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Device used to slow or stop a moving object by generating eddy currents

An eddy current brake, also known as an induction brake, Faraday brake, electric brake or electric retarder, is a device used to slow or stop a moving object by generating eddy currents and thus dissipating its kinetic energy as heat. Unlike friction brakes, where the drag force that stops the moving object is provided by friction between two surfaces pressed together, the drag force in an eddy current brake 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 develops circular electric currents called eddy currents induced in it by the magnetic field, as described by Faraday's law of induction. By Lenz's law, 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 motion, proportional to its velocity. The kinetic energy of the moving object is dissipated as heat generated by the current flowing through the electrical resistance of the conductor. In an eddy current brake the magnetic field may be created by a permanent magnet or an electromagnet. With an electromagnet system, the braking force 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 brake shoe surfaces to wear, eliminating replacement as with friction brakes. A disadvantage is that since the braking force is proportional to the relative velocity of the brake, the brake has no holding force when the moving object is stationary, as provided by static friction in a friction brake, hence in vehicles it must be supplemented by a friction brake. In some cases, energy in the form of momentum stored within a motor or other machine is used to energize any electromagnets involved. The result is a motor or other machine that rapidly comes to rest when power is removed. Care must be taken in such designs to ensure that components involved are not stressed beyond operational limits during such deceleration, which may greatly exceed design forces of acceleration during normal operation. 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 powered tools quickly when power is turned off, and in electric meters used by electric utilities.

Disk electromagnetic brakes are used on vehicles such as trains, and power tools such as circular saws, to stop the blade quickly when the power is turned off. A disk eddy current brake consists of a conductive non ferromagnetic metal disc (rotor) attached to the axle of the vehicle's wheel, with an electromagnet located with its poles on each side of the disk, so the magnetic field passes through the disk. The electromagnet allows the braking force to be varied. When no current is passed through the electromagnet's winding, there is no braking force. When the driver steps on the brake pedal, current is passed through the electromagnet windings, creating a magnetic field. The greater the current in the winding, the greater the eddy currents and the stronger the braking force. Power tool brakes use permanent magnets, which are moved adjacent to the disk by a linkage when the power is turned off. The kinetic energy of the vehicle's motion is dissipated in Joule heating by the eddy currents passing through the disk's resistance, so like conventional friction disk brakes, the disk becomes hot. Unlike in the linear brake below, the metal of the disk passes repeatedly through the magnetic field, so disk eddy current brakes get hotter than linear eddy current brakes. Japanese Shinkansen trains had employed circular eddy current brake system on trailer cars since 100 Series Shinkansen. The N700 Series Shinkansen abandoned eddy current brakes in favour of regenerative brakes, since 14 of the 16 cars in the trainset used electric motors. In regenerative brakes, the motor that drives the wheel is used as a generator to produce electric current, which can be used to charge a battery, enabling the energy to be reused.

Most chassis dynamometers and many engine dynos use an eddy current brake as a means of providing an electrically adjustable load on the engine. They are often referred to as an "absorber" in such applications. Inexpensive air cooled versions are typically used on chassis dynamometers, where their inherently high inertia steel rotors are an asset rather 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) loading or to motor the engine for starting or motoring (downhill simulation). Since they do not actually absorb 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 additional cost and complexity. In contrast, high end AC motor dynamometers cleanly return the engine's power to the grid.

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 electrical coils positioned along the rail, which are being magnetized alternating as south and north magnetic poles. This magnet does not touch the rail, but is held at a constant small 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 force by friction of a brake shoe with the rail). (Unlike mechanical brakes, which are based on friction and kinetic energy, eddy current brakes rely on electromagnetism to stop objects from moving.) It works the same as a disk eddy current brake, by inducing closed loops of eddy current in the conductive rail, which generate counter magnetic fields which oppose the motion 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, 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 disk brake in which each section of the disk passes repeatedly through the brake, the rail doesn't get as hot as a disk, so the linear brake can dissipate more energy and have a higher power rating than disk brakes. The eddy current brake does not have any mechanical contact with the rail, thus no wear, and creates neither noise nor odor. The eddy current brake is unusable at low speeds, but can be used at high speeds for emergency braking and service braking. 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. Modern roller coasters use this type of braking. To avoid the risk posed by power outages, they utilize permanent magnets instead of electromagnets, thus not requiring a power supply. This application lacks the possibility of adjusting braking strength as easily as with electromagnets.

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

In typical experiments, students measure the slower time of fall of the magnet through a copper tube compared with a cardboard tube, and may use an oscilloscope to observe the pulse of eddy current induced in a loop of wire wound around the pipe when the magnet falls through.