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I would like to make a transformer model with a time-variant mutual inductance coefficient. Does it work at (K1 L1 L2 var)?

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  • Why do people think this is a bad question?
    – us2012
    Commented Aug 20, 2013 at 11:53
  • @us2012 Dunno take a +1 from me and I'm interested in the answer.
    – Andy aka
    Commented Aug 20, 2013 at 18:38
  • @Andyaka It's not my question, but I'm interested in the answer, too.
    – us2012
    Commented Aug 20, 2013 at 23:07
  • @us2012 oops. Hope the OP comes back with an answer!
    – Andy aka
    Commented Aug 21, 2013 at 7:05

1 Answer 1

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For a standard inductor, unfortunatley it looks like you can not. The internal model of an inductor (non chan) looks roughly like (taken from the ltwikis site about transformers):

enter image description here

Note how the K factor is used to split the inductance between the mutually coupled and the leakage inductance. In theory if it would be just the K statement not supporting any input variables you could use that one to directly set the inductance values.

Unfortunately, although it accepts a lot of functions to set the inductance value, inductors can not depend in any way on any time varying functions.

The same seems to hold true for chan inductors, though I never played enough with them to be absolutely sure. It seems that most (all?) spice dialects do need to calculate some internal numbers for an inductor before starting a transient analysis. Probably nobody bothered to do this on demand for each timestep (or probably this is even too computationally expensive).


It is evidently evaluating the factor at the start/before op point search, as indicated by the following message in the log when you try to set it outside the allowed range:

Reducing mutual coupling coefficient from 3 to 1.
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  • Inability to use coupled inductors is also true for behavioural inductors (Flux=f(x)). Also, a minor note, even if you can specify, or if LTspice says 1 for coupling, in fact it's 0.999999... (a limited range), but it's so close to 1 that it can be regarded as such. Commented Sep 21, 2016 at 12:17

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