The present disclosure generally relates to electrical transformers. In particular, it relates to medium frequency transformers having an air gap between windings and having at least one fasting element for positioning the windings in respect to each other.

Cooling is a critical aspect in the design of electrical transformers, as heat is inevitably generated by the current flowing through the windings. At the same time, electrical insulation is one of the critical aspects in the design of medium frequency transformers, in particular the insulation between the high voltage (HV) and low voltage windings (LV). The difference in potential between high voltage and low voltage windings dictates the minimal electrical insulation required, which in turn is dependent on the distance between the windings and the insulation material.

Introducing components of solid material - such as an internal bobbin - in an air gap between the windings decreases the gap distance, and hence the dielectric strength of the air insulation allowing the electric field to permeate in the solid insulator. Conventionally, bobbins are created wherein the radial strength of the bobbin is exerted outwards from the bobbin on the winding, and which are placed between an inner winding and an outer winding. Thus, the material of the bobbin is located for a significant part in the gap between the inner and outer windings. This requires the air gap to be larger between the high voltage and low voltage windings in order to compensate for the presence of the solid material of the bobbin.

Considering an air-cooled solution for a scenario with significant core-and winding-caused losses, a conventional bobbin also inhibits efficient air-flow and reduces the local heat transfer coefficient on the portion of the windings directed towards or in contact with the bobbin. Especially medium frequency transformers typically have demanding insulation requirements, and are of small overall size due to their high operating frequency. Therefore, the insulating air gap becomes a limiting factor when trying to achieve the aim of shrinking the transformer size.

DE 80 07 711 U1 discloses a bobbin framework for a transformer having elongated inner and outer supporting portions for carrying inner and outer windings. The windings are being spaced apart unalterable from each other by an air gap. In view of the above, there is a need for a transformer design with a bobbin avoiding the disadvantages of the known solutions.

The problems mentioned above are at least partly solved by a transformer according to claim 1.

In a first aspect, an air-cooled transformer is provided, which comprises a core, an inner winding, an outer winding having an outer boundary face on its outside face, an air gap between the inner winding and the outer winding, a bobbin comprising a dielectric material, located adjacent the outer boundary face of the outer winding and supporting the outer winding and wherein the bobbin comprises supporting portions which extend into the air gap. The transformer is characterized in that the bobbin is mounted to the core via at least one fastening element and the fastening element is adapted for positioning the bobbin and the outer winding with respect to the inner winding.

Concentric transformers according to embodiments allow the reduction of the average size of a dielectric-filled gap between the windings, thus reducing overall size, while maintaining good insulation properties between low voltage and high voltage windings. An outer winding is supported mainly from outside, wherein the support has only minimal interference with a gap between the windings. Also, by reducing or omitting solid dielectric material provided between the inner and outer winding in embodiments, cooling properties are improved, while a coolant can access an enhanced surface of the windings. Further, the stray inductance of transformers according to embodiments may be quickly and easily adjusted.

Further aspects, advantages and features of the present invention are apparent from the dependent claims, the description and the accompanying drawings.

A full and enabling disclosure, including the best mode thereof, to one of ordinary skill in the art is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein:

• Fig. 1 schematically shows a perspective view of a transformer according to embodiments;
• Fig. 2 schematically shows a side view of the transformer of Fig. 1;
• Fig. 3 schematically shows a top view of a transformer according to embodiments;
• Fig. 4 schematically shows a perspective view of a bobbin for a transformer according to embodiments;
• Fig. 5 schematically shows a side view of the bobbin of Fig. 4;
• Fig. 6 schematically shows a top view of the bobbin of Fig. 4.

Reference will now be made in detail to various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet further embodiments. It is intended that the present disclosure includes such modifications and variations.

Within the following description of the drawings, the same reference numbers refer to the same components. Generally, only the differences with respect to the individual embodiments are described. When several identical items or parts appear in a figure, not all of the parts have reference numerals in order to simplify the appearance.

The systems and methods described herein are not limited to the specific embodiments described, but rather, components of the systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. Rather, the exemplary embodiment can be implemented and used in connection with many other applications.

Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

According to embodiments, an air insulated transformer is provided. It comprises at least two windings provided on a ferromagnetic core, with an air gap between the windings. The inner winding has an outer face directed towards the outer winding, and the outer winding has an inner face directed towards the inner winding, and an outside face directed outwards, with respect to a center of the outer winding. The outer winding is supported by a bobbin. The bobbin is a permanent frame for the wire, acting to form the shape of the winding and ease assembly of the windings into or onto the core. The bobbin is located adjacent an outer boundary face of the outer winding. Thereby, supporting portions of the bobbin may extend into the air gap between the inner winding and outer winding. In embodiments, the air gap has regions with a greater distance and regions with a smaller distance between the inner winding and the outer winding. Preferably, in embodiments in which supporting portions of the bobbin extend into the air gap, they extend into the air gap only in regions having a larger distance between the inner and outer winding.

By minimizing or, in some embodiments, entirely avoiding the presence of dielectric bobbin material in the air gap between the inner and outer windings, the space requirement between the windings may be minimized, while maintaining predefined insulation properties. Differently said, embodiments allow to reduce or minimize the air gap while maintaining defined isolation properties. Minimizing the air gap also means that outer dimensions of the outer winding can be reduced, and thus the outer dimensions of the transformer as a whole can be reduced.

Fig. 1 shows a fluid-cooled transformer 10 according to embodiments. The transformer has a shell-type core 20 having three limbs 22, 23, 24. A longitudinal axis L is defined by the extension of the middle limb 23 of the core 20, around which longitudinal axis L - in the following also called axis L or middle axis L - an inner winding 30 is wound. An outer winding 40 surrounding the inner winding 30 has an outer boundary face 42 on its outside face. Between the inner winding with the longitudinal axis L and the outer winding, an air gap 50 is provided, which typically protrudes in a circumferential direction around the inner winding 30 and its longitudinal axis L. A bobbin 60, forming the shape of the outer winding 40, comprising a dielectric material, is located adjacent the outer boundary face 42 of the outer winding 40 and supports the outer winding 40 from its outside face, differently said from a region radially outwards from the outer winding 40. Typically, the bobbin 60 surrounds the outer boundary face 42 of the outer winding 40 in a circumferential direction about longitudinal axis L. In embodiments, supporting portions 70 of the bobbin 60 protrude on an inner side 41 of the outer winding 40 radially outwards from longitudinal axis L and parallel to longitudinal axis L.

As depicted in Fig. 2, the air gap 50 is protruding in a circumferential direction around the inner winding 30 and the longitudinal axis L. In other words, the air gap 50 surrounds the inner winding 30 and forms a space between inner winding 30 and outer winding 40. Due to the substantially cylindrical geometry with rectangular ground surface of the inner winding 30 and the outer winding 40, at least one first region 52 with a first distance d₁ between the inner winding 30 and the outer winding 40, and at least one second region 54 with a second distance d_s between the inner winding 30 and outer winding 40 are provided. The supporting portions 70 of the bobbin 60 may extend into the air gap 50, preferably only in the at least one first region 52, while the first distance d₁ of the first region 52 is larger than the second distance d_s of the second region 54. In Fig. 2, the transformer 10 exemplarily has four regions 52 with a larger first distance d₁. Four supporting portions 70, located in corner regions of the bobbin 60 of substantially rectangular cross section, support the outer winding 40, wherein the supporting portions 70 are mounted to that part of the bobbin 60 which protrudes along the outer boundary face 42 of the outer winding. In Fig. 2, as the depicted bobbin 60 has a substantially rectangular cross section perpendicular to the longitudinal axis L, there are four regions 52 with a larger first distance d₁ and also four regions 54 with a smaller second distance d_s, of which only two are visible in Fig. 2. The regions 54 of smaller distance compared to the regions 52 are formed by parallel sides of the inner winding 30 and by parallel sides of the outer winding 30, each winding 30, 40 of substantially rectangular cross section.

In embodiments, the air gap 50 between the inner and outer winding may also be substantially or entirely free of dielectric material of the bobbin 60. The supporting portions 70, as shown in Fig. 1 and Fig. 2, extending from the bobbin into the air gap are omitted in this case. In order to support the outer winding, the bobbin 60 is mounted to the bobbin structure by other means. In embodiments, this may be realized by an adhesive, e.g. a resin, more specifically an epoxy or polyester resin, which fixes the outer winding 60 to the bobbin located around its outer boundary face 42. That is, the outer winding 40 is glued to the bobbin located on its outer boundary face 42. Thereby, the single wires of the windings are typically immersed by the glue or resin, i.e. they are completely covered by it. In that case, the bobbin 60 supports outer winding 40 entirely from its outside, without any part of the bobbin extending into the air gap 50 between the inner winding 30 and the outer winding 40.

As is shown with respect to Fig. 1 and Fig. 2, in embodiments the bobbin 60 is typically mounted to the core 20. This may be realized by at least one fastening element 100. The fastening element 100 is adapted for positioning the bobbin 60 and the outer winding 40 with respect to the inner winding 30 in a direction along the longitudinal axis L. By changing the relative position of the bobbin 60 with respect to the inner winding along the longitudinal axis L, a stray inductance of the transformer 10 may be adjusted.

Thereby, in embodiments as shown in Fig. 2, the fastening element 100 typically extends from the bobbin 60 towards the core 20 in a radial direction outwards with respect to the longitudinal axis L. The at least one fastening element 100 of the bobbin 60 is typically movably mounted to at least one outer limb 22, 24 of the core 20. This allows an adjustment of the bobbin 60, and thus also the outer winding 40 supported by it, in a direction parallel to the longitudinal axis L to adjust the stray inductance of the transformer. In Fig. 2, four fastening elements 100 are shown, of which two are mounted to each limb 22, 24, typically the outer limbs 22, 24 of a shell type core. Each fastening element 100 typically embraces the respective limb 22, 24. Also, the fastening elements 100 may only partially embrace or surround the limbs 22, 24, for example they may be designed to have a clamp shape, e.g., resembling a C, and may for example only surround one half of the limb 22, 24. It goes without saying that the skilled person can easily find a number of variations of fastening elements, which are regarded to fall into the scope of the present disclosure.

In embodiments, the core 20 typically comprises a non-conductive ferrite material or a laminated silicon steel.

In Fig. 3, two geometrical planes a and b are shown, which delimit the inner winding 30 and the outer winding 40 in a direction of the longitudinal axis L. The supporting portions 70 of the bobbin 60 (not shown in Fig. 3, refer to Fig. 1 and 2) typically protrude in the air gap 50, that is in the space between the inner winding 30 and the outer winding 40, between the two planes a and b.

In Fig. 4, a bobbin 60 of a transformer 10 according to embodiments is shown. The bobbin 60 comprises four substantially rectangular segments 110, 112, 114, 116. Each segment 110, 112, 114, 116 extends between two neighboring supporting portions 70 of a pillar-like, elongated shape. The segments 110, 112, 114, 116 together form the cage-like bobbin 60 with rectangular cross section. In embodiments, the cage-like bobbin 60 has a cube-like shape, wherein the segments 110, 112, 114, 116 form four of the six side faces of the cube, and wherein two opposite side faces are left out. In embodiments, the number of rectangular segments can differ, such that the bobbin 60 has an n-polyedric cross section , with n being, for example, 4, 5, 6, or 8. Fig. 5 and Fig. 6 show a side view and a top view of the bobbin 60 of Fig. 4.

In embodiments, the core 20 of transformer 10 may also be a closed-core type or D-type, different from the embodiments of Fig. 1 to 3 showing a shell-type or E-type shape. In this case, the bobbin extends around one of the two limbs of the closed-core transformer, and is typically mounted via a fastening element 100 to the second, other limb.

10

Transformer

20

Core

22, 23, 24

Limbs

30

Inner Winding

40

Outer Winding

42

Outer Boundary Face

50

Air Gap

52

First Regions

54

Second Regions

60

Bobbin

70

Supporting Portions

100

Fastening Element

110, 112, 114, 116

Rectangular Elements