T.O. 33B-1-1
4-5
SECTION II
FACTORS EFFECTING EDDY CURRENTS
4.2
GENERATION OF AND FACTORS AFFECTING EDDY CURRENTS.
4.2.1
Generation of Eddy Currents.
4.2.1.1
Primary Electromagnetic Field.
Eddy currents are generated when a varying magnetic field penetrates an electrically conductive material. The usual
source of the varying magnetic field is the electromagnetic field produced by a coil carrying an alternating electric
current. This field is called electromagnetic because it has both electric and magnetic field components; however only
the magnetic component is involved in the generation of eddy currents. The rate at which the electromagnetic field
varies is called the frequency. The strength of the electromagnetic field at the surface of the conductor depends on the
coil size and configuration, the amount of current through the coil, and the distance from the coil to the surface.
4.2.1.2
Induction of Eddy Currents.
As the electromagnetic field from a coil penetrates a conductor, it generates eddy currents parallel to the surface of the
part and at right angles to the direction of the applied field (Figure 4-3). The frequency of eddy current flow is the
same as the electromagnetic field.
Figure 4-3. Primary and Secondary Magnetic Fields in Eddy Current Inspection.
