Definition, Characteristics, and Origin of Eddy Currents
– Eddy currents are loops of electric current induced in conductors by changing magnetic fields or relative motion between a conductor and a magnetic field.
– They flow in closed loops within conductors, perpendicular to the magnetic field.
– The magnitude of the current is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux.
– Eddy currents create a magnetic field that opposes the change in the magnetic field that created them, according to Lenz’s law.
– Eddy currents dissipate energy as heat in the material, causing energy loss in AC machinery.
– The term ‘eddy current’ comes from analogous currents observed in water, causing localized areas of turbulence known as eddies.
– Eddy currents can take time to build up and persist for long times in conductors due to their inductance.
– François Arago was the first person to observe eddy currents in 1824.
– Emil Lenz stated Lenz’s law in 1834, which describes the direction of induced current flow in an object.
– Léon Foucault is credited with discovering eddy currents in 1855 and using them for non-destructive testing in 1879.
Explanation of Eddy Currents
– Eddy currents are induced in a conductive metal plate as it moves under a magnet.
– The Lorentz force of the magnetic field on the electrons in the metal induces a sideways current, creating a drag force on the sheet.
– The magnetic field creates counter magnetic fields generated by the circular motion of the charges.
– Eddy currents can be understood as a result of the Lorentz force on the free charge carriers (electrons) in the metal sheet.
– The magnetic field of the magnet acts on the electrons moving sideways, causing a current that circulates through the sheet.
Applications of Eddy Currents
– Eddy currents are used in eddy current brakes to quickly stop rotating power tools.
– They are a cause of energy loss in AC machinery, requiring special construction to minimize them.
– Eddy currents are used in induction heating furnaces and equipment to heat objects.
– They are also used in eddy-current testing instruments to detect cracks and flaws in metal parts.
– Eddy currents have applications in non-destructive testing, sorting, and material analysis.
Properties and Effects of Eddy Currents
– Eddy currents generate heat and electromagnetic forces.
– The heat can be used for induction heating.
– The electromagnetic forces can be used for levitation and braking.
– Eddy currents can cause power loss in transformers.
– Eddy currents can be minimized with thin plates or lamination of conductors.
– Skin effect occurs in fast-changing magnetic fields.
– The magnetic field does not fully penetrate the interior of the material.
– Increased frequency of the same field value increases eddy currents.
– Penetration depth can be calculated using specific equations.
– Power lost due to eddy currents can be calculated using a specific equation.
– The equation considers peak magnetic field, thickness/diameter, frequency, resistivity, and density.
– The equation is valid under quasi-static conditions.
– The diffusion equation models the effect of eddy currents.
– It starts with the differential form of Ampère’s Law.
– The equation involves magnetizing field, current density, electric field, and conductivity.
– Gauss’s law for magnetism is used to simplify the equation.
– Faraday’s law and the magnetization of the material are also considered.
Specific Applications of Eddy Currents
– Eddy current brakes use the drag force created by eddy currents to slow or stop moving objects.
– Eddy current brakes have no mechanical wear since there is no contact with a brake shoe or drum.
– Eddy current brakes are often used in combination with mechanical brakes, such as on overhead cranes.
– Eddy current separators can separate aluminum cans from other metals by utilizing repulsion effects.
– Strong handheld magnets can induce coins to be pushed slightly ahead, even if the coin is not magnetic.
– Dropping a strong magnet down a copper tube results in a dramatically slow descent.
– Eddy currents are used in coin-operated vending machines to detect counterfeit coins or slugs.
– Eddy current separation can separate waste streams into ferrous and non-ferrous scrap metal.
– Eddy currents are used in proximity sensors to observe the vibration and position of rotating shafts.
– Eddy current techniques are commonly used for nondestructive examination (NDE) and condition monitoring of metallic structures. Source: https://en.wikipedia.org/wiki/Eddy_current
In electromagnetism, an eddy current (also called Foucault's current) is a loop of electric current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnetic field. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material. When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid.
By Lenz's law, an eddy current creates a magnetic field that opposes the change in the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material. Thus eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores or ferrite cores to minimize them. Eddy currents are also used to heat objects in induction heating furnaces and equipment, and to detect cracks and flaws in metal parts using eddy-current testing instruments.
