WO2019234681A1

WO2019234681A1 - Magnetic amplification rotor

Info

Publication number: WO2019234681A1
Application number: PCT/IB2019/054725
Authority: WO
Inventors: James Klassen; Javier FERNANDEZ-HAN
Original assignee: Genesis Robotics And Motion Technologies, LP
Priority date: 2018-06-07
Filing date: 2019-06-06
Publication date: 2019-12-12

Abstract

The present disclosure provides a rotor body which utilises a combination of bridges and openings in order to improve the structural strength of the rotor while reducing magnetic flux leakage. The rotor can be used in electric machines with any number of poles comprising thin rotor laminations. Embodiments shown here by example are generally high pole count, but the principles disclosed can be applied to any number of poles and any size of motor. As such, each layer is provided with a back iron, bridging all of the slots, in order to provide the required degree of structural integrity. At the inner or outer diameter of the rotor body, for a given slot, most of the layers include an opening, while the remaining layers include a bridge.

Description

MAGNETIC AMPLIFICATION ROTOR

FIELD OF THE DISCLOSURE

The present disclosure relates to a rotor for an electric machine. More specifically the disclosure relates to rotors comprising layered laminations with improved strength and reduced magnetic flux leakage.

BACKGROUND

Electric machines typically comprise a stator and a rotor which rotates with respect to the stator. The rotor and stator contain a plurality of magnets, creating a magnetic coupling between the two components. By modifying an electric current entering the stator, the magnetic field between the rotor and the stator changes. This change in magnetic field causes the rotor to rotate, creating a physical output resulting from an electrical input. Alternatively, by physically rotating the rotor, the magnetic field fluctuates, which induces an electric current in the stator. Therefore, a physical input can be used to generate an electrical output.

In general, rotors are formed by fixing together a plurality of laminated disks. Laminated construction is often used in stators to reduce eddy currents. It can also be used in rotors to create parts from thin sheets of metal, which has the benefit of allowing punch press forming. This is a low-cost construction method which is faster and less expensive than machining.

Each disk is stamped from a thin sheet of magnetically susceptible material and then glued or otherwise attached to other disks. The combined disks form a cylindrical rotor body. Holes within each of the disks are aligned to form slots, such that magnets can be inserted into the rotor body.

In magnetic amplification rotors, the slots within the rotor body are arranged to hold the magnets such that poles with the same polarity face toward each other. In this manner, the material between each slot acts to amplify the magnetic flux and direct it toward the stator. When designing rotors, two conflicting requirements must be borne in mind. Firstly, due to the high torque produced by electric machines, each laminated disk of the rotor is required to meet certain structural requirements, in order to ensure that the rotor body does not physically fail.

However, when material is added to a rotor lamination in order to increase the structural strength, magnetic flux leakage increases. This introduces inefficiencies in the system, which can reduce the available torque.

As such, there is a need to find a balance between increasing the strength of the rotor and decreasing magnetic flux leakage.

In high pole-count motors, the amount of material holding the magnets in place is generally less than that of similarly sized low pole-count motors. As such, structural integrity of the rotor body becomes a more prominent issue.

US patent number 5,889,346 discloses a low pole-count rotor in which the majority of the laminated disk layers comprise six individual segments which are stacked on top of segments of other layers. Each of the individual segments in these layers are unattached from other segments within the layer. A small number of layers comprise an alternative type of laminated disk, in which the individual segments are connected to each other via bridges. In this manner, the circumferential stresses generated within the rotor body are focussed at each of the bridges. This minimises the amount of flux leakage within the rotor body while maintaining a certain level of structural integrity.

SUMMARY OF THE DISCLOSURE

In a first aspect, the present disclosure provides a rotor for an electric machine, comprising : a plurality of laminate sheets arranged in a stacked configuration to form a rotor body; wherein each laminate sheet defines an inner diameter and an outer diameter, and has a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters; the laminate sheets are further arranged such that the magnet- retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet; each laminate sheet has a back iron, formed around an edge of the laminate; and the plurality of laminate sheets are configured such that, between each slot in the rotor body and the inner or outer diameter, at least one of the laminate sheets includes an opening between a respective magnet-retaining hole and the inner or outer diameter and at least one of the laminate sheets includes a bridge between a respective magnet-retaining hole and the inner or outer diameter, such that one or more openings are formed between each slot and the inner or outer diameter.

In a second aspect, the present disclosure provides a rotor for an electric machine, comprising : a plurality of laminate sheets arranged in a stacked configuration to form a rotor body; wherein each laminate sheet defines an inner diameter and an outer diameter, and has a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters; the laminate sheets are further arranged such that the magnet-retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a permanent magnet; for each laminate sheet, openings are formed between one or more of the plurality of magnet-retaining holes and the inner or outer diameter, and bridges are formed between the remaining magnet-retaining holes and the inner or outer diameter, such that each laminate sheet comprises one or more openings and one or more bridges at the inner or outer diameter; the laminate sheets are further arranged such that they are circumferentially staggered, such that each slot has one or more openings and one or more bridges at the outer or inner diameter.

In a third aspect, the present disclosure provides a method of manufacturing a rotor for an electric machine, comprising the steps of: forming a plurality of laminate sheets, each laminate sheet defining an inner diameter and an outer diameter, a back iron adjacent to the inner diameter or the outer diameter, and having a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters; stacking the plurality of laminate sheets together to form a rotor body, such that the magnet- retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet; fixing the plurality of laminate sheets to each other; and locating magnets within the slots, wherein, the plurality of laminate sheets are configured such that, between each slot in the rotor body and the inner or outer diameter, at least one of the laminate sheets includes an opening between a respective magnet-retaining hole and the inner or outer diameter and at least one of the laminate sheets includes a bridge between a respective magnet-retaining hole and the inner or outer diameter, such that one or more openings are formed between each slot and the inner or outer diameter.

In a fourth aspect, the present disclosure provides a method of manufacturing a rotor for an electric machine, comprising the steps of: forming a plurality of laminate sheets, each laminate sheet defining an inner diameter and an outer diameter, and having a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters; stacking the plurality of laminate sheets together to form a rotor body, such that the magnet-retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet; fixing the plurality of laminate sheets to each other; and locating magnets within the slots, wherein, for each laminate sheet, openings are formed between one or more of the plurality of magnet-retaining holes and the inner or outer diameter, and bridges are formed between the remaining magnet-retaining holes and the inner or outer diameter, such that each laminate sheet comprises one or more openings and one or more bridges at the inner or outer diameter; the laminate sheets are further arranged such that they are circumferentially staggered, such that each slot has one or more openings and one or more bridges at the outer diameter.

In a fifth aspect, the present disclosure provides a method of manufacturing a rotor for an electric machine, comprising the steps of: forming a plurality of laminate disks from one or more laminate sheets, each disc having a central opening and an inner and outer diameter, and each disc further having a plurality of magnet-retaining holes arranged around its periphery between the inner and outer diameters; stacking the plurality of laminate disks together to form a rotor body, such that the magnet-retaining holes are aligned to form slots in the rotor body; and removing material from the inner and/or the outer diameter of the rotor in order form openings between at least some of the slots and the inner diameter and/or outer diameter. Other features of the disclosure are described in the accompanying description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described, by way of example only, in connection with the attached drawings, in which :

FIGURE 1 shows a plan view of a rotor body in accordance with an embodiment the present disclosure;

FIGURE 2 shows a perspective view of the rotor body of FIGURE 1;

FIGURE 3 shows a perspective view of a section of the rotor body of FIGURE 2;

FIGURE 4 shows a plan view of an unbridged disk in accordance with an embodiment of the present disclosure;

FIGURE 5 shows a plan view of a bridged disk in accordance with an embodiment of the present disclosure;

FIGURE 6 shows a perspective view of a section of a rotor body in accordance with a further embodiment of the present disclosure;

FIGURE 7 shows a plan view of a section of a disk in accordance with an embodiment of the present disclosure;

FIGURE 8 shows a plan view of a section of a laminated disk with curved inner edges in accordance with embodiments of the present disclosure;

FIGURE 9 shows a perspective view of a section of a rotor body with tapered magnet slots in accordance with embodiments of the present disclosure; FIGURE 10 shows a perspective view of a rotor body with helical slots including a cross-section aspect, in accordance with embodiments of the present disclosure;

FIGURE 11 shows a rotor body with a press-fit cylinder in accordance with embodiments of the present disclosure;

FIGURE 12 shows a perspective view of the rotor body of FIGURE 11, including a cross-sectional aspect;

FIGURE 13 shows a rotor body for use as an outer rotor in accordance with an embodiment of the present invention;

FIGURE 14 shows a perspective view of a section of the rotor body of FIGURE 13;

FIGURE 15 shows a plan view of a laminated disk in which some flux resistors are formed as complete openings in accordance with an embodiment;

FIGURE 16 shows a perspective view of the laminated disk of FIGURE 15 as part of a rotor;

FIGURES 17a and 17b show perspective view of a method of forming a rotor;

FIGURES 18a and 18b show a perspective view of a method of forming a rotor, continued from FIGURES 17a and 17b;

FIGURES 19a to 19c show a plan view of the step of removing material from the outer diameter of the rotor;

FIGURE 20 shows a perspective view of the rotor of FIGURES 17a to 19c, with staggered slots;

FIGURE 21 shows a flow diagram of a method according to the present invention; and FIGURE 22 shows a flow diagram of a method according to the present invention. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present disclosure provides a rotor body which utilises a combination of bridges and openings in order to improve the structural strength of the rotor while reducing magnetic flux leakage. The rotor can be used in electric machines with any number of poles comprising rotor laminations. Embodiments shown here by example are generally high pole count, but the principles disclosed can be applied to any number of poles and any size of motor. For example, embodiments described herein can be applied to high pole count rotors comprising 56 poles, 60 poles, 64 poles or more. In the present specification, a high pole count machine is generally considered to be one with at least 56 poles. Further examples of high pole count machines include those with between 80 and 100 poles, and preferably 86 poles. As such, each layer is provided with a back iron, bridging all of the slots, in order to provide the required degree of structural integrity. At the inner or outer diameter of the rotor body, for a given slot, most of the layers include an opening, while the remaining layers include a bridge. This may be achieved by using identical layers, in which, for example, every fourth mag net- retaining hole has a bridge. By staggering the layers in a circumferential direction, it is possible to create a mix of openings and bridges along the length of each slot. Alternatively, some of the layers may include openings across the outer diameter of every magnet-retaining hole, and some layers may include bridges across the outer diameter of every mag net- retaining hole. By combining these two 'bridged' and 'unbridged' layers, the desired effect of minimal flux leakage between posts can be achieved while the bridges are radially thick enough such that they can be manufactured and are resistant to buckling . By selecting an appropriate bridge thickness, the overall resistance to buckling is greater than a rotor in which every layer of every slot has a bridge.

Figure 1 shows a top-view of a rotor body 101. The rotor body 101 comprises a plurality of laminated disks 102 arranged in a stacked configuration. The disks 102 may be stamped or punched from a thin sheet of magnetically susceptible material, such as steel. It is important that the material used to produce the laminated disks 102 is of a high stiffness, so as to resist deformation by the torque generated by the motor. Punching is generally suitable for processing materials with a thicknesses as high as 12mm. As such, the laminated disks

102 may be as thick as 12mm. However, the laminated disks 102 may be any thickness, such as between 0.1mm and 12mm. In one embodiment, the laminated disks 102 may be 0.5mm thick.

Each laminated disk 102 is generally ring-shaped, defining an inner diameter

103 and an outer diameter 104. Each laminated disk 102 comprises a plurality of circumferentially distributed magnet-retaining holes 105, which are each arranged to receive a permanent magnet. The material between each magnet- retaining hole 105 is the post 106, which is arranged to enable magnetic flux to flow through the laminated disk 102.

The rotor body 101 is designed such that, when the magnets are inserted in the magnet-retaining holes 105, they are circumferentially polarised, with the north pole of each magnet facing toward the north pole of an adjacent magnet and the south pole of each magnet facing toward the south pole of an adjacent magnet. In this manner, each laminated disk 102 comprises an even number of magnet-retaining holes 105 and an even number of posts 106.

In this arrangement, the posts 106 amplify the magnetic flux from the magnets and direct it toward the outer diameter 104 which, in use, is adjacent a stator. By amplifying the magnetic flux of the magnets and minimising magnetic flux leakage elsewhere in the laminated disk 102, the efficiency and/or torque of a corresponding electric machine can be increased.

Owing to the arrangement of the magnets, magnetic flux at a pole of one magnet is forced towards the edges of the rotor by the adjacent magnet having an identical pole, also facing that magnet. Ideally, this flux should be directed across the airgap towards the stator at the outer diameter. However, the flux tries to wrap around the magnet back to the opposing pole on the other side of the magnet. The back iron, and bridges formed at the outer diameter, facilitate this process. The process by which flux does not reach the stator, and instead makes its way back to the magnet, is called flux leakage.

The laminated disk 102 also comprises a back iron 107, located around the inner diameter 103. The back iron 107 connects adjacent posts 106, and increases the overall structural integrity of the laminated disk 102. As the back iron 107 is constructed from the same magnetically susceptible material as the rest of the laminated disk 102, it provides a path through which magnetic flux can connect to other portions of the laminated disk 102. In this case, the back iron

107 introduces a magnetic flux path between adjacent posts 106. This magnetic flux leakage introduces inefficiencies into the system. However, these inefficiencies are outweighed by the structural benefit provided by the back iron 107. By forming a back iron 107 within the laminated disk 102, the laminated disk can be formed from a unitary piece of material.

In order to reduce the magnetic flux leakage between posts 106 of the laminated disk 102, flux resistors 108 are positioned at the base of each magnet-retaining hole 105, adjacent the inner diameter 103. The flux resistors

108 are holes in the laminated disk 102 which introduce a high resistance to the linkage of magnetic flux through that portion of the laminated disk 102. As a result of the flux resistors 108, much of the magnetic flux which would have passed between posts 106 instead finds that its preferred path is across the airgap between the rotor and the stator at the outer diameter 104.

In Figures 1 and 2, the flux resistors 108, 208 are shown at the end of every magnet-receiving hole 105, 205. As an alternative, some of the round flux resistors may be omitted, such that only some of the magnet-receiving holes have flux resistors at their ends. For example, the flux resistors may alternate, such that every other magnet-receiving hole has a flux resistor at its end. In embodiments where flux resistors are not formed at every magnet-receiving hole, the flux resistors may be larger than shown in Figures 1 and 2, to compensate for the reduction in the number of resistors. As a further alternative, some layers may have flux resistors, while others may not. For example, the layers may alternate between ones having flux resistors, and one not having flux resistors. These two alternatives may also be combined, such that not every layer has flux resistors, and the layers that do have flux resistors, only some of the magnet-receiving holes have flux resistors.

Although a degree of flux leakage between the magnets can be tolerated at the back iron 107, 207, flux leakage at the airgap-side of the rotor 101, 201 is more of a problem. In order to direct as much flux as possible towards the stator through the airgap, openings would ideally be provided at the outer diameter- side of every magnet-retaining hole 105, 205. By doing this, an opening would be formed between the posts 106, 206, reducing flux leakage to beneficial levels. However, in some rotor configurations, such as, for example, a high-pole count motor with relatively tall and thin rotor posts 106, 206, and in which very high levels of torque are generated, such an arrangement would not provide sufficient resistance to bending of the posts 106, 206 in the tangential/circumferential direction.

One solution to this problem is to introduce bridges between every magnet- retaining hole 105, 205 and the outer diameter 104, 204. However, in order to reduce the radial thickness of the bridges such that the flux leakage is acceptably low, the bridges become structurally weak and difficult to manufacture. In other words, by including a bridge between the ends of every post 106, 206 at every magnet-retaining hole 105, 205 and the outer diameter 104, 204, the bridges would be too thin (radially) to be manufactured, if it is to provide adequate magnetic saturation, and too weak in circumferential compression to provide adequate resistance to buckling of the bridge in the circumferential direction. On the other hand, the bridges, if located between every post 106, 206 on every layer, would be too thick (radially) to provide the required magnetic saturation and flux linkage resistance, if they are thick enough to provide the desired manufacturability and/or resistance to circumferential buckling.

It has been shown using finite element analysis (FEA) that when a bridge between the magnet retaining hole 105, 205 and the outer diameter 104, 204 between every post 106, 206 on every layer is thick enough to be manufactured, a torque reduction (compared to having no bridges) of around 50% or more is demonstrated. In order to overcome this problem, the rotor 101, 201 is arranged such that some of the magnet-retaining holes 105, 205 have bridges 109, 209 at the outer diameter 104, 204, and such that some of them have openings 110, 210. The openings 110, 210 act to restrict the connection of magnetic flux between adjacent posts 106, 206, near the outer diameter 104, 204. By reducing the total number of bridges 109, 209 on the rotor 101, 201, the overall amount of flux leakage between posts 106, 206 through the bridges 109, 209 is reduced. This enables each bridge 109, 209 to be manufactured with a greater radial thickness that allows for easier manufacturability and greater structural integrity.

As with the back iron 107, 207, the bridges 109, 209 introduce a magnetic flux path between adjacent posts 106, 206. However, by forming each bridge 109, 209 such that it is radially thin, they can be magnetically saturated, such that the level of magnetic flux leakage can be significantly reduced.

By 'magnetically saturated', it is meant that the magnetic flux connecting through each bridge 109, 209 is maximised for a given volume of material. As the rate of magnetic flux linkage through that volume is dramatically reduced above the saturation point of the material, flux above that saturation point will connect to the next post 6more like as if the bridge 109, 209 did not exist. Therefore, by decreasing the magnetic flux linkage cross section between the posts 106, 206, by reducing the volume of the bridge 109, 209 through which it can travel, the level of flux leakage can be reduced.

By using bridges 109, 209 between the posts 106, 206 at the airgap on periodic posts 106, 206 and then rotationally staggering the laminated disks 102, 202 and adhering the layers together, the parts have increased manufacturability for a given overall rotor structural integrity in terms of tangential rigidity and bridge-buckling resistance. By using a radially thicker back iron 107, 207 and introducing flux resistors 108, 208, the same flux resistance of a radially thinner back iron 107, 207 can be achieved with much higher bending stiffness of the rotor posts 106, 206 in the circumferential direction. The combination of these geometries allows a rotor 101, 201 of this construction to achieve a useful level of magnetic amplification while still maintaining the necessary manufacturability to allow low cost production and high enough structural strength and stiffness, especially in the circumferential direction, to achieve the required torque and long service life by avoiding fatigue failures of the bridges 109, 209 or the back iron 107, 207.

The relationship between the radial thickness of the bridges and the buckling strength of the bridges in the circumferential direction is as follows:

R² X B = S , where R is the radial thickness of each bridge 109, 209, B is the fraction of bridged holes 105, 205 and S is the total buckling strength of all bridges 109, 209 in the laminated disk 102, 202.

As an example, a first (reference) layer has a first radial thickness of 1, and all holes 105, 205 include bridges 109, 209. This leads to a reference buckling strength of 1 (as l² multiplied by 1 is 1).

However, by halving the number of bridges 109, 209 and doubling the thickness of each bridge 109, 209, the total buckling strength of the bridges 109, 209 doubles (as 2² multiplied by 0.5 is 2).

Similarly, by quartering the number of bridges 109, 209 and quadrupling the thickness of each bridge 109, 209 relative to the reference layer, the total buckling strength of the bridges 109, 209 is four times higher (as 4² multiplied by 0.25 is 4).

As such, by reducing the number of bridges 109, 209 and increasing the thickness of each bridge 109, 209, the overall buckling strength of the bridges 109, 209 increases significantly for a given overall flux leakage at the opening between the posts 106, 206, when compared to having bridges 109, 209 between every post 106, 206 on every layer. In addition to the buckling strength benefit, the manufacturability of the components is also increased, enabling manufacturing in some cases, and reducing manufacturing cost and complexity in other cases.

It has also been shown, through analysis, that magnetic flux leakage between the posts 106, 206 via the back iron 107, 207 is less detrimental to magnetic motor performance than is a metallic connection between the posts 106, 206 at the airgap (i.e. at the outer diameter 104, 204 in an internal rotor configuration). For this reason, embodiments of the present disclosure have a relatively thick bridging between the posts 106, 206 at the inner diameter 103, 203 and still provide the desired amplification of the flux at the airgap. By forming the laminated disk 102, 202 such that the back iron 107, 207 is radially thicker than the bridges 109, 209, the majority of the flux leakage in the laminated disk 102, 202 occurs away from the airgap where it has less of an effect on the flux at the airgap, thus reducing the detrimental effect on the torque.

The preferred ratio of the thickness of the back iron 107, 207 to the thickness of the bridges 109, 209 is between 1.5 : 1 and 2: 1, 2: 1 and 2.5: 1, 2.5: 1 and 3: 1, 3: 1 and 4: 1, 4: 1 and 5 : 1, or 5 : 1 or greater.

In the embodiment shown in Figure 1, the laminated disk 102, 202 comprises 60 magnet-retaining holes 105, 205 and posts 106, 206. One in every four magnet-retaining holes 105, 205 has a bridge 109, 209 adjacent the outer diameter 104, 204. However, it is to be appreciated that the specific number of magnet-retaining holes 105, 205, posts 106, 206 and bridges 109, 209 will depend on the design of the electric machine. For example, the laminated disk 102, 202 may contain 56, 64 or any other number of magnet-retaining holes 105, 205. Correspondingly, the laminated disk 102, 202 may contain 14, 16 or any other number of bridges 109, 209 which corresponds to one in every four magnet-retaining holes 105, 205 having a bridge 109, 209. Of course, the ratio of bridges 109, 209 to openings 10, 210 may differ. For example, instead of four-to-one, the ratio of bridges 109, 209 to openings 110, 210 may be three- to-one, two-to-one, five-to-one or any other ratio. Due to the discrete number of posts 106, 206 in the rotor 101, 201 and the discrete number of electromagnets in a corresponding stator, there exists a set of preferred, low-energy orientations in which the rotor 101, 201 and the stator will rest, when no external forces are being exerted upon them. In use, the rotor 101, 201 will tend to 'detent' between these preferred orientations, causing the rotor 101, 201 to rotate with variable speed. This phenomenon is known as 'cogging'. In order to avoid cogging, preferred rotor arrangements attempt to achieve a circumferentially uniform magnetic field at the airgap between the rotor 101, 201 and a corresponding stator. In this manner, in preferred embodiments, the bridges 109, 209 of the rotor 101, 201 are distributed such as to achieve a circumferentially uniform magnetic field. Rotationally staggering the bridges, therefore, has two benefits. It reduces the cogging of the device, and it distributes the circumferentially stiffening effect of the bridges equally to all of the slots after assembly of all the layers.

Referring now to Figures 2 and 3, in accordance with an embodiment of the present disclosure, there is shown the rotor body 201, 301 comprising a plurality of laminated disks 202, 302. Each of the laminated disks 202, 302 of the rotor body 201, 301 is identical to that described with respect to Figure 1. In this manner, when manufacturing the rotor body 201, 301, the process only requires the stamping/punching of a single type of laminated disk 202, 302. The laminated disks 202, 302 are stacked on top of each other, and aligned, such that the magnet-retaining holes 205, 305 of the disks 202, 302 form slots 211, 311, within which magnets can be retained. The laminated disks 202, 302 are attached to each other by conventional means, such as gluing, pinning or bolting. However, the laminated disks 202, 302 are not stacked in a circumferentially identical manner.

As shown in Figures 2 and 3, the laminated disks 202, 302 of the rotor body

201, 301 are stacked such that the bridges 209, 309 of each laminated disk

202, 302 are adjacent to an opening 210, 310 of the adjacent laminated disk 202, 302. Specifically, the embodiment of Figure 2 shows that adjacent laminated disks 202 are rotationally offset by an angle equating to the distance between two adjacent slots 211 (which is the same as the angle between adjacent posts 206). As such, bridges 209 of each subsequent laminated disk 202 are positioned at the outer diameter 204 of an adjacent slot 211, creating a 'stairs' or’staggered’ pattern.

The particular arrangement shown in Figures 2 and 3 is merely exemplary. It is to be understood that the laminated disks 202, 302 are rotationally offset from each other, such that bridges 209, 309 of one or more laminated disks are misaligned from bridges 209, 309 of other laminated disks 209, 309, thus distributing the structural improvements provided by the bridges 209, 309 across the outer diameter 204, 304 of the rotor body 201, 301. In a preferred arrangement, bridges 209, 309 of one laminated disk 202, 302 are at least misaligned from bridges 209, 309 of adjacent laminated disks 202, 302.

As shown in Figures 2 and 3, the flux resistors 208, 308 of each laminated disk 202 ,302 are rotationally positioned such that the flux resistors 208, 308 of all laminated disks 202, 302 align with any other flux resistor 208, 308.

Although Figure 2 shows an embodiment comprising 44 laminated disks 202, it is to be understood that any number of laminated disks 202 can be used to form the rotor body 201, depending on the specific application.

Referring now to Figures 4 to 6, there is shown a further embodiment of the disclosure which differs from the embodiment shown in Figures 1 to 3. In essence, the embodiment of Figures 4 to 6 comprises lamination disks 402 of two types: unbridged disks 412; and bridged disks 413.

Referring to Figure 4, an unbridged disk 412 is of a similar structure to the laminated disk 102 of Figures 1 to 3. As with the above, the unbridged disk 412 is formed from a thin sheet of magnetically susceptible material, in the shape of a ring with an inner diameter 403 and an outer diameter 404. The unbridged disk 412 comprises a plurality of magnet-retaining holes 405 and posts 406 between the magnet-retaining holes 405. The unbridged disk 412 also comprises a back iron 407, around the inner diameter 403, with flux resistors 408 located at the base of each magnet-retaining hole 405, adjacent the inner diameter 403. However, in contrast to the laminated disks 102 of Figures 1 to 3, the unbridged disk 412 of Figure 4 does not comprise any bridges 109 between the magnet- retaining holes 405 and the outer diameter 404 (i.e., between adjacent posts 406 at the outer diameter 404). Instead, the unbridged disk 412 has an opening 410, such that adjacent posts 406 are not physically connected adjacent the outer diameter 404. In this manner, the unbridged disk 412 has a high resistance to magnetic flux flow between posts 406, adjacent the outer diameter 404. However, this high magnetic flux resistance comes at the cost of structural integrity.

Referring now to Figure 5, a bridged disk 513 is of a similar structure to the laminated disk 102 of Figures 1 to 3 and the unbridged disk 412 of Figure 4. The bridged disk 513 is formed from a thin sheet of magnetically susceptible material, in the shape of a ring with an inner diameter 503 and an outer diameter 504. The bridged disk 513 comprises a plurality of magnet-retaining holes 505 and posts 506 between the magnet-retaining holes 505. The bridged disk 513 also comprises a back iron 507, around the inner diameter 503, with flux resistors 508 located within the back iron 507, adjacent the inner diameter 503.

Figure 5 shows an alternative arrangement of flux resistors 508, located at the base of the posts 506 adjacent the inner diameter 503. As will be appreciated, this alternative can be applied to any of the embodiments discussed above.

In contrast to the unbridged disk 412 of Figure 4, the bridged disk 513 of Figure 5 comprises bridges 509 between each magnet-retaining hole 505 and the outer diameter 504 (i.e., connecting adjacent posts 506 at the outer diameter 504). In this manner, the bridged disk 513 has greater structural integrity than the unbridged disk 512, but this causes the bridged disk 513 to have a higher level of magnetic flux leakage.

With reference to Figure 6, the unbridged disk 612 and the bridged disk 613 are stacked together to form a rotor body 614. In this embodiment, the structural benefits provided by the bridged disk 613 are used to improve the structural integrity of the unbridged disks 612. Further, forming the rotor body 614 from mostly unbridged disks 612, the magnetic flux leakage within the rotor body 614 is kept to a reasonable level. For example, as shown in Figure 6, the rotor body 614 comprises two unbridged disks 612 for every bridged disk 613. Further, the unbridged disks 612 and bridged disks 613 may be of differing axial thicknesses. However, it is to be understood that any ratio of unbridged disks 612 to bridged disks 613 may be used. Further, any thickness of disks may also be used.

It is preferable that the bridged disks 613 and unbridged disks 612 are bonded or mechanically secured together by some means which transfers tangential loads which result from magnetic forces between the stator and the rotor, from the unbridged 612 disks to the bridged disks 612 so that the bridges 609 provide rigidity for all of the disks. It should also be understood that there is another non-intuitive benefit to staggering intermittent bridges 609 between posts 606. Specifically, the glue layer between each of the laminated disks 602 also serves as a flux resistor in the axial direction from layer to layer. As a result, there is a greater flux resistance that results from flux in a non-bridged post 606 which must travel axially across one or more glue layers to get to a bridge 609 in the flux path between posts 606. As a result, the same overall minimum cross- section of bridging will have higher flux resistance in the geometry disclosed here, as compared to a bridge 609 on every layer between every post 606 with the same overall minimum bridge cross-section. Here, 'minimum bridge cross- section' means the thinnest part of the bridge 609, which is where the highest flux resistance to flux linkage will be created and where it is necessary to perform flux measurements.

The ideal ratio of bridged disks 613 to unbridged disks 612 will be the result of analysis and experimentation for each specific application. It has been found by the inventor that somewhere having between 10% and 50% of the total number of disks being bridged disks 613 has the desired effect sufficiently increasing the structural rigidity of the rotor posts 606 to adequately reduce bending stress at the base of the posts 606 while maintain necessary buckling resistance of the bridges 609 and the necessary manufacturability of the bridges 609. However, greater or lesser percentages may also work for some applications. Figure 6 also shows magnets 615, inserted into the slots 611 of the rotor body 614. As discussed with reference to Figures 1 to 3, the magnets 615 are arranged such that like poles face toward each other. In this manner, the north polarised face of each magnet 615 faces toward the north polarised face of an adjacent magnet 615. Similarly, the south polarised face of each magnet 615 faces toward the south polarised face of an adjacent magnet 615. This creates a magnetically amplified post 606 between the magnets 615.

As shown in Figure 6, the unbridged disks 612 and the bridged disks 613 may comprise optional dowel holes 623. The dowel holes 623 are used to attach a fastening means 624 (e.g. a dowel or a rod), which extends between each of the disks. In this manner, the fastening means 624 acts to maintain the alignment between disks and acts as a conduit for spreading distributing torsional stress from unbridged disks 612 to bridged disks 613.

With reference to Figure 7, there is shown a close up view of laminated disk 702 in accordance with the embodiment shown in Figures 1 to 3. As discussed above, bridges 709 extend between posts 706, adjacent to the outer diameter 704. This increases the structural rigidity of the laminated disk 702. In a preferred embodiment, the bridges 709 are made from steel, as with the rest of the laminated disk 702.

In general, steel is very strong when it comes to resisting tensional forces, even when the steel is thin. However, thin pieces of steel are prone to buckling when subjected to compressional forces.

In light of the above, it is preferable that the bridges 709 extend substantially straight between posts 706 rather than tangentially in an arc. By substantially straight, it is meant that a straight line 716 can be drawn through the bridge 709, as shown in Figure 7. In ensuring that the bridges 709 are substantially straight, despite the curvature of the outer edge of the laminate disk 702, the bridges 709 provide higher resistance to compressional forces as compared to curved bridges 709. Of course, the bridges 709 may be curved at one or more edges, or thicker at one or more ends, so long as it is possible to draw a straight line 716 through the bridge 709. A variety of outer diameter shapes can be used, for example, such as to reduce the effects of cogging through slight curvature of the outer diameter of the rotor from post to post. Regardless of the shape to be used, it is preferable to maintain the straight line connection between the posts 706 through the bridge 709 as described above.

Although the bridge structure above is discussed with reference to the embodiment disclosed in Figures 1 to 3, it is clear that the same applies to the other embodiments of the present disclosure.

With reference to Figure 8, there is shown a close up view of a bridged disk 813 in accordance with the embodiment shown in Figures 4 to 6. As discussed above, the magnet-retaining holes 805 are arranged to receive a magnet 815, which causes magnetic flux to connect between two magnets 815, one on each side of each post 806, through the posts 806.

As shown in Figure 8, the inner edges of the magnet-retaining holes 805 may be curved. In this manner, the magnets 815 are prevented from abutting the edges of the magnet-retaining hole 805 which are nearest the inner diameter 803 and the outer diameter 804, thus forming a space in the magnet-retaining hole 805.

The space formed in the magnet-retaining hole 805 is filled with a potting material, which is used to hold the magnet 815 in position. The potting material is preferably non-magnetic, so as not to interfere with the magnetic flux flowing through the bridged disk 813. The potting material also preferably has a high melting temperature, so as not to melt in use. For example, the potting material may be Torlon ® or Polyether ether ketone (PEEK).

By using a potting material, the magnet 815 is securely held in position by the polymer, reducing stress caused to the thin material of the bridged disk 813 relative to not using a potting material.

Further, in using a potting material, it is possible to secure the magnet 815 in position in a non-magnetised state. This prevents the magnetic forces of the magnet 815 disfiguring the bridged disk 813 while attempting to locate it in the magnet-retaining hole 805. Once the magnet 815 is held in position by the potting material, it can be magnetised. This is especially preferential when using thin laminates, as they are highly fragile. A method of locating a magnet 815 in a magnet slot 805 using a potting material is described below.

Of course, although potting of the magnets 815 has been discussed with reference to the embodiments disclosed in Figures 4 to 6, it is to be appreciated that this may equally apply to the other embodiments of the present disclosure.

Figures 9 to 12 show examples in which every disk 902, 1002, 1102, 1202 is a bridged disk 913, 1013, 1113, 1213. These Figures are being used to show specific concepts which may be applied to disks 902, 1002, 1102, 1202 and rotor body arrangements of any type, especially those described above. Rotor bodies 925, 1025, 1125 ,1225 in which every disk 902, 1002, 1102, 1202 is fully bridged do not form part of the present invention, and are merely shown here as a vehicle for displaying other concepts.

With reference to Figure 9, there is shown a close up view of a rotor body 925. As discussed above, the magnet-retaining holes 905 are arranged to receive a magnet 915. In use, the rotor body 925 is rotated within an electric machine at high speed. This rotation causes the magnets 915 to move, within the magnet- retaining hole 905, towards the outer diameter 904 of the bridged disks 913.

In this embodiment, in order to prevent the magnets 915 from moving, and the ensure good contact between both faces of the magnets 915 and both faces of the respective magnet-retaining holes 905, the magnet-retaining holes 905 are tapered toward the outer diameter 904 of the bridged disk 913. In other words, the circumferential width of the magnet-retaining holes 905 decreases towards the outer diameter 904. This reduction in width of the magnet-retaining holes 905 causes the magnets 915 to become wedged within the magnet-retaining holes 905.

Further, in tapering the magnet-retaining holes 905 such that they are thinner toward the outer diameter 904, the posts 906 are consequently circumferentially thicker toward the outer diameter 904. This increase in thickness of the posts 906 toward the airgap causes a reduction in magnetic flux leakage at the back iron 907, as the preferred magnetic flux path is toward the airgap. Of course, the opposite effect would occur if the rotor were situated external to the stator such that the airgap is located adjacent the inner diameter 903.

Although the tapered geometry of the magnet-retaining holes 905 is discussed with reference to bridged disks 913, it is to be understood that the magnet- retaining holes 905 of the other embodiments of the disclosure may also be tapered.

With reference to Figure 10, there is shown a sectional view of rotor body 1025. The section view of Figure 10 shows an example of magnets 1015 being located within slots 1011.

In this embodiment, once the magnets 1015 are located within the slots 1011, each of the bridged disks 1013 can be slightly misaligned, so that the slots 1011 form a generally helical shape. In performing this misalignment, each of the bridged disks 1013 will abut the magnets 1015, so as to wedge them into position. After being misaligned, the bridged disks 1013 can be permanently fixed, so as to maintain the abutment of the magnets 1015. In this manner, the magnets 1015 are more securely held in position within the slots 1011.

With reference to Figures 11 and 12, there is shown an embodiment of the disclosure in which a rotor body 1125, 1225 further comprises a press-fit cylinder 1117, 1217. The press-fit cylinder may be used in conjunction with any of the embodiments described herein.

To improve the structural rigidity of the rotor body 1125, 1225, a cylinder or rod 1118, 1218 of a non-magnetic material such as aluminium is press fit into the inner diameter 1103, 1203 of the rotor body 1125, 1225.

This cylinder 1118, 1218 is preferably thermally fit onto the inner diameter 1103, 1203 of the rotor body 1125, 1225. The cylinder 1118, 1218 can also have a flange 1119, 1219 at one end to provide axial location of the disks 1102, 1202 at one end. Figures 11 and 12 show a ring 1120, 1220 fixed to the cylinder 1118, 1218 using fasteners 1121, 1221 (in this case, bolts) to attach the ring 1120, 1220 to the cylinder 1118, 1218.

The flange 1119, 1219 of the cylinder 1118, 1218, and the ring 1120, 1220 at the other end, serve to compress the laminated disks 1102, 1202 in the axial direction and also to retain the magnets 1115, 1215 in the slots in the axial direction.

The cylinder 1118, 1218 may also be used to mount bearings or a bearing race 1122, 1222 directly or via an element attached to the cylinder, such as the ring 1120, 1220 (as also shown in Figure 12).

The flange 1119, 1219 and ring 1120, 1220 may be large enough in diameter to hold the magnets 1115, 1215 in position, preventing them from coming loose from their respective slots 1111, 1211. For example, large enough to engage a few millimetres of the bottom of the magnets 1115, 1215.

Another way to reduce the leakage across the bridges 1109, 1209 is to fully saturate them and preferably also the end of the posts 1106, 1206 nearest the outer diameter 1104, 1204.

The greater the radial length of the magnets 1115, 1215, the more saturated the posts 1106, 1206 will be at the airgap (for a given thickness of bridge 1109, 1209). For this reason it is beneficial to use magnets 1115, 1215 which have an aspect ratio of radial length to circumferential thickness of 2: 1 or greater, 2.5: 1 or greater, 3: 1 or greater, 3.5: 1 or greater, 4: 1 or greater, 5: 1 or greater.

Longer magnet-retaining holes 1105, 1205 also enable the use of longer, more powerful magnets 1115, 1215 which may saturate the back iron 1107, 1207 and restrict flux through it. Simulation results indicated that, in certain arrangements, magnet-retaining holes 1105, 1205 featuring a magnet 1115, 1215 aspect ratio greater than 2 : 1 provided a reduction in flux leakage similar to the level of reduction in magnetic flux leakage provided by placing circular flux resistors 1108, 1208 in the back iron 1107, 1207. The embodiments discussed above, with reference to Figures 1 to 12, are described with reference to an electric machine in which the rotor 101 is arranged to be located radially inside a stator, i.e., closer to the axis of rotation. However, in an alternative embodiment, shown in Figures 13 and 14, the rotor 1301, 1401 is arranged to be located radially outside the stator, such that the stator will be closer to the axis of rotation of the rotor 1301, 1401.

The embodiment of Figures 13 and 14 is largely the same as those described above. However, in contrast to the above, the back iron 1307, 1407 of the rotor 1301, 1401 of this embodiment is located adjacent to the outer diameter 1304, 1404. Further, the bridges 1309, 1409 of this embodiment are located adjacent to the inner diameter 1303, 1403. This is because, in this embodiment, the stator is arranged to be located adjacent to the inner diameter 1303, 1403 of the rotor 1301, 1401. Therefore, in order to minimise flux leakage in regions close to the stator, the bridges 1309, 1409 and openings 1310, 1410 are adjacent to the stator.

It is to be noted that the preferred and alternative features listed above with respect to the embodiments of Figures 1 to 12 apply equally to the embodiment of Figures 13 and 14.

For example, although Figures 13 and 14 show a staggered arrangement of disks 1302, 1402, akin to the embodiments of Figures 1 to 3, it is clear that unbridged disks 1312, 1412 and bridged disks 1313, 1413, as in Figures 4 to 6, may also be used in combination with an outer rotor.

Figures 15 and 16 show a further embodiment of a rotor body 1501, 1601. In this embodiment, some of the flux resistors 1508, 1608 may be replaced with an opening 1510, 1610 between a respective magnet-receiving hole 1505, 1605 and the inner edge of the rotor (or the outer edge in embodiments in which the airgap is provided at the inner edge). In this example, the flux resistors and the openings are formed in an alternating arrangement, from one magnet-receiving hole to the next. As such, the back iron is not a continuous part of the disk, but is instead interrupted by the openings. The laminated disks may then be staggered in such a way that an opening 1510, 1610 in the back iron 1507, 1607 of one layer is adjacent to a closed/circular flux resistor 1506, 1608 of an adjacent layer, as shown in Figure 16. By staggering the laminated disks 1502, 1602 in this way, adjacent layers can provide structural support to each other where openings are formed. In the example shown in Figures 15 and 16, the openings at the outer edge are also formed in an alternating arrangement with the bridges. An opening at the outer edge is aligned with a flux resistor at the inner edge. Furthermore, openings at the inner edge are offset from openings at the outer edge.

In the embodiment shown in Figures 15 and 16, the magnet-retaining holes 1505, 1605 which comprise an opening 1510, 1610 at, for example, the outer diameter 1504, 1604, do not comprise an opening 1510, 1610 at the inner diameter 1503, 1603. As such, each laminated disk 1502, 1602 is formed as a signal integral disk, which may be made from a single sheet of material.

Figures 15 and 16 show an embodiment in a 1 : 1 ratio of openings 1510, 1610 to circular flux resistors 1508, 1608 is used. However, it is to be understood that this is merely an example. For example, the flux resistors may be formed such that three adjacent magnet-retaining holes include flux resistors, followed by a single opening, with that pattern being repeated. This is a 3 : 1 resistor to opening ratio. Other resistor-to-opening ratios may be used, such as 2: 1, 1 :2 and 1 :3.

In an exemplary embodiment, relating to Figures 1 to 3 and Figure 21, manufacturing of the rotor can be accomplished as follows.

In step 2101, individual disks 102 are formed from a larger piece of material by means of at least one of: machining, laser cutting, punching, fine blanking or etching. The disks 102 may initially nor be circular or annular but may start as a sheet of arbitrary shape and machined at some stage of the process to be disks or annular disks. The disks 102 are then coated with an adhesive. Each of the disks 102 are then coaxially aligned such that they can be formed into a rotor body 101. Once the disks 102 are coaxially aligned, in step 2102, they are rotated such that they form the staggered arrangement described above with reference to Figures 1 to 3. In this manner, it is preferable that the rotor body 101 has a consistent number of bridges 109 on every slot 111, with no two bridges 109 in the same slot 111 being formed from adjacent disks 102.

After being rotationally staggered, in step 2103, the disks 102 are pressed together to form the rotor body 101. In a preferred embodiment, the disks 102 are heat activated, such that the rotor body 101 becomes a single part, with each post 106 of each disk 102 being structurally supported by the back iron 107 and the bridges 109.

Once formed, the rotor body 101 is secured to a hub/shaft (for an inner rotor) or the inner diameter of a cylinder (for an outer rotor). The rotor 101 is then set into position with respect to the stator of the electric machine.

As a final step, step 2104, the magnets 115 are inserted axially into the slots 111 of the rotor 101. The magnets 115 are arranged such that the north pole of each magnet 115 faces the north pole of an adjacent magnet 115, and the south pole of each magnet 115 faces the south pole of an adjacent magnet 115.

In one embodiment, when the magnets 115 are inserted into the rotor 101, they are already magnetised. However, in a preferred embodiment, the magnets 115 are magnetised in situ, i.e., once they are located within the rotor 101. In this manner, the magnetic field of each of the magnets 115 does not cause damage to the rotor 101 by interacting with unintended portions of it. In this preferred embodiment, once the demagnetised magnets 115 are located within the rotor 101, they are heated to their Curie temperature and then subjected to a magnetic field in order to magnetise them.

As discussed above, the magnets 115 can be held within the slots 111 by means of the bridges 109 and/or by providing a tapered edge to each magnet-retaining hole 105. In a preferred embodiment, the magnets 115 are further held in position by means of a potting material. The potting material is preferably a material with a high melting temperature, such that it does not melt in use. For example, the potting material may be Torlon ® or Polyether ether ketone (PEEK).

In embodiments in which the laminated disks 102 are staggered, such that flux resistors 108 at the inner diameter 103 or the outer diameter 104 are adjacent to openings 110, the potting material may also be used to further secure adjacent laminated disks 102 to each other. In this manner, because the openings 110 are larger than the flux resistors 108, the potting material, in a molten state, is able to fill the openings 110 and abut the back iron 107 around an adjacent flux resistor 108. In forming the potting material in this manner, it acts as a kind of 'rivet', preventing laminated disks 102 of the rotor 101 from moving axially away from each other. In order to act as a 'rivet', the potting material needs to form a continuous rod through any number of flux resistors 108, as long as the rod has wider portions at each end defined by openings 110. Alternatively, a material other than the potting material may be used to hold the laminated disks 102 together through the flux resistors 108.

In the preferred method of forming a rotor 101, the magnets 115 are located within the slots 111 of the rotor 101 in a demagnetised state, as discussed above. Before the magnets 115 are magnetised, the potting material is injected into the slots 111 to backfill them. The potting material is then allowed to cool, holding the magnets 115 in position within the slots 111. After the potting material has cooled sufficiently, the magnets 115 are heated to their Curie temperature and subjected to a magnetic field in order to magnetise them. By preparing the rotor 101 in this way, the magnets 115 are much easier to work with, as the rotor 101 is provided with further structural integrity before the magnetic fields of the magnets 115 are introduced.

In an alternative embodiment, the magnets 115 can be magnetised as the potting material is injected into the slots 111. In this manner, the heat from the potting material can be used to bring the magnets 115 to the Curie temperature.

In a further embodiment, after the magnets 115 have been located within the slots 111 and held in place with a potting material, the bridges 109 of each disk 102 are mechanically thinned. In this manner, the structural integrity provided by the potting material can be used to offset the structural integrity lost when thinning the bridges 109. In this manner, flux leakage at the bridges 109 is reduced (compared to non-thinned bridges 109) due to the decreased saturation point of the thinner bridges 109. In another embodiment, the bridges 109 are completely removed to form openings 110, after the magnets 115 are secured in place. This may be performed using a mechanical tool or laser etc.

The methods of forming a rotor, as set out above, are discussed with respect to the embodiment of Figures 1 to 3. However, it is clear that the methods may equally apply to the embodiment of Figures 4 to 6, by instead forming two types of disk 412, 413 and arranging them in the manner described above.

The method steps described above are arranged in an exemplary order of performance. However, it is to be understood that the method steps may be performed in a multitude of different orders depending on the requirements. For example, the magnets 115 may be inserted into the slots 111 at any stage, once the rotor body 101 has been formed by stacking the disks 102. Similarly, the disks 102 may be coated with an adhesive after they have been coaxially aligned.

In a further embodiment, shown in Figures 17a to 19c and 22, a method of manufacturing a rotor comprises the following steps:

1. Punching or otherwise forming the laminated disks 1702, 1802, 1902 to form fully closed slots 1711, 1811, 1911 (step 2201);

2. Stacking (step 2202) the laminated disks 1702, 1802, 1902 together to form the rotor body;

3. Permanently or temporarily securing (step 2203) the laminated disks 1702, 1802, 1902 together;

4. Inserting non-magnetised magnets 1715, 1815, 1915 (step 2204) (also referred to as magnetisable elements 1715, 1815, 1915); 5. Inserting potting material to hold the magnets 1715, 1815, 1915 in place (step 2205);

6. Mechanically removing at least a portion of the laminated disks 1702, 1802, 1902 (and unrequired potting material) in order to reduce or remove the bridges 1709, 1809, 1909, either by cutting openings 1710, 1810, 1910 into the bridges 1709, 1809, 1909 or by removing material in order to reduce the inner diameter or the outer diameter of the laminated disks 1702, 1802, 1902 (step 2206); and

7. Heating the magnets 1715, 1815, 1915 to their Curie temperature and magnetising them with a high strength magnetic field pulse (step 2207).

The step of mechanically removing the material around the laminated disks 1702, 1802, 1902 may be performed using a lathe or a mill or any other kind of mechanical material removing means. Using a lathe or a mill results in a high material removal rate as well as producing a precise airgap due to the diameter of the rotor being concentric with the rotor shaft or centre axis.

The fully closed slots 1711, 1811, 1911, punched into the laminated disks 1702, 1802, 1902, may be formed such that they have a notch, adjacent the bridges 1709, 1809, 1909 at the inner or outer diameter. In this manner, when removing material from the disks 1702, 1802, 1902, the notches facilitate the formation of the openings 1710, 1810, 1910, as it requires the removal of less material from the rotor 1701, 1801, 1901.

In accordance with the embodiments described above, and as shown in Figure 20, the removal of the material around the laminated disks 2002 may be performed such that openings 2010 are formed on alternate slots, with openings being formed in slots which are not adjacent each other. The slots may be formed in every other slot, every third slot, every fourth slot etc., as described above with respect to earlier embodiments. Embodiments of the present disclosure generally focus on high pole-count rotors. However, it is to be understood that the disclosure can also be applied to rotors with lower pole-counts.

Embodiments of the present invention comprise a design geometry that is equally applicable to electric machines of any size such as, for example, small motors for use in robotics environments or very large generators for use in domestic electricity generation. In one embodiment, the rotor body 101 has a diameter of between 200mm and 300mm. In a preferred embodiment, the rotor body 101 has a diameter of 250mm.

Further, embodiments of the present disclosure discuss the device in terms of a rotor 101 located radially inside a stator, such that the outer diameter of the rotor 101 is adjacent to an airgap between the rotor 101 and the stator. However, it is to be understood that this is merely exemplary and that, equally, the rotor 101 may be radially outside of the stator, such that the airgap is located adjacent to the inner diameter of the rotor 101. For an outer rotor, features which are defined with reference to the outer diameter below will instead relate to the inner diameter and features which are defined with reference to the inner diameter will instead relate to the outer diameter.

In the above described embodiments, the rotor is described in connection with laminated disks. The laminated disks may also be referred to as laminate sheets.

A series of numbered clauses will now be defined. These clauses may be combined in the manner defined, or in any reasonable combination, as would be clear to the skilled person.

1. A rotor for an electric machine, comprising :

a plurality of laminate sheets arranged in a stacked configuration to form a rotor body; wherein

each laminate sheet defines an inner diameter and an outer diameter, and has a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters; the laminate sheets are further arranged such that the magnet- retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet;

each laminate sheet has a back iron, formed around an edge of the laminate; and

the plurality of laminate sheets are configured such that, between each slot in the rotor body and the inner or outer diameter, at least one of the laminate sheets includes an opening between a respective magnet-retaining hole and the inner or outer diameter and at least one of the laminate sheets includes a bridge between a respective magnet-retaining hole and the inner or outer diameter, such that one or more openings are formed between each slot and the inner or outer diameter.

2. The rotor of clause 1, wherein the back iron is formed around the inner diameter or outer diameter of each laminate sheet.

3. The rotor of clause 2, wherein each laminate sheet further comprises a plurality of posts, located between each magnet-retaining hole, and the back iron is connected to each post.

4. The rotor of any preceding clause, wherein the plurality of laminate sheets are further arranged such that each bridge of each laminate sheet is adjacent to an opening of an adjacent laminate sheet.

5. The rotor of any preceding clause, wherein at the outer diameter or inner diameter of each slot, a plurality of openings and bridges a reformed, and the openings and bridges are formed in an alternating configuration.

6. The rotor of clause 5, wherein for each slot, there are more laminate sheets having openings than laminate sheets having bridges.

7. The rotor of clause 6, wherein for each slot, every fourth laminate sheet includes a bridge. 8. The rotor of any preceding clause, wherein the bridges are thinner than the back iron.

9. The rotor of clause 8, wherein the bridges to back iron thickness has a ratio of at least 1 : 1.5.

10. The rotor of any preceding clause, wherein the rotor is a high pole- count rotor.

11. The rotor of any preceding clause, wherein each laminate sheet is formed from a unitary piece of material.

12. The rotor of any preceding clause, wherein the rotor has a diameter of between 200mm and 300mm.

13. The rotor of any preceding clause, wherein each laminate sheet is the shape of a ring, defined by the inner and outer diameters.

14. The rotor of any preceding clause, wherein each of the laminate sheets further comprise a plurality of flux resistors, positioned in the back iron, the flux resistors restricting the flow of magnetic flux through the back iron.

15. The rotor of clause 14, wherein the plurality of flux resistors are located between each magnet-retaining hole and the inner or outer diameter.

16. The rotor of any preceding clause, wherein each bridge is substantially straight.

17. The rotor of any preceding clause, wherein each of the laminate sheets further comprise a plurality of dowel holes, arranged to receive a fastening means, wherein the dowel holes are preferably located adjacent to the outer diameter or inner diameter. 18. The rotor of any preceding clause, wherein the magnet-retaining holes of each of the laminate sheets are tapered, such that they are

circumferentially thinner adjacent to the outer diameter.

19. The rotor of any preceding clause, wherein the magnet-retaining holes of each of the laminate sheets have chamfered inner corners.

20. The rotor of any preceding clause, wherein each sheet is configured such that a plurality of the magnet-retaining holes include bridges at the outer diameter or inner diameter, and a plurality of the magnet-retaining holes include openings at the outer diameter or inner diameter.

21. The rotor of clause 20, wherein each sheet is configured such that the bridges are evenly distributed around the outer diameter, and at least two magnet-retaining holes with openings are positioned between two magnet- retaining holes having bridges.

22. The rotor of clause 21, in which the laminate sheets are configured such that adjacent sheets are staggered, in a circumferential direction, such that bridges of two adjacent sheets are located across two circumferentially adjacent magnet-retaining holes.

23. The rotor of clauses 20 to 22, wherein the arrangement of bridges and openings is the same on every sheet.

24. The rotor of any preceding clause, wherein at least one of the laminate sheets has bridges across all of the magnet-retaining holes, and at least one of the laminate sheets has openings at the outer diameter or inner diameter of each magnet-retaining hole.

25. The rotor of clause 24, wherein there are a plurality of sheets including bridges across all magnet-retaining holes, and a plurality of sheets having opening at the outer diameter or inner diameter of each magnet-retaining hole, and the sheets having bridges are evenly distributed amongst the sheets having openings. 26. The rotor of any preceding clause, further comprising a plurality of magnets, each magnet provided within a respective magnet-retaining hole.

27. The rotor of clause 26, wherein the magnets are circumferentially polarised.

28. The rotor of clause 27, wherein each magnet is polarised in the opposite direction, circumferentially, to an adjacent magnet.

29. The rotor of clause 28, wherein each laminate sheet further comprises a plurality of posts, between each magnet-retaining hole, and the magnets adjacent each post have the same pole facing a respective post.

30. A rotor for an electric machine, comprising :

each laminate sheet defines an inner diameter and an outer diameter, and has a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters;

the laminate sheets are further arranged such that the magnet- retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a permanent magnet;

for each laminate sheet, openings are formed between one or more of the plurality of magnet-retaining holes and the inner or outer diameter, and bridges are formed between the remaining magnet-retaining holes and the inner or outer diameter, such that each laminate sheet comprises one or more openings and one or more bridges at the inner or outer diameter;

the laminate sheets are further arranged such that they are

circumferentially staggered, such that each slot has one or more openings and one or more bridges at the inner or outer diameter.

31. The rotor of clause 30, wherein the openings and bridges are arranged such that, in a circumferential direction, one or more openings is formed in an alternating arrangement with one or more bridges. 32. The rotor of clause 31, wherein the alternating arrangement is such that, in a circumferential direction, one bridge is alternated with a plurality of openings.

33. The rotor of clause 31, wherein the plurality of laminate sheets are arranged such each bridge of each laminate sheet is adjacent to an opening of an adjacent laminate sheet.

34. The rotor of clauses 30 to 33, wherein each laminate sheet further comprises a plurality of posts, formed between each magnet-retaining hole, and a back iron, formed around the diameter opposing the diameter in which the openings and bridges are formed, the back iron forming a connection between respective adjacent posts.

35. The rotor of clause 34, wherein each of the laminate sheets further comprise a plurality of flux resistors, in the back iron, the flux resistors restricting the flow of magnetic flux through the back iron.

36. The rotor of clause 35, wherein the plurality of flux resistors are located between each magnet-retaining holes and the respective diameter.

37. The rotor of any of clauses 30 to 36, wherein each bridge is

substantially straight.

38. The rotor of any of clauses 30 to 37, wherein each of the laminate sheets further comprise a plurality of dowel holes, arranged to receive a fastening means, wherein the dowel holes are preferably located adjacent to the outer diameter or inner diameter.

39. The rotor of any of clauses 30 to 38, wherein the magnet-retaining holes of each of the laminate sheets are tapered, such that they are circumferentially thinner adjacent to the outer diameter or inner diameter. 40. The rotor of any of clauses 30 to 39, wherein the magnet-retaining holes of each of the laminate sheets have chamfered inner corners.

41. The rotor of any of clauses 30 to 40, in which each laminate sheet is in the shape of a ring.

42. The rotor of any of clauses 30 to 41, wherein the arrangement of openings and bridges for each laminate sheet is identical.

43. The rotor of any of clauses 30 to 42, wherein each of the plurality of laminate sheets is formed from a unitary piece of material.

44. The rotor of any of clauses 30 to 33, wherein for each laminate sheet, the openings and bridges are formed at the inner and outer diameters.

45. The rotor of clause 44, wherein the openings and bridges are formed in an alternating arrangement, at both the inner and outer diameters.

46. The rotor of clause 45, wherein a magnet-retaining hole having an opening at either the inner diameter or the outer diameter, may not also have an opening at the opposing diameter.

47. An electric machine comprising a rotor as defined in any preceding clause.

48. An electric machine of clause 47, further comprising a stator and a plurality of magnets located in each magnet-retaining hole of the rotor.

49. An electric machine according to clause 48, wherein the magnets are circumferentially polarised and each magnet is polarised in the opposite direction, circumferentially, to an adjacent magnet.

50. An electric machine according to clause 49, wherein the flux generated by the magnets is directed radially towards the stator. 51. A method of manufacturing a rotor for an electric machine, comprising the steps of:

forming a plurality of laminate sheets, each laminate sheet defining an inner diameter and an outer diameter, a back iron adjacent to the inner diameter or the outer diameter, and having a plurality of magnet-retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters;

stacking the plurality of laminate sheets together to form a rotor body, such that the magnet-retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet; fixing the plurality of laminate sheets to each other; and

locating magnets within the slots,

wherein, the plurality of laminate sheets are configured such that, between each slot in the rotor body and the inner or outer diameter, at least one of the laminate sheets includes an opening between a respective magnet- retaining hole and the inner or outer diameter and at least one of the laminate sheets includes a bridge between a respective magnet-retaining hole and the inner or outer diameter, such that one or more openings are formed between each slot and the inner or outer diameter.

52. A method of manufacturing a rotor for an electric machine, comprising the steps of:

forming a plurality of laminate sheets, each laminate sheet defining an inner diameter and an outer diameter, and having a plurality of magnet- retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters;

locating magnets within the slots,

wherein, for each laminate sheet, openings are formed between one or more of the plurality of magnet-retaining holes and the inner or outer diameter, and bridges are formed between the remaining magnet-retaining holes and the inner or outer diameter, such that each laminate sheet comprises one or more openings and one or more bridges at the inner or outer diameter;

the laminate sheets are further arranged such that they are

53. A method of manufacturing a rotor for an electric machine, comprising the steps of:

forming a plurality of laminate disks from one or more laminate sheets, each disc having a central opening and an inner and outer diameter, and each disc further having a plurality of magnet-retaining holes arranged around its periphery between the inner and outer diameters;

stacking the plurality of laminate disks together to form a rotor body, such that the magnet-retaining holes are aligned to form slots in the rotor body;

and

removing material from the inner and/or the outer diameter of the rotor in order form openings between at least some of the slots and the inner diameter and/or outer diameter.

54. The method of clause 53, wherein the step of forming further comprises forming notches in at least some of the magnet-retaining holes, the notches formed between the holes and the inner diameter and/or outer diameter.

55. The method of clause 54, wherein the openings formed by the removal of material are formed in the locations of the notches.

56. The method of any of clauses 53 to 55, wherein the step of forming the plurality of laminate discs is performed by punching.

57. The method of any of clauses 53 to 56, wherein the openings in the rotor restrict magnetic flux flow.

58. The method of any of clauses 53 to 57, further comprising : locating a plurality of magnetisable elements within respective slots of the plurality of slots;

securing the magnetisable elements within their respective slots by applying a potting material, the potting material having a melting temperature which is above a desired operational temperature of the rotor.

59. The method of clause 54, wherein the magnetisable elements are magnetised after being located within the slots. 60. The method of clauses 58 or 59, wherein securing the magnetisable elements comprises over-moulding a plastic material onto the laminate disks.

61. The method of any of clauses 53 to 60, wherein removing the material comprises turning the rotor on a lathe, or removing material using a laser.

Claims

1. A rotor for an electric machine, comprising :

the laminate sheets are further arranged such that the magnet- retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet;

each laminate sheet has a back iron, formed around an edge of the laminate; and

2. The rotor of claim 1, wherein the back iron is formed around the inner diameter or outer diameter of each laminate sheet.

3. The rotor of claim 2, wherein each laminate sheet further comprises a plurality of posts, located between each magnet-retaining hole, and the back iron is connected to each post.

4. The rotor of any preceding claim, wherein the plurality of laminate sheets are further arranged such that each bridge of each laminate sheet is adjacent to an opening of an adjacent laminate sheet.

5. The rotor of any preceding claim, wherein at the outer diameter or inner diameter of each slot, a plurality of openings and bridges a reformed, and the openings and bridges are formed in an alternating configuration; and optionally wherein for each slot, there are more laminate sheets having openings than laminate sheets having bridges.

6. The rotor of any preceding claim, wherein the bridges are thinner than the back iron, and the bridges to back iron thickness has a ratio of at least 1 : 1.5.

7. The rotor of any preceding claim, wherein the rotor is a high pole-count rotor, the pole count optionally being at least 56, 60, 64 or 86.

8. The rotor of any preceding claim, wherein the rotor has a diameter of between 200mm and 300mm.

9. The rotor of any preceding claim, wherein each laminate sheet is the shape of a ring, defined by the inner and outer diameters.

10. The rotor of any preceding claim, wherein each of the laminate sheets further comprise a plurality of flux resistors, positioned in the back iron, the flux resistors restricting the flow of magnetic flux through the back iron, and the plurality of flux resistors are located between each magnet-retaining hole and the inner or outer diameter.

11. The rotor of any preceding claim, wherein each of the laminate sheets further comprise a plurality of dowel holes, arranged to receive a fastening means, wherein the dowel holes are preferably located adjacent to the outer diameter or inner diameter.

12. The rotor of any preceding claim, wherein the magnet-retaining holes of each of the laminate sheets are tapered, such that they are circumferentially thinner adjacent to the outer diameter.

13. The rotor of any preceding claim, wherein each sheet is configured such that a plurality of the magnet-retaining holes include bridges at the outer diameter or inner diameter, and a plurality of the magnet-retaining holes include openings at the outer diameter or inner diameter, and optionally wherein each sheet is configured such that the bridges are evenly distributed around the outer diameter, and at least two magnet-retaining holes with openings are positioned between two magnet-retaining holes having bridges.

14. The rotor of claim 13, in which the laminate sheets are configured such that adjacent sheets are staggered, in a circumferential direction, such that bridges of two adjacent sheets are located across two circumferentially adjacent magnet-retaining holes.

15. The rotor of any preceding claim, wherein at least one of the laminate sheets has bridges across all of the magnet-retaining holes, and at least one of the laminate sheets has openings at the outer diameter or inner diameter of each magnet-retaining hole.

16. The rotor of claim 15, wherein there are a plurality of sheets including bridges across all magnet-retaining holes, and a plurality of sheets having opening at the outer diameter or inner diameter of each magnet-retaining hole, and the sheets having bridges are evenly distributed amongst the sheets having openings.

17. The rotor of any preceding claim, further comprising a plurality of circumferentially polarised magnets, each magnet provided within a respective magnet-retaining hole, and optionally wherein each magnet is polarised in the opposite direction, circumferentially, to an adjacent magnet.

18. A rotor for an electric machine, comprising :

the laminate sheets are further arranged such that the magnet- retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a permanent magnet; for each laminate sheet, openings are formed between one or more of the plurality of magnet-retaining holes and the inner or outer diameter, and bridges are formed between the remaining magnet-retaining holes and the inner or outer diameter, such that each laminate sheet comprises one or more openings and one or more bridges at the inner or outer diameter;

the laminate sheets are further arranged such that they are

19. The rotor of claim 18, wherein the openings and bridges are arranged such that, in a circumferential direction, one or more openings is formed in an alternating arrangement with one or more bridges.

20. The rotor of claim 19, wherein the alternating arrangement is such that, in a circumferential direction, one bridge is alternated with a plurality of openings.

21. The rotor of claims 18 to 20, wherein each laminate sheet further comprises a plurality of posts, formed between each magnet-retaining hole, and a back iron, formed around the diameter opposing the diameter in which the openings and bridges are formed, the back iron forming a connection between respective adjacent posts.

22. The rotor of any of claims 18 to 21, in which each laminate sheet is in the shape of a ring.

23. The rotor of any of claims 18 to 20, wherein for each laminate sheet, the openings and bridges are formed at the inner and outer diameters, and optionally wherein the openings and bridges are formed in an alternating arrangement, at both the inner and outer diameters.

24. The rotor of claim 23, wherein a magnet-retaining hole having an opening at either the inner diameter or the outer diameter, may not also have an opening at the opposing diameter.

25. An electric machine comprising :

a rotor as claimed in any preceding claim;

a stator; and

a plurality of magnets located in each magnet-retaining hole of the rotor.

26. An electric machine according to claim 25, wherein the flux generated by the magnets is directed radially towards the stator.

27. A method of manufacturing a rotor for an electric machine, comprising the steps of:

locating magnets within the slots,

28. A method of manufacturing a rotor for an electric machine, comprising the steps of:

forming a plurality of laminate sheets, each laminate sheet defining an inner diameter and an outer diameter, and having a plurality of magnet- retaining holes, arranged circumferentially around the laminate sheet, between the inner and outer diameters; stacking the plurality of laminate sheets together to form a rotor body, such that the magnet-retaining holes are substantially aligned to form a plurality of slots in the rotor body, each slot configured to receive a magnet; fixing the plurality of laminate sheets to each other; and

locating magnets within the slots,

the laminate sheets are further arranged such that they are

29. A method of manufacturing a rotor for an electric machine, comprising the steps of:

stacking the plurality of laminate disks together to form a rotor body, such that the magnet-retaining holes are aligned to form slots in the rotor body; and

30. The method of claim 29, wherein the step of forming further comprises forming notches in at least some of the magnet-retaining holes, the notches formed between the holes and the inner diameter and/or outer diameter, and optionally wherein the openings formed by the removal of material are formed in the locations of the notches.