# Discussion Forum

## Time-varying single conductor coil excitation

Hello,

I am trying to model the magnetic field around a single-conductor induction coil in a 3D-model in COMSOL 6.1. I want to enter a time-varying coil excitation in the Coil node, in my case 50A for the first 90s then 0A for the next 60s, to simulate that the induction is turned off. I am using two study steps: a Coil Geometry Analysis and then a frequency-transient step in which I define the frequency (400kHz in my case). When I put a fixed value of current for the excitation in the Coil node, the model runs well. But when I add the current as a function of time, the model runs until the first step (0.15s) and then never converges. I left it run overnight and it was still blocked there. I tried different function, like 50step(t), 50(t<90), etc., but it always blocks the same way. Even if I reduce the total time (let's say 1s at 50A then 1s at 0A), the model faces the same problem (after step 1 at 0.01s in that case).

Any clue on how to solve this issue?

Thanks!

3 Replies Last Post 2024年2月26日 GMT-5 17:35

Posted: 3 months ago
Updated: 3 months ago

You need to either take more direct control of your time-steps, specify a smoother transition between on/off current states, or both. There are a variety of ways to do this.

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara

Posted: 3 months ago
Updated: 3 months ago

Thank you for your answer. I increased the on/off transition using a smoothing with 2nd derivative and a transition zone of 1s. This did not solve the problem.

Could you expand on what you mean by take a more direct control? So far, I use an implicit solver type with BDF method. Maximum BDF order is 5 and minimum is 1. I tried to force the initial step to better either large (1s) or small (0.001s), but the issue remains. Should I keep BDF method? Or should I switch to explicit? Thank you in advance.

Posted: 3 months ago
Updated: 3 months ago

Consider setting "strict" or "manual" time stepping. If you still can't fix it after a number of tries, consider posting your model to the forum (see "how to reduce MPH-file size" in the small blue print on this page, if necessary) so that those who read here can take a closer look into the details.

-------------------
Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara