WO2007141489A2

WO2007141489A2 - Magnetic core of an electric machine having anisotropic material embedded in isotropic material

Info

Publication number: WO2007141489A2
Application number: PCT/GB2007/002017
Authority: WO
Inventors: Frank Moeller; Karl-Heinz Knoerzer
Original assignee: Nexxtdrive Limited
Priority date: 2006-06-02
Filing date: 2007-05-31
Publication date: 2007-12-13
Also published as: GB0610933D0; WO2007141489A3

Abstract

A core for use in an electric machine includes an anisotropic region of magnetic material embedded in isotropic magnetic material. The anisotropic region is formed of a plurality of laminae of grain-oriented magnetic material formed of cold-rolled steel sheets. The isotropic region is formed of a mixture of resin and iron powder.

Description

- I -

Electric Machine

The present invention relates to a core and a method for making a core for use in an electric machine.

It is known to provide electric machines having magnetic cores with a laminated structure. Use of such magnetic cores may be advantageous in terms of cost, as sections of the core may be produced by stamping therefore facilitating easier production. Efficiency of an electric machine can also be enhanced using a magnetic core having a laminated structure when the laminations are electrically insulated from each other, as this leads to reduced losses resulting from eddy currents.

It is also known that the magnetic properties of laminae made of suitable types of cold rolled steel or iron strips can be desirable. These properties include anisotropic magnetic permeability that is determined by the grain orientation following rolling, and where permeability is lowest in the direction of a given alignment arising from the rolling operation. Aligned appropriately, this can help to keep the magnetic flux concentrated within the material, reducing losses due to stray electrical fields.

Soft iron generally has isotropic magnetic properties and hence exhibits a certain amount of stray magnetic field, thus leading to a reduction in efficiency. Cold rolled iron however, can have a very low magnetic permeability in the direction of roll and is therefore widely used for producing concentrated electric fields.

Figure 1 shows a cross-section of a stator of a known radial flux machine. The radial flux machine has a magnetic core 10 that is made of a plurality of stamped metal members arranged in a stack. The stamped metal members include a yoke portion 12 mechanically and magnetically coupled to poles 14. Each pole 14 is provided with respective coil windings 16 in the slots 18 defined between the poles.

The configuration shown in Figure 1 allows for complex configurations of poles and slots to be manufactured economically as the sheet metal used may be stamped. However, stamping from cold rolled iron can result in non-optimal electrical performance, as each of the poles will have the same grain orientation, but when placed in situ in a rotary electric machine, will be arranged at different angles and will therefore result in different magnetic reluctances. In this situation, steps would need to be taken to randomise the grain structure of such metal stampings to diminish their anisotropic qualities.

Shown in Figure 2 is a conventional axial flux machine where a rotor 28 is located between two parallel disc-shaped stators 20, 20' with poles formed on the stator facing inwards to the rotor. The stators 20, 20', and poles 24, 24', 24a, 24a' are connected to the respective stators mechanically and magnetically via respective yokes 22, 22'. The poles and yokes are manufactured as unitary bodies and are formed from sintered iron or a mixture of iron powder and resin, thereby resulting in a magnetic material having substantially uniform magnetic properties.

The sintering process used to obtain this magnetic material is relatively complex and expensive and forming the magnetic material from an iron and resin mixture results in a material having relatively poor magnetic and thermal properties.

Figure 3 shows another form of an axial flux machine. This axial flux machine has the same general configuration of that shown in Figure 2. However, as shown in Figure 3, the poles are formed as individual discrete pieces 34 which are attached to respective backing plates 32, by means of fasteners 33. In the example shown in Figure 3, these fasteners are machine screws. The backing plates 32 can be made from spirally wound steel strips function as back iron to complete the magnetic circuits. As described with respect to Figure 2, the rotor rotates between the pairs of poles.

The poles 34 shown in Figure 3 have uniform magnetic properties and may, for example, be made of sintered iron or a mixture of iron and resin.

The provision of discrete poles, as shown in Figure 3 is advantageous as windings 36 can be assembled to the pole pieces prior to attachment of the poles to the backing plates. - Z -

The present invention seeks to provide improved magnetic cores for all types of electric machine.

According to a first aspect of the present invention, there is provided a core for use in an electric machine comprising an anisotropic region of magnetic material embedded in isotropic magnetic material.

Use of anisotropic magnetic material, in itself, increases the permeability of a magnet. However, the use of anisotropic material, by its nature, may result in air gaps being formed in the core. For example, if the anisotropic region is formed of a plurality of grain-oriented laminae, then air gaps having very low permeability may be formed between the various laminae. The presence of air gaps within the anisotropic region reduces the overall permeability of the region resulting in reduced efficiency of the magnetic material.

The present inventors have recognised that if these air gaps are replaced with an isotropic magnetic material, despite this isotropic material having a lower relative permeability than the anisotropic region, then the permeability of the core once again increases, resulting in overall increased efficiency of the magnetic material.

The anisotropic region may be formed of any suitable means. However, preferably, the anisotropic region is formed of a plurality of laminae of grain- oriented magnetic material, for example cold-rolled steel sheets.

Preferably, the isotropic region is formed of a mixture of resin and iron powder and use of the mixture of resin and iron powder enables the laminae to be set in place.

The ratio of iron powder within the resin should be as high as possible to result in the best overall properties. For example, a ratio of 90% iron suspended in 10% resin has been found to be effective.

Typically, the core forms part of a magnetic circuit of an electric machine. Preferably, grains in the steel sheets are oriented in substantially the same direction along the length of each grain-oriented steel sheet and the laminae are arranged such that the permeability of the core is maximised in directions transverse to a flat side of the steel sheets.

This ensures maximum permeability in the direction of windings around the pole, when in use in a rotary electric machine.

In an embodiment, the laminae may be arranged in the form of a ring.

Alternatively, and/or additionally, the laminae of at least one section of the core are arranged substantially parallel to one another.

In an embodiment, the core forms part of a pole for use in a rotary electric machine.

Preferably, the pole has a first surface that, in use, is arranged to abut a rotor/stator of a rotary electric machine, and a second surface that faces the other of the stator/rotor. However, it will be appreciated that any other known orientation of pole with respect to a rotor/stator combination may be used.

Preferably, the laminae extend from the first surface of the pole towards the second surface in a substantially parallel direction, such that the region of maximum permeability radiates outwards from a direction transverse to the path between the first surface and the second surface. More preferably, at least one of the laminae extends further than the remainder of the laminae in a direction deviating from the substantially parallel direction.

This results in an improved flux distribution at the second surface without the need to provide a correspondingly large cross-section of magnetic material at the extreme edges.

Preferably, the laminae are arranged in substantially concentric layers.

In an embodiment, the mixture is arranged to adhere the pole to the rotor/stator of the rotary electric machine. This prevents the need for cumbersome and expensive fastening pieces.

In an alternative embodiment, the core forms part of a stator for use in a radial flux machine.

In a further embodiment, the core forms part of a transformer.

Various types of transformer core may be formed using the core of the present invention. Where the transformer core is designed to be formed in two parts, the mixture may be used to adhere the two parts together.

The present invention also extends to an electric machine comprising a core as described above.

The present invention further extends to a method of forming a core for use in an electric machine, said core having an anisotropic region of magnetic material embedded in an isotropic magnetic material, the method comprising the steps of: arranging the magnetic material forming the anisotropic region according to the function of the core; distributing the isotropic magnetic material within the anisotropic region; and applying heat and/or pressure to set the resultant structure.

Any means to distribute the isotropic material through the anisotropic region may be used, however, the method may further comprise the step of agitating the core to ensure even distribution of the isotropic material within the anisotropic material.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying diagrams, in which:

Figure 1 shows a cross-section of a known radial flux machine; Figure 2 shows a cross-section of a known axial flux machine; Figure 3 shows a cross-section of an alternative known axial flux machine; Figure 4a shows a plan view of a first embodiment of a pole piece of the present invention;

Figure 4b shows a cross-section through A-A of Figure 4a;

Figure 5 shows a view of a stator with poles for use in an axial flux machine;

Figure 6a shows a plan view of a second embodiment of a pole piece of the present invention;

Figure 6b shows a cross-section through A-A of Figure 5a;

Figure 7a shows a plan view of a third embodiment of a pole piece of the present invention;

Figure 7b shows a cross-section through A-A of Figure 6a; Figure 8a shows an embodiment of a stator of the present invention;

Figure 8b shows a cross-section through A-A of Figure 7a;

Figure 9 shows a transformer having a C-shaped core of the present invention; and

Figure 10 shows a transformer having an E-shaped core of the present invention.

The present invention is applicable in general to electric machines, for example, motors, generators and transformers, having a core formed of a plurality of laminae. For simplicity, the invention will hereinafter be described generally in terms of its use in a rotary electric machine, for example a motor or generator. However, where applicable, examples of its use in other electric machines will be given.

Figures 4a, 4b, 6a, 6b, 7a and 7c show embodiments of poles that include cores of the present invention. As previously described, both radial and axial flux machines are known in the art and further description of the operation of such machines is not given. However, both radial and axial flux machines, in the simplest embodiments, include a rotor, stator and poles.

Put simply, the magnetic flux generated in a radial flux machine extends in a radial direction from the axis of rotation. By comparison, the magnetic flux generated in an axial flux machine extends along the axis of rotation of the rotor.

The poles shown in Figures 4a, 4b, 6a, 6b, 7a and 7c may be adapted for use with either radial or axial flux machines. However, for the purposes of describing the present invention, Figures 4 and 6 will be described in relation to their use with an axial flux machine and Figure 7 will be described in relation to its use with a radial flux machine.

Each of the embodiments shown has a core 144 made up of a plurality of laminae. The laminae forming the core are strips of grain-oriented steel and are aligned to result in a core having anisotropic properties. In the present invention, the anisotropic region is made up of steel sheets formed by cold rolling to orient the grains. Preferably, the grains are oriented in substantially the same direction along the length of the respective steel sheets. However, it will be appreciated that this area of anisotropic magnetic material may be achieved by any other suitable means.

Each specific embodiment will now be described in turn.

Figures 4a and 4b show a pole 40 for use in an axial flux machine. The pole 40 has a first face 41 that, when placed in a rotary electric machine, defines one end of an air gap in use, and a second face 43 that will abut the frame or stator of an axial flux machine.

As set out above, the pole 40 has a core 144 made up of a plurality of laminae of grain-oriented steel strips. The steel strips 44 are embedded in an iron powdered resin mixture and extend from the second face 43 towards the first face 41 of the pole. Embedding the laminae in the iron powdered resin mixture allows the desired surface typography of the pole pieces to be obtained to result in the desired flux dimensions. In the embodiment shown in Figure 4a, the steel strips are arranged with the grain orientation aligned in substantially the same direction to present a low magnetic permeability path between the first and second faces of the pole, and a relatively high permeability in a direction transverse to the path between the first and second faces. In this respect, the laminae effectively channel their magnetic energy parallel to the grain structure. Thus, when in use, windings for creating a magnetic circuit will be arranged around the pole, between the first and second faces, such that the permeability will be strongest in the direction of the windings. The core is formed of as many steel strips as it is possible to locate in the region without compromising its anisotropic properties. However, the arrangement of the laminae, for example, as shown in Figure 4a results in air gaps 48 between the respective laminae and between the ends of the laminae and the respective faces of the pole. These air gaps have a much lower permeability compared to the steel sheets, and the presence of the air gaps in the core results in an overall decrease in efficiency of the core. Replacing these air gaps with an isotropic magnetic material, having a higher relative permeability than the air gaps increases the overall efficiency of the core and results in the maximum potential permeability in the direction of magnetic flux.

The isotropic magnetic material may be formed of a mixture of iron powder and resin. Pressure and moderate temperatures are applied to the mixture when in position to set the structure around the steel sheets. The ratio of iron powder within the resin should be as high as possible to result in the best overall magnetic properties, for example, a mixture of approx 90 % iron suspended in 10 % resin may be used.

As shown, a first end of each of the steel strip members abuts the second face 43, while the opposite ends 44a extend towards the first face 41 of the pole. In an embodiment of this pole (not shown) the steel strip members may all be the same length. However, as shown in Figure 4b, the opposite ends 44a of some of the outer strips may extend beyond the ends of the remainder of the strips. The additional sections of these elongated strips are angled away from the common axis of alignment of the remainder of the strips. This results in an improved flux distribution at the first face 41 of the pole without the need to provide a correspondingly large cross-section of magnetic material at the extreme edges.

Where required, and as shown in Figure 4b, a through bore 46 is provided through the centre of the pole to accommodate a fastener (not shown). Such a pole can be located, for example, in an axial flux machine as shown in Figure 3. Using this axial flux machine as an example, the second face 43 could be arranged to abut the surface of the stator facing the rotor (or both surfaces if the rotor is provided between two stators) with the first face 41 of the pole defining an edge of the air gap between the pole and the rotor. Several poles could be arranged along the edge of a disc shaped stator with an outer edge 49 of the pole 40 located adjacent the circumference of the stator as shown in Figure 5.

The bore 46 in the embodiment shown is manufactured as part of the casting process of the pole. However, in an alternative embodiment, the bore may be provided subsequent to the casting process by, for example, drilling through a region that has purposefully been provided free of laminations.

In the embodiment shown in Figures 4a and 4b, the strips are of a generally planar configuration with the exception of the angled ends 44a previously referred to. This arrangement can provide a good filling density over the entire cross-section of the pole pieces, with strips of appropriate size being provided to fill all available space. This arrangement is particularly useful for the construction of poles for axial flux machines which have a non-rectangular shape. During the manufacturing process, the resin powder mixture fills all cavities and crevices and also surrounds the laminae.

Figure 6 shows a second embodiment of a pole 50. Again, this pole will be described in relation to its use in an axial flux machine. The pole 50 has a core 144 again made of a plurality of laminae 52. As can be seen in Figure 6a, the laminae 52 of this embodiment are arranged in substantially concentric layers. In this respect, it will be appreciated that although the layers of steel are shown in Figure 6a to be circular, they may not be circular in the mathematical sense.

A core of this structure may conveniently be fabricated by continuously winding a long strip of grain orientated steel in the form of a spiral, leaving an area in the centre through which a bore may be formed. Once the spiral has been formed, a slot 151 is cut into the core to avoid eddy currents which would otherwise result. This slot is then filed with the iron powder resin mixture.

Alternatively, the spiral formation may be formed by obtaining several strips of steel and arranging them in generally concentric layers.

The embodiment shown in Figure 6a shows each of the laminae extending to the same length. However, it will be appreciated that the lengths could be varied to achieve the effect shown in Figure 4a by, for example, tapering the length of the steel strip towards its end to result in the coil being wider towards its outside. Once the coil is formed, these elongated laminae could be pressed such that they extend in an angle away from the angle or orientation of the remainder of the laminae.

As before, the strips (laminae) are arranged such that the grain orientation produces a low magnetic permeability between the first and second surfaces, with a greater relative permeability in a transverse direction. As described with reference to Figure 4, the external topography of the pole piece shown in Figure 6 is defined by a cast iron dust resin portion 52. Furthermore, a bore 56 may be provided for attachment to a yoke of a stator. Poles as shown in Figure 6 may be arranged on a disc-shaped stator as described with reference to Figure 5.

Figure 7 shows an embodiment of a pole 160 for use with a radial flux machine. As stated previously, although the embodiment shown is specific to use with a radial flux machine, it will be appreciated that a similar arrangement of core, with minor modifications to the pole itself, could be used with an axial flux machine.

The pole 160 has a first face 61 that, when located in a radial flux machine, defines one edge of an air gap between the rotor and stator of the machine. A second face 63 is arranged to engage the yoke or rotor/stator of the machine (depending on the machine type).

The pole has a core 144 made of a plurality of laminae in the form of substantially planar sheets of grain-oriented steel. The sheets are arranged accordingly to result in substantially parallel grain orientation.

The portion 62 of the pole piece which defines the edge of the air gap when in use is formed using iron powdered resin mixture. As described with reference to Figure 4, the outer sheets of the core of the pole piece shown in Figure 7b are elongated in comparison to the remainder of the laminae. The end sections 64a of these outer strips are inclined at an angle to the axis of - U -

orientation of the remainder of the strips to assist in obtaining the desired flux distribution at the extremities of the pole.

As with the previous embodiments, a bore 66 is provided to accommodate a fastener (not shown) for fastening the pole piece to the relevant part of the radial flux machine.

An example of a yoke to which the pole described in Figure 7 may be attached is shown in Figure 8a. From this diagram it can be seen that the pole may be attached to the inner or outer edge of the ring. The embodiment shown in Figure 7b is clearly designed for being attached to the outer edge of the ring, as can be seen the nature of the curved faces 60, 63. However, by reversing the curved faces, i.e. changing the convex surface to a concave surface, the pole could be adapted for use on the inner edge of the ring. In either case, the second face 63 would be arranged to abut the inner/outer edge of the ring (yoke). Where the ring forms the stator and the poles are arranged around the inner edge of the ring, the first face would define the edge of the air gap formed between the poles and a rotor located inside the ring. Alternatively, where the ring forms the rotor and the poles are arranged around the outer edge of the ring, the first face would define the edge of the air gap formed between the poles and a stator located outside the ring.

In each of the embodiments of poles shown, the laminae do not extend through the isotropic region of iron powder resin mixture forming the first surface of each respective pole piece. It will, however, be appreciated that it would be possible for at least some of the laminae to extend to this surface.

Figure 8 shows a stator 70 for use in a radial flux machine. The stator 70 is in the form of a ring 72 made up of a plurality of turns of wound steel strip 73 to form a spring-like coil of steel strip. A flat face of the wound strip is orientated at 90° to the winding axis. When viewed in cross-section, or from a side of the coil, the wound steel strip forms a plurality of concentric laminae stacked together.

When manufactured, the strip 73 is wound from a substantially straight strip. The action of conforming the strip to the concentric shape naturally leads to compression of the radially inner region of the steel strip and tension in the radially outer region. The tension in the outer region leads to a natural "thinning" of the outer edge of steel strip. Thus, when viewed in cross-section, as seen in Figure 8b, gaps are formed between the laminae along the outer edge 171 and the previously rectangular cross-section of the laminae is distorted to a trapezoidal cross-section.

To compensate for the consequential reduction in magnetic permeability caused by these gaps, the ring is impregnated with the iron powder resin mixture, thereby filling the air gaps at the radially outer edge and reducing the reluctance.

The strips are specifically arranged for the grain orientation to be aligned to result in a relatively low magnetic reluctance in a radial direction.

In an embodiment, the poles 74, 76 shows in Figure 8a are produced during the impregnation operation by casting using the iron powder resin, allowing impregnation of the ring and fabrication of the poles to be carried out in a single operation. In this respect, use of the epoxy resin results in the mixture having an adhesive quality. During impregnation, the mixture is set in place and as a result, the poles may be attached to the ring without requiring the use of additional fastening means.

In this respect, it may be possible to cast the iron powder resin to the finished size, or it may be preferable to cast the items somewhat oversize and to finish them as required using a suitable machine known to those skilled in the art.

Figures 9 and 10 show a core as described above in use in two forms of transformer. Specifically, Figure 9 shows a C-shaped core of a transformer and Figure 10 shows an E-shaped core of a transformer.

As can be seen in both Figures 9 and 10, the core is formed of a plurality of laminae 82, 92 of grain-oriented steel, forming the anisotropic region of the core. The laminae are embedded in the iron powder and resin mixture described above. Furthermore, to prevent eddy losses occurring in the core, the mixture is also provided between respective ends 84, 94 of the laminae where required. Again, as a result of the adhesive properties of the epoxy resin, and in accordance with the nature of the mixture once heat has been applied, the mixture also serves to join the component pieces of the transformer together, thus obviating the need for screws or alternative means for fixing the core together. .

The method for making a core as described above will vary in accordance with its intended purpose. However, in general terms, the laminae may be arranged in the required manner. Following this, the core may be impregnated with the iron powder and resin mixture and vibrated to ensure even distribution of the mixture, and to ensure that all gaps are filled by the mixture. Then, or during distribution of the mixture, the core may be subjected to high pressure and moderate temperatures, to cure the assembly and to ensure that the configuration of the core is set in its intended form.

Embodiments of the various poles have been described for use specifically with a stator. However, the skilled person will appreciate that the poles may be adapted in accordance with the type of rotor or stator used in a particular type of machine.

It will be appreciated that modifications to and amendments of the embodiments as described and claimed may be made within the scope of this application.

Claims

CLAlMS

1. A core for use in an electric machine comprising an anisotropic region of magnetic material embedded in isotropic magnetic material.

2. A core as claimed in claim 1 , wherein the anisotropic region is formed of a plurality of laminae of grain-oriented magnetic material.

3. A core as claimed in claim 2, wherein the laminae of grain-oriented magnetic material are formed of cold-rolled steel sheets.

4. A core as claimed in any preceding claim, wherein the isotropic region is formed of a mixture of resin and iron powder.

5. A core as claimed in Claim 4, wherein the mixture of resin and iron powder is arranged to set the laminae in place.

6. A core as claimed in any preceding claim, wherein the core forms part of a magnetic circuit of an electric machine.

7. A core as claimed in any of claims 3 to 6, wherein grains in the steel sheets are oriented in substantially the same direction along the length of each grain-oriented steel sheet.

8. A core as claimed in any of claims 3 to 7, wherein the laminae are arranged such that the permeability of the core is maximised in directions transverse to a flat side of the steel sheets.

9. A core as claimed in any preceding claim, wherein the laminae are arranged in the form of a ring.

10. A core as claimed in any of claims 1 to 8, wherein at least one section of the laminae of at least one section of the core are arranged substantially parallel to one another.

11. A core as claimed in any preceding claim, wherein the core forms part of a pole for use in a rotary electric machine.

12. A core as claimed in claim 1 1 , wherein the pole has a first surface that, in use, is arranged to abut a rotor/stator of a rotary electric machine, and a second surface that faces the other of the stator/rotor.

13. A core as claimed in claim 12, wherein the laminae extend from the first surface of the pole towards the second surface in a substantially parallel direction, such that the region of maximum permeability radiates outwards from a direction transverse to the path between the first surface and the second surface.

14. A core as claimed in claim 13, wherein at least one of the laminae extends further than the remainder of the laminae in a direction deviating from the substantially parallel direction.

15. A core as claimed in any of claims 11 to 14, wherein the laminae are arranged in substantially concentric layers.

16. A core as claimed in any of claims 12 to 15, wherein the mixture is arranged to adhere the pole to the rotor/stator of the rotary electric machine.

17. A core as claimed in any of claims 1 to 10, wherein the core forms part of a stator for use in a radial flux machine.

18. A core as claimed in any of claims 1 to 10, wherein the core forms part of a transformer.

19. A core as claimed in claim 18, wherein the core is formed of two parts that are arranged to be adhered together by the mixture.

20. An electric machine comprising a core as described in any of claims 1 to 19.

21. A method of forming a core for use in an electric machine, said core having an anisotropic region of magnetic material embedded in an isotropic magnetic material, the method comprising the steps of: arranging the magnetic material forming the anisotropic region according to the function of the core; distributing the isotropic magnetic material within the anisotropic region; and applying heat and/or pressure to set the resultant structure.

22. A method of forming a core as claimed in claim 21 , wherein the step of distributing the isotropic magnetic material further comprises agitating the core to ensure even distribution of the isotropic material within the anisotropic material.

23. A method of forming a core as claimed in any of claims 21 or 22, wherein the anisotropic region is formed of a plurality of laminae of grain- orientated steel.

24. A core substantially as hereinbefore described with reference to diagrams 4 to 10.