GB2589582A

GB2589582A - Electrical generator

Info

Publication number: GB2589582A
Application number: GB1917583.5A
Authority: GB
Inventors: Peter Devereux Christopher
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-12-02
Filing date: 2019-12-02
Publication date: 2021-06-09
Anticipated expiration: 2039-12-02
Also published as: GB201917583D0; GB2589582B

Abstract

An axial flux permanent magnet electrical generator 10 has two rotors 22a, 22b either side of a stator 12 that comprises winding bundles 18 wrapped around an annular disk core 14. Each rotor comprises permanent magnets 26 annularly arranged on a disk core 24. An upper surface profile of each magnet 26 facing the stator is contoured with a non-uniform height such that a gap 34 between the magnet and stator is not uniform across the area of the magnet. The magnet height can be a minimum at a width that is tangential to a radius of the rotor. The upper surface profile of the magnet can be stepped or curved and define a non-circular ellipse, a half-ellipse, or a sinusoid function. The magnet height can vary in only one circumferential direction across the magnet or across more than one direction. The windings can be equal in number; matching the arrangement of the magnets; and be trapezoidal in shape.

Description

Electrical Generator

FIELD OF THE INVENTION

This invention relates to an electrical generator and in particular to an electrical generator of the Axial Flux Permanent Magnet (AFPG) type

BACKGROUND OF THE INVENTION

Electrical generators can generally be classified in two types: Permanent magnet (PM) type and electrically excited magnet type.

PM generators have advantages over other machines. These are compact in size, more robust and have high power output and torque density. PM generators can be classified into three types: Radial Flux PM generators (RFPM), Axial Flux PM generators (AFPM) and Transverse Flux PM generators (TFPM). The three types have different structures and are characterized in terms of the direction of the magnet flux within the generator. In Axial Flux machines, the flux circulations are in the axial direction. In radial flux machines, the flux circulations are in the radial direction. In transverse flux machines, the flux circulations are in the transverse direction.

Figs. 1 and 2 schematically illustrate the comparative stator 12 and rotor 22 configurations for an AFPM generator and a RFPM generator respectively. The AFPM generator illustrated is a dual rotor single stator machine. As can be seen, the stator disk is sandwiched axially between the two rotors disks, and the three are coaxially aligned and axially spaced from each other by a small gap or separation. They form an axial stack. They all have substantially the same outer radius. The magnets of the two rotors thus each face the stator windings in the axial direction, meaning that the flux lines are in the axial direction. By contrast, for the radial flux machine, the rotor is a circular disk concentrically inset within an outer annular stator ring. It rotates radially inside the stator circumference. Thus the magnets face the stator windings along the radial direction, meaning this is the direction of the flux circulations 13.

AFPM generators have advantages over RFPM in terms of efficiency, size and cost effectiveness. The reasons are outlined below.

A first reason is related to the magnet orientation. Magnets in AFPM generators are located further away from the centre axis compared to RFPM machines, due to which AFPMs have a larger moment arm. Furthermore, a double rotor, single stator AFPM configuration in particular has the magnets on both sides of the stator and so has a larger interface area between the magnets and stator coils. This dual rotor design is not possible in the same way for RFPM machines.

A second reason has to do with flux path. Axial Flux generators are more efficient in terms of the electromagnetic interaction. The path of the magnetic flux lines is longer in RFPM generators as compared to AFPM generators. The flux path of both generators shown in Fig. 3 and 4. Each shows the stator 12 and rotor 22 parts, and the magnetic flux lines 13 from the magnets 26. In AFPM generators (Fig. 3) the flux path is very short. In a double rotor single stator design for example, the flux from a first magnet 26a on the first rotor, passes the stator 12 core to the second magnet 266 on the second rotor. In RFPM generators (Fig. 4) the flux 13 from the first magnet 26a goes to the stator tooth 11, then to another tooth via the stator 12 core and then to the next magnet 26b. As the flux in RFPM magnet must make a bend, its path is effectively 2-dimentional. This leads to greater dissipation of magnetic energy and lower output power.

A further reason has to do with the windings. In RFPM generators, the active winding length (within which electrical induction can take place in use) is less than that in AFPM generators because half of the winding is on the exterior side of the stator and the magnet interaction is only with the one interior side half the winding. In AFPM generators, and especially in a double rotor, single stator topology, magnets can be provided on both sides of the stator, and so both sides of the winding can be utilized during use for voltage generation. This improves efficiency. The winding 18 arrangements for AFPM and RFPM generators are illustrated schematically in Fig. 5 and Fig. 6 respectively. As can be seen, both sides of the winding 18 in Fig.5 interact with the flux 13, while only one sides does in Fig. 6. Another reason has to do with cooling. In the case of radial flux machines, the heat has to be transported through the stator to the outside of the machine. But steel is not a very good heat conductor compared to other metals or other conduction materials. The coil overhang is also difficult to cool, because it is not directly in contact with the machine casing.

In the case of axial flux machines, the cooling is better because the windings are directly in contact with the exterior outside casing. The casing may be made for instance of aluminium, which conducts heat very well. As a result, the windings of axial flux machines stay cool while the resistance of the copper windings remains low. This results again in a much higher efficiency.

The relative advantages and disadvantages of the different types of generators are summarized in the table below.

Type Advantages Disadvantages Remarks I. Good efficiency * Greater in volume as TFPM has poor quality of output as compared Transverse Flux 2. High torque compared to to other machines. It is Permanent Magnet density AFPM. also not as compact as (TFPM) 3. Light weight, but greater volume. * Poor quality of output (more torque ripples) AFPM, * Longer axial length I. Good efficiency * Very heavy in weight as Although RFPNI has good performance in Radial Flux 2. Good output compared to general, it is very Permanent Magnet RFPM quality, * others Larger in size More axial length heavy in weight and * larger in volume as compared to AFPM.

I. Good output quality * Attraction between the AFPM is compact in size, has least axial Axial Flux 2. Light in weight magnets and length and is light in Permanent Magnet 3. Less torque stator core weight. AFPM also has AFPM ripples leads to good output quality.

4. Compact in size cogging torque 5. Least axial length

Table I

A significant drawback of AFPM type generators is the phenomenon of magnetic attraction between the magnets and the stator core material. This attraction leads to a drag effect which reduces the mechanical power of the device. It also leads to cogging torque. Cogging torque is a form of electrical noise in the generator system which arises from the magnetic interaction between the permanent magnets and the material of the stator core.

This effect of cogging torque reduces the machine efficiency. Cogging toque also contributes to the undesirable effect of torque ripple which manifest as a periodic increase or decrease in output torque as the generator rotor rotates. Another potential disadvantage is a relatively complex manufacturing process.

Improvements in the field of electrical generators for providing high power output while avoiding or ameliorating one or more of the above problems would be of advantage.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided an electrical generator, comprising: at least one stator, comprising a core in the form of the an annular disk, and an annular arrangement of winding bundles, each wrapped around the annular disk core at a respective circumferential location; at least two rotors, arranged on respectively axially opposite sides of the stator, and each having a respective core in the form of an annular disk, and each having a respective major surface arranged facing a surface of the stator disk core; each rotor comprising an annular arrangement of permanent magnets mounted on a major surface of the rotor core, between the rotor and stator, and each of the magnets having a respective upper surface facing the stator and separated from the stator by a gap having gap height; wherein an upper surface profile of each magnet is contoured with a nonuniform surface height such that a size of said gap between the magnet upper surface and the stator is non-uniform across the area of the magnet The electrical generator according to embodiments of the present invention is an Axial Flux generator, meaning that the direction of magnetic flux is axial (relative to the stator and rotor, i.e. parallel with the axial axis of the rotor and/or stator). It is a generator more particularly of the Axial Flux Permanent Magnet (AFPM) type. It comprises at least one stator which has a disk core (about which winding bundles are wrapped). The solid disk core is preferably of the slotless core type, meaning that it does not comprise slots within which the winding bundles fit. It is therefore preferably continuous in the circumferential direction meaning uninterrupted across its major surfaces for example. However, this is not essential and it may comprise a slotted stator core in other examples.

As discussed above, generators of this variety are able to produce the highest output torque and can also be provided in the smallest form factor and lightest weight.

However, as discussed above, AFPNI type generators have the disadvantage of increased cogging torque due to interaction between the permanent magnets and the material of the stator core.

Embodiments of the present invention propose to overcome the problem of cogging torque through adjusting a contouring of the upper surface profile of the permanent magnets of the rotors. In particular, it is proposed to provide an upper surface profile which is contoured with a non-uniform height such that a gap On the axial direction) between the magnet upper surface and the stator is non uniform across the area of the magnet.

Cogging torque is dependent upon the air gap flux density and the change of air gap reluctance with the change in the position of rotor. By adjusting the magnet upper surface so that the air gap is non-uniform across the magnet surface, the air gap flux density can be reduced across certain portions of the magnet area, which disrupts the physical interactions which lead to the cogging torque. There is also as a consequence only a small change in air gap reluctance with respect to rotor position. Cogging torque is reduced as a result. As a result of this, it is possible to reduce the size of the unit while increasing the output of the generator or alternator considerably.

In particular, in embodiments of the present invention, the efficiency of generator is increased up to 7% as a result of the new magnet shape (keeping all other parameters of generator equal). Embodiments are thus able to provide higher output power for the given size and weight of the machine than previously known generators. For example, in one example embodiment, a generator of outer diameter of approximately 170 mm can provide a power output of 8.5 kW.

The respective major surface of each rotor is preferably arranged in opposition to (facing) a major surface of the stator core. In this way, the magnets of the rotors are oppositionally (or radially) aligned with the winding bundles of the stator during rotary operation of the generator. Reference to a major surface of the stator or rotor cores in this C) context means a surface parallel with both the radial and circumferential axes/dimensions of the rotor or stator core.

Contoured in this context means that the surface profile of the magnet upper surface is not flat; it varies in height. This provides the non-uniform gap between the magnet and the stator.

The height of the magnet extends from a base of each magnet (the base being coupled to or adjacent to the rotor core surface), to the upper (major) surface of magnet facing away from the rotor core and toward the stator. The height is the smallest magnet dimension.

A non-uniform gap in this context means that the spacing defined by the gap (the distance between the magnet upper surface and the stator) is non-uniform across the area of the magnet.

The gap may be understood as referring to a gap between the upper surface of each magnet and a surface of the stator which is (axially) closest to the magnet upper surface (a most proximal surface or neighbouring surface of the stator). This may typically be an upper surface of each of the winding bundles facing in the direction of the rotor. The winding bundles project axially outward from the core disk of the stator. The gap permits relative rotary movement between the rotor and stator. Thus the gap may be understood in some examples as the gap between a magnet upper surface and a plane containing the axially lower-most surfaces of the winding bundles of the stator (i.e. the surfaces of the winding bundles facing axially toward the relevant rotor).

The gap size or height is the gap spacing distance between the magnet upper surface and a surface (e.g. axially lower-most surface) of the stator along the axial direction (defined by the axial axis of the rotor or stator).

The rotors and stator are co-axial; they share an axial (z) axis.

Each winding bundle is located covering a respective circumferential portion of the stator disk. The windings may be drum windings.

According to one or more embodiments, each magnet may have a length and a width, the length substantially aligned with a radial direction of the rotor, and the width substantially tangential to a radial direction of the stator, and wherein a gap height minimum is located at a centre (or substantially or approximately at the centre) of the magnet width, e.g. a maximum of the magnet height is at a centre of the width of the magnet.

The gap height minimum may correspond to a maximum point or line of the magnet upper surface height for example.

The upper surface height may curve monotonically away from a single maximum point or line in some examples.

In some examples, there may be a minimum of the gap height located at one or more edges of the magnet, for example one or more of the side edges of the magnet meaning the edges at either side of the width of the magnet.

By having the air gap length at a minimum in the middle of the magnet (and optionally at a maximum at the edges of the magnets), the air-gap flux is reduced while keeping the magnet close to the copper winding. Thus means that that the cogging torque is minimized without reducing significantly the flux linkage to stator winding. The cogging torque may be understood as arising substantially from the magnetic attraction between the magnets and the stator core. The stator core may be exposed in the circumferential spaces between neighbouring windings. Thus in practice, cogging torque may substantially be associated with attraction between the edges of each magnet and the stator core, since, when a magnet is circumferentially aligned with a windings, the edges are the part closest the exposed portion of the stator core. So, by providing the central point of the magnet upper surface the minimum distance from the winding, and preferably also declining toward the side edges of the curve, the air gap distance for all of the other points on the magnet increases. The rise in effective air gap length decreases air gap flux, which in turns reduces the overall cogging torque.

Preferably the surface height may decrease away from the maximum point or line toward the edges. Thus the air gap increases gradually in a pattern, and the air-gap rises from the middle point to the edges According to one or more embodiments, the upper surface profile of each magnet may be curved, i.e. the upper surface height may vary smoothly.

In a preferred group of examples, the upper surface profile of each magnet may be curved so as to define a portion of a non-circular ellipse. Preferably the upper surface profile is shaped to define a half-ellipse, preferably extending from one edge of the magnet to an opposing edge of the magnet. Preferably it extends from one side edge to the other, i.e. preferably across a direction parallel with a width of the magnet. In other words the major axis of the ellipse has equal length to and is parallel with the width dimension of the magnet.

An ellipse means a non-circular ellipse, i.e. for which the eccentricity is greater than zero, i.e, for which the two focal points are not the same.

The upper surface profile defines the portion of the ellipse across at least one cross-section of the magnet. Preferably it is across a cross-section cut parallel with a width dimension of the magnet.

Preferably the highest point of the ellipse may be aligned with a centre of the magnet upper surface, e.g. a centre along the width dimension. In an ellipse shape, the very side edges of the ellipse fall in height more rapidly and the parts toward the middle decline in height more gradually compared for example to a semi-circle or an arc shape. As a consequence, an ellipse shape means that the height of the surface stays higher over a larger portion of the magnet area than is the case with a semi-circle or arc shape for example, while the height is lost more rapidly at the very edges. This is optimum for reducing cogging torque, since axial separation from the winding can be kept relatively small over a large part of the winding area, thus maximising flux linkage and so output power, and gap height made large at the magnet edges where the cogging torque phenomenon substantially arises. Thus, an ellipse shape may provide the optimum balance between reducing cogging torque while maximising output power.

However an ellipse shape is just one possible embodiment. Advantages are achieved by any upper surface profile for which the gap distance or magnet upper surface height is non-uniform.

According to one or more embodiments, the upper surface profile of each magnet may be curved according to a sinusoidal function.

It may be curved so as to define a half of a sinusoid for example (i.e. one half of a sine cycle), or a full sinusoid for example. The sinusoid curvature profile may extend for example from one edge of the magnet to the other, e.g.one side edge to the other across a width dimension of the magnet.

According to another group of embodiments, the upper surface profile of each magnet may be stepped in height, i.e. exhibits discrete changes in height.

According to any embodiment, the height of each magnet upper surface may vary in only one direction across the magnet, and may exhibit zero variation in a direction perpendicular to said one direction.

Said one direction may for example be a direction substantially tangential to a radial dimension of the stator or rotor core. Said one direction may be a direction substantially parallel with a circumferential dimension of the stator or rotor core.

However in other embodiments, the height of the upper surface of each magnet may vary in more than one direction across the magnet.

According to preferred embodiments, the number of permanent magnets may match the number of winding bundles.

By providing the same number of permanent magnets as winding bundles, this means that, in use, during rotary operation, each winding bundle can be in electromagnetic interaction with a respective one magnet. This maximises power output, since all winding bundles can be in a state of electromagnetic induction simultaneously at regular rotational positions.

The annular arrangement of the permanent magnets for each rotor may match the annular arrangement of the winding bundles. For example, it mirrors the arrangement.

This means that when the rotor and stator are suitably rotated relative to one another, the position of each magnet is circumferentially aligned with a position of a respective winding. For example, the angular spacing between bundles matches the angular spacing between magnets. The circumferential spacing between magnets may match the circumferential spacing between the winding bundles.

In preferred examples, the annular disk of the stator core is uninterrupted in the circumferential direction. In other words, the annular disk is continuous and delimits no holes or slots or spacings. More particularly, each major surface of the stator may be uninterrupted or continuous, i.e. comprises no gaps or slots or holes or discrete breaks of any kind. The stator core is thus in this case a slotless stator core.

In preferred examples, the magnets may be rare earth magnets (i.e. formed of one or more rare earth materials or elements).

In preferred examples, each of the winding bundles may have a trapezoid shape, i.e. a trapezoid outer or outline shape, e.g. a trapezoid cross section across a plane parallel the plane of the rotor.

The annular arrangement of magnets of each rotor preferably alternate in their polarity along the annular direction, i.e. circumferentially neighbouring magnets are of opposite magnetic polarity.

In an advantageous set of examples, the stator core may be formed of grain oriented steel In AFPM generators, the flux path is linear (one-dimensional). As a result, it is possible to use grain oriented steel material as the stator core material. Grain oriented steel produces fewer electrical losses compared to an iron core material and provides overall generator efficiency increases of up to 2%. For example, the grain orientation may be aligned with the axial direction of the generator.

The generator comprises at least one stator and a pair of rotors arranged on axially alternate sides of the stator. However, in further embodiments, the generator can comprise more than one stator and/or more than two rotors in order to increase total output power of the generator. For example, another stator could be provided axially adjacent one of the rotors (on the opposite axial side as the first stator), and a further rotor axially adjacent the further stator. This arrangement would then comprise two stators and three rotors. This is just one example, and generators in accordance with embodiments may comprise any number of rotors and stators, for instance more rotors and stators to increase output power further.

Examples in accordance with a further aspect of the invention provide a method of generating electrical power using an electrical generator in accordance with any example or embodiment outlined above or described below, or in accordance with any claim of this application, the method comprising driving rotary motion of each rotor relative to the at least one stator.

Preferably both of the at least two rotors are rotated in the same rotary direction simultaneously. Preferably both are rotated in parallel at the same speed, and with their respective magnets angularly aligned. In some cases the two rotors may be yoked together, so that the two rotate together as one. Where more than two rotors are provided, all of the rotors may in some embodiments be rotated together in parallel at the same speed and with their respective magnets angularly aligned. All of the rotors may be yoked together in

some examples.

Examples in accordance with a further aspect of the invention provide a method of using an electrical generator in accordance with any example or embodiment outlined above or described below, or in accordance with any claim of this application, for generating electrical power within one or more of a wind energy generator; a petrol or diesel generator, e.g. a compact petrol or diesel generator; a mobile generator for charging electric vehicles; a water powered turbine generator; and a mobile on-vehicle generator, e.g, for refrigeration, for use in making road repairs, or for powering a compressed air source.

These represent example applications only.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Fig. 1 shows the structure of an Axial Flux Permanent Magnet (AFPM) generator; Fig. 2 shows the structure of a Radial Flux Permanent Magnet (RFPM) generator, Fig 3 shows the flux direction for an AFPM generator; Fig 4 shows the flux direction for a RFPM generator; Fig 5 shows the flux path through the windings of an AFPM generator; Fig. 6 shows the flux path through the windings of a RFPM generator; Fig. 7 shows a side elevation of an example generator according to one or more embodiments; Fig. 8 shows a perspective view of the example generator of Fig. 7; Fig. 9 shows a top down view of the example generator of Fig. 7; Fig. 10 shows the non-uniform upper surface height of a magnet according to one or more embodiments; Fig. 11 shows a top-down view of a magnet included in an example generator according to one or more embodiments; Figs. 12-14 show views of the stator of the generator of Fig. 7; Fig. 15 shows a winding bundle from the generator of Fig. 7; Fig. 16 illustrates dimensions of a stator of the generator of Fig. 7; Fig. 17 shows the magnetic flux directions for magnets of an example generator according to one or more embodiments; Fig 18a shows a flux density diagram of the example generator of Fig 7; Fig 18b shows a heat map of the example generator of Fig. 7; Fig. 19 shows directions of flux circulations of the example generator of Fig 7 in operation; Fig. 20 shows a circuit diagram of the coils of the generator of Fig. 7; Fig. 21 shows a first example non-uniform upper surface profile; Figs. 22-24 show views of an example magnet having an upper surface profile shaped to define an ellipse across two directions of the magnet; Figs. 25 and 26 show a flux density diagram and heat map for an example generator incorporating the magnets of Figs. 22-24; Figs. 27-29 show views of an example magnet having an upper surface profile shaped in a sinusoidal fashion; Figs. 30-31 show a flux density diagram and heat map for an example generator incorporating the magnets of Figs. 27-29; Figs. 32-36 show views of an example magnet having an upper surface profile shaped in a bi-directional skewed curve shape; Figs. 37 and 38 show a flux density diagram and heat map for an example generator incorporating the magnets of Figs. 32-36; Figs. 39-42 show views of an example magnet having an upper surface profile shaped in a stepped curve or spline shape; Figs. 43-44 show a flux density diagram and heat map for an example generator incorporating the magnets of Figs. 39-42; and Figs. 45-47 show views of an example magnet having an upper surface profile shaped in a stepped linear peak shape.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

The invention provides an axial flux permanent magnet electrical generator with at least two rotor parts and at least one stator part, and wherein an upper surface profile of each permanent magnet on each rotor is contoured to have a non-uniform height to thereby define a non-uniform gap between the magnet upper surface and the lower surfaces of the windings on the stator situated opposite. The consequence of this is reduced cogging torque since the air flux density is made to vary across the area of the magnet which interferes with the physical magnetic interaction with the stator core which leads to the cogging torque. In preferred examples, the magnet upper surface height may decline from a maximum located at or towards the centre of the magnet toward the side edges of the magnet.

Preferably the windings follow a similar annular arrangement to the magnets so that when suitable rotated, each magnet aligns circumferentially with a corresponding winding bundle opposite, with side edges of the magnet approximately corresponding to edges of the winding. By having the magnet upper surface height decline from a maximum in the middle of the magnet to a lower level (e.g. a minimum) toward the side edges, this means the flux density is highest in the central portion of the winding bundle (so flux linkage can be maximized), but minimum at the edges of the winding bundle, i.e. close to where the stator core becomes exposed as the winding bundle ends. It is here that magnetic attraction with the stator core may be maximum and so a major source of cogging torque. At this point the gap is largest, so magnetic flux density smallest, so attraction is minimized and cogging torque reduced.

However, it has been found that any magnet surface which has a non-uniform height and so varies the spacing between the magnet and windings opposite will disrupt the magnet-stator core attraction and so reduce cogging torque.

The proposed electrical generator is an Axial Flux Permanent Magnet type generator (AFPM). As a consequence, its size can be adjusted according to the required application. In particular, it is possible to add more rotors or stators, e.g. two, three or more disks (plates) in combination, to obtain different power outputs while maintaining a relatively compact size. AFPM generators can be classified according to the different number of rotors and stators as well as different types of stators. In preferred embodiments, the generator is a double rotor single stator generator meaning that it has at least two rotors and at least one stator, but more than one stator and more than two rotors may be provided in further embodiments In some embodiments, the generator is a slotless axial flux generator. Slotless means that there are no slots in the stator core (for windings to fit into or wrap around).

However, this is not essential and a slotted stator core is also a possibility in accordance with further embodiments.

To explain further, within the field of AFPM generators, three different kinds of stator can be considered: 1 Slot-less stator: the stator comprises a core disk and winding bundle wires are wound on the core.

2 Slotted Stator: the stator comprises a core disk with teeth or slots on or in the stator core, for receiving or locating windings. The windings are wound around the stator and held within the slots or teeth.

3. Coreless Stator: the stator comprises no core; the stator part consists only of the windings. The windings simply sit between the two rotors without any central body running through them.

Comparison of the different stators with advantages and disadvantages is given below.

Stator Type Advantages Disadvantages Slotted More robust structure with smaller effective air gap. Undesired electrical effects such as flux ripple, tooth losses, and cogging torque.

Different flux distribution levels can be achieved because the slots allow for different winding structures.

Slotless The undesired electrical effects of slots are eliminated which leads to higher efficiency overall. Relatively low mechanical strength.

Coreless Since there is no stator core the stator-rotor forces of attraction are not present. Lower power density compared to slotted and Lower weight of the machine due to the absence of iron core. slotless. Also stator fixing b problems and larger air-gap.

Table 2

Embodiments of the present invention aim to minimise detrimental electrical effects such as cogging torque, flux ripple and low quality power output, while achieving a high power output for a small generator size Thus, in preferred embodiments, a generator having a slotless stator core is used as this type is optimum for achieving these requirements.

However, this is not essential and further embodiments may employ a slotted stator, as a slotted stator may achieve a higher torque density as compared to slotless, and problems of cogging torque can be at least partially ameliorated by the non-uniform magnet surface height.

A coreless generator is not preferred since this results in lower power density (i.e. low power output for a given generator size).

As discussed, embodiments incorporate a new shape of magnet to minimize the attraction between the magnets and the stator core material, and therefore minimise the cogging torque. This makes the generator more efficient.

As discussed above, the other types of generator are not able to achieve a power output as high as an AFPNI generator, and in particular as a generator comprising at least two rotors axially sandwiching a solid core stator.

Furthermore, the non-uniform magnet surface height is also able to achieve higher power output than other topologies in particular a flat topology. This is because the reduction in cogging torque and magnet attraction with the stator core means that for the same dimensions and other parameters of the generator, a non-uniform surface height can provide higher output torque. As will be elaborated in more detail below, other magnet surface topologies are for example not able to achieve more than around 7.5 kVA (equivalent to KW) power in a generator of dimension 172 mm x 65 mm. Furthermore, to achieve that power output, an RFPM generator must be constructed with a form factor which is both heavier in weight and larger in outer dimension (particularly the axial length) than the AFPNI. Although a TFPM generator can be made with lower weight than a RFPM generator, its axial length is still longer and the torque quality lower than for the AFPM generator for the same output power.

A first example generator in accordance with one or more embodiments of the invention is illustrated in Figs. 7-16. Fig. 7 shows a side elevation view of the generator and Fig. 8 shows a perspective view of the generator. Fig. 9 shows a top-down view of the generator.

The generator 10 comprises a stator 12 part, the stator comprising a core 14 in the form of an annular disk. The stator further comprises an annular arrangement of winding bundles 18, each wrapped around the annular disk core 14 at a respective circumferential location.

The generator further comprises a pair of rotor parts 22a, 22b, arranged on axially opposite sides of the stator, and each having a respective core 24 in the form of an annular disk. Each rotor core has a respective major surface 28 (of the respective core) arranged facing a surface 30a, 30b of the stator disk core 14.

Each of the rotors 22 comprises an annular arrangement of permanent magnets 26 mounted on said respective major surface 28 of the rotor core, between the rotor and stator 12. Each of the magnets has a respective upper surface 32 facing the stator and separated from the stator by a gap 34 An upper surface 32 profile of each magnet 26 is contoured with a non-uniform surface height (H) such that a size of said gap 34 between the magnet upper surface 32 and the stator 12 is non-uniform across the area of the magnet In the preferred embodiment shown in Figs. 7-16, the annular arrangement of the permanent magnets 26 for each rotor 22a, 22b matches the annular arrangement of the winding bundles 18 of the stator 12. In other words, the number of permanent magnets matches the number of winding bundles, and the angular spacing between the magnets on the rotor is approximately the same as the angular spacing between the winding bundles on the stator. The rotor core 24 for each rotor preferably has the same or substantially the same inner and outer diameter as the stator core 14. The two rotor cores 24 and the stator core are co-axially aligned (meaning they are aligned along a common axial axis), and axially spaced relative to one another. The direction of the axial axis is labelled by arrow z in Fig. 7. A major surface 28 of each rotor core 24 is arranged opposing (facing) a respective major surface 30a, 30b (see Fig. 13) of the stator core.

As discussed, each of the magnets 26 has an upper surface 32 which has a non-uniform height so that a gap 34 between the magnet upper surface 32 and the stator 12 is of non-uniform height. This is shown in more detail in Fig. 10, which schematically illustrates the positioning of a single magnet 26 relative to a lower surface of a winding 18 coupled to the stator core 14 opposite it. Fig. 11 shows a top down view of a single one of the magnets 26.

Between the upper surface 32 of the magnet and a lower-most surface of the winding 18 opposite the magnet is a gap 34. The size or height of the gap or the spacing defined by it varies across the area of the magnet. This is caused by the non-uniform height, H, of the upper surface 32 of the magnet 26. In this example, the magnet has an upper portion with height fit and a lower (base) portion with height h2. The total height H = h1 + h,. Only the height of the upper portion varies in this particular example.

In this example, the total height is non-uniform across a direction parallel with a width, w, of the magnet. The width direction is indicated in Figs. 10 and 11 by dimension arrow w. The length, 1, extends perpendicular to the width and is indicated in Figs. 10 and 11 by dimension arrow /. The width extends between two side edges 62a, 62b of the magnet In the assembled generator device, the width dimension of each magnet is substantially aligned with a tangential direction of the rotor, meaning a direction tangential to a radial dimension of the rotor. In some examples, the width may instead be aligned with a circumferential dimension of the rotor; for example each magnet may have end edges which are curved to follow the curvature of the rotor core.

In the particular example shown, the magnet upper surface 32 height H varies across only the width direction, w, meaning that the height is uniform along the length direction, 1. However, in other examples, the height may vary across more than one direction across the magnet 26.

A non-uniform spacing between each magnet and the stator produces improved output by reducing cogging torque. Torque ripples are also reduced.

As discussed, there are two side rotating parts 22 in the generator 10. The direction of rotation of the rotating parts in operation is preferably the same (either both clockwise or both anti-clockwise).

A plurality of permanent magnets 26 is mounted on a major surface of the annular disk core 24 of each rotor 22, between the rotor and stator 12.

The magnets may be rare earth magnets (made of rare earth elements). According to one advantageous example, the magnets may be Neodymium (NdFeB) magnets, for example, NdFeB-N48SH magnets. N48SH magnets can work in a temperature up to 150 degrees Celsius. This represents just one non-limiting example however.

In some examples, the rotor core 24 may be formed of steel. By way of non-limiting example, this may be steel 50JN1000. This material has good durability and magnetic flux density properties. However, other materials, such as other iron alloys or any other material, may be used.

In accordance with one or more examples, the magnets may be for instance cast or machined. They may be formed of rare earth materials.

Fixing of the magnets may depend upon the manufacturing of the magnets. If the magnets are iron coated, they may by way of example be fixed on the rotor yoke (core) 24 disk with a liquid solution (which is used to fix iron and steel).

The rotors 22 have surface mounted magnets only on an inner side of the rotor core or yoke disk 24, i.e. the side facing axially inward toward the stator 12. The rotor core disk is sometimes referred to as the back iron.

Each rotor 22 may by way of example comprise 12 poles (magnets) mounted to the rotor core 24. The rotor yoke (or core) thickness or depth may be approximately 9 mm in this example, or more broadly, between approximately 7 mm and 11 mm.

According to the specific example shown in Figs. 7-16, the rotor core 24 (or yoke) dimensions for each rotor are 78mm outer radius and 45.03mm inner radius. The thickness of each rotor yoke is 9 mm. These are example dimensions only and are in no way limiting to the inventive concept.

The rotor core outer diameter may according to one or more embodiments be less than 100 mm, for example between 50 mm and 100 mm, for example between 60 mm 10 and 90 mm, for example between 70 mm and 80 mm.

Figs. 12-14 illustrate views of the stator 12 and its composite parts.

The single stator comprises a stator core 14 in the form of an annular disk. This is shown by itself in Fig. 13. The stator core comprises first 30a and second 30b major surfaces on opposite axial sides of the disk.

An annular arrangement of winding bundles 18 is disposed around the core.

This arrangement is shown in Fig. 14.

Each winding bundle is wrapped around the stator core disk 14 at a respective circumferential location. The circumferential and angular spacing between neighbouring winding bundles is the same for all winding bundles. Each winding bundle includes windings wrapped substantially along the radial direction around a circumferential portion of the disk, from inside the core disk to outside and then back inside again.

According to one advantageous example, the stator 12 has twelve winding bundles or coils 18 (each formed for example from a copper conductor, e.g. 8 AWG/10 AWG) wound on the stator core. In advantageous examples the winding bundle shape is trapezoidal and the winding type is drum winding. Fig. 15a shows a front view of a winding bundle 18. Fig. 15b shows a side view of a winding bundle 18.

By way of one advantageous example, each winding bundle 18 may include between 30 and 40 turns, for example 37 turns. These may for example be formed of copper. The stator core is a slotless disk. This means that it comprises no slots or detents or gaps within which the windings sit Instead the core is continuous around the circumferential direction. In particular, major surfaces of the stator (meaning in this disclosure surfaces parallel with the radial dimension of the core) are uninterrupted and are continuous around the core.

The stator core 14 may in some examples have the same dimensions as to the rotor core or yoke 24.

According to one or more examples, the stator core 14 may be formed from steel, for example steel 50.IN1000. This is just one example only.

Fig. 16 illustrates example dimensions of a stator core 14 according to one or more embodiments. According to one example, the stator core may have an outer radius of 78 mm, and an inner radius of 45.03 mm. This corresponds to an outer diameter d of approximately 156 mm, and an inner diameter c of approximately 90.06 mm. The thickness or depth may be 7 mm width.

In advantageous examples, the magnet is trapezoid shaped, as previously illustrated in Fig. 11 above. Thus, there are two end edges 63a, 63b with differing widths. A smaller width end edge 63a (inner end edge) may have a width of approximately 17 mm, a larger end edge 63b (outer end edge) may have a width of approximately 38.66 mm.

The maximum height of the magnet may between approximately 5 mm and 15 mm, for example between 5 mm and 10 mm, for example approximately 8 mm. The minimum height may be between approximately 2 mm and 10 mm, for example approximately 5 mm.

A radial distance, a, from a centre of the stator core 14 to the inner edge 63a may be approximately 36mm, and a radial distance, h, to an outer end edge 63b may be 20 approximately 82 mm These all represent purely illustrative dimensions and are in no way limiting to the inventive concept.

Fig. 17 schematically illustrates magnet orientations and flux directions for the two rotors 22a, 22b. In preferred embodiments, the two rotors are yoked together such that they rotate together as one unit in parallel. The magnets for the two rotors are preferably aligned circumferentially with one another, such that each magnet of the first rotor is arranged axially opposite to a magnet of the second rotor. The magnet arrangement is preferably of the NN type, meaning that magnets opposite one another axially have the same polarity. Circumferentially adjacent magnets on the same rotor preferably have different polarity, such that polarity alternates around the circumferential direction. This is illustrated in Fig. 17.

Fig. 18a shows a flux density diagram for an example generator according to one or more embodiments. The shaded areas indicate the areas of maximum flux density. The maximum flux density is 1.8 T in this example. It is these high flux density regions in-between neighbouring magnets and coil bundles that leads to cogging torque. The nonuniform magnet upper surface profile of embodiments of the present invention means that this flux density between magnets and windings may be reduced.

Fig. 18b shows a heat map for an example generator 10 according to embodiments. The shaded regions indicate regions of maximum heat. Heat is maximum where the flux density is maximum. Hence, the maximum heat producing areas are the rotor core (or yoke) 24 and stator core 14. The stator core has less heat as compared to the rotor core. Hence, the maximum heat in the generator is being produced by the rotor core as maximum flux passes through the rotor core. The maximum heat areas are the regions in-between neighbouring magnets 26 and coil bundles 18.

As the generator is rotated at high speed in operation, heat depends upon magnetic flux density and air friction.

Fig. 19 illustrates the directions of flux circulations in the generator. The arrows indicate the direction of flux circulation when the generator is in operation, with the rotors rotating relative to the stator.

Fig. 20 shows a circuit diagram illustrating an example circuit arrangement for the winding bundles 18 (or coils) of the stator 12.

In the example discussed above, there are twelve coils on the stator. By way of example, there may be for instance 37 turns in each coil, making a total 444 turns in the overall single phase machine. Such a generator is able to produce an 8.5 KW output power

for example.

As shown in Fig. 20, the coils are connected in series.

The connections of the twelve coils with each other and with a load are shown in Fig. 20.

According to the particular example generator discussed above (with the example dimensions discussed above), the achievable output voltage (root mean square output voltage) is 231 V, the (root mean square) output current is 32 A, and the output power is 8.5 KW. This is for a generator operated at 400 Hz, 4000 rpm rotation frequency.

The overall weight of the machine may be between approximately 50 and 10 kg in some examples, for example approximately 8 kg, for example 8.6 kg.

As discussed, the non-uniform magnet upper surface height enables reduction of togging torque and consequently enables higher output power.

To illustrate this, comparative electrical specifications in terms of output characteristics for a standard generator with flat magnet upper surfaces and a generator according to an embodiment of the invention (with non-uniform upper surface) are set out in Table 3 and Table 4 below. The data presented were generated based on software simulations The two generators tested were the same in all respects except for the magnet upper surface shape.

The results shows a roughly 7% improvement in output characteristics of a non-uniform surface height magnet generator compared with a flat magnet generator.

With Loading Shape Vpeak Vrms Ppeak Prms 'peak I rms Flat Magnet 309V 211V 13645W 7880W 44A 30A Non-uniform Magnet 331V 233V 15072W 8471W 47A 33A

Table 3

Without Loading Shape Vpeak VI IIIS Flat Magnet 317V 216V Non-uniform Magnet 345 V 241 V Tabk 4 Vpeak is peak voltage output, V., is the root mean square value of the voltage output, Ppeal, is the peak power output, and Pr ms is root mean square of the power output, 'peak is the peak current, and I is the root mean square output current.

The non-flat magnet generator was able to achieve an output power of approximately 8.5 KW compared with just 7.9 KW for the flat magnet generator.

The magnets for the two tested generators differ in their upper surface shape (curved top with overhang versus flat top) and the volume of magnet also differs slightly due to the overhang and the non-uniform surface height. There is a consequent difference in weight between the two generators of approximately 6 -7 grams difference per magnet.

For the particular example tested for the tables 3 and 4 above, the magnets each had an upper surface profile which was contoured to follow a half-ellipse shape, extending from one side edge 62a of the magnet to the other side edge 62b of the magnet across the width dimension of the magnet.

However, this is just one example magnet shape, and other shapes are possible in accordance with further embodiments, as will be discussed below.

There are different options for the shape of the magnet upper surface profile. In a first general group of embodiments, the upper surface profile may be curved In preferred examples the upper surface profile may curve away from a maximum height in a central region of the magnet to a minimum height at the edges of the magnet, e.g at side edges.

An example is illustrated in Fig. 21.

For example, a maximum point 92 of the magnet surface height H may be located at a centre of the magnet width, w (see Fig. 11). The magnet upper surface 32 height decreases smoothly from the maximum 92 in the centre to a minimum level 94 at either side edge 62a, 62b of the magnet. Thus the gap 34 height is at a minimum 92 in the centre and at a maximum 94 at opposing side edges of the magnet in this example, and the magnet upper surface height H is at a maximum in the centre and at a minimum at either side edge.

In particular, as shown in Fig. 11, each magnet has a length, /, and a width, the length substantially aligned with a radial direction of the rotor 22 or stator, and the width substantially tangential to a radial direction of the rotor 22 or stator and wherein a gap height minimum is located at a centre of the magnet width, w.

According to preferred examples, and as shown in the example of Fig. 21, the magnet upper surface 32 height declines symmetrically on either side of the central maximum height point 92. In other words, the downward curvature from the central maximum height point to the first side edge 62a is exactly symmetrical with the downward curvature from the central maximum height point to the second side edge 63b.

According to one set of examples, and as illustrated in Fig. 21, the upper surface profile may be curved in such a way that from maximum height point 92 of magnet upper surface 32 to the edges 62a, 62b of magnet, every proceeding point is such that there is no two points at the same height and the distance between every two points of the curve is same. Hence, effectively, a uniform change in the air-gap is achieved and a very smooth curvature results.

According to one advantageous set of embodiments, the upper surface profile may follow the shape of a portion of a (non-circular) ellipse. This example is shown in the example generator illustrated in Figs. 7-16 above. The ellipse shape was shown in detail in Fig. 10.

The magnet upper surface profile in this example is shaped to define a half-ellipse, extending from one side edge 62a of the magnet to an opposing side edge 62b of the magnet In other words, the half ellipse has a major axis equal to the width, w, of the magnet and minor axis equal to hi.

The ellipse may instead extend between opposing end edges 63a, 63b of the magnet in other examples. In some examples, the upper surface may be shaped to define a portion of an ellipse which is less than half As shown, the magnet has an upper portion with height hj and a lower (base) portion with height 112. The total height H = h1 + h2.

A maximum point of the magnet surface height H is located at a centre of magnet width, w, in this example. The magnet upper surface 32 height decreases smoothly from the maximum in the centre to a minimum level at either side edge 62a, 62b of the magnet. Thus the gap 34 height is at a minimum in the centre and at a maximum at opposing side edges of the magnet in this example, and the magnet upper surface height His at a maximum in the centre and at a minimum at either side edge.

In this example, the height of each magnet upper surface varies in only one direction across the magnet (the width direction), and exhibits zero variation in a direction perpendicular to said one direction. However, in other examples, the height may vary in more than one direction across the magnet area.

In this example, the magnet height varies along the width direction. In the assembled generator, this corresponds to a direction substantially tangential to a radial dimension of the stator or rotor core. It may in some examples corresponds to a direction substantially parallel with a circumferential dimension of the stator or rotor core.

In advantageous examples, the maximum height of the magnet may between approximately 5 mm and 15 mm, for example between 5 mm and 10 mm, for example 30 approximately 8 mm. The minimum height may be between approximately 2 mm and 10 mm, for example approximately 5 mm.

By providing an elliptical top curvature on upper faces of the magnets, the air gap flux density is reduced and there is only a small change in air gap reluctance with respect to rotor position. As a result, the cogging torque is reduced and hence the efficiency of machine is increased.

In particular, the non-uniform magnet upper surface height means that the magnet flux in the air gap is more sinusoidally distributed and increases the effective air gap 34 length toward the magnet edges. The air gap length for the non-uniform surface magnet is not the same over each pole (each magnet). In particular, in this example, it is at a minimum in the middle of the magnet and at a maximum at the edges of the magnets. This means that the average air-gap flux can be reduced while keeping the magnet close to the winding 18 for as large a part of the magnet area as possible. This enables reducing cogging torque without substantial loss to the flux linkage with the stator winding.

Thus, in the non-uniform magnet upper surface (e.g. elliptical top), the central point of ellipse has the minimum distance from the winding bundle opposite 18, and, moving outward toward the edges of the curve, the gap distance increases gradually to the edges The rise in effective air gap length decreases air gap flux, which in turns reduces the overall cogging torque.

The ellipse shape for the magnet upper surface profile is particularly advantageous because of its relatively gradual rate of height decline from the centre point. In comparison for example to an arc or a semicircle shape, an ellipse remains at a higher level over a larger area of the magnet, and then declines more steeply toward the edges.

An aim of the present invention is to achieve maximal output power and minimum electrical noise. For that, it is optimum to keep the magnets 26 close to the coils 18. However, when the magnets are close to the coils they begin attracting the metallic material in the stator, this leading to cogging toque. A non-uniform magnet upper surface counters this by increasing the gap size across a portion of the magnet area, thereby decreasing the flux density and thus the attraction with the stator core. However, as discussed above, most optimally, the gap distance should increase fastest toward the magnet edges (where the windings are coming to an end, and so flux linkage is lower) where the reduced flux density has less impact on total flux linkage. A rapid change in the curve shape reduces cogging toque but may not be optimum in terms of maximising output power as it may cause a reduction in the output power. It may have some fringing effect on the corners also.

An elliptical top magnet shape enables keeping the magnets close to the coils over a larger area, and the drop down of the magnet to the corners is smoother (i.e. less rapid). As a result, output power is not reduced significantly and the cogging torque is also reduced.

Although the minimum point is at a centre of the width in this example, this is not essential and it may be offset from the centre in other examples.

In the example discussed above, the upper surface of each magnet varies in height across only one direction of the magnet upper surface (width-wise). The height does not vary in the direction perpendicular to the height direction, i.e. there is no variation in the height along the length direction. The magnet upper surface curves to follow the shape of a half ellipse across the width direction, the half ellipse extending from one side edge 62a of the magnet to the other side edge 62b of the magnet. Thus, in other words, the magnet upper surface profile describes a half ellipse across a cross-section of the magnet across a plane parallel with the width of the magnet.

However, in other examples, the height may vary in more than one direction across the magnet.

According to one or more embodiments, the upper surface profile of each magnet may be curved to follow the shape of a respective ellipse across two perpendicular directions of the magnet In particular, the upper surface profile may follow one ellipse shape across a width direction and another ellipse shape across the length direction. The two ellipse shapes may be the same or different in terms of their major and minor axis and their foci.

This example is illustrated in Figs. 22-24.

Fig. 22 shows a cross-sectional view of the magnet 26 with upper surface contoured in an elliptical shape across both the width, w, and perpendicular length, 1, directions.

Fig. 23 shows a number of different views of the magnet 26 according to this set of examples. A top-down view of the magnet is shown in the centre. The magnet has a trapezoid outer shape. Side elevations views of the magnet 26 from an inner edge end 63a, an outer edge end 63b, and a side edge 62a of the magnet are shown adjacent to the relevant edges. It can be seen that the upper surface profile of the magnet follows an ellipse shape across both its width (across the outer 63b and inner 63a edge ends) and along its length (visible across the side edge 62a). The magnet upper surface hence effectively exhibits an elliptical convex shape.

Fig. 24 shows a side view of an example rotor 22 comprising an annular arrangement of magnets 26 having the dual-directional ellipse shape in accordance with this example.

In accordance with one specific example, the depth or thickness of the rotor core 34 may be approximately 9 mm +1-0.5 mm.

Fig. 25 shows a flux density diagram for a generator incorporating magnets having the dual-direction ellipse shape upper surface profile. The shaded areas indicate regions of peak flux density.

Fig. 26 shows a heat map for a generator incorporating magnets having the dual-direction ellipse shape upper surface profile. The shaded areas indicate regions of maximal heat generation during operation.

Although an ellipse shape for the magnet upper surface profile has been discussed above, an ellipse shape is not essential. Any upper surface profile defining a nonuniform upper surface height achieves benefits of reduced cogging toque.

According to one set of advantageous embodiments, the upper surface profile of each magnet may be curved according to a sinusoidal function. For example, the upper surface profile may be contoured so as to define a half sinusoid (e.g. half a cycle of a sinusoidal function) across at least one direction of the magnet upper surface.

An example is illustrated in Figs. 27 and 28.

Fig. 27 shows a side elevation view of the magnet from an inner end edge 63a of the magnet. It can be seen that the upper surface profile curves in a sinusoidal fashion. A maximum of the sinusoid is not in this case located at a centre of the width, w, of the magnet but offset from the centre. This can be see also in Fig. 28 which shows a top-down view of the magnet 26 in the centre of the figure, and shows a view from an outer edge 63b end of the magnet at the top. From this it can be seen that the magnet upper surface varies in height across the width dimension, w, of the magnet, following a sinusoidal type curve, and with a peak in height offset from the centre of the magnet width.

Fig. 28 also shows a side elevation view from a side edge 62a of the magnet. According to one specific example, a maximum height, H, of the magnet may be approximately 8 mm in this example.

Fig. 29 shows a side view of an example rotor 22 comprising an annular arrangement of magnets 26 mounted on the rotor core 24, the magnets having the sinusoidal upper surface profile according to the present example.

In accordance with one specific example, the depth or thickness of the rotor 30 core 34 may be approximately 9 mm +1-0.5 mm.

The magnet 26 upper surface profile may be curved to follow a sinusoid pattern across only direction of the magnet, or may follow respective sinusoid shapes across two dimensions of the upper surface. The sinusoids may be the same or different across each dimension.

Fig. 30 shows a flux density diagram for a generator incorporating magnets having the sinusoidal shape upper surface profile. The shaded areas indicate regions of peak flux density.

Fig. 31 shows a heat map for a generator incorporating magnets having the sinusoidal shape upper surface profile. The shaded areas indicate regions of maximal heat generation during operation.

According to a further set of examples, each magnet 26 upper surface may follow a skewed curve shape in which the upper surface varies smoothly in height across both the width and length dimensions of the magnet between a lowest and highest point on one end edge of the magnet to a different lowest and highest point on the opposite end edge.

This example is illustrated in Figs. 32-35, Fig. 32 shows a view from an inner end edge 63a of the magnet.

A clearer view of the magnet upper surface contouring is shown in Figs. 3334, Fig. 33 shows a side elevation view of the magnet from the inner end edge 63a of the magnet, and Fig. 34 shows a side elevation view of the magnet from the outer end edge 63b of the magnet. As indicated in Fig. 35, the inner and outer end edges are at opposing ends of the length of the magnet, and correspond to the shorter and longer of the two end edges.

As shown in Figs. 33 and 34, along each end edge 63a, 63b of the magnet 26 is a respective lowest height point and highest height point. Along the inner end edge 63a, the highest point 74 is approximately centrally located along the edge. The lowest height point 72 is located at one corner of the end edge. Along the outer end edge 63b, the highest height point 78 and lowest height point 76 are located at respective corners of the outer end edge 63b. However, the specific locations of the highest and lowest height points along each end edges is not critical.

The height of the magnet upper surface varies along the width direction, w, and along the length direction, 1, of the magnet, i.e, between the side edges 62a, 62b and the end edges 63a, 63b. The height varies along each end edge between the respective highest and lowest points, and varies between the end edges along the length of the magnet. This results in a skewed curve shape over the area of the magnet resulting from the curvature from the high and low points of one end edge to the low and high points of the other end edge.

Preferably the low and high points of the two end edges are not aligned with one another along the length dimension.

Fig. 35 shows a top-down view of the magnet 26 in the centre of the figure and shows side elevation views from the outer end edge 63 and the side edge 62a respectively.

The views are shown adjacent to the respective edge. The resultant skewed upper surface profile can be seen from the side edge 62a elevation view.

Fig. 36 shows a side view of an example rotor 22 comprising an annular arrangement of magnets 26 mounted on the rotor core 24, the magnets having the skewed curve upper surface profile according to the present example.

In accordance with one specific example, the depth or thickness of the rotor core 34 may be approximately 9 mm 0.5 mm.

Fig. 37 shows a flux density diagram for a generator incorporating magnets having the skewed curve upper surface profile. The shaded areas indicate regions of peak flux density.

Fig. 38 shows a heat map for a generator incorporating magnets having the skewed curve upper surface profile. The shaded areas indicate regions of maximal heat generation during operation.

Examples discussed above have related to a generator in which the upper surface profile for the magnets is curved. However, in further examples, the magnet upper surface profile may be stepped in height.

An example is illustrated in Figs. 39-41. In this example, an outer envelope shape of the other upper surface profile is curved, but wherein the envelope curvature is formed by discrete steps. The upper surface profile may follow a spline function according to 20 one or more examples.

A maximum of the upper surface height, H, is at a middle of the width, w, of the magnet 26. Fig. 39 shows am elevation view from the inner end edge 63a of the magnet. Fig. 40 shows a top-down view of the magnet in the centre of the figure and shows elevation views from the outer end edge 63b and side edge 62a respectively. The elevation views of the two edges are shown adjacent the respective edges.

Fig. 41 shows the steps 82 of the upper surface profile in closer detail. In the particular example illustrated, each step is shown as having a length of approximately 1.5 mm. However, this is by way of example only.

Fig. 42 shows a side view of an example rotor 22 comprising an annular arrangement of magnets 26 mounted on the rotor core 24, the magnets having the stepped upper surface profile according to the present example.

In accordance with one specific example, the depth or thickness of the rotor core 34 may be approximately 9 mm +/-0.5 mm.

Fig. 43 shows a flux density diagram for a generator incorporating magnets having the stepped upper surface profile according to the present example. The shaded areas indicate regions of peak flux density.

Fig. 44 shows a heat map for a generator incorporating magnets having the stepped upper surface profile according to the present example. The shaded areas indicate regions of maximal heat generation during operation.

According to a further set of examples, the upper surface profile of each magnet may be stepped and wherein an outer envelope shape of the upper surface profile defined by the steps follows a linear peak. By linear peak is meant that the upper surface profile declines in height linearly away from a maximum point or line. The height maximum may be at a centre of the magnet, for example at a centre of the magnet width. The height may be at a minimum at opposing edges of the magnet, for example at opposing side edges 62a, 62b of the magnet.

An example is illustrated in Fig. 45-47.

Fig. 45 shows an elevation view from an inner end edge 63a of the magnet The linear incline of the upper surface profile envelope toward the centre maximum point can be seen. Fig. 46 shows a top-down view of the magnet in the middle of the figure, and shows an elevation view from an outer end edge 63b of the magnet and from the side edge 62a of the magnet. The elevation views are shown adjacent the respective edges.

Fig. 47 shows the steps forming the linear incline outer envelope. In this particular example, the steps are illustrated as each having a width of 1.05 mm and a height of 0.17 mm. This is by way of example only.

In each of the above examples, the upper 63a and lower 63b (inner and outer) edges of the magnets 26 are shown as straight (linear). However, in further examples, the upper and lower edges may be curved. For example they may be curved in an arcuate fashion, and may be curved to follow the curvature of the rotor core disk to which they are mounted. For example, the upper edge of each magnet may be curved to follow the curvature of the outer circumference of the rotor core annular disk and the lower edge may be curved to follow the curvature of the inner circumference of the rotor core annular disk.

In examples described above, generators having a single stator and two rotors have been discussed. However, according to variants, any of the above example embodiments may be expanded to include more than one stator and/or more than one rotor. By adding additional stators or rotors the output power is increased proportionately with only a modest increase the outer dimensions.

By way of example, a so-called Multi Stack AFPM generator may be provided in accordance with one or more embodiments which comprises greater numbers of stators or rotors, the rotors and stators stacked along the axial direction, with the stack alternating axially between stators and rotors.

By way of example, any of the above discussed embodiments might be expanded or adapted to comprise two rotors and three stators, by placing a further two stators on axially opposite sides of the dual-rotor single stator structure discussed in embodiments above (see e.g. Fig. 7 above). Hence here, the two rotors are each positioned axially between two stators, with a middle stator being shared by both rotors.

By way of further example, any of the above discussed embodiments might be expanded or adapted to comprise three rotors and two stators, for example by adding a further stator and further rotor stacked axially above or adjacent one of the rotors of the dual-rotor single stator stnicture discussed above (e.g. Fig. 7). Hence here, three rotors rotate and each of the two stators is sandwiched between two rotors (one of the rotors shared by the two stators).

Generators in accordance with embodiments of the present invention are able to achieve a high power output within a relatively compact form factor. This is in part due to the use of an Axial Flux Permanent Magnet (AFPM) structure for the generator which, as discussed above, provides maximal power output for the given outer dimensions, and also optimum quality of power output. The problem of cogging torque associated with AFPM generators is ameliorated through the employment of permanent magnets having upper surfaces which are contoured to have a non-uniform height across their area. This disrupts the magnetic interactions between the magnets and the stator core which normally lead to cogging torque. As a result, efficiency of the generator is improved, and the generator is able to produce a higher output power for a given set of generator dimensions.

Furthermore, due to the reduced flux density across portions of the magnets (as a result of the variable magnet-winding gap height), heat generation from the generator is reduced.

In view of the above, generators in accordance with embodiments of the present invention are ideally suited for applications which call for relatively high levels of power generation within a compact space.

Example applications for which a generator according to the present invention may therefore be particularly beneficial include -Wind energy electrical generators; - Compact emergency petrol or diesel generators, - Mobile emergency generators for charging electric vehicles; - Water powered turbine electrical generators; - Auxiliary mobile on-vehicle generators for refrigeration, road repairs, compressed air sources or other uses Examples in accordance with a further aspect of the invention provide a method of generating electrical power using an electrical generator in accordance with any example or embodiment outlined above or described below, or in accordance with any claim of this application, the method comprising driving rotary motion of each rotor relative to the stator.

Preferably both rotors are rotated in the same rotary direction simultaneously. Preferably both are rotated in parallel at the same speed, and with their respective magnets angularly aligned. In some cases the two rotors may be yoked together, so that the two rotate together as one.

Examples in accordance with a further aspect of the invention provide a method of using an electrical generator in accordance with any example or embodiment outlined above or described below, or in accordance with any claim of this application, for generating electrical power within one or more of: a wind energy generator; a petrol or diesel generator, e.g. a compact petrol or diesel generator; a mobile generator for charging electric vehicles; a water powered turbine generator; and a mobile on-vehicle generator, e.g. for refrigeration, road repairs, or compressed air sources.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. If a computer program is discussed above, it may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS: An electrical generator (10), comprising: at least one stator (12), comprising a core (14) in the form of an annular disk, and an annular arrangement of winding bundles (18), each wrapped around the annular disk core at a respective circumferential location; at least two rotors (22), arranged respectively on axially opposite sides of the stator, and each having a respective core (24) in the form of an annular disk, and each having a respective major surface (28) arranged facing a surface (30a, 30b) of the stator disk core (14); each rotor comprising an annular arrangement of permanent magnets (26) mounted on a major surface of the rotor core, between the rotor and stator, and each of the magnets having a respective upper surface (32) facing the stator and separated from the stator by a gap (34); wherein an upper surface (32) profile of each magnet is contoured with a nonuniform surface height (H) such that a size of said gap (34) between the magnet upper surface and the stator is non-uniform across the area of the magnet.
2. A generator as claimed in claim 1, wherein each magnet has a length and a width, the length substantially aligned with a radial direction of the rotor or stator core, and the width substantially tangential to a radial direction of the rotor or stator core, and wherein a gap height minimum is located at a centre of the magnet width.
3. A generator as claimed in claim 1 or 2, wherein the upper surface profile of each magnet is curved.
4. A generator as claimed in claim 3, wherein the upper surface profile of each magnet is curved so as to define a portion of a non-circular ellipse.
5. A generator as claimed in claim 4, wherein the upper surface profile is shaped to define a half-ellipse, preferably extending from one edge of the magnet to an opposing edge of the magnet.
6 A generator as claimed in claim 3, wherein the upper surface profile of each magnet is curved according to a sinusoidal function.
7. A generator as claimed in claim 1 or 2, wherein the upper surface profile of each magnet is stepped in height.
8. A generator as claimed in any of claims 1-7, wherein the height of each magnet upper surface varies in only one direction across the magnet, and exhibits zero variation in a direction perpendicular to said one direction 9. A generator as claimed in claim 8, wherein said one direction is a direction substantially tangential to a radial dimension of the stator or rotor core, and/or wherein said one direction is a direction substantially parallel with a circumferential dimension of the stator or rotor core.10. A generator as claimed in claim 9, wherein the height of the upper surface of each magnet varies in more than one direction across the magnet 11. A generator as claimed any of claims 1-10, wherein the number of permanent magnets matches the number of winding bundles.12. A generator as claimed in claim 11, wherein the annular arrangement of the permanent magnets for each rotor matches the annular arrangement of the winding bundles.13. A generator as claimed in any of claims 1-12, wherein the annular disk of the stator core is uninterrupted in the circumferential direction.14. A generator as claimed in any of claims 1-13, wherein each of the winding bundles has a trapezoid shape.15. A generator as claimed in any of claims 1-14, comprising more than two rotors and/or more than one stator.16. A method of generating electrical power using an electrical generator according to any of claims 1-15, the method comprising driving rotary motion of each rotor relative to the stator.17. A method of using an electrical generator according to any of claim 1-15 for generating electrical power within one or more of: a wind energy generator; a mobile generator for charging electric vehicles; a water powered turbine generator, and a mobile on-vehicle generator, for example for refrigeration, road repairs, or compressed air sources.Amendments to the claims have been filed as folio CLAIMS: An electrical generator (10), comprising: at least one stator (12), comprising a core (14) in the form of an annular disk, and an annular arrangement of winding bundles (18), each wrapped around the annular disk core at a respective circumferential location; at least two rotors (22), arranged respectively on axially opposite sides of the stator, and each having a respective core (24) in the form of an annular disk, and each having a respective major surface (28) arranged facing a surface (30a, 30b) of the stator disk core (14); each rotor comprising an annular arrangement of permanent magnets (26) mounted on a major surface of the rotor core, between the rotor and stator, and each of the magnets having a respective upper surface (32) facing the stator and separated from the stator C\I by a gap (34); CO wherein an upper surface (32) profile of each magnet is contoured with a non-uniform surface height (H) such that a size of said gap (34) between the magnet upper surfaceCOand the stator is non-uniform across the area of the magnet, and wherein the upper surface profile of each magnet is curved so as to define a portion of a non-circular ellipse.2. A generator as claimed in claim 1, wherein each magnet has a length and a width, the length substantially aligned with a radial direction of the rotor or stator core, and the width substantially tangential to a radial direction of the rotor or stator core, and wherein a gap height minimum is located at a centre of the magnet width.3. A generator as claimed in claim 1 or 2, wherein the upper surface profile is shaped to define a half-ellipse, preferably extending from one edge of the magnet to an opposing edge of the magnet.4. A generator as claimed in any of claims 1-3, wherein the height of each magnet upper surface varies in only one direction across the magnet, and exhibits zero variation in a direction perpendicular to said one direction.5 A generator as claimed in claim 4, wherein said one direction is a direction substantially tangential to a radial dimension of the stator or rotor core, and/or wherein said one direction is a direction substantially parallel with a circumferential dimension of the stator or rotor core.6. A generator as claimed in claim 5, wherein the height of the upper surface of each magnet varies in more than one direction across the magnet 7. A generator as claimed any of claims 1-6, wherein the number of permanent magnets matches the number of winding bundles.C\I 8. A generator as claimed in claim 7, wherein the annular arrangement of the CO permanent magnets for each rotor matches the annular arrangement of the winding bundles.CO
9. A generator as claimed in any of claims 1-8, wherein the annular disk of the I-20 stator core is uninterrupted in the circumferential direction.
10. A generator as claimed in any of claims 1-9, wherein each of the winding bundles has a trapezoid shape.
11. A generator as claimed in any of claims 1-10, comprising more than two rotors and/or more than one stator.
12. A method of generating electrical power using an electrical generator according to any of claims 1-11, the method comprising driving rotary motion of each rotor relative to the stator.
13. A method of using an electrical generator according to any of claims 1-12 for generating electrical power within one or more of a wind energy generator; a mobile generator for charging electric vehicles; a water powered turbine generator, and a mobile on-vehicle generator, for example for refrigeration, road repairs, or compressed air sources.