Environmental Engineering Reference
In-Depth Information
Eddy Currents
When an alternating current of relatively high frequency is applied to a coil close to
a sheet of steel, the resulting magnetic fi eld induces eddy currents in the steel
(Fig.
13.10
). These eddy currents in the turn produce a magnetic fi eld that tends to
cancel the original fi eld; and the total magnetic fi eld is the vector sum of the two
fi elds. A measure voltage would be induced in a sensor coil situated in the magnetic
fi eld. The generation of eddy currents is, at relatively high frequencies, a near-
surface effect; so, if the surface of the steel adjacent to the coil is damaged, the
magnitude of the eddy currents will be reduced; and, consequently, the total mag-
netic fi eld will be increased. This will result in a variation in the voltage in the sen-
sor coil. A fl aw in the sheet of metal, on the surface away from the coils, will not be
detected; and, depending upon its distance from the surface, a cavity within the
sheet will not infl uence the eddy currents either.
Fig. 13.10
Eddy-current principle. Courtesy Schlumberger
Tool Principle
The measuring sondes of pipe analysis type tools consist of an iron core with the
pole pieces of an electromagnet at each end, and multiple sensor pads arrayed
between the pole pieces (Fig.
13.11
). The arrays are arranged to ensure complete
coverage of the inner surface of the casing. Each of the pads contains a transmitting
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