Magnetic core for electrical inductive apparatus

Abstract

A multiple-legged magnetic core constructed from groups of laminations, with at least two of the groups having different permeabilities. The majority of the magnetic core is constructed of laminations having one permeability. A group of laminations having a lower permeability is positioned in each of the outer legs or in the yoke portions which connect the outer legs to the remainder of the magnetic core. The effective permeability of the outer legs is thereby reduced and the flux density throughout the core is substantially balanced.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, in general, to electrical inductive apparatus and, more specifically, to magnetic cores for power transformers.

2. Description of the Prior Art

Five-legged magnetic cores are sometimes used in power transformers when the advantages thereof are desirable. However, widespread use of five-legged cores does not exist since there are certain disadvantages in the use of conventional five-legged cores. One important disadvantage is caused by additional losses due to third harmonic fluxes which flow in the magnetic core.

Five-legged magnetic cores have four windows or openings in which the winding structures are located. The openings near each end of the core are smaller than either of the other two openings of the core. The larger openings are required since a portion of the windings disposed around two of the core legs is located therein, whereas a portion of the winding around only one core leg is located in each of the smaller openings.

Flux flowing through a core leg which is adjacent to an outer core leg divides and flows across two paths. One path encircles the adjacent large opening and the other path encircles the adjacent smaller opening. Since the lengths of the flux paths differ, and since the amount of flux flowing through a path is dependent upon the length of the path, more flux will flow in the outside leg path if the cross-sectional area and the permeability of the paths are the same.

A reduction in harmonic flux and the losses attributable thereto can be realized by balancing the magnetic paths so that the same amount of flux density will be developed in the materials of each path. Reducing the cross-sectional area of the outside path as taught by many prior art arrangements decreases the flux flowing therethrough, but the flux density, which is the quantity which governs saturation, does not change since it is proportional to both the flux and the cross-sectional area. While such area decreasing arrangements have been used in the prior art to reduce the amount of flux in the outside path, the ability of such arrangements to balance the flux density is minimal. Therefore, it is desirable and it is an object of this invention, to provide a five-legged magnetic core in which the flux paths around the windings disposed thereon are balanced, that is, have substantially the same flux density in each path.

Four-legged cores are sometimes used in large single-phase transformers. The windings of such transformers are positioned around the inner legs of the core, thereby requiring a larger opening between the two inner core legs than between the inner and outer core legs. Consequently, the flux density in each core leg is not equal and losses caused by saturation of a portion of the magnetic core reduce the efficiency thereof. Therefore, it is desirable, and it is another object of this invention, to provide a four-legged magnetic core in which the flux paths around the windings disposed thereon have substantially the same flux density.

SUMMARY OF THE INVENTION

There is disclosed herein new and useful four and five-legged magnetic cores which have unique geometry and material combinations which provide balanced magnetic paths with substantially equal flux density throughout the cores. At least a portion of the outer leg of each magnetic core contains magnetic laminations which have different permeabilities. The value of permeability and the amount of such laminations is selected to decrease the effective permeability of the magnetic path which traverses the outer leg. Thus, the flux is reduced in the outer leg without reducing the area thereof. The reduction in flux provides a flux density in the outer leg which is substantially equal to the flux density in the remainder of the magnetic core, thus reducing unequal saturation of the core.

BRIEF DESCRIPTION OF THE DRAWING

Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawing, in which:

FIG. 1 is a view of a five-legged transformer constructed according to this invention;

FIG. 1A is a view of a four-legged transformer constructed according to this invention;

FIG. 2 is a portion of a multiple-legged magnetic core illustrating quantities used to calculate the desired permeability ratio of the magnetic materials in the core;

FIG. 3 is a partial view of a magnetic core member illustrating an arrangement for changing the permeability of the magnetic materials; and

FIG. 4 is a partial, cross-sectional view of a magnetic core constructed according to this invention illustrating the junction between the laminations which have different permeabilities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout the following description, similar reference characters refer to similar elements or members in all of the figures of the drawing.

Referring now to the drawing, and to FIG. 1 in particular, there is shown a five-legged magnetic core 10 constructed according to this invention. The core 10 includes the inner core legs 12, 14 and 16, the outer core legs 18 and 20, and the core yokes 22 and 24. Each portion of the magnetic core 10 is constructed from a plurality of magnetic laminations stacked together to provide the desired dimensions. The phase windings 26, 28 and 30 are disposed around the inner legs 12, 14 and 16, respectively, and induce flux, when energized, into the legs and yokes of the magnetic core 10. The outer legs 18 and 20 contain segments 32 and 34, respectively, each of which is constructed of a different magnetic material than the remainder of the leg. The magnetic material comprising the segments 32 and 34 has a different permeability than that of the magnetic core materials in the remainder of the core 10.

The purpose of the segments 32 and 34 is to increase the effective length of the megnetic path through the outer leg. FIG. 2 illustrates the portion of the magnetic core 10 which will be used to describe the functions of the magnetic segment 32. The function of the magnetic segment 34 is similar to that of the segment 32.

The flux which flows through the inner leg 12 due to current in the winding thereon divides and travels through the flux paths 36 and 38. The letter-designated dimensions shown in FIG. 2 illustrate that the width a of the opening 40 is smaller than the width c of the opening 42. Thus, the length of the path 36 is physically shorter than the length of the path 38. If the permeabilities in each path were the same, the reluctance of the shorter path 36 would be less than that of the path 38. Since the same magnetomotive force produces the flux in both paths, the flux in the path 36 would be greater than that in the path 38 if the permeabilities are the same. In order to lower the flux density in the path 36 to that of the highest flux density in the path 38, the segment 32 of magnetic material is inserted to increase the effective length of the path 36, thereby decreasing the flux and the flux density along the path 36. It is this change in the permeability of the path 36 which effectively changes the length thereof.

Conventional arrangements which "balance" multiple-legged magnetic cores usually decrease the area of the outer legs and/or the yoke portion connecting the outer legs to the other portions of the magnetic core. Such arrangements decrease the flux through the outer leg but, since the area has been reduced also, do not appreciably change the flux density because of the relationship between flux, area and flux density.

An approximation of the permeability required for the segment 32 can be obtained by an analysis of the dimensions of the core. Referring to FIG. 2, the permeability of the magnetic material in the segment 32 may be assigned the value .mu.2, and the permeability of the other magnetic material in the outer leg 18 and in the other portions of the magnetic core may be assigned the value .mu.1.

The flux in the material on both sides of the junction 42 is equal, thus the flux density in both of these regions is equal. Since the flux density B is equal to the product of the permeability and the magnetic field intensity H, the following equations may be written:

B.sub.1 = .mu..sub.1 h.sub.1

b.sub.2 = .mu..sub.2 h.sub.2

b.sub.1 = b.sub.2 ##EQU1## The subscripts indicate the portion of the magnetic path 36 at which the quantities exist.

By using Maxwell's equations, and by integrating around the magnetic path 36, the exciting current I.sub.2 which produces the flux through path 36 may be calculated. Thus, ##EQU2## The exciting current I.sub.1 can be found by a similar method. Thus,

.phi..sub.1 H.sup.. d1 = 2H.sub.3 (c+b+2p) = I.sub.1

where H.sub.3 is the magnetic field intensity around the path 38.

I.sub.2 = I.sub.1 = I = exciting current. ##EQU3##

For the core to be balanced, "B.sub.1 " must be equal to "B.sub.3, " that is, the flux density in paths 36 and 38 must be equal. ##EQU4## which, when simplified can be written as ##EQU5##

This equation can be used to determine the approximate length d of the segment 32 of magnetic material having a permeability of .mu..sub.2. Since c is greater than a, the ratio .mu..sub.1/.mu..sub.2 of the permeabilities is limited and must be greater than ##EQU6## otherwise d would have to be larger than the length of the portions of the magnetic path 36 which are not common with the magnetic path 38.

Calculations made according to the foregoing may have to be changed somewhat to compensate for non-considered variables, such as three-phase operation, non-linearity of flux density over the entire cross-section, leakage flux, etc. An entirely experimental method may also be used to determine the amount of permeability change which is needed to balance the flux density throughout the core.

Although FIG. 2 has been used in discussing a five-legged magnetic core, the same principle discussed applies to four-legged magnetic cores. In using FIG. 2 for analyzing a four-legged core, only one outer leg is not illustrated compared to one outer and one inner leg for the five-legged core analysis.

The segment 32 of magnetic material can be placed anywhere along the magnetic path 36 except in the leg 12. The permeability of the magnetic material comprising the segment 32 must be different than that of the other magnetic materials in the core. The difference in permeability may be obtained by using a different magnetic material, by using the same basic material which has been processed differently, or by various other methods.

FIG. 1A is a view of a four-legged magnetic core 110 with single-phase windings 112 and 114 disposed thereon. The core 110 includes the outer legs 116 and 118, the inner legs 120 and 122 and the core yokes 124 and 126. The outer legs contain the segments 128 and 130 which are constructed from a magnetic material which has a lower permeability than the other magnetic material in the core 110 to provide the effective increase in magnetic paths as hereinbefore discussed.

One convenient arrangement for obtaining the desired permeability is illustrated in FIG. 3. The grain orientation of the laminations 46 and 48 is aligned in the direction indicated by the arrows 50 and 52. The laminations 44, which may be used to construct the segment 32 shown in FIGS. 1 and 2, have their grain orientation aligned with the arrow 54 which is displaced by an angle .theta. from the direction of the arrows 50 and 52. The laminations 44, 46 and 48 can be constructed of the same magnetic material which has been cut at different angles with respect to the grain orientation. In many magnetic materials, the angle .theta. can be less than 10.degree. for a change in permeability of several magnitudes.

The segment 32 of magnetic material may take many different forms in the path 36. FIG. 4 illustrates an arrangement wherein the laminations 56 are step-lapped with the laminations 58 and 60. The laminations 56 can be constructed of magnetic material which has been cut at an angle to its grain orientation. The arrangement shown in FIG. 4 provides a stronger core than an arrangement which uses butt-type joints between the different laminations. It is also within the contemplation of this invention that some of the laminations 56 may have different grain orientations to provide a segment having the desired overall permeability. By using this invention to change the effective permeability in the outside legs of multiple-legged cores, the area of the core members may be kept constant and the flux density throughout the cores can be balanced. Such an arrangement reduces the losses caused by the harmonic fluxes produced when a portion of the magnetic core becomes saturated.

Since numerous changes may be made in the above-described apparatus, and since different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawing, shall be interpreted as illustrative rather than limiting.

Claims

I claim as my invention:

1. A magnetic core comprising:

at least first and second laminated inner core legs suitable for the placement of windings there-around;

first and second laminated outer core legs; and

first and second laminated cores yokes which magnetically connect together the inner core legs and the outer core legs;

said first and second laminated outer core legs respectively providing first and second magnetic paths with the core yokes, with said first and second paths each containing a first group of laminations having a first predetermined permeability and a second group of laminations having a second and different permeability.

2. The magnetic core of claim 1 wherein the first magnetic path traverses the first outer core leg, the first inner core leg, and the portions of the core yokes connecting said legs, and wherein a third magnetic path traverses the first and second inner core legs and the portions of the core yokes connecting said legs, with the permeability of the laminations in the second group of laminations of the first outer core leg having a value of permeability which makes the effective length of the first and third magnetic paths substantially equal.

3. The magnetic core of claim 1 wherein the first and second groups of laminations are joined together with a step-lapped joint.

4. The magnetic core of claim 1 wherein the first group of laminations is constructed of steel laminations having a grain orientation aligned parallel to the edges of the lamination, and the second group of laminations is constructed of steel laminations having a grain orientation aligned obliquely to the edges of the lamination.

5. A five-legged magnetic core comprising:

first, second and third laminated inner core legs;

first and second laminated outer core legs; and

first and second laminated core yokes which magnetically connect together the inner core legs and the outer core legs;

said first and second outer magnetic core legs containing laminations having a lower permeability than the permeability of substantially all the laminations in the remainder of the magnetic core.

6. A four-legged magnetic core comprising:

first and second laminated inner core legs;

first and second laminated outer core legs; and

first and second laminated core yokes which magnetically connect together the inner core legs and the outer core legs;

said first and second outer magnetic core legs containing laminations having a lower permeability than the permeability of substantially all the laminations in the remainder of the magnetic core.