Independent vector control of asymmetrical nine-phase machines by means of series connection

The interest in multi-phase machines for high performance applications has been growing in recent years due to their potential advantages over three-phase machines. One of the possibilities of taking advantage of the additional degrees of freedom that exist in multi-phase machines is the series connection of several multi-phase machines, with the supply coming from a single multi-phase inverter. Among the different choices, the quasi nine-phase machine with three sets of three-phase windings spatially displaced by 20 degrees, is particularly attractive for high-power applications. This paper discusses the possibility of connecting a number of such asymmetrical nine-phase machines and explains how independent control can be achieved although a single nine-phase inverter is used as the supply. Mathematical derivations and simulation proof of independent vector control, achievable with this system, are provided for the series connection of three nine-phase machines and a three-phase machine. The existence of the independent control of the machines is fully verified both in torque and speed mode using indirect rotor-flux oriented control principles