Design of a 2/6-Pole Combined Pitch Wound Rotor Brushless Doubly-Fed Generator and its Comparative Study With an 8-Pole Doubly-Fed Induction Generator

The brushless doubly-fed generator (BDFG) is especially well-suited for constant-frequency, constant-voltage power generation in small hydropower plants utilizing variable speed hydraulic turbines. This paper presents a novel wound rotor design that combines long and short pitch coils to consider both the power winding (PW) and control winding (CW) pole pair pitches selection, thereby improving the chording factors and enhancing the overall winding factors through harmonic analysis. First, this study investigates the rotor speed regulation characteristics, analyzes the composition and magnitude of the radial electromagnetic forces density under harmonic-free condition, presents the stator PW and CW connection methods. Subsequently, the principle of employing combined pitch coils is elaborated to further increase conductor utilization and mitigate harmonic contents. A practical design example for 1/3 pole-pair is provided, including detailed implementation steps, along with a comparative analysis of coupling coefficients across different rotor configurations. Furthermore, finite element (FE) analysis of the 2/6-pole BDFG and 8-pole doubly-fed induction generator (DFIG) with identical frame sizes are conducted to evaluate the difference in operational characteristics of the generators under the same load and rotor speed range. Finally, the experimental results validate the superiority of the proposed 2/6-pole BDFG rotor design across diverse loads and speeds.