GB2635560A

GB2635560A - Stator assembly

Info

Publication number: GB2635560A
Application number: GB2317632.4A
Authority: GB
Inventors: Boubaker Nadhem; Raimondi Giovanni
Original assignee: Safran Electrical & Power Uk Ltd
Current assignee: Safran Electrical & Power Uk Ltd
Priority date: 2023-11-17
Filing date: 2023-11-17
Publication date: 2025-05-21
Also published as: WO2025104325A1

Abstract

A stator assembly for an electric machine includes a back iron 110 disposed around a longitudinal axis and teeth 121 configured to be mounted on the back iron to extend radially therefore. The teeth are configured to receive stator windings 131. A barrier 111 extends radially from the back iron and is provided integrally therewith. The barrier is configured to separate stator windings on an adjacent pair of teeth. The circumferential width of the barrier is less than two thirds of the circumferential width of a first tooth of the pair of teeth. The circumferential width of the barrier, e.g. mean circumferential width, may be less than two ninths of the circumferential width of the first tooth. The circumferential width of the first tooth may be measured at a winding receiving region of the first tooth or may be the mean circumferential width along the winding receiving region. The teeth may define a slot and the barrier may extend from the back iron across the radial extent of the slot. The back iron may comprise FeSi and the teeth may comprise FeCo. An axial end of the back iron may overhang the teeth in the axial direction. The teeth may comprise four groups, each group occupying a quarter of a circle, and the stator may have four barriers configured to separate adjacent groups of teeth. The stator assembly may be used in an electric machine for an aircraft.

Description

STATOR ASSEMBLY

Technical Field

The invention relates to a stator assembly for an electric machine for use in an aircraft.

Background of the Invention

Electric machines can be used for electrical power generation or to provide motive power in aircraft. An electric machine is typically formed of an assembly of magnetic circuit components comprising a stator, and a rotor configured to rotate within a bore of, or around the outer diameter of, the stator. Rotation of the rotor relative to the stator causes interaction of the magnetic field generated by the rotor with windings provided on the stator, generating an induced electromotive force (EMF) and/or electrical current. In a permanent magnet generator, the rotor's magnetic field is produced by permanent magnets, which induces an AC voltage in the stator windings as the stator windings pass through the moving magnetic field of the permanent magnet. In a motor, an alternating current supplied to the windings of a stator can induce torque in a rotor.

The performance of conventional stators can be hindered by the inefficiency with which heat generated in the windings is transferred away from the stator teeth. The reliability of conventional stators can be hindered by the ability of faults to be propagated throughout the entire stator.

There exists a need for an improved stator assembly.

Summary of the Invention

According to a first aspect of the present invention, there is provided a stator assembly for an electric machine, the stator assembly comprising any or all of the following features: a back iron disposed around a longitudinal axis; a plurality of teeth configured to be mounted on the back iron to extend radially therefrom, the plurality of teeth being configured to receive stator windings; and a barrier extending radially from the back iron and provided integrally therewith; wherein the barrier is configured to separate stator windings on an adjacent pair of teeth of the plurality of teeth; and wherein the circumferential width of the barrier is less than two thirds of the circumferential width of a first tooth of the pair of teeth.

The stator assembly advantageously provides a mechanical barrier between adjacent windings in a given slot. This brings fault-tolerant capabilities because the barrier can prevent a fault from propagating along all of the windings around the stator. In particular, the barrier can prevent faults from propagating to windings in another channel, thereby reducing the likelihood of complete shutdown of the electric machine. By constructing the barrier integrally with the back iron, there is better thermal exchange between the windings in the slot and the back iron and the housing.

Furthermore, in contrast to arrangements in which the teeth and back iron are formed from an integral component, providing the plurality of teeth as mountable on the back iron results in a stator assembly that can be manufactured with improved material yield, cost effectiveness and precision tolerances. For example, this arrangement facilitates more efficient needle winding of the individual stator teeth prior to assembly. There is also provided a greater degree of freedom in terms of the relative dimensions of the components. Furthermore, different materials may be selected for the teeth and the back iron, allowing the material properties to be tailored for each component.

The circumferential width of the barrier may be less than six tenths, preferably less than half, preferably less than two fifths, preferably less than a third, preferably less than one quarter, preferably less than two ninths, of the circumferential width of the first tooth.

This provides a barrier that is relatively narrow compared to the teeth, thereby occupying less space in the stator assembly and advantageously resulting in a lighter and more cost-effective electric machine, while also increasing the space available for windings to be received around the teeth.

The circumferential width of the first tooth may be measured at a winding-receiving region of the first tooth.

The circumferential width of the first tooth may be the mean circumferential width along the winding-receiving region.

The circumferential width of the barrier may be the mean circumferential width of the barrier.

The adjacent pair of teeth may define a slot therebetween. The barrier may extend from the back iron across the radial extent of the slot.

This provides a barrier configured to separate adjacent windings along their entire radial extent, thereby improving the resistance to fault propagation.

The back iron and the plurality of teeth may be configured such that the plurality of teeth are mountable on the back iron in a direction parallel to the longitudinal axis.

This provides a convenient way of assembling the stator, such that each tooth can be wound with a winding before being mounted axially onto the back iron.

The back iron may comprise FeSi. At least one of the plurality of teeth may comprise FeCo.

This combination of materials provides a balance between magnetic properties, structural properties and cost. FeSi has good mechanical stiffness for its cost, while FeCo is more expensive but has better magnetic properties.

At least one axial end of the back iron may overhang at least one of the plurality of teeth in the axial direction.

This advantageously provides a mechanical shoulder which can be used to precisely locate an end-cap of the stator assembly.

A first tooth of the adjacent pair of teeth may comprise a first stator winding wound therearound, and a second tooth of the adjacent pair of teeth may comprise a second stator winding wound therearound, and wherein the slot defined between the first tooth and the second tooth may have a winding fill factor of at least 40%.

By using this fill factor, the overall electromagnetic properties are increased relative to the size and weight of the electric machine into which the stator assembly is incorporated.

The outer diameter of the back iron may be less than 100 cm.

The stator assembly may comprise a plurality of barriers and a plurality of channels, each channel of the plurality of channels may comprise at least one tooth of the plurality of teeth, wherein a barrier of the plurality of barriers may be provided between adjacent channels so as to separate stator windings belonging to different channels.

The plurality of teeth may comprise four groups of teeth, each group occupying a quarter of a circle around the longitudinal axis. The stator assembly may comprise four barriers configured to separate adjacent groups of teeth.

By providing a barrier particularly between adjacent channels, the risk of a fault propagating from one channel to another is reduced. As such, even if a fault were to be propagated between windings in a given channel, the barrier would isolate an otherwise functioning channel from being affected by the fault.

According to another aspect of the present invention, there is provided an electric machine for an aircraft, the electric machine comprising: a stator assembly as described hereinabove; and a rotor configured to rotate with respect to the stator assembly about the longitudinal axis thereof.

According to another aspect of the invention, there is provided an aircraft comprising the electric machine as described hereinabove.

Brief Description of the Drawings

Further features and advantages of the present invention will become apparent from the following description of embodiments thereof, presented by way of example only, and by reference to the drawings, in which: Figure 1 is a sectional view of a stator assembly according to an embodiment of the present invention; Figure 2 is a section view of part of a stator assembly according to an embodiment of the present invention; Figure 3 is a perspective view of part of a stator assembly according to an embodiment of the present invention; Figure 4 is an exploded view of a stator assembly according to another embodiment of the present invention; Figure 5 is a schematic diagram of an aircraft according to an embodiment of the present invention.

Detailed Description

In examples described herein, there is disclosed a stator assembly for an electric machine of the type suitable for use in an aircraft. The electric machine may be used as a generator to produce electrical power from a rotational input and may be used as a motor for the aircraft's propulsion. The electric machine has a stator and a rotor configured to rotate with respect to the stator. The stator assembly is formed from a back iron surrounding the rotational axis of the electric machine. The back iron is an annulus extending longitudinally along the rotational axis. The stator assembly includes stator teeth for receiving stator windings. The stator windings, which may be copper wire, can be wound around at least some of the stator teeth. In conventional arrangements, the stator is formed of a series of stacked laminations, wherein each lamination includes a layer of the back iron and the teeth. In contrast, the stator teeth described herein are not formed integrally with the back iron, but are instead mountable on the back iron through an attachment interface, for example by a dovetail connection. The stator teeth are arranged so as to extend radially from the back iron.

The stator assembly further includes at least one barrier formed integrally with the back iron. As such, a single lamination of the back iron can include the barrier or barriers. A barrier extends radially from the back iron between an adjacent pair of stator teeth. In this manner, the stator windings around an adjacent pair of teeth can be separated by the barrier. The barrier is relatively narrow compared to a stator tooth. The width of the barrier in the circumferential direction (or the hoop direction) is less than two thirds of that of a stator tooth. The barrier provides a more fault-tolerant stator assembly by helping to prevent a fault from propagating between adjacent stator windings.

Figure 1 illustrates a sectional view of a stator assembly 100. The stator assembly 100 is comprised in an electric machine and has a longitudinal axis 101 which coincides with the rotational axis of the electric machine when assembled therewith. The stator assembly 100 comprises a back iron 110 and a plurality of teeth 121. In the illustrated arrangement, the back iron 110 is formed of a continuous annulus disposed around the longitudinal axis 101. In this case, the back iron 110 is configured to define a complete circumference in which there are no breaks. The laminations that form the back iron 110 may form one integral circumferential piece, either as individual laminations, or as a stack of laminations. In alternative arrangements, the back iron 110 may be segmented into two or more segments, preferably at least four equally sized segments. In certain instances, it is preferable to minimise the number of components making up the back iron 110. In some examples, the back iron 110 may be separated into fewer than eight, seven, six, five, four or three segments.

The back iron 110 can comprise a plurality of core laminations (not shown). Each core lamination comprises an annulus of back iron material. The back iron 110 can thus comprise a plurality of core laminations stacked in the direction of the longitudinal axis 101 and bonded together to form a laminated stator core. The back iron 110 may comprise FeSi, FeCo, or any other suitable electrical steel. It will be understood that the bonding of the core laminations may a chemical bonding (such as glue), a mechanical connection provided by interlocking components, a welded joint, or any other suitable bonding technique. In an alternative arrangement, the back iron 110 may be formed by coiling a single continuous metal strip of the back iron material. In other words, the back iron 110 may be formed by providing a single continuous strip of core material and arranging the strip in a helical manner about the longitudinal axis 101 of the stator assembly 100 in order to form the plurality of core laminations made from a single strip of material. The resulting back iron 110 may be described as slinky shaped, with any desired projecting or interlocking features as described herein provided to the back iron.

The stator assembly 100 further comprises at least one barrier 111. The barrier 111 extends radially from the back iron 110 and is provided integrally therewith. In this respect, the barrier 111 is formed of the same continuous material as the back iron 110.

Where the back iron 110 is formed of laminations, the barrier 111 in a given core lamination is integrally formed with the back iron 110 in that lamination. In the arrangement shown, a plurality of barriers 111 extend radially from the back iron 110 towards the longitudinal axis 101. In other words, the barriers 111 can extend radially inwards from the back iron 110.

The plurality of teeth 121 are configured to be mounted on the back iron 110 to extend radially therefrom. That is to say, the plurality of teeth 121 are not formed integrally with the back iron 110, but are instead formed of a separate material. The plurality of teeth 121 can be formed of a plurality of tooth laminations (not shown). Each tooth can comprise a plurality of tooth laminations stacked along a direction parallel to the longitudinal axis 101 and bonded together to form a laminated tooth stack. The plurality of teeth 121 are configured to receive stator windings of the stator assembly 100. The plurality of teeth 121 includes a first tooth 121a and a second tooth 121b, which is adjacent to the first tooth 121a when mounted on the back iron 110. The first tooth 121a and the second tooth 121b define a slot 123 therebetween. In the arrangement shown, the barrier 111 extends from the back iron 110 into the slot 123. The plurality of teeth may comprise FeSi, FeCo, or any other suitable electrical steel.

The back iron 110 may have an outer diameter that is less than 100 cm, preferably less than 90 cm, preferably less than 80 cm, preferably less than 70 cm, preferably less than 60 cm, preferably less than 50 cm. The back iron 110 may have an outer diameter that is in the range of 10 cm to 50 cm, preferably in the range of 20 cm to 40 cm, preferably in the range of 25 cm to 35 cm.

The plurality of teeth 121 may comprise any number of teeth, for example the plurality of teeth 121 may comprise: three teeth, six teeth, nine teeth, 12 teeth, 15 teeth, 18 teeth, 21 teeth, 24 teeth, 27 teeth, 30 teeth, 33 teeth, at least 42 teeth, at least 48 teeth, at least 60 teeth, or more. Multiples of three teeth can be particularly useful for three phase machines. The skilled person will appreciate that the disclosure extends to a stator assembly 100 having any number of teeth, including any integer number encompassed in the above ranges, and which may be fewer than 60 or more than 60. The plurality of teeth 121 may be distributed evenly around the longitudinal axis 101. As such, the angular pitch between adjacent teeth may be 6° (i.e., 360° divided by the number of teeth). In one example, for an arrangement in which 60 teeth are provided, the stator assembly 100 may comprise 60 barriers 111 interspersed evenly with the plurality of teeth 121. More generally, each slot defined between an adjacent pair of teeth may be provided with a barrier 111. The skilled person will appreciate that the disclosure extends to a stator assembly 100 having a different number of barriers. For example, there may be half as many barriers as stator teeth, such that a barrier is provided in alternate slots. The stator assembly 100 may comprise a plurality of channels (or stars), wherein the stator windings associated with each channel are electrically connected to dedicated power electronics.

Figure 2 illustrates a sectional view of part of the stator assembly 100 including stator windings. As discussed in relation to Figure 1, the stator assembly has a back iron 110 with at least one barrier 111 extending radially therefrom, and a first tooth 121a and a second tooth 121b mounted to the back iron 110 and extending radially therefrom. In the arrangement shown, stator windings 131 are provided on the plurality of teeth. The stator windings 131 can be formed of electrically conductive wire, for example copper, wound around a respective tooth. The illustrated arrangement shows a concentrated winding arrangement in which wire is wound around a single tooth. In other arrangements, a distributed winding arrangement may be employed, in which a conductor is provided through a first slot in a first axial direction before being returned through a second slot in a second axial direction opposite the first axial direction, wherein the second slot is remote from (e.g. not adjacent to) the first slot.

At least one tooth of the plurality of teeth 121 may comprise tooth tips. The tooth tips may be provided as ledges extending circumferentially from a distal end of the tooth. The tooth tips are configured to secure the stator winding wound around the tooth. In the arrangement shown, the first tooth 121a comprises tooth tips 122a extending in opposite circumferential directions from the distal end of the tooth 121a with respect to the back iron 110. Similarly, the second tooth 121b comprises tooth tips 122b extending in opposite circumferential directions from the distal end of the tooth 121b with respect to the back iron 110. The tooth tips 122a, 122b may be configured to provide a gap therebetween in which the barrier 111 can at least partially occupy. As such, the stator assembly 100 may be configured so as to prevent direct contact between the barrier 111 and any of the plurality of teeth 121.

The first tooth 121a has a first winding 131a wound therearound and the second tooth 121b has a second winding 131b wound therearound. As such, Figure 2 shows a double winding arrangement in which a slot 123, being defined between adjacent stator teeth, is occupied by wire from the two different windings, i.e. from the first winding 131a and the second winding 131b. The barrier 111 is therefore configured to separate the first winding 131a from the second winding 131b in the slot 123. In the arrangement shown, the majority of the space defined by the slot 123 is occupied by the windings. The slot fill factor is preferably at least 40%, preferably at least 50%, preferably at least 60%.

The plurality of teeth 121 are mountable on the back iron 110 by a connection interface. At least one tooth of the plurality of teeth may comprise a first connection portion and the back iron may comprise at least one second connection portion, wherein the first connection portion is configured to mate with the second connection portion. The first connection portion may be a male connection portion and the second connection portion may be a female connection portion, or vice versa. In the arrangement shown, the plurality of teeth 121 are mounted to the back iron 110 by a dovetail joint. In this respect, at least one tooth, for example the first tooth 121a, may comprise a dovetail pin 124 and the back iron 110 may comprise a dovetail slot 114 provided on the inner circumferential surface of the back iron 110. In this way, the first tooth 121a is configured to be mounted on the back iron 110 in an axial direction, i.e. parallel to the longitudinal axis 101.

As described above, the barrier 111 extends radially from the back iron 110. The barrier 111 has a first side facing the first tooth 121a and a second side facing the second tooth 121b, wherein the first side is substantially parallel to the second side. In other words, the barrier 111 comprises a substantially rectangular cross-section. In some arrangements, the distal end of the barrier (i.e., the end furthest from the back iron 110) is curved to form a smooth transition from the first side to the second side. The barrier 111 may have a high aspect ratio in the sectional plane (i.e. the plane normal to the rotational axis 101), such that its length in the radial direction is significantly greater than its width in the circumferential direction. The aspect ratio may be at least four, preferably at least 8, preferably at least 12, preferably at least 16.

The barrier 111 may subtend an angle of less than 2°, preferably less than 1.5°, preferably less than 1° with respect to the longitudinal axis 101. The barrier 111 may subtend an angle less than one fifth, preferably less than one sixth, preferably less than one eighth of the pitch angle between adjacent teeth 121a, 121b. In the arrangement shown, the barrier 111 subtends an angle of approximately 0.6° with respect to the longitudinal axis 101, which is approximately one tenth of the pitch angle between adjacent teeth 121a, 121b.

The barrier 111 may have a radial length substantially similar to that of the plurality of teeth 121. The barrier 111 has a circumferential width less than that of a tooth 121a of the plurality of teeth 121. In particular, the barrier 111 has a circumferential width less than two thirds of that of a tooth 121a. Preferably, the barrier 111 is as narrow as possible so that the space dedicated to stator windings is maximised, while being sufficiently wide to provide a robust barrier suitable for preventing faults propagating between adjacent stator windings. The barrier 111 preferably has a circumferential width less than six tenths, preferably less than half, preferably less than two fifths, preferably less than a third, preferably less than one quarter, preferably less than two ninths of the circumferential width of the first tooth 121a. In the arrangement shown, the barrier 111 has a circumferential width approximately one fifth of that of the first tooth 121a.

The proportions above can be applied to the respective angles subtended by the barrier 111 compared to the angles subtended by the first tooth 121a and/or to the slot 123. For example, the barrier 111 may subtend an angle less than two thirds of that subtended by a tooth 121a. Preferably, the barrier 111 subtends an angle that is less than six tenths, preferably less than half, preferably less than two fifths, preferably less than a third, preferably less than one quarter, preferably less than two ninths of the angle subtended by a tooth 121a with respect to the longitudinal axis 101. The barrier 111 may have a circumferential width less than two thirds of the circumferential width of the slot 123, the slot 123 being defined as the total space between the adjacent teeth 121a, 121b including the barrier 111. The barrier 111 may have a circumferential width less than six tenths, preferably less than half, preferably less than two fifths, preferably less than a third, preferably less than one quarter, preferably less than two ninths of the circumferential width of the slot 123. Furthermore, the barrier 111 may subtend an angle less than two thirds of the angle subtended by the slot 123. Preferably, the barrier 111 subtends an angle that is less than six tenths, preferably less than half, preferably less than two fifths, preferably less than a third, preferably less than one quarter, preferably less than two ninths of the angle subtended by the slot 123 with respect to the longitudinal axis 123.

Both the barrier 111 and the plurality of teeth 121 can comprise a substantially rectangular cross section in a winding region, which is defined by a notional annulus of the stator assembly in which the stator windings are configured to be received (e.g., excluding the tooth tips and the connection interface, and excluding the rounded end of the barrier 111).

As such, the circumferential width is the same when measured at any radial distance from the longitudinal axis 101. The disclosure extends to a barrier and teeth whose sides are non-parallel such that the circumferential width may change depending on where it is measured. As such, references to the "circumferential width" of the barrier/tooth may encompass the arithmetic mean of all circumferential widths of the barrier/tooth measured in the winding region.

Figure 3 shows a perspective view of part of the stator assembly 100 without any windings on the teeth 121. As discussed above, the teeth 121 are formed separately from the back iron 110. This provides more freedom in designing the stator assembly such that the teeth 121 and the back iron 110 may have different lengths in a direction parallel to the longitudinal axis 101. For example, the plurality of teeth 121 may be shorter in the axial direction such that the back iron 110 overhangs the plurality of teeth 121 in the axial direction. In the illustrated arrangement, the back iron 110 and the barrier 111 are together formed of core laminations, while each tooth of the plurality of teeth 121 is formed from tooth laminations. The overall axial length of the stacked tooth laminations is less than the overall axial length of the stacked core laminations, such that the stator assembly 100 comprises a shoulder 102. The shoulder 102 is configured to precisely locate an end-cap of the stator assembly 100.

As discussed above, the material of the back iron 110 (and therefore the barrier 111) and the plurality of teeth may be an electrical steel such as FeSi or FeCo. In a preferable arrangement, the back iron 110 comprises FeSi and the plurality of teeth 121 comprise FeCo. At least one barrier 111 may be coated with an electrically insulating material, which may be a polyimide film. Furthermore, the connection interface between the back iron 110 and at least one tooth 121a may incorporate an electrically insulating material, such as an oxidation layer.

Figure 4 shows an exploded view of another embodiment of a stator assembly 200, wherein components corresponding to the stator assembly 100 have been given corresponding reference numerals. The stator assembly 200 comprises a back iron 210 disposed around a longitudinal axis 201, and a plurality of teeth 221 configured to be mounted on the back iron 210 to extend radially therefrom. The main difference between the stator assembly 200 of Figure 4 and the stator assembly 100 of Figures 1-3 is in the distribution of barriers. In the arrangement shown, instead of providing a barrier on the back iron between every position at which the plurality of teeth are mounted, only four barriers 211a, 211b, 211c, 211d are provided. The barriers 211a, 211b, 211c, 211d are distributed evenly around the longitudinal axis 201 such that adjacent barriers, for example a first barrier 211a and a second barrier 211b, are separated by a pitch angle of 90°. This arrangement may be particularly suitable for a stator assembly 200 having four segregated channels (stars) 204.

In the arrangement shown, the stator teeth 221 are grouped consecutively into four channels 204a, 204b, 204c, 204d. The teeth belonging to each channel are configured to be received in the back iron 210 between adjacent barriers. For example, the teeth in the first channel 204a are configured to be received in the back iron 210 between the first barrier 211a and the second barrier 211b. This separates the teeth (and therefore the stator windings) of the first channel 204a from the teeth of the second channel 204b and the fourth channel 204d. As such, this arrangement provides a stator assembly in which a barrier is provided between adjacent channels.

It will be appreciated that the general concept of matching the number of channels to the number of barriers extends to different numbers of channels and barriers other than four.

For example, an alternative arrangement may have six channels and six barriers, each being spaced 60° from neighbouring barriers.

Figure 5 is a schematic diagram illustrating an aircraft 1. The aircraft 1 comprises a driving or driven element 2 and an electric machine 4 connected to the driving or driven element 2 by a drive shaft 3. The electric machine 4 comprises the stator assembly 100. A rotor 5 is connected to the drive shaft 3 and configured to rotate within the stator assembly 100. It will be understood that the stator assembly in Figure 5 may be the stator assembly according to any embodiment described hereinabove.

While the foregoing has described a stator assembly for an electric machine of the type having a rotor configured to rotate within the stator assembly, the disclosure also extends to the opposite configuration wherein the rotor rotates around the stator assembly. The majority of the features discussed above are applicable to this alternative arrangement, except that the at least one barrier is provided on the back iron to extend radially outwards instead of radially inwards with respect to the longitudinal axis, and the plurality of teeth are mounted on the outer circumferential surface of the back iron, instead of the inner circumferential surface.

Various modifications, whether by way of addition, deletion and/or substitution, may be made to all of the above described embodiments to provide further embodiments, any and/or all of which are intended to be encompassed by the appended claims.

Claims

Claims 1. A stator assembly for an electric machine, the stator assembly comprising: a back iron disposed around a longitudinal axis; a plurality of teeth configured to be mounted on the back iron to extend radially therefrom, the plurality of teeth being configured to receive stator windings; and a barrier extending radially from the back iron and provided integrally therewith; wherein the barrier is configured to separate stator windings on an adjacent pair of teeth of the plurality of teeth; and wherein the circumferential width of the barrier is less than two thirds of the circumferential width of a first tooth of the pair of teeth.
2. The stator assembly according to claim 1, wherein the circumferential width of the barrier is less than six tenths, preferably less than half, preferably less than two fifths, preferably less than a third, preferably less than one quarter, preferably less than two ninths, of the circumferential width of the first tooth.
3. The stator assembly according to claim 1 or claim 2, wherein the circumferential width of the first tooth is measured at a winding-receiving region of the first tooth.
4. The stator assembly according to claim 3, wherein the circumferential width of the first tooth is the mean circumferential width along the winding-receiving region.
5. The stator assembly according to any preceding claim, wherein the circumferential width of the barrier is the mean circumferential width of the barrier.
6. The stator assembly according to any preceding claim, wherein the adjacent pair of teeth define a slot therebetween and wherein the barrier extends from the back iron across the radial extent of the slot.
7. The stator assembly according to any preceding claim, wherein the back iron and the plurality of teeth are configured such that the plurality of teeth are mountable on the back iron in a direction parallel to the longitudinal axis.
8. The stator assembly according to any preceding claim, wherein the back iron comprises FeSi and at least one of the plurality of teeth comprises FeCo.
9. The stator assembly according to any preceding claim, wherein at least one axial end of the back iron overhangs at least one of the plurality of teeth in the axial direction.
10. The stator assembly according to any preceding claim, wherein a first tooth of the adjacent pair of teeth comprises a first stator winding wound therearound, and a second tooth of the adjacent pair of teeth comprises a second stator winding wound therearound, and wherein the slot defined between the first tooth and the second tooth has a winding fill factor of at least 40%.
11. The stator assembly according to any preceding claim, wherein the outer diameter of the back iron is less than 100 cm.
12. The stator assembly according to any preceding claim, wherein the stator assembly comprises a plurality of barriers and a plurality of channels, each channel of the plurality of channels comprising at least one tooth of the plurality of teeth, wherein a barrier of the plurality of barriers is provided between adjacent channels so as to separate stator windings belonging to different channels.
13. The stator assembly according to any preceding claim, wherein the plurality of teeth comprises four groups of teeth, each group occupying a quarter of a circle around the longitudinal axis, and wherein the stator assembly comprises four barriers configured to separate adjacent groups of teeth.
14. An electric machine for an aircraft, the electric machine comprising: a stator assembly according to any of claims 1 to 13; and a rotor configured to rotate with respect to the stator assembly about the longitudinal axis thereof.
15. An aircraft comprising the electric machine according to claim 14.