Manufacturing method of stator core using split core

Abstract

To perform annealing on a structure 3 in which split cores 1 arranged in an annular shape are held by an outer cylindrical ring 2 using a shrink fitting method without performing annealing on the split cores themselves, thereby alleviating stress within the split cores with fewer annealing processes, when the outer cylindrical ring holds the split cores arranged in a ring shape therein in a manufacturing method of a stator core for a rotating electric machine.SOLUTION: A manufacturing method for a stator core for a rotating electric machine, which is formed by an outer cylindrical ring holding a plurality of roughly fan-shaped split cores arranged in a ring shape therein includes: stacking and joining pieces of material formed by pressing an electromagnetic steel sheet to form the split cores; then arranging the plurality of split cores which have not been annealed, in a ring shape inside the outer cylindrical ring which has been heated and expanded in diameter by thermal expansion, cooling the outer cylindrical ring, and performing shrink fitting to form a core assembly in which the split cores are held in a ring shape by the outer cylindrical ring; and then annealing the core assembly.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a stator for a rotating electric machine, and more particularly to a method for manufacturing a stator core by arranging split cores in an annular shape. [Background technology]

[0002] Stator cores for rotating electrical machines are made of electromagnetic steel sheets, and residual stress in the electromagnetic steel sheets increases iron loss and reduces energy efficiency, so various techniques have been proposed to reduce residual stress in the electromagnetic steel sheets as much as possible after they have been formed into the shape of a stator core. For example, Patent Document 1 proposes that, in order to improve the iron loss characteristics of a core formed by laminating non-oriented electromagnetic steel sheets, the core be annealed at a temperature of 800°C or higher, and that the annealing atmosphere be a non-oxidizing or reducing atmosphere with an oxygen or moisture concentration of less than 1%, or a PH2O / PH2 of 0.1 or less. Patent Document 2 proposes a method of laminating steel sheets punched with a die or the like from slits cut into strips from an original sheet, temporarily fixing the laminated steel sheets with a simple jig, and annealing the laminated steel sheets at 700°C or higher to remove distortion caused by the punching process and restore their magnetic properties. Patent Document 3 proposes a method of punching a strip-shaped steel sheet from which multiple divided core sheets can be obtained, leaving some of the outlines of the divided core sheets intact as connecting portions while separating the other outlines from the surrounding area. After annealing the strip-shaped steel sheet in this state, the connecting portions are cut to separate the individual divided core sheets from the strip-shaped steel sheet. The separated divided core sheets are stacked to form divided cores, joined together, and then a stator core is formed by combining the divided core sheets into an annular shape. Patent Document 4 proposes that, in order to eliminate iron loss due to crimping in laminated cores, expanded core pieces with dummy piece portions provided on the core pieces are aligned and stacked, placed on a jig for positioning, and then the stacked dummy piece portions are removed before joining the stacked core pieces to form a laminated core. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 11-332183 [Patent Document 2] Patent Publication No. 2005-287134 [Patent Document 3] Patent Publication No. 2009-38915 [Patent Document 4] Patent Publication No. 2015-149884 [Patent Document 5] Patent Publication No. 2001-95181 [Non-patent literature]

[0004] [Non-Patent Document 1] Nippon Steel Technical Report No. 401 2015 Scale behavior during hot rolling [Non-patent document 2] Japan Institute of Metals, Vol. 16, No. 11, 1977: Descaling of Steel Plates Summary of the Invention [Problem to be solved by the invention]

[0005] 1(A) to 1(D), one method for forming a stator core by arranging split cores (generally fan-shaped member pieces) in an annular arrangement is to arrange the split cores 1 in an annular arrangement inside an annular outer cylindrical ring 2 that has been heated and expanded by thermal expansion, and then cool the outer cylindrical ring in this state, thereby holding the split cores 1 in an annular arrangement by the contracted outer cylindrical ring 2. Regarding this method, in the past, when manufacturing split cores, steel sheet pieces that have been press-formed in a mold were generally stacked and then annealed to remove internal stress. However, when the shrink fitting method was subsequently used, the outer cylindrical ring contracted to tightly hold the split cores arranged inside it, compressing each of the split cores, which resulted in new internal stress being generated within the cores. In order to remove this newly generated internal stress within the core, it is necessary to anneal the outer cylindrical ring holding the annularly arranged split cores again. In this case, the annealing process must be performed at least twice, making the manufacturing process inefficient.

[0006] In view of the above circumstances, the main object of the present invention is to alleviate residual stress within the stator core with fewer annealing processes by not performing annealing on the split cores themselves, but rather performing annealing on the structure in which the split cores, arranged in an annular shape by shrink fitting, are held inside the outer cylindrical ring by the shrink fitting method as described above, thereby reducing the stress remaining within the stator core with fewer annealing processes. [Means for solving the problem]

[0007] According to the present invention, the above-mentioned object is achieved by a method for manufacturing a stator core for a rotating electric machine, which is formed by holding a plurality of substantially fan-shaped divided cores annularly arranged inside an outer cylindrical ring, the method comprising: a split core forming step of laminating and joining member pieces formed by pressing electromagnetic steel sheets to form the split core; a shrink fitting process in which the split cores, which have not been annealed after being formed in the split core forming process, are arranged annularly inside the outer cylindrical ring, which has been heated and expanded in diameter by thermal expansion, and the outer cylindrical ring is cooled to hold the split cores in an annular arrangement by the outer cylindrical ring, thereby forming a core assembly; an annealing step of subjecting the core assembly to an annealing treatment; A method including This is achieved by:

[0008] In the above configuration, the "rotating electric machine" may be any type of rotating electric machine (generator, electric motor, motor-generator) that uses a stator wound with a coil. The "split core" is a member having a roughly sector-shaped yoke portion and teeth portion, which is formed by dividing an annular stator core into multiple parts along the radial direction. As described above, it is formed by stacking and joining pieces formed by pressing electromagnetic steel sheets. The "electromagnetic steel sheet" may be a steel sheet of a soft magnetic material commonly used for stator cores in this field. The joining may be achieved by any method that allows annealing, such as crimping (see Patent Document 5). The "external cylindrical ring" is a metallic material that can hold multiple split cores in an annular arrangement by shrink fitting. The "external cylindrical ring" is a member formed into an annular shape from a strip-shaped member (typically about 2 mm thick) made of a metallic material whose linear thermal expansion coefficient is equal to or greater than that of electromagnetic steel sheets (typically 11 to 13). The "annealing treatment" may be carried out in a manner that is carried out to remove stress within the split core made of magnetic steel sheets.

[0009] As already mentioned, in the past, split cores formed by stacking and joining member pieces formed by pressing electromagnetic steel sheets were annealed before being held in an outer casing ring, whereas in the present invention, after forming the split cores by stacking and joining member pieces, the split cores are not annealed, but a core assembly is formed by shrink fitting in which multiple split cores are held in an annular array by an outer casing ring, and only then is this core assembly annealed to relieve stress within the split cores in the core assembly. With this configuration, in the process of forming a stator core from electromagnetic steel sheets, annealing to relieve stress within the stator core only needs to be performed once, thereby improving the efficiency of the stator core manufacturing process.

[0010] In the method of the present invention, the outer ring may be formed from any metallic material capable of holding the multiple split cores in an annular arrangement as described above. Specifically, elongated steel sheets, such as hot-rolled steel sheets or cold-rolled steel sheets, are advantageous in terms of their low cost. In this regard, depending on the type of material for the outer ring, an oxide film (black scale) may be easily formed on the surface of the outer ring under the temperature conditions during the annealing treatment in the presence of oxygen (see Non-Patent Documents 1 and 2). Such black scale on the outer ring may fall into the gap between the rotor and the stator during operation of the electric machine, causing rotor rotation problems. Therefore, it is preferable to prevent the formation of black scale during the annealing treatment. Therefore, in the method of the present invention, the annealing step of exposing the core assembly to a temperature at which an oxide film is easily formed on the surface of the outer ring may be preferably performed in an inert gas atmosphere. Examples of the inert gas include nitrogen gas, argon gas, and the like. The temperature at which an oxide film is likely to form on the surface of the outer tubular ring (the transformation point of the material of the outer tubular ring) is, for example, in the range of 350 to 570°C for hot-rolled steel sheet or cold-rolled steel sheet. Therefore, when the core assembly is exposed to a temperature range of such transformation point in the above-mentioned annealing treatment, it is preferable that an inert gas is introduced into the space (annealing furnace) in which the core assembly is placed, and the air in the space is replaced with the inert gas.

[0011] In the method of the present invention, the outer ring may be made of a metal material that does not form an oxide film, such as stainless steel or titanium, in addition to hot-rolled or cold-rolled steel. In this case, the replacement with an inert gas during the annealing process is unnecessary, which is advantageous. The outer ring may also be made of a non-magnetic metal material (austenite stainless steel, titanium), which is advantageous in that magnetic flux does not leak to the outer ring during operation of the electric machine, thereby reducing losses due to eddy currents. [Effects of the Invention]

[0012] Thus, according to the configuration of the present invention, when a stator core for a rotating electric machine is formed by using a shrink fitting method to hold split cores arranged in a ring shape inside an outer cylindrical ring, annealing is not performed after the split cores are formed and before shrink fitting. Instead, annealing is performed only after the split cores are surrounded by the outer cylindrical ring using the shrink fitting method to form a core assembly.As a result, stress relief within the core assembly consisting of the split cores fitted with the outer cylindrical ring can be achieved with fewer annealing treatments, thereby improving energy efficiency and cost efficiency in the manufacture of stator cores.

[0013] Other objects and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention. [Brief explanation of the drawings]

[0014] [Figure 1] 1(A) and 1(B) are schematic diagrams of a split core and a stator core to which this embodiment is applied. (A) is a perspective view of the split core, (B) is a plan view of a state in which multiple split cores are arranged in an annular shape inside an outer cylindrical ring whose diameter has expanded due to thermal expansion, (C) is a plan view of a state in which the outer cylindrical ring has shrunk and is holding the multiple split cores arranged in an annular shape, and (D) is a perspective view of the stator core (core assembly). [Figure 2]FIG. 2(A) is a schematic diagram of a furnace illustrating each step in the annealing process in the method of this embodiment, and FIG. 2(B) is a diagram showing an example of the change in temperature of the core assembly over time in the annealing process in the method of this embodiment. [Explanation of symbols]

[0015] 1... divided core, 2... outer cylindrical ring, 3... core assembly, 10... annealing furnace, 12... shutter BEST MODE FOR CARRYING OUT THE INVENTION

[0016] The present invention will now be described in detail with reference to some preferred embodiments thereof with reference to the accompanying drawings, in which like reference numerals indicate like parts.

[0017] Overview of the stator core manufacturing process As shown in FIG. 1 , a stator core for a rotating electric machine manufactured using the method of this embodiment is formed by arranging a number of approximately sector-shaped split cores 1 in an annular arrangement and surrounding and holding the outer periphery of each split core 1 with an outer cylindrical ring 2. The split cores 1 are formed by stamping a plurality of pieces of material from electromagnetic steel sheets into the planar shape of the split cores 1 using press forming, stacking the pieces, and joining them using a joining process that can withstand an annealing process, such as crimping. Conventionally, the split cores 1 are annealed after the stacking or joining process. However, in this embodiment, the split cores 1 are not annealed, but instead, a core assembly 3 is formed by shrink fitting the multiple split cores 1 in an annular arrangement inside the outer cylindrical ring 2, and then the core assembly 3 is annealed. This annealing simultaneously relieves stresses that remained in the split cores 1 during their formation and stresses that occurred in the split cores 1 due to the shrink fitting process with the outer cylindrical ring 2, thereby achieving improved iron loss.

[0018] Selection of outer ring In the method of this embodiment, the outer cylindrical ring 2 may be any annular member made of a metal material capable of surrounding and holding the outer peripheries of multiple split cores arranged in an annular arrangement by shrink fitting. In the shrink fitting method, as shown in Figures 1(B) and 1(C), the outer cylindrical ring 2 is heated (T↑) and expanded by thermal expansion. Multiple split cores 1 are arranged in an annular arrangement inside the expanded outer cylindrical ring 2. In this state, the outer cylindrical ring 2 contracts when cooled (T↓), clamping the annularly arranged split cores 1 from their outer peripheries and holding them in place. Therefore, the linear thermal expansion coefficient of the outer cylindrical ring 2 must be equal to or greater than the linear thermal expansion coefficient of the electromagnetic steel sheet forming the split cores, so that its inner diameter is larger than the outer periphery of the split cores 1 in a thermally expanded state and smaller than the outer periphery of the split cores 1 at room temperature. Typically, the linear thermal expansion coefficient of an electromagnetic steel sheet is 11 to 13, so the outer cylindrical ring 2 is formed from a metal material with a linear thermal expansion coefficient of about 12 to 15. The outer cylindrical ring 2 may be formed by cutting a strip of material from a metal plate with a thickness of about 2 mm and bending it into a ring shape.

[0019] Specifically, the material of the outer cylindrical ring 2 may be a drawn steel plate such as a hot-rolled steel plate or a cold-rolled steel plate, or a non-magnetic metal material such as stainless steel or titanium.

[0020] In this regard, hot-rolled steel sheets and cold-rolled steel sheets generally have the advantage of being relatively inexpensive. In particular, hot-rolled steel sheets have the advantage of being even cheaper than cold-rolled steel sheets, and cold-rolled steel sheets have the advantage of being stronger than hot-rolled steel sheets. However, when hot-rolled steel sheets and cold-rolled steel sheets are exposed to transformation temperatures of 350 to 570°C in the presence of oxygen, an oxide film (black scale) is easily formed on the surface. Therefore, when a hot-rolled steel sheet or cold-rolled steel sheet is used for the outer casing ring 2, black scale is formed on the surface of the outer casing ring 2 when the annealing process is performed in air, raising the temperature to nearly 800°C. Such black scale on the outer casing ring 2 may fall into the gap between the rotor and the stator during operation of the rotating electric machine, causing poor rotation of the rotor. Therefore, in this embodiment, when a hot-rolled steel plate or a cold-rolled steel plate is used for the outer tubular ring 2, in the annealing process, when the temperature of the core assembly 3 passes through the transformation point of the hot-rolled steel plate or the cold-rolled steel plate, an inert gas such as nitrogen gas or argon gas may be fed into the furnace in which the core assembly 3 is placed, to replace the air in the furnace with the inert gas (to drive out oxygen), thereby preventing oxidation of the surface of the outer tubular ring 2 of the core assembly 3.

[0021] On the other hand, if a non-magnetic metal material such as stainless steel (austenitic) or titanium is used for the outer cylindrical ring 2, magnetic flux does not leak to the outer cylindrical ring 2 during operation of the electric machine, so eddy currents do not occur in the outer cylindrical ring 2, which is advantageous in that iron loss is reduced accordingly. Furthermore, in the case of stainless steel or titanium, even if annealing is performed in air, black scale does not form on the surface of the outer cylindrical ring, so there is also the advantage that the above-mentioned inert gas substitution is not necessary. However, stainless steel, titanium, etc. are more expensive than hot-rolled steel sheet or cold-rolled steel sheet.

[0022] Annealing treatment In this embodiment, as already mentioned, the annealing treatment is first performed after the split cores 1 are formed on the core assembly 3 in which the outer cylindrical ring 2 is fitted onto the outer periphery of the split cores 1 arranged in an annular shape by the shrink fitting method.

[0023] The annealing process is performed by placing the core assembly 3 on a belt conveyor and passing it through a plurality of furnaces 10, each set at a different temperature, as shown schematically in FIG. 2(A), thereby raising and lowering the temperature of the core assembly 3. The temperatures indicated for each furnace (i) to (vii) in the diagram are examples of the temperatures maintained in each furnace. FIG. 2(B) schematically shows the time that the core assembly 3 is placed in each furnace and the temperature of the core assembly 3 at that time. Shutters 12 that close the interior of each furnace 10 may be provided at the front and rear of each furnace 10, and may be opened when the core assembly 3 enters or leaves the furnace, and closed when the core assembly 3 passes through the furnace.

[0024] 2(A) and 2(B), specifically, in the annealing process, first, in the first furnace (i), the core assembly 3 is heated from room temperature to approximately 400°C as a preheating treatment, to remove water vapor from the surface of the electromagnetic steel sheets and within the furnace, which can cause rust. Next, in furnace (ii), the core assembly 3 is maintained at approximately 400°C to remove press oil from the electromagnetic steel sheets (if the oil remains and carbonizes, it will cause magnetic flux conduction and generate iron loss). In addition, in this furnace, the air inside the furnace may be replaced with an inert gas NAG by introducing it to dry the electromagnetic steel sheets to prevent rust and to prevent the formation of black scale on the outer casing ring when the outer casing ring is made of a hot-rolled or cold-rolled steel sheet. Next, in the furnace (iii), the core assembly 3 is heated to the annealing temperature (in this furnace too, the air inside the furnace may be replaced with the inert gas NAG by feeding in the inert gas NAG), and in the furnace (iv), the core assembly 3 is held at the annealing temperature for a while (for example, about 60 minutes) for annealing, i.e., for recrystallization of the electrical steel sheets.

[0025] After annealing is completed, the core assembly 3 is transferred to a furnace (v) where cooling treatment is initiated. In this furnace (v) and the subsequent furnaces (vi) and (vii), if the outer casing ring is made of a hot-rolled steel sheet or a cold-rolled steel sheet, the temperature of the core assembly 3 passes through the transformation temperature range of the hot-rolled steel sheet or the cold-rolled steel sheet, so in order to prevent the formation of black scale on the outer casing ring, inert gas NAG is introduced to replace the air in the furnace with inert gas NAG. Next, in furnace (vi), the core assembly 3 is maintained at about 470°C for a certain period of time to dry the magnetic steel sheets to prevent condensation on them due to moisture that flows in from outside the furnace when the furnace shutter is finally opened and closed, and to anneal the outer casing ring if it is made of hot-rolled steel sheet or cold-rolled steel sheet, and then in furnace (vii), the temperature of the core assembly 3 is cooled to 200°C (when cooled to 200°C, no condensation will form on the core assembly 3, so the supply of inert gas may continue until then). From 200°C, the core assembly 3 may be cooled by natural cooling outside the furnace.

[0026] Thus, the above-mentioned single annealing treatment can relieve the stress that remained in the split core 1 when the split core 1 was formed and the stress that occurred in the split core 1 due to the shrink fitting treatment with the outer cylindrical ring 2. The cooling speed of the core assembly 3 may be increased by increasing the amount of cooling air in the furnace (v).

[0027] In the above-described stator core configuration, the number of divided cores arranged annularly may be any number, such as 2 to 24. The method of the present embodiment may be applied to the manufacture of stator cores not only for synchronous machines but also for induction machines and switched reluctance motors.

[0028] The above description has been made in relation to the embodiments of the present invention, but it will be apparent that many modifications and changes will be readily apparent to those skilled in the art, and the present invention is not limited to the above-described exemplary embodiments, but can be applied to various devices without departing from the concept of the present invention.

Claims

1. A method for manufacturing a stator core for a rotating electric machine, which is formed by holding a plurality of divided cores arranged annularly inside an outer cylindrical ring, the method comprising: a split core forming step of laminating pieces formed by pressing electromagnetic steel sheets to form the split cores; a shrink fitting process in which a plurality of the split cores that have not been subjected to annealing treatment are arranged annularly inside the outer cylindrical ring that has been heated and expanded in diameter by thermal expansion, and the outer cylindrical ring is cooled to hold the split cores in an annular arrangement state by the outer cylindrical ring, thereby forming a core assembly; an annealing step of subjecting the core assembly to an annealing treatment; A method comprising:

2. 2. The method according to claim 1, wherein the annealing step of exposing the core assembly to a temperature at which an oxide film is likely to form on the surface of the outer cylindrical ring is carried out in an inert gas atmosphere.

3. 3. The method of claim 2, wherein said outer ring is formed from hot rolled steel.

4. 3. The method of claim 2, wherein said outer ring is formed from cold rolled steel.

5. 2. The method of claim 1, wherein said outer ring is formed from a non-magnetic metallic material.

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