###### Abstract  
The torque performance of the interior permanent-magnet synchronous motor (IPMSM) must be further improved to satisfy the growing demand of aircraft drive application. To this end, this article focuses on the design optimization of the IPMSM structure in the aircraft drive systems to improve the torque density and reduce the torque ripple. A special fractional-slot winding and \(\
abla\)-type magnetic-pole rotor topology are proposed as the optimized IPMSM structure compared with the structure of an existing motor. The simulations of the original and optimized structures at different current values reveal the variance of the torque in the average and ripple, mechanical and external characteristics, efficiency and steady-state temperature. The performance of an optimized prototype is analyzed by experimental testing, and the results show that an optimized motor has a higher torque density and lower torque ripple than the original one at the same speed and rated power, but it also has a higher temperature rise. However, the temperature rise value is acceptable in the experimental testing condition, so the validity of the design optimization method for the proposed structure is verified. design optimization; fractional-slot winding; interior permanent-magnet synchronous motor; high torque density; low torque ripple; \(\
abla\)-type magnetic-pole rotor topology +
Footnote †: journal: Applied Sciences  
significant advantages compared with an integer-slot winding, such as a lower torque ripple [8]. Furthermore, the use of a fractional-slot winding increases the leakage inductance, thus reducing the cogging torque, and can increase the fault tolerance capability [9; 10]. The mutual inductance and fault tolerance of the PMSM can be influenced by a single- or double-layer winding topology [11]. As for the rotor pole structure, the PMSM can be classified as a surface-mounted PMSM (SPMSM) [12; 13] and an interior PMSM (IPMSM); the latter one provides higher per-unit inductances and thus field-weakening capability [14]. The rotor magnetic-pole topology of the IPMSM can be taken as the optimal design method, and the currently known topology types are the \"conventional type\", \"U-type\", \"V-type\", etc. The comparisons of four different rotor topologies for a distributed winding IPMSM are analyzed and reveal that the \"V-type\" has the lowest magnet mass, the \"\(\overline{W}\)-type\" has the largest \(d\), \(q\) inductances and a higher rated torque than the others and the \"segmented type\" has a wider range of constant power speed operations than the \"conventional type\" [15]. In general, the IPMSM used in electrified vehicles with a \"conventional type\" permanent-magnet rotor has great low-speed performance while having a poor capacity under flux-weakening control [16]. Considering both factors mentioned above and high efficiency in the overall driving range, the \"V-type\" is the better option [17; 18]. The cross-magnetization effect can be significantly reduced by the \"\(\
abla\)-type\" rotor structure; thus, the \(d\)-axis inductance is decreased, which can increase the output torque under the overload flux-weakening control in comparison with the \"V-type\". In high-speed and high-torque-density applications, the \"\(\
abla\)-type\" is the best design among the \"V-type\", \"2V-type\" and \"\(\
abla\)-type\" [19]. The torque ripple can also be reduced by the \"\(\
abla\)-type\" IPMSM with an asymmetric spoke [20]. The performance of the micro IPMSM with a \"conventional type\" and \"spoke type\" has been investigated, and the results show that the \"spoke type\" has a better output torque, while the \"conventional type\" has a lower torque ripple [21]. Figure 1 shows the four different rotor magnetic-pole topologies. Figure 1: Four different rotor magnetic-pole topologies: (**a**) conventional type; (**b**) segmented type; (**c**) V-type; and (**d**) U-type.