TECHNICAL FIELD

The present disclosure relates to a rotor. Particularly, but not exclusively, the present disclosure relates to an electric machine rotor for use in an electric machine that can be used as a motor or a generator.

Aspects of the invention relate to a rotor, to an electric machine, and to a vehicle.

BACKGROUND

It is known to use one or more electric machine in a vehicle. Such electric machines may operate as motors or as generators. Electric machines may operate as traction motors for propelling a vehicle such as an automobile, van, truck, motorcycle, boat, or aeroplane. Electric machines may be used in place of, or in addition to, an internal combustion engine.

Such electric machines comprise a stator and a rotor. One type of known electric machine comprises a stator and a rotor as part of a permanent magnet synchronous motor. An air gap is maintained between the rotor and the stator with the rotor being located within the confines of the stator. The stator is a stationary element of the electric machine which may comprise a plurality of slots within which electrical windings are located. The rotor is a rotating element of the electric machine allowing a transfer of electrical energy input into the motor to a mechanical output, such as the rotation of a driveshaft of a vehicle.

The rotor may comprise a plurality of laminations of a ferromagnetic material to form a rotor iron. Magnets are embedded in the rotor to form a plurality of rotor poles. The magnets are permanent magnets and generate a magnetic flux. A rotor pole has a direct axis, or d-axis, aligned to the permanent magnet flux, and quadrature axes, or q-axes, denoted as a +q-axis and a -q-axis, arranged transverse to the direction of the rotor pole (i.e. transverse to the d- axis). The angular extent of each rotor pole, which is the included angle between the +q-axis and the -q-axis, is referred to as a pole step.

The vehicle may, for example, comprise a battery electric vehicle (BEV), a plug-in hybrid electric vehicle (PH EV) or a hybrid electric vehicle (HEV) where the electric machine is a traction motor for the vehicle. It is desirable to have the lightest possible traction motor with optimised energy conversion from an electrical energy input to a mechanical energy output.

It is an aim of the present invention to address one or more of the disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide a rotor, an electric machine, and a vehicle as claimed in the appended claims.

According to an aspect of the present invention there is provided a rotor for an electric machine, the rotor comprising: a rotational axis; a plurality of poles, each pole comprising: a first permanent magnet arrangement disposed symmetrically about a d-axis of the pole in a first layer, the first permanent magnet arrangement having a centre at a first radial distance from the rotational axis of the rotor, the first permanent magnet arrangement having a width along a first axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor; a second permanent magnet arrangement disposed symmetrically about a first q-axis of the pole in a second layer, the second permanent magnet arrangement having a centre at a second radial distance from the rotational axis of the rotor, the second radial distance being radially inset from the first radial distance, the second permanent magnet arrangement having a width along the first q-axis and orthogonal to the rotational axis of the rotor; and a third permanent magnet arrangement disposed symmetrically about a second q-axis of the pole in the second layer, the third permanent magnet arrangement having a centre at the second radial distance from the rotational axis of the rotor, the third permanent magnet arrangement having a width along the second q-axis and orthogonal to the rotational axis of the rotor; wherein the rotor comprises continuous rotor material, along a radial axis extending from the rotational axis of the rotor, between the first permanent magnet arrangement and a central shaft aperture of the rotor.

An advantage of this aspect of the invention is that this arrangement of the magnets may provide an optimal torque to magnet weight ratio. An advantage of this aspect of the invention is that this arrangement of the magnets may help to distribute stresses more evenly across the rotor. An advantage of this aspect of the invention is that the arrangement may be configured to maintain mechanical integrity of the rotor at high rotational speeds. An advantage of this aspect of the invention is that the arrangement may be effective in controlling flux density in the rotor.

The first permanent magnet arrangement may have a span of between eighty eight and one hundred and twenty eight electrical degrees. The first permanent magnet arrangement may have a span of one hundred and eight electrical degrees. This provides the advantage of an optimal torque to magnet weight ratio.

The first permanent magnet arrangement may be separated in the radial direction from the second permanent magnet arrangement, and the first permanent magnet arrangement may be separated in the radial direction from the third permanent magnet arrangement.

At least one of the first permanent magnet arrangement, the second permanent magnet arrangement, and the third permanent magnet arrangement, may comprise a single magnet slot housing one or more magnet.

At least one of the first permanent magnet arrangement, the second permanent magnet arrangement, and the third permanent magnet arrangement, may comprise a plurality of magnet slots, each magnet slot housing one or more magnet separated from an adjacent slot by a bridge.

The or each magnet slot may comprise a plurality of segmented magnets. This provides the advantage of reducing eddy current losses in the rotor.

The first permanent magnet arrangement may be located in a first magnet slot, the first magnet slot may comprise: a first portion of the first magnet slot with substantially the same shape and size as the first permanent magnet arrangement; a second portion of the first magnet slot, at a first widthwise end of the first portion of the first magnet slot, extending towards the first q- axis, the second portion of the first magnet slot providing a first air void of the first magnet slot; and a third portion of the first magnet slot, at a second widthwise end of the first portion of the first magnet slot, extending towards the second q-axis, the third portion of the first magnet slot providing a second air void of the first magnet slot.

This provides the advantage of reducing torque ripple. This also provides the advantage of helping to avoid magnet demagnetisation. This arrangement may also have structural benefits by avoiding the occurrence of high stress in the areas around the ends of the permanent magnet arrangements. The second permanent magnet arrangement may be located in a second magnet slot, the second magnet slot may comprise: a first portion of the second magnet slot with substantially the same shape and size as the second permanent magnet arrangement; and a second portion of the second magnet slot, at a first widthwise end of the first portion of the second magnet slot, extending towards an outer circumference of the rotor, the second portion of the second magnet slot providing a first air void of the second magnet slot.

This provides the advantage of reducing torque ripple. This also provides the advantage of helping to avoid magnet demagnetisation. This arrangement may also have structural benefits by avoiding the occurrence of high stress in the areas around the ends of the permanent magnet arrangements.

The third permanent magnet arrangement may be located in a third magnet slot, the third magnet slot may comprise: a first portion of the third magnet slot with substantially the same shape and size as the third permanent magnet arrangement; and a second portion of the third magnet slot, at a first widthwise end of the first portion of the third magnet slot, extending towards an outer circumference of the rotor, the second portion of the third magnet slot providing a first air void of the third magnet slot.

This provides the advantage of reducing torque ripple. This also provides the advantage of helping to avoid magnet demagnetisation. This arrangement may also have structural benefits by avoiding the occurrence of high stress in the areas around the ends of the permanent magnet arrangements.

A first rotor pole may have a first adjacent pole sharing the second permanent magnet arrangement and a second adjacent pole sharing the third permanent magnet arrangement.

The first permanent magnet arrangement, second permanent magnet arrangement, and third permanent magnet arrangement, may comprise NdFeB magnets.

According to an aspect of the present invention there is provided an electric machine comprising a rotor according to any preceding aspect and a stator.

According to an aspect of the present invention there is provided a vehicle comprising an electric machine according to any preceding aspect. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a cross sectional image of a rotor according to an embodiment of the invention;

Figure 2 illustrates a single pole of an eight pole rotor according to an embodiment of the invention;

Figure 3 illustrates a magnified view of the single pole illustrated in Figure 2;

Figure 4 shows illustrates the flux flow in a single pole of an eight pole stator shown alongside a section of a stator of an electric machine, according to an embodiment of the invention; and

Figure 5 illustrates a vehicle according to an embodiment of the invention.

DETAILED DESCRIPTION

Examples of the present disclosure relate to a rotor. In particular, examples of the present invention relate to an eight pole rotor with a pole step of forty five mechanical degrees, though it will be understood that a different number of poles, such as six, may be provided. Nonlimiting examples will now be described with reference to accompanying Figures 1 to 5, where the figures illustrate a rotor 10, an electric machine 300, and a vehicle 400. The rotor 10 is intended for use in an electric machine 300, as partially illustrated in Figure 4, where the electric machine 300 comprises the rotor 10 and a stator 212 with an air gap 214 therebetween. The stator 212 comprises a plurality of slots extending radially inwardly to support electrical windings. For example, the electric machine 300 may comprise forty eight slots and eight rotor poles. The electric machine 300 may provide the function of a motor and/or generator for operation in a vehicle 400. For example, the electric machine 300 may be a traction motor for an electric vehicle 400.

With reference to Figure 1 , there is shown a rotor 10 with a plurality of poles, which in this example comprises eight poles 12-1 , 12-2, 12-3, 12-4, 12-5, 12-6, 12-7, 12-8, each pole being identical to each other pole. The rotor 10 may form part of an electric machine 300, which may be used, for example, as a traction motor for a vehicle 400. The rotor 10 has a rotational axis 14 about which the rotor 10 is arranged or configured to rotate. The rotor 10 may have a central shaft aperture 50 configured to receive a shaft such as a driveshaft, such that the driveshaft is positioned at least partially through the central shaft aperture 50.

Figure 5 illustrates a vehicle 400 having a first electric machine 300-1 for driving one or more front wheel of the vehicle 400 and a second electric machine 300-2 for driving one or more rear wheel of the vehicle 400. In other embodiments the vehicle 400 may comprise only a single electric machine 300, arranged or configured to drive one of one or more front wheel of the vehicle 400 or one or more rear wheel of the vehicle 400. At a vehicle axle the electric machine 300 may be arranged to drive both wheels, either directly or through other transmission components. Other arrangements may have one electric machine 300 arranged or configured to drive each wheel of the vehicle 400.

Figure 2 illustrates a single pole 12 of an eight pole rotor 10. Pole 12 is a segment of rotor 10 bounded by a first quadrature axis, or +q-axis, 66 and a second quadrature axis, or -q-axis, 68. The +q-axis 66 and the -q-axis 68 define the lateral boundaries of the pole 12. Pole 12 comprises a first permanent magnet arrangement 16 disposed symmetrically about, and substantially orthogonal to, a direct axis, or d-axis, 18 of the pole 12 in a first layer 20.

The first permanent magnet arrangement 16 has a centre 22 at a first radial distance 48 from the rotational axis 14 of the rotor 10.

The first permanent magnet arrangement 16 has a width 24 along a first axis 26 orthogonal to the d-axis 18 and orthogonal to the rotational axis 14 of the rotor 10. Each pole 12 also comprises a second permanent magnet arrangement 28 disposed along the first q-axis, or +q-axis, 66 of the pole 12 and a third permanent magnet arrangement 40 disposed along the second q-axis, or -q-axis, 68 of the pole 12 to form a second layer 30.

The second permanent magnet arrangement 28 has a centre 32 at a second radial distance 34 from the rotational axis 14 of the rotor 10. The centre 32 is at, or on, the +q-axis 66. The second permanent magnet arrangement 28 is shared with an adjacent pole, and is centred on the +q-axis, as illustrated in Figure 1 . That is, the second permanent magnet arrangement 28 is symmetrical about the +q-axis 66.

The second permanent magnet arrangement 28 has a width 36 along the +q-axis 66 and orthogonal to the rotational axis 14 of the rotor 10.

The width 24 of the first permanent magnet arrangement 16 is greater than the width 36 of the second permanent magnet arrangement 28.

The third permanent magnet arrangement 40 has a centre 42 at the second radial distance 34 from the rotational axis 14 of the rotor 10. The centre 42 is at, or on, the -q-axis 68. The third permanent magnet arrangement 40 is shared with an adjacent pole, and is centred on the -q-axis, as illustrated in Figure 1. That is, the third permanent magnet arrangement 40 is symmetrical about the -q-axis 68.

The third permanent magnet arrangement 40 has a width 46 along the -q-axis 68 and orthogonal to the rotational axis 14 of the rotor 10.

The width 24 of the first permanent magnet arrangement 16 is greater than the width 46 of the third permanent magnet arrangement 40.

The second radial distance 34 is radially inset from the first radial distance 48. That is, the second radial distance 34 is smaller than the first radial distance 48, such that the centre 32 of the second permanent magnet arrangement 28 and the centre 42 of the third permanent magnet arrangement 40, forming the second layer 30, are closer to the rotational axis 14 of the rotor 10 than the first layer 20, and in particular closer to the rotational axis 14 of the rotor 10 than the centre 22 of the first permanent magnet arrangement 16. The centre 58 of the second layer 30 along a second axis 56 intersecting the centre 32 of the second permanent magnet arrangement 28 and the centre 42 of the third permanent magnet arrangement 40 is a second radial distance 38 from the rotational axis 14 of the rotor. The centre 58 of the second layer 30 is closer to the rotational axis 14 of the rotor 10 than the centre 22 of the first permanent magnet arrangement 16 of the first layer 20. The centre 32 of the second permanent magnet arrangement 28 and the centre 42 of the third permanent magnet arrangement 40 are equal in distance from the rotational axis 14 of the rotor 10.

In the arrangement illustrated in Figure 2, where the pole 12 is one of eight poles in an eight pole electric machine 300, the centre 32 of the second permanent magnet arrangement 28 is at a ninety degree electrical angle with respect to the centre 22 of the first permanent magnet arrangement 16, and the centre 42 of the third permanent magnet arrangement 40 is at a ninety degree electrical angle with respect to the centre 22 of the first permanent magnet arrangement 16 and at a hundred and eighty degree electrical angle with respect to the centre 32 of the second permanent magnet arrangement 28.

The rotor 10 comprises continuous rotor material, along a radial axis, for example the d-axis 18, extending from the rotational axis 14 of the rotor 10, between the first permanent magnet arrangement 16 and a central shaft aperture 50 of the rotor 10. The radial axis may be any axis extending from the rotational axis 14 of the rotor to any point along the width 24 of the first permanent magnet arrangement 16 between a first end 60 of the first permanent magnet arrangement 16 and a second end 62 of the first permanent magnet arrangement 16. It can therefore be observed that the rotor 10 is continuous, or comprises continuous rotor material, in the area encompassed by the radial axes between the rotational axis 14 of the rotor and each end 60, 62 of the first permanent magnet arrangement 16 along the width 24 of the first permanent magnet arrangement 16.

The first permanent magnet arrangement 16 may have a span of between eighty eight and one hundred and twenty eight electrical degrees, equating, in an eight pole machine to a span of between twenty two and thirty two mechanical degrees. In the embodiment of Figure 2, the first permanent magnet arrangement 16 has a span of one hundred and eight electrical degrees, equating to a span of twenty seven mechanical degrees.

The first permanent magnet arrangement 16 is separated in the radial direction from the second permanent magnet arrangement 28, and the first permanent magnet arrangement 16 is separated in the radial direction from the third permanent magnet arrangement 40. That is, the second permanent magnet arrangement 28 and the third permanent magnet arrangement 40 are physically separated from the first permanent magnet arrangement 16 in a radial direction. In other words, the outer most extremities of the second permanent magnet arrangement 28 and the third permanent magnet arrangement 40 are inboard or radially further inward of any part of the first permanent magnet arrangement 16. Such an arrangement, as illustrated in Figure 2, may help to focus the magnetic flux at the d-axis 18.

Therefore, the arrangement as described above, and as shown as an example only in Figure 2, may reduce interruptions or discontinuities in the magnetic flux present in the air gap 214 between the rotor 10 and the stator 212. The resulting magnetic flux established in the air gap 214 may change progressively in a circumferential direction across the, or each, rotor pole 12. At least in certain embodiments, the topology of the permanent magnets in each rotor pole 12 generates a magnetic flux in the air gap 214 having a magnitude which is generally sinusoidal in form. The magnitude of the magnetic flux may, for example, be greatest at or proximal to the d-axis 18 and smallest at, or proximal to, the q-axis 66, 68 of the rotor pole. The magnitude of the magnetic flux in the air gap 214 may be substantially zero at the q-axis 66, 68. This sinusoidal variation in the magnitude of the magnetic flux is repeated for each of the rotor poles 12.

By arranging the magnets in the abovementioned ranges and with the abovementioned separations, a high torque to magnet weight ratio is observed, whilst minimising the number of magnets used, and stresses are distributed more evenly across the rotor 10. In particular the arrangement of Figure 2 provides an optimal torque to magnet weight ratio. The distribution of the permanent magnet arrangements 16, 28, 40 provides mechanical integrity at high rotational speeds as are observed in electric machines 300 used as traction motors in vehicles 400.

In some embodiments, at least one of the first permanent magnet arrangement 16, the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, may comprise a single magnet slot housing one or more magnet. Therefore one or more of the permanent magnet arrangements 16, 28, 40 may comprise a single slot housing a single magnet, which can reduce manufacturing complexity and therefore cost to manufacture.

In alternative embodiments, at least one of the first permanent magnet arrangement 16, the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, may comprise a single magnet slot housing a plurality of magnets. Therefore, one or more of the permanent magnet arrangements 16, 28, 40 may comprise a single slot housing two or more magnets in a segmented magnet array, that is, a plurality of segmented magnets. By having segmented magnets in a slot, eddy current losses in the rotor 10 can be reduced.

In alternative embodiments, at least one of the first permanent magnet arrangement 16, the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, may comprise a plurality of magnet slots, each magnet slot housing one or more magnet separated from an adjacent slot by a bridge.

In alternative embodiments, at least one of the first permanent magnet arrangement 16, the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, may comprise a plurality of magnet slots, each magnet slot housing a plurality of magnets, separated from an adjacent slot by a bridge.

In the arrangement shown in Figure 2, the first permanent magnet arrangement 16 has a thickness along the d-axis 18 which is greater than the thickness of the second permanent magnet arrangement 28 along an axis 52 perpendicular to the +q-axis 66 and greater than the thickness of the third permanent magnet arrangement 40 along an axis 54 perpendicular to the -q-axis 68.

In other embodiments, the thickness of the first permanent magnet arrangement 16, the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, may be the same. The thickness of the first permanent magnet arrangement 16, the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, may vary dependent on the size of the rotor 10.

For a traction motor 300 for a vehicle 400, the thickness of the first permanent magnet arrangement 16 may be, for example, between 2.5 mm and 5 mm.

Figure 3 illustrates a magnified section of the pole 12 of Figure 2. In Figure 2, the first permanent magnet arrangement 16 is located in a first magnet slot 70. The first magnet slot 70 comprises a first portion 72 with substantially the same shape and size as the first permanent magnet arrangement 16 such that the magnet or magnets of the first permanent magnet arrangement 16 are arranged or configured to fit into the first magnet slot 70 without a gap, or with a minimal gap, between the magnet or magnets and the first portion 72 of the first magnet slot 70. The first magnet slot 70 also comprises a second portion 74 of the first magnet slot 70, at a first widthwise end 76 of the first portion 72 of the first magnet slot 70, extending towards the +q-axis 66.

The second portion 74 of the first magnet slot 70 provides a cavity, which may be in the form of a first air void 78 of the first magnet slot 70.

The first magnet slot 70 also comprises a third portion 80 of the first magnet slot 70, at a second widthwise end 82 of the first portion 72 of the first magnet slot 70, extending towards the -q-axis 68.

The third portion 80 of the first magnet slot 70 provides a cavity, which may be in the form of a second air void 84 of the first magnet slot 70.

The inclusion of air voids 78, 84 can provide a reduction in torque ripple over a similar arrangement without air voids 78, 84. The air voids 78, 84 further help to avoid magnet demagnetisation and can assist in reducing the occurrence of high stress concentrations in the areas around the ends of the first permanent magnet arrangement 16.

The cavities, or air voids 78, 84, provided as part of the first magnet slot 70, may comprise flat ends with a tail element 75, 85. The flat ends each may lie on a radial axis, each radial axis extending from the rotational axis 14 of the rotor 10 towards the outer perimeter or circumference 64 of the rotor 10.

The tail elements 75, 85 allow for better distribution of material stresses and therefore help to avoid stress concentrations in the rotor 10. These features may be of significant benefit to the first magnet slot 70, as the first permanent magnet arrangement 16 is larger, and therefore has greater mass, than the second permanent magnet arrangement 28, and the third permanent magnet arrangement 40, and is also further away from the rotational axis 14 of the rotor 10 than those other permanent magnet arrangements 28, 40, such that the first permanent magnet arrangement 16 has a higher moment of inertia than the other permanent magnet arrangements 28, 40 causing greater stress in the rotor 10.

The second permanent magnet arrangement 28 is located in a second magnet slot 86. The second magnet slot 86 comprises a first portion 88 with substantially the same shape and size as the second permanent magnet arrangement 28 such that the magnet or magnets of the second permanent magnet arrangement 28 are arranged or configured to fit into the second magnet slot 86 without a gap, or with a minimal gap, between the magnet or magnets and the first portion 88 of the second magnet slot 86.

The second magnet slot 86 also comprises a second portion 90 of the second magnet slot 86, at a first widthwise end 92 of the first portion 88 of the second magnet slot 86, extending outwards along the +q-axis 66 towards the outer perimeter or circumference 64 of the rotor 10.

The second portion 90 of the second magnet slot 86 provides a cavity, which may be in the form of a first air void 94, of the second magnet slot 86.

The inclusion of an air void 94 can provide a reduction in torque ripple over a similar arrangement without an air void 94. The air void 94 further helps to avoid magnet demagnetisation and can assist in reducing the occurrence of high stress concentrations in the areas around the end of the second permanent magnet arrangement 28.

The cavity, or air void, 94, provided as part of the second magnet slot 86, may comprise a narrow portion 96 adjacent the first widthwise end 92 of the first portion 88 and a bulbous portion 98 distal from the widthwise end 92 of the first portion 88. The bulbous portion 98 may extend further from the +q-axis 66 than the second permanent magnet arrangement 28. The bulbous portion 98 may be a similar radial distance from the rotational axis 14 of the rotor 10 as the cavity, or air void 78, provided as part of the first magnet slot 70, thereby providing a flux constriction between the cavity, or air void, 94 provided as part of the second magnet slot 86, and the cavity, or air void, 78, provided as part of the first magnet slot 70, thereby optimizing the rotor 10 to minimise torque ripple.

The third permanent magnet arrangement 40 is located in a third magnet slot 118. The third magnet slot 118 comprises a first portion 120 with substantially the same shape and size as the third permanent magnet arrangement 40 such that the magnet or magnets of the third permanent magnet arrangement 40 are arranged or configured to fit into the third magnet slot 118 without a gap, or with a minimal gap, between the magnet or magnets and the first portion 120 of the third magnet slot 118.

The third magnet slot 118 also comprises a second portion 122 of the third magnet slot 118, at a first widthwise end 124 of the first portion 120 of the third magnet slot 118, extending outwards along the -q-axis 68 towards the outer perimeter or circumference 64 of the rotor 10. The second portion 122 of the third magnet slot 118 provides a cavity, which may be in the form of a first air void 126, of the third magnet slot 118.

The inclusion of an air void 126 can provide a reduction in torque ripple over a similar arrangement without an air void 126. The air void 126 further helps to avoid magnet demagnetisation and can assist in reducing the occurrence of high stress concentrations in the areas around the end of the third permanent magnet arrangement 40.

The cavity, or air void, 126, provided as part of the third magnet slot 118, may comprise a narrow portion 128 adjacent the first widthwise end 124 of the first portion 120 and a bulbous portion 130 distal from the widthwise end 124 of the first portion 120. The bulbous portion 130 may extend further from the -q-axis 68 than the third permanent magnet arrangement 40. The bulbous portion 130 may be a similar radial distance from the rotational axis 14 of the rotor 10 as the cavity, or air void 84, provided as part of the first magnet slot 70, thereby providing a flux constriction between the cavity, or air void, 126 provided as part of the third magnet slot 118, and the cavity, or air void, 84, provided as part of the first magnet slot 70, thereby optimizing the rotor 10 to minimise torque ripple.

The second magnet slot 86 and third magnet slot 118 are mirror images of each other, reflected through the d-axis 18. The first cavity, or air void, 94 of the second magnet slot 86 is a mirror image of the first cavity, or air void, 126 of the third magnet slot 118. These air voids 94, 126 are larger than those of the first magnet slot 70, which assists in reducing self flux linkage. Since these air voids 94, 126 extend with a significant component in the radial direction, their effect on the structural performance of the rotor 10 is less than the effect on the structural performance of the rotor 10 exerted by the air voids 78, 84 of the first permanent magnet arrangement 16.

Furthermore, the first widthwise end 92 of the first portion 88 of the second magnet slot 86 and the first widthwise end 124 of the first portion 120 of the second magnet slot 118 are radially inset from the first permanent magnet arrangement 16 such that there is no overlap, in a radial direction, of the first permanent magnet arrangement 16 with either of the second permanent magnet arrangement 40 and the third permanent magnet arrangement 40.

The cavities, or air voids, 78, 84, 94, 126 at the end of the respective magnet slots 70, 86, 118 comprise a through hole extending through, or substantially through the rotor 10. The cavities 78, 84, 94, 126 may extend parallel or substantially parallel to a longitudinal axis of the rotor 10, that is parallel to the rotational axis 14 of the rotor. The cavities 78, 84, 94, 126 act as flux barriers to channel flux in an advantageous direction, to increase torque and efficiency of the electric machine 300, as illustrated by the flux lines shown in Figure 4. In some embodiments the cavities 78, 84, 94, 126 may be filled with air to form the air voids. In other embodiments, the cavities 78, 84, 94, 126 may be filled with a non-conducting material, and/or another low magnetic permeability material.

The magnets in the rotor 10 may be chosen from any permanent magnet materials depending on the application of the electric machine that the rotor 10 is intended to be a part of. In particular, many electric machines 300 may use low cost magnetic materials, such as hard ferrites. Other electric machines may use aluminium nickel cobalt (AINiCo) or samarium cobalt

(SmCo) as the magnetic material.

In some embodiments one or more of the first permanent magnet arrangement 16, second permanent magnet arrangement 28, and third permanent magnet arrangement 40, each may comprise one or more neodymium iron boron (NdFeB) magnets.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.