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Currents of Foucault

Currents of Foucault

Foucault currents are the currents induced in masses of conducting metal that are immersed in a variable magnetic field or that, in motion, through a constant or variable magnetic field. In any case, it is the variation of the magnetic flux that generates these currents. The phenomenon was discovered by the French physicist Jean Bernard Léon Foucault in 1851.

In high frequency: using cores with magnetic materials that have low electrical conductivity (such as ferrite).

Foucault currents create energy losses through the Joule effect. More specifically, these currents transform useful forms of energy, such as kinetics, into unwanted heat, so that it is generally a useless, if not harmful, effect. At the same time, they reduce the efficiency of many devices that use variable magnetic fields, such as iron core transformers and electric motors. These losses can be minimized considerably.

Description of the phenomenon of Foucault currents

Foucault currents are caused by the movement (or variation) of the magnetic field that passes through a conductor. The relative motion generates the circulation of electrons, that is, the current, in the conductor, according to Faraday's law. These electrons, moving in vortices, in turn generate a magnetic field in the opposite direction to the variation of the applied magnetic field (see Lenz's law). The phenomenon is accentuated:

  • with the increase of the applied magnetic field (if it is sinusoidal with the square of the amplitude)
  • With the increase in conductor conductivity traversed by the magnetic field.
  • With an increasing relative speed between the magnetic field and the conductor.
  • if the magnetic field is periodically variable with the increase of its frequency (if it is sinusoidal with a law proportional to the square of the frequency)

In this case, the greater the intensity of the vortex currents that develop and the stronger the magnetic field they generate (and oppose the original magnetic field).

The current that develops in the conductor has a rotating shape because the electrons are subject to the Lorentz Force, which is perpendicular to the direction of the electrons in motion. Therefore, they rotate to the right or to the left, depending on the direction of the applied field and the variation of the field increasing or decreasing. The resistivity of the conductor dampens these currents.

Foucault currents generate energy losses when the conductor is heated (Joule effect). This phenomenon in many applications is negative because this generation of heat has no useful effect. For example, a decrease in efficiency is determined in transformers and electric motors.

These losses can be attenuated by choosing a magnetic core that has a low conductivity (for example: ferrite, silicon steel) or subdividing the magnetic core into thin layers, electrically insulated (lamination). In this way, the electrons can not cross the insulating layer between the laminations and the area enclosed by their path is reduced.

Thus, the greater the number of laminations per unit area, parallel to the applied magnetic field, the greater the reduction of dispersed currents. Current losses of parasites are not always an unwanted phenomenon.

Foucault current applications

  • The magnetic brake used in trains and attractions for amusement parks. In the first case, during braking, a magnetic field is applied to the metal wheel by an electromagnet that generates the leakage currents in the wheel. These currents find resistance to flow through the metal generating heat and this increases friction, allowing a more intense braking with less chance of wheel slip on the tracks. In the second case, permanent magnets are used, through which passes a blade of a good conducting metal (copper or aluminum).
  • Recycling of waste: it is used to separate the aluminum cans, inducing a magnetic field in them.
  • Superconductors. Superconductors generate currents without losses. The scattering currents that are produced are equal and opposite to the external magnetic field, so they are null, allowing magnetic levitation.
  • Non-destructive structural tests (NDT)

     

    Dispersion currents are commonly used for non-destructive testing and to examine defects in a large number of metallic structures, for example: heat exchangers, fuselages and other structural parts of aircraft.
  • Generate heat in induction furnaces.
  • Induction cookers.
  • Electronic systems of currency recognition (vending machines).
  • Dynamic microphones.
  • Proximity sensors.
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Last review: November 27, 2018

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