Influence of rotor pole number on electromagnetic performance of novel variable flux reluctance machine with DC-field coil in stator

This paper investigates the influence of rotor pole number on the electromagnetic performance of variable flux reluctance machine, which employs a doubly salient structure but with a DC-field coil identically located for each phase. The flux-linkage, back-EMF, torque capability, unbalanced magnetic force and the capability of adjusting air-gap flux density are investigated and compared in the 6-stator pole machines with optimized 4-, 5-, 7- and 8-rotor poles. As one of non-magnet machines, this type of machine has the benefit of low cost and robust structure. It shows that sinusoidal back-EMF and flux linkage waveform are achieved in 5- and 7-rotor pole machines. However, in 4- and 8-rotor pole machines the back-EMF is similar to the conventional DC-excited or PM doubly salient machine, in which high even order harmonics can be observed in the back-EMF. The significant high order harmonics can result in higher torque ripple in the 4- and 8-rotor pole machine. In terms of torque density, the 5-rotor pole machine exhibits the highest owing to the short flux path, and the 8- rotor pole machines produces the lowest torque. In addition, both 5- and 7-rotor pole machines exhibit unbalanced magnetic force due to the odd rotor pole number, which can lead to high vibration and noise, although it can be avoided by doubling the stator and rotor pole numbers, e.g. 12-stator poles, 10- and 14-rotor poles.