The performance of fault-tolerant modular permanent magnet machines depends on the proper selection of the pole and slot numbers which result in negligible coupling between phases. The preferred slot and pole number combinations eliminate the effect of low-order harmonics in the stator magnetomotive force and thereby the vibration and stray loss are reduced. In this paper, three external rotor machines with identical machine dimensions are designed with different slots per phase per pole ratios. A simulation study is carried out using finite element analysis to compare the performance of the three machines in terms of machine torque density, ripple torque, core loss, and machine efficiency. A mathematical model based on the conventional-phase-model approach is also used for the comparative study. The simulation study is extended to depict machine performance under fault conditions. 1. Introduction Thanks to their high power density, fault tolerant permanent magnet brushless drives have shown promise in critical applications, such as in the electric vehicles (EV), aerospace, and automotive sectors [1]. Multiphase modular permanent magnet machines comprise magnetically and physically isolated phases, each having one-per-unit inductance which limits the short circuit current to its rated value under fault conditions. Hence, a fault in any phase will not affect others. Many papers address the design and the operating principle of fault-tolerant permanent magnet (PM) machines [2, 3]. These features have made the multiphase modular PM machine a strong candidate in the field of fault tolerant applications [4–6]. Optimum control under healthy and faulted scenarios is proposed to minimize the machine torque ripple [7, 8]. Favorable slot and pole number combinations for fault-tolerant PM machines have been proposed in [3], which ensure inherently negligible coupling between phases. For a five-phase modular machine, there are three favorable slot and pole number combinations [3], namely, 20 slots/14 poles, 20 slots/18 poles, 20 slots/22 poles and their multiples. In [3], the first two combinations are compared in terms of flux distribution and core loss. In [9], a machine with the second combination is designed by means of the star of slots [10] and using a double-layer fractional-slot winding. Current control strategies, in case of one-phase or two-phase faults are proposed. An analytical model has been adopted to individuate the most suitable current references, without increasing the motor or inverter cost. In [11]; a comparison between two 5-phase PM machine
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