Editorial Feature

What are the Applications of Eddy Currents?

Updated by Reginald Davey 03/03/2023

Understanding electrical and magnetic fields is a key part of the design of multiple products in the 21st century. This article will discuss eddy currents, a specialized type of electric field, and their uses in a range of research and practical commercial applications.

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What are Eddy Currents?

Eddy currents are specialized electric fields that are generated in the presence of a magnetic field and are often seen as a secondary field. An Eddy current can be produced by both electromagnets and permanent magnets, as well as transformers.

They are also produced by the relative motion generated when a magnet is located next to a conducting material. Eddy currents are used in specific applications including non-destructive testing.

As mentioned, eddy currents are electric fields that are generated by a magnetic field, but the strength of an eddy current field is proportional to the strength of the magnetic field, as well as the area of the field and the change in flux.

Field strength is inversely proportional to the resistivity of the material it is generated in/around. As such, there are many things that can affect an eddy current, which also means there are many ways in which they can be tuned.

Eddy currents are generated because a secondary magnetic field is created that opposes the initial magnetic field. This secondary magnetic field opposes the initial magnetic field and reacts against it, which in turn generates a circular electrical field.

Magnetic fields are often depicted as circular lines where the magnetic field flows, and the electric field lines essentially move in a counter-clockwise direction to the original magnetic field lines.

Eddy currents are, therefore very localized electrical fields near the initial magnetic field, but they are induced on a material that is in close proximity to the magnet, not on the magnet itself.

Application of Eddy Currents

The two common application areas are non-destructive testing (NDT) and applications that use the principles of electromagnetic damping. While these are two of the key application areas, eddy currents also have some minor application areas as well.

The minor application areas where eddy currents are utilized include use in energy meters, automobile speedometers, induction alternating current (AC) motors, free-fall devices, as a way of detecting coins in vending machines, to increase temperatures uniformly in induction furnaces, and in proximity/displacement sensors.

Non-Destructive Testing (NDT)

A main application of eddy currents are used is in non-destructive testing (NDT.)  This is one of the most common electromagnetic NDT methods to see if a material or surface has any discontinuities, cracks or faults without damaging the material itself.

NDT methods are used widely throughout the construction and civil engineering industries to analyze various materials, but eddy currents have found the most use on tubular and bar-shaped materials. However, any structural issues that are circumferentially orientated struggle to be picked up due to how the flux interacts with the spherical nature of the tubing.

Eddy current testing is a popular and simple NDT method. Eddy current testing uses an alternating current coil to generate an electromagnetic field on the surface of the material that is being analyzed.

Because of the nature of the test, the test sample must be conductive in nature. A small secondary current is generated on the surface of the test material and because the flow is counter-clockwise to the initial current, an eddy current is generated.

Any discontinuities, voids, or imperfections (unless circumferential) will be picked up because the eddy current density will change, and this is a measurable and detectable change. While it is often used on circular materials, the coils can also be adapted to detect any issues with flat materials—and there are no issues with certain discontinuities being undetectable due to the geometry of the material.

Electromagnetic Damping Applications

Electromagnetic damping is a force that is automatically generated when the eddy current is in close contact to a conductive or magnetic object. Electromagnetic damping is the occurrence of a damping force (i.e., a reduction in motion) when an eddy current and another magnetic field interact with each other.

Electromagnetic damping is, therefore, a resistive force that causes a conductive or magnetic object in the field of the eddy current to slow down without physically touching it.

In terms of real-world uses, it is a principle used on rail brakes to help high-speed rail carriages stop at certain points, as the interaction between the eddy current and conductive materials slows the motion of the train without the need for physical brakes.

One major advantage of using eddy currents in rail brakes is that they are not affected by rain, which makes braking smoother. However, these types of brakes cannot bring a vehicle to a full stop, so the use of other types of braking systems is essential in rail vehicles.

Another use is in the design of galvanometers, devices that measure small electrical currents, where eddy currents can be used to counter the deflections of the galvanometer so that the coil used in the galvanometer reaches equilibrium.

It enables the accuracy of the galvanometer to be determined as the deflections cannot normally be read until the testing process and the coil used comes to a rest, at which point, the galvanometer is no longer at equilibrium.

Magnetic damping and eddy currents can also be used in recycling centers to separate metals from other materials. This works for all types of metal, not just ferromagnetic types such as iron and steel.

Eddy currents are also exploited in applications such as portable metal detectors which are used by the military to detect weapons, explosives, and landmines. Another use is in induction stoves and cooktops. The application of eddy currents in multiple technologies is widespread.

In Summary

Eddy currents are electric currents generated in the presence of a magnetic field. Currently, they are applied in some key industrial and research applications, such as non-destructive testing and rail brakes. Several everyday objects exploit the application of eddy currents, such as vending machines AC motors, and sensors.

An intimate understanding of how eddy currents are generated is therefore essential for the development of a multitude of research, industrial, and commercial applications.

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Liam Critchley

Written by

Liam Critchley

Liam Critchley is a writer and journalist who specializes in Chemistry and Nanotechnology, with a MChem in Chemistry and Nanotechnology and M.Sc. Research in Chemical Engineering.

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