GB2095041A - Magnetic cores - Google Patents

Abstract

A curved lamination magnetic core comprises magnetic strip material forming a laminated closed loop, the laminations being bonded together over a first part of the circumference (above the line 4,4) and unbonded over a second, major part (below that line). A single cut (5) extends through all the laminations at a position in the first, bonded part of the circumference. The position of the cut is such that the closed loop can be sprung open at the cut by flexing in the second, unbonded part of the circumference without any substantial deformation of the bonded laminations in the first, bonded part adjacent to the cut. In this way the core can be assembled with a pre- wound electrical coil in a single simple operation without requiring two joints, and an abrasion-cut joint can be used without machining treatment. <IMAGE>

Description

SPECIFICATION Magnetic cores This invention relates to magnetic cores for transformers and other electric inductive apparatus. More particularly, it relates to cores of the kind formed by bending one or more than one strip of magnetic material (usually grain-oriented silicon steel) into a loop, for example by winding into a spiral or by pre-curving individual laminaticns into appropriate sizes of loop and nesting them together.

It is difficult and expensive to wind electrical coils directly onto a core of this kind, especially as efficiency of the finished apparatus requires the core window to be filled as full as possible, and it is normal practice to pre-wind the electrical coils and appropriately joint the magnetic cores to allow subsequent assembly.Two conventional core types are distinguished by the form of jointing: in a "C-core" the laminations (which are normally bonded together) are cut in two places by straight cuts extending through all the laminations and the cut surfaces are machined ("linished") to form two G or J-shaped parts that can be assembled with minimum magnetic discontinuity; in a "distributed gap core" the individual laminations are cut in at least one place and the cuts in adjacent laminations are spaced apart around the circumference, so that the magnetic effects of the joints is minimised, and if there is only one joint per lamination they are individually sprung open and threaded through the electrical coil one by one.

Proposals have been made to form cores with only one joint, either distributed or planar, so positioned that the core can be sprung open as a whole sufficiently to enable the electrical coil to be slid into place. Examples are to be found in British Patent Specifications 622538, 624137, 769461 and 834165 (all published between 1 949 and 1960). The attraction of this approach is that magnetic properties should be improved compared with a C-core by the elimination of half the joints and without the tedious assembly process required by single-joint distributed-gap cores. It has however proved difficult when seeking to put these proposals in practice to form a joint that will re-close without difficulty and yet be adequately closed.

In accordance with the invention, magnetic core comprises one or more than one strip of magnetic material bent into a loop and forming a laminated structure of closed loop form, the laminations being bonded together over a first part of the circumference of the closed loop and unbonded over a second, major part of the closed loop and a single cut extending through all the laminations at a position in the first part of the circumference such that the closed loop can be sprung open at the cut by flexing of the laminations in the second part of the circumference, without any substantial deformation of the bonded laminations in the first, bonded part adjacent to the cut to allow assembly with a pre-wound electrical coil.

Normally it is preferable that the whole of the circumference of the loop is formed by only one first, bonded part and one second, unbonded part, and preferably the first, bonded part (or the sum of all such parts) does not exceed about one quarter of the total circumference.

The closed loop may be of various shapes, but in most cases will be "rectangular", comprising straight parallel limbs joined by yokes that are either roughly semicircular or else roughly flat apart from rounded corners. In such cases the inner end of the cut is preferably in one of the yokes, just sufficiently clear of a corner to avoid any risk of injury to the inner face of the adjacent limb and the direction of the cut is preferably either substantially parallel to the limbs or else inclined outwardly by less than 30 (preferably less than 10 ). In this case bonding is preferably confined to the cut yokes and a short adjacent part of each of the limbs.

Bonding can be effected with conventional adhesives, but the more flexible ones (say those that can be peeled from a substrate) are much preferred.

Cutting is preferably effected by a flexible abrasive wheel, as subsequent machining steps are then not normally required; but a saw or milling cutter could be used. Since the cutting operation will remove material, allowance has to be made in the initial shape of the core, for example a rectangular core would need to be wound on a slightly tapered mandrel and cut through the longer yoke.

As with conventional curved-strip cores, joints will normally be held closed, after final assembly, by means of strapping or an adhesive; and more complex structures can be made by assembling a number of basic cores of the form described.

Cores in accordance with the invention are relatively robust, and do not for example require special packing.

In the accompanying drawings, Figure 1 shows one form of magnetic core in accordance with the invention, made by winding a single continuous strip of grain-oriented silicon steel onto a mandrel that is slightly wider at the top (as drawn) than the bottom so as to form a 'spiral' winding with so-far slightly diverging limbs 1 joined by a lower yoke 2 and a slightly longer upper yoke 3. This upper yoke and a short adjacent length of each limb (down to the level 4) is bonded (e.g. by inverting and dipping in a liquid bonding resin) and the remainder left unbonded to preserve maximum flexibility. A single cut 5 is formed in the upper yoke parallel to the limbs and about 0.75 mm inside the innerface 6 of the adjacent limb. The width of cut is such that after bonding or otherwise securing to close up the joint, the limbs become parallel.

Figure 2 shows two such cores 7, 7 as finally assembled about an electrical coil 8 (shown diagrammatically) and secured with a common strap 9. This is of unconventional design (though conventional strapping could be used) and utilises a tensioning buckle 10 shown in Figure 3. The buckle consists of a yoke 11 and a split pin 12.

One end 13 of the strap 9 (which may be of soft iron, non-magnetic metal, or suitable plastics material) is attached to the buckle by looping round it and, preferably after making two turns around the core as shown, the opposite end 14 is passed through the slot of the split pin 1 2. A few turns of the split pin is then all that is required to generate adequate tension in the strap.

Claims (5)

Claims

1. A magnetic core comprising one or more than one strip of magnetic material bent into a loop and forming a laminated structure of closed loop form, the laminations being bonded together over the first part of the circumference of the closed loop and unbonded over a second, major part of the closed loop and a single cut extending through all the laminations at a position in the first part of the circumference such that the closed loop can be sprung open at the cut by flexing of the laminations in the second part of the circumference, without any substantial deformation of the bonded laminations in the first, bonded part adjacent to the cut to allow assembly with a prewound electrical coil.

2. A magnetic core in accordance with Claim 1 which is rectangular with straight parallel limbs joined by yokes, wherein the inner end of the cut is in one of the yokes, just clear of a corner of the core.

3. A core as claimed in Claim 2 in which the direction of the cut is substantially parallel to the limbs.

4. A core as claimed in Claim 2 or Claim 3 in which bonding of the laminations is confined to the cut yoke and a short adjacent part of each of the limbs.

5. A magnetic core substantially as described with reference to and as shown in either Figure 1or Figures 2 and 3 of the accompanying drawing.