Bearing for an electrical machine

Abstract

A wound-rotor electrical machine (100, Fig. 1) bearing 700A (also 130, Fig.1 and 230, Fig. 8) comprising electrically conductive rotor 320 and stator 310 parts, which act as terminals or plates of a capacitor, separated by a dielectric or an electrolyte. Each part has a connection which provides current, perhaps from an AC supply (250, Fig. 8), to the rotor through the capacitive bearing (230, Fig. 8). The bearing may include rotatable elements 330 such as balls or rollers in an axially central region (350, Fig. 4B), alongside distal regions (360, Fig. 4B). The central region may contain lubricant and be sealed (375, Fig. 4B) from the distal regions which contain electrolyte, or lubricant may be shared between regions (Fig. 4C). The rolling elements may: have a coating of a lubricant; be electrically insulating or have an insulating coating. The electrolyte may form a double layer supercapacitor (Fig. 5, 620 Fig. 6). The rotor and stator parts may have opposing surfaces 315, 325, of equal or unequal (Fig. 7B) areas. The surfaces may have: interlocking protrusions 317, 327; a non-corrosive coating, perhaps of titanium. The rotor may have a rectifier (240, Fig. 8) and permanent magnets (290, Fig. 9).

Description

Field of the Invention

[0001] The invention relates generally to a bearing for an electrical machine and an electrical machine, more specifically to a bearing for an externally excited electrical machine and an externally excited electrical machine. Background

[0002] Electrical machines, including electrical motors and electrical generators, can take many different forms. Each of these forms have particular advantages and disadvantages, and so may be suitable for different purposes and applications. Examples of different forms of electrical motor include induction motors, permanent magnet motors, and externally excited electrical motors. Each of these types of motors can generate torque from an input AC current, but do so in different ways.

[0003] For example, induction motors create a rotating magnetic field to induce a current in conductors through electromagnetic induction, while permanent magnet motors utilise permanent magnets to create a fixed magnetic field which aligns itself with an oscillating magnetic field to create torque. In contrast, externally excited electrical motors use an external power source to pass a current through rotor windings to create a stationary magnetic field, while an oscillating magnetic field produced by stator windings causes the rotor to rotate via an electromotive force.

[0004] While externally excited synchronous electrical machines are useful in many contexts, existing implementations have a significant number of drawbacks which limit their use to a comparatively small number of applications. The present invention addresses at least some of the issues presented by existing externally excited synchronous electrical machines. Summary of the Invention

[0005] Aspects of the invention are set out in the accompanying claims.

[0006] According to a first aspect of the invention, there is provided a bearing for an electrical machine, the bearing comprising: a rotor part configured to be fixable to a rotor of an electrical machine or a rotor shaft of the rotor of the electrical machine, the rotor comprising rotor windings; and a stator part configured to be fixable to a housing of the electrical machine; wherein the rotor part and stator part are each formed of an electrically conductive material and are arranged to act as opposing terminals of a capacitor or a double-layer supercapacitor; wherein the bearing includes a dielectric material or an electrolyte located between the rotor part and the stator part; wherein the rotor part comprises an electrical connection for providing an input current for the rotor windings, or receiving an output current from the rotor windings; and wherein the stator part comprises an electrical connections for receiving an input current for the bearing, or providing an output current from the bearing.

[0007] As such, power can be effectively transferred to rotor windings of an electrical machine through capacitive coupling using a bearing. Therefore, the use of brushes in the electrical machine can be eliminated (i.e. a brushless electrical motor is provided). As such, a wider range of cooling method may be used in the electrical machine (e.g. ATF cooling), and a separate chamber for said brushes is not required, thereby reducing the size and weight of the electrical machine, and reducing maintenance requirements. Moreover, the drag of the electrical machine is reduced, and the overall cost of the electrical machine is reduced, as fewer components are required. Moreover, the structure of the bearing is such that the capacitance of the bearing is sufficiently large in order to allow capacitive coupling power transfer to occur at specific voltage frequencies, thereby increasing the current density within the rotor windings, and hence the strength of the magnetic field produced, thereby increasing the output of the electrical machine.

[0008] In examples, the bearing comprises a plurality of rotatable elements arranged between the rotor part and the stator part in a central region of the bearing, and wherein the bearing further comprises one or more distal regions located adjacent to the central region in an axial direction. As such, the capacitance of the bearing may be increased through the use of one or more distal regions, in which opposing surfaces of the stator part and rotor part may be arranged to oppose one another.

[0009] In some cases, the rotatable elements are formed of an electrically insulating material and / or comprise an electrically insulating coating, cover or seal. As such, current can be prevented from passing conductively from the stator part to the rotor part, such that the bearing functions as a capacitor. In addition, current may be prevented from flowing between the bearing stator part and the electrical machine stator housing itself, and current may be prevented from flowing between the rotor part of the bearing and the rotor shaft or rotor body. Instead, an insulated electrical connection may be made between the power supply and the stator part of the input side bearing, between the rotor part of the input side bearing and rectifier circuitry, between the rectifier circuitry and the rotor part of the output side bearing, and between the stator part of the output side bearing and the power supply.

[0010] In some cases, the rotatable elements (e.g. bearing balls or rollers) comprise a coating of a lubricating material. Accordingly, the rotation of the rotor part with respect to the stator part is able to occur with reduced friction, thereby increasing the efficiency of the bearing.

[0011] Advantageously, the bearing may include the electrolyte located between the rotor part and the stator part, and wherein the combination of the rotor part, the stator part and the electrolyte are configured to cause the bearing to function as a supercapacitor. As the bearing functions as a supercapacitor, the capacitance of the bearing may be significantly increased. This not only improves the efficiency of power transfer to the rotor windings, but allows the bearing itself to be reduced in size, as the size of the distal regions may be reduced. This reduces the capacitance of the bearing, but is compensated for by the increase in capacitance provided by operating the bearing as a supercapacitor. Furthermore, the required frequency for efficient power transfer to the rotor windings may be reduced.

[0012] In some cases, the central region of the bearing is hermetically sealed from two distal regions of the bearing located on either side of the central region in an axial direction, and wherein the two distal regions each comprise an electrolyte located between the rotor part and the stator part, and wherein the central region comprises a lubricant located between the rotor part and the stator part. Accordingly, the capacitance of the bearing may be increased while promoting the smooth rotation of the rotor part of the bearing relative to the stator part.

[0013] Alternatively, in some cases the central region of the bearing is in fluid communication with two distal regions of the bearing located on either side of the central region in an axial direction, wherein the bearing comprises a lubricant located between the rotor part and the stator part. Accordingly, a bearing is provided with high capacitance but which is arranged to have smooth, low-friction rotation of the relative parts.

[0014] In certain examples, the lubricant is an automatic transmission fluid, which provides simplicity as automatic transmission fluid may be used in cooling or lubrication of other components in the electrical machine. This also reduces sealing requirements for the bearing.

[0015] In some cases, the bearing comprises sealing components arranged at axial ends of the bearing. Accordingly, liquid-based cooling may be used for the stator and rotor windings of the electrical machine (e.g. within a housing of the electrical machine), which allows for more effective cooling of the electrical machine.

[0016] Advantageously, the rotor part and the stator part each include an opposing surface, wherein the opposing surface of the rotor part is arranged to oppose the opposing surface of the stator part, and wherein the opposing surfaces of the rotor part and stator part have complimentary shapes. As such, the capacitance of the bearing may be increased.

[0017] In some cases, the opposing surfaces each comprise a coating of a non-corrosive material. Accordingly, an electrolyte may be used to increase the capacitance of the bearing, without causing degradation of the bearing due to corrosion. An example of such a non-corrosive material is titanium or stainless steel.

[0018] Advantageously, each opposing surface may include a plurality of protrusions and recesses, wherein the protrusions and recesses of the opposing surfaces of the rotor part and stator part are arranged to interlock one another. As such, the capacitance of the bearing may be increased without increasing the size of the bearing in an axial direction, thereby minimising the size of the electrical machine as a whole.

[0019] In certain examples, a surface area of the opposing surface of the rotor part is greater than a surface area of the opposing surface of the stator part. This may further increase the capacitance of the bearing.

[0020] According to a second aspect of the invention, there is provided an electrical machine comprising: a stator comprising stator windings; a rotor comprising rotor windings; and a first bearing as described above, the first bearing being arranged between the rotor and the housing of the electrical machine, wherein the rotor part of the first bearing is fixed to the rotor of the electrical machine, and wherein the stator part of the first bearing is fixed to the housing of the electrical machine; wherein the electrical connection of the rotor part of the first bearing is electrically connected to the rotor windings (via rectifier circuitry to provide a DC current to the rotor windings), and wherein the electrical connection of the stator part of the first bearing is configured to be electrically connected to an AC current source, or return path or ground. The electrical machine, according to the second aspect, may be reduced in size through the use of the bearing while providing efficient power transfer to the rotor windings, thereby increasing the efficiency of the electrical machine as a whole.

[0021] In some cases, the electrical machine further comprises a second bearing as described above, the second bearing being arranged between the rotor and the housing of the electrical machine, wherein the rotor part of the second bearing is fixed to the rotor of the electrical machine, and wherein the stator part of the second bearing is fixed to the housing of the electrical machine; wherein the electrical connection of the rotor part of the first bearing is electrically connected to the rotor windings via rectifier circuitry, and wherein the electrical connection of the stator part of the second bearing is configured to be electrically connected to a return path or ground; wherein the electrical connection of the stator part of the first bearing is configured to be electrically connected to an AC current source; and wherein the electrical connection of the rotor part of the first bearing is electrically connected to the rotor windings via rectifier circuitry. Accordingly, current can be efficiently transferred to the rotor windings in a manner which minimises the size of the electrical machine.

[0022] In some examples, the rotor further comprises rectifier circuitry configured to convert a first AC current provided by the electrical connection of the rotor part of the first bearing to a DC current to be provided to the rotor windings. As such, AC power can be capacatively supplied to the rotor, before being converted to DC current for supply to the rotor windings. As such, a constant current can be supplied to the rotor windings, leading to the generation of a constant magnetic field.

[0023] In certain examples, the electrical connection of the rotor part is configured to receive the first AC current, wherein the AC current has a predefined set frequency; and wherein the stator windings are arranged to receive a second current AC current, wherein the second current AC current is variable in frequency. As such, the AC current supplied to the bearings may have a set frequency which maximises the efficiency of the wireless power transfer via the bearings, while the frequency of the current supplied to the stator windings may be adjusted to suit the requirements of the electrical machine, such as the rotational speed of the rotor of the electrical machine.

[0024] In some examples, the electrical machine further comprises: first power circuitry configured to provide the first AC current to the electrical connection of the stator part of the first bearing. Additionally, in some cases the electrical machine further comprises: second power circuitry configured to provide the second AC current to the stator windings, wherein the second power circuitry is configured to control a frequency and / or peak amplitude of the second AC current according to a power demand for the electrical machine.

[0025] In certain examples, the rotor and / or stator additionally comprise a plurality of permanent magnets. Accordingly, the excitation magnetic field within the electrical machine can be further increased, thereby increasing the efficiency of the electrical machine. Alternatively, the permanent magnets may allow the excitation magnetic field within the electrical machine to be decreased in some cases, as the permanent magnetic flux provided by the permanent magnets may counteract the excitation magnetic field from the rotor windings, particularly at higher rotation speeds. Brief Description of the Drawings

[0026] Embodiments of the invention will now be described, by way of example only, with reference to the following figures.

[0027] In accordance with one (or more) embodiments of the present invention the Figures show the following:

[0028] Figure 1 depicts an electrical machine having stator windings and rotor windings, where the rotor windings are supplied with a DC current through brushes.

[0029] Figure 2 depicts an example electrical machine according to the present disclosure, including a plurality of bearings arranged to act as capacitors in order to provide current to the rotor windings of the electrical machine.

[0030] Figure 3A illustrates a view of a bearing according to an example of the present disclosure, looking along an axial direction of the bearing.

[0031] Figure 3B illustrates a cross-sectional view of the bearing shown in Figure 3A along the line A-A shown in Figure 3A.

[0032] Figure 4A illustrates a cross-sectional view of a bearing according to a further example of the present disclosure.

[0033] Figure 4B illustrates a cross-sectional view of a bearing according to a further example of the present disclosure.

[0034] Figure 4C illustrates a cross-sectional view of a bearing according to a further example of the present disclosure.

[0035] Figure 5 illustrates a close-up view of the bearings shown in Figures 4B and 4C, showing how the bearing behaves as a supercapacitor.

[0036] Figure 6 illustrates an equivalent circuit diagram for powering the rotor windings of an electrical machine via two supercapacitors.

[0037] Figure 7A illustrates a cross-sectional view of a bearing according to a further example of the present disclosure.

[0038] Figure 7B illustrates a cross-sectional view of a bearing according to a further example of the present disclosure.

[0039] Figure 8 illustrates a circuit diagram for providing current to rotor windings of an electrical machine via capacitors according to examples of the present disclosure.

[0040] Figure 9 illustrates an example hybrid excitation electrical machine according to the present disclosure.

[0041] Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field. As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”. The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by these examples. It will also be recognised that the invention covers not only individual embodiments but also combination of the embodiments described herein.

[0042] The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc, other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future. Detailed Description

[0043] Figure 1 shows an example of an externally excited electrical motor 100. The motor 100 includes a housing 110 within which stator laminations 111 are mounted. The motor 100 additionally includes a rotor shaft, to which rotor laminations 121 are mounted. The rotor shaft 120 (and hence rotor laminations 121) are configured to be rotatable relative to the housing 110 and stator laminations 111, with bearings 130 arranged between the housing 110 and the rotor shaft 120. The housing 110 defines a chamber within which the stator laminations 111 and rotor laminations 121 are located. Stator windings including and extending between end windings 112 may be fixed to the stator laminations 111, and rotor windings =including and extending between end windings 122, may be fixed to the rotor laminations.

[0044] The motor additionally includes a secondary chamber 140 located within the housing 110, where the secondary chamber 140 is hermetically sealed from the chamber 115. The secondary chamber 140 includes a plurality of slip rings 143 arranged on the rotor shaft 120, and brushes 141 arranged to contact the slip rings 143 in order to pass electrical current between the brushes 141 and the slip rings 141. More specifically, an electrical current may pass from the brush 141A to slip ring 143A and vice versa, and an electrical current may pass from slip ring 143B to brush 141B and vice versa. As such, electrical current may be passed from an external power source 150 to the rotor shaft 120 via the brushes 141 and slip rings 143. In the example of Figure 1, the slip ring 143A is electrically connected to a first end winding 122A of the rotor windings, and the slip ring 143B is connected to a second end winding 122B of the rotor windings. As such, a DC current may pass from an external power source 150 through brush 141A, slip ring 143A, the rotor windings, slip ring 143B, and brush 141B, where the output current may exit the electrical machine 100 (i.e. return to the power source 150). In this way, the rotor windings may be provided with a DC current of fixed amplitude by an external power source 150. The rotation of the rotor causes an oscillation in the electrical field of the electrical machine with respect to the stator. Accordingly, by additionally providing a moving (e.g. rotating or oscillating) magnetic field through supplying a current to the stator windings, the rotor may be made to rotate through an electromotive force, thereby generating output torque through the rotor shaft 120.

[0045] As briefly discussed above, the brushes 141 and slip rings 143 are located within the secondary chamber 140 which is sealed from the chamber 115 provided by the housing 110. The secondary chamber 140 is provided to prevent liquids (such as lubricants and / or coolants) within the chamber 115 from becoming located between the brushes 141 and the slip rings 143. Such liquids may be electrically insulating and as such may sever the electrical connection between the brushes 141 and the slip rings 143 if located therebetween. The presence of the secondary chamber 140 and the brushes 141 and slip rings 143 therein increases the size of the electrical machine. An alternative approach is to use forced air cooling, which may avoid the need for a secondary chamber 140, however such machines suffer from reduced cooling performance.

[0046] Accordingly, in order to avoid the need for a separate chamber 140, as well as the need for the slip rings 141 and brushes 143, wireless power transfer may be used to provide an electrical current to the rotor shaft 120, and hence rotor end windings 122. Figure 2 shows an electrical machine 200 according to an example of the present disclosure. The electrical machine 200 includes a housing 210 which defines a chamber 215 within which stator laminations 211 (which may be fixed to the housing 210 e.g. directly or indirectly) and rotor laminations 221 are located. Rotor windings may be affixed to the rotor shaft 220, e.g. via the rotor laminations 221, which passes through the housing 210. Stator windings may be affixed to the stator laminations 211, and include end windings 212, and the rotor windings include end windings 222. The electrical machine 200 additionally includes bearings 230 located between the housing and the rotor shaft 220. The bearings 230 are arranged to allow the rotor shaft 220 (and hence rotor windings) to rotate relative to the housing 210 (and hence stator windings). The rotor windings and stator windings are not shown in full for ease of illustration, but extend between the respective end windings for the rotor windings and stator windings.

[0047] According to examples of the present disclosure, one or more of the bearings 230 of the electrical machine 200 are arranged to act as capacitors to be used for wireless power transfer to the rotor end windings 222. That is, bearing 230A may be arranged to act as a capacitor to allow power transfer from a power source 250 to the rotor end winding 222A, and / or bearing 230B may be arranged to act as a capacitor to allow power transfer from the power source 250 to the rotor end windings 222B. More specifically, capacitor 230A may be electrically connected to an AC power source 250 and to rectifier circuitry 240 which converts the AC current to DC current (note that the connection between the capacitor 230A and rectifier circuitry 240 in Figure 2 is shown as not extending fully to the rectifier circuitry 240 only for ease of illustration, and it should be appreciated that the capacitor 230A is electrically connected to the rectifier circuitry 240. The rectifier circuitry 240 is connected to the rotor windings (i.e. connected to both end windings 222), and capacitor 230B may be electrically connected to the rectifier circuitry 240, such that power can flow from the power source 250 to the bearing 230A, through the rectifier circuitry 240, rotor windings, back to the rectifier circuitry 240, and bearing 230B, where the output current may exit the electrical machine 200 (i.e. return to the power source 250). The power source 250 may provide an AC current which is converted to a DC current by the rectifier circuity 240 (e.g. affixed to the rotor or located within the rotor or rotor shaft 220), before passing through the rotor windings . The rectifier circuitry 240 may similarly ensure that the output (return) current is also AC. While both bearings 230 may act as capacitors in the example of Figure 2, in some examples only one of bearings 230A, 230B may be arranged to act as a capacitor, while the other bearing is a conventional bearing which does not act as a capacitor. In such examples, other power transfer means (such as a slip ring and a brush, or other wireless power transfer means such as electromagnetic induction) may be used in place of the non-capacitor bearing in order to transfer power to the rotor windings.

[0048] The use of capacitive power transfer for the rotor shaft 220 means that no physical contacts (e.g. brushes) between the rotor shaft 220 and power supply circuitry are required. This provides greater reliability and reduces the size of the electrical machine 200, while allowing the windings of the electrical machine 200 to be cooled using non-electrically conductive liquids (such as automatic transmission fluid (ATF)). Furthermore, by using the bearings 230 as capacitors, the overall size of the electrical machine 200 can be further reduced by using fewer components.

[0049] The bearings 230 may take a variety of forms. For example, the bearings 230 may be a cylindrical roller bearings, linear bearings, deep groove ball bearings, or any other variety of bearing. An example bearing 300 is shown in Figure 3A, looking along the axial direction of the bearing 300. The bearing 300 includes a stator part 310 configured to be attached to the housing 210 of the electrical machine 200, and a rotor part 320 configured to be attached to the rotor shaft 220 of the electrical machine 200. That is, the bearing has a central cavity 340 configured to interface with the rotor shaft 220 of the electrical machine 200. The bearing 300 may be formed in two or more pieces for initial placement around the rotor shaft 220, where the two or more pieces may be fastened together around the rotor shaft 220 using fixing means (such as one or more clamps). Alternatively, the bearing 300 may be formed in a single piece and may be slid into place from an end of the rotor shaft 220.

[0050] Arranged between the stator part 310 and the rotor part 320 are one or more rotatable elements 330 (e.g. in the form of balls or rollers) which allow the rotor part 320 to rotate relative to the stator part 310. The stator part 310 includes an inner surface 315 configured to at least partially oppose an inner surface 325 of the rotor part 320 as opposing plates of a capacitor. That is, the inner surface 315 and inner surface 325 may be arranged parallel to one another at least for a portion of their total surface areas. The bearings 230 may be formed of an electrically insulating material (such as a ceramic material), and the inner surface 315 and inner surface 325 may be electrically conductive (e.g. formed of an electrically conductive material, or coated in an electrically conductive material). The stator part 310 may additionally include one or more electrical connections (not shown) for connecting the inner surface 315 of the stator part 310 to the power source 250, while the rotor part 320 may include one or more electrical connections (not shown) for connecting the inner surface 325 of the rotor part 320 to the rotor windings. The rotatable elements 330 are formed of an electrically insulating material (e.g. a ceramic material), or are at least partially coated in an electrically insulating material (e.g. an inner cavity proximal to the rotatable portions of the bearing may be coated with an electrically insulating material. Accordingly, current is prevented from passing conductively from the stator part 310 to the rotor part 320 via the rotatable elements 330.

[0051] Figure 3B shows a view of the bearing 300 shown along the line A-A in Figure 3A. Figure 3B shows the stator part 310 with its inner surface 315 and the rotor part 310 with its inner surface 325, with the rotatable elements 330 arranged between the stator part 310 and the rotor part 320. The central cavity 340 is also shown. The inner surfaces 315, 325 define a channel 380, which is open and in which lubricants maybe located. The lubricant may be any suitable lubricant, such as automatic transmission fluid, or any other high-viscosity liquid, such as a grease.

[0052] The bearing 300, includes a central region 350 (i.e. central axial region of the bearing 300) in which the rotatable elements 330 are located. In the central region 350, the inner surfaces 315, 325 are shaped to oppose the rotatable elements 330, in order to allow the rotor part 320 to move relative to the stator part 310. The bearing 300 additionally includes one or more distal regions 360 (i.e. distal axial regions of the bearing 300) located adjacent the central region in an axial direction. In the distal regions 360, the inner surfaces 315 and 325 are closer to one another than in the central region 350 and are arranged parallel to one another. While Figure 3B shows a distal region 360 either side of the central region 350, it should be appreciated that the region containing the rotatable elements 330 may be located in a distal region of the bearing 300. As such, the bearing 300 may be generally considered to include a first region in which the rotatable elements 330 are located, and one or more second regions in which the inner surfaces 315, 325 are arranged opposing (i.e. parallel to) one another. The remainder of the application discusses the central region 350 and distal region 360, however at each occurrence of these terms, it should be appreciated that the central region 350 may be any first region, and the distal region 360 may be any second region.

[0053] In having the inner surfaces 315, 325 opposing one another as shown in Figure 3B, the bearing 300 may be used as a capacitor. The capacitance created by the opposing inner surfaces 315, 325 of the bearing 300 may in some cases be too small for certain uses (i.e. on the order of pF or nF). The optimum AC current frequency for efficient capacitive wireless power transfer depends upon the capacitance of the capacitor. Accordingly, if the opposing inner surfaces 315, 325 of the bearing 300 result in a small capacitance (e.g. on the order of pF or nF), the optimal AC current frequency will be high (e.g. on the order of MHz or GHz). Such a high frequency may in some cases be difficult to achieve within the context of wireless power transfer in electrical machines. Accordingly, approaches are provided herein for increasing the capacitance of the bearing 300.

[0054] Figure 4A shows an example bearing 400A according to an example of the present disclosure. The bearing 400A includes a similar layout to the bearing 300 shown in Figures 3A and 3B, with like reference numerals representing like components. In addition to the components and elements shown in Figures 3A and 3B, the bearing 400A includes sealing components 375 which seal the channel 380 defined by the inner surfaces 315, 325. More specifically, the sealing components 375 seal (e.g. hermetically seal) the distal region 360 of the channel 380 from the central region 350. As such, the central region 350 is sealed to include a closed channel. Accordingly, a lubricant may be located within the central region 350 without leaking into the distal regions 360, which may lead to a decrease in capacitance in the distal regions 360. The sealing components 375 may be any suitable sealing components for sealing the channel 380 of the bearing 400A, such as a rubber seal or a ceramic tube, or any other suitable sealing component. The sealing components may in some cases be inserted into the channel 380, as shown in Figure 4A, and / or may extend along an outer surface (e.g. outer axial or circumferential / radial surface) of the bearing in order to electrically insulate the bearing, as well as seal the channel 380. Furthermore, as shown in Figure 4A the channel 380 may include a dielectric material 385 located between the inner surfaces 315, 325. For example, the dielectric material 385 may be in contact with one or both of the inner surfaces 315, 325. The dielectric material 385 may be any suitable dielectric material for use in the bearing 400A as a capacitor. One example of a dielectric material is Barium titanate, however other materials may be used. The presence of the dielectric material 385 increases the capacitance of the bearing 400A, thereby reducing the optimal frequency for wireless power transfer, thereby increasing the power that can be transferred to the rotor windings.

[0055] The bearing 400A represents just one example implementation for providing increased capacitance. Figure 4B shows another example bearing 400B with increased capacitance. The bearing 400B includes a similar layout to the bearing 300 shown in Figures 3A and 3B, with like reference numerals representing like components. The bearing 400B also includes the sealing components 375 of the bearing 400B of Figure 4A (e.g. a rubber seal or ceramic tube). The bearing 400B additionally comprises second sealing components 370 arranged to seal the channel 380 from the outside. More specifically, the second sealing components 370 are arranged at a periphery of the channel to seal the distal regions 360 from the environment. As such, the central region 350 includes a closed channel, and the distal regions 360 each include a closed channel. Within the central portion 350, a lubricant may be located, as discussed above in relation to the bearing 400A of Figure 4A. Furthermore, if the distal regions 360 include a dielectric material 385 (as in Figure 4A), the second sealing component liquids 370 prevents exposure of the dielectric material 385 to other fluids located within the housing 110 of the electrical machine. The sealing components 370 may be any suitable sealing components for sealing the channel 380 of the bearing 400A, such as a rubber seal or ceramic tube, or any other suitable sealing component.

[0056] In addition, one or more of the distal regions 360 may include an electrolyte (for example instead of a dielectric material 385). An electrolyte is a liquid comprising ions (i.e. positive ions and negative ions). The electrolyte may be any aqueous solution suitable for use as an electrolyte in a supercapacitor, such as sodium sulphide, or sodium chloride, however other aqueous solutions are contemplated. The presence of the electrolyte within the distal region 360 between the inner surfaces 315, 325 causes the bearing 400B to behave as a supercapacitor (i.e. a double-layer capacitor). Accordingly, the capacitance of the bearing 400B may be significantly increased, thereby reducing the optimal frequency for wireless power transfer, thereby increasing the power that can be transferred to the rotor windings. In order to prevent corrosion due to the electrolyte, the inner surfaces 315, 325 may be coated in (or formed of) a non-corrosive material, such as titanium, however other non-corrosive materials may be used. Moreover, the axial length of the bearing 400B may be reduced, as the reduction in capacitance caused by reducing the axial length is more than compensated for by the increased capacitance of the bearing as a supercapacitor.

[0057] The bearing 400B represents just one example implementation for providing increased capacitance through configuration as a supercapacitor. Figure 4C shows another example bearing 400C arranged to behave as a supercapacitor. The bearing 400C includes a similar layout to the bearing 400B shown in Figure 4B, with like reference numerals representing like components. The bearing 400C does not, however, include the sealing components 375 arranged to seal the central region 350 from the distal regions 360. Accordingly, a single channel exists within the bearing 400C between the sealing components 375. In some cases, the rotatable elements 330 may be coated with a lubricating coating. As such, a liquid lubricant may not be used. Accordingly, the channel 380 may comprise an electrolyte, such that the electrolyte is located within both the distal regions 360 and the central region 350. The presence of the electrolyte within the channel 380 between the inner surfaces 315, 325 causes the bearing 400C to behave as a supercapacitor. Accordingly, the capacitance of the bearing 400C may be significantly increased, thereby reducing the optimal frequency for wireless power transfer, thereby increasing the power that can be transferred to the rotor windings.

[0058] The functioning of the bearing as a supercapacitor is described in more detail in relation to Figure 5. Figure 5 shows the stator part 310, the inner surface 315 of the stator part 310, the rotor part 320, and the inner surface 325 of the rotor part 320. Furthermore, the channel 380 between the inner surfaces 315, 325 is filled with an electrolyte comprising ions 510 (i.e. positive ions 510B, and negative ions 510A). When connected to the power source 250, the stator part 310 may be held as a higher potential (i.e. voltage) than the rotor part 320. As such, the inner surface 315 of the stator part 310 may be considered to be relatively positively charged, while the inner surface 325 of the rotor part 320 may be considered to be relatively negatively charged. Accordingly, the negative ions 510A in the electrolyte migrate towards the stator part 310, and the positive ions 510B within the electrolyte migrate towards the rotor part 320. However, as the ions 510 remain dissolved within the electrolyte, current does not flow between the stator part 510 and the rotor part 520. Accordingly, the negative ions 510A are maintained opposite the positively charged stator part 310 (or inner surface 315 thereof), while the positive ions 510B are maintained opposite the negatively charged rotor part 320 (or inner surface 325 thereof), thereby creating effective capacitors at the inner surfaces 315, 325. Accordingly, a supercapacitor (i.e. double-layer capacitor) is created within the bearing. If the power source 250 is reversed (as in AC current) and the relative voltage of the inner surfaces 315, 325 is reversed, the ions 510 move to the opposite inner surfaces 315, 325.

[0059] Figure 6 shows an equivalent circuit diagram for an electrical machine having two bearings acting as supercapacitors. Within a first bearing, a first supercapacitor 620A is formed. A first capacitor 621A is created between the stator part and the electrolyte, the electrolyte acts as a resistor 622A, and a second capacitor 623A is created between the electrolyte and the rotor part. Current may then pass through a diode bridge formed by diodes 640A-D, which converts the AC current from the power supply 250 to DC current which is supplied to the rotor windings 630. Figure 6 shows the rotor windings 630 as having both an inductive component and a resistive component. Current (that is, AC current) may additionally pass from the diode bridge through a second bearing, where a second supercapacitor 620B is formed. A third capacitor 623B is created between the rotor part and the electrolyte, the electrolyte acts as a resistor 622B, and a fourth capacitor 621B is created between the electrolyte and the stator part. The current may then return to the power source 250.

[0060] Figure 7A shows a bearing 700A according to a further example of the present disclosure. The bearing 700A includes a similar layout to the bearing 400C shown in Figure 4C, with like reference numerals representing like components, however it should be noted that the example of Figure 7A is also compatible with arrangements similar to those in Figure 4A including dielectric materials 385, and with arrangements similar to those in Figure 4B including sealing components 375, or other arrangements which do not include an electrolyte or a dielectric material between the stator part 310 and the rotor part 320. In the example of Figure 7A, the inner surfaces 315, 325 are shaped in the distal regions 360 to include one or more protrusions 317 and / or recesses. In particular, rather than the inner surfaces 315, 325 extending in an axial direction, the inner surfaces 315, 325 may extend in alternating radial directions (i.e. have a plurality of radially extending portions 390 (radial portions)) with one or more axial portions (extending in an axial direction) therebetween. The inner surfaces 315, 325 may be arranged to oppose each other (i.e. parallel to one another), as shown in Figure 7A, such that the protrusions 317, 327 of the inner surfaces 315, 325 are arranged to interlock one another. Moreover, in some examples the channel 380 may end at a radial edge of the bearing 700A, rather than an axial edge (as in the preceding examples). In including the one or more protrusions 317, 327 in the inner surfaces 315, 325, the surface area of the capacitor is increased, thereby increasing the capacitance of the bearing 700A. In addition, as shown in Figure 7A, as the channel 380 opens at an outer radial surface of the bearing, the sealing component 370 may extend across the outer radial surface in order to electrically insulate the bearing 700A in addition to sealing the channel 380.

[0061] Figure 7B shows a further example bearing 700B according to the present disclosure. The bearing 700B includes a similar layout to the bearing 400B shown in Figure 4B, with like reference numerals representing like components. In the bearing 700B of Figure 7B, the inner surfaces 315, 325 include protrusions 317 and / or recesses in a similar manner to the bearing 700A, however the inner surface 315 of the stator part 310 has a different total surface area to the inner surface 325 of the rotor part 320. For example, as shown in Figure 7B, the inner surface 325 of the rotor part 320 may not include any protrusions or recesses (in other words the inner surface 325 of the rotor part 320 may not include any radially extending portions. As such, the inner surface 325 of the rotor part 320 may have a smaller surface area than the inner surface 315 of the stator part 310. As another example, the inner surface 325 of the rotor part 320 may have protrusions as in Figure 7A, however the protrusions 317 of the stator part may have a larger axial length than the protrusions 327 of the rotor part 320, such that the inner surface 315 of the stator part 310 has a larger surface area than the inner surface 325 of the rotor part 320, which may further increase the capacitance of the bearing 700B. Alternatively, in some examples, the inner surface 325 of the rotor part 320 may have a larger surface area than the inner surface 315 of the stator part 310. It should be noted that the inner surfaces 315, 325 may have substantially any number of protrusions 317 and recesses 327.

[0062] Electrical machines according to the present disclosure may include one or more of the bearings described herein. Moreover, in some cases an electrical machine may include two bearings according to the same example described herein, or may include multiple different types of example bearing. As discussed briefly with respect to Figure 2, the rotor of an electrical machine including any of the bearings described herein may include rectifier circuitry 240 configured to converts AC current to DC current, such that power is transferred to the rotor as AC current, but is provided to the rotor windings as DC current. Figure 8 shows a simplified circuit diagram of this arrangement. The AC power source 250 is connected to a first bearing 230A according to the present disclosure. The AC power source is connected to the stator part 310A of the first bearing 230A, which is configured to act as a capacitor. Accordingly, current is permitted to flow from the rotor part 320A of the first bearing 230A to the rotor 220. In the rotor 220, the AC current passes to the rectifier circuitry, which includes four diodes 820A-D arranged in an H-bridge, and may additionally include a smoothing capacitor 830 configured to smooth the resulting DC current. The DC current passes through the rotor windings 630 to the diode bridge and then to the rotor part 320B of the second bearing 230B, which is configured to act as a capacitor. Accordingly, current is permitted to flow from the rotor part 310B of the second bearing 230B to the rotor 220. In this way, power may be wirelessly transferred to the rotor 220 via the bearings according to the present disclosure, in order to pass a current through the rotor windings and provide an efficient externally excited electrical machine 200. As in Figure 6, Figure 8 shows the rotor windings 630 as having both an inductive component and a resistive component.

[0063] The power source 250 may comprise electrical inverter circuitry configured to provide an AC current. Similarly, the power source arranged to provide current to the stator windings of the electrical machine 200 when operating as an electrical motor may also comprise electrical inverter circuitry. The power source 250 arranged to provide a current to the rotor windings may be separate from a power source arranged to provide current to the stator windings of the electrical machine 200 when operating as an electrical motor. In particular, while the power source for the stator windings of the electrical motor 200 may be variable in frequency in order to adjust the output of the motor according to a desired torque, the power source 250 may be arranged to provide a constant-frequency current. That is, while the amplitude of the current provided by the power source 250 varies with time, the frequency of the output current is constant. The frequency of the current provided by the power source 250 may be predetermined in order to provide maximal power transfer efficiency, based on the capacitance of the bearing(s) 230, and the resistance and / or inductance of the rotor windings.

[0064] While Figure 2 and the above examples describe the use a bearing according to the present disclosure within an externally excited electrical machine, it should be appreciated that said bearings may be used within a variety of forms of electrical machine. For example, the externally excited electrical machine may be an electrical motor, or an electrical generator. Furthermore, the electrical machine may be an electrical machine where an excitation magnetic field is provided solely by passing a current through rotor windings, or a so-called hybrid excitation electrical machine where a combination of rotor windings and permanent magnets are used to provide an excitation magnetic field. An example is shown in Figure 9, which depicts a hybrid excitation electrical machine 900. The electrical machine 900 has a similar structure as the electrical machine 200 of Figure 2, with like reference numerals representing like components. In addition, the electrical machine 900 includes permanent magnets 290 arranged to provide a constant magnetic field to excite the rotor windings . For example, the electrical machine 900 may include one or more permanent magnets 290 located within (or on) the rotor of the electrical machine 900 and / or the electrical machine 900 may include one or more permanent magnets (not shown) within (or on) the stator of the electrical machine 900. As such, a further excitation magnetic field may be provided, and as such the bearings according to the present disclosure should be seen as being compatible with a variety of forms of electrical machine.

[0065] According to the present disclosure, there has therefore been provided an electrical machine and a bearing for an electrical machine are provided, where the bearing is arranged to function as a capacitor in order to provide current to rotor windings of the electrical machine. The bearing may include a dielectric material or an electrolyte between the plates of the capacitor bearing to increase capacitance. As such, the overall size of the electrical machine can be reduced, as well as the number of components and complexity for maintenance purposes. Moreover, the current density within the electrical machine can be increased, thereby increasing the efficiency of the electrical machine.

Claims

1. A bearing for an electrical machine, the bearing comprising:a rotor part configured to be fixable to a rotor of an electrical machine or a rotor shaft of the rotor of the electrical machine, the rotor comprising rotor windings; anda stator part configured to be fixable to a housing of the electrical machine;wherein the rotor part and stator part are each formed of an electrically conductive material and are arranged to act as opposing terminals of a capacitor;wherein the bearing includes a dielectric material or an electrolyte located between the rotor part and the stator part;wherein the rotor part comprises an electrical connection for providing an input current for the rotor windings, or receiving an output current from the rotor windings; andwherein the stator part comprises an electrical connections for receiving an input current from the bearing, or providing an output current from the bearing.

2. The method according to claim 1, wherein the bearing comprises a plurality of rotatable elements arranged between the rotor part and the stator part in a central region of the bearing, and wherein the bearing further comprises one or more distal regions located adjacent to the central region in an axial direction.

3. The method according to claim 2, wherein the rotatable elements are formed of an electrically insulating material and / or comprise an electrically insulating coating.

4. The method according to claim 2 or claim 3, wherein the rotatable elements comprise a coating of a lubricating material.

5. The method according to any preceding claim, wherein the bearing includes the electrolyte located between the rotor part and the stator part, and wherein the combination of the rotor part, the stator part and the electrolyte are configured to cause the bearing to function as a supercapacitor.

6. The method according to any of claims 2-5, wherein the central region of the bearing is hermetically sealed from the one or more distal regions of the bearing, and wherein the one or more distal regions comprise an electrolyte located between the rotor part and the stator part, and wherein the central region comprises a lubricant located between the rotor part and the stator part.

7. The method according to any of claims 2-5, wherein the central region of the bearing is in fluid communication with the one or more distal regions of the bearing, wherein the bearing comprises a lubricant located between the rotor part and the stator part.

8. The method according to claim 7, wherein the lubricant is an automatic transmission fluid.

9. The method according to any preceding claim, wherein the bearing comprises sealing components arranged at axial ends of the bearing, or a radial end of the bearing.

10. The method according to any preceding claim, wherein the rotor part and the stator part each include an opposing surface, wherein the opposing surface of the rotor part is arranged to oppose the opposing surface of the stator part,11. The method according to claim 10, wherein the opposing surfaces of the rotor part and stator part have complimentary shapes.

12. The method according to claim 10 or claim 11, wherein the opposing surfaces each comprise a coating of a non-corrosive material.

13. The method according to claim 12, wherein the non-corrosive material is titanium.

14. The method according to any of claims 10-13, wherein each opposing surface includes a plurality of protrusions and recesses, wherein the protrusions and recesses of the opposing surfaces of the rotor part and stator part are arranged to interlock one another.

15. The method according to any of claims 10-14, wherein a surface area of the opposing surface of the rotor part is greater than a surface area of the opposing surface of the stator part.

16. An electrical machine comprising:a stator comprising stator windings;a rotor comprising rotor windings; anda first bearing according to any preceding claim, the first bearing being arranged between the rotor and the housing of the electrical machine, wherein the rotor part of the first bearing is fixed to the rotor of the electrical machine, and wherein the stator part of the first bearing is fixed to the housing of the electrical machine;wherein the electrical connection of the rotor part of the first bearing is electrically connected to the rotor windings, and wherein the electrical connection of the stator part of the first bearing is configured to be electrically connected to an AC current source, or return path or ground.

17. The electrical machine according to claim 16, further comprising a second bearing according to any of claims 1-15, the second bearing being arranged between the rotor and the housing of the electrical machine, wherein the rotor part of the second bearing is fixed to the rotor of the electrical machine, and wherein the stator part of the second bearing is fixed to the housing of the electrical machine;wherein the electrical connection of the rotor part of the first bearing is electrically connected to the rotor windings, and wherein the electrical connection of the stator part of the first bearing is configured to be electrically connected to a return path or ground; andwherein the electrical connection of the stator part of the first bearing is configured to be electrically connected to an AC current source.

18. The electrical machine according to any of claims 16 or 17, wherein the rotor further comprises rectifier circuitry configured to convert a first AC current provided by the electrical connection of the rotor part of the first bearing to a DC current to be provided to the rotor windings.

19. The electrical machine according to claim 18, wherein the electrical connection of the rotor part is configured to receive the first AC current, wherein the AC current has a predefined set frequency; andwherein the stator windings are arranged to receive a second current AC current, wherein the second current AC current is variable in frequency.

20. The electrical machine according to claim 19, further comprising:first power circuitry configured to provide the first AC current to the electrical connection of the stator part of the first bearing.

21. The electrical machine according to claim 19 or claim 20, further comprising:second power circuitry configured to provide the second AC current to the stator windings, wherein the second power circuitry is configured to modify a peak amplitude of the second AC current according to a power demand for the electrical machine.

22. The electrical machine according to any of claims 16-21, wherein the rotor additionally comprises a plurality of permanent magnets.

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