Stator assembly and method of assembly

Abstract

A stator assembly for an electric machine comprises a stator core 13 having a plurality of teeth14-n. On each tooth is disposed a first winding 25-n comprising at least one first coil; and a second winding 30-n comprising at least one second coil. Each first coil comprises a first, formed electrical conductor. The formed first conductors may be inserted axially during a stage of manufacture. Each second coil comprises a second electrical conductor formed of multistrand wire, for example Litz wire. The second winding may be installed in a second operation. The first and second coils may be concentrated coils. The arrangement addresses the problem of flux leakage in the stator (250, fig.7) and hence offers reduced power losses in a motor for an electric vehicle.

Description

TECHNICAL FIELD The present disclosure relates to a stator assembly and method of assembly. Aspects of the invention relate to an assembly, an electric machine, a vehicle and a method of assembly a stator. BACKGROUND A challenge of the automotive industry is to extend the range of electrified vehicles. Many factors can be considered in order to increase the available range. One such factor is the reduction of powertrain losses. One such loss associated with a vehicular powertrain is the loss produced in the electric motor windings. Solutions can be considered to reduce secondary mechanisms of loss, such as proximity losses. Losses in an electric machine occur due to flux leakage in the stator. Known solutions to flux leakage, such as using a multistrand winding may reduce slot fill. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a stator assembly; an electric machine; a vehicle; and a method of assembling a stator assembly as claimed in the appended claims. According to an aspect of the present invention there is provided a stator assembly for an electric machine, the stator assembly comprising a stator core comprising a plurality of teeth, the plurality of teeth each comprising a first winding comprising at least one first coil; and a second winding comprising at least one second coil, wherein the at least one first coil comprises a first electrical conductor; the at least one second coil comprises a second electrical conductor; the first electrical conductor comprises a formed electrical conductor; and the second electrical conductor comprises a multistrand wire. According to another aspect of the present invention there is provided a stator assembly for an electric machine, the stator assembly comprising a stator core comprising a plurality of teeth, the plurality of teeth each comprising a first winding comprising at least one first coil; and a second winding comprising at least one second coil, wherein the at least one first coil comprises a first electrical conductor; and the at least one second coil comprises a second electrical conductor, and the first electrical conductor comprises a single core wire, and the second electrical conductor comprises a multistrand wire. The at least one first coil and the at least one second coil may be co-axial. The first coil may disposed in a first region of each of the plurality of teeth; and the second coil may be disposed in a second region of each of the plurality of teeth; and wherein the first and second regions may be radially offset from each other. The second region may be radially inset from the first region. The or each first coil may be disposed at or proximal to a base of each of the plurality of teeth; and the or each second coil may be disposed at or proximal to a distal end of each of the plurality of teeth. The first 1 coil may have a first cross-sectional area in transverse section; and the second coil may have a second cross-sectional area in transverse section; wherein the first cross-sectional area may be larger than the second cross-sectional area. Each of the plurality of teeth may comprise a crown. The first coil may be a concentrated coil or a distributed coil, and the second coil may be a concentrated coil or a distributed coil. According to a further aspect of the present invention there is provided an electric machine comprising a stator assembly as described herein. According to a further aspect of the present invention there is provided a vehicle comprising one or more electric machines as described herein. According to a further aspect of the present invention there is provided a method of assembling a stator core having at least a first tooth, the method comprising forming a first winding on the stator core, the first winding comprising at least one first coil disposed on the first tooth; and forming a second winding on the stator core, the second winding comprising at least one second coil disposed on the first tooth; wherein the second winding is formed on the stator core after forming the first winding on the stator core. The first winding may comprise at least one formed electrical conductor, the method may comprise inserting the at least one formed electrical conductor in an axial direction. The second winding may comprise at least one wire, the method may comprise winding the at least one wire around the first tooth. The method may comprise forming the first coil at or proximal to a base of the first tooth; and forming the second coil at or proximal to a distal end of the first tooth. According to a further aspect of the present invention there is provided a stator assembly for an electric machine, the stator assembly comprising a stator core comprising a plurality of teeth; a first winding comprising at least one first coil, the first coil being a first concentrated coil; and a second winding comprising at least one second coil, the second coil being a second concentrated coil; the stator assembly comprising a plurality of stator poles, the stator poles each being associated with a respective one of the plurality of teeth; wherein one or more of the stator poles comprises: one or more of the first coils and one or more of the second coils. The first winding may be composed of a first electrical conductor and the second winding may be composed of a second electrical conductor. The first electrical conductor may comprise a formed electrical conductor or a wire having a single core. The second electrical conductor comprises a multistrand wire. The first electrical conductor may have a first cross-sectional area and the second electrical conductor may have a second cross-sectional area; wherein the first cross-sectional area may be greater than the second cross-section area. According to a further aspect of the present invention there is provided an electric machine comprising a stator assembly as described herein. According to a further aspect of the present invention there is provided a vehicle comprising one or more electric machines as described herein. According to a further aspect of the present invention there is provided a method of assembling a stator core having a plurality of teeth, the method comprising forming a first winding on the stator core, the first winding comprising a first distributed coil; and forming a second winding on the stator core, the second winding comprising a second distributed coil; wherein: the first distributed coils are distributed across two or more of the teeth of the stator core; the second distributed coils are distributed across the same two or more teeth of the stator core as the first distributed coils; and the second winding is formed on the stator core after forming the first winding on the stator core. The second winding may comprise at least one wire. The wire forming the second winding may be inserted radially into a stator slot through a slot opening. The first coil(s) and the second coil(s) may be disposed on the same tooth. The teeth form a plurality of slots in the stator core. The first coil(s) and the second coil(s) may both be disposed in one or more of the slots. The first coil and the second coil may be co-axial. The first coil and the second coil may be arranged co-axially on one of the plurality of teeth. The axis of the first coil and the second coil may be aligned with a central axis of the associated tooth. The first concentrated coil may comprise a plurality of coils arranged co-axially. The second concentrated coil may comprise a plurality of coils arranged co-axially. The coils in the first and second concentrated coils may be arranged concentrically. The first winding may be composed of a first electrical conductor. The second winding may be composed of a second electrical conductor. The first electrical conductor may be different from the second electrical conductor. The first electrical conductor may comprise or consist of a formed electrical conductor or a wire having a single core. The formed electrically conductor may be installed on the stator core in an axial direction. The first electrical conductor may have cross-section which is circular, elliptical or rectangular. The first electrical conductor may have a first cross-sectional area. The second electrical conductor may have a second cross-sectional area. The first cross-sectional area may be larger than the second cross-sectional area. The first electrical conductor may be a “high-fill” conductor which offers improved filling of the stator slot (compared to the second electrical conductor). The second electrical conductor may comprise or consist of a multistrand wire. The second electrical conductor may be a “low-loss” conductor which offers reduced flux leakage (compared to the first electrical conductor). The second winding may be disposed in the stator core proximal to the air gap. At least in certain embodiments, this may reduce flux leakage in a radially inner portion of the stator assembly (closest to the rotor in the assembled electric motor). The first coil and the second coil may be radially offset from each other. The second coil may be disposed radially inwardly of the first coil. The second electrical conductor may comprise or consist of an electrically conductive wire. The wire may have a cross-section which is circular, elliptical or rectangular. The wire may be wound around one or more of the plurality of teeth. The wire may be installed on the stator tooth in a radial direction. The wire may consist of a single core. Alternatively, the wire may be a multistrand wire comprising a plurality of strands. The multistrand wire may form a low-loss multistrand winding. The wire may, for example, comprise a plurality of film-insulated strands. The strands may, for example, be twisted or braided. The strands may be compressed or compacted, for example into a rectangular profile. The first electrically conductive wire may comprise a Litz wire, for example a twisted, braided or compacted Litz wire. The stator assembly may comprise a third winding comprising at least one third concentrated coil. The third concentrated coil may be disposed on the tooth. According to a further aspect of the present invention there is provided a stator assembly for an electric machine, the stator assembly comprising a stator core comprising a plurality of teeth; and a first winding comprising at least one first coil and a second winding comprising at least one second coil, the first coil and the second coil being disposed on one or more of the plurality of teeth. The first winding may be composed of a first electrical conductor and the second winding may be composed of a second electrical conductor. The first electrical conductor may comprise or consist of a formed electrical conductor or a single core wire. The second electrical conductor may comprise or consist of a multistrand wire. The first coil and the second coil may be disposed on the same tooth. The first coil may comprise or consist of a first concentrated coil. Alternatively, the first coil may comprise or consist of a first distributed coil. The second coil may comprise or consist of a second concentrated coil. Alternatively, the second coil may comprise or consist of a second distributed coil. The first electrical conductor has a first cross-sectional area and the second electrical conductor has a second cross-sectional area. The first cross-sectional area may be greater than the second cross-section area. The teeth in the stator core may extend in a substantially radial direction. The first coil may be disposed in a first region of each tooth. The second coil may be disposed in a second region of the tooth. The first and second regions may be radially offset from each other. The first coil may be offset from the second coil in a radial direction. The second region may be radially inset from the first region. The first region may be disposed proximal to a base or root of the tooth. The second region may be disposed at or proximal to a tip of the tooth. The second region may be disposed proximal to a slot opening. The second coil may be disposed radially inwardly of the first coil. The first coil may be disposed at or proximal to a base of each tooth. The second coil may be disposed at or proximal to a distal end of each tooth. The first coil may comprise a first central axis. The second coil may comprise a second central axis. The first and second central axes may be aligned with each other such that the first and second coils are arranged co-axially. The first and second coils may be co-axial with a radial axis of an associated tooth of the stator. The first concentrated coil may have a first cross-sectional area in transverse section. The second concentrated coil may have a second cross-sectional area in transverse section. The first cross-sectional area may be larger than the second cross-sectional area. The transverse section of the first concentrated coil and / or the second concentrated coil may extend substantially perpendicular to a radius of the stator core. According to a further aspect of the present invention there is provided a stator assembly for an electric machine, the stator comprising: a stator core comprising a plurality of teeth; a first winding comprising at least one first concentrated coil; and a second winding comprising at least one second concentrated coil; wherein the first and second windings are configured such that the first concentrated coil or one of the first concentrated coils and the second concentrated coil or one of the second concentrated coils are disposed on one of the plurality of teeth. The first and second windings may be configured such that the first concentrated coil or one of the first concentrated coils and the second concentrated coil or one of the second concentrated coils are arranged to form a set, the first and second concentrated coils in each set may be disposed on one of the plurality of teeth. The or each set may comprise or consist of one first concentrated coil and one second concentrated coil. The first concentrated coil and the second concentrated coil may form a pair. The first and second concentrated coils in each pair being disposed on one of the plurality of teeth. According to a further aspect of the present invention there is provided a stator assembly for an electric machine, the stator assembly comprising: a first winding comprising a plurality of first coils, the first coils being first distributed coils; a second winding comprising a plurality of second coils, the second coils being second distributed coils; wherein the first distributed coils and the second distributed coils are radially offset from each other. For example, the second distributed coils may be disposed radially inwardly of the first distributed coils. The stator assembly may comprise a plurality of stator poles. The stator poles may each be composed of axials portion of two of the first distributed coils and axial portions of two of the second distributed coils. The first winding may be composed of a first electrical conductor and the second winding may be composed of a second electrical conductor. The first electrical conductor may comprise or consist of a formed electrical conductor. The second electrical conductor may comprise or consist of a multistrand wire. According to a further aspect of the present invention there is provided an electric machine comprising a stator assembly as described herein. According to a further aspect of the present invention there is provided a vehicle comprising one or more electric machines as described herein. According to a further aspect of the present invention there is provided a method of assembling a stator core having at least a first tooth; the method comprising: forming a first winding on the stator core, the first winding comprising at least one first coil disposed on the first tooth; and forming a second winding on the stator core, the second winding comprising at least one second coil disposed on the first tooth; wherein the second winding is formed on the stator core after forming the first winding on the stator core. The first coil may comprise a first concentrated coil or a first distributed coil. The second coil may comprise a second concentrated coil or a second distributed coil. At least in certain embodiments, the first coil may comprise a first concentrated coil and the second coil may comprise a second concentrated coil. The first winding may comprise at least one formed electrical conductor. The method may comprise inserting the at least one formed electrical conductor in an axial direction. The first winding may comprise at least one wire. The method may comprise winding the at least one wire around the first tooth. The method may comprise forming the first concentrated coil at or proximal to a base of the first tooth. The method may comprise forming the second concentrated coil at or proximal to a distal end of the first tooth. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of a vehicle comprising an electric drive unit having an electric machine in accordance with an embodiment of the present invention; 6 Figure 2 shows a longitudinal sectional view of the electric machine shown in Figure 1; Figure 3 shows a longitudinal sectional view of the electric machine shown in Figure 1; Figure 4 shows a schematic representation of a quadrant of a stator of the electric machine in accordance with an embodiment of the present invention; Figure 5 shows a block diagram illustrating a method of assembly of the stator assembly in accordance with an embodiment of the present invention; Figure 6 shows a schematic representation of a quadrant of a stator of an electric machine in accordance with a further embodiment of the present invention; and Figure 7 illustrates a flux leakage effect in a conventional stator of an electric machine. DETAILED DESCRIPTION Losses in an electric machine can occur due to flux leakage in the stator. A section of a stator 204 of an electric machine 201 is shown in Figure 7. The stator 204 comprises a plurality of teeth 214 extending radially inwardly. The stator 204 comprises a winding configured to form a plurality of stator poles 221-n. The winding comprises a plurality of coils 235-n disposed in respective stator slots 219-n. The coils 235-n may be concentrated coils or distributed coils. A series of normal flux lines are represented by dashed lines 245 in Figure 7. The normal flux lines extend in a substantially radial direction through the teeth 214-n. In addition to the normal flux lines 245 in the teeth 214-n, flux leakage occurs the stator slots 219-n. The flux leakage is represented by continuous lines 250 in Figure 7. The flux leakage across the stator slots 219-n may result in losses of the electric machine 201. The proximity losses in the conductors are caused due to flux leakage outside the main magnetic circuit, as illustrated in Figure 7. When the leakage flux travels through the stator slots 219-n (where the winding conductors are located), local eddy currents will be generated inside these conductors which will effectively increase the conductor resistance. Thus, normal Joule effects will be modified by the increase of this resistance. In a radial flux machine, the resulting proximity loss effects are more prominent in the region close to the air gap than in the region behind the stator yoke. This effect is proportional to the currentcarrying conductor cross-section. An electric machine 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures. As illustrated in Figure 1, the electric machine 1 has particular application as an electric drive unit (EDU) in a vehicle V, such as an automobile, a utility vehicle or a tractor unit. In use, the EDU generates a force to propel the vehicle V. The EDU may be used independently, for example in a battery electric vehicle (BEV) application; or in conjunction with an internal combustion engine (not shown), for example in a hybrid electric vehicle (HEV) application or a plug-in hybrid electric vehicle (PHEV) application. It will be understood that the electric machine 1 may be used in other applications. As shown in Figure 2, the electric machine 1 comprises a housing 2, a rotor assembly 3, a stator assembly 4 and a drive shaft 5. The electric machine 1 is described herein with reference to a longitudinal axis X about which the drive shaft 5 rotates. The rotor assembly 3 comprises a rotor (core) 6 which is mounted to the drive shaft 5 (shown in Figure 3). The rotor 6 is fixedly mounted to the drive shaft 5 such that the rotor 6 and the drive shaft 5 rotate together. An air gap G is maintained between the rotor 6 and the stator 7 assembly 4. The rotor 6 is made up of a plurality of laminations of a ferromagnetic material to form a rotor iron (or rotor core). The electric machine 1 in the present embodiment is a permanent magnet synchronous motor. As shown in Figure 3, the rotor 6 comprises a plurality of rotor poles 9-n. The rotor poles 9-n each extend radially outwardly from the longitudinal axis X. The rotor poles 9-n each have a direct axis dr-n and a quadrature axis qr-n. The rotor poles 9-n have an equal angular spacing (i.e. a pitch) between the direct axes dr-n of adjacent rotor poles 9-n. The rotor poles 9-n in the present embodiment each comprise at least one permanent magnet 10 mounted in the rotor 6. For example, the rotor poles 9-n may each comprise one, two or three magnets 10 (represented in Figure 3 by dashed lines). The magnets 10 may be radially offset from each other within each rotor pole 9-n. In the present embodiment, each rotor pole 9-n comprises three (3) permanent magnets 10A, 10B, 10C located in respective magnet receiving apertures 11 (a), 11 (b), 11 (c). The rotor 6 may comprise less than or more than six (6) rotor poles 9-n. As shown in Figure 4, the stator assembly 4 comprises a stator core 12 composed of a plurality of laminations of a ferromagnetic material. The stator core 12 is generally cylindrical and extends along the longitudinal axis X of the electric machine 1. The stator core 12 comprises an annular section 13 and a plurality of teeth 14-n. The annular section 13 forms a back iron of the stator core 12. The teeth 14-n extend radially inwardly from the annular section 13. The teeth 14-n each have a central radial axis R-n. The teeth 14-n each comprise a base portion 15-n and a distal end portion 16-n. As shown in Figure 4, a crown 17-n is disposed at a radially inner end of each of the teeth 14-n. The crowns 17-n are formed integrally with the teeth 14-n in the present embodiment. The crowns 17-n comprise circumferential extensions disposed on opposing sides of the distal end of each tooth 14-n. A plurality of stator slots 19-n are formed between the teeth 14-n. A slot opening 20-n is formed between the crowns 17-n formed on adjacent teeth 14-n. Put another way, pairs of teeth 14-n form a slot opening 20-n between opposing crowns 17-n. The slot opening 20-n is a narrowing of the stator slot 19-n, where the crowns 17-n of the adjacent teeth 14-n serve to reduce the width of the stator slot 19-n. The stator assembly 4 comprises a first winding 25-n and a second winding 30-n configured to form a plurality of stator poles 21-n. The first winding 25-n comprises a plurality of first concentrated coils 35-n; and the second winding 30-n comprises a plurality of second concentrated coils 40-n. The first concentrated coils 35-n and the second concentrated coils 40-n each comprise a concentrated winding. The first concentrated coils 35-n and the second concentrated coils 40-n are disposed on the respective teeth 14-n of the stator assembly 4 to form the stator poles 21-n. The first concentrated coils 35-n each comprise a plurality of co-axial first coils, each first coil being electrically insulated from the adjacent coil(s). The second concentrated coils 40-n each comprise a plurality of co-axial second coils, each second coil being electrically insulated from the adjacent coil(s). The first and second concentrated coils 35-n, 40-n are arranged in sets, each set consisting of one each of the first and second concentrated coils 35-n, 40-n. Each set of the first and second concentrated coils 35-n, 40-n is disposed on the same tooth 14-n. The first and second concentrated coils 35-n, 40-n are offset from each other in a radial direction. Put another way, the first coil may be disposed in a first region, and the second coil in a second region, wherein the first and second regions are radially offset from one another. The second region may be radially inset from the first region. The first winding 25-n comprises a first electrical conductor. The first electrical conductor in the present embodiment comprises a plurality of formed electrical conductors. The formed electrical conductors are formed in the desired shape prior to insertion into the stator slots 19-n. The formed electrical conductors may be inserted into the stator slots 19-n in an axial direction, for example from an axial end of the stator core 12. The use of formed electrical conductors may increase the conductor area in the region of the stator slot 19-n proximal to the annular region 13 of the stator core 12. Thus, the stator assembly 4 may have a higher slot fill than a corresponding stator assembly having a single winding. The second winding 30-n comprises a second electrical conductor. The second electrical conductor in the present embodiment is in the form of a wire. The wire may have a single (solid) core; or may comprise multiple strands. In the present embodiment, the second winding 30-n comprises a multistrand wire having a plurality of electrically insulated strands which may be twisted, braided or compacted. The second winding 30-n in the present embodiment comprises a Litz wire. The multistrand wire has a substantially circular cross section in the present embodiment. In a variant, the multistrand wire may have a rectangular crosssection. The multistrand wire may be compacted to provide the desired cross-section. The second winding 30-n is wound around the respective teeth 14-n, for example by inserting the wire through the slot opening 20-n in a radial direction. In a variant, the first winding 25-n may comprise a wire having a single (solid) core or multiple strands. The wire may be wound around the teeth 14-n to form the first winding 25-n. Alternatively, the second winding 30-n may comprise a formed electrical conductor. The first electrical conductor has a larger cross-sectional area than the second electrical conductor. The first electrical conductor is a “high-fill” conductor which offers improved filling of the stator slot (compared to the second electrical conductor). The second electrical conductor is a multistrand wire. The second electrical conductor is a “low-loss” conductor which offers reduced flux leakage (compared to the first electrical conductor). This may reduce flux losses proximal to the air gap between the rotor 6 and the stator assembly 4. The configuration of the first and second concentrated coils 35-1,40-1 on each tooth 14-1 is substantially the same. For brevity, the configuration of the first and second windings 25-1,30-1 disposed on a first tooth 14-1 of the stator core 12 will now be described. It will be understood that the configuration of the first and second windings 25-n, 30-n on the other teeth is substantially the same. The first and second concentrated coils 35-1, 40-1 on the first tooth 14-1 collectively form a first pair of concentrated coils. The first concentrated coil 35-1 comprises a plurality of formed electrical conductors which are axially inserted into the stator slots 19-1 disposed on opposing sides of the first tooth 14-1. The formed electrical conductors are then electrically connected to each other, for example by crimping or welding, to form the first winding 15-1. The second concentrated coil 40-1 comprises a multistrand wire, such as Litz wire, having a plurality of electrically insulated strands. The wire is wound around the first tooth 14-1 to form the second concentrated coil 40-1. As shown in Figure 4, the second concentrated coil 40-1 9 is radially inset (that is, towards the core) from the first concentrated coil 35-1. During assembly, the second concentrated coil 40-1 is formed on top of the first concentrated coil 35-1. The first concentrated coil 35-1 is disposed around the base portion 15-n of the first tooth 14-1; and the second concentrated coil 40-1 is disposed around the distal end portion 16-n of the first tooth 14-1. The first concentrated coil 35-1 is disposed at or proximal to a base of the first tooth 14-1 proximal to the annular section 13 of the stator core 12. The second concentrated coil 40-n is disposed at or proximal to a distal end of the first tooth 14-1 or proximal to the slot opening 20-1. The first and second concentrated coils 35-1,40-1 on each tooth 14-1 have different compositions. The first winding 25-1 is formed from a first electrical conductor comprising formed electrical conductors. Thus, the first concentrated coil 35-1 comprises a formed electrical conductor having a single (solid) core. The second winding 30-1 is formed from a second electrical conductor comprising a multistrand wire. Thus, the second concentrated coil 40-1 comprises a multistrand wire. The cross-sectional areas of the first and second electrical conductors which form the first and second concentrated coil 35-1,40-1 are different from each other. In the present embodiment, the first electrical conductor has a larger cross-sectional area than the second electrical conductor. By using a first electrical conductor having a larger cross-sectional area, the conductance area in a radially outer region (i.e. proximal to the annular region 13) of the stator slot 19-n may be increased. It will be understood that references to the cross-sectional area of a multistrand wire refer to the combined cross-sectional area of all of the strands (as opposed to the cross-sectional area of individual strands.) The first and second concentrated coils 35-1,40-1 have respective first and second outer profiles (when viewed along the radial axis R-1 of the first tooth 14-1). The first and second profiles in the present embodiment are different from each other. In particular, the first concentrated coil 35-1 has a first profile which is larger than the second profile of the second concentrated coil 40-1. At least in certain arrangements, this arrangement increases the conductor area in the stator slots 14-n, particularly in the region proximal to the annular region 13. The stator assembly 4 may comprise a plurality of electrical phases which are selectively energized to drive the rotor assembly 3. The first concentrated coil 35-n on each tooth 14-n is connected to one or more first concentrated coil 35-n on other teeth 14-n in conventional manner to provide the appropriate electrical phases for operation of the electric machine 1. Similarly, the second concentrated coil 40-n on each tooth 14-n are connected to the second concentrated coil 40-n on other teeth 14-n in conventional manner to provide the appropriate electrical phases for operation of the electric machine 1. As noted above, the region close to the air gap between the rotor 6 and the stator assembly 4 is particularly subject to proximity loss effects. By forming the second winding 30-n from a multistrand wire, at least in certain embodiments the proximity losses in the region proximal to the air gap may be reduced. These reductions may result from the low loss properties of the multistrand wire; and / or the revised geometry of the second winding 30-n resulting from the use of a multistrand wire. Alternatively, or in addition, the first winding 25-n are composed of a formed electrical conductor which may increase the conductor area in the region proximal to the annular region 13, thereby increasing slot fill and reducing associated losses. The 10 arrangement of the first and second windings 25-n, 30-n may also facilitate assembly of the stator assembly 3. For example, the first winding 25-n may be installed with relative ease due to the use of the formed electrical conductors. Since the second winding 30-n is disposed on top of the first winding 25-n, the winding of the second winding 30-n may be simplified as the winding machine does not have to be capable of inserting the electrical conductor on the bottom of the stator slots 19-n. The assembly of the stator assembly 4 will now be described. The first winding 25-n is installed on the stator core 12 in a first operation. During this first operation the first concentrated coil 35-n is provided on one or more of the teeth 14-n of the stator assembly 4. The second winding 30-n is installed on the stator core 12 in a second operation. During this second operation the second concentrated coil 40-n is provided on one or more of the teeth 14-n of the stator assembly 4. The or each second concentrated coil 40-n is installed after the or each first concentrated coil 35-n. The first and second concentrated coils 35-n, 40-n may be installed on each tooth 14-n in sequence. In a preferred arrangement, the first concentrated coil 35-n are installed on each tooth 14-n in the first operation; and then the second concentrated coil 40-n are wound onto each tooth 14-n in the second operation. The first concentrated coil 35-n comprise a plurality of formed electrical conductors which are inserted into the stator slots 19-n in an axial direction. In this arrangement, the first concentrated coil 35-n are not inserted through the slot openings 20-n. Furthermore, the available conductor area in the stator slots 19-n may be increased using formed electrical conductors to form the first concentrated coil 35-n. The second winding 30-n is wound around the teeth 14-n to form the second concentrated coils 40-n. The wire forming the second winding 30-n is inserted radially into the stator slots 19-n through the slot openings 20-n. The radial insertion of the second windings 30-n is facilitated by the location of the second concentrated coil 40-n on top of the first concentrated coil 35-n. The reach (radial extent) of the winding apparatus may be reduced as the second concentrated coil 40-n is formed on top of the first concentrated coil 35-n at the distal end of the tooth 14-n (proximal to the slot opening 20-n), rather than on the annular section 13 of the stator core 12. A flow diagram 100 illustrating assembly of the stator assembly 4 is shown in Figure 5. The assembly process is started (BLOCK 105). A plurality of formed electrical conductors are inserted into each of the stator slots 19-n (BLOCK 110). The formed electrical conductors are inserted axially, for example from a first axial end or a second axial end of the stator core 12. The formed electrical conductors are then connected to each other using a welding or crimping process to form the first concentrated coil 35-n (BLOCK 115). As described herein, the first concentrated coil 35-n is a concentrated winding consisting of a plurality of co-axial coils around each tooth 14-n. Multistrand wire is then wound around each tooth 14-n to form the second concentrated coil 40-n (BLOCK 120). The multistrand wire is wound on top of the first concentrated coil 35-n already disposed in each of the stator slots 19-n. An insert, such as an annular member or partition, may optionally be inserted on top of the first concentrated coil 35-n to provide a platform or guide for the second concentrated coil 40-n. As described herein, the second concentrated coil 40-n is also a concentrated winding consisting of a plurality of co-axial coils around each tooth 14-n. The assembly of the stator assembly 4 is completed (BLOCK 125). An electric machine 101 in accordance with a second embodiment of the present invention will now be described with reference to Figure 6. Like reference numerals are used for like components, albeit incremented by 100 for clarity. The electric machine 101 comprises a stator assembly 104 comprises a first winding 125-n and a second winding 130-n configured to form a plurality of stator poles 121-n. A quadrant of the stator assembly 104 is shown in Figure 6. The first winding 125-n comprises one or more first distributed coils 135-n; and the second winding 130-n comprises one or more second distributed coils 140-n. The first distributed coils 135-n and the second distributed coils 140-n each comprise a distributed coil winding. The first distributed coils 135-n and the second distributed coils 140-n are formed around two or more teeth 114-n which are spaced apart from each other (i.e. distributed) around the stator core 112 to form the stator poles 121-n. The first distributed coils 135-n are electrically insulated from the adjacent coil(s). The second distributed coils 140-n are electrically insulated from the adjacent coil(s). The first and second distributed coils 135-n, 140-n are arranged in sets, each set consisting of one each of the first and second distributed coils 135-n, 140-n. Each set of the first and second distributed coils 135-n, 140-n is disposed across the same teeth 114-n. The first and second distributed coils 135-n, 140-n are offset from each other in a radial direction. The first distributed coils 135-n are formed around a base portion 115-n of the teeth 114-n; and the second distributed coils 140-n are formed around a distal end portion 116-n of the teeth 114-n. An insert, such as an annular member or partition, may optionally be inserted on top of the first distributed coil 135-n to provide a platform or guide for the second distributed coil 140-n. The first winding 125-n comprises a first electrical conductor. The first electrical conductor in the present embodiment comprises a plurality of formed electrical conductors. The formed electrical conductors are formed in the desired shape prior to insertion into the stator slots 119-n. The formed electrical conductors may be inserted into the stator slots 119-n in an axial direction, for example from an axial end of the stator core 112. The use of formed electrical conductors may increase the conductor area in the region of the stator slot 119-n proximal to the annular region 113 of the stator core 112. Thus, the stator assembly 104 may have a higher slot fill than a corresponding stator assembly having a single winding. The second winding 130-n comprises a second electrical conductor. The second electrical conductor in the present embodiment is in the form of a wire. The wire may have a single (solid) core; or may comprise multiple strands. In the present embodiment, the second winding 130-n comprises a multistrand wire having a plurality of electrically insulated strands which may be twisted, braided or compacted. The second winding 130-n in the present embodiment comprises a Litz wire. The multistrand wire has a substantially circular cross section in the present embodiment. In a variant, the multistrand wire may have a rectangular crosssection. The multistrand wire may be compacted to provide the desired cross-section. The second winding 130-n is wound around the respective teeth 114-n, for example by inserting the wire through the slot opening 120-n in a radial direction. In a variant, the first winding 125-n may comprise a wire having a single (solid) core or multiple strands. The wire may be wound around the teeth 114-n to form the first winding 125-n. Alternatively, or in addition, the second winding 130-n may comprise a formed electrical conductor. The assembly of the stator assembly 104 will now be described. The first winding 125-n is installed on the stator core 112 in a first operation. The first distributed coils 135-n are distributed across two or more of the teeth 114-n of the stator core 112. The second winding 130-n is installed on the stator core 112 in a second operation. The second distributed coils 140-n are distributed across the same two or more teeth 114-n of the stator assembly 104 as the first distributed coils 135-n. The second distributed coils 140-n are formed after the first distributed coil 135-n. The first distributed coils 135-n are installed on the teeth 114-n in the first operation; and then the second distributed coils 140-n are wound onto the teeth 114-n in the second operation. The first distributed coil 135-n comprise a plurality of formed electrical conductors which are inserted into the stator slots 119-n axial direction. In this arrangement, the first distributed coils 135-n are not inserted through the slot openings 120-n. Furthermore, the available conductor area in the stator slots 119-n may be increased using formed electrical conductors to form the first distributed coil 135-n. The second winding 130-n is wound around the teeth 114-n to form the second distributed coils 140-n. The wire forming the second winding 130-n is inserted radially into the stator slots 119-n through the slot openings 120-n. The radial insertion of the second windings 130-n is facilitated by the location of the second distributed coil 140-n on top of the first distributed coil 135-n. The reach (radial extent) of the winding apparatus may be reduced as the second distributed coil 140-n is formed on top of the first distributed coil 135-n at the distal end of the teeth 114-n (proximal to the slot opening 120-n), rather than on the annular section 113 of the stator core 112. It will be understood that the stator assembly 4; 104 described herein may be utilised in a range of different types of electric motor. For example, the stator assembly 4; 104 may be used in an induction motor, a permanent magnet motor, a wound rotor synchronous machine, or a switched reluctance machine. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A stator assembly for an electric machine, the stator assembly comprising:a stator core comprising a plurality of teeth, the plurality of teeth each comprising:a first winding comprising at least one first coil; anda second winding comprising at least one second coil, wherein:the at least one first coil comprises a first electrical conductor;the at least one second coil comprises a second electrical conductor;the first electrical conductor comprises a formed electrical conductor; and the second electrical conductor comprises a multistrand wire.

2. A stator assembly as claimed in claim 1, wherein the at least one first coil and the at least one second coil are co-axial.

3. A stator assembly as claimed in any preceding claim, wherein the first coil is disposed in a first region of each of the plurality of teeth; and the second coil is disposed in a second region of each of the plurality of teeth; and wherein the first and second regions are radially offset from each other.

4. A stator assembly as claimed in claim 3, wherein the second region is radially inset from the first region.

5. A stator assembly as claimed in any one of the preceding claims, wherein the or each first coil is disposed proximal to a base of each of the plurality of teeth; and the or each second coil is disposed proximal to a distal end of each of the plurality of teeth.

6. A stator assembly as claimed in any of claim 1 to claim 4, wherein the or each first coil is disposed at a base of each of the plurality of teeth; and the or each second coil is disposed at a distal end of each of the plurality of teeth.

7. A stator assembly as claimed in any preceding claim, wherein the first coil has a first cross-sectional area in transverse section; and the second coil has a second cross-sectional area in transverse section; wherein the first cross-sectional area is larger than the second cross-sectional area.

8. A stator assembly as claimed in any preceding claim, wherein each of the plurality of teeth comprises a crown.

9. A stator assembly as claimed in any preceding claim, wherein the first coil is a concentrated coil or a distributed coil, and the second coil is a a concentrated coil or a distributed coil.

10. An electric machine comprising a stator assembly as claimed in any one of the preceding claims.

11. A vehicle comprising one or more electric machines as claimed in claim 9.

12. A method of assembling a stator core having at least a first tooth, the method comprising: forming a first winding on the stator core, the first winding comprising at least one first coil disposed on the first tooth; andforming a second winding on the stator core, the second winding comprising at least one second coil disposed on the first tooth;wherein the second winding is formed on the stator core after forming the first winding on the stator core.

13. A method as claimed in claim 12, wherein the first winding comprises at least one formed electricalconductor, the method comprising inserting the at least one formed electrical conductor in an axial direction.

14. A method as claimed in claim 12 or claim 13, wherein the second winding comprises at least one wire, the method comprising winding the at least one wire around the first tooth.

15. A method as claimed in any one of claims 12, 13 or 14, the method comprising forming the first coil at or proximal to a base of the first tooth; and forming the second coil at or proximal to a distal end of the first tooth.

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