Torque density improvement in a six-phase induction motor with third harmonic current injection

The use of six-phase induction motor for industrial drives presents several advantages over the conventional three-phase drive such as improved reliability, magnetic flux harmonic reduction, torque pulsations minimization and reduction on the power ratings for the static converter. For these reasons, six-phase induction motors are beginning to be a widely acceptable alternative in high power applications. A typical construction of such drives includes an induction machine with a dual three-phase connection, where two three-phase groups are spatially shifted thirty electrical degrees, a six-leg inverter and a control circuit. By controlling the machine's phase currents, harmonic elimination and torque ripple reduction techniques could be implemented. This paper describes a technique of injecting third harmonic zero sequence current components in the phase currents, which greatly improves the machine torque density. Analytical, finite element and experimental results are presented to show the system operation and to demonstrate the improvement on the torque density.