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Synchronous reluctance (Syn-Rel) machines with embedded permanent magnets (PMs) are research hotspots in variable speed motor drives due to their robust rotor structure and wide constant power speed range (CPSR). In this paper, the potential of PM-assisted Syn-Rel machine to be next generation heavy duty traction motor solution has been investigated, with special attention put on one key geometric parameter, i.e., airgap length. Careful machine design and optimization has been conducted based on geometric parametrization including airgap length variation, for 15000rpm peak speed and 350kW peak power output. In low speed operations, the influence of airgap length on different torque components has been analyzed in detail based on the frozen permeability method. In field weakening region, the variation trend of several key parameters such as output power, torque ripple, and power losses have been investigated along with airgap length. It is found that with high electric and magnetic loading, reducing the electromagnetic airgap length is not always beneficial. There exists a suitable airgap length value to comprehensively balance torque/power density, cooling capability, efficiency and reliability. Numerical FEA and experimental tests of the prototype are combined to verify the conclusions.

In recent years, synchronous reluctance (Syn-Rel) machines are research hotspots in variable speed motor drives due to their robust rotor structure and wide constant power speed range (CPSR). More practically, when kVA limitation is considered, embedded permanent magnets (PMs) have been widely adopted in Syn-Rel rotors to further increase power density as well as power factor. In this paper, the authors have investigated the potential of PM-assisted Syn-Rel machine to be the next generation electrical propulsion motor topology for automotive application, with special attention put on a key geometric parameter, i.e., airgap length. In MTPA region, the influence of airgap length on different torque components has been analyzed in detail based on the frozen permeability method. In field weakening region, the variation trend of several key performance such as peak power, iron loss and torque ripple have been investigated along with airgap length. It is found that with specifically high level of electric and magnetic loading, there exists optimal value of airgap length to achieve high power density based on specific cooling and kVA limitation. Numerical FEA and experimental tests are associated to validate the conclusions.