A system including an x-ray scanner gantry having: a housing； a gantry gear； a rotor； a generator mounted on the rotor； and first and second generator gears connected to or engaged with one or more axles of the generator. The second generator gear is engaged with the gantry gear. A motor rotates the intermediate gear via a motor gear. A second actuator actuates the motor gear to engage the motor with the rotor. A control module operates in first and second modes and: while in the first mode, engages the intermediate gear to the first generator gear via the first actuator to rotate, via the motor gear, the intermediate gear and as a result the first generator gear； and while in the second mode, engage the motor to the rotor via the second actuator to rotate, via the motor gear, the rotor and as a result the second generator gear.
