a Macroscopic Analysis of Eddy Currents in Nonferrous Metals
Abstract
The original purpose of this research was to determine if the signals produced by the eddy currents induced into nonferrous metals as they pass through a static magnetic field could be used to distinguish between these metals. First, it was shown that for samples of the same size and shape, the maximum negative response produced by a Halleffect sensor varied directly with the conductivity of the sample material. This was true for all shapes tested including rectangles, disks, rings, and cylinders. Samples of aluminum, brass, copper, lead, and zinc were easily distinguished from each other using the maximum negative response measured. The largest dimension of any sample tested was 4 inches, but algorithms could be developed for larger samples according to the statistics. The correlation coefficients for all sets of data collected in a randomized factorial design experiment were greater than 0.96. An algorithm was developed which correctly predicted the form of the response of the sensing apparatus to the passing of a thin copper ring through the static magnetic field. This involved writing an expression for the magnetic field produced by the eddy currents in the ring as the ring dropped from above, through, and beyond the static magnetic field. The inductive character of the nonferrous metals was incorporated into the model by introducing convolution. The currents produced by the induced emf were convolved with the residual decaying eddy currents to produce the net current. The model was responsive to the time constant associated with the conductivity, size, and shape of the samples. With convolution included, the simulated response produced by the model developed herein agreed well with the actual response measured. A new expression for the distribution of eddy currents in a nonferrous ring as it passes through a static magnetic field was developed to support the experimental findings. The new current distribution expression has the form of a fourthorder exponential function of the wall width.
 Publication:

Ph.D. Thesis
 Pub Date:
 January 1992
 Bibcode:
 1992PhDT.......138H
 Keywords:

 RECYCLING;
 ALUMINUM;
 BRASS;
 COPPER;
 LEAD;
 ZINC;
 Engineering: Electronics and Electrical; Physics: Electricity and Magnetism; Environmental Sciences