19-11-2017, 11:14 AM
20-11-2017, 11:44 AM
Foucault currents (also called eddy currents) are electric current loops induced within the conductors by a changing magnetic field in the conductor due to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an electromagnet or AC transformer, for example, or by a relative movement between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop and the rate of change of the flow, and inversely proportional to the resistivity of the material.
According to Lenz's law, a parasitic current creates a magnetic field that opposes a change in the magnetic field that created it, and therefore the parasitic currents react again at the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to parasitic currents induced on the surface by the moving magnetic field. This effect is used in parasitic current brakes that are used to quickly stop rotating power tools when they are turned off. The current flowing through the conductor resistance also dissipates the energy in the form of heat in the material. Therefore, eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, motors and electric generators and other AC machines, which require a special construction such as laminated magnetic cores or ferrite cores for minimize them. Foucault currents are also used to heat objects in induction heating ovens and equipment, and to detect cracks and defects in metal parts using eddy current test instruments.