Torque density enhancement of 6/4 variable flux reluctance machine with 2nd harmonic current injection

Variable flux reluctance machines (VFRMs) are developed as magnetless electrical machines. To extend the application of VFRMs, the enhancement of torque density is a key during machine design. In this paper, a novel 2nd harmonic current injection method is developed for torque density enhancement for 6-stator-slot/4-rotor-tooth (6/4) VFRM. By using analytical method, the optimal current profile with injected 2nd harmonic current is obtained. The average torque of 6/4 VFRM is improved by 20% with the proposed method under all load conditions. Moreover, the proposed current profile has fixed harmonic proportions and is easily applicable to machines with different specifications. All the conclusions are confirmed by both finite element analyses and experimental results.