The present invention relates to a spacer for forming a cooling vent between two sets of windings in a rotor of a rotating electrical machine, and to a rotor comprising a plurality of such spacers. The present invention relates in particular but not exclusively to a spacer which can form a cooling vent which extends in an axial direction and a radial direction through the windings of a rotor of a salient pole design.

Rotating electrical machines generally comprise a rotor and a stator which are arranged such that a magnetic flux is developed in an airgap between the two. In a rotating machine of a salient pole design, the rotor has a plurality of poles which extend radially outwards, on which a conductor is wound. An electrical current flowing in these windings causes a magnetic flux to flow across the airgap between the rotor and the stator.

In operation, losses may occur due to, for example, resistance in the windings and in losses in the pole body. These losses result in heat being created within the machine. Therefore, many machines include a fan for forcing air flow through the machine to provide cooling. Air flow through the machine is generally in an axial direction. The main paths for the air flow are through the rotor/stator airgap, and through an air gap between the stator core and the stator frame.

In a typical machine, a greater proportion of the cooling air passes around the stator rather than the rotor. It would therefore be desirable to increase the proportion of air flow through the rotor, in order to achieve more balanced cooling. However, improving the rotor’s cooling performance has been found to be challenging. This is due in part to the fact that a small rotor/stator air gap is necessary for good electromagnetic performance.

WO 2020/240173 A1 , the subject matter of which is incorporated herein by reference, discloses a rotor for a rotating electrical machine of a salient pole design in which a cooling vent is provided between a first, inner set of rotor windings and a second, outer set of rotor windings. The cooling vent extends in an axial direction from one end of the rotor pole to the other, and in a radial

1 direction through the whole of the windings. In one embodiment the cooling vent is formed by a plurality of discontinuous spacers. The spacers comprise two parallel walls held apart by spacing elements to create air passages.

It has been found that providing a cooling vent which extends through the rotor windings in an axial direction and a radial direction in the manner disclosed in WO 2020/240173 may help to improve the thermal performance of the machine. However, it is necessary for the spacers to present a surface to the windings in order to support the windings and to prevent them from drooping into the cooling vent. This may limit the amount of air flow which is able to contact the windings.

The machine rating is typically determined by the actual temperature rise of the rotor and stator. Therefore, the cooling efficiency of the machine may be an important design consideration. It would therefore be desirable to provide a spacer which can improve the cooling efficiency which is achieved.

According to one aspect of the present invention there is provided a spacer for forming an axial cooling vent between two sets of windings in a rotor of a rotating electrical machine, the spacer comprising a plurality of air passages for axial air flow through the spacer, wherein the air passages are at least partially open towards the windings.

The present invention may provide the advantage that, by providing a spacer in which the air passages are at least partially open to the windings, air flow through the spacer may contact the windings. This may help to improve the cooling efficiency of the design.

The spacer is preferably arranged to form an axial cooling vent which extends through the whole of the windings in an axial direction. This may help to ensure that, in use, the rotor is cooled along its whole length, and may avoid the need for complex air flow paths through pole shoes or other parts of the rotor.

Preferably the air passages extend from one end of the spacer to an opposite end of the spacer. Thus, each air passage may have an entry point and an exit point which are at opposite ends of the spacer. Preferably the spacer is arranged

2 such that, when it is in place on the rotor, the air passages extend through the whole of the spacer in an axial direction. This may facilitate the formation of a cooling vent which extends through the whole of the windings in an axial direction.

The spacer is preferably of a type in which an axial cooling vent through the windings is formed by a plurality of such spacers, spaced apart axially through the rotor. Thus, the spacer may be arranged to extend through only part of the windings in an axial direction. For example, the spacer may extend through less than 50%, 30%, 20% or 10% of the rotor in an axial direction. This may allow air flow between the spacers to contact the windings, which may help to improve the cooling efficiency.

Preferably the spacer is arranged to extend through substantially the whole of the windings in a radial direction. For example, the spacer may extend through at least 50%, 70%, 80% or 90% of the windings in a radial direction. Thus, the spacer may form a cooling vent which extends through the whole of the windings in an axial direction and a radial direction. By extending the spacer through the windings in a radial direction, rather than a tangential direction, the centrifugal force which acts on the spacer may be reduced. This may help to reduce any tendency of the spacer or the windings to slip or deform as the rotor rotates and/or may allow the spacer to present a lower profile to air flow through the cooling vent. Furthermore, a plurality of separate spacers may be used to form the cooling vent, thereby allowing air flow between the spacers to contact the windings, or the gaps between the spacers may be increased.

The spacer may also extend beyond the windings in a radial direction. For example, the spacer may extend into a space under the windings (towards the shaft) and/or above the windings (towards the pole shoe). This may help to ensure that the spacer supports the full windings, and may allow the same spacer to be used on a number of different rotor winding designs.

In order to allow the spacer to extend through the whole of the windings in a radial direction, the spacer may have a height which is greater than its width. Thus, when the spacer is in place on the rotor, it preferably has a height in the

3 radial direction which is greater than its width in the tangential direction. For example, the height may be at least two or three times the width.

Preferably the spacer comprises air passages which are at least partially open towards the windings on both sides of the spacer. For example, some air passages may be provided on one side of the spacer and some on the other. Furthermore, some air passages themselves may be at least partially open towards the windings on both sides of the spacer. This may allow air flow to contact both sets of windings, which may help to improve the cooling efficiency.

In a preferred embodiment, the spacer comprises a plurality of ribs which define the air passages. Preferably, an air passage is defined between two adjacent ribs. The ribs are preferably continuous through the width of the spacer (in a tangential direction, away from the rotor pole, when the spacer is in place on the rotor). Thus, the ribs may allow mechanical load transfer between the two sets of windings. However, the air passages between the ribs are at least partially open towards the windings, to allow air flow to contact the windings.

The spacer preferably comprises at least one support member arranged to support the ribs and/or to hold them in a spaced relationship so as to define the air passages.

In one embodiment, the support member is a central support member. In this case, a plurality of ribs may be provided on each side of the central support member. The plurality of ribs may define air passages which are open to the windings along the length of the spacer. Thus, in this embodiment, the air passages may be open towards the windings through the length of the spacer in a direction of air flow (an axial direction when the spacer is in place on the rotor). The support member may be continuous through the length of the spacer in the direction of air flow.

As discussed above, the ribs may be continuous through the width of the spacer, and thus may be arranged to support the windings. However, as the windings typically run in an axial direction, if the ribs were also to run in an axial direction, then there could be a risk of conductors in the windings slipping into the air

4 passages. Thus, the ribs may run in a direction having an axial component and a non-axial component. For example, the ribs may be zig-zagged or curved as they run in an axial direction. This may allow the ribs to support the windings and help to prevent conductors slipping into the air passages.

In the above embodiment, the ribs may be either continuous or discontinuous in the direction of air flow (axial direction). Where the ribs are discontinuous, they may be interleaved at the centre of the spacer in the direction of air flow. This may help to avoid pooling of resin in the spacer, while ensuring that the windings are adequately supported.

In another embodiment of the invention, at least one support member is provided on each side of the spacer. In this case, each support member may present a surface to the windings on one side of the spacer or the other. Thus, the support members themselves may be arranged to support the windings, as well as to hold the ribs in a spaced relationship.

In one arrangement, each support member may link two adjacent ribs. For example, the support members may alternate from one side of the spacer to the other in a radial direction (perpendicular to a direction of air flow), with each support member linking two adjacent ribs. In this arrangement, the ribs preferably run in a direction having an axial component and a non-axial component (for example, they may be zig-zagged or curved) to allow them to support the windings. The support members may be either continuous or discontinuous through the length of the spacer in a direction of air flow (an axial direction).

In another arrangement, each support member may extend through the whole of the spacer in a direction perpendicular to air flow (a radial direction). In this arrangement, the spacer may comprise a first support member in a first part of the spacer and a second support member in a second part of the spacer, in a direction of air flow (axial direction). The first and second support members may be on opposite sides of the spacer. Thus, the first support member may be arranged to support windings on one side of the spacer and the second support member may be arranged to support windings on the other side of the spacer.

5 In the above arrangement, since the support members extend through the whole of the spacer in a radial direction, the support members may be able to support the windings. Thus, in this arrangement, the ribs may be straight, and may extend through the spacer in a substantially axial direction. This may help to optimise air flow through the windings.

In any of the above embodiments, the spacer may comprise a middle part (in a direction of air flow) with no support member, and the middle part of the spacer may be open to the windings on both sides of the spacer. Thus, the spacer may comprise a part in which the air passages are open to the windings on both sides of the spacer. This may help to maximise the contact of air flow with the windings, thereby helping to improve the cooling efficiency.

In any of the above embodiments, the spacer may comprise means for supporting and/or locating the spacer. For example, the spacer may comprise at least one foot, which may be arranged to locate the spacer on a winding support. Preferably two feet are provided, in which case the feet may fit either side of a winding support. This may help to ensure that the spacer is correctly located when it is placed on the rotor and may help to prevent movement of the spacer whilst the rotor is being wound.

In any of the above embodiments, the spacer may comprise a deflector for deflecting air flow from an interpolar axial channel into air passages in the spacer. This may help to increase the amount of air flow through the axial cooling vent, thereby helping to improve the cooling efficiency. The deflector may be, for example, part of a foot, which may also be used to help locate the spacer and/or hold it in place.

According to another aspect of the invention there is provided a rotor for a rotating electrical machine, the rotor comprising rotor windings and a plurality of spacers in any of the forms discussed above. The spacers are preferably spaced apart in an axial direction through the rotor windings. This may allow air flow to contact the windings between the spacers.

6 The rotor may comprise a plurality of salient poles, and the windings may be wound on a salient pole. Preferably the windings comprise side windings running axially though the rotor, and end windings extending around the ends of the rotor. In this case the spacers are preferably located in the side windings. However, the spacers disclosed herein may also be used with other types of rotor.

Preferably the rotor windings comprise a first set of windings and a second set of windings on the same rotor pole, and the axial cooling vent is formed between the two sets of windings. Preferably the first set of windings is beneath the second set of windings, in a direction away from the pole. The spacers preferably form a cooling vent for air flow in an axial direction through the rotor windings.

Preferably the cooling vent extends in an axial direction from one end of the rotor pole to the other and/or through the whole of the windings in a radial direction. Thus, the spacers may extend through substantially the whole of the windings in a radial direction.

The rotor may further comprise a plurality of winding supports for supporting the undersides of the windings. In this case, the spacers may be located on the winding supports. For example, the spacers may comprise locating features such as feet which locate the spacers on the winding supports. This may help to ensure that the spacers are correctly located and held in place during winding of the rotor.

If desired, three or more sets of windings may be provided on each pole, with a plurality of spacers between two adjacent sets. Thus, a plurality of axial cooling vents may be provided through each pole.

According to another aspect of the invention there is provided a rotor for a rotating electrical machine, the rotor comprising: a plurality of salient poles; rotor windings wound on the salient poles; and a plurality of spacers located in the rotor windings of each salient pole, the plurality of spacers forming an axial cooling vent between two sets of the rotor windings,

7 wherein each of the spacers comprises a plurality of air passages for axial air flow through the spacer, and the air passages are at least partially open towards the windings.

According to another aspect of the invention there is provided a rotating electrical machine comprising a spacer or a rotor in any of the forms described above. The machine may comprise means such as a fan for causing air flow through the machine. Air flow is preferably in an axial direction through the machine.

According to another aspect of the invention there is provided a method of winding a rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles, the method comprising: winding a pole with a first set of windings; providing a plurality of spacers on the first set of windings; and winding a second set of windings on the plurality of spacers such that the spacers form an axial cooling vent through the windings, wherein each spacer defines a plurality of air passages for axial air flow through the spacer, and the air passages are at least partially open towards the windings.

Features of one aspect of the invention may be provided with any other aspect. Apparatus features may be provided with method aspects and vice versa.

In the present disclosure, terms such as “radially”, “axially”, “tangentially” and “circumferentially” are generally defined with reference to the axis of rotation of the electrical machine, unless the context implies otherwise.

Preferred embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a radial cross section through part of a rotating electrical machine;

Figure 2 shows parts of a rotor in an embodiment of the invention;

Figure 3 shows an axial cross section through part of the rotor of Figure 2; Figure 4 shows a previously considered spacer block;

8 Figure 5 shows a spacer in an embodiment of the invention;

Figure 6 shows a spacer in another embodiment of the invention;

Figure 7 shows a spacer in another embodiment of the invention;

Figure 8 shows a spacer in another embodiment of the invention;

Figure 9 shows a spacer in another embodiment of the invention;

Figure 10 shows part of a rotor with the spacers of Figure 9 in place;

Figure 11 shows part of an assembled rotor;

Figure 12 illustrates air flow through a spacer; and

Figure 13 shows another embodiment of a spacer.

Figure 1 is a radial cross section through part of a rotating electrical machine.

The machine comprises a rotor 2 located inside a stator 3 with an air gap 4 between the two. The rotor 2 is mounted on a shaft with an axis of rotation indicated by the dashed line 5. The rotor 2 is wound with rotor windings 6. The stator 3 comprises a stator core with slots on its inner circumference in which are wound stator windings 7. The stator 3 is contained within a stator frame 8. A shaft-driven fan 9 is located at the drive end of the machine, in order to draw cooling air through the machine. This air flow is predominately in an axial direction through the rotor/stator air gap and the stator/frame air gap, as indicated by the arrows in Figure 1. If desired, an external, independently driven fan or fans or any other appropriate means of forcing air through the machine could be used instead of a shaft driven fan.

Figure 2 shows parts of a rotor in an embodiment of the present invention. Referring to Figure 2, the rotor comprises a rotor body 10 mounted on a shaft 12. The rotor body is formed from a plurality of laminated sheets of metal stacked together to create a rotor of the required axial length. The rotor body 10 comprises a plurality of salient poles 14, each of which extends radially outwards from the centre of the rotor body. Each salient pole is wound with rotor windings 16. The windings 16 are in the form of a coil comprising a conductor such as copper wire which is wound around the pole 14. The windings 16 include side windings which run in a substantially axial direction along the length of the rotor, and end windings which run in a substantially tangential direction around the end of the rotor. Retaining wedges 18 are provided at spaced locations along the side windings. The retaining wedges 18 press against the side windings of two

9 adjacent poles in order to hold the windings in place. Rotor winding support bars 20 are also provided. The rotor winding support bars 20 run in an axial direction through the rotor and extend outwards at each end of the rotor in order to support the end windings. In this example the rotor has four poles, although of course different machines may have a different number of poles.

Figure 3 shows an axial cross section through part of the rotor of Figure 2. Referring to Figure 3, each salient pole 14 includes a main body 22 and a pole shoe 24 on the leading and trailing edge. The pole shoes 24 assist in retaining the windings 16 against centrifugal force as the rotor rotates. The retaining wedges 18 fit under the tips of two adjacent pole shoes 24. L-shaped winding supports 26 are provided beneath the windings of two adjacent poles, to support the underside of the windings. A bottom, interpolar axial channel 28 extends through the rotor between two adjacent poles beneath the windings and the winding supports 26. In addition, damper bars 30 are provided which run in an axial direction through the poles.

In the arrangement of Figure 3, the rotor windings on each pole comprise a first, inner set of windings 32 and a second, outer set of windings 34, with an air gap 36 between the two. The air gap 36 is formed in place of one or more layers (or partial layers) of windings which would otherwise be present at or towards the centre of the windings. In this embodiment, the air gap 36 extends substantially the whole of the way around the rotor, through both the side windings and the end windings. The air gap also extends through the whole of the windings in a substantially radial direction, from the side of the windings adjacent to the winding supports 26 to the side of the windings adjacent to the pole shoes 24. The air gaps 36 form axial cooling vents, that is, vents which run through the rotor windings in an axial direction (perpendicular to the plane of the paper in Figure 3). The axial cooling vents allow air to flow axially through the rotor windings. The axial cooling vents may be of the type disclosed in WO 2020/240173 A1 , the subject matter of which is incorporated herein by reference.

In the arrangement of Figures 2 and 3, a plurality of spacers is provided between the first set of windings 32 and the second set of windings 34 in order to create the air gaps 36. The spacers are provided at spaced locations in an axial

10 direction through the rotor windings. The spacers extend through substantially the whole of the windings in a radial direction. The spacers may also extend radially beyond the windings, for example, into a space under the windings (towards the shaft) and/or above the windings (towards the pole shoe).

Forming an air gap in place of one or more layers of windings can allow a reduced number of turns to be used in the windings. Since the windings are typically formed from copper wire, this may result in cost savings. However, the reduction in the number of windings may lead to a higher current and therefore may require improved cooling to ensure that temperature limits are not exceeded. This can be achieved by providing air flow through the axial cooling vent, which increases the surface area of the windings which is exposed to the air flow and introduces air flow to the hottest part of the windings. However, size of the cooling vent and design of the spacer is important in order to maximise the cooling benefit while keeping the rotor winding losses low.

Figure 4 shows a previously considered spacer block for creating cooling vents through rotor windings. Referring to Figure 4, the spacer block 40 comprises two parallel side walls 41 , 42, two end walls 43, 44 and an internal wall 45. The side walls are held apart by the end walls and the internal wall to create air passages 46, 47. The spacer block is designed to sit between an inner set of windings and an outer set of windings, with each of the side walls 41 , 42 lying against a respective set of windings. When the spacer block 40 is in place, the air passages 46, 47 run in an axial direction through the side windings.

The spacer of Figure 4 is effective at providing a mechanical structure between the two separate sets of windings. However, it has been found the spacer may partially restrict the flow of air and/or reduce contact of air flow with the windings, which may reduce the cooling efficiency.

Figure 5 shows a spacer in an embodiment of the invention. The spacer is designed to be inserted between an inner and outer set of windings in the rotor of a rotating electrical machine in order to create axial cooling vents through the rotor windings, such as those discussed above with reference to Figures 2 and 3. Referring to Figure 5, the spacer 50 extends in three orthogonal directions: x, y

11 and z. When the spacer is in place on the rotor, the x-direction is a generally axial direction, the y-direction is a generally tangential direction (tangential to a circumferential direction), and the z-direction is a generally radial direction (a direction parallel to the direction in which a salient pole extends). The spacer is substantially rectangular when viewed in the x-z plane. The depth of the spacer in the x-direction is chosen to ensure sufficient mechanical strength to separate two sets of rotor windings. The width of the spacer in the y-direction is chosen so as to create axial cooling vents of the desired width between the two sets of windings. The height of the spacer in the z-direction is chosen such that the spacer extends the whole of the way through the windings in the z-direction (radially).

The spacer 50 comprises central support member 52 with a plurality of ribs 54, 56 on each side. In this embodiment, the central support member 52 and the ribs 54, 56 on either side of the central support member each has a width in the y- direction of approximately one third of the total width of the spacer. The ribs 54, 56 are provided at the same locations on either side of the central support member in the y-direction. Thus, the spacer is continuous from one side to the other in the y-direction at the locations of the ribs 54, 56. This allows mechanical load transfer between the inner windings and the outer windings when the spacer is in place in the rotor. Air channels 58 are defined between two adjacent ribs 54, 56. In this embodiment, six air channels are provided on each side of the spacer. The air channels 58 allow air to flow in a generally axial direction through the rotor windings.

In the arrangement of Figure 5, the ribs 54, 56 have a zig-zag pattern. Thus, the ribs run in a direction which, as well as having an axial component (in the x- direction), also has a non-axial component (in the z-direction). In this embodiment, when viewed in the x-direction (i.e. , axially), the ribs first have a negative component in the z-direction, then a positive component in the z- direction, and then a further negative component in the z-direction. By arranging the ribs in a zig-zag pattern, the ribs are able to support the windings and reduce the risk of conductors in the windings falling into the channels.

12 A spacer design with ribs on either side of a central support member such as that shown in Figure 5 can allow cooling air to contact the windings as it passes through the spacer. This design may therefore provide more efficient cooling than the previously considered designs, while at the same time providing mechanical load transfer between the inner windings and the outer windings and preventing the windings from falling into the channels.

It will be appreciated that various different zig-zag patterns are possible. For example: a different number of ribs could be provided; a different number of changes in direction could be provided; the order of the positive and negative y- direction components could be changed; and the relative thicknesses of the ribs 54, 56 and the central section 52 could be changed. Furthermore, a range of thicknesses of the spacer could be used, and other arrangements could include for example widening (nor narrowing) the space created in the rotor windings.

It has been found that, while the design shown in Figure 5 may offer improvements over the previously considered designs, the relatively small sizes of the air channels through the spacers may restrict air flow and thus may limit the cooling benefit.

Figure 6 shows a spacer in another embodiment of the invention. The spacer of Figure 6 is designed to be inserted between two sets of windings in a rotor in a similar way to the spacers of Figures 4 and 5.

In the arrangement of Figure 6, rather than providing a central support member with a plurality of ribs on each side, the spacer 60 has a corrugated cross section in the y-z plane. Thus, the spacer may be viewed as comprising a plurality of ribs 62 which are connected on one side (in the y-direction) by a first set of support members 64 and on the other side by a second set of support members 65. The first and second support members 64, 65 alternate between one side of the spacer and the other side through the spacer in the z-direction. As a consequence, a first set of air passages 66 is defined between adjacent ribs on one side of the spacer, and a second set of air passages 68 is defined between adjacent ribs on the other side of the spacer. The air passages 66 on one side are interleaved with the air passages 68 on the other side in the z-direction. The

13 first and second support members 64, 65 are integrated with the ribs 62, and function to hold the ribs in a spaced relationship in the z-direction. The first and second support members 64, 65 also function to support rotor windings on respective sides of the spacer.

In the arrangement of Figure 6, the ribs 62 zig-zag in the x-direction, in a similar way to the ribs of Figure 5. Thus, in the spacer of Figure 6, the ribs (and air channels) run in a direction which, as well as having an axial component (in the x- direction), also has a non-axial component (in the z-direction). This can reduce the risk of conductors in the windings falling into the channels.

When compared to the spacer of Figure 5, the spacer 60 of Figure 6 can be considered to have “combined” air passages of twice the width (in the y- direction), with a support member 64, 65 located on one side or the other (rather than a central support member). Each of the air passages 66, 68 has a width in the y-direction which is approximately two thirds of the total width of the spacer.

It has been found that the increased width of the air passages can improve air flow and thus may bring a cooling benefit to the machine. However, a disadvantage of the spacer of Figure 6 is that there is more limited contact of the air flow with the windings.

Figure 7 shows a spacer in another embodiment of the invention. The spacer of Figure 7 is designed to be inserted between two groups of windings in a rotor in a similar way to the spacers of the previous embodiments.

Referring to Figure 7, the spacer 70 comprises a plurality of ribs 72 with air passages 78 defined between the ribs, in a similar way to the spacer of Figure 6. However, in the arrangement of Figure 7, the ribs are connected by first support members 74 and second support members 75 at one end of the spacer in the x- direction, and by third support members 76 and fourth support members 77 at the other end of the spacer in the x-direction. The first and third support members 74, 76 are provided on one side of the spacer in the y-direction, while the second and fourth support members 75, 77 are provided on the other side of the spacer in the y-direction. The support members 74, 75, 76, 77 are integrated with the ribs 72, and function to hold the ribs in a spaced relationship in the z-direction.

14 The support members also function to support rotor windings on respective sides of the spacer. However, in the arrangement of Figure 7, the support members 74, 75, 76, 77 only extend part way along the spacer in the x-direction. The centre of the spacer in the x-direction is without support members, so that holes are provided through the entire spacer in the y-direction. As a consequence, the air passages 74 are opened up to the windings on both sides of the spacer in the y-direction.

The arrangement of Figure 7 provides “combined” air passages with an increased width in the y-direction, which may help to improve air flow. Furthermore, opening up the air passages 74 to the windings on both sides of the spacer allows more contact between the air and the windings, which may improve the cooling efficiency. In addition, alternating the support members between one side of the spacer and the other may allow more even cooling of the windings.

In the arrangement of Figure 7, the ribs 72 zig-zag in the x-direction, in a similar way to the ribs of Figures 5 and 6. Thus, in the spacer of Figure 7, the ribs (and air channels) run in a direction which, as well as having an axial component (in the x-direction), also has a non-axial component (in the z-direction). This can reduce the risk of conductors in the windings falling into the channels.

If desired, the spacer of Figure 5 could be provided with a middle part with no support member and which is open to the windings on both sides of the spacer, in a similar way to that of Figure 7.

In the embodiments described above, the ribs zig-zag in the x-direction in order to support the windings. However, a problem which has been identified with a zig zag rib design such as that of Figures 5 to 7 is that, when the rotor is impregnated with resin, there is a risk of resin being trapped and pooling in the bends in the air passages. This may cause restrictions or blockages in the air passages, reducing the cooling efficiency. Furthermore, the changes in direction of the air passages due to the zig-zag design may hinder airflow.

15 Figure 8 shows a spacer in another embodiment of the invention. The spacer of Figure 8 is designed to be inserted between two groups of windings in a rotor in a similar way to the spacer of the previous embodiments.

Referring to Figure 8, the spacer 80 comprises a central support member 82 with a plurality of ribs on each side, in a similar way to the spacer of Figure 5.

However, in the arrangement of Figure 8, on each side of the central support member 82 there is a first set of ribs 84 in a first part of the spacer in the x- direction (axially), and a second set of ribs 86 in a second part of the spacer in the x-direction. Thus, the ribs 84, 86 are discontinuous in the x-direction. The ribs on one side of the central support member 82 are co-located with the ribs on the other side in the y-direction. Air channels 88 are defined between two adjacent ribs.

When viewed in the x-direction, the first set of ribs 84 initially run substantially in the x-direction, with little or no component in the z-direction. The ribs then curve upwards as they extend in the x-direction. Thus, as the ribs 84 extend in the x- direction they have an increasing component in the z-direction. At the same time, the thickness of the ribs in the z-direction decreases. The ends of the first set of ribs 84 are interleaved with the beginnings of the second set of ribs 86 at the centre of the spacer in the x-direction. The second set of ribs 86 are substantially the same as the first set of ribs 84, with a 180° rotation about the y-axis. When viewed in the x-direction, the second set of ribs 86 initially run in a direction which has a component in the z-direction as well as the x-direction. The second set of ribs 86 then curve around so as to run substantially in the x-direction, with little or no component in the z-direction, as they approach the end of the spacer in the x- direction.

Using a discontinuous curved rib design in the way shown in Figure 8 may help to prevent pooling of resin in the air channels when the rotor is impregnated. Thus, this arrangement may help to ensure that the air channels remain open. At the same time, the curved ribs help to prevent drooping of conductors into the air channels. Furthermore, co-locating the ribs on one side of the central section with the ribs on the other side in the y-direction can allow mechanical load transfer between the two groups of windings. However, the curved rib design of

16 Figure 8 may not bring some of the benefits of larger air passages and/or opening up the air passages to the windings as the designs of Figures 6 and 7.

It will be appreciated that various different curved ribs designs are possible. For example: a different number of ribs could be provided in the x-direction and/or the z-direction; the ribs could curve in the opposite direction; and the relative thicknesses of the ribs 84, 86 and the central support member 82 could be changed.

Figure 9 shows a spacer in another embodiment of the invention. The spacer of Figure 9 is designed to be inserted between two groups of windings in a rotor in a similar way to the spacers of the previous embodiments.

Referring to Figure 9, the spacer 90 in this embodiment comprises a plurality of ribs 92 which extend through the whole of the spacer in the x-direction (axially). The ribs run parallel to the x-direction with little or no component in the z- direction. The ribs 92 are continuous through the spacer in the y-direction. Air channels 94 are defined between two adjacent ribs 92 in the z-direction.

The spacer 90 of Figure 9 includes a first support member 95 in a first portion of the spacer in the x-direction. The first support member 95 extends through substantially the whole of the spacer in the z-direction, through approximately one third of the spacer in the x-direction, and through approximately one third of the spacer in the y-direction. A second support member 96 is provided in the final third portion of the spacer in the x-direction. The second support member 96 is provided on the opposite side of the spacer to the first support member 95 in the y-direction. The second support member 96 extends substantially through the whole of the spacer in the z-direction, through approximately one third of the spacer in the x-direction, and through approximately one third of the spacer in the y-direction. The first and second support members 95, 96 are integrated with the ribs 92, and function to hold the ribs in a spaced relationship in the z-direction.

The first and second support members 95, 96 also function to support rotor windings on respective sides of the spacer. The centre of the spacer in the x- direction is without a support member.

17 Since the first and second support members 95, 96 are provided on respective sides of the spacer in the y-direction, the air passages 94 have a width in the y- direction which is approximately two thirds of the total width of the spacer. This may help with cooling efficiency, by providing larger air passages for air flow. Furthermore, in the centre section of the spacer, the air passages 94 are opened up to the windings on both sides of the spacer. This allows more contact between the air and the windings. In addition, since the air passages 94 are essentially straight and run in a substantially axial direction, they may allow relatively unrestricted air flow in an axial direction through the machine. Thus, the design of Figure 9 may help to further improve the cooling efficiency. In addition, there are no bends in the air passages in which resin could collect.

Also shown in Figure 9 are two feet 98 at the bottom of the spacer in the z- direction. The feet 98 are designed to sit either side of a winding support when the spacer is in place on the rotor. This helps to ensure that the spacers are correctly located and to prevent movement of the spacers whilst winding the rotor. The feet 98 have a sloped edge 99 which may help to deflect air flow from the bottom, interpolar axial channel into the axial cooling vent through the rotor windings as will be explained below. Feet such as those shown in Figure 9 could also be used with a spacer in any of the other embodiments. Alternatively, the spacer of Figure 9 could be provided without feet.

It will be appreciated that various modifications to the design of Figure 9 are possible. For example: a different number of ribs could be provided; rather than being straight, the ribs could be zig-zagged or curved; the relative thicknesses (in the y-direction) of the ribs 92 and the support members 95, 96 could be changed; and the relative dimensions of the support members 95, 96 in the x-direction could be changed.

Overall, the spacer design of Figure 9 may be preferred over those of the previous embodiments, as it may provide optimal air flow and cooling and reduce the risk of resin pooling. It also is a relatively simple design which can be easily moulded. However, any of the other embodiments, or modifications thereof, could be used as appropriate.

18 Figure 10 shows part of a rotor with the spacers 90 of Figure 9 in place.

Referring to Figure 10, the rotor comprises a rotor body 10 mounted on a shaft 12. The rotor body comprises a plurality of salient poles 14, each of which extends radially outwards from the centre of the rotor body. Each salient pole is wound with rotor windings. In Figure 10, the rotor is shown partially wound with first, inner set of windings 32. L-shaped winding supports 26 are provided beneath the windings radially. A bottom, interpolar axial channel 28 extends through the rotor between two adjacent poles beneath the windings and the winding supports 26.

In the arrangement of Figure 10, a plurality of spacers 90 are provided at spaced locations in an axial direction through the rotor. The spacers located on top of the first, inner set of windings 32 in the direction of winding (i.e. , a direction which is tangential to the circumferential direction, corresponding to the y-direction of the spacer). Each spacer 90 is located on top of a winding support 26 in the radial direction. The feet 98 of the spacers fit around the winding support 26. This helps to locate the spacers and prevent movement of the spacers whilst winding the second, upper set of windings. The spacers 90 extend through substantially the whole of the windings in the radial direction. Once the spacers 90 have been placed on the rotor, the rotor can be wound with a second, upper set of windings on top of the first set. The spacers 90 create axial cooling vents through the rotor windings.

Figure 11 shows part of the assembled rotor with the upper set of windings 34 and the winding wedges 18 in place. The arrow in Figure 11 shows the entry of air flow into an axial cooling vent 36 through the rotor windings.

It has been found that the spacers 90 of Figures 9 to 11 may provide the following features and advantages:

• The first and second support members provide a full surface to support the rotor windings. Each surface extends approximately one-third of the depth of the spacer in the axial direction.

19 • The ribs allow mechanical load transfer between the inner and outer groups of windings. The ribs are in compression only (no bending or shearing of the material) which may help to provide a resilient design.

• The total cross section available for air flow is approximately 33% of the total spacer cross section.

• The straight air passages allow relatively unrestricted air flow paths.

• Air is able to make direct contact with the rotor windings.

• The middle section of the spacer is open on both sides to allow maximum air contact with the windings, and to maximise the air passage cross section.

• The spacer is sufficiently open to reduce the risk of resin collecting and “pooling” within the spacer during rotor impregnation. This would reduce the area available to the air flow. The rotor may need to be rotate-cured or cured at an angle to further avoid pooling (these are known production methods).

• The feet provide a location feature to span the winding support. This may aid location and help prevent the spacer from moving whilst winding the rotor.

Figure 12 illustrates air flow through the spacer when the machine is in operation. The dashed arrows indicate axial air flow through the air passages in the spacer. In addition, the bottom arrow shows how the sloped edge 99 is used to deflect air flow from the bottom, interpolar axial channel 28 into the air passages in the spacer. This helps to reduce the amount of air which escapes from the axial cooling vent 36 into the bottom interpolar channel 28. Thus, this may help to increase the total amount of air flow through the axial cooling vent.

Figure 13 shows another embodiment of a spacer. The spacer 100 of Figure 13 is similar to that of Figures 9 to 12. However, in the arrangement of Figure 13, a first, larger foot 102 is provided on one side of the spacer, and a second smaller foot 104 is provided on the other side of the spacer. The second foot 104 has an essentially rectangular cross section in the x-z plane. However, the first foot 102 has an essentially triangular cross section in the x-z plane, with a sloping edge 103.

20 In use, the spacer 100 is placed on the rotor such that the first foot 102 is upstream and the second foot 104 is downstream relative to the direction of air flow. The two feet 102, 104 are used to locate the spacer on the winding support and to prevent movement during winding, in a similar way to the spacer of Figures 9 to 12. However, in the spacer of Figure 13, the first foot 102 extends further into the bottom interpolar channel 28 than the second foot. The sloped edge 103 deflects air flow from the interpolar channel 28 into the air passages in the spacer. By extending the foot 102 further into the interpolar channel, more air flow may be deflected into the air passages in the spacer. Thus, this arrangement may help to increase the total amount of air flow through the axial cooling vent.

If desired, the feet 102, 104 of Figure 13 could be used with any of the other embodiments described above.

The spacers described above may be manufactured from any suitable material. However, an electrically insulating material is preferred so that the spacer can make direct contact with the windings without the addition of an insulation barrier. For example, the spacers may be manufactured from a heat resistant plastic, such as a polyamide. In one embodiment, the spacer is moulded using a glass filled Nylon 66 material. However, alternative materials and/or alternative manufacturing methods, including machining the spacer from solid or casting the spacer, could be used instead.

Preferred embodiments of the invention have been described above by way of example only, and modification in detail are possible. For example, a range of different spacer thicknesses may be used in order to widen or narrow the space created in the rotor windings. Features of one embodiment may be used with any other embodiment. Other modifications will be apparent to the skilled person within the scope of the claims.

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