Eddy current

Eddy current (English eddy current) is called a current is induced in an extended electrical conductor in a time-varying magnetic field or in a moving conductor in a time-invariant, but spatially inhomogeneous magnetic field. The name was chosen because the induced current lines such as vertebrae are self-contained and have no fixed orbits.

Description

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

If the conductor has a finite electrical resistance heats up. This eddy current losses corresponds to the case of time-varying magnetic field deviates from 90 ° phase shift between current and voltage in the stimulating coil, or in the case of the relative motion between the field and conductor a braking force. The power scales linearly, the power dissipation quadratically with the frequency or speed, as far as the skin effect is negligible. In particular, the force vanishes when the relative velocity between the field and conductor zero.

An exact calculation of the current distribution and the forces requires the solution of Maxwell's equations for applied geometries by numerical methods.

Practical applications and Countermeasures

Amplitude and phase of the eddy currents

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

The shielding effect of non-ferromagnetic metal housing against magnetic fields due to eddy currents in the body, its magnetic field partially compensates for the incident fields. The quantity of shielding effectiveness is measured by the size of shielding effectiveness. It is good if the eddy currents are somewhat muted by electrical resistance.

Attenuation of the currents results in a phase shift relative to the generated field. When a shorted turn around the shaded pole motor shaded pole generated transverse to the main field and together with it a spinner, a delayed field. When powered with alternating current and traction magnet contactors prevents short-circuit turn is a part of the armature, that the tensile force is zero periodically. Strictly speaking, the current in the shorted no eddy current, because to him his way is dictated by the shape of the conductor.

Force effect

A rapidly increasing inhomogeneous magnetic field repels good conductors. This is for material forming of sheet metal and used the gauss rifle.

The braking force between a conductor and an inhomogeneous magnetic field is exploited in the eddy current brake of railway vehicles in free-fall towers, ergometers and for damping mechanical vibrations during scanning tunneling microscope and in moving coil instruments.

In contrast, when the magnetic field is moving faster than the ladder, it takes this with. Again, the force is proportional to the differential speed. In asynchronous motors, which can be designed as a linear motor, the rotating or traveling electromagnetic field is generated. In mechanical speedometers and tachometers, a rotating permanent magnet deflects an aluminum disc against a spring force. Work in a similar eddy current separators are separated with which, for example, aluminum cans or foil from waste.

In Ferraris electricity meters eddy currents act both driving and damping: A word formed from the variables to be measured current and voltage traveling magnetic field drives an aluminum disk, whose rotation is strongly damped by a permanent magnet.

Heating

Inductive heating of the metal ( for example, in melting furnaces, induction cooking hobs, for the inductive hardening) using an alternating magnetic field. Also, the getter of electron tubes is inductively heated by passing the flask, to evaporate it.

Often associated with the heating losses are an undesirable side effect in the use of alternating magnetic fields. As a measure against the eddy current losses ( see other iron losses ) iron cores of transformers and electric motors will not run massive, but " laminated ". The packages usually enameled electric sheets are thereby oriented parallel to the magnetic field lines, so that the eddy current paths are interrupted ( see figure). At high frequencies, ferrites or powder cores are used for transformers and chokes.

Head for higher frequency applications are made to avoid the displacement current ( skin effect) of interlaced, isolated, parallel strands (HF litz wire).