Nonlinear harmonic amplitudes in air coils above and below a steel plate as a function of tensile strength via finite-element simulation

One way to measure nonlinear harmonic amplitudes associated with a steel plate is to measure the secondary electromotive forces (EMFs) in air coils above and below the plate when the field is generated by ac currents in primary coils also above and below the plate. The flux density inside the plate is not being measured in this case, but rather the flux density of the stray field outside the material. The harmonic behavior of this stray field is different from the harmonic behavior of the flux density inside the material itself. Indeed, as a demonstration of this different behavior, we present finite-element modeling (FEM) for the given configuration using ANSYS EMAG 2D in the transient mode (with eddy currents turned on in the plate). It is found, after Fourier transforming the computed EMF in the secondary coils, that the third harmonic actually increases with increasing dislocation density and decreasing grain size (or, equivalently, with increasing tensile strength). This behavior is opposite in trend to what is found for the flux density inside the material from prior modeling or from experiment. An explanation for this is provided in the paper. The increase in the third harmonic is not only predicted by the FEM, but also by measurements with air coils, as cited in this paper.