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 In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

as heat. Unlike friction brakes, where the drag force that stops the moving object is provided by

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

between two surfaces pressed together, the drag force in an eddy current brake is an electromagnetic force between a

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

and a nearby

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...

object in relative motion, due to eddy currents induced in the conductor through

electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force, electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1 ...

. A conductive surface moving past a stationary magnet develops circular

electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...

s called eddy currents induced in it by the

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

, as described by Faraday's law of induction. By

Lenz's law Lenz's law states that the direction of the electric current Electromagnetic induction, induced in a Electrical conductor, conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in t ...

, 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

of the moving object is dissipated as

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

generated by the current flowing through the

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...

of the conductor. In an eddy current brake the magnetic field may be created by a

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, c ...

or an

electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire (likely copper) wound into a electromagnetic coil, coil. A current through the wire creates a magnetic ...

. With an electromagnet system, the braking force can be turned on and off (or varied) by varying the

in the electromagnet windings. Another advantage is that since the brake does not work by

, there are no

brake shoe A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational ...

surfaces to

wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...

, 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 Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

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 coaster A roller coaster is a type of list of amusement rides, amusement ride employing a form of elevated Railway track, railroad track that carries passengers on a roller coaster train, train through tight turns, steep slopes, and other elements, usua ...

s, 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.

Mechanism and principle

An eddy current brake consists of a conductive piece of metal, either a straight bar or a disk, which moves through the magnetic field of a magnet, either a

or an

. When it moves past the stationary

, the magnet exerts a

drag force In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...

on the metal which opposes its motion, due to circular

s called eddy currents induced in the metal by the

. Note that the conductive sheet is not made of

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...

metal such as iron or steel; usually copper or aluminum are used, which are not attracted to a magnet. The brake does not work by the simple attraction of a ferromagnetic metal to the magnet. See the diagram at right. It shows a metal sheet ''(C)'' moving to the right under a magnet. The magnetic field ''(B, green arrows)'' of the magnet's north pole ''N'' passes down through the sheet. Since the metal is moving, the

magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the we ...

through the sheet is changing. At the part of the sheet under the leading edge of the magnet ''(left side)'' the magnetic field through the sheet is increasing as it gets nearer the magnet. From Faraday's law 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 field through the sheet is decreasing, inducing a clockwise eddy current in the sheet. Another way to understand the action is to see that the free

charge carrier In solid state physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. ...

s (

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s) in the metal sheet are moving to the right, so the magnetic field exerts a sideways force on them due to the

Lorentz force In electromagnetism, the Lorentz force is the force exerted on a charged particle by electric and magnetic fields. It determines how charged particles move in electromagnetic environments and underlies many physical phenomena, from the operation ...

. Since the velocity ''v'' of the charges is to the right and the magnetic field ''B'' is directed down, from the right hand rule the Lorentz force on positive charges ''qv''×''B'' is toward the rear in the diagram (to the left when facing in the direction of motion of the sheet) This causes a current ''I'' toward the rear under the magnet, 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 magnet again. The mobile

s in the metal, the

s, actually have a negative charge, so their motion is opposite in direction to the conventional current shown. As described by Ampere's circuital law, each of these circular currents creates a counter magnetic field (''blue arrows''), which in accordance with

opposes the change in magnetic field, causing a drag force on the sheet which is the braking force exerted by the brake. At the leading edge of the magnet ''(left side)'' by the right hand rule the counterclockwise current creates a magnetic field pointed up, opposing the magnet's field, causing a repulsive force between the sheet and the leading edge of the magnet. In contrast, at the trailing edge ''(right side)'', the clockwise current causes a magnetic field pointed down, in the same direction as the magnet's field, creating an attractive force between the sheet and the trailing edge of the magnet. Both of these forces oppose the motion of the sheet. The

which is consumed overcoming this drag force is dissipated as heat by the currents flowing through the resistance of the metal, so the metal gets warm under the magnet. The braking force of an eddy current brake is exactly proportional to the velocity ''V'', so it acts similar to viscous friction in a liquid. The braking force decreases as the velocity decreases. When the conductive sheet is stationary, the magnetic field through each part of it is constant, not changing with time, so no eddy currents are induced, and there is no force between the magnet and the conductor. Thus an eddy current brake has no holding force. 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 same for both.

Disk eddy current brakes

Disk electromagnetic brakes are used on vehicles such as trains, and power tools such as

circular saw A circular saw or a buzz saw, is a power-saw using a toothed or Abrasive saw, abrasive disk (mathematics), disc or blade to cut different materials using a rotary motion spinning around an Arbor (tool), arbor. A hole saw and ring saw also use ...

s, to stop the blade quickly when the power is turned off. A disk eddy current brake consists of a conductive non-

metal disc (

rotor ROTOR was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...

) attached to the axle of the vehicle's wheel, with an

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

s, 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 Joule heating (also known as resistive heating, resistance heating, or Ohmic heating) is the process by which the passage of an electric current through a conductor (material), conductor produces heat. Joule's first law (also just Joule's law), ...

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 The , colloquially known in English as the bullet train, is a network of high-speed railway lines in Japan. It was initially built to connect distant Japanese regions with Tokyo, the capital, to aid economic growth and development. Beyond lon ...

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 brake Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerati ...

s, 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.

Dynamometer eddy current absorbers

Most chassis

dynamometer A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed ( RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dyna ...

s 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

Linear eddy current brakes are used on some rail vehicles, such as trains. They are used on

s, 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

of the moving vehicle is converted to heat by the eddy current flowing through the

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 A Technical Specification for Interoperability (abbreviated as TSI) is a text provided for in European Directive 2016/797 adopted by the European Parliament and the Council of the European Union on the interoperability of the European rail system ...

) 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

s use this type of braking. To avoid the risk posed by

power outage A power outage, also called a blackout, a power failure, a power blackout, a power loss, a power cut, or a power out is the complete loss of the electrical power network supply to an end user. There are many causes of power failures in an el ...

s, they utilize

s instead of electromagnets, thus not requiring a power supply. This application lacks the possibility of adjusting braking strength as easily as with electromagnets.

Lab experiment

physics education Physics education or physics teaching refers to the education methods currently used to teach physics. The occupation is called physics educator or physics teacher. Physics education research refers to an area of pedagogical research that seek ...

a simple experiment is sometimes used to illustrate eddy currents and the principle behind magnetic braking. When a strong

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 An oscilloscope (formerly known as an oscillograph, informally scope or O-scope) is a type of electronic test instrument that graphically displays varying voltages of one or more signals as a function of time. Their main purpose is capturing i ...

to observe the pulse of eddy current induced in a loop of wire wound around the pipe when the magnet falls through.

Notes

References

* * * * * * {{DEFAULTSORT:Eddy Current Brake Brakes Electromagnetic components Electrodynamics Railway brakes Physics experiments Electromagnetic brakes and clutches fr:Frein à courants de Foucault nl:Wervelstroomrem