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More services...Coil excitation: Current vs Voltage
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5 Replies
Last post: February 2, 2011 10:49pm UTC
July 16, 2010 3:03pm UTC
Coil excitation: Current vs Voltage
Hi,
I am new to Comsol 4.0 and I am trying to understand how to modelize an electro-magnet. I used the "Magnetic Fields (mf)" physical interface. I get two different results depending whether I use current or voltage in the coil excitation of the "Multi-Turn Coil Domain Settings". The problem is that both simulation should give the same result.
Here's my setup: 1 [in] iron core, 4 [in] copper coil and 2.5 [in] length. The coil is made of 3450 turns of 21 AWG wire (fill factor taken into account) rated at 1.2 [A] max and 42 [Ohm/km]. According to my calculation, the length of the wire is
688 [m] = N * L_average_of_1_turn
= N * 2Pi * {(R_coil - r_core)/2+r}
= N * Pi* (D_coil + D_core)/2
So the total inductance of the coil should be
29 [Ohm] = 688 [m] * 42 [Ohm/km].
With the relation V=RI, I should have a voltage of 35 [V] to have a current of 1.2 [A]. So the simulation shoud give me the same results whether I use voltage or current in the "Coil excitation" section of the "Multi-Turn Coil Domain".
The thing is that 35 [V] gives me 372.5 [Tesla] and 1.2 [A] gives me 0.061 [Tesla]. What could explain that big difference?
The file is attached to this message.
Thank you very much.
Alexandre
Student in mechanical engineering
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July 16, 2010 6:56pm UTC in response to Alexandre Savard
Re: Coil excitation: Current vs Voltage
Hi
read carefully the doc and test out the three options in 2D axi, fo the same geometry
a) single rectangle normal surface current ,
b) single turn coil and,
c) multi turn coil.
There are subtile differences, as COMSOL adds in new variables to compensate for the induced voltage in the single and multiturn coil BC's. There are also differences w.r.t skin effects in harmonic frequency sweeps.
You will then probably better understand the differences, these have been common eror sources for coil FEM analysis
Have fun Comsoling
Ivar
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August 25, 2010 8:23pm UTC in response to Alexandre Savard
Re: Coil excitation: Current vs Voltage
Hi Alexandre,
I am also struggling to understand how to correctly setup up a single domain multi-turn coil that can models skin and proximity effects. Have you been able to figure out how the definition of the current density for a voltage-driven coil is derived? I would greatly appreciate any information you might be able to share.
Regards,
Johan
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August 26, 2010 1:16am UTC in response to Johan Gustafsson
Re: Coil excitation: Current vs Voltage
I simply ended up defining the current density of the coil as it was the limits of my problem. I modelized a rectangle in a 2D axi-symetric setup and added en external current density with the mf physics.
I'm sorry I can't help you much.
If you feel what I did could help, don't hesitate!
Best luck!
Alexandre
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August 26, 2010 6:19am UTC in response to Johan Gustafsson
Re: Coil excitation: Current vs Voltage
Hi,
as explained in another thread, I'd like to go even a step further and define turn by turn, using litz cable with individually isolated wire strands. Our coils probably have only a few turns (maybe 10), and I will have to see local effects.
So I am deeply interested as well, and will keep investigating and posting results.
Matthias
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February 2, 2011 10:49pm UTC in response to Alexandre Savard
Re: Coil excitation: Current vs Voltage
Hi,
I am new to Comsol 4.0 and I am trying to understand how to modelize an electro-magnet. I used the "Magnetic Fields (mf)" physical interface. I get two different results depending whether I use current or voltage in the coil excitation of the "Multi-Turn Coil Domain Settings". The problem is that both simulation should give the same result.
Here's my setup: 1 [in] iron core, 4 [in] copper coil and 2.5 [in] length. The coil is made of 3450 turns of 21 AWG wire (fill factor taken into account) rated at 1.2 [A] max and 42 [Ohm/km]. According to my calculation, the length of the wire is
688 [m] = N * L_average_of_1_turn
= N * 2Pi * {(R_coil - r_core)/2+r}
= N * Pi* (D_coil + D_core)/2
So the total inductance of the coil should be
29 [Ohm] = 688 [m] * 42 [Ohm/km].
With the relation V=RI, I should have a voltage of 35 [V] to have a current of 1.2 [A]. So the simulation shoud give me the same results whether I use voltage or current in the "Coil excitation" section of the "Multi-Turn Coil Domain".
The thing is that 35 [V] gives me 372.5 [Tesla] and 1.2 [A] gives me 0.061 [Tesla]. What could explain that big difference?
The file is attached to this message.
Thank you very much.
Alexandre
Student in mechanical engineering
Try V=IZ instead and it will work. Then again, I have no idea about how or what your core is doing as I did not see your attached file.
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