Medium-frequency (MF) transformers are used in multi-system power converters (converter modules) together with boost converters. Conventional MF transformers are generally characterized by relatively large dimensions and relatively great weight.

Known rectangular and cylindrical designs, for example, have a power-to-weight ratio of about 0.4-0. 6 g/W at 6-12 KHz at a transmission capacity between 30 and 100 kW.

Due to the large dimensions and the relatively large specific weight, known MF transformers are connected in series after MF modules and generally cooled by air and/or water. Due to the heat generation, additional cooling plates are sometimes necessary, which take up further space in the electrical supply cabinet.

However, even known MF transformers of extremely low volume and weight are not suitable for direct installation in MF power converter modules, especially in the vicinity of IGBTs (Insulated Gate Bipolar Transistors).

Installation in a power converter module presupposes that the primary and secondary windings around the magnetic or ferrite core can be brought into the cooling air flow of the module cooler. The design and volume of known transformers precludes this. The windings in known transformers are supported and positioned in close proximity around the earthed core in order to obtain a good degree of efficiency. This results in the danger of partial discharges in the gaps between the windings, spacers and core.

Moreover, the alternative of arranging the MF transformer outside the electrical supply cabinet in aspirated atmospheric air necessitates additional protection against dirt contamination and is disadvantageous.

An object of the invention is to design an MF transformer of low volume and weight that may be integrated in known system converters (power converters).

The solution to this task according to the present invention is provided by the features according to claim 1.

The MF transformer according to the invention can, thanks to its low volume and weight, be integrated into known system converters (power converter modules).

Additional installation space for the transformer outside the power converter module is then no longer required.

A power-to-weight ratio of 0.2 to 0.25 g/W at 6-12 kHz and a transmission capacity between 30 and 100 kW can be achieved with a transformer according to the invention.

The primary and secondary windings of the MF transformer and the insulation which reliably decouples and isolates them are arranged in a thermally and electrically insulated winding encapsulation, which is kept completely clear of the core so that there are air gaps ranging between 3-10 mm on all sides between the winding encapsulation and core.

Mechanical metallic mountings and supports such as clamp bars, bolted joints etc. used in conventional transformers to space the windings and cores are dispensed with. The core potential can float between the primary and secondary winding without partial discharges or arcing occurring. The dielectric displacement currents cause such a small charge in the core that they present no danger, even in the absence of protection against physical contact.

The relatively large air gap between the winding encapsulation and the core, which can be between 3 and 10 mm, allows efficient cooling of the core and of the winding of the MF transformer in the air flow of the power converter cooler. Consequently, the winding and connections of the transformer are arranged in the air duct of the power

converter module in such a way that there are no points from which an arc can spring.

This also contributes towards a particularly compact design.

In a preferred embodiment, the winding encapsulation is itself designed as a mounting for the core, in which the core (magnetic circuit) is placed.

Assembly of the MF transformer is preferably simplified by the fact that the magnetic part consists of part-cores, put together so that they enclose the primary and secondary windings. To assemble the transformer, the part-cores, to which adhesive is preferably applied in advance, only need to be pushed sideways into the winding encapsulation.

This enables the winding encapsulation with the windings to be manufactured as a winding block, separately from the core.

In a preferred embodiment, the core consists of 2 pairs of U-shaped half-cores, with the end faces of two half-cores facing each other. The two pairs of U-shaped half-cores form a single core with a common central limb, around which the winding encapsulation is arranged. Alternatively, the core can also consist of U-and I-shaped part-cores.

To secure the core, angle plates embracing it are provided extending from the outer surface of the winding encapsulation. The core is preferably glued to the angle plates of the winding encapsulation. This allows the core to be securely retained without the use of electrically-conductive parts, and uninterrupted air gaps to be maintained between the winding encapsulation and the core. One advantage is that under the temperature changes due to the different moduli of elasticity, the core expands and contracts less than the winding encapsulation, so the specific expansion and contraction forces are low. One of the reasons for this is that the modulus of elasticity of the angle-plate shaped part of the encapsulation is lower by several powers of ten than that of the core material. The entire construction is more impact-and vibration-resistant than conventional core constructions. The same applies as regards freedom from noise.

It is advisable for the angle plates to incorporate ribs as stiffness. Clamps can be arranged on the end faces of the winding encapsulation for further lateral support of the core and as protection during assembly.

The electrical connections of the transformer are preferably arranged in shoulders at the end faces of the winding encapsulation. The preferably lateral connections of the transformer into the shoulders can advantageously be used as hanging points when casting the winding encapsulation.

The primary and secondary windings can comprise one or more part-windings. Primary and secondary conductors can also comprise sub-conductors.

The winding encapsulation is preferably a casting which hermetically encloses a primary and a secondary winding. Together with the windings it constitutes a block for mounting the core. The casting is preferably resin, preferably epoxy resin with thermally-conductive extenders, preferably aluminium nitride and/or silanised quartz powder and/or insulated metal particles, unless this impairs the casting insulation properties.

To achieve a stable, thin-walled and hermetically-sealed and mechanically stable winding encapsulation, the windings are preferably wrapped in fibre, preferably glassfibre cloth.

In a further, particularly preferred embodiment, the cooling of the MF transformer is improved by cooling plates projecting sideways from the end faces of the primary and/or secondary windings, inserted in between the windings. The cooling plates are preferably cast in position, gas-and water-tight, and insulated by a thin insulating layer with a low thermal resistance and good heat transfer characteristics, preferably a polyester coating.

A further improvement to the cooling is effected preferably with cooling plates in thermal contact with the primary and/or secondary winding, and placed on the outer

surface of the windings. They can be in electrical contact with the conductors, for example by being soldered to them. Apart from that, the conductors are preferably given a thickened winding insulation in this area. The cooling plates on the outer surface of the windings can also be thermally connected with, but electrically insulated from, the conductors.

The primary and secondary windings are preferably single or multiple stacked block windings. They are preferably foil conductors, particularly copper foil, with layers of insulation material, e. g. mica, placed in between them.

The volume and weight and electrical losses of the MF transformer are also reduced, regardless of the presence of an encapsulated winding with air gaps in the core spaces, by the fact that the conductors of the primary and/or secondary windings comprise sub- conductors, which are transposed foil conductors.

The foil conductors preferably have lateral cut-outs. They are wound flat over one another in such a way that part of one sub-conductor is alternately underneath and above part of another sub-conductor. Two sub-conductors connected in parallel are provided preferably. However, more than two sub-conductors in a winding, transposed and connected in parallel, is also possible. An insulation layer is preferably laid in between the sub-conductors.

Transposing the foil windings has the advantage that with a compact design, it is not necessary to bring connecting wires out from the sides of the windings. The transposition reduces the impedance of the winding, which results in lower losses which in turn results in reduced heat generation. This means the transformer can be made smaller overall, and the performance increased.

To suppress partial discharges, even at relatively high voltages, it has proved particularly advantageous, irrespective of the design of the transformer, to insert layers of electrically-conductive material between the windings at the end faces of the primary and/or secondary windings in such a way that several layers of the primary and/or

secondary windings are covered at the side, at least at the end face of the winding. The layers of slightly electrically-conductive dielectric materials above and below the insulating layers of the block winding have the effect that no major differences in electrical potential, which could result in partial discharges, occur in the winding encapsulation at the end faces of the windings towards to the floating cores. The slightly electrically-conductive dielectric material is preferably crepe tape. <BR> <BR> <P>To minimise eddy current losses in the windings, "air gaps"are preferably provided at<BR> all separation points in the core (magnetic circuit). In this connection, "air gap"is understood as a point where the magnetic field enters and leaves the core halves (surfaces), the distance between which is defined by a foil and the thickness of the adhesive layer inside the glued joint.

An embodiment of the invention is described in greater detail below, with reference to the drawings.

These are: Fig. 1 a perspective view of the medium-frequency transformer ; Fig. 2 the MF transformer in Fig. 1 viewed in the direction of arrow II; Fig. 3 the MF transformer in Fig. 1 viewed in the direction of arrow III ; Fig. 4 a perspective view of the core of the transformer in Fig. 1; Fig. 5 a section through part of the winding encapsulation of the transformer in Fig. 1 ; Fig. 6 a perspective view of another embodiment of the MF transformer ; Fig. 7 the transformer in Fig. 6 viewed in the direction of arrow II;

Fig. 8 the transformer in Fig. 6 viewed in the direction of arrow III; Fig. 9 a partial section through the transformer in Fig. 6; Fig. 10 a partial section through an alternative embodiment of the transformer in Fig. 6; Fig. 11 a section through part of the winding encapsulation of the transformer in Fig. 6; Fig. 12 a perspective view of the transposed foil winding; and Fig. 13 a perspective view of an alternative embodiment of the core.

Fig. 1 shows a perspective view of the medium-frequency transformer, while Figs. 2 and 3 show the side elevations. The MF transformer comprises a winding encapsulation 1 with an essentially rectangular cross section and rounded corners. The winding encapsulation 1 consists of a primary winding, and a secondary winding divided into an upper and a lower layer (1/211/2), and is a block that hermetically encloses the windings. It is designed as a mounting for the core 2 of the transformer.

Fig. 4 shows a perspective view of the core 2 of a first embodiment. The core 2 is composed of two pairs of identical half-cores 2a, 2b, which are placed in the winding encapsulation 1 of the transformer with their ends facing each other. To fix the half- cores 2a, 2b in position, the two opposite sides of the winding encapsulation 1 are formed as angle plates 4 above the core 2, which together with the winding encapsulation embrace and hold the outer limbs of the core. The angle plates 4 are integral parts of the winding encapsulation. Two ribs 5 can be provided on the angle plates 4 for stiffness.

The external surfaces 6 of the core 2 are glued to the internal surfaces 7 of the winding units 4, such that between the internal surfaces 8 of the core 2 and the winding encapsulation 1, on the one side, air openings 3 are left and, on the other side, the surfaces 6 and 7 can also be filled in with glue. As a result, the core 2, except for the surfaces 6 and 7, is at all times freely suspended in the winding.

At the frontal surfaces of the winding encapsulation 1 two extensions 8 in each case are fitted to the upper and lower side of the core 2, on which the core may rest during gluing but which are also distanced from the frontal sides of the winding encapsulation.

The electrical connections 9 and 10 of the primary and secondary winding are located in the extensions 11 and 12 at the frontal sides of the spooling winding. Brackets 13 may be attached to the extensions 11 and 12 of the winding encapsulation, to additionally hold the core to the side, although this is not necessary. The core may be held sufficiently firmly and in the winding encapsulation by the glue alone, with little or no unwanted noise.

All parts for fixing the core consist of non-conducting material, so that the core can float freely in terms of voltage. The core, in contrast to conventional transformers, is not earthed. The preference is for ferrite cores or nanocrystal or amorphous cores.

The extensions 11 and 12 include seals (not shown) which seal against the openings through the supporting plate of the module and are used for the mechanical fastening of the transformer. For further support, moulded bushes can also be provided.

Because of its small volume or small mode of construction and its low weight, the transformer can be installed in the cooling current of a medium frequency rectifier module. It is installed in such a way that the air flow flows over all surfaces and flows through the air opening 3.

Figure 5 shows a section through the winding encapsulation 1 in the area of one of the two core windows. The winding encapsulation comprises the primary winding 14 and

the secondary winding 15, which consists of two subsidiary windings 15 a and 15 b.

The wires are copper foil wires 16, which are wound in an essentially square form with the interim layer of a winding insulation 17.

The winding is composed of an internal first secondary subsidiary winding 15 a, an intermediate primary winding 14 and an external secondary winding 15 b. The windings are externally switched to the extensions 11 and 12 of the winding encapsulation 1. The windings 14,15 a and 15 b are separated by medium frequency scale layer insulations 18 a and 18 b preferably made of mica. On the frontal sides of the individual winding layers above or below the mica insulation, bands 19 of electrically conducting material, preferably semiconductor"crepe bands", are introduced, which envelope the winding layers of the primary winding 14 and the first and second secondary subsidiary winding 15 a and 15b in the area of the frontal sides. The central crepe band 19, for example, is shifted to the side in such a way that on the one hand it is below the layer insulation 18 a and above the external winding and on the other hand above the layer insulation 18 b and below the internal winding. As a result, partial discharges in the area of the frontal sides, the layer insulation and in the area of the thin-walled edges of the foils are avoided.

All windings are enveloped in a fixed and hermetically sealed manner by a casting block from a resin, preferably epoxy resin with filling materials which are capable of transmitting heat. The filling materials capable of transmitting heat are for preference aluminium nitride and/or silanised quartz powder or metal particles insulated at their upper surface by oxidisation, provided that the moulding insulation properties are not impaired by the metal particles. The windings 14,15 a and 15 b are enveloped by a gross grain glass silk band 20, as a result of which the winding encapsulation is extremely stable and resistant to heat and cold shock.

Figures 6 and 8 show another execution example of the medium frequency transformer.

This form of execution is different from the execution example described with reference to Figures 1-6 in as much as it is provided with additional cooling sheets.

On mutually facing sides of the mantle surface of the winding encapsulation 1 above and below the core 2 in each case a cooling sheet 21 which is essentially U-shape is added to the windings. The middle spar of the cooling sheets 21, for example, is soldered to the foil wire.

Figure 9 shows a section through the medium frequency transformer in the area of a cooling sheet 21. For insulation, the cooling sheet is covered with powder. The cooling sheets 21 can, however, also be enveloped by the casting of the winding encapsulation 1 (Figure 10).

Further cooling sheets 22 are provided at the frontal sides of the winding encapsulation 1 below the core 2. Figure 11 shows a section through the winding encapsulation in the area of the cooling sheet 22. The cooling sheet 22 stretches through the primary winding 14. The parts of the cooling sheets projecting from the winding encapsulation 1 are either insulated with a polyester coating or embedded in the casting of the winding encapsulation 1. For illustration, Figure 11 shows on the left side a cooling sheet 22 a with polyester coating and on the right side a cooling sheet 22 b embedded in the casting resin. In practice, however, the same insulation material is used on both sides.

The cooling sheets 22 are bent to match the bending of the windings. They may be, for example, copper bending parts or aluminium profile units. Depending on the mode of construction, cooling sheets 21 can also be provided only at the mantle surfaces or cooling sheets 2 only at the frontal surfaces of the winding encapsulation 1.

Figure 12 gives a perspective illustration of two crossed conductors 23 and 24 of the primary and/or secondary winding. The copper conductors have a thickness of approximately 0.1-0. 2 mm at 10 kHz. In the area of the crossover, the conductors 23 and 24 have a reinforced cross section 23 a and 24 a for better discharge of heat. The conductors in each case on their mutually facing sides have a U-shape indent 25 and 26.

Between the conductors there is an insulation layer 27, which is also indented. The conductors 23 and 24 are arranged in such a way that they each lie alternately below or above the insulation layer 27.

This type of crossover is in principle possible with any number of conductors, whereby the conductors and insulation layers must be indented appropriately.

Figure 13 shows an alternative embodiment of the core 28, which is different from the embodiment of Figure 4 by virtue of the fact that the core is composed of two pairs of a U-shape 28 a and an I-shape 28 b semi-core. The U-shape and the I-shape semi-core are in each case glued to each other with the formation of air openings 29 of a defined thickness to control the magnetisation current. The gluing of the semi-cores at their mutual points of contact 30 takes place via intermediate U-shaped foil spacing layers 31 made, for example, of insulating material, whereby the thickness of the foils defines the width of the air openings. The foil spacing layers 31 are placed into the still pasty glue, before the semi-cores are put together. As a result, a highly stable binding is obtained.

Eddy current formation and development of heat in the related windings is to a large extent prevented by this type of binding.

It is an advantage if the core has a relatively large number of openings, each of which is however relatively narrow. As such, the U-form semi-cores 28 b, for their part, may consist of several subsidiary cores 28c and 28d, which in turn are glued to each other at their points of contact 30 with an interim layer of U-shape distance foils 31 of defined thickness. Figure 13 shows the core 28 before the gluing of the relevant semi-cores 28 a and 28 b.