Phase voltage harmonic imbalance in asymmetrical multiphase machines with single neutral point

Abstract

Multiphase (n-phase) machines are often designed with l sub-windings on stator, each having k phases, and the machine is typically operated with l isolated neutral points. However, such a machine can also operate with a single neutral point, which improves the fault-tolerant properties. When a machine is inverter supplied, low order harmonics may be present due to the low switching frequency and nonlinear inverter properties. Moreover, low order zero-sequence harmonics can be deliberately injected to increase dc bus voltage utilisation. This paper investigates a phenomenon that has not been reported so far in relation to asymmetrical multiphase machines with a single neutral point, namely that the presence of balanced low order harmonics in leg voltages produces unbalanced phase voltage harmonics and consequently unbalanced phase current harmonics. By analysing the neutral point (common mode) voltage harmonics, imbalance in the phase voltage harmonics is explained. Analytical expressions for neutral point voltage harmonics and phase voltage harmonics are provided for asymmetrical machine configurations with a single neutral point having arbitrary numbers of sub-windings and sub-winding phases. Theoretical considerations are verified using simulations and experiments with asymmetrical twelve- and nine-phase loads with a single neutral point, respectively.