Permanent magnets thermal operation limits in a hybrid excited synchronous machine used on wide speed applications

In variable speed drives thermal phenomena interfere with electromagnetic ones. Moreover, when it comes to electrical machines excited through permanent magnets (PMs), beside the common problems related to the stator winding insulation damage (at high levels of current densities) and mechanical stress (due to centrifugal forces while high speed is attempted) a supplementary issue appears: the PM irreversible demagnetization risk, since its operating point varies with the temperature. The goal of this paper is to discuss on the thermal safe operation limits of a hybrid excited synchronous machine (HESM). Analytical calculation, numerical computation through finite element method (FEM), accompanied by tests, will reveal the behavior of the proposed machine and will validate the applied design procedure in terms of safe operation.