GB2615741A

GB2615741A - Electrical machine

Info

Publication number: GB2615741A
Application number: GB2201273.6A
Authority: GB
Inventors: Shi Juntao; Deodhar Rajesh; Umemura Chiaki
Original assignee: IMRA Europe SAS
Current assignee: IMRA Europe SAS
Priority date: 2022-02-01
Filing date: 2022-02-01
Publication date: 2023-08-23
Also published as: GB202201273D0

Abstract

A stator 210 and rotor 220 for an electrical machine 200, the stator and rotor comprising a housing 212, 222 and one or more first chambers 216, 226, formed in the housing. Stator 214 and rotor 224 windings are located within the chambers where the chambers include a first surface configured to define a first boundary of an air gap 230 of the machine. The windings are separated from the first surface of the chambers, where the chambers comprise a ferrofluid, where the ferrofluid is located at least between the windings and the surface of the first chambers. The housing may be formed of a first material and the first surface of the chambers may be formed of a second material having a higher magnetic permeability than the first material. The chambers are arranged coaxially with the one or more windings. The stator or rotor may comprise a second chamber formed in the housing, where the ferrofluid contained in the first chambers is a first ferrofluid, and where the second chamber comprises a second ferrofluid, where the second chamber is arranged coaxially with the first chamber, with channels connecting the chambers in a radial direction. The ferrofluids may have different volumetric concentrations of magnetic particles.

Description

Electrical Machine

Field of the Invention

The invention generally relates to electrical machines, more specifically to approaches for increasing the output of electrical machines.

Background

As the world transitions away from fossil fuels and to cleaner, renewable sources of power, electrical machines are becoming increasingly important. Electrical machines include electric generators to generate electricity in, for example, a wind turbine or for regenerative braking applications in vehicles. Alternatively, electric motors, such as induction motors, can use electrical energy to generate torque to power vehicles, aeroplanes and many other machines.

With the use of electrical machines becoming increasingly prevalent in the modern word, improving the performance and efficiency of these electrical machines is an area of intense research. However, there are a number of separate ways in which performance or efficiency might be improved.

Certain types of electrical machine, such as resonant induction motors, can produce significant quantities of heat due to comparatively large currents in the windings of the motor. This heat decreases the efficiency of the motor and can potentially lead to component damage. As such, managing the temperature of electrical machines is crucial.

Furthermore, while traditional electrical machines are often provided with a ferromagnetic core, some more modern electrical machines are provided without such a ferromagnetic core, but rather with a non-magnetic core, or a so-called 'air-core'. In such air-core electrical machines, leakage of magnetic flux from the electrical machine can be greater than in traditional electrical machines with ferromagnetic cores. Such magnetic flux leakage can lead to decreases in performance as well as electromagnetic interference. Accordingly, managing magnetic flux leakage is essential in electrical machines, in particular air-core electrical machines.

The present inventors have identified an improved approach for cooling an electrical machine, reducing magnetic flux leakage from the electrical machine, and increasing the output of an electrical machine.

Summary of the Invention

Aspects of the invention are set out in the accompanying claims.

In a first aspect of the invention there is provided a stator for an electrical machine, the stator comprising: a housing; and one or more first chambers formed in the housing; and one or more windings located within the one or more first chambers; wherein the one or more first chambers includes a first surface configured to define a first boundary of an air gap of the electrical machine, wherein the one or more windings are separated from the first surface of the one or more first chambers; wherein the one or more first chambers comprise a ferrofluid, wherein the ferrofluid is located at least between the one or more windings and the first surface of the one or more first chambers.

In a second aspect of the invention there is provided a rotor for an electrical machine, the rotor comprising: a housing; and one or more first chambers formed in the housing; and one or more windings located within the one or more first chambers; wherein the one or more first chambers includes a first surface configured to define a first boundary of an air gap of the electrical machine, wherein the one or more windings are separated from the first surface of the one or more first chambers; wherein the one or more first chambers comprise a ferrofluid, wherein the ferrofluid is located at least between the one or more windings and the first surface of the one or more first chambers.

The ferrofluid helps to cool the windings via thermomagnetic convention, as well as providing electromagnetic shielding to prevent leakage of magnetic flux outside the electrical machine or into the core of the electrical machine. In addition, the presence of ferrofluid in the region between the stator windings and rotor windings (that would conventionally be part of the air gap) may increase the magnetic permeability in the region between the stator windings and rotor windings. This may lead to higher mutual-inductances between the stator windings and rotor windings, higher torque, larger output power, and increased efficiency.

In some embodiments, the housing is formed of a first material and the first surface of the one or more first chambers is formed of a second material having a higher magnetic permeability than the first material. As such, the magnetic permeability in the air-gap region can be further increased, despite the windings being enclosed within chambers.

In some embodiments, the one or more first chambers are arranged coaxially with the one or more windings. In other words, the winding may be circumferentially arranged about the axis of rotation of the electrical machine at a constant radial position and the first chamber may enclose the windings. In other examples, the one or more windings may be arranged with particular windings located at the same circumferential position but with different radial position, such that the chamber extends in a radial direction.

Advantageously, a radial distance between the first surface of the one or more first chambers and the one or more windings is between 0.3mm and 0.4mm. This range is optimal for increasing the magnetic permeability within the air gap region (between the stator and rotor windings) without excessively reducing the size of the physical air gap between the stator and rotor, or excessively increasing the distance between the stator and rotor windings, both of which may lead to performance and/or operating difficulties.

In some embodiments, the stator or rotor further comprises a second chamber formed in the housing, wherein the ferrofluid contained in the one or more first chambers is a first ferrofluid, and wherein the second chamber comprises a second ferrofluid; and wherein the second chamber is arranged coaxially with the first chamber. This provides additional electromagnetic shielding, thereby preventing magnetic flux leakage outside of the electrical machine.

Advantageously in some embodiments, the stator rotor further comprise one or more channels connecting the one or more first chambers and the one or more second chambers, the one or more channels having a length in a radial direction of the stator or rotor. In this manner, the first ferrofluid and the second ferrofluid may be the same. This promotes the circulation of the ferrofluid within and between the first and second chambers thereby improving the rate at which heat is carried away from the windings, and as such improving the cooling of the windings and the electrical machine in general.

Further advantageously, the one or more channels of the rotor may be angled with respect to the radial direction of the rotor. The centrifugal force exerted on the ferrofluid within the first chamber of the rotor during the operation of the electrical machine promotes the exchange (convection) of the ferrofluid between the first chamber and the second chamber via the one or more channels.

In some embodiments, the one or more first chambers and the one or more second chambers are not in fluid communication with one another, wherein the ferrofluid included in the one or more first chambers is a first ferrofluid, and wherein the second chamber comprises a second ferrofluid having a volumetric concentration of magnetic particles different to that of the first ferrofluid. As such, the volumetric concentrations first and second chambers may each be optimised for improved shielding or thermomagnetic convection (and hence cooling).

Advantageously, the volumetric concentration of magnetic particles of the first ferrofluid is between 2% and 4%, and the volumetric concentration of magnetic particles of the second ferrofluid is between 8% and 18%. As such, the ferrofluid within the second chamber may be optimised for more effective electromagnetic shielding, while the ferrofluid within the first chamber may be optimised for more effective cooling. However, the second chamber will still provide some degree of cooling to the stator/rotor, and the first chamber will still provide some degree of shielding to the stator/rotor.

In some embodiments, the first chamber includes two or more windings and one or more barriers between the two or more windings, wherein the one or more barriers are formed of a non-magnetic material. Advantageously, the two or more windings are arranged to receive electrical current of different phases from a power supply. As such, any increase in mutual inductance between the windings due to the ferrofluid can be reduced or eliminated.

In some examples, the stator comprises three windings each arranged to receive a different phase of a three-phase electrical current from the power supply. As such, the benefits described herein are applicable to three-phase electrical machines that produce rotating magnetic fields, without excessive increase in mutual inductance between the three different phased windings. However, the techniques described herein are additionally applicable to substantially any other type of electrical machine.

In some examples, the housing is formed of a non-magnetic material. As such, the weight of the electrical machine may be reduced while providing the advantages discussed herein.

Advantageously, the housing may further comprise a lamination arranged coaxially with the first chamber. Accordingly, additional electromagnetic shielding may be provided which prevents leakage of magnetic flux outside the electrical machine or into the core of the electrical machine.

In some examples, the first surface of the one or more chambers includes one or more radially extending protrusions on which the windings are mounted. In this way, the ferrofluid within the chamber may be in contact with a large proportion of the surface area of the windings, thereby further promoting heat transfer from the windings to the ferrofluid and as a result promoting thermomagnetic convection. In other words, chamber may include a boundary surface including a circumferentially extending portion and a plurality of radially extending portions. The windings are mounted to the radially extending portions so as to be removed from the circumferentially extending portion of the boundary surface.

In a third aspect there is provided an electrical machine comprising: a stator as described above; and a rotor as described above. An air gap of the electrical machine may be defined by the first surfaces of the one or more chambers of the stator and rotor. Advantageously, the length of the air gap in a radial direction may be between 0.5mm and 0.7mm.

In some embodiments, the electrical machine is an air-core electrical machine. In other embodiments, the electrical machine includes a core formed of a ferromagnetic material. As such, the features and benefits discussed above are applicable to substantially any type of electrical machine.

Brief Description of the Drawings

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

In accordance with one (or more) embodiments of the present invention the Figures show the following: Figure 1 shows an example of a conventional electrical machine.

Figure 2 shows a cross-section of an example of an electrical machine according to the

present disclosure.

Figure 3A shows a close-up view of the electrical machine of Figure 2.

Figure 3B shows a close-up of a cross-section of an electrical machine according to an example of the present disclosure.

Figure 4 shows a close-up of a cross-section of an electrical machine according to an

example of the present disclosure.

Figure 5 shows a close-up of a cross-section of an electrical machine according to an example of the present disclosure.

Figure 6 shows a close-up of a cross-section of an electrical machine according to an example of the present disclosure.

Figure 7 shows a close-up of a cross-section of an electrical machine according to an

example of the present disclosure.

Figure 8 shows a close-up of a cross-section of an electrical machine according to an example of the present disclosure.

Figure 9 shows a close-up of a cross-section of an electrical machine according to an

example of the present disclosure.

Figure 10 shows a stator of an electrical machine according to an example of the present disclosure.

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.

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

Figure 1 is cross-section of an example of a conventional electrical machine 100. The electrical machine 100 includes a stator 110 and a rotor 120 where the rotor 120 is configured to rotate relative to the stator 110. The stator 110 and rotor 120 are mounted coaxially with the axis of rotation into the page. The stator 100 includes a stator body 122 (or housing) and stator windings 114 mounted on or in the stator body 112. The rotor 120 includes a rotor body 122 (or housing) and rotor windings 124 mounted on or in the rotor body 122. The electrical machine 100 includes an air gap 130 between the stator 110 and the rotor 120 to allow the rotor 120 to rotate relative to the stator 120.

In the electrical machine 100 of Figure 1, an electrical current passes through the stator windings 114 to create a rotating magnetic field. The electrical current may pass through the stator windings 114 with three phases, however other modes of operation, such as single or dual-phase operation, are equally as applicable. This magnetic field induces an opposing current in the rotor windings 124, causing the rotor windings 124 to produce their own magnetic field, creating a torque on the rotor 120, thereby causing the rotor 120 to rotate. The size of the torque on the rotor 120 depends on the strength of the interaction between the magnetic fields of the stator windings 114 and the rotor windings 124.

Increasing the amount of the current in the stator windings 114 increases the strength of the magnetic field and increases the torque on the rotor, however increasing the current in the stator windings 114 causes greater heating of the stator windings 114. Furthermore, with a stronger magnetic field, magnetic flux will leak outside the electric machine 100 as well as into the core of the electrical machine 100 to a greater extent. Accordingly, increasing the current in the stator windings 114 to increase torque is an inefficient process, as well as potentially causing electromagnetic interference with other electrical devices (due to the leakage of magnetic flux).

Figure 2 is an example electrical machine 200 according to the present disclosure. The electrical machine 200 includes a stator 210 and a rotor 220 in a similar manner to the electrical machine 100 of Figure 1. The stator 200 includes a stator body 212 (or housing) and stator windings 214 mounted on or in the stator body 212. The rotor 220 includes a rotor body 222 (or housing) and rotor windings 224 mounted on or in the rotor body 222. The electrical machine 200 includes an air gap 230 between the stator 210 and the rotor 220 to allow the rotor 220 to rotate relative to the stator 220. Figure 2 shows the stator 210 as having six stator windings 214 and the rotor 220 as having six rotor windings 224, however it is appreciated that substantially any number of windings (or permanent magnets) may be provided on the stator 210 and the rotor 220. The rotor body 222 may be formed of a ferromagnetic material, such as iron, a non-magnetic material, or the rotor may be hollow (i.e. the electrical machine may be an air-core electrical machine). Furthermore, while the electrical machine 200 shown in Figure 2 is an electric induction motor, the techniques discussed in relation to the electrical machine 200 of Figure 2, and the entirety of this disclosure, may be applied to substantially any type of electrical machine.

The stator 210 additionally includes a chamber 216 that includes a ferrofluid, where the stator windings 214 may be located within the chamber 216. A ferrofluid is a colloidal fluid of magnetic particles. In the example of Figure 2, the rotor 220 also includes a chamber 226 that includes a ferrofluid, where the rotor windings 224 may be located within the chamber 226. The chambers 216 and 226 may extend along the full axial length of the stator 210 and rotor 210 (i.e. into the page), and also extend in a circumferential direction of the electrical machine 200. The ferrofluid within the chamber 216 may have the same composition as the ferrofluid within the chamber 226, or these ferrofluids may have different compositions. As the ferrofluid itself is magnetic, the ferrofluid acts as a shield to prevent leakage of magnetic flux outside the electrical machine 200 or into the core of the electrical machine 200. Accordingly, the efficiency of the electrical machine 200 is increased, and electromagnetic interference is prevented.

Furthermore, as the stator windings 214 may be located within the chamber 216 and the rotor windings 224 may be located within the chamber 226, the ferrofluid may help to cool the respective windings 214, 224. The ferrofluid flows within the chambers 216, 226 due to the phenomenon of thermomagnetic convection. That is, the heating of the ferrofluid by the windings 214, 224 will not be perfectly uniform and as such, a temperature gradient will be created within the ferrofluid, causing varying magnetic susceptibility within the ferrofluid. This in combination with the magnetic field created by the windings of the electrical machine 200, creates a Kelvin body force within the ferrofluid, leading to thermomagnetic convection.

Accordingly, the ferrofluid flows within the respective chambers 216, 226 without using a pump to cause circulation of the ferrofluid. Accordingly, the chambers 216, 226 may be a closed chamber that is not in fluid communication with any other component or the external environment. The circulation of the ferrofluid within the chambers 216, 226 helps to transfer heat away from the windings 214, 224. In some examples, the electrical machine 200 may be provided with a heat exchanger to transfer heat away from the ferrofluid within the channel(s) 214, 224 and improve cooling of the electrical machine 200. Such a heat exchanger may be arranged proximate to at least a portion of one or more of the channels 216, 226.

In this manner, the use of a ferrofluid within the chamber 216, 226 acts not only to reduce the leakage of magnetic flux from the electrical machine 200, but also to improve cooling of the windings 214, 224 of the electrical machine 200. Furthermore, in preventing leakage of magnetic flux outside the electrical machine 200, the amount of magnetic flux within the electrical machine 200 is increased, thereby increasing the strength of the magnetic interaction between the stator 110 and the rotor 120, causing the amount of torque to be increased for a given electrical current input to the stator windings 214.

Figure 3A is a close-up of an electrical machine 200A that is identical to that of Figure 2.

Figure 3A is shown from the same perspective as Figure 2, but shows only a small section of each of the stator 210 and the rotor 220. As shown in Figure 3A, the windings 214, 216 are located within the respective chambers 216, 226. The chambers 216, 226 each include a boundary surface 218, 228 where each of the boundary surfaces 218, 228 define a boundary of the air gap 230. In other words, the air gap is defined as the gap between the boundary surface 218 of the ferrofluid chamber 216 of the stator 110 and the boundary surface 228 of the ferrofluid chamber 226 of the rotor 220.

As can be seen in Figure 3A, the boundary surface 218 is separated from the stator windings 214, and is located between the stator windings 214 and the air gap 230. In other words, the stator windings 214 are not in physical contact with the boundary surface 218 (the stator windings 214 do not abut the boundary surface 218), such that ferrofluid is located between the stator windings 214 and the boundary surface 218. Similarly, the boundary surface 228 is separated from the rotor windings 224, and is located between the rotor windings 224 and the air gap 230. In other words, the rotor windings 224 are not in physical contact with the boundary surface 228 (the rotor windings 224 do not abut the boundary surface 228), such that ferrofluid is located between the rotor windings 224 and the boundary surface 228.

In this manner, the presence of ferrofluid in the region between the stator windings 214 and the rotor winding 224 (that would conventionally be part of the air gap) may increase the magnetic permeability in the region between the stator windings 214 and rotor windings 224.

This may lead to higher mutual-inductances between the stator windings 214 and rotor windings 224, higher torque, larger output power, and increased efficiency.

The distance between the stator winding 214 and the boundary surface 218 may, in some examples, be approximately 0.3mm-0.4mm. Similarly, the distance between the rotor winding 224 and the boundary surface 228 may, in some examples, be approximately 0.3mm-0.4mm. Furthermore, the distance between the boundary surfaces (i.e. the width of the air gap 230) may, in some examples, be approximately 0.5mm-0.7mm. As such, the total distance between the stator windings 214 and the rotor windings 224 may, in some examples, be 1.3mm. As such, the total distance between the stator windings 214 and the rotor windings 224 may be larger than for conventional electrical machines.

The stator windings 214 and rotor windings 224 may be bonded to the stator 210 and rotor 220 respectively via carbon fibre. For example, a plurality of layers of carbon fibre (such as three layers) may be provided with glue therebetween in order to aid bonding of the stator windings 214 and rotor windings 224 to the stator 210 and rotor 220 respectively. The windings 214, 224 may be bonded to either the stator or rotor bodies 212, 222, or may be bonded to the boundary surfaces 218, 228.

In some examples, the boundary surfaces 218, 228 may be formed of a material with a comparatively high magnetic permeability. In other words, the boundary surfaces 218, 228 may be formed of a material with a magnetic permeability that is larger than the material from which the stator body 212 and/or the rotor body 222 are formed. Accordingly, the enclosure of the windings 214, 224 within the chambers 216, 226 does not reduce the mutual inductance between the windings 214, 224, and as such the utilisation of a ferrofluid as described herein may lead to an overall increase in mutual inductance.

In some examples, the boundary surfaces may include spines for bonding to the windings, as shown in Figure 3B. Figure 3B shows an example electrical machine 200B that is substantially identical to the electrical machine of Figure 3A, except with alternative boundary surfaces 218A, 228A as well as the manner in which the windings 214, 224 are mounted within the chambers 216, 226. In the electrical machine 200B, the boundary surfaces 218A, 228A take the form of a spline-shaped sleeve. As such, the boundary surface 218A, 228A may extend in a circumferential direction of electrical machine and may additionally have a plurality of radially extending protrusions 218B, 228B to which the windings 214, 224 are mounted. While Figure 3B shows the boundary surface 218A, 228A as having a discontinuous surface due to the presence of the radially extending protrusions 2188, 228B, the radially extending protrusions 2188, 228B may in some examples extend from the chamber-side of the boundary surface 218A, 228A such that the boundary surface 218A, 228A has a continuously circular circumference at a constant radius.

The mounting of the windings to the radially extending protrusions 2188, 2288 of the boundary surface 218A, 228A allows the ferrofluid within the chamber 216, 226 to be in contact with a larger percentage of the surface area of the windings 214, 224 than in the electrical machine 200A of Figure 3A. That is, the windings 214, 224 may be removed from the stator/ rotor body 212, 222 as shown in Figure 3B, such that the ferrofluid is in contact with nearly all of the surface area of the windings 214, 224. Accordingly, heat transfer from the windings to the ferrofluid can be enhanced, thereby further promoting thermomagnefic convection within the ferrofluid.

The boundary surfaces 218A, 228A, including the radially extending protrusions 218B, 228B, may be formed of titanium or a titanium alloy, or any other material with comparatively high magnetic permeability. In this way, the windings 214, 224 can be removed from the majority of the boundary surface 218A, 228A, meaning that the total distance between the stator windings 214 and the rotor windings 224 may be larger than for conventional electrical machines, but with high magnetic permeability in the region between the windings.

While the boundary surfaces 218A, 228A of the electrical machine 2008 are not shown in combination with other examples described herein, it should be appreciated that the boundary surfaces 218A, 228A discussed in relation to Figure 3A may be used in combination with any of the examples discussed in the present disclosure.

Figure 4 shows an electrical machine 400 according to a further example of the disclosure. Where common reference numerals are used as for components already discussed in relation to previous figures, these reference numerals refer to the substantially similar components, except where explained otherwise. As shown in Figure 4, the stator 210 of the electrical machine 400, includes a second chamber 217 comprising a ferrofluid, where the second chamber 217 may be located further from the air gap 230 in a radial direction of the electrical machine 400 than the chamber 216. The ferrofluid within the second chamber 217 may have the same composition as the ferrofluid within the chamber 216, or these ferrofluids may have different compositions. In addition, the rotor 220 of the electrical machine 400 may include a second chamber 227 comprising a ferrofluid, where the second chamber 227 may be located further from the air gap 230 in a radial direction of the electrical machine 400 than the chamber 216. The second chambers 217, 227 extend in a circumferential direction of the electrical machine 500 and may extend along the full axial length of the stator 210 and rotor 210 (i.e. into the page). The ferrofluid within the second chamber 227 may have the same composition as the ferrofluid within the chamber 226, or these ferrofluids may have different compositions. The second chamber 217 and the second chamber 227 are located further from the air gap 230 than the chamber 216 and the chamber 226 respectively.

The ferrofluid within the second chambers 217, 227 may provide additional magnetic shielding to prevent magnetic flux from escaping outside of the electrical machine 400. Therefore, in some examples the ferrofluid within the second chambers 217, 227 may have a higher volumetric concentration of magnetic particles than the ferrofluid within the chambers 216, 226. As an example, the ferrofluid within the chambers 216, 226 may have a volumetric concentration of magnetic particles of 2%-4%, and the ferrofluid of the second chambers 217, 227 may have a volumetric concentration of magnetic particles of 8%-18%. These values balance factors such as viscosity and magnetic susceptibility of the ferrofluid to achieve optimised shielding or cooling.

By having a higher volumetric concentration of magnetic particles, the ferrofluid within the second chambers 217, 227 provides greater magnetic shielding than the ferrofluid within the chambers 216, 226, but has poorer thermomagnetic convection properties. In other words, the level of convention of the ferrofluid within the second chambers 217, 227 due to thermomagnetic properties is reduced due to the increased volumetric concentration of magnetic particles. However, as the second chambers 217, 227 are not located proximate to the stator windings 214 or rotor windings 224, the poorer thermomagnetic convection properties are not particularly disadvantageous. Furthermore, by providing a ferrofluid with comparatively higher volumetric concentration of magnetic particles in the second chambers 217, 227, the volumetric concentration of magnetic particles in the ferrofluid of the chambers 216, 226 may be comparatively lower, which may yield improved thermomagnetic convection properties, but poorer shielding properties. Therefore, the innermost chambers 216, 226 (the chambers 216, 226 closest to the air gap 230) may be optimised for improved thermomagnetic convection, while the outermost chambers 217, 227 (the chambers 217, 227 that are further from the air gap 230) may be optimised for improved magnetic shielding.

As such, by utilising two ferrofluid chambers in the stator 210 and/or rotor 220, with different volumetric concentrations of magnetic particles, optimised shielding and thermomagnetic convection (and hence cooling) may be achieved.

Figure 5 shows an electrical machine 500 according to a further example of the disclosure. Where common reference numerals are used as for components already discussed in relation to previous figures, these reference numerals refer to the substantially similar components, except where explained otherwise. The electrical machine 500 includes second chambers 217, 227 in a similar manner to the electrical machine 400 of Figure 4. However, the second chambers 217 and 227 of the electrical machine 500 are in fluid communication with the chambers 216 and 226 respectively via one or more channels 213. 223. Channels 213, 223 each extend in a radial direction of the electrical machine 500 and may be formed in the stator body 212 and rotor body 222 respectively.

In the stator 210, a buoyancy force aids the exchange (convection) of the ferrofluid between the first chamber 216 and the second chamber 217 (or vice versa) via the one or more channels 213. This promotes the circulation of the ferrofluid within and between the chambers 216, 217, thereby improving the rate at which heat is carried away from the stator windings 214, and as such improving the cooling of the stator windings 214 and the stator 210 in general.

The channel(s) 223 that extend between the first chamber 226 and second chamber 227 of the rotor 220 may be angled with respect to a radial direction of the rotor 220, as shown in Figure 5. Accordingly, the centrifugal force exerted on the ferrofluid within the first chamber 226 of the rotor 226 during the operation of the electrical machine 500 promotes the exchange (convection) of the ferrofluid between the first chamber 226 and the second chamber 227 via the one or more channels 223. This promotes the circulation of the ferrofluid within and between the chambers 226, 227, thereby improving the rate at which heat is carried away from the rotor windings 224, and as such improving the cooling of the rotor windings 224 and the rotor 220 in general. Alternatively, the channels 223 of the rotor 220 may be arranged parallel to a radial direction of the rotor 220 Figure 6 shows an electrical machine 600 according to a further example of the disclosure. Where common reference numerals are used as for components already discussed in relation to previous figures, these reference numerals refer to the substantially similar components, except where explained otherwise. The electrical machine 600 is substantially similar to the electrical machine 500 of Figure 5, however the stator 210 and rotor 220 of the electrical machine 600 include barriers 219, 229 located within the chambers 216, 226 and between respective stator windings 214 or rotor windings 224. The barriers 214, 224 may extend in a radial direction of the electrical machine 600 across the radial width (i.e. width in a radial direction of the electrical machine 600) of the chambers 216, 226.

The barriers 219, 229 may be formed of a non-magnetic material and may be located between stator windings 214 or rotor windings of different electrical phases. In particular, as is well known electrical machines may be arranged to have multiple electrical phases (e.g. two or three electrical phases), such that electrical current through particular stator windings and/or rotor windings may have a different phase to one another. By placing a non-magnetic barrier 219, 229 between adjacent windings 214, 224 of different electrical phases, any increase in mutual inductance between adjacent windings 214, 224 of different electrical phases can be reduced. That is, the mutual inductance between adjacent windings 214, 224 may be increased due to the use of the ferrofluid within the chambers 216, 226 in which the windings 214, 224 are also located. This can lead to reduced output (current or torque) of the electrical machine. By providing a non-magnetic barrier 219, 229 between adjacent winding 214, 224 of different electrical phases, this increase in mutual inductance can be reduced or eliminated. Accordingly, any reduction in output current or torque for the electrical machine 600 as a result of the use of a ferrofluid (to provide the above-stated advantages) can be minimised or avoided.

In the example of Figure 6, the stator windings 214 and rotor windings 224 are wound such that all adjacent stator windings 214 have different electrical phases and all adjacent rotor windings 224 have different electrical phases. In other words, the windings 214, 224 are wound in a sequential manner. For example, for a three-phase electrical machine with phases A, B and C, the windings 214, 224 are wound such that a winding of phase A is adjacent to windings of phase B and C, a winding of phase B is adjacent to windings of phase A and C, and a winding of phase C is adjacent to windings of phase A and B. Hence, in the electrical machine 600 of Figure 6, barriers 219, 229 are provided between all adjacent windings.

Figure 7 shows an example electrical machine 700 with a different winding arrangement of the stator windings 214 and rotor windings 224. As shown in Figure 7, the stator windings 214 and/or rotor windings 224 may have a different winding arrangement to that shown in Figure 6, such that a different barrier 219, 229 arrangement is provided. As just one example, the windings 214, 224 may be arranged in pairs according to their phase, such that windings of the same phase are arranged adjacent to one another. For example, for a three-phase electrical machine, a winding of phase A is adjacent to another winding of phase A and a winding of either phase B or C, a winding of phase B is adjacent to another winding of phase B and a winding of either phase A or C, and a winding of phase C is adjacent to another winding of phase C and a winding of either phase A or B. With such an example winding arrangement, the arrangement of barriers 219, 229 shown in Figure 7 may be used, whereby a barrier 219, 229 is arranged between adjacent windings 214, 224 of different phase, but not between adjacent windings 214, 224 of the same phase. Accordingly, any increase in mutual inductance between adjacent windings of different electrical phase can be minimised with minimal increased in weight of the electrical machine 700. The winding arrangement of the example electrical machine 700 is just one example, and the windings 214, 224 may be arranged in substantially any other arrangement such as in threes (where a barrier would be located every third windings) or any other grouping of windings.

Figure 8 shows an electrical machine 800 according to a further example of the disclosure. Where common reference numerals are used as for components already discussed in relation to previous figures, these reference numerals refer to the substantially similar components, except where explained otherwise. The stator 210 of the electrical machine 800 includes a third chamber 215 comprising a ferrofluid, where the third chamber 215 may be located further from the air gap 230 in a radial direction of the electrical machine 800 than the chamber 216 and second chamber 217. The third chamber 215 may be formed in the stator body 212 and may not be in fluid communication with the first chamber 216 or the second chamber 217. The rotor 220 of the electrical machine 800 may also include a third chamber 225 comprising a ferrofluid, where the third chamber 227 may be located further from the air gap 230 in a radial direction of the electrical machine 800 than the chamber 226 and second chamber 227. The third chamber 225 may be formed in the rotor body 222 and may not be in fluid communication with the first chamber 226 or the second chamber 227.

The third chambers 215, 225 extend in a circumferential direction of the electrical machine 800 and may extend along the full axial length of the stator 210 and rotor 210 (i.e. into the page). In some examples, the electrical machine 800 may be provided without the barriers 219, 229 described above in relation to Figures 6 and 7.

The third chambers 215, 225 may provide additional magnetic shielding to prevent magnetic flux from escaping outside of the electrical machine 800 in a manner similar to the second chamber 217 of the electrical machine 400 of Figure 4, as described above. Therefore, in some examples the ferrofluid within the third chambers 215, 225 may have a higher volumetric concentration of magnetic particles than the ferrofluid within the chambers 216, 226 and second chambers 217, 227. As an example, the ferrofluid within the chambers 216, 226 and second chambers 217, 227 may have a volumetric concentration of magnetic particles of 2%-4%, and the ferrofluid of the third chambers 215, 225 may have a volumetric concentration of magnetic particles of 8%-18%.

By having a higher volumetric concentration of magnetic particles, the ferrofluid within the third chambers 215, 225 provides greater magnetic shielding than the ferrofluid within the chambers 216, 226 and second chambers 217, 227, but has poorer thermomagnetic convection properties. In other words, the level of convention of the ferrofluid within the third chambers 215, 225 due to thermomagnetic properties is reduced due to the increased volumetric concentration of magnetic particles. However, as the third chambers 215, 225 are not located proximate to the stator windings 214 or rotor windings 224, the poorer thermomagnetic convection properties are not particularly disadvantageous. Furthermore, by providing a ferrofluid with comparatively higher volumetric concentration of magnetic particles in the third chambers 215, 225, the volumetric concentration of magnetic particles in the ferrofluid of the chambers 216, 226 and second chambers 217, 227 may be comparatively lower, which may yield improved thermomagnetic convection properties, but poorer shielding properties. Therefore, the innermost chambers (the chambers 216, 226 and second chambers 217, 227, which are both closer to the air gap 230 than the third chambers 215, 225) may be optimised for improved thermomagnetic convection, while the outermost chambers 215, 225 (the chambers 215, 225 that are further from the air gap 230) may be optimised for improved magnetic shielding. As such, by utilising two different ferrofluid compositions in the stator 210 and/or rotor 220, with different volumetric concentrations of magnetic particles, optimised shielding and thermomagnetic convection (and hence cooling) may be achieved. Moreover, the electrical machine 800 of Figure 8 allows this optimisation to be achieved while still providing the improved ferrofluid circulation and winding cooling as discussed above in relation to Figure 5.

Figure 9 shows an electrical machine 900 according to a further example of the disclosure.

Where common reference numerals are used as for components already discussed in relation to previous figures, these reference numerals refer to the substantially similar components, except where explained otherwise. One or both of the stator 210 and rotor 220 of electrical machine 900 may additionally include a lamination 213, 223 of a magnetic material that is located further from the air gap than chambers 216, 226 and Cif provided) second chambers 217, 227 and/or third chambers 215, 225. The laminations 213, 223 extend in a circumferential direction of the electrical machine 900 and may extend along the full axial length of the stator 210 and rotor 210 (i.e. into the page). In some examples, the electrical machine 900 may be provided without the barriers 219, 229 described above in relation to Figures 6 and 7.

The laminations 213, 223 are formed of a magnetic material and as such provide additional electromagnetic shielding to prevent leakage of magnetic flux outside the electrical machine 900 or into the core of the electrical machine 900. Accordingly, the efficiency of the electrical machine 900 is increased, and electromagnetic interference is prevented. As an example, the laminations 213, 223 may each have a thickness On a radial direction of the electrical machine 900) of approximately 0.1mm, with a total thickness of all laminations of approximately 2mm. As such, the thickness of the laminations 213, 223 may be smaller than conventional shielding laminations due to the presence of the ferrofluid(s) within the chambers 216, 226 and, if provided, the second chambers 217, 227 and third chambers 215, 225. Accordingly, as the laminations 213, 223 can be thinner than in conventional shielding techniques, any weight increase from the provision of the laminations 213, 223 can be minimised, and iron loss from the provision of the laminations 213, 223 can be minimised.

The electrical machines illustrated and described in Figures 2-9 may be so-called air-core electrical machines, or may have non-magnetic cores. However the techniques described above are equally applicable to electrical machines with cores made of ferromagnetic materials. Figure 10 illustrates a stator 1000 of a ferromagnetic-core electrical machine. The stator 1000 includes a stator body 1000 formed of a series of axially stacked laminations, as with conventional ferromagnetic-core electrical machines. The stator body 1010 includes a plurality of slots (or chambers) 1020 in which windings 1040 are located. In the present example, the slots 1020 extend in a radial direction of the stator 1000 and the windings 1040 are wound in a radial manner, whereby particular turns of the windings 1040 are located at the same circumferential position of the stator 1000 but at different radial locations. In other words, particular turns of the windings 1040 are located further from the axis of rotation of the electrical machine than other turns of the windings 1040. However, the same winding arrangement as shown in Figures 2-9 may also be sued. The stator 1000 shown in Figure 10 includes six slots 1020 for the purposes of illustration, however it is appreciated that the stator 1000 may include substantially any number of slots.

As shown in Figure 10, the slots 1020 each include an end region 1030 that is located within the slot 1020 but is located closer to an air gap of the electrical machine (i.e. an innermost peripheral surface of the stator for an electrical machine where the stator is external to the rotor) than the windings 1040. In other words, the end region 1030 is located between the windings 1040 and the air gap of the electrical machine. In some examples, the circumferential width of the slot 1020 is smaller at the end region 1030 of the slot 1020 in order to hold the windings 1040 in place, however this is not shown in Figure 10.

The stator 1000 of Figure 10 includes a first surface 1050 located at the boundary between the air gap of the electrical machine and the end region 1030 of the slot 1020. In this way, the first surface 1050 seals the slot 1020 such that the slot 1020 is not in fluid communication with the air gap of the electrical machine. As such, a ferrofluid can be added into the slot 1020, such that the end region 1030 of the slot may be filled with a ferrofluid without the ferrofluid leaking into the air gap of the electrical machine.

Accordingly, the advantages of utilising a ferrofluid within the same chamber as the windings (as described above in relation to Figures 2-9) can be achieved in other types of electrical machine, such as electrical machines with ferromagnetic cores. Specifically, the ferrofluid within the slots 1040 undergoes thermomagnetic convection when heated, thus causing the ferrofluid to flow and draw heat away from the windings 1040. Furthermore, the presence of ferrofluid in the end region 1030 that would conventionally be empty may increase the magnetic permeability in the region between the stator windings 1040 and rotor windings. This may lead to higher mutual-inductances between the stator windings 1040 and rotor windings (not shown), higher torque, larger output power, and increased efficiency.

The first surface 1050 that seals the slot 1040 may be formed of a material with a comparatively high magnetic permeability. For example, the first surface 1050 may be a ferromagnetic material and may in some examples be the same material at the laminations of the stator body 1010, or a different material with a higher magnetic permeability than the laminations of the stator body 1010. In this way, the magnetic permeability in the air-gap region can be further increased, despite the stator windings 1040 being enclosed within slots 1020. The stator 1000 of Figure 10 may be further combined with additional features described in relation to the electrical machines described above in relation to Figures 2 and 9.

Accordingly, from one perspective there has been described a stator and/or rotor for an electrical machine is provided with a chamber filled with a ferrofluid in which one or more windings is located. The chamber and windings are arranged such that ferrofluid is located at least between the windings and the air gap of the electrical machine. The stator and/or rotor are useable in both air-core electrical machine and iron core electrical machines.

Claims

Claims 1. A stator for an electrical machine, the stator comprising: a housing; and one or more first chambers formed in the housing; and one or more windings located within the one or more first chambers; wherein the one or more first chambers includes a first surface configured to define a first boundary of an air gap of the electrical machine, wherein the one or more windings are separated from the first surface of the one or more first chambers; wherein the one or more first chambers comprise a ferrofluid, wherein the ferrofluid is located at least between the one or more windings and the first surface of the one or more first chambers.
2. A rotor for an electrical machine, the rotor comprising: a housing; and one or more first chambers formed in the housing; and one or more windings located within the one or more first chambers; wherein the one or more first chambers includes a first surface configured to define a first boundary of an air gap of the electrical machine, wherein the one or more windings are separated from the first surface of the one or more first chambers; wherein the one or more first chambers comprise a ferrofluid, wherein the ferrofluid is located at least between the one or more windings and the first surface of the one or more first chambers.
3. The stator or rotor according to claim 1 or claim 2, wherein the housing is formed of a first material and the first surface of the one or more first chambers is formed of a second material having a higher magnetic permeability than the first material.
4. The stator or rotor according to any preceding claim, wherein the one or more first chambers are arranged coaxially with the one or more windings.
5. The stator or rotor according to any preceding claim, wherein a radial distance between the first surface of the one or more first chambers and the one or more windings is from 0.3mm to 0.4mm.
6. The stator or rotor according to any preceding claim, further comprising a second chamber formed in the housing, wherein the ferrofluid contained in the one or more first chambers is a first ferrofluid, and wherein the second chamber comprises a second ferrofluid; and wherein the second chamber is arranged coaxially with the first chamber.
7. The stator or rotor according to claim 6, further comprising one or more channels connecting the one or more first chambers and the one or more second chambers, the one or more channels having a length in a radial direction of the stator or rotor.
8. The rotor according to claim 7, wherein the one or more channels are angled with respect to a radial direction of the rotor.
9. The stator or rotor according to claim 6, wherein the one or more first chambers and the one or more second chambers are not in fluid communication with one another, wherein the ferrofluid included in the one or more first chambers is a first ferrofluid, and wherein the second chamber comprises a second ferrofluid having a volumetric concentration of magnetic particles different to that of the first ferrofluid.
10. The stator or rotor according to claim 9, wherein the volumetric concentration of magnetic particles of the first ferrofluid is between 2% and 4%, and the volumetric concentration of magnetic particles of the second ferrofluid is between 8% and 18%.
11. The stator according to any of claims 1, 3-7, and 9-10, or the rotor according to any of claims 2-10, wherein the first chamber includes two or more windings and one or more barriers between the two or more windings, wherein the one or more barriers are formed of a non-magnetic material
12. The stator according to claim 11, wherein the two or more windings are arranged to receive electrical current of different phases from a power supply.
13. The stator according to claim 12, wherein the stator comprises three windings each arranged to receive a different phase of a three-phase electrical current from the power 35 supply.
14. The stator according to any of claims 1, 3-7, and 9-13, or the rotor according to any of claims 2-11, wherein the housing is formed of a non-magnetic material.
15. The stator according to any of claims 1, 3-7, and 9-14, or the rotor according to any of claims 2-11 and 13-14, wherein the housing further comprises a lamination arranged coaxially with the first chamber.
16. The stator or rotor according to any previous claim, wherein the first channel is arranged to extend in a radial direction of the stator or rotor, and wherein the stator or rotor includes a plurality of windings arranged adjacent one another in a radial direction of the stator or rotor.
17. The stator or rotor according to any previous claim, wherein the first surface of the one or more chambers includes one or more radially extending protrusions on which the 15 windings are mounted
18. An electrical machine comprising: a stator according to any of claims 1,3-7, and 9-17; and a rotor according to any of claims 2-11 and 13-17.
19. The electrical machine according to any of claims 18-19, wherein the electrical machine is an air-core electrical machine.
20. The electrical machine according to any of claims 18-19, wherein the electrical machine includes a core formed of a ferromagnetic material.