GB2169147A - Cores for electrical machinery - Google Patents

Abstract

A region 16 of at least one lamination forming part of a core for a high power transformer is scribed, preferably by a laser, at an angle to the rolling direction so as to modify the reluctance in that area. By suitably choosing the areas so scribed, the flux can be guided in desired directions through the core. <IMAGE>

Description

SPECIFICATION Cores for electrical machinery The present invention relates to coresforelectrical machinery 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 ( 10)[001] orientation expressed by Miller indices. This designation indicatesthatthe(110) plane, ofthe 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 mannerthat the direction of magneticflux through the core of the transformer is coincident with the rolling direction.

There is shown in Figurel(a)wherethe 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 Figure 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 Figure 1 (b) andthe magneticflux path is shown by arrows C in Figure 1 (c) which also shows the overlapping regions ofthe laminations at the joints byshowingthe 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 is demonstrates lower values of specific power loss per unit weight of core than other types of electrical steel. The steel has very close alignment ofthe preferential easily magnetised direction ofthe polycrystalline structure along the rolling direction of the steel strip 7 as showm in Figure 2. As a consequence of the method of producing this type of steel,the mean grain size ofthe 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 polyc- rystalline 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 ofthe electrical steel material. This is illustrated in Figure 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 oftransition ofthe magnetic direction termed domain walls 11, and which extend completelythrough the material thickness. With well aligned crystals the pattern demonstrated by the domains is simple.Theytakethe shape of rectangular parallelpipeds and adjacent domain volumes have directions of magnetisation 10 which are opposite to each other. As the size ofthe grains is increased the width ofthese 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 ofthe external fieid expand at the expense of the other domains. Thus, with large domains with a corresponding small numberofdomain walls 1 1,the domain walls move further and faster to accommodate a magnetic field increment than the case where there are small domaimsanda large number of domain walls. In the former case the higher velocity ofthe domain walls gives rise to a higher value of the eddy current component of no-load loss.

As illustrated in Figure 2(b), however, on grains misorientated from the (110)[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 p 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 predominant process is one of rearrangement of the main domains, as explained above, atthe 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 ofthe main domain structure. High levels of magnetic field are required to nucleate sufficent closure domains to initiate magnetisation of the steel strip/core by rearrangement ofthe 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 thatthey 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 ofclosure 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 ofthe main domains and a reduction in the number closure domains. Many commercially The drawings originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

available grain-orientated electromagnetic steel sheets have such a coating.

Mechanical, or physical, scribing transverse to the sheet rolling direction is anothertechnique 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 domaims and thus increase the number of such domains. The disadvantages of mechanical scribing are, however,thatthe insulative coating is damaged and the surface ofthe steel sheet becomes uneven.

These factors tend to produce increased interlaminar losses in a tra(Ç)ròrmer manufactured from a steel so treated.

It has also been proposed to irradiate the grainorientated 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 coatings (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 coreforelectrical machinery, such as transformers, which has been laser-scribed in a direction either transverse or parallel to the rolling direction ofthe 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 ofthetransformer, magnetic flux traverses around the core and at the joint regions, that isasthe cornerjoints and the T-joints the magnetic flux turn 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. For example, a core 13 such as that shown in Figure 3, which is a 5-limb core, has magneticflux paths as shown by the arrows 14 and 15.

In this case the cross flux shown by arrow 15 increases the flux magnitude and hence the power loss in the outeryoke region.

It is thus an object of the present invention to decreasethe overall power loss by guiding the magneticflux in desired directions within the core.

Accordingly, the invention provides a laminated coreforelectrical machinery wherein at least one of the laminations has been scribed in at least one region thereof at an angle to the rolling direction so asto modifythe reluctance of that region as compared to the restofthe laminations in such a way that magnetic flux passing along the lamination,when it is in use, is guided in a desired direction.

Preferably, the scribing is done in the same direction as the direction of magnetisation of the closure domains in the volume ofthe material when the region is in the elongated limb or yoke portions and is done in the direction of magnetisation of the main domains in the joint regions. This increasesthe reluctance ofthe material in that region and so guides the flux away from the region. Preferably, also, the rest ofthe lamination is also scribed at such an angle that the reluctance is decreased, so as to enhance the guiding effect.

In the preferred embodiment of the invention, the scribing is carried out by means of a laser beam but another means maybe used ifdesired.

The invention will now be more fully described, by way of example, with reference to Figure 4 of the drawings of which: Figure 1 shows the conventional method of manufacturing a core for a transformer; Figure 2 shows a magnetic structure of grain orientated electromagnetic sheet steel; Figure 3 shows a conventional five limb core with the directions of magnetic flux indicated; and Figure 4shows part of a five limb core similarto that of Fig u re 3 but with ascribed region according to the invention.

Thus,the core in Figure4hasa region 16which has been scribed using a laser beam at 45c to the rolling direction, this being the angle of alignment of the closure domains. The region 16 is a straight strip-like region running along part of the length ofthe limb 18.

Due to the scribing, the number of closure domains increases and thus the reluctance ofthe region of magnetisation in the rolling direction increases and the flux is guided into regions ofthe limb on either side of the region 16. These other regions have been scribed transverse to the rolling direction so asto increase the numberof main domains and thus decrease the reluctanceto magnetisation in the rolling direction.

In the joint region 19 ofthe core, as shown outlined by dotted lines, the region 16 continues as region 17 but in the case the scribing is transverse to the rolling direction so as to increase the reluctance of this region to magnetisation which is not in the directon of rolling.

The rest ofthe joint region 19 is scribed in the direction of magnetisation ofthe closure domains to reduce the power loss in this region. Further details ofthis may be obtained from our copending application No.8529485 (TF/2465).

By scribing a region ofthe core according to the invention as described above, the reluctance ofthe core is modified in such a waythatthe cross-flux 15 is guided in the desired direction and reduces the power loss.

Claims (6)

1. A laminated core for electrical machinery, wherein at least one ofthe laminations has been scribed in at least one region thereof at an angle to the rolling direction so asto modify the reluctance ofthat region as compared to the rest ofthe laminations in such a way that magnetic flux passing along the lamination, when it is in use, is guided in a desired direction.

2. Alaminatedcoreforelectrical machinery according to Claim 1, wherein the scribing is done in the same direction as the direction of magnetisation of the closure domains in the volume ofthe material whenthe region is in the elongated limb or yoke portions and is done in the direction of magnetisation ofthe main domains inthe joint regions.

3. A laminatedcoreforelectrical machinery according to Claim 2, wherein the rest ofthe lamination is scribed at such an angle that the reluctance is decreased.

4. A laminated core for electical machi nery accord- ing to Claim 1, wherein the scribing is carried out by means of a laser beam.

5. A laminated core for electrical machinery substantially as herein described with reference to Figures 1 to 4 ofthe accompanying drawings.

6. An electrical machine incorporating a laminated core according to any preceding Claim.