the screw unit 8 the iron core unit is formed by laminate units of different levels and different width each of the laminate units belonging to the same level is determined to have a preset size and shape according to the relative positions of the main column 1 the side column 2 the upper and the lower yokes 3 4 through calculation including computing a width of the joint portion a sectional area of the iron core a column spacing and a height of a window when cutting laminate units of the same level are cut into two types of laminate units based on a two level connection requirement then the laminate units of different levels are overlapped in sequence and in order in this embodiment one set of laminate units has two laminate units in which the first level connecting joint portion 5 and the second level connecting joint portion 6 are alternately positioned when placing the laminate units into position the critical characteristic according to the preferred embodiment of the present invention the joint portion which is increased is ensured to has a reserved width requirement in which the width of the joint portion is not less than a standard deviation and is not larger than a center distance of the joint portion between two adjacently positioned laminate.