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External U vs. I

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Hello,
I'm quite new to COMSOL. I generally have doubts where to use external U vs. I or external I vs. U when I want to insert my designed model into a circuit. I think in any of those cases I have to choose the excitation method accordingly, right? Is there a guideline for that?
What exactly is the difference? I tried to compute the model "inductor-in-circuit" in the model library once using the external U vs. I and the other time using external I vs. U. the computation of the model with external U vs. I takes a lot longer but converges and eventually finishes, however the resulting plots for the magnetic flux density norm are very different. So I don't understand how these choices affect the physical reality of the problem and why are the results different?

5 Replies Last Post 16.11.2016, 10:37 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago 19.03.2013, 18:44 GMT-4
Hi

check carefully the doc about U vs I and I vs U. Normally COMSOL will only allow you to connect a CIR "connector" if there is an appropriate Terminal or current source in the other physics

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Good luck
Ivar
Hi check carefully the doc about U vs I and I vs U. Normally COMSOL will only allow you to connect a CIR "connector" if there is an appropriate Terminal or current source in the other physics -- Good luck Ivar

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Posted: 1 decade ago 20.03.2013, 04:16 GMT-4
Hi,

Could you please let me know what exactly this document on U vs I and I vs U is?

Thanks,
Hi, Could you please let me know what exactly this document on U vs I and I vs U is? Thanks,

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago 20.03.2013, 05:19 GMT-4
Hi

I used only the "active help" to find quite some explanations, try a search on "U vs I"

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Good luck
Ivar
Hi I used only the "active help" to find quite some explanations, try a search on "U vs I" -- Good luck Ivar

Sven Friedel COMSOL Employee

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Posted: 1 decade ago 20.03.2013, 09:26 GMT-4
Dear Bahar,

the difference is:

U vs. I is typically measures a current in your PDE model and assigns a source of a prescribed voltage there.
I vs. U is typically measures a voltage in your PDE model and assigns a source of a prescribed current there.

For instance a lumped port in the magnetic fields interface can only be excited with a current, because
voltage is not a variable that is solved for nor contained in the model. Hence you would need to define an I vs. U because you need to assign a current. In many other interfaces, e.g. electrical currents, where you could assign both a driving voltage or a current on a "normal boundary condition", you can use either U vs. I or I vs. U.

Sven
Dear Bahar, the difference is: U vs. I is typically measures a current in your PDE model and assigns a source of a prescribed voltage there. I vs. U is typically measures a voltage in your PDE model and assigns a source of a prescribed current there. For instance a lumped port in the magnetic fields interface can only be excited with a current, because voltage is not a variable that is solved for nor contained in the model. Hence you would need to define an I vs. U because you need to assign a current. In many other interfaces, e.g. electrical currents, where you could assign both a driving voltage or a current on a "normal boundary condition", you can use either U vs. I or I vs. U. Sven

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Posted: 8 years ago 16.11.2016, 10:37 GMT-5
Thanks,

When it says arbitrary voltage value put in the External I vs U block. and then I try to excite my device model using a pulsed Voltage source from cir.. What is the impact of the value of voltage given in External I vs U block.

Why is that the model is not getting excited when I used pulsed voltage source values .

< PS : I am using frequency domain study>
Thanks, When it says arbitrary voltage value put in the External I vs U block. and then I try to excite my device model using a pulsed Voltage source from cir.. What is the impact of the value of voltage given in External I vs U block. Why is that the model is not getting excited when I used pulsed voltage source values . < PS : I am using frequency domain study>

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