Transformer and transformer manufacturing method

Abstract

The present invention provides a transformer and a transformer manufacturing method that can reduce the size of the transformer, or improve its characteristics if the size of the transformer is the same, and reduce manufacturing costs. The transformer 10 of the present invention comprises a coil 20 formed by winding a copper wire 21 in an annular shape such that opposing straight portions are included, an insulation sheet 30 having an electrically insulating property and wrapped around straight portions of the coil, and a cylindrical core 40 made of a grain-oriented electromagnetic steel plate 41 and rolled on top of the insulating sheet. The insulation sheet has a hook piece 31 formed at a wrapping end 33 positioned on the outer peripheral side, or at a portion on the outer peripheral side when the insulation sheet is wrapped around the straight portions of the coil. The core has an opening 42 formed at a rolling start tip 43 on the inner peripheral side of the grain-oriented electromagnetic steel plate. The hook piece is engaged with the opening.

Description

TECHNICAL FIELD

[0001] The present invention relates to a transformer wherein a grain-oriented electromagnetic steel plate is rolled on a coil via an insulation material, and a method of manufacturing the transformer.BACKGROUND ART

[0002] A transformer for converting voltage or current is known. The transformer comprises a coil wound into an oval shape with rounded corners and a core of a grain-oriented electromagnetic steel plate, and the core is rolled onto the coil in a cylindrical shape. (See Patent Document 1, for example).

[0003] The coil and core must be electrically insulated. So, the coil is wound on a resin bobbin of an insulation material, and the core is rolled onto the outer circumference of the resin bobbin.

[0004] To roll the core onto the resin bobbin, the rolling start tip of the core must be engaged with the resin bobbin. Therefore, the rolling start tip of the core has a claw piece formed by bending the grain-oriented electromagnetic steel plate inwardly at a right angle, and the resin bobbin is formed with a recess into which the claw piece fits. The claw piece is configured to fit into the recess, when the core is rolled onto the resin bobbin.PRIOR ART DOCUMENT (S)PATENT DOCUMENT

[0005] Patent Document 1: Japanese Patent Application Publication H08-51034SUMMARY OF THE INVENTIONPROBLEM TO BE SOLVED BY THE INVENTION

[0006] Since resin bobbins are molded from resin material, their thickness must be between 0.8 mm and 1.5 mm to provide the predetermined properties such as strength and insulated heat-resistant property. That is, the thickness of the bobbin in the radial direction is 1.6 mm to 3.0 mm. The resin bobbin having such thickness increases the size of transformers. If the size of the transformer is limited, the diameter of the copper wire used for the coil must be reduced depending on the thickness of the resin bobbin. This results in increased losses due to temperature rise and also in increased voltage fluctuation property.

[0007] Manufacturing resin bobbins incurs significant costs, including production equipment such as molding dies. Besides, resin bobbins cannot be easily modified to the change of the size or shape of transformers.

[0008] In addition, the core has a claw piece formed by bending the tip of the grain-oriented electromagnetic steel plate. However, the claw piece must be bent in a separate process after the stamping process of forming the core or the core tip in a tapered trapezoidal shape. Thus, the number of steps in the core production process increases.

[0009] The object of the present invention is to provide a transformer and a manufacturing method that can reduce the size of the transformer, or improve its characteristics if the size of the transformer is the same, and reduce manufacturing costs.MEANS TO SOLVE THE PROBLEMS

[0010] The transformer of the present invention comprises: a coil formed by winding a copper wire in an annular shape such that opposing straight portions are included; an insulation sheet having an electrically insulating property, the insulation sheet being wrapped around the straight portions of the coil; and a cylindrical core made of a grain-oriented electromagnetic steel plate, the cylindrical core being rolled on top of the insulating sheet.

[0011] The insulation sheet has a hook piece formed at a wrapping end positioned on an outer peripheral side, or at a portion positioned on the outer peripheral side when the insulation sheet is wrapped around the straight portions of the coil, the core has an opening formed at a rolling start tip positioned on an inner peripheral side of the grain-oriented electromagnetic steel plate, and the hook piece is engaged with the opening.

[0012] The hook piece may have a protruded shape extending from the wrapping end of the insulation sheet.

[0013] A wrapping direction of the insulation sheet and a rolling direction of the core may be opposite to each other.

[0014] The hook piece may be formed on a portion positioned on the outer peripheral side when the insulating sheet is wrapped around the straight portions of the coil, and may have a protruded shape created by a notch formed on the wrapping end side, the protruded shape extending toward the wrapping end side or the wrapping start tip side of the insulation sheet.

[0015] The protruded shape of the hook piece extends toward the wrapping end side of the insulation sheet, and the wrapping direction of the insulating sheet and the rolling direction of the core are opposite to each other.

[0016] The protruded shape of the hook piece extends toward the wrapping start tip side of the insulation sheet, and the wrapping direction of the insulating sheet and the rolling direction of the core are the same direction.

[0017] A method of manufacturing a transformer comprises: a step of preparing a coil by winding a copper wire in an annular shape such that opposing straight portions are included; a step of producing an insulation sheet having an electrical insulation property and for wrapping the straight portions of the coil, including forming a hook piece at a wrapping end of the insulation sheet, or at a portion positioned at an outer peripheral side of the insulation sheet when the insulation sheet is wrapped around the straight portions, a step of producing a preliminary rolled core, including subjecting a grain-oriented electromagnetic steel plate strip material to a stamping process to punch out a grain-oriented electromagnetic steel plate with an opening at a rolling start tip, rolling the grain-oriented electromagnetic steel plate into a cylindrical shape such that the rolling start tip of the grain-oriented electromagnetic steel plate is positioned on an inner peripheral side, and subjecting the rolled grain-oriented electromagnetic steel plate to an annealing heat treatment, a step of wrapping the insulation sheet around the straight portions of the coil such that the hook piece is positioned at an outer periphery of the insulation sheet, a step of disposing a rotatable roller of roller means into a rolling center of the preliminary rolled core and placing the preliminary rolled core in a juxtaposed position in the vicinity of the straight portions on which the insulation sheet is wrapped, a first pullout step, including rotating the roller to pull out a rolling end of the grain-oriented electromagnetic steel plate to form a large ring surrounding the preliminary rolled core and the insulation sheet, a step of temporarily fixing the rolling end of the grain-oriented electromagnetic steel plate to a peripheral surface of the large ring, a second pullout step, including rotating the roller to rotate the preliminary rolled core, and pulling out the grain-oriented electromagnetic steel plate until the rolling start tip of the grain-oriented electromagnetic steel plate is within the large ring, a step of hooking the opening of the grain-oriented electromagnetic steel plate to the hook piece of the insulation sheet, a step of withdrawing the roller from the large ring; a step of obtaining a core, including reducing the diameter of the large ring of the grain-oriented electromagnetic steel plate via a restoring force, and rolling the grain-oriented electromagnetic steel plate onto the insulation sheet wrapped around the straight portions of the coil; and a step of fixing the rolling end of the grain-oriented electromagnetic steel plate to a peripheral surface of the core.

[0018] The hook piece may have a protruded shape extending in the direction opposite to the rolling direction of the core.

[0019] The step of producing the preliminary rolled core involves forming the rolling start tip in a tapered trapezoidal shape, during the stamping process, simultaneously with punching out the grain-oriented electromagnetic steel plate with the opening at the rolling start tip.EFFECTS OF THE INVENTION

[0020] According to the transformer and the transformer manufacturing method, an insulation sheet is used instead of a resin bobbin, and the insulation portion can be made thinner, so that the core diameter can be made smaller and the weight of the transformer can be reduced. If the size of transformers is the same, the diameter of copper wire can be made larger, which enables the suppression of losses due to temperature rise and reduces voltage fluctuation characteristics.

[0021] The insulation sheet can be purchased as a commercial product and easily made to the desired size. Therefore, it is more readily adaptable to design changes than resin bobbins. Additionally, unlike resin bobbins, the insulation sheet eliminates the need for injection molding dies and molding equipment. Consequently, manufacturing costs can be significantly reduced.

[0022] The opening of the core can be formed simultaneously when punching out the core tip into a trapezoidal shape during the stamping process, thereby preventing an increase in the number of processes.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Fig. 1 shows (a) a front view, (b) a bottom view, and (c) a rear view of the transformer according to one embodiment of the present invention. Fig. 2 is a cross-sectional view along line A-A in Fig. 1. Fig. 3 shows (a) a front view, (b) a bottom view, and (c) a rear view of the coil. Fig. 4 shows (a) a front view, (b) a bottom view, and (c) a rear view of the coil having the straight portions wrapped by an insulating sheet. Fig. 5 is a perspective view of the straight portions of the coil wrapped by the insulating sheet. Fig. 6 is a cross-sectional view along line B-B in Fig. 4 (c). Fig. 7 is a plan view of the insulation sheet. Fig. 8 is a plan view showing the process of producing the insulating sheet from an elongate insulation sheet strip. Fig. 9 is a side view of the preliminary rolled core formed by rolling the grain-oriented electromagnetic steel plate. Fig. 10 is a plan view of the grain-oriented electromagnetic steel plate before rolling. Fig. 11 is a plan view showing the process of producing the grain-oriented electromagnetic steel plate from an elongate grain-oriented electromagnetic steel plate strip. Fig. 12 is an explanatory diagram showing steps of rerolling the preliminary rolled core onto the straight portions of the coil wrapped with the insulation sheet. Fig. 13 is a side view of roller means. Fig. 14 shows the procedure for fitting the opening of the core in the hook piece of the insulating sheet. Fig. 15 is a plan view showing another embodiment of the hook piece of the insulating sheet. Fig. 16 shows the procedure for fitting the opening of the core in the hook piece of the insulating sheet shown in Fig. 15 (a). Fig. 17 shows the procedure for fitting the opening of the core into the hook piece of the insulating sheet shown in Fig. 15 (b). Fig. 18 is a cross-sectional view of a transformer including the core portion of (a) an example of the invention and (b) a comparative example using a resin bobbin as the insulating material. Fig. 19 is a graph showing the offset voltage of the waveform of alternating current. Fig. 20 shows the placement of the microphones used for noise measurement, wherein (a) is a front view and (b) is a bottom view. Fig. 21 is a cross-sectional view of a transformer with a core portion having an insulating sheet wrapped in an almost circular shape. Fig. 22 is a cross-sectional view of the transformer with a core portion having a coil wound into a nearly circular shape in cross section. MODE FOR CARRYING OUT THE INVENTION

[0024] One embodiment of transformer 10 according to the present invention will be described below with reference to the drawings.<Overall Outline of Transformer 10>

[0025] Fig. 1 shows transformer 10 according to an embodiment of the present invention wherein (a) is a front view, (b) is a bottom view, and (c) is a rear view. Fig. 2 is a cross-sectional view along line A-A in Fig. 1. Transformer 10 of the present invention comprises a coil 20 wound in an annular form, an insulation sheet 30 wrapped on the outer periphery of the coil 20, and a cylindrical core 40 of a grain-oriented electromagnetic steel plate 41 rolled via the insulation sheet 30.<Coil 20>

[0026] The coil 20 of transformer 10 is, for example, a single coil that shares part of a primary coil and a secondary coil, and is formed by winding copper wire 21, such as enameled copper wire with an insulating coating (Step of Producing Coil 20). For the coil wherein the primary coil and the secondary coil are separated, the primary coil and secondary coil are insulated before winding. Coil 20 can be wound into an approximately rectangular shape with rounded corners and having straight portions 22, 22 facing each other, as shown in Fig. 3. Straight portions 22, 22 of coil 20 are the portions wrapped by the insulation sheet 30 and rolled by the core 40. In Fig. 3, the straight portions 22, 22 of coil 20 are contiguous with arc-shaped curved portions 23. The curved portion 23 is not limited to the arc-shape and the curvature factor shown in this figure, but may include straight portions.

[0027] Coil 20 can be wound into a shape with a substantially hexagonal cross-section, as shown in Fig. 2. A cross section of coil 20 may be other shapes, e.g., approximately octagonal or other polygonal shapes, approximately circular shape (as shown in Figs. 21 and 22), approximately elliptical shape.

[0028] After completing the coil 20, a fixing means, such as tape 25, is applied to one or more places of the wound copper wire, as shown in Fig. 3, to prevent the copper wire 21 from coming loose and to maintain the shape of the coil.<Insulation Sheet 30>

[0029] As shown in Figs. 4 to 6, the straight portions 22, 22 of the coil 20 are wrapped by the insulation sheet 30. The insulation sheet 30 is a thin sheet member composed of an electrically insulating material, as shown in Fig. 7. For example, the insulation sheet 30 may be an aramid paper (Nomex (trademark) manufactured by DuPont) woven with aramid fibers made of aromatic polyamide resin, or may be a polyester film.

[0030] The insulation sheet 30 is preferably 0.1 mm to 0.5 mm in thickness, and more preferably 0.2 mm to 0.4 mm in thickness.

[0031] Insulation sheet 30 has a width that can be wrapped around the straight portion 22 of the coil 20 and is wider than the grain-oriented electromagnetic steel plate 41, which constitutes the core 40. To ensure creepage distance between the coil 20 and the core 40, the width of the insulation sheet 30 is preferably at least 5 mm (2.5 mm on each side) wider than the width of the grain-oriented electromagnetic steel plate 41. In addition, the insulation sheet 30 should have a length that is long enough to cover the outer circumference of the straight portion 22 more than one turn, i.e., a length that is greater than the outer circumference length of the straight portion 22 to ensure the insulation effect. In the embodiment shown in Fig. 6, the insulation sheet 30 has a length that is one circumference plus one side of the nearly hexagonal coil 20. The insulation sheet is preferably long enough to be wrapped around the coil 20 about 1.2 to 2 times. The wrapped form of insulation sheet 30 is not limited to a substantially hexagonal shape that is tightly wrapped on the peripheral surface of a substantially hexagonal-shaped coil 20, as shown in Fig. 6. The insulation sheet 30 may be wrapped around the peripheral surface of the substantially hexagonal-shaped coil 20 in a nearly circular shape, as shown in Fig. 21, which is described later.

[0032] As shown in Figs. 4 to 7, the insulation sheet 30 has a hook piece 31 engageable with the core 40 and formed in the vicinity of the wrapping end 33, which is positioned on the outer periphery side of the insulation sheet 30 when the insulation sheet 30 is wrapped. Hook piece 31 is a protruded shape formed at a portion of the insulation sheet 30. Hook piece 31 may have a protruded shape extending in the direction away from the wrapping end 33 of insulation sheet 30, as shown in Fig. 7, or may have a protruded shape created when notches 34a, 34b are formed in the insulation sheet 30, as shown in Fig. 15, which is described later.

[0033] When viewed the hook piece 31 from above, its shape may be, for example, a substantially rectangular shape, a substantially trapezoidal shape with a tapered tip (see Figs 5 and 7), a substantially arcuate shape, or a substantially triangular shape. The tapered tip of hook piece 31 facilitates ease of engagement with an opening 42 of the grain-oriented electromagnetic steel plate 41 and also provides the engaged strength.

[0034] The size of the hook piece 31 is preferably about 5 mm to 30 mm in width and about 5 mm to 20 mm in length, to ensure that the hook piece has a predetermined strength when engaged with the opening 42 of the grain-oriented electromagnetic steel plate 41. A width of 10 mm to 20 mm and a length of 7 mm to 15 mm are more preferable.

[0035] The insulation sheet 30 with hook piece 31 having a configuration described above can be formed from an elongate insulation sheet strip 30a, as shown in Fig. 8 (Step of Producing Insulation Sheet 30). For example, the insulation sheet 30 can be produced by pulling out the insulation sheet strip 30a in the rolled state and then stamping the area indicated by the letter C in Fig. 8, using a Thomson die. Thus, the hook piece 31 can be created at the central portion in the width direction in a single step, and the insulation sheet 30 can be produced to a predetermined length. As shown in Figs. 7 and 8, a cut-out trace 35, left from forming the hook piece 31 of the subsequent insulation sheet, remains at the wrapping start end 32 of the insulation sheet 30.<Step of Wrapping Insulation Sheet 30>

[0036] The produced insulation sheet 30 is wrapped around the straight portions 22 of the coil 20, as shown in Figs 4 to 6. The insulation sheet 30 is wrapped around the coil 20 such that the hook piece 31 is positioned on the outer circumference, and then is secured by a fixing means, such as tape 37, to prevent it from coming loose. The tape 37 should be applied while avoiding the hook piece 31, so that the hook piece 31 protrudes outward due to the rigidity of the insulation sheet 30, as shown in Fig. 6. To make the hook piece 31 protrude further outward, it is desirable to attach the tape 37 so as to press down on both outer sides of the hook piece 31, as shown in Figs. 4 and 5. Applying the tape 37 on both outer sides of the hook piece 31 imparts rigidity to the hook piece 31. Consequently, when the hook piece 31 fits into an opening 42 of the grain-oriented electromagnetic steel plate 41 (described next), it prevents the insulation sheet 30 from loosening and the hook piece from following, even if the hook piece 31 is pulled by the core 40.<Preliminary Rolled Core 45>

[0037] Core 40 is provided on the insulation sheet 30 wrapped around the straight portions 22 of the coil. The core 40 comprises the grain-oriented electromagnetic steel plate 41 rolled onto the insulation sheet 30, as shown in Fig. 1.

[0038] This core 40 is prepared by rerolling the preliminary rolled core 45 onto the insulation sheet 30. The preliminary rolled core 45 is prepared by rolling the grain-oriented electromagnetic steel plate 41 in a cylindrical shape, as shown in Fig. 9, and then subjected to an annealing heat treatment. The grain-oriented electromagnetic steel plate 41 has a through opening 42 at the rolling start tip 43, which is positioned on the inner side when the plate 41 is rolled up, as shown in Fig. 10. This opening 42 is engageable with the hook piece 31 of the insulation sheet 30. The opening 42 is wide enough to fit at least the hook piece 31, and is preferably 1 to 5 mm wider than the width of the hook piece 31.

[0039] Grain-oriented electromagnetic steel plate 41, preferably, the rolling start tip 43 and the rolling end 44 are formed into tapered, generally trapezoidal shapes (tapered portions 43a, 44a), as shown in Fig. 10. This provides strength and prevents distortion during rolling operation.

[0040] Grain-oriented electromagnetic steel plate 41 can be formed from an elongate grain-oriented electromagnetic steel plate strip 40a, as shown in Fig. 11. To reduce the number of processes, it is desirable to apply a stamping process to the area indicated by letter D in Fig. 11 to form the tapered portion 44a on the rolling end side 44 of the grain-oriented electromagnetic steel plate 41, and the tapered portion 43a and the opening 42 on the rolling start tip side 43 of the following grain-oriented electromagnetic steel plate 41a, in a single working process.

[0041] Creating an opening in a grain-oriented electromagnetic steel plate is generally considered undesirable because it can cause a deterioration in properties. However, in the case of the present invention, only a portion of the tip of the grain-oriented electromagnetic steel plate 41 is removed. Therefore, the properties of the grain-oriented electromagnetic steel plate are not affected by the opening 42.

[0042] The preliminary rolled core 45 is produced as follows: The grain-oriented electromagnetic steel plate 41 with the opening 42 is rolled such that the rolling start tip 43 (the opening 42 is formed there) is on the inner circumference and the rolling end 44 is on the outer circumference, as shown in Fig. 9, and then is subjected to heat treatment in an annealing heat treatment process (Step of Producing Preliminary Rolled Core). The inner diameter of the preliminary rolled core 45, i.e., the diameter of the rolled center 45a, is arranged to conform to the outer diameter of the insulation sheet 30 wrapped around the straight portions of the coil 20 or to the maximum outer diameter. At a later process, after rolling the grain-oriented electromagnetic steel plate 41 of the preliminary rolled core 45 onto the outer circumference of the insulation sheet 30, the core 40 is tightened in the rolling direction to remove any slack of the core 40. At that time, it is necessary to check whether the core 40 is tightened until the outer diameter becomes the same as the preliminary rolled core 45. For this reason, prior to the above-mentioned annealing heat treatment process, it is desirable to mark a straight line 46 in the radial direction on the preliminary rolled core 45, as shown in Fig. 9, so that it can be used as a sign showing a tightened state in the subsequent process. The straight line 46 is marked using materials that can withstand the heat of the annealing heat treatment process. Additionally, to prevent the preliminary rolled core 45 from loosening or disengaging during the annealing heat treatment, it is preferable to wind a wire such an iron-based or brass-based wire that can withstand the heat treatment temperature around the outer periphery of the core 45 after the grain-oriented electromagnetic steel plate 41 is rolled. Alternatively, a plate material or a fixture can be used to fix the core 45 and maintain its shape as well.<Rerolling of Preliminary Rolled Core 45; Manufacturing of Transformer 10>

[0043] The grain-oriented electromagnetic steel plate 41 of the preliminary rolled core 45 is rolled onto the outer periphery of the insulation sheet 30 wrapped around the straight portions 22 of the coil 20, whereby a core 40 is produced. Rerolling of the preliminary rolled core 45 can be performed using a rolling apparatus 50.

[0044] The rolling apparatus 50 may comprise a coil fixing means 51 for holding coil 20 and roller means 53 for holding and pulling out the preliminary rolled core 45, as shown in Fig. 12 (a). To facilitate ease of understanding of the explanation, Fig. 12 shows the grain-oriented electromagnetic steel plate 41 in cross-section, to make the opening 42 visible.

[0045] The coil fixing means 51 may be, for example, a clamp or clip, and holds the straight portions 22 of the coil 20 in an almost vertical position. Preferably, the coil fixing means 51 holds portions such as the curved portion 23, i.e., portions other than the straight portions 22 in Figs. 1 and 3, where the insulation sheet 30 is not provided.

[0046] Preliminary rolled core 45 is disposed in roller means 53 comprising an inner roller 54 and an outer roller 55. Fig. 13 shows a side view of the roller means 53. Inner roller 54 and outer roller 55 are substantially hollow or solid cylinders, and their outer peripheries are covered with slip-resistant means 54a, 55a such as rubber material (e.g., NBR: nitrile butadiene rubber) or polyurethane resin material. As shown in the figure, the inner roller 54 has a smaller diameter than the outer roller 55. However, these rollers may have the same diameter, or the inner roller may have a larger diameter than the outer roller. The inner roller 54 and outer roller 55 are biased toward each other in their approaching direction. One of the inner roller 54 and outer roller 55 is rotatably driven in the direction of pulling out the preliminary rolled core 45, i.e., in the direction of arrow R in Fig. 12 (a), via a drive mechanism such as motor (not shown). The other of the inner roller 54 and outer roller 55 may be configured to follow a rotation as the preliminary rolled core 45 is pulled out. In the present embodiment, the inner roller 54 is configured to move up and down in order to withdraw the inner roller 54 out of the large ring 48 of the grain-oriented electromagnetic steel plate 41 during the withdrawing step which is described below.

[0047] The coil 20 and the preliminary rolled core 45 are disposed. As shown in Fig. 12 (a), the coil 20 with the insulation sheet 30 wrapped around the straight portions 22 is disposed in the coil fixing means 51. The rolled center 45a of the preliminary rolled core 45 is inserted into the inner roller 54. The preliminary rolled core 45 is placed between the inner roller 54 and the outer roller 55, in a juxtaposed position with the straight portion 22 of the coil 20 (Step of Placing Preliminary Rolled Core 40). One of the two straight portions of the coil 20 is placed on the roller side. The preliminary rolled core 45 is placed such that the rolling direction of the preliminary rolled core 45 is opposite to the wrapping direction of the insulation sheet 30, as shown in Fig. 12 (a). That is, when the insulation sheet 30 is wrapped clockwise, the preliminary rolled core 45 is positioned so that the rolling direction of the grain-oriented electromagnetic steel plate 41 is counterclockwise (from the inner circumference toward the outer circumference).

[0048] From this state, the inner roller 54 or the outer roller 55 is rotated in the direction of arrow R in Fig. 12 (b) to rotate the preliminary rolled core 45 in the direction of arrow E, whereby the rolling end 44 of the preliminary rolled core 45 is pulled outward, passes between the straight portions 22, 22, and pulled back along the outer circumferential surface of the preliminary rolled core 45. As a result, the grain-oriented electromagnetic steel plate 41 that was pulled out forms a large ring 48 surrounding the preliminary rolled core 45 and the insulating sheet 30 (First Pullout Step). Thus, the large ring 48 is formed by expanding the diameter from the state of the preliminary rolled core 45, while maintaining an almost circular shape. Therefore, the stress due to mechanical distortion remaining even after the annealing heat treatment is low, and the deterioration of magnetic properties, such as iron loss in the transformer, is suppressed.

[0049] The rolling end 44 pulled out from the preliminary rolled core 45 passes between the inner roller 54 and the outer roller 55, as shown in Figure 12 (b), and the rolling end 44 that has passed through is temporarily fixed to the peripheral surface of the preliminary rolled core 45 by a fixing means such as tape 47 (Step of Temporarily Fixing the Rolling End 44 of the Preliminary Rolled Core 45).

[0050] From this state, as shown in Figure 12 (c), the inner roller 54 or the outer roller 55 is further rotated to pull out the grain-oriented electromagnetic steel plate 41 from the preliminary rolled core 45 (Second Pullout Step).

[0051] The inner roller 54 or the outer roller 55 is rotated until the grain-oriented electromagnetic steel plate 41 of the preliminary rolled core 45 is completely included in the large ring 48, as shown in Fig. 12 (d).

[0052] Next, the opening 42 formed at the rolling start tip 43 of the grain-oriented electromagnetic steel plate 41 of the large ring 48 is put into the hook piece 31 of the insulating sheet 30. In Fig. 12 (d), the rolling start tip 43 is positioned on the side of rollers 54, 55. From this position, the large ring 48 is rotated, as shown in Fig. 12 (e). The grain-oriented electromagnetic steel plate 41 has a restoring force that tends to return to the original shape of the preliminary rolled core 45, which is smaller in diameter than the large ring 48. Therefore, the rolling start tip 43 is curled inward and abuts against the insulating sheet 30, as shown in Fig. 12 (e) and its enlarged view of Fig. 14 (a). Since the insulation sheet 30 comprises the hook piece 31, the rolling start tip 43 moves in the direction of arrow F in Fig. 14 (a) by further rotating the large ring 48, and the hook piece 31 fits into the opening 42, as shown in Fig. 12 (e) and Fig. 14 (b) (Hooking Step).

[0053] In the state where the hook piece 31 is fitted into the opening 42, when tape 47 is peeled off and the inner roller 54 is withdrawn (for example, the inner roller 54 is moved downward), the large ring 48 decreases in diameter due to the restoring force of a residual stress remaining even after annealing heat treatment, as shown in Fig. 12 (f), and then the grain-oriented electromagnetic steel plate 41 is rolled onto the insulating sheet 30 (Rolling Step). The grain-oriented electromagnetic steel plate 41 rolled onto the insulation sheet 30 is tightened until it has the same outer diameter as the preliminary rolled core 45 shown in Fig. 9. If the straight line 46 marked on the preliminary rolled core 45 shown in Fig. 9 is not in alignment, it is necessary to tighten the sheet 41 by manual operation or using a tightening aid or other means until the line 46 is brought into alignment. In this case, the tightening force should not exceed the straight line 46 because exceeding it generates a new stress to cause mechanical distortion and deteriorates the magnetic properties.

[0054] In the tightened state, the rolling end 44 of the grain-oriented electromagnetic steel plate 41 is fixed to the peripheral surface of the core 40 by spot welding 49, etc., as shown in Figs. 1 (a) and (c) (Rolling End Fixing Step). Next, the coil fixing means 51 is operated to release the retention of the coil 20.

[0055] Then, the preliminary rolled core 45 is rerolled also onto the insulating sheet 30 wrapped around another straight portion 22, in the same manner, and the rolling end 44 is fixed to produce the transformer 10 as shown in Fig. 1. After that, the transformer is typically impregnated with a resin varnish for moisture protection, regardless of its shape or form. This treatment is a common practice for finishing the transformer and is also applied to the transformer of the present invention.

[0056] According to the above-described manufacturing method, the grain-oriented electromagnetic steel plate 41 is expanded in diameter to the large ring 48 from the state of the preliminary rolled core 45 while maintaining a substantially circular shape, and then is rerolled onto the straight portions 22 to produce a core 40, while maintaining the substantially exact circular shape. The stress caused on the grain-oriented electromagnetic steel plate 41 by mechanical distortion after the annealing heat treatment is small, so that the deterioration of the transformer's magnetic properties, mainly iron loss, is suppressed.

[0057] The transformer 10 of the present invention uses an insulation sheet 30 to insulate the coil 20 and the core 40, instead of a resin bobbin. Since the insulation sheet is thinner than the resin bobbin, the insulation portion can be thinner, allowing the transformer to be smaller. If the transformer 10 is produced in the same size as the transformer using the resin bobbin, the copper wire having a larger diameter can be used, so that losses due to temperature rise are suppressed and voltage fluctuation characteristics are stabilized.

[0058] Insulation sheet 30 can be purchased as a commercial product and easily produced to the desired size, which allows more flexibility in design changes compared to a resin bobbin. Additionally, the insulation sheet eliminates the need for injection molding dies and equipment to produce resin bobbins, thereby significantly reducing the manufacturing costs of transformers.

[0059] The opening 42 for engagement can be created in the grain-oriented electromagnetic steel plate 41 during the formation of the tip of the grain-oriented electromagnetic steel plate 41, simultaneously with stamping the grain-oriented electromagnetic steel plate 41 into an almost trapezoidal shape using a general-purpose method. No additional process is required to create the opening 42, accordingly.<Different Embodiment of Hook Piece 31>

[0060] Fig. 15 shows another embodiment of hook pieces 31a and 31b of the insulating sheet 30. The hook piece 31 of the embodiment shown in Fig. 7 has a protruded shape extending from the edge of the wrapping end 33 of the insulation sheet 30. In Fig. 15, hook pieces 31a and 31b are created by notches 34a and 34b formed at the portion where the insulation sheet 30 is located on the outer peripheral side when the insulating sheet 30 is wrapped around the straight portion 22 of the coil 20, i.e., at the position that is from the wrapping end 33 to the circumferential length of the straight portion 22.

[0061] Fig. 15 (a) shows the insulation sheet 30 having a hook piece 31a that is formed by a notch 34a and extends toward the wrapping end side 33. Fig. 15 (b) shows the insulation sheet 30 having a hook piece 31b that is formed by a notch 34b and extends toward the direction opposite to that of Fig. 15 (a), i.e., toward the wrapping start tip side 32. The hook pieces 31a and 31b are both approximately triangular with pointed tips, but may also be approximately trapezoidal, rectangular, arc-like in shape, or other shapes.

[0062] The protruding direction of the hook piece 31a of the insulation sheet 30 shown in Fig. 15 (a) is in the same direction as the protruding direction of the hook piece 31 of the insulation sheet 30 shown in Figs. 4 to 7. Therefore, when the grain-oriented electromagnetic steel sheet 41 of the preliminary rolled core 45 is arranged such that the rolling direction from the inner circumference to the outer circumference is opposite to the wrapping direction of the insulating sheet 30, the hook piece 31a of the insulation sheet 30 can be engaged with the opening 42, in the same manner as shown in Fig. 14. To explain more specifically, the large ring 48 is slightly rotated in a state where the rolling start tip 43 of the grain-oriented electromagnetic steel plate 41 is in contact with the peripheral surface of the insulating sheet 30, as shown in Fig. 12 (e), and then the opening 42 is engaged with the hook piece 31a, as shown in Figs. 16 (a) and (b). The subsequent steps are the same as those described above.

[0063] The protruding direction of the hook piece 31b in Fig. 15 (b) is opposite to that of the above embodiment. Therefore, to engage the opening 42 with the hook piece 31b, the wrapping direction of the insulation sheet 30 and the rolling direction of the preliminary rolled core 45 are arranged to be in the same direction. Referring to Fig. 17 (a), it can be seen that the insulation sheet 30 is wrapped in the opposite direction to that shown in Fig. 16 (a). Therefore, when the large ring 48 is slightly rotated in a state where the rolling start tip 43 of the grain-oriented electromagnetic steel plate 41 is in contact with the peripheral surface of the insulating sheet 30, as shown in Fig. 12 (e), the opening 42 can be engaged with the hook piece 31b of the insulation sheet 30, as shown in Figs. 17 (a) and (b). The subsequent steps are identical to those described above.EXAMPLE

[0064] The transformer 10 of the present invention and the transformer 70 for comparison were prepared. The inventive transformer 10 uses an insulation sheet 30 shown in Fig. 18 (a) as the insulation material, and the comparative transformer 70 uses a resin bobbin 72 shown in Figure 18 (b) as the insulating material. The produced transformers 10 and 70 are single-phase and single-winding transformers with a rated capacity of 2 kVA.

[0065] The coil 20 of the present invention is formed by winding a copper wire 21 with a diameter of 2.3 mm for 200 turns and has an output tap provided after 100 turns. Insulation sheet 30 is 0.25 mm in thickness and has a hook piece 31 shown in Fig. 7. As shown in Fig. 18 (a), the insulation sheet 30 is provided on the coil by wrapping one round and one side. Core 40 is prepared by rerolling the preliminary rolled core 45, which was formed by rolling a 0.23 mm thick grain-oriented electromagnetic steel plate 41 for 60 turns in an inner diameter of 40 mm, onto the insulation sheet 30 in a manner shown in Figs. 12 and 14.

[0066] On the other hand, the comparative example uses a resin bobbin 72 with a thickness of 0.8 to 1.5 mm to maintain the shape of the coil. The coil 71 is prepared by winding a copper wire having a diameter of 2.1 mm, which is thinner than that of the inventive example, with the same number of turns as in the invention. A core 74 is identical to the core 40 of the inventive example and is rolled onto the outer periphery of the resin bobbin 72. As shown in Fig. 18 (b), the resin bobbin 72 and the grain-oriented electromagnetic steel plate 75 are engaged by fitting an inwardly curved claw piece 76 formed on the rolling start tip of the grain-oriented electromagnetic steel plate 75, into a recess 73 formed on the peripheral surface of the resin bobbin 72. For the comparative transformer 70, if the preliminary rolled core is rerolled around the resin bobbin 72 in the same manner as shown in Fig. 12, the inner roller 54 in Fig. 13 is required to provide a recess at the center in the longitudinal direction of the inner roller 54 to escape the claw piece 76.

[0067] The inventive example can use a thinner insulation sheet 30 with higher insulating properties, such as an aramid sheet and resin molded sheet of polyester. Therefore, if the core 40 of the inventive transformer 10 and the core 74 of the comparative transformer 70 are the same in size, the coil 20 of the present invention can use a wire diameter of 2.3 mm, whereas the comparative example uses a wire diameter of 2.1 mm. The increased wire diameter of the inventive transformer reduces the DC resistance value, and copper loss by 15%, which is calculated as the square of the current multiplied by the resistance. The reduced copper loss suppresses temperature rise and improves voltage fluctuation rate.

[0068] If the comparative transformer 70 is prepared using the coil 71 wound with 2.3 mm wire for 200 turns to have the properties of the comparative transformer 70 identical to those of the inventive transformer 10, the core 74 must be made by rolling a grain-oriented electromagnetic steel plate for 60 turns with an inner diameter of 44 mm. In that case, the outer diameter of the core 74 is 72 mm and is 4 mm larger than the outer diameter of the core 40 rolled with an inner diameter of 40 mm in the inventive example, concerning the distance "d" as shown in Fig. 18. Since the amount of the grain-oriented electromagnetic steel plate to be used for core 74 increases, the weight of the core 74 increases by approximately 7.4%. Thus, when the transformer 70 of the comparative example is prepared with the same wire diameter and number of turns as those of the present invention, the product of the comparative example becomes larger and heavier.

[0069] The above description is intended to explain the invention and should not be construed as limiting or restricting the scope of the invention as recited in the claims. The present invention is not limited to the above-mentioned examples, and various modifications can be made within the technical scope recited in the claims.

[0070] For example, the hook piece 31 may be formed from another sheet-like member and adhered to the insulation sheet 30. The hook piece 31 shown in Figs. 7 and 15 may also be a form folded back outward.

[0071] In the latest solar power conditioners, systems that utilize storage devices such as batteries to convert into household commercial power are becoming increasingly available. For example, in Japan, transformers are used as a function of converting 200 V AC to 100 V AC. However, when converted, an offset voltage of approximately 0 to 0.3 V occurs during the conversion from DC (direct current) to AC (alternating current), as shown in Fig. 19, causing magnetic flux imbalance and resulting in an increased noise from the transformer. As the input voltage of the transformer is constant at the commercial power supply voltage, the maximum magnetic flux density remains unchanged, and the magnetostrictive vibration strength inherent to the grain-oriented electromagnetic steel plate remains unchanged. However, the voltage increases on the positive side and decreases on the negative side. Therefore, an imbalance in the magnetic flux density is caused, resulting in the grain-oriented electromagnetic steel plate being more prone to mechanical vibration and increasing noise.

[0072] The transformer 10 of the present invention has a magnetic circuit structure that eliminates a butt portion in the core 40, thereby reducing noise generated by the magnetostrictive vibrations of the grain-oriented electromagnetic steel plate 41. In addition, the hook structure of the insulating sheet 30 is so designed to tighten up the grain-oriented electromagnetic steel plate 41. This minimizes the gap between the adjacent stacks of the rolled grain-oriented electromagnetic steel plate 41, thereby suppressing mechanical vibration of the grain-oriented electromagnetic steel plate 41 and reducing noise. Table 1 shows the results of measurement. Fig. 20 shows the microphone 80 placed for the measurement of noise. Table 1Transformer of the inventive example35-38 dBTransformer of the comparative example40-50 dBBackground noise: 26 dB

[0073] When the grain-oriented electromagnetic steel plate 41 is tightened, it should be tightened up to the same state before being provided on the coil 20. Excessive tightening can cause mechanical distortion, which degrades the magnetic properties and increases losses.<Wrapping Method of Insulation Sheet 30>

[0074] In Fig. 6, the insulating sheet 30 is tightly wrapped around the nearly hexagonal shaped coil 20 to form a roughly hexagonal tubular shape. In Fig. 21, the insulating sheet 30 is wrapped around the peripheral surface of an approximately hexagonal coil 20 to form a cross-sectional shape that is a nearly circular shape, particularly, a hexagonal shape with arc-shaped sides.<Embodiment of Different Shape of Coil 20>

[0075] Fig. 22 is a cross-sectional view showing the core 40 of the transformer 10 including the coil 20 wound in a roughly circular shape. In Fig. 22, the coil 20 is wound in a roughly circular shape. This increases the space factor of the coil 20, compared to a roughly hexagonal shape. In addition, the insulation sheet 30 is wrapped around the coil 20 in a substantially circular shape.

[0076] As in Figs. 21 and 22, the gap between coil 20 and insulating sheet 30 and the gap between insulation sheet 30 and core 40 are made smaller. So, the inner side of the core 40 can be compressed, and noise caused by mechanical vibration is suppressed.DESCRIPTION OF REFERENCE SIGNS

[0077] 10Transformer 20Coil 22Straight portion 30Insulation sheet 31Hook piece 40Core 41Grain-oriented electromagnetic steel plate 42Opening

Claims

1. A transformer comprising: a coil formed by winding a copper wire in an annular shape such that opposing straight portions are included; an insulation sheet having an electrically insulating property, the insulation sheet being wrapped around the straight portions of the coil; and a cylindrical core made of a grain-oriented electromagnetic steel plate, the cylindrical core being rolled on top of the insulating sheet.

2. The transformer according to claim 1 wherein the insulation sheet has a hook piece formed at a wrapping end positioned on an outer peripheral side, or at a portion positioned on the outer peripheral side when the insulation sheet is wrapped around the straight portions of the coil, the core has an opening formed at a rolling start tip positioned on an inner peripheral side of the grain-oriented electromagnetic steel plate, and the hook piece is engaged with the opening.

3. The transformer according to claim 2 wherein the hook piece has a protruded shape formed on the wrapping end side of the insulation sheet.

4. The transformer according to claim 3 wherein a wrapping direction of the insulation sheet and a rolling direction of the core are opposite to each other.

5. The transformer according to claim 4 wherein the hook piece is formed on the portion positioned on the outer peripheral side when the insulation sheet is wrapped around the straight portions of the coil, and the protruded shape is created by a notch formed on the wrapping end side and extends toward the wrapping end side or the wrapping start tip side of the insulation sheet.

6. The transformer according to claim 5 wherein the protruded shape of the hook piece extends toward the wrapping end side of the insulation sheet, and the wrapping direction of the insulating sheet and the rolling direction of the core are opposite to each other.

7. The transformer according to claim 5 wherein the protruded shape of the hook piece extends toward the wrapping start tip side of the insulation sheet, and the wrapping direction of the insulating sheet and the rolling direction of the core are the same direction.

8. A method of manufacturing a transformer comprising: a step of preparing a coil by winding a copper wire in an annular shape such that opposing straight portions are included; a step of producing an insulation sheet having an electrical insulation property and for wrapping the straight portions of the coil, including forming a hook piece at a wrapping end of the insulation sheet or at a portion positioned at an outer peripheral side of the insulation sheet when the insulation sheet is wrapped around the straight portions; a step of producing a preliminary rolled core, including subjecting a grain-oriented electromagnetic steel plate strip material to a stamping process to punch out a grain-oriented electromagnetic steel plate with an opening at a rolling start tip, rolling the grain-oriented electromagnetic steel plate into a cylindrical shape such that the rolling start tip of the grain-oriented electromagnetic steel plate is positioned on an inner peripheral side, and subjecting the rolled grain-oriented electromagnetic steel plate to an annealing heat treatment; a step of wrapping the insulation sheet around the straight portions of the coil such that the hook piece is positioned at an outer periphery of the insulation sheet; a step of disposing a rotatable roller of roller means into a rolling center of the preliminary rolled core and placing the preliminary rolled core in a juxtaposed position in the vicinity of the straight portions on which the insulation sheet is wrapped; a first pullout step, including rotating the roller to pull out a rolling end of the grain-oriented electromagnetic steel plate to form a large ring surrounding the preliminary rolled core and the insulation sheet; a step of temporarily fixing the rolling end of the grain-oriented electromagnetic steel plate to a peripheral surface of the large ring; a second pullout step, including rotating the roller to rotate the preliminary rolled core, and pulling out the grain-oriented electromagnetic steel plate until the rolling start tip of the grain-oriented electromagnetic steel plate is within the large ring, a step of hooking the opening of the grain-oriented electromagnetic steel plate to the hook piece of the insulation sheet; a step of withdrawing the roller from the large ring; a step of obtaining a core, including reducing the diameter of the large ring of the grain-oriented electromagnetic steel plate via a restoring force, and rolling the grain-oriented electromagnetic steel plate onto the insulation sheet wrapped around the straight portions of the coil; and a step of fixing the rolling end of the grain-oriented electromagnetic steel plate to a peripheral surface of the core.

9. The method of manufacturing a transformer according to claim 8 wherein the hook piece has a protruded shape extending in the direction opposite to the rolling direction of the core.

10. The method of manufacturing a transformer according to claim 9 wherein the step of producing the preliminary rolled core involves forming the rolling start tip in a tapered trapezoidal shape simultaneously with punching out the grain-oriented electromagnetic steel plate with the opening at the rolling start tip, during the stamping process.

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