Are you doubling the flux to allow for looking only at a quarter? This is in your calculation of L. S.R.H. Hoole

Hello Mr.Hoole

In fact I am trying to reduce the geometry and subsequently mesh elements to get faster runs. 

when I divide the complete model to half (considering that I keep the drive current and number of turns same), the inductance(L) decreases to half but this is not the same for the coil resistance. That is why I think there should be a problem in my model. Hope I could use any of your kind advises.

Regards,

Hi Ehsan,

When you cut the geometry in half (or into a quarter) in order to reduce the number of mesh elements, it's important that this symmetry operation maintains the true geometry of the original object, and that the appropriate boundary condition is enforced at the plane of symmetry. If you have a multi-turn coil, I'm not sure that you can invoke symmetry at all, so I think you might not be able to utilise a half or quarter model to represent the original coil. This might explain your results.  

Regards,

Ray Simpkin

Hi, change the conductivity of wire or change the diameter of wire in the coil, it may work, good luck.

Hi Dear Ray,

For nearly two months I have been trying to run my simulations with whole model geometry. In fact, I am not able to use a complete geometry anymore due to the complexity of the coil I am working on. I read a comsol blog and subsequently decided to reduce the geometry, You could find it at:

http://www.comsol.com/blogs/exploiting-symmetry-simplify-magnetic-field-modeling

This writer started with a simple numerical multi-turn coil, however I think this could be applicable for a coil over a conductor since the same boundary conditions can be used for symmetry planes of conductor. From the other hand, the results of L seems to be promising for half and quarter model, the problem insists on the R part.

Regards,

Ehsan

Hi Mr. Shahram,

Those values inserted in simulation are mostly taken from a  experimental reference work which is done by SK Burk. The title of his works is:  

"A benchmark problem for computation of ΔZ in eddy-current nondestructive evaluation (NDE)"

And being honest, I don't know how changing the coil turns or conductivity  could help on matching the results, I really appreciate it if you could help me on this issue by providing a more detailed explanation. Thanks in advance.

Regards,

Ehsan

Hi Ehsan,

Thanks for the link to the comsol blog on the use of symmetry which I read with interest. This actually confirms what I thought might be happening. From what is described in the blog, the multi-turn coil feature in comsol appears to use what is usually called the 'current sheet' approximation. This replaces the current flowing through the multi-turn coil with a uniform current sheet that flows around the coil perimeter. This is an approximation which, once in place, permits the use of symmetry as a means to reduce the model size.

However, this approximation is only good for tightly wound coils and only for estimating the coil's inductance. The reason is that the inductance is primarily determined by the magnetic flux external to the coil (giving rise to the external inductance) which depends on the total current flowing in the wire. Therefore, using a uniform current sheet doesn't alter the result for the inductance too much from that obtained using the actual coil geometry.

However, the internal inductance (that due to the magnetic flux inside the wire) and - most importantly for what you are looking at - the coil's resistance will not be accurately determined by the current sheet approximation.

To get an accurate simulation of the resistance, the spatial distribution of current in the actual wire geometry must be detemined. In an isolated single turn of circular wire, this distribution of current will be cylindrically symmetric and follow the same kind of distibution as the electric field, that is, it will be concentrated near the surface of the wire and decay exponentially into the wire - the familiar 'skin effect'.

When additional turns of wire are added and brought close together (as in a tightly wound coil) the current distribution in the wire will be altered due to the 'proximity effect' which results from the Lorentz Force on the electrons in the wire due to the magnetic field in neighbouring parts of the coil. The net result is that the current gets concentrated in the parts of the wire cross section that are nearest to the other turns. The current is then no longer cylindrically symmetric within each turn of wire. This proximity effect therefore increases the resistance over and above that due to the skin effect since the current now flows in a reduced cross sectional area.

The proximity effect isn't modelled by the current sheet approximation. Therefore, one cannot utilise symmetry in the comsol multi-turn coil model for calculating resistance (or internal inductance).  A real multi-turn coil doesn't have symmetry with regard to the actual current distribution in the wire.

Regards,

Ray