Eddy current brake
An eddy current brake is a wear-free brake that uses the eddy currents of magnetic fields in moving metal discs ( rotors ) for braking or swords.
The principle: moves a metal plate in an inhomogeneous external magnetic field, as in their tensions and in consequence, eddy currents are induced, which in turn own the external magnetic field according to Lenz's law create opposing magnetic fields which slow down the plate eventually. The same applies if, conversely, the source of the external magnetic field, such as a permanent magnet or electromagnet, is moved via an electrically conductive surface, such as a railroad track - what matters is only the relative motion between the field and the electric conductor ( see Fig. ).
The electrical resistance of the metal plate forms the eddy currents a resistive load, which converts the kinetic energy of the magnet or the plate into heat. The magnetization of the metal plate on the other hand, which plays a role in the functioning similar hysteresis is irrelevant for the induction of an eddy current brake, the only decisive factor is the electrical conductivity.
The idea of the eddy current brake was patented in 1892.
Control
The strength of the braking effect depends on several parameters:
- Conductivity of the brake disc: the induced currents are directly proportional to the electrical conductivity of the material used. A copper plate is therefore strongly decelerated as an identical steel disc.
- Direction of magnetic field: the greatest braking effect is obtained when the magnetic field of the movable disc passes through vertically.
- Air gap, the larger the airgap, the smaller is the maximum braking action.
- Shape of the disk: disks with peripherally comb-shaped structure or cracks have reduced braking efficiency, since the annular eddy currents can not form a large scale.
- Area under the exciter: the area is smaller at the pole, the less the braking.
- Speed: The braking action is highly dependent on the relative speed between the panel and plate.
- Coil current: the higher the current flowing through the magnet current, the stronger the magnetic field, and thus the braking force.
A rotating disc by a static magnetic field brakes (e.g. permanent magnet), the wafer is always slower. The standstill theoretically never reaches quite - However - due to the decrease of the braking force with the relative speed. An eddy current brake is not suitable as a parking brake.
Conversely, this effect provides a natural ABS. This trait can be influenced by a changing magnetic field, then even possible to generate movement, such as the asynchronous motor with squirrel-cage rotor or electricity meters after the Ferraris principle.
Formation of eddy currents
The metal plate is so decelerated both at the inlet as well as when re- emerging from the magnetic field in their movement. The kinetic energy is according to Ohm's law converted into thermal energy ( The copper plate is heated ).
Applications
Rail vehicles
As standard, the linear eddy current brake in Europe came from the 2000 that entered service in ICE 3 train sets used for the first time. In contrast to the case of fast moving trains usual magnetic rail brake, the magnetic field is along rather than transverse to the rail produced. The iron core of the electromagnet does not rely on, but is supported by access to the wheel set bearing about 7 mm above the rail level. The problem here is the skin effect, the forces due to the high frequencies the eddy current on the outer edges of the rail cross-section. This should have resulted in the development time for annealing of the rail surface. In the eddy current brake, the dissipated braking energy is converted into heat in the rails. The eddy current brake is not subject to wear and works independently of the coefficient of friction between wheel and rail (eg leaves on the rail).
Eddy current brakes are used on the ICE 3, especially at high speeds; However, they can also act until shortly before the stoppage. To use it, sets a route must be specially upgraded, in particular to avoid disturbances of the safety systems. In its entirety, that is, also for service brake, eddy current brake of the ICE in Germany just comes down to the high-speed lines Cologne-Rhine/Main and Nuremberg -Ingolstadt used as heat- induced sufficient tolerance only through the use of slab track warping of the tracks can be ensured.
In many parts of the road on which the ICE 3 network, the eddy current brake can be used only during rapid braking. They can be counted on the brake mass percentages of the train and allowed ( under LZB ), a speed of 160 km / h On some routes that are not upgraded for the use of eddy current brakes for emergency braking, it will not be counted against the brake mass percentages of the train, so the maximum speed is limited ( under PZB ) to 140 km / h.
Before use in the ICE 3 trains travel the system of eddy current brake has been performed on the test vehicles ICE V and ICE S for series production. Testing of the eddy current brake of the ICE 3 as emergency and full service brake under LZB procedure was performed from July 2001 on the Rhine Valley line between Baden -Baden and Offenburg, later during the commissioning of the high-speed line Cologne-Rhine/Main. In the same year reversed a single ICE 3 train between Munich and Hamburg, in which the eddy current brake was switched for service braking at higher speeds.
The eddy current brake of InterCityExperimental was not initially transferred due to problems with the infrastructure to the later ICE - series trains.
Wherein the rotating eddy-current brake, the rail is used as an electromagnet and current in the wheels of the train induced their magnetic interaction with the electromagnet, and which respond to brake the vehicle. This brake is currently used only in test vehicles.
Power brake
The eddy current brake is used as a power brake on engine dynamometers. It is used for braking a device under test ( combustion engine, electric motor). The advantages of the good controllability of the size and use as a performance measurement system are the disadvantages of the power dissipation in the form of heat and the only passive mode against ( candidate can not be towed ).
Fitness Equipment
In exercise equipment, especially in quality ergometers, the load is controlled by electrically adjustable eddy current brakes. Through the use of microprocessors to be controlled at varied according to different parameters.
Rides
In the faster and higher and higher roller coasters and free-fall towers increasingly linear eddy current brakes are used. Especially in comparison to the classic, based on the effect of friction brake shoes gentle onset of braking, no wear, and the safety of these brake systems led to this trend. Eddy current brake with permanent magnets do not require electricity. Therefore, they work perfectly even during a power failure.
Depending on the car type of roller coasters, the metal swords (mostly copper alloys) side or mounted underneath the car. The swords mounted on the distance between the permanent magnet move to the brake points through. Some of the magnets can be folded away after braking to allow easier Next train is traveling.
If very high speeds reached and thus used many or long brakes, the magnets are often mounted on the roller coaster car and the metal swords on the rail. The reason is that magnets are significantly more expensive than the brake swords to buy and so less of it is needed. This combination can be found especially at launch roller coasters, some of which travel at speeds of up to 214 km / h and where conventional friction brakes would have too high wear. Also on the acceleration path brake swords are mounted in such orbits, the braking in the event of a faulty launch the train while rolling back if it does not pass the subsequent rise. The brake swords are then lowered during the acceleration phase or folded down and placed on the braking position after passing by the train again. Retractable brake swords are especially characteristic of " Accelerator Coaster " by Intamin. Also launching roller coasters that are accelerated by a linear motor and to require the magnets on the car, have this combination.
In free-fall towers, the swords are mounted vertically on the tower and attach the magnets to the passenger carrier. The swords are usually mounted so that on the way to the gondola towards more swords are present first a few, down to achieve an approximately constant increasing braking force.
Commercial Vehicles
The advantage of wear -free retarder is also utilized in the commercial vehicle sector for trucks. The best-known manufacturers are Voith, Telma and Knorr. As an alternative to eddy current brake retarder also be installed which operate hydraulically. Some manufacturers try to summarize alternator, starter motor and eddy current brake in one unit.
Meters
In a moving coil instrument is the moving coil - which moves in a permanent magnetic field - wound onto an aluminum frame. In this, therefore, arise in pointer movements eddy currents, which jerky pointer movement damps.
When Ferraris meters ( electrical energy ) generate fixed current and voltage coil eddy current, which sets a rotating aluminum disc in motion. The disc also passes through the magnetic field of a strong permanent magnet, the eddy currents excited in her. The interaction of the braking and driving forces causes a uniform and the measured value ( electrical active power) proportional rotary motion of the disk.