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

Due to the special doubly-salient structure, flux-reversal permanent magnet (FRPM) machines typically suffer from relatively large torque and speed ripples, as well as 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 a smooth torque. In this paper, an improved configuration of FRPM machine is proposed by introducing a small space-gap between the two adjacent magnets belonging to the same stator tooth to improve the symmetry of phase back-EMF waveform. The influence of the small space-gap on phase back-EMF waveform is evaluated by employing 2D finite element analysis (FEA), and consequently, an optimal value of space-gap for a more sinusoidal back-EMF waveform is obtained.