Electromagnetic brake
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Electromagnetic brakes, also called eddy current brakes, seek to retard motion or cause deceleration in a moving system. Unlike conventional brakes, which operate by causing friction between moving parts, in an electromagnetic brake kinetic energy is converted to heat without contact between the moving parts.
[edit] Construction and operation
A typical electromagnetic brake is composed of a metal disc (rotor) attached to a rotating axis and electromagnets or permanent magnets positioned to generate a magnetic field intersecting the disc. The electromagnet allows control of the braking action by varying the strength of the magnetic field; to produce a braking force, electric current is passed through the electromagnets. As the metal disc rotates inside the magnetic field, electric currents, called eddy currents are induced inside it. Those currents then generate a magnetic field in opposition to the original field thus creating a force which acts to decelerate the rotating disc.
Heat is created in the disc as a direct result of the electrical resistance of the disk material and the current induced in it; this heat represents the kinetic energy being dissipated and is analogous to the heat generated by a conventional friction brake.
Because the induced current is proportional to the speed of the disc, the braking torque decreases as the disc decelerates resulting in a smooth stop. Electromagnetic brakes of this fashion are therefore unable to completely stop a moving part or to keep it still. A small friction brake might be coupled for that purpose. Exception is use to slow an engine, as once below a certain speed (engine specific) the engine will stall.
Regenerative braking is not used with brakes of this type since the induced current is dissipated as heat and is not practical to recover.
A similar type of braking is used in modern roller coasters, which differs only in that permanent magnets are used instead of electromagnets (and that the brakes are linear rather than rotary). This means that the brakes require no electricity and that the braking force cannot be adjusted. It doesn't need to be adjusted, since it is engineered into the design of the coaster.
[edit] References
- Sears, Francis Weston, Zemansky, Mark W. (1955). University Physics, 2nd, Reading, MA: Addison-Wesley.
- Siskind, Charles S. (1963). Electrical Control Systems in Industry. New York: McGraw-Hill, Inc.. ISBN 0-07-057746-3.
- How it worksde:Wirbelstrombremse
