This paper compares alternative rotor designs for the brushless doubly-fed machine (BDFM) using finite element analysis to estimate their operating ranges and characteristics. Two types of rotors are considered: induction (with cage, nested layer, double layer and isolated layer) and reluctance rotors, so a total of five rotors are investigated. Grid-connected converters are connected to the control windings to give a variable voltage and frequency control, with the power windings connected to the grid for constant voltage and frequency connection. The machine then can operate to in a similar manner to the doubly fed induction machine as used in many wind turbines. The results show that the induction rotors benefit from simpler converter topology and smoother power output. The isolated layer of induction rotor gives smallest torque ripple.