What are eddy currents?

Usually, people that move cause something. Is it the same for electrons? The answer is: yes. Their movement, which is a current — the eddy current — causes a magnetic alternating field on their part, which opposes the stimulating magnetic field and weakens it. This magnetic alternating field created by the eddy currents is called secondary field, which occurs as a result of the stimulating magnetic field. The stimulating magnetic alternating field is the primary field.

The primary magnetic alternating field is usually created by coils carrying alternating current. These ensure that the primary field enters the material and stimulates the eddy currents. This causes "stimulated" electrons to move back and forth on curved lines. The higher the electrical conductibility of the material, the better the functioning of the eddy current generation.

The eddy currents themselves are not passive and create a secondary magnetic alternating field due to their movement, which weakens the primary field and can thus be detected outside of the material. The secondary field can thus give evidence of the behavior of the eddy currents on the inside of the material.

Eddy currents are electrons moving back and forth on curved lines in the material. These electrons can serve as "spies", so to speak — indicating whether all is as it should be within the material, or whether there are any disturbances. For example, such a disturbance could be a crack in a metal. For the moving electrons this could be a big obstacle which could block their way, so that they would need to find a different route. Coming back to the comparison with a large city, this could for example be a blocked road.

Now how does such an obstacle affect the electrons in the material? The electrons cannot move across the obstacle, so there is no eddy current here. This also means that there is no secondary magnetic field at the site of the obstacle, which in turn also means the primary stimulating field is not weakened. This implies that flaws in an electrically conductive material can be detected via the amplitude of the magnetic alternating field resulting from the overlay of stimulating field and secondary field.

If the electrons are forced to make a detour due to the obstacle, which requires additional time, this means that — compared to the uninterrupted case — there is a time shift between the primary and the secondary magnetic field. This is called a displacement of phase between the two fields. This can also be measured.

For finding flaws in the material, amplitudes and phases of magnetic field changes caused by the eddy currents are thus available. Different flaws in the material obviously cause different behavior of the eddy currents and their signals, which makes it possible to differentiate between various flaws. For example, material losses or wall attenuations due to corrosion provide different signal responses than cracks. For this reason, the application of material tests with eddy currents can be quite versatile.

The measuring probes used in eddy current technology normally consist of a transmitting coil for creating the stimulating primary field, and a receiving coil for measuring the resulting magnetic field. For this reason, the production of coils is relatively simple and cost-effective.

Electrical voltages and currents in the frequency range up to a few megahertz are used. Usually, the analog signals from the probes are transformed to digital signals, so that the eddy current signals can be analysed quickly and easily. As these are electromagnetic signals, their effect is quick, and high test speeds can be achieved. The signals and their processing and analysis enables the creation of fully automatic eddy current test systems for online or inline testing of work pieces during production.