Discussion Forum

Hi,

I have some questions on the magnetic field interface, for the multi-turn coil module in order to model 2D axi-symmetric coils, coil+shield assemblies, and then evaluate R and L at different frequencies. These questions include coil excitation methods and how to accurately interpret them. How would you compare results to those obtained empirically from say an impedence analyzer. etc etc.

Please find the attached models Coil_axysmmetric2D_DCandAC.mph, CoilandShield_axysmmetric2D_DCandAC.mph.

My questions are as follows and for the models listed:

Coil_axysmmetric2D_DCandAC.mph
1) For the magnetic fields interface, if we are solving Ampere’s law, (j*omega*sigma-omega^2*epsilon0)*Avector+curl(curlAvector)/mu0 = Je
in the frequency regime, using the multi-turn coil domain, why don’t I get the same value of (say) the magnetic flux density norm when I excite the coil using a current excitation of 28 mA (please see underlined model) as compared to a voltage excitation of Vcoil=0.47516 V (which equals Vcoil = Icoil X Rcoil = 28mA X 16.97ohms). How do both these excitations arrive at different values of A ?phi. If this is because of mf.Vind_1 then how is mf.Vind_1 calculated, i.e. if mf.Vind_1=root.mod1.mf.intmtcd1(-300*mf.Ephi*r*pi/mf.coilDomainArea_1), and mf.Ephi= -Aphit, where Aphit is calculated from Ampere’s law above, is it not the case of the chicken and the egg ? I am trying to understand how the solution works here i.e. the sequence of evaluation. I empirically verified this model of an 150 turn coil, with a current excitation of 28 mA at a freq of 175 KHz. for Hz (magnetic field z component) at a given z distance from the coil. They matched to within 1 dB, which shows good agreement b/n the model and experimental data. However, the question of voltage excitation remains.
2) Can I estimate that Zcoil= mf.Rcoil + j*omega* mf.Lc_1 ?


CoilandShield_axysmmetric2D_DCandAC.mph
1) Again, for the magnetic fields interface, if we are solving Ampere’s law, (j*omega*sigma-omega^2*epsilon0)*Avector+curl(curlAvector)/mu0 = Je
in the frequency regime, using the multi-turn coil domain. When I excite the 150 turn coil using a current excitation of 28 mA @ 175 KHz (please see underlined model) and evaluate the inductance of the coil+shield assembly, I obtain a close empirical match. mf.Lc_1 is the exact same value that you would obtain if the real coil+shield assembly were hooked to an impedance analyzer in the lab (the impedance analyzer lumps the value of Zdut into Ldut and Rdut based on the topology that is chosen). Obviously mf.Lc_1 is lower for the coil+shield assembly of this model example as compared to mf.Lc_1 as evaluated by the coil only model example (Coil_axysmmetric2D_DCandAC.mph), indicating that the shield is indeed loading the field of the coil.
2) However I cannot use Zcoil= mf.Rcoil + j*omega* mf.Lc_1 now because mf.Rcoil is still the resistance of the coil only. How would I be able to measure the total impedance of the coil + shield assembly (as I would obtain if I were to use an impedance analyzer) ?
3) Constant Current Vs. Constant voltage coil excitation in the multi-turn coil domain:
(a) For a Constant current excitation, progressing from a Coil only model to a Coil+Shield model with all coil parameters, including the coil excitation current remaining the same, would the loading of the shield on the coil field be represented by, say, the difference in Hz(magnetic field component in the z direction) between the two models ? Would I be right in saying that we cannot observe a reflected impedance effect from the shield loading when the coil is excited by a constant current.
(b) For a Constant voltage excitation, progressing from the Coil only model to the Coil+Shield model with all coil parameters, including the coil excitation voltage remaining the same, how would the addition of the shield reflect in the model. Would Vind increase so that Je is now lower and hence generate a lower field ? What is the sequence of evaluation here ?

Thanks for your time. Your replies and advise is greatly appreciated.


Regards,

Venkat