Technical Field

The invention relates to radial flux electrical machines and components thereof, in particular a coil, a stator and a rotor, and a method of assembly.

Background

Electrical machines, including electric motors and electric generators, are already very widely used. However, concerns over our reliance on and the pollution caused by the fossil fuels that power internal combustion engines is creating political and commercial pressures to extend the use of electrical machines to new applications, and to expand their use in existing ones. Electrical machines are increasingly being used in vehicles, such as electric cars, motorbikes, boats and aircraft. They are also used in energy generation applications, for example generators in wind turbines.

In order to meet the needs of these applications, it will be necessary to design electrical machines that have both suitable performance properties, such as speed and torque, and high efficiency. The efficiency of electrical machines is critically important in almost all applications: it can, for example, both increase an electric vehicle’s range and decrease the required battery capacity. Decreasing the required battery capacity can in turn decrease the weight of the vehicle, which leads to further efficiency gains. Another important aspect of electric machine design is the size and shape. It is an advantage of electric machines, over internal combustion engines for example, that they can be provided in a number of sizes and shapes to suit different applications.

One known type of electrical machine is the radial flux machine. As the name suggests, the direction of the lines of magnetic flux that are cut during the operation of a radial flux machine is perpendicular to the axis of rotation of the machine. This is in contrast to axial flux machines, in which the direction of the lines of magnetic flux that are cut during the operation of the machine is parallel to the rotation axis of the machine.

Summary of the Invention

Embodiments described herein provide a conductive coil, a rotor and a stator for a radial flux machine comprising a plurality of conductive coils which provide for high machine efficiencies, ease of manufacture and good heat conduction from the coils to the stator housing which aids cooling. In addition, embodiments described herein provide an electric machine comprising such a rotor and stator, as well as devices comprising such an electric machine.

Throughout this disclosure, unless otherwise qualified, terms such as “radial”, “axial”, “circumferential” and “angle” are used in the context of a cylindrical polar coordinate system (r, &, z) in which the direction of the axis of rotation of the electrical machine is parallel to the z-axis. That is, “axial” means parallel to the axis of the rotation (that is, along the z- axis), “radial” means any direction perpendicular to the axis of rotation, an “angle” is an angle in the azimuth direction &, and “circumferential” refers to the azimuth direction around the axis of rotation.

Terms such as “radially extending” and “axially extending” should not be understood to mean that a feature must be exactly radial or exactly parallel to the axial direction. To illustrate, while it is well-known that the Lorentz force experienced by a current carrying conductor in a magnetic field is at a maximum when the direction of the current is exactly perpendicular to the direction of the magnetic flux, a current carrying conductor will still experience a Lorentz force for angles less than ninety degrees. Deviations from the parallel and perpendicular directions will therefore not alter the underlying principles of operation.

It will be understood that whilst aspects of the present invention are described with respect to an electric motor, it will be appreciated that it is also possible for the arrangements described herein to function as an electric generator.

The invention is defined in the independent claims to which reference should now be made. Preferred features are set out in the dependent claims.

According to a first aspect, a coil for use in a yokeless radial flux motor is provided. The coil comprises a plurality of windings and has a first portion and a second portion. The windings at the first portion are stacked adjacent to one another such that the first portion is substantially planar in a first plane and the windings at the second portion are stacked adjacent to one another such that the second portion is substantially planar in a second plane. The first plane and the second plane intersect along a first axis, such that the first portion and second portion are offset in a circumferential direction about the first axis and are radially offset with respect to one another from the first axis. The first portion and second portion are configured to carry an electrical current in an opposite direction to one another and in a direction substantially parallel to the first axis. As described, a plurality of windings make up the coil. The first portion and the second portion of the coil are configured to be the “active” portions that interact with a magnetic field of a rotor. They are parallel lengthwise in an axial direction and angled together and offset in a radial direction so that, when viewed along an axial direction, they form an offset “V” shape.

The offset planar winding portions allow a high density of such coils to be stacked together to form a stator. In particular, the features that allow a dense stacking of the coils are the circumferential and radial offsets of the first and second portions, as well as the angled planes of the first and second portions.

Using a plurality of insulated winding turns mitigates skin and proximity effects in the first and second portions. This is because the cross-section of each winding turn is smaller and, given that the winding turns are series connected, the current is deterministically governed to flow over the full radial extent of each portion. This reduces heating, since the current is spread more evenly through the conductive cross-section.

Optionally, the coil is comprised of a single continuous length of wire or other type of conductor that is bent or formed into the shape described above.

This forming may be done for each coil individually prior to the assembly of the coils into a stator. This allows the coils to be rapidly formed and then assembled into a stator as needed.

Optionally, the first portion and the second portion are connected by one or more connecting portions.

The connecting portions may provide structural support allowing the coils to be formed independently from the stator as well as providing an electrical connection between the first and second portions. The connecting portions may be formed of windings for the coil. Each connecting portion may have any shape but may preferably be substantially semi-circular or rectangular such that the outer part of the coil is a half-disk or rectangular surface.

Optionally, the connecting portions comprise a bend or a fold such that the first plane and the second plane intersect along the first axis or wherein the connecting portions are planar. The coils having bent connecting portions allows the coils to be easily manufactured. For example, the coil can be manufactured (e.g. wound) with the first and second planes being parallel, and then subsequently one or more bends or folds can be incorporated into the connecting portion such that the first and second planes intersect along a first axis.

Alternatively, the coils can be formed with flat or planar connecting portions. This requires a bend to be incorporated into the first and/or second portions to achieve the first and second planes intersecting along a first axis. Advantageously, such a coil is easy to assemble with other coils to form a stator. For example, the planar connecting portion can easily be slotted into a slot in an annular ring. This can allow multiple coils to quickly, easily and cheaply be assembled into a stator.

Optionally, the coil further comprises a first terminal portion connected to the first portion and a second terminal portion connected to the second portion. The first and second terminal portions are configured to be connected to one or more of another coil, a busbar or a power supply.

The terminal portions of each coil allows the coil to be connected to one or more other coils in series, or in parallel, using a common busbar.

Optionally the busbar may be located on the inner radius of a stator housing, and may in particular be located on an inner ring of a stator housing ring on an inner cylindrical surface of the stator.

This can allow the coils to be electrically arranged to form a number of groups of coils and/or stator poles. Alternatively, the coil can be connected to a power supply (or socket for subsequent connection to a power supply).

The terminal portions may extend substantially perpendicular to the first axis of the coil, for example allowing connection to the busbar, which may be located on an inner ring of a stator housing ring on an inner cylindrical surface of the stator. The terminal portions may extend in the same perpendicular direction, or in some cases in opposite perpendicular directions. Perpendicular extending terminal portions allow for very simple connection of the coils.

Optionally, the first terminal portion corresponds to a first end of a wire and the second terminal portion corresponds to a second end of the wire. In this case, the terminal portions are intrinsically formed from the wire used to form the windings of the coil for ease of manufacturing.

Optionally the windings at the first portion and the second portion are stacked adjacent to one another such that the first portion and the second portion are substantially planar and one winding thick.

Using coils that are a single winding thick, that is, a single conductor (e.g. wire) thick, allows the coils to be stacked very densely in a stator. This in turn provides for more flexibility with groupings of the coils compared to thicker coils that cannot be stacked as closely together.

According to a second aspect, a stator for use in a yokeless radial flux motor is provided. The stator comprises a plurality of coils according to the first aspect. The stator has a central axis configured to be coaxial with an axis of rotation of the radial flux motor, and each of the plurality of coils are arranged circumferentially about the central axis. The plurality of coils are arranged such that the first axis of each coil is parallel to the central axis of the stator and such that the second portion of a given coil overlaps circumferentially with the first portion of a first adjacent coil and the first portion of the given coil overlaps circumferentially with the second portion of a second adjacent coil.

The stator of the second aspect provides a very dense arrangement of coils that is easy and cheap to manufacture. In particular, the coils of the first aspect can be manufactured independently and then assembled together to form the stator. This is in contrast to the usual method of manufacturing a stator which comprises winding a conductor around teeth on a stator yoke.

It is noted that the stator of the second aspect is yokeless. However, this does not necessarily mean that the stators are ironless. Optionally the stator may have flux guides inserted into the spaces formed by the coils of the stator and so the stator is a yokeless (but not ironless) slotted stator.

Optionally, the second portion of the given coil overlaps circumferentially with the first portion of multiple adjacent coils in a first circumferential direction, and the first portion of the given coil overlaps circumferentially with the second portion of multiple adjacent coils in a second circumferential direction. That is, each coil overlaps circumferentially with multiple coils in each circumferential direction. Having each coil overlapping multiple other coils in each circumferential direction in this manner allows for a very compact stacking of the coils in the stator. It also allows for short- chording the coils to produce more sinusoidal back-emf and more sinusoidal flux density waveforms in the air gap regions. For example, short-chording by 1 slot for 6 slots per pole reduces the 5th and 7th harmonics considerably.

Optionally, each coil is arranged such that the first axis of each coil is coaxial with the central axis of the stator. That is, the first and second planes of each coil, in which the first and second portions of each coil respectively lie, are radial planes, i.e. the central axis of the stator passes through both the first and second planes of each coil in the stator.

This ensures that the whole of the first portion of each coil will enter and exit the magnetic fields of the rotor in synchronisation, as will the whole of the second portion, provided that the boundaries of the magnetic fields produced by the rotor are also radial.

Optionally, the coils are provided in a plurality of groups, and each group comprises two or more adjacent coils. These groups correspond to a pole of the stator.

Connecting up multiple coils in this manner to form the poles of the stator means that the stator can easily be configured to have a desired number of poles. Furthermore, each additional coil per group increases the number of slots per pole per phase by one, which can reduce losses and therefore improve efficiency. Advantageously, the number of coils per group can be scaled with the radius of machine.

Optionally, the stator is configured to be used with a multi-phase power supply having N phases, where the number of coils in each group is an integer multiple of N.

Optionally, the coils in each group are arranged in N subgroups of adjacent coils, each subgroup comprising an equal number of coils connected in series and configured to be connected to a different phase of the multi-phase power supply.

Connecting the coils in this manner means that the current through each coil varies in such a manner as to efficiently make use of the multiphase power supplies varying current.

The coils within a subgroup may be integrally formed together, or the subgroup may be formed by connecting, in series, a plurality of separate coils. The connection may be made using a ferrule, by brazing or by welding, for example. Separate coils may be formed by winding, bonding and forming conductors which can be performed using known winding techniques that are relatively cheap to implement. Integrally forming the coils may be expensive but may also allow for more complex topologies that cannot be achieved or are difficult to achieve by usual winding techniques. Furthermore, with integrally-formed coils, the number of constituent parts of a stator can be reduced, simplifying assembly.

Optionally, the stator further comprises a plurality of common busbars. Every second subgroup configured to be connected to a given phase of the multi-phase power supply is connected to one of the common busbars.

Optionally, the multi-phase power supply is a three phase power supply such that N is 3.

Optionally, the stator further comprises a plurality of flux guides. Spaces may be formed between adjacent first and second portions of respective coils, and the flux guides may optionally be disposed within these spaces.

Flux guides between the first and second portions of overlapping coils increase flux density whilst keeping the stator compact.

Optionally, each of the flux guides are discreet. That is, each flux guide is an individual component that is inserted into a space between coils, rather than the flux guides all being formed from a single piece around which the coils are wound, for example.

Having each flux guide be a discreet piece allows for them to easily be inserted into the spaces formed between coils during manufacture. This allows a stator with to be quickly and easily manufactured and provides a good magnetic efficiency.

Optionally, the flux guides have a uniform thickness. Optionally, each flux guide is identical.

This is advantageous because, whilst the flux guides will occupy an annular sector of a circle (i.e. the space in which they are placed will be wider towards the outside of the stator than the inside due to the radial nature of the first and second portions of the coil), the flux guides will saturate at their thinnest point. Furthermore, a uniform thickness simplifies assembly of the stator during the insertion of the flux guides as it does not matter which way round the flux guides are inserted.

Optionally, the spaces formed between adjacent first and second portions of respective coils include spaces of a first type, the spaces of a first type being formed between adjacent first and/or second portions of coils in different groups of coils. Optionally, the spaces formed between adjacent first and second portions of respective coils include spaces of a second type, the spaces of a second type being formed between adjacent first and/or second portions of coils in the same group of coils.

Optionally, the plurality of coils are divided into a first plurality of coils and a second plurality of coils, wherein the first plurality of coils are arranged circumferentially at a first radius from the central axis and the second plurality of coils are arranged circumferentially at a second radius from the central axis, the second radius being greater than the first radius. Optionally, the first and second pluralities of coils are concentric and/or the first and second pluralities of coils are arranged such that they do not overlap in a radial direction.

Optionally, the coils of the second plurality of coils are thicker than the coils of the first plurality of coils. Optionally, this is due to the windings comprising a thicker wire or other conductor, and/or because two or more layers of windings are used to form the first and second portions.

Having two or more pluralities of coils in this manner allows for a better use of circumferential space in the stator. This is because at larger radii, the circumferential distance spanned by a given angular distance is greater. Hence, the coils will be less densely packed at the outside of the stator compared to the inside. A single ring of coils spanning a certain radial distance will have more space at the outer radii than two rings of coils that together span the same radii as more coils can be included in the outer ring to make use of the additional space. Additionally or alternatively to more coils being included in the outer ring, the coils of the outer ring can use thicker conductors allowing for greater current without producing as much heat, and hence making the motor more efficient.

Optionally, the first and second portions of each coil are connected by one or more connecting portions, and the one or more connecting portions of each coil are held in one or more support rings disposed coaxially with the central axis.

Arranging the coils in support rings in this manner allows for an easy assembly of the stator by simply slotting each pre-formed coil into place within the support ring. In particular, the coils can form a structure into which the flux guides, such as lamination packs, can be placed. This allows for the stator to be manufactured quickly, and also with a high degree of accuracy which improves the efficiency of the electrical machine. Additionally, the number of flux guides and, correspondingly, slots per pole per phase of the stator can be readily increased and readily scales with the radius of the electric machine. Increasing the number of slots per pole per phase can make the circumferential, spatial magnetic flux density within the stator and the two machine airgaps more sinusoidal, with lower harmonic distortion. For sinusoidally varying phase currents, the average torque that is produced by the electrical machine results more from the interaction of the fundamental magnetic field components and not from the harmonic components. This is advantageous because harmonic components in the circumferential spatial magnetic flux density result in larger eddy currents in the permanent magnets of the rotors, which causes higher losses and increased heating. Furthermore, any additional harmonic components in the winding magnetomotive force distribution can cause increased losses in the flux guides.

According to a third aspect, a rotor for use in a radial flus flux motor is provided. The rotor has an axis of rotation and comprises a plurality of magnet pairs circumferentially disposed about the axis of rotation. Each magnet pair comprises a first magnet and a second magnet, with the magnets oriented to present opposite poles to one another and configured to rotate about the axis of rotation in synchronism. Each first magnet is located at a first radius from the axis of rotation and each second magnet is located at a second radius from the axis of rotation, wherein the second radius is larger than the first radius so as to define an annular space between the first magnets and the second magnets for receiving a stator.

Using such a rotor configuration, also called a dual-rotor, in a radial flux motor means that a yoke is not required and, because the flux in the core segments is all in a radial direction, grain-oriented electric steel laminations can be used to achieve higher flux density and lower loss in the teeth.

Optionally, adjacent first magnets and adjacent second magnets are oriented to have opposite polarities. In this case, each pair of magnets defines a pole-pair of the rotor.

Optionally, each first magnet has a first curvature and a first thickness and each second magnet has a second curvature and a second thickness; such that a magnetic field with a sinusoidal flux density is produced between the magnets around the axis of rotation.

Using magnets that are different to accommodate for the fact they are at different radii so as to provide a sinusoidal magnetic field between them is advantageous for improving the efficiency of a motor that the rotor is used in. It is to be understood that the magnetic field may not be perfectly sinusoidal due to the discreet nature of the magnets, but will be a good approximation of a sinusoidal field.

Optionally, the magnetic field between the first and second magnets of each magnet pair occupies a sector of the annulus about the axis of rotation defined between the first and second radii.

According to a fourth aspect, a radial flux motor is provided. The radial flux motor comprises a stator according to the second aspect. Optionally, the motor may also, or alternatively, comprise a rotor according to the third aspect. It is not a requirement that the stator according to the second aspect be used in combination with the rotor of the type according to the third aspect; other suitable rotor designs including designs of conventional type may be used.

The angle by which each corresponding first and second portions of the coils are pitched apart may be different than the pole pitch of the radial flux motor defined by the permanent magnets. While the angle by which each pair of first and second portions is pitched apart may be the same as the pole pitch, using a different angle facilitates long-chording or short- chording of the winding. Optionally, the angle by which each pair of first and second portions may be pitched apart is less than the pole pitch. Using a smaller angle allows short-chording, which can be used to further reduce harmonics in the stator field.

Optionally, the first and second magnets of the rotor and the coils of the stator are configured such that current flowing along the first and second portions of each coil generates a torque acting in the same direction.

According to a fifth aspect, a wheel is provided.

Optionally, the wheel comprises the rotor of the third aspect. The wheel is configured to be fitted over a stator according to the second aspect.

For example, in such a wheel the second magnets of the rotor may be integral to or provided within the wheel rim. Such a wheel may be relatively easy and cheap to change or replace, whilst still allowing individual control of the wheel through a dedicated motor.

Optionally, the wheel rim acts as a back-iron for the second magnets of the rotor. This can provide an efficient return path for the primary magnetic flux without requiring additional ferromagnetic material, thus increasing the efficiency of the motor without increasing its weight. Alternatively, the wheel comprises the radial flux motor of the fourth aspect.

Having both the rotor and stator integral to the wheel can provide a wheel having a dedicated motor to provide individual control that is quick to change and can be provided as a sealed unit, increasing durability.

According to a sixth aspect, an electric vehicle is provided. The electric vehicle comprises one or more wheels of the fifth aspect, and if appropriate an equal number of stators of the second aspect.

According to a seventh aspect, a method of manufacturing a stator for use in a yokeless radial flux motor is provided. The method comprises: providing a plurality of coils according to the first aspect; stacking the plurality of coils circumferentially about a central axis; and connecting each coil to one or more other coils and/or to an electrical terminal to form a plurality of stator poles.

Because the coils of the first aspect have a defined structure independently from the stator, they can be manufactured separately. With the shape described above, they can then quickly and easily be stacked to form a ring about a central axis and electrically connected as required to form a stator.

Optionally, when the first and second portions of each coil are connected by one or more connecting portions, the step of stacking the plurality of coils circumferentially about the central axis comprises inserting the connecting portions of each coil into a slot in one or more support rings disposed coaxially with the central axis.

Using a support ring into which the connecting portions are slotted provides a very simple and quick way for the coils to be assembled into a stator. In particular, this method of manufacturing a stator is much simpler and cost effective compared to winding the coils around the stator in situ.

Optionally, subsequent to at least the step of stacking the plurality of coils circumferentially about an axis, the method further includes the step of inserting one or more flux guides into spaces formed between the first and second portions of the coils. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently. Furthermore, whilst the invention is generally described with respect to its implementation in an electric motor, it will be understood that the components and principles herein can also equally be applied to other forms of electric machine, such as an electric generator.

Brief Description of the Drawings

Embodiments of the invention will now be further described by way of example only and with reference to the accompanying figures in which:

Figure 1 is a schematic diagram of the shape of a coil according to the first aspect;

Figure 2 is also a schematic diagram of the shape of a coil according to the first aspect;

Figure 3 is a perspective view of a stator according to the second aspect;

Figure 4 is a perspective view of a radial flux motor according to the fourth aspect;

Figure 5 is a plan, or axial, view of a radial flux motor according to the fourth aspect;

Figure 6 is a side, or radial, view of a radial flux motor according to the fourth aspect;

Figure 7 is a cross section view of a radial flux motor according to the fourth aspect along line 600 of Figure 6.

Figure 8 is a flow diagram illustrating a method of manufacturing a stator according to the seventh aspect.

Like reference numbers are used for like elements throughout the description and figures.

Detailed Description Coil

The present disclosure includes a coil for use in a yokeless radial flux motor. Figure 1 illustrates an example of the shape of such a coil 100. As can be seen, the coil 100 comprises two portions which are each substantially planer - a first portion 102 and a second portion 104. The first portion 102 is substantially planar in a first plane, whilst the second portion 104 is substantially planar in a second plane. The first and second planes are preferably not parallel, and so the first portion 102 and the second portion 104 of the coil 100 are angled with respect to one another, as will be discussed in more detail below with respect to Figure 2.

In addition to the first portion 102 and the second portion 104, the coil 100 illustrated in Figure 1 comprises two connecting portions 106. These connecting portions 106 connect the first portion 102 and the second portion 104 both structurally and electrically. In order to hold the first portion 102 and the second portion 104 at the required angle with respect to each other, the connecting portions 106 illustrated in Figure 1 each comprise a bend 108.

In between the first portion 102, the second portion 104 and the connecting portions 106 can be intermediate portions 110. These serve as transitions between the connecting portions 106 and the first portion 102 or the second portion 104. The intermediate portions 110 comprise the necessary bends and/or curves required such that the connecting portions 106, the first portion 102 and the second portion 104 have the desired shape and orientations as described herein.

The orientation of the first portion 102 and the second portion 104 of the coil 100 with respect to each other is illustrated in Figure 2. As shown in this figure, the first portion 102 extends parallel to line 200 between two intermediate portions 110 and lies in a first plane illustrated by lines 202 and 204, whilst the second portion 104 extends parallel to line 200 between two different intermediate portions 110 and lies in a second plane illustrated by lines 212 and 214. The intermediate portions 110 then diverge from the first and second planes to connect with the connecting portions 106. The first and second planes intersect along a line 200, also called a first axis of the coil, and the planes are angled with respect to each other. The angle by which the first portion 102 and the second portion 104 are pitched apart will be referred to as the coil span y.

This orientation of the first portion 102 and the second portion 104 with respect to each other and the first axis of the coil 200 means that, when in use in an electric machine as described herein, the first portion 102 and the second portion 104 are configured to carry current perpendicular (or substantially so) to the magnetic field created by a rotor of the electric machine. It is the current flowing perpendicular to the magnetic field through the first portion 102 and the second portion 104 that generates the majority of the electromagnetic forces that act within the electric machine. For this reason, the first portion 102 and the second portion 104 may also be referred to as active portions, as they are configured to interact with the magnetic field of a rotor.

As well as being angled with respect to one another, the first portion 102 and the second portion 104 are offset from line 200 with respect to one another. That is, they are each a different distance from line 200, or radially offset with respect to one another from the first axis. In particular, the edge of the first portion 102 that is furthest from line 200 is closer to line 200 than the edge of the second portion 104 that is closest to line 200.

The first portion 102 and the second portion 104 both extend in a lengthwise direction in the first and second planes respectively, parallel to each other in this direction and also to the first axis of the coil, illustrated by line 200. Each of the first portion 102 and the second portion 104 also has a second dimension that lies within the first and second plane respectively. This second dimension, referred to as a height, extends in the direction of lines 202 and 204 for the first portion 102 and in the direction of lines 212 and 214 for the second portion 104. The height and length of the first portion 102 and the second portion 104 define the area of each portion 102, 104 of the coil 100 that lies within each of the first and second planes respectively. The third dimension of the first portion 102 and the second portion 104 is referred to as the width or thickness, and lies in a direction perpendicular to the first and second planes respectively.

A coil 100 according to any aspects of the present disclosure, for example having the shape described with respect to Figures 1 and 2, may be made from a plurality of windings. These windings are of an electrically conductive material, such as copper wire having an insulating coating on the outside. Forming a coil 100 from a plurality of windings is a well- known technique in the art, and will not be described in detail herein. Nevertheless, particular aspects that relate to implementation with a coil 100 having the shape described herein will be discussed in more detail.

The plurality of windings of the coil 100 may comprise a number of windings, also known as turns or winding turns. In this case, a path along the conductor through the coil (e.g. the path of an electrical current) may be as follows: a first terminal portion, the first portion, a first intermediate portion, a first connecting portion, a second intermediate portion, the second portion, a third intermediate portion, a second connecting portion, a fourth intermediate portion, the first portion, the first intermediate portion, the first connecting portion, the second intermediate portion, the second portion, ... , a second terminal portion. The ellipses represents that the number of windings or turns in the plurality of windings can vary and is not limited by the present invention. For example, there may be two, five, ten or another number of turns or windings in the plurality of windings. The first and second terminal portions may allow for an electrical and optionally a mechanical connection to another component, such as another coil, busbar or power supply.

Throughout the coil 100 generally, but within the first portion 102 and the second portion 104 in particular, each of the plurality of windings are stacked adjacent to one another such that the first portion 102 and the second portion 104 are both substantially planar. That is, a conductor that corresponds to a given turn of the plurality of windings in the first portion 102 extends in a lengthwise direction within the first portion 102, and is adjacently stacked next to two (except for the first and last turns) other turns that also lie within the first plane and extend in a lengthwise direction to form the first portion 102, such that these two other turns are either side of the given turn in a height-wise direction.

In some cases, the coil 100 may be a single conductor thick. That is, the coil 100, including the first portion 102 and the second portion 104, may comprise a single stack of conductors (i.e. windings) stacked adjacent to one another in a height-wise direction. Alternatively, the coil 100 may be multiple conductors thick, for example two, three or more conductors thick. In this case, multiple stacks of windings may be adjacent to one another, such that as well as having a conductor stacked adjacently above and below a given (non-peripheral) conductor, the given conductor may also have one or even two conductors adjacent to it in a width-wise direction.

The terminal portions of the coil 100 are not shown in Figures 1 or 2, which merely illustrate the overall shape of the coil. However, it will be appreciated that they may extend from the coil 100 in a direction for connection to one or more busbars or other coils. The terminal portions may be extensions of a wire used in the windings of the coil 100, and may extend from different portions of the coil 100. In one exemplary case, the terminal portions may extend from one or both of the connecting portions 106. This may be desirable because it provides a clear path (when the coil is stacked with other coils in a stator, as illustrated in Figure 3, for example) in a direction parallel to the first axis 200, which may be a direction of a busbar to which the coil 100 is to be connected when assembled into an electric machine. In this case, at least one of the terminal portions may extend in a direction parallel to, or substantially parallel to, first axis 200. The other terminal portion may be configured to be connected to another coil. In this case, the terminal portion may extend in a circumferential direction about the first axis 200, such that when the coil 100 is stacked with other coils in a stator the terminal portion is pointing in the direction of, and can be connected to, an adjacent coil.

Stator

According to another aspect of the disclosure, a plurality of coils, such as those described with respect to Figures 1 and 2, are arranged to form a stator. Elements of such a stator are illustrated in Figure 3.

Figure 3 illustrates a stator 300, comprising a plurality of coils 100, the coils having the shape illustrated in Figures 1 and 2. The stator 302 has a central axis 302, about which the coils 100 are arranged. The central axis 302 of the stator 300 is configured to be coaxial with an axis of rotation of a rotor designed to cooperate with the stator 300 in the form of a radial flux electric motor.

As can be seen in Figure 3, each coil 100 is oriented such that the first and second portions of the coil 100 extend in an axial direction between the two connecting portions 106, the axial direction being a direction parallel to the central axis 302.

The coils 100 are stacked together to form an annular or ring shape, such that the first axis 200 of each coil 100 lies within the centre of the annulus or ring shape and is parallel to the central axis 302. Optionally, the first axis 200 of each coil 100 is disposed close to the central axis 302, or is even coaxial with the central axis 302. In the case that the first axis 200 of each coil is coaxial with the central axis 302 of the stator, the first portion 102 and second portion 104 of each coil 100 will lie in planes extending radially from the central axis 302. In other words, the first portion 102 and second portion 104 of each coil 100 will extend radially from the central axis 302 in a height-wise direction and will extend parallel to the central axis 302 in a length-wise direction, and circumferentially about the central axis 302 in a width-wise direction.

With respect to each other, each of the coils 100 overlaps with at least the coils either side in a circumferential direction. That is, the second portion of a given coil 310 will overlap with the first portion of a first coil 311 and the first portion of that given coil 310 will overlap with the second portion of a second coil 312, whereby the first coil 311 and second coil 312 are disposed adjacent to, and either side of, the given coil 310. In some implementations, each coil will overlap multiple coils in each circumferential direction. For example, the second portion of a given coil may overlap the first portions of two, three or even more coils in a first circumferential direction, and the first portion of the given coil may overlap the second portions of an equal number coils in a second circumferential direction.

In this manner, the coils 100 in the stator 300 may be tightly packed, as illustrated in Figure 3. In particular, it is noted that the first portions 102 and the second portions 104 of the coils 100 are radially offset. That is, the first portion 102 of each coil 100 is at a smaller radial distance from the central axis 302 than the second portion 104, such that they can overlap in a circumferential direction as described above. This facilitates stacking of the coils in the radial and circumferential direction, which provides for flexibility in the span (pitch) between each pair of first 102 and second 104 portions, and also improves the structural rigidity of the complete winding owing to the interlocking nature of the coils.

Within the stator, the coils may be provided in a plurality of groups. Each group may comprise an integer number of adjacent coils 100, for example two or more coils. Each group may correspond to a pole of the stator 300. The coils 100 in each group may also be arranged into a number of sub-groups, with each sub-group being configured to be connected to a different phase of a multi-phase power supply. The number of coils 100 in each subgroup should be equal, preferably a multiple of two, and the coils 100 in each subgroup are connected in series to one another. Hence, for a three-phase power supply, there should be an integer multiple of three coils 100 per group, evenly split between three subgroups. More generally, with M coils 100 per subgroup and N phases, each group will consist of M x N coils, where M and N are both integers.

For example, turning briefly to Figure 5, coils 100a and 100b may be in the same pole of the stator, but connected to different phases of the power supply. Therefore, coils 100a and 100b are in the same group but different subgroups. Coil 100c is in a different pole of the stator to coils 100a and 100b, and is therefore in a different group and subgroup to coils 100a and 100b.

Optionally, the multi-phase power supply is a three-phase power supply, i.e. N = 3. However, embodiments of the invention can be used with a power supply having a different number of phases. The stator 300 may also comprise a plurality of common busbars (not shown) that are connected to the coils. The busbars may be annular, and/or may comprise a plurality of busbar sections. For each phase of the multi-phase power supply, every second coil of the stator that is connected to said phase may be connected to a common busbar. In this way, the windings may be divided into two interleaved portions that connect half of the total number of coils per phase to one of two phase busbars. The coils may be connected to the power supply via the common busbars.

Whilst the busbars may be positioned at any location around the coils (such as at either axial end, or radially within or outside of the coils), in one example all of the busbars and busbar portions (e.g., a busbar for each phase and a star connection busbar) may be disposed at one axial end of the stator 300. This may have a number of advantages, including making the manufacture of the stator 300 more simple as all of the connections that have to be made between the coils and the busbars are at one end of the stator 300. Another advantage may be that nothing is positioned between the rotors 404 (see Figures 4 to 7, discussed below) and the stator 300, enabling the rotors 404 to be as close to the stator as possible to increase the efficiency of the electric machine. Furthermore, as such a stator may work with rotors 404 at an inner radius and an outer radius, the inner and outer portions of the rotor 404 can be connected together at the opposite axial end of the electric machine to that at which the busbars are disposed. Such an arrangement allows the rotor arrangement to fit snugly around the stator 300 whilst enabling an easy electrical connection to be made to the busbars (such as from a controller of the electric machine).

When stacked together in the manner described above, the coils 100 in the stator 300 may form a plurality of spaces between adjacent first portions 102 and second portions 104 of different coils 100. The spaces can be seen in Figure 7, which shows a cross section of the stator and rotor arrangement of Figures 4 to 6 along line 600.

The spaces can be grouped into two types: spaces of a first type are defined between adjacent first portions 102 and second portions 104 of coils 100 of different groups; spaces of a second type are defined between adjacent first portions 102 and second portions 104 of different coils 100 within the same group (i.e. between first portions 102 and second portions 104 of two coils that are connected together in series).

The spaces, both the first type and the second type, may be a circumferential space and define a volume that is substantially radially extending and may be elongate in the radial direction. Within these spaces, flux guides 304 can be disposed, such as electric steel laminations. Flux guides 304 channel the flux radially between corresponding magnetic poles on opposing rotors. These flux guides 304 are configured to increase the magnetic flux density of the stator for a given arrangement of permanent magnets. For example, the flux guides may have high magnetic permeability in at least the radial direction.

Optionally the flux guides 304 may be individual, discreet items. For example, each flux guide 304 that is disposed in one of the spaces may be a separate steel lamination pack. Each flux guide 304 may also have a uniform thickness in at least a radial direction. Each flux guide 304 may be identical to the other flux guides 304 of the stator 300. This can facilitate easy construction of the stator 300, by allowing the flux guides to simply be slotted into place. Furthermore, by having the flux guides 304 have a uniform thickness and be identical, construction of the stator 300 is further simplified because the flux guides 304 may have two or more orientations in which they can be inserted into the spaces between the coils 100.

In Figure 7, it can be seen how the first and second portions of the coils are offset circumferentially, and how the coils stack together. In Figure 7, the first and second portions of three adjacent coils are labelled. The first coil has a first portion 102a and a second portion 104a, the second coil has a first portion 102b and a second portion 104b, and the third coil has a first portion 102c and a second portion 104c. The first portion 102a of the first coil is offset circumferentially with the second portion 104a of the first coil, as are first portions 102b and 102c with second portions 104b and 104c of the second and third coils respectively. In between the first portions 102 and the second portions 104 are the spaces, with flux guides 304 disposed within. As noted above, in the case that the first and second coils are connected in series, that is they are in the same group, then the spaces between the first portions 102a and 102b and between the second portions 104a and 104b are spaces of a second type. In the case that coil 100c is in a different group from coils 100a and 100b, then the spaces between the first portions 102b and 102c and between the second portions 104b and 104c are spaces of a first type.

It will be noted that spaces of the first type and second type may align with one another in a radial direction. For example, a space of a first type between two first portions 102 may align with a space of a second type between two second portions 104, or vice versa. Similarly, spaces of the first type may align with other spaces of the first type, and spaces of the second type may align with other spaces of the second type. A single flux guide 304 may be positioned in multiple, e.g. two, spaces aligned in this manner, as is shown in Figure 7. Here, each flux guide 304 is positioned in a space between adjacent second portions 104 and also in a space between adjacent first portions 102. These spaces may be of a first type, of a second type, or a combination thereof.

The stator may be configured so that, in use, current flows in the same direction along adjacent first portions 102 and second portions 104 of the same group, i.e. groups separated by one of the second type of spaces. This avoids the current flowing in these adjacent first portions 102 and second portions 104 being counter-productive to torque production.

Optionally, the stator may consist of two or more concentric rings of coils. For example, the stator may consist of a first ring, such as the stator 300 illustrated in Figure 3, and a second similar ring, having a larger radius such that the first ring fits concentrically within the second ring. That is, the radially outermost portion of the first ring must be at a smaller radial distance from the central axis of the stator than the radially innermost portion of the second ring.

The first ring comprises a first plurality of coils, whilst the second ring comprises a second plurality of coils. It will be appreciated that, due to having a larger radius, the second ring has a greater circumference. This greater circumference could be achieved by having more coils in the outer ring than in the inner ring, i.e. in the second plurality of coils compared to the first plurality of coils. Alternatively, the second ring could have the same number coils as the first ring, but with larger spaces between the coils. Larger (i.e. thicker in a circumferential direction) flux guides could then be placed in the second ring compared with the flux guides placed in the first ring. Alternatively, the same flux guides as in the first ring could be used in the second ring, but multiple flux guides could be placed in each space. For example, a single flux guide could be placed in each space in the inner ring but two flux guides could be placed in each space in the outer ring. Alternatively or additionally, thicker coils could be used in the outer ring compared to the coils used in the inner ring. The coils may be made thicker by using a thicker conductor to make the coils, such as using a wire with a larger diameter than that used to make the coils of the inner ring. Additionally to, or instead of, using a thicker conductor, the coils of the outer ring may be made thicker by being multiple conductors thick, as discussed previously herein.

Optionally the coils of the outer ring may be made thicker than the coils of the inner ring such that they occupy the same angular distance from the central axis of the stator. In this manner, the spaces between the first and second portions of each ring can be aligned, and each flux guide can be inserted through both rings, rather than requiring separate flux guides to be inserted into the inner ring and the outer ring. This is advantageous as it allows the flux guides to be inserted together after all of the coils have been positioned, rather than requiring the flux guides for the first ring to be inserted after the coils of the first ring are positioned, and then the flux guides for the second ring to be inserted after the coils of the second ring are positioned around the first ring.

Any of the stators described above may be formed by holding the coils in place using one or more support rings. The support rings may be annular and coaxial with the central axis of the stator, and may be formed of one or more sections. In particular, the support rings may hold each coil via a connecting portion of the ring. If the coils comprise two connecting portions, the support ring may hold the coils via one or both of the connecting portions. Supporting the coils in this manner using support rings can effectively extract heat from the windings that is generated during operation of an electric machine. This helps to increase the efficiency of the electric machine and to allow it to provide higher output power continuously.

Optionally, multiple support rings are provided. For example, a first support ring may be provided to hold a first connecting portion of the coils and a second support ring may be provided to hold the second connecting portion of the coils. In the case when there is more than one ring of coils, individual support rings may be provided for each ring, or the support ring or rings may hold coils from multiple rings.

Having the one or more support rings at the axial end or ends of the stator in this manner means that the stator can be supported without increasing the radius of the overall electric machine. Furthermore, no material is required to be positioned between the magnets of the rotor and the coils of the stator, minimising the air gap and increasing the efficiency of the electric machine. It also provides for a convenient manner in which the stator can be manufactured, with the individual coils being slotted into at least one of the support rings which holds the coils in place whilst discreet laminated flux guides (which do not provide structural support) are then introduced between the coils. The assembly of the support rings, coils and laminated flux guides can then be potted, for example, in a suitable resin. Supporting the coils using the support rings in such a yokeless manner, rather than using the core to support the coils, enables a denser packing of coils compared to traditional methods.

As will be understood by those skilled in the art, the stators described herein are yokeless.

A yoke is an additional structural element present in some stators for guiding lines of magnetic flux between opposite poles of the rotor magnetic field. That is, the yoke completes the magnetic circuits. Since the stator can be for use in machines utilising a rotor having a dual rotor configuration, as discussed below, there is no need for a yoke to complete the magnetic circuits because the flux is unidirectional. Having a yokeless stator reduces the overall weight of the radial flux machine, which is greatly beneficial in many practical applications. In addition, it improves efficiency since there are no losses attributed to a varying flux density in a yoke region. Whilst it is noted that the present disclosure relates to yokeless stators, this does not necessarily mean that the stators are ironless. In particular, optionally the stators described herein may have flux guides inserted into the spaces formed by the coils of the stator. Hence, some of the stators described herein may be considered to be yokeless (but not ironless), slotted stators.

When compared to a stator for an axial flux electric machine in which the flux runs in an axial direction and hence the first and second (active) portions of a coil are in a radial direction, a stator for a radial flux electric machine as described herein typically has more space within the stator. For example, looking at Figure 3 it can be seen that the connecting portions 106 are disposed at the axial ends of the stator 300. However, in a similar axial flux electric machine, half of the connecting portions are at an inner radius because of the radial alignment of the first and second portions. This leads to a greater density of connecting portions and reduces the space between them. Furthermore, there is more space within the stator, in particular in between and within the coils, because the flux guides can be of rectangular cross-section and the coils cane also be of rectangular crosssection and both can disposed in a radial direction. (Unlike with the axial flux machine where the flux guides are wedge shape.)

In a radial flux electric machine, such as those described herein, the additional space within the stator 300 can be utilised for cooling the stator 300. For example, cooling means can be included within the stator 300, such as between the connecting portions 106 of adjacent coils 100. The cooling means may be provided at one or both ends of the stator 300. Such cooling means may comprise channels for a coolant to pass through (such as a liquid or gaseous coolant), thermally conductive inserts, an application of a thermal paste, some combination thereof, or another means.

In one example, the cooling means comprise air channels that are configured to use atmospheric air (i.e. , the cooling system is not a closed system) to cool the stator. This may be particularly suited to radial flux electric machines due to the increased space available for the cooling means, and so a less efficient coolant (air, as opposed to a denser liquid coolant) may be used. Using atmospheric air in this manner can lead to additional benefits, such as a reduction in the complexity and weight of the resultant electric machine. This is because additional cooling components such as a liquid coolant itself, radiators, heat exchangers and the like can be omitted as cool air can be drawn from the atmosphere during use and heated air can simply be expelled to remove heat. Such an electric machine may be particularly suited for use in aviation, for example.

Rotor

The invention also provides a rotor for use with a stator of the invention. Figures 4 to 7 illustrate a rotor according to an example of the invention, in position around a stator.

Figure 4 illustrates a perspective view of an electric motor 400, whilst Figure 5 illustrates a plan view along axis 402 and Figure 6 illustrates a side view along a radial direction. The electric motor 400 comprises a stator 300 and a rotor 404. The stator 300 may be, for example, that described in Figure 3, comprising a plurality of coils 100.

The rotor 404 has an axis of rotation 402, which is coaxial with the central axis 302 of the stator 300 and may also be coaxial with the first axis 200 of each of the coils 100. The rotor also comprises a plurality of magnets arranged in pairs circumferentially disposed about the axis of rotation.

Each pair of magnets has a first magnet 406 and a second magnet 408. Each of the first magnets 406 are arranged about the axis of rotation 402 at a first radius, while each of the second magnets 408 are arranged about the axis of rotation 402 at a second radius, the second radius being larger than the first radius. The first 406 and second 408 magnets in each pair are disposed opposite each other, the first magnet 406 being positioned on the inside of the stator 300 and the second magnet 408 being positioned on the outside of the stator 300 when the rotor 404 is in position. The first 406 and second 408 magnets in each pair have their poles oriented such that opposite poles are facing each other. In this manner, the magnetic field between the first 406 and second 408 magnets of each magnet pair occupies a sector of the annulus about the axis of rotation 402 defined between the first and second radii, with the magnetic field lines between the first 406 and second 408 magnets being radial in direction. Optionally, adjacent first magnets have alternating orientations of their poles, as do adjacent second magnets. Optionally, and as illustrated in Figure 4, the first 406 and second 408 magnets are curved. Optionally the thickness of the first magnets 406 and the second magnets 408 may be different. The thicknesses and shapes should be chosen to produce a magnetic field with a sinusoidal flux density between the magnets around the axis of rotation. It will be appreciated that in practice this will be an approximate sinusoidal magnetic field, due to the discreet nature of the magnets used.

The rotor is configured such that the first magnets 406 and second magnets 408 rotate about the axis of rotation 402 in synchronism, that is, with the same angular velocity, such that they do not move relative to one another.

Rotors of this type can be referred to as dual-rotors, comprising an inner and outer rotor. This rotor, in a radial flux motor, has the benefit that a laminated yoke is not required as discussed previously and, because the flux in the core segments is all in a radial direction, grain-oriented electric steel laminations can be used as flux guides to achieve higher flux density and lower losses in the stator.

In the case where the stator comprises more than one ring of coils, e.g. two rings comprising two pluralities of coils, the rotor described above comprising pairs of magnets may be employed, with the first magnet 406 being disposed inside the innermost ring of the stator, and the second magnet 408 being disposed outside the outermost ring of the stator. Alternatively, additional rings of magnets may be provided between each ring of coils in the stator. For example, a stator with two rings of coils may have a rotor with three rings of magnets. In this case, the magnets are not arranged in pairs, but in groups of three aligned radially, with alternating polarities such that the magnetic field between each magnet in the trio is in a radial direction.

It will be appreciated by those skilled in the art that the rotor may contain back-irons on the inner-most and outer-most surfaces of the magnets to provide efficient return paths for the primary magnetic flux.

Electric Motor

The stator and rotor described above, particularly those illustrated in Figures 4 to 7, may be used in an electric motor, in particular a yokeless radial flux motor.

In use, a current is passed through the coils, and when the current passes along the first and second portions of each coil it will be running in an axial direction whilst the magnetic field between the first and second magnets of the rotor will be in a radial direction. Hence, the current will be flowing perpendicular to the magnetic field, leading to a torque being generated. This torque can act to turn the rotor and stator relative to one another.

Preferably, the coils are arranged in the stator with respect to the magnets in the rotor such that current flowing along the first and second portions of each coil generates a torque acting in the same direction.

The coil span y can be the same as or different (less or more) than the pole pitch a (defined by the angle between two lines extending from the axis 402 to the centres of the permanent magnets of the rotor). Preferably the coil span y is less than the pole pitch a. For example, y may be approximately 5/6 of a. By making y less than a, short-chording of the winding can be implemented, which reduces the spatial harmonic content of the winding magnetomotive force (mmf).

The circumferential (angular) separation of the centres of two adjacent permanent magnets of the rotor defines the pole pitch a of a radial flux motor incorporating the rotor. It is noted that the span of the permanent magnets /3 may be the same as or less than the pole pitch a of the rotor. The span /3 of the magnets may be less than the pole pitch a. In an example, /3 is approximately 3/4 of a. The ratio of p to a can be chosen to reduce the circumferential, spatial harmonic distortion of the permanent magnet flux density in the stator.

It will be appreciated by the skilled person that other components of the motor will also be present that are not described in detail herein, such as a drive shaft, end plates, bearings and the like. These aspects of the motor are well known, and can be implemented in accordance with standard techniques.

The electric motor can be incorporated into other apparatuses. In particular, it is envisaged that the electric motor can be incorporated into a wheel. Due to the radial flux arrangement, the motor of the invention is particularly suited to such a use because the stator can easily be attached to, for example, a vehicle and both rings of magnets on the rotor can be attached to a wheel. This requires a simple mount for the magnets of the rotor, having a “II” shaped cross section that fits over the stator. In particular, the wheel rim can act as a back- iron for the outer ring of magnets of the rotor. This can provide an efficient return path for the primary magnetic flux without requiring additional metal, thus increasing the efficiency of the motor without increasing its weight. Optionally the stator may be integral to a vehicle whilst the wheel comprises the rotor. Alternatively the wheel may comprise both the stator and the rotor, with the part holding the stator configured to be attached to another device, such as a vehicle body.

- Stator Manufacture

The features and construction of the conductive coils described above provide for particularly efficient and effective manufacture of a stator that includes a plurality of circumferentially distributed coils. Of particular significance is the fact that the coils themselves provide a structure into which flux guides, for example in the form of laminated flux guides (e.g., lamination packs), can be provided. This makes placing of the lamination packs in the stator assembly a comparatively straightforward and precise exercise, especially compared to many known manufacturing techniques which may involve winding coils around bobbin-like structures which house lamination packs, and then separately securing (using glue, for example) the wound bobbin-like structures into a stator housing.

A method 800 of manufacturing a stator is illustrated in Figure 8. At step 802, a plurality of coils as described herein are provided. At step 804, the coils are stacked circumferentially about a central axis of the stator. At step 806, the coils are electrically connected as desired to form a stator having the correct number of poles and so on. In particular, the coils are connected to one or more other coils and/or to an electrical terminal. Step 808 is an optional step, and at this step one or more flux guides are inserted into spaces formed between the first and second portions of the coils.

Optionally, the stacking of the plurality of coils comprises inserting connecting portions of each of the coils into a respective slot in one or more support rings disposed coaxially with the central axis, as discussed above. The support ring may take the forms described herein, and can hold the coils in place. In this manner, the coils and support ring can provide a rigid structure of the stator into which flux guides can be inserted.

Described above are a number of embodiments with various optional features. It should be appreciated that, with the exception of any mutually exclusive features, any combination of one or more of the optional features are possible.