What are eddy currents?
Professor Dr.-Ing. Dieter Stegemann
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 behaviour 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 the already mentioned 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
he frequency range up to a few megahertz are used. Usually, the analogue 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.
Der Einsatz von Wirbelströmen
für die zerstörungsfreie Werkstoffprüfung Dieter Stegemann,
em. ord. Professor Dr.-Ing. Fakultätfü rMaschinenbau
Leibniz Universität Hannover