Optimal Selective Harmonic Elimination PWM for Dual Three-phase PMSM Under Low Switching Frequency

Selective harmonic elimination pulse-width modulation (SHEPWM) strategy selects appropriate switch angles to shape the output voltage waveforms to eliminate the unwanted harmonics from the Fourier series of voltage waveforms, thereby reducing phase current harmonics. The conventional SHEPWM eliminates 6k ± 1(k=1,2,3$\ldots$) harmonics (i.e. $5^{\mathrm{t}\mathrm{h}},\ 7^{\mathrm{t}\mathrm{h}}$, 1$1^{\mathrm{t}\mathrm{h}}$, 1$3^{\mathrm{t}\mathrm{h}},\ldots$) to reduce the current harmonics in dual three-phase machines. However, in dual three-phase machines, the 12k ± 5(k=0,1,2,3$\ldots$) order harmonics mapped to the Z1Z2 subspace, especially $5^{\mathrm{t}\mathrm{h}},\ 7^{\mathrm{t}\mathrm{h}}$, 1$7^{\mathrm{t}\mathrm{h}}$, and 1$9^{\mathrm{t}\mathrm{h}}$ harmonics, can generate current harmonics more significantly than the 12k ± 1(k=1,2,3$\ldots$) order harmonics of the $\alpha \beta$ subspace, since the impedance in the $\alpha \beta$ subspace is much higher than the Z1Z2 subspace. Hence, the 12k ± 1(k=1,2,3, $\ldots$) order harmonics should have higher weighted factors on current total harmonic distortion (THD) than the 12k ± 5(k=0,1,2,3, $\ldots$) order harmonics. In this paper, an optimal SHEPWM considering different weighting factors of different harmonics on current THD is proposed. The proposed SHEPWM can significantly reduce current distortion significantly compared to the conventional SHEPWM under low switching frequency condition, as verified by simulations.