PURPOSE: To reduce the exciting current of a three-phase U-shaped butted type core by a method wherein the constitution of the core is simplified by butted-concentration of both ends of three pieces of the double laminated cores, which are deformed into a U shape and are performed a thermal treatment, and a heat-resisting insulating film, on both faces of which a magnetic fluid is bonded, is inserted in the butted parts.

CONSTITUTION: Two pieces of big and small ring-shaped cores, on each of which an oriented silicon steel strip is wound around helically and concentraically, are presse from three directions on the outside and U-shaped core legs 1, 1' and 1" are made by performing a thermal treatment on the cores. At this time, gaps C and D are generated between both ends of the interior of an outside core A and both ends of an inside core B. The core legs 1 and 1" are placed in parallel to each othr, the core leg 1' is placed between the core legs 1 and 1" in the reverse direction to that of the core legs 1 and 1", a bolt is penetratingly inserted in the gaps C and D, and the outside and inside cores A and B are integrally formed in one body. According to this constitution, the magnetic characteristics of the three-phase U-shaped butted type core can be improved by setting the butted areas at the prescribed value, because the magnetic path length of each core leg becomes equal and the ratio of the core width of each phase to the winding thickness becomes 1:1.6 (rectangular section), and an assembling of the core is also simplified. Moreover, the butted parts are lessened, no-load current is reduced and the exciting current of the core is also reduced as a heat-resisting insulating film, on both faces of which a magnetic fluid is bonded, is inserted in the butted parts.