VARIABLE ANGLE SCRAPLESS TRANSFORMER CORE CENTRAL LEG

[0001] FIELD OF THE INVENTION

[0002] The invention relates to transformers and more particularly, to transformers having a stacked core and methods of making the same with reduced waste and improved core performance.

[0003] BACKGROUND OF THE INVENTION

[0004] A stacked transformer core is comprised of thin metallic laminate plates, such as grain oriented silicon steel. This type of material is used because the grain of the steel may be groomed in certain directions to reduce the magnetic field loss. The plates are stacked on top of each other to form a plurality of layers. A stacked core is typically rectangular in shape and can have a rectangular or cruciform cross-section. A front view of a conventional three leg stacked core 10 for a three phase transformer is shown in FIG. 1 . The core 10 comprises an upper yoke 12, a lower yoke 14, an inner leg 16, and first and second outer legs 18, 20. A pair of windows 22 are disposed between the inner leg 16 and the first and second outer legs 18, 20, respectively. Wire coils (not shown) are mounted to the inner leg 16 and the first and second outer legs 18, 20, respectively.

[0005] The upper yoke 12 comprises a stack of plates 24, the lower yoke 14 comprises a stack of steel plates 26, the first outer leg 18 comprises a stack of plates 28 and the second outer leg 20 comprises a stack of plates 30. The plates 24, 26 of the upper and lower yokes 12, 14 have opposing ends that form joints with opposing ends of the plates 28, 30 of the first and second outer legs 18, 20, respectively. A V-shaped upper notch 32 is formed in each of the plates 24 of the upper yoke 12 and a V-shaped lower notch 36 is formed in each of the plates 26 of the lower yoke 14. The upper notches 32 form an upper groove 38 in the upper yoke 12, while the lower notches 36 form a lower groove 40 in the lower yoke 14. The size of the individual plates 24-30 vary depending on the stacking technique used to assemble the core 10. The inner leg 16 comprises a stack of plates 42. Each of the plates 42 has an upper tined end 42a formed by a pair of miter cuts and a lower tined end 42b formed by a pair of miter cuts. The upper and lower tined ends 42a, b of the plates 42 provide the inner leg 1 6 with upper and lower tined ends 16a, b, which are adapted for receipt in the upper and lower grooves 38, 40 of the upper and lower yokes 12, 14, respectively.

The manufacture of the conventional core 10 described above results in a significant amount of steel being cut away and discarded. For example, during the manufacture of the inner leg 16, four pieces of steel must be cut away from each plate 42 to provide the plate 42 with tined ends.

There is a need to provide a stacked transformer core and method of making the same that reduces the amount of steel that is scrapped and, thus, wasted.

SUMMARY OF THE INVENTION

An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a transformer core having a plurality of first and second outer leg plates; a plurality of inner leg plates, each having opposing, V-shaped ends having a certain internal angle; and a plurality of upper yoke plates and a plurality of lower yoke plates. Each of the upper and lower yoke plates has an outer side and an inner side with a V-shaped notch defined to extend from the inner side such that a vertex thereof is disposed substantially at the outer side. Each V-shaped notch has an internal angle substantially equal to the certain internal angle. The inner leg plates, the upper and lower yoke plates, and the first and second outer leg plates are joined to form first and second outer legs from the first and second outer leg plates, respectively; an upper yoke and a lower yoke from the upper and lower yoke plates, respectively; and a central leg from the inner leg plates, with each V-shaped end of the inner leg plates being joined with an associated V-shaped notch of each of the upper and lower yoke plates. [0011] In accordance with another aspect of the invention, a method provides upper and lower yokes of a transformer core. The method provides a rectangular strip of steel. The strip is cut to define yoke plates such that each yoke plate has an end cut at about a 64 degree angle and an opposing end cut at an angle of about 45 degrees. Pairs of yoke plates are arranged in abutting relation to define a V-shaped notch in the pair of yoke plates, with the V-shaped notch having an internal angle of about 52 degrees. A plurality of abutted pairs of yoke plates are stacked to define an upper yoke, and a plurality of abutted pairs of yoke plates are stacked to define a lower yoke.

[0012] Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings wherein like numbers indicate like parts, in which:

[0015] FIG. 1 shows a front elevational view of a prior art transformer core.

[0016] FIG. 2 shows a front elevational view of a transformer core constructed in accordance with an embodiment of the invention.

[0017] FIG. 3 shows an enlarged view of a portion of central leg of the core of FIG. 2, shown spaced above a lower yoke. [0018] FIG. 4 shows a strip of steel and the cutting sequence for the top and bottom yokes of the core of FIG. 2.

[0019] DETAILED DESCRIPTION OF THE EXEMPLARY EMBODI MENTS

[0020] With reference to FIG. 2, a core 1 00 is shown for a transformer, such as a distribution transformer. The transformer may be an oil-filled transformer, i.e., cooled by oil, or a dry-type transformer, i.e., cooled by air. The core 1 00 has a rectangular shape and generally comprises an upper yoke 1 02, a lower yoke 104, first and second outer legs 1 06, 1 08 and a central leg 1 1 0 disposed generally centrally between the outer legs. Upper ends of the first and second outer legs 1 06, 1 08 are connected to first and second ends of the upper yoke 102, respectively, while lower ends of the first and second outer legs 1 06, 1 08 are connected to first and second ends of the lower yoke 1 04. The central leg 1 1 0 is disposed about midway between the first and second outer legs 1 06, 1 08 and has an upper end connected to the upper yoke 1 02 and a lower end connected to the lower yoke 1 04. With this construction, two windows 1 1 2 are formed between the central leg 1 1 0 and the first and second outer legs 1 06, 108. Three coil windings 1 1 1 are mounted to the core 1 00 to from a transformer.

[0021] With reference to FIG. 4, a rectangular strip of grain-oriented silicon steel 1 09 is shown together with the cutting sequence of the strip 1 09 to define stacks (#1 , #2) of upper and lower yoke plates. As shown, an end of stack #1 is cut at an angle A of 64 degrees while the opposing end thereof is cut at an angle B of 45 degrees. The next cut in the strip 1 09 is at 64 degrees (defining stack #2) and the next cut is at 45 degrees (defining more for stack #1 ). This cutting sequence is continued until the necessary stacks of upper and lower yoke plates are provided. Since the strip 1 09 is cut at multi-angles to provide the upper and lower yokes as explained below, no scrap is generated.

[0022] Returning to FIG. 2, the upper yoke 1 02 is defined by yoke plates 1 02a, 1 02b (of

FIG. 4) arranged so that the 64 degree cut ends join n abutting relation at vertex 103 (requiring yoke plate 1 02a to be flipped over). The lower yoke 1 04 is arranged in a similar manner using yoke plates 104a and 104b. The upper yoke 102 has an inner side 105a and an outer side 107a and the lower yoke 104 has an inner side 105b and an outer side 107b. The upper yoke 102 comprises a stack of the yoke plates 114, while the lower yoke 104 comprises a stack of the yoke plates 116. Both the plates 114 and the plates 116 are arranged in groups. In one exemplary embodiment of the present invention, the groups are groups of seven, but for ease of illustration, four groups are shown and described herein. Of course, groups of different numbers may be used. Each of the plates 114, 116 is composed of grain-oriented silicon steel and has a thickness in a range of from about 7 mils to about 14 mils, with the particular thickness being selected based on the application of the transformer. The plates 114, 116 each have a unitary construction and are trapezoidal in shape. The plates 114 have the same width to provide the upper yoke 102 with a rectangular cross-section and the plates 116 have the same width to provide the lower yoke 104 with a rectangular cross-section. However, the lengths of the plates 114 are not all the same and the lengths of the plates 116 are not all the same. More specifically, the lengths within each group of plates 114 are different and the lengths within each group of plates 116 are different. The pattern of different lengths is the same for each group of plates 114 and the pattern of different lengths is the same for each group of plates 116. The difference in lengths within each group permits the formation of multi-step lap joints with plates 118, 120 of the first and second outer legs 106, 108. With reference to FIG. 2, with plates 102a and 102b abutting at vertex 103, an upper V-shaped notch 122 is defined in each inner side 105a of the plates 114 of the upper yoke 102 and a lower V-shaped notch 124 is formed in each inner side 104a of the plates 116 of the lower yoke 104. Each V-shaped notch 122, 124 extends from inner side 105a or 105b to the vertex 103 at the outer side 107a or 107b. The upper interior edges 126 (defining the notches 122) in adjacent plates 114 of the upper yoke 102 have different depths for forming vertical lap joints with upper ends 127 of inner leg plates 128 of the central inner leg 110, as will be described more fully below. Similarly, and as best shown in FIG. 3, the lower interior edges 130-130c in adjacent plates 116 -116c of the lower yoke 104 have different depths for forming vertical lap joints with lower ends 129 of the inner leg plates 128 of the central leg 110, as will be described more fully below. In the embodiment, the internal angle C of each V-shaped notch 122, 124 is approximately 52 degrees.

[0024] The first outer leg 106 comprises a stack of the plates 118, while the second outer leg 108 comprises a stack of the plates 120. Both the plates 118 and the plates 120 are arranged in groups of the same number as the plates 114, 116. Each of the plates 118, 120 is composed of grain-oriented silicon steel and has a thickness in a range of from about 7 mils to about 14 mils, with the particular thickness being selected based on the application of the transformer. The plates 118, 120 each have a unitary construction and are trapezoidal in shape. In each of the plates 118, 120, opposing ends of the plate are mitered at oppositely- directed angles of about 45 degrees, thereby providing the plate 118, 120 with major and minor side edges. The plates 118 have the same width to provide the first outer leg 106 with a rectangular cross-section and the plates 120 have the same width to provide the second outer leg 108 with a rectangular cross- section. However, the lengths of the plates 118 are not all the same and the lengths of the plates 120 are not all the same. More specifically, the lengths within each group of plates 118 are different and the lengths within each group of plates 120 are different. The pattern of different lengths is the same for each group of plates 118 and the pattern of different lengths is the same for each group of plates 120. The difference in lengths within each group permits the formation of the multi-step joints with the plates 114, 116 of the upper and lower yokes 102, 104, as will be described more fully below.

[0025] Referring now to FIG.3, there is shown an enlarged view of an end 117 of the lower end of the central leg 110 spaced from the lower yoke 104. The central leg 110 comprises a first stack 125 of inner leg plates 128 and a second stack 127 of inner leg plates 128. In each of the first and second stacks 125, 127, the inner leg plates 128 are arranged in groups of the same number as the plates 114, 116. The first and second stacks 125, 127 abut each other along a seam 131 that extends in the longitudinal direction of the central leg 110. Upper ends of the first and second stacks 125, 127 are disposed in the upper notch 122 of the upper yoke 102 and lower ends of the first and second stacks 125, 127 are disposed in the lower notch 124 of the lower yoke 104. The inner leg plates 128 form vertical multi-step lap joints with the plates 114, 116 of the upper and lower yokes 102, 104, as will be described further below. The inner leg plates 128 may all have the same length if the joints are offset by vertically shifting the inner leg plates 128. Alternately, the inner leg plates 128 may have a plurality of different lengths if the joints are offset by the different lengths of adjacent inner leg plates 128. Each of the inner leg plates 128 has a unitary construction and is trapezoidal in shape. Each of the inner leg plates 128 is composed of grain- oriented silicon steel and has a thickness in a range of from about 7 mils to about 14 mils, with the particular thickness being selected based on the application of the transformer 100. When the lower end of the central leg 110 is assembled to the lower yoke 104, the V-shaped ends 129 of first, second, third and fourth inner leg plates 128a, 128b, 128c and 128d abut (form joints with) the V-shaped lower interior edges 130, 130a, 130b and 130c of first, second, third and fourth plates 116, 116a, 116b, and 116c of the lower yoke 104, respectively. Each V-shaped end 129 has an internal angle D of about 52 degrees to mate with the corresponding notch 124 in the plates 116-116c. The first through fourth inner leg plates 128a- 128d are vertically offset such that lower ends thereof are located successively farther downward. In order to accommodate these differences in length, the lower interior edges of the plates 116-116c are cut successively deeper. With this construction, the first plate 116 overlaps the joints between the second inner leg plates 128b and the second plate 116a, the second plate 116a overlaps the joints between the third inner leg plates 128c and the third plate 116b, and the third plate 116b overlaps the joints between the fourth inner leg plates 128d and the fourth plate 116c. Although not shown, additional groups of the plates 116 and inner leg plates 128 are provided and repeat the pattern of the first through fourth plates 128a-128d and the first through fourth plates 116-116c. In this manner, multi-step lap joints are formed between the plates 116 of the lower yoke 104 and the inner leg plates 128, with plates 116 of the lower yoke 104 overlapping plates 128. [0027] Since the lower ends of the first through fourth inner leg plates 1 28a-1 28d are located successively farther downward, upper ends of the first through fourth inner leg plates 1 28a-1 28d are located successively farther downward. As a result, the upper interior edges 1 26 (and, thus, the upper notches 1 22) of the plates 1 14 within each group are successively shallower, which is the inverse of the lower yoke 1 04. With this construction, vertical multi-step lap joints are formed between the plates 1 14 of the upper yoke 1 02 and the inner leg plates 128 in the manner similar to that described above with regard to the lower yoke 104.

[0028] It should be appreciated that the inner leg plates 1 28 may be offset differently so as to have plates 1 14 of the upper yoke 1 02 overlapping inner leg plates 1 28, and inner leg plates 1 28 overlapping plates 1 1 6 of the lower yoke 1 04. In addition, the inner leg plates 1 28 may be offset to form a seven or other number step lap joint pattern, instead of the four step lap joint pattern.

[0029] In the embodiment where the inner leg plates 1 28 have different lengths, such as four different lengths, vertical multi-step lap joints are formed between the plates 1 14, 1 1 6 of the upper and lower yokes 1 02, 1 04 in a manner similar to that described above, however, the upper interior edges 1 26 (and thus the upper notches 122) of the plates 1 14 of the upper yoke 1 02 may have the same arrangement as the lower interior edges 1 30 (and thus the lower notches 1 24) of the plates 1 1 6 of the lower yoke 1 04 with regard to depth, because there is no vertical shifting of the inner leg plates 1 28.

[0030] Plates of the first and second outer legs 1 06, 1 08 are joined in a multi-step lap joint arrangement with plates the upper and lower yokes 1 02, 1 04 in a manner similar to the joining of the central yoke 1 1 0 to the upper and lower yokes 102, 1 04, described above. Cutting and joining of the outer legs 1 06, 1 08 is described in U.S. Patent No. 7,877,861 B2, the content of which is hereby incorporated by reference into this specification.

[0031] In the embodiment, each end 1 1 7 of each stack 1 25, 1 27 of the central leg 1 1 0 is cut at about 64 degree angles from a strip of steel with no scrap and each the top and bottom yokes 1 02, 1 04 are each cut at 64 degree angles to define the V- shaped notch 1 22, 1 24 and at 45 degrees to define the yoke ends. This provides upper and lower yokes with no scrap steel being generated.

[0032] The embodiment of the core 1 00 has been described using a step-lap arrangement that provides a joint configuration in which lamination packet pieces are not of equal lengths but are sequential cut, with each lamination being of unequal length, typically 1 /8" steps, or any step length. It can be appreciated that the core 100 can be arranged in a butt-lap arrangement, which is meshing of pieces having the same length.

[0033] The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.