EP1676284B1

EP1676284B1 - Core for a controllable inductive device

Info

Publication number: EP1676284B1
Application number: EP04775086A
Authority: EP
Inventors: Espen Haugs; Frank Strand
Original assignee: Magtech AS
Current assignee: Magtech AS
Priority date: 2003-10-14
Filing date: 2004-10-13
Publication date: 2011-11-30
Anticipated expiration: 2024-10-13
Also published as: PT1676284E; GB0324092D0; WO2005036568A1; CA2537700A1; CN1868008B; EA200600771A1; ES2378160T3; HK1092937A1; CN1868008A; CA2537700C; BRPI0415384A; ATE535920T1; EA008971B1; EP1676284A1; GB2407214A; JP2007508711A; KR101082576B1; JP4630873B2; KR20070023623A

Abstract

A variable inductor device comprises a magnetic core with first and second coaxial and concentric cylindrical members 101, 102 formed from magnetic material which are arranged with a high permeability direction and a lower permeability direction. The ends of the cylindrical members 101,102 are magnetically interconnected by end coupling members 105, 106. A first winding 103 is wound around both of the magnetic cylinders with an axis perpendicular to that of the cylinders. A second winding 104 is wound around at least one of the cylindrical members and has an axis that coincides with that of the cylinders. The first winding 103 may provide a magnetic field which is aligned with the high permeability direction of the magnetic core and the second winding 104 may be orthogonal to the first winding and provide a magnetic field which is aligned with a lower permeability direction, or vise versa. A magnetic coupler device 105, 106 and a magnetic structure are also claimed. The magnetic coupler 105, 106 may be formed from anisotropic magnetic sheet or wire material and arranged to form a toroidal member for magnetically coupling two coaxial and concentric cylindrical magnetic core members. A non-magnetic spacer material may be used between the cylinders and the end couplers. The magnetic structure has two windings which are orthogonal to one another and act on a closed magnetic core formed from anisotropic material. The variable inductor device provides a low loss variable inductor for high power applications.

Description

The present invention relates to a controllable inductor, and more particularly a controllable inductor comprising first and second coaxial and concentric pipe elements comprising anisotropic material, wherein said elements are connected to one another at both ends by means of magnetic end couplers, a first winding wound around both said magnetic pipe elements, a second winding wound around at least one of said magnetic pipe elements, where a winding axis for the first winding is perpendicular to an axis of at least one of the magnetic pipe elements, wherein a winding axis of the second winding coincides with the axis, wherein, when energized, the first winding generates a magnetic field in a first direction that coincides to a direction of a first magnetic permeability, wherein, when energized, the second winding generates a magnetic field in a second direction that coincides to a direction of a second magnetic permeability, and wherein the first magnetic permeability is substantially higher than the second magnetic permeability.
There is a long standing interest in using a control field to control a main field in an inductive device.
US 4, 210, 859 describes a device comprising an inner cylinder and an outer cylinder joined to one another at the ends by means of connection elements. In this device the main winding is wound around the core and passes through the cylinder's central aperture. The winding axis follows a path along the cylinder's periphery. This winding creates an annular magnetic field in the cylinder's wall and circular fields in the connection elements. The control winding is wound around the cylinder's axis. It will thus create a field in the cylinder's longitudinal direction. As usual in magnetic materials, the core's permeability is changed by the action of the control current applied to the control winding. Because the cylinders and the connection elements are made of the same material, the rate of change of permeability is the same in both types of elements. Consequently, the magnitude of the control field must be limited to prevent saturation of the core and decomposition of the control field. As a result, the control range of this inductor is thus limited, and the device, in US 4, 210, 859 has a relatively small volume that limits the device's power handing capability.
Other devices include controlled permeability of only part of the main flux path. However, such an approach dramatically limits the control range of the device. For example, US 4,393,157 describes a variable inductor made of anisotropic sheet strip material. This inductor comprises two ring elements joined perpendicularly to one another with a limited intersection area. Each ring element has a winding. The part of the device where magnetic field control can be performed is limited to the area where the rings intersect. The limited controllable area is a relatively small portion of the closed magnetic circuits for the main field and the control field. Part of the core will saturate first (saturation will not be attained simultaneously for all parts of the core because different fields act upon different areas) and this saturation will result in losses generated by stray fields from the main flux. Partial saturation results in a device with a very limited control span.
United States Patent Application US 2 883 604 discloses a magnetic frequency changer where a single core of grain oriented material with input windings placed upon the core parallel and perpendicular to the direction of grain orientation are provided. This is used to provide a magnetic frequency changer either as a frequency divider or a frequency multiplier.
United States Patent US 3 351 860 discloses changing the reactance of an aerial circuit to provide automatic tuning by providing a control signal coupled to a winding in a ferrite pot core to vary the inductance of a controlled winding.
International Patent Application WO0190835 discloses a magnetically influenced current or voltage regulator comprising a body which is composed of a magnetisable material and provides a closed, magnetic circuit, at least one first electrical conductor wound about the body of a first main winding and at least one second electrical conductor wound about the body of a second main winding. The winding axis for the main winding is at right angles to the winding axis for the control winding with the object of providing orthogonal magnetic fields in the body and thereby controlling the behaviour of the magnetisable material relative to the field in the main winding by means of the field in the control winding.
Thus, the prior art lacks a means to control permeability in a core for substantial power handling capability without introducing considerable losses. The shortcomings of the prior art effect all inductive device geometries, and in particular, curved structures made of sheet strip metal because considerable eddy currents and hysteresis losses occur in these types of curved structures.
The invention addresses these shortcomings and can be implemented in a low loss controllable inductive device suitable for high power applications. Generally, the invention can be used to control the magnetic flux conduction in a rolling direction by controlled domain displacement in a transverse direction.
The present invention provides a core for a magnetic controllable inductor according to appended Claim 1. Preferred embodiments are set out in the dependent claims.
As will be explained in the following, the permeability of grain-oriented material in the rolling direction is controlled by employing a control field in the transverse direction. A controllable inductive device of grain-oriented steel can be magnetized in the transverse direction. In an embodiment, a controllable inductor comprising first and second coaxial and concentric pipe elements is provided. The elements are connected to one another at both ends by means of magnetic end couplers. A first winding is wound around both said elements, and a second winding is wound around at least one of said elements. The winding axis for the first winding is perpendicular to the elements' axes and the winding axis of the second winding coincides with the elements' axes. The first and second magnetic elements are made from an anisotropic magnetic material such that the magnetic permeability in the direction of a magnetic field introduced by the first of the windings is significantly higher than the magnetic permeability in the direction of a magnetic field introduced by the second of the windings. In a version of this embodiment, the anisotropic material is selected from a group consisting of grain-oriented silicon steel and domain controlled high permeability grain oriented silicon steel.
In one embodiment, the magnetic end couplers are made of anisotropic material and provide a low permeability path for the magnetic field created by the first winding and a high permeability path for the magnetic field created by the second winding. The controllable inductor may also include a thin insulation sheet situated between magnetic pipe element edges and the end couplers.
There is also described in the following a controllable magnetic structure that includes a closed magnetic circuit. The closed magnetic circuit includes a magnetic circuit first element, and a magnetic circuit second element. Each of the magnetic circuit elements is manufactured from an anisotropic material having a high permeability direction. The controllable magnetic structure also includes a first winding which is wound around a first portion of the closed magnetic circuit, and a second winding which is oriented orthogonal to the first winding. The first winding generates a first magnetic field in the high permeability direction of the first circuit element and the second winding generates a second field in a direction orthogonal to the first field direction when the respective windings are excited (i.e., energized).
In one example, the controllable magnetic structure includes a first circuit element that is a pipe member and a magnetic circuit second element that is an end coupler that connects a first pipe member to a second pipe member. In a version of this example, the first pipe member and the second pipe member are located coaxially around an axis and the high permeability direction is an annular direction relative to the axis. Additionally, the second high permeability direction can be in a radial direction relative to the axis. In another version of this example, the controllable magnetic structure is manufactured from grain-oriented material. In yet another version of this example, the controllable magnetic structure is an inductor.
In another example, insulation is located in the closed magnetic circuit between the magnetic circuit first element and the magnetic second element. In another example, the magnetic circuit second element has a volume that is 10% to 20% of the volume of the magnetic circuit first element.
In accordance with the invention, a core is provided for a magnetic controllable inductor. The core includes first and second coaxial and concentric pipe elements and each pipe element is manufactured from an anisotropic magnetic material. An axis is defined by each pipe element and the pipe elements are connected to one another at both ends by means of magnetic end couplers. In addition, the core presents a first magnetic permeability in a first direction parallel to the axes of the elements that is significantly lower than a second magnetic permeability in a second direction orthogonal to the elements' axes. Further, in the core of the embodiment, a third magnetic permeability exists in the coupler in an annular direction relative to the axes of the elements and a fourth magnetic permeability exists in the coupler in a radial direction relative to the axes of the elements. The fourth magnetic permeability is substantially greater than the third magnetic permeability.
In a version of this embodiment, first and second pipe elements are made of a rolled sheet material comprising a sheet end and a coating of an insulation material. In another version, the first pipe element includes a gap in the third direction parallel to the axes of the elements and the first and second pipe elements are joined together by means of a micrometer thin insulating layer in a joint located between the first and second pipe elements. In a further version, an air gap extends in an axial direction in each pipe element and a first reluctance of a first element equals a second reluctance of the second element. In one example, the insulation material is selected from a group consisting of MAGNETITE-S and UNISIL-H.
A magnetic coupler device connects first and second coaxial and concentric pipe elements to one another to provide a magnetic core for a controllable inductor. The magnetic end couplers are manufactured from anisotropic material and provide a low permeability path for magnetic field created by the first winding and a high permeability path for magnetic field created by a second winding. In a version of this embodiment, the magnetic coupler includes grain-oriented sheet metal with a transverse direction that corresponds to the grain-oriented direction of pipe elements in an assembled core. In addition, the grain-oriented direction corresponds to the transverse direction of the pipe elements in the assembled core to assure that the end couplers get saturated after the pipe elements. In a version of this embodiment, the magnetic end couplers are manufactured from a single wire of magnetic material. In another version of this embodiment, the magnetic end couplers are manufactured from stranded wires of magnetic material.
The magnetic end couplers may be produced by a variety of means. In one embodiment, the end couplers are produced by rolling a magnetic sheet material to form a toroidal core. The core is sized and shaped to fit the pipe elements, and the cores are divided into two halves along a plain perpendicular to the material's Grain Orientation (GO) direction. Additionally, the end coupler width is adjusted to make the segments connect the first pipe element to the second pipe element at the pipe element ends. In another embodiment, the magnetic end couplers are produced from either stranded or single wire magnetic material wound to form a torus and the torus is divided into two halves along a plane perpendicular to all the wires.
In another embodiment, the invention implements a variable inductive device with low remanence, so that the device can easily be reset between working cycles in AC operation and can provide an approximately linear, large inductance variation.
The invention will now be described in detail by means of examples illustrated in the following drawings.

Figure 1 shows a sheet of magnetic material and the relative position of the rolling and axial direction.
Figure 2 shows a rolled core and the rolling and axial directions defined in it.
Figure 3 shows a sheet of grain oriented material and the grain and transverse directions defined in it.
Figure 4 shows a rolled core of grain oriented material, and the grain and transverse directions defined in it.
Figure 5 shows the relative positions of the different directions in a pipe element.
Figure 6 shows schematically a part of a device according to an embodiment of the invention.
Figure 7 shows the device according to the embodiment of Figure 6.
Figure 8 shows sectional view of the device shown in Figure 7.
Figure 9 shows the position of thin insulation sheets between the magnetic end couplers and the cylindrical cores of a device according to an embodiment of the invention.
Figure 10 shows production of magnetic end couplers based on magnetic sheet 10 material.
Figure 11 shows a torus for production of magnetic end couplers based on strands of magnetic material.
Figure 12 shows a cross section of torus shaped magnetic material for production of magnetic end couplers.
Figure 13 shows the grain and transverse direction in magnetic end couplers described herein.
Figure 14 shows a view of a torus for production of magnetic end couplers whose shape is adjusted to fit pipe elements in accordance with an embodiment of the invention.
Figure 15 shows a torus produced with magnetic wire.
Figure 16 shows a cross-sectional view of the torus of Figure 15.
Figure 17 shows the domain structure in grain oriented material.

Sheet strip material is used in production of magnetic cores. These cores can be made for example, by rolling a sheet of material into a cylinder or by stacking several sheets together and then cutting the elements which will form the core. It is possible to define at least two directions in the material used to produce the "rolled" cores, for example, the rolling direction ("RD") and the axial direction ("AD").
Figures 1 and 2 show a sheet of magnetic material and a rolled core respectively. The rolling and the axial direction (RD, AD) are shown in these Figures. As shown in Figure 2, the rolling direction of a rolled core follows the cylinder's periphery and the axial direction coincides with the cylinder's axis.
Material that has magnetic characteristics that vary depending upon the direction in the material is referred to as anisotropic. FIGS. 3 and 4 show directions defined in a sheet of grain-oriented anisotropic material. Grain oriented ("GO") material is manufactured by rolling a mass of material between rollers in several steps, together with the heating and cooling of the resulting sheet. During manufacture, the material is coated with an insulation layer, which affects a domain reduction and a corresponding loss reduction in the material. The material's deformation process results in a material where the grains (and consequently the magnetic domains) are oriented mainly in one direction. The magnetic permeability reaches a maximum in this direction. In general, this direction is referred to as the GO direction. The direction orthogonal to the GO direction is referred to as the transverse direction ("TD")·UNISIL and UNISIL-H, for example, are types of magnetic anisotropic materials. In one embodiment, the grain oriented material provides a substantially high percentage of domains available for rotation in the transverse direction. As a result, the material has low losses and allows for improved control of the permeability in the grain oriented direction via the application of a control field in the tranversal direction TD.
Other types of anisotropic material are the amorphous alloys. The common characteristic for all these types is that one can define an "easy" or "soft" magnetization direction (high permeability) and a "difficult" or "hard" magnetization direction (low permeability). The magnetization in the direction of high permeability is achieved by domain wall motion, while in the low permeability direction, magnetization is achieved by rotation of the domain magnetization in the field direction. The result is a square m-h loop in the high permeability direction and a linear m-h loop in the low permeability direction (where m is the magnetic polarization as a function of the field strength h). Further, in one embodiment, the m-h loop in the transverse direction does not show coercivity and has zero remanence. In this description, the term GO is used when referring to the high permeability direction while the term transverse direction ("TD") is used when referring to the low permeability direction. These terms will be used not only for grain oriented materials but for any anisotropic material used in the core according to an embodiment of the invention. In one embodiment, the GO direction and the RD direction are in the same direction. In a further embodiment, the TD direction and the AD direction are the same direction. In another embodiment, the anisotropic material is selected from a group of amorphous alloy consisting of METGLAS Magnetic Alloy 2605SC, METGLAS Magnetic Alloy 2605SA1, METGLAS Magnetic Alloy 2605CO, METGLAS Magnetic Alloy 2714A, METGLAS Magnetic Alloy 2826MB, and Nanokristallin R102. In still a further embodiment, the anisotropic material is selected from a group of amorphous alloys consisting of iron based alloys, cobalt based alloys and iron-nickel based alloys.

Although the use of anisotropic material is described, other materials may be used provided that they have a suitable combination of the following characteristics: 1) high peak magnetic polarization and permeability in the RD; 2) low losses, 3) low permeability in the TD; 4) low peak magnetic polarization in the TD; and 5) rotation magnetization in the transverse direction. Table 1 includes a partial list of materials in which the sheet strip may be implemented and some of the characteristics of the materials that are relevant to one or more embodiments of the invention.

TABLE I

Material	Bmax at 800 A/m	Loss at 1.5T, 50Hz	Material Type	Thickness
Unisil-H 103-27-P5	1.93T	0.74W.kg	grain oriented	0,27 mm
Unisil-H 105-30-P5	1.93T	0.77W/kg	grain oriented	0,30 mm
NO 20 grade	1.45T	2.7W/kg	non-oriented		0,2 mm
Unisil M 140-30-S5	1.83T	0.85W/kg Max permeability is approx. 6000	grain oriented	0,3 mm
Unisil 140-30-S5, AC magnetization curve in the transverse direction	1.4T (1.15T at 120 A/m)	Max permeability is approx. 800

Figure 5 shows an example of a pipe element in a variable inductance according to an embodiment of the invention. Because this element is made by rolling a sheet of anisotropic material, one can define the rolling direction (RD), the axial direction (AD), the high permeability (GO) direction, and the low permeability (TD) direction. The relative positions of these directions in the element are shown in Figure 5. The pipe element can have any cross section because the shape of the cross section will simply depend on the shape of the element around which the sheet is rolled. If the sheet is rolled on a parallel piped with square cross section, the pipe element will have a square cross section. Similarly, a sheet rolled on a pipe with an oval cross section will be formed into a pipe with an oval cross section. In one embodiment, the pipe element is a cylinder.
Figure 6 shows schematically a part of an embodiment of a device 100 according to the invention. This device 100 comprises a first pipe element 101 and a second pipe element 102, where the elements are connected to one another at both ends by means of magnetic end couplers. For clarity, the magnetic end couplers are not shown in this leisure. A first winding 103 is wound around elements 101 and 102 with a winding axis perpendicular to the elements' axes. The magnetic field (Hf, Bf) created by this winding when activated will have a direction along the element's periphery i.c.7 an annular direction relative to the elements' axes. A second winding 104 is wound around clement 102 with a winding axis parallel to the elements' axes. The magnetic field created by this winding when activated (Hs, Bs) will have a direction parallel to the elements' axes, i.e., an axial direction relative to the elements' axes. In one embodiment, the winding axis of the second winding 104 is coincident the elements' axes. In another embodiment, the elements' axes are not coincident to one another.
If we combine the windings and magnetic fields of Figure 6 with the rolled material core of Figure 5, a device 100 according to one embodiment of the invention results. In a version of this embodiment, the magnetic permeability in the direction of a magnetic field (Hf, Bf) introduced by the first winding 103 (i.e., the direction of GO, RD) is significantly higher than the magnetic permeability in the direction of a magnetic field (Hs, Bs) introduced by the second winding 104 (i.e., the direction of TD, AD).
In one embodiment, the first winding 103 constitutes the main winding and the second winding 104 constitutes the control winding. In this embodiment, the main field (Hf, Bf) is generated in the high permeability direction (GO, RD) and the control field (Hs, Bs) is generated in the low permeability direction (TD, AD).
Minimum losses result when anisotropic material is used to provide the device 100 as described with reference to Figures 5 and 6. These results are achieved regardless of whether the device 100 is employed in a linear application or a switched application. In a linear application, the device 100 is switched on and remains in a circuit as an inductance. In a switched application the device 100 is used for connecting and disconnecting another device to a power source.
Low losses allow the device 100 to be employed in high power applications, for example, applications in circuits that can employ transformers ranging from a few hundred kVA to several MVA in size.
As shown in Equation 44) the power handling capacity of the core is dependent on the maximum blocking voltage Ub at high permeability and the maximum magnetizing current Im at the minimum value of the controlled permeability. $Ps = Ub \cdot Im$
If the magnetizing current and the blocking voltage arc expressed as functions of the magnetic field density Em, the apparent power 1's can be expressed as: $Ps = π \cdot f \cdot {Bm}^{2} \cdot \frac{Vj}{μ}$
Where Vj is the volume of the main flux path in the core, no is the permeability of free space, and µ_r is the relative permeability of the core. Equation 45) shows that the power handling capacity is related to both the volume of the core and the relative permeability of the core. At very high permeability the magnetizing current is at its lowest level and only a small amount of power is being conducted.
It is clear from Equation 45) that the apparent power Ps per volume unit of the core is related to the relative permeability µ_r. For two similar cores, where the minimum relative permeability of the first core is half the minimum relative permeability of a second core, the first core's apparent power is twice as large as the second core. Thus, the power handling of a given core volume is limited by the minimum relative permeability of the core volume.
Accordingly, in one embodiment, the volume of the magnetic end couplers is approximately 10-20% of the main core but the magnetic end coupler volume can be further lowered to ½ or ¼ of that depending on the construction of the core, and the necessary power handling capacity. In one such embodiment, the volume of magnetic end couplers is 5%-10% of the volume of the main core. In yet another embodiment, the volume of the magnetic end couplers is 2.5% - 5% of the volume of the main core.
A novel phenomonological theory of the magnetization curves and hysteresis losses in grain oriented (GO) laminations is described in an article entitled, "Comprehensive Model of Magnetization Curve, Hysteresis Loops, and Losses in Any Direction in Grain-Oriented Fe-Si", by Fiorillo et al. which published in IEEE Transactions on Magnetics, vol. 38, NO. 3, May 2002 (hereinafter "Fiorillo et al."). Fiorillo et al. provides theoretical and experimental proof of the fact that the volume that evolves with magnetization in the transverse direction is occupied for magnetization in the rolling direction. Thus, the article demonstrates that it is possible to control permeability in one direction by means of a field in another direction.
Fiorillo et al. also provides a model of the processes in a GO material. It presents, for example, a model that includes magnetization curves, hysteresis loops, and energy losses in any direction in a GO lamination. The model is based on the single crystal approximation and describes that the domains evolve in a complex fashion when a field is applied along the TD. Referring to Figure 17, a GO sheet comprises a pattern of 180 domain walls basically directed along the RD.
The demagnetized state (Figure 17a) is characterized by magnetization Js directed along [001] and [001]. When a field is applied in the TD (Figure 17b), the basic 180° domains transform, through 90° domain wall processes, into a pattern made of bulk domains, having the magnetization directed along [100] and [010] (i.e. making an angle of 45° with respect to the lamination plane). When this new domain structure occupies a fractional sample volume the macroscopic magnetization value is: $J_{90} = \frac{J_{s}}{\sqrt{2}} v_{90}$

J ₉₀ =: Magnetization in TD
J _s =: Magnetization in RD
v ₉₀ =: Fractional sample volume

The maximum magnetization obtainable at the end of the magnetization process is J90 = 1.42 Tesla and further increase is obtained by moment rotations of domains.
Fiorillo et al. also shows that the volume of the sample occupied by 180° domains decreases because of the growth of the 90° domains. Thus, permeability or flux conduction for fields applied in the rolling direction can be controlled with a control field and controlled domain displacement in the transverse direction.
The magnetization behaviour in the transverse direction in GO steel is described in "Magnetic Domains" by Hubert et al., Springer 2000, pages 416-417 and 532- 533. Control of the domain displacement in the transverse direction to control permeability in the rolling direction is most favourable primarily because motions of the 180 walls are avoided when a field is applied perpendicular to the 180 walls. Thus, the main field does not affect the orthogonal control field, in already TD magnetized volumes.
In contrast to GO steel where the magnetization mechanism in GO direction and the TD differ, the magnetization of non-oriented steel consists primarily of 180 domain wall displacements; therefore, the controlled volume is continuously affected by both the main field and the control field in non oriented steel.
Figure 7 shows a device 100 according to an embodiment of the invention. The Figure shows first pipe element 101, first winding 103, and the magnetic end couplers 105,106. The anisotropic characteristic of the magnetic material for the pipe elements has already been described, it consists of the material having the sot's magnetization direction (GO) in the rolling direction (RD).
The pipe elements are manufactured by rolling a sheet of GO material. In one embodiment, the GO material is high-grade quality steel with minimum losses, e.g., Cogent's Unisil HM105-30P5.
The permeability of GO steel in the transverse direction is approximately 1-10% of the permeability in the GO direction, depending on the material. As a result, the inductance for a winding which creates a field in the transverse direction is only 1-10% of the inductance in the main winding, which creates a field in the GO direction, provided that both windings have the same number of turns. This inductance ratio allows a high degree of control over the permeability in the direction of the field generated by the main winding. Also, with control flux in the transverse direction, the peak magnetic polarization is approx. 20% lower than in the GO direction. As a result, the magnetic end couplers in the device according to an embodiment of the invention are not saturated by the main flux or by the control flux, and are able to concentrate the control field in the material at all times.
To prevent eddy current losses and secondary closed paths for the control field, in one embodiment, an insulation layer is sandwiched between adjacent layers of sheet material. This layer is applied as a coating on the sheet material. In one embodiment, the insulation material is selected from a group consisting of MAGNETITE and MAGNETITE-S. However, other insulating material such as C and C-6, manufactured by Rembrandtin Lack Ges. m.b. H, and the like may be employed provided they are mechanically strong enough to withstand the production process, and also have enough mechanical strength to prevent electrical short circuits between adjacent layers of foil. Suitability for stress relieving annealing and poured aluminium sealing are also advantageous characteristics for the insulating material. In one embodiment, the insulation material includes organic/inorganic mixed systems that are chromium free. In another embodiment, the insulation material includes a thermally stable organic polymer containing inorganic fillers and pigments.
Figure 8 is a sectional view of the device 100. In this embodiment, the first pipe element 101 comprises a gap 107 in the element's axial direction located between a first layer and a second layer of the first pipe layer. The main function of gap 107 is to adapt the power handling capacity and volume of material to a specific application. The presence of an air gap in the core's longitudinal direction will cause a reduction in the core's remanence. This will cause a reduction in the harmonic contents of the current in the main winding when the permeability of the core is lowered by means of a current in the control winding. A thin insulation layer is situated in the gap 107 between the two parts of element 101. In a version of this embodiment,
the magnetic end couplers are not divided into two parts.
Figures 9-16 relate to different examples of the magnetic end couplers. In one embodiment, the material used for the magnetic end couplers is anisotropic. In a version of this embodiment, the magnetic end couplers provide a hard magnetization (low permeability) path for the main magnetic field Hf, that is created by the first winding 103. The control field Hs the field created by the second winding 104 (not shown in Figure 7), will meet a path with high permeability in the magnetic end couplers and low permeability in the pipe elements.
The magnetic end couplers or control-flux connectors can be manufactured from GO-sheet metal or wires of magnetic material with the control field in the GO direction and the main field in the transverse direction. The wires may be either single wires or stranded wires.
In one embodiment, the magnetic couplers arc made of GO-steel to ensure that the end couplers do not get saturated before the pipe elements or cylindrical cores in the TD, but instead, concentrate the control flux through the pipe elements. In another embodiment, the magnetic couplers are made of pure iron.
We will now describe the magnetic field behaviour in the end couplers in an embodiment of the device corresponding to Figure 7. Initially, that is, when the second winding or control winding 104 is not activated, only a very small fraction (approx. 0.04-0.25%) of the main field Hf enters the magnetic end couplers' volume because of the very low permeability in the main field direction (TD) in the magnetic end coupler. The permeability in the main field direction Hf, TD is from 8 to 50 through the end coupler depending on the construction and material used. As a result, the main flux Bf goes in the volume of the pipe elements or cylindrical cores 101, 102. Additionally, the concentration of the main flux allows the main cores' 101, 102 permeability to be adjusted downward to approximately 10.
The control flux-path (Bs in Figures 6 and 7) goes up axially within one of the pipe elements' 101, 102 core wall and down within the other element's core wall and is closed by means of magnetic end couplers 105, 106 at each end of the concentric pipe elements 101, 102.
The control flux (B) path has very small air gaps provided by thin insulation sheets 108 between the magnetic end couplers 105, 106 and the circular end areas of the cylindrical cores (Figure 9). This is important to prevent creation of a closed current path for the transformer action from the first winding 103 through the "winding" made by the first and the second pipe elements 101,102 and the magnetic end couplers 105, 106.
As previously mentioned, the magnetic end couplers in one embodiment of the invention are made of several sheets of magnetic material (laminations). Reference is now made to Figures 10-14, which are helpful for understanding the present invention. Figure 10 shows a magnetic end coupler of GO sheet steel and the pipe elements 101 and 102 seen from above. Each segment of the end coupler 105 (for example, segments 105a and 105b) is tapered from a radially inward end 110 to a radially outward end 112, where the radially inward end 110 is narrower than the radially outward end 112. Directions GO and TD are shown in Figure 10 as they apply to each segment 105a, 105b of the end coupler. A portion of the end coupler 105 on the left and the right sides of Figure 10 has been removed to show sheet ends 114 of the inner core 102 and the outer core 101. Figure 11 shows a torus shaped member 116, which when cut into two parts, provides the magnetic end couplers. Figure 12 shows a cross section of the torus and the relative position of the sheets (e.g. laminations) 105' of magnetic material. Figures 12 and 13 show the GO direction in the magnetic end couplers, which coincides with the direction of the main field. Figure 14 shows how the size and shape of the magnetic coupler segment 105a is adjusted to insure that the coupler connects the first pipe element 101 (outer cylindrical core) to the second pipe element 102 (inner cylindrical core) at each end. In figure 14 radially inward end 110 is narrower than radially outward end 112.
In another background example, which is helpful for understanding the invention, shown in Figure 15, the same type of segments is made using magnetic wire. Production of end couplers using stranded or single wire magnetic material. The toroidal shape formed by the magnetic material is cut into two halves as indicated by cross section A-A in Figure 15. Figure 16 shows how the ends of the magnetic wires provide entry and exit areas for the magnetic field Hf. Each wire provides then a path for the magnetic field Hf.
To be able to increase the power handled by the controllable inductive device, the core can be made of laminated sheet strip material. This will also be advantageous in switching where rapid changes of permeability are required.

Claims

A core for a magnetic controllable inductor, comprising:
first (101) and second (102) coaxial and concentric pipe elements, each pipe element (101, 102) comprising an anisotropic magnetic material and defining an axis;

wherein the pipe elements (101, 102) are connected to one another at both ends by means of magnetic end couplers (105, 106), and

wherein the core presents a first magnetic permeability in a first direction orthogonal to the axes of the elements (101, 102) significantly higher than a second magnetic permeability in a second direction parallel to the elements' (101, 102) axes;

the core being characterised in that:
a third magnetic permeability exists in the coupler (105, 106) in the annular direction relative to the axes of the elements (101, 102), and a fourth magnetic permeability exists in the coupler (105, 106) in a radial direction relative to the axes of the elements (101, 102), wherein the fourth magnetic permeability is substantially greater than the third magnetic permeability.
A core according to claim 1, wherein the first (101) and second (102) pipe elements are made of a rolled sheet material comprising a sheet end and a coating of an insulation material.
A core according to claim 1, wherein the magnetic end couplers (105, 106) are made from grain oriented (GO) steel or iron.
A core according to claim 1 or 3, where the magnetic end couplers (105, 106) comprise:
a plurality of segments (105a, 105b), each segment (105a, 105b) having a radially outward end (112) and a radially inward end (110);

each segment being tapered from the radially inward end (110) to the radially outward end (112); and

the radially inward end (110) being narrower than the radially outward end (112).
A core according to claim 1, the first pipe element (101) comprising:
a first layer (101);

a second layer (101); and

a gap (107) in a third direction parallel to the axes of the elements,

wherein the first layer (101) and the second layer (101) of the first pipe element (101) are joined together by means of a micrometer thin insulating layer in a joint (107) located between the first and second layers.
A core according to claim 1, further comprising:
an air gap (107) extending in an axial direction in each pipe element (101, 102), and

wherein first reluctance of the first element (101) equals a second reluctance of the second element (102).
A controllable inductor (100) comprising a core according to claim 1, said controllable inductor further comprising:
a first winding (103) wound around both said magnetic pipe elements; and

a second winding (104) wound around at least one of said magnetic pipe elements (101, 102), wherein a winding axis for the first winding (103) is perpendicular to an axis of at least one of the magnetic pipe elements (101, 102);

wherein a winding axis of the second winding (104) coincides with the core axis;

wherein:
the first winding (103), when energized, is adapted to generate a magnetic field in a first direction that coincides to a direction of a first magnetic permeability;

the second winding (104), when energized, is adapted to generate a magnetic field in a second direction that coincides to a direction of a second magnetic permeability;

the first magnetic permeability is substantially higher than the second magnetic permeability; and

the magnetic end couplers (105, 106) are made of an anisotropic material and adapted to provide a low permeability path for the magnetic field created by the first winding (103) and a high permeability path for the magnetic field created by the second winding (104).
The controllable inductor (100) according to claim 7, wherein the anisotropic material is selected from a group consisting of grain oriented silicon steel and domain controlled high permeability grain oriented silicon steel.
A controllable inductor according to claim 7, wherein the magnetic end couplers (105, 106) are made from grain oriented (GO) steel or iron.
A controllable inductor according to claim 7 or 9, where the magnetic end couplers (105, 106) comprise:
a plurality of segments (105a, 105b), each segment (105a, 105b) having a radially outward end (112) and a radially inward end (110);

each segment being tapered from the radially inward end (110) to the radially outward end (112); and

the radially inward end (110) being narrower than the radially outward end (112).
The controllable inductor according to claim 7, further comprising a thin insulation sheet (108) situated between magnetic pipe (101, 102) element edges and the end coaplers (105, 106).
The controllable inductor according to claim 7, wherein a volume of the magnetic end couplers (105, 106) is 10-20% of the volume of the magnetic pipe elements (101, 102).
The controllable inductor according to claim 7, wherein a volume of the magnetic end couplers (105, 106) is 25-50% of the volume of the magnetic pipe elements (101, 102).
The controllable inductor of claim 7, wherein the magnetic field direction introduced by the first winding (103) is in an annular direction relative to the axis of at least one of the elements (101, 102).
The controllable inductor of claim 7, wherein the magnetic field direction introduced by the second winding (104) is in a direction parallel to the axis of at least one of the elements (101, 102).