Motor

Abstract

A motor 100 comprises a housing 1, 2, 7, 8, 15, 16 with an internal space; a rotor 45 rotatable around an axis D; a stator 40 arranged around the rotor and being operable to generate a rotating magnet

Description

Technical Field The present disclosure relates to a motor and a method of assembling the same. More particularly, but not exclusively, the present disclosure relates to a motor with its stator being sealed by a magnetic-field modulating structure. The magnetic-field modulating structure achieves highly-reliable sealing of the stator, without increasing the electromagnetic air gap between the rotor and stator of the motor or reducing the electromagnetic output performance of the motor. Therefore, the motor is suitable for use in underwater applications with strict sealing requirements (e.g., underwater operation robots and hydropower applications). Background Stator sealing technology is a critical factor in ensuring the safe operation of underwater motors. The underwater motors may be used in various applications, such as submersibles, underwater robots, and offshore oil drilling equipment. The underwater motors often need to operate under high water pressure and in corrosive water environments. Effective stator sealing can prevent water and / or other liquid from entering the stator of the underwater motor, thereby avoiding short circuits or corrosion of electrical components and ensuring the normal operation of the motor. Effective stator sealing can also significantly reduce the frequency of motor repairs and replacements due to water damage, thereby reducing the maintenance costs and downtime of underwater equipment. Therefore, the stator sealing technology for underwater motors is also fundamental for the overall performance, safety, and economic efficiency of underwater equipment. With the increasing demand for ocean development and underwater applications, its importance is becoming more prominent. It is generally desirable to provide highly-reliable stator sealing to a motor, without negatively affecting the motor's electromagnetic performance. It is an object of the present disclosure, among others, to provide such a motor. Summary According to a first aspect of the present disclosure, there is provided a motor comprising: a housing comprising an internal space; a rotor rotatable around an axis; a stator arranged around the rotor and being operable to generate a rotating magnetic field; a modulator arranged, in a radial direction perpendicular to the axis, between the rotor and the stator, wherein the modulator is configured to modulate the rotating magnetic field, thereby inducing the rotor to rotate around the axis; and wherein: the modulator is arranged to separate the internal space into a first chamber and a second chamber, wherein the stator is arranged within the first chamber, and at least a part of the 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 modulator not only modulates the magnetic field between the stator and the rotor, but can also be used to provide static sealing to the first chamber (where the stator is located). As compared to dynamic sealing, the static sealing of the first chamber has improved reliability and provides a higher protection level for the stator. By modulating the magnetic field between the stator and the rotor, the modulator becomes part of the magnetic circuit between the stator and the rotor, and thus the presence of the modulator does not increase the electromagnetic air gap of the motor. Accordingly, the modulator can be made with sufficient radial thickness, so as to improve the structural strength of the modulator and the sealing reliability of the stator, while maintaining the electromagnetic output performance of the motor. With the expression “the stator is arranged within the first chamber”, it is meant that the entirety of the stator is within the first chamber as enclosed by the housing and the modulator, and that the stator itself does not form any wall defining the first chamber. In other words, the stator is not exposed to the second chamber in any way. It would be appreciated that the rotor is completely outside of the first chamber. It would further be appreciated that the stator and the rotor may have different lengths along the axis, and thus the stator may only be around the rotor at some of the axial positions of the motor. The stator may be of a tubular shape surrounding the rotor, or alternatively may comprise a plural discrete portions arranged around the rotor. It would be understood that the stator remains stationary relative to the housing during operation of the motor. With the expression “a modulator arranged, in a radial direction perpendicular to the axis, between the rotor and the stator”, it is meant that the modulator is arranged within an air gap between an outer surface of the rotor and an inner surface of the stator. It would be understood that the modulator is spaced apart from the rotor. That is, an air gap may exist between the modulator and an outer surface of the rotor along the radial direction. It would further be understood that the first chamber is of a tubular shape, and that the first chamber surrounds the second chamber, at at least some of the axial positions of the motor. The motor may be a permanent magnet motor. The modulator may comprise magnetic components and non-magnetic components, which are arranged in an alternating manner along a circumferential direction of the modulator. The magnetic components may also be referred to as ferromagnetic pole pieces. It would be understood that the magnetic components are made of a magnetic material and are configured to modulate the magnetic field between the stator and the rotor. At least one of the magnetic components may extend along a direction which is parallel to the axis. In other words, at least one of the magnetic components may be of an elongated shape. A longitudinal axis of the at least one of the magnetic components may be parallel to the axis. Each of the magnetic components may extend along a direction which is parallel to the axis. When viewed in a radial plane which is perpendicular to the axis, at least one of the magnetic components may have a central angle a, and at least one of the non-magnetic components may have a central angle p, and optionally 0.3 <a / (a+P) <0.7. It would be understood that the “central angle”, as per its common technical meaning, refers to an angle whose apex is on the center of the modulator (e.g., the axis) and whose legs / sides are radii intersecting the modulator at two ends of a magnetic or non-magnetic component. In other words, the central angle is subtended by the magnetic or nonmagnetic component at the axis. More preferably, a / (a+P) may be within a range of from 0.4 to 0.6. The value of a / (a+P) may be equal to 0.5. Each of the magnetic components may have the same central angle a. Each of the non-magnetic components (between two adjacent magnetic components) may have the same central angle p. The stator may comprise a stator core which includes a plurality of projecting teeth, and a plurality of windings wound around the projecting teeth. The plurality of windings, when energised, may generate the rotating magnetic field. The stator core may be fixedly attached to the housing. When viewed in a radial plane which is perpendicular to the axis, each of the projecting teeth may have a central angle, and all of the magnetic components of the modulator may be arranged within the spans of the central angles of the projecting teeth. For each projecting tooth, at least two of the magnetic components may be arranged within the span of its central angle. The number of magnetic components within the span of the central angle of each projecting tooth may be the same. When viewed in the radial plane, for at least one of the projecting teeth, the magnetic components arranged within the span of its central angle may be symmetrically distributed with respect to a centreline of the respective projecting tooth. When viewed in the radial plane, for at least one of the projecting teeth, legs of its central angle may coincide with outermost edges of the magnetic components arranged with the span of its central angle. When viewed in the radial plane: the at least one of the projecting teeth may comprise a straight edge which intersects an edge of one of the magnetic components arranged with the span of its central angle, at a point on an outer surface of the modulator. An angle 0 formed between the straight edge and a tangent to the outer surface of the modulator at the point may be within a range of from 15° to 60°. More preferably, the angle 0 may be within a range of from 20° to 45°. Most preferably, the angle 0 may be around 25°. The projecting teeth may have inner surfaces which face the rotor, and all of the magnetic components of the modulator may be attached to the inner surfaces of the projecting teeth. The rotor may comprise a rotor core and a plurality of permanent magnets attached to the rotor core. The plurality of permanent magnets may include a total number of pole pairs, Pr. The modulator may include a total number of magnetic components, Zs. The plurality of windings may includes a total number of pole pairs, Ps. Zs, Pr and Ps may satisfy the relationship of Zs = Ps ± Pr. Pr may be greater than Ps. Advantageously, with the presence of the modulator and by making Pr greater than Ps, the motor can achieve self-deceleration based upon a transmission ratio of Pr:Ps as compared to a conventional permanent magnet motor. The magnetic components of the modulator, the stator core and the permanent magnets of the rotor may have the same length along the axis. The magnetic components of the modulator, the stator core and the permanent magnets of the rotor may be aligned at both ends along the axis. The magnetic components may comprise laminated sheets of silicon steel, which are stacked along a direction which is parallel to the axis. The laminated sheets of silicon steel may be welded together. At least one 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. The non-metal material may comprise one or more of resin, ceramic, fiberglass and polymer. The modulator may be of a tubular shape. The modulator may be of a hollow cylindrical shape. A central axis of the tubular shape may be the rotational axis of the rotor. The modulator may be arranged around the rotor (at at least some of the axial positions of the motor). The modulator may be arranged to remain stationary relative to the housing during operation of the motor. The modulator may be fixedly attached to the housing. The modulator may be fixedly attached to the stator. The rotor may protrude outside of the housing, and the motor may further comprise a dynamic seal between the rotor and the housing. According to a second aspect of the present disclosure, there is provided a method of assembling a motor, wherein the motor comprises a housing, a rotor, a stator and a modulator, the method comprising: attaching the stator to a peripheral wall of the housing; installing the modulator such that the stator is arranged around the modulator, wherein the modulator comprises a through hole; inserting the rotor into the through hole; and attaching a first end cover of the housing to a first end of the peripheral wall and a first end of the modulator, and attaching a second end cover of the housing to a second end of the peripheral wall and a second end of the modulator, such that the modulator separates the internal space of the housing into a hermetically sealed first chamber and a second chamber, with the stator arranged within the first chamber and with at least a part of the rotor arranged within the second chamber. The stator may be operable to generate a rotating magnetic field, and the modulator may be configured to modulate the rotating magnetic field, thereby inducing the rotor to rotate around an axis. The rotor may be supported by the first end cover and the second end cover of the housing. It would be understood that the first end and the second end of the peripheral wall (or the modulator) are axial ends along a rotational axis of the rotor. A central axis of the through hole may be the rotational axis of the rotor. When viewed in a radial plane which is perpendicular to the rotational axis of the rotor, the stator, the modulator and the rotor may be concentric. Where appropriate any of the features described above in relation to the first aspect of the present disclosure may be applied to the second 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 according to a first embodiment of the present disclosure; Figure 2 schematically illustrates a radial cross-sectional view of the motor of Figure 1; Figure 3 shows an enlarged view of a stator tooth and a part of a modulator in the motor of Figure 2; Figure 4 schematically illustrates a radial cross-sectional view of a motor according to a second embodiment of the present disclosure; Figure 5 shows an enlarged view of a stator tooth and a part of a modulator in the motor of Figure 4; and Figure 6 shows processing steps of a method for assembling a motor 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 3 schematically illustrate a motor 100 according to a first embodiment of the present disclosure. With reference to Figure 1, the motor 100 includes a housing, a stator 40, a magnetic-field modulating structure 11 (“modulator” hereinafter), and a rotor 45 for driving an external load. These components of the motor 100 are described below in more detail. The motor 100 has a central axis D, which is also the rotational axis of the rotor 45. 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 the axis D. Figure 1 is an axial cross-sectional view, where the motor 100 is cut along a plane defined by the axis D and a radial direction (not shown) of the motor 100. The plane along which the motor 100 is cut is parallel to the XY plane. The housing of the motor 100 includes a non-drive end outer cover 1, a non-drive end cap 2, a drive end cap 15, a drive end outer cover 16, a peripheral wall 8 extending between the end caps 2, 15, and a junction box 7. In an example, the peripheral wall 8 is generally of a cylindrical shape although other shapes are possible. The junction box 7 is fixedly attached to the peripheral wall 8 (e.g., by welding or with bolts). The peripheral wall 8 has holes formed therein. Lead wires of motor windings exit the housing through the holes of the peripheral wall 8 and the junction box 7 and are then connected to an external motor driver circuit (not shown). The non-drive end outer cover 1 is fixedly connected to the non-drive end cap 2 (e.g., via bolts). The non-drive end cap 2 is fixedly connected to the left end of the peripheral wall 8 (e.g., by bolts and / or interference fit). The drive end cap 15 is fixedly connected to the right end of the peripheral wall 8 (e.g., by bolts and / or interference fit). The drive end outer cover 16 is fixedly connected to the drive end cap 15 (e.g., via bolts). The above described components of the housing are equipped with sealing grooves at the interfaces / joints there-between. Sealing rings 6 (shown as black dots in Figure 1) are installed in the sealing grooves to achieve static sealing between the various components of the housing. The housing encloses an internal space, in which the stator 40 and the modulator 11 are located. The stator 40 includes a stator core 10 and a plurality of windings 9 attached to the stator core 10. The stator 40 (in particular the stator core 10) is fixedly connected to the peripheral wall 8 (e.g., through an interference fit). Figure 2 is a radial cross-sectional view of the motor 100 which is obtained at a cutting line A-A’ as shown in Figure 1. Referring to Figure 2, the stator core 10 includes a tubular portion 31 which extends continuously along a circumferential direction F of the motor 100, and a plurality of projecting teeth 32 extending radially inwards from the tubular portion 31 towards the axis D. In the following description, the expression “circumferential” refer to the circumferential direction F. It would be understood that the tubular portion 31 and the projecting teeth 32 may be integrally formed. Slots 33 are formed between neighbouring projecting teeth 32. In the example of Figure 2, the stator core 10 features semi-open parallel teeth 32 with slots 33, although other arrangements of the stator core are possible. For example, the stator core 10 may be separated into a plurality of discrete portions individually attached to the peripheral wall 8. In the example provided by Figure 2, the plurality of projecting teeth 32 have identical dimensions. One of the projecting teeth 32 is shown in the enlarged view of Figure 3. Referring to Figures 2 and 3, each of the projecting teeth 32 has a center line (e.g., C1, C2 in Figure 2, Ci in Figure 3), which extends along a respective radial direction of the motor 100, and each projecting tooth has mirror symmetry with respect to its center line. Referring to Figure 3, each projecting tooth 32 has a tapered end portion 36 and a neck portion 34 connected between the tapered end portion 36 and the tubular portion 31. The width of the tapered end portion 36 increases from the neck portion 34 along the radial direction. As shown in Figure 3, the inner-most surface of the tapered end portion 36 (which faces the modulator 11) is smooth and arc-shaped. In the example provided Figures 2 and 3, the inner-most surfaces of the tapered end portions 36 of all projecting teeth 32 form a cylindrical shape (with gaps between neighboring projecting teeth 32) which is concentric with the tubular portion 31. The stator windings 9 are placed in the slots 33 and are wound around the neck portions 34 of the projecting teeth 32. The tapered end portions 36 of the projecting teeth 32 prevent any radial movement of the windings 9.In an example, the windings 9 form three phases, although other numbers of phases are possible. The lead wires of the windings 9 are drawn out through the junction box 7 and connected to the external motor driver circuit. Referring back to Figure 1, the rotor 45 includes a rotor core 13 which is of a tubular shape, permanent magnets 12 fixedly attached to an outer surface of the rotor core 13, and a rotor shaft 4 which passes through a central bore of the rotor core 13 and is also fixedly attached to the rotor core 13. The fixed attachment means that relative rotation between any two of the permanent magnets 12, the rotor core 13 and the rotor shaft 4 is prevented. The permanent magnets 12 may be embedded into or surface-mounted on the outer surface of the rotor core 13. In the example of Figure 1, the rotor shaft 4 is fixedly connected to the rotor core 13 through a shaft key 14 and interference fit, although other arrangements are possible. The rotor 45 also includes a compress ring 5 which is fixedly attached to the rotor shaft 4 (e.g., through interference fit) and abuts an axial end of the rotor core 13. The compression ring 5 serves to prevent axial movement of the rotor core 13 (i.e., along the -X direction). It would be understood that the rotor 45 may be implemented in any suitable way, and is not limited to the particular example described above. The rotor 45 is supported by the non-drive end cap 2 and the drive end cap 15 of the housing. In particular, the non-drive end of the rotor shaft 4 is supported by the non-drive end cap 2 through a bearing 3. The drive end of the rotor shaft 4 is supported by the drive end cap 15 through a bearing 17. The bearings 3,17 allow the rotor shaft 4 to rotate relative to the housing of the motor 100. The rotor 45 is generally concentric with the stator 40. There is an air gap between the inner surface of the stator 40 (i.e., the innermost surfaces of the tapered end portions 36) and an outer surface of the rotor 45 (i.e., the surface of the permanent magnets 12), when viewed along a radial direction of the motor 100. Magnetic flux paths between the stator windings 9 and the permanent magnets 12 are oriented in the radial direction of the motor 100. Each of the drive end cap 15 and the drive end outer cover 16 comprises a hole, through which the rotor shaft 4 protrude over the housing. A skeleton oil seal 18 is installed around the central hole of the drive end outer cover 16 to provide dynamic sealing between the rotor shaft 4 and the drive end outer cover 16. Other types of dynamic seals may be used to replace the skeleton oil seal 18. Dynamic sealing means that there is a relative moment at one or more sealed interfaces. In the example of Figure 1, the relative movement takes place between the drive end outer cover 16 and the rotor shaft 4. Generally, the bearings 3, 17 are not water-proof and therefore do not provide any sealing function, although other arrangements are possible. The modulator 11 provides highly-reliable sealing to the stator 40, while improving the motor's electromagnetic performance. This is described below in more detail. Referring to Figures 1 and 2, the modulator 11 is arranged, in the radial direction, between the rotor 45 and the stator 40. In particular, the modulator 11 is placed within the central hole of the stator 40 and is surrounded by the projecting teeth 32. The modulator 11 itself is of a tubular structure (e.g., a hollow cylindrical structure). The rotor 45 is further placed within the central hole of the modulator 11. In other words, the modulator 11 is placed within the tubular air gap between the rotor 45 and the stator 40. When viewed in the radial plane YZ, the stator 40, the modulator 11 and the rotor 45 are concentric. In the example illustrated by Figures 1 and 2, the modulator 11 is fixedly attached to the stator core 10, the non-drive end cap 2, and the drive end cap 15 through for example adhesive bonding. In particular, a part of the outer surface of the modulator 11 (at a middle portion of the modulator 11 along the axis D) is fixedly attached to the inner-most surfaces of the tapered end portions 36 of the projecting teeth 32. The inner surface of the modulator 11 is fixedly connected to the non-drive end cap 2 and the drive end cap 15 at both axial ends of the modulator 11. As described below in more detail, the modulator 11 itself is made impermeable. The interfaces between the modulator 11 and each of the non-drive end cap 2 and the drive end cap 15 are also made impermeable by suitable means (e.g., through adhesive bonding and curing). In this way, the modulator 11 separates the internal space of the housing into a first chamber 50 and a second chamber 60 (Figure 1). The first chamber 50 is of a tubular shape, and is enclosed by the peripheral wall 8 (which defines the outer circumferential boundary of the chamber 50), the modulator 11 (which defines the inner circumferential boundary of the chamber 50) and parts of the non-drive end cap 2 and the drive-end cap 15 (which define the axial boundaries of the chamber 50). The second chamber 60 is generally surrounded by the first chamber 50, and is enclosed by the modulator 11, the non-drive end outer cover 1, the non-drive end cap 2, the drive end cap 15 and the drive end outer cover 16. The entirety of the stator 40 is located within the first chamber 50, while the rotor 45 is largely located within the second chamber 60. Since the modulator 11 is fixedly attached to the housing of the motor 100, there is no relative movement between the modulator 11 and the housing during rotation of the rotor 45. Therefore, the first chamber 50 is statically sealed. Static sealing generally achieves a lower rate of leakage and has higher sealing reliability than dynamic sealing. The stator 40 is fully enclosed within the statically-sealed first chamber 50, which provides a high protection level for the stator 40. It would be appreciated that the interfaces between the modulator 11 and the housing may be sealed in a different manner than that described above. The second chamber 60 remains dynamically sealed through the use of the skeleton oil seal 18. The modulator 11 is further designed to modulate the magnetic field between the stator 40 and the rotor 45. Referring to Figure 2, the modulator 11 includes magnetic components 19 and non-magnetic components 20, which are arranged in an alternating manner along the circumferential direction F. The magnetic components 19 are made of a magnetic material (e.g., a soft / metal magnetic material) and are used for modulating the magnetic field between the stator 40 and the rotor 45. The non-magnetic components 20 are made of a non-magnetic material. It would be understood that the magnetic material has a much higher magnetic permeability than the non-magnetic material. Preferably, the non-magnetic material is a non-metallic material which is not electrically conductive, e.g., resin, ceramic, fiberglass, polymer etc.. The use of non-metallic material for the non-magnetic components 20 is beneficial in that it reduces the eddy current formed in the modulator 11 during operation of the motor 100, thereby reducing the eddy current loss and accordingly improving the motor performance. The magnetic components 19 may also be referred to as ferromagnetic pole-pieces. Referring to Figures 1 and 2, each of the magnetic components 19 is of an elongated shape extending along a direction parallel to the axis D. In other words, the longitudinal axis of each magnetic component 19 is parallel to the axis D. In an example, each of the magnetic components 19 is made by welding laminated sheets of silicon steel together. The laminated sheets of silicon steel may be stacked along a direction which is parallel to the axis D. The non-magnetic components 20 may be made by casting. When casting the non-magnetic components 20, the pre-fabricated magnetic components 19 serve as metal inserts that are held in place by additional equipment and cast integrally with the non-magnetic components 20. During the casting process, the material of the nonmagnetic components 20 may also be used to seal any potential gaps between the laminated sheets of silicon steel included within the magnetic components 19. Further, while it is not shown in Figure 1 or 2, the material of the non-magnetic components 20 may form a protective coating covering the surfaces of the magnetic components 19 (e.g., the surfaces which face the rotor 45 and / or the stator 40), thereby improving the impermeability of the modulator 11. Alternatively, a different coating material (e.g., resin) may be used to form a protective sleeve around the modulator 11, thereby making the modulator 11 completely impermeable. Referring again to Figure 1, the axial length of the magnetic components 19 in the modulator 11, the axial length of the stator core 10, the axial length of the permanent magnets 12 as well as the axial length of the rotor core 13 are identical. Further, the axial ends of the above components are aligned with one another as well. By the expression “aligned with”, it is meant that the location(s) of the ends of the above components on the axis D are substantially coincident when viewed along a direction (e.g., the Y or Z axis) perpendicular to the axis D. In particular, within a region of the modulator 11 that overlap the stator core 10 and the permanent magnets 12 when viewed along the Y / Z axis, the magnetic components 19 and non-magnetic components 20 are alternately arranged in the circumferential direction F. However, outside such region of the modulator 11, there is no magnetic components 19 at all and the modulator 11 is made of non-magnetic material only. This arrangement is beneficial for reducing the leakage of magnetic flux between the stator 40 and the rotor 45, thereby optimizing the modulation effect provided by the modulator 11 and improving the electromagnetic performance of the motor 100. Referring to Figure 2, when viewed in the radial plane, all of the magnetic components 19 are attached to the projecting teeth 32 of the stator 10, in particular, the inner-most surfaces of the tapered end portions 36 of the projecting teeth 32. In the example of Figure 2, the projecting teeth 32 have the same dimension. Thus, the same number (e.g., two in Figure 2) of magnetic components 19 are attached to each projecting tooth 32. As shown in Figure 2, each of the projecting teeth 32 has a central angle y. The central angle is an angle whose apex (vertex) is the center of a circle (e.g., the axis D in the view of Figure 2) and whose legs (sides) are radii intersecting the circle in two distinct points (e.g., the two ends of a projecting tooth 32 along the circumferential direction F). All of the magnetic components 19 are arranged within the spans of the central angles y of the projecting teeth 32. In other words, only non-magnetic components 20 are arranged at the openings of the slots 33. Further, as shown by Figure 2, for each projecting tooth, legs of its central angle y coincide with the circumferential edges of the magnetic components arranged with the span of its central angle y. Figure 3 provides an enlarged view showing the magnetic components 19 and one projecting tooth 32 in detail. In this example, two magnetic components 19_1, 19_2 are attached to the inner-most surface of the tapered end portion 36, with a single nonmagnetic component 20_1 between them. It can be seen that the magnetic components 19_1,19_2 are symmetrically distributed with respect to the centreline Ci of the tooth 32. Further referring to Figure 3, the tapered end portion 36 includes two straight edges 24, 28 which extend between its inner-most curved surface and the neck portion 34. The left-side edge of the magnetic component 19_1 intersects the straight edge 24. The rightside edge of the magnetic components 19_2 intersects the straight edge 28. In other words, the circumferential boundaries of the two magnetic components 19_1, 19_2 associated with the projecting tooth 32 are coincident with the circumferential boundaries of the projecting tooth 32 at the outer surface of the modulator 11. The above described spatial arrangement between the projecting teeth 32 and the magnetic components 19 is beneficial for reducing the leakage of magnetic flux between the stator 40 and the rotor 45, thereby optimizing the modulation effect provided by the modulator 11 and improving the electromagnetic performance of the motor 100. Referring again to Figure 3, the intersection between the straight edge 24 and the leftside edge of the magnetic component 19_1 defines a tangent to the outer surface of the modulator 11. The angle 0 between the straight edge 24 and this tangent is with a range of from 15°to 60°, with a preferred range of from 20°to 45° and a most-preferred value of 25°. This particular range of 0 is useful for optimizing the modulation effect achievable by the modulator 11, thereby improving the motor's output performance. Further referring to Figure 3, the central angle spanned by each magnetic component 19 is defined as a, and the central angle spanned by each non-magnetic component 20 (between adjacent magnetic components 19) is defined as p. In an example, the magnetic components 19 have the same central angle a, and the non-magnetic components 20 also have the same central angle p. A ratio Rg, which is equal to a / (a+P), is preferably between around 0.3 and around 0.7. More preferably, the ratio Rg is between around 0.4 and around 0.6. In the example of Figure 2, Rg is equal to 0.5. It has been found by the inventors of the present disclosure that the value of Rg directly affects the modulation effect achievable by the modulator 11, and that the preferred range of between 0.3 and 0.7 optimises the modulation effect achievable by the modulator 11, thereby improving the motor's output performance. To further optimise the modulation effect achievable by the modulator 11, the total number of magnetic components 19, Zs, included within the modulator 11 shall preferably satisfy Equation (1) below: Zs = Pr±Ps (1) Pr is the number of pole pairs included within the permanent magnets 12 of the rotor 45. Ps is the number of pole pairs included within the stator windings 9. In the example illustrated by Figures 1 to 3, Ps is four, Pr is fourteen, and Zs is eighteen. Hence Zs=Pr+Ps. It would further be understood that Zs = 3607(a+P). The working principle of the motor 100 is as follows: the stator windings 9 are energized to generate a rotating magnetic field. As the rotating magnetic field passes through the modulator 11, the magnetic components 19 of the modulator 11 modulate the rotating magnetic field. The modulated rotating magnetic field interacts with the magnetic field generated by the permanent magnets 12 of the rotor 45, producing torque. This torque drives the rotor 45 to rotate, thereby providing output torque. Due to the presence of the modulator 11, the motor 100 can achieve a built-in deceleration function. In particular, when the relationship set by Equation (1) is met, the rotational speed of the motor 100 is equal to 60f / Pr, where f is the winding current frequency and Pr is the number of pole pairs included within the permanent magnets 12. In contrast, for a conventional permanent magnet motor where the modulator 11 does not exist, the rotational speed of the motor is equal to 60f / Ps, where f is still the winding current frequency and Ps is the number of pole pairs included within the stator windings. Therefore, when operating with the same f and the same Ps, a rotational speed of the conventional permanent magnet motor over the rotational speed of the motor 100 is equal to Pr: Ps. In other words, by making Pr greater than Ps, the motor 100 can readily achieve self-deceleration based upon a transmission ratio of Pr:Ps as compared to the conventional permanent magnet motor. 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 mechanical gears. The motor 100 achieves the self-deceleration effect by using the magnetic modulation effect provided by the modulator 11 without utilizing any mechanical gear. Due to the reduced rotational speed, the motor 100 can produce high torque density at the rotor shaft 4. Therefore, the motor 100 is particularly suitable for applications requiring high sealing performance in low-speed operation scenarios. Generally speaking, the air gap between the stator and the rotor of a conventional motor is an important parameter for the electromagnetic performance of the motor, and increasing the radial length of the air gap would typically have a negative impact to the electromagnetic performance of the conventional motor. While the modulator 11 is placed within the air gap between the stator 40 and the rotor 45, the modular 11 is able to maintain the output performance of the motor 100. This is because the modulator 11 (due to the presence of the magnetic components 19) becomes part of the magnetic circuit between the stator 40 and the rotor 45 due to the magnetic field modulation effect provided thereby, and thus the thickness of modulator 11 along the radial direction does not become a real part of the motor air gap. Therefore, the introduction of the modulator 11 does not effectively increase the electromagnetic air gap of the motor 100. Accordingly, it is possible to increase the thickness of the modulator 11, so as to increase the structural strength of the modulator 11, without negatively impacting the output performance of the motor 100. For a motor with an outer diameter of around 200mm, the thickness of the modulator 11 used within the motor may reach a level of between 10mm and 15mm. This level of thickness allows the modulator 11 to be made with relatively high structural strength, thereby improving the sealing reliability of the stator 40. As shown by Figure 2, the radial thickness of the modulator 11 is substantially equal to the radial thickness of the magnetic components 19 or the non-magnetic components 20. By suitably selecting the non-magnetic material (e.g., high strength resin), the modulator 11 becomes a structural component with relatively high structural strength rather than a thin film. This is also beneficial for improving the structural reliability of the modulator 11 and the sealing reliability of the stator 40. In the example illustrated by Figures 1 to 3, the modulator 11 is directly attached to the inner surfaces of the projecting teeth 32 of the stator 40. It would be appreciated that other arrangements are possible. For example, the modulator 11 may be attached to the inner surface of the stator 40 via another component (such as a protective sleeve around the outer surface of the modulator 11). In any event, the modulator 11 is spaced apart from the outer surface of the rotor 45, allowing the rotor 45 to rotate without any mechanical contact with the inner surface of the modulator 11. As shown by Figures 1 and 2, the entirety of the stator 40 is within the first chamber 50 as fully enclosed by the housing and the modulator 11, and that the stator 40 itself does not form any wall defining the first chamber 50. In other words, none of the stator 40 is ever exposed to the second chamber 60. In modern motors, it is common to make the stator core 10 by laminating silicon steel sheets, so as to reduce core eddy current losses in the stator core. Often there are still gaps between the laminated silicon steel sheets of the stator core, and fluid is able to pass through the gaps. Therefore, directly exposing any surface of the stator core 10 to the second chamber 60 (which is dynamically sealed) may significantly lower the sealing reliability of the stator 40. Figures 4 and 5 schematically illustrate a motor 100A according to a second embodiment of the present disclosure. Elements of the motor 100A that are identical to those of the motor 100 are identified using the same labels. Elements of the motor 100A that correspond to, but are different from those of the motor 100 are labelled using the same numerals but with a letter ‘A’ for differentiation. The features and advantages described above with reference to the first embodiment are generally applicable to the second embodiment. The stator 40 of the motor 100A is identical to that of the motor 100. However, the modulator 11A differs from the modulator 11 in that the number of magnetic components 19A arranged on the inner surface of each stator tooth is three (rather than two). Therefore, the modulator 11A includes twenty-seven (i.e., Zs) magnetic components 19A in total. The rotor 45A differs from the rotor 45 in that the total number of pole pairs included within the permanent magnet 12A of the rotor 45A is twenty-three (Pr). With the number of pole pairs included within the stator windings 9 being four (Ps), the motor 100A still satisfies the relationship of Zs=Pr±Ps. It would be understood that other values of Zs, Pr and Ps are possible. Figure 5 shows an enlarged view of one projecting tooth 32 with its associated magnetic components 19A, within the motor 100A. It can be seen that three magnetic components 19A_1, 19A_2, 19A_3 are attached to the inner-most surface of the tapered end portion 36, with non-magnetic components 20A_1, 20A_2 between neighboring ones of the magnetic components. The magnetic components 19A_1, 19A_2, 19A_3 are still symmetrically distributed with respect to the centreline Ci of the tooth 32. All other features of the modulator 11 as described above equally apply to the modulator 11 A. Figure 6 schematically illustrates processing steps of a method for assembling a motor (e.g., the motor 100 or 100A). The motor comprises a housing (e.g., the wall 8, the junction box 7, and the caps / covers 1, 2, 15, 16), a rotor (e.g., the rotor 45 or 45A), a stator (e.g., the stator 40) and a modulator (e.g., the modulator 11 or 11 A). At step M1, the stator is attached to a peripheral wall (e.g., the peripheral wall 8) of the housing. In an example, the stator is fixedly attached to the peripheral wall through for example interference fit. Other attaching means is possible. At step M2, the modulator is installed such that the stator is arranged around the modulator. The modulator comprises a through hole. In an example, the modulator may be of a hollow cylindrical shape. The modulator may be installed by fixedly attaching (e.g., through adhesive bonding and curing) an outer surface of the modulator to an inner surface of the stator. At step M3, the rotor is inserted into the through hole. In this way, the modulator is arranged around (an outer circumferential surface of) the rotor. At step M4, a first end cover (e.g., the non-drive end cap 2 and / or the non-drive end outer cover 1) of the housing is attached to a first end of the peripheral wall and a first end of the modulator. At step M5, a second end cover (e.g., the drive end cap 15 and / or the non-drive end outer cover 16) of the housing is attached to a second end of the peripheral wall and a second end of the modulator. Steps M4 and M5 are performed in a way such that the modulator separates the internal space of the housing into a hermetically sealed first chamber (e.g., the first chamber 50) and a second chamber (e.g., the second chamber 60), with the stator arranged within the first chamber and with at least a part of the rotor arranged within the second chamber. In an example, the “first end” and “second end” refer to the left end and the right end as shown in Figure 1. The first / second end cover may be fixedly attached to the first / second end of the peripheral wall (e.g., by using bolts and / or interference fit) with the relevant interface statically sealed (e.g., by using sealing rings). Similarly, the first / second end cover may be fixedly attached to the first / second end of the modulator (e.g., by adhesive bonding and curing) with the interface statically sealed. The step M4 may comprise an optional step of supporting the rotor by the first end cover. The step M5 may comprise an optional step of supporting the rotor by the second end cover. A central axis of the through hole may be the rotational axis of the rotor. It would be appreciated that steps M1 to M5 may take place according to a sequence which is different from the sequence of description. 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’ etc. are made with reference to conceptual illustrations of a motor such as that shown in the appended drawings. These terms are 5 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 when in an orientation as shown in the accompanying drawings. Although the disclosure has been described in terms of preferred embodiments as set 10 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 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 15 any appropriate combination with any other feature disclosed or illustrated herein.

Claims

:

1. A motor comprising:a housing comprising an internal space;a rotor rotatable around an axis;a stator arranged around the rotor and being operable to generate a rotating magnetic field;a modulator arranged, in a radial direction perpendicular to the axis, between the rotor and the stator, wherein the modulator is configured to modulate the rotating magnetic field, the modulated rotating magnetic field inducing the rotor to rotate around the axis; andwherein:the modulator is arranged to separate the internal space into a first chamber and a second chamber, wherein the stator is arranged within the first chamber, and at least a part of the rotor is arranged within the second chamber;the housing and the modulator are configured such that the first chamber is hermetically sealed;the modulator comprises magnetic components and non-magnetic components, which are arranged in an alternating manner along a circumferential direction of the modulator, the modulator including a total number of magnetic components, Zs;the stator comprises a stator core which includes a plurality of projecting teeth, and a plurality of windings wound around the projecting teeth, the plurality of windings including a total number of pole pairs, Ps; andthe rotor comprises a rotor core and a plurality of permanent magnets attached to the rotor core, the plurality of permanent magnets including a total number of pole pairs, Pr, and wherein Zs = Ps ± Pr.

2. The motor of claim 1, wherein at least one of the magnetic components extends along a direction which is parallel to the axis.

3. The motor of claim 1 or 2, wherein, when viewed in a radial plane which is perpendicular to the axis, at least one of the magnetic components has a central angle a, and at least one of the non-magnetic components has a central angle p, and wherein 0.3 <a / (a+P) <0.7.24 10 254. The motor of claim 3, wherein a / (a+P) is equal to 0.5.5 5. The motor of any preceding claim, wherein, when viewed in a radial plane whichis perpendicular to the axis, each of the projecting teeth has a central angle, and all of the magnetic components of the modulator are arranged within the spans of the central angles of the projecting teeth.10 6. The motor of claim 5, wherein, for each projecting tooth, at least two of themagnetic components are arranged within the span of its central angle.

7. The motor of claim 5 or 6, wherein, when viewed in the radial plane, for at least one of the projecting teeth, the magnetic components arranged within the span of its 15 central angle are symmetrically distributed with respect to a centreline of the respective projecting tooth.

8. The motor of any one of claims 5 to 7, wherein, when viewed in the radial plane, for at least one of the projecting teeth, legs of its central angle coincide with outermost 20 edges of the magnetic components arranged with the span of its central angle.

9. The motor of claim 8, wherein, when viewed in the radial plane:the at least one of the projecting teeth comprise a straight edge which intersects an edge of one of the magnetic components arranged with the span of its central angle, 25 at a point on an outer surface of the modulator; andan angle 0 formed between the straight edge and a tangent to the outer surface of the modulator at the point is within a range of from 15° to 60°.

10. The motor of any preceding claim, wherein the projecting teeth have inner 30 surfaces which face the rotor, and all of the magnetic components of the modulator are attached to the inner surfaces of the projecting teeth.35 11. The motor of any preceding claim, wherein Pr is greater than Ps.24 10 2512. The motor of any preceding claim, wherein the magnetic components of the modulator, the stator core and the permanent magnets of the rotor have the same length along the axis.

513. The motor of claim 12, wherein the magnetic components of the modulator, the stator core and the permanent magnets of the rotor are aligned at both ends along the axis.10 14. The motor of any preceding claim, wherein the magnetic components compriselaminated sheets of silicon steel, which are stacked along a direction which is parallel to the axis.

15. The motor of any preceding claim, wherein at least one of the non-magnetic 15 components is made of a non-metal material.

16. The motor of claim 15, wherein the non-metal material comprises one or more of resin, ceramic, fiberglass and polymer.20 17. The motor of any preceding claim, wherein the modulator is of a tubular shape.

18. The motor of any preceding claim, wherein the modulator is arranged to remain stationary relative to the housing during operation of the motor.25 19. The motor of any preceding claim, wherein the modulator is fixedly attached tothe stator.

20. The motor of any preceding claim, wherein the rotor protrudes outside of the housing, and the motor further comprises a dynamic seal between the rotor and the 30 housing.

21. A method of assembling a motor, wherein the motor comprises a housing, a rotor, a stator and a modulator, the method comprising:attaching the stator to a peripheral wall of the housing;24 10 25installing the modulator such that the stator is arranged around the modulator, wherein the modulator comprises a through hole;inserting the rotor into the through hole; andattaching a first end cover of the housing to a first end of the peripheral wall and5 a first end of the modulator, and attaching a second end cover of the housing to a second end of the peripheral wall and a second end of the modulator, such that the modulator separates the internal space of the housing into a hermetically sealed first chamber and a second chamber, with the stator arranged within the first chamber and with at least a part of the rotor arranged within the second chamber; wherein:10 the modulator comprises magnetic components and non-magneticcomponents, which are arranged in an alternating manner along a circumferential direction of the modulator, the modulator including a total number of magnetic components, Zs;the stator comprises a stator core which includes a plurality of projecting15 teeth, and a plurality of windings wound around the projecting teeth, the pluralityof windings including a total number of pole pairs, Ps; andthe rotor comprises a rotor core and a plurality of permanent magnets attached to the rotor core, the plurality of permanent magnets including a total number of pole pairs, Pr, and wherein Zs = Ps ± Pr.20s

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