What Are Eddy Currents?

Just as turbulence forms when a body moves through air, when a magnetic field is found to be changing close to a conductor, eddy currents form within the conductor. These currents form a mechanism of loss for devices that operate with magnetic fields, which can be a good thing (e.g., magnetic braking) or a bad thing (e.g., transformers) to the operation of the device.

Phenomenon

Leon Foucault discovered the phenomenon of eddy currents in 1851. The mechanism through which they are formed is the tendency of a changing magnetic field to induce a current that opposes the change. When such a changing field is close to a solid conductor, little current loops form within the conductor, electrons traversing small circles that, in turn, generate a magnetic field which opposes the change.

Effect on Operation

Since the induced magnetic field will be of nearly opposite magnitude to the initial field, it will generate the equivalent of a frictional force. Such forces will convert energy from kinetic and magnetic to thermal energy. Such a conversion of energy can be considered good or bad in the operation of magnetic devices that generate strong fields close to conductors.

Unfavorable Effects

The biggest example of the unfavorable effect of eddy currents is in transformers. Transformers work by running a current at a given voltage through one current loop which creates a magnetic field. This field will induce a current and voltage in another loop, and through differences in the loops, voltages can be converted up or down as needed. However, when eddy currents are generated in the cores of these transformers, power escapes the conversion and the resulting voltage may not be as close to the target as intended.

Solutions for Unfavorable Effects

The best solution to minimize eddy currents is to use laminated cores. Instead of a solid piece of metal, laminated cores are made up of dozens of layers of the material, each layer insulated from each other. This limits the area within which an eddy current can form, and while it does not eliminate them, they are greatly reduced in size and their ability to leech energy is vastly compromised.

Favorable Uses for Eddy Currents

The formation of eddy currents is not necessarily a bad thing. Since eddy currents convert magnetic energy into heat, they have found uses as induction heaters. Their response to the kinetic motion of magnets also lends their use to electromagnetic braking. Since the eddy currents generate magnetic fields, these fields will push against the incoming magnets, in effect slowing them down. This action results in the smooth slowing and damping of the motion of magnets close to a conductor, with negligible physical wear.

References

• Eddy Currents; Jiří Lammeraner; 1966.