Irreversible demagnetization risk of rotor-surface-mounted permanent magnets at rated and short-circuit modes in two-pole multi-megawatt machines

Abstract

The flux density distribution of a rotor-surface-magnet two-pole 4 MW permanent-magnet synchronous machine with a solid rotor core is studied in three different operating modes: rated operation, two- and three-phase short circuits. Demagnetizing field strength by the armature reaction at the rated operation of multimegawatt two-pole machines can be significant, and it should be ensured that coercivity in the magnets is not decreased due to irreversible demagnetization. Significant demagnetization field can take place during the transient of two- or three-phase short circuit. In this work, the capability of the magnets to withstand short-circuits in multi-megawatt two-pole machines regarding the risk of irreversible demagnetization is investigated. To evaluate the role of the solid rotor core in short-circuit tolerance of the machine, short-circuit tests were performed in transient 2D Finite Element Method (FEM) model and the results were compared with similar tests when the rotor yoke is replaced with a traditional lamination stack.