Eddy current brake
An eddy current brake, like a conventional friction brake, is a device used to slow or stop a moving object by dissipating its kinetic energy as heat. However, unlike electro-mechanical brakes, in which the drag force used to stop the moving object is provided by friction between two surfaces pressed together, in an eddy current brake the drag force is an electromagnetic force between a magnet and a nearby conductive object in relative motion, due to eddy currents induced in the conductor through electromagnetic induction.
A conductive surface moving past a stationary magnet will have circular electric currents called eddy currents induced in it by the magnetic field, due to Faraday's law of induction. By Lenz's law, the circulating currents will create their own magnetic field which opposes the field of the magnet. Thus the moving conductor will experience a drag force from the magnet that opposes its motion, proportional to its velocity. The electrical energy of the eddy currents is dissipated as heat due to the electrical resistance of the conductor.
In an electromagnetic brake the magnetic field may be created by a permanent magnet, or an electromagnet so the braking force can be turned on and off or varied by varying the electric current in the electromagnet's windings. Another advantage is that since the brake does not work by friction, there are no brake shoe surfaces to wear out, necessitating replacement, as with friction brakes. A disadvantage is that since the braking force is proportional to velocity the brake has no holding force when the moving object is stationary, as is provided by static friction in a friction brake, so in vehicles it must be supplemented by a friction brake.
Eddy current brakes are used to slow high-speed trains and roller coasters, to stop powered tools quickly when power is turned off, and in electric meters used by electric utilities.
Construction and operation
Circular eddy current brake
Electromagnetic brakes are similar to electrical motors; non-ferromagnetic metal discs (rotors) are connected to a rotating coil, and a magnetic field between the rotor and the coil creates a resistance used to generate electricity or heat. When electromagnets are used, control of the braking action is made possible by varying the strength of the magnetic field. A braking force is possible when electric current is passed through the electromagnets. The movement of the metal through the magnetic field of the electromagnets creates eddy currents in the discs. These eddy currents generate an opposing magnetic field (Lenz's law), which then resists the rotation of the discs, providing braking force. The net result is to convert the motion of the rotors to heat in the rotors.
Japanese Shinkansen trains had employed circular eddy current brake system on trailer cars since 100 Series Shinkansen. However, N700 Series Shinkansen abandoned eddy current brakes in favour of regenerative brakes, since 14 of the 16 cars in the trainset used electric motors.
Linear eddy current brake
The principle of the linear eddy current brake has been described by the French physicist Foucault, hence in French the eddy current brake is called the "frein à courants de Foucault".
The linear eddy current brake consists of a magnetic yoke with electrical coils positioned along the rail, which are being magnetized alternating as south and north magnetic poles. This magnet does not touch the rail, as with the magnetic brake, but is held at a constant small distance from the rail (approximately 7 mm).
When the magnet is moved along the rail, it generates a non-stationary magnetic field in the head of the rail, which then generates electrical tension (Faraday's induction law), and causes eddy currents. These disturb the magnetic field in such a way that the magnetic force is diverted to the opposite of the direction of the movement, thus creating a horizontal force component, which works against the movement of the magnet.
The braking energy of the vehicle is converted in eddy current losses which lead to a warming of the rail. (The regular magnetic brake, in wide use in railways, exerts its braking force by friction with the rail, which also creates heat.)
The eddy current brake does not have any mechanical contact with the rail, and thus no wear, and creates no noise or odor. The eddy current brake is unusable at low speeds, but can be used at high speeds both for emergency braking and for regular braking.[1]
The TSI (Technical Specifications for Interoperability) of the EU for trans-European high-speed rail recommends that all newly built high-speed lines should make the eddy current brake possible.
The first train in commercial circulation to use such a braking system has been the ICE 3.
Modern roller coasters also use this type of braking, but in order to avoid the risk posed by potential power outages, they utilize permanent magnets instead of electromagnets, thus not requiring any power supply, however, without the possibility to adjust the braking strength as easily as with electromagnets.
Lab experiment
In physics education a simple experimental configuration is sometime used to illustrate Lenz's law. When a magnet is dropped down a conducting pipe, eddy currents are induced in the pipe, and these retard the descent of the magnet. As one set of authors explained
- If one views the magnet as an assembly of circulating atomic currents moving through the pipe, [then] Lenz’s law implies that the induced eddies in the pipe wall counter circulate ahead of the moving magnet and co-circulate behind it. But this implies that the moving magnet is repelled in front and attracted in rear, hence acted upon by a retarding force.[2]
Laboratory work ranges from comparison of time-of-fall with a magnet in a cardboard tube, to oscilloscope reading of current in a loop wound around the pipe,[3] to use of multiple magnets.[4]
See also
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Wikimedia Commons has media related to Eddy current brakes. |
- Telma retarder – an eddy current brake system made by Telma, a company that is part of the Valeo group
- Regenerative braking – generates electricity rather than heat and hence is usually more energy efficient
- Electromagnetic brakes (or electro-mechanical brakes) – use the magnetic force to press the brake mechanically on the rail
- Linear induction motor can be used as a regenerative brake
Notes
- ↑ "Wirbelstrombremse im ICE 3 als Betriebsbremssystem hoher Leistung" ("Eddy-current brake in the ICE 3 as high-efficiency service brake system", by Jürgen Prem, Stefan Haas, Klaus Heckmann, in "electrische bahnen" Vol 102 (2004), No. 7, pages 283ff
- ↑ M.H. Partori & E.J. Morris (2006) "Electrodynamics of a magnet moving through a conducting pipe" Canadian Journal of Physics 84:253–71
- ↑ C. S. Maclatchy, P, Backman, L. Bogan (1993) "A quantitative magnetic braking experiment", American Journal of Physics 61:1096
- ↑ G. Ireson & J. Twidle (2008) "Magnetic braking revisited: Activities for the undergraduate laboratory", European Journal of Physics 29:745–51
References
- K.D. Hahn, E.M. Johnson, A. Brokken, & S. Baldwin (1998) "Eddy current damping of a magnet moving through a pipe", American Journal of Physics 66:1066–66.
- M.A. Heald (1988) "Magnetic braking: Improved theory", American Journal of Physics 56: 521–2.
- Y. Levin, S.L. Da Silveira & F.B. Rizzato (2006) "Electromagnetic braking: A simple quantitative model", American Journal of Physics 74:815–17.
- Sears, Francis Weston; Zemansky, Mark W. (1955). University Physics (2nd ed.). Reading, MA: Addison-Wesley.
- Siskind, Charles S. (1963). Electrical Control Systems in Industry. New York: McGraw-Hill, Inc. ISBN 0-07-057746-3.
- H.D. Wiederick, N. Gauthier, D.A. Campbell, & P. Rochan (1987) "Magnetic braking: Simple theory and experiment", American Journal of Physics 55:500–3.
External links
- Eddy-current braking: a long road to success. Railway Gazette International article on ICE experience of eddy braking.
- Video of magnet falling through copper pipe
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