WO2015170100A1

WO2015170100A1 - Cooling assembly for rotating electrical machine

Info

Publication number: WO2015170100A1
Application number: PCT/GB2015/051334
Authority: WO
Inventors: Robert Mitchell Rolston; Daniel CIOCHINA; Adnan MEHMEDOVIC
Original assignee: Cummins Generator Technologies Limited
Priority date: 2014-05-08
Filing date: 2015-05-07
Publication date: 2015-11-12
Also published as: GB201618417D0; GB2540499A; GB2540499B; GB201408127D0

Abstract

A cooling assembly for a rotating electrical machine is disclosed. The cooling assembly comprises a plenum (40, 42; 60; 70) located within the machine housing (26, 28). The plenum distributes air flow produced by an external fan (36, 38) within the interior of the machine. The plenum comprises a first chamber, and is arranged to receive airflow via a second chamber. The second chamber may be formed from an external plenum, or by an internal partition within the plenum. An air inlet duct (62, 72) may admit air tangentially into the plenum. This may improve the distribution of airflow within the machine.

Description

COOLING ASSEMBLY FOR ROTATING ELECTRICAL MACHINE

The present invention relates to a cooling assembly for a 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 is usually arranged to produce a magnetic field which crosses an air gap between the rotor and stator. In many machines the magnetic field is produced by passing an electrical current through rotor windings. In the case of a generator, when the rotor is rotated by a prime mover, the rotating magnetic field causes an electrical current to flow in the stator windings, thereby generating the output power. In the case of a motor, an electrical current is supplied to the stator windings and the thus generated magnetic field causes the rotor to rotate.

When the machine is in operation, currents passing through the rotor and stator 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.

Known techniques for cooling a rotating electrical machine involve mounting a fan on the rotor shaft. Shaft-mounted fans are fixed-speed devices which deliver the same quantity of cooling air irrespective of the cooling demand. As a result the fans are sized for the worst-case scenario and hence usually deliver more air-flow and consume more power than is actually required in most operating conditions.

As an alternative to shaft-mounted fans, it has been proposed to use remote fans which are not mounted to the rotor shaft. Remote fans can be driven

independently by electric motors. This can allow the air flow, and hence the power consumption, to be minimised in line with actual cooling requirements.

WO 2009/045958 discloses a fan assembly which is provided independently of the rotating field assembly. The fan assembly comprises four individual fans bolted to a mounting plate. However, a problem with remote fans is that they may not provide all of the cooling effects which would be achieved with shaft-mounted fans. In particular, remote fans may not distribute air uniformly around the machine circumference and/or may not distribute air as effectively towards some components such as stator end-windings.

According to a first aspect of the present invention there is provided a cooling assembly for a rotating electrical machine, the cooling assembly comprising a plenum for distributing air flow produced by an external fan within an interior of the machine, the plenum comprising a first chamber which is arranged to receive airflow via a second chamber.

The present invention may provide the advantage that, by providing a plenum for distributing air flow produced by an external fan within an interior of the machine, it may be possible to use an external fan while still achieving at least some of the cooling effects which would be obtained with an internal shaft-mounted fan.

Providing a second chamber may facilitate distribution of air flow.

Preferably the plenum is arranged to distribute air flow circumferentially around the interior of the machine. In order to help achieve this, the plenum may comprise an annular chamber. The annular chamber may form a continuous loop around the machine. Thus air may flow circumferentially around the axis of the machine through the annular chamber. Alternatively, rather than an annular chamber, a chamber which extends part-way around the machine could be used, or a plurality of chambers could be provided at different locations around the machine.

The second chamber may be may be annular or partially annular, or some other shape. The second chamber may be formed by an interior wall in the plenum, or may be formed by a second, separate plenum.

The second chamber may be arranged to distribute air flow circumferentially around the plenum. In order to help achieve this, the first chamber and the second chamber may be adjacent to each other for at least part of the way circumferentially around the axis of the machine. This may facilitate

circumferential distribution of air flow.

The air flow in the machine preferably has an axial component (i.e. a component which is in the direction of the axis of the machine). This may facilitate air flow through the machine, for example, through the air gap between the rotor and the stator. The air flow may also have a radial component and/or another component such as a circumferential component. The second chamber may be arranged to divert air flow so as to increase an axial component of the air flow in the plenum. This may help to direct airflow axially through the machine.

In some embodiments the external fan is arranged to blow air towards the machine. In these embodiments the plenum may be arranged to receive air flow from the fan and direct it to the machine. In other embodiments, the fan is an exhaust fan which is arranged to draw air through the machine. In these embodiments the plenum may be arranged to receive air flow from an exterior of the machine, for example, from the atmosphere, and to direct airflow drawn by the fan through the machine.

The plenum may comprise a plurality of outlets for distributing air flow within the interior of the machine. By distributing air flow within the interior of the machine, different parts of the machine may be cooled.

The plenum may comprise a plurality of nozzles for directing air flow towards the machine. This may help to ensure that air is directed to those areas which are most likely to experience the highest temperatures, such as stator windings and rotor windings. The plurality of nozzles may be distributed circumferentially about the plenum. The size, shape and positioning of the nozzles may be adjusted in order to achieve the required distribution of air flow.

Some of the nozzles may be arranged to direct air flow in a different direction to other nozzles. For example, the air flow produced by some of the nozzles may have a different radial component than that of other nozzles. This can allow different parts of the machine to be cooled.

In one example, some nozzles produce air flow having an outwards radial component, while other nozzles produce air flow having an inwards radial component. In both cases the air flow may also have an axial component. This may facilitate cooling of both rotor windings and stator windings. Thus the plenum may comprise a first set of nozzles for directing air flow towards a stator of the machine, and a second set of nozzles for directing air flow towards a rotor of the machine.

The nozzles may be arranged to create jets of air. The jets of air may impinge on surfaces such as rotor windings and stator windings, and may enhance

convection cooling on these surfaces.

Preferably the plenum is located inside a machine housing. This can help to distribute air to internal components such as rotor windings and stator windings.

In some configurations, the interior space within the machine may be limited, which may limit the space available for the plenum. If the cross-section of the plenum is too small, the air velocity may be too high, which may lead to excessive pressure drops and variations in flow distribution.

In one embodiment of the invention, the plenum is an internal plenum which is located inside the machine housing, and the second chamber is formed by an external plenum located outside of the machine housing. This may help to improve the flow distribution, particularly where limited space is available for the internal plenum. Preferably the external plenum is arranged to transfer air flow between an exterior of the machine and the internal plenum. For example, the external plenum may be arranged to distribute air flow circumferentially around the internal plenum. The external plenum may extend all or part of the way around the exterior of the machine. In some cases the external fan may be mounted radially outwards of the machine. Thus air flow from the fan may be in a substantially radial direction. However it may be desirable for the air flow to have at least an axial component, in order for it to pass more readily through the machine. Thus the external plenum may be arranged to divert air flow so as to increase an axial component of the air flow. For example, the external plenum may be arranged to divert air flow from a substantially radial and/or circumferential direction to a substantially axial direction. The cooling assembly may further comprise a plurality of transfer ports for connecting the external plenum with the internal plenum. The transfer ports may pass through the machine housing. The size, shape and positioning of the transfer ports may be adjustable in order to achieve the required distribution of air flow.

In another embodiment of the invention, the plenum comprises an interior wall which divides the plenum into the first chamber and the second chamber. Thus in this embodiment the two chambers are each part of the plenum. The two chambers may be annular or partly annular, and may extend all or part of the way around the machine. Preferably the two chambers are located adjacent to each other as they extend around the axis of the machine. The plenum may comprise further internal walls and/or chambers if desired.

In this embodiment the internal wall is preferably arranged to separate inlet and outlet air flows. The internal wall is preferably perforated, in order to allow air flow from one chamber to the other. Preferably airflow from one chamber to the other is in a substantially axial direction.

In the above embodiment the plenum may be located within the machine housing, and may be arranged to receive air flow directly from outside the machine (rather than through an external plenum). In order to help achieve this, the cooling assembly may comprise an inlet duct for connecting the plenum to an exterior of the machine. For example, the inlet duct may be connected to the fan or (in the case of an exhaust configuration) the inlet duct may be connected to the atmosphere. The inlet duct may be arranged to admit air tangentially into the plenum. This may help to achieve a relatively smooth path for air flow, and thus may help to avoid pressure drops.

Where air is admitted tangentially into the plenum, it may have a significant circumferential component. While this may be useful to ensure that air flow is distributed circumferentially about the machine, it may be preferred for air flow within the machine to be in a substantially axial direction. Thus the plenum may comprise means for diverting air flow to increase an axial component of the air flow. For example, the plenum may comprise means for diverting air flow from a substantially circumferential direction to a substantially axial direction.

In order to help divert air flow, the internal wall may comprise flaps for directing air flow through the perforations. The flaps may be arranged to divert air flow to increase an axial component of the air flow. For example, the flaps may be arranged to divert air flow from a substantially circumferential direction to a substantially axial direction. The flaps may be on either the entrance or the exit side of the perforations, or a combination of the two.

Alternatively or in addition, the plenum may comprise flaps adjacent to the outlets. The flaps may be arranged to divert air flow, for example, from a substantially circumferential direction to a substantially axial direction. The flaps may be located on either the entrance or the exit side of the outlets, or a combination of the two.

According to another aspect of the present invention there is provided a cooling assembly for a rotating electrical machine, the cooling assembly comprising a plenum for distributing air flow produced by an external fan within an interior of the machine, and an inlet duct arranged to admit air tangentially into the plenum. This aspect is linked to the first aspect by the common inventive concept of ensuring sufficient circumferential distribution of airflow within the plenum.

The plenum may comprise a single chamber, or two or more chambers divided by one or more internal walls. The plenum may comprise flaps arranged to divert air flow from a substantially circumferential direction to a substantially axial direction. The flaps may be located, for example, at the outlets of the plenum.

In any of the above arrangements the plenum may be arranged for connection to a plurality of fans, either via inlet/outlet ducts or via one or more external plenums, or both. This may help to achieve more even distribution of air flow, and may provide some redundancy in case of failure or servicing of a fan.

Preferably the cooling assembly further comprises at least one fan for producing airflow through the plenum. The fan may be an electrical fan. This may allow control of the fan speed to achieve the desired amount of air flow. Control means may be provided for controlling the speed of the fan. The control means may be responsive, for example, to a sensed temperature which may be a machine temperature and/or an ambient temperature, and/or machine load.

Preferably the cooling assembly is arranged such that, in operation, a pressure within the plenum is higher than a pressure within the interior of the machine.

According to another aspect of the present invention there is provided a rotating electrical machine comprising a cooling assembly in any of the forms described above. The machine may comprise a rotor and a stator, and the cooling assembly may be arranged to direct air flow towards the rotor and the stator. For example, the cooling assembly may be arranged to direct air flow towards stator windings and rotor windings. Preferably the machine comprises a machine housing and the or each plenum is located inside the housing.

The cooling assembly may be provided at one end of the machine, or a cooling assembly may be provided at each end of the machine. In the latter case the cooling assemblies need not be the same, and each may be in any of the forms described above.

The fan may be mounted at the same end of the machine as the plenum, for example when in a blow configuration. Alternatively the fan may be mounted at the opposite end of the machine from the plenum, for example when in an exhaust configuration. Where two or more fans are provided, they may both be mounted at the same end as or opposite end to the plenum, or some combination of the two. Alternatively, in an exhaust configuration, the fan and/or exhaust may be mounted in the middle of the machine with an inlet at one or both ends. Any combination of these configurations may be used.

According to another aspect of the present invention there is provided a method of cooling a rotating electrical machine, the method comprising distributing air flow produced by an external fan within an interior of the machine through a plenum, the plenum comprising a first chamber which receives airflow via a second chamber.

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 specification terms such as "circumferential", "radial" and "axial" are generally defined with reference to the axis of the rotating electrical machine about which the rotor rotates.

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

Figure 1 shows a cut away through a known rotating electrical machine; Figure 2 shows a cross-section through part of a rotating electrical machine in an embodiment of the invention;

Figure 3 is a cross-section through part of the machine of Figure 2;

Figure 4 is an end view of an internal plenum ;

Figure 5 is an isometric view of a rotating electrical machine in an embodiment of the invention; Figure 6 shows part of a cooling assembly in accordance with another embodiment of the invention; and

Figure 7 is a cross section through a plenum in another embodiment of the invention.

Figure 1 shows a cut away through a known rotating electrical machine.

Referring to Figure 1 , the machine comprises a rotor 10 mounted on a shaft 12. The rotor includes rotor windings 14 which in this example are wound on salient poles. The rotor 10 is located inside a stator 16. The stator includes stator windings 18 which pass through stator slots. The stator windings include overhangs 19 where the windings pass around the outside of the stator from one slot to another. An exciter 20 is mounted on the rotor, and provides a rotating DC voltage for supply to the rotor windings 14. The shaft is supported at either end by bearings 22, 24. The open ends of the machine are protected by housings 26, 28, each of which forms part of an end adaptor. A terminal box 30 is connected to the housing 26, and is used to provide connections to the exciter 20 and the stator windings 18.

In the arrangement shown in Figure 1 , axial fans 32, 34 are mounted on the shaft on either side of the rotor 10. The fans 32, 34 have blades which rotate with the shaft, and provide the following cooling functions:

Push air through the machine core

Distribute air uniformly around machine circumference

Distribute air towards the stator overhangs (end-windings) to ensure adequate cooling. This takes place mainly by natural spillage from the tips of the rotating blades.

The shaft-mounted fans of Figure 1 rotate at the same speed as the rotor, and thus deliver the same quantity of cooling air irrespective of the cooling demand as defined by machine load and ambient conditions. As a result the fans are sized for the worst-case scenario and hence usually deliver more air-flow and consume more power than is actually required in most operating conditions. However, simply replacing the shaft-mounted fans with remote-mounted fans blowing un- controlled air into the end-adaptors may not provide all of the cooling functionality of shaft-mounted fans. In particular, such remote mounted fans may not provide uniform air distribution, nor distribution of air towards the stator overhangs.

Embodiments of the present invention relate to techniques for improving the delivery of cooling air by using remote-mounted cooling fans, while still providing at least some of the cooling functionality of shaft-mounted cooling fans.

Figure 2 shows a cross-section through part of a rotating electrical machine in an embodiment of the invention. Parts which correspond to those shown in Figure 1 are given the same reference numerals, and are not described further.

Referring to Figure 2, the electrical machine is provided with external fans 36, 38 which are mounted on the outside of the machine (i.e. outside of the housings 26, 28). Each of the fans 36, 38 is driven independently by an electric motor. This can allow the amount of air flow, and hence the power consumption, to be adjusted in accordance with the cooling requirements of the machine under different operating conditions. Air flow from the external fans is distributed to the inside of the machine by means of internal plenums 40, 42. In the arrangement of Figure 2, the internal plenums 40, 42 are provided inside the machine housings 26, 28, on either side of the rotor 10. Thus the internal plenums 40, 42 are located in approximately the same positions as the internal fans 32, 34 shown in Figure 1 . Each of the internal plenums 40, 42 is "doughnut" shaped, and forms an annular chamber around the inside of the machine. The internal plenums include outlet nozzles which distribute airflow around the machine. The outlet nozzles are positioned so as to achieve similar cooling functions as would be provided by an internal fan.

The internal plenums 40, 42 are connected to the external fans 36, 38 via external plenums 44, 46. The external plenums 44, 46 are located outside the housings 26, 28, and direct air flow from the fans 36, 38 to the internal plenums 40, 42. To achieve this, a number of transfer ports are provided passing through the housings 26, 28 to connect the external plenums 44, 46 with the internal plenums 40, 42. The external plenums provide a more even distribution of air flow around the circumference of the machine than would otherwise be the case. The size and location of the transfer ports may be adjusted to achieve the required distribution of air. In the present embodiment the external plenums 44, 46 extend roughly halfway around the machine, although this may be varied as required.

In the arrangement of Figure 2, the external plenums 44, 46 are provided due to the limited amount of space available for the internal plenums 40, 42. While it would be possible to connect the fans 32, 34 directly to the internal plenums 40, 42, if the internal plenum cross-section is too small, the circumferential air velocity may be too high leading to excessive pressure drops and variations in flow distribution. Thus in this arrangement the external plenums 44, 46 are provided to help distribute air flow circumferentially around the internal plenums 40, 42. However it should be noted that the external fans 32, 34 may be connected directly to the internal plenums 40, 42 without the use of external plenums.

Figure 3 is a cross-section through part of the machine of Figure 2. Air flow through the machine is indicated with arrows. Referring to Figure 3, the fan 36 is located radially outwards of the machine. Thus air flow is initially in a radial direction from the fan 36 towards the external plenum 44. In the external plenum 44 the air is distributed circumferentially, and is diverted from a radial direction to an axial direction. Air then flows axially through the transfer ports and into the internal plenum 40. The internal plenum 40 directs air to the stator overhang windings 19 and the rotor windings 14 at multiple positions around the

circumference of the machine, as indicated by the arrows.

Figure 4 is an end view of an internal plenum, showing schematically the positions of the outlet nozzles. Referring to Figure 4, the plenum 40 comprises a first set of outlet nozzles 50 and a second set of outlet nozzles 52. The first set of outlet nozzles 50 directs air towards the stator overhangs 19 and the air gap between the rotor and the stator, while the second of outlet nozzles 52 directs the air towards the rotor windings 14. The outlet nozzles create jets of cooling-air which impinge on the rotor and stator end-windings, and enhance the convection cooling on these surfaces. The size and location of the nozzles can be adjusted to achieve the appropriate cooling at the required locations around the machine. Figure 5 is an isometric view of a rotating electrical machine incorporating external fans and an internal plenum. Referring to Figure 5, in this arrangement a first set of fans 36, 37 is provided on one side of the machine, and a second set of fans 38, 39 is provided on the other side of the machine. The fans 36, 37 are in fluid communication with the external plenum 44, and the fans 38, 39 are in fluid communication with the external plenum 46. In this example the external plenums 44, 46 extend approximately halfway around the outside of the machine due to space limitations. However, where appropriate, the external plenums may extend all of the way around the machine (i.e. through 360°), or through any other angle.

By providing two fans on each side of the machine, air can be distributed more evenly than if a single fan were used. The two fans also provide some

redundancy in case one fan should fail or require servicing.

In practice, any appropriate number of fans could be used, including a single fan at each end, and three or more fans at each end. If desired, a different number of fans could be used at each end. The fans may be of the same or similar size, or differently sized as appropriate. The fans may be individually controllable to adjust the cooling in accordance with the operating conditions of the machine. Where appropriate the fans could be connected directly to an internal plenum without the use of an external plenum. Figure 6 shows part of a cooling assembly in accordance with another

embodiment of the invention. Referring to Figure 6, the cooling assembly comprises a plenum 60 which fits inside the machine housing. The plenum comprises an air inlet duct 62 which connects to an external fan or to the atmosphere. A first set of outlets 64 is provided for directing air flow towards the machine's stator overhang windings, while a second set of outlets 66 is provided for directing airflow towards the machine's rotor windings.

In the arrangement of Figure 6, air flows directly from into the plenum 60 via the tangential inlet duct 62. This results in a relatively smooth transition to a circumferential air flow around the plenum. This arrangement may therefore avoid some of the pressure drops that may occur in an external plenum due to changes in flow direction.

The arrangement shown in Figure 6 may result in the air having a significant circumferential velocity depending upon the cross-sectional area available for the plenum. As a result the exit air-jets from the outlets 64, 66 may also have a large circumferential component. In order to limit the circumferential velocity

component, each of the outlets 64, 66 may be provided with a flap 68. The flaps 68 help to direct air flow from a circumferential direction to an axial direction. In Figure 6 the flaps 68 are shown on the entrance side of the outlets 64, 66, but they may also be located on the exit side.

Figure 7 is a cross section through a plenum in another embodiment. In this embodiment an internal perforated wall (partition) is provided inside the plenum to separate the inlet and outlet flows.

Referring to Figure 7, the plenum 70 includes an interior wall 74 which divides the plenum into two chambers. An inlet duct 72 directs incoming air into the plenum on one side of the interior wall 74. The interior wall 74 comprises perforations 76, which allow air to flow from one side of the wall to the other. Air then passes through outlets 78 in the plenum, and into the interior of the machine. The backpressure created by the internal, perforated wall significantly reduces the circumferential flow and hence maximises the axial flow velocity through the outlets.

In the arrangement of Figure 7, each of the perforations 76 has a flap 80 which helps to direct air flow from a circumferential direction to an axial direction. Each of the outlets 78 also has a flap 82 to further direct the air flow in an axial direction. In Figure 7 the flaps 80, 82 are shown on the entrance side of the perforations 76 and outlets 78, but they may also be located on the exit side.

Although a single inlet duct is shown in Figures 6 and 7, if desired two or more inlet ducts could be provided at different locations around the circumference of the plenum. In this case each of the inlet ducts may be connected to its own external fan. The plenums described above may be connected to a fan which pushes air through the plenum. Alternatively the plenums may be used with exhaust- mounted remote fans which pull rather than push air through the machine. In this case the plenum may be at the inlet and hence the air-flow may be drawn through the plenum into the machine and then into the fan. This option may simplify the design of the air inlet system on each end-adaptor. There are potential space limitations around each end-adaptor which limit the space available to house the air inlet manifold system including filter housings and fans. By moving the remote fans to the cooling-air exhaust, the cooling-air inlet design can be simplified and reduced in size. If desired, a combination of push fans and exhaust fans could be used.

In the above description, preferred features of the invention have been described with reference to various embodiments. It will be appreciated that features of one embodiment may be used with any other embodiment. Furthermore, the invention is not limited to these embodiments, and variations in detail may be made within the scope of the appended claims.

Claims

1 . A cooling assembly for a rotating electrical machine, the cooling assembly comprising a plenum for distributing air flow produced by an external fan within an interior of the machine, the plenum comprising a first chamber which is arranged to receive airflow via a second chamber.

2. A cooling assembly according to claim 1 , wherein the plenum is arranged to distribute air flow circumferentially around the interior of the machine.

3. A cooling assembly according to claim 1 or 2, wherein the plenum comprises an annular chamber.

4. A cooling assembly according to claim 3, wherein the annular chamber forms a continuous loop around an axis of the machine.

5. A cooling assembly according to any of the preceding claims, wherein the second chamber is arranged to distribute air flow circumferentially around the plenum.

6. A cooling assembly according to any of the preceding claims, wherein the first chamber and the second chamber are adjacent to each other for at least part of the way circumferentially around an axis of the machine.

7. A cooling assembly according to any of the preceding claims, wherein the second chamber is arranged to divert air flow so as to increase an axial component of the air flow in the plenum.

8. A cooling assembly according to any of the preceding claims, wherein the plenum is arranged to direct airflow from the fan to the machine.

9. A cooling assembly according to any of claims 1 to 7, wherein the plenum is arranged to direct airflow drawn by the fan through the machine.

10. A cooling assembly according to any of the preceding claims, wherein the plenum comprises a plurality of outlets.

1 1 . A cooling assembly according to any of the preceding claims, wherein the plenum comprises a plurality of nozzles for directing air flow towards the machine.

12. A cooling assembly according to claim 1 1 , wherein some of the nozzles are arranged to direct air flow in a different direction to other nozzles.

13. A cooling assembly according to claim 1 1 or 12, wherein the air flow produced by some of the nozzles has a different radial component than that of other nozzles.

14. A cooling assembly according to any of claims 1 1 to 13, wherein the plenum comprises a first set of nozzles for directing air flow towards a stator of the machine, and a second set of nozzles for directing air flow towards a rotor of the machine.

15. A cooling assembly according to any of claims 1 1 to 14, wherein the nozzles are arranged to create jets of air.

16. A cooling assembly according to any of the preceding claims, wherein the rotating electrical machine comprises a machine housing, and the plenum is located inside the machine housing.

17. A cooling assembly according to claim 16, wherein the second chamber is formed by an external plenum located outside of the machine housing.

18. A cooling assembly according to claim 17, wherein the external plenum is arranged to transfer air flow between an exterior of the machine and the internal plenum.

19. A cooling assembly according to claim 17 or 18, wherein the external plenum is arranged to distribute air flow circumferentially around the internal plenum.

20. A cooling assembly according to any of claims 17 to 19, wherein the external plenum is arranged to divert air flow so as to increase an axial component of the air flow.

21 . A cooling assembly according to any of claims 17 to 20, further comprising a plurality of transfer ports for connecting the external plenum with the internal plenum.

22. A cooling assembly according to claim 21 , wherein the transfer ports pass through the machine housing.

23. A cooling assembly according to any of claims 1 to 16, wherein the plenum comprises an interior wall which divides the plenum into the first chamber and the second chamber.

24. A cooling assembly according to claim 23, wherein the internal wall is perforated.

25. A cooling assembly according to claim 23 or 24, wherein the cooling assembly comprises an inlet duct for connecting the plenum to an exterior of the machine.

26. A cooling assembly according to claim 25, wherein the inlet duct is arranged to admit air tangentially into the plenum.

27. A cooling assembly according to any of claims 23 to 26, wherein the plenum comprises means for diverting air flow to increase an axial component of the air flow.

28. A cooling assembly according to claim 27, wherein the internal wall is perforated and comprises flaps for directing air flow through the perforations.

29. A cooling assembly according to claim 28, wherein the flaps are arranged to direct air flow from a substantially circumferential direction to a substantially axial direction.

30. A cooling assembly for a rotating electrical machine, the cooling assembly comprising a plenum for distributing air flow produced by an external fan within an interior of the machine, and an inlet duct arranged to admit air tangentially into the plenum.

31 . A cooling assembly according to claim 30, wherein the plenum comprises flaps arranged to divert air flow from a substantially circumferential direction to a substantially axial direction.

32. A cooling assembly according to any of the preceding claims, wherein the plenum is arranged for connection to a plurality of fans.

33. A cooling assembly according to any of the preceding claims, further comprising at least one fan for producing airflow through the plenum.

34. A cooling assembly according to claim 33, wherein the fan is an electrical fan.

35. A cooling assembly according to claim 34 further comprising control means for controlling the speed of the fan.

36. A rotating electrical machine comprising a cooling assembly according to any of the preceding claims.

37. A machine according to claim 36, wherein the machine comprises a rotor and a stator, and the cooling assembly is arranged to direct air flow towards the rotor and the stator.

38. A machine according to claim 36 or 37, wherein a cooling assembly is provided at each end of the machine.

39. A machine according to any of claims 36 to 38, wherein the machine comprises a housing and the plenum is located inside the housing.

40. A method of cooling a rotating electrical machine, the method comprising distributing air flow produced by an external fan within an interior of the machine through a plenum, the plenum comprising a first chamber which receives airflow via a second chamber.