A rotor air cooling system, comprising: an outer housing; an inner housing configured to be inserted into the outer housing, the inner housing comprising an open backend and further a cover arranged to enclose the inner housing and the outer housing upon the inner housing being inserted into the outer housing; a stator attached to an interior side of the inner housing; a main rotor body comprised in the inner housing, the main rotor body being arranged around a rotor shaft arranged to extend through the inner housing and the outer housing, wherein cooling air axially enters the main rotor body in an inlet end via the open backend of the inner housing, passes through cavities of the main rotor body, and exits the main rotor body in an outlet end; a water jacket arranged between the inner housing and the outer housing, the water jacket being formed by an exterior side of the inner housing being arranged with a radially protruding first flange and a radially protruding second flange around the exterior side, the first and second flange (16, 17) being arranged to abut an interior side of the outer housing, thereby forming a casing surrounding the inner housing where water can be accommodated; a heat exchanger for dissipating heat from the cooling air exiting the main rotor body, the heat exchanger being integrated with the outer housing; wherein the air exiting the main rotor body is radially guided, via an air outlet of the inner housing being aligned with an air outlet of the outer housing, through the heat exchanger before being arranged to be radially guided via an air inlet of the outer housing being in fluid communication with the open backend of the inner housing to the air inlet of the main rotor body for recirculation of the cooled air through the main rotor body, the rotor air cooling system further comprising: an impeller arranged around the rotor shaft at an outlet end of the main rotor body to facilitate radial guiding of the cooling air exiting the main rotor body at the outlet end. an outer housing; an inner housing configured to be inserted into the outer housing, the inner housing comprising an open backend and further a cover arranged to enclose the inner housing and the outer housing upon the inner housing being inserted into the outer housing; a stator attached to an interior side of the inner housing; a main rotor body comprised in the inner housing, the main rotor body being arranged around a rotor shaft arranged to extend through the inner housing and the outer housing, wherein cooling air axially enters the main rotor body in an inlet end via the open backend of the inner housing, passes through cavities of the main rotor body, and exits the main rotor body in an outlet end; a water jacket arranged between the inner housing and the outer housing, the water jacket being formed by an exterior side of the inner housing being arranged with a radially protruding first flange and a radially protruding second flange around the exterior side, the first and second flange (16, 17) being arranged to abut an interior side of the outer housing, thereby forming a casing surrounding the inner housing where water can be accommodated; a heat exchanger for dissipating heat from the cooling air exiting the main rotor body, the heat exchanger being integrated with the outer housing; wherein the air exiting the main rotor body is radially guided, via an air outlet of the inner housing being aligned with an air outlet of the outer housing, through the heat exchanger before being arranged to be radially guided via an air inlet of the outer housing being in fluid communication with the open backend of the inner housing to the air inlet of the main rotor body for recirculation of the cooled air through the main rotor body, the rotor air cooling system further comprising: an impeller arranged around the rotor shaft at an outlet end of the main rotor body to facilitate radial guiding of the cooling air exiting the main rotor body at the outlet end.