Back-EMF waveform optimization of flux-switching permanent magnet machines

Abstract

Due to the nature of salient poles in both stator and rotor, flux-switching permanent magnet (FSPM) machines typically suffer from severe torque ripples, causing undesired acoustic noise and vibration, especially at low speeds. As one of the main sources of torque ripples, harmonics in phase back electro-motive-force (EMF) should be suppressed as much as possible in order to produce smooth torque. In this paper, the back-EMF waveform of FSPM machines is investigated and analyzed, and the definition of coil group (CG) is newly introduced. Then, based on the number of (CGs), three novel approaches, namely shifting the stator teeth, shifting the rotor teeth and stepped skewing of the rotor, are proposed by counteracting the specific harmonic component, which can remarkably improve the back-EMF waveform. Besides, based on the analytical expression of back-EMF, the optimal design of these three methods is investigated and obtained, which can be generally extended to other FSPM machines. What's more, finite element analysis (FEA) is employed to verify the validation of this optimal design by setting a 12-stator-slots/10-rotor-pole FSPM machine as an example. It turns out to be that though these three approaches can improve the back-EMF waveform, they don't do work all the time. In fact, either shifting stator or rotor teeth is effective only when the number of CGs is 2, while for the stepped skewing of rotor, it can always do work for the machines having arbitrary CGs number. In addition, it should be noted that the first two approaches will inevitably result in unbalanced magnetic force (UMF).