Comparison of three permanent magnet structures with respect to torque production by means of analytical field calculations

In this paper, three different permanent magnet arrays are compared for application on the rotor of an energy storage flywheel. The design goals are to reach as high a torque as possible for a given magnet volume while the no-load losses are to be minimized as well. Since Halbach arrays result in a more gradual change of magnetic flux density in the airgap of a machine, they were considered as an alternative for the radially magnetized array to lower the eddy current losses in the winding of a machine with a slotless stator without compromising the torque production. In this paper, the conventional radially magnetized array is compared with both a discrete Halbach array and an ideal Halbach array with respect to torque production. An equivalent current density in the airgap is used to calculate the electromagnetic torque. The results are presented for the torque as a function of the ratio of magnet thickness to total airgap length.