Eddy current
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An eddy current (also known as Foucault current) phenomenon discovered by a French physicist Léon Foucault in 1851, is caused by a moving magnetic field intersecting 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 change in the external magnetic field (see Lenz's law). The stronger the magnetic field, or greater the electrical conductivity of the conductor, the greater the currents developed and the greater the opposing force. This is used to great effect in movement to electricity converters such as electrical generators and dynamic microphones. It can also be used to induce a magnetic field in aluminum cans, which allows them to be separated easily from other recyclables. Friction within the conductor, or resistance, cause a dragging effect that has been used for braking and damping. Superconductors allow perfect, lossless conduction which creates eddy currents that entirely cancel the external magnetic field, allowing magnetic levitation. Eddy currents are also the root cause of skin effect in conductors carrying AC current.
Eddy currents are used for braking at the end of some roller coasters; this mechanism has no mechanical wear, and produces a very precise braking force. Typically heavy copper plates extending from the car are moved between pairs of very strong permanent magnets.
Eddy currents create losses through Joule heating, and they reduce the efficiency of many devices that use changing magnetic fields such as iron core transformers and alternating current motors. They are minimized by selecting core materials that have low electrical conductivity or by using thin sheets (laminations) of magnetic material.
Use of eddy current techniques is a common method for the nondestructive examination (NDE) and condition monitoring of a large variety of metallic structures, including heat exchanger tubes, aircraft fuselage, and aircraft structural components.
An analogous eddy current is seen in water when dragging an oar, localised areas of turbulence give rise to vortices, which persist.
Some examples of uses of eddy currents:
- Electromagnetic brakes
- Metal detectors
- Induction cooker
- Traffic detection systems
- Vending machines (detection of coins)
- Safety Hazard and defect detection applications
- Electric meters (Electromechanical Induction Meters)
- Displacement sensors
- Eddy-current testing
- Eddy current adjustable-speed drives
[edit] References
- Fitzgerald, A. E., Kingsley, Charles Jr. and Umans, Stephen D. (1983). Electric Machinery, 4th ed., Mc-Graw-Hill, Inc., page 20. ISBN 0-07-021145-0.
- Sears, Francis Weston, Zemansky, Mark W. (1955). University Physics, 2nd ed., Reading, MA: Addison-Wesley, pages 616-618.de:Wirbelstrom
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