Hvirvelstrømmene

Hvirvelstrømmene, also known as eddy currents, are electrical currents induced in conductive materials by a changing magnetic field. This phenomenon was first described by French physicist Léon Foucault in 1851, hence the alternative name Foucault currents. When a conductor is exposed to a varying magnetic flux, such as when it moves through a magnetic field or is placed near an electromagnet with alternating current, electromagnetic induction occurs. This induction generates circulating currents within the conductor itself.

The magnitude of these induced currents depends on the strength of the magnetic field, the rate of change of the magnetic field, the conductivity of the material, and the geometry of the conductor. Hvirvelstrømmene can lead to several effects. One significant effect is Joule heating, where the flow of current through the resistance of the material generates heat. This can be a desired outcome, as seen in induction cooktops and industrial heating applications, or an undesirable consequence, leading to energy loss and overheating in transformers and motors.

Conversely, hvirvelstrømmene can also be used to create braking forces. In electromagnetic braking systems, such as those used in trains and roller coasters, the motion of a conductive disc or rail through a magnetic field induces hvirvelstrømmene, which oppose the motion, thus slowing down the vehicle. They are also utilized in non-destructive testing methods, like eddy current testing, to detect flaws or variations in conductive materials. To minimize unwanted energy losses due to hvirvelstrømmene in applications like transformers, the conductive cores are typically laminated, meaning they are constructed from thin, insulated sheets of material.