WO2002047238A1

WO2002047238A1 - Electrical machine magnetic cores

Info

Publication number: WO2002047238A1
Application number: PCT/GB2001/005426
Authority: WO
Inventors: Lawrence Haydock; Neil Brown
Original assignee: Newage International Limited
Priority date: 2000-12-06
Filing date: 2001-12-06
Publication date: 2002-06-13
Also published as: AU2002220912A1

Abstract

An external annular stator (110) for an electrical machine inlcudes an interconnected series of stator modules (120) and an interconnected series of pole piece modules (130). Each stator module (120) includes an annular portion (111) of the stator (110) with a respective pole (112) projecting radially inwardly therefrom. In an assembly operation, each module (120) of the stator (110) is arranged to receive a pre-wound coil on the respective pole (112) thereof and then to have the interconnected series of pole piece modules (130) placed across ends of the poles (112). The series of stator modules (120) and the series of pole piece modules (130) are then arranged to be formed into the annular stator (110). Each pole piece module (130) is joined to the or each pole piece module stator (130) by a bridge of reduced thickness (139) relative to the pole piece modules (130). The reduced thickness of each bridge (139) renders it more readily saturable by a working flux of the electrical machine. Thus, the working flux tends to pass radially through each pole (112), rather than circumferentially through the pole piece modules (130).

Description

ELECTRICAL MACHINE MAGNETIC CORES

This invention is directed towards components of electrical machines. More particularly, it is directed towards stators or rotors of electrical machines.

Electrical machines, such as motors and alternators, conventionally include a stator and a rotor. These two components are often mounted coaxially, with an internal rotor being positioned inside an external stator. An external stator may include electrical windings that are positioned in slots between poles that project radially inwards from an annular portion of the stator. The slots usually have a narrow opening that leads to a wider cavity and are termed "semi-closed slots". In some cases this may lend each slot the appearance of a teardrop. An advantage of semi-closed slots is a reduction in the reluctance variation around the stator and hence a reduction in the incidence of "cogging" of the rotor relative to the stator. It will be appreciated that inserting windings into each slot is made difficult by the narrow opening.

Stators and rotors are conventionally comprised of stacks of laminations of a ferrous material, each lamination being stamped or otherwise cut out from sheet material. The annular nature of stators and rotors results in a significant proportion of the sheet material being wasted. This is undesirable.

EP-A-0871282 discloses a stator comprised of interconnected stator modules, each module including a segment of a yoke of the stator and a pole projecting radially inwardly therefrom. Each pole has a pair of bendable prongs at a radially innermost end. The prongs initially project radially inwards from the end such that a pre-wound coil is fitable to each pole. The prongs are then bent so as to extend substantially circumferentially in opposite directions, thereby forming a pole piece on the end of the pole.

However, bending the prongs has the effect of altering the electromagnetic properties thereof so as to reduce the permeability of the prongs and increase iron losses therein. Bending also causes the prongs to lack concentricity with an axis of rotation of the machine, leading to a large and irregular airgap between the prongs and an adjacent rotor. This increases the reluctance of the airgap and reduces the performance of the machine disclosed in EP-A-0871282.

It is an object of this invention to address these problems.

According to an aspect of this invention there is provided an interconnected series of pole piece modules for attaching to one of a stator and a rotor of an electrical machine so that each module of the series is at an end of a respective pole of the stator or the rotor so as to form a pole piece, wherein each pole piece module is joined to the or each juxtaposed pole piece module by a bridge which, in use, is more readily saturable by a working flux of the electrical machine than is each pole piece.

The stator or the rotor may be that of a linear electrical machine.

The electrical machine may be a radial flux electrical machine and each pole piece module may be arcuate such that when it forms a respective pole piece of the stator or the rotor, a radially inner surface of each pole piece is concentric with an axis of rotation of the electrical machine.

According to another aspect of this invention there is provided a stator for an electrical machine, the stator including an annular portion with a series of circumferentially distributed poles projecting radially therefrom, wherein the stator is formed from a series of interconnected, circumferentially distributed stator modules, each module including a segment of the annular portion and a respective one of the poles, wherein the stator includes an interconnected series of circumferentially distributed pole piece modules, each module of the series being at an end of a respective pole so as to form a pole piece, wherein each pole piece module is joined to one or both respective juxtaposed pole piece modules by a bridge which, in use, is more readily saturable by working flux of the electrical machine than is each pole piece.

According to a further aspect of this invention, there is provided a rotor for an electrical machine, the rotor including an annular portion with a series of circumferentially distributed poles projecting therefrom, wherein the rotor is formed from a series of interconnected, circumferentially distributed rotor modules, each module including a segment of the annular portion and a respective one of the poles, wherein the rotor includes an interconnected series of circumferentially distributed pole piece modules, each module of the series being at an end of a respective pole so as to form a pole piece, wherein each pole piece module is joined to one or both respective juxtaposed pole piece modules by a bridge which, in use, is more readily saturable by a working flux of the electrical machine than is each pole piece.

According to a still further aspect of this invention, there is provided a method of assembling an annular stator or rotor of an electrical machine including the steps of: a) forming a series of interconnected stator modules or rotor modules of the electrical machine, each stator or rotor module having an arcuate portion with a respective pole projecting radially therefrom; b) presenting a series of pre-wound coils to the series of stator or rotor modules and fitting each coil around a respective one of the poles; c) forming a series of interconnected pole piece modules; d) presenting the series of pole piece modules to the series of stator or rotor modules such that the pole piece modules are at the ends of the poles; e) forming the stator or rotor modules, the coils and the pole piece modules into an annulus, each pole piece lying at a radially inner end of a respective one of the poles; d) attaching the pole piece modules to the stator or rotor modules; wherein each pole piece module is joined to one or both respective juxtaposed pole piece modules by a bridge which, in use, is more readily saturable by a working flux of the electrical machine than is each pole piece.

Each bridge may be thinner in a radial direction than is each pole piece. Each bridge may have an arcuate and concave radially innermost edge. Each bridge may have an arcuate and concave radially outermost edge.

Each pole is preferably arranged to receive a respective coil thereon in a sliding fit. Each pole is preferably of a substantially constant cross section along its radial length.

Preferably at least one of the pole piece modules includes a pole portion projecting radially outwards therefrom, and at least one of the poles of the rotor or stator is absent a corresponding pole portion, said pole piece module and said pole being arranged to be radially juxtaposed thereby providing relative location of the pole piece modules with respect to the rotor or stator.

At least one stator or rotor module may have a male connector part and at least one respective juxtaposed module may have a cooperating female connector part and the two parts are arranged to be interlocking thereby. The male connector part and the female connector part may form a "puzzle lock" type of connection.

The modules may be made by powder metallurgy.

Laminations usually have a grain orientation dictated by rolling operations that were performed on the sheet material from which they are cut. This grain orientation, if it does not lie in the same direction as lines of magnetic flux that pass through the lamination, may also prove undesirable.

The grain structure of the material of the stator or rotor, and of the poles and the pole pieces may be orientated so as to aid the passage of magnetic flux through the pole pieces and the poles in a radial direction, and through the stator or rotor in a circumferential direction At least one pole may be integral with the body portion. At least one pole and/or at least one piece may be formed by powder metallurgy. At least one pole may be arranged for attachment to the body portion. The at least one pole

that is arranged for attachment to the body portion may be so arranged by the

pole having one of a co-operating male and female connecting part and the body portion having the other of the co-operating male and female connecting part, the two parts being arranged to be interlocking thereby. The male and female connecting parts may be connected by means of a mitred joint; the male and female connecting parts may be connected by means of a puzzle lock type arrangement. The at least one of the poles that is arranged for attachment to the body portion may include a v-shaped male part and the body portion may include a co-operating v-shaped female part, whereby to form a mitred joint and thereby facilitating the passage of magnetic flux between that pole and the body portion.

Specific embodiments of this invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows lines of modules for forming into a stator;

Figure 2 shows the modules of Figure 1 formed into an annular stator; Figure 3 shows modules of the another stator; Figure 4 shows modules of a further stator; and

Figure 5 shows detail of the stator Of Figure 4.

Figure 1 shows modules 120 for assembly into a stator 110 of an electrical machine (not shown). Each module 120 is substantially T-shaped so as to have a yoke portion 111 and a stem portion 112 projecting therefrom. An edge of the yoke portion 111 remote from the stem portion 112 and an opposite edge of the stem portion 112 are arcuate about a respective centre displaced from each module 120 in the direction of projection of the respective stem portion 112. In a non-assembled state, the modules 120 lie side-by-side in a substantially straight line so that each module 120 is in juxtaposition with a respective two other modules 120, apart from a respective module 120 at each end of the line, each of those two modules being in juxtaposition with one respective other module 120. Each module 120 is integrally connected to the respective juxtaposed modules 120 or module 120 at a respective side edge of the yoke portion 111. One of the modules 120 at one end of the line has a stem portion 112 that is approximately half as long as each of those of the other modules 120.

The straight line of modules 120 is comprised of a stack of substantially identical laminations (not shown). Each lamination is stamped from a sheet of ferrous material (not shown), such as electrical steel. It will be appreciated that the shape of the line of modules 120 lends itself to efficient use of the sheet of material. The T-shape of the modules 120 allows successively inverted and closely situated laminations to be stamped out from the sheet, thereby reducing wastage.

To assemble the stator 110, pre-wound "bobbins" of electrical windings (not shown) are placed between and around the stem portions 112 of juxtaposed modules 120. It is envisaged that this may be achieved by presenting a line of such bobbins to the line of modules 120 in a direction perpendicular to the line of bobbins and the line of modules 120, the two lines being parallel. In this way, each of the bobbins can be fitted to the respective stem portion 112 in a single, linear operation. Each bobbin is an interference fit on the respective stem portion 112. In this embodiment, the two sides of each stem portion 112 are parallel to one another and so the width of each stem portion is constant along its length. However, it will be appreciated that the stem portion 112 may be tapered towards the end thereof remote from the yoke portion 111 of the respective module 120 and that a bobbin with a corresponding tapered central aperture may be provided such that a good, close fit between the bobbin and the stem portion 112 is achieved. A close fit between each bobbin and the respective stem portion 112 is desirable for efficiently inducing a magnetic field in that stem portion 112 when a current is passed through the electrical winding of the bobbin.

Pole piece modules 130 are also provided. Each pole piece module 130 is

arcuate. The pole piece modules 130 are positioned in a similar side-by-side linear arrangement as the modules of the stator 120: juxtaposed pairs of pole piece modules 130 being integrally attached to each other. One of the pole piece modules 130 includes a pole piece top portion 135 projecting therefrom. The pole piece top portion 135 of the one pole piece module 130 is for cooperating with the half-length stem portion 112 of the one module 120. The pole piece top portion 135 is placed inside the bobbin that is situated around the one module 120 that has the half-length stem portion 112 such that the pole piece top portion 135 abuts the half-length stem portion 112 of the one module 120. The pole piece top portion 135 is an interference fit inside the bobbin that is situated around the one module 120. In fitting the pole piece top portion 135 of the one pole piece module 130 in this way, the other pole piece modules 130 extend across ends of the stem portions 112 of the line of modules 120. As

is shown in Figure 1, the line of pole piece modules 130 is shorter than the line of modules 120.

Each module 120 with the respective bobbin mounted thereon is then displaced angularly relative to the respective juxtaposed modules 120 or module 120 such that, in combination, the modules 120 form an annulus with each module 120 being in juxtaposition with two respective modules 120. At the same time, each pole piece module 130 is also displaced angularly relative to the respective juxtaposed pole piece modules 130 or pole piece module 130, such that, in combination, the pole piece modules 130 form an annulus inside the annulus formed by the modules 120, each pole piece module 130 being adjacent a respective stem portion 112 of a respective module 120. In other words, the line of modules 120 and the line of pole piece modules 130, having been juxtaposed, are "rolled-up". The arrangement arrived at is shown in Figure 2. Each of the two modules 120 that was at a respective end of the line of modules 120 are attached to each other by a fixing operation such as welding or gluing. Each pole piece module 130 is then attached to the respective module 120 by a fixing operation such as welding or gluing. Thus, the yoke portions 111 of the modules 120 form a radially outer annulus of the stator 110, the stem portions 112 form poles 112 protecting radially inwardly therefrom and each pole piece module 130 forms a pole piece 130 on a respective one of the poles 112.

To reduce stress concentrations in the modules 120 during the aforementioned angular displacement, a radius is provided in material that connects the side edges of each juxtaposed and interconnected pair of modules 120. An opposed pair of radii 136 and 138 are also provided between each juxtaposed and interconnected pair of pole piece modules 130. The radii 136 and 138 similarly reduce stress concentrations in the pole piece modules 130 during angular displacement

In addition to this, however, the radii 136 and 138 are advantageous in reducing the thickness of material 139 that connects pairs of juxtaposed pole pieces modules 130. This connecting material 139 may be considered as a bridge 139. In use of the stator 110 as a component of an electrical machine (not shown), this reduction in thickness lowers the saturation point of the bridge 139. As a result, once the bridge 139 is saturated, magnetic flux tends to flow in the desired direction: through each pole piece 130 and through the respective adjacent pole 112 in a substantially radial direction, rather than circumferentially in a magnetic short circuit through the pole pieces 130, as would be the case were the opposed pair of radii 136 and 138 not provided and the thickness of the bridge 139 not reduced thereby.

Figure 3 shows another stator 210 for an electrical machine, such as an alternator. The other stator 210 includes a substantially annular portion 212 with a plurality of poles 214, in this case four, projecting radially inwards therefrom. The other stator is comprised of a plurality of modules 220, again, in this case four. Each module is comprised of a stack of substantially identical module laminations of a ferrous material such as electrical steel. It will be appreciated that the use of laminations in the construction of electrical machines is known. Each module 220 is a section of the other stator and is substantially identical to each other module 220. Each module 220 has two edges that are in the annular portion 212 of the other stator 210, a clockwise- most edge 221 and an anti-clockwise-most edge 222. Each edge is provided with a respective male or female connector of the "puzzle lock" type that are similar to those used to connect juxtaposed pieces of a jigsaw picture. The clockwise-most edge 221 of each module 220 is provided with a female connector 224 and an anti-clockwise-most edge 222 of each module is provided with a male connector 226.

To assemble the other stator, a pre-wound "bobbin" electrical winding (not shown) is placed around the respective pole 214 of each module 220. Each module 220 is then attached to two juxtaposed modules by locating the male connector 226 of the anti-clockwise-most edge 222 of each module 220 in the female connector 224 of the clockwise-most edge 221 of a respective juxtaposed module 220. Each module 220 is then permanently attached to the two respective juxtaposed modules 220 by a fixing operation such as welding or gluing. Although not shown in Figure 3, it is envisaged that the other stator 210 would also be provided with an interconnected series of pole piece modules similar to that 130 of the stator 110 described above with reference to Figures 1 and 2.

Figure 4 shows a stator 410 for an electrical machine. The stator includes an annular portion 420 and a plurality of poles 430. In this example there are eight poles 430. The poles 430 are circumferentially distributed with a substantially constant angular pitch and project radially inwards from an inner face of the annular portion 420. Both the annular portion 420 and the poles 430 are each made up from a respective plurality of laminations.

An interconnected series of pole piece modules 435, similar to that 130 of the stator 110 described above with reference to Figures 1 and 2 is also provided. The series of pole piece modules 435 is located radially inside the poles, so as to form a respective pole piece on a radially inner end of each pole 430.

For each of the annular portion 420 and the poles 430, the respective laminations are substantially identical and are stacked axially with reference to the annular portion 420. An outermost end of each of the poles 430 includes a v-shaped point of substantially constant cross section in a radial plane with respect to the annular portion 420. A respective corresponding v-shaped recess is provided in the innermost face of the annular portion 420 to receive a v- shaped outermost end of a respective pole 430. The outermost ends of each pole 430 is fixed in the respective recess by a fixing operation such as welding or gluing.

The laminations which make up the annular portion 420 and the poles 430 are stamped from sheet material that is ferrous, such as electrical steel. It is known that sheet material that has undergone a rolling operation may have a grain structure that is orientated in a particular direction. This direction is usually parallel to the direction of rolling. Each of the laminations that goes to make up the annular portion 420 is stamped from the sheet material as a substantially linear strip, the grain orientation being in the direction of the length of the strip. The strip is then formed into the respective annular lamination so that the grain orientation is in a circumferential direction. Magnetic flux passes through the annular portion 420 of the stator 410 in a substantially circumferential direction and through the poles 430 in a substantially radial direction. The alignment of the grain orientation with the direction of the magnetic flux facilitates that passage of the flux. Figure 5 shows in detail a part of the annular portion 420 and one pole 430. Also shown are lines of magnetic flux 440 that may be considered representative of the excitation field of an electrical machine. The v-shaped outermost end of the pole 420 and the v-shaped recess of the annular portion 420 further enable the grain orientation of material of the stator 410 to be substantially aligned with the direction of the magnetic flux. The laminations which go to make up the poles 430 are similarly stamped from the sheet material. The laminations are so stamped such that the grain orientation is in the direction of the length of the lamination, that direction corresponding to a radial direction in the respective pole 430.

The pole piece modules 435 are similarly formed such that the grain orientation thereof lies in a substantially radial direction.

Rather than forming each lamination that goes to make up the annular portion 420 from a linear strip into an annulus, it is also envisaged that the annular portion 420 of the stator may be comprised of a number of circumferentially distributed stator modules as described hereinbefore, each stator module having attached thereto a respective pole 430 with a v-shaped outermost end. The laminations that would go to make up the stator modules would be stamped from the sheet material such that the grain orientation is substantially tangential where the respective pole is attached to the stator module. This would result in the annular portion of the stator having a grain portion that is substantially circumferential.

Claims

1. An interconnected series of pole piece modules for attaching to one of a stator and a rotor of an electrical machine so that each module of the series is at an end of a respective pole of the stator or the rotor so as to form a pole piece, wherein each pole piece module is joined to the or each juxtaposed pole piece module by a bridge which, in use, is more readily saturable by a working flux of the electrical machine than is each pole piece.

2. An interconnected series of pole piece modules according to claim 1 wherein each pole piece module is arcuate such that when it forms a respective pole piece of the stator or the rotor, a radially inner surface of each pole piece is concentric with an axis of rotation of the rotor of the electrical machine.

3. A stator for an electrical machine, the stator including an annular portion with a series of circumferentially distributed poles projecting radially therefrom, wherein the stator is formed from a series of interconnected, circumferentially distributed stator modules, each module including a segment of the annular portion and a respective one of the poles, wherein the stator includes an interconnected series of circumferentially distributed pole piece modules, each module of the series being at an end of a respective pole so as to form a pole piece, wherein each pole piece module is joined to one or both respective juxtaposed pole piece modules by a bridge which, in use, is more readily saturable by working flux of the electrical machine than is each pole piece.

4. An rotor for an electrical machine, the rotor including an annular portion with a series of circumferentially distributed poles projecting therefrom, wherein the rotor is formed from a series of interconnected, circumferentially distributed rotor modules, each module including a segment of the annular portion and a respective one of the poles, wherein the rotor includes an interconnected series of circumferentially distributed pole piece modules, each module of the series being at an end of a respective pole so as to form a pole piece, wherein each pole piece module is joined to one or both respective juxtaposed pole piece modules by a bridge which, in use, is more readily saturable by a working flux of the electrical machine than is each pole piece.

5. Apparatus according to any one of claims 2 to 4 wherein each bridge is thinner in a radial direction than is each pole piece.

6. Apparatus according to claim 5 wherein each bridge has an arcuate and concave radially innermost edge.

7. Apparatus according to claim 5 or claim 6 wherein each bridge has an arcuate and concave radially outermost edge.

8. Apparatus according to any one of the preceding claims wherein each pole is arranged to receive a respective coil thereon in a sliding fit.

9. Apparatus according to claim 8 wherein each pole is of a substantially constant cross section along its radial length.

10. Apparatus according claim 3 or claim 4, or any one of claims 5 to 9 when appended to claim 3 or claim 4, wherein at least one of the pole piece modules includes a pole portion projecting radially therefrom, and at least one of the poles of the rotor or stator is absent a corresponding pole portion, said pole piece module and said pole being arranged to be radially juxtaposed thereby providing relative location of the pole piece modules with respect to the rotor or stator.

11. A method of assembling an annular stator or rotor of an electrical machine including the steps of: a) forming a series of interconnected stator modules or rotor modules of the electrical machine, each stator or rotor module having an arcuate portion with a respective pole projecting radially therefrom; b) presenting a series of pre-wound coils to the series of stator or rotor modules and fitting each coil around a respective one of the poles; c) forming a series of interconnected pole piece modules; d) presenting the series of pole piece modules to the series of stator or rotor modules such that the pole piece modules are at the ends of the poles; e) forming the stator or rotor modules, the coils and the pole piece modules into an annulus, each pole piece lying at a radially inner end of a respective one of the poles; f) attaching the pole piece modules to the stator or rotor modules; wherein each pole piece module is joined to one or both respective juxtaposed pole piece modules by a bridge which, in use, is more readily saturable by a working flux of the electrical machine than is each pole piece.