Energy saving device capable of rotating by half cycle and reversely returning by aid of gravity

Disclosed is an energy saving device capable of rotating by a half cycle and reversely returning by the aid of gravity is characterized in that a transmission shaft of a motor rotor is controlled to rotate in the front half cycle of each periodic operation at first, a first rotary shaft is driven by a first variable-speed gear mechanism to rotate in a single direction, and the motor rotor is synchronously driven to ascend from the lowest position of a circumference to the highest position of the circumference, and the circumference is defined by using the first rotary shaft as a circle center and a support arm as the radius, so that rotation by a half cycle is completed; and the rear half cycle of the periodic operation is carried out, at the moment, the transmission shaft of the motor rotor is controlled to stop rotating, and the motor rotor reversely returns from the highest position to the lowest position in the same half-cycle path by the aid of the gravity of the motor rotor. Accordingly, the periodic ha