Design of a low torque ripple fractional-slot interior permanent magnet motor

Among the Permanent Magnet (PM) machines, the Interior PM (IPM) machine is proposed, despite of a strong non linear behaviour and a complex design, as a good candidate thanks to its interesting peculiarities, i.e., higher torque in flux-weakening operation, higher fault-tolerance, and ability to adopt low cost PMs. A second trend in designing PM machines concerns the adoption of fractional-slot (FS) non-overlapped coil windings, which reduce the end winding length and consequently the Joule losses and the cost. Therefore, the adoption of IPM machine with a FS winding aims to combine both advantages: high torque and efficiency in a wide operating region. However, the combination of an anisotropic rotor and a FS winding stator causes some problems. The interaction of the MMF harmonics due to the stator current interact with the rotor anisotropy causing a very high torque ripple. This paper illustrates a procedure to design an IPM motor with FS winding exhibiting a low torque ripple. The design strategy is based on two consecutive steps: at first, the winding is optimized by taking a multi-layer structure, then the rotor geometry is optimized by adopting a non-symmetric structure. As an example, a 12-slot 10-pole IPM machine is considered, achieving a torque ripple lower than 1.5% at full load.