Cooling system

Abstract

A cooling system 500 for a stator of an electric machine, the cooling system comprising a plurality of independent cooling arrangements 12, 14, 16, 18 arranged in use to cool segments 20 of the stator

Description

Field and Background The present invention relates to cooling systems for a stator of an electric machine, such as an electric motor for an electrically powered aircraft. More particularly, the present invention relates 5 to cooling stator segments of an electric machine. Demand placed on electric machines has been increasing in recent years. For instance, electric machines are used in various modern transportation forms. As an example, while electrically powered aircraft, for example that use electric machines such as propulsive motors, are not widespread and are not used in commercial contexts (e.g. 48 or 96 or more passenger aircraft 10 and / or CS-25 related aircraft), such use remains an area of cutting-edge research and interest at least in part due to the associated environmental benefits. Existing cooling systems and approaches towards cooling of stators of electric machines are not suitable for the complexities and considerations involved with the increased demand placed on electric machines, for example their use within electrically powered aircraft. 15 The cooling systems discussed herein provide improved cooling performance while providing increased redundancy and safety in the event of failure. Summary Aspects of the invention are set out in the accompanying claims. Viewed from a first aspect, there is provided a cooling system for a stator of an electric machine, the cooling system comprising: a plurality of independent cooling arrangements arranged in use to cool segments of the stator, wherein each cooling arrangement comprises an inlet arranged in use to receive coolant, and a cooling circuit arranged in use to communicate the coolant from the inlet to an outlet such that in use heat is transferred from a respective segment of the stator to the coolant. Hence, the cooling arrangements for cooling segments of the stator are independent of each other. This independent cooling arrangement provides various advantages. Indeed, by utilising independent cooling arrangements for different stator segments, the number of stator segments cooled by the same cooling circuit and thus same circulating coolant is reduced compared to an arrangement without independent cooling arrangements for different stator segments. This results in improved cooling performance. Further, by providing independent cooling arrangements, redundancy is provided. In the event of a failure of a given cooling arrangement, other cooling arrangements can continue to operate to provide cooling to the stator. For example, a given cooling arrangement may become damaged or non-operational such that coolant is no longer able to flow therein. However, the other independent cooling arrangements, which may not be damaged or affected by the failure that affected the given cooling arrangement due to their independence, can continue to provide cooling to the stator. This increases the likelihood that the electrical machine can remain operational even in the event of a failure associated with the cooling system. In the case of an electric motor for an electrically powered aircraft, propulsion can still be provided even in the event of failure, which may be required for a given stage of flight. Therefore, safety in the event of failure is increased. The present cooling systems may seem counter-intuitive because in some implementations they may result in increased cooling system complexity and weight, however, the present inventors have realised that the increase to system redundancy, safety and cooling performance is particularly advantageous for meeting the increased demand placed on cooling systems and electric machines. In some examples, the plurality of independent cooling arrangements is arranged in use to cool different segments of the stator, for example, different coil winding portions of the stator. In some examples, a segment of the stator corresponds to a coil winding portion of the stator. In some examples, the cooling circuits of the plurality of cooling arrangements are arranged in use to provide independent communication of coolant between a respective inlet and outlet. In some examples, the cooling circuits are arranged to provide closed-loop communication of coolant between the respective inlet and outlets. Hence, the cooling arrangements are independent of one another in the sense that the coolant flow path, i.e. coolant circuit, from inlet to outlet in a respective cooling arrangement is independent or closed-loop (i.e. arranged so as not to provide coolant communication between cooling circuits of different cooling arrangements). In some examples, the cooling circuit of each cooling arrangement comprises a plurality of cooling devices spaced substantially equidistant around a central void, the central void arranged in use to receive a rotor of the electric machine. Thus, each cooling arrangement may provide a plurality of cooling devices. The cooling devices may be arranged in use to cool the different segments of the stator. The cooling devices may take the form of cooling blocks or heatsink-like structures, as discussed further below. In the event of failure of a segment of the electric machine (i.e. a pole pair), this equidistant spacing of cooling devices allows for efficient integration of the cooling system with a rotor of the electric machine, and also provides a balanced load about the perimeter of the void. In some examples, the cooling devices of the plurality of cooling arrangements form a ring of cooling devices around the central void. Hence, the weight of the cooling arrangements can be spread around the central void. Further, a close positioning of cooling devices and stator segments can be achieved. The ring may be circular or any closed loop in shape, such as polygonal. In some examples, the cooling circuit of each cooling arrangement comprises a pair of cooling devices arranged on opposite sides of the central void. Hence, a given cooling arrangement may provide cooling devices arranged on opposite sides of the central void. This provides redundancy while providing a more symmetrical temperature distribution of the stator coils across the stator, thereby preventing damage to the stator and increasing stator lifetime. Further, in the event of a failure associated with a given cooling arrangement, as the failed cooling arrangement provides cooling devices arranged on opposite sides of the central void, the load on opposite sides of the central void is balanced and thus the load of the rotor (or motor shaft in an electric motor example) is balanced. This reduces adverse and asymmetrical loading of the electric machine. While in some examples the cooling circuit of each cooling arrangement may comprise a pair of cooling devices arranged on opposite sides of the central void, it will be appreciated that a subset (i.e. one or more) of the cooling arrangements may be provided with pairs of cooling devices arranged in this manner. It will also be appreciated that the cooling circuits of each or more cooling arrangements may comprise a plurality of cooling devices arranged substantially equidistant around the central void. For example, three or four or more cooling devices may be provided for each cooling arrangement, and the advantages associated with the equidistant spacing (i.e. the load balancing, particularly in the event of failure of a given cooling arrangement) can still be realised. In some examples, for a given cooling arrangement, the plurality of cooling devices is arranged in series. One or more or each of the cooling arrangements may have (one or more or all) cooling devices arranged in series. By having cooling devices arranged in series, it is meant that in use coolant flows through a first cooling device and then through a second cooling device. In other words, coolant flows through the first cooling device and cools the segment of the stator associated with the first cooling device, and then the coolant is provided (i.e. via a conduit) to the second cooling device where the coolant then flows through the second cooling device and cools the segment of the stator associated with the second cooling device. Thus, in some examples, a first cooling device of the plurality of cooling devices is arranged to receive the coolant from the inlet, and a second cooling device of the plurality of cooling devices is arranged to receive the coolant from the first cooling device. Hence, for a given cooling arrangement, one or more or each of the cooling devices of that given cooling arrangement may be arranged in series, such that coolant flows through each of the cooling devices in turn. This can reduce the space required for the cooling system and can increase the efficiency of integration of the cooling system with the electric machine. This also improves the thermal stability of the system and in some cases avoids balancing of the coolant flows. The inlet of each cooling arrangement may be arranged to receive coolant from a common coolant source, or from separate coolant sources. The coolant source may be external from the cooling system. The coolant may be a cryogenic fluid. Examples of cryogenic fluid include liquid hydrogen or helium, but it will be appreciated that other cryogenic fluids may be used. In other examples, the coolant is water, oil, water-glycol, or gas such as ambient air. The outlet of each cooling arrangement may be arranged in use to provide the coolant (after having been used in the cooling system) to a further downstream component or for use in other downstream components, such as for heat recovery or to provide heating or coolant for other components. For example, the coolant output from the outlet may then be used in one or more fuel cells. The present techniques are not particularly limited in these respects. In some examples, the cooling devices are arranged around the central void such that a given cooling device is adjacent cooling devices of a different cooling arrangement from the given cooling device. In some examples, the cooling devices are arranged such that cooling devices of different cooling arrangements alternate around the perimeter of the central void. Hence, cooling devices of the same cooling arrangement are spaced from each other by cooling devices from other cooling arrangements. As discussed, this provides a balanced load about the rotor shaft in the event of a failure of a given cooling arrangement. In some examples, each cooling device is arranged to cool a different coil winding portion of the stator associated with different phases of the electric machine. In some examples, each cooling device is arranged to cool a coil winding portion of the stator associated with multiple phases of the electric machine. Hence, the cooling devices may cool a coil winding portion of the stator associated with a single phase of the electric machine or a plurality of phases of the electric machine. Hence, the cooling system can be adapted depending on implementation. The coil winding portion may correspond to one or more coil windings of the stator. In some examples, each cooling device is arranged in use to conductively communicate heat away from a different coil winding portion of the stator. For example, as a result of coolant flow, heat from the coil winding portion can be transferred to coolant flowing within the cooling device and transferred away from the cooling device. In some examples, each cooling device is arranged in use to mechanically connect with a different coil winding portion of the stator. In some examples, the cooling devices may abut or physically contact the coil winding portions of the stator. This can increase the heat transfer between the coil winding portions and the cooling devices. In some examples, the cooling devices of the plurality of cooling arrangements form a first ring of cooling devices around the central void, wherein each cooling arrangement comprises a second plurality of cooling devices spaced substantially equidistant around the central void and forming a second ring of cooling devices, the second ring of cooling devices spaced in a longitudinal direction of the electric machine from the first ring of cooling devices. Hence, the present cooling system may comprise two rings of cooling devices that are spaced along an axis (e.g. the rotor shaft axis) of the electric machine. As a result, different portions of the electric machine in a longitudinal direction of the electric machine can be cooled by the cooling system. In particular, the coils of the stator can be cooled from both front and rear / end faces and thus the symmetry of the temperature distribution of the coils can be increased. Further, excess temperature gradients across the coils in a front / rear direction of the stator can be reduced. This increases cooling performance and also increases component lifespan. In some examples, the first ring of cooling devices is arranged in use to cool a front end of the stator, and the second ring of cooling devices is arranged in use to cool a rear end of the stator. Hence, different ends of the stator can be cooled by the cooling system. A front end of the stator may refer to an end of the stator from which the rotor shaft extends. In other implementations, this may be reversed. In some examples, for a given cooling arrangement, a first cooling device of the second plurality of cooling devices is arranged in use to receive the coolant from a cooling device of the plurality of cooling devices, and a second cooling device of the second plurality of cooling devices is arranged to receive the coolant from the first cooling device of the second plurality of cooling devices. Hence, for a given cooling arrangement, the series arrangement of cooling devices continues between the first and second longitudinally spaced rings. A cooling device of the first ring may be fluidly connected to a cooling device of the second ring by a conduit or pipe for example. As a result, coolant may first be introduced to the cooling devices of the first ring, and then communicated to the cooling devices of the second ring. The present inventors have identified that more heat is typically generated at a front end of the stator due to heat-generating components typically being located at the front side, and so advantageously, the coolant is introduced first to the cooling devices of the first ring which may be located at the front end of the stator. This means that when the coolant is at its coldest (because it has not yet passed through other cooling devices), the coolant can more effectively cool the front end components of the stator (which typically generate more heat). Hence, overall cooling performance can be increased. In some examples, each (or one or more) cooling device comprises an internal channel for communicating the coolant between an input of the cooling device and an output of the cooling device, the internal channel arranged in use such that heat is transferred from coil windings of the stator to the coolant communicated in the internal channel. In some examples, the cooling devices are cooling blocks. Hence, heat can be efficiently and effectively transferred away from the stator. The cooling devices of the same cooling arrangement may be connected by one or more conduits for communicating the coolant between the cooling devices of the same cooling arrangement. In some examples, at least two conduits connecting the cooling devices are made from different material. Hence, the cooling system can be adapted for specific thermal conductivity and expansion requirements. Further, ease of assembly is increased as this example allows for the use of a variety of off-the-shelf components, such as pipe couplings. In some examples, the coolant is a cryogenic fluid. For example, the coolant may be liquid hydrogen or helium. In other examples, the coolant is water, oil, or water-glycol. In some examples, the coolant is gas, such as ambient air. In some examples, the electric machine is an electric motor arranged to provide propulsive power to an electrically powered aircraft. Hence, as discussed herein, the cooling system may be for an electrically powered aircraft. In some examples, the cooling system comprises three or four cooling arrangements. In some examples, each cooling arrangement comprises four cooling devices, arranged in two pairs of cooling devices, the first pair provided as part of the first ring and the second pair provided as part of the second ring. The present inventors have identified such arrangements provide an advantageous balance between cooling and redundancy and weight. However, it will be appreciated that the present techniques are not particularly limited in this respect and that the number of cooling arrangements and the number of cooling devices of each cooling arrangement may vary depending on implementation and design. Viewed from a second aspect, there is provided an electrically powered aircraft comprising the cooling system described herein. Viewed from a third aspect, there is provided a stator for an electric motor, the stator comprising: a plurality of coils; and the cooling system described herein for cooling the plurality of coils. Viewed from a fourth aspect, there is provided an electric motor for providing propulsive power to an electrically powered aircraft, the electric motor comprising: a rotor; a stator arranged around the rotor; and the cooling system as described herein for cooling the stator. Viewed from a fifth aspect, there is provided a propulsion system for an electrically powered aircraft, the system comprising: the electric motor described above configured to provide a plurality of phases; a plurality of inverters, each associated with one of the plurality of phases, the plurality of inverters for receiving electrical power, converting the electrical power, and providing the electrical power to the electric motor. Viewed from a further aspect, there is provided a method of operating the cooling system described herein to cool a stator of an electric machine. The method may comprise providing coolant to each independent cooling arrangement to cool a respective segment of the stator. Viewed from a further aspect, there is provided a method of forming the cooling system described herein. The method may comprise forming a plurality of independent cooling arrangements, wherein each cooling arrangement comprises an inlet arranged in use to receive coolant, and a cooling circuit arranged in use to communicate the coolant from the inlet to an outlet such that in use heat is transferred from a respective segment of a stator to the coolant. Other aspects will also become apparent upon review of the present disclosure, in particular upon review of the Brief Description of the Drawings, Detailed Description and Claims sections. Brief Description of the Drawings Examples of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic view of an example cooling system as described herein; Figure 2 shows a schematic view of an example cooling system having two cooling arrangements, each providing a pair of cooling devices arranged around a stator, as described herein; Figure 3 shows a schematic view of an example cooling system having three cooling arrangements, each providing three cooling devices arranged around a stator as described herein; Figure 4 shows a schematic view of an example cooling system having two cooling arrangements, each providing four cooling devices arranged in two pairs across a front ring and a rear ring, as described herein; Figure 5 shows a schematic view of an example cooling system in use to cool segments / coil winding portions of the stator as described herein; Figure 6 shows an example cooling device in-situ with a coil winding portion of a stator as described herein; Figure 7 shows an example cooling system in-situ with an electric machine as described herein; and Figure 8 shows an example propulsion system as described herein. While the disclosure is susceptible to various modifications and alternative forms, specific example approaches are shown by way of example in the drawings and are herein described in detail. It should be understood however that the drawings and detailed description attached hereto are not intended to limit the disclosure to the particular form disclosed but rather the disclosure is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention. It will be recognised that the features of the aspects of the invention(s) described herein can conveniently and interchangeably be used in any suitable combination. It will also be recognised that the invention covers not only individual embodiments but also combinations of the embodiments that have been discussed herein. Detailed Description An invention described herein relates to cooling systems for electric machines, and in particular for an electric motor for an electrically powered aircraft. While the present invention relates generally to stators of various electric machines (such as a motor or generator) for various uses, the present invention is particularly well-suited to use with electric machines for electrically powered aircraft or other implementations using cryogenic fluid as a coolant. A particular use for this invention may be in an aircraft with an electrically drivable motor or a drivable motor that is at least partially electrically driven. For example, the propulsion in the aircraft in which the power systems disclosed herein are used may be fully or partially electrically powered. Indeed, it will be appreciated that the term ‘electrically powered aircraft’ refers to an aircraft that is at least partially electrically powered (and may also be at least partially combustion powered). Partially powered aircraft may use thrust provided in part by electrical means and in part by combustion means. This invention may be used in a fully or partially combustion powered aircraft. The electrical and combustion aspects may be provided by one or by a few fuels. In some examples a cryogenic fuel may be used. For example, a high energy density power source arrangement, such as a fuel cell arrangement, may comprise a cryogenic fuel arrangement arranged to provide fuel for power generation. Various example cooling systems will now be described with reference to the figures. It will be appreciated that the example cooling systems may include additional components not shown in the figures, that components shown may be omitted, and that various features of the cooling systems may be combined. Figure 1 shows an example cooling system 100 according to the present techniques. Cooling system 100 includes a plurality of independent cooling arrangements 2a, 2b, which are arranged in use to cool different segments of a stator (not shown). Each cooling arrangement 2a, 2b includes an inlet 4a, 4b to receive coolant in use, a cooling circuit 6a, 6b to communicate the coolant from the inlet 4a, 4b to an outlet 8a, 8b in use, such that heat is transferred from the segments of the stator to the coolant. As can be seen from figure 1, cooling arrangements 2a and 2b are independent. In other words, the respective cooling circuits 6a, 6b between the inlets 4a, 4b and outlets 8a, 8b are independent of one another, i.e. between inlet 4a, 4b and outlet 8a, 8b they are closed-loop and coolant is not transferred between the different cooling arrangements 2a, 2b. However, it will be appreciated that each cooling arrangement 2a, 2b may be supplied with coolant from the same source and / or may provide coolant from the outlet to the same place. It will further be appreciated that figure 1 schematically shows the cooling arrangements 2a, 2b from a high-level and that in some examples the cooling arrangements contain various discrete components (such as cooling devices) disposed along the cooling circuit 6a, 6b. In this way, ‘cooling arrangement’ refers to the collection of the cooling circuit and cooling devices that may be present. It will also be appreciated that the number, shape, and arrangement of the cooling arrangements 2 is not particularly limited, and that the cooling arrangements 2 may be located proximally to or in contact with the respective segments of the stator which are to be cooled. As discussed above, by providing independent cooling arrangements, redundancy, cooling performance, and safety in the event of failure of a cooling arrangement is increased. The present cooling systems having cooling devices as part of their cooling circuits will now be described with reference to figure 2. Figure 2 shows an example cooling system 200, which includes cooling arrangements A, B, which may correspond to the cooling arrangements 2a, 2b of figure 1. In figure 2, cooling system 200 is in-situ and arranged to cool a stator 10 of an electric machine, such as an electric motor of an electrically powered aircraft. Figure 2 thus shows an end-on view of the stator 10. It will be appreciated that the stator 10 may include a plurality of coil winding portions and may be divided into different stator segments, and that the coil windings may change direction at the end of the stator (not shown). Stator 10 includes a central void 10a for receiving a rotor or rotor shaft. As shown in figure 2, cooling devices 12a, 12b, 14a, 14b are arranged around the central void 10a of the stator 10. Cooling arrangement A includes a pair of cooling devices 12a, 12b (labelled as A), connected by a conduit (shown by the connecting arrow). Cooling device 12a includes an inlet (shown by the inwards arrow) and cooling device 12b includes an outlet (shown by the outwards arrow). The inlet of cooling device 12a may correspond to the inlet 4a of figure 1, and the outlet of cooling device 12b may correspond to the outlet 8a of figure 1. The cooling circuit 6a of figure 1 is formed from the coolant path from the inlet of cooling device 12a, the cooling device 12a, the conduit to cooling device 12b, the cooling device 12b, and the outlet of cooling device 12b. Similarly, cooling arrangement B includes a pair of cooling devices 14a, 14b (labelled as B), connected by a conduit (shown by the connecting arrow). Cooling device 14a includes an inlet (shown by the inwards arrow) and cooling device 14b includes an outlet (shown by the outwards arrow). The inlet of cooling device 14a may correspond to the inlet 4b of figure 1, and the outlet of cooling device 14b may correspond to the outlet 8b of figure 1. The cooling circuit 6b of figure 1 is formed from the coolant path from the inlet of cooling device 14a, the cooling device 14a, the conduit to cooling device 14b, the cooling device 14b, and the outlet of cooling device 14b. The cooling devices 12a, 12b, 14a, 14b cool the stator. These cooling devices may comprise an internal channel to communicate the coolant from their inlet to outlet. This internal channel is arranged in use to carry coolant and support the transfer of heat from the stator to the coolant in the internal channel. The internal channel may be serpentine and thus may maximise a surface area in contact or proximity to the stator. The cooling action is described in more detail with reference to figure 6 further below. As shown in figure 2, the cooling arrangements A, B are independent as the coolant circuits (shown by the arrows) are independent of one another and form two separate, closed-loop subsystems. This provides redundancy in the event that one of the cooling arrangements fails. For example, a coolant leak in cooling arrangement A does not result in coolant leakage from cooling arrangement 14. In this example, the cooling devices of each cooling arrangement are connected in series. Hence, coolant from cooling device 12a is then provided to cooling device 12b (i.e. by the conduit described above and shown by the arrow). That is to say, coolant used in cooling device 12a is output and then provided to cooling device 12b for use in cooling device 12b. This is also the case for cooling devices 14a, 14b. As discussed above, the series connection of cooling devices increases the likelihood that there will be a balanced load around the rotor shaft in the event of a failure associated with a cooling arrangement. Further, as shown in figure 2, the cooling devices 12a, 12b, 14a, 14b are arranged substantially equidistant around the central void 10a, in that the distance around the void 10a between adjacent cooling devices is largely the same, thereby forming a ring of cooling devices. In other words, the cooling devices are approximately symmetrical around the central void 10a. This balances the load / mass of the cooling devices in a symmetric manner around the central void 10a / rotor shaft. It will be appreciated that the shape of the stator may take a variety of forms, and indeed that the ring of cooling devices may take a variety of closed loop shapes, and in some cases may be a circular ring, but in other cases may be square-shaped, triangular, hexagonal, etc. The present techniques are not particularly limited in this respect. Also, as shown in figure 2, cooling devices 12a, 12b of the same cooling arrangement (labelled as A), are arranged on opposite sides of the central void 10a. This is also the case for cooling devices 14a, 14b (labelled as B). In otherwords, cooling devices of the same cooling arrangement may be diametrically opposed from one another across the central void 10a or may be arranged at opposing clocking positions about the central void 10a or axis of the stator (e.g. a rotor axis). Further, as shown by the A - B - A - B arrangement of cooling devices around the central void 10a, a given cooling device is adjacent cooling devices of a different cooling arrangement. For example, cooling device 12a of cooling arrangement A is adjacent cooling devices 14a, 14b of cooling arrangement B. In this way, cooling devices of different arrangements alternate around the perimeter of the central void 10a. This is advantageous because, in the event of a failure of a cooling device, the load is balanced about the central void 10a. For example, should cooling device 14a fail, cooling device 14b would no longer be supplied with coolant. A given cooling device may be arranged to cool a group of M coils of the stator (i.e. each cooling device may cool M coils), which may correspond to a coil winding portion of the stator. The cooling device may be in contact with the coils, and thus heat can be transferred via conduction. This provides efficient heat transfer from the stator coil windings to the coolant in the cooling devices. It will be appreciated that the contact between the cooling devices and the coils of the stator may be direct or indirect. For example, in cases, the cooling devices may physically abut the coils, and in other cases one or more intermediate components or layers may be provided, for example to enhance heat transfer, such as thermal paste or heat pipes etc. A given cooling device may be arranged to cool a coil winding portion of the stator 10 associated with multiple phases of the electric machine. For example, multiple phases per cooling device or one phase per cooling device. In some cases, each cooling device may cool different coil winding portions associated with different phases of the electric machine, or each cooling device may cool different coil winding portions, each coil winding portion associated with multiple phases of the electric machine. The number of cooling devices is not particularly limited, as described in relation to figure 3. Figure 3 shows a similar cooling system 300 to figure 2, where three independent cooling arrangements are provided (the cooling devices of these cooling arrangements labelled as A, B and C), and each cooling arrangement having three cooling devices. This provides increased redundancy. The discussion of figure 2 is not repeated for figure 3. It will be appreciated that each cooling arrangement does not necessarily require only a pair of cooling devices on a face / end of a stator. It will further be understood that the cooling arrangements of the present cooling systems do not require the same number of cooling devices, and that in some examples the number of cooling devices between cooling arrangements may be different. Like in figure 2, figure 3 includes cooling devices of different cooling arrangements connected by conduits (shown by the arrows), and a cooling device of each of the different cooling arrangements includes an inlet to receive coolant and introduce coolant to the respective coolant circuit, and another cooling device of each of the different cooling arrangements includes an outlet to output coolant from the cooling circuit. A stator may have a front end and a rear end. These ends may be separated in the longitudinal direction of the stator / rotor shaft. The front end may be the end of the stator where a load is connected to the rotor in use. For example, a front end of a stator for an electric motor may be the end of the stator where a rotor shaft rotates in use and provides rotational movement to a load. The present cooling systems may be arranged to cool both the front end and the rear end of a stator. This is shown in figure 4. Figure 4 shows a similar cooling arrangement (and cooling devices) as figure 2, but for both the front end and the rear end of the stator 10. In this way, the cooling devices are arranged in two rings, each ring arranged in use to cool an end of the stator 10. The second ring may be arranged in a similar manner to the first ring. While a cylindrical stator 10 having circular front and rear ends is shown in figure 4 in the dotted lines, it will be appreciated that the shape of the stator may be varied (and may be rectangular, square, polygonal in cross-section), and that the coil winding portions at the end faces of the stator 10 may be arranged in a shape other than a circular ring (such as any polygonal shape). An example coolant flow path for cooling arrangement A will now be described. As can be seen from the arrows (showing coolant communication paths / conduits), for cooling devices A (i.e. from the cooling arrangement A), a first cooling device 12a of the front ring receives coolant at an inlet (shown by the inward arrow), cools a segment of the stator 10 (i.e. a coil winding portion of the stator 10), and then communicates this to the diametrically opposed cooling device 12b. Cooling device 12b then cools another segment of the stator 10 and then communicates the coolant (in a conduit etc.) to cooling device 12c which is arranged on the second ring of cooling devices at the rear end of the stator 10. Cooling device 12c then cools another segment of the stator 10 and then communicates the coolant to cooling device 12d. Cooling device 12d then cools a further segment of the stator 10 and the coolant is then provided to an outlet of cooling device 12d (shown by the outward arrow). The coolant flow path for cooling arrangement B (and cooling devices labelled as B) is arranged in a similar and corresponding manner. In this way, the coils of the stator 10 are cooled from both ends of the stator 10 and the symmetry of the temperature distribution of the coils is increased. Hence, large temperature gradients across coils (both across a single coil and across different coils) can be reduced, thereby providing more effective cooling, reducing the likelihood of damage and increasing the lifetime of the stator. As shown, the same number of cooling devices may be provided at the front end as the rear end of the stator 10. This provides a balanced load. However, it will be appreciated that this may be varied depending on implementation. An example cooling system arranged to cool coil winding portions of a stator according to the present techniques will now be described with reference to figure 5. Figure 5 shows a plurality of segments of a stator 20 (i.e. stator 10 discussed previously). The plurality of segments 20 are arranged in a ring, and it will be appreciated that the plurality of segments 20 include a number of coil winding portions as shown in figure 5 by the repeating units. Each segment may include one or more coil winding portions, and each cooling device may cool one or more coil winding portions. As shown in the example of figure 5, each cooling device cools three coil winding portions. As also shown in figure 5, the coil windings extend in the front-rear direction of the stator, and the coil windings change direction at the front / rear faces of the stator (i.e. where the cooling devices are located in this example). Figure 7, discussed further below, shows a similar arrangement. In the example of figure 5, cooling system 500 includes four independent cooling arrangements (the cooling devices of these cooling arrangements being denoted by 12a-d, 14a-d, 16a-d, and 18a-d), providing cooling devices that are arranged in two rings of cooling devices. For each cooling arrangement, on the front end, two cooling devices are arranged in use to cool respective segments of the stator / coil winding portions. For example, a first cooling arrangement includes cooling devices 12a and 12b at the front end, and cooling devices 12c and 12d at the rear end (i.e. arranged in two rings of cooling devices). Cooling device 12a provides an inlet for receiving coolant, and a conduit connects cooling device 12a to cooling device 12b. Cooling device 12b provides the coolant to cooling device 12c via a further conduit, which is located on the ring at the rear end of the stator. Cooling device 12c provides the coolant to cooling device 12d, and cooling device 12d includes an outlet for outputting the coolant. Cooling devices 14a-d, 16a-d, and 18a-d are arranged in a similar manner. In this way, cooling system 500 provides a front ring of cooling devices (formed from 12a-b, 14a-b, 16a-b, and 18a-b) and a rear ring of cooling devices (formed from 12c-d, 14c-d, 16c-d, and 18c-d) arranged to cool segments of the stator. Hence, both a front end / face and rear end / face of the stator can be effectively cooled by the cooling system 500, and excess temperature gradients across the coils of the stator can be reduced. As described, the coolant is introduced to cooling system 500 at a ring of cooling devices corresponding to the front side of the stator. The present inventors have identified that more heat is typically generated at the front end of the stator due to heat-generating components typically being located at the front side. Thus, advantageously, the coolant is introduced first to the cooling devices of the front ring, so that the coolant is coldest when passing through cooling devices of the front ring so as to more effectively cool the front end components of the stator. The cooling devices, as shown in figure 5, may be arcuate. This provides increased surface area in contact with an annular stator arrangement. However, the cooling devices may take other shapes. The cooling mechanism and example arrangement of a cooling device in relation to the coil windings of a stator according to the present techniques will now be described with reference to figure 6. Figure 6 shows a cross-section view of a cooling device 22 arranged to cool a coil winding portion 24 of a stator (such as stator 10). This is an example cross-section of a cooling device and coil winding portion taken along the front-rear direction of figure 5. As indicated by the vertical lines, coil winding portion 24 includes coil windings arranged to extend along the longitudinal / rotor shaft axis of the stator. Where the coil windings of the coil winding portion 24 change direction at each end of the stator, one or more cooling devices are arranged to cool the coil winding portion 24. It will be appreciated that a segment of a stator may include one or more coil winding portions. In the example of figure 6, cooling device 22 is arranged to cool a coil winding portion of the stator having three discrete units, although this may vary depending on implementation. Further, cooling device 22 may be arranged to cool a coil winding portion associated with a single phase of the electric machine or multiple phases of the electric machine. For example, each unit of the coil winding portion 24 of figure 6 may be associated with the same phase of the electric machine, or with different phases of the electric machine (or one or more phases). Cooling device 22 may be a cooling block or heatsink or the like. Cooling block 22 may be formed from a heat conductive material. As shown by the arrows, coolant is received by a cooling device 22 inlet / input, and the coolant is communicated through an internal channel 26 to an output of the cooling device 22. During use, heat is transferred from the coil winding portions 24 to the coolant, thereby cooling the coil winding portions. The internal channel 26 may be serpentine to increase the length of the internal channel 26 between the inlet and outlet of the cooling device 22 and to increase heat transfer. Cooling device 22 may be connected to the coil winding portion 24 in a variety of ways. For example, via a mechanical connection such as by bolts, rivets, etc., either directly or indirectly (i.e. with the additional of an intermediate layer or component). In some examples, the cooling device 24 may physically contact / abut the coil winding portion 24, or may be spaced from the coil winding portion 24 while still allowing heat transfer. The present techniques are not particularly limited in this respect. Figure 7 shows an example cooling system 700 arranged in use to cool an electric machine 28 according to the present techniques. Electric machine 28 may be any electric machine having a stator arrangement, for example an electric motor or generator. In the example of figure 7, electric machine 28 is an electric motor having the stator 10 and a rotor 30 (the rotor 30 having a central rotor shaft 30a). In use, the rotor 30 is caused to rotate. In some examples, electric machine 28 is a propulsive motor for an at least partially electrically powered aircraft. Thus, in some examples, the rotor 30 may be provided with one or more fan blades. Cooling system 700 may correspond to cooling system 500 of figure 5. Figure 7 thus shows the arrangement of figure 5 arranged in situ. It will be appreciated that in figure 7, the front ring of cooling devices can be seen, while the rear ring cannot be seen due to casing of the stator 10. The conduits connecting the front and rear rings may run through the casing of the stator 10 in a longitudinal / axial direction of the stator 10 as shown. Integration of the present cooling systems with a propulsion system for an electrically powered aircraft will now be described with reference to figure 8. Figure 8 shows a propulsion system 850 for an electrically powered aircraft. Propulsion system 850 includes electric motor 28 (i.e. electric motor 28 of figure 7). Electric motor 28 includes stator 10 and rotor 30. Cooling system 800 is arranged to cool segments of the stator 10 as discussed herein. In this example, a front ring of cooling system 800 includes two pairs of cooling devices of diametrically opposed cooling devices, and the rear ring is the same (not shown). Electric motor 28 is arranged to provide a plurality of phases. For example, electric motor 28 may be a three-phase electric motor. Propulsion system 850 includes a plurality of inverters 32. Each inverter of the plurality of inverters 32 is associated with a different phase of the electric motor 28. The plurality of inverters 32 are each configured to receive electrical power, convert the electrical power into a form suitable for the electric motor 28, and provide the converted electrical power to the electric motor 28. As discussed above, a given cooling device of the cooling system 850 may be arranged to provide cooling to one or more coil winding portions associated with one or more phases of the electric motor 28. Propulsion system 850 may be integrated into an at least partially electrically powered aircraft to provide propulsion. For example, the propulsion system 850 may also include one or more fuel cell arrangements (for example hydrogen fuel cells) arranged in use to provide the electrical power to the electric motor 28 (via the plurality of inverters 32). The propulsion system 850 may include a fuel tank for supplying fuel to the one or more fuel cells. In some examples, the fuel is hydrogen and the hydrogen is stored in the fuel tank as a liquid cryogen. The propulsion system 850 may also include a battery arrangement, and super-capacitor arrangement in addition to the one or more fuel cell arrangements to provide additional power. In this way, improved cooling systems for cooling stator segments have been described, which 5 may be particularly advantageous for propulsive electric motors, such as electric motors for electrically powered aircraft.

Claims

1. A cooling system for a stator of an electric machine, the cooling system comprising: a plurality of independent cooling arrangements arranged in use to cool segments of the stator, wherein each cooling arrangement comprises an inlet arranged in use to receive coolant, and a cooling circuit arranged in use to communicate the coolant from the inlet to an outlet such that in use heat is transferred from a respective segment of the stator to the coolant.

2. The cooling system of claim 1, wherein the cooling circuits of the plurality of cooling arrangements are arranged in use to provide independent communication of coolant between a respective inlet and outlet.

3. The cooling system of claims 1 or 2, wherein the cooling circuit of each cooling arrangement comprises a plurality of cooling devices to cool the segments of the stator and spaced substantially equidistant around a central void, the central void arranged in use to receive a rotor of the electric machine.

4. The cooling system of claim 3, wherein the cooling devices of the plurality of cooling arrangements form a ring of cooling devices around the central void.

5. The cooling system of any of claims 3 to 4, wherein the cooling circuit of each cooling arrangement comprises a pair of cooling devices arranged on opposite sides of the central void.

6. The cooling system of claim 5, wherein for a given cooling arrangement, cooling devices of the pair of cooling devices are arranged substantially diametrically opposed from each other across the central void.

7. The cooling system of any of claims 3 to 6, wherein for a given cooling arrangement, the plurality of cooling devices is arranged in series.

8. The cooling system of any of claims 3 to 7, wherein a first cooling device of the plurality of cooling devices is arranged to receive the coolant from the inlet, and a second cooling device of the plurality of cooling devices is arranged to receive the coolant from the first cooling device.

9. The cooling system of any of claims 3 to 8, wherein the cooling devices are arranged around the central void such that a given cooling device is adjacent cooling devices of a different cooling arrangement from the given cooling device.

10. The cooling system of any of claims 3 to 9, wherein each cooling device is arranged to cool a different coil winding portion of the stator associated with different phases of the electric machine.

11. The cooling system of any of claims 3 to 9, wherein each cooling device is arranged to cool a coil winding portion of the stator associated with multiple phases of the electric machine.

12. The cooling system of any of claims 3 to 11, wherein each cooling device is arranged in use to conductively communicate heat away from a different coil winding portion of the stator.

13. The cooling system of any of claims 3 to 12, wherein each cooling device is arranged in use to mechanically connect with a different coil winding portion of the stator.

14. The cooling system of any of claims 3 to 13, wherein the cooling devices of the plurality of cooling arrangements form a first ring of cooling devices around the central void, wherein each cooling arrangement comprises a second plurality of cooling devices spaced substantially equidistant around the central void and forming a second ring of cooling devices, the second ring of cooling devices spaced in a longitudinal direction of the electric machine from the first ring of cooling devices.

15. The cooling system of claim 14, wherein the first ring of cooling devices is arranged in use to cool a front end of the stator, and the second ring of cooling devices is arranged in use to cool a rear end of the stator.

16. The cooling system of any of claims 14 to 15, wherein for a given cooling arrangement, a first cooling device of the second plurality of cooling devices is arranged in use to receive the coolant from a second cooling device of the plurality of cooling devices, and a second cooling device of the second plurality of cooling devices is arranged to receive the coolant from the first cooling device of the second plurality of cooling devices.

17. The cooling system of any of claims 3 to 16, wherein each cooling device comprises an internal channel for communicating the coolant between an input of the cooling device and an output of the cooling device, the internal channel arranged in use such that heat is transferred from coil windings of the stator to the coolant communicated in the internal channel.

18. The cooling system of any of claims 3 to 17, wherein the cooling devices are cooling blocks.

19. The cooling system of any of claims 3 to 18, wherein at least two conduits connecting the cooling devices are made from different material.

20. The cooling system of any preceding claim, wherein the coolant is a cryogenic fluid.

21. The cooling system of any preceding claim, wherein the electric machine is an electricmotor arranged to provide propulsive power to an electrically powered aircraft.

22. An electrically powered aircraft comprising the cooling system of any of claims 1 to 21.

23. A stator for an electric motor, the stator comprising:a plurality of coils; andthe cooling system for cooling the plurality of coils according to any of claims 1 to 21.

24. An electric motor for providing propulsive power to an electrically powered aircraft, the electric motor comprising:a rotor;a stator arranged around the rotor; andthe cooling system for cooling the stator according to any of claims 1 to 21.

25. A propulsion system for an electrically powered aircraft, the system comprising:the electric motor of claim 24 configured to provide a plurality of phases;a plurality of inverters, each associated with one of the plurality of phases, the plurality of inverters for receiving electrical power, converting the electrical power, and providing the electrical power to the electric motor.s

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