What is Eddy current?

zxteh

can u tell me in detail about the Eddy current......

king11_yann

不可以用英文哦～这是中文论坛～An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a moving (or changing) magnetic field intersects a conductor, or vice-versa. The relative motion causes a circulating flow of electrons, or current, within the conductor. These circulating eddies of current create electromagnets with magnetic fields that oppose the effect of the applied magnetic field (see Lenz's law). The stronger the applied magnetic field, or greater the electrical conductivity of the conductor, or greater the relative velocity of motion, the greater the currents developed and the greater the opposing field.

It is important to appreciate that eddy currents are created when a conductor moves across a constant, uniform magnetic field, as well as when a stationary conductor encounters a varying magnetic field. Both effects are present when a conductor moves through a varying magnetic field, as is the case at the top and bottom edges of the magnetised region shown in the diagram. Eddy currents will be present wherever the conducting object, which is moving, experiences a magnetic field, and not just at the boundaries. However, in some geometries, transient eddy currents can cause charges to collect on the extremities of the object and these charges then produce electric fields that oppose any further flow of current.

The swirling current set up in the conductor is due to electrons experiencing a Lorentz force that is perpendicular to their motion. Hence, they veer to their right, or left, depending on the direction of the applied field and whether the strength of the field is increasing or declining. The resistivity of the conductor acts to damp the amplitude of the eddy currents, as well as straighten their paths. Lenz's law encapsulates the fact that the current swirls in such a way as to create an induced magnetic field that opposes the phenomenon that created it. In the case of a constant, uniform applied field, the induced field will always be in the opposite direction to that applied. The same will be true when a varying external field is increasing in strength. However, when a varying field is falling in strength, the induced field will be in the same direction as that applied, in order to oppose the decline.

Eddy currents create losses through Joule heating. More accurately, eddy currents transform useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. Hence they reduce the efficiency of many devices that use changing magnetic fields, such as iron-core transformers and electric motors. They are minimized by selecting magnetic core materials that have low electrical conductivity (eg ferrites) or by using thin sheets of magnetic material, known as laminations. Electrons cannot cross the insulating gap between the laminations and so are unable to circulate on wide arcs. Charges gather at the lamination boundaries, in a process analogous to the Hall effect, producing electric fields that oppose any further accumulation of charge and hence suppressing the flow of eddy currents. The shorter the distance between adjacent laminations (ie the greater the number of laminations per unit area, perpendicular to the applied field), the greater the suppression of eddy currents.

The loss of useful energy is not always undesirable, however, as there are some practical applications. One is in the brakes of some trains. During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The eddy currents meet resistance as they flow through the metal, thus dissipating energy as heat, and this acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.

The term eddy current comes from analogous currents seen in water when dragging an oar: localised areas of turbulence known as eddies give rise to persistent vortices.