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 30 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.