US3546358A - Shielded electrical inductive apparatus - Google Patents

Description

0 United States Patent 1111 3,546,358

[72] Inventor meter y u [56] References Cited Nurnberg, Germany UNITED STATES PATENTS [21] A pp 715,620 1,697,142 1/1929 Roller l74/35(.4)UX liiled d $32 3 3 1,942,575 1/1934 Shapiro 174/35(.3)x

meme FOREIGN PATENTS Aktlengesel [73] :Z'Ig 315,372 4/1930 Great Britain 336/84 390,500 4/1933 Great Britain... 336/84 a corporation of Germany [32] Priority April 7, 1967 632,125 11/1949 Great Br1ta1n 336/84 [33] Germany Primary Examiner-Darrell L. Clay [31 1 No. 5109230 Attorneys-A. T. Stratton, F. E. Browder and Donald R.

Lackey ABSTRACT: Electrical inductive apparatus having a flux producing component disposed within a casing formed of a metallic, magnetic material, and shielding means disposed [54] igi i f g g INDUCTWE between the flux producing component and the casing. The 6 cm 11 D h m shielding means is formed of at least one layer of a woven 8 fabric, with the fabric strands which run parallel with one [52] [1.8. CI. 174/35, another in a first direction being formed of metallic wire. The 336/84 fabric strands which run parallel with one another in a second [51] lnt.Cl. ..H0lf15/04, direction, and which are woven with the metallic strands of l-l05k 9/00 the first direction, are formed either of an electrical insulating olsell'ch material, or of metallic wires which are electrically insulated from the metallic wires of the first direction.

PATENTED DEC-81976 SHEET 1 OF 2 J l/ J l J l4 FIGI FIG. 20.

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lllllllll lllllll I llllllll SHEET 2 OF 2 o, o J l llllllll l llllllll l I lllllll l I llllllJ 1 SHIELDED ELECTRICAL INDUCTIVE APPARATUS BACKGROUNDOF THE INVENTION 1. Field of the Invention The invention relates in general to means for shielding stray magnetic fields associated with electrical inductive apparatus such as transformers and rotating electrical machines.

2. Description of the Prior Art Stray fluxes produce appreciable eddy-current losses in massive component members and walls of large surface area such as occur, for example, in electrical machines. These losses can be reduced by employing different types of wellknown stray-flux shields. Of course, observable losses also occur in the screens themselves If, for example, copper or aluminum shields are employed, they must have an appreciable thickness in order to satisfy the shielding requirements. By employing magnetically permeable sheet-material screens, such as sheet iron, the formation of sheet from thin layers, and hence subdividing the said sheet into mutually, electrically insulated layers-produces a reduction in the longitudinal losses caused by the stray fluxes traversing the sheet surface. The appreciable transverse losses which arise from the entry and exit of the stray flux into and out of the plane of the sheet can only be reduced by subdividing the sheet material into individual electrically insulated strips. This possibility can only be employed to a limited extent because sheet material can only be slit and machined to breadths of about 1 cm. in an economical manner.

SUMMARY OF THE INVENTION Briefly, the disadvantages of prior art shielding are obviated, according to the teachings of the invention, by employing multilayered woven wire fabric for shielding purposes.

The entry and exit losses become vanishingly small using surfaces made from parallel'wires, for in this case similar sheet-surfaces are present which are subdivide into the finest insulated strips. In addition, the losses in the longitudinal direction of the plane surface can be reduced in this manner, mainly because wire can be produced which is much thinner than sheet. Thus, both the longitudinal and transverse losses become smaller, the thinner the wire employed.

The technological advance provided by the invention when compared with the the prior art procedure of subdividing sheet into the thinnest of strips lies, in the main, in the use of less costly products, namely woven wire cloth, whose commercially available forms can be made to suit the proposed .usage in an optimum manner.

As a woven wire fabric, it is possible to employ woven iron in which at least one direction of the wire fibers is electrically insulated, or a fabric can be used in which the fibers in one direction consist of an insulating material, while the other consists of iron. The characteristic form of the weave ensures good and uniform electrical insulation of the iron wires.

The insulating fibers can be made of plastics, such as nylon, for example. Exceptionally good space-filling can be obtained when the iron wires are packed together as closely as possible. This situation can be obtained if, for example, the thickness of the insulating fibers is appreciably smaller than that of the iron wires. Good space utilization also signifies that the total space employed by the screening device is filled to the greatest possible extent with magnetically effective iron. The better the space-usage, the less space is occupied by the screening device. This can be of very great significance when it is required to produce the most compact form of electrical machines.

Iron wire-synthetic fiber woven fabrics, such as those already employed for other purposes, will serve to produce compound-packaging of several layers of parallel wires. In this regard, the warp may consist of iron wire and the woof, for example, of nylon threads, or the reverse. The fabric faces can easily be finished in many ways and joined together with resins to form multilayer sheets. This compound sheet material should display great improvements in fatigue strength by com- III parison with glued (cemented) sheets because there are no outstanding planes of preferred strength and an altogether more uniform material is produced.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 provides a schematic representation of a magnetically permeable shield into which penetrates a stray flux (stray magnetic field);

FIGS. 2a and b show schematically the magnetic flux and the eddy-current flow pattern in a round iron wire;

FIGS. 30 and b show schematically the transverse magnetic circulation in parallel iron wires;

FIG. 4 provides a schematic representation of a commercially available woven wire fabric;

FIGS. 5a and b show schematically woven iron fabrics having improved space-filling in accordance with the invention;

FIG. 6 shows in graphicalform the ratio of the eddy-current losses in wire to the the losses in sheet material or to the losses in sheet material subdivided into strips during longitudinal magnetic circulation as a function of the wire diameter of a woven wire fabric plate;

FIG. 7 shows in graphical form the ratio of the eddy-current losses in wire to the losses in sheet material, or to the losses in sheet material subdivided into strips, during transverse magnetic circulation, as a function of the wire diameter of a woven wire fabric plate; and

FIG. 8 is a plan view of electrical inductive apparatus, shielded according to the teachings of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown a schematic representation of the general case of a prior art magnetically permeable shielding plate into which is passing a stray flux (side view). The flux forced into the shielding plate is transmitted longitudinally and then emerges again. Shielding plate 11 is subdivided into several layers 12. The path taken by the stray flux I is shown by the arrows 13. The regions of transverse magnetic circulation (flow) are denoted by 14, while-15 denotes those regions in the shielding plate where the magnetic circulation is longitudinal.

FIGS. 20 and 2b show in a schematic manner the paths taken by the magnetic flux and the eddy-current in an iron wire of circular cross section. In FIG. 2a, the magnetic flux 21 in the longitudinal direction of an iron wire 23 is indicated by the arrow 22. In FIG. 2b, the circular-shaped eddy-current produced around the longitudinal axis of the iron wire by the magnetic flux are denoted by 24.

The transverse magnetic circulation in parallel iron wires in different relative positions is shown schematically in FIGS. 3a and 3b. The wires are denoted by 31 and the lines of magnetic flux by 32. I

FIG. 4 shows in cross section a schematic representation of a commercially available woven wire fabric with insulating fibers. The iron wires are denoted by 41 and the insulating fibers by 42.

FIGS. 5a and 5b show woven wire fabrics which, in accordance with the invention, are suitable for improving the use of space on account of their design. Thus, for example, FIG. 5a shows a woven wire fabric with a ratio of wire thickness of wire 51 to fiber thickness of insulating fiber 52 of 4:1. FIG. 5b shows that wires 53 can be satisfactorily aligned by weaving in the form of flat wire and held in place, the said wires being converted to the finest of strips by rolling, such a degree of fineness being unobtainable by the slitting of sheet material. Such flat wires, made from thicker material, can produce the same reduction in the longitudinal losses as correspondingly thinner wire of circular section, whereby the otherwise most greatly weakened and vanishingly small transverse losses are increased somewhat. An additional advantage is that of a higher space-filling. This is because of flat wire,

considered in cross section, corresponds to a series of parallel fine wires which are not insulated from each other. In this case, again, the insulating fibers are identified by the number 54.

It is possible to obtain an increase in the space-utilization of the device made in accordance. with the invention by employing other methods such as, for example, by different forms of weaving. The wire warp can be made closer. In such a case, the woof must consist of very fine insulating threads each consisting of several fibers, if necessary, to retain their strength, instead of the woof consisting, in customary fashion, of a single or two-ply fiber which corresponds closely in thickness to that of the wire. In addition, by heating of multiple layers of woven-wire fabric until softening of the insulating fibers consisting of, for example, thermoplastic synthetic fibers, and by simultaneous or subsequent pressing, it is possible to reduce the distance between the wire-planes (layers) set by the synthetic fibers. The layers can thus be baked together. By this means, it is possible to improve the strength of the material because the bonding-system is now virtually free from layerorientation. The combining of the strips of woven material to form a composite sheet can be effected in the usual manner as in the production of conventional plied materials such as, for example, resin bonded fabrics, This can be further extended or improved by the interposition of layers of thermoplastic films or thermoplastic fabrics or thermoplastic flakes or thermoplastic mats or by the scattering of thermo or Duroplastic powder or thermo or Duroplastic shavings on the surfaces and finally pressing with the application of heat, or by the use of glues or other bonding materials, such as, for example, by soaking with a casting resin. During the the process of pressing the sheets together, it is possible to ensure a minimum residual separation by means of woven-in or inserted insulating fibers of higher thermal resistance or by electrically insulating the wires, as conventionally provided in the case of sheet material. The materials consisting of layers of woven-wire fabric can also be bonded together by means of sprayed-on liquified thermoplastics.

If the strips of woven fabric are placed on top of each other with the nap running in the same direction and then bonded into a multiply sheet, one obtains two preferred magnetic directions for a material incorporating iron wire in both the warp and the woof, whereas, only one preferred direction of magnetization is obtained in the wire-direction for a material having an iron wire warp and an insulating fiber woof. Thus a preferred direction can be provided for cases where the stray flux is in a known, constant direction. At the same time, the best magnetic use is made of the iron cross section in the shielding device.

By superimposing layers with their naps aligned in different directions, it becomes possible to provide several magnetically preferred directions, corresponding to the wire directionsas would be required for wandering stray fluxes or fluxes of different direction.

The preferred magnetic directions do not have to be rectilinear. Rather, they can follow the most advantageous curved paths consistent with the prevailing conditions becauseespecially prior to the bonding the multilayered composite-the woven wire fabric can just as easily be warped in the plane of the sheet as bent at right-angles to it,

By adding iron powder to the above above-mentioned bonding media, it is possible to attain an improvement in the mag netic conduction perpendicular to the direction of the wires, from layer to layer or from one wire direction to another.

The above-described methods of bonding can also be employed in any arbitrary combination.

FIG. 6 gives a graphical representation of the ratio of the eddy-current losses P,, in the wire to the losses P,, which occur either in sheet material or sheet material which has been subdivided into strips, the ratio being for longitudinal magnetic flux and drawn as a function of the wire-diameter D of a wire in a woven wire fabric plate. The wire-diameter D in mm. is plotted as abscissa and the ratio P, /P,, as ordinate. The numerals associated witheach line in the graphs refer to the parameter, that is, to the thickness in mm. of the sheet material or to the strips ofsheet material. I

From the graphs shown in FIG. 6 it is easy to read off the reduction in the losses by the use of woven wire fabrics with longitudinal magnetic flux. Thus, the longitudinal losses are reduced by about l percent, for example, by employing woven wire fabric with a wire diameter of 0.5 mmv by comparison with sheet material having a thickness of5 mm. With the same thickness of wire and sheet, the losses are reduced by about 40 percent.

FIG. 7 provides a graphical representation ofthe ratio of the eddy-current losses P,, in the wire to the losses P in sheet or in sheet subdivided into strips, the ratio being for transverse magnetic flux and shown as a function of the diameter D of a wire in a woven wire plate. Here, again, the wire diameter is taken in mm. as the abscissa while the ordinate is plotted with the ratio P /P The reference numbers on the curves again refer to the parameter which in this case can be the breadth in cm. of the sheet material or of the strips.

As far as the transverse losses are concerned, entire sheets and laminated stacks of sheet are equivalent. A significant reduction could be obtained in the transverse losses by subdividing the sheet-layers into strips. From the graphs shown in FIG. 7, one can see the advantage to be gained by employing woven wire fabric instead of strips of sheet material for shielding. For example, with a strip width of 10 mm. and a wire diameter of0.5 mm. a loss-reduction P,1/P,, of about l percent is obtained. In the comparisons made in accordance with FIGS. 6 and 7, equal volumes and equal average inductions for sheet and wire are taken as a basis, as well as average property values for single-ply dynamo and steel sheet.

FIG. 8 is a plan view of electrical inductive apparatus with its cover removed, such as a transformer or reactor, illustrating how the teachings of the invention may be applied. Electrical inductive apparatus 80 includes a flux producing component 82, which in this instance is illustrated as being a core-winding assembly of a polyphase transformer or reactor ofthe core-form type, having electrical winding assemblies 84, 86, and 88 disposed in inductive relation with a magnetic core 90. The core-winding assembly 82 is disposed within a tank or casing 92 which is formed of a metallic, magnetic material, such as steel. In order to shield the tank walls from stray magnetic flux, and thus prevent objectionable energy losses and excessive heating of the tank walls, shielding means 94, 96, 98 and 100 are disposed between the core-winding assembly 82 and the sidewalls of the casing 92. Similar shielding means may be disposed to shield the bottom and cover of the casing, if necessary.

Shielding means 94, 96, 98'and 100 are formed according to the teachings of the invention, each having one or more layers of plies of a woven fabric, in which metallic wire is used to form the strands which run parallel with one another in a first direction. The strands which are woven with the strands of the first direction, are either formed of an electrical insulating material, or of metallic wires which are electricallyinsulated from the metallic wires of the first direction.

As illustrated in FIG. 8, the shielding means may be secured in the desired location relative to the inside of the casing by suitable anchors 102 which are secured to the casing and project outwardly therefrom to engage suitable openings in the shielding means. The shielding means may be a plurality of sheets, as illustrated, or one continuous sheet may be used, depending upon the size of the apparatus to be shielded, and the economically practical maximum dimensions of the sheetlike shielding means.

While apparatus 80 shown in FIG. 8 is illustrated as being a polyphase transformer or reactor of the core-form type, it is to be understood that the teachings of the invention may be beneficially applied to single-phase transformers and reactors of the core and shellform types, and to polyphase transformers and reactors of the shell-form type, as well as generally to any encased flux producing component whose casing is to be shielded from stray magnetic flux.

Although the invention, has been described using magnetic wires, such as iron or steel, for the strands of the woven fabrics, which shield the casing of a flux producing component by shunting the flux away'from the casing, it is within the scope of the invention to use nonmagnetic metallic wires, such as aluminum or copper, and shield the casing by setting up a counter flux which opposes the flux from the flux producing component, and, therefore, reduces the total amount of flux reaching'the casing. Or, a combination of magnetic and nonmagnetic metallic wires may be used.

lclaim:

1. Electrical inductive apparatus comprising:

a magnetic flux producing component; l

a metallic casing;

said flux producing component being disposed within said casing; 1

and shielding means disposed between said flux producing component and at least a predetermined portion of said metallic casing; and

said shielding means consisting of a plurality of layers of a woven fabric, said woven fabric having a plurality of metallic strands running in a first direction, woven with a plurality of thermoplastic insulating strands running in a second direction, the material of said thermoplastic insulating strands bonding the plurality of layers of woven fabric together.

2. The electrical inductive apparatus of claim 1 wherein the metallic strands are magnetic.

3. The electrical inductive apparatus of claim 1 wherein the thickness of the metallic strands is greater than the thickness of the insulating strands.

4. The electrical inductive apparatus of claim 1 wherein the metallic strands are substantially flat, having first and second dimensions wherein the first dimension is substantially greater than the second dimension.

5. The electrical inductive apparatus of claim 1 wherein the first and second directions of the strands run in the same directions in each of the layers of the woven fabric, with respect to the shielding means.

6. The electrical inductive apparatus of claim I wherein the first and second directions of the strands, in at least two of the plurality of layers of woven fabric, run in different directions, relative to the shielding means.

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