Fault-tolerant permanent magnet machines with mixed-layer fractional-slot concentrated-windings

Many safety-critical applications require compact yet fault-tolerant electrical machines. In this paper, to maximize the fault tolerant capability, a double layer (DL) fractional slot concentrated winding (FSCW) 9-slot/8-pole (9s/8p) permanent magnet (PM) machine is transformed into a mixed-layer (ML) FSCW 9s/8p PM machine by removing one coil from each phase and re-designing the stator structure. A comparison on the electromagnetic performance of the machines, such as winding factor, open-circuit air-gap flux density, phase back-electromotive force, average torque against copper loss, torque ripple and cogging torque, has been conducted. Also, the factors that affect the fault tolerant capability of the machines, such as phase self-inductances, mutual inductance and d- and q-axis inductances as well as three phase short circuit (SC) performance have been investigated. It is found that ML 9s/8p machine can achieve higher winding factor, more than 35% higher self-inductance and a near zero mutual inductance that is only 1.1% of its DL counterpart. This helped the ML 9s/8p to have better fault tolerance and smaller three phase SC current, while having identical average torque (albeit with higher torque ripple) and comparable copper loss. The finite element results are validated by experiments using a prototype machine.