Brushless Doubly-Fed Generator with Reconfigured 3Y/3Y Pole-Changing Rotor Winding Using Unequal Turn Coils

The single winding pole-changing rotor structure can be applied in a brushless doubly-fed generator (BDFG) to couple two magnetic fields produced by stator. However, the traditional scheme employing equal turn coils may result in an asymmetric distribution of the magnetomotive force (MMF) phase angles among the parallel branches within each phase. Through comprehensive circuit analysis, it is further revealed that three branches exhibit distinct characteristics in terms of the slots number and spatial arrangement, leading to unbalanced current amplitudes. The structural asymmetries and current imbalance may significantly enrich magnetic field harmonic contents, amplify electromagnetic vibration and noise, generate parasitic circulating current, and increase copper losses. To address these limitations, an improved scheme with unequal turn coils is developed by reconfiguring the rotor wiring mode. The operational characteristics of BDFG with equal turn, unequal turn, and improved schemes are investigated through finite element (FE). Analysis results imply that the improved scheme significantly reduces the MMF angles discrepancies, ensures balanced current distribution across all branches, and simplifies the circuit connection. Finally, experimental results of a prototype BDFG validate the superior of the proposed scheme.