Wound iron core and wound iron core transformer

Abstract

Provided is a wound iron core excellent in the suppression effect of temperature rise around a joint part.　The wound iron core has a winding shape in which a plurality of grain-oriented electrical steel sheets are wound and stacked in a sheet thickness direction, and has a flat surface part and a corner part adjacent to the flat surface part. At least a part of the flat surface part has the joint part in which end surfaces in a winding direction of each grain-oriented electrical steel sheet are butted against each other. A gap length of the joint part is 0.10 mm or more, and an average height Rc of the end surface is 50 μm or less.

Description

Wound cores and wound core transformers

[0001] The present invention relates to wound cores and wound core transformers.

[0002] Grain-oriented electrical steel sheets, which have a crystalline structure in which the <001> orientation, the axis of easy magnetization of iron, is highly concentrated in the rolling direction of the steel sheet, are used for transformer cores. Transformers are broadly classified into stacked core transformers and wound core transformers based on their core structure. Stacked core transformers have a core made of stacked steel sheets cut to a specified shape. On the other hand, wound core transformers have a core made of wound steel sheets. There are various characteristics required for transformer cores, but low iron loss is particularly important.

[0003] Low iron loss is also an important characteristic required of grain-oriented electrical steel sheets, which are used as core materials. Furthermore, in order to reduce the excitation current in transformers and thereby reduce copper loss, the grain-oriented electrical steel sheets used as materials must also have a high magnetic flux density. Generally, the higher the degree of orientation concentration in the Goss orientation, the higher the magnetic flux density of the grain-oriented electrical steel sheets. Grain-oriented electrical steel sheets with high magnetic flux density have low hysteresis loss and excellent iron loss characteristics. In order to reduce the iron loss of grain-oriented electrical steel sheets, it is effective to not only increase the degree of orientation concentration in the Goss orientation, but also to reduce impurities in the steel.

[0004] However, there are limits to controlling crystal orientation and reducing impurities. Therefore, technologies have been developed that introduce nonuniformity into the surface of grain-oriented electrical steel sheets using physical methods to refine magnetic domains and reduce iron loss, i.e., magnetic domain refinement technologies. For example, Patent Documents 1 and 2 describe heat-resistant magnetic domain refinement techniques in which linear grooves of a predetermined depth are formed on the surface of the steel sheet. Patent Document 1 describes a groove formation method using a gear-type roll. Patent Document 2 also describes a groove formation method using an etching process. These techniques have the advantage that the magnetic domain refinement effect is not lost even when heat treatment is performed, and they can also be applied when stress relief annealing is performed during iron core production.

[0005] In wound cores with joints, the use of grain-oriented electrical steel sheets with reduced iron loss achieved through the physical techniques described above significantly reduces iron loss in areas other than the joints. However, due to the structure of the wound core, iron loss at the joints is known to be very high, regardless of the properties of the grain-oriented electrical steel sheets used. This is because the magnetic resistance of the gaps formed at the joints is so high that magnetic flux does not pass through the gaps but instead passes to the adjacent steel sheets in the out-of-plane direction, resulting in in-plane eddy current loss. The large iron loss at the joints is converted into thermal energy, causing a temperature rise around the joints. This temperature rise around the joints can lead to a deterioration in the insulating properties of the insulating coating applied to the surface of the grain-oriented electrical steel sheets, ultimately leading to dielectric breakdown. Dielectric breakdown can lead to burnout of the transformer core, so it is essential to ensure that this temperature rise does not cause a decrease in insulation.

[0006] One method for suppressing temperature rise around the joints is to improve the cooling capacity of the iron core. Patent Document 3 proposes a method in which one or more of the wound electromagnetic steel sheets are wound so that they are offset in the width direction perpendicular to the longitudinal direction relative to the electromagnetic steel sheets forming the other layers over the entire longitudinal length, thereby increasing the contact area with the oil or air cooling medium. Another method for suppressing temperature rise around the joints is to suppress iron loss increases at the joints. Patent Document 4 proposes a method in which the positions of the joints (cut portions) are dispersed in the winding direction to reduce the magnetic flux density around the joints and suppress iron loss increases. While these methods can suppress temperature rise around the joints to some extent, further suppression of temperature rise is required to increase design freedom in terms of magnetic flux density, iron core shape, etc.

[0007] Japanese Patent Publication No. 62-53579 Japanese Patent No. 2895670 International Publication No. 2022 / 092101 International Publication No. 2016 / 006314

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a wound core that is excellent in the effect of suppressing temperature rise around the joint.

[0009] The temperature rise around the joints of wound cores is caused by the extremely large magnetic resistance of the gaps formed at the joints, which causes magnetic flux to pass through the gaps and into adjacent steel sheets in the out-of-plane direction, resulting in in-plane eddy current loss. Eliminating the gaps at the joints is a very effective way to suppress this in-plane eddy current loss. However, because there are errors in the processing accuracy during core manufacturing and the accuracy of repeated winding, eliminating the gaps at the joints is difficult in practice when productivity is taken into consideration.

[0010] Therefore, we investigated ways to improve the cooling capacity of the iron core by actively passing insulating oil, which also serves as a cooling medium, through the gap at the joint.

[0011] Using an AEM Unicore manufacturing machine, wound cores were fabricated. Each core had two 45-degree bends at each corner, measuring 250 mm long x 250 mm wide x 100 mm deep, and weighed approximately 20 kg. The core material was a 0.23 mm thick grain-oriented electrical steel sheet with a magnetic flux density B8 of 1.91 T and iron loss W17 / 50 of 0.88 W / kg. Here, B8 is the magnetic flux density at a magnetizing force of 800 A / m, and W17 / 50 is the iron loss at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T. The wound cores were joined using a step-lap method, with only the gap length at the joint varied between 0.01 and 20 mm. The number of laminated layers was 200, and the primary and secondary coils had 40 turns. The excitation conditions were a frequency of 50 Hz and a magnetic flux density of 1.7 T.

[0012] The configuration of the wound core that is the subject of the present invention will now be described with reference to FIG. 9 . As shown in FIG. 9 , the wound core 1 is formed by winding multiple grain-oriented electromagnetic steel sheets M in the sheet thickness direction. When viewed from the side, the wound core 1 has a wound shape. Note that, here, "side view" refers to viewing the wound core 1 in the width direction of the grain-oriented electromagnetic steel sheets M that make up the wound core 1. The wound core 1 has a joint C where end faces E of the grain-oriented electromagnetic steel sheets M in the winding direction (lengthwise direction of the grain-oriented electromagnetic steel sheets M) are butted together. A gap G is formed in the joint C. In the present invention, the length (spacing) L of the gap G formed in the joint C is referred to as the gap length of the joint.

[0013] Next, the wound core prepared above was placed in the tank of an insulating oil circulation system as shown in Figure 1, and the temperature of the insulating oil flowing into the tank and the temperature of the insulating oil discharged from the tank were measured when the insulating oil was excited under the above excitation conditions. ２ The insulating oil circulation system shown in Figure 1 includes a wound core 1 and insulating oil 2. The insulating oil circulation system simulates a wound core transformer equipped with the wound core 1 and insulating oil 2.

[0014] Figure 2 shows the relationship between the gap length at the joint in the wound core and the temperature difference between the insulating oil flowing into the tank and the insulating oil discharged from the tank (hereinafter referred to as the inflow / outflow oil temperature difference). It was observed that the inflow / outflow oil temperature difference increases as the gap length at the joint increases. Figure 3 shows the relationship between the gap length at the joint and the iron loss of the wound core transformer when excited under the above excitation conditions. It was observed that the iron loss increases as the gap length at the joint increases. The increase in iron loss was particularly noticeable when the gap length at the joint was 10 mm or more.

[0015] Given the increase in iron loss, the following two reasons are thought to be the cause of the increase in the inflow / outflow oil temperature difference: (1) As the gap length of the joint increases, iron loss at the joint increases, and this thermal energy is transmitted to the insulating oil. (2) As the gap length of the joint increases, it becomes easier for the insulating oil to pass through the gap at the joint, and the thermal energy around the joint is more easily transmitted to the insulating oil (improved cooling capacity). Here, taking cause (2) into consideration, we decided to explore a method for efficiently passing the insulating oil through the gap at the joint.

[0016] To facilitate the passage of air or liquid through pipes, it is important to minimize energy loss (pressure loss). Therefore, we considered that the unevenness of the end faces of grain-oriented electrical steel sheets (hereinafter simply referred to as steel sheet end faces), which form the wall surfaces of the gaps at the joints, could have an effect. Therefore, we flattened the steel sheet end faces and investigated the change in the inflow / outflow oil temperature difference. Here, we performed wire cutting to flatten the steel sheet end faces. Generally, wire cutting is more likely to produce a flat surface than conventional shear cutting. Using the grain-oriented electrical steel sheets described above, we fabricated wound cores using the aforementioned method after processing the end faces with both wire cutting and shear cutting. We then placed the fabricated wound cores in the tank of the insulating oil circulation system described above, and investigated the relationship between the gap length at the joints and the inflow / outflow oil temperature difference when the cores were excited under the aforementioned excitation conditions.

[0017] The relationship between the gap length at the joint and the temperature difference between the inflow and outflow oil for each processing method is shown in Figure 4. With wound cores made from grain-oriented electrical steel sheets processed by wire cutting, the temperature difference between the inflow and outflow oil increased, particularly when the gap length at the joint was 0.10 mm or more, compared to wound cores made from grain-oriented electrical steel sheets processed by conventional shear cutting.

[0018] The reason for the increase in the inflow / outflow oil temperature difference is thought to be that the wire cutting process reduced the unevenness of the steel plate end surface, thereby reducing pressure loss and making it easier for the insulating oil to pass through the gap at the joint, thereby improving the cooling capacity of the wound iron core.

[0019] Therefore, in order to improve the cooling capacity of the wound core, it is sufficient to reduce the unevenness of the steel sheet end surfaces that form the wall surfaces of the gap at the joint and set the gap length at the joint to 0.10 mm or more. Furthermore, when considering both the iron loss characteristics and cooling capacity of the wound core, it is considered more preferable that the gap length at the joint be 0.10 to 10 mm.

[0020] Next, to clarify the relationship between the roughness of the steel sheet end surface and the inflow / outflow oil temperature difference, the roughness of the steel sheet end surface was varied. Here, the roughness of the steel sheet end surface was varied by adjusting the roughness of the emery paper used when polishing the steel sheet end surface, as well as the load and time during polishing. Using the aforementioned grain-oriented electrical steel sheet as the raw material, the roughness of the steel sheet end surface was varied by polishing with emery paper, and then a wound core was fabricated using the aforementioned method. The fabricated wound core was then placed in the tank of the aforementioned insulating oil circulation system, and the relationship between the roughness of the steel sheet end surface and the inflow / outflow oil temperature difference was investigated when excited under the aforementioned excitation conditions. The gap length at the joint of the wound core was 0.10 mm. The roughness of the steel sheet end surface was measured in the area of ​​the sheet thickness × total width using a 3D laser roughness meter, and the average height Rc and arithmetic mean roughness Ra were calculated, respectively. In the present invention, the average height Rc and arithmetic mean roughness Ra are values ​​measured in accordance with JIS B 0601:2013.

[0021] Figure 5 shows the relationship between the average height Rc of the steel sheet end face and the inflow / outflow oil temperature difference. When the average height Rc of the steel sheet end face is 50 μm or less, the inflow / outflow oil temperature difference increases significantly, improving cooling performance. Figure 6 shows the relationship between the arithmetic mean roughness Ra of the steel sheet end face and the inflow / outflow oil temperature difference. No correlation was observed between the arithmetic mean roughness Ra of the steel sheet end face and the inflow / outflow oil temperature difference. These results demonstrate the importance of controlling the average height Rc of the steel sheet end face. While the reason for the observed correlation between the average height Rc of the steel sheet end face and the cooling performance of the wound core and the arithmetic mean roughness Ra of the steel sheet end face is not clear, it is presumed as follows: In other words, the arithmetic mean roughness Ra is the average of the absolute values ​​of the heights of each concave and convex portion from the reference plane, and is less susceptible to the influence of scratches on the measurement surface or external deposits attached during core manufacturing. On the other hand, since the average height Rc uses the average height of each pair of adjacent concave and convex portions, it is possible to sensitively reflect their influence. Because pressure loss is also affected by such defects and external deposits, it is believed that a correlation with the cooling capacity of the wound core was found only in the average height Rc of the steel sheet end face.

[0022] It was thought that the kinematic viscosity of the insulating oil also had an effect on the reduction of pressure loss. A wound core was manufactured using the above-mentioned grain-oriented electrical steel sheet as the material and by the above-mentioned method. The manufactured wound core was then placed in the tank of the above-mentioned oil circulation system and excited under the above-mentioned excitation conditions. Here, the kinematic viscosity of the insulating oil at 40°C was set to 1 to 100 mm ２ The relationship between the kinematic viscosity of the insulating oil and the temperature difference between the inlet and outlet oil was investigated by changing the oil temperature within the range of 0.10 mm / sec. The gap length at the joint and the average height Rc of the steel plate end faces were set to 0.10 mm and 50 μm, respectively, which are the conditions that result in the greatest pressure loss among the conditions for which the effectiveness has been confirmed.

[0023] Figure 7 shows the relationship between the kinematic viscosity of insulating oil and the temperature difference between the inlet and outlet oil. ２ / sec or less, the inflow / outflow oil temperature difference became larger. ２ It can be said that a higher cooling capacity can be obtained in a wound core transformer using insulating oil of 1000 kJ / sec or less. The kinematic viscosity of the insulating oil is preferably 10 mm / sec or less, at which the inflow / outflow oil temperature difference is saturated. ２ / sec or less.

[0024] There are two methods for manufacturing wound cores: one is to wind steel sheets and then press the corners to a predetermined curvature to form a rectangular shape, and the other is to form the corners by stacking steel sheets that have been bent in advance. Cores manufactured using the former method are called Trancocores, and those manufactured using the latter method are called Unicores or Duocores. Figure 10 shows schematic diagrams of Unicores and Trancocores. Here, we compared the core cooling capacity of wound core transformers using Unicores and Trancocores.

[0025] For the Unicore, an AEM Unicore manufacturing machine was used to produce two Unicores with two 45-degree bends at each corner, measuring 500 mm long x 500 mm wide x 100 mm deep, and a total weight of approximately 100 kg. One of the Unicores was subjected to stress relief annealing at 825 °C for 3 hours in a DX gas atmosphere, while the other was not. For the Trancocore, the Unicore was wound around a mandrel to a size of 500 mm long x 500 mm wide x 100 mm deep, a total weight of approximately 100 kg, and then pressed and stress relief annealed under the same conditions as the Unicore that had undergone stress relief annealing. The material used was a 0.23 mm thick, heat-resistant, magnetic domain refined grain-oriented electrical steel sheet with properties of B8 = 1.93 T and W17 / 50 = 0.75 W / kg. The joining method for the wound core was step lap, and the gap length of the joining portion was changed in the range of 0.01 to 15 mm. The processing conditions were adjusted so that the average height Rc of the steel sheet end face was 30 μm. The number of laminated sheets was 500, and the number of turns of the primary and secondary coils was 40 turns. The excitation conditions were a frequency of 60 Hz and a magnetic flux density of 1.7 T. As shown in Figure 1, the kinematic viscosity at 40°C was 5 mm ２ The wound core was placed in the tank of an oil circulation system using insulating oil of 1 / 2 sq. m / s, and the oil temperature flowing into the tank and the oil temperature discharged from the tank were measured when the system was excited under the above excitation conditions.

[0026] Figure 8 shows the relationship between the gap length at the joint and the inflow / outflow oil temperature difference for each wound core. The Unicore had a larger inflow / outflow oil temperature difference than the Trancocore. The Unicore without stress relief annealing also had a larger inflow / outflow oil temperature difference than the Trancocore with stress relief annealing. The reason for this is believed to be as follows: Because large localized strain is introduced into the Unicore, deformation twins remain even after stress relief annealing, resulting in a lower magnetic permeability in the winding direction than the Trancocore. This is thought to be due to an increase in the amount of magnetic flux passing through adjacent out-of-plane steel sheets around the joint, resulting in increased iron loss around the joint. When stress relief annealing is not performed, the magnetic permeability in the winding direction is lower, so the amount of magnetic flux passing through adjacent out-of-plane steel sheets is thought to be the highest of the three conditions, resulting in the largest inflow / outflow oil temperature difference. These results suggest that wound core transformers using Unicore or Duocore with bent corners can achieve higher cooling capacity.

[0027] The present invention is based on the above findings, and the gist of the present invention is as follows: [1] A wound core having a wound shape in which a plurality of grain-oriented electromagnetic steel sheets are wound and overlapped in the sheet thickness direction, having a flat portion and a corner portion adjacent to the flat portion, and having a joint in at least a part of the flat portion where end faces in the winding direction of the grain-oriented electromagnetic steel sheets are butted together, the gap length of the joint being 0.10 mm or more, and the average height Rc of the end faces being 50 μm or less. [2] The wound core according to [1], having a bent portion in the corner portion. [3] A wound core transformer comprising the wound core according to [1] or [2] and insulating oil. [4] The insulating oil has a kinetic viscosity at 40°C of 50 mm ２ / sec or less.

[0028] According to the present invention, it is possible to provide a wound core that is excellent in the effect of suppressing temperature rise around the joint.

[0029] According to the present invention, it is possible to effectively suppress temperature rises near the joints of the wound core, and to prevent deterioration of the insulating properties of the insulating coating due to temperature rises, thereby increasing the design freedom of the transformer, such as by increasing the design magnetic flux density or reducing its size.

[0030] FIG. 1 is a schematic diagram of the insulating oil circulation system used in the experiment. FIG. 2 is a graph showing the relationship between the gap length at the joint and the inflow / outflow oil temperature difference. FIG. 3 is a graph showing the relationship between the gap length at the joint and the transformer iron loss W17 / 50. FIG. 4 is a graph showing the relationship between the gap length at the joint and the inflow / outflow oil temperature difference for each wound core fabricated using grain-oriented electrical steel sheets processed by wire cutting and shear cutting. FIG. 5 is a graph showing the relationship between the average height Rc of the steel sheet end face and the inflow / outflow oil temperature difference. FIG. 6 is a graph showing the relationship between the arithmetic mean roughness Ra of the steel sheet end face and the inflow / outflow oil temperature difference. FIG. 7 is a graph showing the relationship between the kinematic viscosity of insulating oil at 40°C and the inflow / outflow oil temperature difference. FIG. 8 is a graph showing the relationship between the gap length at the joint and the inflow / outflow oil temperature difference when using Unicore and Trancocore. FIG. 9 is a schematic diagram explaining the configuration of a wound core. FIG. 10 is a schematic diagram (side view) showing Unicore and Trancocore. FIG. 11 is a schematic diagram showing the positions where thermocouples were attached (temperature measurement positions) in the example.

[0031] As described above, in order to effectively suppress the temperature rise around the iron core joint, the following requirements must be met.

[0032] <Wound core> [A] A wound shape in which multiple grain-oriented electromagnetic steel sheets are wound and overlapped in the sheet thickness direction, with a flat portion and a corner portion adjacent to the flat portion. [B] At least a part of the flat portion has a joint where the end faces of the grain-oriented electromagnetic steel sheets in the winding direction are butted together, with the gap length of the joint being 0.10 mm or more, more preferably 0.10 to 10 mm. [C] The average height Rc of the end faces is 50 μm or less, more preferably 10 μm or less.

[0033] The method for manufacturing the wound core is not particularly limited, and known methods can be used, for example. More specifically, Unicore and Duocore, which have bent corners, can be manufactured using a Unicore manufacturing machine manufactured by AEM. In this case, when a design drawing is input into the manufacturing machine, the steel sheet is sheared and bent to the size specified in the design drawing, and processed steel sheets are produced one by one. The wound core can be manufactured by winding and stacking these processed steel sheets (raw materials) in the thickness direction. After manufacturing the wound core, stress relief annealing may or may not be performed.

[0034] On the other hand, TrancoCore, which has curved corners, is produced by unloading a coil of directional electromagnetic steel sheet, cutting it into individual turns, winding it around a circular winding die, and winding it in the thickness direction. After that, forming dies are pressed against the inside and outside of the wound core material, and after forming it into the desired wound shape, stress relief annealing is performed.

[0035] In either manufacturing method, the method for controlling the gap length of the joint is not particularly limited. For example, when shearing the grain-oriented electrical steel sheet as the raw material for each turn, the gap length of the joint can be controlled within the range of the present invention by setting the length in the winding direction slightly shorter than the designed turn length. The gap length of the joint can also be controlled within the range of the present invention by adjusting the binding force or tightening force of the binding bands or tightening jigs that secure the wound grain-oriented electrical steel sheets together. As described above, in the present invention, the gap length of the joint is 0.10 mm or more. The gap length of the joint is preferably 0.50 mm or more. The gap length of the joint is preferably 20 mm or less, more preferably 15 mm or less, even more preferably 10 mm or less, and even more preferably 5 mm or less.

[0036] The method for controlling the average height Rc of the steel sheet end faces is not particularly limited. For example, in the case of shearing, which is a common processing method, the average height Rc of the steel sheet end faces can be controlled within the range of the present invention by adjusting the clearance, adjusting the surface roughness of the shear blade, or brush grinding after shearing. Furthermore, even in the case of processing using a laser beam, the average height Rc of the steel sheet end faces can be controlled within the range of the present invention by adjusting the thermal energy introduced into the steel sheet by changing the laser output or scanning speed. It is more preferable to incorporate a process for measuring the average height Rc of the steel sheet end faces into the wound core manufacturing process. As described above, in the present invention, the average height Rc of the steel sheet end faces is 50 μm or less. The average height Rc of the steel sheet end faces is preferably 25 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. On the other hand, the lower limit of the average height Rc of the steel sheet end faces is not particularly limited, but as an example, the average height Rc of the steel sheet end faces is 0.5 μm or more.

[0037] In the present invention, when manufacturing a wound core, as long as the gap length of the joint and the average height Rc of the steel sheet end faces are controlled within the ranges of the present invention, there are no particular limitations on the core structure, such as the core size and joining method. Furthermore, if the wound core has bent portions at the corners, there are no particular limitations on the core structure, such as the bending angle and number of bent portions at the corners.

[0038] As an example, the method for manufacturing a wound core of the present invention can include the steps of measuring the average height Rc of the end faces of grain-oriented electrical steel sheets, preparing as raw materials a plurality of grain-oriented electrical steel sheets whose average end face height Rc is within the range of the present invention, and winding and stacking the grain-oriented electrical steel sheets prepared as raw materials in the above steps to form a wound core. In the step of forming the wound core, the gap length of the joint is controlled within the range of the present invention, for example, as described above.

[0039] <Grain-oriented electrical steel sheet> In the present invention, the grain-oriented electrical steel sheet used as the material for the wound core is not particularly limited. The thickness of the grain-oriented electrical steel sheet may be selected appropriately depending on the application. The thickness may be, for example, 0.15 mm or more, or may be 0.18 mm or more. The thickness may be, for example, 0.35 mm or less, or may be 0.27 mm or less. The thickness may be, for example, in the range of 0.15 to 0.35 mm, or may be in the range of 0.18 to 0.27 mm.

[0040] The method for producing grain-oriented electrical steel sheets is also not particularly limited. For example, a slab having a known chemical composition is heated to 1150 to 1450°C, hot-rolled, and then, if necessary, hot-rolled sheet annealed at 900 to 1150°C. Subsequently, the slab is cold-rolled once or twice or more times with intermediate annealing, decarburization annealed at 700 to 900°C in a wet hydrogen atmosphere, and, if necessary, further nitriding is performed. An annealing separator is then applied, finish annealed at 1000 to 1200°C, and an insulating coating is formed at approximately 900°C. As an example, grain-oriented electrical steel sheets can be produced using the above method. Furthermore, grain-oriented electrical steel sheets can also be subjected to magnetic domain refinement treatment using distortions or grooves by known methods. When distortion relief annealing is performed during wound core production, groove-type magnetic domain refinement treatment is preferably performed.

[0041] <Wound Core Transformer> The wound core transformer of the present invention includes the wound core and insulating oil. As described above, the insulating oil has a kinematic viscosity of 50 mm at 40°C. ２ The insulating oil preferably has a kinematic viscosity of 10 mm / sec or less at 40°C. ２ The lower limit of the kinematic viscosity is not particularly limited, but as an example, the kinematic viscosity is 1 mm ２ / sec or more.

[0042] Next, the present invention will be specifically described based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to these examples. The embodiments of the present invention can be appropriately modified within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0043] Example 1: A wound core weighing approximately 40 kg and with a capacity of 30 kVA was fabricated using a 0.23 mm thick grain-oriented electrical steel sheet (heat-resistant, magnetic domain-refined material) with grooves formed on the surface and magnetic properties of B8 = 1.88 T and W17 / 50 = 0.68 W / kg. The wound core was fabricated as follows: The coil-shaped grain-oriented electrical steel sheet was unwound and wound onto a circular winding die, cutting each turn. The steel sheet was cut 0.03 to 13.00 mm shorter than the designed circumference length to vary the gap length at the joint. The cutting was performed using a laser. The laser processing conditions were a single-mode fiber laser with a deflection speed of 10 m / sec and a laser power output of 0.5 to 10.0 kW, thereby varying the average height (Rc) of the steel sheet end face. The steel sheet is then wound up, and the core material is pressed with a forming die on the inside and outside of the wound steel sheet in the thickness direction to form the final wound shape. ２ The wound cores were then subjected to stress relief annealing under gas atmosphere conditions to produce wound cores. Table 1 shows the difference between the designed circumference length and the actual cutting length, the gap length at the joint of each wound core, the laser power, and the average height Rc of the steel sheet end faces forming the joint of each wound core. The wound cores were then unwound and the coils were assembled. At the same time, a thermocouple was attached around the joint to measure the temperature around the joint. The thermocouple was attached at the center of the width of the center joint of the wound core (the joint at the center of the core thickness), 5 mm from the steel sheet end faces forming the joint (see Figure 11). After the coil was assembled, the wound cores were placed in a tank and insulating oil was poured into them to produce wound core transformers. The dynamic viscosity of the insulating oil was varied by using different insulating oils. Table 1 also shows the dynamic viscosity of the insulating oil, transformer loss, and the temperature around the joint. The transformers were excited at 50 Hz and 1.7 T. The transformer loss and the temperature around the joint were measured by monitoring the temperature of the insulating oil and after the temperature became constant (after thermal equilibrium).

[0044] In Nos. 1 to 12, the gap length at the joint was short and outside the range of the present invention, resulting in high temperatures around the joint under all conditions. The reason why the average height Rc of the steel sheet end face changed with laser power is thought to be because, as the power increased, the amount of melting at the laser-processed steel sheet end face increased, and the molten material scattered and re-adhered to the processed surface. For Nos. 13 to 24 and Nos. 25 to 36, it was found that within the range of the average height Rc of the steel sheet end face of the present invention, the temperature rise around the joint was suppressed compared to cases outside the range of the present invention. Furthermore, it was found that a lower average height Rc of the steel sheet end face and a lower kinematic viscosity of the insulating oil resulted in lower temperatures around the joint and higher cooling capacity. Furthermore, the increase in transformer loss due to an increase in the gap length at the joint was minimized, demonstrating the achievement of both transformer characteristics and cooling capacity. For Nos. 37 to 48, the gap length at the joint was large, resulting in lower transformer loss compared to Nos. 13 to 36. However, it can be seen that when the average height Rc of the steel sheet end face is within the range of the present invention, the temperature rise around the joint is sufficiently suppressed, and the object of the present invention is achieved.

[0045]

[0046] Example 2: A wound core with a weight of approximately 500 kg and a capacity of 500 kVA was fabricated using a 0.30 mm thick grain-oriented electrical steel sheet with magnetic properties of B8 = 1.93 T and W17 / 50 = 1.05 W / kg. The wound core was fabricated as follows: Using an AEM Unicore manufacturing machine, the steel sheet was sheared to the designed circumference length, the corners were pre-bent, and the wound core was then wound in the thickness direction to produce a wound core. Each corner of the wound core had two 45-degree bends. The average height Rc of the steel sheet end faces was also varied by changing the clearance during shearing. After the core was unwound and the coil was installed, cable ties were applied to secure the wound grain-oriented electrical steel sheet together. The tightening pressure of the cable ties was 0.01 to 1.00 kg / mm. ２The wire tension was changed within the specified range, and the gap length at the joint was varied. Furthermore, while the core was unwound and the coil was assembled, a thermocouple was attached around the joint, and the temperature around the joint was measured, as in Example 1. After the coil was assembled, the wound core was placed in a tank, and insulating oil was poured into it to form a wound-core transformer. The dynamic viscosity was varied by using different insulating oils. Table 2 shows the tightening pressure of the cable ties, the gap length at the joint, the clearance during shearing, the average height Rc of the steel sheet end faces, the dynamic viscosity of the insulating oil, the transformer loss, and the temperature around the joint. Here, the transformer was excited, and the transformer loss and the temperature around the joint were measured, as in Example 1.

[0047] In Nos. 37 to 48, the gap length at the joint was short and outside the range of the present invention, resulting in high temperatures around the joint under all conditions. For Nos. 13 to 24 and Nos. 25 to 36, when the average height Rc of the steel sheet end faces was within the range of the present invention, the temperature rise around the joint was suppressed compared to cases outside the range of the present invention. It was also found that a lower average height Rc of the steel sheet end faces and a lower kinematic viscosity of the insulating oil resulted in lower temperatures around the joint and higher cooling capacity. Furthermore, the increase in transformer loss due to an increase in the gap length at the joint was minimized, demonstrating that both transformer characteristics and cooling capacity were achieved. For Nos. 1 to 12, the gap length at the joint was long, resulting in lower transformer loss compared to Nos. 13 to 36. However, when the average height Rc of the steel sheet end faces was within the range of the present invention, the temperature rise around the joint was sufficiently suppressed, demonstrating that the objectives of the present invention were achieved.

[0048]

Claims

1. A wound core having a wound shape in which a plurality of grain-oriented electromagnetic steel sheets are wound and overlapped in the sheet thickness direction, the wound core having a flat portion and a corner portion adjacent to the flat portion, at least a part of the flat portion having a joint where the end faces in the winding direction of the grain-oriented electromagnetic steel sheets are butted against each other, the gap length of the joint being 0.10 mm or more, and the average height Rc of the end faces being 50 μm or less.

2. A wound core according to claim 1, wherein said corner portions have bent portions.

3. A wound core transformer comprising the wound core according to claim 1 or 2 and insulating oil.

4. The kinetic viscosity of the insulating oil at 40°C is 50 mm ２ 4. The wound core transformer according to claim 3, wherein the winding current is equal to or less than 1000 Hz / sec.

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