12 13 wound therearound respectively the hall motor 1 is further provided with a predetermined number of permanent magnets 14 15 a rotor assembly 16 a rotating shaft 18 and a motor case 21 the rotor assembly 16 is composed of a pair of cylindrical rotors 22 23 having vertical walls 22a 23a respectively which are brought into contact with each other in alignment on the outside thereof and welded together to form an elongated cylindrical rotor assembly 16 having a united vertical wall 24 the united vertical wall 24 has a boss 25 fixedly attached to the center thereof which is in turn secured to the rotating shaft 18 each of the rotors 22 23 has a pair of permanent magnets 14 15 which are semicircular in vertical section attached to the inner side of the cylindrical wall thereof opposite to each other namely as particularly shown in fig4 the rotor assembly 16 has two sets of permanent magnets 14 15 of respective polarities n s attached to the inner side thereof opposite to each other each extending over a predetermined angular region u03c6 about 120 u00b0 in such a manner that the rotor assembly 16 has a pair of predetermined angular regions 16a about 60 u00b0 left unmagnetized opposite to each other thus the rotor assembly 16 is structured to have the predetermined angular portions u03c6 producing a magnetic flux and the predetermined angular portions 16a producing no magnetic flux as shown in fig2 and 5 the rotor assembly 16 composed of two rotors 22 23 is opened axially in opposite directions and so arranged as.