A system for ventilating a rail traction motor by purified air, said system comprising a centrifugal fan surrounded by a circular housing concentric to the rotation axis of the fan and partially enclosing said fan, an opening within a top of the circular housing serving as an inlet for the ambient air, a second opening in a bottom of the circular housing serving as an outlet for the purified air for flow towards the motor, a plane sheet disposed transversely with respect to the rotation axis of the fan intermediate of said top and bottom openings dividing the inside volume of the circular housing into an upstream chamber connected to the air inlet and including the fan, and into a downstream chamber connected to the outlet of the air to the motor, the plane sheet being pierced by a central hole communicating the upstream and downstream chambers and having openings situated at the external periphery of the plane sheet except for a sector, the housing having a side exhaust outlet to communicate the exterior of the housing and the part of the upstream chamber corresponding to the sector, said exhaust outlet serving for the exhaust of the impure air, a separator element being disposed in the sector facing the exhaust outlet but set back radially of said exhaust outlet towards the fan and forming a passage for the exhaust of impure air between the separator element and the housing leading to said housing side exhaust outlet.