Electric machine cooling system comprising an electric machine having an inner rotor and an outer stator, the inner rotor being rotatable about an axis of rotation and having or forming an inner hollow shaft; and a cooling cartridge arranged inside and coaxial to the inner hollow shaft of the inner rotor, the cooling cartridge being interchangeably attached to the inner rotor so as to be rotatable with the inner rotor about the axis of rotation; wherein the cooling cartridge includes an inner coolant tube having an internal coolant duct configured to lead coolant from an inlet opening of the inner coolant tube to a closed dead-end of the cooling cartridge, and an outer coolant sleeve, the outer coolant sleeve being arranged between the inner coolant tube and the inner rotor and comprising a plurality of coolant channels which are fluidically connected with the internal coolant duct via at least one connection channel (28A, 28B) in a region of the closed dead-end and which are configured to lead the coolant from the at least one connection channel (28A, 28B) to at least one outlet opening of the outer coolant sleeve; the electric machine cooling system further comprising a closing cap shaft being aligned with and arranged adjacent to the cooling cartridge so as to secure the cooling cartridge inside the inner hollow shaft of the inner rotor; wherein the closing cap shaft provides an inlet passage fluidically connected with the inlet opening of the inner coolant tube of the cooling cartridge for leading the coolant from a main coolant input to the internal coolant duct; and wherein the closing cap shaft provides at least one outlet passage fluidically connected with the at least one outlet opening of the outer coolant sleeve of the cooling cartridge for leading the coolant from the plurality of coolant channels to a main coolant output; characterized in that the cooling cartridge is a polymer cartridge and the closing cap shaft comprises at least one lubrication path that fluidically connects the at least one outlet passage with a first bearing arrangement for lubricating the first bearing arrangement. an electric machine having an inner rotor and an outer stator, the inner rotor being rotatable about an axis of rotation and having or forming an inner hollow shaft; and a cooling cartridge arranged inside and coaxial to the inner hollow shaft of the inner rotor, the cooling cartridge being interchangeably attached to the inner rotor so as to be rotatable with the inner rotor about the axis of rotation; wherein the cooling cartridge includes an inner coolant tube having an internal coolant duct configured to lead coolant from an inlet opening of the inner coolant tube to a closed dead-end of the cooling cartridge, and an outer coolant sleeve, the outer coolant sleeve being arranged between the inner coolant tube and the inner rotor and comprising a plurality of coolant channels which are fluidically connected with the internal coolant duct via at least one connection channel (28A, 28B) in a region of the closed dead-end and which are configured to lead the coolant from the at least one connection channel (28A, 28B) to at least one outlet opening of the outer coolant sleeve; an inner coolant tube having an internal coolant duct configured to lead coolant from an inlet opening of the inner coolant tube to a closed dead-end of the cooling cartridge, and an outer coolant sleeve, the outer coolant sleeve being arranged between the inner coolant tube and the inner rotor and comprising a plurality of coolant channels which are fluidically connected with the internal coolant duct via at least one connection channel (28A, 28B) in a region of the closed dead-end and which are configured to lead the coolant from the at least one connection channel (28A, 28B) to at least one outlet opening of the outer coolant sleeve; the electric machine cooling system further comprising a closing cap shaft being aligned with and arranged adjacent to the cooling cartridge so as to secure the cooling cartridge inside the inner hollow shaft of the inner rotor; wherein the closing cap shaft provides an inlet passage fluidically connected with the inlet opening of the inner coolant tube of the cooling cartridge for leading the coolant from a main coolant input to the internal coolant duct; and wherein the closing cap shaft provides at least one outlet passage fluidically connected with the at least one outlet opening of the outer coolant sleeve of the cooling cartridge for leading the coolant from the plurality of coolant channels to a main coolant output; characterized in that the cooling cartridge is a polymer cartridge and the closing cap shaft comprises at least one lubrication path that fluidically connects the at least one outlet passage with a first bearing arrangement for lubricating the first bearing arrangement.