GB2613810A

GB2613810A - Subsea axial flux electrical machine

Info

Publication number: GB2613810A
Application number: GB2118165.6A
Authority: GB
Inventors: Walker Vahid
Original assignee: Walker Subsea Engineering Ltd
Current assignee: Walker Subsea Engineering Ltd
Priority date: 2021-12-15
Filing date: 2021-12-15
Publication date: 2023-06-21
Also published as: GB202118165D0

Abstract

An axial flux machine 200 for operating in a subsea environment, the machine comprising a sealed housing 101 for containing a pressurised fluid, at least one stator 102, 103, and at a least one rotor 104 within the housing, and a pressure compensator 106 comprising a first fluid port 115 and a second fluid port 116. The first port is in fluid communication with an environment external to the housing and the second fluid port in fluid communication with the inside of the housing, the pressure compensator arranged to pressurise fluid inside the housing via the second fluid port in response to the fluid pressure at the first port. A fluid conduit 117 may be provided connecting the second port of the compensator with a fluid port of the housing. The compensator may be integrally formed with an end plate of the housing. The compensator may comprise a chamber comprising a first 112 and second part 113 separated by a moveable seal 114. The seal may be a piston or diaphragm. A resilient member 114 may apply force to the seal to generate additional pressure at the second fluid port where the member is a spring. Sensors may also be provided for pressure, oil condition, temperature and vibration. The housing fluid may be synthetic oil, distilled water or water-glycol.

Description

Subsea Axial Flux Electrical Machine

Technical Field

The present invention relates to axial flux electrical machines for operating in subsea 5 environments.

Background

Axial flux electrical machines are a well-known class of electrical machine that are becoming increasingly popular in applications such as electric vehicles and power 10 generation.

In axial flux electrical machines, the direction of magnetic flux between the rotor and stator is parallel to the axis of rotation. This is in contrast radial flux electrical machines where the direction of magnetic flux between the rotor and stator is perpendicular to the axis of rotation.

Various types of axial flux electrical machine exist including twin stator single rotor and single stator twin rotor topologies (and multiples thereof). For example, W02008/003987 A2 discloses an axial flux electrical machine that uses a twin-stator single rotor topology.

Axial flux electrical machines are preferrable to radial flux electrical machines in certain applications for several reasons including their higher torque and power density.

However, existing axial flux electrical machines are not suited to operating in subsea environments because they are prone to seawater ingress when submerged and are not capable of withstanding high levels of hydrostatic pressure without sustaining damage to their housing and internal components.

It is an object of the invention to provide an axial flux electrical machine that obviates or mitigates one or more of the above-described disadvantages.

Summary of the Invention

In accordance with a first aspect of the present invention there is provided an axial flux electrical machine for operating in a subsea environment. The axial flux electrical machine comprises a sealed housing for containing a pressurised fluid; at least one stator and at least one rotor located within the housing; and a pressure compensator. The pressure compensator comprising a first fluid port and a second fluid port, the first fluid port in fluid communication with an environment external to the housing and the second fluid port in fluid communication with the inside of the housing, the pressure compensator arranged to pressurise fluid inside the housing via the second fluid port in response to the fluid pressure at the first fluid port.

Optionally, the axial flux electrical machine further comprises a fluid conduit fluidly connecting the second fluid port of the pressure compensator with a fluid port of the housing.

Optionally, the pressure compensator is integrally formed with the housing.

Optionally, the pressure compensator is integrally formed with an end plate of the housing.

Optionally, the pressure compensator comprises a chamber comprising a first part and a second part separated by a moveable seal, wherein the first part is fluidly connected to the first fluid port and the second part is fluidly connected to the second fluid port.

Optionally, the moveable seal is provided by a piston or a diaphragm.

Optionally, the pressure compensator comprises a resilient member arranged to apply force to the moveable seal to generate additional pressure at the second fluid port and thereby generate a predetermined overpressure of fluid within the housing.

Optionally, the resilient member is a spring.

Optionally, the predetermined overpressure is between 0.1 and 1 bar.

Optionally, the axial flux electrical machine further comprises a shaft connected to and arranged to rotate with the at least one rotor, the shaft supported within the housing by at least one bearing, wherein the inside of the at least one bearing is exposed to fluid present within the housing.

Optionally, a surface of the at least one stator is secured to a mounting plate within the housing, and wherein the surface of the at least one stator comprises one or more grooves providing a path for pressurised fluid within the housing to flow across the surface of the at least one stator between the at least one stator and the mounting plate.

Optionally, the one or more grooves comprise annular grooves. Optionally, the one or more grooves comprise radial grooves.

Optionally, the axial flux electrical machine further comprises a sensor housing located within the housing, the sensor housing sealed to prevent pressurised fluid within the housing from entering it.

Optionally, the sensor housing is arranged to maintain a pressure at or close to standard atmospheric pressure during use.

Optionally, the axial flux electrical machine further comprises one or more sensors located within the housing.

Optionally, the one or more sensors comprise at least one of: an oil condition sensor, a pressure sensor, a temperature sensor and a vibration sensor.

Optionally, the housing is filled with fluid.

Optionally, the fluid is synthetic oil or distilled or deionised water or water-glycol.

Advantageously, embodiments of the present invention provide an axial flux electrical machine that can operate in the harsh conditions of subsea environments by avoiding seawater ingress and by withstanding high levels of hydrostatic pressure. The subsea pressure compensator can maintain fluid within the housing at a pressure that is at or slightly higher than the ambient pressure in the environment outside the housing.

Pressurising fluid inside the housing in this way balances the hydrostatic forces experienced by the housing and ensures that seawater does not leak into the housing.

Additionally, the fluid inside the housing is in direct contact with the components of the device that heat up during operation such as the stators. This means that the fluid inside the housing can be used to cool the components of the device when the device is connected to a suitable cooling system. In certain embodiments, this can avoid the need for cooling galleries to be provided on the outside of the housing to cool the stators.

Advantageously, certain embodiments of the present invention provide an axial flux electrical machine that can be used in various subsea devices including subsea construction, propulsion, winch, and compressor devices. Advantageously, certain embodiments of the present invention provide an axial flux electrical machine that can operate at depths of up to 6000m for extended periods of time.

Advantageously, in certain embodiments the subsea pressure compensator is integrally formed with the housing. Advantageously, in such embodiments an external pressure compensator is not required. This means that the device can be more compact.

Advantageously, in certain embodiments a surface of the stator includes grooves arranged to provide a path for pressurised fluid within the housing to contact the surface of the stator. Advantageously, this can help ensure that the housing is entirely filled with fluid so that no air pockets remain between components. Advantageously, this can also improve cooling of the stator by allowing fluid to flow along the surface of the stator.

Advantageously, in certain embodiments a sensor housing is located within the housing. The sensor housing is sealed to prevent pressurised fluid within the housing from entering it. Advantageously, this means that the sensor housing can provide an environment within the device that is filled with air at or close to standard atmospheric pressure. This means that conventional "off the shelf" sensors can be used within the device without the sensors being damaged by the extreme hydrostatic pressure or by being immersed in the fluid present within the housing.

Various further features and aspects of the invention are defined in the claims.

Brief Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings where like parts are provided with corresponding reference numerals and in which: Figure 1 provides a simplified schematic diagram of an axial flux electrical machine in accordance with certain embodiments of the present invention; Figure 2 provides a simplified schematic diagram of a further axial flux electrical 10 machine which includes an on-board subsea pressure compensator in accordance with certain embodiments of the present invention; Figure 3 provides a simplified schematic diagram of an axial flux electrical machine which includes a sensor housing in accordance with certain embodiments of the present invention; Figure 4a provides a simplified diagram of a surface of a stator in accordance with certain embodiments of the present invention that can be used in an axial flux electrical machine; and Figure 4b provides a simplified diagram showing a cross-section of the stator of Figure 4a secured to a mounting plate.

Detailed Description

Figure 1 provides a simplified schematic diagram of an axial flux electrical machine in accordance with certain embodiments of the present invention.

The axial flux electrical machine 100 can operate as a motor or a generator. In certain embodiments, the axial flux electrical machine 100 is configured to operate underwater in subsea environments down to a depth of at least approximately 6000m. The axial flux electrical machine 100 can also operate in freshwater environments.

The axial flux electrical machine 100 comprises a housing 101, a first stator 102, a second stator 103, a rotor 104 connected to a drive shaft 105, and a pressure compensator 106.

The housing 101 is fluidly sealed such that it can contain a pressurised fluid within it.

In certain embodiments, the housing 101 is arranged to withstand an internal pressure of approximately 10MPa (the approximate hydrostatic pressure at 1000m depth). In certain embodiments, the housing 101 is arranged to withstand an internal pressure of approximately 60MPa (the approximate hydrostatic pressure at 6000m depth). The housing 101 can be composed of a suitable material such as stainless steel, for example duplex stainless steel, or titanium. The housing 101 typically has a substantially cylindrical shape.

In use, the inside of the housing 101 is filled with fluid. Typically, the fluid is a suitable oil such as synthetic oil although other suitable non-conducting liquids such as distilled or deionised water or water-glycol can be used. The fluid completely fills the inside of the housing 101 such that there are substantially no air pockets present within the housing 101.

The housing 101 comprises an end plate 107 shaped to accommodate an end of the drive shaft 105. The housing 101 comprises a first fluid port 108 and a second fluid port 109. The first fluid port 108 and second fluid port 109 are in fluid communication with the inside of the housing 101. The first fluid port 108 and second fluid port 109 can be used to connect fluid pressurisation or cooling components to the housing 101.

The first stator 102 and second stator 103 are secured in place within the housing 101 on either side of the rotor 104.

The rotor 104 is a disc that contains a series of permanent magnets that, in use, 5 interact with the magnetic field generated by the first stator 102 and second stator 103 to rotate the rotor 104. The rotor 104 is connected to the drive shaft 105 such that rotation of the rotor 104 causes a corresponding rotation of the drive shaft 105.

The drive shaft 105 is supported by a first bearing 110 and a second bearing 111. The first bearing 110 and second bearing 111 are open type bearings. This means that the inside of the first bearing 110 and second bearing 111 is in contact with fluid present within the housing 101. Advantageously, using open bearings rather than sealed bearings avoids air pockets inside the bearings which would cause high mechanical forces on the bearings when the pressure of fluid inside the housing is high.

The pressure compensator 106 is arranged to apply pressure to fluid present within the housing 101 corresponding to the ambient pressure outside the housing 101. The pressure compensator 106 is a subsea pressure compensator.

The subsea pressure compensator 106 comprises a chamber. The chamber comprises a first part 112 and a second part 113 separated by a moveable seal 114. In use, the first part 112 typically contains sea water and the second part 113 contains the same type of fluid as is present inside the housing 101 (for example synthetic oil).

The subsea pressure compensator 106 comprises a first fluid port 115 fluidly connecting the first part 112 with the environment external to the housing. The first fluid port 115 is open such that fluid present within the environment external to the housing (typically sea water) can enter the first part 112.

The subsea pressure compensator 106 comprises a second fluid port 116 fluidly connecting the second part 113 with the inside of the housing 101. In this embodiment the second fluid port 116 is fluidly connected to the inside of the housing 101 via a fluid conduit 117 connected between the second fluid port 116 of the subsea pressure compensator 106 and the corresponding fluid port 108 of the housing 101.

The moveable seal 114 is provided by the head of a piston which defines the boundary between the first part 112 and the second part 113. The moveable seal 114 prevents fluid from passing between the first part 112 and the second part 113 but transfer pressure between the fluid present in the first part 112 and the second part 113. In this way, the subsea pressure compensator 106 is arranged to pressurise fluid inside the housing 101 via the second fluid port 116 in response to the fluid pressure at the first fluid port 115.

The subsea pressure compensator 106 also comprises a resilient member 118 provided by a spring. The resilient member 118 is arranged to apply a force to the moveable seal 114 to urge the moveable seal 114 towards the second part 113 and thereby to generate an additional predetermined overpressure at the second fluid port 116. In certain embodiments, the predetermined overpressure is between 0.1 and 1 bar.

It will be understood that other suitable types of pressure compensator can be used.

The second fluid port 109 can be connected to a suitable cooling system (not shown).

In certain embodiments, the cooling system comprises a pump and a heat exchanger. The heat exchanger can be provided by copper coils wrapped around the housing 101. The pump can pump fluid from inside the housing 101 through the heat exchanger and back into the inside of the housing 101.

The components located within the housing 101 including the first stator 102, second stator 103, rotor 104, drive shaft 105 and any associated plastic components, seals or cables are configured to withstand the design pressure of the housing 101.

Before use, the housing 101 is filled with a suitable fluid such as synthetic oil until 30 substantially no air pockets remain within the housing 101. The second part 113 of the subsea pressure compensator 106 is also filled with the same fluid. The housing 101 is sealed such that fluid is prevented from entering or leaving the housing 101.

The axial flux electrical machine 100 will now be described in use in an example where it is located in a subsea environment. The axial flux electrical machine 100 can be used as a motor or generator in various types of equipment including for example hydraulic turbines, propulsion devices, winches, pumps and compressors.

The axial flux electrical machine 100 is underwater at a suitable depth. Seawater has entered the first part 112 of the subsea pressure compensator 106 via the first fluid port 115. The seawater present in the first part 112 urges the moveable seal 114 such that it applies pressure to the second part 113. The pressure applied by the seawater to the second part 113 corresponds with the ambient pressure outside the device.

Additional pressure is applied to the moveable seal 114 by the resilient member 118 to generate an overpressure in the second part 113 relative to the ambient pressure. Typically, the overpressure generated by the resilient member 118 is approximately 0.1-1 bar, or more preferably 0.5 bar.

By virtue of the fluid connection between the second part 113 and the inside of the housing 101, fluid present inside the housing 101 is pressurised.

In this way, the pressure of fluid within the housing is controlled so that it corresponds with the ambient pressure (plus a small over pressure contribution from the resilient member 118). As the device is moved to a different depth below sea level, the fluid pressure within the housing is balanced with the ambient pressure.

Pressurising the inside of the housing 101 in this way balances the hydrostatic forces experienced by the housing 101. This means that the housing 101 can be designed to withstand considerably less mechanical stress compared with if it was not pressurised. Additionally, pressurising the inside of the housing 101 to at or slightly above the ambient pressure ensures that seawater does not leak into the housing 101 and potentially cause damage to components of the axial flux electrical machine 100.

The stators 102 103 and rotor 104 operate in a conventional manner to rotate the drive shaft 105 when operating as a motor or to generate electricity from the rotating drive shaft 105 when operating as a generator, as will be understood by the skilled person.

The axial flux electrical machine 100 has a single stage, single rotor twin stator topology. It will however be understood that various axial flux electrical machine topologies can be used. For example, one or more additional single rotor twin stator stages can be added to increase the power and torque output of the device, or a different topology such as a single stator twin rotor topology can be used.

The axial flux electrical machine 100 typically includes additional components such as one or more electrical connection ports located on the outside of the housing 101.

In certain embodiments, all of the components located within the housing 101 are solid state such that they do not contain any air pockets.

The housing 101 and the components located inside it are arranged to enable fluid to entirely fill the housing 101. In certain embodiments, the inside surface of the housing 101 is shaped to encourage fluid flow through the housing 101. Additionally, in certain embodiments fluid flow passageways are provided within the housing 101 to ensure fluid is able to reach all of the spaces within the housing 101.

In certain embodiments, the materials used for components of the axial flux electrical machine are selected so that components that make direct physical contact are composed of different compatible materials to avoid galvanic corrosion or cold welding of components. For example, a component made from 316L stainless steel is secured to a component made from duplex steel.

Figure 2 provides a simplified schematic diagram of a further axial flux electrical machine which includes an on-board subsea pressure compensator in accordance with certain embodiments of the present invention.

The axial flux electrical machine 200 substantially corresponds with the axial flux electrical machine of Figure 1 except as otherwise described and depicted.

The axial flux electrical machine 200 comprises a housing 201. The housing 201 comprises an end plate 202 shaped to accommodate an end of the drive shaft 203.

In contrast with the axial flux electrical machine of Figure 1, the axial flux electrical machine 200 comprises a pressure compensator 204 that is integrally formed with the housing 201. In this embodiment, the pressure compensator 204 is integrally formed with the end plate 202. Advantageously, the end plate 202 provides a convenient location for the pressure compensator 204 due to the additional space around the end plate 202. However, it will be understood that the pressure compensator 204 could be integrally formed with other parts of the housing 201. The pressure compensator 204 is a subsea pressure compensator.

In contrast with the axial flux electrical machine of Figure 1, the subsea pressure compensator 204 comprises a diaphragm 205 that acts as a moveable seal to separate first and second parts of the subsea pressure compensator 204. The first part is fluidly connected to a first fluid port 206 and the second part is fluidly connected to a second fluid port 207.

The axial flux electrical machine 200 operates in substantially the same manner as the axial flux electrical machine of Figure 1. The subsea pressure compensator 204 regulates the pressure inside the housing 201 so that it corresponds with the ambient 20 pressure outside the housing 201.

Advantageously, providing an axial flux electrical machine 200 with an integral subsea pressure compensator can reduce the overall size of the axial flux electrical machine 200. Additionally, using a diaphragm as part of the subsea pressure compensator 204 can further reduce the size of the axial flux electrical machine 200.

In certain embodiments, the subsea pressure compensator 204 comprises a resilient member such as a spring arranged to apply a force to the diaphragm 205 to increase the pressure of fluid within the housing 201 to above the ambient pressure.

It will be understood that other suitable types of pressure compensator can be integrated with the housing 201.

Figure 3 provides a simplified schematic diagram of an axial flux electrical machine which includes a sensor housing in accordance with certain embodiments of the present invention.

The axial flux electrical machine 300 substantially corresponds with the axial flux electrical machine of Figure 1 except as otherwise described and depicted. For clarity, the subsea pressure compensator is not shown in Figure 3. It will be understood that other subsea pressure compensators as described herein can be provided as part of the device.

The axial flux electrical machine 300 comprises a housing 301. In contrast with the axial flux electrical machine of Figure 1, the housing 301 comprises a sensor housing 302. The sensor housing 302 is located inside the housing 301.

The sensor housing 302 is fluidly sealed such that pressurised fluid within the housing 301 is prevented from entering it. The inside of the sensor housing 302 can be maintained at or close to standard atmospheric pressure. The sensor housing 302 contains a suitable fluid such as air. Advantageously, this means that standard off the shelf (non-solid-state) sensors and computing devices can be located inside the housing 301.

The sensor housing 302 is typically composed of a suitable material such as stainless steel, for example 316L or duplex stainless steel. The sensor housing 302 is typically sealed using metal to metal contact and an 0-ring. The sensor housing 302 is arranged to withstand the hydrostatic forces present within the housing 301 during use.

The axial flux electrical machine 300 comprises a plurality of sensors located within the sensor housing 302 including a first sensor 303, a second sensor 304 and a third sensor 305. The axial flux electrical machine 300 also comprises a computing device 306 located within the sensor housing 302 and connected to the first sensor 303, second sensor 304 and third sensor 305.

The sensors 303 304 305 can be used to monitor the condition of the axial flux electrical machine 300 during use. In certain embodiments, the computing device 306 is arranged to wirelessly transmit data collected by the sensors to a remote computing device.

It will be understood that various numbers and types of sensors can be used. For example, in certain embodiments an oil condition sensor (for example based on fluid dielectric strength), a pressure sensor, a temperature sensor and/or a vibration sensor can be used.

It will be understood that a sensor housing of a type described with reference to Figure 3 can be provided in other axial flux electrical machines described herein.

Figure 4a provides a simplified diagram of a surface of a stator in accordance with 15 certain embodiments of the present invention. The stator can be used in an axial flux electrical machine of a type described herein.

In use, the stator 400 is secured in a fixed position inside the housing of an axial flux electrical machine immediately adjacent to a rotor. The stator 400 includes electrical 20 windings which generate a magnetic field which causes the rotor to rotate.

The stator 400 comprises a mounting surface 401 that is arranged to be directly secured to a mounting plate within the housing of an axial flux electrical machine using suitable fasteners. Advantageously, securing the stator 400 to a mounting plate within the housing rather than directly to the housing means that the fasteners do not penetrate the housing. This reduces the potential for fluid to leak out of the housing.

Figure 4b provides a simplified diagram showing a cross-section of the stator 400 of Figure 4a secured to a mounting plate 402 via fasteners 403404.

As shown in Figure 4b, a small gap is present between the stator 400 and the mounting plate 402 when they are secured together. As described, to avoid a build-up of hydrostatic pressure it is important that the housing, including the spaces between components, is completely filled with pressurised fluid.

In order to avoid air pockets adjacent to the mounting surface 401, the mounting surface 401 comprises grooves which provide a path for pressurised fluid within the housing to flow across the mounting surface 401 between the mounting surface 401 and the mounting plate 402. The grooves are provided by channels cut into the mounting surface 401. In certain embodiments the grooves have a substantially semicircular cross-section.

Returning to Figure 4a, the stator 400 comprises a series of grooves on its mounting 10 surface 401 arranged to provide fluid flow passageways between the stator 400 and the mounting plate 402.

The mounting surface 401 comprises a first annular groove 405, a second annular groove 406, a third annular groove 407 and a fourth annular groove 408. The annular 15 grooves are spaced radially apart from each other on the mounting surface 401.

The mounting surface 401 also comprises a plurality of radial grooves including first radial groove 409 and second radial groove 410 extending in a radial direction and fluidly connecting one or more of the annular grooves.

The mounting surface 401 comprises one or more through apertures 411 that are aligned with the grooves to provide a path for fluid to pass through the stator 400 and into the grooves and from the grooves back through the stator 400.

Example fluid paths through the grooves are shown by the arrows in Figure 4a.

Advantageously, the grooves can reduce or eliminate air pockets present on the mounting surface 401. This avoids a build-up of hydrostatic pressure within the housing and also improves the flow of fluid around the stator 400 to increase cooling 30 of the stator 400.

It will be understood that alternative arrangements of grooves could be provided in the mounting surface 401. For example, a different number of grooves or a different pattern of grooves on the mounting surface can be provided.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar lo features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations).

It will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be lo made without departing from the scope of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope being indicated by the following claims.

Claims

CLAIMS1. An axial flux electrical machine for operating in a subsea environment, the axial flux electrical machine comprising: a sealed housing for containing a pressurised fluid; at least one stator and at least one rotor located within the housing; and a pressure compensator comprising a first fluid port and a second fluid port, the first fluid port in fluid communication with an environment external to the housing and the second fluid port in fluid communication with the inside of the housing, the lo pressure compensator arranged to pressurise fluid inside the housing via the second fluid port in response to the fluid pressure at the first fluid port.
2. An axial flux electrical machine as claimed in claim 1, further comprising a fluid conduit fluidly connecting the second fluid port of the pressure compensator with a fluid port of the housing.
3. An axial flux electrical machine as claimed in claim 1, wherein the pressure compensator is integrally formed with the housing.
4. An axial flux electrical machine as claimed in claim 3, wherein the pressure compensator is integrally formed with an end plate of the housing.
5. An axial flux electrical machine as claimed in any previous claim, wherein the pressure compensator comprises a chamber comprising a first part and a second 25 part separated by a moveable seal, wherein the first part is fluidly connected to the first fluid port and the second part is fluidly connected to the second fluid port.
6. An axial flux electrical machine as claimed in claim 5, wherein the moveable seal is provided by a piston or a diaphragm.
7. An axial flux electrical machine as claimed in any previous claim, wherein the pressure compensator comprises a resilient member arranged to apply force to the moveable seal to generate additional pressure at the second fluid port and thereby generate a predetermined overpressure of fluid within the housing.
8. An axial flux electrical machine as claimed in claim 7, wherein the resilient member is a spring.
9. An axial flux electrical machine as claimed in claim 7 or claim 8, wherein the predetermined overpressure is between 0.1 and 1 bar.
10. An axial flux electrical machine as claimed in any previous claim, further comprising a shaft connected to and arranged to rotate with the at least one rotor, 10 the shaft supported within the housing by at least one bearing, wherein the inside of the at least one bearing is exposed to fluid present within the housing.
11. An axial flux electrical machine as claimed in any previous claim, wherein a surface of the at least one stator is secured to a mounting plate within the housing, and wherein the surface of the at least one stator comprises one or more grooves providing a path for pressurised fluid within the housing to flow across the surface of the at least one stator between the at least one stator and the mounting plate.
12. An axial flux electrical machine as claimed in claim 11, wherein the one or 20 more grooves comprise annular grooves.
13. An axial flux electrical machine as claimed in claim 11 or claim 12, wherein the one or more grooves comprise radial grooves.
14. An axial flux electrical machine as claimed in any previous claim, further comprising a sensor housing located within the housing, the sensor housing sealed to prevent pressurised fluid within the housing from entering it.
15. An axial flux electrical machine as claimed in claim 14, wherein the sensor 30 housing is arranged to maintain a pressure at or close to standard atmospheric pressure during use.
16. An axial flux electrical machine as claimed in claim 14 or claim 15, further comprising one or more sensors located within the housing.
17. An axial flux electrical machine as claimed in claim 16, wherein the one or more sensors comprise at least one of: an oil condition sensor, a pressure sensor, a temperature sensor and a vibration sensor.
18. An axial flux electrical machine as claimed in any previous claim, wherein the housing is filled with fluid.
19. An axial flux electrical machine as claimed in claim 18, wherein the fluid is synthetic oil or distilled or deionised water or water-glycol.