Open Access

Carsten Fräger, Francesco Quattrone

• Brushless doubly-fed induction generators (BDFIG) are currently being investigated for renewable energy generation, e.g. with wind turbines. They are an alternative to doubly-fed induction generators with slip rings and brushes. BDFIG work with two rotating fields with different numbers of pole pairs. There are two separate windings in the stator for the two numbers of pole pairs or one combined winding for the simultaneous generation of the two rotating fields with separate terminals. The rotor has a closed winding that is linked to both rotating fields and whose current generates both rotating fields simultaneously. The calculation of BDFIG is quite complicated because the current distribution in the rotor winding depends on the induced voltages from the two stator fields, the mutual inductances between the different rotor branches, and the connection of the rotor branches. Furthermore, the slotting effects and the stray field have an influence on the behavior of the BDFIG. In this paper, the theory of rotation fields is used to analyse the rotor winding and the inductive connection to the stator windings. The equations for calculating the operating behavior of the BDFIG are presented in this paper. The calculated operating characteristics are compared to measurements on an experimental machine. The calculation results show good match with the measurement results. The result is a verified calculation method, which is suitable as a basis for further investigations, e.g. for the design of generators with a higher output power.