# Eddy current

Eddy current is the name given to a current that is induced in an extended electrical conductor in a magnetic field that changes over time or in a moving conductor in a temporally constant but spatially inhomogeneous magnetic field. The name was chosen because the induction streamlines are self-contained like vortices.

## description

If a metallic disc falls in a straight line through a magnetic field that is perpendicular to it, this induces eddy currents in the disc, which in turn generate two magnetic fields that slow the disc down. ( I : conventional current direction)

Eddy currents in turn generate a magnetic field which, according to Lenz's rule, counteracts the change in the field. As a result, the current is displaced from the center of the conductor at high frequencies and large cross-sections ( skin effect ).

If the conductor has a finite electrical resistance, it heats up. In the case of a magnetic field that changes over time, these eddy current losses correspond to a phase shift between current and voltage in the exciting coil that differs from 90 °, or a braking force in the case of relative movement between field and conductor. The force scales linearly, the power loss scales squarely with the frequency or the speed, provided that the skin effect is negligible. In particular, the force disappears when the relative speed between field and conductor is zero.

An exact calculation of the current distribution and the acting forces requires solving Maxwell's equations , for application-oriented geometries using numerical methods .

## Uses and countermeasures

### Amplitude and phase of the eddy currents

The eddy current testing is used for non-destructive material testing , and material characterization and based on the measurement of the amplitude and phase of eddy currents.

The shielding effect of non-ferromagnetic metallic housings against alternating magnetic fields is based on eddy currents in the housing, the magnetic field of which partially compensates for the fields. The quantity of the shielding effect is recorded with the shielding attenuation quantity . It is good when the eddy currents are slightly dampened by electrical resistance.

Attenuation of the currents leads to a phase shift in relation to the generating field. In the shaded pole motor , a short-circuit winding around the shaded pole creates a delayed field across the main field and, together with it, a rotating field. In the case of contactors and pull magnets operated with alternating current, a short-circuit winding around part of the armature prevents the pulling force from periodically becoming zero. Strictly speaking, the current in the short-circuit winding is not an eddy current because its path is predetermined by the shape of the conductor.

### Force effect

Force effects of magnetic fields generated by eddy currents using the example of a metal plate moving across the field lines of an external magnetic field

A rapidly increasing inhomogeneous magnetic field repels closed good conductors. This is used for magnet forming and electromagnetic pulse welding and with the Gauss rifle .

The braking force between a moving, closed conductor and a magnetic field is used in eddy current brakes in rail vehicles , free-fall towers, bicycle ergometers and to dampen mechanical vibrations in scanning tunnel microscopes and moving- coil measuring mechanisms .

Even if the magnetic field moves in relation to the conductor, a force is created and the conductor may be moved. Here, too, the force is proportional to the differential speed. In asynchronous motors , which can also be designed as linear motors , the rotating or traveling field is generated electromagnetically. In mechanical speedometers and tachometers , a rotating permanent magnet deflects an aluminum disc against a spring force. Eddy current separators work similarly , with which z. B. aluminum cans or foils are separated from waste.

In Ferraris electricity meters , eddy currents have both a driving and a dampening effect: a magnetic traveling field formed from the quantities to be measured, current and voltage, drives an aluminum disc, the rotation of which is strongly dampened by a permanent magnet.

### warming

Eddy currents in the iron block (above) and in laminated sheets (below)

Inductive heating of metal (e.g. in melting furnaces, cookware on induction hobs , workpieces for inductive hardening or thermal shrinking) uses an alternating magnetic field. In the production of electron tubes , a small cup or ring is inductively heated through the glass bulb in order to vaporize the getter inside . It is necessary that the metal trough to be heated, including its contents, is at least flat, is sufficiently electrically conductive in the area to be heated and is easily accessible from other, in particular flat, metal fittings of the tube.

Often the losses associated with the heating are an undesirable side effect when using alternating magnetic fields. As a measure against eddy current losses (for others see under iron losses ), iron cores of transformers and electric motors are not made solid, but rather " laminated ". The packages of mostly enamel-insulated electrical steel sheets are oriented parallel to the magnetic field lines so that the possible large eddy current paths are interrupted (see illustration); only smaller eddy currents can develop in the individual sheets, but their power loss is low. At high frequencies, ferrites or powder cores are used for transformers and chokes .

Conductors for high-performance oil transformers are designed as twisted conductors (Röbeldraht) with a cross-section made up of two adjacent stacks of insulated flat wires (reduces eddy current losses in the copper), which are continuously stacked, transposed, i.e. twisted successively, which creates flexibility for winding . The magnet windings in residual current and line circuit breakers are sometimes also made up of bundles of isolated individual conductors.

To avoid current displacement ( skin effect ), conductors for higher-frequency alternating currents consist of intertwined, isolated, parallel-connected individual conductors ( HF litz wire ).

## history

François Arago (1786–1853) (1824: "rotatory magnetism") made the first observations , supplemented and explained by Michael Faraday (1791–1867).

Heinrich Lenz established Lenz's rule in 1834 .

The French Léon Foucault (1819–1868) is credited with discovering eddy currents in 1855.

As a result, it was possible to work specifically to avoid undesired eddy currents and to use eddy currents for material analysis.

Eddy currents were first used in 1879 by David E. Hughes for metallurgical tests as a basis for sorting material.