MAGNETIC CORE AND TRANSFORMER INCLUDING A MAGNETIC CORE

Abstract

 The invention is about a magnetic core, said magnetic core comprising a stack of ferromagnetic laminations that are cut or stamped from metallic sheets and where one or more lamination types comprise at least one longitudinal segment oriented along a preferred magnetic direction of the metallic sheet, also called easy direction, and at least one transverse segment whose direction is orthogonal or almost orthogonal to that of the longitudinal segment, wherein the effective magnetic width of the transverse segment is larger than the effective width of the longitudinal segment such that the magnetic flux density in the transverse segment is lower than in the longitudinal segment. The invention is also about a transformer with such a magnetic core.

Description

[0001] The invention is about a magnetic core, said magnetic core comprising a stack of ferromagnetic laminations that are cut or stamped from metallic sheets and where one or more lamination types comprise at least one longitudinal segment oriented along a preferred magnetic direction of the metallic sheet, also called easy direction, and at least one transverse segment whose direction is orthogonal or almost orthogonal to that of the longitudinal segment, according to the preamble of claim 1.

[0002] The invention also is about a transformer, particularly a current transformer or a voltage transformer or a power transformer, including a magnetic core.

[0003] The invention deals with magnetic cores manufactured by stacking layers of thin ferromagnetic laminations and comprising at least one lamination type composed of segments oriented along different in-plane directions, typically orthogonal. One common design of such laminated core is made from interleaved stacks of E-shaped laminations capped with I-shaped laminations, leading to its name of EI core as shown in Fig. 1. Other common constructions are UI, EE, FF, LL, and DU cores as also described in the standard IEC60740-1:2005.

[0004] Each ferromagnetic lamination is electrically insulated from its neighbouring laminations by thin passivation and/or insulation layer(s) in order to confine eddy currents to paths that enclose a minimum amount of magnetic flux, and so greatly reduce the related energy losses. The laminations are cut to suitable geometries from metallic strips or sheets. The metallic alloys can be electrical steels, nickel-iron alloys, iron-cobalt alloys, and amorphous materials as described in the IEC60404 set of standards, or other type of alloys such as nanocrystalline materials. Ferromagnetic strips may feature anisotropic magnetic properties along the in-plane directions depending on the fabrication techniques and on further processing such as thermal annealing, magnetic annealing, and domain refining.

[0005] The electrical steels and mainly the silicon steels are very popular materials in the construction of magnetic cores due to their excellent compromise between magnetic properties, wide availability, and relatively low cost. The term silicon steels is employed here to refer to electrical steels based on an iron alloys which contain a certain amount of silicon, typically up to 7% silicon. Silicon steels feature high magnetic permeability, reasonable low losses, and the highest saturation magnetic flux density among commonly available materials, up to around 2 T. Silicon steel strips or sheets are usually fabricated using cold rolling and can be classified in two major categories: grain-oriented and non-oriented. The grain-oriented silicon steel strips are processed in such a way that the magnetic properties are optimized along the rolling direction, based on a tight control of the crystal orientation relative to the rolling direction. However, enhancing the magnetic properties along the rolling direction is achieved at the expense of the magnetic properties along the transverse direction and the materials exhibit strong anisotropy. Non-oriented silicon steels are produced without particular processing to control the crystal orientation in the plane of the strips and are generally perceived as being anisotropic. However, their magnetic properties are slightly better along the rolling direction than along the transverse direction and they are thus also anisotropic even though their anisotropy is much less pronounced than that of grain-oriented silicon steels.

[0006] The magnetic cores are typically used in transformers such as power transformers, voltage transformers, or current transformers. The reluctance, the magnetic coercivity, and the eddy currents of the core are critical parameters affecting the performance of the transformers such as their operation range, efficiency, and accuracy. The eddy currents depend on operating conditions such as the value of the magnetic flux and operation frequency, and on core properties such as the thickness of the laminations and the resistivity of the ferromagnetic material. The magnetic core is optimized for given operating conditions by selecting the appropriate material and strip thickness. The reluctance of the magnetic core depends on the magnetic permeability of the ferromagnetic material and on the construction type and geometry of the core. The magnetic coercivity is a material property, however, it is shown in this invention that it is possible to influence the effective coercivity of the core by the geometry of the laminations.

[0007] The focus of the invention is on stacked cores comprising laminations featuring segments oriented along two orthogonal or almost orthogonal directions, such as U, E, F, L, and DU laminations. Such laminations comprise segments that are typically longer in one direction than in the other and we refer to the longer segment(s) of one lamination as longitudinal segment(s). The segment(s) of the lamination oriented orthogonal to the longitudinal segment(s) are referred to as transverse segment(s).

[0008] Common examples of stacked cores are UI, EI, EE, EF, LL, and DU. The stacked cores could be divided into two categories based on whether the magnetic flux can follow a closed path within one single layer of the stack. The first category includes magnetic cores where a closed flux path is possible in one layer of laminations, typically represented by UI, EI, EE, EF, and LL core types. We refer to this category as closed-path-layer cores. The second category includes cores where at least two layers of laminations are needed to properly close the path of the magnetic flux, because the air gap present in one single lamination layer is generally too high. We refer to this second category as open-path-layer cores being typically represented by DU cores. Equivalent DE cores are also known, however, it is believed that they are generally less common that the DU cores.

[0009] EI cores are built from multiple layers of laminations, each layer containing one E-type lamination closed by one I-type lamination. For lowest magnetic reluctance of the core, the laminations are stacked in one-by-one interleaved manner, that is consecutive layers of laminations are rotated at 180 degrees with respect to each other, as shown in Fig. 1. Reduced interleaving, for example two-by-two, or no interleaving at all may also be employed to reduce the assembly cost of the cores at the expense of higher magnetic reluctance. An example of UI core with two-by-two interleaved stacking is shown in Fig. 2.

[0010] Interleaved constructions allow additional paths of the magnetic flux between the laminations from adjacent layers when the core is not operated close to the magnetic saturation limit of the ferromagnetic material. The cross-layer paths have much lower magnetic reluctance than the in-layer paths because the exchange areas are considerable larger, often by orders of magnitude, and the air gaps are smaller. The air gaps that can be practically achieved between the lamination layers is often several times smaller than the air gaps between the laminations within the same layer as they are less sensitive to manufacturing tolerances and because the stack can be easily compressed using simple mounting procedures. The cross-layer paths result in significantly lower magnetic reluctance and higher effective permeability of the core. If the magnetic saturation limit of the ferromagnetic material is approached the cross-layer paths are blocked because of flux crowding and little advantage is offered then by an interleaved arrangement.

[0011] Open-path-layer cores can only be used in interleaved stacks, as interleaved layers are necessary to close the path of the magnetic flux. An example of DU core with one-by-one interleaved stacking is shown in Fig. 3. The effect of flux crowding in open-path-layer cores is to reduce the flux exchange area between adjacent laminations but never to suppress it completely.

[0012] Stacked cores are traditionally designed such that the magnetic flux density is equal or approximately equal in all branches of the core. This is achieved by keeping the magnetic area constant through the magnetic core, implying that within one branch of the magnetic circuit the laminations' segments have equal effective magnetic width. The following relationship applies thus to the EI and UI magnetic cores from Fig. 1 and Fig. 2:

[0013] The EI core example from Fig. 1 is for single phase application and the width of the middle segment of the E lamination is double the width of the outer segments, because the magnetic flux is generated in the middle segment and it closes through both outer segments. In EI cores for 3 phase applications, also known as 3UI cores, the width of all segments is equal.

[0014] In open-path-layer cores, such as DU and DE cores, one transverse segment conducts the magnetic flux coming from its own stack layer but also from adjacent layer(s). In DU cores, the number of transverse segments in one core stack is equal to half the number of longitudinal segments. The effective magnetic width of the transverse segment relative to one stack layer is thus equal to half of the physical width of the segment:

[0015] In order to keep the magnetic area in the transverse branch equal to the magnetic area in the longitudinal branch, the width of the transverse segment must be double the width of the longitudinal segment:

[0016] This approach of keeping constant or almost constant magnetic area along the magnetic circuit of stacked cores has been deeply established in common design and manufacturing practice. It is recommended in standards like the IEC60740-1:2005 and in design guidelines for laminations for transformers and inductors.

[0017] Laminations are typically produced from ferromagnetic strips or sheets by cutting or stamping. The ferromagnetic strips or sheets may feature superior magnetic properties along a preferred in-plane direction, generally called easy direction. The magnetic properties along the transverse direction are inferior compared to the easy direction leading to anisotropic behavior. Silicon steel strips or sheets are by far the most widely employed materials for the fabrication of laminations in stacked cores and exhibit strong anisotropy. The easy direction in silicon steel strips or sheets is typically given by the rolling direction.

[0018] Even non-oriented silicon steel strips feature significant magnetic anisotropy, contrary to common misconceptions. For example, according to datasheet values of the non-oriented silicon steel material M235-35A from ThyssenKrupp Steel, the magnetic losses are between 91% and 24% higher in the transverse direction as compared to the rolling direction, for magnetic induction levels comprised between 0.5 T and 1.7 T respectively, at 50 Hz. The corresponding permeability is between 280% and 38% smaller in the transverse direction. The saturation magnetic induction at 1000 A/m is also smaller in the transverse direction by around 6% compared to the rolling direction.

[0019] The anisotropy of grain-oriented silicon steel strips is significantly stronger. The magnetic properties are purposely optimized along the rolling direction at the expense of the transverse direction. Grain oriented silicon steels are further divided into conventional grades and high permeability grades, as also described in the standard IEC60404-8-7. The high permeability grades feature further improved crystal orientation relative to the rolling direction to improve even more their magnetic properties in the rolling direction, resulting in even stronger anisotropy. The material grade M155-35S5 is the poorest grade of grain-oriented silicon steel covered by the standard IEC60404-8-7. At 50 Hz, the magnetic losses of the M155-35S5 material are around 4 times higher in transverse direction than in rolling direction, while the permeability is more than 10 times lower. Furthermore the saturation magnetic induction measured at 1000 A/m in transverse direction is less than 1.4 T, while in rolling direction is above 1.8 T. The differences between the magnetic properties in transverse and rolling directions are even greater for the superior material grades, especially for the high permeability grades.

[0020] As most ferromagnetic strips or sheets are anisotropic, the dominant direction of the laminations is aligned along the easy direction of the material. Straight shaped laminations consisting of mainly one single segment, such as I-shape, are straightforward to align along the easy direction of the material. In the case of laminations consisting of multiple segments, the longitudinal segments are oriented along the easy direction of the material as shown in Fig. 4 for E, U, and L type laminations. Other types of multi-segment laminations such as F, DU, or DE type would be similarly oriented. The orientation of the multi-segment laminations is the result of a compromise: the best magnetic properties are ensured for the dominant segment(s) and less good magnetic properties are left to the transverse segment(s). Because of lower permeability,lower saturation induction and higher magnetic losses in the transverse direction, the transverse segment(S) act as a bottleneck in the path of the magnetic flux. The relatively poor magnetic properties in transverse direction impact the performance of the magnetic core and cause lower effective permeability, lower inductance, lower saturation induction and higher losses. The operating range of the magnetic core is therefore reduced, magnetic losses are increased, and the accuracy is degraded.

[0021] So it is the objective of the current invention to provide a transformer with a magnetic core featuring lower cost, lower losses, wider operating range, and better accuracy.

[0022] The objective is achieved by a transformer with a magnetic core according to claim 1. So the effective magnetic width of the transverse segment is purposely larger than the effective width of the longitudinal segment such that the magnetic flux density in the transverse segment is lower than in the longitudinal segment.

[0023] According to an advantageous embodiment, the effective magnetic width of the transverse segment is larger than the effective width of the longitudinal segment by a factor greater than or equal to 1.1.

[0024] According to an advantageous embodiment, the magnetic core features one lamination per stacking layer, such as DU or DE lamination, and the said lamination comprises two longitudinal segments connected via a transverse segment to form a U-like shape, wherein the physical width of the transverse segment is larger than the physical width of the longitudinal segments by a factor greater than or equal to 2.2. More than two longitudinal segments are possible, for example, the DE lamination features three longitudinal segments.

[0025] According to an advantageous embodiment, the magnetic core features two or more laminations per stacking layer where at least one lamination type, such as U, E, F, or L laminations, comprises one transverse segment connected to at least one longitudinal segment, wherein the physical width of the transverse segment is larger than the physical width of the longitudinal segment(s) by a factor greater than or equal to 1.1.

[0026] According to an advantageous embodiment, the laminations are made from cold-rolled electrical steel strip or sheets containing silicon, the silicon content being typically comprised between 0.5 % and 7 %, and where the preferred magnetic direction corresponds to the rolling direction.

[0027] According to an advantageous embodiment, the ratio between the effective magnetic width of the transverse segment and the effective width of the longitudinal segment is comprised between 1.2 and 4.

[0028] According to an advantageous embodiment, the laminations are made from cold-rolled grain-oriented electrical steel.

[0029] According to an advantageous embodiment, the ratio between the effective magnetic width of the transverse segment and the effective width of the longitudinal segment is comprised between 1.3 and 4.

[0030] According to an advantageous embodiment, the lamination featuring multiple segments are matched to laminations featuring mainly one segment, such as I-shaped laminations, wherein the main segment is oriented along the preferred magnetic direction of the metallic sheet.

[0031] According to an advantageous embodiment, the width of the I-like lamination is approximately comprised between the width of the longitudinal segment and the width of the transverse segment of the multi-segment lamination.

[0032] According to an advantageous embodiment, at least one lamination is provided with cuts or notches where the ends of the matching laminations are inserted to provide improved alignment of the laminations and additional possible paths for the magnetic flux to travel between the laminations.

[0033] According to an advantageous embodiment, the I-like laminations are provided with cuts or notches and where the matching laminations can be for example E- or U-shaped.

[0034] According to an advantageous embodiment, the I-like laminations are partly or completely enclosed between the longitudinal segments of the matching laminations, which can be for example E- or U-shaped.

[0035] According to an advantageous embodiment, the laminations are provided with features such as chamfers or angles to improve their alignment and facilitate the assembly of the magnetic core.

[0036] According to an advantageous embodiment, the laminations of the magnetic core are provided with features for fixation purposes such as holes or notches, whose position may be aligned or shifted with respect to the longitudinal segments of the multi-segment laminations.

[0037] According to an advantageous embodiment, the laminations of the magnetic core are fixed in position using welds, screws, rivets, clips or clamps.

[0038] The invention will be described in greater detail by description of eight embodiments with reference to the accompanying drawings, wherein

Figure 1

shows an example of EI core construction known from the art, with one-by-one interleaved stacking,

Figure 2

shows an example of a UI core construction known from the art, with two-by-two interleaved stacking,

Figure 3

shows the orientation of E, U, and L type laminations with respect to the easy direction of the magnetic sheet,

Figure 4a and 4b

shows an example of UI (fig. 4a) and DU (fig. 4b) laminations according to the invention,

Figure 5a and 5b

shows an illustration of possible flux paths between the E and I laminations within the same layer of the stack at high magnetic induction levels in EI cores known from the art (fig. 5a) and according to the invention (fig. 5b),

Figure 6a and 6b

shows an example of UI (fig. 6a) and EI (fig. 6b) laminations according to the invention, where the I laminations are provided with notches,

Figure 7a, 7b, 7c

shows three examples according to the invention, where the I laminations are provided with notches and the U and I laminations are provided with chamfers or angles for improved assembly,

Figure 8a, 8b, 8c

shows three examples of UI laminations according to the invention, where the I lamination is contained within the branches of the U laminations,

Figure 9a, 9b

shows an example of laminations according to the invention, EI type in fig. 9a and UI-type in fig. 9b, provided with fixation holes placed in between the longitudinal segments of the multi-segment lamination.

[0039] In the figures, elements having the same or a similar functional purpose have the same reference sign.

[0040] Figure 1 shows on the left hand side an EI core lamination sheet 1 a in a first orientation, on the right hand side an EI core lamination sheet 1 b in a second orientation which is 180° turned with respect to the first orientation, and in the middle section a side view on a stack made of various core laminations, alternately a first orientation 1 a and a second orientation 1 b stacked above each other. The EI core lamination 1 a has two longitudinal segments 2a, 2b, each having a physical width d, and a middle segment 5 having a larger physical width e, connected with each other by a transverse segment 3 having a physical width h. The E-structure is covered on top with an I - segment 4 having a physical width f.

[0041] Figure 2 shows on the left hand side an UI core lamination sheet 6a in a first orientation, on the right hand side an UI core lamination sheet 6b in a second orientation which is 180° turned with respect to the first orientation, and in the middle section a side view on a stack made of various core laminations, alternately two first orientation sheets 6a and two second orientation sheets 6b stacked above each other. The UI core lamination 6a has two longitudinal segments 7a, 7b, each having a physical width d, connected with each other by a transverse segment 8 having a physical width h. The U-structure is covered on top with an I - segment 9 having a physical width f.

[0042] Figure 3 shows the orientation of, from left to right, E, U, and L type laminations 10, 11, 12 with respect to the easy direction of the magnetic sheet. Indicated is also the transverse direction of the magnetic sheet, each by a double-arrow. The E type lamination 10 corresponds to the lamination 1a shown in figure 1, the U type lamination 11 corresponds to the lamination 6a shown in figure 2, the L-type lamination has a first leg 13 and a second leg 14.

[0043] The present invention provides a method to drastically improve the performance of stacked cores by optimizing the design of multi-segment laminations manufactured from ferromagnetic strips or sheets with anisotropic properties. The method consists in increasing the effective magnetic area of the transverse branches of the core with respect to the magnetic area of the longitudinal branches such that the poorer magnetic properties are at least partly compensated by the greater magnetic area. The larger magnetic area in the transverse branches results in lower reluctance and lower magnetic induction level. The lower reluctance of the transverse branches reduces the overall reluctance of the core and increases the effective permeability and the inductance of the core. The lower magnetic induction level reached in the transverse branches relative to the longitudinal branches results in lower magnetic losses but also in higher saturation limit of the core. By properly scaling the magnetic area of the core branches, the saturation induction level of the ferromagnetic material in the easy direction can be reached. The magnetic core is thus more efficient and can be operated over wider range of the magnetic induction due to the higher inductance and saturation level. The operation range is effectively extended both towards smaller and greater magnetic induction levels as compared to traditional construction of magnetic cores. The improvement is of high interest both for the construction of inductors and transformers. For inductors, it is possible to significantly reduce the total losses and to increase the inductance value and the operational current range. For power transformers, it is possible to significantly reduce the total losses and increase the power ratings. For instrument transformers like current or voltage transformers, much better accuracy and measurement range can be reached.

[0044] The effective magnetic area of the transverse branches is increased by enlarging the effective width of the transverse segments of the laminations with respect to the longitudinal segments. In close-path-layer cores, such as EI or UI, the following equation can be introduced using the notations from Fig. 4

where k is the scaling factor and is greater than 1.1.

[0045] In open-path-layer cores, such as DU and DE cores, the effective magnetic width of the transverse segment relative to one stack layer is equal to half of the physical width of the segment, and the scaling according to the invention leads to the following equation:

[0046] Examples of UI and DU cores according to the invention are shown in Fig. 4a and 4b, respectively.

[0047] The scaling factor k is selected according to the anisotropy of the material from which the laminations are produced but also according to design targets such as operation range, accuracy, losses, size, and cost. For materials with weak anisotropy it shall ensure that the width of transverse segments is larger than that of the longitudinal segments in order to bring a tiny amount of compensation even when accounting for production tolerances and variations, for example higher than 1.1. For materials with strong anisotropy such as grain-oriented silicon steels the optimum scaling factor would be higher, however excessively high scaling factors would result in too large core size and the core would no longer be competitive. A very good range for the scaling factor was found to lie between 1.1 and 4, depending on materials and applications.

[0048] The present invention allows reducing the negative effects of flux crowding. Flux crowding occurs when the magnetic induction B approaches the saturation limit of the material B_s, e.g. B > B_s /2. The low reluctance flux paths between alternating layers of laminations are then reduced or even completely blocked. When flux crowding occurs in close-path-layer cores, the path of the magnetic flux is constrained mainly through the air gap between the laminations in one single stack layer which features relatively high reluctance. The path of the magnetic flux over the air gap between I and U laminations is shown in Fig. 5a. In UI cores according to the invention, the magnetic flux can travel between the I and U laminations from one layer via the U laminations from the adjacent layers even when the core is saturated. This is exemplified in Fig. 5b, where the additional flux paths enabled by the invention are shown. Fig. 5b shows a side view on a stacked core made up of core laminations as shown in figure 4a. In stacked cores according to the invention, the effect of flux crowding is thus to reduce the flux exchange areas between alternating layers of laminations but not to suppress them like in cores according to prior art. The inductance of close-path-layer cores according to the invention can be increased by more than 100% at high magnetic induction values close to magnetic saturation. At high magnetic induction levels, the invention also improves significantly the inductance of open-path-layer cores where the cross-layer flux exchange areas are larger as compared to the traditional constructions, especially in the case of flux crowding.

[0049] The laminations consisting of mainly one single segment, such as I-shaped laminations, are aligned along the easy direction of the material. Even though they are aligned along the easy axis of the material, their width can be increased to become closer to the width of the transverse segments in the matching laminations. The single-segment laminations can be provided with cuts or notches 26, 35a, 35b where the ends of the matching laminations are inserted to provide additional possible paths for the magnetic flux to travel between the laminations. This embodiment of the invention is exemplified in figures 7a and 7b for UI and EI cores. The example core lamination shown in figure 6a is derived from the one shown in figure 4a. The example core lamination shown in figure 6b is the equivalent for a EI type core according to the invention. One further advantage of the cuts or notches 26, 35a, 35b is to allow improved alignment of the laminations for the construction of the magnetic core. Improving the alignment allows lower air gaps between the laminations in one stack layer, while the flux exchange area between the in-layer laminations is also increased by the cuts or notches. The reluctance of the in-layer flux path can thus be drastically decreased, by values up to 50% and even above. Furthermore, using cuts or notches allows both increasing the width of the I laminations and providing additional cross-layer flux exchange paths even when flux crowding occurs. Increasing the width of the I laminations provides the possibility that they also take part of the magnetic flux going through the transverse segment which has higher reluctance and losses. The reluctance and the losses of the core caused by the transverse segments of the laminations are thus further reduced.

[0050] The laminations can be provided with features such as chamfers or angles 36a, 36b, 37a, 37b, 38a, 38b, 39a, 39b in order to improve their alignment, facilitate their assembly, and/or to ensure minimum air gaps between the laminations. Possible chamfers and angles are shown in figures 7a, 7b, 7c for UI laminations, being also applicable to other types of laminations.

[0051] The single-segment laminations can also be partly or completely enclosed between the longitudinal segments of the matching laminations, which can be for example E- or U-shaped. Several examples of I laminations enclosed between the branches of U laminations are shown in figures 8a, 8b, 8c, but other arrangements are also possible.

[0052] The lamination stack can be stabilized by various fixation techniques such as welding, elastic clips, rigid or elastic clamps, screwing, riveting, etc. The laminations can be provided with features allowing suitable alignment and/or fixation such as notches or holes. Because the width of the transverse segments is oversized according to the invention, the position of the fixation features is not constrained to the corners of the core or to the regions where the branches of the core intersect. They can also be placed misaligned with respect to the branches of the core, allowing reducing the number of fixations. Such examples are shown in figures 9a and 9b, where the fixation holes 40, 40a, 40b, 40c, 40d, 40e are placed in-between the ends of the longitudinal branches 16a, 16b resp. 31 a, 31 b. The EI core from Fig. 9a is an example for a three-phase application, also known as 3UI core. In this particular example, the length of the longitudinal segments 31 a, 31 b of the E lamination is not equal to the length of the middle segment 32a, a feature that may be used to purposely balance or unbalance the magnetic behavior of the core.

[0053] The embodiments presented here allow greatly improving the magnetic properties of stacked cores including multi-segment laminations. The magnetic losses and the magnetic reluctance of the core can be reduced by more than 50%, at least over some ranges of the magnetic induction. The inductance of the core is accordingly improved while the operating range of the core is extended both towards small and high levels of the magnetic induction. Increasing the width of the transverse branches of the core may appear to cause larger size of the core and increased material usage. However, the drastic improvement of the magnetic properties allows scaling down the full core such that at comparable volume to a traditional core it still exhibits far superior magnetic performance. The superior magnetic performance allows reducing the cost and mass of the winding such that a core according to the invention would provide both lower cost and better performance compared to traditional cores.

[0054] The core described here can be employed to build inductors or transformers with superior performance and lower cost. It can be advantageously used both for the construction of power transformers or instrument transformers such as current and voltage transformers.

List of reference signs

1a	EI core sheet, first orientation
1b	EI core sheet, second orientation
1c	Side view of EI core stack with one by one interleaved stacking
2a, 2a'	Longitudinal segment
2b, 2b'	Longitudinal segment
3, 3'	Transverse segment
4, 4'	I-segment
5, 5'	Middle segment
6a	UI core, first orientation
6b	UI core, second orientation
6c	Side view of UI core stack with two by two interleaved stacking
7a, 7a'	Longitudinal segment
8, 8'	Transverse segment
9, 9'	I-segment
10	E-type lamination with magnetic anisotropy
11	U-type lamination with magnetic anisotropy
12	L-type lamination with magnetic anisotropy
13	Transverse segment
14	Longitudinal segment
15, 15a, 15b, 15c, 15d, 15e, 15f	UI-type lamination core
16a, 16b	Longitudinal segment
17	Transversal segment
18, 18'	I-segment
18a, 18b, 18c	I-segment, inserted between longitudinal segments
20	DU type lamination core
21a, 21b	Longitudinal segment
22	Transversal segment
25	UI type lamination core with notch
26	Notch part of I-segment
30, 30a, 30b	EI type lamination core with notch
31a, 31b	Longitudinal segment
32, 32a	Middle segment
33	Transversal segment
34	I-type segment
35a, 35b	Notch part of I-segment
36a, 36b, 37a, 37b	Chamfered corner
37a, 37b	Chamfered corner
38a, 38b	Angled step
39a, 39b	Angled step
40, 40a, 40b, 40c, 40d,40e	Fixation hole
d	physical width of longitudinal segment
h, h1	physical width of transverse segment
f	Physical width of I - segment
f1	Physical width of notch-portion of I-segment
f2	Physical width of non-notched portion of I-segment
e	Physical width of middle segment
g1	physical width of transversal segment

Claims

1. A magnetic core, said magnetic core comprising a stack of ferromagnetic laminations (15, 20) that are cut or stamped from metallic sheets and where one or more lamination types (15, 20) comprise at least one longitudinal segment (16a, 16b) oriented along a preferred magnetic direction of the metallic sheet, and at least one transverse segment (17) whose direction is at least almost orthogonal to that of the longitudinal segment (16a), characterized in that the effective magnetic width of the transverse segment (17) is larger than the effective width of the longitudinal segment (16a) such that the magnetic flux density in the transverse segment (17) is lower than in the longitudinal segment (16a).

2. A magnetic core according to claim 1, where the effective magnetic width of the transverse segment (17) is larger than the effective width of the longitudinal segment (16a) by a factor greater than or equal to 1.1.

3. A magnetic core according to claim 1 or 2, where the magnetic core features one lamination per stacking layer, and the said lamination comprises two longitudinal segments (16a, 16b) connected via a transverse segment (18) to form a U-like shape, wherein the physical width (h₁) of the transverse segment (17) is larger than the physical width (d) of the longitudinal segments (16a, 16b) by a factor greater than or equal to 2.2.

4. A magnetic core according to claim 1 or 2, where the magnetic core features two or more laminations per stacking layer where at least one lamination type comprises one transverse segment (17) connected to at least one longitudinal segment (16a), wherein the physical width (h₁) of the transverse segment (17) is larger than the physical width (d) of the at least one longitudinal segment (16a) by a factor greater than or equal to 1.1.

5. A magnetic core according to any one of claims 1 to 4, where the laminations are made from cold-rolled electrical steel strip or sheets containing silicon, the silicon content being typically comprised between 0.5 % and 7 %, and where the preferred magnetic direction corresponds to the rolling direction.

6. A magnetic core according to any one of claims 1 to 4, where the ratio between the effective magnetic width of the transverse segment (17) and the effective width (d) of the longitudinal segment is comprised between 1.2 and 4.

7. A magnetic core according to claim 6, where the laminations are made from cold-rolled grain-oriented electrical steel.

8. A magnetic core according to claim 7, where the ratio between the effective magnetic width of the transverse segment (17) and the effective width of the longitudinal segment (16a) is between 1.3 and 4.

9. A magnetic core according to any one of claims 4 to 8, where the laminations featuring multiple segments are matched to laminations featuring mainly one segment, wherein a main segment is oriented along the preferred magnetic direction of the metallic sheet.

10. A magnetic core according to any one of claims 4 to 9, where at least one lamination is provided with cuts or notches (26) where the ends of the matching laminations are inserted to provide improved alignment of the laminations and additional possible paths for the magnetic flux to travel between the laminations.

11. A magnetic core according to any one of claims 1 to 10, where the laminations are provided with chamfers (36a, 37a) or angles (38a, 39a) to improve their alignment and facilitate the assembly of the magnetic core.

12. A magnetic core according to any one of claims 1 to 11, where the laminations of the magnetic core are provided with holes (40, 40a) or notches for fixation purposes.

13. A transformer having a magnetic core according to any of the preceding claims.

Drawing