steel two adjacent ends of the two curved braking plates 21 are pivotally connected via pivoting means such as pivot shafts 20 a and 20 b to predetermined positions on an outer circumferential surface of the stator 20 and the other two adjacent ends of the two curved braking plates 21 are left suspended so that outer circumferential surfaces of the curved braking plates 21 face toward an inner circumferential surface of the rotor 11 by regulating an air clearance between the outer circumferential surfaces of the curved braking plates 21 and the inner circumferential surface of the rotor 11 a magnitude of a braking force of the braking assembly may be controlled the curved braking plates 21 are fixedly provided on respective outer circumferential surfaces with a plurality of permanent magnets 22 the permanent magnets 22 have curved outer surfaces facing toward the circumferential inner surface of the rotor 11 in the present invention there are permanent magnets 12 continuously arranged around an outer surface of the rotary central shaft 10 so that the north poles and the south poles alternate around the rotary central shaft 10 a first power generating coil 23 is provided on the stator 20 at a position close to the permanent magnets 12 in addition the stator 20 also has a pulling mechanism 30 provided thereon for regulating the air clearance between the curved braking plates 21 and the rotor 11 the pulling mechanism 30 includes two pulling cords 34 a 34 b which are fixedly connected at respective one end to free ends 20 c 20 d of the two curved.