Capacitor arrangement for electrical machine

Abstract

A synchronous electric machine comprising a stator, a wound rotor, and two ring shaped excitation capacitors 250, which have: an axially fixed conductive plate 510 on the rotor; an axially movable or

Description

[0001] The present invention relates to generally an electrical machine having a particular capacitor arrangement, and more specifically to an externally excited electrical motor comprising rotor windings arranged to receive an excitation voltage from a power supply. 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 (also referred to as an externally excited synchronous machine (EESM)) 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 EESMs 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, there is provided an electrical machine comprising: a stator comprising stator windings; a rotor comprising rotor windings configured to receive an excitation voltage from a power supply; and a plurality of ring capacitors arranged about an axis of rotation of the electrical machine; wherein each ring capacitor comprises a fixed conductor affixed to the rotor, a movable conductor that is movable in an axial direction of the electrical machine, and a first dielectric material arranged between the fixed conductor and the movable conductor; wherein for each of the ring capacitors the fixed conductor is electrically connected to the rotor windings, and the movable conductor is configured to receive the excitation voltage from the power supply; wherein the rotor further comprises a chamber and a radially extending inlet channel configured for supply of liquid to the chamber; and wherein the chamber, radially extending inlet channel, and movable conductor of each ring capacitor are arranged such that rotation of the rotor is configured to generate a centrifugal hydraulic pressure on the liquid causing compression of each of the plurality of ring capacitors in the axial direction, such that during rotation of the rotor the fixed conductor abuts the first dielectric material, and the first dielectric material abuts the movable conductor. In this way, an electrical machine is provided including capacitors for wireless power transfer to the rotor, which maintain a large capacitance by maintaining contact during rotation of the rotor, and which avoid excessive friction, without requiring precise manufacturing tolerances. Moreover, a large capacitance can be provided with a comparatively compact capacitor, and without requiring a separate chamber for electrical components, such that the overall size of the electrical machine can be small.

[0007] It should be appreciated that while the rotor windings of the electrical machine receive a DC excitation voltage, the stator windings may receive a driving voltage which causes rotation of the rotor. This driving voltage may be an AC voltage and may be a single-phase driving voltage, or a multi-phase driving voltage, such as a three-phase driving voltage. The driving voltage may in some cases be generated from the same power supply as that used to provide the excitation voltage to the rotor windings, or different power supplies may be used. Moreover, it should be appreciated that the movable conductor of the ring capacitors receive an AC excitation voltage, and that if the power supply provides a DC excitation voltage, inverter circuitry may be provided (either within the electrical machine or outside the electrical machine) which converts the DC excitation voltage to an AC excitation voltage. In addition, the excitation voltage provided to the rotor windings may be a DC excitation voltage, and as such rectifier circuitry may be located within the rotor of the electrical machine which converts the AC excitation voltage to a DC excitation voltage to be provided to the rotor windings. Both the AC and DC excitation voltage may be commonly referred to as an excitation voltage, with the details regarding whether the excitation voltage is AC or DC at any given time omitted for brevity. In addition, the stator and rotor described above are arranged about the same axis of rotation, e.g. a rotor shaft. Furthermore, while the inlet channel for the chamber is described as radial, it should be appreciated that this may mean that the channel may extend only in the radial direction, or may extend in the radial direction and a circumferential direction.

[0008] According to some examples, the movable conductor may be electrically connected to a respective stator conductor affixed to the stator, and wherein the movable conductor may be configured to receive the excitation voltage via the respective stator conductor. As such, the movable conductor may be provided with the excitation voltage in order to wirelessly transfer power to the rotor.

[0009] In some examples, the electrical connection between the movable conductor and the stator conductor is a conductive connection, wherein a radially outermost edge of the movable conductor abuts the stator conductor. As such, the connection between the movable conductor and the stator conductor may not require additional components, such that the capacitor may be compact and lightweight.

[0010] In some alternative examples, the electrical connection between the movable conductor and the stator conductor is a capacitive connection, wherein a second dielectric material is located between the movable conductor and the stator conductor at a radially outermost edge of the movable conductor. In this manner, the total capacitance between the stator and the rotor may be increased, resulting in efficient wireless power transfer. The second dielectric material may be affixed to the movable conductor, or may be affixed to the stator conductor. Accordingly, design and construction flexibility is provided.

[0011] According to certain examples, the first dielectric material may be affixed to the fixed conductor. Alternatively, in some examples the first dielectric material may be affixed to the movable conductor, and the first dielectric may have an outermost radius less than an outermost radius of the movable conductor. Accordingly, design and construction flexibility is provided, while allowing the movable conductor to be electrically connected to the stator conductor.

[0012] Advantageously, the fixed conductor may comprise one or more outlet slits in fluid communication with the chamber, wherein the one or more outlet slits are arranged at one or more respective circumferential locations of the fixed conductor, wherein the one or more outlet slits extend along a full radial length of the fixed conductor, and wherein the one or more outlet slits extend only partially along an axial length of the fixed conductor. Alternatively, the first dielectric material may comprise one or more outlet slits in fluid communication with the chamber, wherein the one or more outlet slits are arranged at one or more respective circumferential locations of the first dielectric material, wherein the one or more outlet slits extend along a full radial length of the first dielectric material, and wherein the one or more outlet slits extend only partially along an axial length of the first dielectric material. As such, the liquid may be able exit the chamber, allowing for liquid circulation. In addition, this may allow the liquid to be used for one or more additional purposes.

[0013] According to certain examples, the one or more outlet slits are in fluid communication with one or more of: one or more end windings of the rotor windings, one or more end windings of the stator windings, and one or more bearings of the electrical machine. Accordingly, the liquid may be used for cooling and / or lubrication of one or more other components of the electrical machine. As such, the overall weight of the electrical machine may be reduced, as only a single liquid may be required for multiple uses. In some cases, the liquid is an oil for use as a coolant and / or lubricant, such as ATF.

[0014] In some examples, the stator comprises one or more electrically non-conductive components arranged to constrain the movable conductors of the plurality of ring capacitors to respective axial regions. As such, the movable conductors may be constrained to a particular axial region, while still being free to move within this axial region. The one or more electrically non-conductive components may include one or more protrusions arranged to constrain both of the movable conductors of the plurality of ring capacitors to their respective axial regions. In some cases the inlet channel may be formed in the non-conductive component, and in particular the inlet channel may pass through the one or more protrusions.

[0015] Advantageously, the movable conductor may comprises a protrusion at a particular circumferential location on a radially outermost surface of the movable conductor, wherein the protrusion may be arranged to interface with a recess in at least one or more electrically non-conductive components of the stator, wherein the interfacing of the protrusion and the recess may limit rotation of the movable conductor. As such, rotation of the movable conductor may be restricted, such that a reliable electrical connection between the movable conductor and the stator conductor may be achieved, while reducing friction on the movable conductor.

[0016] In certain examples, the movable conductor is electrically connected to a respective stator conductor affixed to the stator, and wherein the movable conductor is configured to receive the excitation voltage via the respective stator conductor, and the stator conductor is arranged within the recess in the at least one electrically non-conductive component. As such, a reliable electrical connection between the movable conductor and stator conductor may be achieved.

[0017] According to some examples, the movable conductor is a substantially ring-shaped conductor formed of a single piece of electrically conductive material having two discrete circumferential ends, wherein the stator is arranged to exert a radial compression force on the movable conductor such that the two ends of the movable conductor are configured to abut one another. Accordingly, the electrical machine is assemble such that the movable conductor is under compression. The movable conductor therefore exerts a radially outward force on the stator conductor, such that the movable conductor remains in electrical contact with the stator conductor, despite being free to move in an axial direction. In some cases, a recess in at least one or more electrically non-conductive components of the stator receives protrusions at each of the two ends of the movable conductor. As such, the two ends may be further prevented from separating from one another. Moreover, while the above example defines that the two ends of the movable conductor abut one another when placed within the electrical machine, it should be appreciated that this may not necessarily be the case and that, more generally, the two ends may be closer to one another than when not under a radial compressive force.

[0018] Advantageously, a surface area of the fixed conductor arranged to abut the first dielectric material may be greater than a surface area of the movable conductor arranged to abut the first dielectric material. As such, the capacitance of the capacitor may be increased, resulting in more efficient wireless power transfer, without substantially increasing the size of the capacitor. Brief Description of the Drawings

[0019] Embodiments of the invention will now be described, by way of example only, with reference to the following figures.

[0020] In accordance with one (or more) embodiments of the present invention the figures show the following:

[0021] Figure 1 depicts an electrical machine having stator windings and rotor windings, where the rotor windings are supplied with a DC current through brushes.

[0022] Figure 2 depicts an externally excited electrical machine comprising two capacitors according to an example of the present disclosure.

[0023] Figure 3 depicts a circuit diagram for an externally excited electrical machine according to an example of the present disclosure.

[0024] Figure 4 illustrates electrical connections for the capacitors of the electrical machine according to an example of the present disclosure.

[0025] Figure 5 illustrates a close-up view of the capacitors of the electrical machine according to an example of the present disclosure.

[0026] Figure 6 illustrates a fixed conductor or dielectric material according to examples of the present disclosure.

[0027] Figure 7 illustrates a movable conductor, dielectric material, and insulating component according to an example of the present disclosure.

[0028] Figure 8 illustrates a close-up view of an anti-rotation mechanism for a movable conductor according to an example of the present disclosure.

[0029] Figure 9 illustrates a movable conductor and dielectric material for a capacitor before and after installation according to an example of the present disclosure.

[0030] Figure 10 illustrates a close-up view of the capacitors of the electrical machine according to a further example of the present disclosure.

[0031] Figure 11 illustrates a movable conductor and dielectric materials for a double capacitor before and after installation according to an example of the present disclosure.

[0032] 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 4 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.

[0033] 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

[0034] Figure 1 shows an example of an externally excited electrical motor 100 (i.e. externally excited synchronous motor (EESM)). 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.

[0035] 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 141 A, 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.

[0036] 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 5 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.

[0037] 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. Like reference numerals across Figures 1 and 2 denote common components, unless otherwise stated. As can be seen from Figure 2, the electrical machine 200 does not include the brushes 141 and slip rings 143 of the electrical machine 100 of Figure 1, and instead includes two capacitors 250A and 250B arranged to be connected to a power supply (not shown). The capacitors 250 allow for wireless power transfer, and prevent the need for two separate chambers within the electrical machine, such that the electrical machine may include only a single chamber 215, in which the windings are located. The capacitors 250 are ring capacitors about the axis of rotation of the electrical machine 200 (i.e. about the rotor shaft 120) and are arranged axially apart from one another.

[0038] Figure 3 illustrates a circuit diagram of an externally excited electrical motor such as that shown in Figure 2. A DC power supply 310 (such as a battery e.g. for a vehicle) may be connected to inverter circuitry 320 (such as an H-bridge e.g. comprising four transistor switches), and provides electrical power to rotor windings 340 of an electrical machine via a rectifier 330 (e.g. comprising four diodes). The electrical power may be provided to the rotor windings 340 via the two capacitors 250, located electrically between the inverter circuitry 320 and rectifier circuitry 330, shown in Figure 2. That is, the capacitors 250 receive electrical power from power supply 310 via the inverter circuitry 320, and the capacitors provide electrical power to the rotor windings 340 via the rectifier circuitry. The two capacitors 250 include a first capacitor 250A and a second capacitor 250B. While the power supply 310 provides a DC voltage / current, the inverter 320 converts the DC voltage / current to an AC voltage / current, which is provided to the capacitors 250A, 250B. The AC voltage / current may then be converted to a DC voltage / current, which is provided to the rotor windings 340. Figure 4 illustrates a close-up view showing the connectivity of the capacitors 250A, 250B. As can be seen from Figure 4, the power source 310 connects to each of the capacitors 250A, 250B (via inverter circuitry which is not shown for clarity), which are connected to the rotor windings 122 (via rectifier circuitry, which is not shown for clarity, but may be located within / on the rotor, such as in the rotor shaft or elsewhere on the rotor).

[0039] The optimum AC current frequency for efficient capacitive wireless power transfer depends upon the capacitance of the capacitor. Therefore, in order to achieve efficient wireless power transfer using capacitors at conventional operating frequencies for an electrical motor, a comparatively large capacitance is desired for the capacitors 250. In order to achieve such a large capacitance, a material with high permittivity (i.e. a dielectric material) should be located between conductors (i.e. electrodes) of the capacitor, with the conductors in contact with the dielectric material. If either or both of the conductors are not in contact with the dielectric material, such that air is present between the conductors, the overall permittivity between the conductors is significantly reduced, thereby significantly reducing the capacitance of the capacitor and efficiency of the wireless power transfer.

[0040] In the present context, the capacitors are used to transfer power from the stator to the rotor, where the rotor windings are located such that the rotor windings can be supplied with electrical power. As such, one of the conductors of the capacitor would generally be affixed to the rotor, while the other conductor would generally be affixed to the stator, such that in use the conductors of the capacitor would rotate relative to one another. As such, either the stator conductor or rotor conductor must be movable with respect to the dielectric material and the other conductor, in order to achieve wireless power transfer in this way. This presents a number of difficulties such that, within the context of an electrical motor, particularly for a vehicle, manufacturing a capacitor in which contact between the conductors and the dielectric material is maintained during use is challenging.

[0041] Firstly, precise manufacturing tolerances are required to provide a capacitor in which the conductors are both in contact with the dielectric material, without excessive friction which would increase wear on the conductors and dielectric material. Moreover, even with such precise tolerances, the motor can be subject to external forces (e.g. due to movement of the vehicle, e.g. over bumps in the road surface) which can cause the conductor(s) to temporarily (or permanently) move apart from the dielectric material. This results in a decrease in the capacitance of the capacitor, and therefore a loss of power. Furthermore, the use of additional mechanisms to force the conductors and dielectric material together presents its own challenges. For example, any form of spring-based mechanism to force the conductors and dielectric material together would generally either cause excessive friction between the conductor(s) and the dielectric material, or would also suffer from the possibility of temporary loss of contact due to external forces. In addition, spring-based mechanisms would require precise manufacturing tolerances in order to achieve appropriate force levels.

[0042] According to the present disclosure, a capacitor arrangement is provided which addresses these identified challenges. Figure 5 illustrates a cross-sectional view of a top portion of ring capacitors 250A, 250B of electrical machine 200. The first capacitor 250A is formed of a fixed conductor 510A, a movable conductor 530A, and a dielectric material 520A arranged between the fixed conductor 510A and the movable conductor 530A. The second capacitor 250B is formed of a fixed conductor 510B, a movable conductor 530B, and a dielectric material 520B arranged between the fixed conductor 510B and the movable conductor 530B. As shown in Figure 4, the fixed conductors 510A, 510B of capacitors 250A, 250B are each connected to the rotor windings (specifically the rotor end windings 122). Furthermore, as shown in Figure 4, the movable conductors 530A, 530B are electrically connected to the power supply 310 (i.e. the movable conductors 530A, 530B are configured to receive an excitation voltage from the power supply 310). An AC excitation voltage may be provided to the capacitors 250A, 250B, however the power supply 310 (e.g. vehicle battery) may supply a DC voltage which is converted to an AC voltage using inverter circuitry (as shown in Figure 3).

[0043] The movable conductors 530A, 530B are each movable in an axial direction of the electrical machine (i.e. in the left-right direction of Figure 5). That is, the movable conductors 530A, 530B are not affixed to the rotor and are also not affixed to the stator of the electrical machine, and as such are able to move in an axial direction relative to the rotor and / or the stator. The fixed conductors 510A, 510B are affixed to the rotor, and the dielectric material 520A, 520B may be affixed to either the fixed conductor 510A, 510B (i.e. the dielectric material 520 rotates relative to the movable conductor 530A, 530B) or affixed to the movable conductor 530A, 530B (and as such the fixed conductor 510A, 510B may rotate relative to the dielectric material 520A, 520B) for the respective capacitors 250A, 250B. The dielectric material 520A, 520B is arranged between the fixed conductor 510A, 510B and the movable conductor 530A, 530B in an axial 7 direction for the respective capacitors 250A, 250B. The movable conductor 530A, 530B may be electrically connected to a stator conductor 540A, 540B which is affixed to the stator, such that the movable conductor 530A, 530B may receive the excitation voltage from the power supply 310 via the stator conductor 540A, 540B. For example, the electrical connection between the movable conductor 530A, 530B and stator conductor 540A, 540B may be a conductive connection, such that the movable conductor 530A, 530B abuts (i.e. is in direct contact with) the stator conductor 540A, 540B. However, it should be appreciated that other implementations are possible whereby the movable conductor receives the excitation voltage without requiring the stator conductors 540A, 540B, such as via one or more electrical contacts (in particular if the movable conductor 530A, 530B is arranged to have its rotation and / or axial translation limited).

[0044] According to the present disclosure, two capacitors 250A, 250B are provided, each including a respective movable conductor 530A, 530B, and a chamber 570 is defined between the movable conductors 530A, 530B. The chamber 570 may be in fluid communication with one or more radially extending inlet channels 550 configured to supply a liquid to the chamber 570 (as shown by the horizontal and vertical arrows within the channel 550). The liquid may be an electrically non-conductive liquid, such as an oil. The chamber 570, channel(s) 550, liquid and movable conductors 530A, 530B are arranged such that rotation of the rotor causes compression of each of the plurality of ring capacitors 250A, 250B in the axial direction, such that during rotation of the rotor, the fixed conductor 510A, 510B abuts the dielectric material 520A, 520B, and the dielectric material 520A, 520B abuts the movable conductor 530A, 530B. In particular, the rotation of the rotor generates a centrifugal force on the liquid within the channel 550, causing increased hydraulic pressure within the chamber 570 in an axial direction. This increased hydraulic pressure during rotation results in a net force (shown by the horizontal arrows within the chamber 570) on the movable conductors 530A, 530B in an axial direction towards their respective fixed conductors 510A, 510B. This increased hydraulic pressure causes the components of the capacitors 250A, 250B to remain in contact with one another during rotation of the rotor. In other words, rotation of the rotor creates a compression force on the components of the 250A, 250B.

[0045] Therefore, according to the present disclosure, the components of the capacitors 250A, 250B are compressed together with increased pressure during rotation of the rotor (as compared with when the rotor is not rotating). As such, the requirement for precise manufacturing tolerances for the capacitor is reduced as the capacitor components are movable relative to one another. Furthermore, loss of contact between the capacitor components is prevented due to the hydraulic pressure creating a compression force between the capacitor components, and excessive friction is prevented as the strength of the compression force is automatically adjusted according to the rotational speed of the rotor.

[0046] The electrical machine 200 may include one or more non-electrically conducting (i.e. insulating) components configured to hold one or more other components in place. For example, the electrical machine 200 may include one or more first insulating components 561 (which forms part of the stator) to which the stator conductors 540A, 540B may be affixed. These one or more first insulating components 561 (in combination with the stator conductors 540A, 540B) may limit radial movement of the movable conductors 530A, 530B. Furthermore, the electrical machine 200 may include one or more additional insulating components (which each form part of the rotor) to which the fixed conductors 510A, 510B may be affixed. For example, the electrical machine may include a second insulating component 562 (which forms part of the rotor), to which the fixed conductor 510B of the second capacitor 250B is affixed, and a third insulating component 563 (which forms part of the rotor) to which the fixed conductor 510A of the first capacitor 250A is affixed. It should be appreciated, however, that the insulating components 562, 563 shown in 8 Figure 5 are just one example arrangement and that functionality of the second and third insulating components 562, 563 could be achieved using a single insulating component or more than two insulating components.

[0047] The electrical machine 200 may in some examples include one or more insulating components arranged to limit axial movement of the moveable conductors 530A, 530B. For example, as shown in Figure 5, the second insulating component 562 may include one or more protrusions 580 arranged to limit axial movement of the moveable conductors 530A, 530B beyond a particular axial location in a particular axial direction. In this way, while the volume of the chamber 570 is changeable due to movement of the moveable conductors 530A, 530B, the minimum volume of the chamber 570 is set based on the location of the protrusion(s) 580, as the moveable conductors 530A, 530B are unable to move past the protrusion(s) 580.

[0048] As mentioned above, the liquid present within the chamber 570 may be an electrically non-conductive liquid, such as an oil. The liquid may, in some cases, be further utilised within the electrical machine as a coolant or a lubricant. In particular, the chamber may include one or more outlet channels to allow the liquid to leave the chamber to be used elsewhere within the electrical machine. For example, the fixed conductor 510 or the dielectric material 520 may include one or more channels (i.e. slits) extending in a radial direction of the electrical machine, and along a full radial length of the fixed conductor 510 or the dielectric material 520 which allow the liquid to pass through the fixed conductor 510 or the dielectric material 520 towards one or more other components for lubrication or cooling. In other words, the channels within the fixed conductor 510 or the dielectric material 520 may act as outlet channels for the chamber 570. Figure 6 illustrates a cross-section of a fixed conductor 510 taken along line B-B in Figure 2. The fixed conductor 510 includes a plurality of channels / slits 515. The channels 515 may extend only partially through the a fixed conductor 510, such that the a fixed conductor 510 may be a single piece of electrically conductive material. While Figure 6 shows four channels 515, it should be appreciated that substantially any number of channels may be used, for example based on desired flow characteristics of the liquid. Furthermore, while in Figure 6 the channels 515 extend radially at a common circumferential location, the channels 515 may in some cases extend both radially and circumferentially, such that the outermost point of the channel 515 is located at a different circumferential location than the innermost point of the channel 515. While Figure 6 illustrates the presence of one or more slits 515 within the a fixed conductor 510, it should be appreciated that the slit(s) 515 may instead be formed in the dielectric material 520 in substantially the same manner, as discussed above. In particular, in examples where the dielectric material 520 is affixed to the movable conductor 530, the channels may be formed in the fixed conductor 510, however if the dielectric material is affixed to the fixed conductor 510, the channels may be formed in either the fixed conductor 510 or the dielectric material 520.

[0049] The flow of liquid through the channels is illustrated in Figure 5, where the dashed vertical line within the dielectric materials 520A, 520B indicated flow of the liquid out of the chamber 570, where the liquid may then be used for cooling or lubrication purposes for one or more other components of the electrical machine 200, such as for cooling the rotor end windings and / or the stator end windings, and / or for lubricating one or more bearings. One or more additional channels may be provided which guide the liquid to one or more other components of the electrical machine for cooling and / or lubrication.

[0050] Moreover, the movable conductors 530 and chamber 570 may be arranged such that when the rotor is not rotating, the movable conductors form a seal with the one or more insulating components (e.g. the first insulating component 561 and / or second insulating component 562), such that the liquid cannot reach (and pass through) the slits 515. One or more sealing components may be used for each capacitor 250A, 250B to achieve this seal. However, the movable conductors 530 and chamber 570 may also be arranged such that when the rotor rotates, the axial hydraulic pressure caused by the centrifugal force on the liquid causes the movable conductors 530 to move axially to compress the respective capacitors 250. This may create one or more openings which allow the liquid to pass to the outlet channels of the chamber 570. The outlet channels may comprise the one or more slits 515 discussed above, or may not include the one or more slits 515 (in either the fixed capacitor 510 or dielectric material 520). In having the outlet channels of the chamber 570 sealed when the rotor is not rotating, the liquid can be prevented from draining from the chamber 570 when the rotor is not rotating.

[0051] Figure 7 illustrates a cross-sectional view of the dielectric material 520, movable conductor 530 and first insulating component 561 along line A-A shown in Figure 2. In the present example, the dielectric material 520 is affixed to the movable conductor 530, however as mentioned above the dielectric material 520 may instead be affixed to the fixed conductor 510. As can be seen, the movable conductor 530 includes one or more protrusions 535 at one or more circumferential positions on the movable conductor 530. As can be seen from Figure 7, the dielectric material 520 may have a smaller outer radial diameter than the movable conductor 530. The protrusion 535 shown in Figure 7 is located at a radially outermost location on the movable conductor 530 and extends outwards in a radial direction. As can be seen from Figure 7, this protrusion 535 is arranged to interface with a recess 565 in the first conductive component 561 (of the stator). A close-up view of this arrangement is shown in Figure 8.

[0052] As can be seen in Figure 8, the first conductive component 561 includes one or more recesses 565 arranged to receive the protrusion(s) 535 in the movable conductor 530. The combination of the recess(es) 565 and protrusion(s) 535 limits (i.e. restricts) rotation of the movable conductor 530 (and dielectric material 520, if the dielectric material 520 is affixed to the movable conductor 530). In addition, the stator conductor 540 may in some cases be located within the recess(es) 565, as shown in Figures. As such, a reliable electrical connection between the movable conductor 530 and the stator conductor 540 may be ensured.

[0053] As can also be seen from Figure 7, the movable conductor 530 (and dielectric material 520, if the dielectric material 520 is affixed to the movable conductor 530) may be substantially ring shaped and formed of a single piece of material, but may have two distinct ends such that the movable conductor 530 is substantially C-shaped. This is illustrated in more detail in Figure 9. The left-hand side of Figure 9 illustrates the movable conductor 530 (and dielectric material 520) having a first end 531 and a second end 532 located at opposite circumferential ends of the movable conductor 530. The movable conductor 530 may be manufactured such that in the absence of an external compression force, the two ends 531,532 of the movable conductor 530 are separated from another, as shown in the left-hand side of Figure 9. However, the movable conductor 530 and electrical machine 200 may be arranged such that once assembled a radial compression force is exerted on the movable conductor 530 (e.g. by the first non-conducting component 561) in order to cause the two ends 531, 532 to abut one another (or be closer to one another), as shown in the right-hand side of Figure 9. This compression force may, in some cases be exerted by the recess 565 in the first insulating component 561 on the protrusion 535 of the movable conductor 530, as shown in Figure 8. This additionally helps to maintain a constant electrical connection between the movable conductor 530 and the stator conductor 540. Furthermore, while the protrusion(s) 535 in the movable conductor 530 is shown as being located at (i.e. divided across) the two ends 531, 532 of the movable conductor 530, in some cases the protrusion(s) 535 may be located elsewhere on the movable conductor 530.

[0054] It should be noted that while a protrusion 535 (and interfacing recess 565) are illustrated in a number of figures of the present disclosure, such a mechanism may in some cases not be included. In particular, the use of a protrusion 535 and recess 565 limits rotation of the movable conductor 530. This may be particularly useful if the stator conductor 540 is only located in a particular circumferential region (e.g. within the recess 565). However, in some cases the stator conductor 540 may extend along the full circumferential length of the (compressed) movable conductor 530. Accordingly, it may not be necessary to limit rotation of the movable conductor 530 in order to maintain a reliable electrical connection between the movable conductor 530 and stator conductor 540, and as such the protrusion 535 and recess 565 may not be provided.

[0055] As discussed above, the connection between the movable conductor and the stator conductor may be an electrically conductive connection, however in other cases the connection between the movable conductor and the stator conductor may be a capacitive connection. An example, of this arrangement is shown in Figure 10, where like reference numerals denote similar components to those already discussed. In the example of Figure 10, a second dielectric material 690A, 690B is located between the movable conductor 630A, 630B and the stator conductor 640A, 640B, thereby forming a third capacitor 250C (formed by the movable conductor 630A, second dielectric material 690A, and stator conductor 640A) and a fourth capacitor 250D (formed by the movable conductor 630B, second dielectric material 690B, and stator conductor 640B). Accordingly, the overall capacitance between the stator and rotor may be increased, as two capacitors are arranged in series.

[0056] As shown in Figure 10, the radially outermost surface of the movable conductor 630 may have a larger axial length than a main body of the movable conductor 630, thereby increasing a surface area of the capacitors 250C, 250D, thereby increasing the capacitance of capacitors 250C, 250D, and as such further increasing the capacitance between the rotor and the stator. The second dielectric material 690 may either be affixed to the movable conductor 630 or affixed to the stator conductor 640.

[0057] Figure 11 illustrates a cross-sectional view of the movable conductor 630, dielectric material 520, and second dielectric material 690 depicted in Figure 10. In a similar manner as discussed above in relation to Figure 9, the movable conductor 630 (and dielectric material 520 and / or second dielectric material 690, if affixed to the movable conductor) may be substantially ring shaped and formed of a single piece of material, but may have two distinct ends such that the movable conductor 630 is substantially C-shaped. The left-hand side of Figure 11 illustrates the movable conductor 630 (and dielectric material 520 and second dielectric material 690) have a first end 631 and a second end 632 located at opposite circumferential ends of the movable conductor 630. The movable conductor 630 may be manufactured such that in the absence of an external compression force, the two ends 631,632 of the movable conductor 630 are separated from another, as shown in the left-hand side of Figure 11. However, the movable conductor 630 and electrical machine may be arranged such that once assembled a radial compression force is exerted on the movable conductor 630 (e.g. by the first non-conducting component 661) in order to cause the two ends 631, 632 to abut one another (or be closer to one another), as shown in the right-hand side of Figure 11. The compression force exerted on the movable conductor 630 improves the reliability of the contact between the components of the capacitors 250C, 250D.

[0058] While in the example of Figure 9 (in which the connection between the movable conductor 530 and stator conductor 540 is conductive) the compression force may be exerted by the recess 565 on the protrusion 535, in the present example the recess 565 and protrusion 535 may not be present. In particular, as the connection between the movable conductor 630 and stator conductor 640 is capacitive, the compression force may be exerted by the stator conductor 640 on the movable conductor 630 across the entire circumferential length of the stator conductor 640 / movable conductor 630. This increases the overall surface area of the capacitors 250C, 250D, thereby increasing the capacitance of the capacitors 250C, 250D. As such, the overall capacitance between the stator and rotor may be further increased.

[0059] It should be appreciated that various modifications may be made to the above examples within the scope of the present disclosure. As just one example, in some cases, the dielectric material and one of the fixed conductor and movable conductor (in particular the conductor to which the dielectric material is affixed) may have a larger surface area than the other conductor of the capacitor. For example, while the fixed conductor is depicted throughout the application as having a flat surface arranged on an axial plane, it should be appreciated that this may not necessarily be the case. For instance, the fixed conductor may include one or more protrusions which extend in axial direction towards the movable conductor, and the dielectric material may have corresponding recesses arranged to receive the protrusions of the fixed conductor. Similarly, the movable conductor may include one or more protrusions which extend in axial direction towards the fixed conductor, and the dielectric material may have corresponding recesses arranged to receive the protrusions of the movable conductor. Accordingly, the surface area of the capacitor can be increased, thereby increasing the overall capacitance.

[0060] Moreover, it should be appreciated that various materials may be used for the components and elements discussed herein, and that the present disclosure is not limited to any specific materials. For example, any dielectric material suitable for use in a capacitor may be used as the dielectric material or the second dielectric material. The dielectric material and second dielectric material may be formed of the same material or different materials. Furthermore, the fixed conductors, movable conductors and stator conductors may be formed of any electrically conductive material suitable for use in a capacitor. The fixed conductors, movable conductors and stator conductors may be formed of the same materials, or may be formed of different materials. Furthermore, the non-conductive components may be formed of any electrically insulating material suitable for holding electrically conductive components in place. The various non-conductive components may be formed of the same insulating material or from different insulating materials. Furthermore, the liquid used to exert hydraulic pressure on the movable conductors may be any suitable electrically non-conductive liquid. An example of such a liquid is an oil, such as automatic transmission fluid (ATF), however other liquids may be used. The liquid may also be an electrically non-conductive liquid that is suitable for use as a lubricant or coolant (such as ATF).

[0061] Accordingly, from one perspective, there has been provided an externally excited electrical machine is provided incorporating wireless power transfer to rotor windings of the electrical machine. The electrical machine includes two capacitors each having a conductor which is movable in an axial direction. A chamber comprising a liquid is defined between the movable conductors, and an inlet channel is arranged to provide liquid to the chamber. Rotation of the rotor exerts a centrifugal force on the fluid, hydraulic pressure which exerts an axial force on the movable conductors, thereby compressing the components of the two capacitors.

Claims

1. An electrical machine comprising:a stator comprising stator windings;a rotor comprising rotor windings configured to receive an excitation voltage from a power supply; anda plurality of ring capacitors arranged about an axis of rotation of the electrical machine;wherein each ring capacitor comprises a fixed conductor affixed to the rotor, a movable conductor that is movable in an axial direction of the electrical machine, and a first dielectric material arranged between the fixed conductor and the movable conductor;wherein for each of the ring capacitors the fixed conductor is electrically connected to the rotor windings, and the movable conductor is configured to receive the excitation voltage from the power supply;wherein the rotor further comprises a chamber and a radially extending inlet channel configured for supply of liquid to the chamber; andwherein the chamber, radially extending inlet channel, and movable conductor of each ring capacitor are arranged such that rotation of the rotor is configured to generate a centrifugal hydraulic pressure on the liquid causing compression of each of the plurality of ring capacitors in the axial direction, such that during rotation of the rotor the fixed conductor abuts the first dielectric material, and the first dielectric material abuts the movable conductor.

2. The electrical machine according to claim 1, wherein the movable conductor is electrically connected to a respective stator conductor affixed to the stator, and wherein the movable conductor is configured to receive the excitation voltage via the respective stator conductor.

3. The electrical machine according to claim 2, wherein the electrical connection between the movable conductor and the stator conductor is a conductive connection, wherein a radially outermost edge of the movable conductor abuts the stator conductor.

4. The electrical machine according to claim 2, wherein the electrical connection between the movable conductor and the stator conductor is a capacitive connection, wherein a second dielectric material is located between the movable conductor and the stator conductor at a radially outermost edge of the movable conductor.

5. The electrical machine according to claim 4, wherein the second dielectric material is affixed to the movable conductor.

6. The electrical machine according to any preceding claims, wherein the first dielectric material is affixed to the fixed conductor.

7. The electrical machine according to any of claims 1-5, wherein the first dielectric material is affixed to the movable conductor, and wherein the first dielectric has an outermost radius less than an outermost radius of the movable conductor.

8. The electrical machine according to claim 7, wherein the fixed conductor comprises one or more outlet slits in fluid communication with the chamber, wherein the one or more outlet slits are arranged at one or more respective circumferential locations of the fixed conductor, wherein the one or more outlet slits extend along a full radial length of the fixed conductor, and wherein the one or more outlet slits extend only partially along an axial length of the fixed conductor.

9. The electrical machine according to claim 7, wherein the first dielectric material comprises one or more outlet slits in fluid communication with the chamber, wherein the one or more outlet slits are arranged at one or more respective circumferential locations of the first dielectric material, wherein the one or more outlet slits extend along a full radial length of the first dielectric material, and wherein the one or more outlet slits extend only partially along an axial length of the first dielectric material.

10. The electrical machine according to claim 7 or claim 9, wherein the one or more outlet slits are in fluid communication with one or more of: one or more end windings of the rotor windings, one or more end windings of the stator windings, and one or more bearings of the electrical machine.

11. The electrical machine according to any preceding claim, wherein the stator comprises one or more electrically non-conductive components arranged to constrain the movable conductors of the plurality of ring capacitors to respective axial regions.

12. The electrical machine according to any preceding claim, wherein the movable conductor comprises a protrusion at a particular circumferential location on a radially outermost surface of the movable conductor, wherein the protrusion is arranged to interface with a recess in at least one or more electrically non-conductive components of the stator, wherein the interfacing of the protrusion and the recess limits rotation of the movable conductor.

13. The electrical machine according to claim 12, wherein the movable conductor is electrically connected to a respective stator conductor affixed to the stator, and wherein the movable conductor is configured to receive the excitation voltage via the respective stator conductor, andwherein the stator conductor is arranged within the recess in the at least one electrically non-conductive component.

14. The electrical machine according to any preceding claim, wherein the movable conductor is a substantially ring-shaped conductor formed of a single piece of electrically conductive material having two discrete circumferential ends, wherein the stator is arranged to exert a radial compression force on the movable conductor such that the two ends of the movable conductor are configured to abut one another.

15. The electrical machine according to any preceding claim, wherein the liquid is an oil foruse as a coolant and / or lubricant.

16. The electrical machine according to any preceding claim, wherein a surface area of the fixed conductor arranged to abut the first dielectric material is greater than a surface area of the movable conductor arranged to abut the first dielectric material.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:11 02 26CLAIMS1. An electrical machine comprising:a stator comprising stator windings;a rotor comprising rotor windings configured to receive an excitation voltage from a power supply; anda plurality of ring capacitors arranged about an axis of rotation of the electrical machine;wherein each ring capacitor comprises a fixed conductor affixed to the rotor, a movable conductor that is movable in an axial direction of the electrical machine, and a first dielectric material arranged between the fixed conductor and the movable conductor;wherein for each of the ring capacitors the fixed conductor is electrically connected to the rotor windings, and the movable conductor is configured to receive the excitation voltage from the power supply;wherein the electrical machine further comprises a chamber, and the rotor further comprises a radially extending inlet channel configured for supply of liquid to the chamber in a radially outwards direction; andwherein the chamber, radially extending inlet channel, and movable conductor of each ring capacitor are arranged such that rotation of the rotor is configured to generate a centrifugal hydraulic pressure on the liquid causing compression of each of the plurality of ring capacitors in the axial direction, such that during rotation of the rotor the fixed conductor abuts the first dielectric material, and the first dielectric material abuts the movable conductor.

2. The electrical machine according to claim 1, wherein the movable conductor is electrically connected to a respective stator conductor affixed to the stator, and wherein the movable conductor is configured to receive the excitation voltage via the respective stator conductor.

3. The electrical machine according to claim 2, wherein the electrical connection between the movable conductor and the stator conductor is a conductive connection, wherein a radially outermost edge of the movable conductor abuts the stator conductor.

4. The electrical machine according to claim 2, wherein the electrical connection between the movable conductor and the stator conductor is a capacitive connection, wherein a second dielectric material is located between the movable conductor and the stator conductor at a radially outermost edge of the movable conductor.

5. The electrical machine according to claim 4, wherein the second dielectric material is affixed to the movable conductor.

6. The electrical machine according to any preceding claims, wherein the first dielectric material is affixed to the fixed conductor.

7. The electrical machine according to any of claims 1-5, wherein the first dielectric material is affixed to the movable conductor, and wherein the first dielectric has an outermost radius less than an outermost radius of the movable conductor.

8. The electrical machine according to claim 7, wherein the fixed conductor comprises one or more outlet slits in fluid communication with the chamber, wherein the one or more outlet slits are arranged at one or more respective circumferential locations of the fixed conductor, wherein the one or more outlet slits extend along a full radial length of the fixed conductor, and wherein the one or more outlet slits extend only partially along an axial length of the fixed conductor.11 02 269. The electrical machine according to claim 7, wherein the first dielectric material comprises one or more outlet slits in fluid communication with the chamber, wherein the one or more outlet slits are arranged at one or more respective circumferential locations of the first dielectric material, wherein the one or more outlet slits extend along a full radial length of the first dielectric material, and wherein the one or more outlet slits extend only partially along an axial length of the first dielectric material.

10. The electrical machine according to claim 7 or claim 9, wherein the one or more outlet slits are in fluid communication with one or more of: one or more end windings of the rotor windings, one or more end windings of the stator windings, and one or more bearings of the electrical machine.

11. The electrical machine according to any preceding claim, wherein the stator comprises one or more electrically non-conductive components arranged to constrain the movable conductors of the plurality of ring capacitors to respective axial regions.

12. The electrical machine according to any preceding claim, wherein the movable conductor comprises a protrusion at a particular circumferential location on a radially outermost surface of the movable conductor, wherein the protrusion is arranged to interface with a recess in at least one or more electrically non-conductive components of the stator, wherein the interfacing of the protrusion and the recess limits rotation of the movable conductor.

13. The electrical machine according to claim 12, wherein the movable conductor is electrically connected to a respective stator conductor affixed to the stator, and wherein the movable conductor is configured to receive the excitation voltage via the respective stator conductor, andwherein the stator conductor is arranged within the recess in the at least one electrically non-conductive component.

14. The electrical machine according to any preceding claim, wherein the movable conductor is a substantially ring-shaped conductor formed of a single piece of electrically conductive material having two discrete circumferential ends, wherein the stator is arranged to exert a radial compression force on the movable conductor such that the two ends of the movable conductor are configured to abut one another.

15. The electrical machine according to any preceding claim, wherein the liquid is an oil for use as a coolant and / or lubricant.

16. The electrical machine according to any preceding claim, wherein a surface area of the fixed conductor arranged to abut the first dielectric material is greater than a surface area of the movable conductor arranged to abut the first dielectric material.11 02 26

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