re: NST modeling paper correction

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Peter,
          I think you make an excellent point:

On 20 Mar 01, at 17:31, Tesla list wrote:

> Original poster: "Peter Lawrence by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> Tero,
>      my understanding (quite possibly mistaken) is that a "current
> limited" transformer (Neon Sign Transformer being an example) does not
> have a constant mutual inductance between primary and secondary as
> that must vary as magnetic flux does or does not go through the shunts
> (shunts are what differentiates a current limited transformer from the
> more common type). Since K is a function of mutual inductance, it too
> cannot be a constant.
> 
> A typical current vs voltage graph of a current limited transformer
> shows almost a straight line from rated-volts-at-zero-amps to
> zero-volts-at-rated-amps. Am I wrong in assuming that this can only be
> the case if the "K" and/or mutual inductance of the transformer is
> different at these end points and all the points in between.
> 
> As near as I can tell, neither Tero's , nor Terry Fritz' microsim,
> model try to account for the variable mutual inductance of an NST. If
> I'm wrong here please point it out, I've tried reading Tero's web
> page, but cannot see anywhere where there might be a variable mutual
> inductance.
> 
> I am curious because I'm trying to wind my own current-limited
> transformer on a core with shunts that I bought at a second-hand
> electronics parts store, and I'm still working out the math for how
> many primary and secondary turns I need and what sizes of wire to use.

One presumes that at zero loading, k is pretty close to 1 so a 
straight turns ratio proportional to voltage ratio gives the required 
turns. The current ratios would dictate the shunting requirements I 
would think.
 
> So far I have checked my 6-20, and 7.5-30 NSTs, and neither one comes
> even close to the calculations in the power-transformer web page:
>         http://members.tripod-dot-com/~schematics/xform/xformer1.htm
> I'm planning on extrapolating from my NSTs rather than using the info
> in the power-transformer web page. My TCs are all static-gap, so a
> current limited transformer is the only way to go (I've already tried
> using MOTs, and in my experience they put out way too much current for
> a static gap).

The MOTs I've measured have strange characteristics and don't seem to 
lend themselves to phase-shifted type PFC. The primary current was 
essentially a low duty cycle spike on each half cycle positioned in 
the middle of the voltage halversine and the readings I got for a 
single unloaded transformer showed peak currents hitting 20A from the 
230VAC mains.

Regards,
Malcolm