Analysis and Design Recommendations to Mitigate Demagnetization Vulnerability in Surface PM Synchronous Machines

A design approach is presented to mitigate demagnetization vulnerability in surface permanent magnet (PM) synchronous machines by the proper selection and design of stator windings and rotor configurations. First, a comparative analysis of the stator demagnetizing magnetomotive forces (MMFs) and leakage inductances for surface PM machines equipped with integral-slot distributed windings (ISDW) and fractional-slot concentrated windings (FSCW) is performed under the constraint of equal magnet flux linkage. Finite element analysis is used to build confidence in the predicted variation of flux density over the magnet surfaces in the two machines. The higher leakage inductance in the FSCW machine reduces the negative impact of its higher peak demagnetizing MMF, but not enough to offset the ISDW machine's advantage of lower demagnetizing MMF. Subsequently, two convenient metrics are proposed to evaluate the relative amplitude of the peak demagnetizing MMF and potential rotor temperature rise due to eddy-current losses. This study shows that the higher peak demagnetizing MMF applied by the stator winding currents and rich spatial harmonics that increase rotor losses makes the FSCW-PM machine more vulnerable to rotor demagnetization compared with the ISDW-PM machine. Finally, it is shown that the FSCW-PM machines with slot-per-phase-per-pole values of ½ offer appealingly low values for both defined demagnetization metrics.