Infrared measurements of induced eddy currents in sheet conductors

An infrared scanning system is used to visualize eddy current distributions in thin sheet-like conductors. Results show that the infrared technique can detect eddy current related temperature changes as small as 0.2°C induced by alternating, pulsed and convective magnetic fields. The effects of magnetic field frequency are examined as well as a comparison of alternating and pulsed field induction of currents. Comparison of infrared measurements with results from the more conventional search coil technique show good agreement. A theoretical model is developed which can predict the induced currents and Joule heating thermal patterns in the large skin depth limit. This theoretical model includes heat conduction effects and shows that isotherm patterns are close to current density patterns when heat conduction can be neglected. Thus the method works well for pulsed current applications. The model is applied to the induction of currents in an electrodynamic levitation problem of a long rectangular coil over a sheet conductor track. The numerical predictions of eddy current thermal patterns show good agreement with the results from the infrared technique. These results show an increase in eddy current density when the lift coil moves close to the edge of the conductor track. Eddy current densities in linear motor secondaries have also been detected using this method. This experimental method should have applications to magnetic levitation and magnetic fusion technologies.