Electromagnetic brake
Electromagnetic brakes, also called eddy current brakes, seek to retard motion or cause deceleration in a moving system. Unlike conventional brakes, which operate by causing friction between moving parts, in an electromagnetic brake kinetic energy is converted to heat without contact between the moving parts.
Construction and operation
A typical electromagnetic brake is composed of a metal disc (rotor) attached to a rotating axis and electromagnets or permanent magnets positioned to generate a magnetic field intersecting the disc. The electromagnet allows control of the braking action by varying the strength of the magnetic field; to produce a braking force, electric current is passed through the electromagnets. As the metal disc rotates inside the magnetic field, electric currents, called eddy currents are induced inside it. Those currents then generate a magnetic field in opposition to the original field thus creating a force which acts to decelerate the rotating disc.
Heat is created in the disc as a direct result of the electrical resistance of the disk material and the current induced in it; this heat represents the kinetic energy being dissipated and is analogous to the heat generated by a conventional friction brake.
Because the induced current is proportional to the speed of the disc the braking torque decreases as the disc decelerates resulting in a smooth stop. Electromagnetic brakes of this fashion are therefore unable to completely stop a moving part or to keep it still. A small friction brake might be coupled for that purpose. Except when used to slow an engine, as once below a certain speed (engine specific) the engine will stall.
Regenerative braking is not used with brakes of this type since the induced current is dissipated as heat and is not practical to recover.