Winding arrangement for an electrical machine

The present invention relates to windings for electrical machines, and in particular to a wave winding arrangement and a method of manufacture thereof.

Electrical machines can be categorized as motors or generators. The construction of electrical motors and generators is similar with many electrical machines being able to operate as both a motor and a generator.

Permanent magnet electrical machines have a winding that moves relative to the magnets. There are many different configurations of permanent magnet electrical machines. The winding may be part of the stator of the machine with the magnets attached to the rotor, or the winding may be part of the rotor with the magnets attached to the stator. Permanent magnet electrical machines may be brushed or brushless.

The geometry of an electrical machine can be categorized by the direction of the flux that the winding is exposed to. Axial flux machines have a disc shaped winding exposed to a magnetic flux that flows substantially parallel to the axis of rotation of the machine. The axial flux may be provided by two disc shaped sets of magnets with the winding positioned in the gap between the magnet sets. One of the sets of magnets may be replaced with iron only, to provide a return path for the magnetic field. In some applications there may be only one set of magnets without any iron on the other side of the winding. Radial flux machines have a cylindrically shaped winding exposed to a radially directed magnetic flux, which may be provided by two cylindrical sets of magnets, one cylindrical set of magnets and an iron cylinder to provide a return path for the magnetic field, or one cylindrical set of magnets alone. Similarly, conical flux machines have a conically shaped winding with magnets providing flux normal to the surface of the conical shaped winding. Linear electrical machines have a flat rectangular shaped winding that moves linearly relative to one or two rectangular sets of magnets that provide flux normal to the rectangular shaped winding.

The winding of a traditional electrical machine has conductors wound onto a laminated iron core. The iron core is primarily used to reduce the reluctance of the magnetic circuit, but it serves a secondary purpose of providing a supporting structure and thermal path for heat generated in the winding.

Ironless machines have windings without any ferromagnetic material. When optimised, an ironless machine can have extremely low spinning losses, no cogging torque, high peak torque and minimal torque ripple. Ironless machines find applications in servo drives, hard disk drives and have significant potential for use in vehicle drives or other applications that require low spinning losses and high transient torque. With the introduction of high energy rare earth permanent magnets, the ability to produce high performance ironless machines became a reality.

In order to obtain the maximum performance from an ironless machine, it is necessary to maximise the amount of conducting material, typically copper, in the winding. This is referred to as the “fill factor”. An electric machine has an active volume that is dedicated to be filled by the active portions of the winding conductors. In an axial flux ironless machine with two sets of magnets, this active volume is the volume between the faces of the two sets of magnets. The fill factor is defined as the proportion of the active volume taken up by conducting material.

In ironless machines, the conductors in the winding are exposed to the full magnetic field and so to avoid excessive eddy currents in the winding the conductors are typically made from fine strands of insulated copper wire. However, reducing the copper wire diameter to reduce eddy currents also makes it difficult to manufacture a winding with a high fill factor because of the thickness of the insulation on the wire and the gaps between the wire strands. Machines with poor fill factor require large magnetic content to achieve acceptable performance, and machines with poor fill factor typically suffer from poor thermal conductivity due to the excessive quantity of resin required to construct the winding.

“Litz wire” is a conductor made from a bundle or bundles of strands of fine copper wire. The wire strands have a thin film of insulation, and the entire bundle is surrounded by an insulating layer made from textile yarn, tape or other materials. Litz wire has several configurations, known as “types”. Type-1 Litz wire is comprised of a single bundle of strands of copper wire twisted together within the outer insulating layer. Type-2 Litz wire is comprised of multiple sub-bundles of strands of copper wire. The strands of each sub-bundle are twisted together, like Type-1 Litz wire, and the sub-bundles themselves are then twisted together within the outer insulating layer. Type-2 Litz wire does not have any additional insulating layers surrounding the sub-bundles. Type-3 Litz wire is also comprised of multiple sub-bundles of strands of copper wire twisted together within the outer insulating layer but unlike Type-2 Litz wire, each sub-bundle has its own outer insulating layer. The sub-bundles of Type-3 Litz wire may comprise a single twist of strands, like Type-1 Litz wire, or multiple twists of strands like Type-2 Litz wire. As supplied, Litz wire is flexible.

Windings for ironless machines using conductors made from Type-1 or Type-2 Litz wire are known, but use of Type-3 Litz wire is not known. The paper titled “Design of an in-wheel motor for a solar-powered electric vehicle” by V. S. Ramsden, B. C. Mecrow and H. C. Lovatt, published in the IEE Proceedings, “Electric Power Applications”, 1998, 145, No 5, 402-408, discloses an ironless direct drive wheel motor with a winding made from Type-2 Litz wire. International Patent Publication WO 2004/010561 A1 (Hamilton Sundstrand) also discloses the use of Litz wire to make a winding for an electric machine, and shows an example using Type-2 Litz wire.

Many methods of manufacturing windings for ironless machines have been proposed or used. U.S. Pat. No. 5,744,896 (Kessinger et al) discloses a method of manufacturing a winding for an ironless machine in which the winding is made up of a large number of individually wound coils of wire strands. The coils are assembled in a mould and wired together. The disadvantages of this method are that each coil must be individually electrically connected and small variations between each coil affect the overall performance of the machine.

An alternative to individually wound coils is to arrange the conductors of the winding into a wave pattern. The conductors of a wave pattern winding are wave shaped, meaning that each conductor progresses around the winding, periodically crossing the magnetic field, without looping back on itself. Each wave shaped conductor has a plurality of legs that are the active part of the conductor directly exposed to the magnetic field and end turns that connect the legs. It is desirable to minimise the length of the end turns in order to minimise resistance losses in the machine, and to minimise the weight and size of the winding. Compact end turns can be difficult to achieve in a multiphase wave pattern winding because the end turns overlap each other.

Some wave pattern windings have two layers, with the legs of each conductor alternating between the two layers. U.S. Pat. No. 5,319,844 (Huang et al) discloses a radial air gap motor having a two layer wave pattern winding. However, this motor has flux concentrators positioned in the gaps between the legs of the wave shaped conductors, rather than being a purely ironless motor.

The winding of the motor disclosed in the above referenced paper “Design of an in-wheel motor for a solar-powered electric vehicle” is manufactured by arranging Litz wire into a two layer wave pattern then bonding the Litz wire with resin in a mould to form a rigid structure. The wave pattern is formed by winding the Litz wire directly in the mould, or winding the Litz wire onto a temporary former, such as a board with pegs, then transferring the wave pattern into the mould. Whilst this method of manufacturing a winding is well suited to light weight solar-powered vehicles, a winding having a higher copper fill factor than this prior art method provides is desirable for higher power applications.

U.S. Pat. No. 6,649,844 (Kusumtoto et al) discloses a method of preforming a wave shaped conductor from a bundle of strands of copper wire. The end turns of the wave shaped conductors disclosed in this patent are straight and lie in the plane of the wave shape, resulting in a square wave shaped conductor. These square wave shaped conductors are arranged into a wave pattern with large gaps between adjacent conductor legs. This method of forming a wave pattern winding is directed to electric machines having an iron core to fill the large gaps between the conductor legs of the winding. However, this method is not well suited to manufacturing ironless wave pattern windings because the square wave shaped conductors result in an inefficient packing of the end turns once the gap between the legs of the conductors is reduced, as is necessary for an efficient ironless winding arrangement.

It is an object of the present invention to provide an improved winding arrangement for an electric machine.

In a first aspect, the present invention consists of a winding for an electrical machine, comprising at least one conductor, the conductor comprising multiple strands of wire and an outer insulating layer, and having a wave shape comprising a plurality of legs connected by end turns, with at least a portion of at least one end turn following a curved path, characterised in that the cross section of the end turn at least one point on the curved path, viewed normal to the curved path, is thicker in a radial direction than an axial direction, the radial and axial directions being with respect to the instantaneous centre of curvature of the curved path at the point.

In a second aspect, the present invention consists of a preformed conductor for use in the manufacture of a winding for an electrical machine, the conductor comprising multiple strands of wire and an outer insulating layer, the conductor having a self supporting wave shape comprising a plurality of legs connected by end turns, characterised in that at least a portion of a least one end turn follows a curved path.

In a third aspect, the present invention consists of a method of manufacturing a winding for an electrical machine, the winding comprising at least two conductors arranged into a wave pattern, each conductor comprising multiple strands of wire and an outer insulating layer, the method comprising preforming the conductors such that they each have a self supporting wave shape, the wave shape comprising a plurality of legs connected by end turns, characterised in that the method further comprises weaving the preformed conductors together to form the wave pattern.

In a fourth aspect, the present invention consists of a method of manufacturing an ironless winding for an electrical machine, comprising arranging at least one conductor, having multiple strands of wire and an outer insulating layer, into a wave pattern then bonding the wave pattern to form a rigid structure, characterised in that, prior to arranging the wave pattern, each conductor is preformed, from a flexible state, such that each preformed conductor has a self supporting wave shape.

Some further variations of the invention are defined in the Claims.

FIG. 1 is a cross sectional view of an axial flux electric motor, having an ironless stator winding, in accordance with the present invention, manufactured by a method in accordance with the present invention.

FIG. 2 shows the winding of the motor shown in FIG. 1 ;

FIG. 3 is a sectioned view of the winding shown in FIG. 2 ;

FIG. 4 schematically shows the wave pattern of the winding shown in FIG. 2 ;

FIG. 5 schematically shows a pair of wave shaped conductors of the winding shown in FIG. 2 ;

FIG. 6 schematically shows a single wave shaped conductor of the winding shown in FIG. 2 ;

FIG. 7 shows a single wave shaped conductor of the winding shown in FIG. 2 ;

FIG. 8 is an enlarged view of part of the wave shaped conductor shown in FIG. 7 ;

FIG. 9 is a sectional view of a length of Type-3 Litz wire;

FIGS. 10 and 11 show two halves of a die set used to preform the conductors of the winding shown in FIG. 2 ;

FIG. 12 shows the die set of FIGS. 10 and 11 mounted in a press;

FIGS. 13 and 14 show how Litz wire is incrementally fed through the die set of FIGS. 10 and 11 to preform the wave shaped conductor shown in FIG. 7 ;

FIG. 15 shows the wave shaped conductor of FIG. 7 laid out straight;

FIG. 16 shows the wave shaped conductors being weaved together to form the winding shown in FIG. 2 ;

FIG. 17 is a cross sectional view of a second embodiment of an axial flux electric motor, having a tapered ironless stator winding, in accordance with the present invention, manufactured by a method in accordance with the present invention;

FIG. 18 is a partial side view of the winding of the motor shown in FIG. 17 ;

FIG. 19 is a schematic side view of the winding shown in FIG. 2 ;

FIG. 20 is a side view of one outer end turn of the wave shaped conductor shown in FIG. 8 , viewed along XX-XX of FIG. 8 ;

FIG. 21 schematically shows a wave pattern winding arrangement in accordance with the present invention in which the legs of the conductors are skewed to the radial direction; and

FIG. 22 is a cross sectional view of a radial flux electric motor, having an ironless stator winding, in accordance with the present invention, manufactured by a method in accordance with the present invention.

The present invention will be described in detail primarily as applied to axial flux motors but the method of manufacturing the winding described, and the resulting winding arrangement, is equally applicable to other configurations of electrical machines, such as radial or conical flux machines, generators, and linear machines, regardless of whether the winding forms the stator or the rotor, or the machine has brushes or is brushless.

Referring to FIG. 1 , axial flux electric motor 1 comprises a disc shaped ironless stator 2 positioned between two disc shaped sets of permanent magnets 4 that are mounted on backing plates 5 . Backing plates 5 are attached to a hub 6 that is supported for rotation around stub axle 9 by bearings 10 . Magnets 4 , backing plates 5 and hub 6 make up the rotor 3 of motor 1 . The stub axle 9 is attached to the motor housing 8 , and the stator 2 is attached to the housing 8 by a clamp plate 7 . Motor 1 can be used as a direct drive wheel motor for a vehicle by attaching the motor housing 8 to the suspension of the vehicle, and mounting a wheel onto the motor hub 6 .

The stator 2 is comprised of a wave pattern winding 13 bonded together by encapsulating it in a resin 14 to form a rigid structure. Referring to FIG. 2 , winding 13 is made up of six wave shaped conductors 15 a , 15 b , 15 c , 15 d , 15 e and 15 f . Motor 1 is a 3-phase motor with the conductors 15 a - f arranged as three pairs 15 a and 15 b , 15 c and 15 d , and 15 e and 15 f . Each pair of conductors is connected in series or parallel such that each pair of conductors is electrically in phase. Each conductor 15 a - f makes one turn of the winding 13 such that all the conductors 15 a - f enter and exit the winding 13 at approximately the same location on the outer circumference of the winding 13 .

As shown in FIG. 3 , winding 13 has two layers 16 a and 16 b . FIG. 4 schematically shows how the wave pattern of winding 13 is arranged. Each conductor 15 a - f is shown as a solid line when it is in the front layer 16 a , and a dashed line when it is in the back layer 16 b . FIG. 5 schematically shows a single pair of conductors 15 a - b . Each conductor 15 a - f has the same shape and FIG. 6 schematically shows the shape of a single conductor 15 a . Conductor 15 a has a wave shape, in that it never loops back on itself as it progresses around the winding 13 , with radially extending legs 17 connected by end turns 18 a and 18 b . As conductor 15 a progresses around winding 13 , its legs 17 alternate between the two layers 16 a and 16 b . To clearly show how the wave pattern is arranged, in FIGS. 4 and 5 the legs 17 of each pair of conductors 15 a - b , 15 c - d and 15 e - f are schematically shown as slightly circumferentially offset from each other. In the actual winding 13 , the legs 17 of each pair of conductors 15 a - b , 15 c - d and 15 e - f are circumferentially aligned with each other.

FIG. 19 schematically shows a partial side view of winding 13 . Referring to FIGS. 4 , 5 , 6 and 19 , as each conductor 15 a - f progresses clockwise around the winding 13 , the outer end turns 18 a always progress from the front layer 16 a to the back layer 16 b , and the inner end turns 18 b always progress from the back layer 16 b to the front layer 16 a . As such all of the outer end turns 18 a are skewed in the same direction with respect to the axis of the motor and never cross over each other. Likewise, all the inner end turns 18 b are skewed in the same direction, opposite to that of the outer end turns 18 a.

FIGS. 7 and 8 show an actual wave shaped conductor 15 a of winding 13 . The remaining conductors 15 b - f are the same shape as conductor 15 a . Referring to FIG. 8 , the legs 17 of conductor 15 a have a rectangular cross section as indicated by the broken outline 23 . The corners of the rectangular section are shown as being sharp for clarity of description. In practice the corners may be rounded with the cross sectional shape of legs 17 then being approximately rectangular. The use of a rectangular cross sectional shape of legs 17 results in winding 13 having a significantly higher fill factor, than if the winding was made with legs having a substantially circular cross section.

Referring to FIG. 8 , each outer end turn 18 a of conductor 15 a is curved such that it follows a three dimensional curved path as indicated by broken line 19 a . The curved path 19 a is helical about axis 24 a . FIG. 20 is a side view of one outer end turn 18 a of conductor 15 a , which shows that axis 24 a is approximately parallel to, and mid way between layers 16 a and 16 b . Referring again to FIG. 8 , similarly, each inner end turn 18 b is curved such that it follows a three dimensional curved path as indicated by broken line 19 b . The curved path 19 b is helical about axis 24 b . Axis 24 b is also approximately parallel to, and mid way between layers 16 a and 16 b.

The end turns 18 a and 18 b are flattened as will be described by reference to the cross sectional shape at a single point on each end turn, as follows. Broken outline 22 a shows the cross section of an outer end turn 18 a at a single point 46 a on the curved path 19 a , viewed normal to the curved path 19 a . Point 47 a is the instantaneous centre of curvature of path 19 a at point 46 a . Reference numeral 21 a indicates the thickness of cross section 22 a in a radial direction with respect to the instantaneous centre of curvature 47 a , and reference numeral 20 a indicates the thickness in an axial direction with respect to centre 47 a . The radial direction is defined by a line through points 46 a and 47 a , and the axial direction is defined as being perpendicular to the radial direction and to the tangent direction of path 19 a at point 46 a . Cross section 22 a is approximately elliptical in shape such that its radial thickness 21 a is thicker than its axial thickness 20 a.

Similarly, broken outline 22 b shows the cross section of an inner end turn 18 b at a single point 46 b on the curved path 19 b , viewed normal to the curved path 19 b . Point 47 b is the instantaneous centre of curvature of path 19 b at point 46 b . Reference numeral 21 b indicates the thickness of cross section 22 b in a radial direction with respect to the instantaneous centre of curvature 47 b , and reference numeral 20 b indicates the thickness in an axial direction with respect to centre 47 b . The radial direction is defined by a line through points 46 b and 47 b , and the axial direction is defined as being perpendicular to the radial direction and to the tangent direction of path 19 b at point 46 b . Cross section 22 b is approximately elliptical in shape such that its radial thickness 21 b is thicker than its axial thickness 20 b.

The shape of the end turns 18 a and 18 b results in a compact end turn arrangement with relatively short end turns compared with other wave pattern windings. The helical path combined with the flattening of the end turns allows the end turns to closely nest together. Each end turn 18 a , 18 b must circumferentially overlap its adjacent end turns 18 a , 18 b for a significant proportion of its length, and the relatively small axial thickness 20 a , 20 b of the end turns 18 a , 18 b minimises the overall axial thickness of winding 13 at the end turns.

The wave shaped conductors 15 a - f are made from Type-3 Litz wire. FIG. 9 is a sectional view of a length of Type-3 Litz wire 25 . Litz wire 25 has many copper wire strands 26 and an outer insulating layer 27 made from textile yarn, tape, or other suitable material. The wire strands 26 are arranged as multiple sub-bundles 28 twisted together, with each sub-bundle 28 having its own outer insulating layer 29 . Each sub-bundle 28 is made up of multiple smaller sub-bundles 30 twisted together, with each smaller sub-bundle 30 being made up of multiple wire strands 26 twisted together. The wire strands 26 typically have a thin insulating coating, but it is necessary for the sub-bundles 28 to have outer insulating layers 29 to ensure they are electrically separated, as the thin insulating coating is easily broken. Litz wire is flexible, even though it may not fully spring back if bent sharply. Litz wire is usually supplied as a roll. As an example, the copper wire strands may have a diameter of 0.3 to 0.4 mm.

FIGS. 10 and 11 show two halves 33 , 34 of a die set used to preform the wave shaped conductors 15 a - f from Type-3 Litz wire 25 . One half 33 of the die set has a punch 35 and the other half 34 has a cavity 36 . When the die sets halves 33 and 34 are placed together there is a gap between the punch 35 and the cavity 36 that has the shape of four legs 17 , two inner end turns 18 b , and one outer end turn 18 a of a conductor 15 a - f.

FIG. 12 shows the die halves 33 , 34 mounted in a simple press 39 that has a cylinder 40 to open and close the die set. FIGS. 13 and 14 show how Litz wire 25 is incrementally fed through the die halves 33 , 34 to preform the wave shaped conductor 15 a . Referring to FIG. 13 , a portion of the Litz wire 25 is bent and placed in the die cavity 36 . The die halves 33 , 34 are then closed and pressurised together by cylinder 40 such that the original substantially circular cross section of the portion of the Litz wire 25 in the cavity 36 is deformed and compacted to preform a portion of the wave shaped conductor 15 a . Referring to FIG. 14 , the Litz wire 25 is then incremented through the die cavity 36 to preform the next portion of the wave shaped conductor 15 a . Even though the die cavity 36 extends for approximately two wavelengths of the wave shaped conductor 15 a , the Litz wire 25 is only incremented by one wavelength between pressings to ensure correct spacing of the wave shape.

As a result of pressing the Litz wire 25 as it is preformed into a wave shape, the wave shape of conductor 15 a is substantially self supporting in that it will maintain its wave shape without needing to be supported, as it is handled. The die cavity 36 and punch 34 are shaped to curve the conductor 15 a as it is preformed, with a radius equal to that of the final winding 13 . However, the self supporting preformed wave shaped conductor 15 a is still flexible to an extent that it can be laid out straight as shown in FIG. 15 .

To avoid the Litz wire 25 springing back after pressing, significant compaction pressure is required to permanently compact and deform the Litz wire 25 into the shape of the rectangular sectioned legs 17 and the arcuate end turns 18 a and 18 b . However, the compaction pressure should not be excessive or the Litz wire 25 may be damaged. As an example, a compaction pressure of the order of 10 MPa may be used.

The process of incrementally feeding the Litz wire 25 through the die set 33 , 34 may be automated. In other not shown embodiments of the invention, a die set may be used that is large enough to preform the entire length of a wave shaped conductor in one pressing. Alternatively, the wave shaped conductors may be preformed by using rollers or a machine similar to a CNC spring bending machine to press the Litz wire.

To manufacture the complete winding 13 , the six identical wave shaped conductors 15 a - f are individually preformed as described above. The conductors 15 a - f are then progressively woven together as shown in FIG. 16 . This is made possible by the conductors 15 a - f retaining a degree of flexibility after they are preformed. Optionally, one or more threads may also be woven into the winding 13 to assist with tying the winding 13 together.

The winding 13 is then placed in a mould (not shown). A vacuum is applied to the mould and resin 14 is pressurised into the winding 13 to encapsulate it and bond it together, thereby forming the stator 2 . The mould may have several pegs that protrude into the gaps between the conductor legs 17 to ensure that the conductors 15 a - f are properly circumferentially aligned. Optionally, the winding 13 may be pressed after it is placed in the mould to further compact it.

Winding 13 has a high fill factor compared with other known windings made from Litz wire, such as the motor disclosed in the above referenced paper “Design of an in-wheel motor for a solar-powered electric vehicle”, primarily due to the rectangular cross section of the legs 17 of the wave shaped conductors 15 a - f , and the compaction of the Litz wire 25 as it is pressed in the die set 33 , 34 . The fill factor of a winding manufactured as described above will typically be in the range of 40% to 70%.

Windings in accordance with the present invention may be made from multi-strand conductors other than Litz wire. However, an advantage of using conductors made from Litz wire compared with using a conductor with multiple wire strands that are not twisted together, is that the twisted arrangement of the Litz wire means that each wire strand is near the surface of the conductor at various points along its length, allowing it to conduct heat to the surface of the conductor at those points. The overall thermal performance of the winding is therefore improved.

An advantage of using conductors made from Type-3 Litz wire 25 rather than Type-1 or Type-2 Litz wire is that the insulated sub-bundles 28 can be connected in series. This means that a single turn of Type-3 Litz wire 25 around the winding can provide multiple electrical turns, and therefore it is not necessary to have multiple physical turns of a conductor to provide multiple electrical turns, which makes the winding easier to manufacture. Furthermore, it is possible to actively switch the connection of the sub-bundles 29 between series and parallel.

FIG. 17 shows a second embodiment of an axial flux electric motor 1 a in accordance with the present invention. Motor 1 a is the same as motor 1 shown in FIG. 1 except that the stator 2 a and its winding 13 a are tapered such that the active portion of the winding 13 a in the axial gap between magnets 4 a becomes thinner as the radius increases. The magnet sets 4 a have a corresponding conical shape such that the same small gap is maintained between the magnets 4 a and the stator 2 a as the radius varies.

Referring to FIG. 18 , the legs 17 a of each wave shaped conductor 15 aa - af of winding 13 a are tapered in the plane of the wave shape of conductors 15 aa - af such that they become thinner as the radius decreases. This is illustrated by the thickness indicated by reference numeral 42 being less than the thickness indicated by reference numeral 43 .

Reference numeral 44 indicates the gaps between adjacent legs 17 a . The taper of the legs 17 a in the plane of the wave shaped conductors 15 aa - 15 af minimises the gaps 44 between the legs 17 a . As shown in FIG. 18 , the gap 44 does not vary with the radial distance from the centre of the winding 13 a , and in practice the adjacent legs 17 a may contact each other such that there is no gap. In contrast, if the legs have a constant thickness, such as legs 17 of motor 1 described above, then the gap between the legs will increase as radial distance from the centre of the winding increases. This minimisation of the gaps 44 between the legs 17 a provided by the tapering further improves the fill factor.

Like the motor shown in FIG. 1 , the wave shaped conductors 15 aa - af of motor 1 a are made from Type-3 Litz wire 25 . The substantially constant cross sectional area of Litz wire 25 results in the Legs 17 a also tapering in a direction transverse to the wave shape such that they become thinner as the radius increases, thus resulting in the tapering of the stator 2 a and winding 13 a shown in FIG. 17 .

The winding 13 a is manufactured using the same method as described for motor 1 above. The only difference is that the punch 35 and cavity 36 of the die set 33 , 34 are shaped differently to preform the tapered shape of legs 17 a.

Whilst the present invention is primarily directed to the manufacture of ironless windings, it is also applicable to windings having flux concentrators. These flux concentrators are made from a ferro-magnetic material, such as iron, and can be placed in the gaps between the legs of the wave shaped conductors after they have been weaved together, but before bonding the winding. Metglass may also be used for the flux concentrators.

Whilst the embodiments of the axial flux motor described above have windings in which the legs of the wave shaped conductors are radially oriented with respect to the centre of the winding, other not shown embodiments may have windings in which the conductor legs are skewed to the radial direction. In this case, each pair of conductor legs will tend to cross each other as viewed in the plane of the winding, however each pair of legs can still be considered as being circumferentially aligned together. This alternative winding arrangement is schematically shown in FIG. 21 . FIG. 21 shows a wave pattern winding 13 b , in a similar manner to FIG. 4 . The legs 17 b of each conductor 15 ba to 15 bf are skewed to the radial direction, and the legs 17 b of each pair of conductors 15 ba and 15 bb , 15 bc and 15 bd , and 15 be and 15 bf cross over each other. The performance of the winding 13 b is reduced by having the legs 17 b of the conductors 15 ba to 15 bf skewed but this is compensated for by the resulting reduced length of the end turns 18 ba , 18 bb , compared with the arrangement shown in FIG. 4 .

FIG. 22 is a cross sectional view of a radial flux electric motor 1 c , comprising a cylindrical ironless stator 2 c positioned between two cylindrical sets of permanent magnets 4 c that are mounted on backing cylinders 5 c . The inner backing cylinder 5 c is mounted on a hub 6 c , and the outer backing cylinder 5 c is mounted on a separate carrier 49 that is bolted to the hub 6 c . Magnets 4 c , backing plates 5 c , hub 6 c and carrier 49 make up the rotor 3 c of motor 1 c . The hub 6 c is supported for rotation around stub axle 9 c by bearings 10 c . The stub axle 9 c is attached to the motor housing 8 c , and the stator 2 c is attached to the housing 8 c by a clamp plate 7 c . Motor 1 c can be used as a direct drive wheel motor for a vehicle by attaching the motor housing 8 c to the suspension of the vehicle, and mounting a wheel onto the motor hub 6 c . The stator 2 c is comprised of a wave pattern winding 13 c bonded together by encapsulating it in a resin 14 c to form a rigid structure.

The cylindrical wave pattern winding 13 c is similar in construction to the disc shaped winding 13 shown in FIG. 2 . Winding 13 c is comprised of six wave shaped conductors that are preformed and woven together in the same manner as winding 13 , to form a winding pattern having two cylindrical layers. The end turn regions 50 a and 50 b of the stator 2 c must be thicker than the active region of the stator 2 c , between the magnets 4 c , to accommodate the additional radial thickness at the end turns of the winding 13 c . To allow assembly of motor 1 c , end turn region 50 a is biased radially inwards, and end turn region 50 b is biased radially outwards. The end turns of the wave shaped conductors used to make up winding 13 c are preformed into a three dimensional curved shape to suit this biasing of the end turn regions 50 a , 50 b . Motor 1 c is assembled by fitting the hub 6 c , with the inner set of magnets 4 c attached, onto the stub axle 9 c , then sliding the stator 2 c over the inner set of magnets 4 c and clamping it to the housing 8 c . Then, the carrier 49 , with the outer set of magnets 4 c attached, is slid over the stator 2 c and bolted to the hub 6 c.

In other not shown embodiments of the invention, Litz wire with a heat bondable coating may be used so that the winding arrangement can be bonded together prior to final encapsulation in resin.

In other not shown embodiments of the invention, the winding may be bonded by methods other than using a vacuum mould to encapsulate it in a resin. For example, the winding may be bonded and encapsulated by injection moulding. Materials other than resin may also be used, such as thermoplastics or polyurethane.

In other not shown embodiments of the invention, two preformed wave shaped conductors may be formed on the one continuous length of Litz wire. This may reduce the number of connections that need to be made at the ends of the wave shaped conductors.

The term “comprising” as used herein is used in the inclusive sense of “including” or “having” and not in the exclusive sense of “consisting only of”.