Transformer ratios

[jim.bloor@smiths-aerospace.com]

Can anyone tell me whether it is possible to calculate the expected 
output voltages from a transformer with three windings. I can model 
two winding transformers OK and get reasonable results but my 
attempts with three windings are not so successful.

The transformer has a primary and two secondaries. The stainless 
steel core material has a non-linear BH curve. Does this affect the 
calculation? Using the excellent note on 'Calculating Transformer 
Inductances with Femm' by Cornelius Paul, I've calculated the Self 
Inductance, Mutual Inductance and Leakage Inductance and eventually 
the 'k' value for each pair of coils in turn, ignoring the third 
coil. The inductance values are reasonable but the 'k' values do not 
match the practical values obtained by a considerable margin. What 
should I being doing with the third coil whilst calculating for the 
other two?

Any help would be much appreciated.

Jim Bloor