Re: NST Saturation during TC use?

To: tesla-at-pupman-dot-com
Subject: Re: NST Saturation during TC use?
From: "Ed Phillips" <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 12 Apr 2000 18:32:27 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> 
> Hi Mark,
> 
>         You have the capacitor hooked across the NST and you find a point
were the
> current jumps to super high levels as you turn up the variac.  This was
> discussed a few months back.  We think it is due to the magnetic shunts in
> the NST saturating and basically taking themselves out of the circuit so
> that the NST looses its current limiting ability.  I am now putting fuses
> on my NST inputs to prevent them from frying under this condition.  I also
> am playing with taking advantage of this to get more power out of NST
> without blowing them up ;-)  You definitely don't want to run an NST at
> those high currents for any length of time.  The gap firing does prevent
> this from being a problem in normal operation.  The firing allows the
> shunts to desaturate so they stay in control.  However, you can push it ;-)
> 
> Cheers,
> 
>         Terry
> 
> At 01:52 AM 04/12/2000 -0500, you wrote:
> >Hello, all (again)
> >
> >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
> >

	I think the effect is due to something different, at least as far as
I've observed it here.  When running a 15 kV, 60 ma transformer with a
great big 0.006 mfd 40 kV mica capacitor I noticed a similar effect when
I cranked the primary voltage up from zero.  My theory, which I think is
correct, is that at some primary voltage the voltage-varying inductance
of the core became series-resonant with the capacitor ("matched" case)
and the output voltage went very high (experimentally verified).  Net
effect is that the primary input looked like a low-impedance series
resonant circuit.  Notice that, in this case, the capacitor was smaller
than the "matched" value for 15 kV, 60 ma.