# Discussion Forum

## Electric resistance of a coil?

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RSS feed   |   Turn on email notifications   |   3 Replies   Last post: September 25, 2011 9:16am UTC

W. Wong

September 24, 2011 2:58pm UTC

Electric resistance of a coil?

After creating the geometry (the coil), assigning a material (copper) and assigning electric potentials, the current through the coil can be calculated. I suppose this is calculated from the potentials and from the resistance. I also suppose that the resistance is determined from the conductivity, length and cross-sectional surface. I want to know the value of the total resistance.

My question is: how do I make COMSOL V4.1 display the value of the total electric resistance of the entire coil?

Ivar Kjelberg

September 24, 2011 3:07pm UTC in response to W. Wong

Re: Electric resistance of a coil?

Hi

The resistance of the coil is calculated from the full solution of your equations, hence not only the DC resistance from the resisitivity and the section and length but also from potential resisitivity changes due to Temperature, but also, particularly in AC mode the impedance will depend on back EMF fields from the magnetic behaviour, as well as the effect of the magnetic field on the current circulating in the coil.

Therefore it's important to know how you define your coil: as a bulk slab section and define a number N of turns, or a more sophisticated equation taking into effect the N separate coils wires/turn (see the doc onthe physics behind the new v4.2 BC for ACDC single and multiple coil BC's).

It's worth to play with the ACDC tutorial examples and verification models to get used to these different effects, such as the skin effect etc.

Once you have solved your system, i.e. by defining a given voltage, you can integrate the current over the coil section to get the TOTAL current, then i.e. divide by N to get an AVERAGE current (but do note that this average value might differ from the varying current density plot you will see for your solution, all depends on how you define your coli BC ;)

For the DC value you can and should always do your hand cacluations to be sure all BC are coherent.

--
Good luck
Ivar

W. Wong

September 24, 2011 4:28pm UTC in response to Ivar Kjelberg

Re: Electric resistance of a coil?

The coil geometry that I am using is defined using Helix, 6 turns, major radius 2 m, minor radius 2 m, axial pitch 0 m and radial pitch 10 m. In other words, it is a flat spiral coil. Furthermore, the voltage that is assigned is shaped like an overdamped oscillation, so (rapidly) changing, but in a constant direction. So I should not have to deal with the skin effect, right?

Can the electric resistance be displayed DIRECTLY on the screen AT ALL in COMSOL? The purpose of this simulation is to verify a mathematical model (in which the electric resistance R of the entire coil is inserted). The idea is to use COMSOL to determine the value of R more accurately. Determining the resistance by hand therefore beats the purpose of this COMSOL model.

Ivar Kjelberg

September 25, 2011 9:16am UTC in response to W. Wong

Re: Electric resistance of a coil?

Hi

If I read you correct, you have a 6 turn helix of radus 2 meters, and pitch 10 meter radially, but you do not say the section of your coil wire, this gives only the length, if you resolve the length of a flat spiral.

The DC resistance of a constant section wire is R[Ohm] = rho_electric[Ohm/m] * Section[m^2] / Length[m]

The fact that you have an overdamped pulse does NOT mean you do NOT have skin effects, (and variations of) during the transient and at DC level (careful with my double negations, I have been speaking Franch for too long ;)

COMSOL as such calculates densities and NOT absolute values, to get total average or absilute values you ned to integrate or average the COMSOL variables over the relevant boundary(ies). This is by design, it is how FEM applies, we calcualte on the small mesh elements dx*dy*dz but we humans mostly work with the absolute values. This requires some careful thoughts wen you define your model and when you inteprete the results.

The "true" resistance is the ratio between the voltage drop between two boundaries and the curent flowing trough one boundary (assuming curent conservation between the two boundaries considered) this might vary with time etc (impedance) in particularly when you have magnetic effects acting on the current (moving electrons)

--
Good luck
Ivar

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