A drive mechanism for a door operator comprises a drive member and a driven member. The drive member is adapted to be operably connected between a motor assembly for rotating the drive member and a door closer assembly rotating with the driven member. The drive member and the driven member both include a protrusion. The driven member protrusion moves in the free space defined by driving surfaces of the drive member protrusion. Rotation of the drive member from a first angular orientation to a second angular orientation causes rotation of the driven member for powered opening of the door. The driven member protrusion moves in the free space without engaging the protrusion surfaces when the door is opened manually and allowed to close.