# Re: [TCML] Model for NST

```Hi Carl,

```
You can model the behavior of the shunt as an inductor in series with the primary of an ideal transformer. The inductor approximates the leakage inductance created by the shunt. The leakage inductance can be estimated as follows (with numbers for a 15/60 shown):
```
```
1. Estimate output impedance of the NST by taking faceplate ratings of Vout and Iout (open circuit and short circuit).
```
Zout = 15000/0.06 = 250,000 ohms

2. Estimate turns ratio (N) by taking Vout/Vin

N = 15000/120 =  125

3. Calculate the reflected primary-side impedance:

Zpri = Zout/N^2 = 16 ohms

4. Calculate the equivalent primary-side leakage inductance:

Lpri = Zpri/(2Pi*F)

For F = 60 Hz,

Lpri = 42.4 mH

```
You can also add primary and secondary winding resistances to the model for a bit more accuracy.
```
Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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Carl Noggle wrote:
```
```Hey---

Does anyone have a good Spice-type model for a NST with magnetic shunt?
I can't figure out who to make one, and even the consultant for my
analysis program is at a loss.

Thanks for the help---

Carl
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```
```

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