Forces on Particles in Time-Varying Magnetic Fields

Electrodynamic sorting is a process that sorts metals based on conductivity, density, and geometry. The process works by inducing electrical eddy currents within particles placed in a time-varying magnetic field. For the special case of a perfect, uniform sphere, an approximate equation can be used to predict the net force under a linear magnetic gradient. This paper explores the accuracy of that model by measuring the net force on spherical samples of copper, brass, and aluminum with varying sizes and excitation frequencies. Results consistently show strong agreement with the approximate models over all conditions. We also explore several non-spherical geometries, including cylinders, cubes, and disks. We found that they could be modeled as equivalent spheres, given an appropriate radius, and had reasonable accuracy over frequency.