The present invention relates to a hub for a rotating electrical machine, and in particular a hub for connecting a coupling disc to a shaft of the rotating electrical machine.

Rotating electrical machines such as motors and generators generally comprise a rotor mounted on a shaft and arranged to rotate inside a stator. The rotor comprises a rotor core which holds rotor windings or permanent magnets. The rotor produces a rotating magnetic field which crosses an air gap between the rotor and the stator. The stator comprises a stator core which holds stator windings which combine with the rotating magnetic field.

In the case of an electrical generator, the rotor may be coupled to a prime mover such as an internal combustion engine. In order to achieve this, a coupling disc may be provided for connecting the rotor to a rotating component of the prime mover such as an engine flywheel. The coupling disc may be mounted on a hub which is itself mounted on the rotor shaft.

When the machine is in operation, currents passing through the stator and/or rotor windings, as well as other factors such as friction and windage losses, may cause the machine to heat up. Therefore many machines, particularly those of a larger design, require some form of cooling. This may be achieved by providing a fan for forcing airflow through the machine. The fan is typically mounted on the shaft, in front of the stator windings.

In known generator designs, the fan is assembled by pressing the fan onto the shaft. However, this requires the assembly of pressing tooling and the use of pressing equipment, which may lead to a cumbersome manufacturing process. Furthermore, if the fan is pressed onto the shaft, it may be difficult to remove the fan from the shaft after the rotor has been assembled, which may make the machine difficult to service. For example, it may be necessary to replace the whole rotor if the fan is broken, which may be time consuming and expensive. It has been proposed to mount the fan on a hub which is itself mounted on the shaft. This may facilitate assembly and allow the fan to be more easily replaced if it is broken. However, this requires two separate hubs to be provided, adding to the component count and the assembly time. Furthermore, the fan hub and the coupling hub may have an angle requirement, and it is not easy to control the angle from a manufacturing point of view.

As an alternative, it has been proposed to provide a single hub for coupling both the fan and the coupling disc to the shaft. This may avoid the need to assemble two separate hubs to the shaft, which may simplify assembly and reduce the number of parts. However, in such an arrangement, the hub needs to be long enough in an axial direction to position the coupling disc at a sufficient distance from the fan to allow access to coupling bolts. Furthermore, the outside diameter of the hub needs to be sufficiently large to accommodate bolts which can provide the required load transfer characteristics between the prime mover and the shaft. Thus, this arrangement may result in a hub which is bulky, has a high moment of inertial, and/or which is expensive to manufacture.

It would therefore be desirable to provide an arrangement which can facilitate assembly and servicing of the rotor while optimising the size, mass and cost of the hub.

According to one aspect of the present invention there is provided a hub for connecting a coupling disc to a shaft of a rotating electrical machine, wherein: the hub is arranged to connect a fan to the shaft; and the hub comprises a surface with at least one recess.

The present invention may provide the advantage that a single component can be used to couple both the fan and the coupling disc to the shaft, thereby facilitating manufacture. Furthermore, by providing at least one recess in a surface of the hub, the amount of material in the hub can be reduced. This may help to reduce the mass of the hub and the cost of the hub.

The fan is preferably arranged to draw cooling air through the machine when the machine is in operation. The coupling disc is preferably arranged to couple the shaft to a prime mover (via the hub). For example, the coupling disc may be arranged to couple the shaft to a rotating component of an engine, such as an engine flywheel. Thus, the hub may be arranged to be connected to a coupling disc for coupling the rotating electrical machine to a prime mover; and the hub may be arranged to be connected to a fan for drawing cooling air through the rotating electrical machine.

The prime mover is preferably arranged to drive the shaft of the rotating electrical machine via the coupling disc and the hub. The hub is preferably arranged to transfer torque between the coupling disc and the rotating electrical machine.

The hub is preferably able to transfer sufficient torque to allow the rotating electrical machine to be driven.

The hub may comprise a mating surface for engagement with the coupling disc. The mating surface may be, for example, an end surface of the hub axially. The hub may comprise a plurality of bolt holes for connecting the coupling disc to the hub. The bolt holes may extend axially into the hub. The bolt holes may be sized and/or located to allow load transfer between the coupling disc and the shaft.

The hub may comprise at least one surface for engagement with the fan. The hub may comprise a plurality of bolt holes for connecting the fan to the hub.

The hub is preferably arranged such that the fan and the coupling disc are spaced apart axially. For example, the hub may be arranged such that the coupling disc can be mounted at one end of the hub and the fan can be mounted at the other end of the hub. This can allow access to bolts for connecting the coupling disc to a prime mover.

The hub is preferably arranged such that the fan can be assembled onto the hub from the same end of the hub as the coupling disc. For example, the hub may be arranged such that the coupling disc can be assembled from one end (the drive end) of the hub and then slid axially along the hub to the other end (the non-drive end) before being connected to the hub. This may facilitate assembly, by allowing the fan to be assembled after the hub has been connected to the shaft. Furthermore, this may facilitate servicing or replacement of the fan. The hub is preferably arranged such that the fan is removably mountable on the hub. For example, the hub may comprise a plurality of bolt holes for mounting the fan on the hub, and the fan may be removable by removing the bolts. This can facilitate servicing or replacement of the fan after the machine has been assembled.

The hub preferably comprises an inner (radially inwards) surface for engagement with the shaft. The inner surface is preferably substantially cylindrical. The hub may be assembled onto the shaft, for example, by pressing.

The at least one recess is preferably arranged to reduce the mass of the hub. Thus, when the hub is assembled in the rotating electrical machine, the mass of the hub is preferably less than it would have been without the recesses.

In one embodiment, the hub comprises an annular recess at one end (axially) of the hub. The annular recess may extend part way through an interior of the hub in an axial direction. The annular recess may also extend radially outwards from an axis of the hub and/or a radial position at which an inner surface of the hub (for example, a surface for engagement with the shaft) is located. By providing an annular recess, the mass of the hub and the amount of material used may be reduced.

Preferably, the annular recess is provided at an opposite end of the hub from the coupling disc, which may be the fan end of the hub. This may allow the mass of the hub to be reduced and the cost of the material to be reduced, while still providing sufficient load transfer ability between the coupling disc and the shaft.

In one embodiment, the annular recess is provided in an end surface of the hub (for example the non-drive end). In this case, the annular recess may be open at one end of the hub. This may facilitate manufacture of the hub, for example using a casting and/or machining process.

Where the annular recess is provided in an end surface of the hub, it may be desirable to reduce any vortex loss which might otherwise occur in the recess. Thus, in this embodiment, the hub may be arranged to receive an annular disc which at least partially closes the annular recess. For example, the hub may comprise a plurality of bolt holes for attaching the annular disc to the hub. The bolt holes may also be used for attaching the fan to the hub.

In another embodiment, the annular recess is at least partially closed at one end of the hub. For example, the hub itself may include an end wall which at least partially closes the annular recess at one end of the hub. Thus, in this embodiment, the annular recess may be provided in the interior of the hub, and may extend radially outwards from an inner surface of the hub. This feature may be included in the hub, for example, as part of a casting process.

In either of the above embodiments, in the assembled machine, the annular recess may form an annular chamber inside the hub. By providing such an annular chamber, the mass of the hub and amount of material may be reduced, while maintaining an outside diameter of the hub.

In one embodiment, the hub comprises a flange for connecting the fan to the hub. The flange is preferably provided at one end of the hub axially, which is preferably the opposite end from the coupling disc. The flange may comprise a plurality of bolt holes for connecting the fan to the hub.

In one embodiment, the flange comprises a plurality of recesses. The recesses may be provided, for example, (circumferentially) between two adjacent bolt holes. The recesses may be for example in the form of cut-outs in the flange. Thus, the recesses may extend radially into the flange from an outside surface of the flange and/or axially through the flange. The recesses may be spaced circumferentially about the flange. This embodiment may further reduce the mass of the hub, the rotational inertia and the amount of material used.

In another embodiment, which may be provided in combination with any of the above embodiments, the hub comprises a plurality of linear recesses in an outer (radially outwards) surface of the hub. The linear recesses may be in the form of troughs or channels in the outer surface of the hub. For example, the hub may be substantially cylindrical, and the linear recesses may be provided in an outer cylindrical surface of the hub. The linear recesses may run in an axial direction from one end of the hub (such as the drive end) at least part way through the hub. The linear recesses may be provided (circumferentially) between two adjacent bolt holes for connecting a coupling disc to the hub. This may allow the mass of the hub to be reduced and the cost of the material to be reduced, while still providing sufficient load transfer capability between the coupling disc and the shaft.

If desired, a flange could be provided at one end of the axial channels for connecting the fan to the hub. However, in a preferred embodiment, the linear recesses in the hub are open at both ends axially. In this embodiment, a bolt hole for connecting the coupling disc to the hub may be provided (circumferentially) between two adjacent linear recesses at one end of the hub, and a bolt hole for connecting the fan to the hub may be provided (circumferentially) between two adjacent linear recesses at the other end of the hub. Each of the bolt holes may extend axially into the hub. This may avoid the need for a flange, and thus may help to further reduce the mass of the hub and the cost of the material.

For example, the hub may be considered as comprising a protrusion between two adjacent linear recesses (circumferentially). In this case a bolt hole for connecting the coupling disc to the hub may be provided at one end of the protrusion and a bolt hole for connecting the fan to the hub may be provided at the other end of the protrusion. The protrusions may extend radially outwards and run axially at least part way through the hub.

In one embodiment, the fan is provided with a plurality of inward protrusions for connecting the fan to the hub. In this case, the fan may be arranged such that, during assembly, it can be slid along the hub with the protrusions in linear recesses on the hub. This may facilitate assembly, by allowing the fan to be assembled from the drive end of the hub and then slid along the hub to a position where it can be attached to the hub. The inward protrusions may comprise bolt holes for connecting the fan to the hub. Thus, according to another aspect of the invention, there is provided a fan assembly comprising: a hub in any of the forms described above; and a fan arranged to be mounted on the hub, wherein the fan comprises a plurality of inward protrusions for connecting the fan to the hub, and the fan is arranged such that, during assembly, the fan can be slid axially along the hub with the protrusions in linear recesses on the hub.

If desired, in the above embodiment, it would be possible for the hub to include a flange for connecting the fan to the hub. The flange may be provided at the end of the linear recesses in an axial direction. In this case, once the fan has been slid along the hub, it may be bolted to the flange.

However, in a preferred embodiment, the linear recesses are open at both ends (axially). In this case, the hub and fan may be arranged such that the inward protrusions on the fan can be slid through the linear recesses, and, once the protrusions have passed through the open ends of the linear recesses, the fan can be indexed (rotated) with respect to the hub to align the protrusions with bolt holes on the hub. The bolt holes are preferably provided between the linear recesses on the hub. The fan can then be connected to the hub using bolts which pass through the protrusions and into the bolt holes in the hub. This arrangement may help to minimise the mass of the hub and the cost of the material, while allowing the fan to be assembled from the coupling disc end (drive end) of the hub and allowing the fan to be removed or replaced for servicing.

According to another aspect of the invention there is provided a hub for connection to a shaft of a rotating electrical machine, wherein: the hub is arranged to be connected to a coupling disc for coupling the rotating electrical machine to a prime mover; the hub is arranged to be connected to a fan for drawing cooling air through the rotating electrical machine; and the hub comprises a surface with at least one recess. According to another aspect of the invention there is provided a hub assembly comprising a fan, a coupling disc, and a hub in any of the forms described above.

According to another aspect of the invention there is provided a rotor assembly comprising a rotor, a shaft, a fan, a coupling disc, and a hub in any of the forms described above. The coupling disc is preferably arranged such that it is able to transfer torque from a prime mover via the hub to the rotor, to cause the rotor to rotate. The fan is preferably arranged such that it is able to draw cooling air through the rotating electrical machine.

According to a further aspect of the invention there is provided a rotating electrical machine comprising a stator and a rotor assembly as described above.

Corresponding methods may also be provided. Thus, according to another aspect of the invention there is provided a method of assembling a fan and a coupling disc on a shaft of a rotating electrical machine, the method comprising pressing a hub onto the shaft, connecting a fan to the hub, and connecting a coupling disc to the hub, wherein the hub comprises a surface with at least one recess.

The method may comprise assembling the fan from the same end of the hub as the coupling disc. The method may comprise sliding the fan axially along the hub, indexing the fan with respect to the hub, and then attaching the fan to the hub.

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” and “circumferentially” are generally defined with reference to the axis of rotation of the shaft unless the context implies otherwise. The coupling disc end of the hub is generally referred to as the drive end, while the fan end of the hub is generally referred to as the non-drive end. 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 shows parts of a previously considered rotor assembly;

Figure 2 is a cross section through part of a rotor assembly;

Figures 3(A) and 3(B) are perspective views of a hub in an embodiment of the invention;

Figure 4 shows part of a rotor assembly with a fan and a coupling disc attached to a hub in an embodiment of the invention;

Figures 5(A) and 5(B) are perspective views of a hub in another embodiment of the invention;

Figure 6 is a cutaway view of part of a rotor assembly in an embodiment of the invention;

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

Figure 8 is a cross-section through the hub of Figure 7 when in place on a rotor shaft;

Figure 9 shows part of a rotor assembly with the hub of Figures 7 and 8;

Figure 10 is a perspective view of a hub in another embodiment of the invention;

Figure 11 shows the hub of Figure 10 together with a fan and coupling disc;

Figures 12 and 13 show a hub in another embodiment of the invention;

Figure 14 shows a fan for use with the hub of Figures 12 and 13;

Figure 15 to 17 show part of the hub and fan at different stages of assembly;

Figure 18 shows part of a rotor assembly with the fan of Figure 14;

Figure 19 is a cross-section through part of the rotor assembly; and Figure 20 illustrates air flow through the rotor assembly of Figure 19.

Figure 1 shows parts of a previously considered rotor assembly for a rotating electrical machine. Referring to Figure 1 , the rotor assembly comprises a main rotor 10 and an exciter rotor 12, both of which are mounted on a shaft 14. The main rotor 10 comprises a plurality of salient poles, each of which is wound with rotor windings 16. The rotor windings 16 include side windings which run in a substantially axial direction along the length of the main rotor, and end windings which run in a substantially tangential direction around the end of the main rotor. Rotor winding support bars 32 run in an axial direction through the main rotor and extend outwards at each end in order to support the end windings. The exciter rotor comprises exciter windings 18 which are connected to the rotor windings 16 via rotating diodes 20. The main rotor 10 is designed to fit within a main stator (not shown), and the exciter rotor is designed to fit within an exciter stator (not shown). A shaft-mounted fan 22 is provided for drawing cooling air through the machine. The rotor assembly is connectable to a prime mover by means of a coupling disc 24. For example, in the case of an internal combustion engine, the coupling disc 24 may connect to the engine flywheel. Non-rotating parts of the electrical machine are connectable to the prime mover by means of an adaptor (not shown). The adaptor is typically connected between the machine frame and the flywheel housing, and surrounds the fan 22. The adaptor may be in any form, such as any of those disclosed in WO 2019/243829, the subject matter of which is incorporated herein by reference.

In the arrangement of Figure 1 , a hub 26 is provided for connecting the coupling disc 24 to the shaft 14. The hub 26 is generally cylindrical and is shrunk onto the shaft 14. The coupling disc 24 is provided at the drive end of the hub 26 and is connected to hub by a plurality of bolts 28. The fan 22 is connected to the shaft 14 using a separate hub.

In operation, the rotor assembly is caused to rotate by the prime mover via the coupling disc 24 and the hub 26. Thus, the coupling disc 24 and hub 26 are part of the rotary load path for power transfer into the electrical machine. Excitation for the main rotor windings 16 is provided by the exciter rotor 12 via rotating diodes 20. A rotating magnetic flux produced by the main rotor 10 intersects with windings in the main stator (not shown) to produce the electrical output. Cooling air is drawn through the machine by the fan 22. Air flow is generally in an axial direction, with some of the air flow passing through channels between adjacent rotor poles.

The arrangement shown in Figure 1 is a single bearing rotor assembly in which a bearing 30 is provided at the non-drive end. The drive end of the rotor assembly is supported by bearings within the prime mover. However, a two-bearing rotor assembly, with bearings at each end, could be used instead.

Figure 2 is a cross section through part of the drive end of the rotor assembly of Figure 1 . Referring to Figure 2, it can be seen that the hub 26 is provided on the shaft 14, and the coupling disc 24 is connected to the hub 26 using bolts 28. The size of the hub 26 as well as the size and locations of the bolts 28 are chosen to provide the require load transfer characteristics between the coupling disc 24, the hub 26 and the shaft 14. The fan 22 is mounted on the shaft 14 using a separate hub 34. A gap g is provided between the fan 22 and the coupling disc 24 to allow access to bolts for connecting the coupling disc to the engine flywheel.

It has been found that the hub arrangement shown in Figures 1 and 2 may suffer from a number of limitations. For example, it is necessary to provide a separate hub 34 for connecting the fan to the shaft. The fan needs to be assembled onto the shaft before the hub 26 and the coupling disc, which may complicate assembly and servicing. The hub needs to be sufficiently long in an axial direction to space the fan away from the coupling disc to allow access. Furthermore, the outside diameter of the hub needs to be sufficiently large to accommodate bolts which can provide the required load transfer between the prime mover and the shaft. This may result in a hub with a relatively large mass and high material costs.

Embodiments of the present invention provide an integrated hub which can couple both a fan and a coupling disc to the shaft of a rotating electrical machine. This can facilitate assembly and reduce the risk of incorrect installation. The hub is designed so that the fan can be removed from the hub after assembly, to allow servicing or replacement of the fan. The hub is designed to optimise the amount of material used, thereby reducing its mass and reducing the cost of the material.

Figures 3(A) and 3(B) show two perspective views of a hub in one embodiment of the invention. The hub of Figures 3(A) and 3(B) is designed to fit onto the shaft of a rotor such as that shown in Figure 1 . Referring to Figures 3(A) and 3(B), the hub 40 comprises a hub body 42 and a flange 44. The hub body 42 is generally cylindrical, and has a cylindrical inner surface 46 which is designed to engage with the rotor shaft. The hub 40 is an integrated hub which is designed to connect both a fan and a coupling disc to the shaft.

Referring to Figure 3(A), the hub body 42 has an end surface 48 at one end of the hub axially (the drive end). The end surface 48 provides a mating surface for engagement with a coupling disc for coupling the electrical machine to a prime mover. The end surface 48 comprises a plurality of bolt holes 50. The bolt holes 50 are provided at spaced locations circumferentially around the hub and extend into the hub body 42 in an axial direction to a predetermined depth. The bolt holes 50 are arranged to align with corresponding bolt holes in the coupling disc. The bolt holes 50 are tapped in order to engage with bolts which connect the coupling disc to the hub. The number of bolt holes 50, the size of the bolt holes 50 and the radius at which they are located are chosen to provide the required torque transmission characteristics between the prime mover and the electrical machine.

Referring to Figures 3(A) and 3(B), the flange 44 is provided at the other end axially of the hub from the end surface 48 (the non-drive end). The flange 44 is an annular flange and extends radially outwards from the hub body 42. The flange 44 is used to mount a fan onto the hub 40. The flange comprises a plurality of bolt holes 52 for connecting the fan to the hub. The bolt holes 52 are provided at spaced locations circumferentially around the flange 44 and extend through the flange in an axial direction. The bolt holes 52 are arranged to align with corresponding bolt holes in the fan. The bolt holes 52 for connecting the fan can be smaller than the bolt holes 50 for connecting the coupling disc due to the lower torque transmission requirements.

Figure 4 shows part of a rotor assembly with a fan and a coupling disc attached to the hub. Referring to Figure 4, the rotor assembly includes rotor shaft 14, hub 40, fan 54 and coupling disc 24. The hub 40 is located on the rotor shaft 14.

The fan 54 comprises an inlet ring 55, a back plate 56, a plurality of fan blades 57 and an inner mounting ring 58. The mounting ring 58 is used to mount the fan 54 on the hub 40. The mounting ring 58 comprises a plurality of bolts holes 59 which are spaced circumferentially about the radially inwards side of the fan. The bolt holes 59 are arranged to align with the bolt holes 52 in the flange 44. The fan 54 is attached to the hub 40 using bolts which pass through the bolt holes 52 in the flange 44 and into the bolt holes 59 in the fan (or vice versa).

The coupling disc 24 comprises an outer pitch of holes 25 for connecting the coupling disc to the engine flywheel, and an inner pitch of bolt holes for connecting the coupling disc 24 to the hub 40. The coupling disc 24 is connected to the hub 40 using bolts 28 which pass through the inner bolt holes in the coupling disc and into the bolt holes 50 in the hub body 42. The size of the bolts 28, the number of bolts, and their location in a radial direction are chosen to provide the required torque transmission characteristics between the engine and the electrical machine.

During assembly, the hub 40 is first pressed onto the rotor shaft 14. The fan is then assembled from the drive end and slid axially along the hub body 42 until it engages with the flange 44. The fan 54 is then connected to the hub 40 using bolts which pass through the bolt holes 52 in the flange 44 and into the bolt holes 59 in the fan(or vice versa). The coupling disc 24 is then attached to the hub using the bolts 28 which pass through the coupling disc and into the bolt holes 50.

The arrangement of Figures 3 and 4 allows the fan to be assembled from the drive end of the hub and then bolted to the hub. This can avoid the need to press the fan or a fan hub onto the shaft, which can facilitate assembly and reduce the risk of incorrect assembly. Furthermore, the fan can be easily disconnected and replaced in case of damage, which can facilitate maintenance and reduce maintenance costs.

Referring back to Figure 3(B), it can be seen that the end surface, axially, of the hub at the non-drive end comprises an annular recess 60. The annular recess 60 is located radially inwards of the flange 44 and extends axially into the hub to a predetermined depth. The annular recess 60 reduces the axial length of the cylindrical inner surface 46, and thus reduces the length of the engagement between the hub 40 and the shaft 14. In this embodiment, the depth of the annular recess 60 is such that it does not reach the ends of the bolt holes 50 for connecting the coupling disc to the hub.

The effect of the annular recess 60 is to remove some of the material from the inside of the hub. This can reduce the weight of the hub and reduce the hub material cost, while allowing the coupling disc to be located away from the fan. The annular recess 60 is provided at the non-drive end of the hub, which helps to ensure that sufficient load transfer capability is maintained between the coupling disc and the shaft.

Referring to Figures 3(A) and 3(B), it can also be seen that the hub 40 comprises a plurality of linear recesses 62 in the outer surface of the hub body 42. The linear recesses extend radially inwards from the outer surface of the hub body 42. Each linear recess 62 forms a channel or trough between two adjacent bolt holes 50 in a circumferential direction. The linear recesses 62 run axially from the end face 48, part way along the hub body 42. The linear recesses 62 remove some of the material from the hub, thereby reducing its weight and the cost of the hub material. In this embodiment, the linear recesses have a substantially U-shaped cross section with a curved surface. However, if desired, different shapes and/or straight surfaces could be used.

Since the linear recesses 62 are provided between the bolt holes 50 circumferentially, the outside diameter of the hub at locations radially outwards of the bolt holes 50 remains the same as if the linear recesses were not present. This can allow the bolts 28 to be sized and located at a radius which can provide the required torque transmission characteristics between the engine and the electrical machine.

In the arrangement of Figures 3 and 4, the annular recess 60 extends axially into the hub until a point which is before the ends of the linear recesses 62. Thus, the annular recess 60 does not overlap with the linear recesses 62 in an axial direction. This ensures that there is sufficient material between the annular recess 60 and the linear recesses 62 to maintain the strength of the hub. Furthermore, in this embodiment, the annular recess 60 has an outside diameter which is larger than the inside diameter of the pitch of holes 50. Thus, the annular recess 60 extends axially into the hub to a point which is before the ends of the holes 50. For example, the linear recesses 62 may extend axially along the outer surface of the hub body 42 for a distance which is at least as far as the depth of the bolt holes 50, and the annular recess 60 may extend axially into the hub until a point which is before the ends of the linear recesses 62. However, it will be appreciated that any of these dimensions and relationships may be varied as required.

The hub 40 of Figures 3 and 4 may be made from any suitable material, for example, a metal such as a ferrous metal or an aluminium-based alloy. The hub may be manufactured, for example, using a casting process and/or machining.

Figures 5(A) and 5(B) are perspective views of a hub in another embodiment.

The hub of Figures 5(A) and 5(B) comprises a hub body 42 and a flange 44 which are similar to those of the hub of Figures 3(A) and 3(B). The hub of Figures 5(A) and 5(B) includes an annular recess 60 and linear recesses 62 which remove some of the material. However, in the arrangement of Figures 5(A) and 5(B), the flange 44 also has recesses 45 in locations circumferentially between adjacent bolt holes 52. The recesses 45 are in the form of cut-outs in the flange 44. Thus, in the locations of the recesses 45, the outer diameter of the flange 44 is reduced. In this case, the diameter at the inside of the recesses 45 is approximately equivalent to the outside diameter of the hub body 42, although other configurations are also possible. The recesses 45 further reduce the mass of the hub, its rotational inertia, and the amount of material used.

Figure 6 is a cutaway view of part of a rotor assembly in one embodiment. Referring to Figure 6, the rotor assembly includes rotor shaft 14, hub 40, fan 54 and coupling disc 24. The hub 40 is located on the shaft 14. The coupling disc 24 is attached to the hub 40 using bolts 28. The fan 54 is attached to the hub 40 using bolts 64 which pass through the bolt holes 52 in the flange 44 and into the bolt holes 59 in the fan 54. In this embodiment, the bolt holes 59 in the fan 54 are tapped in order to engage with the bolts 64.

Also shown in Figure 6 is an annular disc 66. The annular disc is located at the non-drive end of the hub 40, facing the rotor 10. The annular disc 66 has an inner edge with a diameter which is slightly larger than the diameter of the shaft 14, and an outer edge with a diameter which is approximately equal to the outer diameter of the flange 44. The annular disc 66 is attached to the end face of the flange 44 using the same bolts 64 which are used to attach the fan 54 to the flange 44. The bolts 64 pass through bolt holes in the annular disc 66, through the bolt holes 52 in the flange, and into the bolt holes 59 in the fan 54. However, it will be appreciated that if desired different bolts could be used to attach the annular disc to the hub.

The annular disc 66 functions to cover the annular recess 60 in the end face of the hub. This can help to reduce vortex loss due to eddy currents in the space created by the annular recess 60. Thus, the annular disc 66 can help to ensure smooth air flow from the rotating electrical machine to the fan 54, which may help to optimise the cooling.

The annular disc 66 may be a single piece, in which case it is assembled onto the shaft before the fan 54. Alternatively, the annular disc may be divided into two or more parts, in which case it can be assembled after the fan.

Figure 7 shows a hub in another embodiment of the invention. Referring to Figure 7, in this embodiment, the hub itself comprises an end wall 68 which replaces the separate annular disc 66 of Figure 5. The end wall 68 may be formed, for example, as part of a casting process. The end wall 68 can help to reduce vortex loss due to eddy currents in a similar way to the annular disc 66 of Figure 6.

Figure 8 is a cross-section through the hub of Figure 7 when in place on the rotor shaft. Referring to Figure 8, in this embodiment the end wall 68 closes the annular recess 60 when the hub is on the shaft 14, in order to reduce or avoid vortex loss. This creates an annular chamber inside the hub. The annular chamber reduces the mass of the hub and the amount of material which is used in its manufacture, while allowing the coupling disc to be located away from the fan. The annular chamber is at the non-drive end of the hub, which helps to ensure that sufficient load transfer capability is maintained between the coupling disc and the shaft. In this embodiment, the shaft 14 includes a flange 69 which engages with the end wall 68 of the hub. The flange 69 helps to prevent air flow into the chamber created by the annular recess 60. The flange 69 also helps to locate the hub on the shaft.

Figure 9 shows part of a rotor assembly with the hub of Figures 7 and 8. Referring to Figure 9, the rotor assembly comprises main rotor 10, fan 54 and coupling disc 24. The fan 54 and the coupling disc 24 are mounted on a hub in the way described above. It can be seen that the end wall 68 allows smooth air flow from the rotor 10 into the fan 54. In particular, air flow through interpolar channels beneath the windings 16 in the rotor 10 is directed into the fan 54 by the end wall 68. This helps to optimise cooling efficiency. At the same time, the chamber created inside the hub by the annular recess 60 reduces the mass of the hub and the amount of material used.

Figure 10 is a perspective view of a hub in another embodiment. The hub of Figure 10 is similar to that of Figures 7 to 9 and includes an end wall 68 at the non-drive end. The hub of Figure 10 includes an annular recess 60 and linear recesses 62 which remove some of the material. However, in the arrangement of Figure 10, the flange 44 also has recess 45 in locations circumferentially between adjacent bolt holes 52. The recesses 45 are in the form of cut-outs in the flange 44, in a similar way to those described above with reference to Figures 5(A) and 5(B). The recesses 45 further reduce the mass of the hub and the amount of material used.

Figure 11 shows the hub of Figure 10 with the fan and coupling disc in place. The fan 54 is connected to the coupling disc using bolts which pass through the holes 52 in the flange 40 and into the holes 59 in the fan 54.

Figure 12 shows a hub in another embodiment of the invention. The hub of Figures 12 is designed to engage with a rotor shaft in a similar manner to the hub of Figures 3 to 11 . Referring to Figure 12, the hub 70 is generally in the form of a hollow cylinder, and has a cylindrical inner surface 72 which is designed to engage with the rotor shaft. One end of the hub 70 has a mating surface 74 for engagement with a coupling disc. The mating surface 74 comprises a plurality of bolt holes 76. The bolt holes 76 are provided at spaced locations circumferentially around the hub and extend into the hub in an axial direction to a predetermined depth, in a similar way to the arrangement of Figures 3 to 11 .

In the arrangement of Figure 12, a plurality of linear recesses 78 are provided in the outer surface of the hub 70. Each linear recess 78 forms a channel between two adjacent bolt holes 76 circumferentially. In this embodiment, the linear recesses 78 run continuously through the hub in an axial direction and are open at each end. The linear recesses 78 form radial protrusions 80 which run axially through the hub.

As in the previous embodiments, the linear recesses 78 remove some of the material from the hub, thereby reducing its weight and the cost of the hub material. However, in the arrangement of Figure 12, the hub does not a have flange for attaching the fan. Instead, the hub includes a portion 82 at the nondrive end which is recessed with respect to the maximum outside diameter of the hub (for example, to the same depth as the linear recesses 78). The fan is attached to the hub using the protrusions 80 as will be explained below.

Figure 13 shows the hub of Figure 12 in place on a rotor shaft. Referring to Figure 13, the hub 70 comprises a plurality of bolt holes 84 in the protrusions 80 at the non-drive end of the hub. The bolt holes 84 extend into the protrusions 80 in an axial direction. The bolt holes 84 are used to attach a fan to the hub. Also shown in Figure 13 is an annular recess 86 at the non-drive end of the hub. The annular recess 86 removes some of the material from the hub, in a similar way to the annular recess in the hub of Figures 3 to 11 .

Figure 14 shows a fan for use with the hub of Figures 12 and 13. Referring to Figure 14, the fan 90 comprises an inlet ring 91 , a back plate 92, a plurality of fan blades 93 and an inner mounting ring 94. The mounting ring 94 comprises a plurality of protrusions 95 which extend radially inwards. The protrusions 95 are provided at spaced locations circumferentially about the inner ring 94. Each of the protrusions 95 comprises a bolt hole which extends axially into the protrusion. The bolt holes are used to connect the fan 90 to the hub 70. During assembly, the hub 70 of Figures 12 and 13 is first pressed onto the rotor shaft 14. The fan 90 is then assembled to the hub 70 from the drive end. The inward protrusions 95 on the fan 90 are arranged to fit into the linear recesses 78 on the hub 70. This allows the fan to be slid axially along the hub, with the protrusions 95 on the fan located inside the linear recesses 78 on the hub.

Figure 15 shows part of the hub and fan during assembly with the fan 90 slid part way along the hub 70. As can be seen from Figure 15, the protrusions 95 on the fan 90 fit into the linear recesses 78 between the protrusions 80 on the hub. This allows the fan 90 to be slid axially along the hub 70. The fan 90 is slid along the 70 until the protrusions 95 exit the linear recesses 78 at the non-drive end.

Figure 16 shows part of the hub and fan once the protrusions 95 have exited the linear recesses 78. At this point, the protrusions 95 are located radially outwards of the recessed portion 82 of the hub 70. The fan 90 can then be indexed (rotated) around the hub 70 until the holes 96 in the protrusions 95 on the fan are aligned with the holes 84 in the hub 70.

Figure 17 shows part of the hub and fan once the fan 90 has been indexed around the hub 70. At this point, the holes 96 in the protrusions 95 are aligned with the holes 84 in the hub 70. The fan 90 can then be bolted to the hub 70 using bolts which pass through the holes 96 in the fan and into the holes 84 in the hub. The coupling disc is then attached to the hub in the manner described above.

Figure 18 shows part of a rotor assembly with the fan of Figures 14 to 17 in place. Referring to Figure 18, the rotor assembly comprises main rotor 10, rotor shaft 14 and fan 90. The fan 90 is mounted on the shaft 14 using a hub 70 in the manner described above with reference to Figures 12 to 17.

In the arrangement of Figure 18, an annular disc 97 is provided at the non-drive end of the hub. The annular disc 97 has an inner edge with a diameter which is slightly larger than the diameter of the shaft 14, and an outer edge with a diameter which is approximately equal to the inner diameter of the inner ring 94 of the fan 90. The annular disc 97 helps to ensure smooth air flow from the rotor 10 into the fan 90, which helps to optimise cooling efficiency. At the same time, the chamber created inside the hub 70 by the annular recess reduces the mass of the hub and the amount of material used.

The annular disc 97 is attached to the hub 70 using bolts 98. The bolts 98 pass through holes in the annular disc 97, through the holes 96 in the protrusions 95 on the fan, and into the holes 84 in the hub 70. Thus, in this arrangement, the same bolts 98 are used to attach both the annular disc 97 and the fan 90 to the hub. However, it will be appreciated that different bolts could be used if desired.

Figure 19 is a cross-section through part of the rotor assembly. Referring to Figure 19, the hub 70 is pressed onto the shaft 14. The fan 90 and the annular disc 97 are attached to the hub 70 using bolts 98. The coupling disc 24 is attached to the hub 70 using bolts 28. The annular recess 86 forms a chamber inside the hub which is closed by the annular ring 97.

Figure 20 illustrates some of the air flow through the rotor assembly of Figure 19. Referring to Figure 20, in operation, the fan 90 draws air through the machine. Air flow is in a generally axial direction. Some of the air flow is in channels between two adjacent rotor poles beneath the rotor windings 16. The air flow from these interpolar channels is diverted outwards by the annular disc 97 towards the fan 90, as shown by the dashed arrow. This prevents vortex loss due to eddy currents in the space created by the annular recess 86. Thus, the annular disc 97 helps to ensure smooth air flow from the rotating electrical machine to the fan 90.

The arrangements described above can facilitate assembly and servicing by providing a single hub for connecting a fan and a coupling disc to the rotor shaft, while at the same time optimising the amount of material used in the hub, thereby reducing the rotating mass and the cost of material.

Embodiments of the present invention have been described above by way of example only, and variations in detail will be apparent to the skilled person. Although embodiments of the invention have been described with reference to a synchronous generator, the hubs disclosed herein may be used with any type of rotating electrical machine with a fan and a coupling disc, including any type of motor or generator.