Analysis of a Novel Stator Winding Structure Minimizing Harmonic Current and Torque Ripple for Dual Six-Step Converter-Fed High Power AC Machines

Many attempts have been made to expand the application of ac drives to high-power levels. One way to use ac drives at high-power levels is to use multiple power converters together to supply power to a large machine. For a multiple converter combination scheme, the six-step converter is competitive due to its robust structure and simplicity of control. In our previous study, an ac drive with dual stator windings fed by dual six-step converters was proposed. Two sets of electrically isolated windings are placed in the stator slots, and one set is shifted from the other by 30° in space. Two converters are used to power the dual windings, and the phase voltages from the two converters are also shifted 30° from each other in time. The 5th and 7th harmonics of the airgap flux and rotor currents are reduced dramatically, and the output torque from the drive is superior to that of the drive which is singly fed from a six-step converter. However, for each individual six-step converter, the 5th and 7th harmonic currents are found to be substantial and the functioning of the power converter evidently is deteriorated. In this paper, we propose a novel stator winding structure and control strategy to solve the converter problem. Principles associated with the new stator winding are discussed and a practical implementation presented.