Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
  • H01F41/02—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets
  • H01F41/0206—Manufacturing of magnetic cores by mechanical means
  • H01F41/0233—Manufacturing of magnetic circuits made from sheets
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F27/00—Details of transformers or inductances, in general
  • H01F27/24—Magnetic cores
  • H01F27/245—Magnetic cores made from sheets, e.g. grain-oriented
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F27/00—Details of transformers or inductances, in general
  • H01F27/33—Arrangements for noise damping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
  • H01F41/02—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets

Definitions

• the present invention relates to a laminated core and a manufacturing method of a laminated core. This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2022-159532, filed on October 3, 2022 , the entire contents of which are incorporated herein by reference.
• Patent Literature 1 has disclosed that a damping steel sheet is partially interposed between a plurality of laminated electrical steel sheets.
• Patent Literature 2 has disclosed that after processed grooves are formed in the sheet surface of each electrical steel sheet, a plurality of electrical steel sheets are laminated so as to prevent the sheet surfaces with no processed grooves formed therein from overlapping each other. Further, Patent Literature 2 has disclosed that an adhesive resin is applied to end surfaces of a plurality of the laminated electrical steel sheets.
• the laminated core of the present invention includes: a plurality of blocks, each of the blocks including a plurality of laminated steel sheets, in which the plural blocks include block facing end surfaces, the block facing end surface is, of end surfaces of the block, an end surface that is located at a position facing the another block each other, at each of at least one pair of the block facing end surfaces, (A) Expression below is satisfied, and the pair of block facing end surfaces is the two block facing end surfaces that are arranged at positions facing each other. 1 ⁇ Ra D / Ra S ⁇ 12
• the number, shape, size, and arrangement of the blocks are determined according to the specifications of a device including the laminated core. Therefore, the number, shape, size, and arrangement of the blocks are not limited to those illustrated in Fig. 1 , as an example.
• the end surfaces of the plural blocks 110a to 110e include block facing end surfaces 111a to 111p.
• the block facing end surfaces 111a to 111p of the blocks 110a to 110e are end surfaces located at positions facing another block each other of the end surfaces of the blocks 110a to 110e.
• the end surface of the block 110a includes the block facing end surfaces 111a to 111d.
• the end surface of the block 110b includes the block facing end surfaces 111e to 111h.
• the end surface of the block 110c includes the block facing end surfaces 111i to 111j.
• the end surface of the block 110d includes the block facing end surfaces 111k to 111l.
• the end surface of the block 110e includes the block facing end surfaces 111o to 111p.
• the laminating-direction surface roughness Ra(D) is measured, at a pair of block facing end surfaces being a calculation target for (1) Expression, on a virtual straight line passing through the middle position between both ends of the block facing end surface when the block facing end surface is viewed from the laminating direction of the steel sheet having an end surface that forms part of the block facing end surface.
• the laminating direction of the steel sheets having end surfaces that form part of the block facing end surfaces 111a to 111p is the z-axis direction.
• the middle position of both ends of the block facing end surface is obtained at each of the steel sheets in the laminating direction (z-axis direction).
• virtual straight lines 201a to 201p are obtained, as illustrated in Fig. 2A to Fig. 2E .
• the representative value of the plural (200 in the previously-described example) laminating-direction surface roughnesses Ra(D) measured in the above manner is set as the laminating-direction surface roughness Ra(D) of the one block facing end surface.
• the representative value is, for example, an arithmetic mean value.
• the median value or the like may be used in place of the arithmetic mean value.
• the laminating-direction surface roughness Ra(D) of the other block facing end surface of the pair of block facing end surfaces being a calculation target for (1) Expression is also calculated in the same manner as the block facing end surface Ra(D) of the one block facing end surface.
• the laminating-direction surface roughnesses Ra(D) of one and the other of a pair of block facing end surfaces are calculated as above at each of the block facing end surfaces 111a and 111i, the block facing end surfaces 111b and 111l, the block facing end surfaces 111c and 111m, the block facing end surfaces 111d and 111n, the block facing end surfaces 111e and 111k, the block facing end surfaces 111f and 111j, the block facing end surfaces 111g and 111o, and the block facing end surfaces 111h and 111p.
• Fig. 3A to Fig. 3E are views each illustrating one example of a measurement position of a roughness curve for calculating the in-plane-direction surface roughness Ra(S).
• the x-y coordinates illustrated in Fig. 3A to Fig. 3E correspond to the x coordinate and the y coordinate of the x-y-z coordinates illustrated in Fig. 1 .
• Fig. 3A and Fig. 3D are views illustrating one example of the measurement position of the roughness curve for calculating the in-plane-direction surface roughness Ra(S) of the steel sheets having end surfaces that form part of the block facing end surfaces 111a to 111d and 111e to 111h included in the blocks 110a and 110b.
• Fig. 3B and Fig. 3E are views illustrating one example of the measurement position of the roughness curve for calculating the in-plane-direction surface roughness Ra(S) of the steel sheets having end surfaces that form part of the block facing end surfaces 111i to 111j and 111k to 111l included in the blocks 110c and 110d.
• Fig. 3A and Fig. 3D are views illustrating one example of the measurement position of the roughness curve for calculating the in-plane-direction surface roughness Ra(S) of the steel sheets having end surfaces that form part of the block facing end surfaces 111i to 111j and 111k to 111l included in the blocks 110
• 3C is a view illustrating one example of the measurement position of the roughness curve for calculating the in-plane-direction surface roughness Ra(S) of the steel sheets having end surfaces that form part of the block facing end surfaces 111m to 111p included in the block 110e.
• the direction vertical to the main magnetic flux direction (or rolling direction) and the laminating direction of the steel sheet is referred to as the width direction as necessary.
• the roughness curve for calculating the in-plane-direction surface roughness Ra(S) is measured, at a sheet surface of the steel sheet having an end surface that forms part of a pair of block facing end surfaces being a calculation target for (1) Expression, on a virtual straight line that passes through the middle position in the width direction of the steel sheet and extends along the main magnetic flux direction (or rolling direction) of the steel sheet.
• the main magnetic flux direction (or rolling direction) of the steel sheets that form part of the block facing end surfaces 111i to 111j, 111k to 111l, and 111m to 111p is the x-axis direction (the direction of the double-headed arrow line illustrated in Fig. 1 ). Further, the width direction of the steel sheets that form part of the block facing end surfaces 111i to 111j, 111k to 111l, and 111m to 111p is the y-axis direction.
• the virtual straight line passing through the middle position in the width direction (x-axis direction) of the steel sheet having end surfaces that form part of the block facing end surfaces 111a to 111d and extending along the main magnetic flux direction (or rolling direction, y-axis direction) of the steel sheet is a virtual straight line 301a.
• the virtual straight line passing through the middle position in the width direction (x-axis direction) of the steel sheet having end surfaces that form part of the block facing end surfaces 111e to 111h and extending along the main magnetic flux direction (or rolling direction, y-axis direction) of the steel sheet is a virtual straight line 301d.
• the virtual straight line passing through the middle position in the width direction (y-axis direction) of the steel sheet having end surfaces that form part of the block facing end surfaces 111m to 111p and extending along the main magnetic flux direction (or rolling direction, x-axis direction) of the steel sheet is a virtual straight line 301c.
• this virtual straight line is divided into a plurality of lines depending on the shape of the steel sheet in some cases.
• the roughness curve is measured on any one of the plural divided virtual straight lines.
• the roughness curve may be measured on the plural virtual straight lines as if these plural virtual straight lines were not divided (namely, as if each spacing between the plural virtual straight lines was 0 (zero)).
• in-plane-direction measurement positions the positions of the virtual straight lines 301a to 301e where the roughness curve (roughness curve element) for calculating the in-plane-direction surface roughness Ra(S) is measured are referred to as in-plane-direction measurement positions as necessary.
• the plural steel sheets forming one block facing end surface of the pair of block facing end surfaces being a calculation target for (1) Expression are extracted one by one, and then the in-plane-direction surface roughness Ra(s) of the extracted steel sheet is measured at the in-plane-direction measurement position.
• the in-plane-direction surface roughness Ra(S) may be measured at the in-plane-direction measurement position of the pre-laminated steel sheet that is used to form the block including the one block facing end surface. This measurement is performed for each of all of the steel sheets that form the one block facing end surface. For example, when the number of laminated sheets is 200, 200 in-plane-direction surface roughnesses Ra(S) are measured for one block facing end surface.
• the representative value of the plural (200 in the previously-described example) in-plane-direction surface roughnesses Ra(S) measured in the above manner is set as the in-plane-direction surface roughness Ra(S) of the one block facing end surface.
• the representative value is, for example, an arithmetic mean value.
• the median value or the like may be used in place of the arithmetic mean value.
• the in-plane-direction surface roughness Ra(S) of the other block facing end surface of the pair of block facing end surfaces being a calculation target for (1) Expression is also calculated in the same manner as the in-plane-direction surface roughness Ra(S) of the one block facing end surface.
• the roughness curve (roughness curve element) is measured.
• the roughness curve (roughness curve element) is measured, for example, for each of the plural steel sheets that form one block facing end surface. Further, the roughness curve (roughness curve element) is measured at each of the laminating-direction measurement position and the in-plane-direction measurement position.
• the representative value of the plural (200 in the previously-described example) roughness curves measured in the above manner is set as the roughness curve for calculating the laminating-direction surface roughness Ra(D) of the one block facing end surface.
• the representative value of the plural (200 in the previously-described example) roughness curves is calculated by calculating the representative value of the values at the same positions in the sheet thickness direction for each of the plural roughness curves.
• the representative value is, for example, an arithmetic mean value.
• the median value or the like may be used in place of the arithmetic mean value. Further, when the plural steel sheets have different sheet thicknesses, for example, a roughness curve obtained by connecting the plural (200 in the previously-described example) roughness curves may be used as the roughness curve (roughness curve element) for calculating the laminating-direction surface roughness Ra(D).
• the roughness curve for calculating the in-plane-direction surface roughness Ra(S) is measured for each of all of the steel sheets having end surfaces that form part of one block facing end surface, for example. For example, when the number of laminated sheets is 200, 200 roughness curves are measured as the roughness curve for calculating the in-plane-direction surface roughness Ra (S) for one block facing end surface.
• the representative value of the plural (200 in the previously-described example) roughness curves measured in the above manner is set as the roughness curve for calculating the in-plane-direction surface roughness Ra(S) of the steel sheet having an end surface that forms part of the block facing end surface.
• the measurement magnification is set to, for example, 200 times so that the size of one field of view is, for example, 500 ⁇ m ⁇ 500 ⁇ m.
• the measurement magnification is preferably 100 times or more, and more preferably 500 times to 700 times.
• the in-plane-direction surface roughness Ra(S) and the laminating-direction surface roughness Ra(D) calculated for the same block facing end surface are used to calculate Ra(D)/Ra(S). Then, whether or not the calculated Ra(D)/Ra(S) satisfies (1) Expression is checked.
• the method for satisfying (1) Expression may be any method as long as it is capable of adjusting the roughness of the end surface of the steel sheet.
• the roughness of the end surface of the steel sheet is adjusted, for example, by any one of grinding, cutting, and polishing.
• the roughness of the end surface of each of the steel sheets may be adjusted. Further, the roughness of the end surface of each of the steel sheets cut into the planar shapes of the blocks 110a to 110e may be adjusted. For example, when shearing each of the steel sheets, the roughness of the end surface of each of the steel sheets may be adjusted by controlling the clearance between upper and lower blades of a shearing machine. Further, after laminating the plural steel sheets, the roughnesses of the end surfaces corresponding to the block facing end surfaces 111a to 111p may be adjusted. Further, any two or more of these methods may be combined.
• the value obtained by dividing the number of crystal grains on the steel sheet facing end surface by the length of the steel sheet facing end surface is referred to as the number of crystal grains per unit length on the steel sheet facing end surface, as necessary.
• the number of crystal grains on the steel sheet facing end surface is set as n (grains).
• the length of the steel sheet facing end surface is set as L (mm). Then, the number of crystal grains per unit length on the steel sheet facing end surface is n/L (grains/mm).
• the arithmetic mean value of the calculated numbers of crystal grains per unit length on the steel sheet facing end surface n/L was calculated as the number of crystal grains per unit length on the steel sheet facing end surface n/L at the one block facing end surface.
• the present inventors have found out that the noise level of the laminated core 100 begins to change significantly due to vibrations at the butt parts (block facing end surfaces 111a to 111p) of the blocks 110a to 110e when the number of crystal grains per unit length on the steel sheet facing end surface n/L reaches 0.5 (grains/mm).
• the present inventors have found out that at at least one of the block facing end surfaces (preferably both of the block facing end surfaces) of the pairs of block facing end surfaces 111a and 111i, 111b and 111l, 111c and 111m, 111d and 111n, 111e and 111k, 111f and 111j, 111g and 111o, and 111h and 111p, (3) Expression below is satisfied, thereby making it possible to inhibit vibration of the laminated core 100. n / L ⁇ 0.5
• this embodiment is configured to satisfy 1 ⁇ Ra(D)/Ra(S) ⁇ 12 at each of at least one pair of block facing end surfaces (for example, the pair of block facing end surface 111a and 111i). Therefore, it is possible to provide a laminated core that is capable of inhibiting vibration without using a material different from the steel sheet. Further, 1 ⁇ Ra(D)/Ra(S) ⁇ 12 is changed to 6 ⁇ Ra(D)/Ra(S) ⁇ 8, thereby making it possible to more reliably improve the effect of inhibiting noise of the laminated core 100.
• the number of crystal grains per unit length on the steel sheet facing end surface n/L is set to 0.5 or less. Therefore, it is possible to further inhibit noise of the laminated core.
• a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, a decarburizing annealing step, a nitriding treatment (nitriding annealing) step, and a batch annealing step were performed in this order under the manufacturing conditions illustrated in Table 2.
• a nitriding treatment (nitriding annealing) was performed on a cold-rolled steel sheet after decarburizing annealing in a mixed atmosphere of hydrogen-nitrogen-ammonia.
• the amount of nitriding was adjusted by adjusting the flow rate of ammonia through ammonia nitriding.
• an annealing separating agent containing, as its main component, magnesia or alumina was applied to the cold-rolled steel sheet, and then batch annealing was performed thereon.
• Several types of annealing separating agents with different mixing ratios of components containing the main component were used as the annealing separating agent.
• an annealing temperature during batch annealing and a holding time at the annealing temperature were adjusted.
• the crystal grain diameter of the steel sheet after batch annealing was controlled by the amount of nitriding in the nitriding treatment and the annealing temperature and the holding time during the batch annealing.
• a laminated core 100 having the shape illustrated in Fig. 1 was manufactured using the steel sheet of Material type A as a material.
• the present inventors found out that there is a direct proportional relationship between Ra(D)/Ra(S) illustrated in (1) Expression and a clearance.
• the clearance between upper and lower blades of a shearing machine was controlled based on these findings, to thereby adjust the roughness of the end surface of the steel sheet of Material type A.
• Each of the blocks 110a to 110e was manufactured by laminating the steel sheets of Material type A having the planar shapes of the blocks 110a to 110e.
• plural sets of the blocks 110a to 110e having the block facing end surfaces 111a to 111p whose roughnesses are mutually different were manufactured as the set of the blocks 110a to 110e.
• plural sets of the blocks 110a to 110e were manufactured in the same manner.
• the blocks manufactured from the steel sheets of Material types A to E are referred to as blocks of Material types A to E as necessary.
• the block 110a, the block 110b, the block 110c, the block 110d, and the block 110e are referred to as an upper block 110a, a lower block 110b, a left block 110c, a center block 110d, and a right block 110e as necessary.
• the blocks 110a to 110e manufactured from the steel sheets of the same material type were combined to manufacture the laminated core 100.
• the laminated core 100 manufactured in this manner is referred to as a laminated core 100 of Material types A to E as necessary.
• the left block 110c, the center block 110d, the right block 110e, the upper block 110a, and the lower block 110b were manufactured from the steel sheets of different types as the block, and the blocks 110a to 110e were combined to manufacture a laminated core 100.
• a laminated core 100 of Material types A and B, a laminated core 100 of Material types A and D, a laminated core 100 of Material types B and C, and a laminated core 100 of Material types B and E were manufactured.
• the laminated core 100 of Material types A and B is a laminated core in which the left block 110c, the center block 110d, and the right block 110e are made of the steel sheet of Material type A and the upper block 110a and the lower block 110b are made of the steel sheet of Material type B.
• the laminated core 100 of Material types A and D is a laminated core in which the left block 110c, the center block 110d, and the right block 110e are made of the steel sheet of Material type A and the upper block 110a and the lower block 110b are made of the steel sheet of Material type D.
• the laminated core 100 of Material types B and C is a laminated core in which the left block 110c, the center block 110d, and the right block 110e are made of the steel sheet of Material type B and the upper block 110a and the lower block 110b are made of the steel sheet of Material type C.
• the laminated core 100 of Material types B and E is a laminated core in which the left block 110c, the center block 110d, and the right block 110e are made of the steel sheet of Material type E and the upper block 110a and the lower block 110b are made of the steel sheet of Material type B.
• the width (length in the y-axis direction), the height (length in the x-axis direction), and the thickness (length in the z-axis direction) of the laminated core 100 were 750 mm, 750 mm, and about 41 mm respectively. Further, the widths (length in the x-axis direction) of the upper block 110a and the lower block 110b and the widths (length in the y-axis direction) of the left block 110c, the center block 110d, and the right block 110e were 150 mm.
• the lengths L (L1 to L15) of the steel sheet facing end surfaces of the steel sheets of Material types A to E having the planar shapes of the blocks 110a to 110e were measured with a vernier caliper. Further, the steel sheet facing end surfaces (shear surfaces) of these steel sheets were corroded with a 5% nital solution for 100 to 300 seconds. Thereafter, these steel sheet facing end surfaces were observed with an industrial microscope BX53M manufactured by OLYMPUS CORPORATION, to thereby count the number of crystal grain boundaries present in the steel sheet facing end surfaces. Then, the number of crystal grains on the steel sheet facing end surface n was calculated by adding 1 to the counted number.
• the number of crystal grains per unit length on the steel sheet facing end surface n/L was calculated from the length L of the steel sheet facing end surface and the number of crystal grains on the steel sheet facing end surface n obtained from the same steel sheet.
• the number of crystal grains per unit length on the steel sheet facing end surface n/L of each of all of the steel sheets that form the blocks 110a to 110e was calculated.
• the arithmetic mean value of the numbers of crystal grains per unit length on the steel sheet facing end surface n/L was calculated at each of the block facing end surfaces 111a to 111p.
• the arithmetic mean value of the number of crystal grains per unit length on the steel sheet facing end surface n/L calculated at one block facing end surface was set as the number of crystal grains per unit length on the steel sheet facing end surface n/L at the block facing end surface.
• the laminating-direction surface roughness Ra(D) and the in-plane-direction surface roughness Ra(S) were calculated in accordance with JIS B 0601: 2013.
• each of the block facing end surfaces 111a to 111p of each of the laminated cores 100 the plural steel sheets forming the block facing end surface were extracted one by one, and the laminating-direction surface roughness Ra(D) of the extracted steel sheet was measured at the laminating-direction measurement positions (positions on the virtual straight lines 201a to 201p) using a one-shot 3D shape measuring machine (model name: VR-6000) manufactured by KEYENCE CORPORATION.
• the arithmetic mean value of the laminating-direction surface roughnesses Ra(D) of the block facing end surface calculated from the plural steel sheets forming the same block facing end surface was calculated as the laminating-direction surface roughness Ra(D) of the block facing end surface.
• each of the block facing end surfaces 111a to 111p of each of the laminated cores 100 the plural steel sheets forming the block facing end surface were extracted one by one, and the in-plane-direction surface roughness Ra(S) of the extracted steel sheet was measured at the in-plane-direction measurement positions (positions on the virtual straight lines 301a to 301e) using a one-shot 3D shape measuring machine (model name: VR-6000) manufactured by KEYENCE CORPORATION.
• the arithmetic mean value of the in-plane-direction surface roughnesses Ra(S) of the block facing end surface calculated from the plural steel sheets forming the same block facing end surface was calculated as the in-plane-direction surface roughness Ra(S) of the block facing end surface.
• Ra(D)/Ra(S) of the block facing end surface was calculated from the laminating-direction surface roughness Ra(D) and the in-plane-direction surface roughness Ra(S) at the same block facing end surface.
• Such calculation of Ra(D)/Ra(S) was performed at all of the block facing end surfaces 111a to 111p of all of the laminated cores 100 manufactured as described above.
• Table 3 and Table 4 illustrate the values of n/L, Ra(D)/Ra(S), and noise of each of the laminated cores 100 obtained in this manner.
• 111a to 111p indicate the block facing end surfaces 111a to 111p illustrated in Fig. 1 , Fig. 2A to Fig. 2E , Fig. 3A to Fig. 3E , and Fig. 4A to Fig. 4E .
• the noise was lower in Numbers 25, 29, 33, 37, and 41 compared to Number 21.
• the noise was lower in Numbers 26, 30, 34, 38, and 42 compared to Number 22.
• the noise was lower in Numbers 27, 31, 35, 39, and 43 compared to Number 23.
• the noise was lower in Numbers 28, 32, 36, 40, and 44 compared to Number 24.
• the noise reduction effect was significant. That is, regarding Material type A, the noise was lower in Numbers 4, 6, 8 to 13, 18, and 19 to 20 compared to Numbers 2, 3, 5, 7, 16, 17, and 19. Regarding Material type B, the noise was lower in Numbers 25, 29, and 33 compared to Numbers 37 and 41. Regarding Material type C, the noise was lower in Numbers 26, 30, and 34 compared to Numbers 38 and 42. Regarding Material type D, the noise was lower in Numbers 27, 31, and 35 compared to Numbers 39 and 43. Regarding Material type E, the noise was lower in Numbers 28, 32, and 36 compared to Numbers 40 and 44.
• the noise reduction effect was significant. That is, regarding Material type A, the noise was lower in Numbers 8, 9, 11, 12, 19, and 20 compared to Numbers 2 to 7, 10, 13, and 16 to 18, and the noise was lower in Numbers 8, 9, 11, and 12 compared to Numbers 19 and 20.
• the noise was lower in Number 29 compared to Numbers 25, 33, 37, and 41.
• the noise was lower in Number 30 compared to Numbers 26, 34, 38, and 42.
• the noise was lower in Number 31 compared to Numbers 27, 35, 39, and 43.
• the noise was lower in Number 32 compared to Numbers 28, 36, 40, and 44.
• the noise was lower in Numbers 45 and 46 (Material types A and B) compared to Number 2 (Material type A).
• the noise was lower in Number 47 (Numbers B and C) compared to Numbers 21 (Material type B) and 22 (Material type C).
• the noise was lower in Number 48 (Numbers B and E) compared to Numbers 21 (Material type B) and 26 (Material type E).
• the present invention can be used in devices including a core, for example.

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• 2023
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