Influence of Pole-pair Combinations on the Characteristics of the Brushless Doubly Fed Induction Generator

The brushless doubly fed induction generator (BDFIG) is an alternative to the doubly fed induction generator (DFIG), widely used in wind turbines which avoids the need for brush gear and slip rings. The choice of pole numbers for the two stator windings present in the BDFIG sets the operating speed range but also affects the torque capability, magnetizing currents and back iron depth. Analytical expressions are developed for these effects and a comparison is made between the BDFIG and the conventional DFIG. The torque capabilities and magnetizing currents are not strongly dependent on the choice of pole numbers but the back iron depth is significantly affected. The torque density of the BDFIG is somewhat reduced compared to a similarly sized DFIG but magnetizing currents per unit torque are the same. However, the required back iron depths are greater. The work also shows that multi-megawatt machines are expected to work within the desired range of power factors at acceptable efficiencies.