Comparative Analysis of Single-Star and Dual-Star Permanent-Magnet Synchronous Machines

The performance of a fractional-slot concentrated-winding PMSM with two different winding diagrams, namely, single and dual star, is investigated. Firstly, the air-gap flux density for these two cases in terms of rotating fields is studied, which provides insight into the essential difference between these two windings. Then, a computationally efficient model taking into account saturation and spatial effects using FE-obtained look-up tables is developed. Using the model, the control and the offline design of the controllers are carried out based on the two-axis reference frame model for both machines and the vector space decomposition for dual-star one. The comparison is made in terms of terminal current harmonics, torque ripple, power density, and closed-loop control dynamic performance. It is shown that the dual-star machine has higher power density and less torque ripple, but higher 5th and 7th current harmonics. Moreover, the effect of one imperfection which may come with the dual-star winding diagram, namely imperfect electrical phase shift between the respective currents or voltages of two stars, on the performance of the machine is studied. It is concluded that the imperfection can deteriorate the closed-loop dynamic performance.