LtSpice transformer - confusion

[Svein_B]

For simple modelling of SE OPT and IT I have used the standard method with inductors and a K directive.
The inductance ratio is the same as the transformer's impedance ratio - as described in the help files.

When I try to use the same method with a center-tapped primary I do not get the result I expected.

The voltage ratio works out as 1.4142 times that of the to the same transformer with a single primary. Consequently the impedance ratio is doubled. When I make the primary 10H+10H I get the same result as a single winding of 40H.


What am I doing wrong ? 😕


See the illustration below which is modelled with no loss or DC resistance to illustrate the point.

Svein.

 

[dave slagle]

Hey,

2 - 20Hy inductors in series on a common core nets you 80hy. In order to get the CT version to have 40hy's end to end each of the coupled inductors needs to be 10hy. (which is why your 10hy test gave you the proper results)

dave

 

[kenpeter]

10H in series with 10H, on separate cores, gives you 20H.
10H in series with 10H, in phase, on the same core, gives you 40H.
10H in series with 10H, out of phase, on the same core, gives you 0H.

 

[Svein_B]

Thanks guys!

Of course, we double the number of windings, and the inductance increases with the square of windings.

😱

Svein.