Eddy currents are the currents induced in the masses of conductive metals 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).
Eddy currents create energy losses through the Joule effect. More specifically, these currents transform useful forms of energy, such as kinetics, into unwanted heat, so it is generally a useless effect, if not harmful. In turn, they decrease the efficiency of many devices that use variable magnetic fields, such as iron-core transformers and electric motors. These losses can be minimized considerably.
How do eddy currents work?
Eddy 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 of the conductivity of the conductor crossed by the magnetic field.
- With an increasing relative speed between the magnetic field and the conductor.
- if the magnetic field is periodically variable with increasing frequency (if it is sinusoidal with 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 moving electrons. 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.
Eddy currents generate energy losses by heating the conductor (Joule effect). This phenomenon in many applications is negative because this generation of heat has no useful effect. For example, in transformers and electric motors a decrease in efficiency is determined.
These losses can be mitigated by choosing a magnetic core that has a low conductivity (eg ferrite, silicon steel) or by subdividing the magnetic core into thin, electrically insulated layers (lamination). In this way, the electrons cannot cross the insulating layer between the laminations and the area enclosed by their path is reduced.
So the greater the number of laminations per unit area, parallel to the applied magnetic field, the greater the reduction in stray currents. Current parasite losses are not always an unwanted phenomenon.
Examples where eddy currents are used
- The magnetic brake used in train and amusement park rides. In the first case, during braking, a magnetic field is applied to the metal wheel by an electromagnet that generates leakage currents in the wheel. These currents meet resistance to flow through the metal generating heat and this increases friction, allowing more intense braking with less chance of wheel slip on the tracks. In the second case, permanent magnets are used, through which a blade of a good conductive metal (copper or aluminum) passes.
- Electromagnetic levitation trains using linear motors.
- Waste recycling: used to separate aluminum cans, inducing a magnetic field in them.
- Superconductors. Superconductors generate currents without losses. The stray currents that are produced are equal and opposite to the external magnetic field, so they are zero, which allows magnetic levitation.
- Non-destructive structural testing (NDT). Stray currents are commonly used for non-destructive testing and for examining defects in a large number of metal structures, for example: heat exchangers, airframes, and other structural parts of aircraft.
- Generate heat in induction furnaces.
- Induction cookers.
- Electronic coin recognition systems (vending machines).
- Dynamic microphones.
- Proximity sensors.
