Active radial magnetic bearing combined with a back-up bearing

A radial active magnetic bearing with back-up bearing between a rotary shaft and a casing, comprising a laminated ferromagnetic stator core fast with the casing, electromagnetic windings on the stator core, a laminated ferromagnetic rotor armature fast with the rotary shaft and facing said stator core to define an air gap therewith, at least one radial sensing means for detecting a radial position of the rotary shaft and control means for receiving signals produced by said sensing means for controlling via said radial active magnetic bearing, radial play between machine components fast with the casing and machine components fast with the rotary shaft, wherein said bearing further comprises at least one back-up bearing, constituted by a first ring fast with said laminated ferromagnetic rotor armature, and a second ring fast with said laminated ferromagnetic stator core, offset with respect to the first ring and defining with said first ring a free space whose length in the radial direction is about one half the length of said air gap between the stator core and the rotor armature, wherein said first and second rings are made of a material selected from the group consisting of composite materials and fluorinated thermoplastic resins.