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Eddy currents are specialized electric fields which 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 and 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, which are discussed in this article.
As mentioned, eddy currents are an electric field which is 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. It 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 which is in close proximity to the magnet, but not on the magnet itself.
The two common application areas are in non-destructive testing (NDT) and applications that use the principles of electromagnetic damping. While these are two of the key application areas, we’re going to focus on them more below, but 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)
Eddy currents are used in 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 NDT method with a simple operation. 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, it is an eddy current that is generated.
Any discontinuities, voids or imperfection (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. Another use is in the design of galvanometers, devices which 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 come to a rest, at which point, the galvanometer is no longer at equilibrium.