Abstract: This device, comprising a main motor (1), a transmission mechanism with an epicylic gear train comprising an input member (3) driven by the main motor, an output member (5), and a reaction member (7) locked in rotation when the main motor is operating,is characterised in that it furthermore comprises an auxiliary motor (10), a speed-reduction mechanism (11) with at least one stage, comprising an input member (13) driven by the auxiliary motor, and an output member (19) adapted in order to drive in rotation the reaction member (7) of the epicylic gear train, means (21, 22) being provided in order to release the reaction member and in order to lock in rotation the input member of the epicyclic gear train when the main motor is at a standstill and when the auxiliary motor must operate.