WO2005022718A1

WO2005022718A1 - Laminated stator with cooling fins

Info

Publication number: WO2005022718A1
Application number: PCT/GB2004/003708
Authority: WO
Inventors: William Brian Turner; Philip David Bend
Original assignee: Newage International Limited
Priority date: 2003-09-01
Filing date: 2004-09-01
Publication date: 2005-03-10
Also published as: GB0320449D0; GB2419747A; GB0602638D0

Abstract

A stator (200) of an alternator is made up of a number of laminations (210). Each lamination (210) has a body that is substantially annular, but that includes a number of cooling fin portions (270) projecting radially outward therefrom, such that when a group (280) of the laminations are axially-stacked, the cooling fin portions (270) combine to make up complete cooling fins. To form the stator (200), several of the groups (280) of laminations are axially juxtaposed, but with each group (280) being angularly staggered with respect to the group or groups (280) juxtaposed therewith. Thus, staggered rows of cooling fins are formed on the outside of the stator (200). To minimize material, each lamination (210) does not include any cooling fin portions (270) that project beyond two sides of a notional square that just encloses the body of the lamination (2 10). However, cooling fin portions (270) are provided on each lamination (210) that projects beyond the other two sides of that notational square.

Description

A Stator

This invention relates to a rotor and a stator for an electrical machine. In a preferred embodiment, this invention relates to a rotor and a stator for a radial-flux, rotary, alternator.

During operation of an electrical machine that includes a rotor and stator, each comprising a ferrous metal core with windings thereon or therearound, electrical currents in the windings and eddy currents induced in the cores result in heating of the rotor and the stator. This heating is disadvantageous in that the machine rating, that is to say the power density of the electrical machine, decreases with increasing temperature.

It is an object of this invention to address this problem.

According to a first aspect of this invention, there is provided a rotor or a stator for an electrical machine, the rotor or the stator having a plurality of cooling fins shaped and arranged such that airflow between and adjacent the cooling fins is disrupted thereby and turbulence of the airflow thereby encouraged.

This is advantageous in maximising heat transfer from the rotor or the stator to air in the airflow, via the cooling fins, and therefore in cooling the rotor or the stator.

The cooling fins are preferably arranged such that at least certain of the cooling fins are in the way of substantially straight paths between others of the cooling fins such that airflow between and adjacent the cooling fins is disrupted thereby and turbulence of the airflow thereby encouraged. The cooling fins are preferably arranged into at least two substantially parallel rows of cooling fins, one row being staggered with respect to the other row. The one row is preferably staggered with respect to the other row such that cooling fins of the other row are adjacent spaces between cooling fins of the one row. The rows of cooling fins are preferably arranged so as to extend substantially across the direction of airflow during operation of the electrical machine. In a radial flux, rotary, embodiment of the invention, the rows of cooling fins preferably extend substantially circumferentially.

The cooling fins are preferably provided on a rotor core of the rotor or on a stator core of the stator, each being preferably formed from a ferrous material, such as electrical steel. The rotor core or stator core may be formed as a one-piece construction, for example by way of powder metallurgy. Preferably, however, the rotor core or stator core comprises a plurality of laminations formed from sheeting of the ferrous material, the laminations including integral cooling fins or integral cooling fin portions projecting from a body of the rotor or stator, the cooling fin portions of one lamination cooperating with those of at least one other lamination to form the cooling fins when those laminations are juxtaposed to form at least part of the rotor or stator.

Preferably, the laminations are arranged such that at least some of the cooling fins or cooling fin portions are formed from material that would otherwise be discarded. For example, at least some laminations may have a body with a substantially circular periphery and at least some of the cooling fins or cooling fin portions may be formed from material within a respective notional rectangle of the sheet material which just encloses the body. Conveniently, each lamination may be of the same shape and configuration as each other lamination. The rotor or stator preferably includes a plurality of groups of juxtaposed laminations, the laminations in each group being aligned such that each cooling fin portion of each lamination is aligned with a respective cooling fin portion in each other lamination of the group, such that the aligned cooling fin portions form a cooling fin, and a series of cooling fins in side-by-side arrangement are formed. Preferably, juxtaposed pairs of groups of laminations are arranged such that the cooling fins of one of the groups are staggered with respect to the cooling fins of the other group.

Preferably, at least some of the cooling fins or cooling fin portions project beyond the notional rectangle that just encloses the body of the rotor or stator. Although this is disadvantageous in that it is not the most efficient use of the sheet material, this is advantageous in that, otherwise, there would be juxtaposed parts of juxtaposed groups that were devoid of cooling fins due to their being adjacent a "side" of the notional rectangle, resulting in there being little or no disruption of airflow over such parts. The laminations may be arranged such that some lengths of the periphery thereof include cooling fins or cooling fin portions that project beyond the respective notional rectangle and such that other lengths do not, the some lengths being angularly displaced from the other lengths by the amount by which juxtaposed groups are angularly displaced or staggered.

In a preferred embodiment, the stator is an annular stator and is provided with fixing means receiving portions for receiving fixing means for fixing the laminations relative to one another, the fixing means receiving portions being arranged to provide for the fixing of each group relative to a respective other group in a plurality of angular positions separated by a substantially constant pitch angle, each lamination being arranged such that cooling fin portions thereof are separated from (not necessarily) juxtaposed spaces between cooling fin portions thereof by the amount of the pitch angle. For example, the annular stator may be provided with four sets of fixing means receiving portions and arranged such that each group of laminations is positionable in four different angular positions with respect to other groups of laminations with a pitch angle of 90 degrees therebetween.

The fixing means receiving portions are preferably mounting sockets. Preferably, the mounting sockets have a curved, concave surface for receiving a fixing bar for welding to the stator. The mounting sockets may be shaped so as to receive bars of a rectangular or any suitable polygonal cross-section. Preferably the mounting sockets are arranged such that flux leakage from the stator to the fixing means therethrough is minimized. The mounting sockets may be arranged such that contact between the sockets and the fixing means is minimized. The mounting sockets may be arranged such that the curved, concave surfaces each have a recess therein. The recess is advantageous in providing an airgap between the stator and the fixings means when the fixing means is received in the mounting sockets.

Indentations in the rotor or stator may be provided adjacent the cooling fins. Indentation portions may be provided in rotor laminations or stator laminations.

Preferably, the cooling fins are arranged such that heat transfer from the rotor or stator is substantially uniform thereby minimizing temperature differences between different parts of the rotor or stator.

According to another aspect of this invention there is provided an electrical machine including the rotor and the stator.

Preferably, the electrical machine is a radial flux, wire wound, rotary alternator. The electrical machine may be a motor.

A specific rotor and stator in which the invention is embodied are now described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of part of an electrical machine; Figure 2 is a plan view of a lamination of a rotor of the electrical machine; Figure 3 is a perspective view of a plurality of rotor laminations; Figure 4 is a plan view of a lamination of a stator of the electrical machine; Figure 5 is a perspective view of a plurality of stator laminations; and Figure 6 is a perspective view of part of a plurality of stator laminations; and Figure 7 is a plan view of detail of an alternative electrical machine.

Figure 1 shows a part 10 of a wire wound, radial flux, rotary alternator. The part 10 shown in Figure 1 is a representative cross section through the alternator that includes a section of a rotor 100 of the alternator and a stator 200 of the alternator. The stator 200 is an annular, slotted, radially-outer stator that surrounds the rotor 100. The rotor 100 is a radially-inner, four pole, salient pole rotor. Both the stator 200 and the rotor 100 are formed of many axially-stacked laminations. Figure 2 is a plan view of one of the laminations 110 that goes to make up the rotor 100. The rotor lamination 110 is cut from a thin sheet of electrical steel, such as electrical sheet steel that might normally be used for forming laminations for use in conventional electrical machines. In this embodiment, the thickness of the rotor lamination 110 is 0.65 mm, but it is envisaged that any commonly-used thickness of electrical machine laminations may be used. The rotor lamination 110 may be thought of as a thin cross-sectional slice through the rotor 100 and so has portions that will go to make up various parts of the rotor 100 when several of the rotor laminations 110 are axially-stacked on top of one another. Specifically, the rotor lamination 110 has a radially-inner annular portion 120 with four integral salient pole piece portions 130 projecting radially-outwards therefrom. The pole piece portions 130 are circumferentially distributed with' a substantially constant angular pitch of 90 degrees. The outside of the annular portion 120 is shaped so as to have fin portions 140 projecting therefrom in each of the four areas that are between a respective circumferentially-juxtaposed pair of the pole piece portions 130. Thus, there are four fin portion arrangements 145, one in each of these areas. Of the four fin portion arrangements 145, each arrangement 145 is the same as one other arrangement 145 and different to the remaining two other arrangements 145, so that there are two different configurations of fin portion arrangements 145. For a first one of these arrangements 145, there are two radially- outwardly projecting fin portions 140 that are positioned close together. In the other configuration, there are also two radially-outwardly projecting fin portions 140, but these are positioned further apart from one another. In each configuration, there is a centrally- positioned cut out 150 from the annular portion 120. The fin portion arrangements 145 that are radially opposite one another are of the same configuration, but the fin portions 140 that are adjacent one another are not of the same configuration.

To form the rotor 100, many of the rotor laminations 110 are axially stacked together. Stacked rotor laminations 110 are shown in Figure 3. With reference to Figure 3, a number of the rotor laminations 110 are axially stacked to form a group of laminations 170. In this exemplary embodiment the number of stacked rotor laminations 110 used to form a group 170 is 16. In the group 170, the rotor laminations 110 are all angularly aligned with one another such that fin portion arrangements 145 of like configuration are on top of one another. Thus, the respective fin portions 140 of each of the rotor laminations 110 collectively form fins 160. Several groups 170 of axially stacked rotor laminations 110 are formed in this way. The actual number of groups is determined be the desired axial length of the rotor 100 (together with the number or laminations in each group) . For simplicity, however, only four groups 170 are shown in Figure 3 and will be described herein. The four groups 170 of rotor laminations 110 are also axially stacked, but are not stacked so as to be aligned with one another. Instead, the groups are stacked such that, if a first group 170a is considered to be a reference group 170a, the next, second, group 170b stacked thereon is rotated out of alignment with the first, reference, group by an angle of 90 degrees. The third group 170c is stacked on the second group 170b so as to be rotated by 180 degrees with respect to the first, reference, group 170a, thereby aligning it with the first group 170a, but not the second group 170b. Finally, the fourth group 170d is stacked on the third group 170C so as to rotated relative to the first group 170a by 270 degrees, thereby aligning it with the second group 170b, but not with the first 170a and third 170c groups.

This manner of stacking the groups 170 of rotor laminations 110 results in the fins 160 of each group 170 being staggered with respect to the fins 160 of the or each juxtaposed group 170. This is because a 90 degree angular pitch between juxtaposed groups 170 results in fins 160 arranged in one of the configurations described above with reference to Figure 2 being axially juxtaposed with those 140 arranged in the other configuration. Thus, the fins 160 of axially-juxtaposed groups 170 of rotor laminations 110 are staggered.

When all the groups are so stacked, the respective pole piece portions 130 of each of the rotor laminations 110 collectively form pole pieces 135 of the rotor 100. A core of the rotor 100 is thereby assembled and is ready for receiving fixing means of a known form, such as retaining bolts and nuts, or may be welded to fix the rotor laminations 110 and the groups 170 thereof relative to one another. Alternatively, or additionally, fixing may be achieved by welding along a channel 155 formed by juxtaposed recesses 150 of that rotor laminations 100. Once fixed in this manner the rotor 100 may receive its windings 180 and be mounted on its mountings.

Figure 4 is a plan view of one of the laminations 210 that goes to make up the stator 200. The stator lamination 210 is also, in similarity with the rotor lamination 110 described above with reference to Figure 2, cut from a thin sheet of electrical steel such as that from which a stator lamination for a conventional electrical machine might be cut. The stator lamination 210 may be thought of as a thin slice through the stator 200 of the electrical machine in a direction normal to its axis. The stator lamination 210 therefore has portions that will go to make up various parts of the stator 200 when several of the stator laminations 210 are axially-stacked on top of one another. Specifically, the rotor lamination 210 includes a generally annular portion 220 with eight integral mounting socket portions 230 projecting radially outwards therefrom. The mounting socket portions 230 each have an outwardly-facing concave and generally arcuate surface 240, but with a part of which being recessed from the remainder thereof to form a concave arcuate recess 250. The mounting socket portions 230 are not evenly distributed about the circumference of the generally annular portion 220 of the stator 200. Instead, the mounting socket portions 230 are grouped together into four adjacent pairs and each pair is spaced from each circumferentially adjacent pair by 90 degrees. Thus, the periphery of the annular portion 220 of the stator lamination 210 is divided up into eight sections by the eight mounting socket portions 230: four longer sections 260 and four shorter sections 265. These sections 260, 265 will be referred to in more detail below. In each pair of mounting socket portions 230, however, the two mounting socket portions 230 face 90 degrees away from each other and such that each mounting socket portion 230 in each pair faces in the same direction as the adjacent mounting socket portion of the respective adjacent pair. Mounting socket portions such as those 230 of the present embodiment are known. In common with such known mounting socket portions, the present portions 230 do not project beyond the edges of a notional square into which the annular portion 220 of the stator just fits. The reason for this is to maximize the number of the stator laminations that can be cut from a given area of the sheet material.

Returning to the eight peripheral sections 260, 265 of the stator lamination 210, each section 260,265 is shaped so as to have a number of fin portions 270 projecting outwardly therefrom. On two 260a of the long sections 260 that are radially opposite one another, the fin portions 270 do not project beyond the edge of the notional square referred to above that just encapsulates the annular portion 220 of the stator lamination 210. Thus, there are no fin portions 270 at a mid point of each of these sections 260a, and the fin portions 270 project further from the annular portion 220 the further away from this mid point they are positioned. Similarly, none of the fin portions 270 that project from the four shorter sections 265 project beyond the edge of this square. However, the fin portions 270 that project from the other two 260b of the long sections 260 of the periphery of the annular portion 220 of the stator lamination 210 do project slightly beyond the notional square. The reason for this will be described below. As a result of this, the stator laminations 210 of this embodiment must be cut from parts of sheet material that are spaced further apart that would be the case if all the fin portions 270 did not project beyond the notional square. This results in a degree of inefficiency in that the amount of sheet material that is not used to form stator laminations 210 is increased. However this does give rise to advantageous effects, as is described further below.

It will therefore be appreciated that the fin portions 270 that project from each of the four longer and radially_τopposite peripheral sections 260 of the annular portion 220 of the stator lamination 210 are arranged in one of two configurations: in one configuration they project further than in the other. These two configurations also differ, however, in the positioning of the respective fin portions 270 along the respective peripheral section 260. If the fin portions 270 in one configuration were positioned in the same way as those in the other configuration (which they are not), each fin portion in one configuration would be 90 degrees from a respective fin portion 270 in the other configuration. However, in the present embodiment, the positioning is such that each fin portion 270 in one configuration is 90 degrees from a respective space between two juxtaposed fin portions 270 in the other configuration. In other words, one configuration is slightly staggered with respect to the other.

The fin portions 270 that project from each of the four shorter peripheral sections 265 are also arranged in one of two different configurations. Again, those fin portions 270 that are radially opposite are arranged in the same configuration; and, again, each configuration is such that there is the staggering described above.

To form the stator 200, many of the stator laminations 210 are axially stacked together. Stacked stator laminations 210 are shown in Figure 5. With reference to Figure 5, a number of the stator laminations 210 are axially stacked to form a group 280 of the laminations 210. In this embodiment, the number of stacked stator laminations 210 used to form a group 280 is 16. In the group 280, the stator laminations 210 are all angularly aligned with one another such that the mounting socket portions 230 of each stator lamination 210 are each aligned with a respective mounting socket portion 230 of each other stator lamination 210; and the fin portions 270 of each stator lamination 210 are similarly each aligned with a respective fin portion 270 of each other stator lamination 210. Thus, the respective mounting socket portions 230 of each stator lamination 210 collectively form mounting sockets 235; and the respective fin portions 270 of each stator lamination 210 collectively form fins 275.

Several groups 280 of axially-stacked stator laminations are formed. The actual number of groups 280 is determined by the desired axial length of the stator 200. For simplicity, however, only four groups 280 are shown in Figure 5 and will be described herein. i similarity with the groups 170 of rotor laminations described with reference to Figure 3, the groups 280 of stator laminations are axially stacked and, again, are not so stacked so as to be aligned with one another. They are instead stacked in the same manner as the groups 170 of rotor laminations 110; that is, with a first group 280a of stator laminations 210 at a referential 0 degrees, a second group 280b stacked thereon at 90 degrees, a third group 280c stacked on the second group 280c at 180 degrees and a fourth group 280d stacked on the third group 280c at 270 degrees.

This manner of stacking the groups 280 of stator laminations 210 results in the fins 275 of each group 280 being staggered with respect to the fins 275 of the or each juxtaposed group 280. This is because a 90 degree angular pitch between juxtaposed groups 280 results in fins 275 arranged in a first one of the configurations described above with reference to Figure 4 being axially juxtaposed with those 275 arranged in the other configuration. This is true of both the fins 275 along each of the longer peripheral sections 260 and the fins 275 along the shorter peripheral sections 265. Thus, the fins 275 of axially-juxtaposed groups 280 of stator laminations 210 are staggered.

It is now that the advantageous effect referred to above may be seen that results from having fin portions 270 on one pair of radially opposite longer sections 260a project outside the notional square that just encloses the annular portion 220 of the stator laminations 210. With reference to Figure 6, the 90 degree staggering of axially juxtaposed groups 280 of stator laminations 210 results in the longer sections 260a in each group 280 that have fin portions 270 that project beyond the notional square, being axially juxtaposed with those long sections 260a in the adjacent group 280 that have fin portions 270 that do not project beyond the notional square. Thus, by having some fin portions 270 that project beyond the notional square, a situation is avoided in which there would be areas extending axially along the outside of the stator that were without fins 275 thereon.

With reference to Figure 1, when stacked as described above, the groups 280 of stator laminations 210 are fixed relative to one another. This is achieved by placing a fixing bar 12 inside each mounting socket 235, against the concave arcuate surfaces 240 of the stator laminations 210, and welded in position. The provision of the concave arcuate recess 250 in each stator lamination 210 results in there being an airgap 16 between the main body of the stator 200 and the fixing bars 12.

The stator 200 receives its windings in a conventional manner.

In operation of the electrical machine, air is caused to flow in a generally axial direction over the surfaces of the rotor 100 and the stator 200. Although not shown in the drawings it is envisaged that a co-axially mounted cooling fan may be provided on an axle on which the rotor 100 is mounted to generate this airflow. flowing over the surfaces of the rotor 100 and the stator 200 in an axial direction, air of the airflow flows between and around the fins 160 of the rotor 100 and the fins 275 of the stator 200. The staggered arrangement of the fins 160,275 causes the airflow therebetween and therearound to tend to be turbulent, rather than laminar. This increases heat transfer from the fins 160,275 to air of the airflow, which cools the rotor 100 and the stator 200.

During operation, magnetic fields are set up in the main body of the stator 200. Flux of these fields has a tendency to leak from the main body of the stator 200 to adjacent ferrous material. The provision of the airgaps 16 between the main body of the stator and the fixing bars 12 minimises flux leakage to the bars 12.

It should be noted that, because, amongst other reasons, the mounting socket portions 230 do not include any fin portions 270 on the concave, generally arcuate surfaces 240 thereof, sections of the annular portion 220 of the stator laminations 210 that include the socket portions 270 would have fewer fin portions 230 that similarly-sized sections of the annular portion 220 that do not include socket portions 230. For this reason, the fin portions 230 between the adjacent pairs of mounting socket portions 230 or those adjacent the socket portions 230 may be more densely packed and may project further than the other socket portions 230. This is to encourage heat transfer from the stator 200 so as to maintain different parts of the stator 200 at substantially the same temperature or at least to minimize temperature gradients between such parts.

As may be seen in Figure 1, a cover 14 is provided around the outside of the stator 200 and the fixing bars 12. Although, in the embodiment illustrated in Figure 1, the cover 14 follows a straight path between adjacent ones of the fixing bars 12, in an alternative embodiment shown in Figure 7, the cover 14 more closely follows the outside of the stator 200. This is advantageous in guiding airflow inside the cover 14 over the fin portions 270 of the stator 200.

Claims

1. A stator for an electrical machine, the stator having a plurality of cooling fins shaped and arranged such that airflow between and adjacent the cooling fins is disrupted thereby and turbulence of the airflow thereby encouraged, wherein the cooling fins are provided on a stator core of the stator that comprises a plurality of laminations formed from sheeting of a ferrous material, each lamination having a body with a substantially circular periphery and integral cooling fins or integral cooling fin portions projecting from that body, the cooling fin portions of one lamination cooperating with those of at least one other lamination to form the cooling fins when those laminations are juxtaposed to form at least part of the stator, and wherein each lamination is arranged to have no cooling fins or cooling fin portions that project beyond two opposite sides of a notional square that just encloses the body, but has cooling fins or cooling fin portions that project beyond the other two sides of that square.

2. A stator according to claim 1, wherein the cooling fins are arranged such that at least certain of the cooling fins are in the way of substantially straight paths between others of the cooling fins such that airflow between and adjacent the cooling fins is disrupted thereby and turbulence of the airflow thereby encouraged.

3. A stator according to any preceding claim, wherein the cooling fins are arranged into at least two substantially parallel rows of cooling fins, one row being staggered with respect to the other row.

4. A stator according to claim 3, wherein the one row is preferably staggered with respect to the other row such that cooling fins of the other row are adjacent spaces between cooling fins of the one row.

5. A stator according to claim 3 or claim 4, wherein the rows of cooling fins are arranged so as to extend substantially across the direction of airflow during operation of the electrical machine.

6. A stator according to any one of claims 3 to 5, wherein the electrical machine is a radial flux, rotary, electrical machine and the rows of cooling fins extend substantially circumferentially.

7. A stator according to any preceding claim, wherein the laminations are arranged such that at least some of the cooling fins or cooling fin portions are formed from material that would otherwise be discarded.

8. A stator according to any preceding claim, wherein each lamination is of the same shape and configuration as each other lamination.

9. A stator according to any preceding claim, wherein the stator includes a plurality of groups of juxtaposed laminations, the laminations in each group being aligned such that each cooling fin portion of each lamination is aligned with a respective cooling fin portion in each other lamination of the group, such that the aligned cooling fin portions form a cooling fin and a series of cooling fins in side-by-side arrangement are formed.

10. A stator according to claim 9, wherein juxtaposed pairs of groups of laminations are arranged such that the cooling fins of one groups of each pair are staggered with respect to the cooling fins of the other group in that pair by angular displacement between the groups of each pair.

11. A stator according to claim 9 or claim 10, wherein the laminations are arranged such that some lengths of the periphery thereof include cooling fins or cooling fin portions that project beyond the respective notional square and such that other lengths do not, the cooling fins or cooling fin portions of the some lengths being angularly displaced from the cooling fins or cooling fin portions of the other lengths by the amount by which juxtaposed groups are angularly displaced.

12. A stator according to claims 9 to 11, wherein the stator is an annular stator and is provided with fixing means receiving portions for receiving fixing means for fixing the laminations relative to one another, the fixing means receiving portions being arranged to provide for the fixing of each group relative to each other group in a plurality of angular positions angularly displaced from one another by multiples of a substantially constant pitch angle, each lamination being arranged such that at least some juxtaposed cooling fin portions thereof are each separated from a respective space between juxtaposed cooling fin portions thereof by the amount of the pitch angle.

13. A stator according to claim 12, wherein the annular stator is provided with four sets of fixing means receiving portions and arranged such that each group of laminations is positionable in four different angular positions with respect to other groups of laminations with a pitch angle of 90 degrees therebetween.

14. A stator according to claim 13, or according to any preceding claim and wherein the stator includes fixing means receiving portions, wherein the fixing means receiving portions are mounting sockets for receiving a fixing bar for welding to the stator, the mounting sockets being arranged such that flux leakage from the stator to the fixing means therethrough is minimized.

15. A stator according to claim 14, wherein the mounting sockets arranged such that contact between the sockets and the fixing means is minimized.

16. A stator according to claim 14 or 15, wherein the mounting sockets each have a respective curved, concave surface for receiving the fixing means and are arranged such that the curved, concave surfaces each have a recess therein.

17. A stator according to any preceding claim, wherein indentations in the stator body are provided adjacent the cooling fins.

18. A stator according to any preceding claim, wherein the cooling fins are arranged such that heat transfer from the stator is such that temperature differences between different parts of the stator are minimised.

19. A stator according to claim 18, wherein the surface area of each cooling fin and/or the distance between juxtaposed cooling fins is so as to minimize said temperature differences.

20. A stator according to claim 19 wherein at least one pair of juxtaposed cooling fins are more closely spaced than at least one other pair of juxtaposed cooling fins.

21. An electrical machine including a stator according to any preceding claim.