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Voltage source in AC

Posted Oct 25, 2012, 2:49 a.m. EDT Low-Frequency Electromagnetics Version 4.3a 4 Replies

Nicola Gagliardi
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Hi.
i'm studing an inductor to analyze inductance and resistence od the model.
I use electrical circuit and i work with AC frequency of about 20kHz and i have some questions:

1)Vsrc in voltage source is a rms value(effective value)?
2)Why i have to add a resistence in series with my circuit to work? So if i use only voltage source and External IvsU the program doesn't work. It works only if i add a resistence after the external IvsU (serie).

Thanks
Nicola
4 Replies Last Post Oct 25, 2012, 9:23 a.m. EDT
Andrea Ferrario COMSOL Employee

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Posted: 1 decade ago Oct 25, 2012, 9:07 a.m. EDT
Hi Nicola,

1) the value you enter in Vsrc is the amplitude. It is multiplied by a phase factor given computed from the value you write in the Phase textbox. It is also possible to write a complex value in Vsrc. The actual frequency-domain voltage source (phasor) is computed as Vsrc*exp(i*phase), so if you want to specify a complex phasor directly you can simply write it in Vsrc and leave phase = 0.

2) The External I vs U and U vs I features act, respectively, as a current-controlled voltage source, and as a voltage-controlled current source. So if your circuit is made by a voltage source and a I vs U only, you are effectively connecting two voltage sources in parallel and this can cause an inconsistency. The solution can be to add a small series resistance, as you did, or use the dual entity (U vs I), if the connected object supports it (e.g. Coil features in the AC/DC module can be connected to both entities).

--
Andrea Ferrario
Electromagnetics Group
COMSOL AB
Hi Nicola, 1) the value you enter in Vsrc is the amplitude. It is multiplied by a phase factor given computed from the value you write in the Phase textbox. It is also possible to write a complex value in Vsrc. The actual frequency-domain voltage source (phasor) is computed as Vsrc*exp(i*phase), so if you want to specify a complex phasor directly you can simply write it in Vsrc and leave phase = 0. 2) The External I vs U and U vs I features act, respectively, as a current-controlled voltage source, and as a voltage-controlled current source. So if your circuit is made by a voltage source and a I vs U only, you are effectively connecting two voltage sources in parallel and this can cause an inconsistency. The solution can be to add a small series resistance, as you did, or use the dual entity (U vs I), if the connected object supports it (e.g. Coil features in the AC/DC module can be connected to both entities). -- Andrea Ferrario Electromagnetics Group COMSOL AB
Nicola Gagliardi
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Posted: 1 decade ago Oct 25, 2012, 9:15 a.m. EDT
Hi Andrea,
and thanks for your answears.

Vsrc amplitude means that i have to multiply for sqt(2) in order to reach an effetive values? I mean 3000V in DC of Vrc are 3000*sqrt(2) in AC right (in order to have compatible power losses)?

Thanks
Nicola
Hi Andrea, and thanks for your answears. Vsrc amplitude means that i have to multiply for sqt(2) in order to reach an effetive values? I mean 3000V in DC of Vrc are 3000*sqrt(2) in AC right (in order to have compatible power losses)? Thanks Nicola
Andrea Ferrario COMSOL Employee

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Posted: 1 decade ago Oct 25, 2012, 9:21 a.m. EDT
Yes, correct.

--
Andrea Ferrario
Electromagnetics Group
COMSOL AB
Yes, correct. -- Andrea Ferrario Electromagnetics Group COMSOL AB
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 25, 2012, 9:23 a.m. EDT
Hi

perhaps you still need to have both physics self constained, and a CIR physics with only a source and no sink (resistance) is probably then not accepted.

I would have to try (I did and it seems that my comment above stands, but adding a 1MOhm resistance is not going to change much), never though of that, But also, a simple AC source you can write it out within the other physics without the extra CIR part.

Normally the V source in most physics is the amplitude, no the rms, but that needs to be rechecked for the CIR, might be different

Well other werre quicker than me here that's good ;)

--
Good luck
Ivar
Hi perhaps you still need to have both physics self constained, and a CIR physics with only a source and no sink (resistance) is probably then not accepted. I would have to try (I did and it seems that my comment above stands, but adding a 1MOhm resistance is not going to change much), never though of that, But also, a simple AC source you can write it out within the other physics without the extra CIR part. Normally the V source in most physics is the amplitude, no the rms, but that needs to be rechecked for the CIR, might be different Well other werre quicker than me here that's good ;) -- Good luck Ivar

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