Eddy current brake

Eddy current brake
Eddy current brake A linear eddy current brake in a german ICE 3 high-speed string in action An eddy current brake, besides known as an induction brake, electric brake or electric retarder, is a device used to slow or stop a moving aim by dissipating its kinetic energy as heat. Unlike clash brakes, where the drag coerce that stops the moving object is provided by friction between two surfaces pressed together, the dredge force in an eddy current brake is an electromagnetic storm between a magnet and a nearby conductive object in relative motion, due to eddy currents induced in the conductor through electromagnetic trigger. A conductive surface moving past a stationary attraction develops circular electric currents called eddy currents induced in it by the magnetic battlefield, as described by Faraday ‘s law of trigger. By Lenz ‘s police, the circulating currents create their own magnetic field that opposes the field of the magnet. Thus the moving conductor experiences a drag force from the magnet that opposes its apparent motion, proportional to its speed. The kinetic energy of the moving object is dissipated as heat generated by the current menstruate through the electrical underground of the conductor. In an eddy stream brake the magnetic field may be created by a permanent wave attraction or an electromagnet. With an electromagnet system, the braking storm can be turned on and off ( or varied ) by varying the electric current in the electromagnet windings. Another advantage is that since the brake does not work by friction, there are no bracken shoe surfaces to wear, eliminating surrogate as with friction brakes. A disadvantage is that since the braking push is proportional to the relative speed of the bracken, the brake has no holding force when the moving object is stationary, as provided by electrostatic friction in a friction brake, hence in vehicles it must be supplemented by a friction brake.

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Eddy current brakes are used to slow high-speed trains and roller coasters, as a complement for friction brakes in semi-trailer trucks to help prevent brake wear and overheating, to stop power tools quickly when world power is turned off, and in electric meters used by electric utilities .

mechanism and principle [edit ]

Eddy current brake A metal sheet moving to the right under a magnet, illustrating how a linear eddy current bracken works. In this drawing the magnet is drawn separated apart from the sheet to reveal the vectors ; in an eddy current brake the magnet is normally located as conclusion to the sheet as possible .Eddy current brake A round or disk eddy stream brake An eddy current brake consists of a conductive piece of metallic, either a straight cake or a disk, which moves through the magnetic field of a magnet, either a permanent magnet or an electromagnet. When it moves past the stationary attraction, the magnet exerts a haul power on the alloy which opposes its gesture, due to circular electric currents called eddy currents induced in the metal by the magnetic field. Note that the conductive tabloid is not made of ferromagnetic alloy such as iron or steel ; normally copper or aluminum are used, which are not attracted to a attraction. The brake does not work by the simple attraction of a ferromagnetic alloy to the magnet. See the diagram at right. It shows a metallic sheet (C) moving to the veracious under a magnet. The magnetic field (B, green arrows) of the magnet ‘s north pole N passes down through the sheet. Since the metallic is moving, the magnetic magnetic field through the sail is changing. At the depart of the plane under the leading border of the magnet (left side) the charismatic field through the sheet is increasing as it gets nearer the magnet. From Faraday ‘s jurisprudence of induction, this field induces a counterclockwise flow of electric current (I, red), in the sheet. This is the eddy current. In contrast, at the trailing edge of the magnet (right side) the magnetic playing field through the sheet is decreasing, inducing a clockwise eddy current in the sail. Another manner to understand the natural process is to see that the dislodge charge carriers ( electrons ) in the alloy plane are moving to the right, so the charismatic field exerts a crabwise pull on them due to the Lorentz military unit. Since the speed v of the charges is to the right and the magnetic airfield B is directed down, from the right field hired hand rule the Lorentz force on plus charges qv × B is toward the rise in the diagram ( to the leave when face in the steering of movement of the sheet ) This causes a current I toward the raise under the attraction, which circles around through parts of the sheet outside the magnetic field in two currents, clockwise to the right and counterclockwise to the left, to the front of the attraction again. The mobile cathexis carriers in the metal, the electrons, actually have a damaging charge, so their movement is opposite in direction to the conventional current shown. ascribable to Ampere ‘s circuital law, each of these circular currents creates a counter magnetic field ( blue arrows ), which due to Lenz ‘s law opposes the change in charismatic field, causing a drag impel on the sail which is the braking force exerted by the brake. At the leading edge of the attraction (left side) by the right hand rule the counterclockwise current creates a magnetic sphere pointed improving, opposing the magnet ‘s field, causing a hideous force between the sheet and the leading boundary of the attraction. In contrast, at the trailing boundary (right side), the clockwise current causes a charismatic field pointed down, in the same steering as the magnet ‘s field, creating an attractive force between the sheet and the trailing edge of the attraction. Both of these forces oppose the motion of the sheet. The kinetic energy which is consumed overcoming this drag pull is dissipated as hotness by the currents flowing through the electric resistance of the alloy, so the alloy gets warm under the attraction. The braking wedge of an eddy current brake is precisely proportional to the speed V, so it acts like to syrupy clash in a fluent. The braking force decreases as the speed decreases. When the conductive sheet is stationary, the charismatic discipline through each part of it is ceaseless, not changing with time, so no eddy currents are induced, and there is no force between the attraction and the conductor. Thus an eddy current bracken has no holding pull. Eddy current brakes come in two geometries :

  • In a linear eddy current brake, the conductive piece is a straight rail or track that the magnet moves along.
  • In a circular, disk or rotary eddy current brake, the conductor is a flat disk rotor that turns between the poles of the magnet.

The physical working principle is the like for both .

Disk eddy current brakes [edit ]

Disk electromagnetic brakes are used on vehicles such as trains, and baron tools such as circular saw, to stop the blade promptly when the might is turned off. A disk eddy current brake consists of a conductive non- ferromagnetic metallic magnetic disk ( rotor ) attached to the axle of the vehicle ‘s roulette wheel, with an electromagnet located with its poles on each side of the disk, so the magnetic field passes through the magnetic disk. The electromagnet allows the braking force to be varied. When no stream is passed through the electromagnet ‘s tortuous, there is no braking force. When the driver steps on the brake bicycle, current is passed through the electromagnet windings, creating a magnetic plain. The greater the current in the hoist, the greater the eddy currents and the stronger the braking force. Power tool brakes use permanent magnets, which are moved adjacent to the phonograph record by a linkage when the power is turned off. The energizing energy of the vehicle ‘s motion is dissipated in Joule heat by the eddy currents passing through the disk ‘s resistance, so wish conventional friction disk brakes, the harrow becomes hot. Unlike in the linear brake below, the alloy of the phonograph record passes repeatedly through the magnetic field, so harrow eddy stream brakes get hotter than linear eddy current brakes. japanese Shinkansen trains had employed round eddy stream brake system on dawdler cars since 100 Series Shinkansen. The N700 Series Shinkansen abandoned eddy stream brakes in privilege of regenerative brakes, since 14 of the 16 cars in the trainset used electric motors. In regenerative brakes, the centrifugal that drives the bicycle is used as a generator to produce electric current, which can be used to charge a battery, enabling the energy to be reused .

Dynamometer eddy current absorbers [edit ]

A 6-minute ‘ how-it-works video recording ’ tutorial explaining how engine-dynamometer and human body dyno eddy-current absorbers work. Most human body dynamometers and many locomotive dynos use an eddy-current brake as a means of providing an electrically adjustable load on the engine. They are much referred to as an “ absorber ” in such applications. cheap air-cooled versions are typically used on chassis dynamometers, where their inherently high-inertia steel rotors are an asset preferably than a liability. conversely, performance engine dynamometers tend to utilize low-inertia, high RPM, liquid-cooled configurations. Downsides of eddy-current absorbers in such applications, compared to expensive AC-motor based dynamometers, is their inability to provide stall-speed ( zero RPM ) load or to motor the engine – for starting or motoring ( downhill model ). Since they do not actually absorb department of energy, provisions to transfer their radiated heat out of the test cell area must be provided. Either a high-volume air-ventilation or water-to-air heat exchanger adds extra price and complexity. In contrast, high-end AC-motor dynamometers flawlessly return the engine ‘s baron to the grid .

Linear eddy current brakes [edit ]

Eddy current brake Goliath made by Eddy current brakes on the curler coastermade by Intamin, at Walibi Holland ( Netherlands )

Linear eddy current brakes are used on some rail vehicles, such as trains. They are used on roller coasters, to stop cars smoothly at the end of the ride. The linear eddy current brake consists of a magnetic yoke with electric coils positioned along the rail, which are being magnetized alternate as south and north magnetic poles. This magnet does not touch the rail, but is held at a ceaseless little distance from the rail of approximately 7 mm ( the eddy current brake should not be confused with another device, the magnetic brake, which exerts its braking coerce by clash of a brake shoe with the rail ). It works the lapp as a magnetic disk eddy stream brake, by inducing close loops of eddy current in the conductive rail, which generate counter magnetic fields which oppose the movement of the gearing. The kinetic energy of the moving vehicle is converted to heat by the eddy current flowing through the electric electric resistance of the rail, which leads to a warming of the rail. An advantage of the linear brake is that since each section of rail passes only once through the magnetic field of the brake, in contrast to the magnetic disk brake in which each section of the harrow passes repeatedly through the bracken, the rail does n’t get equally hot as a magnetic disk, so the linear brake can dissipate more energy and have a higher office rat than phonograph record brakes. The eddy current brake does not have any mechanical contact with the rail, thus no wear, and creates neither noise nor smell. The eddy stream brake is unserviceable at moo speeds, but can be used at high speeds for emergency brake and avail brake. [ 1 ] The TSI ( technical Specifications for Interoperability ) of the EU for trans-European high-speed railing recommends that all newly built high-speed lines should make the eddy current brake potential. The foremost train in commercial circulation to use such a brake system has been the ICE 3. Modern roller coasters use this type of braking. To avoid the risk posed by ability outages, they utilize permanent magnets rather of electromagnets, frankincense not requiring a office issue. This application lacks the hypothesis of adjusting braking lastingness vitamin a easily as with electromagnets .

Lab experiment [edit ]

In physics education a childlike experiment is sometimes used to illustrate eddy currents and the principle behind charismatic brake. When a strong magnet is dropped down a vertical, non-ferrous, conducting shriek, eddy currents are induced in the shriek, and these retard the origin of the magnet, so it falls slower than it would if free-falling. As one set of authors explained

If one views the magnet as an assembly of circulating nuclear currents moving through the shriek, [ then ] Lenz ’ second law implies that the induce 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 movement and attracted in rear, hence acted upon by a retarding push. [ 2 ]

In distinctive experiments, students measure the slower clock of fall of the magnet through a copper pipe compared with a cardboard tube, and may use an oscilloscope to observe the pulse of eddy stream induced in a coil of wire wind around the pipe when the attraction falls through. [ 3 ] [ 4 ]

See besides [edit ]

  • Dynamic braking – either rheostatic (dissipating the train’s energy as heat in resistor banks within the train, or regenerative where the energy is returned to the electrical supply system)
  • 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 [edit ]

  1. ^ “ Wirbelstrombremse im ICE 3 als Betriebsbremssystem hoher Leistung ” ( “ Eddy-current brake in the ICE 3 as high-efficiency service brake arrangement ”, by Jürgen Prem, Stefan Haas, Klaus Heckmann, in “ electrische bahnen ” Vol 102 ( 2004 ), No. 7, pages 283ff
  2. ^Partovi, M Hossein; Morris, Eliza J (2006). “Electrodynamics of a magnet moving through a conducting pipe”. Canadian Journal of Physics. 84 (4): 253–71. arXiv:physics/0406085Bibcode:2006CaJPh..84..253P. doi:10.1139/p06-065.
  3. ^MacLatchy, Cyrus S; Backman, Philip; Bogan, Larry (1993). “A quantitative magnetic braking experiment”. American Journal of Physics. 61 (12): 1096. Bibcode:1993AmJPh..61.1096M. doi:10.1119/1.17356.
  4. ^Ireson, Gren; Twidle, John (2008). “Magnetic braking revisited: Activities for the undergraduate laboratory”. European Journal of Physics. 29 (4): 745. Bibcode:2008EJPh…29..745I. doi:10.1088/0143-0807/29/4/009.

References [edit ]

  • 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: 521–522.
  • Y. Levin, F. L. da Silveira, F. B. Rizzato (2006) “Electromagnetic braking: A simple quantitative model”, American Journal of Physics 74:815–817.
  • Sears, Francis Weston; Zemansky, Mark W. (1955). University Physics
  • Siskind, Charles S. (1963). Electrical Control Systems in IndustryISBN 978-0-07-057746-6.
  • H. D. Wiederick, N. Gauthier, D. A. Campbell, P. Rochan (1987) “Magnetic braking: Simple theory and experiment”, American Journal of Physics 55:500–503.
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Category : Car Brakes