Geometrical enhancements to permanent magnet flux sources: application to energy efficient magnetocaloric refrigeration systems

The research reported here shows that a magnetic flux density B far in excess of the magnetic remanence B_R of a permanent magnet material can be achieved by arranging permanent magnets in the form of hollow cylindrical shells (magic rings), or hollow spherical shells (magic spheres) with additional modifications. In this paper, we report on three additional modifications to enhance the magnetic flux density in the pole gap, i) by changing the cross sectional shape of the permanent magnet array into an ellipse instead of a circle, ii) by including a high remanence flux concentrator close to the air gap and iii) by surrounding the magnet array with a soft magnetic shell which acts as a flux return path. In this way flux densities in excess of 3 Tesla can be achieved in a 23.5 mm²×15.2 mm pole gap using NdFeB magnets with remanence of only 1.2 Tesla. The enhanced magnetic flux density can be used for magnetic refrigeration systems, which require sources of magnetic field typically above 1.5 Tesla