Winding arrangement for an electrical machine

Abstract

A stator (Fig.1, 105) or rotor (Fig.1, 155) for an electrical machine (Fig.1, 100) comprises a stator or rotor body, and windings (Fig.1, 140) mounted on the body. The one or more windings is arranged

Description

Field of the Invention The present invention relates to arrangements for windings for an electrical machine, more specifically to arrangements for windings incorporating a heat pipe. Background As the world transitions away from fossil fuels and to cleaner, renewable sources of power, electrical machines are becoming increasingly important. Electrical machines include electric generators to generate electricity in, for example, a wind turbine or for regenerative braking applications in vehicles. Alternatively, electric motors, such as induction motors, can use electrical energy to generate torque to power vehicles, aeroplanes and many other machines. With the use of electrical machines becoming increasingly prevalent in the modern word, improving the performance and efficiency of these electrical machines is an area of intense research. However, there are a number of separate ways in which performance or efficiency might be improved. The output of an electrical motor can be increased by increasing the current density used to power to motor. However, this inevitably results in greater heating of the electrical machine and consequently performance losses and increased risk of component failure. Reliability is of the utmost concern for modern electrical machines, particularly in motor vehicles, and as such further developments are required to improve upon existing techniques. The present inventors have identified an arrangement which allows for improved cooling of electrical machines in a manner which allows higher currents and minimises performance losses. Summary of the Invention Aspects of the invention are set out in the accompanying claims. According to a first aspect there is provided a stator for an electrical machine, the stator comprising: a stator core; and one or more stator windings mounted to the stator body, wherein the one or more stator windings are arranged in a plurality of turns, wherein the plurality of turns of the one or more windings are arranged in slots formed in the stator body; wherein for a subset of the plurality of turns in each of the slots the one or more windings comprises a heat pipe. According to a second aspect there is provided a rotor for an electrical machine, the rotor comprising: a rotor core; and one or more rotor windings mounted to the rotor core, wherein the one or more rotor windings are arranged in a plurality of turns, wherein the plurality of turns of the one or more windings are arranged in slots formed in the rotor body; wherein for a subset of the plurality of turns in each of the slots the one or more windings comprises a heat pipe. According to the first and second aspects, the windings of the electrical machine can be more effectively cooled, while also maintaining the performance of the electrical machine. In particular, the use of heat pipes improves cooling, however the use of heat pipes for only a subset of the windings allows the number of turns in a slot to be maximised. As such, due to the improved cooling and the maximisation of the number of turns, the output power per unit volume of the electrical machine can be increased. The heat pipe is a conductor having an internal sealed channel, the channel comprising a fluid. This improves the cooling performance as the fluid is able to vaporise from a liquid to a vapour and absorb thermal energy, before cooling and condensing at a cooler region of the heat pipe, releasing thermal energy. The channel further comprises a wick, wherein the wick is arranged to provide for a capillary effect in the fluid. Accordingly, a capillary effect for the fluid is facilitated, allowing the fluid to flow in liquid form from a cooler region of the heat pipe to a warmer region. As such, the cooling performance of the heat pipe, and therefore the windings, is improved. Advantageously, a turn of the plurality of turns in a respective slot that is closest to an air gap of the electrical machine may comprise a heat pipe. Due to the reduced quantity of conductor in the heat pipe as compared with a solid conductor, the AC resistance in the windings is reduced. In some examples, the turn of the plurality of turns that is closest to an air gap of the electrical machine is arranged to at least partly define an air gap of the electrical machine. In other words, no empty region exists between the innermost windings and the air gap. This allows for greater space utilisation within the slots, meaning the number of turns, and hence the output of the electrical machine, can be maximised, without substantially increasing AC resistance. The plurality of turns of the one or more windings are arranged in a slot formed in the rotor core. As such, the turns of the windings are arranged so as to increase the output of the electrical machine. For example, respective turns of the plurality of turns in a respective slot are located at a different radial position of the stator, and / or respective turns of the plurality of turns in a respective slot are located at a different circumferential position of the stator. Advantageously, the plurality of turns in a respective slot may be arranged with heat pipes and solid conductors in an alternating arrangement. Furthermore, the plurality of turns in a respective slot may be arranged with a repeating pattern of heat pipes and solid conductors. As such, a heat pipe may improve the cooling of neighbouring solid conductors. In some examples, the ratio of solid conductors to heat pipes in a respective slot is at least 2 to 1. Preferably, the ratio of conductors to heat pipes in a respective slot may be approximately 3 to 1. Accordingly, the cooling performance of the electrical machine may be improved without needing to reduce the number of turns in the windings. Advantageously, the plurality of turns in a respective slot may be arranged such that a concentration of heat pipes is configured to be greater closer to an air gap of the electrical machine. By increasing the overall packing factor closer to the air gap, AC resistance in the windings can be reduced. In some examples, one or more of the plurality of turns includes an end winding portion, wherein the end winding portion comprises a solid conductor. As such, the windings may include a heat pipe only in the region of the stator rotor that interfaces with the air gap of the electrical machine. This improves cooling performance of the windings in the hottest regions. According to a third aspect, there is provided an electrical machine comprising a stator and / or a rotor according to any aspect or example described above. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, with reference to the following figures. In accordance with one (or more) embodiments of the present invention the figures show the following: Figure 1 illustrates an example of an electrical machine. Figure 2 illustrates a stator for an electrical machine according to an example of the present disclosure. Figure 3 illustrates a rotor for an electrical machine according to an example of the present disclosure. Figure 4A illustrates a side-view of a heat pipe according to an example of the present disclosure. Figure 4B illustrates a cross-sectional view of the heat pipe of Figure 4A along line A-A. Figure 4C illustrates a cross-sectional view of the heat pipe of Figures 4A and 4B along line B-B. Figures 5A and 5B illustrate example winding arrangements for an electrical machine according to an example of the present disclosure. Figure 6 illustrates example winding arrangements for an electrical machine according to an example of the present disclosure. Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field. As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”. The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by these examples. It will also be recognised that the invention covers not only individual embodiments but also combination of the embodiments described herein. The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future. Detailed Description Figure 1 illustrates an example of an electrical machine 100. The electrical machine 100 includes a stator 105 and a rotor 155, where the rotor 155 is arrange in use to rotate relative to the stator 105, and where an air gap 150 exists between the stator 150 and the rotor 155. This electrical machine 100 is an external stator 105 electrical machine 100, such that the stator 105 is arranged further from the axis of rotation than the rotor 155. The stator 105 includes a stator core 110 that extends in an axial direction of the electrical machine 100 (i.e. along the axis of rotation of the electrical machine, which in Figure 1 is into the page). The stator core 110 may be formed of a plurality of laminations, or may be formed of a solid core. The stator core may be formed of substantially any material, including ferromagnetic materials such as iron, or non-magnetic materials, such as a plastic or a ceramic material. The stator 105 further includes a plurality of slots 120 formed in the stator core 110. The slots 120 extend in a radial direction of the stator 105 (as shown in Figure 1) and also extend in an axial direction of the stator 105. The plurality of slots 120 include stator windings 140. The stator windings 140 are solid conductors and are arranged in turns across the plurality of slots 120, and may be wound around teeth of the stator core 110 which define the slots 120. The slots 120 in Figure 1 include four turns of the windings 140, however any number of turns may be used e.g. based on the size of the slots 120 and the windings 140. The stator 105 shown in Figure 1 includes six slots 120 for ease of illustration, however the stator 105 may include substantially any number of slots 120, including a much greater number of slots. Furthermore, as shown in Figure 1, the slots each include an empty region 130 that is arranged to be proximal to an air gap 150 of the electrical machine. In other words, the empty region 130 is located between the radially innermost winding / turn 140 and the air gap 150 of the electrical machine. Slots 120 include empty region 130 in order to reduce the level of alternating current (AC) resistance in the windings 140. As magnetic flux in electrical machines is greater closer to the air gap, 150 the AC resistance is higher in windings 140 closer to the air gap 150. Therefore, in order to maintain the AC resistance in the windings 140 at manageable levels, the empty region 130 is provided. The rotor 155 of the electrical machine 100 has a similar configuration to the stator 105. That is, the rotor 155 includes a rotor core 160 in which slots 170 are formed. Rotor windings 190 are arranged in turns in the slots 170 in the same way as the stator windings 140. Furthermore, the slots 170 also include an empty region 180 in a similar manner to the empty region 130 in the slots 120 of the stator 105. While both the stator 105 and rotor 155 of the electrical machine 100 are shown as having electrical windings 140, 190, it should be appreciated that this is just one example of an electrical machine configuration. For example, instead of windings the stator or rotor may include permanent magnets, such that only one of the stator and rotor includes electrical windings, depending on the particular type of electrical machine. Electrical machines 100 including stators 105 and / or rotors 155 such as those shown in Figure 1 can be challenging to cool effectively, at least in part due to the aforementioned AC resistance. Therefore, the output of electrical machines such as this is generally limited to avoid hot spots and general damage to the electrical machine 100 through excess heating. Figure 2 shows a stator 200 for an electrical machine according to an example teaching of the disclosure. The stator 200 includes a stator core 210 and slots 220 in a similar manner to the stator 105 of Figure 1. In particular, the stator core 210 extends in an axial direction of the stator 200 (i.e. along the axis of rotation of the electrical machine, which in Figure 2 is into the page). The stator core 210 may be formed of a plurality of laminations, or may be formed of a solid core, or may include only a scaffolding or support to which stator windings 240 are fixed. The stator core 210 may be formed of substantially any material, including ferromagnetic materials such as iron, or non-magnetic materials, such as a plastic or ceramic material. The slots 220 extend in a radial direction of the stator 200 (as shown in Figure 2) and also extend in an axial direction of the stator 200. The slots 220 include stator windings 240 that are wound in a plurality of turns, where different turns of the stator winding 240 are arranged at different radial positions in the same slot, in a similar manner to the stator 105 of Figure 1. The stator windings 240 may be wound around teeth of the stator core 210 which define the slots 220. The slots 220 in Figure 2 include four turns of the windings 240, however any number of turns may be used e.g. based on the size of the slots 220 and the windings 240. The stator 200 shown in Figure 2 includes six slots 220 for ease of illustration, however the stator 200 may include substantially any number of slots 220, including a much greater number of slots. As shown in Figure 2, at least some of the stator windings 240 (e.g. a subset of the stator windings 240) are heat pipes that include a sealed internal channel 250 comprising a fluid. In the example of Figure 2, two of the four windings per slot shown are heat pipes, however various arrangements of windings 240 may be utilised according to the present disclosure, as will be explained in more detail in relation to Figures 5A and 5B. The structure of the heat pipes is discussed in more detail in relation to Figures 4A-4C. Figure 3 shows a rotor 300 having a corresponding structure to the stator 200 of Figure 2. Rotor 300 includes a rotor core 310 and slots 320 formed in the rotor core 310. Rotor windings 340 are located in the slots 320 in the same manner as stator windings 240 are located in slots 220. Furthermore, the rotor windings 340 include heat pipes having a sealed internal channel 350, as shown in Figure 3. As such, all the techniques of this disclosure (unless explicitly stated) are equally as applicable to both stators and rotors. Furthermore, an electrical machine may include both a stator and rotor according to the example teachings of the present disclosure, or only the stator or rotor of the electrical machine may incorporate the teachings of the present disclosure. Furthermore, it should be appreciated that the techniques of the present disclosure are applicable to a variety of types of electrical machines, such as resonant electrical machines, conventional (i.e. non-resonant) electrical machines, ferromagnetic core (i.e. iron-core) electrical machines, non-magnetic core electrical machines, and coreless (i.e. air-core) electrical machines. The use of heat pipes as windings 240 in the manner shown in Figure 2 and discussed above improves the cooling performance of the windings. In particular, windings generate heat due to the passing of current through the windings. Traditional electrical machines cool the windings by cooling an end portion (i.e. end winding) of the windings (e.g. with a coolant liquid), which allows the windings’ heat to conductively dissipate away from the centre of the winding, to the end winding, to the coolant fluid. However, these techniques generally do not cool the centre of the windings as effectively (i.e. fast) as may be desired. As mentioned above, a heat pipe is a conductive material (i.e. a conductor, such as copper) which includes a sealed internal channel which includes a fluid. Figure 4A shows an external view of a heat pipe 400, such as the heat pipes shown in Figure 2. The heat pipe is shown extending from left to right in an axial direction (i.e. along the axis of rotation of the electrical machine). Figure 4B shows a cross-sectional view of the heat pipe 400 looking along line A-A. In other words, in Figure 4B the left-right direction is the radial direction, the up-down direction is the radial direction, and the in-out direction is the axial direction. As shown in Figure 4B, the heat pipe 400 includes a sealed internal channel 410 formed in a conductor 420, such as copper. The channel includes a fluid (which may be referred to as a working fluid). An example of the working fluid is water, however other fluids may be used. The channel 410 also includes a wick 430 located on the internal surface of the channel 410. The wick 430 is a structure that facilitates (i.e. allows for) capillary action by the liquid phase of the fluid in the internal channel 410 (i.e. the wick 430 enables a capillary effect in the fluid, whereby the liquid is absorbed into the wick at a cool region of the heat pipe 400 and transferred to a warmer region of the heat pipe 400, where the liquid vaporises). The wick 430 may take on a variety of different structures, such as a sintered metal power or grooves formed in the conductor 420. Figure 4C shows a cross-sectional view of the heat pipe 400 looking along line B-B. That is, Figure 4C is shown from the same angle as Figure 4A, but depicts a cross-sectional view of the heat pipe 400. As can be seen, the channel 410 may extend along a majority of the heat pipe, with regions of solid conductor located at the extremities of the heat pipe in the length (axial) direction. The wick 430 is not shown in Figure 4C for ease of illustration. As mentioned above, when in operation the heat pipe 400 may provide for improved cooling performance as compared to solid conductors, as will be explained. Before being put into use, the working fluid located in the channel 410 may begin as a liquid. When the heat pipe 400 is heated, the working fluid evaporates to a vapour, thereby absorbing thermal energy. The vapour then migrates along the channel 410 to a cooler region of the channel, which may, for example, be externally cooled. At this cooler region, the vapour condenses to a liquid, releasing thermal energy. The liquid may be absorbed by the wick 430 and the thermal energy is transferred from the fluid to the conductive material 420 via the wick 430. The liquid then transfers to the warmer region of the heat pipe 400 via the wick 430 (due to the capillary effect), and then vaporises to a vapour at the warmer region. In this manner, heat can be effectively transferred from a warm region of the heat pipe to a cooler region. The cooling performance of a heat pipe is significantly greater than a metal conductor of similar dimensions. The end winding region(s) of the heat pipes can be cooled to aid in the transfer of heat way from the heat pipe. That is, one or both axial ends of the heat pipe may be cooled (for example via a coolant liquid) to transfer heat away from the windings. However, the cooling performance is dependent on the packing factor of the heat pipe 400. The packing factor defines the fraction of the cross-sectional area of the heat pipe 400 that is occupied by the channel 410, otherwise known as the vapour area (i.e. the area of the heat pipe occupied by the fluid / vapour). In other words, the larger the area of the channel 410, the higher the packing factor. A packing factor of zero would mean that there is no internal channel 410. The cooling performance of the heat pipe depends on the packing factor / vapour area, as well as the pressure within the wick 430 area. Generally, the higher the packing factor the greater the cooling performance. However, when utilised as an electrical winding for an electrical machine (as in Figures 2 and 3), a higher packing factor increases the direct-current (DC) resistance of the conductor 420, as the total cross-sectional area through which current can flow is reduced, thereby generating more heat in the presence of a given current. Therefore, the packing factor may be chosen to optimise the performance of the electrical machine, as will be discussed in relation to Figures 5A and 5B. The arrangement of the slots 220 and windings 240 of the example stator 200 of Figure 2 do not include the empty regions 130 shown in Figure 1. In other words, a radially innermost surface of the radially innermost winding 240 is located in substantially the same radial location as an innermost surface of the stator core 210. This is shown in more detail in Figures 5A and 5B which show example winding arrangements for a stator (such as in Figure 2) or a rotor (such as in Figure 3). Figure 5A and 5B respectively show first and second example winding arrangements for a slot 520 of a stator or rotor, such as those shown in Figures 2 and 3 respectively. The windings include heat pipes 530. For example, a subset (i.e. some but not all) of the windings take the form of heat pipes 530, and the remaining windings are solid conductors 510. As shown in Figures 5A and 5B, the slot 520 interfaces directly with the air-gap 540 of the electrical machine. The slots 520 of Figures 5A and 5B include ten turns of windings arranged at the same circumferential position but at different radial locations. While ten windings are shown here, it should be appreciated that any number of windings may be used. As discussed above, including heat pipes as windings improves the cooling performance of the windings. However, heat pipes 530 have a larger footprint than solid conductors. As such, if a slot 520 can fit ten solid conductors 510, the slot 520 would fit fewer numbers of heat pipes, due to their increased size (i.e. the number of turns of the winding would need to decrease). Therefore, a subset of the windings may be implemented as heat pipes 530, while the remainder of the windings are solid conductors 510. The number of turns of the windings can therefore be maintained, thereby maintaining the output levels of the electrical machine. Furthermore, the heat pipes 530 may cool not only the winding that is itself a heat pipe 530, but also neighbouring solid conductor 510 windings. In some implementations, the winding closest to the air gap 540 may be a heat pipe 530. In electrical machines, the magnetic flux is greatest closer to the air gap 540, where magnetic flux creates AC resistance in conductors. However, as heat pipes 530 have reduced quantities of conductive material relative to solid conductors 510, the amount of AC resistance is reduced by choosing the winding closest to the air gap 540 (i.e. where magnetic flux is greatest) as a heat pipe 530. Moreover, because of the reduced AC resistance when choosing the winding closest to the air gap 540 as a heat pipe 530, this heat pipe 530 can be located closer to the air gap. That is, the empty region 130 shown in the slots 120 of Figure 1 may not be present in stators and rotors of the present disclosure. In this way, a surface of the heatpipe 530 may at least partially define the air gap 540. For example, a surface of the heat pipe 530 facing the air gap 540 may be located as a same radial position as a surface of the stator or rotor core that defines the air gap 540. As such, more space is made available within the slot 520 to locate windings, which can be used e.g. to maximise the number of turns, or increased the size of the windings, thereby increasing the output of the electrical machine. The spacing of the heat pipes 530 among the windings of the slot 520 can be set in a number of different ways. For example, as shown in Figure 5A, the heat pipes 530 may be generally evenly spaced throughout the slot 520. Alternatively, as shown in Figure 5B, the slot 520 may include a higher concentration of heat pipes 530 closer to the air-gap 540. This increases the overall packing factor of the windings closer to the air-gap where the magnetic flux, and therefore AC resistance, is greatest. Accordingly, by arranging the windings in this way, the output of the electrical machine can be maximised by improving cooling while reducing losses. It should be appreciated that the arrangements shown in Figures 5A and 5B are only examples and that other arrangements are possible. Furthermore, different number of heat pipes and turns may be used in a given slot 520. In addition, the ratio of solid conductors 510 to heat pipes 530 in a slot may be a range of values, for example at least 2 to 1, and in some cases approximately 3 to 1. This range of ratios provides for an optimal balance between reducing AC resistance, minimising increases in DC resistance, and improving cooling performance, thereby maximising the output of the electrical machine. Furthermore, in some examples the packing factor of the heat pipes 530 may be different depending on the location of the heat pipes within the slot 520. For example, heat pipes 530 closer to the air gap 540 may have a higher packing factor to reduce AC resistance and improve cooling, while heat pipes further from the air gap 540 may have a smaller packing factor. Figures 5A and 5B show arrangements of a slot 520 where the turns of the windings are arranged radially i.e. where each turn is arranged at a different radial position in the slot. However, other winding arrangements are possible within the scope of the present disclosure. For example, Figure 6 shows an alternative winding arrangement for a slot 600 of a stator or rotor. The slot 620 includes a plurality of first windings 610 having a first orientation, wherein the turns of the first windings 610 are arranged in a circumferential direction. In other words, successive turns of the first windings 610 are arranged at different circumferential position, but at the same radial position. The first windings 610 also extend in an axial direction of the electrical machine (i.e. into the page in Figure 6). The first windings 610 may include only solid conductors, only heat pipes, or both solid conductors and heat pipes (as shown in Figure 6). In addition to the first windings 610, one or more second windings 630 are provided having a second orientation. In the example of Figure 6, the second windings 630 have an orientation that is perpendicular to the orientation of the first windings 610 (i.e. rotated 90 degrees about an axial direction of the electrical machine). The second winding 630 is arranged closer to the airgap 640 of the electrical machine than the first windings 610. As discussed above, magnetic flux is greatest closer to the air gap 640 and thus the provision of a heat pipe as the second winding 630 reduces AC resistance, thereby improving the output of the electrical machine. As a heat pipe, the second winding 630 may improve the cooling performance of not only itself, but also the neighbouring first windings 610. In general, it should be appreciated that the techniques of the present disclosure are applicable to a variety of types of electrical machines, including electrical motors and generators, and more generally to any form of electrical machine that includes at least one of stator or a rotor that includes windings. Therefore, from one perspective there has been described stators, rotors and electrical machines, where the windings are arranged in turns in a slot of the rotor and / or stator, where a subset of the turns are heat pipes, and the remaining turns are solid conductors. A turn of the windings closest to an air gap of the electrical machine may be a heat pipe, and said turn may at least partially define the air gap.

Claims

1. A component for an electrical machine, the component being one of a stator or a rotor, the component comprising:a body; andone or more windings mounted to the body, wherein the one or more windings are arranged in a plurality of turns, wherein the plurality of turns of the one or more windings are arranged in slots formed in the body of the component, and wherein each of the plurality of turns of the windings extends in an axial direction of the electrical machine; andwherein for a subset of the plurality of turns in each of the slots the one or more windings are a heat pipe, wherein the heat pipe is a conductor having an internal sealed channel, the channel comprising a fluid and a wick, wherein the wick is arranged to provide for a capillary effect in the fluid.

2. The component according to claim 1, wherein the component is a rotor.

3. The component according to claim 1, wherein the component is a stator.

4. The component according to any preceding claim, a turn of the plurality of turns in a respective slot that is closest to an air gap of the electrical machine comprises a heat pipe.

5. The component according to claim 4, wherein the turn that is closest to the air gap is arranged to at least partly define the air gap of the electrical machine.

6. The component according to any preceding claim, wherein respective turns of the plurality of turns are located at a different radial position in a respective slot of the component.

7. The component according to any preceding claim, wherein respective turns of the plurality of turns in a respective slot are located at a different circumferential position of the component8. The component according to any preceding claim, wherein the plurality of turns in a respective slot are arranged with heat pipes and solid conductors in an alternating arrangement.

9. The component according to any preceding claim, wherein the plurality of turns in a respective slot are arranged with a repeating pattern of heat pipes and solid conductors.

10. The component according to any preceding claim, wherein the ratio of solid conductors to heat pipes in a respective slot is at least 2 to 1.

11. The component according to claim 10, wherein the ratio of conductors to heat pipes in the respective slot is approximately 3 to 1.

12. The component according to any preceding claim, wherein the plurality of turns in a respective slot are arranged such that a concentration of heat pipes is configured to be greater closer to an air gap of the electrical machine.

13. The component according to any preceding claim, wherein one or more of the plurality of turns includes an end winding portion, wherein the end winding portion comprises a solid conductor.

14. An electrical machine, the electrical machine comprising one or more of:the stator according to any of claims 1 or 3-13; andthe rotor according to any of claims 1, 2, or 4-13.

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