# Re: NST Inductance

• To: tesla-at-pupman-dot-com
• Subject: Re: NST Inductance
• From: Terry Fritz <twftesla-at-uswest-dot-net>
• Date: Sat, 01 Jan 2000 21:22:37 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

```At 11:03 AM 01/01/2000 +0000, "Jim Lux" <jimlux-at-jpl.nasa.gov> wrote:

>Calculate the estimated inductance from the impedance it must have to limit
>the current.  On a 15/30, the impedance has to be 15K/30m = 500 Kohms.
>Using 60 Hz, the inductance (calculated from X = 2 * pi * f * L ==> L =
>X/(2*pi*f) ==> L = x/377) is 500K/377 or, about 1300 H.... somewhat beyond
>the range of your inductance meter...
>
snip....

Hi Jim and All,

I am not sure that is the proper mechanism/model to use with NSTs.  NSTs
bypass much of the magnetic flux to the secondary through the shunts.

Consider this:

The voltage step up of a 15kV NST from 120VAC is 15000/120=125.  Since the
primary to secondary inductance is proportional to this number squared
(125^2=15625), the primary inductance would be 1300H/15625=0.0832H.

That would give the NST on open load current of 120/(2 x pi x 60 x 0.0832)
= 3.83 amps.  The actual open load is around 0.3 amps.

I have always used a standard transformer model with inductance based on
the primary open load current and step up ratio.  Then I used the
transformer's coupling to adjust for the secondary short circuit current.
In actual measurements and testing (especially in complex LTR design work),
with this model always seem to work very well.