Ring Losses Evaluation in Ringed-Pole PM Motors

In this paper, the ringed-pole surface-mounted permanent-magnet machine is investigated from the point of view of the ring losses. The rings around the poles create a high-frequency anisotropy that is useful to detect the rotor position by means of a high-frequency signal injection. However, the presence of the rings causes additional rotor losses. The causes of ring losses are investigated in the paper. The first cause is the high-frequency signal injection itself. The harmonics in the airgap magnetomotive force, which are not synchronous with the rotor and hence induce currents in the rings, are the second source of losses. The third source is the airgap flux fluctuation due to stator slotting. Machines of different sizes and winding types are investigated. A finite-element analysis and an analytical model are used to estimate the ring losses, comparing the results. The influence of the ring cross-sectional area on the ring losses is analyzed, showing that the ring losses can be reduced in certain configurations.