Fixed speed wind turbines have the advantage of being robust and reliable. They allow a direct connection to the electric. The purpose of the article is to study the aerodynamic behaviour and determine the performance of a fixed speed wind turbine. The work presents an analysis method based on the theory of blade element moments (BEM). The variation of aerodynamic parameters is studied for a wide range of wind speeds. A case study is conducted for the design of a wind turbine adapted to the Adrar site which is located in the Algerian Sahara. The results obtained showed that the wind turbine has maximum efficiency just at the design speed. For speeds higher than the design speed, the efficiency is reduced by the stall effect with decreases in torque due to the fall of the lift force. At wind speeds lower than the design value, the thrust effect increases, which puts the rotor under high mechanical stress and blade rotation decreases with low efficiency.