Eddy current dynamometers determine the torque of an engine by creating eddy currents. They are used extensively in the automotive industry to produce braking torque. Also known as eddy current dynos, these devices offer the advantages of low maintenance, high levels of control and simple construction.
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Eddy current dynamometers are a subset of chassis dynamometers and are the most common absorbers, made to fit in most chassis dyno pits. They can also act as brake dynamometers. Unlike inertia dynos, they are able to provide a quick load change rate as well as steady state and controlled acceleration. Many new eddy current dynamometers use cast iron discs to pass through the magnetic field and form eddy currents. Additional materials include copper, aluminum and other conductive metals. Variable electromagnets change the strength of the magnetic field in order to control the amount of braking. The electromagnetic voltage is computer controlled to match the changes in the magnetic field to the applied power output. The range of speed and torque that eddy current dynamometers provide makes these dynos very versatile and ideal for engine testing. They are also useful and effective when testing transmissions, turbines, electric motors, gears, pump and other machines. Dynamometers are used primarily for these purposes and are widespread in manufacturing, production and industrial applications.
Eddy currents are created when a conductor (such as a metal disk) is introduced to a changing magnetic field. A circulating flow of electrons creates its own current within the magnetic field. These currents move in opposing directions from the movement of the disk, creating a repelling or dragging force between the conductor and the magnet. The strength of the currents and therefore the strength of the repelling action increases when the power of the magnetic field is increased, the electrical conductivity of the disk is greater or when the magnetic field changes quickly. The torque of the vehicle is determined through the voltage that was required to create eddy currents that were strong enough to match the output of the vehicle. A certain level of voltage indicates a specific amount of torque. Because eddy currents generate heat, eddy current dynos must be cooled. Most are air cooled but some require external water cooling systems. In other systems, the heat from the currents is used for induction heating. These dynamometers are a heavy duty but cost effective way to produce braking torque. They are also used as load absorbers because they can provide a slight levitation effect or an air cushion to absorb an impact. Because eddy current dynamometers are chassis dynos, they are able to measure torque while the engine is still in the vehicle.
