GB2546255A - Cooling electric machines - Google Patents

Abstract

An electric machine comprising: stator (3, fig 1) having a series of electromagnetic elements (6, fig 1), each electromagnetic element comprising a magnetically susceptible core 25a-c and a coil extending around the core 26a-c, and a housing formed by an outer casing wall 24, inner casing wall 23, backplate 27 and front plate (28, fig 5) defining a fluid chamber around voids, gaps or spaces (38, fig 7) created between the core and coil, permitting cooling fluid to circulate between the coil and core; a rotor (1, 2, fig 1) moveable with respect to the stator and having a series of magnetically active elements (5, fig 1) capable of interacting with a magnetic field generated in the cores of the stator. The fluid enters the machine housing via an inlet port (34, fig 6) and is directed through the housing by a series of outer baffles 37 and inner baffles 36, before exiting the housing via an outlet port (35, fig 6). Cables 32 connect to the coils through ports 31 of the housing.

Description

COOLING ELECTRIC MACHINES

Background of the invention

This invention relates to cooling electric machines such as motors and generators.

There is a need to improve the power density of electric machines such as motors and generators. One reason for this is the increasing focus on electrically driven vehicles, including land vehicles and aircraft. As the power density of electric machines increases, arranging for effective cooling becomes increasingly difficult. Many prior art electric machines can be adequately cooled by air passing over vanes on the exterior of the machines’ casings. As power density increases it becomes advantageous to provide stronger cooling mechanisms, for instance by circulating liquid coolant around the electric machine. This raises additional problems because the interior of the machine must be designed so as to accommodate the presence of coolant without significantly reducing the power of the machine.

One type of electrical machine is the yokeless and segmented armature (YASA) machine. Typical YASA machines are particularly difficult to cool because they lack a stator yoke through which heat can be conducted away from the stator. WO 2010/092400 discloses one approach to cooling an electric machine of the YASA type. Liquid coolant is circulated around a chamber in which it can contact electrical stator coils of the machine. The electrical coils surround iron stator poles. Because the stator coils surround the stator poles, they can be expected to inhibit cooling of the stator poles by the coolant.

It is known to locate pipes to channel fluid next to a coil or iron part of an electric machine.

There is a need for improved mechanisms for cooling electric machines.

Summary of the invention

There is provided an electric machine comprising: a first part having (i) a series of electromagnetic elements, each electromagnetic element comprising a magnetically susceptible core and a coil extending around the core, and (ii) a housing defining a fluid chamber around the voids between the cores and the coils of the electromagnetic elements; a second part movable with respect to the first part and having a series of magnetically active elements capable of interacting with a magnetic field generated in the cores of the electromagnetic elements; the coil of each electromagnetic element being at least partially spaced from the core of the respective electromagnetic element, so as to permit cooling fluid contained in the chamber to circulate through a void defined between the coil and the core.

There is provided an electric machine comprising: a first part having (i) a series of electromagnetic elements, each electromagnetic element comprising a magnetically susceptible core and a coil extending around the core, and (ii) a housing defining a fluid chamber around the voids between the cores and the coils of the electromagnetic elements; a second part movable with respect to the first part and having a series of magnetically active elements capable of interacting with a magnetic field generated in the cores of the electromagnetic elements; the coil of each electromagnetic element being at least partially spaced from the core of one or more adjacent electromagnetic elements, leaving a void therebetween, so as to permit cooling fluid contained in the chamber to circulate through the voids between the coil and the core(s) of the said adjacent electromagnetic element(s).

Where possible the following additional features may apply to either of the above machines.

The machine may be configured to operate as a motor or a generator, or both.

The machine may be configured so that the cooling fluid contacts the coils of the electromagnetic elements in the voids. The machine may be configured so that the cooling fluid contacts the cores of the electromagnetic elements in the voids. The machine may be configured so that the cooling fluid contacts magnetically susceptible material of the cores of the electromagnetic elements in the voids.

The machine may be configured so that cooling fluid circulating through the voids circulates between the coil and the core of each electromagnetic element.

The voids may extend substantially fully around the core of each electromagnetic element, or partially around the core of each electromagnetic element. Each void may extend in a direction generally perpendicular to a rotation axis of the machine. Each void may define a passageway for fluid between an inner plenum of the chamber and an outer plenum of the chamber. Each plenum may extend around a rotation axis of the motor. Each plenum may be interrupted by formations such as baffles which restrict the flow of fluid in through the plenum in a direction generally around the axis.

The machine may comprise formations located in the chamber configured for urging fluid to pass in substantially opposite directions through two neighbouring electromagnetic elements.

The series of electromagnetic elements may be arranged in one of the following configurations: a ring, a linear row, a curvilinear row.

The interior of the chamber may be configured for urging fluid to pass in a radially inward direction through a first electromagnetic element and in a radially outward direction through an electromagnetic element neighbouring the first electromagnetic element.

The housing may comprise an inner wall radially inward of a ring of the electromagnetic elements, an outer wall radially outward of the ring and a pair of side walls disposed on either side of the inner and outer walls.

The cores may extend through the side walls.

The coils may be arranged around the cores so as to induce a magnetic field in the cores directed axially with respect to the ring.

The first part may comprise a further set of magnetically susceptible units, each further magnetically susceptible unit being located next to at least one of the electromagnetic elements for forming part of a return path for flux generated by the electromagnetic element.

The first part may comprise a yoke of magnetically susceptible material joined to the cores.

The voids may be located wholly within the chamber.

The chamber may be a fluid tight chamber. The chamber may be fluid tight with the exception of one or more ports communicating through the boundary of the chamber for conveying fluid into or out of the chamber. Those ports may be coupled to a heat exchanger.

The chamber and the voids may be filled with liquid coolant.

The voids may extend over greater than 40% of the surface area of the cores; thus greater than 40% of the surface area of the cores may be in contact with a void.

In each electromagnetic element the void(s) of that element may extend over at least 50% of the area of that part of the core of that element over which the coil of that element is wound.

The coil of each electromagnetic element may be spaced from the core of the respective electromagnetic element on substantially only a single side of the electromagnetic element. The said side may be an axially-facing side of the electromagnetic element. The said side may be a radially-facing side of the electromagnetic element.

The magnetically active elements may be elements of magnetically susceptible material. The magnetically active elements may be permanent magnets.

The core of each electromagnetic element, or that part of the core that is overlain by the coil of the respective electromagnetic element may be (i) longer in the radial direction than in the axial and/or circumferential directions and/or (ii) longer in the direction in which fluid can flor through the void of the element from one end of the core of the element to the other than in the directions about which the coil covers the core.

The machine may comprise electromagnetic elements that are not provided with voids as described above.

Brief description of the drawings

The present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a schematic exploded view of an axial flux switched reluctance electric machine.

Figure 2 is a view of an electromagnetic element of the machine of figure 1.

Figure 3 illustrates the principle of operation of the machine of figure 1.

Figure 4 is a partial cross-section of an electric machine on a radial plane.

Figure 5 is a partial exploded view of the machine of figure 4.

Figure 6 is a cut-away view of the machine of figure 4 in an axial direction.

Figure 7 is a view similar to figure 4, showing fluid flow paths.

Figure 8 is a cross-section of the machine of figure 4 on a transverse plane, showing fluid paths.

Figure 9 is a partial cut-away view of another electric machine in an axial direction.

Figure 10 is a view of the machine of figure 9 with an outer casing removed, showing offset baffles.

Figure 11 shows alternative fluid paths.

Figure 12 is a partial cross-section of a stator in which only a subset of the cores are wound.

Figure 13 shows examples of winding spacings in the stator of figure 12.

Figure 14 is a partial axial view of the core parts of a radial flux stator.

Figure 15 shows an example of fluid flow in a radial flux stator having a core as in figure 14.

Detailed description

In the machines to be described in the principal embodiments below, there is magnetic interaction between a stator and one or more rotors by virtue of magnetic flux generated by coils in the stator. The coils run around magnetically susceptible core parts of the stator. When an electric current flows in one or more of the coils magnetic polarisation is induced in the corresponding core parts. This polarisation can be used to drive relative motion of the rotor(s) and the stator. The coils are located in a substantially fluid-tight cavity. Each coil is spatially separated from its corresponding core part, leaving a gap therebetween. To cool the machine fluid is passed through the gaps. Configurations such as baffles may be provided within the cavity to promote a thorough circulation of fluid between the coils and the cores. The coils are spaced from the cores in a direction transverse to the direction of magnetic flux between the cores and the rotor(s). The fluid flows around the transverse sides of the cores. This allows the rotor(s) to closely approach the ends of the cores from which the flux extends, improving magnetic coupling between the cores and the rotor(s).

Electric machines of the types described herein may be capable of converting mechanical energy to electrical energy and/or vice versa. When operating to convert mechanical energy to electrical energy these machines are operating as generators. When operating to convert electrical energy to mechanical energy these machines are operating as motors. A single electric machine may be capable of performing both functions.

The electric machine described with reference to figures 1 to 8 is a dual rotor, axial flux, yokeless machine. The principle of operation of the machine will be described with reference to figures 1 to 3, which show the machine of figure 4 in simplified form. The operation of the machine as a motor will be described. Figure 1 is an exploded view of the machine. It comprises a pair of rotors 1, 2 arranged on either side of a stator element 3. The rotors are linked together by a shaft (not shown) and can rotate together with respect to the stator element about axis 4. Each rotor carries a series of magnetisable elements 5 arranged in a ring about the axis 4. The magnetisable elements are positioned so that each magnetisable element on one rotor faces a magnetisable element on the other rotor. The stator element comprises a series of electromagnet elements 6. An electromagnet element is shown in more detail in figure 2. Each electromagnet element comprises a core 7 of magnetically susceptible material, such as iron. A coil 8 of electrically conductive material, such as copper or aluminium wire, winds around the core. The machine is assembled so that there is a gap in the axial direction between the electromagnet elements 6 of the stator and the magnetisable elements 5 of the rotors. When the machine is in operation the coils of the electromagnet elements can be energised to cause the electromagnet elements to interact with the magnetisable elements of the rotors.

One form of interaction is shown in figure 3. Figure 3 is a radial view of a part of the machine showing two electromagnet elements 6a, 6b and two magnetisable elements 5a, 5b. Magnetisable element 5a is on rotor 1 and magnetisable element 5b is on rotor 2. In the configuration shown in figure 3, magnetisable elements 5a and 5b are not maximally aligned with electromagnet elements 6a, 6b. Coil 8a of electromagnet element 6a encircles stator core 7a. Coil 8a is energised so that it induces magnetic polarisation in core 7a of a sense such that a North pole faces element 5a and a South pole faces magnet 5b. Coil 8b of electromagnet element 6b encircles stator core 7b. Coil 8b is energised so that it induces magnetic polarisation in core 7b of a sense such that a South pole faces element 5a and a North pole faces element 5b. With the electromagnet elements 6a and 6b energised in this way a flux path 9 is formed through the electromagnet elements and also through the magnetisable elements of both rotors. The existence of this path means that magnetisable elements 5a and 5b are attracted into alignment with electromagnet elements 6a and 6b in order to minimise the total reluctance of the magnetic path. This effect can be used to drive the rotors to rotate with respect to the stator as indicated at 10. By energising and deenergising selected coils in the appropriate sense at appropriate times, this rotation can be sustained. The locations of the magnetisable elements on the rotors can be arranged so that different pairs of magnetisable elements come into maximal alignment with neighbouring electromagnet elements at different times. This helps to avoid dead spots in the operation of the motor.

In the example described above, each rotor carries magnetically active elements 5 which do not carry remanent magnetisation. In other designs of electrical machine the rotor can carry multiple magnetically active elements which are permanent magnets. In the latter case the rotor can be formed of magnetically susceptible material which can complete a magnetic flux path between two of the permanent magnets. The magnetically active elements on the rotors could alternatively be electromagnets.

Figure 2 shows that there is a gap 11 between the core 7 and the coil 8. This permits coolant to flow between and in direct contact with both the core and the coil, as will be discussed below.

Figures 4 to 6 show in more detail an electric machine of the type described above.

Figure 4 is a partial radial cross-section of the electric machine with the rotors and a front plate removed for clarity. The machine operates to cause rotation of rotors about axis 20. The rotors are mounted on shaft 21. Shaft 21 is supported by bearing 22 with respect to an inner casing wall 23 of the stator, allowing the shaft to rotate freely with respect to the stator about axis 20. Inner casing wall 23 closes the stator on its radially inward side. On its radially outward side the stator is closed by an outer casing wall 24. When the machine is installed for use, it may be attached by means of the outer casing wall, for example by bolts running through mounting holes (not shown) in the outer casing wall.

The inner casing wall 23 and the outer casing wall 24 define an annular cavity between each other. In the annular cavity are a series of electromagnet elements. Each electromagnet element comprises a core, e g. 25a-c and a coil, e.g. 26a-c. The cores are formed of magnetically susceptible, ferromagnetic, material such as iron. Each coil winds around a respective one of the cores. The coils are coils of an electrically conductive material, for example copper or aluminium. The coils can conveniently be formed of wire or another linear conductor which has been wound generally into a helix having a shape and size that can closely surround its corresponding core in directions perpendicular to the rotation axis 20.

The electromagnet elements are arranged in a rotationally symmetric series about the rotation axis 20. The electromagnet elements can be arranged so that each electromagnet element is offset from the next by the same amount around the axis 20, or the adjacent electromagnet elements may be offset from each other by different amounts, for example to reduce torque ripple. To facilitate close packing of the electromagnet elements the cores are conveniently of a truncated wedge shape, so that each core is wider in a circumferential direction at its radially outer end than at its radially inner end. The coils are configured in a similar way.

Each coil is arranged so as to be capable of inducing magnetic polarisation in its corresponding core along a direction parallel to axis 20. For this reason, conveniently the material of each coil extends around the corresponding cores in directions generally perpendicular to axis 20, and the ends of the cores in the axial direction are not covered by the coils. A back plate 27 extends between the inner casing wall 23 and the outer casing wall 24. The back plate extends generally perpendicular to axis 20. The back plate is sealed in a fluid-tight manner against the inner and outer casing walls, and against the rear ends of the cores. A similar front plate 28 (see figure 5) extends on the opposite side of the stator between the inner casing wall 23 and the outer casing wall 24 and is sealed in a fluid-tight manner against the inner and outer casing walls, and against the forward ends of the cores. Hence the front and back plates, together with the inner and outer casing walls, and in this example the axially outer parts of the cores, define a fluid-tight chamber in which the coils and at least the majority of the cores are located. The front and back plates could pass over the axial ends of the cores, but it is preferred that the axial ends of the cores extend through the front and back plates, as shown at 29. This allows for closer proximity between the cores and the rotors. With this design the front and back plates have apertures 33 through which the cores extend (see figure 5) and the front and back plates seal against the side surfaces of the cores, as illustrated at 30 in figure 4.

The back plate 27 includes ports 31 through which electrical cables 32, for driving or drawing current from the coils, pass into the stator chamber.

Figure 5 is an exploded view of the stator and the shaft. It can be seen that the front and back plates 27, 28 include apertures 33 which are configured to seal against the side surfaces of the cores.

The cooling mechanism of the stator will now be described.

Figure 6 is an axial view of the stator. The outer casing wall 24 includes two fluid ports 34, 35. The fluid ports are located diametrically opposite to each other. Port 34 is an inlet port. Port 35 is an outlet port. When the machine is in operation coolant is passed from inlet 34 through the stator chamber and out through outlet port 35. The coolant could then be passed to a heat exchanger to liberate thermal energy captured in the coolant. With the exception of ports 34, 35, the chamber containing the stator coils is fluid-tight.

Efficient cooling of the stator is enhanced by a series of baffles located in the stator chamber. The baffles define sinuous coolant paths between the inlet and the outlet. A first set of inner baffles 36 are located between the inner casing wall and some of the electromagnetic elements. A second set of outer baffles 37 are located between the outer casing wall and some of the electromagnetic elements. The inner baffles resist coolant flow between those electromagnetic elements and the inner casing wall, deflecting fluid radially outwardly. The outer baffles resist coolant flow between those electromagnetic elements and the outer casing wall, deflecting fluid radially inwardly.

Figure 6 shows the locations of the baffles in this embodiment. Each inner baffle is offset in a circumferential direction from the outer baffles. This induces the fluid to flow along the following path: from the inlet (arrow 70), in a circumferential direction (arrows 71) around the exterior of the stator chamber, radially inwards (arrows 72) through some of the electromagnetic elements to the interior of the stator chamber (further circumferential flow at the outer part of the chamber having been impeded by baffle 37a), in a circumferential direction (arrows 73) around the interior of the stator chamber, radially outwards (arrows 74) through some of the electromagnetic elements to the exterior of the stator chamber (further circumferential flow at the inner part of the chamber having been impeded by baffle 36a), around the exterior of the stator chamber in a circumferential direction (arrows 75) and so on. In the lower half of the stator as shown in figure 6 the flow path is a mirror image of the flow path in the upper half. When the fluid is flowing as indicated by arrows 71, further circumferential progress of the fluid is inhibited by baffle 37a. This promotes inward radial flow of the fluid. When the fluid is flowing as indicated by arrows 73, further circumferential progress of the fluid is inhibited by baffle 36a. This promotes outward radial flow of the fluid. In this way, the baffles promote a sinuous flow path through the stator. In this flow path fluid in the vicinity of each electromagnetic element is channelled in a generally radial direction. This is discussed with reference to figure 7 below.

The baffles do not need to seal perfectly against either the electromagnetic elements or the casing walls. However, the better the sealing the more effectively the serpentine flow path is encouraged.

Certain arrangements of the baffles can help to promote uniform cooling. The interior baffles may be regularly spaced in a circumferential direction. The outer baffles may be regularly spaced in a circumferential direction. The inner baffles may be offset circumferentially from the outer baffles. The inner baffles may be offset circumferentially so as to be located mid-way between the neighbouring outer baffles. Inner baffles may be located opposite the fluid inlet and/or the fluid outlet. The preferred number of baffles depends on the characteristics of the stator but, for example, there may be from 2 to 8 of each of the inner and outer baffles.

Part of the fluid path is shown in more detail in figure 7. Each coil runs around the radially inner and outer ends, and around the circumferentially-facing sides, of its respective core. The coil is spaced from its respective core, for example by rigid pillars (not shown) extending between the coil and the core. This leaves a gap 38 between the coil and the respective core over the majority of the interface between the two. The cores are sealed to the front and back plates 27, 28. The coils are located so as to be spaced from one or both of the front and back plates. This arrangement permits fluid to circulate between the radially outer zone of the stator cavity and the radially inner zone of the stator cavity, through the gap between the coil and front/back plates and then through the gap between the coil and the core. For a zone where the fluid flow is radially inwards, fluid circulating in the radially outer zone (arrow 80) passes between the front or back plate and the radially outer part of the coil (arrow 81) and then flows axially so as to be distributed over the core (arrow 82) and radially so as reach the radially inner part of the core (arrows 83). The fluid can flow between the radially inward ends of the core and coil (arrow 84) and then passes between the coil and the front or back plate (arrows 85) to reach the radially inner zone. In regions of the stator where the flow is radially outwards the flow path is analogous but reversed. It will be noted that a substantial part of the coolant path is between a coil and its respective core. The fluid path is such that both the coils and the cores are directly exposed to the coolant. The fluid path runs substantially between each coil and its respective core, preferably between the major surfaces of the coils and their respective cores. This allows the coolant to achieve effective cooling of both the coils and the cores. The coils are preferably closely wound so as to resist or prevent fluid passing between the wires of a coil. In this way each coil confines fluid located within that coil to flow in a generally radial direction.

Effective cooling of the cores is particularly important in a machine of this type. To permit the machine to run at best efficiency there should be a small gap between the cores and the rotors. If the cores vary in temperature then additional gap must be allowed to accommodate thermal expansion of the cores. For this reason, cooling of the cores in the manner described above can be highly advantageous.

Because the interface between each core and its respective coil is substantially occupied by fluid coolant, which can be moving when the machine is in operation, it is possible to maintain a substantial degree of heat isolation between the coils and the cores. The fluid may be driven by a pump or by convection.

Figure 8 is a transverse section on part of the stator, showing the fluid path (arrows 90) extending between the cores and the coils, and indicating that the sense of fluid flow is reversed from external to internal on the opposite side of baffle 36.

Each coil may be wound sufficiently tightly and/or provided with potting so that it provides a substantial resistance to fluid flow through the body of the coil. The body of each coil may be fluid-tight. By having the coils resist fluid flow therethrough the coils can confine fluid to the paths described above, inhibiting the fluid from migrating in a circumferential direction other than in the radially inner and outer zones of the stator cavity. To help resist circumferential fluid flow around the sides of the coils, the coils may be sealed against the front and back plates in their radially central portions, leaving their end regions free for flow as indicated by arrows 81 and 84. Adjacent coils can abut each other to inhibit fluid flow between adjacent coils.

The inner and outer baffles are located between adjacent coils. The baffles are shaped (e.g. tapered at their ends facing the coils) so as to fit snugly against the coils.

Figures 9 and 10 show an alternative embodiment. In this embodiment the fluid flows between the coils and the cores predominantly within the outermost parts of the coils. The outer baffles 50 are staggered so that successive outer baffles are positioned next to (and preferably sealed against) alternate ones of the front and rear plates. This leaves a fluid channel between the baffle and the other one of the front and rear plates. This arrangement encourages the fluid to adopt a sinuous path from front to rear as it moves circumferentially around the outer part of the stator. In the example of figures 9 and 10 successive outer baffles are staggered front to rear. In other embodiments two or more adjacent baffles could be close to one of the front and rear plates and then a succeeding set of two or more baffles could be close to the other of the front and rear plates. The radially inner parts of the coils and cores could be cooled in an analogous way.

Figure 11 illustrates examples of some other flow patterns that could be employed.

Figures 11a to 11c illustrate flow patterns around individual electromagnetic elements. The flow paths are indicated by heavy dotted lines. In figures 11 a and 11 b sealing is configured around parts of the coil and/or core so that fluid flows in at the radially outer side of the core and out at the radially inner side of the core, or vice versa. In figure 11b, sealing 100, 101 is provided partially around the core at each of its axial ends. At one end of the core the sealing is incomplete, leaving a fluid gap, at the radially outer side of the core; and at the other axial end of the core the sealing is incomplete, leaving a fluid gap, at the radially inner side of the core. The gaps communicate with the inner and outer regions of the stator cavity. Where it is present the sealing resists flow between the region surrounding the core and the inner and outer regions of the stator cavity. This arrangement encourages fluid flow around the sides of the core, as indicated. In figure 11c sealing 102, 103 is provided partially around the core at each of its axial ends. At both ends the sealing is incomplete at the radially inner and outer parts of the core, leaving gaps there. The gaps communicate with the inner and outer regions of the stator cavity. Where it is present the sealing resists flow between the region surrounding the core and the inner and outer regions of the stator cavity.

Figures 11 d and 11 e are views of a number of electromagnetic elements from radially externally to the elements. Fluid flow on the exterior side of the elements is indicated by heavy dotted lines. Fluid flow on the interior side of the elements is indicated by double lines in figure 11 d. Figure 11e shows a configuration as in figures 6 and 7. In figure 11 d additional sealing 104, 105 is provided along one side of each electromagnetic element, so as to encourage fluid flow in an axial direction.

In another arrangement the fluid inlet port can communicate with the stator chamber at its radially outer part, and the fluid outlet port can communicate with the stator chamber at its radially inner part. Fluid can then pass in a substantially radial direction between the ports, going through the gaps between the cores and the coils. The flow direction could be reversed.

Figures 12 and 13 illustrate another winding arrangement. Figure 12 is an axial view of part of a stator of an axial flux machine. The stator has cores 110, 111 of magnetically susceptible material. The cores are disposed around an axis 112. In operation the cores interact magnetically with magnetically susceptible elements of a rotor to cause the rotor to rotate relative to the stator about axis 112. Coils 113 are arranged around a subset of the cores. The coils encircle stator elements 111, but lie alongside stator elements 110 without encircling them. The stator elements 110 can provide a return path from flux passing through the rotors if, for example, only a single coil is activated. Figure 13 shows various ways in which the coils can be disposed in such a system. Figure 13a shows an embodiment in which the coil 113 is spaced from the circumferential faces of the core 111 that it surrounds, but abuts the adjacent cores 110 that it does not surround. This leaves spaces 114 through which fluid can flow to preferentially cool core 111. As with the other embodiments of figure 13, a space may or may not be left at 115 between the radial end(s) of the surrounded core 111 and the coil. Figure 13b shows an embodiment in which the coil 113 is spaced from the circumferential faces of the adjacent cores 110 that it does not surround, but abuts the core 111 that it does surround. This leaves spaces 116 through which fluid can flow to preferentially cool cores 110. Figure 13c shows an embodiment in which the coil 113 is spaced from the circumferential face of one of the adjacent cores 110 that it does not surround, leaving a space 117, but abuts the other of the adjacent cores that it does not surround. The coil 113 abuts one circumferential face of the core 111 that it does surround and is spaced from the other such circumferential face, leaving a space 118. Coolant can flow through spaces 117,118. In another embodiment, the coils could be spaced from both cores 110 and 111 on all their circumferential surfaces.

Figures 14 and 15 show how the present cooling mechanism can be applied to a radial flux machine. Figure 4 shows part of the stator iron of an example radial flux machine. The stator iron has a carrier 120, in the shape of a ring. Salients 121 protrude radially from the carrier. The salients act as energisable cores, as will be described further below. The carrier can act as a yoke to return flux from one energisable core to another. Figure 15 shows a part of the stator iron with coils 122 installed on the salients 121. Each coil encircles a respective one of the salients to form an electromagnetic element. Selected coils can be energised to polarise their cores and thereby generate a magnetic flux extending radially from those cores. The machine can be controlled in various ways, but in one example, two adjacent coils can be energised to generate opposite polarities in adjacent cores. On the radially outer side of the stator that flux can pass through a rotor, driving it to rotate about axis 128 relative to the stator. On the radially inner side, the flux path can be completed through the carrier part of the stator iron, acting as a yoke. Figure 15 shows that the coils can be configured to define a space 123 between a coil 122 and the core 121 that it encircles.

In the example of figure 15 there is a space on a single axial face of the core, but there could be spaces left on both axial faces and/or on one or both circumferential faces. The spaces permit fluid to flow between the coils and the cores they surround, one potential fluid path being indicated by arrows 127. This fluid path runs in a serpentine form around the stator, alternating between radially inward and radially outward motion as it progresses around the stator in a circumferential direction. This reversal of the flow path in a radial direction can be encouraged by baffles 124, 125, which can be generally of the type described in relation to figure 4.

It is preferred that the fluid paths are arranged so as to avoid forming pockets of stagnant fluid, where localised overheating could occur. This may be achieved through modelling or through measurement.

The examples described above involve switched reluctance machines. Similar serpentine cooling paths between coils and cores can be made in machines of other types, for example in synchronous reluctance machines, inductance machines, or permanent magnet reluctance machines. A machine to which the present cooling approach is applied could employ axial, transverse or radial flux from the cores to the rotor(s). A machine to which the present cooling approach is applied could be brushed or brushless. One example application is in the armature of a DC brushed motor. A machine to which the present cooling approach is applied could have one, two or more rotors. It could have no yoke (as in the embodiment of figure 4) or could be provided with a yoke (as in the embodiment of figure 15) attached to the stator cores for defining a return flux path between two stator cores.

The inlet and outlet core fluid into and out of the stator cavity could be provided at any any suitable location. They need not be opposite each other. There could be multiple inlets and/or outlets.

The coolant is preferably a liquid coolant such as oil, water, a glycol-based fluid or an advanced heat transfer fluid.

Instead of the stator being fixed, the rotor(s) could be fixed and the stator could move relative to its environment.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims (25)

1. An electric machine comprising: a first part having (i) a series of electromagnetic elements, each electromagnetic element comprising a magnetically susceptible core and a coil extending around the core, and (ii) a housing defining a fluid chamber around the voids between the cores and the coils of the electromagnetic elements; a second part movable with respect to the first part and having a series of magnetically active elements capable of interacting with a magnetic field generated in the cores of the electromagnetic elements; the coil of each electromagnetic element being at least partially spaced from the core of the respective electromagnetic element, so as to permit cooling fluid contained in the chamber to circulate through a void defined between the coil and the core.

2. An electric machine as claimed in claim 1, configured so that the cooling fluid contacts the coils of the electromagnetic elements in the voids.

3. An electric machine as claimed in claim 1 or 2, configured so that the cooling fluid contacts the cores of the electromagnetic elements in the voids.

4. An electric machine as claimed in claim 1 or 2, configured so that the cooling fluid contacts magnetically susceptible material of the cores of the electromagnetic elements in the voids.

5. An electric machine as claimed in any preceding claim, configured so that cooling fluid circulating through the voids circulates between the coil and the core of each electromagnetic element.

6. An electric machine as claimed in any preceding claim, comprising formations located in the chamber configured for urging fluid to pass in substantially opposite directions through two neighbouring electromagnetic elements.

7. An electric machine as claimed in any preceding claim, wherein the series of electromagnetic elements are arranged in one of the following configurations: a ring, a linear row, a curvilinear row.

8. An electric machine as claimed in claim 7, wherein the interior of the chamber is configured for urging fluid to pass in a radially inward direction through a first electromagnetic element and in a radially outward direction through an electromagnetic element neighbouring the first electromagnetic element.

9. An electric machine as claimed in claim 7 or 8, wherein the housing comprises an inner wall radially inward of the ring, an outer wall radially outward of the ring and a pair of side walls disposed on either side of the inner and outer walls.

10. An electric machine as claimed in claim 9, wherein the cores extend through the side walls.

11. An electric machine as claimed in any of claims 7 to 10, wherein the coils are arranged around the cores so as to induce a magnetic field in the cores directed axially with respect to the ring.

12. An electric machine as claimed in any preceding claim, wherein the first part comprising a further set of magnetically susceptible units, each further magnetically susceptible unit being located next to at least one of the electromagnetic elements for forming part of a return path for flux generated by the electromagnetic element.

13. An electric machine as claimed in any preceding claim, wherein the first part comprises a yoke of magnetically susceptible material joined to the cores.

14. An electric machine as claimed in any preceding claim, wherein the voids are located wholly within the chamber.

15. An electric machine as claimed in any preceding claim, wherein the chamber is a fluid tight chamber with the exception of one or more ports communicating through the boundary of the chamber for conveying fluid into or out of the chamber.

16. An electric machine as claimed in any preceding claim, wherein the chamber and the voids are filled with liquid coolant.

17. An electric machine as claimed in any preceding claim, wherein the voids extend over greater than 40% of the surface area of the cores.

18. An electric machine as claimed in any preceding claim, wherein in each electromagnetic element the void(s) of that element extend(s) over at least 50% of the area of that part of the core of that element over which the coil of that element is wound.

19. An electric machine as claimed in any preceding claim, wherein the coil of each electromagnetic element is spaced from the core of the respective electromagnetic element on substantially only a single side of the electromagnetic element.

20. An electric machine as claimed in claim 19, wherein the said side is an axially-facing side of the electromagnetic element.

21. An electric machine as claimed in claim 19, wherein the said side is a radially-facing side of the electromagnetic element.

22. An electric machine as claimed in any preceding claim, wherein the magnetically active elements are elements of magnetically susceptible material.

23. An electric machine as claimed in any preceding claim, wherein the magnetically active elements are permanent magnets.

24. An electric machine comprising: a first part having (i) a series of electromagnetic elements, each electromagnetic element comprising a magnetically susceptible core and a coil extending around the core, and (ii) a housing defining a fluid chamber around the voids between the cores and the coils of the electromagnetic elements; a second part movable with respect to the first part and having a series of magnetically active elements capable of interacting with a magnetic field generated in the cores of the electromagnetic elements; the coil of each electromagnetic element being at least partially spaced from the core of one or more adjacent electromagnetic elements, leaving a void therebetween, so as to permit cooling fluid contained in the chamber to circulate through the voids between the coil and the core(s) of the said adjacent electromagnetic element(s).

25. An electric machine substantially as herein described with reference to the accompanying drawings.