In a door operator having a driving end and a weight carrying end, an electric motor mechanism, a first electromagnetic clutch mechanism, a combination pull-chain disc and brake, a second electromagnetic clutch mechanism are disposed in sequence between the driving end and the weight carrying end, the mechanisms being connected by rotating shafts and a hollow shaft, in which when the electric motor mechanism is electrically powered, the first electromagnetic clutch mechanism moves the hollow shaft to separate from a brake device, and the second electromagnetic clutch mechanism connects the driving end to the weight carrying end; and when the electric motor mechanism is not electrically powered, the first electromagnetic clutch mechanism connects the hollow shaft to the brake device, and the second electromagnetic clutch mechanism separates the driving end from the weight carrying end.