Mechanism and principle Eddy current brake

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a metal sheet moving right under magnet, illustrating how linear eddy current brake works. in drawing magnet drawn spaced apart sheet reveal vectors; in eddy current brake magnet located close sheet possible.



a circular or disk eddy current brake


an eddy current brake consists of conductive piece of metal, either straight bar or disk, moves through magnetic field of magnet, either permanent magnet or electromagnet. when moves past stationary magnet, magnet exerts drag force on metal opposes motion, due circular electric currents called eddy currents induced in metal magnetic field. note conductive sheet not made of ferromagnetic metal such iron or steel; copper or aluminum used, not attracted magnet. brake not work simple attraction of ferromagnetic metal magnet.


see diagram @ right. shows metal sheet (c) moving right under magnet. magnetic field (b, green arrows) of magnet s north pole n passes down through sheet. since metal moving, magnetic flux through sheet changing. @ part of sheet under leading edge of magnet (left side) magnetic field through sheet increasing gets nearer magnet. faraday s law of induction, field induces counterclockwise flow of electric current (i, red), in sheet. eddy current. in contrast, @ trailing edge of magnet (right side) magnetic field through sheet decreasing, inducing clockwise eddy current in sheet.


another way understand action see free charge carriers (electrons) in metal sheet moving right, magnetic field exerts sideways force on them due lorentz force. since velocity v of charges right , magnetic field b directed down, right hand rule lorentz force on positive charges qv×b toward rear in diagram (to left when facing in direction of motion of sheet) causes current toward rear under magnet, circles around through parts of sheet outside magnetic field in 2 currents, clockwise right , counterclockwise left, front of magnet again. mobile charge carriers in metal, electrons, have negative charge, motion opposite in direction conventional current shown.


due ampere s circuital law, each of these circular currents creates counter magnetic field (blue arrows), due lenz s law opposes change in magnetic field, causing drag force on sheet braking force exerted brake. @ leading edge of magnet (left side) right hand rule counterclockwise current creates magnetic field pointed up, opposing magnet s field, causing repulsive force between sheet , leading edge of magnet. in contrast, @ trailing edge (right side), clockwise current causes magnetic field pointed down, in same direction magnet s field, creating attractive force between sheet , trailing edge of magnet. both of these forces oppose motion of sheet. kinetic energy consumed overcoming drag force dissipated heat currents flowing through resistance of metal, metal gets warm under magnet.


the braking force of eddy current brake proportional velocity v, acts similar viscous friction in liquid. braking force decreases velocity decreases. when conductive sheet stationary, magnetic field through each part of constant, not changing time, no eddy currents induced, , there no force between magnet , conductor. eddy current brake has no holding force.


eddy current brakes come in 2 geometries:



in linear eddy current brake, conductive piece straight rail or track magnet moves along.
in circular, disk or rotary eddy current brake, conductor flat disk rotor turns between poles of magnet.

the physical working principle same both.







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