Dc bus utilisation in multiphase VSI supplied drives with a composite stator phase number

Multiphase variable-speed drives are typically supplied from a two-level voltage source inverter (VSI). The number of phases of the machine and hence the legs of the inverter can be either a prime or a composite number. Limits of the dc bus utilisation in the linear modulation region have been recently determined in a general manner for all the cases when the phase number is a prime number. The purpose of this paper is to complement that study by investigating the dc bus utilisation when the number of phases is a composite number. The emphasis is placed on stator windings based on multiple three-phase sub-windings, with a single and with multiple neutral points, in symmetrical and asymmetrical configurations. The results are generalised to other possible composite phase numbers. Theoretical findings are verified by experiments on a symmetrical six-phase system with a single neutral point.