Motor assembly

Abstract

A motor assembly 100 comprises a housing 1, 3, 6, 9, 13, 14 comprising an internal space; an output shaft 16 extending along an axis D; and a motor 30 and an axial flux magnetic gear 40 arranged insid

Description

Technical Field The present disclosure relates to a motor assembly and methods of operating and assembling the same. More particularly, but not exclusively, the present disclosure relates to a motor assembly with its motor being statically sealed by a highly reliable magnetic coupling structure which operates based upon the axial flux magnetic field modulation effect. Background One of the key technical challenges for deep-sea motors and explosion-proof motors for use in hazardous media is the design of the high-protection-grade sealing mechanism for sealing the motors. Compared to conventional motors with a typical protection rating of from IP44 to IP54, the protection rating for the deep-sea motors and / or the explosionproof motors may be increased to the level of IP68. The substantial increase in protection rating significantly raises the demands on the motor sealing mechanism. It is generally desirable to further enhance the reliability and the protection rating of the sealing mechanism for use with deep-sea motors and explosion-proof motors. It is an object of the present disclosure, among others, to provide a motor assembly with such an improved sealing mechanism. Summary According to a first aspect of the present disclosure, there is provided a motor assembly, the motor assembly comprising: a housing comprising an internal space; an output shaft extending along an axis; and a motor and an axial flux magnetic gear arranged inside the housing along the axis, wherein: the motor comprises a stator and a rotor, and the axial flux magnetic gear comprises a first rotor and a second rotor spaced apart from one another along the axis and magnetically coupled to one another, and a modulator arranged between the first and second rotors for modulating a magnetic field between the first and second rotors, and wherein the rotor of the motor is coupled to the first rotor and the second rotor is coupled to the output shaft such that a rotation of the rotor of the motor causes a rotation of the output shaft around the axis; wherein: the modulator is configured to separate the internal space of the housing into a first chamber and a second chamber; the motor and the first rotor are arranged within the first chamber; the second rotor is arranged within the second chamber; and the housing and the modulator are configured such that the first chamber is hermetically sealed. Advantageously, the use of the modulator not only modulates the magnetic field between the first and second rotors, but can also provide static sealing to the first chamber (where the motor is located). As compared to dynamic sealing, the static sealing of the first chamber has an improved reliability and provides a higher protection level for the motor. Further, the use of the axial flux magnetic gear allows the modulator to be easily made with an improved structural strength, as compared to a cantilever-shaped modulator used within a radial flux magnetic gear. This further improves the sealing reliability of the first chamber, and the overall protection rating of the motor may achieve IP68, thereby allowing the motor assembly to be used as a deep-sea motor and / or as an explosionproof motor in hazardous media. In addition, the use of the axial flux magnetic gear allows a rotational axis of the first rotor to be independent from a rotational axis of the second rotor, and does not require the two rotational axes to be strictly coaxial, thereby improving the fault tolerance of the motor assembly. The axial flux magnetic gear may also be referred to as a magnetic coupling structure or a disc-type magnetic gear, in which the magnetic flux path between the first and second rotors is oriented along the rotational axis of the first / second rotor. It would be understood that the modulator of the axial flux magnetic gear remains stationary relative to the housing, during operation of the motor assembly. It would further be understood that the entirety of the motor is located within the first chamber. It would further be appreciated that the first and second chambers are arranged along the axis. The second rotor may be rotatable around the axis. The rotor of the motor, and / or the first rotor may also be rotatable around the axis. A peripheral surface of the modulator may be fixedly attached to a peripheral wall of the housing. The expression “fixedly couple” or “fixedly attach” means that the relevant elements cannot move relative to one another. The modulator and at least a part of the peripheral wall of the housing may be integrally formed. The expression “integrally formed” means that the modulator and at least a part of the housing are structurally integrated into a unitary structure, which is incapable of being easily dismantled without destroy the integrity of the structure. The peripheral wall of the housing may comprise a locking mechanism for mechanically engaging with the modulator to restrict movement of the modulator along the axis. The modulator may extend along a plane which is perpendicular to the axis. In other words, the first and second chambers do not overlap along the axis. Further or alternatively, the first and second rotors do not overlap along the axis. The modulator may comprise a modulation ring and a support frame which supports the modulator ring. The modulation ring may extend along a plane which is perpendicular to the axis. The modulation ring may surround the support frame. The modulation ring and the support frame may be integrally formed. The support frame may comprise a locking mechanism for mechanically engaging with the modulation ring to restrict movement of the modulation ring along the axis. The modulation ring may comprise magnetic components and non-magnetic components arranged in an alternating manner along a circumferential direction of the modulation ring. The magnetic components may also be referred to as ferromagnetic pole pieces. The modulation ring may have a first surface facing the first rotor and a second surface facing the second rotor, with the second surface being opposite to the first surface. A dimension of at least one of the magnetic components at a mid-plane between the first and second surfaces may be greater than the corresponding dimension of the at least one of the magnetic components at the first and / or second surface. In other words, the dimension of at least one of the magnetic components may decrease from the mid-plane towards the first and / or second surface. The mid-plane, the first surface and / or the second surface may be perpendicular to the axis. A dimension of each of the magnetic components at a mid-plane between the first and second surfaces may be greater than the corresponding dimension of the respective magnetic component at the first and / or second surface. At least one of the non-magnetic components may be made of a non-metal material. More particularly, each of the non-magnetic components may be made of a non-metal material. Further or alternatively, at least one of the non-magnetic components may be made of an electrically insulating material. More particularly, each of the non-magnetic components may be made of an electrically insulating material. The non-metal material may comprise one or more of resin, ceramic, fiberglass and polymer. The non-metal material may be configured to seal an interface between the peripheral surface of the modulator and the peripheral wall of the housing. The first rotor may comprise a first core and a first set of permanent magnets. The second rotor may comprise a second core and a second set of permanent magnets. The first and / or second set of permanent magnets may be magnetised using a Halbach array configuration. The modulation ring may comprise a total number of magnetic components, Zs. The first set of permanent magnets may comprise a total number of pole pairs, Pr1. The second set of permanent magnets may comprise a total number of pole pairs, Pr2. Zs, Pr1 and Pr2 may satisfy the relationship of Zs = Pr1 ± Pr2. Pr2 may be greater than Pr1. By making Pr2 greater than Pr1, the motor assembly can readily achieve self-deceleration based upon a transmission ratio of Pr2:Pr1. The output shaft may protrude outside of the housing. The motor assembly may further comprise a dynamic seal between the output shaft and the housing. In other words, the second chamber may be dynamically sealed. The dynamic seal may comprise a skeleton oil seal. The motor assembly may further comprise first and second bearings which are arranged at opposite sides of the modulation ring along the axis. The first bearing may be arranged to rotatably couple at least one of the rotor of the motor and the first rotor to the housing. The second bearing may be arranged to rotatably couple at least one of the second rotor and the output shaft to the housing. Advantageously, the use of the first and second bearings allows the structures of the rotors (including the rotor of the motor, the first rotor and the second rotor) to have strong mechanical strength. With the expression “rotatably couple”, it is meant that the coupled elements can rotate relative to one another. The support frame of the modulator may be fixedly coupled to the housing. The first bearing may be arranged to rotatably couple the rotor of the motor to the support frame of the modulator, and / or the second bearing may be arranged to rotatably couple the output shaft to the support frame of the modulator. The rotor of the motor may be mechanically coupled to the first rotor. The rotor of the motor may be fixedly connected to the first rotor, such that they have the same rotational speed during operation. The second rotor may be mechanically coupled to the output shaft. The second rotor may be fixedly connected to the output shaft, such that they have the same rotational speed during operation. The motor may be a permanent magnet motor. The first chamber may comprise an inert gas. According to a second aspect of the present disclosure, there is provided an underwater apparatus comprising the motor assembly of the first aspect. According to a third aspect of the present disclosure, there is provided a method of operating a motor assembly, wherein the motor assembly comprises a housing comprising an internal space, an output shaft extending along a axis, and a motor and an axial flux magnetic gear which are arranged inside the housing along the axis, the method comprising: energising stator coils of a stator of the motor so as to generate a rotating magnetic filed, thereby causing a rotor of the motor to rotate relative to the stator; driving, by the rotor of the motor, a first rotor of the axial flux magnetic gear to rotate; modulating, by a modulator of the axial flux magnetic gear, a magnetic field between the first rotor and a second rotor of the axial flux magnetic gear, wherein the modulator is arranged between the first and second rotors along the axis; driving, by the first rotor through the modulated magnetic field, a second rotor of the axial flux magnetic gear to rotate; and driving, by the second rotor, the output shaft to rotate around the axis; wherein: the modulator is configured to separate the internal space of the housing into a first chamber and a second chamber which are arranged along the axis; the motor and the first rotor are arranged within the first chamber, and the second rotor is arranged within the second chamber; and the housing and the modulator are configured such that the first chamber is hermetically sealed. According to a fourth aspect of the present disclosure, there is provided a method of assembling a motor assembly, wherein the motor assembly comprises a housing, an output shaft, and a motor and an axial flux magnetic gear, the method comprising: attaching a stator of the motor to a first peripheral wall portion of the housing; coupling a rotor of the motor to a first end cover of the housing; attaching the first end cover to the first peripheral wall portion such that the stator is arranged around the rotor of the motor; coupling a first rotor of the axial flux magnetic gear to the rotor of the motor; attaching a modulator of the axial flux magnetic gear to a second peripheral wall portion of the housing; attaching the first peripheral wall portion to the second peripheral wall portion; coupling a second rotor of the axial flux magnetic gear to the output shaft; coupling the output shaft to a second end cover of the housing; and attaching the second end cover to the second peripheral wall portion; wherein: the output shaft extends along an axis; the motor and the axial flux magnetic gear are arranged along the axis; the first rotor and the second rotor are spaced apart from one another along the axis and magnetically coupled to one another; the modulator is arranged between the first and second rotors to modulate a magnetic field between the first and second rotors; and the rotor of the motor is coupled to the first rotor and the second rotor is coupled to the output shaft such that a rotation of the rotor of the motor causes a rotation of the output shaft around the axis; the first end cover, the first peripheral wall portion, a part of the second peripheral wall portion and the modulator collectively enclose a first chamber which is hermetically sealed; and the motor and the first rotor are arranged within the first chamber; and the second end cover, a remaining part of the second peripheral wall portion and the modulator collectively enclose a second chamber, wherein the second rotor is arranged with the second chamber. Attaching the modulator to the second peripheral wall portion may comprise integrally forming the modulator with the second peripheral wall portion. The modulator may comprise a modulation ring and a support frame which supports the modulator ring, and the modulation ring may comprise magnetic components and nonmagnetic components. Attaching the modulator to the second peripheral wall portion may comprise casting the non-magnetic components while the support frame, the magnetic components and the second peripheral wall portion remain as inserts during the casting process. Where appropriate any of the features described above in relation to any aspect of the present disclosure may be applied to any other aspect of the disclosure. It would also be understood that the terms “first”, “second” etc. are simply used in the present disclosure to label the relevant elements for the ease of description, and do not imply any limitations to the sequence or locations of the relevant elements. Brief Description of the Drawings In order that the disclosure may be more fully understood, a number of embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates an axial cross-sectional view of a motor assembly according to a first embodiment of the present disclosure; Figure 2 schematically illustrates an axial cross-sectional view of an axial flux magnetic gear used in the motor assembly of Figure 1; Figure 3 schematically illustrates radial cross-sectional views of various components of the axial flux magnetic gear shown by Figure 2; Figures 4 and 5 schematically illustrate magnetisation directions of permanent magnets used in the two rotors of the axial flux magnetic gear as shown by Figure 2; Figure 6 schematically illustrates (a) a radial cross-sectional view of a modulation ring used in the axial flux magnetic gear of Figure 2; and (b) a circumferential cross-sectional view of one magnetic component of the modulation ring. Figure 7 schematically illustrates an axial cross-sectional view of a motor assembly according to a second embodiment of the present disclosure; Figure 8 schematically illustrates an axial cross-sectional view of a motor assembly according to a third embodiment of the present disclosure; Figure 9 schematically illustrates an axial cross-sectional view of a motor assembly according to a fourth embodiment of the present disclosure; Figures 10 and 11 schematically illustrate alternative magnetisation directions of permanent magnets used in a rotor of the axial flux magnetic gear; Figures 12 and 13 schematically illustrate further alternative magnetisation directions of permanent magnets used in the rotors of the axial flux magnetic gear; Figure 14 shows processing steps of a method for operating a motor assembly according to an aspect of the present disclosure; Figure 15 shows processing steps of a method for assembling a motor assembly according to an aspect of the present disclosure; In the figures, like parts are denoted by like reference numerals. It will be appreciated that the drawings are for illustration purposes only and are not drawn to scale. Detailed Description of the Preferred Embodiments Figures 1 to 6 schematically illustrate a motor assembly 100 according to a first embodiment of the present disclosure. With reference to Figure 1, the motor assembly 100 includes a housing which encloses a motor 30 and an axial flux magnetic gear 40, and an output shaft 16 which protrudes beyond the housing for driving an external load. These components of the motor assembly 100 are described below in more detail. The output shaft 16 extends along an axis D, which is a central axis of the output shaft 16. The axis D may also be a central axis of the motor assembly 100. In the XYZ coordinates shown in the figures, the X direction is parallel to the axis D. In the following description, the expressions “axial”, “axially” and “axial direction” refer to the axis D. The expressions “radial plane” and “radial cross-section” refer to the YZ plane. The expressions “radial”, “radially” or “radial direction” refer to a direction which is parallel to the YZ plane but intersects with the axis D. Figure 1 is an axial cross-sectional view, where the motor assembly 100 is cut along a plane defined by the axis D and a radial direction (not shown) of the motor assembly 100. The plane along which the motor assembly 100 is cut is parallel to the XY plane. The housing of the motor assembly 100 includes a non-drive end outer cover 1, a nondrive end cap 3, a drive end cap 13, a drive end outer cover 14 and a peripheral wall extending between the end caps 3, 13. Each of the drive end cap 13 and the drive end outer cover 14 comprises a hole, through which the output shaft 16 protrude over the housing. The peripheral wall of the housing is generally of a cylindrical shape, and in this example comprises two peripheral wall portions 6, 9 joined together. The peripheral wall portions 6, 9 are arranged along the axis D and overlap with one another only at the joint therebetween. The peripheral wall portion 9 is fixedly connected to the peripheral wall portion 6 (e.g., by using spigots and bolts). It would be understood that the location of the joint between the peripheral wall portions 6, 9 may be suitably varied, and / or the entire peripheral wall of the housing may be made as a single-piece item. Referring to Figure 1, a junction box 5 is fixed to the peripheral wall portion 6 (e.g., by welding or threaded connection), and lead wires of motor windings exit the housing through the junction box 5. The non-drive end cap 3 is fixedly connected to the non-drive end outer cover 1 (e.g., with bolts). The non-drive end outer cover 1 is fixedly connected to the peripheral wall portion 6 (e.g., with interference fit and / or bolts). The peripheral wall portion 9 is also fixedly connected to the drive end cap 13 (e.g., using interference fit and / or bolts). The drive end outer cover 14 is fixedly connected to the drive end cover 13 (e.g., using bolts). The non-drive end outer cover 1, the non-drive end cap 3, the peripheral wall portions 6, 9, the junction box 5, the drive end cap 13 and the drive end outer cover 14 are equipped with sealing grooves at the interfaces / joints there-between. Sealing rings 4 (shown as black dots) are installed in the sealing grooves to achieve static sealing between the various components of the housing. Within the housing, the motor 30 and the axial flux magnetic gear 40 are arranged side-by-side along the axis D. The motor 30 includes a stator 7 and a rotor 8. The stator 7 is fixedly connected to the peripheral wall portion 6 (e.g., through an interference fit). The stator 7 comprises an annular stator core and a plurality of stator coils which form three phases, although other numbers of phases are possible. The three-phase windings may be embedded in the stator core, and the lead wires of the three-phase windings are drawn out through the junction box 5 and connected to an external motor driver circuit (not shown). The rotor 8 is concentric with the stator 7 and carries permanent magnets 34. The permanent magnets 34 are fixedly attached to an annular body 32 of the rotor 8, and, by way of example, may be embedded into or surface-mounted on the annular body 32. The rotor 8 further comprises a rotor shaft 36 to transmit torque. The rotor shaft 36 is fixedly connected to the annular body 32 (by for example interference fit and / or a keyed connection) such that relative rotation between the rotor shaft 36 and the annular body 32 is prevented. In the example of Figure 1, the motor 30 is a permanent magnetic motor, and the magnetic flux path between the permanent magnets 34 of the rotor 8 and the stator coils of the stator 7 is oriented in the radial direction. It would be appreciated other types of motor with any suitable structure may be used alternatively. The axial flux magnetic gear 40 includes a first rotor 10 and a second rotor 12 spaced apart from one another along the axis D and magnetically coupled to each other, a modulation ring 11, and a support frame 20 which supports the modulation ring 11. The modulation ring 11 is arranged within an air gap between the first and second rotors 10, 12 and does not contact either of the rotors 10, 12. The modulation ring 11 and the support frame 20 may be collectively referred to as a “modulator”. With reference to Figure 2, the first rotor 10 includes a core 23 which carries a set of permanent magnets 24 on a surface facing the modulation ring 11. A radial cross-sectional view of the first rotor 10 along A-A plane perpendicular to the axis D is shown in Figure 3. The second rotor 12 includes a core 26 which carries a set of permanent magnets 25 on a surface facing the modulation ring 11. A radial cross-sectional view of the second rotor 12 along C-C plane perpendicular to the axis D is shown in Figure 3. As shown by Figure 3, the modulation ring 11 has a first surface 42 facing the first rotor 10, and a second opposite surface 44 facing the second rotor 12. Air gaps remain between the first surface 42 and the permanent magnets 24, and between the second surface 44 and the permanent magnets 25. In this way, the rotors 10,12 can freely rotate relative to the modulation ring 11. A radial cross-sectional view of the modulation ring 11 along B-B plane perpendicular to the axis D is also shown in Figure 3. The B-B plane may be a mid-plane between the surfaces 42, 44. As shown in Figure 3, the modulation ring 11 includes magnetic components 28 and nonmagnetic components 27 arranged in an alternating manner along a circumferential direction F of the modulation ring 11. The magnetic components 28 are made of a magnetic material (e.g., a soft magnetic material). The non-magnetic components 27 are made of a non-magnetic material (e.g., resin, ceramic, fiberglass, polymer etc.). Preferably, the non-magnetic material is a non-metallic material. It would be understood that the magnetic material has a much higher magnetic permeability than the nonmagnetic material. In the example provided by Figure 3, the magnetic components 28 and the non-magnetic components 27 are evenly distributed along the circumferential direction F and have the same dimension. It would be understood that other arrangements are possible. For example, the magnetic components 28 may have a different dimension than the non-magnetic components 27 along the circumferential direction F. The magnetic components 28 may also be referred to as ferromagnetic polepieces. In the example of Figure 3, the permanent magnets 24 has 4 pole-pairs, and the permanent magnets 25 has 8 pole-pairs, although other numbers of pole-pairs are possible. The permanent magnets 24 and 25 are magnetised using the Halbach array arrangement. As shown by Figure 4, each pole pair of the permanent magnets 24 is divided into four smaller pieces, and when viewed along the -X direction, the magnetisation directions of the four smaller pieces along a clockwise direction in each pole pair are circumferentially clockwise, X direction, circumferentially counterclockwise, -X direction. As shown by Figure 5, each pole pair of the permanent magnets 25 is also divided into four smaller pieces, and when viewed along the X direction, the magnetisation directions of the four smaller pieces along a clockwise direction in each pole pair are circumferentially clockwise, -X direction, circumferentially counterclockwise, X direction. Magnetising the permanent magnets 24, 25 in accordance with the Halbach array arrangement is useful for focusing the magnetic field towards the modulation ring 11, thereby enhancing torque transmission efficiency and also increasing torque density. It would however be understood that Figure 4 is just an example and that the permanent magnets 24, 25 may be magnetised in a different way. The basic operation principle of the axial flux magnetic gear 40 is that the magnetic components 28 of the modulation ring 11 modulate the magnetic field between the permanent magnets 24 and the permanent magnets 25, such that each of the rotors 10, 12 sees a working space harmonic corresponding to its own number of poles. As a result, a rotation of the first rotor 10 would induce a rotation of the second motor 12 via the modulated magnetic field there-between. There is no mechanical contact between the permanent magnets 24 and the permanent magnets 25. Being an axial flux magnetic gear 40, the magnetic flux path between the rotors 10, 12 is generally oriented along the axial direction D. It is in contrast to a radial flux magnetic gear where the magnetic flux path between the two rotors is along the radial direction. The number Zs of the magnetic components 28 in the modulation ring 11 may satisfy the relationship Zs = Pr1 ± Pr2. Pr1 is the number of pole pairs included within the permanent magnets 24. Pr2 is the number of pole pairs included within the permanent magnets 25. In the example of Figure 3, Pr1 is 4, Pr2 is 8 and Zs is 12. It would be appreciated that other values of Zs, Pr1 and Pr2 are possible. With reference to Figure 1, the modulation ring 11 is fixedly attached to the peripheral wall portion 9 of the housing, and is also fixed attached to the support frame 20. More specifically, an outer periphery of the modulation ring 11 is securely connected to an inner side of the peripheral wall portion 9, and an inner periphery of the modulation ring 11 is securely connected to an outer surface of the support frame 20. This may be done in any suitable way insofar as the joints between the modulator ring 11 and each of the peripheral wall portion 9 and the support frame 20 are sealed. In an example, the modulation ring 11, the support frame 20 and the peripheral wall portion 9 are integrally formed as a single-piece structure (i.e., unitary structure). In this example, the support frame 20 is made of a metal material (preferably with high mechanical strength), such as, Chromium Molybdenum alloy, and the peripheral wall portion 9 is also made of a metal material. Both the peripheral wall portion 9 and the support frame 20 are made with fixing slots 50, 52 (as shown in Figures 1 and 2). The magnetic components 28, the support frame 20 and the peripheral wall portion 9 are preassembled by holding the magnetic components 28 in place (e.g., by using additional equipment) with respect to the support frame 20 and the peripheral wall portion 9. The non-magnetic components 27 are made by casting. When casting the non-magnetic components 27, the assembly of the magnetic components 28, the support frame 20 and the peripheral wall portion 9 serve as metal inserts that are cast integrally with the nonmagnetic components 27. After casting, an outer peripheral edge of the modulation ring 11 is securely fixed in the slots 50, and an inner peripheral edge of the modulation ring 11 is securely fixed in the slots 52. Therefore, the slots 50, 52 act as a locking mechanism for holding the modulation ring 11 in place and preventing the modulation ring 11 from movement along the axis D. During the casting process, the material of the non-magnetic components 27 also seals any interface between the modulation ring 11 and each of the peripheral wall portion 9 and the support frame 20. It would be understood that the slots 50 may include a plurality of discrete (i.e., spaced apart) slots, each of which may engage with a respective magnetic component 28, and that similarly, the slots 52 may include a plurality of discrete slots, each of which may engage with a respective magnetic component 28. Alternatively, it would be appreciated that the slots 50 and / or 52 may be replaced by a continuous groove, which is still useful for preventing axial movement of the modulation ring 11.lt would be also be appreciated that the modulation ring 11 may be mechanically coupled to the peripheral wall portion 9 and the support frame 20 in different ways, and / or the modulator may be made with different materials. Figure 6(a) schematically illustrates a radial cross-sectional view of the modulation ring 11, where the modulation ring 11 is cut along the B-B plane. In the modulation ring 11 made using the casting method as described above, each magnetic component 28 is surrounded by the non-magnetic material which holds the magnetic component 28 in place. In some applications, the main surfaces of the magnetic components 28 (along the surfaces 42, 44) may not be covered by any non-magnetic material. With reference to Figure 6(b), the magnetic components 28 may be shaped so as to restrict / avoid any axial displacement of themselves under the magnetic forces applied by the permanent magnets 24, 25. Figure 6(b) is a circumferential cross-sectional view of one magnetic component 28 (labelled as T in Figure 6(a)), when the magnetic component I is viewed along a radially inward direction E. As shown in Figure 6(b), the magnetic component I has a width W1 along the circumferential direction F of the modulation ring 11 at the midplane B-B, but has a smaller width W2 along the circumferential direction F at the surface 42 or 44. In other words, the width of the magnetic component I decreases from the midplane B-B towards the surfaces 42, 44. Due to this particular shape of the magnetic component I, axial movement of the magnetic component I is restricted, because the non-magnetic material close to the surfaces 42, 44 provides smaller openings (with a width W2) which prevents the wider area of the magnetic component I (with a width W1) to pass through. Other magnetic components 28 may have the same circumferential cross-sectional shape as the magnetic component I. In this way, the magnetic components 28 are held in place even under the magnetic forces exerted by the permanent magnets 24, 25, thereby ensuring that there are always air gaps between the magnetic components 28 and the permanent magnets 24, 25. The rotor shaft 36 of the motor 30 is mechanically coupled to the first rotor 10. Referring back to Figure 1, the mechanical coupling is achieved by fixedly connecting the rotor shaft 36 to a central hole of the first rotor 10 through interference fit and / or a keyed connection. With this fixed connection, a rotation of the rotor shaft 36 would cause a corresponding rotation of the first rotor 10 along the same direction with the same speed. It would be appreciated that other arrangements of mechanical coupling is possible. The non-drive end of the rotor shaft 36 is supported by the non-drive end cap 3, through a bearing 2. The drive end of the rotor shaft 36 is supported by the support frame 20 through a bearing 21. The bearings 2, 21 allow the rotor shaft 36 to rotate relative to the non-drive end cap 3 and the support frame 20. Therefore, the rotor shaft 36 is supported at both of its ends and has a relatively high overall mechanical strength. A bearing support frame 22 is positioned between the first rotor 10 and the bearing 21, and is fixedly connected to the rotor shaft 36 (e.g., through an interference fit). The bearing support frame 22 serves to axially secure the first rotor 10 and the bearing 21. Further referring to Figure 1, the second rotor 12 is mechanically coupled to the output shaft 16. As an example, the mechanical coupling is achieved by fixedly connecting the output shaft 16 to a central hole of the second rotor 12 through interference fit and / or a keyed connection. Consequently, a rotation of the second rotor 12 would cause a corresponding rotation of the output shaft 16 along the same direction with the same speed. It would be appreciated that other arrangements of mechanical coupling is possible. One end of the output shaft 16 is supported by the support frame 20 through a bearing 19. Another end of the output shaft 16 is supported by the drive end cap 13 through a bearing 17. The bearings 17, 19 allow the output shaft 16 to rotate relative to the drive end cap 13 and the support frame 20. Therefore, the output shaft 16 is supported at both of its ends and has a relatively high overall mechanical strength. A bearing support frame 18 is positioned between the bearing 19 and the second rotor 12, and is fixedly connected to the output shaft 16 (e.g., through an interference fit). The bearing support frame 18 axially fix the second rotor 12 and the bearing 19. With reference to Figure 1, it can be seen that support frame 20 is equipped with independent dual bearing installation spaces at its opposite sides, with the bearing 21 installed at its left side and the bearing 19 installed at its right side. This particular structure of the support frame 20 ensures that both the rotor shaft 36 and the output shaft 16 are robustly supported, and that the rotor shaft 36 and the output shaft 16 are independent of each other (meaning that eccentricity or bending in one shaft does not significantly affect the other shaft). A skeleton oil seal 15 is installed around the central hole of the drive end outer cover 14 to provide dynamic sealing between the output shaft 16 and the drive end outer cover 14. Other types of dynamic seals may be used to replace the skeleton oil seal 15. The motor assembly 100 may be operated according to the method of Figure 14. At step S1, the stator coils of the motor 30 are energized to generate a rotating magnetic field, which causes the rotor 8 of the motor 30 to rotate relative to the stator 7. It would be understood that an external motor driver (not shown) supplies power through the junction box 5 to energize the stator coils. At step S2, the rotor 8 of the motor 30 drives the first rotor 10 of the axial flux magnetic gear 40 to rotate. This is caused by the mechanical coupling between the rotor 8 and the first rotor 10. In the example of Figure 1, the transmission ratio between the rotor 8 and the first rotor 10 is 1:1. At step S3, the modulator of the axial flux magnetic gear 40, in particular, the modulation ring 11, modulates a magnetic field between the first rotor 10 and the second rotor 12. At step S4, the first rotor 10 drives, through the modulated magnetic field, the second rotor 12 to rotate. At step S5, the second rotor 12 drives the output shaft 16 to rotate around the axis D. This is caused by the mechanical coupling between the second rotor 12 and the output shaft 16. In the example of Figure 1, the transmission ratio between the second rotor 12 and the output shaft 16 is 1:1. In the example of Figure 3 where Zs = Pr1 + Pr2, the rotors 10, 12 rotate in opposite directions at a fixed transmission ratio of Pr2:Pr1. In other words, the ratio of the angular rotational speed of the rotor 10 over the angular rotational speed of the rotor 12 is equal to Pr2 divided by Pr1 (in the example of Figure 3, 2:1). Therefore, by making Pr2 greater than Pr1, the motor assembly 100 can readily achieve self-deceleration based upon a transmission ratio of Pr2:Pr1. Low speed motors are generally achieved by using mechanical gears, which tend to increase the size of the motor and also affect the lifespans of the motor due to wear and tear at the teeth of the gears. The motor assembly 100 achieves the self-deceleration effect by using the magnetic gear 40, where there is no direct mechanical contact between the rotors 10, 12, and therefore has an improved lifespan. Due to the reduced rotational speed, the motor assembly 100 can achieve high torque at the output shaft 16. When the motor assembly 100 is in operation, the following components move relative to the housing of the motor assembly 100: the rotor 8 of the motor 30, the first rotor 10, the bearing support frame 22, the second rotor 12, the bearing support frame 18, the output shaft 16, and the inner rings of all bearings 2, 21, 19, 17. In particular, these components all rotate around the axis D. Other components of the motor assembly 100, in particular, the modulator (including the modulating ring 11 and the support frame 20) and the stator 7 of the motor 30, remain stationary relative to the housing. The modulator of the axial flux magnetic gear 40 divides the internal space of the housing into two chambers. Referring to claim 1, the chamber on the left side of the modulator (referred to as “first chamber”) is statically sealed, and the chamber on the right side of the modulator (referred to as “second chamber”) is dynamically sealed. Dynamic sealing means that there is a relative moment at one or more sealed interfaces. The second chamber is enclosed by the drive end outer cover 14, the drive end cap 13, a part of the peripheral wall portion 9 and the modulator. In the example of Figure 1, the relative movement takes place between the drive end outer cover 14 and the output shaft 16. Generally, the bearings 17, 19 are not water-proof and therefore do not provide any sealing function, although other arrangement is possible. Static sealing means that there is no relative movement at any sealed interface, and generally achieves a lower rate of leakage than dynamic sealing. The first chamber is enclosed by the non-drive end outer cover 1, the non-drive end cap 3, the peripheral wall portion 6, a part of the peripheral wall portion 9 and the modulator. The static sealing is provided by the sealing rings 4 at the interfaces between some of the above components and the integrally-formed nature of the peripheral wall portion 9 and the modulator. It would be appreciated that the interfaces between the peripheral wall portion 9, the modulation ring 11 and the support frame 20 may be statically sealed in a different manner. The motor 30 (in particular the stator coils) is fully enclosed within the statically-sealed first chamber, which provides a high protection level for the motor 30. Generally, the bearings 2, 21 are not water-proof and therefore do not provide any sealing function, although other arrangement is possible. Therefore, the use of the modulator not only modulates the magnetic field between the first and second rotors 10, 12, but can also provide static sealing to the first chamber (where the motor 30 is located). In other words, the modulator converts the sealing of the motor 30 from dynamic sealing (if without the axial flux magnetic gear 40) into static sealing. Known sealing mechanisms of motors (such as those disclosed by Chinese patent applications CN112357032A, CN113501115A and CN201286043Y) typically use radial flux magnetic gears. A radial flux magnetic gear includes an inner rotor and an outer rotor with a modulation ring positioned in an air gap there-between along the radial direction. Generally speaking, each of the rotors and the modulation ring is of a cylindrical shape, and overlap with one another along the axial direction. Further, the two rotors and the modulation ring are required to be strictly concentric. The known sealing mechanisms have the following issues: (1) its modulation ring is often a cantilever structure, which inherently has poor structural strength; In particular, the cylindrical-shaped modulation ring is only supported at one end, with the other end unsupported; (2) during operation, the cantilever structure of the modulation ring tends to oscillate radially, causing the modulation ring to easily interfere with the inner and outer rotors of the radial flux magnetic gear; (3) it is generally difficult to align the inner and outer rotors of the radial flux magnetic gear concentrically, resulting in increased eccentric forces. The motor assembly 100 of the present disclosure addresses the above problems encountered by known sealing mechanisms by using the axial flux magnetic gear 40. Firstly, the modulator (including the modulation ring 11 and the support frame 20) of the axial flux magnetic gear 40 is generally of a disk shape and no longer has a cantilever structure. The modulator generally extends along the radial plane YZ, and can be easily made with enhanced mechanical strength and structural reliability. Secondly, the entire outer peripheral edge of the modulator is fixedly connected to an inner surface of the peripheral wall portion 9 (e.g., by using a locking mechanism 50 described above). In this way, the modulator is restricted all-around, and cannot easily move along the axial direction D. Further, the inner peripheral edge of the modulation ring 11 is fixedly connected to an outer peripheral surface of the support frame 20 (e.g., by using a locking mechanism 52 as described above). Therefore, the modulation ring 11 remains stationary during operation of the motor assembly 100, and the risk of the modulation ring 11 interfering with the rotors 10, 12 in the air gap there-between is significantly reduced. Thirdly, the design of the present disclosure does not require the axes of the rotors 10, 12 to be strictly coaxial, and therefore improves fault tolerance during the machining and assembly processes of the motor assembly 100. This addresses the issue of eccentricity commonly encountered in the air gaps of known sealing mechanisms. Therefore, by using the modulator of the axial flux magnetic gear 40 to provide static sealing to the motor 30, the motor 30 can be reliably sealed. The motor assembly 100 is also easy to implement in engineering applications due to the improved fault tolerance. The motor protection level achievable by the motor assembly 100 can reach IP68. Accordingly, the motor assembly 100 is especially suitable for use in the field of deep-sea underwater motors for marine equipment, or the field of explosion-proof motors. Explosion-proof motors may be used in equipment (e.g., cement mixing equipment, steelmaking equipment) which operates under high temperatures and / or within dangerous medium (e.g., flammable gases). The motor assembly 100 is suitably for use as explosion-proof motors for the following reasons. Firstly, the motor 30 is axially spaced part from the output shaft 16. While the output shaft 16 may be in direct contact with the high-temperature environment, the motor 30 is not directly subject to the high temperature. Secondly, because the sealing structure of the motor 30 has an enhanced sealing reliability, the risks of the dangerous medium entering the first chamber and getting into contact with the stator 7 are significantly reduced. In some applications, the first chamber (where the motor 30 is located) may be filled with an inert gas (such as, nitrogen), which further reduces the risks of flammable gases entering the first chamber and causing explosion. In addition, as described above, the non-magnetic components 27 of the modulation ring 11 are made of a non-magnetic material which is also a non-metallic material. The use of non-metallic material is beneficial in that it reduces the eddy current formed in the modulation ring 11 during operation of the motor assembly 100, thereby causing the modulation ring 11 to suffer from less severe mechanical vibrations. This further improves the overall mechanical strength of the modulator, and the sealing reliability of the motor 30. Figures 7 to 9 schematically illustrate motor assemblies 100A, 100B and 100C according to second to fourth embodiments of the present disclosure. Elements of the assemblies 100A-C that are identical to those of the assembly 100 are identified using the same labels. Elements of the assemblies 100A-C that correspond to, but are different from those of the assembly 100 are labelled using the same numerals but with a letter ‘A’, ‘B’ or ‘C’ for differentiation. The features and advantages described above with reference to the first embodiment are generally applicable to the second and third embodiments. Referring to Figure 7, the motor assembly 100A differs from the motor assembly 100 in the following aspects: the support frame 20A is no longer equipped with any bearing installation space at its sides; a left-side bearing installation space is provided between the first rotor 10A and the peripheral wall portion 6A of the housing, with a bearing 21A positioned between the first rotor 10A and the peripheral wall portion 6Aalong a radial direction; the peripheral wall portion 9A of the housing has a shortened axial length while the drive end cap 13A has an increased axial length; and a right-side bearing installation space is provided between the second rotor 12A and the drive end cap 13A, with a bearing 19A positioned between the second rotor 12A and the drive end cap 13A along a radial direction. In this way, the drive end of the rotor shaft 36 is supported by the peripheral wall portion 6A of the housing through the first rotor 10A and the bearing 21A, and the non-drive end of the output shaft 16 is supported by the drive end cap 13A through the second rotor 12A and the bearing 19A. The modifications made to the peripheral wall portion 9A and the drive end cap 13A are for simplifying installation of the bearing 19A. Referring to Figure 8, the motor assembly 100B differs from the motor assembly 100 in that the support frame 20B is not equipped with bearing installation space at the left side. Rather, a left-side bearing installation space is provided between the first rotor 10A and the peripheral wall portion 6A of the housing, with a bearing 21A positioned between the first rotor 10A and the peripheral wall portion 6A along a radial direction. As such, the drive end of the rotor shaft 36 is supported by the peripheral wall portion 6A of the housing through the first rotor 10A and the bearing 21A. Referring to Figure 9, the motor assembly 100C differs from the motor assembly 100 in that: the support frame 20C is not equipped with a bearing installation space at its right side. Rather, the peripheral wall portion 9A of the housing has a shortened axial length while the drive end cap 13A has an increased axial length, and a right-side bearing installation space is provided between the second rotor 12A and the drive end cap 13A, with a bearing 19A positioned between the second rotor 12A and the drive end cap 13A along a radial direction. As such, the non-drive end of the output shaft 16 is supported by the drive end cap 13A through the second rotor 12A and the bearing 19A. In the motor assemblies 100A to 100C, each of the rotor shaft 36 and the output shaft 16 is still supported at both ends, and thus maintains its relatively high mechanical strength. It would be understood that the method of operating the motor assembly as shown by Figure 14 also applies to the motor assemblies 100A to 100C. In each of the motor assemblies 100, 100A-100C, the permanent magnets 24, 25 of the first and second rotors 10, 12 may be magnetised in different ways than that described above in relation to Figures 4 and 5. As an example, Figures 10 and 11 show alternative ways of magnetising the permanent magnets 24 of the first rotor 10 using the Halbach array arrangement. Referring to Figure 10, the permanent magnets 24 still have 4 pole-pairs, but in contrast to Figure 3, each pole pair of the permanent magnets 24 is divided into six segments, and when viewed along the -X direction, the magnetisation directions of the six segments along a clockwise direction in each pole pair are: clockwise along the tangential direction upwards by 30°, clockwise along the tangential direction downwards by 30°, X direction, counterclockwise along the tangential direction downwards by 30°, counterclockwise along the tangential direction upwards by 30°, and -X direction. Similarly, the permanent magnets 25 of the second rotor 12 may also have each pole pair divided into six segments (not shown), and when viewed along the X direction, the magnetisation directions of the six segments per pole pair along a clockwise direction are as follows: clockwise along the tangential direction upwards by 30°, clockwise along the tangential direction downwards by 30°, -X direction, counterclockwise along the tangential direction downwards by 30°, counterclockwise along the tangential direction upwards by 30°, and X direction. Referring to Figure 11, the permanent magnets 24 also have 4 pole-pairs, but each pole pair is divided into eight segments, and when viewed along the -X direction, the magnetisation directions of the eight segments along a clockwise direction in each pole pair are: clockwise along the tangential direction upwards by 45°, clockwise in the circumferential direction, clockwise along the tangential direction downwards by 45°, X direction, counterclockwise along the tangential direction downwards by 45°, counterclockwise in the circumferential direction, counterclockwise along the tangential direction upwards by 45°, and -X direction. Similarly, the permanent magnets 25 of the second rotor 12 may also have each pole pair divided into eight segments (not shown), and when viewed along the X direction, the magnetisation directions of the eight segments per pole pair along a clockwise direction are as follows: clockwise along the tangential direction upwards by 45°, clockwise along the circumferential direction, clockwise along the tangential direction downwards by 45°, -X direction, counterclockwise along the tangential direction downwards by 45°, counterclockwise along the circumferential direction, counterclockwise along the tangential direction upwards by 45°, and X direction. It would of course be appreciated that the permanent magnets 24, 25 of the first and second rotors 10, 12 may be magnetised in any suitable way according to the Halbach array arrangement, which provides the magnetic focusing effect as described above. Alternatively, the permanent magnets 24, 25 of the first and second rotors 10, 12 may be magnetised using the conventional N / S pole arrangement which is simple and cost-effective. An example is provided by Figures 12 and 13, with Figure 12 showing the permanent magnets 24 of the first rotor 10, and with Figure 13 showing the permanent magnets 25 of the second rotor 12. In this example, the permanent magnets 24 still have 4 pole-pairs and the permanent magnets 25 still have 8 pole-pairs. However, each polepair is divided into two pieces with magnetisation directions of X and -X directions. Figure 15 schematically illustrates processing steps of a method for assembling a motor assembly (e.g., the motor assemblies 100, 100A to 100C). The motor assembly comprises a housing (e.g., the peripheral wall portions 6, 9 and the caps / covers 1, 3, 13, 14), an output shaft (e.g., the output shaft 16), and a motor (e.g., the motor 30) and an axial flux magnetic gear (e.g., the magnetic gear 40). At step M1, a stator (e.g., the stator 7) of the motor is attached to a first peripheral wall portion (e.g., the peripheral wall portion 6 or6A) of the housing. In an example, the stator is fixedly attached to the first peripheral wall portion, through for example interference fit. At step M2, a rotor (e.g., the rotor 8) of the motor is coupled to a first end cover (e.g., the non-drive end cap 3 and the non-drive end outer cover 1) of the housing. In an example, the rotor of the motor is rotatably coupled to the first end cover, through for example a bearing (e.g., the bearing 2). At step M3, the first end cover is attached to the first peripheral wall portion such that the stator is arranged around the rotor of the motor. In an example, the first end cover is fixedly attached to the first peripheral wall portion (by using, e.g., interference fit and / or bolts) with a static seal provided between the first end cover and the first peripheral wall portion. At step M4, a first rotor (e.g., the first rotor 10, 10A) of the axial flux magnetic gear is coupled to the rotor of the motor. In an example, the first rotor is fixedly coupled / attached to the rotor of the motor, through for example interference fit and / or keyed connection. At step M5, a modulator (e.g., the modulator ring 11 and its support frame 20 or 20A to 20C) of the axial flux magnetic gear is attached to a second peripheral wall portion (e.g., the peripheral wall portion 9) of the housing. The modulator may be fixedly attached to the second peripheral wall portion. In an example, this is done by integrally forming the modulator with the second peripheral wall portion as described above, although other ways are possible. At step M6, the first peripheral wall portion is attached to the second peripheral wall portion. In an example, the first peripheral wall portion is fixedly attached to the second peripheral wall portion (by using, e.g., bolts) with a static seal provided between the first peripheral wall portion and the second peripheral wall portion. At step M7, a second rotor (e.g., the rotor 12 or 12A) of the axial flux magnetic gear is coupled to the output shaft (e.g., the output shaft 16). In an example, the second rotor is fixedly coupled / attached to the output shaft, through for example interference fit and / or keyed connection. At step M8, the output shaft is coupled to a second end cover (e.g., the drive end cap 13 and the drive end outer cover 14) of the housing. In an example, the output shaft is rotatably coupled to the second end cover, through for example a bearing (e.g., the bearing 17). At step M9, the second end cover is attached to the second peripheral wall portion. In an example, the second end cover is fixedly attached to the second peripheral wall portion (by using, e.g., interference fit and / or bolts) with a static seal provided between the second end cover and the second peripheral wall portion. The output shaft extends along an axis (e.g., the axis D). After the motor assembly has been assembled according to steps M1 to M9, the motor and the axial flux magnetic gear are arranged along the axis, and the first rotor and the second rotor are spaced apart from one another along the axis and magnetically coupled to one another. Further, the modulator is arranged between the first and second rotors along the axis to modulate a magnetic field between the first and second rotors. In addition, the first peripheral wall portion is generally arranged between the first end cover and the second peripheral wall portion along the axis. The second peripheral wall portion is generally arranged between the first peripheral wall portion and the second end cover along the axis. Steps M4 and M7 are performed such that a rotation of the rotor of the motor causes a rotation of the output shaft around the axis. Steps M2, M3, M5 and M6 are performed such that the first end cover, the first peripheral wall portion, a part of the second peripheral wall portion and the modulator collectively enclose a first chamber which is hermetically sealed, and the entirety of the motor and the first rotor are arranged within the first chamber. Steps M5 and M9 are performed such that the second end cover, a remaining part of the second peripheral wall portion and the modulator collectively enclose a second chamber, wherein the second rotor is arranged with the second chamber. It would be appreciated that steps M1 to M9 may take place according to a sequence which is different from the sequence of description. For example, steps M1, M2 and M5 are independent of one another and thus may be performed according to any suitable sequence. Preferably, step M3 may be performed after the completion of steps M1 and M2; step M4 may be performed after step M3; step M6 may be performed after the completion of steps M4 and M5; and / or step M9 may be performed after the completion of step M7. Step M9 may be performed simultaneously with or after the completion of step M8. The method may further comprise one or more of the following optional steps: At step M10, at least one of the rotor of the motor and the first rotor is rotatably coupled to the housing via a first bearing (e.g., the bearing 21 or 21A). Step M10 may be performed before step M6 or simultaneously with step M6. In an example, step M10 may comprise rotatably coupling, by the first bearing (e.g., the bearing 21), the rotor (e.g., the rotor 8) of the motor to a support frame (e.g., the support frame 20 or 20C) of the modulator while the support frame is fixedly coupled to the second peripheral wall portion (e.g., the wall portion 9 or 9A) of the housing according to step M5. This step may be performed simultaneously with step M6. This example may be used during the assembly process of the motor assembly 100 or 100C. In an alternative example, step M10 may comprise rotatably coupling, by the first bearing (e.g., the bearing 21A), the first rotor (e.g., the first rotor 10A) to the first peripheral wall portion (e.g., the wall portion 6A) of the housing. This step may be performed before step M6, e.g., simultaneously with step M4. This example may be used during the assembly process of the motor assembly 100A or 100B. At step M11, at least one of the second rotor and the output shaft is rotatably coupled to the housing via a second bearing (e.g., the bearing 19 or 19A). Step M11 may be performed before step M8 or simultaneously with step M8. In an example, step M11 may comprise rotatably coupling, by the second bearing (e.g., the bearing 19), the output shaft to a support frame (e.g., the support frame 20 or 20B) of the modulator while the support frame is fixedly coupled to the second peripheral wall portion (e.g., the wall portion 9) of the housing according to step M5. This step may be performed before step M8. This example may be used during the assembly process of the motor assembly 100 or 100B. In an alternative example, step M11 may comprise rotatably coupling, by the second bearing (e.g., the bearing 19A), the second rotor (e.g., the second rotor 12A) to the second end cover (e.g., the second end cover 13A) of the housing. This step may be performed simultaneously with step M8 after the completion of step M7. This example may be used during the assembly process of the motor assembly 100A or 100C. The terms “having”, “containing”, “including”, “comprising” and the like are open and the terms indicate the presence of stated structures, elements or features but not preclude the presence of additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘left’, ‘right’, ‘clockwise’, ‘anti-clockwise’ and ‘side’ etc. are made with reference to conceptual illustrations of a motor assembly, such as that shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a motor assembly when in an orientation as shown in the accompanying drawings. 5 Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated 10 as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

:

1. A motor assembly, comprising:a housing comprising an internal space;5 an output shaft extending along an axis; anda motor and an axial flux magnetic gear arranged inside the housing along the axis, wherein: the motor comprises a stator and a rotor, and the axial flux magnetic gear comprises a first rotor and a second rotor spaced apart from one another along the axis and magnetically coupled to one another, and a modulator arranged between the first 10 and second rotors for modulating a magnetic field between the first and second rotors, and wherein the rotor of the motor is coupled to the first rotor and the second rotor is coupled to the output shaft such that a rotation of the rotor of the motor causes a rotation of the output shaft around the axis;wherein:15 the modulator is configured to separate the internal space of the housing into afirst chamber and a second chamber;the motor and the first rotor are arranged within the first chamber;the second rotor is arranged within the second chamber; andthe housing and the modulator are configured such that the first chamber is 20 hermetically sealed;the modulator is fixedly attached to a peripheral wall of the housing, the peripheral wall being made of a metal material;the modulator comprises a modulation ring and a support frame which supports the modulation ring;25 the modulation ring comprises magnetic components and non-magneticcomponents arranged in an alternating manner along a circumferential direction of the modulation ring, the non-magnetic components being made of a non-metal material; and the non-metal material is configured to seal an interface between the modulation ring and the peripheral wall of the housing.

302. The motor assembly of claim 1, wherein the modulator and at least a part of the peripheral wall of the housing are integrally formed by casting the non-magnetic components while the support frame, the magnetic components and the at least a part of the peripheral wall remain as metal inserts during the casting process.3524 10 253. The motor assembly of claim 1 or 2, wherein the peripheral wall of the housing comprises a locking mechanism for mechanically engaging with the modulator to restrict movement of the modulator along the axis.5 4. The motor assembly of any preceding claim, wherein the modulator extendsalong a plane which is perpendicular to the axis.

5. The motor assembly of any preceding claim, wherein the modulation ring and the support frame are integrally formed.

106. The motor assembly of any preceding claim, wherein the non-metal material of the non-magnetic components is configured to seal an interface between the modulation ring and the support frame.15 7. The motor assembly of any preceding claim, wherein:the modulation ring has a first surface facing the first rotor and a second surface facing the second rotor, with the second surface being opposite to the first surface; and a dimension of at least one of the magnetic components at a mid-plane between the first and second surfaces is greater than the corresponding dimension of the at least 20 one of the magnetic components at the first and / or second surface.

8. The motor assembly of any preceding claim, wherein the non-metal material comprises one or more of resin, ceramic, fiberglass and polymer.25 9. The motor assembly of any preceding claim, wherein the first rotor comprises afirst core and a first set of permanent magnets, the second rotor comprises a second core and a second set of permanent magnets.

10. The motor assembly of claim 9, wherein the first and / or second set of permanent 30 magnets are magnetised using a Halbach array configuration.

11. The motor assembly of claim 9 or 10 , wherein:the modulation ring comprises a total number of magnetic components, Zs;the first set of permanent magnets comprises a total number of pole pairs, Pr1;24 10 25the second set of permanent magnets comprises a total number of pole pairs, Pr2; andZs = Pr1 ± Pr2.5 12. The motor assembly of claim 11, wherein Pr2 is greater than Pr1.

13. The motor assembly of any preceding claim, wherein the output shaft protrudes outside of the housing, and the motor assembly further comprises a dynamic seal between the output shaft and the housing.1014. The motor assembly of any preceding claim, further comprising first and second bearings which are arranged at opposite sides of the modulation ring along the axis, wherein:the first bearing is arranged to rotatably couple at least one of the rotor of the 15 motor and the first rotor to the housing; andthe second bearing is arranged to rotatably couple at least one of the second rotor and the output shaft to the housing.

15. The motor assembly of claim 14, wherein:20 the support frame of the modulator is fixedly coupled to the housing; andthe first bearing is arranged to rotatably couple the rotor of the motor to the support frame of the modulator, and / or the second bearing is arranged to rotatably couple the output shaft to the support frame of the modulator.25 16. The motor assembly of any preceding claim, wherein the first chamber comprisesan inert gas.

17. An underwater apparatus comprising the motor assembly of any preceding claim.30 18. A method of operating a motor assembly, wherein the motor assemblycomprises a housing comprising an internal space, an output shaft extending along a axis, and a motor and an axial flux magnetic gear which are arranged inside the housing along the axis, the method comprising:energising stator coils of a stator of the motor so as to generate a rotating 35 magnetic field, thereby causing a rotor of the motor to rotate relative to the stator;24 10 25driving, by the rotor of the motor, a first rotor of the axial flux magnetic gear to rotate;modulating, by a modulator of the axial flux magnetic gear, a magnetic field between the first rotor and a second rotor of the axial flux magnetic gear, wherein the 5 modulator is arranged between the first and second rotors along the axis;driving, by the first rotor through the magnetic field, a second rotor of the axial flux magnetic gear to rotate; anddriving, by the second rotor, the output shaft to rotate around the axis;wherein: the modulator is configured to separate the internal space of the housing 10 into a first chamber and a second chamber which are arranged along the axis;the motor and the first rotor are arranged within the first chamber, and the second rotor is arranged within the second chamber;the housing and the modulator are configured such that the first chamber is hermetically sealed;15 the modulator is fixedly attached to a peripheral wall of the housing, the peripheralwall being made of a metal material;the modulator comprises a modulation ring and a support frame which supports the modulation ring;the modulation ring comprises magnetic components and non-magnetic 20 components arranged in an alternating manner along a circumferential direction of the modulation ring, the non-magnetic components being made of a non-metal material; and the non-metal material is configured to seal an interface between the modulation ring and the peripheral wall of the housing.25 19. A method of assembling a motor assembly, wherein the motor assemblycomprises a housing, an output shaft, and a motor and an axial flux magnetic gear, the method comprising:attaching a stator of the motor to a first peripheral wall portion of the housing;coupling a rotor of the motor to a first end cover of the housing;30 attaching the first end cover to the first peripheral wall portion such that the statoris arranged around the rotor of the motor;coupling a first rotor of the axial flux magnetic gear to the rotor of the motor;attaching a modulator of the axial flux magnetic gear to a second peripheral wall portion of the housing;35 attaching the first peripheral wall portion to the second peripheral wall portion;24 10 25coupling a second rotor of the axial flux magnetic gear to the output shaft; coupling the output shaft to a second end cover of the housing; and attaching the second end cover to the second peripheral wall portion;wherein:5 the output shaft extends along an axis; the motor and the axial flux magnetic gearare arranged along the axis; the first rotor and the second rotor are spaced apart from one another along the axis and magnetically coupled to one another; the modulator is arranged between the first and second rotors to modulate a magnetic field between the first and second rotors; and the rotor of the motor is coupled to the first rotor and the 10 second rotor is coupled to the output shaft such that a rotation of the rotor of the motor causes a rotation of the output shaft around the axis;the first end cover, the first peripheral wall portion, a part of the second peripheral wall portion and the modulator collectively enclose a first chamber which is hermetically sealed; and the motor and the first rotor are arranged within the first chamber;15 the second end cover, a remaining part of the second peripheral wall portion andthe modulator collectively enclose a second chamber, wherein the second rotor is arranged with the second chamber;the modulator comprises a modulation ring and a support frame which supports the modulation ring, and the modulation ring comprises magnetic components and non-20 magnetic components, the non-magnetic components being made of a non-metal material; andattaching the modulator to the second peripheral wall portion comprises casting the non-magnetic components while the support frame, the magnetic components and the second peripheral wall portion remain as metal inserts during the casting process.2520. The method of claim 19, wherein attaching the modulator to the second peripheral wall portion comprises integrally forming the modulator with the second peripheral wall portion.30s

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