Inductance of a coil on a thick ferromagnetic metal plate

Abstract

We study the frequency-dependent inductance of a small air-cored coil of wire placed flat upon various ferromagnetic metal plates. The change in the complex inductance of the coil, measured with an HP 4194A impedance analyzer, is reported for frequencies between 1 kHz and 1 MHz. The metal plates consisted of commercially pure (99.7% and 99.9%) Ni, commercially pure (99.9%) Fe, and a suite of medium carbon steels. For the steel plates, inductance changes were consistent with a simple halfspace model that treats the metal as a continuum defined by a conductivity and a relative initial-permeability where these material parameters are isotropic, local, and uniform throughout the plate. The inductance changes for Ni and Fe could not be fit to the half-space model for any values of and , but were consistent with a model that assumes a thin (10 m) surface layer with a significantly reduced permeability-a dead layer. We tested the existence of the hypothetical dead layer in several ways. We found that the inductance increased when the surface was chemically etched (presumably eroding the dead layer) and decreased when the surface was mechanically polished (presumably increasing the dead layer). We also found that the inductance of the Fe and Ni samples decreased substantially over the course of days and months when exposed to air.

Scaling relation for the inductance of a coil on a ferromagnetic half-space

“…Quantitative measurements have been made recently for the inductance of a coil placed next to flat ferromagnetic metal plates [1][2][3]. Results for a suite of low carbon steels were in good agreement with the analytic solutions of Cheng [4] and Dodd and Deeds [5], which describe the metal as a structureless half-space defined by an isotropic and uniform conductivity, 0', and initial relative permeability, Jl.…”

Transient response of a driver-pickup coil probe in transient eddy current testing

“…In particular, it was shown that the interaction between the conductor and the driver coil could be understood and represented as a complex frequencydependent self-inductance coefficient. Previous works make mention of complex inductances [18], but this phenomenon was not clearly described in terms of electromagnetic processes. The analytical expression for this complex coefficient, which accounts for real (inductive) and imaginary (resistive) elements associated with the rod, fell out of the theory naturally.…”

Assessment of the performance of different EMAT configurations for shear horizontal and torsional waves

“…The wave amplitude was predicted without any adjustable parameters, with a 20% accuracy over a 200 kHz range and over a wide range of bias field strength. The main sources of error in the model are the uncertainties in the input values: the intrinsic physical variability of the magnetic properties of ferromagnetic materials, together with their frequency dependencies [15].…”