Eddy Current Power Dissipation in Rare Earth Packed Particle Bed Regenerators for AMRR Applications

Abstract

The magnetic regenerators in an active magnetic regenerative refrigerator (AMRR) are exposed to high fluctuating magnetic fields during operation. The rapid change of field over time can cause large eddy currents to be formed in any conducting material used in the regenerator and cause parasitic heating loads. The magnitude of eddy currents formed depends not only on the field change with time but also the shape factor of the material, its bulk resistivity, and its geometry. To detennine the power dissipation due to eddy currents, specifications from the prototype AMRR being built by the Cryofliel Systems Group at University of Victoria were used. The regenerator consisted of 100 micron diameter dysprosium particles packed into a regenerator bed (120 mm long by 125 mm wide and 50 mm deep), and cycled through a magnetic field fluctuation of 8 Tesla. Eddy current generation estimates were performed based on three models. The first assumed that the particles were electrically isolated from each other. For the specified regenerator, the power dissipated was less than 1 W for frequencies up to 10 Hz. The second model assumed that the regenerator is a solid porous block. This model resulted in power dissipations 3 orders of magnitude greater than model 1. The third model assumed some degree of electrical contact among the particles. Bulk resistivity experiments were conducted to assess the degree of electrical contact between particles and to estimate the power dissipated due to eddy currents. The results show dissipated powers several orders of magnitude less than the insulated particle model over a large range of bed packing pressures. It was expected that this model would be the most realistic and show power dissipations between that shown by the first two model estimates. This discrepancy is explained by the variable packing pressure with bed length characteristic of a packed particle bed. Dissipated end plate pressure leaves particles in the bed middle relatively unloaded and leads to high bulk resistivities. Tests performed with short beds and high packing pressures lead to similar power dissipations and suggests that the insulated particle model is reasonable approximation to assess the magnitude of eddy currents formed in the prototype AMRR packed particle bed.