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The effect of rotor excitation voltage on steady-state stability and output power of a doubly fed induction generator (DFIG) is investigated. Wind turbine mechanical torque-speed characteristics, as well as generator electromagnetic torque-speed characteristics, are considered in the derivation of the steady-state stability region for the magnitude and phase angle of the rotor excitation voltage. To evaluate the maximum output power of a DFIG, detailed expressions for stator power, rotor power, stator loss, rotor loss, and electrical power are derived as functions of the generator speed and the magnitude and phase angle of the rotor excitation voltage. The effect of stator resistance on the magnitude and phase angle of the resultant optimal rotor excitation voltage, which gives maximum output power and minimum loss, is examined. Experimental results are given to demonstrate the effectiveness of the theoretical analysis.