Optimal Third-Harmonic Current Injection for Asymmetrical Multiphase PMSMs

The paper proposes a modelling approach and an optimization strategy to exploit a third harmonic current injection for the torque enhancement in multiphase isotropic PMSMs with non-sinusoidal back-EMFs. The modelling approach is based on a proper vector space decomposition and on the associated rotational transformation, aimed to properly select a set of stator current space vectors to be controlled. It is presented for a generic (i.e. asymmetrical, with an arbitrary angular shift) winding configuration. The injection strategy is related to the choice of a constant synchronous current set, aimed at minimizing the average stator winding losses for a given reference torque by using the 1st and the 3rd spatial harmonics of the air-gap flux density. The optimal solution has been found analytically and has been developed in detail for a selected set of asymmetrical winding configurations. Both the numerical and experimental results are in good agreement with the theoretical analysis.