Eddy Currents

Eddy Currents 

(Foucault currents), the closed electrical currents produced in a massive conductor by a change in the magnetic flux passing through it. Eddy currents are induction currents that are created in a conducting body either because of a change with time of a magnetic field in which the body is situated or because of the body’s motion in a magnetic field, which thereby alters the magnetic flux through the body or some part of it. The more rapid the change in the magnetic flux, the larger the eddy currents.

Unlike the electrical current in wires, which flows along precisely defined paths, eddy currents are contained directly within the conducting mass, thus forming vortex-shaped circuits. These current circuits interact with magnetic flux which they generate. According to Lenz’s law the magnetic field of the eddy currents is in a direction opposite to the change in the magnetic flux that induces these currents.

Eddy currents lead to a nonuniform distribution of the magnetic flux in a cross section of a magnetic circuit. This is explained by the fact that the magnetizing force of the eddy currents, which is acting in the opposite direction to the main flux, is the largest in the center of the section because this part of the cross section is surrounded by a greater number of eddy current circuits. Such an “expulsion” of the flux from the center of the magnetic circuit’s cross section becomes more pronounced with increases in the frequency of the alternating current and in the magnetic permeability of the ferromagnetic material. At high frequencies the flux flows only through a thin surface layer of the core, producing a reduction in the apparent magnetic permeability (as averaged over the cross section). The phenomenon of magnetic flux expulsion from the ferromagnetic material, which changes with increasing frequency, is analogous to the electrical skin effect and is called the magnetic skin effect.

According to the Joule-Lenz law, eddy currents heat the conductors in which they are produced. Consequently they cause energy losses (eddy current losses) in magnetic circuits (in transformer cores, in AC windings, and in the magnetic circuits of machines).

To reduce the energy loss from eddy currents (and the detrimental heating of the magnetic circuits) and their “expulsion” of the magnetic flux from the magnetic materials of the magnetic circuit in machines and AC apparatus, the magnetic circuits are made from separate sheets that are insulated from each other (for example, by a special varnish) rather than from a solid piece of ferromagnetic material (electrical steel). This division into sheets placed at right angles to the direction of the eddy currents restricts the circuits available for the eddy current paths, thus greatly reducing the magnitude of these currents. At very high frequencies, ferromagnetic materials are not suitable for magnetic circuits; in these cases they are made from ferrites, in which eddy cur-rents are practically nonexistent because of the very high resistance of these materials.

When a conducting body moves in a magnetic field, the induced eddy currents cause an appreciable mechanical interaction between the body and the field. This is the principle which, for example, is the basis of the braking action in the moving parts of electrical kilowatt-hour meters, in which an aluminum disk rotates in a permanent magnetic field. In AC machines with a rotating field, a solid metallic rotor is pulled along by the field because of eddy currents induced in it. The interaction of eddy currents with the alternating magnetic field is the basis for various types of pumps for molten metals.

The ejection of nonferromagnetic metal bodies from the field of an AC coil is in the same category of mechanical effects caused by eddy currents.

Eddy currents are also produced in a conductor carrying alternating current, thus creating a nonuniform current distribution through the conductor’s cross section. At the moment when the conductor current is increasing, the induction eddy currents are in the same direction as the primary current at the conductor’s surface, but they are opposite to the current along the axis. As a result, the current inside the conductor is reduced, but at the surface it is increased. High-frequency currents are practically confined to a thin layer at the conductor’s surface; there is no current inside the conductor. This phenomenon is known as electrical skin effect. To reduce eddy current losses, wires of large cross section for AC are divided into separate strands, which are insulated from each other.

Eddy currents are utilized for the melting and surface hardening of metals, and their force effect is used in vibration dampers for the moving parts of instruments and apparatus and in induction brakes (where a massive disk is rotating in the field of an electromagnet).

REFERENCE

Neiman, L. R., and P. L. Kalantarov. Teoreticheskie osnovy elektrotekhniki, 5th ed. Moscow, 1959.