Homopolar electric machine

Abstract

The technology relates to an electric machine for converting between electrical energy and mechanical energy, for example a motor or a generator. The electric machine may be homopolar and comprise a stator comprising an armature winding and a magnetic field generator, and a rotor comprising one or more magnetic rotor portions. The magnetic field generator may generate a magnetic field in the one or more magnetic rotor portions. The magnetic field generator may comprise field coils be formed from superconducting material. The stator may comprise a plurality of magnetic stator portions and the magnetic field generator may generate a magnetic field in the magnetic stator portions. Adjacent pairs of the magnetic stator portions and / or the magnetic rotor portions may be spaced azimuthally.

Description

[0001] HOMOPOLAR ELECTRIC MACHINE

[0002] 1. FIELD OF THE TECHNOLOGY

[0003] The technology generally relates to the field of electric machines. More particularly, the technology relates to the field of electric motors and / or electric generators. Still more particularly, the technology relates to homopolar electric motors and / or homopolar electric generators.

[0004] 2. BACKGROUND TO THE TECHNOLOGY

[0005] An electric motor is a type of electric machine that converts electrical energy into mechanical energy, often in the form of rotational motion. An electric generator is a type of electric machine that converts mechanical energy, often in the form of rotational motion, into electrical energy. Electric motors and generators are used in a myriad number of applications.

[0006] A superconducting electric machine incorporates one or more superconducting elements. Superconductors exhibit zero resistance under the critical temperature and consequently can operate more efficiently than non-superconducting machines.

[0007] A homopolar electric machine is a type of electric machine in which the polarity of the conductor and / or magnetic field poles does not change (e.g. the machine does not require commutation). The architecture of a homopolar electric machine may include both the alternating current (AC) armature winding and direct current (DC) field winding within the stationary part of the machine. The rotor can be constructed simply and, in some cases, with no permanent magnets, coils or electrical connections being needed. This makes the homopolar architecture very simple and robust. Also, the lack of permanent magnets means that the machine can be de-magnetised for service or when not in use.

[0008] Homopolar machines with field coils formed from conventional conductor windings have significant cooling requirements which limit the machine to only a few kilowatts. However, using a superconductor field coil increases efficiency and performance, and megawatt ratings can be achieved. A superconducting field coil can be formed to be compact and relatively simple to package, insulate and cool. Nevertheless, conventional homopolar machines may have a relatively low power factor and a relatively low specific power. Whilst there are many concept designs for electrical aircraft thruster motors, one of the biggest challenges for electric aviation is finding a viable on-board source to supply megawatts of electricity. Except for small aircraft which can use batteries, it is therefore possible that hybrid electrical power architectures will be a key step on the pathway to a fully electric power system. Hybrid systems can still be very low carbon, with SAF or hydrogen powered turbo generators providing the electrical power for thrust. Compact, lightweight, high-efficiency generators are therefore needed, in the multi megawatt class. Homopolar machines are a compelling choice for these applications as, in addition to the advantages already discussed, they enable relatively high rotational speeds that permit direct coupling to the power turbine.

[0009] Other applications for superconducting homopolar machines include large vehicle propulsion, power plants, wind turbines and mobile generators, to name but a few.

[0010] 3. OBJECT OF THE TECHNOLOGY

[0011] There is an ongoing need to provide improvements in various properties of electric machines, including any one or more of efficiency, longevity, power output, power-to-weight ratio, size, weight, transportability, reliability and serviceability. It is an object of the technology to provide an improved electric machine that provides an improvement over existing technology in one or more of these areas. Alternatively, it is an object of the technology to at least provide the public with a useful choice.

[0012] 4. SUMMARY OF THE TECHNOLOGY

[0013] According to some aspects of the technology there is provided an electric machine for converting between electrical energy and mechanical energy. In some forms, the electric machine may be a motor. The motor may convert electrical energy into mechanical energy, for example rotational energy. In other forms, the electric machine may be a generator. The generator may convert mechanical energy, for example rotational energy, into electrical energy.

[0014] According to some aspects of the technology there is provided a homopolar electric machine. The homopolar electric machine may comprise a stator and a rotor, wherein the rotor is configured to rotate relative to the stator. The stator may comprise an armature winding and a magnetic field generator. The rotor may comprise one or more magnetic rotor portions. The magnetic field generator may be configured to generate a magnetic field in the one or more magnetic rotor portions. The magnetic field generator may be formed from one or more lengths of superconducting material.

[0015] According to some aspects of the technology there is provided a homopolar electric machine. The homopolar electric machine may comprise a stator and a rotor, wherein the rotor is configured to rotate relative to the stator. The stator may comprise an armature winding and a magnetic field generator. The magnetic field generator may be configured to generate a magnetic field in one or more magnetic rotor portions of the rotor. The magnetic field generator may comprise a first field coil and a second field coil. The first field coil may be positioned proximate the first longitudinal end of the stator and the second field coil may be positioned proximate the second longitudinal end of the stator. Field coils may each be formed from one or more lengths of superconducting material.

[0016] According to some aspects of the technology there is provided a homopolar electric machine. The homopolar electric machine may comprise a stator and a rotor, wherein the rotor is configured to rotate relative to the stator. The stator may comprise an armature winding and a magnetic field generator. The stator may comprise a plurality of magnetic stator portions. The rotor may comprise a plurality of magnetic rotor portions. The magnetic field generator may be configured to generate a magnetic field in the plurality of magnetic stator portions and the plurality of magnetic rotor portions.

[0017] In some forms, adjacent pairs of the plurality of magnetic stator portions may be spaced azimuthally. In some forms, adjacent pairs of the plurality of magnetic rotor portions may be spaced azimuthally.

[0018] In some forms, a plurality of magnetic circuits may be formed at each end of the electric machine during operation.

[0019] In some forms, the plurality of magnetic stator portions may comprise a plurality of first magnetic stator portions and a plurality of second magnetic stator portions, wherein the plurality of first magnetic stator portions may be positioned proximate a first longitudinal end of the stator and the plurality of second magnetic stator portions may be positioned proximate a second longitudinal end of the stator.

[0020] In some forms, the plurality of magnetic rotor portions may comprise a plurality of first magnetic rotor portions and a plurality of second magnetic rotor portions, wherein the plurality of first magnetic rotor portions may be positioned proximate a first longitudinal end of the rotor and the plurality of second magnetic rotor portions may be positioned proximate a second longitudinal end of the rotor. In some forms, the magnetic field generator may comprise a first field coil and a second field coil. The first field coil may be positioned proximate the first longitudinal end of the stator and the second field coil may be positioned proximate the second longitudinal end of the stator. Field coils may each be formed from one or more lengths of superconducting material.

[0021] According to some aspects of the technology there is provided a homopolar electric machine. The homopolar electric machine may comprise a stator and a rotor, wherein the rotor is configured to rotate relative to the stator. The stator may comprise an armature winding and a magnetic field generator. The stator may comprise a plurality of magnetic stator portions spaced from each other azimuthally, i.e. adjacent pairs of the plurality of magnetic stator portions may be spaced azimuthally.

[0022] According to some aspects of the technology there is provided a homopolar electric machine. The homopolar electric machine may comprise a stator and a rotor, wherein the rotor is configured to rotate relative to the stator. The stator may comprise an armature winding and a magnetic field generator. The rotor may comprise a plurality of magnetic rotor portions spaced from each other azimuthally, i.e. adjacent pairs of the plurality of magnetic rotor portions may be spaced azimuthally.

[0023] According to one aspect of the technology there is provided an electric machine for converting between electrical energy and mechanical energy. The electric machine may comprise a stator and a rotor. The stator may comprise an armature winding of electrically conducting material arranged around an elongate core volume. The stator may further comprise a plurality of magnetic stator portions positioned at least partly on a first radial side of the armature winding with adjacent pairs of the plurality of magnetic stator portions being spaced azimuthally. The stator may comprise a magnetic field generator comprising one or more lengths of superconducting material. The rotor may comprise a rotor body configured to rotate around an axis extending longitudinally through the elongate core volume. The rotor may further comprise a plurality of magnetic rotor portions mounted to the rotor body with adjacent pairs of the plurality of magnetic rotor portions being spaced azimuthally around the rotor body. The plurality of magnetic rotor portions may be positioned on a second radial side of the armature winding. The magnetic field generator may be arranged to generate a magnetic field in the magnetic rotor portions and in the magnetic stator portions when the one or more lengths of superconducting material are energised. In certain forms, the plurality of magnetic stator portions may be configured such that the plurality of magnetic stator portions collectively subtend a total of approximately half of the azimuthal distance around the axis.

[0024] In certain forms, the armature winding may be wound as a multi-phase winding.

[0025] In certain forms, the plurality of magnetic rotor portions may be configured such that the plurality of magnetic rotor portions collectively subtend a total of approximately half of the azimuthal distance around the axis.

[0026] In certain forms, there may be the same number of the magnetic rotor portions as the number of the magnetic stator portions.

[0027] In certain forms, each of the magnetic stator portions may hook partly around the magnetic field generator.

[0028] In certain forms, a plurality of magnetic circuits may be formed when the magnetic rotor portions are azimuthally aligned with the magnetic stator portions. Each of the plurality of magnetic circuits may span between a respective magnetic rotor portion and a respective magnetic stator portion in a substantially radial direction.

[0029] In certain forms, the plurality of magnetic stator portions may be positioned at least partly radially outside the armature winding and the plurality of magnetic rotor portions are position radially inside the armature winding.

[0030] In certain forms, each of the magnetic stator portions may comprise a first surface facing radially inwards and a second surface facing radially inwards, and each of the magnetic rotor portions comprises a first surface facing radially outwards and a second surface facing radially outwards. The magnetic stator portions and the magnetic rotor portions may be configured so that, as the rotor body rotates on the axis, each of the first surfaces of the magnetic stator portions moves through positions in which it is directly adjacent to each of the first surfaces of the magnetic rotor portions and each of the second surfaces of the magnetic stator portions moves through positions in which it is directly adjacent to each of the second surfaces of the magnetic rotor portions. In certain forms, each of the magnetic rotor portions may be configured with the second surface projecting radially further outward than the first surface.

[0031] In certain forms, the plurality of magnetic stator portions may be positioned at least partly radially inside the armature winding and the plurality of magnetic rotor portions are positioned radially outside the armature winding.

[0032] In certain forms, the plurality of magnetic stator portions may comprise a plurality of first magnetic stator portions and a plurality of second magnetic stator portions. The plurality of first magnetic stator portions may be positioned proximate a first longitudinal end of the stator and the plurality of second magnetic stator portions may be positioned proximate a second longitudinal end of the stator.

[0033] In certain forms, the plurality of first magnetic stator portions may be azimuthally offset from the plurality of second magnetic stator portions.

[0034] In certain forms, the plurality of magnetic rotor portions may comprise a plurality of first magnetic rotor portions and a plurality of second magnetic rotor portions. The plurality of first magnetic rotor portions may be positioned proximate a first longitudinal end of the rotor and the plurality of second magnetic rotor portions may be positioned proximate a second longitudinal end of the rotor.

[0035] In certain forms, the plurality of first magnetic rotor portions may be azimuthally offset from the plurality of second magnetic rotor portions.

[0036] In certain forms, the magnetic field generator may comprise one or more field coils. Each of the field coils may loop around the axis.

[0037] In certain forms, the magnetic field generator may comprise a first field coil and a second field coil. The first field coil may be positioned proximate a first longitudinal end of the stator and the second field coil may be positioned proximate a second longitudinal end of the stator.

[0038] In certain forms, each of the field coils may be positioned inside an annular cryostat.

[0039] According to one aspect of the technology there is provided a motor comprising an electric machine as provided in any other aspect of the technology. According to one aspect of the technology there is provided a generator comprising an electric machine as provided in any other aspect of the technology.

[0040] Further aspects of the technology, which should be considered in all its novel aspects, will become apparent to those skilled in the art upon reading of the following description which provides at least one example of a practical application of the technology.

[0041] 5. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] One or more embodiments of the technology will be described below by way of example only, and without intending to be limiting, with reference to the following drawings, in which:

[0043] Figure 1 is a perspective view illustration of an electric machine 100 according to one form of the technology;

[0044] Figure 2 is a side view illustration of the electric machine 100 of Figure 1;

[0045] Figure 3 is an end view illustration of the electric machine 100 of Figure 1;

[0046] Figure 4 is a cross-sectional view illustration of the electric machine 100 of Figure 1;

[0047] Figure 5 is an exploded view illustration of the electric machine 100 of Figure 1;

[0048] Figure 6 is an illustration of magnetic field strength modelling in the electric machine 100 of

[0049] Figure 1;

[0050] Figure 7 is a perspective view illustration of part of the rotor 300 of the electric machine 100 of Figure 1;

[0051] Figure 8 is a perspective view illustration of part of a rotor 300 of an electric machine 100 according to another form of the technology;

[0052] Figure 9 is a perspective view illustration of part of a rotor 300 of an electric machine 100 according to another form of the technology;

[0053] Figure 10 is a perspective view illustration of part of a rotor 300 of an electric machine 100 according to another form of the technology;

[0054] Figure 11 is a cross-sectional view illustration of part of a stator 200 according to one form of the technology;

[0055] Figure 12 is an exploded view illustration of the stator 200 of Figure 11;

[0056] Figure 13 is an exploded view illustration of an electric machine 100 according to another form of the technology; Figure 14 is a cross-sectional view illustration of the electric machine 100 of Figure 13; and

[0057] Figure 15 is an illustration of magnetic field strength modelling in an electric machine 100 according to another form of the technology.

[0058] 6. DETAILED DESCRIPTION OF EXEMPLARY FORMS OF THE TECHNOLOGY

[0059] 6.1. Electric Machine

[0060] Forms of the technology relate to an electric machine 100. An exemplary form of electric machine 100 according to one form of the technology is illustrated in Figures 1 to 5. Another exemplary form of electric machine 100 is illustrated in Figures 13 and 14. A further form is illustrated in Figure 15. Electric machine 100 may comprise a stator 200 and a rotor 300. Each of the stator 200 and the rotor 300 may be formed as an assembly of components. During operation of the electric machine 100, the parts forming the rotor 300 may move relative to the parts forming the stator 200. More particularly, the rotor 300 may rotate around a rotational axis 310. It will be appreciated that the stator 200 may move during operation, for example if the electric machine 100 itself moves, e.g. if a device of which the electric machine 100 forms part moves.

[0061] In the form of Figures 1 to 5, the rotor 300 is positioned generally radially inwardly of the stator 200 while, in the forms of Figures 13 to 15, parts of the rotor 300 are positioned radially outwardly of the stator 200. Other than the inverted nature of the arrangement of the rotor and stator between these forms, the manner of operation of the two forms of the technology is similar.

[0062] Since the shape of the electric machine 100 may be generally cylindrical in its form, terms referencing a cylindrical co-ordinate system will be used throughout this specification. That is, an axis 310 passes through the centre of the rotor 300 and the rotor 300 may rotate around this axis. A direction parallel to this axis is the "longitudinal" direction. The term "radial" refers to a direction directly towards or away from the axis, i.e. in a direction perpendicular to the axis 310. The term "azimuthal" refers to a direction around the axis 310, perpendicular to the radial direction and the longitudinal direction.

[0063] In some forms of the technology, the electric machine 100 may be comprised as part of a motor. A motor is a type of electric machine that converts electrical energy into mechanical energy, for example rotational energy in the case of some forms of the technology such as the electric machine 100 of Figures 1 to 5. In order to function as a motor, electric machine 100 may be configured to be connected to a source of electrical power. More particularly, the source of electrical power may be configured to supply alternating current to an armature winding 210 comprised as part of the stator 200. This may cause the rotor 300 to rotate around the rotational axis through the interaction of the magnetic field produced by the armature winding 210 and the magnetic field produced by the magnetic field generator 260 channelled through the magnetic portions of the rotor 300 and stator 200.

[0064] In some forms of the technology, the electric machine 100 may be comprised as part of a generator. A generator is a type of electric machine that converts mechanical energy, for example rotational energy in the case of some forms of the technology such as the electric machine 100 of Figures 1 to 5, into electrical energy. In order to function as a generator, the rotor 300 of electric machine 100 may be configured to be able to be driven to rotate by some source of rotational movement, e.g. produced by a prime mover. Rotation of the rotor 300 may cause electrical current to be induced in the armature winding 210 through interaction of the magnetic field produced by the magnetic field generator 260 channelled through the magnetic portions of the rotor 300 and stator 200.

[0065] In forms of the technology described, the electric machine 100 may operate as a synchronous machine, e.g. where there is a synchronicity between the rotation of the rotor 300 and the phase of the alternating current supplied or produced.

[0066] 6.2. Stator

[0067] As has already been explained, the stator 200 is comprised of the parts of the electric machine 100 relative to which the rotor 300 rotates during operation. In some forms, the stator 200 may be stationary during operation, although in other forms the stator 200 itself may move, for example if it is carried or otherwise caused to move.

[0068] 6.2.1. Armature Winding

[0069] In certain forms of the technology, the stator 200 may comprise an armature winding 210 of electrically conducting material.

[0070] The armature winding 210 may be arranged around an elongate core volume 215. For example, in the form of the technology shown in Figures 1-5, and as shown particularly in Figure 5, the core volume 215 around which the armature winding 210 is arranged is cylindrical in shape, with the axis of the cylinder being substantially aligned with the rotational axis 310 of the rotor 300. That is, the armature winding 210 may be arranged on the surface of a cylinder (real or imaginary), thus defining a cylindrical core volume 215. In other forms, the armature winding 210 may form an elongate core volume 215 having another shape. As shown in the Figure 5, the direction of the majority of the lengths of conducting material in the armature winding 210 may generally be longitudinal.

[0071] The stator 200 may further comprise a winding support 220 to support the armature winding 210 in position. For example, the armature winding 210 may be wound on or around the winding support 220. The winding support 220 may be an elongate tubular component with an outer or inner surface having the same shape as the desired shape of the armature winding 210. For example, in the form of Figures 1 to 5, the winding support 220 is cylindrical in shape. The winding support 220 may be formed from a non-magnetic material, for example a material having a low relative magnetic permeability. The material used to form the winding support 220 may also be relatively light and strong. In some forms, a polymer or plastic material may be used, for example a fibre-reinforced polymer such as GFRP. The winding support 220 may comprise laminations in order to reduce eddy currents, reduce hysteresis loss and to help avoid overheating.

[0072] Since, in some forms, the armature winding 210 may not be wound around a magnetic material, it may be described as having an air core.

[0073] The armature winding 210 may comprise one or more lengths of an electrically conducting material, for example copper in the form of copper or aluminium wire. In some forms, the winding may comprise litz wire. In some forms, the stator 200 may comprise a cooling mechanism to cool the armature winding 210. The cooling mechanism may comprise an assembly that positions the armature winding 210 in thermal contact with a coolant, for example an ambient temperature liquid (such as water / glycol or a dielectric coolant or transformer oil), liquid nitrogen or cryogenic gaseous helium. In some forms, for example as shown in Figures 11 and 12, the armature winding 210 may comprise longitudinal sections 212 formed from electrically conducting material, which may take the form of a bundle of litz wire, wrapped around a cooling tube 230, through which a coolant flows. In some forms, the cooling tube 230 may have a substantially rectangular cross-section to aid the formation of a substantially rectangular litz bundle around the cooling tube. The rate of flow of coolant through the cooling tube 230 may be configured to allow a relatively small temperature rise along the length of each longitudinal section 212, for example of the order of a few degrees Celsius (e.g. 5°C). This may help avoid issues caused by differential thermal expansion.

[0074] In some forms, the assembly of the longitudinal sections 212 of the armature winding 210 and the cooling tube 230 may be held in position by embedding in epoxy. Each longitudinal section 212 of the armature winding 210 may be so formed to form a series of bars, which are assembled to form a substantially cylindrical shape, with the bars aligned longitudinally, to form the armature winding 210.

[0075] As shown in Figure 12, the end of each cooling tube 230 may be fluidly connected to a cap 232, into which coolant may be fed through feed 234, which may take the form of an opening or pipe, for example. Each feed 234 may be fluidly connected to a manifold 236, which may be substantially annular in shape and supply coolant to a plurality of, for example all, the cooling tubes 230 around the circumference of the stator 200. A supply of coolant may be in turn fluidly connected to the manifold 236.

[0076] As also shown in Figures 11 and 12, the longitudinal sections 212 of the armature winding 210 may be electrically connected together, for example in series, by a plurality of end sections 214. Each end section may comprise one end that is electrically connected to an end of one of the longitudinal sections 212 and another end that is electrically connected to an adjacent end section 214. Each end section 214 may comprise a hook in between its ends, as shown in Figures 11 and 12. This hooked form of the end sections 214 may help to reduce their size, which may be beneficial in order to keep the components that form a magnetic circuit (described below) also relatively small to reduce the overall weight of the electric machine 100. In some forms, the end sections 214 may be formed as a single body of an electrically conductive material, for example as a solid body of copper.

[0077] In certain forms of the technology, the armature winding 210 is configured to be electrically connected to one or more other components. For example, the armature winding 210 may comprise a plurality of terminals for electrical connection to other components. In forms in which the electric machine 100 is comprised as part of a motor, the armature winding 210 may be configured to connect to a source of electrical power, for example an alternating current supply. In forms in which the electric machine 100 is comprised as part of a generator, the armature winding 210 may be configured to connect to a load.

[0078] In some forms, the armature winding 210 may be configured to produce a rotating magnetic field when the armature winding 210 is energised. Conventional ways of configuring the armature winding 210 to achieve this may be used. In some forms, the armature winding 210 may be wound as a multi-phase winding. Any suitable number of phases may be used. In some forms, the armature winding 210 may be wound as a three-phase winding. In other forms, two, four, five or six phase windings may be used. 6.2.2. Magnetic Stator Portions

[0079] In certain forms, the stator 200 may comprise a plurality of magnetic stator portions 250. Each magnetic stator portion 250 may be formed from magnetic material, i.e. a material having a relatively high magnetic relative permeability, for example iron. As will be explained in more detail later, in operation of the electric machine 100, the magnetic stator portions 250 channel the magnetic field generated by the magnetic field generator 260 and form part of a magnetic circuit with magnetic rotor portions 350 of the rotor 300. Consequently, each magnetic stator portion 250 may comprise two surfaces 254 and 255 that are positioned to face complementary surfaces 354 and 355 of the magnetic rotor portions 350 during operation in order to form magnetic circuits. In some forms, the surfaces 254 and 255 face substantially radially inwardly or outwardly while, in other forms, at least one of these surfaces may face in another direction. When a magnetic field is formed in the magnetic stator portions 250, the surfaces 254 and 255 may form opposite poles of a magnet.

[0080] Each magnetic stator portion 250 may be positioned at least partly on one radial side of the armature winding 210. For example, as shown in Figure 4, each magnetic stator portion 250 may comprise a part that is positioned on a radially outward side of the armature winding 210. Further, the magnetic stator portion 250 may be positioned substantially immediately radially adjacent the armature winding 210 on its radially outward side. In this form, each magnetic stator portion 250 may comprise a surface 254 facing radially inwards. In other forms, for example as shown in Figure 14, each magnetic stator portion 250 may comprise a part that is positioned on a radially inward side of the armature winding 210. Again in this form, the magnetic stator portion 250 may be positioned substantially immediately radially adjacent the armature winding 210. In both forms, the surface 254 may be positioned substantially immediately adjacent the armature winding 210. In some forms, the surface 254 may have a shape that substantially lies on a surface of a cylinder while in other forms, the surface 254 may be substantially planar.

[0081] In some forms, each of the magnetic stator portions 250 may comprise an end part positioned longitudinally near the end of the stator 200 that projects radially inwardly or outwardly. In some cases, the end part may hook partly around the field coil 262 that may, in some forms, be comprised as part of the magnetic field generator (as described below). In the form of Figures 1 to 5, the end part of each magnetic stator portion 250 may extend radially inwardly of the field coil 262, and may also extend radially inwardly of the armature winding 210, although in other forms the end part does not extend radially inwardly to this degree. As shown in Figure 4, this end part may comprise a surface 255 facing radially inwards. In some forms, the surface 255 may have a shape that substantially lies on a surface of a cylinder while in other forms, the surface 255 may be substantially planar. The magnetic stator portions 250 may each be configured such that surface 255 is positioned radially further inwardly than surface 254. In other forms, surface 255 may be positioned radially substantially level with, or radially further outwardly than, surface 254. A similar description applies to forms such as shown in Figures 13 to 15, only with the end part of the magnetic stator portion 250 extending radially outwardly instead of radially. In the form of Figure 15, the surface 255 of the end part faces radially outwardly. However, in the form shown in Figures 13 and 14, the surface 255 faces axially. In other forms, the surface 255 may face in another direction.

[0082] Each of the magnetic stator portions 250 may comprise one or more individual magnetic stator members 252 in an assembly. For example, as shown in Figures 4, 5, 13 to 15, each magnetic stator portion 250 may comprise an end member 252a that is positioned at or proximate an end of the stator 200 and comprises surface 255. The end member 252a may comprise, or form part of, the end part described in the previous paragraph. Each magnetic stator portion 250 may additionally comprise another member 252b positioned adjacent the armature winding 210 and comprising surface 254. In some forms one or more other members, for example member 252c, support the other members. Each of the magnetic stator members 252 in the assembly forming the magnetic stator portion 250 may be in substantial abutment with at least one other magnetic stator member 252 so that the magnetic stator members 252 form a connected path of magnetic material forming part of a magnetic circuit (as explained in more detail below) to channel magnetic flux during operation of the electric machine 100.

[0083] In certain forms of the technology, for example as shown in Figures 1 to 5, 13 and 14, adjacent pairs of the magnetic stator portions 250 are spaced from each other azimuthally, i.e. there is a gap between adjacent pairs of magnetic stator portions 250 around the circumference of the stator 200. In some forms of the technology, for example as shown in Figures 1 to 5, 13 and 14, the magnetic stator portions 250 are spaced substantially equally around the circumference of the stator, i.e. the gaps between adjacent pairs of magnetic stator portions 250 are substantially the same. Additionally, the azimuthal distance of each of the magnetic stator portions 250 may be substantially the same. It should be understood that a gap between the magnetic stator portions 250 in this context may mean that there is a space between adjacent pairs of magnetic stator portions 250 that is occupied by something other than another magnetic stator portion. The gap may be empty (or more accurately filled with air or any gas depending on the surrounding environment in which the electric machine 100 is situated) or, in some forms, the gaps between magnetic stator portions 250 may be filled, or partly filled, with another material, for example another solid material, e.g. a substantially non-magnetic material. Incorporating a gap between the magnetic stator portions 250 allows the amount of material in the electric machine 100 to be reduced compared to if the magnetic stator portions substantially occupied the entire azimuthal distance around the axis 310. Since the material forming the magnetic stator portions 250 may be heavy, for example iron, the omission of this material provides substantial weight reduction in the electric machine 100. In addition, it has been found that gaps between the magnetic stator portions 250 may maintain a tolerably high torque in the electric machine 100.

[0084] Each magnetic stator portion 250 subtends an azimuthal distance at the axis 310 and all of the magnetic stator portions 250 collectively subtend a total azimuthal distance at the axis 310. If the magnetic stator portions 250 entirely surrounded the axis 310, that total azimuthal distance would be 360° or all of the azimuthal distance around the axis 310. The actual total azimuthal distance subtended by the magnetic stator portions 250 individually and collectively depends on the azimuthal span of each magnetic stator portion 250 and the number of magnetic stator portions 250, which in turn also determines the azimuthal spacing between magnetic stator portions 250. This may be determined in different forms of the technology in order to achieve the desired characteristics of the electric machine 100, including a desired power-to-weight ratio and torque output. In certain forms, the magnetic stator portions 250 may be configured such that they collectively subtend a total of approximately half, for example half, of the azimuthal distance around the axis 310, i.e. approximately 180°. In such forms, the total azimuthal distance of the magnetic stator portions 250 may be substantially equal to the total azimuthal distance of the spacings between the magnetic stator portions 250. For example, in the case where there are three magnetic stator portions 250 at each end of the electric machine 100, each of the magnetic stator portions 250 subtends an azimuthal distance of approximately 60°, while in forms where there are four magnetic stator portions 250 at each end of the electric machine 100, each of the magnetic stator portions 250 subtends an azimuthal distance of approximately 45°. In other forms, the magnetic stator portions 250 may collectively subtend a proportion of the full azimuthal distance around the axis 310 that is larger than half, for example 50-80% or 60-70%, while in other forms the magnetic stator portions 250 may collectively subtend a proportion of the full azimuthal distance around the axis 310 that is smaller than half, for example 20-50% or 30-40%. The lesser the proportion, the less magnetic material there is in the stator 200, which significantly reduces the weight of the electric machine 100, especially considering that magnetic material (such as iron) is typically heavy. However, the lesser the proportion, the lower the torque is produced. It has been found that the level of torque drops off rapidly as the proportion falls lower than 50%. While the torque increases when the proportion is higher than 50%, the amount of the increase may not be worth the disadvantages associated with the increase in weight of the electric machine 100. In some forms of the technology, the number of magnetic stator portions 250 may be related to the number of magnetic poles formed on the rotor 300. For example, the number of magnetic stator portions 250 may be the same as the number of pairs of magnetic poles (i.e. north-south pairs) formed on the rotor 300. For example, in the case of the forms of electric machine shown in Figures 1 to 6, 13 and 14, that number is three. In some forms of electric machine 100, for example as shown in Figures 1 to 5, 13 and 14, there may be a plurality of magnetic stator portions 250 at each end of the stator 200 (explained further below). In such a case, there may be six magnetic stator portions 250 and six magnetic rotor portions 350 in total. In other forms, the number of magnetic stator portions 250 may be four, five or seven, or double those amounts where there are magnetic stator portions 250 at each end of the stator 200. As will be explained below, the number of pairs of magnetic poles formed on the rotor 300 may be the same as the number of magnetic rotor portions 350.

[0085] In some forms of the technology, the armature winding 210 may be arranged such that the armature winding 210 forms a plurality of slots, with the number of slots being equal to the number of phases or an integer multiple thereof. Each of the magnetic stator portions 250 may be configured so that it subtends an azimuthal distance that corresponds to substantially half of each slot. In such forms, the magnetic stator portions 250 collectively subtend a total of approximately half of the azimuthal distance around the axis 310, as explained earlier.

[0086] In some forms of the technology, for example as shown in Figures 1 to 5, 13 and 14, the plurality of magnetic stator portions 250 comprises two sets of magnetic stator portions 250, each set being positioned at or proximate opposite longitudinal ends of the stator 200. As can be seen in Figures 4 and 13, the proximal ends of the magnetic stator portions 250 in each set may be separated longitudinally along the axis 310 although, as shown in Figures 4 and 14, the separation distance between the proximal ends of each set may be relatively small in comparison to the overall length of the stator 200. The proximal ends of the magnetic stator portions 250 in each set may be separated longitudinally along the axis 310 by a gap and, as with the azimuthal gap between the magnetic stator portions 250 described above, the longitudinal gap may be filled, or partly filled, with any suitable material, including air or a solid material. Also as shown in Figures 1 to 5, 13 and 14, in some forms of the technology, the sets of magnetic stator portions 250 may be azimuthally offset from each other. That is, each of the magnetic stator portions 250 of one of the sets of magnetic stator portions 250 may be azimuthally offset from each of the magnetic stator portions 250 of the other set of magnetic stator portions 250, and vice versa. For example, an azimuthally central region of a magnetic stator portion 250 of the first set may azimuthally align with the centre of the gap between a magnetic stator portion 250 of the second set. This may be true of all of the magnetic stator portions 250. This may mean that, in forms of the technology in which the magnetic stator portions 250 collectively subtend a total of half or less of the azimuthal distance around the axis 310, there are some azimuthal angles at which there is no magnetic stator portion 250.

[0087] While the exemplary forms of electric machine 100 shown in Figures 1 to 5, 13 and 14 comprise a stator 200 in which there is a set of magnetic stator portions 250 at each end, in other forms, the stator 200 may comprise only a single set of magnetic stator portions 250. This may be visualised as a stator similar to one longitudinal half of the stator 200 shown in Figures 1 to 5 or 13 and 14, for example as shown in Figure 15. In each set of magnetic stator portions 250, whether there is one or two sets, each of the magnetic stator portions 250 may be substantially longitudinally aligned with each other, e.g. their proximal and / or distal ends may be in substantial longitudinal alignment respectively.

[0088] 6.2.3. Magnetic Field Generator

[0089] In certain forms of the technology, for example as shown in Figures 1 to 5, 13 to 15, the stator 200 may comprise one or more magnetic field generators 260. The magnetic field generators 260 may each comprise one or more lengths of superconducting material, for example high-temperature superconducting (HTS) material. During operation of the electric machine 100, the one or more lengths of superconducting material may be energised, i.e. supplied with current. Consequently, when the superconducting material is in the superconducting state and energised, each magnetic field generator 260 may generate a magnetic field having a relatively high magnetic field strength for even moderate current supplies to the length of superconducting material. This increases the efficiency of the electric machine 100 compared to forms of the technology in which the magnetic field generator 260 comprises one or more lengths of a normal conductor.

[0090] In certain forms, for example as shown in Figures 1 to 5, 13 and 14, each magnetic field generator 260 may comprise lengths of superconducting material arranged into one or more field coils 262. The field coils may alternatively be referred to as excitation coils or excitation windings. The field coils 262 may be coils of superconducting material that loop around the axis 310, for example all or substantially all of the azimuthal distance around the axis 310. In some forms, each field coil 262 may be arranged to substantially define a plane and the plane may be substantially perpendicular to axis 310. Additionally, or alternatively, each field coil 262 may be substantially circular in plan view and each field coil 262 may be positioned substantially co-axially with axis 310. A diameter of the field coils 262 may in some forms be substantially similar to a diameter of the armature winding 210, while in other forms a diameter of the field coils 262 may be larger or smaller than that of the armature winding 210.

[0091] In some forms, each of the field coils 262 may comprise a coil of superconducting tape or wire and, in some forms may be formed as a double-pancake coil. The tape or wire may be formed of any suitable superconducting material but, in some forms, may be formed of a high-temperature superconductor such as REBCO. Other configurations of windings of superconducting material may be used in other forms of the technology.

[0092] As shown in Figures 1 to 5, 13 and 14, each magnetic field generator 260 may be positioned closely adjacent an end of the armature winding 210. For example, the field coil 262 may be positioned closely adjacent an end of the armature winding 210. Positioning the field coils 262 at or proximate the ends of the electric machine 100 may also provide easier access to the coils for servicing / maintenance than homopolar electric machines where the coil is situated near the centre of the machine. Irrespective of the position of the field coils 262 relative to the longitudinal axis of the electric machine 100, e.g. even in forms where the field coils 262 are not positioned proximate the ends of the electric machine 100, the field coils 262 may still be relatively accessible in comparison to some conventional machines because the field coils may be accessed without needing to remove many other components. For example, in the form illustrated in Figures 1 to 7, the field coil 262 may be accessed upon removal of only end part 252a and / or member 252c, and in the form illustrated in Figures 13 and 14, the field coil 262 may be accessed upon removal of only end part 252a. In contrast, some conventional machines may need to be largely disassembled in order to access the field coil for servicing or replacement.

[0093] As may be seen in Figures 1, 4 and 14, for example, each magnetic field generator 260, for example each field coil 262, may be positioned closely adjacent to some of the magnetic stator portions 250. When the magnetic stator portions 250 comprise a plurality of magnetic stator members 252, each field coil 262 may be positioned closely adjacent to each of the magnetic stator members 252 comprised as part of the adjacent magnetic stator portion 250. As has been explained earlier, each of the magnetic stator portions 250 may hook partly around the respective field coil 262, and an end part of each magnetic stator portion 250 may extend radially inwardly or outwardly of the field coil 262. During operation, when the magnetic field generator 260 generates a magnetic field, the magnetic field is channelled through the magnetic stator portions 250. In addition, the magnetic field generator 260, e.g. the one or more field coils 262, may be positioned sufficiently close to the magnetic rotor portions 350 that, during operation, when the magnetic field generator 260 generates a magnetic field, the magnetic field is channelled through the magnetic rotor portions 350. Consequently, the magnetic stator portions 250, in combination with the magnetic rotor portions 350, each form part of a magnetic circuit which, in operation of the electric machine 100, channel the magnetic field generated by the magnetic field generator 260 when energised.

[0094] In some forms of the technology, the magnetic field generator 260 may be configured to generate magnetic fields at each end of the stator 200. For example, as shown in Figures 1 to 5, 13 and 14, the magnetic field generator 260 may comprise two field coils 262. Each field coil 262 may be positioned proximate respective ends of the stator 200. Each of the field coils 262 may be as described earlier. Each of the field coils 262 may be positioned closely adjacent to a respective set of the magnetic stator portions 250 such that separate magnetic fields are generated in each of the sets of magnetic stator portions 250, one by each of the field coils 262.

[0095] In some forms, the field coils 262 may be arranged such that the direction of flow of current in each of the coils is the same. For example, this may be advantageous in forms such as illustrated in Figures 1 to 5, 13 and 14 in which there are three phases and three pairs of poles / magnetic rotor portions 350 at each end rotationally offset from each other. In other forms, the field coils 262 may be arranged such that the direction of flow of current in each of the coils is different, i.e. the current flows anticlockwise in one of the coils and clockwise in the other. This may be useful in electric machines 100 with a different number of poles, phases and / or degrees of offset between ends of the machine, for example. If the direction of current flow in the field coils 262 is opposite, the direction of the magnetic field formed in the region between the coils 262 will be in the same direction throughout that region. In certain forms, the field coils 262 may both receive current from the same current supply. In certain forms, the magnitude of the current in both field coils 262 may be substantially the same.

[0096] To maintain the one or more lengths of superconducting material at a temperature at which they are in a superconducting state, each of the field coils 262 may be positioned inside an annular cryostat 270. Each cryostat 270 may, for example, be a vacuum-insulated cryostat, although any suitable cryostat may be used. Each cryostat may be thermally coupled to a cryocooler Til, for example through copper conduction plates positioned in abutment with the lengths of superconducting material at the cryocooler TIT An advantage of the magnetic field generator 260 comprising field coils 262 is that, in those forms of the technology, the electric machine does not use permanent magnets to generate the magnetic fields. Permanent magnets that produce magnetic fields of comparable strength would be relatively large in size and may present difficulties for servicing the electric machine 100 and in terms of the mechanical strength needed in the electric machine 100. In contrast, a field coil 262 allows the electric machine 100 to be de-magnetised for service and during assembly. Nevertheless, there may be some situations in which permanent magnets are desirable, or at least tolerable, and in some forms of the technology, the magnetic field generator 260 may comprise one or more permanent magnets.

[0097] In some forms, the lengths of superconducting material in the magnetic field generator 260 may be energised using a flux pump. This may be advantageous as, in these forms, the superconducting material may be energised in a contactless fashion, which may maintain good thermal isolation of the superconducting material and consequently improve the efficiency of operation of the electric machine 100. In other forms, the lengths of superconducting material may be energised by supplying a supply of electric current through connected current leads, for example supply wires. This may reduce the complexity of the design, but increase the thermal load because of heat transfer through the leads.

[0098] 6.2.4. Frame

[0099] In certain forms of the technology, the electric machine 100 may comprise a frame 280 that is structured and arranged to position and secure in place one or more other components of the electric machine 100. The frame 280 may comprise one or more frame members.

[0100] In some forms, for example in the form of the technology illustrated in Figures 1 to 5, the frame 280 comprises a collar 282 and two end caps 284. The collar 282 may be an annular component used in forms of the technology in which there is a set of magnetic stator portions 250 at each and two field coils 262. The collar 282 may be positioned longitudinally centrally and radially outside the magnetic stator portions 250. As shown in Figure 4, in cross-section the collar 282 may be generally T-shaped with the arms of the T having radially inner sides positioned in abutment with radially outer sides of the magnetic stator portions 250 and the leg of the in abutment with the longitudinal ends of the magnetic stator portions 250.

[0101] The end caps 284 may be positioned at the longitudinal ends of the electric machine 100. They may be generally circularly shaped and may include a central opening positioned co-axially with the axis 310 and through which the axle 312 of the rotor 300 may pass. The end caps 284 may each be fastened to the adjacent magnetic stator portions 250.

[0102] The frame members, for example the collar 282 and end caps 284 may be formed from a material with low magnetic relative permeability, for example plastic or polymer (e.g. fibre-reinforced polymer). The material used to form the frame members may be selected to be relatively light and strong to provide structural rigidity to the electric machine 100 without greatly increasing its weight. Positioning a frame member that is formed from a material with low magnetic relative permeability, or part thereof, in between the magnetic stator portions 250 at each end of the electric machine 100 may assist in keeping separate the magnetic circuits that are formed at each end of the electric machine 100 during operation. For example, the leg of the collar 282 serves this function in the example of Figures 1 to 5. The frame members may additionally be formed from an electrically insulating material, which may help to limit eddy current heating that may otherwise be generated from the magnetic rotor portions 350 that are in close proximity.

[0103] In other forms, other arrangements of frame may be used and the collar 282 and end caps 284 illustrated in Figures 1 to 5 are merely exemplary. In some forms, the frame 280 may comprise a housing within which the electric machine 100 is entirely or substantially housed, for example. In addition, the magnetic stator portions 250 may provide significant structural strength and rigidity to the stator 200.

[0104] In some forms, for example forms in which parts of the rotor 300 are positioned radially outside the stator 200 as shown in Figures 13 and 14, the frame 280 may be configured in a different manner. The frame 280 in these forms may comprise a central hollow shaft 286, within which the axle 312 is positioned and able to rotate. The shaft 286 may be mounted to an end cap 284 and, in this form of the technology, there may be only a single end cap 284 at or proximate one end of the electric machine 100. One or more projections 288 may extend radially outwardly from the shaft 286. The projections may be in the form of discs or arms. The magnetic stator portions 250 may be mounted to the projections 288, for example by an interlocking fit or other suitable mounting mechanism.

[0105] 6.3. Rotor

[0106] The rotor 300 may be comprised of the parts of the electric machine 100 that rotate relative to the stator 300 during operation. In certain forms of the technology, the rotor 300 may comprise a plurality of magnetic rotor portions 350 mounted to a rotor body 320. The rotor 300 may be configured to rotate around an axis 310 and in some forms may be configured to rotate on an axle 312 around the axis 310, for example the rotor body 320 may comprise the axle 312 or be rotationally mounted to the axle 312.

[0107] 6.3.1. Magnetic Rotor Portions

[0108] In certain forms, the rotor 300 may comprise a plurality of magnetic rotor portions 350. Each magnetic rotor portion 350 may be formed from magnetic material, i.e. a material having a relatively high magnetic relative permeability, for example iron. In some forms, the magnetic rotor portions 350 may be formed from the same material as the magnetic stator portions 250. The magnetic rotor portions 350 may be positioned sufficiently close to the magnetic field generator 260, e.g. field coils 262, that the magnetic field produced by the magnetic field generator 260 is channelled through the magnetic rotor portions 350 which, together with the magnetic stator portions 250, form a plurality of magnetic circuits.

[0109] As the rotor 300 rotates relative to the stator 200, the magnetic rotor portions 350 rotate relative to the magnetic stator portions 250. The rotor 300 and the stator 200 are configured such that the magnetic rotor portions 350 and the magnetic stator portions 250 form a magnetic circuit for the magnetic field produced by the magnetic field generator 260 at times during the rotation of the rotor 300 when one of the magnetic rotor portions 350 is azimuthally aligned with one of the magnetic stator portions 250. To form such a magnetic circuit, each magnetic rotor portion 350 may comprise two surfaces 354 and 355 that are positioned to face complementary surfaces 254 and 255 of the magnetic stator portions 250 during operation. When a magnetic field is formed in the magnetic rotor portions 350, the surfaces 354 and 355 may form opposite poles of a magnet.

[0110] In addition, in some forms the magnetic rotor portions 350 and the magnetic stator portions 250 may be configured such that the magnetic circuit spans between the magnetic rotor portions 350 and the magnetic stator portions 250 in a substantially radial direction, and in some forms only in a substantially radial direction. That is, lines of magnetic flux spanning across the gap between the magnetic rotor portions 350 and the magnetic stator portions 250 may be in a substantially radial direction and, in some forms, are substantially not in an axial direction. Compared to some forms in which the magnetic rotor portions 350 and the magnetic stator portions 250 are such that the magnetic circuit spans between them in a substantially axial direction, the amount of material in the portions may be reduced, thus saving weight. To enable such a configuration, each magnetic rotor portion 350 may comprise two surfaces 354 and 355 that face substantially radially outwardly or inwardly. These surfaces are positioned to face complementary surfaces 254 and 255 of the magnetic stator portions 250 during operation in order to form magnetic circuits. When a magnetic field is formed in the magnetic rotor portions 350, the surfaces 354 and 355 may form opposite poles of a magnet. Consequently, in some forms, for example the form of Figures 1 to 5, both poles of the magnetic rotor portions 350 may face radially outwardly while, in other forms, for example the form of Figure 15, both poles of the magnetic rotor portions 350 may face radially inwardly.

[0111] In another form of the technology, for example as shown in Figure 14, the magnetic rotor portions 350 and the magnetic stator portions 250 may be configured such that the magnetic circuit spans between one part of the magnetic rotor portions 350 and one part of the magnetic stator portions 250 in a substantially axial direction. In Figure 14, surface 255 of end part 252a faces in a substantially axial direction and magnetic rotor portion 350 comprises a complementary surface 355 also facing in a substantially axial direction, although in the opposite direction.

[0112] Each magnetic rotor portion 350 may comprise at least a part that is positioned on a side of the rotor body 320 facing radially outwardly or inwardly. For example, as shown in Figure 4, each magnetic rotor portion 350 may comprise a part that is positioned substantially immediately radially adjacent the armature winding 210 on a radially inward side thereof. Each magnetic rotor portion 350 may comprise a surface 354 facing radially outwards. In other forms, for example as shown in Figures 14 and 15, each magnetic rotor portion 350 may comprise a part that is positioned substantially immediately radially adjacent the armature winding 210 on a radially outward side thereof. In both forms, the surface 354 may be positioned substantially immediately adjacent the armature winding 210. In some forms, the surface 354 may have a shape that substantially lies on a surface of a cylinder (real or imaginary) while in other forms, the surface 354 may be substantially planar.

[0113] In some forms, each of the magnetic rotor portions 350 may comprise a step or a discontinuity in its radially outwardly or inwardly facing surface. Each magnetic rotor portion 350 may comprise an end part positioned longitudinally near the end of the rotor 300 and the end part may comprise another surface 355 facing radially outwards or inwards and which may be discontinuous with the surface 354. In some forms, the surface 355 may have a shape that substantially lies on a surface of a cylinder (real or imaginary) while in other forms, the surface 355 may be substantially planar. The magnetic rotor portions 350 may each be configured such that surface 355 is positioned radially further inwardly or outwardly than surface 354. For example, in the forms shown in Figures 1 to 5 and 15, the magnetic rotor portions 350 comprise such a step. In other forms, the magnetic rotor portions 350 may not include a step or discontinuity and their radially outer or inner sides may be continuous. In such forms, the surface 354 may be a region at one end of the portion and the surface 355 may be at the other end, both surfaces still forming poles of a magnet in operation. Such a form is shown in Figures 13 and 14.

[0114] The shape of the magnetic rotor portions 350 may be complementary to the shape of the magnetic stator portions 250 so that each of the magnetic rotor portions 350 is able to closely nest with each of the magnetic stator portions 250 with which it is rotationally aligned. In the nesting configuration, the surfaces 254 and 354, and the surfaces 255 and 355 may directly face each other and be closely spaced. The respective parts may be configured so that the spacing between the respective surfaces in the nesting configuration is sufficiently small that there is good magnetic flux linkage between the surfaces but sufficiently large that there is substantially no chance of impact between the surfaces as the rotor 300 rotates with respect to the stator 200 at operational speeds. The spacing between the surfaces 254 and 354 also allows space for the armature winding 210 to be positioned between the surfaces.

[0115] Figures 6 and 15 are illustrations of magnetic field strength modelling within exemplary electric machines 100. The form modelled in Figure 6 has the stator 200 radially outward of the rotor 300, similar to the case of the machine 100 in Figures 1 to 5. The axis of rotation 310 is illustrated at the bottom of the figure. On the left-hand side is shown a magnetic stator portion 250, a magnetic rotor portion 350, a field coil 262 and a winding support 220 modelled as a number of laminations similar to these components as illustrated in Figures 1 to 6. The magnetic circuit looping around the field coil 262 can clearly be seen. On the right-hand side is shown a field coil 262 surrounded only by air. Figure 6 illustrates the significantly stronger magnetic flux densities that are generated in the magnetic circuit comprising the magnetic stator portion 250 and the magnetic rotor portion 350 compared to what is generated in the air. The strong magnetic flux densities inside the magnetic stator portion 250 and magnetic rotor portion 350 are also shown in Figure 15, although the form modelled in this figure has the stator 200 radially inward of the rotor 300, similar to the case of the machine 100 in Figures 13 and 14 (although in Figure 15 the surfaces 255 and 355 face radially outwardly and inwardly respectively whereas in Figures 13 and 14 these surfaces face axially).

[0116] Each of the magnetic rotor portions 350 may comprise one or more individual magnetic rotor members 352 in an assembly. In the forms of the technology illustrated in Figures 4 to 5, 7 to 10, 13 and 14, each magnetic rotor portion 350 comprises a single magnetic rotor member 352, but in other forms of the technology each magnetic rotor portion 350 may comprise a plurality of magnetic rotor members 352 in an assembly. In such forms, each of the magnetic rotor members 352 in the assembly may be in substantial abutment with at least one other magnetic rotor member 352 so that the magnetic rotor members 352 form a connected path of magnetic material forming part of a magnetic circuit to channel magnetic flux during operation of the electric machine 100.

[0117] In certain forms of the technology, for example as shown in Figures 4 to 5, 7 to 10, 13 and 14, adjacent pairs of the magnetic rotor portions 350 are spaced from each other azimuthally, i.e. there is a gap between adjacent pairs of magnetic rotor portions 350 around the circumference of the rotor 300. In some forms of the technology, for example as shown in Figures 4 to 5, 7 to 10, 13 and 14, the magnetic rotor portions 350 are spaced substantially equally around the circumference of the rotor, i.e. the gaps between adjacent pairs of magnetic rotor portions 350 are substantially the same. Additionally, the azimuthal distance of each of the magnetic rotor portions 350 may be the substantially the same. As explained above in relation to the gaps between magnetic stator portions 250, it should be understood that a gap between the magnetic rotor portions 350 in this context may mean that there is a space between adjacent pairs of magnetic rotor portions 350 that is occupied by something other than another magnetic rotor portion. The gap may be empty (or more accurately filled with air or any gas depending on the surrounding environment in which the electric machine 100 is situated) or, in some forms, the gaps between magnetic rotor portions 350 may be filled, or partly filled, with another material, for example another solid material, e.g. a substantially non-magnetic material. In some forms, the azimuthal spacing of the magnetic rotor portions 350 may complement the azimuthal spacing between the corresponding magnetic stator portions 250, e.g. the azimuthal angle between adjacent magnetic rotor portions 350 may be the same as the azimuthal angle between adjacent magnetic stator portions 250. The complementary alignment may mean that, at certain times during the rotation of the rotor 300, all of the magnetic rotor portions 350 may be substantially aligned with respective magnetic stator portions 250 at the same time.

[0118] Each magnetic rotor portion 350 subtends an azimuthal distance at the axis 310 and all of the magnetic rotor portions 350 collectively subtend a total azimuthal distance at the axis 310. The actual total azimuthal distance subtended by the magnetic rotor portions 350 individually and collectively depends on the azimuthal span of each magnetic rotor portion 350 and the number of magnetic rotor portions 350, which in turn also determines the azimuthal spacing between magnetic rotor portions 350. This may be determined in different forms of the technology in order to achieve the desired characteristics of the electric machine 100, including a desired power-to-weight ratio and torque. In certain forms, the magnetic rotor portions 350 may be configured such that they collectively subtend a total of approximately half or less of the azimuthal distance around the axis 310, i.e. approximately 180° or less. In some forms, the magnetic rotor portions 350 may be configured such that they collectively subtend a total of approximately two-thirds, for example 60-75% or two-thirds, of the azimuthal distance subtended by each of the magnetic stator portions 250. For example, if each magnetic stator portion 250 covered 12 stator slots, then each magnetic rotor portion 350 would cover 8 stator slots in such a form. This arrangement may help to reduce harmonics on the output voltage (for a generator) or rotation (for a motor).

[0119] In some forms of the technology, as explained above, the number of magnetic rotor portions 350 may be related to the number of magnetic stator portions 250. For example, the number of magnetic rotor portions 350 may be the same as the number of magnetic stator portions 250. This may also be the number of pairs of magnetic poles formed on the rotor 300. For example, in the case of the electric machines shown in Figures 1 to 6, 13 and 14, in which there are three magnetic stator portions 250 and three magnetic rotor portions 350 at each end of the electric machine, that number is six. In other forms, for example in forms in which there is only a single set of magnetic stator portions 250 and magnetic rotor portions 350, that number may be three. In other forms, the number of magnetic rotor portions 350 may be four, five or seven, or double those amounts where there are magnetic rotor portions 350 at each end of the stator 200.

[0120] In some forms of the technology, for example as shown in Figures 4 and 5, 13 and 14, the plurality of magnetic rotor portions 350 comprises two sets of magnetic rotor portions 350, each set being positioned at or proximate opposite longitudinal ends of the rotor 300. As can be seen in Figures 4 and 5, 13 and 14, the proximal ends of the magnetic rotor portions 350 in each set may be separated longitudinally along the axis 310 although, as shown in Figure 4 and 14, the separation distance between the proximal ends of each set may be relatively small in comparison to the overall length of the rotor 300. The proximal ends of the magnetic rotor portions 350 in each set may be separated longitudinally along the axis 310 by a gap and, as with the azimuthal gap between the magnetic rotor portions 350 described above, the longitudinal gap may be filled, or partly filled, with any suitable material, including air or a solid material. The longitudinal position of the sets of magnetic rotor portions 350 may be aligned with respective sets of magnetic stator portions 350, i.e. the longitudinal gap between the sets of magnetic rotor portions 350 may be substantially the same as the longitudinal gap between the sets of magnetic stator portions 250 and the proximal and / or distal ends of each of the magnetic rotor portions 350 may be substantially longitudinally aligned with proximal and distal ends of respective magnetic stator portions 250. Also as shown in Figures 4 and 5, 13 and 14, in some forms of the technology, the sets of magnetic rotor portions 350 may be azimuthally offset from each other. That is, each of the magnetic rotor portions 350 of one of the sets of magnetic rotor portions 350 may be azimuthally offset from each of the magnetic rotor portions 350 of the other set of magnetic rotor portions 350, and vice versa. In some forms, the azimuthal offset of each set of magnetic rotor portions 350 (i.e. the group of magnetic rotor portions 350 aligned longitudinally on the rotor 300) may complement, e.g. be the same as, the azimuthal offset between the corresponding sets of magnetic stator portions 250. The complementary alignment may mean that, at certain times during the rotation of the rotor 300, all of the magnetic rotor portions 350 may be substantially aligned with respective magnetic stator portions 250 at the same time. For example, an azimuthally central region of a magnetic rotor portion 350 of the first set may azimuthally align with the centre of the gap between a magnetic rotor portion 350 of the second set. This may be true of all of the magnetic rotor portions 350. This may mean that, in forms of the technology in which the magnetic rotor portions 350 collectively subtend a total of half or less of the azimuthal distance around the axis 310, there are some azimuthal angles at which there is no magnetic rotor portion 350. The described offset may help to maintain a relatively high level of interaction between the magnetic fields of the magnetic field generator 260 and the armature winding 210, thus providing significant torque for a larger proportion of a cycle of the rotation of the rotor 300.

[0121] In forms in which there are a plurality of magnetic rotor portions 350 at or proximate each end of the rotor 300, in addition to there being a plurality of magnetic stator portions 250 at or proximate each end of the stator 200, the electric machine 100 comprises, at or proximate each end, separate sets of magnetic circuits formed from the magnetic rotor potions 350 and magnetic stator portions 250 during operation.

[0122] While the exemplary forms of electric machine 100 shown in Figures 1 to 5, 13 and 14 comprises a rotor 300 in which there is a set of magnetic rotor portions 350 at each end, in other forms, for example as shown in Figures 7 to 10, the rotor 300 may comprise only a single set of magnetic rotor portions 350. This may be visualised as a rotor similar to one longitudinal half of the rotor 300 shown in Figures 4 and 5 or, in other forms, in Figures 13 and 14. In each set of magnetic rotor portions 350, whether there is one or two sets, each of the magnetic rotor portions 350 may be substantially longitudinally aligned with each other, e.g. their proximal and / or distal ends may be in substantial longitudinal alignment respectively. 6.3.2. Rotor Body

[0123] The rotor 300 may comprise a rotor body 320. The rotor body 320 may be configured to rotate around the axis 310, for example the rotor body 320 may comprise an axle 312 or be rotationally mounted to the axle 312.

[0124] Exemplary rotor bodies 320 according to different forms of the technology are illustrated in Figures 7 to 10. Each of the examples of rotor bodies 320 comprises a hub portion 322 central to the rotor body 320 and configured to be rotationally mounted on axle 312. In some forms, for example as shown in Figures 7 to 9, the rotor body 320 may comprise arms 324 extending radially outwardly from the hub portion 322. Alternatively, for example as shown in Figure 10, the hub portion 322 may extend far enough radially outwardly to support the magnetic rotor portions 350 directly. In some forms, for example as shown in Figure 7, the rotor body 320 may further comprise one or more rim portions 326 that extend circumferentially around all or part of the circumference of the rotor body 320, the one or more rim portions 326 being connected to radially outward regions of the arms 324. In the example of Figure 7, the rotor body 320 comprises a plurality of rim portions 326, for example three rim portions, arranged in parallel in an orientation perpendicularly to the axis 310 with a spacing between them. One advantage of the forms of the technology that include one or more rim portions 326, for example that of Figure 7, that has been identified is a relatively high stiffness in the out-of-plane / longitudinal direction. This may be useful in some applications, for example aircraft generators that are subject to regular rotations in different directions, e.g. yaw and pitch, which may subject the rotor 300 to precession loads.

[0125] The magnetic rotor portions 350 may be mounted to the rotor body 320. In some forms, for example as shown in Figure 7, each magnetic rotor portion 350 is mounted to the one or more rim portions 326. In some forms, for example as shown in Figures 8 and 9, each magnetic rotor portion 350 is mounted to one or more of the arms 324. The magnetic rotor portions 350 may each be mounted in between two arms 324, as shown in Figure 8 for example. In some forms, for example as shown in Figure 10, each magnetic rotor portion 350 is mounted directly to the hub portion 322.

[0126] A rotor body 320 having a different form is shown in Figures 13 and 14. In this form, a part of the rotor 300 is positioned radially outwardly of the stator 200. In this form, the rotor 300 comprises an axle 312 and a projecting portion 327 projecting radially outwardly from one end of the axle 312. The projecting portion 328 may take a variety of forms, for example a disc or a plurality of arms. A rim portion 326 is connected to a radially outward region of the projecting portion 328 and extends axially away from the projecting portion 326. The rim portion 326 may be on the same axial side of the projecting portion 328 as the axle 312, as shown in Figures 13 and 14. The magnetic rotor portions 350 may be mounted to a radially inner side of the rim portion 326. In some forms, the rotor body 320 may further comprise one or more mount portions 329 projecting outwardly from the inner side of the rim portion 326 and configured to facilitate the mounting of the magnetic rotor portions 350 to the rim portion 326.

[0127] Any suitable mechanism may be used to mount the magnetic rotor portion 350 to the rotor body 320. In some forms, this may be achieved through an interlocking engagement between the magnetic rotor portion 350 and the rotor body 320, for example a fir-tree joint or the like. In some forms, the rotor body 320 may define a plurality of slots, within which one of the magnetic rotor portions 350 is positioned. It will be appreciated that, in view of the high rotational speeds that the rotor 300 may reach during operation, the mounting mechanism may be sufficiently robust to maintain the magnetic rotor portions 350 in position at typical operational speeds of the rotor 300.

[0128] In some forms, the rotor body 320 may be a single-piece, integrally formed component. In other forms, the rotor body 320 may be formed as an assembly of a plurality of components.

[0129] Any suitable material may be used to form the rotor body 320. In some forms, the material is a relatively strong and light material to provide the required strength for the rotor body 320 without adding significant weight. For example, the rotor body 320 may be formed from titanium, carbon, carbon-fibre composite or another fibre composite material.

[0130] In other forms of the technology, the rotor body 320 may be integrally formed with the magnetic rotor portions 350. For example, the rotor 300 may be integrally formed from a magnetic material, for example iron or steel. In comparison to such a rotor, rotors having rotor bodies such as shown in Figures 7 to 10 may reduce the amount of magnetic material used, which may reduce the size of the magnetic circuit and the weight of the rotor 300, although they are more complex in their design.

[0131] 6.4. Control System

[0132] In some forms of the technology, the electric machine 100 is configured to comprise, or configured to be connected to, a control system. The control system may be configured to control the waveform of the current supplied to, or produced by, the armature winding 210 (dependent on whether the electric machine 100 operates as a motor or generator). Conventional current control mechanisms may be used for this purpose, for example waveform generators comprising any processing system or computing device configured to run waveform generator software, including devices not dedicated to this purpose, e.g. general-purpose computing or processing devices.

[0133] The control system may help to improve performance of the electric machine 100. While some synchronous electric machines may work most efficiently when the current waveform is as smooth as possible, the electric machine 100 according to some forms of the technology described herein, particularly the existence of gaps in the magnetic materials of the magnetic stator portions 250 and / or magnetic rotor portions 350, may introduce discontinuities in the waveform, creating complexity in the current signal, including spikes and harmonics, particularly when the speed of operation is variable. Such discontinuities and complexity might create fluctuations in torque and variation in the amount of rotation produced (in a motor) or current produced (in a generator). However, a suitably configured control system may mitigate against these potential issues, helping to realise the advantages provided by the technology, such as an improved power-to-weight ratio.

[0134] 6.5. Other Remarks

[0135] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

[0136] The entire disclosures of all applications, patents and publications cited above and below, if any, are herein incorporated by reference.

[0137] Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour in any country in the world.

[0138] The technology may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features.

[0139] Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth. It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the technology and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present technology.

Claims

CLAIMS1. An electric machine for converting between electrical energy and mechanical energy, the electric machine comprising: a stator and a rotor, wherein the stator comprises: an armature winding of electrically conducting material arranged around an elongate core volume; a plurality of magnetic stator portions positioned at least partly on a first radial side of the armature winding with adjacent pairs of the plurality of magnetic stator portions being spaced azimuthally; and a magnetic field generator comprising one or more lengths of superconducting material, wherein the rotor comprises: a rotor body configured to rotate around an axis extending longitudinally through the elongate core volume; and a plurality of magnetic rotor portions mounted to the rotor body with adjacent pairs of the plurality of magnetic rotor portions being spaced azimuthally around the rotor body, wherein the plurality of magnetic rotor portions are positioned on a second radial side of the armature winding, wherein the magnetic field generator is arranged to generate a magnetic field in the magnetic rotor portions and in the magnetic stator portions when the one or more lengths of superconducting material are energised.

2. An electric machine as claimed in claim 1, wherein the plurality of magnetic stator portions is configured such that the plurality of magnetic stator portions collectively subtend a total of approximately half of the azimuthal distance around the axis.

3. An electric machine as claimed in any one of claims 1 to 2, wherein the armature winding is wound as a multi-phase winding.

4. An electric machine as claimed in any one of claims 1 to 3, wherein the plurality of magnetic rotor portions is configured such that the plurality of magnetic rotor portions collectively subtend a total of approximately half of the azimuthal distance around the axis.

5. An electric machine as claimed in any one of claims 1 to 4, wherein there are the same number of the magnetic rotor portions as the number of the magnetic stator portions.

6. An electric machine as claimed in any one of claims 1 to 5, wherein each of the magnetic stator portions hooks partly around the magnetic field generator.

7. An electric machine as claimed in any one of claims 1 to 6, wherein a plurality of magnetic circuits are formed when the magnetic rotor portions are azimuthally aligned with the magnetic stator portions, wherein each of the plurality of magnetic circuits spans between a respective magnetic rotor portion and a respective magnetic stator portion in a substantially radial direction.

8. An electric machine as claimed in any one of claims 1 to 7 , wherein the plurality of magnetic stator portions are positioned at least partly radially outside the armature winding and the plurality of magnetic rotor portions are position radially inside the armature winding.

9. An electric machine as claimed in claim 8, wherein each of the magnetic stator portions comprises a first surface facing radially inwards and a second surface facing radially inwards, and each of the magnetic rotor portions comprises a first surface facing radially outwards and a second surface facing radially outwards, wherein the magnetic stator portions and the magnetic rotor portions are configured so that, as the rotor body rotates on the axis, each of the first surfaces of the magnetic stator portions moves through positions in which it is directly adjacent to each of the first surfaces of the magnetic rotor portions and each of the second surfaces of the magnetic stator portions moves through positions in which it is directly adjacent to each of the second surfaces of the magnetic rotor portions.

10. An electric machine as claimed in claim 9, wherein each of the magnetic rotor portions is configured with the second surface projecting radially further outward than the first surface.

11. An electric machine as claimed in any one of claims 1 to 7, wherein the plurality of magnetic stator portions are positioned at least partly radially inside the armature winding and the plurality of magnetic rotor portions are positioned radially outside the armature winding.

12. An electric machine as claimed in any one of claims 1 to 11, wherein the plurality of magnetic stator portions comprises a plurality of first magnetic stator portions and a plurality of secondmagnetic stator portions, wherein the plurality of first magnetic stator portions are positioned proximate a first longitudinal end of the stator and the plurality of second magnetic stator portions are positioned proximate a second longitudinal end of the stator.

13. An electric machine as claimed in claim 12, wherein the plurality of first magnetic stator portions is azimuthally offset from the plurality of second magnetic stator portions.

14. An electric machine as claimed in any one of claims 1 to 13, wherein the plurality of magnetic rotor portions comprises a plurality of first magnetic rotor portions and a plurality of second magnetic rotor portions, wherein the plurality of first magnetic rotor portions are positioned proximate a first longitudinal end of the rotor and the plurality of second magnetic rotor portions are positioned proximate a second longitudinal end of the rotor.

15. An electric machine as claimed in claim 14, wherein the plurality of first magnetic rotor portions is azimuthally offset from the plurality of second magnetic rotor portions.

16. An electric machine as claimed in any one of claims 1 to 15, wherein the magnetic field generator comprises one or more field coils, wherein each of the field coils loops around the axis.

17. An electric machine as claimed in claim 16, wherein the magnetic field generator comprises a first field coil and a second field coil, wherein the first field coil is positioned proximate a first longitudinal end of the stator and the second field coil is positioned proximate a second longitudinal end of the stator.

18. An electric machine as claimed in any one of claims 1 to 17, wherein each of the field coils is positioned inside an annular cryostat.

19. A motor comprising the electric machine as claimed in any one of claims 1 to 18.

20. A generator comprising the electric machine as claimed in any one of claims 1 to 18.

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