Comparison of PMSM and Inverter Efficiency for Five-Phase Powertrains in Dependence of Third Harmonic Current Utilization

In dependence of the requirements a phase number higher than three can be feasible. Here, high maximum torque at low speeds and high efficiency over the full operating range is targeted. Five-phase drives offer the possibility to generate torque by utilizing third harmonic current injection. Hence, a reduction of fundamental flux linkage is possible resulting in reduced field-weakening current. The impact of different ratios of fundamental and third harmonic flux linkage on machine and inverter efficiency is investigated based on two machine designs. The design process for stator and rotor, utilizing a third harmonic Halbach array (THHA) to achieve different ratios, is described. To ensure best use of the third harmonic injection a five-phase specific reference current calculation with maximization of output torque per phase current RMS (MTPA13) is introduced. Furthermore, a new modulation method for control of first and third harmonic current with arbitrary relative phase angle is described. The presented methods are combined with a transient phase current calculation and detailed loss calculation including saturation and harmonics due to the machine’s geometry and voltage modulation.