GB2026251A - Electro-magnetic devices - Google Patents

Abstract

An electromagnetic device, e.g. a choke or a current transformer, which includes a core such as a closed ferrite ring 1, has a stress-relieving material 3 between a winding 2 and the core, so that the core can be tightly wound and the stress subsequently relieved by flow of the intermediate material between the turns of the winding. The material-may be one which is softened by heat or one which slowly deforms under pressure. <IMAGE>

Description

SPECIFICATION Electro-magnetic devices This invention relates to electro-magnetic devices and to methods of producing them. When an electro-magnetic core, especially but not exclusively a closed ferrite core, is wound tightly with a heavy conductor there is a risk that the mechanical force exerted by the conductor on the core will result in a change in the permeability of the core, frequently in an unpredictable fashion.

In order to combat this situation the invention provides an intermediate material between the winding and the core which transmits the winding stress to the core but can subsequently be made to collapse so that the stress is reduced or eliminated.

The intermediate material may be such that its structure can be changed by, for example, heat.

Thus, it may be a wax or a plastics material such as low density polythene provided as a washer, shell or former, or as a coating, in a suitable thickness, on which the winding is put down. After winding, the material may be softened, for example by passing a current through the winding to heat it. Alternatively, the intermediate material may be such that it has a characteristic of slowly deforming under pressure, so that the initial stress due to the winding is gradually relieved as the material flows under the pressure into the spaces between the turns of the winding.

One form of the invention is illustrated in the accompanying drawing which is a section through a choke in the form of a toroidally wound ring core of ferrite material. In the drawing a core 1 is in the form of an uninterrupted annulus of ferrite material and may have a typical core section of say 1 Omm. x 5mm. Such a core may form the basis of a transformer or a choke for interference suppression for heavy current apparatus, say for use at currents of the order of 10 amperes, in which case it might be wound with a copper conductor of about 1 .5mm2 cross-section, as indicated at 2 in the drawing. The conductor 2 would conveniently be applied by means of a machine which carries it through the core and wraps is around the core while the latter is rotated so as to distribute the winding around the annular surface.The machine relies on applying considerable tension to the conductor to correctly position the winding on the core section with optimum utilisation of the winding space. The result of this can be to change the permeability of the core by as much as 40%.

However, the provision of an intermediate material in the form of a washer or shell 3 of low density polythene, which may if desired be duplicated at the other face of the core, holds the winding away from the core by a distance equal to the thickness of the washer. In the example, this is of the order of 6.0mm.

When the winding is completed, a current is passed through it for a short time to raise the temperature to the softening point of the polythene. This then flows around the winding and permits the conductors to settle into a position where they are still spaced slightly from the core but closely embrace it. The stress that was previously applied to the core by the winding is thus eliminated or reduced to an accept able value. The spacing needed to relieve the stress may be as little as 0.25mm. but the provision of shells thicker than this allows for individual variations and ensures that the winding does not contact the core.

Although the invention is principally applicable to suppression chokes, it may also find application in other devices such as, for example, cores for current transformers.

1. An electro-magnetic device including a core on which a conductive winding is wound, wherein there is provided between the winding and the core an intermediate layer of material which is capable of subsequent deformation to relieve the stress applied to the core by the winding.

2. A device according to claim 1, wherein the intermediate layer is of a material capable of being softened by heat.

3. A device according to claim 2, wherein the intermediate layer is of low density polythene.

4. A device according to claim 1, wherein the intermediate layer is of a material which deforms slowly under pressure.

5. An electro-magnetic choke or transformer in accordance with any one of the preceding claims and comprising an uninterrupted annulus of ferrite material, a washer or shell of stress-relieving material on at least one end face of said annulus, and a toroidal winding on said core wound over said washer or shell.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electro-magnetic devices This invention relates to electro-magnetic devices and to methods of producing them. When an electro-magnetic core, especially but not exclusively a closed ferrite core, is wound tightly with a heavy conductor there is a risk that the mechanical force exerted by the conductor on the core will result in a change in the permeability of the core, frequently in an unpredictable fashion. In order to combat this situation the invention provides an intermediate material between the winding and the core which transmits the winding stress to the core but can subsequently be made to collapse so that the stress is reduced or eliminated. The intermediate material may be such that its structure can be changed by, for example, heat. Thus, it may be a wax or a plastics material such as low density polythene provided as a washer, shell or former, or as a coating, in a suitable thickness, on which the winding is put down. After winding, the material may be softened, for example by passing a current through the winding to heat it. Alternatively, the intermediate material may be such that it has a characteristic of slowly deforming under pressure, so that the initial stress due to the winding is gradually relieved as the material flows under the pressure into the spaces between the turns of the winding. One form of the invention is illustrated in the accompanying drawing which is a section through a choke in the form of a toroidally wound ring core of ferrite material. In the drawing a core 1 is in the form of an uninterrupted annulus of ferrite material and may have a typical core section of say 1 Omm. x 5mm. Such a core may form the basis of a transformer or a choke for interference suppression for heavy current apparatus, say for use at currents of the order of 10 amperes, in which case it might be wound with a copper conductor of about 1 .5mm2 cross-section, as indicated at 2 in the drawing. The conductor 2 would conveniently be applied by means of a machine which carries it through the core and wraps is around the core while the latter is rotated so as to distribute the winding around the annular surface.The machine relies on applying considerable tension to the conductor to correctly position the winding on the core section with optimum utilisation of the winding space. The result of this can be to change the permeability of the core by as much as 40%. However, the provision of an intermediate material in the form of a washer or shell 3 of low density polythene, which may if desired be duplicated at the other face of the core, holds the winding away from the core by a distance equal to the thickness of the washer. In the example, this is of the order of 6.0mm. When the winding is completed, a current is passed through it for a short time to raise the temperature to the softening point of the polythene. This then flows around the winding and permits the conductors to settle into a position where they are still spaced slightly from the core but closely embrace it. The stress that was previously applied to the core by the winding is thus eliminated or reduced to an accept able value. The spacing needed to relieve the stress may be as little as 0.25mm. but the provision of shells thicker than this allows for individual variations and ensures that the winding does not contact the core. Although the invention is principally applicable to suppression chokes, it may also find application in other devices such as, for example, cores for current transformers. CLAIMS

1. An electro-magnetic device including a core on which a conductive winding is wound, wherein there is provided between the winding and the core an intermediate layer of material which is capable of subsequent deformation to relieve the stress applied to the core by the winding.

2. A device according to claim 1, wherein the intermediate layer is of a material capable of being softened by heat.

3. A device according to claim 2, wherein the intermediate layer is of low density polythene.

4. A device according to claim 1, wherein the intermediate layer is of a material which deforms slowly under pressure.

5. An electro-magnetic choke or transformer in accordance with any one of the preceding claims and comprising an uninterrupted annulus of ferrite material, a washer or shell of stress-relieving material on at least one end face of said annulus, and a toroidal winding on said core wound over said washer or shell.