You can handle this much more effectively in a frequency-transient study, because it is normally not required to resolve the AC-waveform explicitely in coupled thermal-electromagnetic problems.
Have a look into the inductive heating example in the model library.
If you want to insist in explicit time dependent modeling you can write the time dependent function directly into the respective field in your boundary condition. Take care then, to set the time dependent solver to strict time stepping and choose the time stepping in order to resolve the waveform properly.
Cheers
Edgar
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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
