EP1279177A1

EP1279177A1 - A wound transformer core and a method and apparatus for manufacturing thereof

Info

Publication number: EP1279177A1
Application number: EP00921014A
Authority: EP
Inventors: Eliezer Adar
Original assignee: Att Advanced Transformer Technologies (1998) Ltd
Current assignee: Att Advanced Transformer Technologies (1998) Ltd
Priority date: 2000-04-27
Filing date: 2000-04-27
Publication date: 2003-01-29
Anticipated expiration: 2020-04-27
Also published as: RU2002131940A; DE60029640D1; ES2269128T3; WO2001082316A1; AU2000241411A1; DE60029640T2; RU2241271C2; ATE334473T1; EP1279177B1

Abstract

A transformer core made of magnetic strips is presented for use in a power distribution transformer. The transformer core has a desired height and a substantially cylindrical toroidal shape produced by winding a multi-layer structure about a central axis of the toroid in a spiral like manner. Each layer in the structure is composed of a predetermined number of magnetic strips arranged along the central axis with air gaps naturally existing between each two adjacent strips of the layer. A number <i>n</i> of layers in the structure is defined by the magnetic properties of the strips. The layers are shifted with respect to each other a predetermined distance along the central axis such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers of the structure.

Description

A WOUND TRANSFORMER CORE AND A METHOD AND APPARATUS FOR MANUFACTURING THEREOF

FIELD OF THE INVENTION

This invention relates to strip wound transformer cores and method of their manufacturing.

BACKGROUND OF THE INVENTION A transformer is a known electrical device widely used for transferring the energy of an alternating current in the primary winding to that in one or more secondary windings through electromagnetic induction. It typically contains two or more electrical circuits comprising primary and secondary windings, each made of a multi-turn coil of electrical conductors with one or more magnetic cores coupling the coils by transferring a magnetic flux therebetween.

Electrical transformer cores are typically formed of high grain oriented silicon steel laminations. The most common procedure for manufacturing such a transformer is to wind the core independently of a preformed coil or coils with which it will ultimately be linked. To this end, the core is formed with a joint at which the core laminations can be separated to open the core and thus accommodate insertion of the core into the coil window(s). The wound core joints are typically of the so-called step-butt joint or step-lap joint types.

US Patent No. 1,164,288 discloses a technique of fabricating a cylindrical magnetic core for a power transformer. The magnetic core is made from coiled strips, wherein the core is of greater axial dimension than the width of the strip. To manufacture the core, a plurality of layers of the magnetic steel strips is simultaneously coiled to form the cylindrical core. The sum of the width of the strips in each layer is equal to the axial dimension of the core, and at least one longitudinal edge of each strip is staggered is relation to those in adjacent turns of the resultant coil.

It is known from the disclosure in US Patent No. 2,909,742 that in order to obtain a desired height of the transformer core, a number of toroids can be stacked on top of each other. This technique, however, suffers from energy losses caused by unavoidable introduction of unwanted air gaps between each two adjacent toroids. Great advances have been made in amorphous magnetic alloys for use as the core material for transformers because they are lower loss materials as compared to grain steel. However, annealed amoφhous metals become extremely brittle, and thus break under mechanical stress, for example, during the stage of closing the core joints. Various techniques have been developed aimed at facilitating the manufacture of a wound transformer core from amoφhous strips, wherein the core has joints in a localized region thereof that allow the core to be opened to permit insertion into the window of preformed coil structure. These techniques are disclosed for example in the following patents: US 4,789,849; US 4,790,064; US 4,893,400; US 5,398,402; US 5,398,403; US 5,329,270; US 5,347,746, US 5,548,887.

SUMMARY OF THE INVENTION

There is a need in the art to provide a novel method and apparatus for manufacturing a transformer core of a desired height from a plurality of thin strips made of a magnetic material having predetermined magnetic properties, wherein the available width of such strip is less than the desired height of the core.

The main idea of the present invention consists in the use of a desired number of layers of magnetic strips to be simultaneously wound so as to form a substantially cylindrical, toroid-like transformer core of a desired height for carrying a coil block mounted thereon. The construction is such that each layer is formed of a desired number of strips arranged along the longitudinal axis of the core, and the layers of a desired number are specifically arranged with respect to each other. This construction is aimed at providing the optimal distribution of magnetic flux inside the strips in the layers. The number of strips in the layer is dictated by the height of the transformer core and by the available widths of the magnetic strips. The number of layers is dictated by the magnetic properties of the magnetic material of which the strips are made. As for the thickness of the resultant winding, it is dictated by the electrical and mechanical parameters of the transformer, such as the height and cross section of the core, and frequency and rate power of the transformer.

There is thus provided according to one aspect of the present invention a transformer core to be used in a power distribution transformer, the transformer core having a desired height and being of a substantially cylindrical toroidal shape, wherein: - the transformer core is in the form of a multi-layer structure wound about a central axis of the toroid;

- each layer in the structure is composed of a predetermined number of magnetic strips arranged along said central axis with air gaps naturally existing between each two adjacent strips of the layer, the predetermined number of the strips being such that the sum of the widths of said strips is substantially equal to said desired height of the core;

- a required number n of layers in said structure is defined by the magnetic properties of the strips, and the layers are shifted with respect to each other a predetermined distance along said central axis such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers of the structure.

It is known that the commercially available strips made of amoφhous metals are characterized by a working value of magnetic induction, B_w, about 1.35T, and the saturation value of magnetic induction, B_sat, about 1.55T Thus, the number n of the layers should be such that a magnetic flux created in the first (uppermost) layer and flowing along the longitudinal axis of the layer, while reaching an air gap on its way and passing through all other layers in a transverse direction so as to return into the first layer, will not cause the saturation of the magnetic induction in other layers.

Generally speaking, the number of layers should satisfy the following relation: n>BJ(B_sarB_w), n being integer. Considering the above parameters of the commercially available amoφhous strips, the minimal value of n is 7.

The predetermined distance defining the shift between each two adjacent layers is such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers of the structure. As indicated above, the number of strips in the layer is defined by the desired height of the transformer core, namely, the sum of the width of the strips in the layer is substantially equal to the height of the core. It should be understood that, in order to planarize the top and bottom surfaces of the core, the number of strips in the extreme layers of the entire structure (I^s and 7 layers) differs from that of the intermediate layers. The two opposite extreme strips in each of the intermediate layers are of a smaller width than that of the other strips in the layer.

In a resultant winding, wherein each winding turn is formed by the above-described multi-layer structure, each air gap of the intermediate layer in turn is overlapped by the (n-1) strips of other layers. Here, some of the overlapping strips are of the structure in the same turn and the others are of the structure of an adjacent turn.

The strip layers are wound simultaneously being fed from a corresponding number of bobbins (e.g., 7 bobbins). Namely, the bobbins are aligned in an array, each bobbin feeding a corresponding one of the seven strip layers. The strips are fed from the bobbin with the predetermined shift between the layers. To this end, the bobbins may be prepared such that the strips layer wound on each of the bobbins is arranged with respect to the strip layer wound on the other bobbins in a manner corresponding to the arrangement of layers in the resultant core. Alternatively, the bobbins may be identically wound with the strip layers, but mounted with the desired shift with respect to each other. Thus, according to another aspect of the present invention, there is provided a method of manufacturing a transformer core to be used in a power distribution transformer, wherein the transformer core has a desired height and is formed of a multi-layer structure of magnetic strips wound so as to create a resultant substantially cylindrical toroidal winding of the core, the method comprising the steps of:

(i) specifying a number n of layers in the structure in accordance with magnetic properties of the magnetic strips; (ii) preparing each of the layers formed of a predetermined number of parallel magnetic strips mounted with air gaps naturally existing between each two adjacent strips in the layer; (iii) winding said multi-layer structure about a central axis of a mandrel supporting the core during the manufacture, by simultaneously feeding the n layers in a manner that the layers of the core are shifted with respect to each other a predetermined distance along said central axis such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers of the structure. According to yet another aspect of the present invention, there is provided an apparatus for manufacturing a transformer core to be used in a power distribution transformer, wherein the transformer core has a desired height and is formed of a multi-layer structure of magnetic strips wound so as to create a substantially cylindrical toroidal winding of the core, the apparatus comprising:

- a required number of bobbins, each carrying a predetermined number of magnetic strips for a corresponding one of the layers, the strips being wound on the bobbin and arranged along an axis thereof with small air gaps naturally existing between each two adjacent strips, wherein the required number of the layers is defined by magnetic properties of the strips; - a drive assembly for driving the simultaneous movement of the strips layers from the bobbins onto a mandrel supporting the transformer core; and

- a guiding assembly for guiding the winding of the fed layers about a central axis of the mandrel with a required density between the layers, wherein the layers of the core are shifted with respect to each other a predetermined distance along said central axis of the mandrel such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers The present invention may be used for manufacturing a three-phase transformer. A magnetic circuit of such transformer is composed of three transformer cores, each constructed as described above (for carrying three coil blocks, respectively) and two spaced-apart, parallel, plate-like elements attached to the top and bottom surfaces of the transformer cores, respectively. In other words, the transformer cores with the coil blocks mounted thereon are enclosed between the upper and lower plates of the magnetic circuit. The transformer cores are spaced at intervals of 120 degrees about a central vertical axis of the entire transformer structure. The cores are spaced from each other and from the plate-like elements by insulating spacers. All the spacers may be formed of plastic with filler of a magnetic powder with the concentration of 20-50%. The plates and the cores may be formed of amoφhous strips. The plate may be of a substantially triangular shape with rounded edges, or of a circular shape that simplifies the technological process of its manufacture. The plate-like element may be a toroid manufactured similar to that of the transformer core, as described above.

The advantages of the above construction of a three-phase transformer consist of the following. The provision of the plate-like elements of a triangular shape with rounded corners allows for effectively transferring the magnetic flux between the three column-like elementary circuits enclosed between the plates. The provision of the column-like elementary circuits formed by one or more toroids produced by wounding the amoφhous strips, enables to obtain a desired height of the column irrespectively of the limited width of the strip. By appropriately selecting the dimensions of the elements of the magnetic circuit (i.e., the diameter of each transformer core and each of the plate-like elements), the desired properties of the transformer can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1A schematically illustrates a transformer core according to the invention; Fig. IB partly illustrates a cross section of the transformer core of Fig. 1A taken along a plane A-A;

Fig. 2 more specifically illustrates the principles of arranging layers in the core of Figs. 1A-1B;

Fig. 3 schematically illustrates the main components of an apparatus for manufacturing the transformer core of Figs . 1 A- 1 B ; and

Fig. 4 is a schematic illustration of a three-phase transformer utilizing the transformer core of the present invention.

DETAILED DESCRIPTION OF EXAMPLES OF THE INVENTION

Referring to Fig. 1 A, there is illustrated a transformer core 10 to be used in a power distribution transformer (not shown here). The core is in the form of a cylindrically shaped toroid of a desired height L. The toroid 10 is formed by coiling a multi-layer structure 12 of magnetic strips about a central axis 14 of a mandrel 16

(constituting a central axis of the toroid).

As shown in Fig. IB, the multi-layer structure 12 is composed of a plurality of parallel layers - seven layers Lι-L in the present example, which are arranged along an axis peφendicular to the central axis 14. Several strips (ribbons) made for example of a soft ferromagnetic amoφhous alloy form each of the layers: layer Li is formed by strips Si, layer L₂ is formed by strips S , etc. The strips in the layer are arranged in an array extending along the central axis 14, with the unavoidable existence of an air gap, generally 18, between each two adjacent strips. The air gaps of each layer are shifted with respect to the air gaps of the adjacent layer, as will be described more specifically further below. Turning back to Fig. 1A, it should be understood that the resultant winding of the core 10 is composed of a plurality of turns of regularly repeated multi-layer structure 12. The number of turns (i.e., the thickness of the resultant winding) is defined by the required power of the transformer.

Reference is made to Fig 2, more specifically illustrating the principles of the arrangement of layers in the structure 12, and in the resultant winding. As indicated above, the commercially available amoφhous ribbons are typically limited in width (up to 200mm), the width I of the strip being typically much smaller than that required for the height of a transformer core. Therefore, each of the layers in the structure 12 is composed of several strips such that the sum of the widths of the strips (together with the gaps between the strips) is substantially equal to the height of the transformer core.

On the other hand, the amoφhous ribbons have certain magnetic properties, such as the working value of a magnetic induction (e.g., B_w=l.35T), and the saturation value of the magnetic induction (e.g., To optimize the operation of a transformer (i.e., the distribution of a magnetic flux in the structure), the structure 12 is composed of 7 layers. Generally the number n of layers is determined in accordance with the magnetic properties of the amoφhous material, as follows: n>BX(B_sarB_w), n being integer.

As shown in Fig. 2, air gaps exist between each two adjacent strips of the layer. More specifically, strips Si of layer Li are arranged with air gaps 18a, strips S₂ of layer L - with gaps 18b, strips S₃ of layer L₃ - with gaps 18c, strips S₄ of layer L₄ - with gaps 18d, strips S₅ of layer L₅ - with gaps 18e, strips S₆ of layer L₆ - with gaps 18f, and strips S₇ of layer L₇ - with gaps 18g. Gaps of each layer are shifted with respect to those of the adjacent layer a predetermined distance along the central axis 14 such that each of the air gaps in one layer is overlapped by six strips of the other layers (generally, (n-1) strips). For example, gap 18a of layer Li is overlapped by strips S₂-S of layers L₂-L of the same turn of the resultant winding, gap 18c of layer L₃ is overlapped by strips S₄-S and further by strips of Si- S of layers Li and L₂ of an adjacent turn of the resultant winding. As exemplified with respect to layer Li, magnetic flux F (produced by the passage of an electric current through the strip Si) flows across the strip Si, and while reaching the gap 18a, flows through the six strips S₂-S of layer L₂-L₇ overlapping this gap 18a.

It should be understood that the shift between the layers is appropriately selected. For example, considering the equal width of the intermediate strips of the layer (i.e., strips between two opposite extreme strips), the sum of shift distances of all the layers in the structure should not exceed the width of the intermediate strip.

Fig. 3 illustrates the main components of an apparatus 20 for manufacturing the transformer core 10. The apparatus 20 comprises seven bobbins Bι-B (generally, n bobbins), each for carrying a corresponding strip layer to be fed to the mandrel 16. The layers are previously wound onto the bobbins in a manner, which will be described further below, and simultaneously fed onto the mandrel 16, by a suitable driving assembly, which is not specifically shown.

The driving assembly may be of any known suitable kind, and may be associated with the mandrel 16 for driving the revolution thereof, while the bobbins are rotatably mounted on their shafts (not shown) to rotate against the tension of the feeding layers. In order to provide the desired tension of the layers during the coiling procedure, the driving assembly may also be associated with the shafts of bobbins for driving the revolution thereof. The construction may be such that the bobbins are driven together for rotation about the mandrel, which, in this case, is mounted stationary.

Further provided in the apparatus 20 is a guiding assembly 22, comprising one or guiding rollers, generally at 24, and a pair of width limiting rollers 26 accommodated at opposite ends of the mandrel 16 extending normally to the direction of movement of the layers onto the mandrel. As further shown in the figure, the layers are prepared on the bobbins with the corresponding shift between the strips of each two adjacent layers as described above. To this end, either the corresponding arrangements of strips of different layers are previously determined, and the strips are wound on the bobbins accordingly, or identically wound bobbins are prepared and then cut by any suitable cutting tool.

It should be noted, although not specifically shown, that the layers of sufficient width, appropriately shifted with respect to each other, could be wound on the mandrel, and the so produced core then cut at opposite ends. In this case, the bobbing and/or guiding means may be appropriately shifted.

Reference is now made to Fig. 4, illustrating a three-phase transformer 30 utilizing the transformer cores designed as described above. The transformer 30 comprises a magnetic circuit formed by an upper plate-like element 32a, a lower plate-like element 32b, and three parallel identical cores 10 (only two of them being shown in the drawing). The magnetic circuit is arranged such that the plates 32a and 32b are parallel to each other, and the cores 10 serve as supports between the plates, thereby forming a cage-like structure spatially symmetrical about a central axis CA. In the present example, each of the plates 32a and 32b is a toroid, and is made of amoφhous ribbons 34 wound about a central hole 35 to form the planar toroid. Further provided are three coil blocks 36, each mounted on a corresponding one of the cores 10. Each of the coil blocks 36 includes a primary winding 36a and a secondary winding 36b. Thus, each phase of the transformer 30 is formed by the transformer core 10 with the corresponding coil block 36 mounted thereon. The transformer 30 has a modular structure, namely, the plates 32a and 32b, and the cores 10 can be easily assembled together and disassembled. When one of the plates 32a or 32b is removed, the coil blocks 36 can be removed as well, thereby enabling, for example, to repair the coil.

In the present example, each of the plates 32a and 32b has a generally triangular shape with rounded sides and corners. The amoφhous ribbon 34 is made of an alloy having soft ferromagnetic properties. Each of the cores 10 is a toroid manufactured as described above. This construction enables to achieve a desired height of the core 10, notwithstanding the fact that the width of amoφhous ribbon is typically limited. The entire structure is held together with three de-mountable bands 38 (only two of them being seen in the figure), each having a screw (or spider) 40 to tighten the band. Structural members 42 are provided, each located between the corresponding one of the bands 38 and each of the plates 32a and 32b. A base 44 supports the entire structure. An inner, upper surface of the plate 32b is brought into contact with lower surfaces of the cores 10 to transfer magnetic fluxes therebetween, as will be described more specifically further below.

The transformer 30 operates in the following manner. As an electric current passes through each primary winding 36a of the coil block 36, a magnetic flux is generated and propagates along the corresponding core 10 between the upper and lower plates 32a and 32b. Arrows 46, 48 and 50 show fluxes generated in the three cores 10, respectively. The magnetic flux flowing through the column 10 generates an induced voltage in the secondary winding 36b of the corresponding coil block 36. The device having this structure thus functions as a three-phase transformer.

Thus, the electric current, for example, with the working frequency of 50Hz, is supplied from a power source (not shown) to a terminal of coil of the primary winding 36a, and, whilst passing through the coil turns, creates the basic magnetic flux 46. Assuming, for example, that at a given moment the flux 46 flows up. Then, the flux 46 is divided into two identical fluxes 52 and 54 in the plate 32a. These fluxes 52 and 54 flow along two identical portions of the toroidal plate 32a, and, then, flow down through the two other cores 10. The flux 52 changes into flux 48, and the flux 54 changes into the flux 50 passing down through the cores 10. Then, the fluxes 48 and 50 flow along two equal paths of the toroidal plate 32b. Whilst passing along the toroidal plate 32b, the flux 48 changes into a flux 56, and the flux 50 changes into a flux 58. The fluxes 56 and 58 are transferred into the core 10 - In ¬

forming the sum flux 46, which flows up. Thus, the magnetic flux loop is closed. The fluxes of the other phases of the transformer flow in the similar way summing up the total magnetic flux.

The plates 32a and 32b could have a circular shape. In this case, the flux streams 52, 54, 56 and 58 will flow along circular paths therein. In the example of Fig. 4, each of the plates 32a and 32b is shaped like an equilateral triangle with rounded sides and corners. This results in a shorter path for the flux streams in the plates between the cores 10, i.e., the shape of the flux streams is closer to a straight line. This enables to achieve a lower magnetic reluctance, or better conductance of the magnetic flux.

Those skilled in the art will readily appreciate that various modifications and changes can be applied to the preferred embodiments of the invention as hereinbefore exemplified without departing from its scope as defined in and by the appended claims.

Claims

CLAIMS:

1. A transformer core to be used in a power distribution transformer, the transformer core having a desired height and being of a substantially cylindrical toroidal shape, wherein: - the transformer core is in the form of a multi-layer structure wound about a central axis of the toroid;

2. The transformer core according to Claim 1, wherein said magnetic properties of the strip include a working value of a magnetic induction, B_w, and a saturation value of the magnetic induction, B_sat, the required number n of the layers in the structure satisfying the following relation: n>BX(B_sarB_w), n being integer.

3. The transformer according to Claim 1, wherein said magnetic strips are made of amoφhous metals.

4. The transformer core according to Claim 3, wherein the working value of the magnetic induction and the saturation value of the magnetic induction of the amoφhous strip are, respectively, about 1.35T and 1.55T, the number of layers being no less than 7.

5. A method of manufacturing a transformer core to be used in a power distribution transformer, wherein the transformer core has a desired height and is formed of a multi-layer structure of magnetic strips wound so as to create a resultant substantially cylindrical toroidal winding of the core, the method comprising the steps of: (i) specifying a number n of layers in the structure in accordance with magnetic properties of the strips;

(ii) preparing each of the layers formed of a predetermined number of parallel magnetic strips mounted with air gaps naturally existing between each two adjacent strips in the layer;

(iii) winding said multi-layer structure about a central axis of a mandrel supporting the core during the manufacture, by simultaneously feeding the n layers in a manner that the layers of the core are shifted with respect to each other a predetermined distance along said central axis such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers of the structure.

6. The method according to Claim 5, wherein the number n of the layers satisfies the following relation: n>BX(B_sarB_w), wherein n is integer, B_w is a working value of a magnetic induction, and B_sat is a saturation value of the magnetic induction in the magnetic strip.

7. The method according to Claim 5, wherein the preparation of each of the layers comprises winding the strips on a bobbin such that sum of width of the strips is substantially equal to the desired height of the core, the bobbins being aligned in a spaced-apart parallel relationship, such that the layers on the bobbins are shifted with respect to each other said predetermined distance along the axis of the bobbin.

8. The method according to Claim 5, wherein edges of at least some of the wound layers are cut such that extreme strips on said bobbins are of different widths as compared to that of identical intermediate strips on the bobbins.

9. The method according to Claim 5, wherein the preparation of the layers comprises winding identical layers of the strips on bobbins and accommodating the bobbins in a manner to provide the shifting of the layers with respect to each other.

10. The method according to Claim 9, and also comprising the step of cutting opposite ends of the resultant core so as to planarize its top and bottom surfaces.

11. An apparatus for manufacturing a transformer core to be used in a power distribution transformer, wherein the transformer core has a desired height and is formed of a multi-layer structure of magnetic strips wound so as to create a substantially cylindrical toroidal winding of the core, the apparatus comprising:

- a required number of bobbins, each carrying a predetermined number of the magnetic strips for a corresponding one of the layers, the strips being wound on the bobbin and arranged along an axis thereof with small air gaps between each two adjacent strips, wherein the required number of the layers is defined by magnetic properties of the strips;

- a drive assembly for driving the simultaneous movement of the strips layers from the bobbins onto a mandrel supporting the transformer core; and

- a guiding assembly for guiding the winding of the fed layers about a central axis of the mandrel with a required density between the layers, wherein the layers of the core are shifted with respect to each other a predetermined distance along said central axis of the mandrel such that each of the air gaps in one layer is overlapped by (n-1) strips of the other layers

12. The apparatus according to Claim 11, wherein the required number n of the layers satisfies the following relation: n>BX(B_sarB_w), wherein n is integer, B_w is a working value of a magnetic induction, and B_sat is a saturation value of the magnetic induction in the magnetic strip.

13. The apparatus according to Claim 11, wherein the bobbins are aligned in a spaced-apart parallel relationship along an axis peφendicular to the axis of the bobbin, and the layers on the bobbins are shifted with respect to each other said predetermined distance along the axis of the bobbin.

14. The apparatus according to Claim 11, wherein the strips of different layers are arranged on respective bobbins in a similar manner, and the bobbins are aligned in a spaced-apart parallel relationship along an axis peφendicular to the axis of the bobbin, being shifted with respect to each other said predetermined distance along an axis parallel to the axis of the bobbin.

15. The apparatus according to Claim 11, wherein the driving assembly is associated with a shaft of the mandrel for driving revolution thereof.

16. The apparatus according to Claim 11, wherein the driving assembly is associated with the bobbins for providing rotation thereof about the axis of the mandrel.

17. The apparatus according to Claim 11, wherein the guiding assembly comprises width limiting rollers accommodated at opposite ends of the mandrel.

18. A three-phase transformer comprising a magnetic circuit and three coil blocks, wherein the magnetic circuit comprises two spaced-apart, parallel, plate-like elements; and three spaced-apart, parallel transformer cores, each constructed according to Claim 1, and carrying the corresponding one of said three coil blocks and serving for the corresponding one of the three phases, wherein the columns are substantially peφendicular to the plate-like elements and are enclosed therebetween such as to form a spatial symmetrical structure about a central axis of the transformer.