Comparative Study of Radial-Flux Dual-Rotor Fractional-Slot Permanent Magnet Machines with Series and Parallel Magnetic Circuits

In this paper, electromagnetic performances of radial-flux dual-rotor (DR) fractional-slot concentrated-winding permanent magnet (PM) machines with series and parallel magnetic circuits are compared based on the optimised designs using finite-element analysis based genetic algorithm. It is found that under the same copper loss, the series magnetic circuit DRPM machine has higher output torque and lower iron loss due to yokeless structure. The influences of critical design parameters on both machines, e.g., PM thicknesses and pole arcs, inner and outer split ratios, and stator tooth width, are also investigated. It shows that both machines tend to sacrifice the inner rotor in order to enlarge the slot area, and the outer rotor makes a major contribution to the resultant torque.