Reducing the permanent magnet content in fractional-slot concentrated-windings permanent magnet synchronous machines

The efficiency of permanent magnet synchronous machines is investigated as a function of the mass of magnets used, keeping the power and volume constant. A comparison is made between rare earth (NdFeB) magnets and ferrite magnets. For a given type and amount of magnets, the geometry of the machine is optimized and the efficiency map is computed, taken into account iron loss, copper loss, magnet loss and pulse width modulation loss. The amount of permanent magnet material can be reduced significantly with a minor influence on the efficiency and power density. Furthermore, the machine is optimized for several soft magnetic materials. It is observed that the optimal number of poles is higher for low loss materials, and that the efficiency is up to 5% higher than for the highest loss material. For a motor optimized for ferrite magnets, the minimal magnet thickness for high efficiency is about double compared to NdFeB.