An asymmetrical six-phase machine (ASPM) is one of the most common multi-phase machines for applications in high-power and safety-critical drives, like pumps, compressors, railway traction, electric vehicles, ships, and aircraft. The multi-phase machine allows for splitting of the total power across different phases, which can be realized by reducing the phase voltage with the same current, therefore the low-voltage devices with superior characteristics can be used. Except for the above, the machine has higher fault tolerance due to having more phases. This talk discusses optimal pulse-width modulation (PWM) techniques of ASPM so that the low-frequency and switching-frequency harmonic currents and their associated losses are minimized.

 

An ASPM with two isolated neutral points is analyzed in two orthogonal subspaces: energy-transferring and harmonic subspaces. Linear modulation techniques (LMTs) synthesize the desired average voltage vectors in the energy-transferring subspace without exciting the harmonic subspace. Through an innovative approach, our work finds a way to account for all possible LMTs and determine the optimal LMTs with minimum current ripple. Overmodulation (OVM) techniques of ASPM extend the operating region of ASPM in the energy-transferring plane and thus attain higher voltage gain after injecting non-zero average voltage in the harmonic subspace. However due to low impedance in the harmonic subspace, the applied voltage causes large circulating current and copper loss, but this current doesn’t participate in torque production or energy transfer. Hence, the RMS of the applied average voltage is minimized in this work to minimize the RMS of the circulating current. It is further shown in our work that there is more than one PWM sequence that can achieve the optimal injection of non-zero average voltage in harmonic subspace. But they have different high-frequency ripple currents. Henceforth, we found the overmodulation technique with minimum high-frequency RMS current, which attains close to five times reduction in high-frequency RMS currents compared to the state-of-the-art overmodulation technique.