Application Catalog

209 - Magnetic Measurement Analysis Accounting for Eddy Currents


Module: TR



Magnetic properties affect the performance of products composed of magnetic circuits using magnetic material. This requires that the magnetic properties are assessed accurately. The H current method and excitation current method can be used to measure magnetic properties of magnetic material. The excitation current method will be used in this Application Note.
An excitation coil is wrapped around a ring test piece and a current is run through it. Doing this generates a magnetic field inside the excitation coil, and magnetic flux flows though the test piece placed inside the coil. Output voltage will be measured in the B coil wrapped around the ring test piece. Magnetic flux is obtained from the output voltage waveform, and then magnetic flux density is calculated from a cross section of the target. When the excitation current frequency used in the measurement is high, eddy current is generated in the magnetic material, so effects also appear in the waveform of the measured magnetic properties.
This document introduces an analysis model is created based on the excitation current method, and the differences in magnetic properties obtained when the frequency is changed are shown.

Relationship between Frequency and Magnetic Properties (B-H Curve)

Fig. 1 Relationship between the magnetic flux density and the magnetic field

The relationship between the average magnetic flux density and magnetic field of the magnetic materials obtained when changing the frequency of the alternating current passed through the excitation coil to 1(Hz) and 10(Hz) is shown in fig. 1 (B-H curve), respectively. When eddy current is generated, responsiveness is lowered in the measured average magnetic flux density due to eddy current, and a hysteresis loop is drawn.

Magnetic Waveform Produced by B Coil

Fig. 2 History Graph of B Coil Magnetic Flux

The history graph of the B coil magnetic flux when the excitation current frequency is 1(Hz) is shown in fig. 2.

Eddy Current Density Contour Plot

Fig. 3 Current density contour plot

Current density contour plot is shown in fig. 3. Eddy currents concentrate on the outside perimeter of the magnetic material due to the skin effect.