Design Characteristics of Unequal-Turn Sinusoidal Wound Rotor Winding in Brushless Doubly-Fed Induction Generator

Brushless doubly-fed machine (BDFM) is one of the crucial target applications in variable frequency speed regulation systems such as wind turbines due to reducing required maintenance, reliable operation, etc. BDFMs have two stator windings, each with different pole counts where one is directly connected to the grid while the other is used to control synchronous mode operation through a converter. However, the rotor windings need a particular configuration of coils to modulate both magnetomotive forces (MMFs) due to the stator windings and indirectly couple them. Recently, it has been shown that the multi-layer wound rotor winding enables a more efficient BDFM design than nested-loop rotor structures, principally because of simple manufacturing and low harmonic contents due to the winding configuration. This paper presents a model for a wound-rotor brushless doubly-fed induction generator (BDFIG) using a multiple transformation vector space. A new closed-form electromagnetic (EM) torque equation is developed regarding different terms on the power winding, control winding, the interaction of these two, and their modulation. Furthermore, finite element analysis (FEA) simulations are provided for different rotor winding cases to evaluate the analytical equation and the effect of unequal-turn winding on critical parameters such as EM torque production, the unbalanced induced voltage, and the copper utilization. The results show that the sinusoidally distributed winding is necessary for synchronous operation in multilayer rotor windings with lower unbalanced voltage and higher torque density.