Economical core design for electromagnetic devices

A magnetic core for an electromagnetic device is formed from alternating interleaved steel laminations. The core comprises a plurality of core elements comprising legs and yokes oriented substantially quadrature to the legs, such that abutting core elements are in substantially quadrature relation. A plurality of flux deflection zones are defined in regions where flux flows from one core element to an abutting core element. At least one of the layers has at least one core element composed of grain-oriented steel, and the remaining core elements are composed of non-grain-oriented steel, such that at least some flux deflection zones are composed of a substantial amount or substantially entirely of non-grain-oriented steel. Flux flowing in the direction of the grain orientation in the core element(s) composed of grain-oriented steel changes direction to flow through the abutting core element in the flux deflection zone composed of non-grain-oriented steel. This reduces the power losses in flux deflection zones of the core relative to cores formed entirely from grain-oriented steel, because the flux is never flowing perpendicular to the direction of the grains in the steel, while providing a design that is considerably less expensive than cores formed from non-grain-oriented steel with substantially the same level of power losses or lower.

FIELD OF THE INVENTION

This invention relates to electromagnetic devices. In particular, this invention relates to electromagnetic devices with laminated steel cores.

BACKGROUND OF THE INVENTION

Electromagnetic devices such as various kinds of transformers and reactors are widely used in power supply and distribution systems. Reduction of their cost and/or power losses can significantly improve the economic parameters of such power systems.

In electromagnetic devices, power losses in the windings are directly proportional to the square of the loading of the winding. Therefore, power losses in a winding are much lower under low load conditions than under heavy load conditions. To the contrary, power losses in the core of an electromagnetic device having a ferrous core are independent of the load, and therefore power losses do not change significantly as long as the device is connected to the power system. This can be costly, because in many applications the devices are always connected to the power system regardless of whether there is load on them or not.

Conventional methods for reducing losses in a ferrous core have involved the use of higher quality steel for the core. For example, a major advancement in core losses reduction was the introduction of cold rolled grain-oriented steel. Grain-oriented steel has a polycrystalline structure, which provides high permeability and low energy dissipation (power losses) when the magnetic field flows in the direction of the grains.

However, there are two main drawbacks in the use of the grain-oriented steel. The cost of grain-oriented steel is substantially higher than the cost of non-grain-oriented steel; and the power loss in grain-oriented steel is substantially higher when the flux is flowing perpendicular (quadrature) to the direction of the grains than when the flux is flowing in the direction of the grains. As a result, a relatively high power loss is concentrated in the corners of a ferromagnetic core where the flux direction changes and crosses the grain orientation, as illustrated schematically inFIG. 2. The higher the grade of the grain-oriented steel, the higher is the difference between the power losses with flux flowing along the grain orientation versus across the grain orientation.

In order to reduce such power losses in the corners of grain-oriented steel cores, the prior art employed different core configurations such as mitered cores and wound distributed-gap cores. The use of a mitered core allows for some reduction of corner losses, but at significantly greater expense than a conventional grain-oriented steel core. A wound core is even more expensive than a mitered core, and in general for multi-phase devices does not result in any substantial reduction of power losses in the core.

The highest level of core losses reduction is achieved through use of amorphous steel for the core. However, the cost of amorphous steel is extremely high, and as such this core design option is not widely used.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides, for an electromagnetic device, a magnetic core formed from steel laminations, comprising a plurality of core elements, comprising at least two legs, at least two yokes, the yokes being oriented substantially quadrature to the legs, such that abutting core elements are in substantially quadrature relation, a plurality of flux deflection zones defined in regions where flux flows from one core element to an abutting core element, the core elements being formed from alternating interleaved layers, at least one of the layers comprising at least one core element composed of grain-oriented steel, and the remaining core elements being composed of non-grain-oriented steel, such that a plurality of flux deflection zones are composed of a substantial amount or substantially entirely of non-grain-oriented steel, whereby flux flowing in a direction of a grain orientation in the at least one core element composed of grain-oriented steel changes direction to flow through the abutting core element in the flux deflection zone composed of non-grain-oriented steel.

The invention further provides an electromagnetic device, comprising a magnetic core formed from steel laminations, and at least one winding wound over the core, the magnetic core comprising a plurality of core elements comprising at least two legs, at least two yokes, the yokes being oriented substantially quadrature to the legs, such that abutting core elements are in substantially quadrature relation, a plurality of flux deflection zones defined in regions where flux flows from one core element to an abutting core element, the core elements being formed from alternating interleaved layers, at least one of the layers comprising at least one core element composed of grain-oriented steel, and the remaining core elements being composed of non-grain-oriented steel, such that a plurality of flux deflection zones are composed of a substantial amount or substantially entirely of non-grain-oriented steel, whereby flux flowing in a direction of a grain orientation in the at least one core element composed of grain-oriented steel changes direction to flow through the abutting core element in the flux deflection zone composed of non-grain-oriented steel.

A magnetic core for an electromagnetic device comprises a plurality of core elements, including legs12and yokes14arranged quadrature to the legs12.FIGS. 1A and 1Billustrate typical alternating layers10a,10bin a prior art interleaved magnetic core for an electromagnetic device, such as (without limitation) a transformer or reactor. As is well known to those skilled in the art, an interleaved-type laminated magnetic core comprises a series of interleaved laminate layers10a,10bof steel laminations. Each laminate layer10aor10bmay be formed from one laminate of sheet steel or from multiple laminates of sheet steel, depending on design parameters and the gauge of the steel. A typical magnetic core may for example have three 0.014 inch thick steel laminates in each laminate layer10a,10b. In the embodiment illustrated, the core would alternate between the layers10aand10b, in interleaved fashion, to achieve the desired core thickness. As is well known, in this type of magnetic core the joints between core elements are located in different parts of the layers10a,10b, so that in the assembled core with the layers10a,10binterleaved there is no intentional gap between the core legs12(formed from alternating leg components12a,12b) and the yokes14(formed from alternating yoke components14a,14b), which would increase the magnetic reluctance of the core.

FIG. 2is a partial schematic view showing the layer10aof the prior art laminated grain-oriented steel core ofFIGS. 1A and 1B. In the simple case illustrated, the width of the core leg12and the width of the yoke14being equal, angle α3245 degrees. It will be appreciated by those skilled in the art that the magnetic flux in the core10actually changes direction in a less abrupt and more chaotic pattern, and the 45 degree angle shown is an approximation for purposes of illustration only.

The flux passing through the portions of the layer10where the legs12and yokes14abut, hereinafter referred to as the “flux deflection zones”16(i.e. those regions where the flux changes direction in the core), can be represented by two components: a direct component Φd (as shown, in the direction of the yoke14), and a quadrature component Φq (as shown, in the direction of the leg12). Because the yoke14overlaps the leg12in the layer10a, the direct component of the flux Φd is flowing along the grain orientation and the quadrature component of the flux Φq is flowing across the grain orientation.

Power losses created by the flow of the direct component of the flux Φd are defined in losses per pound, as specified by the steel manufacturer. However, the power losses created by the flow of quadrature component of the flux Φq, because it is flowing quadrature to grains in the steel, are much higher than nominal power losses created by the flow of the direct component of the flux Φd, which flows in the direction of the grains in the steel. For example, in M6 type grain-oriented steel, power losses created by the flow of quadrature component of the flux Φq are approximately three times higher than power losses created by the flow of the direct component of the flux Φd flowing in the direction of the grain.

FIG. 3Aillustrates a corner of a butt gap core20according to the invention, wherein the core legs22are formed entirely from grain-oriented steel and the yokes24are formed entirely from non-grain-oriented steel. This type of core is formed from identical layers, thus creating a joint between the legs12and the yokes14. The legs22are formed from grain-oriented steel while the yokes24are formed from non-grain-oriented steel. In such a core design the flux in the core legs22flows along the direction of the grain with low power losses, and flux shifting occurs in the yokes24which are made of non-grain-oriented steel. This design reduces core losses in the flux deflection zones26, since the steel in the flux deflection zones26is non-grain-oriented steel, and also reduces the overall cost of the core because non-grain-oriented steel is substantially less expensive than grain-oriented steel. However, the use of non-grain-oriented steel in this fashion, although reducing power losses in the flux deflection zones26of the core20, may increase power losses in the other parts of the core (specifically in this case, the yokes24) because power losses are substantially greater in non-grain-oriented steel than in grain-oriented steel. Reduction of power losses from the yokes24may be achieved by increasing the width of each yoke24, and the cost savings of the core20will still be significant, if design parameters permit this.

FIG. 3Billustrates a corner of a butt gap core30according to the invention, wherein the core legs32are formed entirely from non-grain-oriented steel and the yokes34are formed entirely from grain-oriented steel. In this embodiment the legs32overlap the ends of the yokes34, and the flux deflection zones36are generally at the ends of the legs32which are formed from non-grain-oriented steel. The principles and operation of this embodiment are similar to those of the embodiment ofFIG. 3A.

FIGS. 4 to 6illustrate various configurations of interleaved-type laminated magnetic cores according to the invention. According to the principles of the invention, the core elements are formed from steel laminate layers, the layers being composed of a combination of grain-oriented steel and non-grain-oriented steel such that the magnetic flux flows along the steel grains of grain-oriented steel within the portions of core elements in which the flux flows substantially along the length of the core element and does not change direction; while the flux deflection zones of the core, i.e. the regions of the core in which the flow of magnetic flux changes direction from a leg to a yoke or from a yoke to a leg, are composed partly or entirely of non-grain-oriented steel. This reduces power losses in the flux deflection zones16between abutting core elements, because the magnetic flux is never flowing quadrature to the direction of the grain orientation in the grain-oriented steel portions of the core elements. The level of core power losses can still be controlled by changing the width of the yokes and/or changing the ratio of the cross-section areas of the grain-oriented steel and non-grain-oriented steels.

In particular,FIGS. 4A and 4Brespectively illustrate the two alternating interleaved laminate layers40a,40bin a three-phase magnetic core40according to the invention. The core40has core elements comprising legs42aand42babutting yokes44arranged in quadrature relation to the legs42aand42b, as is conventional. However, in the first layer40aof the two alternating layers, shown inFIG. 4A, the yokes are formed from components44acomposed of grain-oriented steel (as indicated by the double-headed arrows) disposed between the legs42a, while the legs42aof the layer40aare composed entirely of non-grain-oriented steel. The flux deflection zones46in the layer40aare disposed at the ends of the legs42a, and are thus composed of non-grain-oriented steel. In the second layer40bof the two alternating layers, shown inFIG. 4B, the yokes44are composed entirely of non-grain-oriented steel while the legs42bare composed of grain-oriented steel (as indicated by the double-headed arrows) and extend between the yokes44. The flux deflection zones in the layer40bare disposed at the ends and middle portions of the yokes44, and thus composed of non-grain-oriented steel. When multiple layers40a,40bare assembled into a core40in alternating interleaved fashion, the flux deflection zones46in the core40are entirely composed of non-grain-oriented steel, as shown inFIG. 4C.

Thus, as magnetic flux flows through the non-grain-oriented steel core legs42and reaches a flux deflection zone46of the core40, the flux deflects toward a quadrature orientation substantially within the flux deflection zone46, which is composed of non-grain-oriented steel, and enters the grain-oriented steel yoke components44aalready substantially aligned with the direction of the steel grains. This configuration is also advantageous because the longer components in the layers40a,40bare formed from non-grain-oriented steel, which is less expensive than grain-oriented steel, so the cost of the core40relative to a conventional core having substantially the same power losses is considerably less than the cost of a comparable prior art core composed entirely of grain-oriented steel, for example up to 25% less. Alternatively, the power losses in the core40are substantially less than the power losses in a conventional core of the same cost as the core40.

FIGS. 5A and 5Billustrate laminate layers50a,50bof another configuration of magnetic core50according to the invention. The core50has core elements comprising legs52abutting yokes54arranged in quadrature relation to the legs52. In this embodiment, in the first layer50aof the two alternating layers, shown inFIG. 5A, the yokes54are formed from non-grain-oriented steel and are disposed between the outer legs52which are also formed from non-grain-oriented steel. Only the middle leg52, disposed between the yokes54, is composed of grain-oriented steel (as indicated by the double-headed arrow). The flux deflection zones56in the layer50aare disposed at the ends of the outer legs52and the middle portions of the yokes54, which are composed of non-grain-oriented steel. In the second layer50bof the two alternating layers, shown inFIG. 5B, the yokes54are composed of non-grain-oriented steel while the outer legs52are composed of grain-oriented steel (as indicated by the double-headed arrows) and extend between the yokes54. The flux deflection zones in the layer50bare disposed at the ends of the yokes54and the ends of the middle leg52, and are thus composed of non-grain-oriented steel. When multiple layers50a,50bare assembled into a core50in alternating interleaved fashion, the flux deflection zones56in the core50are entirely composed of non-grain-oriented steel. The core power losses in this option are higher than in core40, but the cost of this core50is substantially reduced in comparison with the cost of the core40.

FIGS. 6A and 6Billustrate a single-phase interleaved magnetic core60according to the invention, with the winding on the middle core leg62. The core60has core elements comprising outer legs61(typically one half the width of the middle leg62) abutting yokes64arranged in quadrature relation to the legs61,62. In this embodiment, in the first layer60aof the two alternating laminate layers, shown inFIG. 6A, the yokes64aare formed from non-grain-oriented steel and are disposed between the outer legs61awhich are also formed from non-grain-oriented steel. Only the middle leg62a, disposed between the yokes64a, is composed of grain-oriented steel (as indicated by the double-headed arrow). The flux deflection zones66in the layer60aare disposed at the ends of the outer legs61aand the middle portions of the yokes64a, which are composed of non-grain-oriented steel. In the second layer60bof the two alternating layers, shown inFIG. 6B, the yokes64are composed of non-grain-oriented steel portions64bextending between a non-grain-oriented steel middle leg62b, while the outer legs61bare composed of grain-oriented steel (as indicated by the double-headed arrows) and extend between the yokes64b. The flux deflection zones66in the layer60bare disposed at the ends of the yoke portions64band the ends of the middle leg62b, and are thus composed of non-grain-oriented steel. When multiple layers60a,60bare assembled into a core60in alternating interleaved fashion, the flux deflection zones66in the core60are entirely composed of non-grain-oriented steel.

FIGS. 7A and 7Billustrate a single-phase interleaved magnetic core70according to the invention, with windings on one or preferably both outer core legs72. The core70has core elements comprising legs72abutting yokes74arranged in quadrature relation to the legs72. In this embodiment, in the first layer70aof the two alternating layers, shown inFIG. 7A, the yokes74are formed from grain-oriented steel (as indicated by the double-headed arrows) and are disposed between the legs72which are formed from non-grain-oriented steel. The flux deflection zones76in the layer70aare disposed at the ends of the legs72which are composed of non-grain-oriented steel. In the second layer70bof the two alternating layers, shown inFIG. 7B, the yokes74are composed of non-grain-oriented steel, while the legs72are composed of grain-oriented steel (as indicated by the double-headed arrows) and extend between the yokes74. The flux deflection zones76in the layer70bare disposed at the ends of the yokes74, and are thus composed of non-grain-oriented steel. When multiple layers70a,70bare assembled into a core60in alternating interleaved fashion, the flux deflection zones76in the core70are entirely composed of non-grain-oriented steel.

The invention thus covers both butt gap core and interleaved core designs. It will be appreciated that the principles of the invention will apply to reduce power losses at a reduced cost even if some, but not all, of the flux deflection zones16are composed entirely of non-grain-oriented steel.

Various embodiments of the present invention having been thus described in detail by way of example, it will be apparent to those skilled in the art that variations and modifications may be made without departing from the invention. The invention includes all such variations and modifications as fall within the scope of the appended claims.