WO2021116649A1

WO2021116649A1 - An electric motor

Info

Publication number: WO2021116649A1
Application number: PCT/GB2020/052562
Authority: WO
Inventors: Tuncay Celik; Yu Chen; Daniel Smith; Abdelhadi BESRI
Original assignee: Dyson Technology Limited
Priority date: 2019-12-13
Filing date: 2020-10-14
Publication date: 2021-06-17
Also published as: GB201918381D0; GB2590384B; GB2590384A; CN114830493A

Abstract

An electric motor (112) includes a housing (205), a stator assembly (198) disposed within the housing (205), and a rotor assembly (116,119) mounted within the housing (205) for rotation within the stator assembly (198). The stator assembly (198) comprises a stator core (199) comprising a first steel having a silicon content exceeding 3.5% and the rotor assembly (116,119) comprises a rotor core (116) comprising a second steel having a silicon content of less than or equal to 3.5%.

Description

AN ELECTRIC MOTOR

Field of the Invention

The present invention relates to an electric motor.

Background of the Invention

Interior permanent magnet electric motors have a rotor assembly comprising a rotor core having a set of circumferentially spaced magnets that generate magnetic poles around a circumference of the rotor core. Stator windings are supplied with current to generate electromagnetic poles on stator teeth, which interact with the magnetic poles of the rotor core in order to cause rotation of the rotor core. Such motors and associated circuitry are often reversible, to allow their use in regenerative braking.

It would be desirable to improve performance (such as power and/or efficiency) of such motors, and/or to reduce the cost of their manufacture.

Summary of the Invention

According to an aspect of the invention, there is provided an electric motor comprising: a housing; a stator assembly disposed within the housing; and a rotor assembly mounted within the housing for rotation within the stator assembly; wherein the stator assembly comprises a stator core comprising a first steel having a silicon content exceeding 3.5% and the rotor assembly comprises a rotor core comprising a second steel having a silicon content of less than or equal to 3.5%. This combination of steels having different silicon contents may allow for improved switching performance, particularly at high switching frequencies, while reducing production costs.

In particular, by utilising a stator core with a relatively high silicon content, more efficient switching may be achieved. However, such relatively high silicon content steel may be more costly than relatively low silicon content steel. It has surprisingly been found that utilising relatively high silicon content steel in both the stator core and the rotor core of an electric motor may offer little efficiency gain in comparison with an electric motor where relatively high silicon content steel is only used in the stator core. Thus the inventors of the present application have found that an electric motor having a stator core comprising a first steel having a silicon content exceeding 3.5% and a rotor core comprising a second steel having a silicon content of less than or equal to 3.5% may offer improved efficiencies without prohibitively increasing cost.

The first steel may have a silicon content exceeding 4%, or exceeding 6%. In particular embodiments, the first steel may have a silicon content of 6.5%, which may provide good switching performance, particularly at high switching frequencies.

The stator core may comprise a plurality of circumferentially spaced stator teeth, wherein at least the teeth are formed from the first steel. This may provide good switching performance, particularly at high switching frequencies.

The stator core may comprise a plurality of circumferentially adjacent segments, each segment comprising one or more of the teeth. The use of multiple segments may allow for more efficient use of material during production.

Each segment may have circumferentially spaced apart end portions, each end portion defining a key that interlocks with a corresponding key defined by an end portion of an adjacent segment. Such an arrangement may assist in assembly of the stator core, and/or may improve structural strength and/or rigidity. Each segment may have exactly one, two, three or ten of the teeth. Depending upon the implementation, these numbers of teeth per segment may provide a good compromise between performance and production cost.

The electric motor may comprise exactly six of the segments.

The electric motor may comprise at least one yoke, the yoke supporting one or more of the segments. The or each yoke may form part of one or more of the segments. In some embodiments, the yoke may encircle the segments.

Each segment may comprise a segment feature that interlocks with a corresponding yoke feature at a radially inner region of the yoke. The use of such segment features may assist in assembly of the stator core, and/or may improve structural strength and/or rigidity.

Each segment feature and its corresponding yoke feature may form a dovetail joint. For example, each segment feature may take the form of a dovetail and each yoke feature may take the form of a recess having a shape complementary to the dovetail. The use of such dovetail joints may improve structural strength and/or rigidity.

The yoke may comprise a third steel. For example, the third steel may be the same as the second steel. Alternatively, the third steel may be different to the first steel and the second steel. The particular steels chosen may be optimised for the specific electrical, magnetic and/or mechanical requirements of each component.

Each segment may comprise a stack of laminations of the steel. Each lamination may be between 0.05 and 0.25mm thick, for example. The use of laminations may reduce eddy currents in use.

The electric motor may be an interior permanent magnet motor. According to another aspect, there is provided an electric vehicle comprising the electric motor of the preceding aspect.

Brief Description of the Drawings

In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a top schematic view of a vehicle comprising two electric drive units

(EDUs);

Figure 2 is a perspective view of an electric motor forming part of the EDU of Figure 1;

Figure 3 is a plan view of a rotor of the electric motor of Figure 2;

Figure 4 is a simplified schematic perspective view of the rotor of Figure 3, with features omitted for clarity;

Figure 5 is a plan view of the rotor of Figures 3 and 4, along with a stator core;

Figure 6 is a plan view of a stator core comprising a plurality of segments;

Figure 7 is a plan view of part of a stator core and a rotor;

Figure 8 is a plan view of part of an alternative stator core and rotor;

Figure 9 is a plan view of part of another alternative stator core and rotor;

Figure 10 is a plan view of part of yet another alternative stator core and rotor;

Figure 11 is a plan view of part of yet another alternative stator core and rotor;

Figure 12 is a plan view of part of yet another alternative stator core and rotor; and Figure 13 is a plan view of part of yet another alternative stator core and rotor.

Detailed Description of the Invention

Unless the specific context suggests otherwise, the word “axial” used in this description refers to the axis about which the rotor is intended to rotate when installed in an electric motor.

Referring to the drawings, Figure 1 shows a vehicle in the form of a car 100. The car 100 includes a front electric drive unit (EDU) 102 and a rear EDU 104. The front EDU 102 drives a pair of front wheels 106 and the rear EDU 104 drives a pair of rear wheels 108. In other embodiments, only a single EDU is employed, driving any desired number of wheels. In yet other embodiments, more than two EDUs may be employed, each driving any desired number of wheels.

The front EDU 102 includes a gearbox 110. An electric motor 112 is mounted to the gearbox 110 and configured to provide drive to an input shaft (not shown) of the gearbox 110. A drive electronics unit in the form of an inverter 114 is mounted to the gearbox 110 and configured to provide drive current to the electric motor 112. In the embodiment shown, the electric motor 112 is mounted to a first lateral side of the gearbox 110, and the inverter 114 is mounted to a second lateral side of the gearbox 110 opposite the first lateral side.

The rear EDU 104 has a similar combination of components, including a gearbox 111, electric motor 113 and inverter 115.

Figure 2 shows the electric motor 112 of front EDU 102 (the electric motor 113 from the rear EDU 104 is identical, and will not be described separately). The electric motor 112 is an interior permanent magnet (IPM) motor, which comprises an IPM rotor core 116 that has permanent magnets embedded within stacks of steel laminations, as described below. The rotor core 116 is mounted to an output shaft 119 that mates with an input shaft (not shown) of the gearbox 110 when the EDU 102 is assembled.

Turning to Figures 3 and 4, in the illustrated embodiment, the rotor core 116 is formed from a plurality of bodies. Each body takes the form of an axial stack 118. Each axial stack 118 is formed from a stack of electrical steel laminations 120. For example, in the illustrated embodiment, the rotor core 116 comprises six axial stacks 118, each of which is formed from around one thousand laminations 120 of electrical steel, each lamination being approximately 0.1mm thick.

The stacks 118 may be angularly aligned with each other. Alternatively, at least some of the stacks 118 may be angularly offset relative to each other, which may smooth torque delivery.

The skilled person will appreciate that in other embodiments, only a single body may be provided. Also, the or each body may be formed from a different number of laminations 120, or may be formed from a single layer of material.

As best shown in Figures 3 and 5, each stack 118 includes a plurality of axially extending apertures 122. In the illustrated embodiment, the apertures 122 extend through the entire stack 118, but in alternative embodiments, the apertures 122 may pass only partly through the stack 118. For example, where laminations 120 are used, the apertures may pass through only some of the laminations 120.

The apertures 122 may be formed by any suitable process. For example, the individual laminations 120 may be cut, stamped, machined or otherwise processed to form the corresponding portion of each aperture 122 before assembly of the laminations 120 to form the stack 118. Alternatively, each aperture 122 may be cut, stamped, machined or otherwise formed in the stack 118 after assembly of the laminations 120. Each aperture 122 comprises, in axial plan, a parallel sided portion having parallel sides. The apertures 122 are disposed in a plurality of aperture pairs 124 spaced circumferentially about the stack 118, as described in more detail below. Each aperture pair 124 is configured such that its apertures 122 are disposed either side of a radius 130 of the rotor core 116. In the illustrated embodiment, there are exactly ten aperture pairs 124.

The apertures 122 of each aperture pair 124 define, in axial plan, a mechanical angle that may be selected to provide a desired magnetic pole.

Each of the apertures 122 receives a magnet 136. The apertures 122 and the magnets 136 they receive are arranged and configured such that the magnetic fields generated by the magnets 136 of each aperture pair 124 generate a magnetic pole at a radially outer surface 138 of the rotor core 116, circumferentially centred at a point 121 where the radius 130 meets an outer edge of the rotor core 116. Where there are ten pairs of aperture pairs 124, ten of the magnetic poles are generated around the outer surface 138 of the rotor core 116.

Optionally, and as shown in the illustrated embodiments, each magnet 136 may be mounted in a holder 137 that is in turn inserted into one of the apertures 122. Each holder 137 can be formed from a resilient material, such as an elastomeric material. In use, each holder 137 covers the ends of its corresponding magnet 136 and extends beyond it. Each magnet 136 and holder 137 pair is a friction fit within its aperture 122.

Optionally, each holder 137 may extend radially inwardly and/or outwardly from its corresponding magnet 136. For example, in the illustrated embodiment (and as best shown in Figure 3), each holder 137 includes a radially inner portion 139 extending beyond its magnet 136, and a radially outer portion 141 extending beyond its magnet 136. The radially inner portion 139 and radially outer portion 141 of each holder 137 help retain the corresponding magnet 136 at a predetermined position within its aperture 122. In the illustrated embodiment, each aperture pair 124 is positioned symmetrically about its corresponding radius 130. In other embodiments, the aperture pairs 124 may be positioned asymmetrically about their respective radii. Different forms of asymmetry may provide improvements in electric motor performance, either when driven as a motor, or used to generate power in regenerative mode. For example, such asymmetry may provide improved efficiency when the motor is turning in one direction, while reducing efficiency in the other direction. The more efficient rotation direction may be used for propelling the vehicle forward, with the less efficient rotation direction being used for reverse.

Figures 5 and 6 show a stator (or stator assembly) 198, which is mounted within, and supported by, a housing 205 of the electric motor 112 shown in Figure 2 (the windings for the stator assembly 198 are omitted for clarity). The rotor core 116 is mounted within the housing 205 for rotation relative to the stator assembly 198. The stator assembly 198 comprises a stator core 199. Sixty circumferentially spaced, axially extending slots 200 are formed in the stator core 199, to define sixty circumferentially spaced, axially extending teeth 201.

In general, the stator comprises a stator core comprising a first steel having a silicon content exceeding 3.5% and the rotor assembly comprises a rotor core comprising a second steel having a silicon content of less than or equal to 3.5%.

The stator core 199 may take a number of different forms, as will now be described with reference to Figures 6 to 13.

For example, Figure 6 illustrates a stator core 199 having six circumferentially adjacent segments 300 that form an annular ring 301. Each segment 300 is arcuate in plan, and includes slots 200, which define ten radially extending teeth 201 per segment 300. Each segment 300 has circumferentially spaced apart end portions 302, each end portion defining a key 304 that interlocks with a corresponding key defined by an end portion of an adjacent segment 300. In this context, “interlocks” means to engage with each other by overlapping or by the fitting together of projections and recesses. The keys 304 may take any suitable form to achieve this interlocking. For example, one end portion 302 of each segment 300 may have a projection, and the other end may have a recess that is complementary in shape to the projection.

In other embodiments, each end portion 302 may have multiple projections and/or recesses, which are complementary to corresponding projections and/or recesses on an adjacent end portion 302. It is also not necessary for the projections and/or recesses to be identical on every segment, but keeping them the same will in many cases simplify design, manufacture and assembly of the stator core 199.

Alternatively, or in addition, the keys 304 may include overlapping regions (not shown) of the end portions 302.

The keys 304 may act primarily to locate the segments 300 relative to each other during assembly. However, in other embodiments, the keys 304 may be configured to interlock more robustly, which may increase mechanical integrity of the stator core as a whole.

The segments may comprise steel with a silicon content exceeding 4%, or, in other embodiments, exceeding 6%. In the illustrated embodiment, the segments 300 comprise steel with a silicon content of 6.5%. This relatively high level of silicon results in lower switching losses, especially at high switching frequencies.

Each segment 300 comprises an axial stack of laminations of the steel. In general, each lamination may be between 0.05 and 0.25mm thick. In the illustrated embodiment, each lamination is 0.1mm thick. Individual laminations may be cut, stamped, machined or otherwise processed to form the corresponding segments 300. Alternatively, the stamping, machining, cutting or other forming process may be performed after the laminations have been stacked.

It will be appreciated that the stator core 199 may comprise any suitable number of segments. In general, six or more segments may be desirable due to reduced manufacturing wastage, but this must be balanced against a reduction in stiffness caused by larger numbers of segments.

While the embodiment of Figure 6 uses segments that are made entirely of steel having a silicon content greater than 3.5%, in other embodiments, only part of each segment may be formed from such steel. For example, the part of each segment around which a conductor is positioned (i.e., the tooth or teeth) may be formed from such steel, while at least some of the rest of the segment may be formed from one more other steels with a lower silicon content. The two (or more) types of steel may be joined in any suitable way, such as by welding, interlocking, or with suitable fasteners. This principle may also be applied to any other embodiment, including those specifically described above and below.

Turning to Figure 7, there is shown an alternative embodiment of the stator core 199 that includes an annular yoke 306. The yoke 306 encircles the annular ring 301 to support the segments 300 from which it is formed. In the embodiment of Figure 7, each segment 300 comprises only a single tooth 201. A plurality of circumferentially spaced recesses 308 are formed along a radially inner region of the yoke 306. Each of the recesses 308 retains one of the segments 300. In this particular embodiment, the segments 300 are not in contact with each other, and therefore do not have key portions 304 as described in relation to the embodiment of Figure 6. The annular ring 301 is therefore not continuous in this embodiment.

The yoke 306 may be formed from a different steel to that used for the segments 300. For example, the yoke 306 may be formed from a steel having a lower silicon content than that used for the segments 300. This may reduce a production cost of the stator, due to reduced material cost (eg, cheaper steel may be used for the yoke 306) and/or production cost (eg, lower silicon steel may be cheaper to produce, because it causes slower wear on cutting dies). For example, the yoke may comprise steel with a silicon content of less than or equal to 3.5%. The yoke 306 may, for example, be formed from the same steel used in production of the rotor core 116. In some embodiments, this may allow production of the rotor and yoke in a single stamping or cutting operation. Even if this is not the case, the ability to use the same material for multiple components may simplify production and stock management. Alternatively, the yoke 306, the rotor core 116 and the segments 300 are all formed from different types of steel, which may be optimised for the specific electrical, magnetic and/or mechanical requirements of each component.

Figure 8 shows a variation on the embodiment of Figure 7, in which like reference signs are used to indicate like features. The primary difference is that the segments 300 of the Figure 8 embodiment extend further in the radially outward direction than those of the Figure 7 embodiment. This increases the radial extent of the segments 300 relative to that of the yoke 306. Where the yoke 306 is formed from a lower silicon steel than the segments 300, increasing the radial extent of the segments 300 may improve switching performance, at a cost of using more relatively high silicon content steel.

Figure 9 shows a variation on the embodiments of Figures 7 and 8, in which like reference signs are used to indicate like features. The primary difference is that the segments 300 of the Figure 9 embodiment do not extend as far in the radially outward direction as those of the Figures 7 and 8 embodiments. This decreases the radial extent of the segments 300 relative to that of the yoke 306. When the yoke 306 is formed from a lower silicon steel than the segments 300, decreasing the radial extent of the segments 300 may reduce the amount of relatively high silicon content steel required, at a cost of reduced high frequency switching performance.

Figure 10 shows an embodiment in which each segment 300 has two teeth 201. Another difference between this embodiment and those shown in Figures 7-9 is that segments 300 in the Figure 10 embodiment are in contact with each other to form a continuous annular ring 301 (i.e., there are no gaps between the segments, as there are in the Figures 7-9 embodiments). In the embodiment of Figure 10, each segment 300 comprises a segment feature that takes the form of a radially extending projection, in this case in the form of a dovetail 314. The dovetail 314 interlocks with a corresponding yoke feature at a radially inner region of the yoke 306. The yoke feature takes the form of a recess 316 in the radially inner region of the yoke 306, the recess 316 being complementary to the dovetail 314. The dovetail joint formed by the dovetail 314 and recess 316 helps locate the segments 300 relative to the yoke 306.

Figure 11 shows a variation on the embodiment of Figure 10, in which like reference signs are used to indicate like features. The primary difference is that the segments 300 of the Figure 11 embodiment extend further in the radially outward direction than those of the Figure 10 embodiment. This increases the radial extent of the segments 300 relative to that of the yoke 306. Where the yoke 306 is formed from a lower silicon steel than the segments 300, increasing the radial extent of the segments 300 may improve switching performance, at a cost of using more relatively high silicon content steel.

Figure 12 shows an embodiment in which each segment 300 has three teeth 201, in which like reference signs are used to indicate like features. As with the embodiment of Figures 10 and 11, the segments 300 are in contact with each other to form the annular ring 301.

Figure 13 shows a variation on the embodiment of Figure 12, in which like reference signs are used to indicate like features. The primary difference is that the segments 300 of the Figure 13 embodiment extend further in the radially outward direction than those of the Figure 12 embodiment. This increases the radial extent of the segments 300 relative to that of the yoke 306. Where the yoke 306 is formed from a lower silicon steel than the segments 300, increasing the radial extent of the segments 300 may improve switching performance, at a cost of using more relatively high silicon content steel.

In the embodiments of Figures 10 to 13, each segment 300 has only one projection in the form of dovetail 314. In other embodiments, any or all of the segments 300 may include two or more of the projections, and/or other locating and/or reinforcing features. Where one or more of the projections (and/or other locating and/or reinforcing features) is provided, it may take the form of a dovetail, similar to that shown in Figures 10-13. Alternatively, any or all of such dovetails may be formed on the yoke, with the complementary recess(es) being formed on the corresponding segment. Any other shape or configuration of locating and/or reinforcing feature may be used in place of, or in addition to, the illustrated dovetail joints.

Optionally, at least one of the annular rings in the stack is angularly offset relative to at least another of the annular rings. For example, laminations in a stack of laminations forming the stator core may be angularly positioned such that the abutments between circumferentially adjacent segments are angularly offset relative to the abutments of segments in adjacent laminations in the stack. This may increase mechanical integrity of the stator core as a whole. Alternatively, or in addition, one or more of the bodies (i.e., stacks of the annular rings) may be radially offset relative to one or more other bodies. This may help smooth torque delivery.

Although the yoke 306 in the described embodiments takes the form of a separate, annular element that completely encircles the segments, in other embodiments a plurality of circumferentially spaced yokes may be provided, each forming part of, and/or supporting, one or more of the segments.

Each stator core 199 may form part of a stator or stator assembly. In each stator assembly, the teeth 201 are wound with conductors (not shown) in a known manner. The conductors may take the form of insulated wire or pins. The conductors may be driven, in use, with current in a controlled manner by the inverter 114 to generate electromagnetic poles that interact with the magnetic poles 121 of the rotor core 116 to generate torque in the rotor core 116. The stator assembly 198, conductor winding and drive current may be, for example, conventional, and so are not described in more detail. Although the invention has been described with reference to a number of specific embodiments, the skilled person will appreciate that the invention may also be embodied in many other forms.

Claims

1. An electric motor, comprising: a housing; a stator assembly disposed within the housing; and a rotor assembly mounted within the housing for rotation within the stator assembly; wherein the stator assembly comprises a stator core comprising a first steel having a silicon content exceeding 3.5% and the rotor assembly comprises a rotor core comprising a second steel having a silicon content of less than or equal to 3.5%; wherein the stator core comprises a plurality of circumferentially spaced stator teeth, wherein at least the teeth are formed from the first steel, and a plurality of circumferentially adjacent segments, each segment comprising one or more of the teeth; and the motor further comprises at least one yoke supporting one or more of the segments, said at least one yoke comprising a third steel.

2. The electric motor of claim 1, wherein the first steel has a silicon content exceeding 4%.

3. The electric motor of claim 1 or 2, wherein the first steel has a silicon content greater than or equal to 6%.

4. The electric motor of any preceding claim, wherein the first steel has a silicon content of 6.5%.

5. The electric motor of any preceding claim, wherein each segment has circumferentially spaced apart end portions, each end portion defining a key that interlocks with a corresponding key defined by an end portion of an adjacent segment.

6. The electric motor of any preceding claim, wherein each segment has exactly one, two, or three of the teeth.

7. The electric motor of any of claims 1 to 5, wherein each segment has exactly ten of the teeth.

8. The electric motor of claim 7, comprising exactly six of the segments.

9. The electric motor of any preceding claim, wherein said at least one yoke forms part of one or more of the segments.

10. The electric motor of any preceding claim, wherein said at least one yoke encircles the segments.

11. The electric motor of any preceding claim, wherein each segment comprises a segment feature that interlocks with a corresponding yoke feature at a radially inner region of the yoke.

12. The electric motor of claim 11, wherein each segment feature and its corresponding yoke feature form a dovetail joint.

13. The electric motor of claim 12, wherein each segment feature takes the form of a dovetail and each yoke feature takes the form of a recess having a shape complementary to the dovetail.

14. The electric motor of any preceding claim, wherein the third steel is different to the first steel.

15. The electric motor of any preceding claim, wherein the third steel is the same as the second steel.

16. The electric motor of any of claims 1 to 13, wherein the third steel is different to the first steel and the second steel.

17. The electric motor of any preceding claim, wherein each segment comprises a stack of laminations of the steel.

18. The electric motor of claim 17, wherein each lamination is between 0.05 and

0.25mm thick.

19. The electric motor of any preceding claim, wherein the electric motor is an interior permanent magnet motor.

20. An electric vehicle comprising the electric motor of any preceding claim.