Eddy current brake


Eddy current brake

An eddy current brake, like a conventional friction brake, is responsible for slowing an object, such as a train or a roller coaster. Unlike friction brakes, which apply pressure on two separate objects, eddy current brakes slow an object by creating eddy currents through electromagnetic induction which create resistance, and in turn either heat or electricity.

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, which then resists the rotation of the discs, providing braking force. The net result is to convert the motion of the rotors into heat in the rotors.

Linear eddy current brake

The principle of the linear eddy current brake has been described bythe French physicist Foucault, that's why in French the eddy currentbrake is called the "frein à courants de Foucault".

The linear eddy current brake consists of a magnetic yoke withelectrical coils positioned along the rail, which are being magnetizedalternating as south and north magnetic poles. This magnet does nottouch the rail, as with the magnetic brake, but is held at a definitesmall distance from the rail (approximately 7 millimeters). It does not movealong the rail, it exerts only a vertical pull on the rail. Let us callthat magnetic force F.

When the magnet is moved along the rail, it generates innon-stationary magnetic field in the head of the rail, which thengenerates electrical tension (Faraday's induction law), and thatcauses eddy currents. These disturb the magnetic field in such a waythat the magnetic force F, mentioned above, is diverted to theopposite of the direction of the movement, thus creating aparallelogram of forces consisting of the remaining vertical force FVand the horizontal force FH, which works against the movement of themagnet.

The braking energy of the vehicle is converted in eddy current losseswhich lead to a warming of the rail.

The regular magnetic brake which is in wide use in railways, exerts itsbraking force by friction with the rail, which also creates heat.

The eddy current brake does not have any mechanical contact with therail, and thus no wear and tear of it, and creates no noise or odor. The eddy current brake is, as should be clear from theabove explanation, unusable at low speeds, but can be used at highspeeds both for emergency braking as well as regular and regulatedbraking."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]

The TSI (Technical Specifications for Interoperability) of the EU fortrans-European high speed rail recommends that all newly built highspeed lines should make the eddy current brake possible.

The first train in commercial circulation to use such a braking is the ICE 3.

Modern roller coasters use this type of braking, but utilize permanent magnets instead of electromagnets. These brakes require no electricity. However, their braking strength cannot be adjusted.

ee also

*Regenerative braking generates electricity rather than heat and hence is usually more energy efficient
*Electromagnetic brakes work differently, by using the magnetic force to press the brake on the rail.
* [http://www.railwaygazette.com/news_view/article/2008/06/8478/a_long_road_to_success.html] Article on ICE's experience of eddy braking.

References

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