Patent GB-621675-A (GB)
Title: Improvements in and relating to electromagnetic induction apparatus and methods of making the same
IPC: H01F41/02
Filed: 1945-05-18
Abstract: 621,675. Making magnetic cores. BRITISH THOMSON-HOUSTON CO., Ltd. May 18, 1945, No. 12537. Convention date, May 22, 1944. [Class 83 (ii)] [Also in Group XXXV] A laminated magnetic core for a transformer or the like is made by assembling on an annular former a main, part annular core section and a bridge or yoke portion from groups of pre-cut strips, removing the assembly from the former and annealing while encircled with a holding band to give a permanent set. A core is made from magnetic strips cut as shown in the three piles 26, 27, 28 in Fig. 2, in the apparatus shown in Fig. 3. In this, two spaced bands 30, 31 bent circular are mounted at their centres on a support 32 with their ends fixed to relatively movable blocks 33, 34. The space between the ends is closed by segments 35, 36. Three fingers 37, 38, 39 in the centre of the rings 30, 31 can be pulled outwards by rods 40, 41, 42. In building up the core, the longest strip 46 of pile 26 is laid round inside bands 30, 31 with a locating pin 51 of support 32 passing through a hole 52 near the centre of the strip. A strip 45 from pile 27 is next laid inside segments 35, 36 and located by a pin 50 passing through a hole 53 in it, with its ends overlapping those of strip 46. The next longest strip 48 is laid inside strip 46 and another short strip 47 inside strip 45, and the core is built to the right thickness using alternately strips from piles 26 and 27 with a few strips 49 from pile 28 placed in alignment with the shorter strips 45, 47, resulting in the formation shown in Fig. 4. Fingers 37, 38, 39 are then pulled outwards against the tension of bands 30, 31 until the short strips such as 45, 47 have room to lie without overlapping, between the ends of the strips such as 46, 48, and the bands 30, 31 are then pulled tight to close the butt joints between the adjacent ends of the long and short strips with the result shown in Fig. 6. Staggering the joint is effected by having holes 52, 53 displaced from the centres of the strips, alternate strips being reversed before placing in position. The strips may be used in groups of two or more with coinciding joints, the staggering being between joints of the groups. The filler strips 49 have holes 54 centrally placed and are long enough to form a magnetic path bridging all the butt joints. Before removing the core, a band 60 is pulled tightly round it between the bands 30, 31 and welded at the ends. It is removed after annealing. If the core is to be rectangular, it is so shaped by driving a wedge 67 between two suitably shaped blocks 65, 66 placed in its window as in Fig. 8. The cores are then annealed in an oven in which they are stacked on their sides under pressure of weights. To assemble the core round the preformed coil 20 of Fig. 10, the smaller strips forming the yoke 25 are pushed, together or a few at a time, from the U-shaped part by pressure applied parallel to the axis of the core window. The U-shaped part 24 is placed round the coil and the yoke restored without bending any strips. Several cores may be placed round each coil, and they may be formed of sheets of different widths to give a cruciform cross-section, Figs. 12 and 15 (not shown). The invention may be applied in a modification in which three coils placed side by side are linked with three cores in the manner described in Specification 553,801, [Group XXXV], Fig. 13 (not shown). Also described are rectangular and circular cores in which the butt joints are not staggered, Figs. 16-18 (not shown), and in which staggered butt joints are formed in the longer sides of a rectangular core which is thus formed of two U-shaped pieces, 140, 141, Fig. 26. In a further modification the strips forming the yoke are sufficiently long to overlap the longer strips of the U-shaped part at one end while making a butt joint at the other. The layers of the yoke 126 thus have spaces between them, which allows the making of an opening to receive fractional turns of a winding, by driving through it a wedge 136 which is retained in position until after annealing, Fig. 22. Specifications 7856/89, [Class 38], 356,066, [Group XXXV], 501,474; 545,229, [Group XXXV], 550,191; 552,778, 621,674, [both in Group XXXV], and 621,676 also are referred to. The Specification as open to inspection under Sect. 91 comprises also an index shearing machine of the kind described in Specification 621,677. This subject-matter does not appear in the Specification as accepted.