A modular motor and magnetic bearing assembly comprising a positioning casing having a plane reference surface, an outer cylindrical reference surface, a central portion provided on an outer face with cooling liquid flow channels, and intermediate portions provided with openings for gaseous fluid entry and exit; a rotor presenting an inner cylindrical reference surface and a plane reference surface; an electric motor; radial magnetic bearings; an axial abutment; and auxiliary mechanical bearings. The modular assembly can then be incorporated in a main casing simply by sliding and it can be connected directly to a functional unit without reworking adjustments of the magnetic bearings.