The invention relates to a method for producing inductive components, especially current transformers for electric power meters.

Current transformers are inductive components that through their design with respect to imaging response and phase error enable precise acquisition of mains current in industrial meters as well as household meters. In combination with evaluation electronics they are now replacing the so-called Ferraris watt-meters (three-phase meters).

It is commonly known that such current transformers as well as other inductive components such as transformers for power supplies, chokes and repeaters can be produced on the basis of magnet cores affixed or molded in plastic housings. Here a magnet core provided with one winding or several windings is inserted in a thermoplastic housing and immersed in a molding resin generally consisting of polyurethane or epoxy resin. This causes numerous problems as regards outlets for the connecting cables from the windings. Providing outlets for the connecting cables through the molded surface demands precise positioning of the cable during the molding process. This generally requires the use of so-called molding pallets with cable fixtures. The use of such molding pallets is very costly.

A further possibility is to fasten the cable in a double groove on the upper edge of the housing. This, however, frequently leads to a discharge of molding resin as a consequence of capillary attraction through the groove.

A cable guide through a hole drilled in the wall of the housing requires additional sealing and hence additional cost which is economically disadvantageous.

The goal of the present invention, therefore, is to present a new method for producing inductive components, especially current transformers, that by a wide margin avoids the above-mentioned problems.

According to the invention this goal is achieved through a method for producing inductive components with the following steps:

• 1. Providing a metallic mold;
• 2. Inserting the magnet core provided with at least one winding in the mold;
• 3. Closing the mold;
• 4. Filling the mold with a molten, hot-melt adhesive material under pressure;
• 5. Defined cooling of the mold;
• 6. Opening the mold and withdrawing the molded inductive component.

This method greatly simplifies the production sequence in comparison to existing technology, which leads to noticeably lower costs.

The molds employed generally consist of aluminum or aluminum alloy, which cost significantly less than the injection molds used for the housing in existing technology. The use of these molds also enables significantly shorter product introduction cycles in comparison to existing technology, as the molds are significantly simpler. This leads to rapid and simple changes for customized versions of the inductive components.

Filling with molten, hot-melt adhesive material is carried out preferably at a pressure of from 0 to 20 bar, preferably from about 10 to 20 bar, with the hot-melt adhesive material consisting of a polyamide-based thermoplastic hot-melt adhesive. In particular this refers to a polyamide-based hot-melt adhesive free from filler material. Use of this polyamide hot-melt adhesive significantly increases opportunities for recycling the inductive components, as only a single plastic is used for these inductive components. In particular several polyamide hot-melt adhesives can be composted. Moreover these thermoplastics do not have to be made from fossil materials such as oil or coal, but can also be made from renewable raw materials (wood resins).

In one version the connections from the winding or windings are led out of the mold in a clearly defined manner before the magnet core is inserted in the mold. Here cables are provided as connections. The cables are led out directly through grooves in the mold, whereby the special process prevents the discharge of hot-melt adhesive.

In another version the connections from the winding or windings in the mold are laid in blind holes. This enables the use of connections with relatively high bend resistance, so that the inventive method can also be used for producing inductive components directly suitable for surface-mount-devices (SMD).

The magnet cores consist preferably of toroidal cores made from metal alloys, in particular toroidal cores made from amorphous or nanocrystalline alloys. The use of such amorphous or nanocrystalline alloys in comparison to the crystalline alloys or ferrite cores used hitherto yields a substantial reduction in volume as well as better technical properties for the types of inductive components mentioned above. The benefit and properties of these amorphous or nanocrystalline alloys are fully described, for example, in EP 0271657 B1.

As shown in the figures, a current transformer 1 produced by the inventive method comprises a magnet core 2 provided with a secondary winding 3. The secondary winding 3 generally consists of several 100 to several 1000 windings. The secondary winding 3 in this current transformer 1 consists of relatively thin wire, that is the wire has a thickness of from 0.05 to 0.25 mm. The ends of the secondary winding 3 shown here are led out as a dual-strand connector 5, so that the current transformer 1 can be connected to a circuit board (not shown).

The magnet core 2 of the current transformer 1 shown here is a toroidal core made from an amorphous alloy. The toroidal core with the secondary winding 3 mounted on it was produced by the inventive low-pressure hot-melt molding method.

Here a mold made from aluminum alloy was provided (not shown). The toroidal core with the secondary winding 3 mounted on it was inserted in this mold, where the secondary winding 3 with its ends in the form of a dual strand connector 5 was led out of the mold. The mold was closed and filled with a molten, polyamide-based thermoplastic hot-melt adhesive under a pressure of about 15 bar. This caused a molded body 4 to surround the magnet core 2. In the region of the opening of the toroidal core the molded body 4 shows an opening 5 open at both ends through which the primary winding (not shown) of the current transformer 1 can be led.

Subsequently the mold underwent a defined cooling. After cooling, the mold was opened and the molded current transformer 1 was withdrawn. After withdrawal of the molded current transformer 1, the sprues were removed.