# What are Eddy Currents?

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Eddy currents are currents which circulate in conductors like swirling eddies in a stream. They are induced by changing magnetic fields and flow in closed loops, perpendicular to the plane of the magnetic field. They can be created when a conductor is moving through a magnetic field, or when the magnetic field surrounding a stationary conductor is varying i.e. anything which results in the conductor experiencing a change in the intensity or direction of a magnetic field can produce eddy currents. The size of the eddy current is proportional to the size of the magnetic field, the area of the loop and the rate of change of magnetic flux, and inversely proportional to the resistivity of the conductor.

Like any current flowing through a conductor, an eddy current will produce its own magnetic field. Lenz’s Law states that the direction of magnetically induced current, like an eddy current, will be such that the magnetic field produced will oppose the change of magnetic field which created it. This resistance created by the opposing magnetic fields is exploited in eddy current braking, which is commonly used as a method of stopping rotating power tools and rollercoasters.

In the diagram below, the conductive metal sheet (representing the moving rollercoaster car or power tool for instance), moves past a stationary magnet. As the sheet moves past the left edge of the magnet, it will feel an increase in magnetic field strength, inducing counter-clockwise eddy currents. These currents produce their own magnetic fields and according to Lenz’s Law, the direction will be upwards i.e. opposing the external magnetic field, creating magnetic drag. At the other edge of the magnet, the sheet will be leaving the magnetic field and the change of field will be in the opposite direction, thus inducing clockwise eddy currents which then produce a magnetic field acting downwards. This will attract the external magnet, also producing drag. These drag forces slow the moving sheet, providing the braking. An electromagnet can be used for the external magnet, meaning it is possible to vary the strength of the braking applied by adjusting the current through the electromagnet’s coils. An advantage of eddy braking is that it is contactless, so results in no mechanical wear. However, eddy braking is not suitable for low speed braking and because the conductor has to be moving, eddy brakes cannot hold objects in stationary positions. Thus, it is often necessary to also use a traditional friction brake.

Eddy currents were first observed in 1824 by scientist and then Prime Minister of France, François Arago. He realized that it was possible to magnetize most conductive objects and was the first to witness rotary magnetism. Ten years later, Lenz’s Law was postulated by Heinrich Lenz, but it wasn’t until 1855 that the French physicist Léon Foucault officially discovered eddy currents. He found that the force needed to rotate a copper disk when its rim is placed between the poles of a magnet, such as a horseshoe magnet, increases and the disk is heated by the induced eddy currents.

The heating effect originates from the transformation of electric energy into heat energy and is used in induction heating devices, like some cookers and welders. The resistance felt by the eddy currents in a conductor causes Joule heating and the amount of heat generated is proportional to the current squared. However, for applications like motors, generators and transformers, this heat is considered wasted energy and as such, eddy currents need to be minimized. This can be achieved by laminating the metal cores of these devices, where each core is made up of multiple insulated sheets of metal. This splits the core in many individual magnetic circuits and restricts the flow of the eddy currents through it, reducing the amount of heat generated through Joule heating.

Eddy currents can also be removed by cracks or slits in the conductor, which break the circuit and prevent the current loops from circulating. This means that eddy currents can be used to detect defects in materials. This is called nondestructive testing and is often used in airplanes. The magnetic field produced by the eddy currents is measured, where a change in the field reveals the presence of an irregularity; a defect will reduce the size of the eddy current, which in turn reduces the magnetic field strength.

Another application of eddy currents is magnetic levitation. Conductors are exposed to varying magnetic fields which induce eddy currents within the conductor and produce a repulsive magnetic field, pushing the magnet and conductor apart. This alternating magnetic field can be caused by relative motion between the magnet and conductor (generally the magnet is stationary and the conductor moves) or with an electromagnet applied with a varying current to vary the magnetic field strength.

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