GB2620419A

GB2620419A - Electric machine rotor

Info

Publication number: GB2620419A
Application number: GB2209972.5A
Authority: GB
Inventors: Devara Prasad; Gupta Vinayak; Kaurav Deepanshu; Pitambar Mahajan Deepak
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2022-07-07
Filing date: 2022-07-07
Publication date: 2024-01-10
Also published as: WO2024008887A1; GB202209972D0

Abstract

A rotor 10 for an electric machine comprising: a rotational axis 14; poles 12, each pole comprising a first magnet 16 disposed symmetrically about a d-axis 18 of the pole in a first layer 20, the first magnet having a centre 22 at a first radial distance 23 from the rotational axis. The first magnet has a width 24 along a first axis 26 orthogonal to the d-axis and orthogonal to the rotational axis. A second magnet is disposed symmetrically about the d-axis in a second layer 30 having a centre 32 at a second radial distance 34 from the rotational axis, the second distance being radially inset from the first distance 23. The second magnet has a width 36 along a second axis 38 orthogonal to the d-axis and orthogonal to the rotational axis, where the width of the first magnet arrangement is greater than the width of the second magnet arrangement. A third magnet 40 is provided having a centre 42 at a third radial distance 44-1 from the rotational axis this distance being between the first and second radial distances, the third magnet extending at an acute angle 50 to the d-axis 18. A fourth magnet arrangement 52 extends at an acute angle 60, where the fourth magnet is symmetrical about the d-axis with the third magnet.

Description

ELECTRIC MACHINE ROTOR

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 as part of a permanent magnet synchronous motor. An air gap is maintained between the rotor and 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 the 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 (PHEV) 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 the d-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 a second axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor, wherein the width of the first permanent magnet arrangement is greater than the width of the second permanent magnet arrangement; a third permanent magnet arrangement, the third permanent magnet arrangement having a centre at a third radial distance from the rotational axis of the rotor, the third radial distance being between the first radial distance and the second radial distance, the third permanent magnet arrangement having a width along a third axis extending at an acute angle to the d-axis and orthogonal to the rotational axis of the rotor; and a fourth permanent magnet arrangement, the fourth permanent magnet arrangement having a centre at the third radial distance from the rotational axis of the rotor, the fourth permanent magnet arrangement having a width along a fourth axis extending at an acute angle to the d-axis and orthogonal to the rotational axis of the rotor, wherein the fourth permanent magnet arrangement is symmetrical, about the d-axis, with the third permanent magnet arrangement.

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 thatthe arrangement may be effective in controlling flux density in the rotor.

The first permanent magnet arrangement may have a span of between ninety six and one hundred and thirty six electrical degrees. The first permanent magnet arrangement may have a span of one hundred and sixteen electrical degrees. This provides the advantage of an optimal torque to magnet weight ratio.

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

At least one of the first permanent magnet arrangement, the second permanent magnet arrangement, the third permanent magnet arrangement, and the fourth 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, the third permanent magnet arrangement, and the fourth permanent magnet arrangement, may comprise a single magnet slot housing a plurality of magnets. This provides the advantage of reducing eddy current losses in the rotor.

At least one of the first permanent magnet arrangement, the second permanent magnet arrangement, the third permanent magnet arrangement, and the fourth 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.

At least one of the first permanent magnet arrangement, the second permanent magnet arrangement, the third permanent magnet arrangement, and the fourth permanent magnet arrangement, may comprise a plurality of magnet slots, each magnet slot housing a plurality of magnets, separated from an adjacent slot by a bridge. This provides the advantage of reducing eddy current losses in the rotor.

The first permanent magnet arrangement and the second permanent magnet arrangement may have the same thickness along the d-axis.

At least a portion of the third permanent magnet arrangement and at least a portion of the fourth permanent magnet arrangement may be separated by a distance, along an axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor, less than the width of the first permanent magnet arrangement. This provides the advantage of focussing the magnetic flux at the d-axis.

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

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

The third portion of the third magnet slot may not overlap the second permanent magnet arrangement in an axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor.

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

The third portion of the fourth magnet slot may not overlap the second permanent magnet arrangement in an axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor.

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

According to an aspect of the present invention there is provided 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. Non-limiting 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. 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.

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 23 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 symmetrically about the d-axis 18 of the pole 12 in a second layer 30, the second permanent magnet arrangement 28 having a centre 32 at a second radial distance 34 from the rotational axis 14 of the rotor 10.

The second radial distance 34 is radially inset from the first radial distance 23. That is, the second radial distance 34 is smaller than the first radial distance 23, such that the second layer 30 is closer to the rotational axis 14 of the rotor 10 than the first layer 20.

The second permanent magnet arrangement 26 has a width 36 along a second axis 38 orthogonal to the d-axis 18 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 15 second permanent magnet arrangement 28.

Each pole 12 also comprises a third permanent magnet arrangement 40, the third permanent magnet arrangement 40 having a centre 42 at a third radial distance 44-1 from the rotational axis 14 of the rotor 10.

The third radial distance 44-1 is a value between the first radial distance 23 and the second radial distance 34. That is, the third radial distance 44-1 is smaller than the first radial distance 23, such that the centre 42 of the third permanent magnet arrangement 40 is 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 third radial distance 44-1 is larger than the second radial distance 34, such that the centre 42 of the third permanent magnet arrangement 40 is further away from the rotational axis 14 of the rotor 10 than the second layer 30, and in particular further away from the rotational axis 14 of the rotor 10 than the centre 32 of the second permanent magnet arrangement 28.

The third permanent magnet arrangement 40 has a width 46 along a third axis 48 extending at an acute angle 50 to the d-axis 18 and orthogonal to the rotational axis 14 of the rotor 10. The width 46 of the third permanent magnet arrangement 40 may be less than the width 24 of the first permanent magnet arrangement 16. The width 46 of the third permanent magnet arrangement 40 may be less than the width 36 of the second permanent magnet arrangement 28.

The acute angle 50 is the angle between the third axis 48 and the d-axis 18 of the pole 12, where the third axis 48 extends from the d-axis 18 of the pole 12 towards an outer perimeter 62 of the pole 12 of the rotor 10 Each pole 12 also comprises a fourth permanent magnet arrangement 52, the fourth permanent magnet arrangement 52 having a centre 54 at the third radial distance 44-2 from the rotational axis 14 of the rotor 10. The third radial distance 44-2 for the fourth permanent magnet arrangement 52 is the same distance as the third radial distance 44-1 for the third permanent magnet arrangement 40. Therefore, the centre 54 of the fourth permanent magnet arrangement 52 is at the same distance from the rotational axis 14 of the rotor 10 as the centre 42 of the third permanent magnet arrangement 40.

The fourth permanent magnet arrangement 52 has a width 56 along a fourth axis 58 extending at an acute angle 60 to the d-axis 18 and orthogonal to the rotational axis 14 of the rotor 10.

The width 56 of the fourth permanent magnet arrangement 52 may be less than the width 24 of the first permanent magnet arrangement 16. The width 56 of the fourth permanent magnet arrangement 52 may be less than the width 36 of the second permanent magnet arrangement 28.

The acute angle 60 is the angle between the fourth axis 58 and the d-axis 18 of the pole 12, where the fourth axis 58 extends from the d-axis 18 of the pole 12 towards an outer perimeter 62 of the pole 12 of the rotor 10 on the opposite side of the d-axis 18 to the third axis 48.

The width 56 of the fourth permanent magnet arrangement 52 is the same as the width 46 of the third permanent magnet arrangement 40. Moreover, the fourth permanent magnet arrangement 52 is symmetrical, about the d-axis 18, with the third permanent magnet arrangement 40.

It may be considered that the third permanent magnet arrangement 40 and the fourth permanent magnet arrangement 52 comprise a third layer 64 of the rotor 10. Alternatively the third permanent magnet arrangement 40 and the fourth permanent magnet arrangement 52 may be considered to be part of the second layer 30, where the second layer is a U-profile magnet arrangement.

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

The second permanent magnet arrangement 28 may have a span of between sixty four and one hundred and four electrical degrees, equating, in an eight pole machine to a span of between sixteen and twenty six mechanical degrees. In the embodiment of Figure 2, the second permanent magnet arrangement 28 has a span of eighty four electrical degrees, equating to a span of twenty one mechanical degrees.

The angle 50 for the third axis 48, along which the third permanent magnet arrangement 40 is positioned, is such that the third permanent magnet arrangement 40 is physically separated from the first permanent magnet arrangement 16 and the second permanent magnet arrangement 28 but radially overlaps both the first permanent magnet arrangement 16 and the second permanent magnet arrangement 28 in order to focus the magnetic flux at the d-axis 18. That is, a portion of the third permanent magnet arrangement 40 is at a radial distance from the rotational axis 14 of the rotor 10 which is less than at least a portion of the second permanent magnet arrangement 28, and a portion of the third permanent magnet arrangement 40 is at a radial distance from the rotational axis 14 of the rotor 10 which is greater than at least a portion of the first permanent magnet arrangement 16. The angle 50 may be between thirty degrees and fifty degrees. For example, the angle 50 may be thirty nine degrees.

In the embodiment of Figure 2, a portion of the third permanent magnet arrangement 40 is inboard, that is closer to the rotational axis 14 of the rotor 10, than any part of the second permanent magnet arrangement 28.

The angle 60 for the fourth axis 58, along which the fourth permanent magnet arrangement 52 is positioned, is such that the fourth permanent magnet arrangement 52 is physically separated from the first permanent magnet arrangement 16 and the second permanent magnet arrangement 28 but radially overlaps both the first permanent magnet arrangement 16 and the second permanent magnet arrangement 28 in order to focus the magnetic flux at the d-axis 18. That is, a portion of the fourth permanent magnet arrangement 52 is at a radial distance from the rotational axis 14 of the rotor 10 which is less than at least a portion of the second permanent magnet arrangement 28, and a portion of the fourth permanent magnet arrangement 52 is at a radial distance from the rotational axis 14 of the rotor 10 which is greater than at least a portion of the first permanent magnet arrangement 16. The angle 60 may be between thirty degrees and fifty degrees. For example, the angle 60 may be thirty nine degrees.

In the embodiment of Figure 2, a portion of the fourth permanent magnet arrangement 52 is inboard, that is closer to the rotational axis 14 of the rotor 10, than any part of the second permanent magnet arrangement 28. This arrangement, coupled with the arrangement of the third permanent magnet arrangement 40 noted above, may lead to a sinusoidal flux to be created in the air gap 214 between the rotor 10 and stator 212 of the electric machine 300.

In order to provide further focussing of the magnetic flux, at least a portion of the third permanent magnet arrangement 40 and at least a portion of the fourth permanent magnet arrangement 52 are separated by a distance, along an axis orthogonal to the d-axis 18 and orthogonal to the rotational axis 14 of the rotor 10, which is less than the width 24 of the first permanent magnet arrangement 16. That is, the distance between the closest points of the third permanent magnet arrangement 40 and the fourth permanent magnet arrangement 52 is less than the width 24 of the first permanent magnet arrangement 40.

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 angular ranges and with the abovementioned separations, a high torque to magnet weight ratio is observed, whilst 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, 52 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, the third permanent magnet arrangement 40, and the fourth permanent magnet arrangement 52, may comprise a single magnet slot housing one or more magnet. Therefore one or more of the permanent magnet arrangements 16, 28, 40, 52 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, the third permanent magnet arrangement 40, and the fourth permanent magnet arrangement 52, may comprise a single magnet slot housing a plurality of magnets. Therefore, one or more of the permanent magnet arrangements 16, 28, 40, 52 may comprise a single slot housing two or more magnets in a segmented magnet array.

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, the third permanent magnet arrangement 40, and the fourth permanent magnet arrangement 52, 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, the third permanent magnet arrangement 40, and the fourth permanent magnet arrangement 52, 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 and the second permanent magnet arrangement 20 have the same thickness along the d-axis 18. In other embodiments the thickness of the first permanent magnet arrangement 16 and the second permanent magnet arrangement 20 may be different to optimise torque generation.

The thickness of the first permanent magnet arrangement 16 and the second permanent magnet arrangement 20 may vary dependent on the size of the rotor. For a traction motor for a vehicle 400, the thickness of the first permanent magnet arrangement 16 and the second permanent magnet arrangement 20 may be, for example, between 2.5 mm and 5 mm. Further, the third permanent magnet arrangement 40 and the fourth permanent magnet arrangement 52 may also have the same thickness as the first permanent magnet arrangement 16and the second permanent magnet arrangement 28.

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 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 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 third permanent magnet arrangement 40.

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 fourth permanent magnet arrangement 52.

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 round ends with a tail or curl element 75, 85. The tail or curl elements 75, 85 allow for better distribution of material stresses and therefore help to avoid stress concentrations in the rotor 10. The tail or curl elements 75, 85 of the cavities, or air voids 78, 84, provided as part of the first magnet slot 70, may overlap the first portion 72 of the first magnet slot 70 along the width 24 of the first permanent magnet arrangement 16 along the first axis 26. 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, the third permanent magnet arrangement 40, and the fourth permanent magnet arrangement 52, and is also further away from the rotational axis 14 of the rotor 10 than those other permanent magnet arrangements 28, 40, 52, such that the first permanent magnet arrangement 16 has a higher moment of inertia than the other permanent magnet arrangements 28, 40, 52 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 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 towards the third permanent magnet arrangement 40.

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 second magnet slot 86 also comprises a third portion 96 of the second magnet slot 86, at a second widthwise end 98 of the first portion 88 of the second magnet slot 86, extending towards the fourth permanent magnet arrangement 52.

The third portion 96 of the second magnet slot 86 provides a cavity, which may be in the form of a second air void 100 of the second magnet slot 86.

The inclusion of air voids 94, 100 can provide a reduction in torque ripple over a similar arrangement without air voids 94, 100. The air voids 94, 100 further help to avoid magnet demagnetisation and can assist in reducing the occurrence of high stress concentrations in 25 the areas around the ends of the second permanent magnet arrangement 28.

The cavities, or air voids 94, 100, provided as part of the second magnet slot 86, may comprise round ends with a tail or curl element 93, 99. The tail or curl elements 93, 99 allow for better distribution of material stresses and therefore help to avoid stress concentrations in the rotor 10. Since stresses at the second permanent magnet arrangement 28 are not as great as those at the first permanent magnet arrangement 16, the cavities, or air voids 94, 100, can be smaller at the second permanent magnet arrangement 28 compared to the first permanent magnet arrangement 16, and can be positioned closer to the third permanent magnet arrangement 40 and the fourth permanent magnet arrangement 52, without detrimentally affecting the structural integrity of the rotor 10. Further, the tail or curl elements 93, 99 of the cavities, or air voids 94, 100, provided as part of the second magnet slot 86, may be less curved than those of the air voids 78, 84, provided as part of the first magnet slot 70, such that they do not overlap the first portion 88 of the second magnet slot 86 along the width 36 of the second permanent magnet arrangement 26 along the second axis 38.

The third permanent magnet arrangement 40 is located in a third magnet slot 102. The third magnet slot comprises a first portion 104 of the third magnet slot 102 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 102 without a gap between the magnet or magnets and the first portion 104 of the third magnet slot 102.

The third magnet slot 102 also comprises a second portion 106 of the third magnet slot 102, at a first widthwise end 108 of the first portion 104 of the third magnet slot 102, extending towards the circumference or outer perimeter 62 of the rotor 10.

The second portion 106 of the third magnet slot 102 provides a cavity, which may be in the form of a first air void 110 of the third magnet slot 102.

The third magnet slot 102 also comprises a third portion 112 of the third magnet slot 102, at a second widthwise end 114 of the first portion 104 of the third magnet slot 102, extending towards the d-axis 18 and, at least in part, towards the centre or rotational axis 14 of the rotor 10.

The third portion 112 of the third magnet slot 102 provides a cavity, which may be in the form of a second air void 116 of the third magnet slot 102.

The second portion 106 of the third magnet slot 102 and the third portion 112 of the third magnet slot 102 have a thickness which is substantially equal to the thickness of the first portion 104 of the third magnet slot 102, that is, a thickness which is substantially equal to the thickness of the magnet or magnets of the third permanent magnet arrangement 40.

The inclusion of air voids 110, 116 can provide a reduction in torque ripple over a similar arrangement without air voids 110, 116. The air voids 110, 116 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 third permanent magnet arrangement 40.

In some embodiments, the third portion 116 of the third magnet slot does not overlap the second permanent magnet arrangement 28 in an axis orthogonal to the d-axis 18 and orthogonal to the rotational axis 14 of the rotor 10.

The fourth permanent magnet arrangement 52 is located in a fourth magnet slot 118. The fourth magnet slot 118 comprises a first portion 120 of the fourth magnet slot 118 with substantially the same shape and size as the fourth permanent magnet arrangement 52 such that the magnet or magnets of the fourth permanent magnet arrangement 52 are arranged or configured to fit into the fourth magnet slot 118 without a gap between the magnet or magnets and the first portion 120 of the fourth magnet slot 118.

The fourth magnet slot 118 also comprises a second portion 122 of the fourth magnet slot 118, at a first widthwise end 124 of the first portion 120 of the fourth magnet slot 118, extending towards the circumference or outer perimeter 62 of the rotor 10.

The second portion 122 of the fourth magnet slot 118 provides a cavity, which may be in the form of a first air void 126 of the fourth magnet slot 118.

The fourth magnet slot 118 also comprises a third portion 128 of the fourth magnet slot 118, at a second widthwise end 130 of the first portion 120 of the fourth magnet slot 118, extending towards the d-axis 18 and, at least in part, towards the centre or rotational axis 14 of the rotor 10 The third portion 128 of the fourth magnet slot 118 provides a cavity, which may be in the form of a second air void 132 of the fourth magnet slot 118.

The second portion 122 of the fourth magnet slot 118 and the third portion 128 of the fourth magnet slot 118 have a thickness which is substantially equal to the thickness of the first portion 120 of the fourth magnet slot 118, that is, a thickness which is substantially equal to the thickness of the magnet or magnets of the fourth permanent magnet arrangement 52.

The inclusion of air voids 126, 132 can provide a reduction in torque ripple over a similar arrangement without air voids 126, 132. The air voids 126, 132 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 fourth permanent magnet arrangement 52.

In some embodiments, the third portion 128 of the fourth magnet slot 118 does not overlap the second permanent magnet arrangement 28 in an axis orthogonal to the d-axis 18 and orthogonal to the rotational axis 14 of the rotor 10.

The third magnet slot 102 and fourth magnet slot 118 are mirror images of each other, reflected through the d-axis 18. The first cavity, or air void 110 of the third magnet slot 102 is a mirror image of the first cavity, or air void 126 of the fourth magnet slot 118 and the second cavity, or air void 116 of the third magnet slot 102 is a mirror image of the second cavity, or air void 132 of the fourth magnet slot 128. These air voids 110, 116, 126, 132 are larger than those of the first magnet slot 70 and second magnet slot 86 which assists in reducing self flux linkage.

Further, each of the second air voids 116 and 132 extend along respective axes 48, 58 towards the d-axis 18 and generally in the direction of the rotational axis 14 of the rotor, closer to the rotational axis 14 of the rotor than any part of the second magnet slot 86. By extending in this way the self flux linkage can be substantially further reduced. Since these air voids 116, 132, 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 of the first permanent magnet arrangement 16 and the second permanent magnet arrangement 28.

The cavities, or air voids, at the end of the magnet slots 70, 86, 102, 118 comprise a through hole extending through, or substantially through the rotor 10. The cavities 70, 86, 102, 118, 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 70, 86, 102, 118 act as flux barriers to channel flux in an advantageous direction, to increase torque and efficiency, as illustrated by the flux lines shown in Figure 4. In some embodiments the cavities may be filled with air to form the air voids. In other embodiments, the cavities 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 the first permanent magnet arrangement 16, second permanent magnet arrangement 28, third permanent magnet arrangement 40, and fourth permanent magnet arrangement 52, 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.

Claims

CLAIMS1. 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 the d-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 a second axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor, wherein the width of the first permanent magnet arrangement is greater than the width of the second permanent magnet arrangement; a third permanent magnet arrangement, the third permanent magnet arrangement having a centre at a third radial distance from the rotational axis of the rotor, the third radial distance being between the first radial distance and the second radial distance, the third permanent magnet arrangement having a width along a third axis extending at an acute angle to the d-axis and orthogonal to the rotational axis of the rotor; and a fourth permanent magnet arrangement, the fourth permanent magnet arrangement having a centre at the third radial distance from the rotational axis of the rotor, the fourth permanent magnet arrangement having a width along a fourth axis extending at an acute angle to the d-axis and orthogonal to the rotational axis of the rotor, wherein the fourth permanent magnet arrangement is symmetrical, about the d-axis, with the third permanent magnet arrangement.
2. A rotor according to claim 1, wherein the first permanent magnet arrangement has a span of between ninety six and one hundred and thirty six electrical degrees.
3. A rotor according to any preceding claim, wherein the second permanent magnet arrangement has a span of between sixty four and one hundred and four electrical degrees.
4. A rotor according to any preceding claim, wherein the first permanent magnet arrangement and the second permanent magnet arrangement have the same thickness along the d-axis.
5. A rotor according to any preceding claim, wherein at least a portion of the third permanent magnet arrangement and at least a portion of the fourth permanent magnet arrangement are separated by a distance, along an axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor, less than the width of the first permanent magnet arrangement.
6. A rotor according to any preceding claim, wherein the first permanent magnet arrangement is located in a first magnet slot, the first magnet slot comprising: 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 third permanent magnet arrangement, 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 fourth permanent magnet arrangement, the third portion of the first magnet slot providing a second air void of the first magnet slot.
7. A rotor according to any preceding claim, wherein the second permanent magnet arrangement is located in a second magnet slot, the second magnet slot comprising: a first portion of the second magnet slot with substantially the same shape and size as the second permanent magnet arrangement; a second portion of the second magnet slot, at a first widthwise end of the first portion of the second magnet slot, extending towards the third permanent magnet arrangement, the second portion of the second magnet slot providing a first air void of the second magnet slot; and a third portion of the second magnet slot, at a second widthwise end of the first portion of the second magnet slot, extending towards the fourth permanent magnet arrangement, the third portion of the second magnet slot providing a second air void of the second magnet slot.
8. A rotor according to any preceding claim, wherein the third permanent magnet arrangement is located in a third magnet slot, the third magnet slot comprising: a first portion of the third magnet slot with substantially the same shape and size as the third permanent magnet arrangement; a second portion of the third magnet slot, at a first widthwise end of the first portion of the third magnet slot, extending towards the circumference of the rotor, the second portion of the third magnet slot providing a first air void of the third magnet slot; and a third portion of the third magnet slot, at a second widthwise end of the first portion of the third magnet slot, extending towards the centre of the rotor, the third portion of the third magnet slot providing a second air void of the third magnet slot.
9. A rotor according to claim 8, wherein the third portion of the third magnet slot does not overlap the second permanent magnet arrangement in an axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor.
10. A rotor according to any preceding claim, wherein the fourth permanent magnet arrangement is located in a fourth magnet slot, the fourth magnet slot comprising: a first portion of the fourth magnet slot with substantially the same shape and size as the fourth permanent magnet arrangement; a second portion of the fourth magnet slot, at a first widthwise end of the first portion of the fourth magnet slot, extending towards the circumference of the rotor, the second portion of the fourth magnet slot providing a first air void of the fourth magnet slot; and a third portion of the fourth magnet slot, at a second widthwise end of the first portion of the fourth magnet slot, extending towards the centre of the rotor, the third portion of the fourth magnet slot providing a second air void of the fourth magnet slot.
11. A rotor according to claim 10, wherein the third portion of the fourth magnet slot does not overlap the second permanent magnet arrangement in an axis orthogonal to the d-axis and orthogonal to the rotational axis of the rotor.
12. A rotor according to any preceding claim, wherein one or more of the first permanent magnet arrangement, the second permanent magnet arrangement, the third permanent magnet arrangement, and the fourth permanent magnet arrangement comprise a plurality of magnets.
13. A rotor according to any preceding claim, wherein the first permanent magnet arrangement, second permanent magnet arrangement, third permanent magnet arrangement, and fourth permanent magnet arrangement, comprise NdFeB magnets.
14. An electric machine comprising a rotor according to any preceding claim and a stator.
15. A vehicle comprising an electric machine according to claim 14.