Stator with cooling structure and electric motor

Abstract

This invention provides a stator and an electric motor having the stator, which is capable of efficient cooling with refrigerant and suppresses enlargement caused by refrigerant inlets and outlets. The stator comprises: a coil; a core having slots for arranging the coil and a refrigerant flow path; a molded resin portion that at least partially embeds the axial end of the coil; and an inlet and an outlet formed within an axial region of the coil end between the axial end face of the core and the axial tip of the axial end of the coil, opening on a side surface of the core where the slots are not formed, and fluidly communicating with the flow path.

Description

Technical Field

[0001] This disclosure relates to a stator having a cooling structure and an electric motor having the stator. Background Technology

[0002] In an electric motor having a stator with coils wound around it and a rotor rotating relative to the stator, in order to remove the heat generated by energizing the coils, a flow path for refrigerant to flow is sometimes formed in the stator (see, for example, Patent Documents 1 to 3).

[0003] In addition, a motor is known to have a molding resin section for molding multiple laminated plates and coils constituting a stator, wherein a flow path for a cooling medium is formed in the molding resin section (see, for example, Patent Document 4).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Publication No. 2018-504881

[0007] Patent Document 2: Japanese Patent Application Publication No. 10-051983

[0008] Patent Document 3: Japanese Patent Application Publication No. 2008-312292

[0009] Patent Document 4: Japanese Patent Application Publication No. 2023-166527 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] In liquid-cooled electric motors that use refrigerant for cooling, inlets / outlets for introducing / exiting refrigerant are required in or near the stator. However, the formation of these inlets / outlets sometimes leads to an increase in the size of the motor, particularly its axial dimension. Therefore, there is a need for a stator and an electric motor that can efficiently utilize refrigerant for cooling and suppress the increase in size caused by the refrigerant inlets / outlets.

[0012] Solution for solving the problem

[0013] One technical solution disclosed herein is a stator, wherein the stator comprises: a coil; a core having a slot for arranging the coil and a refrigerant flow path; a molded resin portion that at least partially embeds the axial end of the coil; and an inlet and an outlet formed in the axial end axial region between the axial end face of the core and the axial tip of the axial end of the coil, and an opening on the side surface of the core where the slot is not formed, fluidly communicating with the flow path.

[0014] Another technical solution disclosed herein is an electric motor having the aforementioned stator and a rotor configured to rotate relative to the stator. Attached Figure Description

[0015] Figure 1 This is a schematic axial sectional view of an electric motor having a stator according to the first embodiment.

[0016] Figure 2 It is along Figure 1 A radial sectional view of line II-II.

[0017] Figure 3 This is a schematic axial sectional view of the stator in the second embodiment.

[0018] Figure 4 This is an enlarged view of the main part of the stator in the first embodiment.

[0019] Figure 5 This is an enlarged view of the main part of the stator in the second embodiment.

[0020] Figure 6 This is an enlarged view of the main part of the stator in the comparative example.

[0021] Figure 7 It is along Figure 4 A radial sectional view of line VII-VII.

[0022] Figure 8 This is a radial sectional view showing a structural example of a cooling flow path.

[0023] Figure 9 This is a radial sectional view showing other structural examples of cooling flow paths.

[0024] Figure 10 This is a radial sectional view showing another example of a cooling flow path.

[0025] Figure 11 This is a radial sectional view showing another structural example of a cooling flow path.

[0026] Figure 12 This is a radial sectional view showing another example of a cooling flow path.

[0027] Figure 13 This is a schematic axial cross-sectional view showing an example of a recess formed in molding resin.

[0028] Figure 14 This is a schematic radial cross-sectional view showing an example of a protrusion formed in molding resin.

[0029] Figure 15 This is a schematic radial sectional view showing an example of a stator with a segmented structure.

[0030] Figure 16 This is a schematic axial sectional view showing an example of a stator formed from multiple electromagnetic steel plates.

[0031] Figure 17 This is a schematic axial sectional view showing an example of an iron core covered with resin.

[0032] Figure 18 This is a schematic axial sectional view showing a structural example of an external rotor type electric motor. Detailed Implementation

[0033] Figure 1 This is an axial sectional view showing the schematic structure of the main parts of the electric motor 2 in the first embodiment. Figure 2 It is along Figure 1 The radial cross-sectional view along line II-II. The motor 2, for example, can be used as a drive source for industrial robots and machine tools, and has a stator 4 and a rotor 8 that rotates about an axis 6 relative to the stator 4. The motor 2 of the first embodiment is a so-called inner rotor type motor in which the rotor 8 is arranged radially inward than the stator 4, but it is not limited thereto.

[0034] The stator 4 has a coil 10 composed of windings and a generally cylindrical core (iron core) 16 having a slot 12 for arranging the coil 10 and at least one refrigerant flow path 14. The core 16 can be formed, for example, by stacking multiple electromagnetic steel plates. In the illustrated example, multiple flow paths 14 are provided at the same angular position as the coil 10, and each flow path 14 extends axially in a straight line within the core 16. The core 16 is housed within a housing 22, and molded resin portions 20 are formed at both axial ends of the core 16. These molded resin portions 20 at least partially (in the illustrated example, entirely) embed the axial ends (hereinafter also referred to as coil ends) 18 of the coil 10 that protrude axially from the core 16 and at least partially fill the slot 12.

[0035] There are no particular limitations on the material used to form the molding resin portion 20, but materials with electrical insulation and high thermal conductivity are preferred, such as epoxy resin. The molding resin portion 20 cooperates with the axial end face of the core 16, defining an inlet 24 for introducing refrigerant into a flow path 14 extending axially within the core 16 and an outlet 26 for discharging refrigerant from the flow path 14. The inlet 24 and outlet 26 are formed within an axial interval L1 (hereinafter also referred to as the coil end axial interval) between the axial tip of the coil end 18 and the axial end face 32 of the core 16, on the surface of the generally cylindrical core 16 where the groove 12 (i.e., the coil 10 not adjacent to the rotor 8) is not formed. Figures 1-2 In the example, the opening is on the outer side.

[0036] Figure 3This is an axial sectional view showing the schematic structure of the main parts of the motor 2a according to the second embodiment. The second embodiment differs from the first embodiment in that the shape of the coil end 18a is different, but other parts can be the same as the first embodiment. Therefore, the corresponding components are labeled with the same reference numerals as in the first embodiment, and detailed descriptions are omitted.

[0037] Figure 4 and Figure 5 These are enlarged views of the axial ends of the motor 2 in the first and second embodiments, respectively. Figure 4 As shown, in the first embodiment, the shortest radial distance d1 of the flow path 14 is longer than the longest radial distance d2 of the coil end 18, but as Figure 5 As shown, in the second embodiment, the shortest radial distance d1 of the flow path 14 is shorter than the longest radial distance d3 of the coil end 18a. In other words, the coil end 18a extends radially towards the inlet 24 or outlet 26 compared to the portion of the flow path 14 furthest from the inlet 24 or outlet 26. More specifically, the coil end 18a extends radially outward from the innermost radial part of the flow path 14. Typically, the coil end is axially compressed during the manufacture of the motor, thus the coil end has a shape that extends radially outward. Therefore, in the second embodiment, the axial section L2 of the coil end can be made shorter than the axial section L1 of the coil end in the first embodiment, resulting in a shorter overall length of the motor. Furthermore, in the second embodiment, the coil end 18a can be made closer to the inlet 24, thus enabling efficient cooling of the coil end 18a, which also serves as a heat source.

[0038] Figure 6 This is an enlarged view of the axial end of the stator 104 of the comparative example. The stator 104 has a coil 110 composed of windings and a generally cylindrical core 116 having a refrigerant flow path 114. The core 116 is housed within a housing 122, and molded resin portions 120 are formed at both axial ends of the core 116 to embed the coil ends 118 of the coil 10 that protrude axially from the core 116.

[0039] The flow path 114 is formed at a position radially inward of the coil 110, and is formed to open on the axial end face, rather than on the outer or inner side of the core 116. However, in such a configuration, the axial length of the molded resin portion 120 is necessarily much longer than the axial section L3 at the end of the coil, resulting in a longer overall length of the stator 104.

[0040] In contrast, in the first and second embodiments, by forming the inlet 24 and outlet 26 within the axial interval L1 or L2 at the end of the coil, the total length of the stator 4 can be made to be approximately the same as or slightly longer than the axial length of the coil 10.

[0041] In the comparative example, the flow path 114 is located radially inward of the coil 110. In this positional relationship, it may be necessary to reduce the volume of the coil 110 based on the volume occupied by the flow path 114. Furthermore, since the flow path 114 is close to the coil 110, cooling the coil 110 is easier, but efficiently cooling the radially outer side of the core 116 is difficult. Moreover, the radially inner side of the stator faces significant dimensional constraints, making it difficult to form a flow path with a large radial cross-sectional area.

[0042] On the other hand, in the first and second embodiments, since the flow path 14 is positioned radially outward from the coil 10, there is no need to reduce the volume of the coil 10 by forming the flow path 14. Furthermore, since the flow path 14 can approach both the radially outward sides of the coil 10 and the core 16, both the coil 10 and the core 16 can be cooled efficiently. Moreover, the flow path 14 is not easily constrained by size, and it is easy to form a flow path with a relatively large radial cross-sectional area, which also contributes to efficient cooling.

[0043] Figure 7 It is along Figure 4 A cross-sectional view along line VII-VII. The stator 4 has an annular refrigerant flow path 30, which is defined by the axial end face of the core 16 and the molded resin portion 20, and is in fluid communication with the inlet 24 or outlet 26. The refrigerant flow path 30 is also in fluid communication with a plurality of straight flow paths 14. By forming an annular refrigerant flow path 30, only one inlet 24 and one outlet 26 are needed, and the refrigerant can flow evenly in each flow path 14.

[0044] Figures 8-12 This indicates the various shapes and configurations of the refrigerant flow path in core 16. Figure 8 In this example, the flow path 14a, which has a circular radial cross-sectional shape, is arranged circumferentially at the same angular position as the slot 12. More specifically, the flow path 14a is formed radially outside the slot 12 and is arranged such that the center of the flow path 14a is located on the line segment 17 representing the circumferential angular position of the slot 12. The flow path 14a is formed relatively close to the coil 10, which also serves as a heat source, thus enabling efficient cooling of the coil 10 using the refrigerant flowing in the flow path 14a.

[0045] exist Figure 9 In the example, the configuration of flow path 14 is the same as that of flow path 14a, but the radial cross-sectional shape of each flow path 14 is an ellipse that is longer in the circumferential direction. This allows the distance from the radially outer side of flow path 14 to the outer surface of core 16 to be greater than... Figure 8 The example is long, and the circumferential magnetic flux is difficult to block, thus suppressing the decrease in motor torque.

[0046] Figure 10In the example, the configuration of flow path 14b is the same as that of flow path 14a or flow path 14, but the radial cross-sectional shape of each flow path 14b is a roughly triangular shape with a vertex whose circumferential angular position is equal to that of the center of groove 12 (more specifically, the vertex is located on line segment 17 representing the angular position of the center of groove 12). Figure 10 In the example, with Figure 9 Similarly, circumferential magnetic flux is less likely to be blocked, thus suppressing torque reduction in the motor, and... Figure 9 Compared to the previous example, it can suppress the radial distance between the inner wall of the flow path 14b and the coil 10, thus enabling efficient cooling of the core 16 and the coil 10.

[0047] exist Figure 11 In the examples, except Figure 8 In addition to the flow path 14a shown, there is also a groove 15a formed on the outer surface of the core 16 and extending axially. The groove 15a cooperates with the outer casing 22 to define the flow path for the refrigerant. Furthermore, to prevent circumferential magnetic flux from being blocked, the groove 15a is preferably formed with its center located between the angular positions of adjacent slots 12; in other words, it is located on the line segment 19 representing the angular position of the center of the tooth 13. Figure 11 In the example, a large amount of refrigerant can flow through two flow paths 14a and 15a, so in addition to efficiently cooling the core 16, it can also suppress the decrease in motor torque.

[0048] exist Figure 12 In the example, the configuration of the flow path and Figure 11 The flow path is the same, but at the same angular position as slot 12. Figure 9 The elliptical flow path 14a shown is not defined by the outer surface of the core 16 and the outer shell 22. Figure 10 Instead of a roughly semi-circular shape like groove 15a, it has a roughly semi-elliptical shape that is longer in the circumferential direction. Figure 12 In the example, with Figure 11 Compared to other examples, circumferential magnetic flux is more difficult to block, thus significantly suppressing the reduction in motor torque.

[0049] Figure 13 This is an enlarged view of the axial end of the motor 2b according to the third embodiment. Parts that can be the same as those in the first embodiment are labeled with the same reference numerals as those in the first embodiment, and detailed descriptions are omitted.

[0050] For example in Figure 5 In the example shown, depending on the conditions (pressure, flow rate, etc.) of the refrigerant introduced through inlet 24, stress concentrates at the boundary 32 between the axial end face of core 16 and the molding resin portion 20. The core 16 or molding resin portion 20 near boundary 32 may experience localized deterioration, or refrigerant may leak from boundary 32. In contrast, in Figure 13 In the example, the molding resin portion 20a has a recess 34 that is radially recessed towards the coil 10 side compared to the portion of the flow path 14 that is radially furthest from the inlet 24a or outlet. More specifically, the molding resin portion 20a has a recess 34 that is radially recessed towards the innermost radial side compared to the innermost radial side of the flow path 14. Therefore, in Figure 13 In the example, stress concentration at boundary 32 is mitigated, which can suppress localized deterioration and refrigerant leakage.

[0051] Figure 14 It means and Figure 7 A similar radial cross-sectional view of the annular flow path 30. Figure 14 In the example, the molded resin portion 20 has a protrusion 36 at the same circumferential angle position as the inlet 24, protruding radially towards the side opposite to the coil (radially outward in the example). The protrusion 36 prevents the refrigerant from contacting the outer diameter surface of the molded resin portion 20 perpendicularly, and facilitates the circumferential flow of the refrigerant in the annular flow path 30. Therefore, it can suppress localized deterioration of the molded resin portion 20, refrigerant leakage, and enable efficient cooling of the stator 4.

[0052] Figure 15 An example is shown where the core of the stator 4 is configured as a split structure. Specifically, the core 16c has: a first core portion 38a having a slot 12 for receiving the coil 10; and a second core portion 38b having an inner diameter surface fixed to the outer diameter surface of the first core portion 38a and forming a flow path 14a. The first core portion 38a and the second core portion 38b can be joined together by thermoforming, bonding, pressing, etc., and can also be separated from each other.

[0053] like Figure 15 As in the example, by designing the core to be divisible into a part with slots and a part with flow paths, the shape and number of flow paths can be changed without changing the shape and number of slots, making it easy to change or upgrade the specifications of the motor.

[0054] Figure 16 This is a schematic axial sectional view showing an example of a core 16 formed from multiple electromagnetic steel sheets 40 of the same shape. Typically, when the core 16 forms the flow path 14, a punching process is performed on the portion of each electromagnetic steel sheet 40 corresponding to the flow path 14 in the direction of arrow 42 before stacking the multiple electromagnetic steel sheets 40. However, this process creates burrs 44 protruding in the punching direction on the electromagnetic steel sheets 40. Especially when electromagnetic steel sheets of different shapes are stacked, local gaps are created in the flow path due to the burrs, and refrigerant may leak from these gaps.

[0055] Therefore, as Figure 16As shown, it is preferable that multiple electromagnetic steel plates 40 are all of the same shape and are stacked in such a way that the directions (punching directions) in which these burrs 44 protrude are all in the same direction 42. In this way, even if there are burrs 44 in each electromagnetic steel plate 40, a gapless flow path 14 can be formed, which can prevent refrigerant leakage.

[0056] Figure 17 This is a schematic axial cross-sectional view showing an example of resin covering the inner surface of the core 16. The side of the core 16 with the groove formed (the inner diameter surface in the example) is covered, for example, with resin 46, the same resin used to form the molding resin portion 20. Furthermore, the resin 46 covering the inner diameter surface of the core 16 is in communication with the molding resin portion 20. By covering the inner diameter surface of the core 16 with resin 46, the inner diameter surface can be protected, and refrigerant leakage can be prevented between adjacent electromagnetic steel plates 40 and between the core 16 and the molding resin portion 20.

[0057] Figure 18 This is a schematic axial sectional view showing a structural example of an external rotor type electric motor. Furthermore, the motor 2d has a stator 4d and an external rotor 8d that rotates radially outward relative to the stator 4d. Additionally, for components in the motor 2d that are functionally identical to those in the motor 2 of the first embodiment, the reference numerals are prefixed with "d" following the reference numerals of the components in the first embodiment, and detailed descriptions are omitted.

[0058] In the external rotor type motor 2d, the inlet 24d and outlet 26d for allowing refrigerant to flow within the refrigerant flow path 14d of the core 16d are formed within the axial section L4 of the coil end between the coil end 18d and the axial end face of the core 16d. These outlets are located on the side surfaces (outer and inner surfaces) of the generally cylindrical core 16 where slots are not formed (i.e., coils 10d not adjacent to the rotor 8d are not positioned). Figure 18 In the example, the opening is on the inner side. Therefore, in an external rotor type motor, the same effect as that of an internal rotor type motor described above can be obtained.

[0059] For example, when Figure 5 When the example is applied to an external rotor type motor 2d, the coil end 18d extends radially inward from the outermost radial direction of the flow path 14d. If... Figure 13 Applying the example to motor 2d, the molded resin part 20d has a recess that is radially recessed outward from the outermost radial direction compared to the flow path 14d. Furthermore, if... Figure 14 Applying this example to the motor 2d, the molded resin part 20d has a protrusion that projects radially inward toward the coil 10d at the same circumferential angular position as the inlet 24d. Furthermore, if... Figure 15Applying the example to the electric motor 2d, the core 16d has: a first core having a slot for receiving the coil 10d; and a second core having an outer diameter surface fixed to the inner diameter surface of the first core and forming a flow path 14d. Furthermore, if... Figure 17 In the example applied to motor 2d, the outer diameter surface of core 16d is covered with the same resin as that forming the molding resin portion 20d, and the resin covering the outer diameter surface of core 16d is in communication with the molding resin portion 20d. This embodiment, applicable to internal rotor type motors, can also be applied to external rotor type motors based on the same idea.

[0060] According to the above embodiment, the inlet and outlet for allowing refrigerant to flow into the core are defined by the cooperation of components typically found in electric motors, such as the core and molding resin. Therefore, it is not necessary to use additional components to form the inlet and outlet. In addition, the inlet and outlet are formed within the axial region of the coil end, so by providing the inlet and outlet, the entire length of the stator does not extend, and a compact electric motor can be constructed.

[0061] The following notes are also disclosed regarding the above-described embodiments and variations.

[0062] (Note 1)

[0063] A stator, wherein,

[0064] The stator has the following features:

[0065] coil;

[0066] The core has slots for arranging the coils and flow paths for the refrigerant;

[0067] A molded resin portion, which at least partially embeds the axial end of the coil; and

[0068] The inlet and outlet are formed in the axial region of the coil end between the axial end face of the core and the axial top end of the axial end of the coil, and are openings on the side of the core where the groove is not formed, and are fluidly connected to the flow path.

[0069] (Note 2)

[0070] According to the stator described in Appendix 1, the inlet and the outlet are defined by the axial end face of the core and the molded resin portion.

[0071] (Note 3)

[0072] According to the stator described in Appendix 1 or 2, the axial end of the coil extends radially toward the inlet or outlet side relative to the portion of the flow path that is radially furthest from the inlet or outlet.

[0073] (Note 4)

[0074] According to any one of Appendices 1 to 3, the stator wherein the molded resin portion has an annular refrigerant flow path fluidly connected to the inlet or the outlet, the refrigerant flow path being fluidly connected to a plurality of the flow paths respectively.

[0075] (Note 5)

[0076] According to any one of Appendices 1 to 4, the stator wherein the flow path extends axially within the core and is positioned at the same circumferential angle as the slot.

[0077] (Note 6)

[0078] According to the stator described in Appendix 5, the radial cross-section of the flow path has an elliptical shape that is longer in the circumferential direction.

[0079] (Note 7)

[0080] According to the stator described in Appendix 5, the radial cross-section of the flow path is approximately triangular in shape with a vertex whose circumferential angular position is equal to that of the center of the groove.

[0081] (Postscript 8)

[0082] According to any one of Appendices 1 to 7, the stator has a groove formed on the surface of the core and extending axially, located between circumferential angular positions of adjacent grooves in the radial section of the core.

[0083] (Note 9)

[0084] According to any one of Appendices 1 to 8, the stator wherein the molded resin portion has a recess that is radially recessed toward the coil side compared to the portion of the flow path that is radially furthest from the inlet or the outlet.

[0085] (Postscript 10)

[0086] According to any one of Appendices 1 to 9, the stator wherein the molded resin portion has a protrusion at the same circumferential angle position as the inlet, protruding radially toward the side opposite to the coil.

[0087] (Postscript 11)

[0088] According to Appendix 1, the stator has: a first core portion having the groove; and a second core portion having an inner diameter surface fixed to the outer diameter surface of the first core portion or an outer diameter surface fixed to the inner diameter surface of the first core portion and forming the flow path.

[0089] (Postscript 12)

[0090] According to any one of Appendices 1 to 11, the stator is wherein the core is composed of a plurality of stacked electromagnetic steel plates, the electromagnetic steel plates being stacked in such a manner that the protruding direction of the burrs of each electromagnetic steel plate is the same.

[0091] (Postscript 13)

[0092] According to any one of Appendices 1 to 12, the stator wherein the side of the core on which the groove is formed is covered with the same material as the material forming the molding resin portion.

[0093] (Postscript 14)

[0094] An electric motor comprising a stator as described in any one of Appendices 1 to 13 and a rotor configured to rotate relative to said stator.

[0095] Explanation of reference numerals in the attached figures

[0096] 2: Electric motor; 4: Stator; 8: Rotor; 10: Coil; 12: Slot; 14, 15: Flow path; 16: Core; 18: Coil end; 20: Molded resin part; 22: Housing; 24: Inlet; 26: Outlet; 30: Annular flow path; 32: Axial end face; 34: Recess; 36: Protrusion; 38a: Inner core; 38b: Outer core; 40: Electromagnetic steel plate; 44: Burr.

Claims

1. A stator, wherein, The stator has the following features: coil; The core has slots for arranging the coils and flow paths for the refrigerant; A molded resin portion, which at least partially embeds the axial end of the coil; and The inlet and outlet are formed in the axial region of the coil end between the axial end face of the core and the axial top end of the axial end of the coil, and are openings on the side of the core where the groove is not formed, and are fluidly connected to the flow path.

2. The stator according to claim 1, wherein, The inlet and outlet are defined by the axial end face of the core and the molded resin portion.

3. The stator according to claim 1 or 2, wherein, The axial end of the coil extends radially toward the inlet or outlet side, relative to the portion of the flow path that is furthest from the inlet or outlet in the radial direction.

4. The stator according to any one of claims 1 to 3, wherein, The molding resin section has an annular refrigerant flow path that is fluidly connected to the inlet or outlet, and the refrigerant flow path is fluidly connected to a plurality of flow paths respectively.

5. The stator according to any one of claims 1 to 4, wherein, The flow path extends axially within the core and is positioned at the same circumferential angle as the groove.

6. The stator according to claim 5, wherein, The radial cross-section of the flow path has an elliptical shape that is relatively long in the circumferential direction.

7. The stator according to claim 5, wherein, The radial cross-section of the flow path is approximately triangular in shape with a vertex whose circumferential angular position is equal to that of the center of the groove.

8. The stator according to any one of claims 1 to 7, wherein, The stator has a groove formed on the surface of the core and extending axially, located between circumferential angular positions of adjacent grooves in the radial section of the core.

9. The stator according to any one of claims 1 to 8, wherein, The molded resin portion has a recess that is radially recessed toward the coil side compared to the portion of the flow path that is radially furthest from the inlet or outlet.

10. The stator according to any one of claims 1 to 9, wherein, The molded resin portion has a protrusion at the same circumferential angle position as the inlet, which protrudes radially toward the side opposite to the coil.

11. The stator according to claim 1, wherein, The core has: a first core portion having the groove; and a second core portion having an inner diameter surface fixed to the outer diameter surface of the first core portion or an outer diameter surface fixed to the inner diameter surface of the first core portion and forming the flow path.

12. The stator according to any one of claims 1 to 11, wherein, The core is composed of multiple stacked electromagnetic steel plates, which are stacked in such a way that the protruding direction of the burrs on each electromagnetic steel plate is the same.

13. The stator according to any one of claims 1 to 12, wherein, The side of the core in which the groove is formed is covered with the same material as the material forming the molding resin portion.

14. An electric motor, wherein, The stator comprising any one of claims 1 to 13 and a rotor configured to rotate relative to the stator.

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