A eddy current brake , also known as an induction brake , electric brake or electric retarder , is a device used slowing or stopping moving objects by removing its kinetic energy as heat. However, unlike friction brakes, in which the drag force that stops the moving object is provided by friction between two muted surfaces, the drag force in the eddy brake current is the electromagnetic force between the magnet and the nearest conductive object in relative motion, since the eddy current is induced in conductors through electromagnetic induction.
The conductive surface moving through the stationary magnet will have a circular electric current called the eddy current induced in it by the magnetic field, as described by Faraday's induction law. With Lenz's law, the circulating currents will create their own magnetic fields that are opposed to the magnetic field. So the moving conductor will experience a drag force from the magnet opposing its movement, proportional to its speed. The kinetic energy of the moving object is dissipated as heat generated by the current flowing through the electrical resistance of the conductor.
In the current eddy brake magnetic field can be made by a permanent magnet, or electromagnet so that the braking force can be switched on and off or varied by varying the electric current in the electromagnetic windings. Another advantage is that the brakes do not work with friction, no brake shoe surfaces that wear out, require replacement, such as friction brakes. The disadvantage is that the braking force is proportional to the relative speed of the brake, the brake does not have holding when the object moves silently, as provided by static friction in the friction brake, so the vehicle must be equipped with friction brake.
Eddy brakes are currently used to slow high-speed trains and roller coasters, as a complement to friction brakes in semi-trailer trucks to help prevent brake wear and overheating, to stop powered devices quickly when power is turned off, and in the power meter used by electric utilities.
Video Eddy current brake
Mechanism and principle
The eddy current brake consists of a conductive piece of metal, either straight rod or disk, which travels through a magnetic magnetic field, either a permanent magnet or an electromagnet. As it travels through the stationary magnet, the magnet provides a drag force to the metal opposing its motion, due to a circular electric current called the eddy current induced in the metal by the magnetic field. Note that conductive sheets are not made of ferromagnetic metal such as iron or steel; usually copper or aluminum is used, which is not interested in magnets. The brakes do not work with the simple appeal of the ferromagnetic metal to the magnet.
See the diagram on the right. This indicates the metal sheet (C) moves to the right under the magnet. The magnetic field ( B, green arrow ) from the magnetic north pole N passes the sheet. As the metal moves, the magnetic flux through the sheets changes. In the piece under the front end of the magnet (left side) the magnetic field through the sheet increases as the magnet draws closer. From Faraday's induction law, this field induces an electric current flow counterclockwise ( I, red ) , in the sheet. This is eddy stream. Conversely, in the trailing edge of the magnetic (right-hand side) the magnetic field through the sheet decreases, pushing the eddy current clockwise across the sheet.
Another way to understand this action is to see that the carrier free charge (electrons) in the metal sheet move to the right, so that the magnetic field gives sideways force on them due to the power of Lorentz. Since the velocity v of cost is to the right and the magnetic field B is directed downwards, from the right hand reigns Lorentz style in the positive charge q v ÃÆ'â € " B leads backwards in the diagram (to the left when facing toward the movement sheet) This causes the current I to the rear below the magnet, which rotates around through the sheet portions outside the magnetic field in two currents, clockwise to the right and counterclockwise to the left , to the front of the magnet again. The transporter of charge moves in a metal, an electron, actually has a negative charge, so its motion is in opposite direction with the conventional current shown.
Due to Ampere's circulation law, each of these circular currents creates a counter magnetic field ( blue arrow ), which due to Lenz's law against magnetic field changes, causes a force barrier to the sheet which is braking force provided by the brakes. At the front end of the magnet (left side) by the right hand dictates the counterclockwise currents creating a magnetic field pointing upwards, opposing the magnetic field, causing the starting force between the sheet and the leading edge of its magnet. Conversely, on the trailing edge (right side) , clockwise currents cause the magnetic field to point downward, in the same direction as the magnetic field, creating an attractive force between the sheet and the trailing edge of its magnet. Both of these styles oppose the sheets of movement. The kinetic energy consumed to overcome this drag force is dissipated as heat by the current flowing through the metal resistance, allowing the metal to warm under the magnet.
The eddy brake current braking force is exactly proportional to the velocity V , so it acts similar to the viscous friction in the liquid. Braking force decreases as speed decreases. When the stationary conductive sheet, the magnetic field through each part is constant, does not change with time, so no eddy current is induced, and there is no force between the magnet and the conductor. Thus the eddy brake currently has no retaining power.
Rem Eddy currently comes in two geometries:
- In brake eddy current linear , the conductive part is a straight rail or a track whose magnet is moving.
- In the circular circle , disk or rotates the current brake eddy, the conductor is the flat disk rotors that rotate between the magnetic poles.
The principle of physical work is the same for both.
Maps Eddy current brake
Eddy current disc brakes
Disk electromagnetic brakes are used on vehicles such as trains, and electrical appliances such as circular saws, to stop the blades quickly when the power is off. The current disk disc brake comprises a conductive non-ferromagnetic (rotor) metal disc mounted on the wheel axle of the vehicle, with an electromagnet located at the poles on each side of the disc, allowing the magnetic field to pass through the disc. The electromagnet allows for varied braking styles. When no current passes through the electromagnetic windings, there is no braking force. When the driver steps on the brake pedal, the current is passed through the electromagnet windings, creating a magnetic field, The larger the current in the winding, the larger the eddy current and the stronger the braking force. The power tool brake uses a permanent magnet, which is moved to a disk adjacent to the linkage when power is turned off. The kinetic energy of the motion of the vehicle is dissipated in Joule heating by the eddy current passing through the disk resistance, so like conventional friction disc brakes, the disk becomes hot. Unlike the linear brakes below, the metal disk passes repeatedly through the magnetic field, so the disk disc brakes now become hotter than the eddy current brake linear.
The Japanese Shinkansen train has been using the eddy eddy eddy brake system on trailer cars since the 100 Series Shinkansen. However, the Shinkansen N700 Series leaves the eddy brakes current and supports regenerative brakes, as 14 of the 16 cars on the trainset use an electric motor. On regenerative brakes, motors that drive the wheels are used as generators to generate electric current, which can be used to charge the battery, so that energy can be used again.
Brake linear eddy current
Eddy linear disc brakes are used on some vehicles that ride on rails, such as trains. They are used on a roller coaster, to stop the car smoothly at the end of the journey.
Eddy linear eddy brakes consist of a magnetic yoke with electric coils positioned along the rails, which are alternating magnets as south and north magnetic poles. This magnet does not touch the rails, but is held at a constant small distance from the rails of about 7 mm (eddy current brakes should not be equated with other devices, magnetic brakes, in widespread use in the railroads, which exerts its braking forced by friction brake shoe with rail ). It works closely with discus disc brakes, by inducing closed-loop eddy currents in a conductive rail, which produces a counter magnetic field that opposes the movement of the train.
The kinetic energy of the moving vehicle is converted to heat by the eddy current flowing through the electrical resistance of the rail, leading to rail heating. The advantage of a linear brake is that every part of the rail passes only once through the brake magnetic field, unlike the disc brakes in which each disk part passes repeatedly through the brake, the rail does not get heat as a disc, so the linear brake can dissipate more energy and have higher power ratings than disc brakes.
The eddy brake currently has no mechanical contact with the rail, and thus does not wear, and does not cause sound or smell. Eddy current brakes can not be used at low speeds, but can be used at high speeds for both emergency braking and for regular braking.
The TSI (Technical Specs for Interoperability) of the EU for high-speed trans-European rail recommends that all newly built high speed lanes should make the eddy brake current possible.
The first train in the commercial circulation to use such a braking system is ICE 3.
Modern roller coasters also use this type of braking, but to avoid the risk posed by potential power outages, they use a permanent magnet instead of an electromagnet, thus requiring no power supply, however, without the possibility of adjusting the braking power as easily as with an electromagnet.
Lab Experiments
In physics education, simple experiments are sometimes used to illustrate the eddy current and the principle behind magnetic braking. When strong magnets are dropped down the vertical, non-iron, conductor, eddy current pipes are induced in the pipe, and this inhibits the drop of the magnet, so it falls more slowly than if it falls freely. As explained by a set of authors
If one looks at a magnet as a series of atomic currents moving through a pipe, Lenz's law implies that the induced vortex on the pipe wall counter circulates in front of the moving magnet and circulates behind it. But this implies that the moving magnet is rejected in the front and pulled back, therefore it is followed up by the decelerating force.
In a typical experiment, the student measures the time slower than the fall of the magnet via a copper tube compared to the cardboard tube, and can use an oscilloscope to observe the eddy pulse currently induced in the loop of the wire wound around the pipe when the magnet falls. through.
See also
- Dynamic braking - either rheostatic (scattering rail energy as heat in bank resistor in rail, or regenerative where energy is returned to power supply system)
- Telma retarder - the current eddy brake system made by Telma, a company that is part of the Valeo group
- Electromagnetic brake (or electro-mechanical brake) - use magnetic force to press brake mechanically on rail
- Linear induction motors can be used as regenerative brakes
Note
References
- K.D. Hahn, E.M. Johnson, A. Brokken, & amp; S. Baldwin (1998) "Eddy current damping of magnets moving through pipes", American Journal of Physics 66: 1066-66.
- M.A. Heald (1988) "Magnetic Braking: Improved theories", American Journal of Physics 56: 521-2.
- Y. Levin, S.L. Da Silveira & amp; F.B. Rizzato (2006) "Electromagnetic Braking: 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). Industrial Electricity Control System . New York: McGraw-Hill, Inc. ISBN: 0-07-057746-3. Ã,
- H.D. Wiederick, N. Gauthier, D.A. Campbell & amp; P. Rochan (1987) "Magnetic braking: simple theory and experiment", American Journal of Physics 55: 500-3.
External links
- Eddy's current braking: a long way to success. Railway Gazette International article about the ICE experience of eddy braking.
- The magnetic video falls through a copper pipe
Source of the article : Wikipedia
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