# Re: How To Measure NST Coupling

```Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Rob,

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I used the method given and it worked fine. I needed the k value at rated voltage for use in some Cres simulations (being off a little this way or that way makes a mess of the model). Anyway, I used a variac as the source for the NST measuring pri I and V at the NST input. I took readings in 10V increments from 10V to 140V. Quite interesting after I graphed the work. Looks like for my 12/60, k is at 0.938 at rated voltage.
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Here's a graph of the data in case anyone is interested.
http://www.classictesla.com/temp/k_nst1260.gif

Take care,
Bart

Tesla list wrote:

```
```Original poster: Rob Maas <robm@xxxxxxxxx>

Tesla list wrote:

The coupling coeff. k can be calculated from the relation

k = SQRT(1 - i_0/i_short)

i_0 is - at a given primary voltage V_p - the primary current drawn
by the transformer when the secundary winding is open

i_short is - again at primary voltage V_p - the primary current, but
now the secundary winding is shortened.

You can make a series of measurements for a certain range of V_p.
You will then observe that in the lower range of V_p the value of
k will rise rather steeply to its nominal value; increasing the
value to the rated value of V_p of the transformer the value of
k will taper off slightly.
Typical value of k for an NST is about k = 0.95
If it is a 230V transformer, the value of k at 50-75 V may be around
k = 0.90; at V_p = 150 V, the value of k may reach its max. value of
say k = 0.955; by increasing V_p to 230 V, k may drop to 0.950.

http://myweb.tiscali.co.uk/teslatutorial/index.htm

for more details.

Success with the measurements!,    Rob

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