Torque ripple reduction for 6-stator/4-rotor-pole variable flux reluctance machines by using harmonic field current injection

The variable flux reluctance machines (VFRMs) are electrically field excited synchronous machines having both field and armature windings on the stator. In order to investigate the torque production of the 6/4 VFRM, an instantaneous torque equation is derived for the first time considering the inductance harmonics by using Fourier series. According to the equation, the undesirable torque ripples having multiples of third harmonic exist due to the inductance harmonics of the field and armature windings. Hence, the harmonic field current injection methods are proposed for the torque ripple reduction. The torque waveforms calculated by 2D-finite element analysis (FEA) are compared with the predicted results from the derived torque equation. Although the methods are proposed at which the magnetic saturation does not occur, the torque ripple can be also reduced in saturation region. Since the proposed method utilizes the machine parameters the influence of the parameter mismatch is investigated. It shows that the torque ripple reduction performance is not affected much unless the fundamental component of nominal inductance is underestimated. The experimental results verify that the proposed approach can effectively reduce the torque ripple in the VFRM.