Aspects of the invention relate to a magnetic core arrangement, in particular for a transformer, comprising at least one core comprising a partially reduced cross-section and/or thickness, in particular ring-like cores, in particular configured for a transformer. Further aspects relate to an inductive device such as an inductor or transformer, including at least one magnetic core arrangement, and an installation unit including at least one of said inductive devices, in particular transformers.

Technical background:

A magnetic core typically comprises one or more pieces of ferromagnetic material with high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical, and magietic devices. Inductive devices and transformers using a magnetic core are known and widely used in electrical equipment. Modem electrical equipment is required to provide increasingly better performance, functionality, operation range, and robustness, leading to technical challenges.

On the one hand, modem electrical equipment requires steadily enhanced performance and accuracy over wider AC operating currents and frequency. For example, wider nominal current ranges are being introduced and multiple functionalities such as protection, control, monitoring and metering are to be fulfilled with one device. Wider operating frequency range is required in order to deal with various rated frequencies, high frequency harmonics, and low frequency or high frequency signals caused by fault conditions. For example, typical rated frequencies in electrical power and distribution equipment are 16.7 Hz, 50 Hz, 60 Hz, but harmonics well above 1 kHz may be present. Transitory signals with very low frequency content, e.g. below 5 Hz, or very high frequency content, e.g. above 10 kHz, are possible in case of fault conditions. Wide operating frequency range is also required in case of power drives and converters.

On the other hand, DC currents may be present in AC power lines because of the type of equipment connected to the grid or because of fault conditions such as short circuits. There is a clear trend that modem power grids must deal with steadily higher DC currents caused by increased usage of equipment such as static VAR compensation, AC to DC power converters, and DC to AC power converters. Inductive devices are thus required to be able to operate under moderate DC currents caused by modem equipment or under severe DC currents caused by fault conditions.

Ideally, inductive devices are desired to provide excellent AC performance at low to medium frequencies but also to withstand high DC currents while providing acceptable AC performance. For example, this applies for current transformers employed for the measurement of electrical current in installation devices, such as protection relays. Such transformers are required to accurately measure AC currents comprised in a very wide range, possibly froml mA and 1000 A, and they must also withstand DC currents up to 100 A or even above without saturating. Furthermore, the current transformers must be ideally compact and lightweight.

Current transformers with excellent AC performance, e.g. accuracy and wide operating frequency range, require ferromagnetic cores with high permeability and low losses. The high permeability causes them to saturate easily when a DC current is applied. Combining great AC performance and strong DC withstand is known to be very challenging and it is often necessary to increase the mass and the cost of the transformer in order to allow for better tradeoffs.

In the present description, we mainly refer to the relative permeability, m _G of some material. It is the ratio between the permeability of a specific medium, m, and that of vacuum, mo, i.e. m _G = m / mo. It is a convenient, unitless parameter commonly employed for describing magnetic materials. Magnetic cores are described here using effective permeability values, also relative. The effective permeability of a magnetic core is employed to fictively represent the core as it was made from a homogenous material occupying the full volume of the core. However, the dimensions of the magnetic core shall not include eventual mechanical casing or encapsulating layer(s) such as polymer coating. It is representative for the magnetic core and it is usually not identical to the permeability of the pure ferromagnetic material but it is also influenced by the construction of the core. It depends on multiple factors related to the construction of the core such as the orientation of the magnetic material, filling ratio, presence of gaps, mechanical stresses or strains, etc. The effective permeability is a practical way to compare cores fabricated using different technologies and/or materials. Experimentally, it can be determined from inductance measurements or from hysteresis measurements (B-H curves), without applying corrections related to the construction of the core. Hysteresis measurements are typically employed to also measure other magnetic properties of the core, such as saturation flux density, Bs, and residual magnetic flux density, B,·. The remanence of the magnetic core may be described and/or defined by using the ratio between the residual magnetic flux density and the saturation flux density B _r /B _s . Again, the magnetic properties of the core are representative for the core and are the result of the construction of the core and of the magnetic properties of the ferromagnetic material(s) employed.

Common solutions known from the art employ ferromagnetic cores with moderate relative permeability, typically comprised between 1000 and 5000. For example, the cores may be based on amorphous alloys, like in particular Co-based amorphous alloys, but nanocrystalline alloys, like in particular Fe-based nanocrystalline alloys, with low permeability also became recently available. Cheaper designs may even employ electrical steels but higher non-linearity errors would be engendered. The moderate permeability ensures some level of DC withstand but only allows limited AC accuracy. Electronic corrections are then applied to improve the amplitude and phase errors of the current transformer, however, the corrections are not straightforward because of temperature and frequency effects. The achievable accuracy is thus limited and unpleasant tradeoffs between AC accuracy and DC withstand are to be considered. Boosting the size of the magnetic cores and windings allows for better tradeoffs between AC accuracy and DC withstand but results in bulky and expensive solutions.

Other solutions known from the art but not commonly employed in practice rely on using a composite core comprising two ferromagnetic cores, where one of the cores comprises at least one gap to lower its effective permeability. Current transformers with composite cores are for example known from JPS58216412 and JP5525270. The composite core features thus two operating regimes: one with high inductance and effective permeability given by the first core when DC currents are not present, and one with low inductance and effective permeability given by the second core when DC currents are present causing the first core to saturate. In order to maximize the AC performance, the relative permeability of the non-gapped core needs to be very high, e.g. above 20000. In order to maximize the DC withstand current, the effective relative permeability of the gapped core needs to be low, e.g. below 2000. Each core is mainly responsible for one operating regime and little overlap exists between the two operating regimes. In practice, this may cause a relatively large mismatch between the two operating regimes and lead to unacceptable non-linear operation of the composite core. In order to ensure acceptable AC perfonnance under DC currents, the relative permeability of the gapped core needs to be sufficiently large, e.g. above 500. In order to ensure sufficient permeability, the total gap width must be very thin, e.g. below 0.25 mm. Furthermore, the gap width must be precise and stable over operating conditions in order to ensure reliable and reproducible performance. The total gap width is equal to the sum of the width of all gaps, taking usually the mean values. For example, when two identical gaps are provided and the total gap width must be 0.1 mm, the width of each gap must be 0.05 mm and cannot be precisely produced using available methods. Fabricating thin gaps with precise width is technically very challenging and the problem becomes increasingly severe as the gap is thinner, for example in the order of 0.1 mm or less. Practical means to fabricate gaps with precise and thin width, e.g. below 0.25 mm, are not known from prior art, e.g. JPS58216412 and JP5525270.

Composite cores presently face limited optimization possibilities related to the mismatch between the operating regimes of the two cores and to the fabrication of thin and precise gaps, making their usage unpopular.

Providing composite cores combining relatively good AC perfonnance and strong DC withstand still faces additional challenges. The two cores would feature very different permeability causing large mismatch between the two operating regimes of the composite core. Large permeability jumps in the order of two decades may lead to unacceptable non-linear operation of the composite core. Even though the capability of the current transformer to withstand high DC currents is a critical merit, the presence of very high DC currents is rare in practice. Actually, the presence of low to medium DC currents is much more common and it is this case which would be mostly affected by non-linear operation of the composite core. Providing moderately high permeability at moderate DC currents would improve the linearity of the composite core and provide enhanced AC performance in such conditions. However, this would require a composite core with very high effective permeability at low DC currents, sufficiently high permeability at moderate DC currents, and low permeability at high DC currents. In order to be of practical interest, the core would also need to be compact, reliable, easy to produce, and cost efficient. Accordingly, the object of the invention is to provide an improved magnetic core arrangement with a relatively high magnetic permeability which has a compact design and can be produced with relatively low efforts.

This object is solved by a magnetic core arrangement according to claim 1, comprising one or more magnetic cores, in particular at least one ring-like core, wherein at least one magnetic core is provided comprising a partially reduced cross-section and/or thickness, wherein the reduction of the cross-section is achieved by providing one or more narrow slit portions and/or grooves through or in its cross-section in order to create a constriction and to locally reduce the cross- sectional area of the magnetic path, wherein the width of the slit or groove is made relatively small such that the magnetic flux can cross the slit or groove, in particular under moderate magnetomotive force.

Further aspects relate to an inductive device such as an inductor or transformer, including at least one core arrangement according to the invention, and an installation unit including at least one of said inductive devices, in particular transformers or inductors.

This magnetic core arrangement according to the invention provides significant benefits over prior art. The use of a thin slit allows modulating the effective permeability of the core depending on the DC current present in the circuit. The desired magnetic behavior of the core can be precisely optimized through the construction of the core and of the slit(s).It is thus possible to provide better AC performance, higher DC withstand, improved stability, and improved reliability.

In a preferred embodiment at least one magnetic core is made from magnetic material, such as ferromagnetic material, with a relatively high relative permeability in order to ensure very good AC performance.

Furthermore, the magnetic core is provided with at least one narrow slit or groove in the cross- section of the core, wherein slit reduces locally the cross-sectional area of the magnetic material in the core causing a constriction of the magnetic path of the core. The slit can be any feature or means comprised by or applied to the magnetic core to provide a magnetic constriction of the magnetic core, wherein the cross-sectional area of the magnetic constriction is smaller than the main cross-sectional area of the magnetic core in the rest of the core, and where the length of the magnetic path of the constriction is significantly smaller than the length of the magnetic path of the core. In a simple embodiment, the length of the magnetic path of the constriction can be considered equal to the width of the slit. The effective permeability of the slit is significantly smaller than the effective permeability of the magnetic constriction.

In an exemplary embodiment the slit can be a cut, a groove, a kerf, a hole, or any other form of partial, thin opening provided in the magnetic path of the core.

When dealing with complex constructions of the magnetic core and/or of the magnetic portion, it can be relevant to consider the effective magnetic area instead of the cross-sectional area of some core or of some portion of the core, in particular.

If the magnetic flux in the core is sufficiently low and magnetic saturation in the constriction is not reached, the constriction provides high magnetic permeability and it is a preferred path for the magnetic flux. In this case, most of the magnetic flux is concentrated through the magnetic constriction and only a tiny relative amount of magnetic flux would cross the slit. The magnetic flux in the core is proportional, among others, to the effective permeability of the core and to the magnetomotive force applied to the core, where the total magnetomotive force shall be considered in the case of multiple windings. The cross-sectional area of the magnetic constriction is smaller than the main cross-sectional area of the magnetic core and causes the magnetic flux density to be larger in the constriction than in the rest of the core.

At moderate magnetomotive force, the magnetic flux reaches a value where magnetic saturation is partly reached in the constriction but not in the rest of the core. The magnetic flux where the constriction starts saturating drops with increasing depth of the slit. Partial saturation may refer to a gradual saturating behavior of the magnetic material itself and/or to a gradual saturation caused by non-homogenous magnetic flux density achieved within and/or near the constriction. In an exemplary embodiment partial magnetic saturation may occur first in a confined space within the constriction, close to the slit, and propagate through the region of the constriction as the magnetic flux increases. The permeability of the magnetic material in the constriction decreases as partial saturation becomes stronger and degrades the flux concentration capability of the magnetic constriction. The magnetomotive force over the constriction and over the slit increases with the magnetic saturation in the constriction, causing the relative amount of magnetic flux that crosses the slit to increase. At moderate to high magnetomotive force, sufficiently strong magnetic saturation is reached in the constriction and causes the incremental magnetic permeability of the material in the constriction to approach the magnetic permeability in the slit. At high magnetomotive force, strong magnetic saturation of the constriction is reached before the rest of the core saturates, and the magnetic core is still operational under further increasing magnetic flux. In this case, the effective permeability of the magnetic core is mainly given by the width of the slit.

The effective magnetic permeability of the slit core decreases with the magnetomotive force allowing to maximize the range of the magnetomotive force where the predominant part of the magnetic core does not saturate. The slit core reaches high effective permeability and high inductance at low magnetomotive force and sufficient effective permeability and sufficient inductance at high magnetomotive force. The effective permeability and the inductance of the slit core drop gradually with the magnetomotive force and enable optimum operation of the slit core over wide excitation range with minimum non-linearity errors. Sufficient inductance and gradual change of the inductance is ensured over the full operation range of the magnetic core if a sufficient amount of magnetic flux can cross the slit, in particular under moderate magnetomotive force and/or moderate to high magnetomotive force.

According to a further embodiment, this may typically be achieved if the width of the slit is much smaller than the length of the magnetic path of the core and if the depth of the slit is larger than the width of the slit, in particular the depth of the slit is considerable larger than the width of the slit. For example, the ratio between the length of the magnetic path of the core and the predominant width of the slit is larger than 150, in particular larger than 300.

Slit arrangements with complex constructions and geometries can be described using the effective width of the slit arrangement which can be the average mechanical width of the slit arrangement or which can be deduced from the magnetic response, e.g. B-H curve; of the core. In a similar way, the effective area of the slit arrangement is considered to be equal to the difference between the main cross-sectional area of the magnetic core and the cross-sectional area of the magnetic constriction. The slit arrangement can be generally characterized by the ratio between the square root of the effective area of the slit arrangement and the effective width of the slit arrangement. The ratio between the square root of the effective area of the slit arrangement and the effective width of the slit arrangement is preferably considerable larger than 1 , For example, the ratio between the square root of the effective area of the slit arrangement and the effective width of the slit arrangement is larger than 10, in particular larger than 25. The effective area of the slit arrangement takes a fraction of the main cross-sectional area of the magnetic core, wherein the fraction can be larger than 0.1, in particular larger than 0.4 of the main cross-sectional area of the magnetic core.

Advantageously, the slit core or grooved core features thus at least two operating regimes, wherein at low DC current the slit core provides high relative permeability overall and ensures very good AC performance and at high DC current, where the constriction saturates but the rest of the core is not saturated, the effective permeability of the slit core is still sufficient for useable AC performance.

Unlike a composite core, the slit or grooved core provides a more gradual transition between the two operation modes because saturation starts at the edge of the constriction and progressively extends through the constriction as the magnetic flux increases.

The operating mode, in which the constriction is only partly saturated, can be seen or understood as a third, intermediary operating regime which occurs at moderate DC currents. The intermediary operating regime of the slit or grooved core provides rather progressive evolution of the effective permeability of the core and of its inductance resulting in softer nonlinear effects.

In comparison, a composite core does not provide such intermediary regime and features abrupt change of the effective permeability, being prone to strong non-linear effects. In a composite core, each operating regime is mainly supported by one core and the saturation of the high permeability core occurs in rather homogenous and sudden manner.

Advantageously, in the slit or grooved core, almost all the magnetic material of the core is involved in conducting the magnetic flux in all operating regimes resulting in improved overlap and smoother transition between the different operating regimes but also resulting in an optimized material usage with regard to the magnetic flux. At low DC current, saturation does not occur in the constriction and the effective permeability and the inductance of the slit or grooved core are very high, being mainly defined by the magnetic material, dimensions, and construction of the magnetic core. For example, the value of the DC current may be considered as low if it is comprised between zero or null and a current value where the inductance of the slit or grooved core is equal to 50% of the inductance value reached at null DC current.

The relatively high inductance value of the slit or grooved core reached at low DC current leads to excellent AC performance. At moderate DC current, partial saturation occurs in the constriction and the permeability and the inductance of the slit or grooved core are high to moderate and depend on the magnitude of the magnetic flux in the core, through a relationship mainly involving the construction and dimensions of the slit and the magnetic material. The high to moderate inductance value leads to good AC performance.

At relatively high DC current, strong saturation occurs in the constriction and the permeability and the inductance of the slit core are moderate, being mainly defined by the width of the slit or groove. A moderate inductance value leads to acceptable AC performance.

The width and the depth of the slit are convenient means to control and/or to engineer the magnetic response of the core in a precise and reproducible manner. Moreover, the dimensions of the slit are very stable as they are not affected by assembly methods or by deformations of the core that often occur when gaps through the complete cross-section of the magnetic path are provided in traditional gapped cores.

Compared to common solutions, the slit or grooved core provides thus improved performance, linearity, reliability, and reproducibility. Furthermore, the slit or grooved core also features lower size and mass because of more efficient usage of the provided magnetic material.

In a further embodiment, in order to ensure useable permeability at high DC current, when the constriction is saturated, the predominant width of the slit or groove is relatively small, preferably less than 0.23 mm.

In further embodiments, for example for producing compact magnetic cores with effective permeability predominantly larger than 500, slits or grooves of approximately 0.1 mm or 0.15 mm are provided. Furthermore, the depth of the slit or groove can be provided in a wide range depending on the design target and in a farther embodiment of the magnetic core the depth of the slit is larger than the width of the slit or groove at least by a factor of 2, in particular at least by a factor of 5 or 10.

According to various embodiments the slit or groove may take from a tiny fraction up to a dominant amount from the cross-section of the magnetic core.

Moreover, according to various embodiments the slit or groove may be provided or applied from any side of the magnetic core, in particular from the inner and/or outer peripheral or circumferential area or surface or from the upper and/or lower side of the magnetic core. In farther embodiments the slit or groove may be applied orthogonal to the magnetic path of the core or under an angle a to the magnetic path of the core, in particular a may be in the range between 45° and 90°. Advantageously, the slit or groove may be intentionally produced at a different angle than orthogonal to the magnetic path of the core in order to provide for additional means to enhance the magnetic performance of the core.

Compared to magnetic cores with partial gaps, the slit or grooved cores are optimized for providing sufficiently high permeability at moderate and high DC currents.

The core arrangement according to the invention provides significantly thinner slit or groove arrangement compared to known magnetic cores with partial gap.

According to the invention advantageously the predominant width of the slit or groove is preferably smaller than 0.23 mm and, in some cases, the minimum width of one slit can be smaller than 0.17 mm.

Furthermore, the core arrangement is optimized to reach high inductance at low to moderate values of the magnetic flux by employing appropriate magnetic materials and core construction.

In a farther embodiment magnetic cores with tape- wound construction are provided, which tend to exhibit an optimal combination between compact size, high permeability, and low losses. For example tape-wound cores made from grain-oriented electrical steel may be used to provide reasonably high inductance. Tape-wound magnetic steel is a more general material family including also grain-oriented electrical steel. The main drawback of grain-oriented electrical steel is that they feature low permeability at low magnetic flux density, considerable magnetic losses, and high remanence.

In a further embodiment, to achieve high magnetic permeability over the full magnetic flux density range and very low losses magnetic cores made from nanocrystalline alloy may be applied.

Nanocrystalline cores can be processed, for example through appropriate thermal and magnetic annealing, to control their magnetic parameters such as permeability and remanence. High inductance of the core arrangement described here can be thus obtained at low to moderated DC currents by including at least one core portion forming a closed magnetic path and being primarily made from nanocrystalline alloy.

In a further embodiment the core arrangement is engineered to provide an effective relative magnetic permeability whose maximum value is larger than 5000, in particular larger than 15000, very low magnetic losses, and low remanence, B _r /B _s , below 0.35 or even 0.2. The core arrangement would thus provide excellent AC performance at low to moderated DC currents without being affected by magnetization shifts caused by large currents occurring in the electrical equipment.

In a further embodiment relatively thin slits or grooves are provided and/or produced by using appropriate slitting means, for example based on saws, abrasive cutting, laser cutting, or micromachining, wherein the aspect ratio of the slit or groove, i.e. the ratio between the depth and the width, which can be achieved using productive and cost efficient manufacturing methods, is limited. For example, manufacturing a slit or groove with the width below 0.23 mm and the aspect ratio larger than 6 or 10 can be technically and commercially challenging. Slits or grooves with lower aspect ratio are usually faster and with minor efforts and/or cheaper to produce.

It may be thus beneficial to provide multiple slits or grooves with reduced depth rather than one slit with larger depth in order to reach a magnetic constriction portion with similar size.

Accordingly in a further embodiment more than one slit or groove are provided in the magnetic core, wherein in particular two slits can be applied in the magnetic core for reaching an equivalent constriction as with one single slit having double the depth, in particular when provided and/or extending to each other at adjacent sides or surfaces of the magnetic core. In a further embodiment, when two slits are provided at the opposite lateral sides of the core by reducing the depth of one slit or groove its aspect ratio is lowered and fabrication is made easier, in particular if the width is relatively thin and/or in the range around 0.1 mm and 0.2 mm. In another embodiment, providing three or more slits would allow to further reduce the depth of each slit. It is thus convenient in a further embodiment to provide a slit arrangement comprising multiple slit portions with lower depth instead of one slit portion with large depth, wherein multiple slit portions may be aligned or slightly misaligned. In particular, multiple slit portions can be applied with intentional misalignments or at different angles in order to engineer the magnetic response of the constriction and/or of the magnetic core.

In another embodiment, the magnetic behavior of the core can be further enhanced by applying multiple slits where the slits have different width.

In a further embodiment two slits with different width are provided, one at each opposite or adjacent lateral side of the magnetic core.

In a further embodiment the slits or grooves may be provided at various sides of the core, wherein the slits may overlap at least partly or totally.

In another embodiment two overlapping slits or grooves are provided in the magnetic core, wherein a thin slit or groove is provided as an extension of a wider slit or groove, wherein the cross-section or profile of the slit or groove provides an almost step-like limb or structure. Such slit or grooved arrangement may also be perceived like one deep slit or grooved arrangement comprising two portions of different width.

In a further embodiment a slit or groove is provided, wherein the width of the slit or groove varies almost continuously, thus the cross-section of the slit or groove has an almost triangular or almost trapezium or conical shape. A slit whose width varies almost continuously, can possibly be perceived as being formed of multiple slit portions, where, for example, each slit portion has the width varying by no more than 10%.

A slit arrangement providing different or varying width values allows, for example, to better control the relationship between the effective permeability and the magnetic flux. The dependence of the slit core inductance on the DC current can be thus better optimized in order to reach better AC accuracy, stronger DC withstand, or both.

Significant design flexibility is provided by a further embodiment and a slit or groove arrangement where the ratio between the maximum width and the minimum width is larger than 1.3.

Depending on the fabrication process and the related tolerances, in a further embodiment it may also be suitable to have a slit arrangement where the difference between the maximum width and the minimum width is larger than 0.05 mm.

In a further embodiment the magnetic core comprises multiple overlapping slits where one pair of two overlapping slits is provided from opposite lateral sides of the core, wherein using and/or providing multiple slits or grooves and different slit or groove widths provides wide design possibilities for shaping and optimizing the magnetic performance of the slit or grooved core.

In a further embodiment the magnetic core arrangement can be further enhanced by combining a first slit or grooved core according to the invention with at least one second magnetic core. Because the slit or grooved core can be optimized for providing both excellent AC performance and strong DC withstand, different constructions for the second core are possible depending on the performance type which needs to be further enhanced. Combining a first slit or grooved core and at least one second magnetic core allows boosting the operating limits of each core by providing complementary performance. In contrast to composite cores known from the art, the slit or grooved core according to the invention also allows achieving significant overlap between the operating regimes of each core. The overlap can be tuned through the design of the slit or grooved core and of the at least one second core in order to avoid non-linear effects and to reach the desired performance at various current levels. Efficient means to tailor the magnetic behavior of the slit or grooved core have been provided in this description.

In an advantageous embodiment the at least one second magnetic core may be for example a further slit core and/or a gapped core and/or a full or solid core without a slit or a gap.

A further slit or grooved core as second core would allow to further enhance the AC performance, or the DC withstand, or both depending on the magnetic material and on the construction of the core and of the slit(s) or groove(s). A gapped core as second core may allow to further enhance the DC withstand of the core arrangement. The gapped core could then be primarily made from a magnetic material with high B _s , such as grain-oriented electrical steel.

A full core as second core may allow to further boost the AC performance of the core arrangement.

Even a combination of the different magnetic core types as described above seems to be possible according to a further embodiment of the magnetic core arrangement.

In a further embodiment a magnetic core arrangement comprising a slit or grooved core and a full or solid core is provided, wherein the full core is preferably made from a magnetic material with high permeability, very low losses, and low remanence such as nanocrystalline alloy.

Furthermore, the slit or grooved core may then be made from a magnetic material primarily optimized for high AC performance or extra strong DC withstand, the latter being best served by a material with high B _s such as grain-oriented electrical steel.

In a further embodiment the magnetic cores may be shaped like a ring or approximately like a ring, however, other geometries such as oval or rectangular are also possible.

Furthermore, in the magnetic core arrangement the magnetic cores may be assembled in concentrically arrangement or in stacked arrangement, wherein in a stacked arrangement the two or more magnetic cores have roughly equal inner diameters and/or outer diameters. In particular, at least two magnetic cores are stacked and feature approximately equal inner diameters and/or outer diameters.

In a further embodiment of the magnetic core arrangement the arranged magnetic cores are fixed together by various means to form or build a composite core, in particular in one piece, for example by using adhesive joining means, tape, clamps, and the like.

In a further embodiment the magnetic core arrangement and/or any arranged magnetic core is provided with a mechanical encapsulation, preferably a polymer layer or coating such as epoxy, which ensures a smooth surface suitable for applying electrical windings without having to enclose the core arrangement in a case. Avoiding a case is suitable for minimizing the dimensions of the inductive device and minimizing the resistance of the secondary winding resulting in better electrical performance.

Advantageously, the slit or groove core may be provided with a mechanical encapsulation prior or after slitting or grooving.

In another embodiment the edges of the slit or groove comprise additional protection means in order to provide a smooth surface of the magnetic core, for example in order to avoid damaging the electrical windings. For example, local coating or adhesive tape may be applied if necessary.

Furthermore, the magnetic core arrangement and/or the magnetic core as described above may be applied to an inductive device like an inductor or transformer. Thus, an inductive device comprising at least one magnetic core arrangement and/or magnetic core and/or at least one electrical winding being configured to be wound around said magnetic core arrangement as described above is covered by and included in said application and claimed also.

A current transformer comprising the described core arrangement provides much better AC accuracy, wider AC current operating range, stronger DC current withstand, reduced size, lower mass, and lower cost.

An installation device comprising one or more inductive devices, in particular one or more current transformers and/or inductors, is covered by and included in said application and claimed also.

Further advantages, features, aspects and details that can be combined with embodiments described herein are evident from the dependent claims, the description and the drawings.

Brief description of the Figures:

The details will be described in the following with reference to the figures, wherein

Fig. 1 two views of a magnetic core arrangement comprising one core where a slit or groove is provided from the outer side from the core;

Fig. 2 two views of a magnetic core arrangement comprising one core where a slit or groove is provided from the lateral side or upper side of the core; Fig 3 two views of a magnetic core arrangement comprising one core where two slits are provided from opposite lateral sides or the upper and lower side of the core;

Fig. 4 two views of a magnetic core arrangement comprising one corewhere two slits with different width are provided, one at each opposite lateral side or the upper and lower side of the core ;

Fig. 5 two views of a magnetic core arrangement comprising one core with two overlapping slits where a thin slit is provided as an extension of a wider slit;

Fig. 6 two views of a magnetic core arrangement comprising one core where a core comprises multiple overlapping slits where one pair of two overlapping slits is provided from opposite lateral sides or the upper and lower side of the core , and

Fig. 7 two views of a magnetic core arrangement comprising two cores wherein a slit core and a full core is provided.

In Fig. 1 a side view Fig. la and a top view Fig. lb of a magnetic core arrangement comprising one core 1 where a slit 11 or groove is provided from the outer side from the core 1 is presented.

According to Fig. 1 a ring-like core 1 is provided comprising a narrow slit portion 11 or groove through the cross-section of the magnetic core 1 in order to create a constriction and to locally reduce the cross-sectional area of the magnetic path, wherein the width of the slit 11 or groove is relatively small such that the magnetic flux can cross the slit 11 , in particular under“moderate” magnetomotive force.

The magnetic core 1 is made from magnetic material, such as ferromagnetic material, with high relative permeability in order to ensure very good AC performance. As long as it does not saturate, the constriction provides high magnetic permeability and it efficiently channels the magnetic flux. The width of the slit is much smaller than the length of the magnetic path of the core and, at low value of the magnetic flux density, the effective permeability of the core is only marginally affected by the constriction. The magnetic flux density is higher in the constriction than in the rest of the core and causes the constriction to start saturating at lower magnetic flux than the rest of the core. The magnetic permeability of the constriction drops drastically under strong saturation and its magnetic behavior approaches that of the slit. Strong saturation of the constriction is reached before the rest of the core saturates and the effective permeability of the magnetic core is mainly given then by the width of the slit. The magnetic flux where the constriction saturates drops with increasing depth of the slit.

The dimensions of the slit or groove allow optimizing the magnetic performance of the core in a precise and reproducible manner. Moreover, the dimensions of the slit or groove are very stable as they are not affected by drifts, in particular with regard to heating or temperature effects, that may occur in traditional gapped cores. The slit or groove core provides thus improved performance, linearity, reliability, and reproducibility. Furthermore, the slit or groove core also features lower size and mass because of more efficient material usage.

In order to ensure useable permeability at high DC current, when the constriction is saturated, the width of the slit or groove is made relatively small, typically less than 0.23 mm. The depth and the width of the slit or groove are defined/provided according to the desired performance of the magnetic core and, for some applications, slits or grooves of approximately 0.1 mm or 0.15 mm are desired. The depth of the slit or groove can be prepared in a wide range depending on the design target and the depth is usually larger than the width, for example at least by a factor of one or two. The slit or may take from a tiny fraction up to a dominant amount from the cross-section of the magnetic core. The slit or groove may be provided at any location and at any side of the core, as for example shown in Fig. 1 and Fig. 2. The slits 11, 12 or grooves shown in the exemplary embodiments of Fig. 1 and Fig. 2 extend along a surface of the magnetic core 1,2 and have almost constant depth. In Fig. 2 a side view Fig.2a and a top view Fig 2b of a magnetic core arrangement comprising one core 2 where a slit 12 or groove is provided from the upper side from the core 1 is presented, wherein the slit 12 or groove is arranged and/or extending in radial direction.

Alternatively it is also possible to vary the depth of the slit or to provide the slit or groove at some edgeof the magnetic core.

The slit or groove may be arranged almost orthogonal to the magnetic path of the core, like shown in Fig. 1 and Fig. 2, or under an angle a to the magnetic path. The slit or groove may be intentionally produced at a different angle than orthogonal to the magnetic path of the core in order to provide for additional means to enhance and to engineer the magnetic response and/or performance of the core. The predominant width of the slit is preferably smaller than 0.23 mm and, in some cases, the minimum width of one slit can be smaller than 0.17 mm.

The aspect ratio of a slit, i.e. the ratio between the depth and the width, which can be achieved using productive and cost efficient manufacturing methods is limited. Slits with lower aspect ratio, in particular below six, are usually faster and cheaper to produce. It may be thus beneficial to provide multiple slits with reduced depth rather than one slit with large depth in order to achieve a similar effective area of the slit arrangement. For example, two slits can be applied in the core for reaching an equivalent constriction as with one single slit having double the depth.

Fig.3 shows an exemplary embodiment of a magnetic core arrangement in a side view Fig.3a and a top view Fig.3b comprising one core 3 where two slits 13a, 13b are provided from opposite lateral sides or rather the upper and lower side of the magnetic core 3 having both a reduced depth compared to the slit 12 of the embodiment according to Fig. 2. Reducing the depth of a slit 13 a, 13b lowers its aspect ratio and makes it easier to fabricate especially if its width is very thin, for example around 0.1 mm or 0.2 mm.

Providing 3 or more slits would allow to further reduce the depth of each slit. It is thus convenient to provide a slit arrangement comprising multiple slit portions with lower depth instead of one slit portion with large depth. Multiple slit portions may also be applied with intentional misalignments or at different angles in order to controlthe magnetic behavior of the constriction and thus of the core.

For example, the magnetic behavior of the core can be further enhanced by applying multiple slits having different width, as shown in Fig. 4. In Fig.4 an exemplary embodiment of a magnetic core arrangement with one magnetic core 4 is shown in a side view Fig.4a and a top view Fig.4b presentation, comprising two slits 14a, 14b with different width are provided, one slit 14a, 14b at each opposite lateral side of the core or rather the upper side and lower side of the core.

Other configurations are also possible by producing the slits at various sides of the core and the slits may overlap partly or totally. In Fig.5 an exemplary embodiment of a magnetic core arrangement with one magnetic core 5 is shown in a side view Fig.5a and a top view Fig 5b presentation, comprising two overlapping and/or nesting slits 15a, 15b where a thin slit 15b is provided as an extension of a wider slit 15a. Such slit arrangement 15a, 15b may also be perceived like one deep slit arrangement comprising two portions of different width. It is also possible according to the invention to provide a slit where the width varies almost continuously.

In Fig.6 an exemplary embodiment of a magnetic core arrangement with one magnetic core 6 is shown in a side view Fig.6a and a top view Fig.6b presentation, comprising multiple overlapping slits 16a, 16b, 16e, 16d where one pair 16a, b; 16c, d of two overlapping slits is provided from opposite lateral sides or rather the upper and lower side of the magnetic core 6. Using multiple slits and different slit widths provides wide design possibilities for engineering and optimizing the magnetic performance of the slit core.

As disclosed in Fig. 7 the core arrangement can be further enhanced by combining a slit magnetic core 7 with a second magnetic core 8. Because the slit magnetic core 7 can be optimized for providing both excellent AC performance and strong DC withstand, different constructions for the second core are possible depending on the performance type which needs to be further enhanced. Combining a slit magnetic core 7 and a second magnetic core 8 allows boosting the operating limits of each core by providing complementary performance. In contrast to composite cores known from the art, the slit core also allows achieving significant overlap between the operating regimes of each core. The overlap can be tuned through the design of the slit core and of the second core in order to avoid non-linear effects and to reach the desired performance at various current levels. Efficient means to tailor the magnetic behavior of the slit core have been provided in this description. The second core may be for example a slit core, a gapped core, or a full / solid core without a slit or a gap as presented in the exemplary embodiment in Fig. 7. A slit or grooved core as second core would allow to further enhance the AC performance, or the DC withstand, or both depending on the magnetic material and on the construction of the core and of the slit(s). The cores comprised in the core arrangement can be fixed together by various means, for example using adhesive joining means, tape, clamps, etc. The cores may be assembled in concentric arrangement or in stacked arrangement. The cores comprised in a stacked arrangement would preferably feature approximately the same inner and outer dimensions.

In Fig. 7 an exemplary embodiment of a magnetic core arrangement with two magnetic cores 7, 8 is shown in a side view Fig.7a and a top view Fig.7b presentation, comprising Fig. 7 shows comprising a slit or grooved core 7 and a full solid core 8. The cores 7, 8 are ring shaped and are assembled in a stacked manner, featuring approximately equal imier and outer diameters.

The magnetic core arrangement as such or any of the provided cores may be provided with a mechanical encapsulation, preferably a polymer layer or coating such as epoxy. This ensures a smooth surface suitable for applying electrical windings without having to enclose the core arrangement in a case.

The slit or grooved core 7 may be provided with mechanical encapsulation prior or after slitting or grooving. The edges of the slit may need additional protection in order to provide a smooth surface of the core, for example in order to avoid damaging the electrical windings. For example, local coating or adhesive tape may be applied if necessary.