Experimental results from a frame size 180 brushless doubly fed (induction) machine (BDFM) fitted with four rotor designs are presented. The machine is intended for use as a variable speed generator, or drive. A per phase equivalent circuit for the machine has been developed and a method of obtaining parameters for the circuit is described. Expressions for the torque as a function of speed have been derived and predictions of machine performance in both self-cascaded and synchronous (doubly fed) modes have been verified experimentally. The work illustrates the link between rotor equivalent circuit parameters and machine performance and a comparison between rotor designs has been made.