Overmodulation Techniques of Asymmetrical Six-Phase Machine With Minimum RMS Current Ripple

An asymmetrical six-phase machine (ASPM) with two isolated neutral points is analyzed in two orthogonal subspaces: energy-transferring and harmonic subspaces. Overmodulation (OVM) techniques of ASPM extend the operating region of ASPM in the energy-transferring plane and thus attain higher voltage gain after injecting nonzero average voltage in the harmonic subspace. But due to low impedance in the harmonic subspace, the applied voltage causes large circulating current and copper loss. Hence, the RMS of the applied average voltage is minimized in the literature. It is also shown in the literature that for a given reference voltage vector in the OVM plane, there is more than one pulsewidth modulation (PWM) sequence that can achieve the minimum injection of nonzero average voltage in harmonic subspace. But they have different high-frequency ripple currents. Although the current ripple is one of the essential performance indices, its study is missing for OVM techniques of ASPM. Hence, this work studies the current ripple performances of the above PWM sequences in the OVM region. One machine parameter, which is the ratio of inductances in energy-transferring and harmonic subspaces, impacts this current ripple. The article finds the sequence with the best ripple performance for a given reference vector in the OVM region and a machine parameter. After that, a PWM technique is proposed, which improves the high-frequency current ripple RMS by 623.67% and 416.53% compared to two existing OVM techniques for a feasible machine parameter. Theoretical analysis is validated through simulation and experiments on a hardware prototype at 4.2 kW power level.