Experimental studies of eddy currents
The thesis is mainly concerned with experimental studies of the heating effects of eddy currents. Sinusoidal and non-sinusoidal eddy current heating losses in non-magnetic conducting discs and spheres, subjected to a uniform field, are studied both theoretically and experimentally. The theory uses two different methods to calculate the power dissipation in the objects, and it employs Fourler series for the non-sinusoidal case. Some approximations for the losses at low frequency have been derived. The experimental study has three main sections. The first deals with the generation of uniform field for inducing eddy currents. Square Helmholtz coils have been designed, constructed and calibrated for this purpose. The second part is concerned with the measurement of the heat generated by eddy currents. A differential thermometer, using two matched therrnistors in a Blumlein bridge circuit, has been designed, built and calibrated. Operating close to balance, with a phase sensitive detector, the thermometer detects differences of 10-5 degrees C. In the third part, the experiment arranged for the eddy current heating measurements is described. The measured and the calculated results are compared, and the agreement was found to be about 2%. In foil wound inductors excited by alternating current, eddy currents together with proximity effect cause a redistribution of the current density across the width of the inductors. The current flow is increased along the edges of the foil and decreased along the centre portion. A thermal demonstration of this phenomenon, which is known as width effect, is described and studied by using the differential thermometer.