T.O. 33B-1-14-5SECTION IIFACTORS EFFECTING EDDY CURRENTS4.2 GENERATIONOFANDFACTORSAFFECTINGEDDYCURRENTS.4.2.1 GenerationofEddyCurrents.4.2.1.1 PrimaryElectromagneticField.Eddy currents are generated when a varying magnetic field penetrates an electrically conductive material. The usualsource of the varying magnetic field is the electromagnetic field produced by a coil carrying an alternating electriccurrent. This field is called electromagnetic because it has both electric and magnetic field components; however onlythe magnetic component is involved in the generation of eddy currents. The rate at which the electromagnetic fieldvaries is called the frequency. The strength of the electromagnetic field at the surface of the conductor depends on thecoil size and configuration, the amount of current through the coil, and the distance from the coil to the surface.4.2.1.2 InductionofEddyCurrents.As the electromagnetic field from a coil penetrates a conductor, it generates eddy currents parallel to the surface of thepart and at right angles to the direction of the applied field (Figure 4-3). The frequency of eddy current flow is thesame as the electromagnetic field.Figure 4-3. Primary and Secondary Magnetic Fields in Eddy Current Inspection.
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