Investigation of Cascaded and Modulated Rotors for Dual-Stator Brushless Doubly-Fed Machines

This article investigates the magnetic field conversion capacities based on the cascaded and modulated rotors including the staggered dual-cage rotor, the wound rotor, the nest-loop rotor, and the salient-pole rotor for the dual-stator brushless doubly-fed machines. The magnetic field conversion factor of the cascaded rotor is newly derived and the magnetic field conversion behaviors of both cascaded and modulated rotors are studied. The cascade rotor offers the advantage of minimizing undesirable magnetic field harmonics due to its specific winding connections, while the modulation rotor offers a compact structure and improved space utilization. The four machines with the same size are analyzed by the finite element method. The results demonstrate that the staggered dual-cage rotor, with low rotor resistance, resolves the problem of high rotor copper loss in the wound rotor, thus improving efficiency. The nest-loop rotor, utilizing the advantages of short-circuit coil modulation and reluctance effect, achieves a higher capacity for magnetic energy conversion compared to the salient-pole rotor. The experimental prototype validates the accuracy of the theoretical analysis.