GB2133932A - Improvements to strip wound magnetic cores - Google Patents

Abstract

An annular magnetic core 16 comprises two strips 20 and 22 of magnetic steel wound in a spiral configuration so that the second strip 22 is disposed between successive turns of the first strip 20. The strips 20 and 22 are separated by a layer of insulating material 14. Similar cores can be produced using three or more strips wound so that successive turns of one strip are separated by the other strips. <IMAGE>

Description

SPECIFICATION Improvements to strip-wound magnetic cores The present invention relates to strip-wound magnetic cores for use in electrical engineering applications involving high power densities and, in particular, to annular cores used in limiting the rate and extent of current increases in conductors during fault conditions.

For example, where it is desired to limit current surges in a dc bus bar, an annular core is placed around it. As the current in the bus bar begins to increase it gives rise to a circulating magnetic field of increasing magnitude around the bus bar. The electro-magnetic properties of the core are such as to oppose the increase in the magnetic field in the core and, consequently, the increase in the current in the bus bar. Magnetic cores for use in such applications are frequently strip-wound, that is, they are formed by winding a thin strip of magnetic steel in a spiral like a clock-spring.

Successive turns of the strip are separated from one another by a layer of insulating material or by an insulating coating provided on the strip. The effect of the increasing magnetic field encountered in surge-limiting applications is to induce a voltage round each turn of the magnetic strip. If high power densities are involved, the induced voltage around each turn may be greater than the maximum voltage that can be withstood by the insulating material between the turns and breakdown may occur. This prevents the core functioning properly.

In practice, it is not possible to solve this problem simply by increasing the insulation between successive turns of the strip. Since the rate of current increase is very fast, for example, the rise may occur over a period of a few microseconds only, the strip used to wind the core must be very thin, for example, of the order of 50 microns, in order to minimise eddy current effects which would otherwise detract from the effectiveness of the magnetic core during the initial stages of a current surge. It is generally impractical to apply interleaved or varnish-type insulation in thicknesses less than 40 microns and the resulting ratio of insulation to effective magnetic material which results is undesirably high.

It is possible to use powder insulation between the core laminations but this form of insulation present problems during the manufacture of the core as it is difficult to handle and to retain in place between the laminations. Magnetic cores for use in other applications are frequently manufactured from double oxide coated steel strip. However the thickness of the oxide coating which can be applied is limited to values which are not adequate to provide the insulation needed in severe current limiting applications.

In accordance with the invention there is provided a magnetic core comprising a plurality of strips wound in a spiral configuration such that the other strip or strips are disposed between successive turns of any one strip; there being between neighbouring strips a layer of insulating material. With this arrangement, two points one turn apart on any one strip are separated by the other strip(s) and by as many layers of insulating material as there are strips. If the core is composed of two strips, for example, the voltage across each layer of insulating material is halved and the risk of breakdown correspondingly reduced.

Embodiments of the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 is a perspective view of a conventional strip-wound annular magnetic core; Figure 2 is a schematic end view of the core of Figure 1 showing how the core is wound; Figure 3 is a schematic end view of an annular magnetic core in accordance with the invention; and Figure 4 is a schematic end view of a second core in accordance with the invention.

The conventional annular core 10 shown in Figure 1 is produced by taking a strip 12 of magnetic steel of thickness of the order of 50 microns and winding it in the manner of a clockspring, that is, in a generally spiral configuration, to form a short cylinder. Successive turns of the spiral strip 12 are separated by a layer of insulating material 14 indicated by means of a dotted line in Figure 2. The insulating material 14 may be in sheet, varnish or powder form and forms a layer about 40 microns thick. Alternatively the insulation may be provided by oxide coating the strip 12, in which case, there are in fact two layers of insulating material, one associated with each turn, separating each turn from its immediate neighbour.

As described above, the effect of the increasing magnetic fields encountered by such cores when used in limiting fault currents is to induce in the core a voltage around each turn of the strip 12, so that there is a voltage across the insulation material 14 between two points on the strip 12 one complete turn apart, for example, points A and B in Figure 2. If the core 10 is being used in an application involving high power densities, the voltage between points A and B may be greater than the maximum voltage that can be withstood by the insulation material so that breakdown occurs between points A and B. For the reasons outlined above, this problem cannot be solved simply by increasing the thickness of the insulating material without detracting from the core's usefulness in surge limiting applications.

The cores 16 and 18 shown in Figures 3 and 4 are intended to resist breakdown when utilised in circumstances where high power densities occur.

The core 16 shown in Figure 3 comprises two similar magnetic steel strips 20 and 22 which are wound together to form a cylindrical core similar to that shown in Figure 1. However, with this arrangement, two points on the same strip one turn apart, for example, points C and D in Figure 3, are separated from one another by the other strip and by two layers of insulating material 14. As a result, the voltage across each layer of insulation 1 4 is half what it was in the core 10 shown in Figures 1 and 2 and the likelihood of breakdown across the insulation 14 is greatly reduced.

Similarly, the core 18 shown in Figure 4 is formed by winding three strips 24, 26 and 28 together. In the core 18, the points E and F one turn apart, on the strip 26, are separated by the strips 24 and 28 and by three layers of insulating material 14. Consequently the voltage across each layer of insulation 14 in the core 18 is one third tne voltage across each layer in the core 10 shown in Figures 1 and 2.

The voltage across each layer nf insulation and, hence, the likelihood of breakdown across each layer, can be further reduced by the use of a larger number of strips in winding the core.

Any technique used to improve the performance of a core wound from a single strip can be applied to the strips used in winding a core according to the invention. These may include, for example, edge-rolling to thin the edges of the strips and prevent edge-to-edge contact, reducing the mechanical stress on the strip material, controlling the strip tension during winding, special oxide or powder coatings and special anneaiing and surface finishing treatments.

In the cores shown in Figures 3 and 4 the points at which the individual strips start and finish are evenly spaced around the inner and outer circumferences of the annul us. Whilst the arrangement is convenient, it is not essential to the invention.

Magnetic cores of any shape which follows a simple two-dimensional closed path, for example, a square or a D-shape, can be wound from a plurality of strips as described above. Such cores can replace conventionally-wound strips in many electrical machines or other applications where breakdown is likely to occur.

Claims (5)

Claims

1. A magnetic core comprising a plurality of strips wound in a spiral configuration such that the other strip or strips are disposed between successive turns of any one strip; there being between neighbouring strips a layer of insulating material.

2. A magnetic core according to claim 1 wherein the strip material is magnetic steel.

3. A magnetic core according to claim 1 or 2 wherein the insulating material is an oxide layer formed on the surface of each strip.

4. A magnetic core according to any preceding claim wherein the strips are so arranged that the core is rotationally symmetrical about a central axis.

5. A magnetic core substantially as hereinbefore described with reference to Figure 3 or Figure 4 of the drawings.