This paper introduces a variable structure robust controller for wind energy conversion systems based on brushless doubly fed reluctance machine. Two simultaneous control goals are tackled: maximization of the wind energy conversion efficiency and minimization of the machine copper losses. The proposed multiple-input multiple-output (MIMO) robust controller is systematically synthesized following a general design method for MIMO nonlinear affine systems. The design is approached through a theoretical framework based on the combination of variable structure techniques and the Lyapunov's theory. Robustness to bounded disturbances and chattering minimization is attained. © 2007 IEEE.