The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, widely used in wind turbines without use of brush gear and slip ring. Rotor design is important for achieving an optimal multi-MW BDFM. To date, nested-loop rotors have been exclusively used in various BDFMs, although they may not be ideal for larger machines. This paper studies, both analytically and experimentally, different rotors for the D160 and D400 frame size BDFMs. An optimization on the rotor equivalent circuit parameters is carried out and verified with different parameter extraction methods. A comparison of the performance of the suggested rotors is also presented.