GB2114373A - Laminated magnetic cores - Google Patents

Abstract

A ferro-magnetic alloy that is wholly or to a substantial degree amorphous is used in making a laminated magnetic core 1. At least one lamina of the alloy is bent into a closed or nearly closed loop; usually a large number of laminae will be nested together with coincident or nearly coincident joints. Because of the high elastic limit of the amorphous alloys, compared with conventional magnetic alloys, the joint can be sprung open, repeatedly if required, for linking with an electrical coil or for other purposes, and without risk of damage to the magnetic properties of the core. <IMAGE>

Description

SPECIFICATION Magnetic cores This invention relates to magnetic cores of the kind in which at least one lamina of magnetic material (and usually many more than one) is bent into a closed or nearly closed loop with a joint that can be sprung open to allow the loop to be linked with an electrical coil.

Cores of this kind have been known for many years. In some cases the laminations are opened one-by-one for linking with the coil, which ailows the joints to be positioned at different places around the core for optimum magnetic performance. More often, for convenience of manufacture, substantial groups of contiguous laminations, or even all the laminations of the core, are arranged with their joints coincident or nearly so, in order that the group, or the core, can be sprung open and linked with the coil in a single operation.

Such cores, as currently known, are difficult to design because the magnetic properties of the laminations from which they are made are strongly dependent on the metallurgical condition of the laminations.

Magnetic properties are optimised by annealing at temperatures of 700"C or higher, and are adversely affected by very small degrees of work hardening, such as may be involved in springing the core open beyond the elastic limit of the material. It is therefore necessary to design the core, and the associated coil, in such a way that the degree of movement needed to spring the core open and link it with the coil is as small as possible and ideally well within the elastic limit; and it is inadvisable to spring the core open more than once.

We have now discovered that certain known but rare materials can be used to make cores of this kind that can be sprung open widely and/or repeatedly without serious detriment to their magnetic properties.

In accordance with the invention, a magnetic core of the kind in which at least one lamina of magnetic material is bent into a closed or nearly closed loop with a joint that can be sprung open to allow the loop to be linked with an electrical coil is distinguished by the fact that the lamina, or at least one of the laminae, is made of a ferromagnetic alloy that is wholly or to a substantial degree amorphous (that is non-crystalline or glassy).

Suitable amorphous alloys and methods of making them are known from the patent literature, comprising British Patents 1447267,1525959, 1540771, 1547461,1548124, 1558151, 1580499,1596909; British Applications 2003772A, 2023653A, 2038358A, 2051 860A; European applications 7062, 21101; and US Patents 3938583, 3939900,3960200, 4036638,4038073,4052201, 4053331,4053333,4056411, 4067732,4081298, 4116728,4150981, 4152144,4184532,418821 1,4217135 and 4219355. Our present preference is for the alloys based on iron-boron-additives or on cobalt-boron-additives, for example the alloys comprising, in atom percent, (a) Fe 78, B 13, Si 9, and (b) Co 58, B 16, Si 11, Ni 10, Fe 5.

As with conventional materials, the core in accordance with the invention is preferably made by winding a spiral from a tape of an appropriate alloy, optionally pressing to a shape with relatively flat-sides, bonding the turns together, cutting through all the turns with an abrasive wheel or other suitable cutting tool, and then closing up the joint, ideally in such a way that the spiral initially formed is converted into a series of nested closed loops, though especially when the strip is thin perfect alignment is unlikely to be achieved.

Alternatively, cut lengths of tape may be individually bent to form nesting loops.

Preferably, in accordance with our prior application No. 8108047 (now published under Serial No.

2095041A), the laminae are bonded together only over a first, minor, part of the circumference of the core and unbonded over a second major portion, and the joint extends through the bonded portion, such that the core can be sprung open by flexing of the laminae in the second part of the circumference without any substantial deformation of the laminations in the bonded part.

Apart from being linked with coils, the cores of the invention can be sprung around a conductor, or group of conductors, for example in a clip-on ammeter or similar current measuring device.

The invention will now be described by way of example and with reference to the accompanying drawing which is a perspective sketch of one form of magnetic core in accordance with the invention, shown with its joint opened.

Example 1 An amorphous tape of the alloy comprising Fe 80, B 14.5, Si 3.5, C 2 (atom percent), 10 mm wide by 0.04 mm thick, was spirally wound to form a toroid with inside and outside diameters of 22 and 35 mm respectively. This was then pressed, in a conventional manner, to form a core 1 with two straight limbs 2, 3 about 22 mm long on opposite sides of a 9 mm wide window 4 and clamped.

The clamped core was next heat-treated in argon, holding at a temperature of 370"C for two hours after which it was cooled to room temperature in the same atmosphere; the clamps could then be removed without any substantial loss of the pressed shape.

One end 5 of the core was then dipped, until the curved yoke and a short length of each limb up to the level 6 indicated in the drawing, was immersed, in a low-viscosity filled expoxide resin in order to bond the turns of the spiral winding together in the immersed portion 5 only. After the resin had hardened, a single cut was made along a line 7 through the bonded yoke, parallel to and just clear of the inside surface of one of the iimbs, using an abrasive wheel, so forming a joint through the core.

The magnetic characteristics of the core 1 were then measured and the results are given below: 50Hz Magnetisation Hrms (alms B (TeslaJ 0.4 0.0009 1.0 0.0025 10 0.045 100 0.45 50Hz Losses B(Tesla) Loss(W/kg) 0.1 0.007 0.5 0.10 1.0 0.30 20kHz Losses 8(Tesla) Loss(W/kg) 0.1 9 0.5 230 1.0 800 The joint formed in the core 1 was opened to produce a gap of 30 mm across the two faces and then closed. This operation was repeated 2000 times and the magnetic properties re-measured after 30, 100,500, 1000 and 2000 repetitions. The magnetic characteristics were found not to have been substantially degraded by these operations.

Example 2 This was similar to Example 1 except that the amorphous tape was formed of an alloy of the composition Fe 78, B 13, Si 9 (atom per cent).

Claims (7)

1. A magnetic core of the kind in which at least one lamina of magnetic material is bent into a closed or nearly closed loop with a joint that can be sprung open to allow the loop to be linked with an electrical coil distinguished by the fact that the lamina, or at least one of the laminae, is made of a ferro-magnetic alloy that is wholly orto a substantial degree amorphous.

2. A core as claimed in Claim 1 comprising a plurality of nested loops of the said alloy.

3. A core as claimed in Claim 1 or Claim 2 in which the said ferro-magnetic alloy includes iron and boron as its major constituents.

4. A core as claimed in Claim 1 or Claim 2 in which the said ferro-magnetic alloy includes cobalt and boron as its major constituents.

5. A core as claimed in any one of the preceding claims in which the laminae are bonded together over a first, minor, part of the circumference of the core and unbonded over a second, major, portion and in which the joint extends through the bonded portion such that the core can be sprung open by flexing of the Iaminae in the second part of the circumference without any substantial deformation of the lamination in the bonded part.

6. A magnetic core substantially as described with reference to Example 1.

7. A magnetic core substantially as described with reference to Example 2.