GB2168202A - Transformer cores - Google Patents

Abstract

A high power transformer has at least one of its laminations scribed, preferably by a laser beam or wire brush, at an angle between 35 DEG and 55 DEG to the rolling direction D of the steel, in the region of the joints and corners 20, 21 so as to increase the number of closure domains in the magnetic structure of the steel and thus minimise the power loss. Central regions may be scried in a direction perpendicular to the rolling direction. <IMAGE>

Description

SPECIFICATION Transformer cores The present invention relates to cores for transformers and particularly to laminated cores for large power transformers.

Usually, a grain-orientated electromagnetic steel sheet is employed for the construction of such cores. The grain-orientated electromagnetic steel sheet is comprised of crystal grains which have a so called Goss texture and which have an (110)[001] orientation expressed by Miller indices. This designation indicates that the (110) plane of the crystal grains are parallel to the sheet surface, while the [001] axis of the crystal grains, i.e. the direction of easy magnetisation, is parallel to the rolling direction. Thus, the magnetic properties of the grain-orientated electromagnetic steel sheet are excellent in the rolling direction and deteriorate with a deviation of angle of magnetisation from the rolling direction.The grain-orientated electromagnetic sheet steel is, therefore cut or blanked in such a manner that the direction of magnetic flux through the core of the transformer is coincident with the rolling direction.

This is shown in Fig. 1(a) where the coiled sheet steel 1 is unwound with the rolling direction shown by the arrow A and a blank 2 of the limbs of a transformer core 3 is cut out with the major lineal dimension along the rolling direction. As shown in Fig. 1(b) the core is composed of several legs 4 and yokes 5 each made up of laminations stacked together and overlapping at the joint regions. The direction of rolling is shown by the arrows B in Fig. 1(b) and the magentic flux path is shown by arrows C in Fig. 1(c) which also shows the overlapping regions of the laminations at the joints by showing the next succeeding layer in dotted outline.

The grain-orientated electrical steel has special magnetic properties which make it amenable for use in transformer cores, since it demonstrates lower values of specific power loss per unit weight of core than other types of electrical steel. The steel has very close alignment of the preferential easily magnetised direction of the polycrystalline structure along the rolling direction of the steel strip 7 as shown in Fig. 2. As a consequence of the method of producing this type of steel, the mean grain size of the polycrystalline matrix 6 is exceedingly large, some 1-2 cm in diameter, and this can offset the lower power loss benefits obtained from the improved alignment of the polycrystalline structure.

The fundamental phenomenon which relates grain size, orientation, state of tensile strain and other material variables, with the observed low values of total power loss under cyclic magnetisation is the magnetic domain structure of the electrical steel material. This is illustrated in Fig. 2(a) where within each single crystallite 8 of the polycrystalline matrix 6 the magnetic moment vectors of individual atoms align up over comparatively large distances such that in typical grains of this material there are well defined magnetically aligned regions called domains 9, which are separated by regions of transition of the magnetic direction termed domain walls 11, and which extend completely through the material thickness. With well aligned crystals the pattern demonstrated by the domains is simple.

They take the shape of rectangular parallelelpipeds and adjacent domain volumes have directions of magnetisation 10 which are opposite to each other. As the size of the grains is increased the width of these main magnetic domains also increases. Large magnetic domains are not, however, conducive to low values of power loss because on applying an external magnetic field, the domains with magnetisation in the same direction as that of the external field expand at the expense of the other domains. Thus, with large domains with a corresponding small number of domain walls 11, the domain walls move further and faster to accommodate a magnetic field increment than the case where there are small domains and a large number of domain walls. In the former case the higher velocity of the domain walls gives rise to a higher value of the eddy current component of no-load loss.

As illustrated in Fig. 2(b), however, on grains mis-orientated from the (1 10)[001], the main domains also have a surface structure of domains called closure domains 12 which are formed to minimise the total magnetic energy of the composite domain structure The closure domains are aligned at an angle 0 to the rolling direction to form a dendritic shaped pattern. When the electrical steel is magnetised along the rolling direction, (as in limbs and yokes), the predominent process is one of rearrangement of the main domains, as explained above, at the expense of the closure domains.However when the steel is magnetised at angles to the rolling direction (as in the core joint regions), the magnetisation process is accomplished by firstly nucleation and subsequently, rearrangement of the closure domain structure at the expense of the main domain structure. High levels of magnetic field are required to nucleate sufficient closure domains to initiate magnetisation of the steel strip/core by rearrangement of the closure domain structure.

Clearly, therefore, to minimise the power loss when the steel is magnetised in the rolling direction, it is desirable to increase the number of main domains so that they are fairly small but aligned with the direction of magnetisation. Similarly, to minimise the power loss when the steel is magnetised at angles to the rolling direction, it is desirable to increase the number of closure domains.

One known method for increasing the number of main domains and thus decreasing the power loss when the steel is magnetised in the rolling direction is to apply a high-stress insulative coating onto the finished grain-orientated electromagnetic steel as taught in U.S.

Patent No. 3996073. Such coatings place the grain-orientated steel strip in tension in the rolling direction of the steel which causes a decrease in the width of the main domains and a reduction in the number of closure domains. Many commercially available grain-orientated electromagnetic steel sheets have such a coating.

Mechanical, or physical, scribing transverse to the sheet rolling direction is another technique that has been found to be effective in reducing the main domain spacing and lowering the power losses. This is because scribing lines across the sheet has the effect of nucleating new domains by creating additional domain walls which subdivide the existing domains and thus increase the number of such main domains. The disadvantages of mechanical scribing are, however, that the insulative coating is damaged and the surface of the steel sheet becomes uneven. These factors tend to produce increased interlaminar losses in a transformer manufactured from a steel so treated.

It has also been proposed to irradiate the grain-orientated steel strip with a laser beam.

The steel is irradiated at or nearly transverse to the direction of rolling and this induces a domain substructure by nucleating new domains in the same way as described above and thus will henceforth be called laser scribing. The scribing can take place in such a way as to mark the steel sheet and thus ablate the insulative coating (see European Patents Nos.

0008385, 0033878 and 0087587) or in such a way that the steel is not marked and the coating is not damaged (see European Patents Nos. 0100038 and 0102732).

In U.K. Patent No. 2062972 there is disclosed a method of producing a core for electrical machinery, such as transformers, which has been laser-scribed in a direction either transverse or parallel to the rolling direction of the steel so as to produce decreased power loss. In all cases, the elongated portions of the core are scribed in only these two directions and the joint regions may or may not be so scribed.

However, during operation of the transformer, magnetic flux traverses around the core and at the joint regions, that is at the corner joints and the Joints, the magnetic flux turns through 90" and is at some point in a direction which is not in the rolling direction. This consequently produces an increase in total power loss. By scribing the joint regions of the core at angles to the direction of rolling, it has been found that this power loss can be decreased. Fig. 3 shows a graph of the percentage change in power loss versus the angle 6 between the direction of scribing and the direction of easy magnetisation or direction of rolling.Fig. 4 shows the sample 13, called an Epstein sample, taken from the corner joint region of a core 17 and showing the rolling direction 14, the direction of scribing 15 and the direction of magnetisation 16.

As can be seen the power loss is at a minimum when the angle O is approximately 45". It will be realised that this angle is related to the angle (0 of the direction of alignment of the closure domains to the rolling direction, since as discussed earlier, when the direction of magnetisation is at angle to the rolling direction, the closure domain structure is rearranged at the expense of the main domains. Thus, by scribing at such an angle as to increase the number of closure domains, the power loss is minimised.

Accordingly, the invention provides a laminated core for a transformer in which at least one of the laminations has been scribed in at least one of its joint regions in the direction of magnetisation of the closure domains in the volume of the material.

Preferably the scribing is produced by a laser beam but it may be produced by any other desired means which may, for example, comprise a wire brush.

The direction of magnetisation of the closure domains is usually between 35 and 55 and is preferably about 45 to the rolling direction.

In a preferred embodiment of the invention, all of the joint regions of each lamination have been scribed in this way and the rest of the lamination has been scribed in a direction transverse to the rolling direction.

Preferably the spacing between the scribing is about 3-4 mm in the joint regions although it may be greater in the limb and yoke portions of the core. The scribing may be linear or it may be in the form of spots arranged in rows and may produce marks on the steel or not, as desired.

The invention will now be more fully described by way of example with reference to Fig. 5 of the drawings, of which: Figure 1 shows the conventional method of manufacturing a core for a transformer; Figure 2 shows the magnetic structure of grain-orientated electromagnetic sheet steel; Figure 3 shows the variation of power loss versus the angle between the direction of easy magnetisation and the direction of scribing for magnetisation at angles to the direction of easy magnetisation; Figure 4 shows diagrammatically the directions and angles described above with reference to Fig. 3; and Figure 5 shows diagrammatically a blank of one lamination for a yoke for a core according to the invention.

Thus, in forming a lamination in accordance with the invention, a blank 2 of the type described above with reference to Fig. 1 for use as one of the laminations forming a yoke 5 of a core 3 is first cut out of the coil 1 in accordance with the standard technique. As is best shown in Fig. 5, the regions 18 and 19 which form the corner joint and T-joint regions respectively, and are outlined in dotted line in Fig. 5, are then scribed at an angle of 45 to the rolling direction D of the blank. This is at the same angle as the angle of orientation of most of the closure domains on the surface of the blank and thus increases the number of these domains. This is shown at 20 and 21 which portions have been enlarged for clarity.

In the region 20 the corner joint region 18 has been scribed using a wire brush at an angle of 45" to the rolling direction in the direction transverse to the joining direction, whereas in the region 21 the corner joint region has been scribed using a laser beam and is shown in both orientations of 45 to the rolling direction. In each case, it will be seen that the part of the blank not forming a joint region has been scribed in a direction transverse to the rolling direction D as is more clearly shown at 23. The laser scribing is linear and has a spacing of about 6mm in the non-joint region as at 23 and about 3-4mm in the joint regions as at 21. The T-joint region 19 is also scribed similarly to that of the corner joint regions 18.

Clearly, therefore, a core formed with laminations scribed in the manner according to the invention will have a reduced power loss compared to cores that have been scribed in the manner known hitherto.

Claims (10)

1. A laminated core for a transformer in which at least one of the laminations has been scribed in at least one of its joint regions in the direction of magnetisation of the closure domains in the volume of the material.

2. A laminated core for a transformer according to Claim 1, wherein the scribing is produced by a laser beam.

3. A laminated core for a transformer according to Claim 1, wherein the scribing is produced by a wire brush.

4. A laminated core for a transformer according to Claim 1, wherein the direction of magnetisation of the closure domains is between 35 degrees and 55 degrees to the rolling direction of the respective lamination.

5. A laminated core for a transformer according to Claims 1, 2, 3, wherein all of the joint regions of each lamination have been scribed in the direction of magnetisation of the closure domains and the rest of the lamination has been scribed in a direction transverse to the rolling direction.

6. A laminated core for a transformer according to Claim 5, wherein the spacing between the scribing is from 3mm to 4mm in the joint regions.

7. A laminated core for a transformer according to Claim 6, wherein the spacing between the scribing in the limb and yoke portions of the core is greater than or equal to the spacing between the scribing in the joint regions.

8. A laminated core for a transformer according to any preceding Claim, wherein the scribing is either linear or in the form of spots arranged in rows.

9. A laminated core for a transformer substantially as herein described with reference to Figs. 1 to 5 of the accompanying drawings.

10. A transformer incorporating a laminated core according to any preceding Claim.