >(when TC is fully set up) until the variac is at about 102V.  I wanted to
> >run two more NSTs for a total of 15/120 (15/60 now), but I'm already maxing
> >out my 10A variac.  I figured that it would run 15A ok for a couple minutes
> >straight, and I may add a small fan in the future.  But I won't even
> >consider running more than 20, even for only a couple seconds (even a 10A
> >variac is starting to get expensive to replace)!
> >
> >I was wondering if this core saturation phenomenon was common, and if the
> >current drops when the gap starts firing (was unable to test when TC was
> >running).
> >
> >Thanks
> >Mark
> >