A permanent magnet rotor for an electric machine

Abstract

A rotor for an electric machine that is formed of a shaft onto which are mounted a plurality of permanent magnets. The shaft has one or more axial grooves formed in the radial outer surface of the shaft. The grooves extend longitudinally along a least a partial longitudinal length of the shaft.

Description

[0001] A Rotor For An Electric Machine

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a rotor for an electric machine, and for an electric machine comprising the rotor.

[0004] BACKGROUND OF THE INVENTION

[0005] Known electric machines, for example brushless motors and generators, comprise a rotor having permanent magnets mounted to a shaft that is arranged concentrically within a stator. The stator comprises a plurality of stator pole teeth arranged circumferentially about an axis of rotation of the machine, where each stator pole tooth has a coil wound there around to form a stator coil stack. Each of the stator coil stacks produce a magnetic field when a current is passed there through to interact with the permanent magnets of the rotor to cause the rotor to rotate about its axis.

[0006] In known electric machines, the shaft of the rotor is comprised of steel, and sometimes surrounded by an iron core. During use, the magnetic fields produced by the stator coil stacks interacts not only with the permanent magnets on the surface of the rotor (to drive the rotor around its axis) but also the steel central core of the rotor. A consequence of this interaction is the production of eddy currents within the shaft of the rotor, which causes the shaft of the rotor to heat and which degrades the performance of the machine.

[0007] We have therefore appreciated the need for an improved rotor for an electric machine.

[0008] SUMMARY OF THE INVENTION

[0009] The present invention provides a rotor for an electric machine, a method of manufacturing a rotor for an electric machine and an electric machine according to the independent claims appended hereto. Further advantageous embodiments are provided in the dependent claims, also appended hereto.

[0010] We describe a rotor for an electric machine, the rotor comprising: a shaft having a diameter, an axial length and an axis of rotation extending along the axial length; and a plurality of permanent magnets mounted to a radial outer surface of the shaft, wherein the shaft comprises one or more axial grooves formed in the radial outer surface of the shaft, the one or more axial grooves comprising one or more grooves extending longitudinally along a least a partial longitudinal length of the shaft. Such an arrangement advantageously reduces eddy currents that would ordinarily be produced in the shaft of the rotor when used in such an electric machine.

[0011] At least one of the one or more axial grooves formed in the radial outer surface of the shaft may be located in an area of the shaft that is between two respective adjacent permanent magnets.

[0012] At least one of the one or more axial grooves formed in the radial outer surface of the shaft may be located in an area of the shaft that is between two opposing magnetic poles of respective adjacent permanent magnets. For example, there may be 4 axial grooves, or 8 axial grooves. Other configurations are possible.

[0013] The rotor may also comprise one or more circumferential grooves formed in the radial outer surface of the shaft that extend at least partially circumferentially around the radial outer surface of the shaft. At least one of the one or more circumferential grooves extends fully circumferentially around the outer surface of the shaft.

[0014] When there are two or more circumferential grooves, the circumferential grooves are separated by a longitudinal gap. For example, when there are three circumferential grooves, each are separated by a longitudinal gap. Each of the longitudinal gaps may be substantially the same, or each of the longitudinal gaps may be different from each other.

[0015] One or more of the circumferential grooves may be located in an area of the shaft that is between two respective adjacent permanent magnets. One or more of the circumferential grooves may be located in an area of the shaft that is between two axial ends of a respective permanent magnet.

[0016] A groove in the outer surface of the shaft may have a depth that is proportional to the diameter of the shaft. The depth of the groove may be between 1% and 20% of the diameter of the shaft, preferably between 5% and 13% of the diameter of the shaft. A groove may be formed between 1 mm and 2mm into the outer surface of the shaft, preferably 1.8mm.

[0017] At least one of the one or more grooves may be filled with a different material to the material of the shaft. The material in the at least one of the one or more grooves may be an adhesive.

[0018] The rotor may also comprise a retention sleeve radially outwards of the permanent magnets for surrounding and retaining the permanent magnets on the surface of the shaft. The retention sleeve may be formed of carbon fibre.

[0019] The rotor may also comprise a first end ring mounted to the outer surface of the shaft adjacent a first axial end of the permanent magnets, and a second end ring mounted to the outer surface of the shaft adjacent a second axial end of the permanent magnets. The first and second end rings may have a radial thickness that is substantially the same as the radial thickness of the permanent magnets. The first and second end rings may be adhered to the outer surface of the shaft.

[0020] In any of the above, the permanent magnets may be adhered to the shaft.

[0021] We also discuss a method of manufacturing a rotor for an electric machine, the rotor comprising a shaft having a diameter, an axial length and an axis of rotation extending along the axial length, and a plurality of permanent magnets, the method comprising: forming one or more axial grooves in the outer surface of the shaft and which extends longitudinally along a least a partial longitudinal length of the shaft; and mounting the plurality of permanent magnets to a radial outer surface of the shaft. Such an arrangement advantageously reduces eddy currents that would ordinarily be produced in the shaft of the rotor when used in such an electric machine. Mounting the plurality of magnets to the radial outer surface of the shaft may comprise: mounting a first end ring on a first position on the shaft; mounting the permanent magnets on the shaft such that a first end of the magnets abuts the first end ring; mounting a second end ring at a second end of the permanent magnets opposing the first end of the permanent magnets such that the second end of the permanent magnets abuts the second end ring.

[0022] The method may further comprise grinding down a radially outer surface of the magnets and the first and second end rings such that they have the same radial thickness as each other. Mounting the first and second end rings may comprise adhering the respective first and second end rings to the radial outer surface of the shaft.

[0023] Mounting the permanent magnets on the shaft may comprise adhering the permanent magnets to the radial outer surface of the shaft.

[0024] The method may also comprise providing a retention sleeve on an outer radial surface of the permanent magnets, the retaining sleeve for retaining the magnets in place on the surface of the shaft. The retention sleeve may be formed of carbon fibre.

[0025] The method may also comprise balancing the rotor.

[0026] The method may further comprise magnetising the permanent magnets after the magnets are mounted to the shaft.

[0027] In any of the above methods, at least one of the one or more axial grooves formed in the radial outer surface of the shaft may be located in an area of the shaft that is between two respective permanent magnets.

[0028] At least one of the one or more axial grooves formed in the outer surface of the shaft may be located in an area of the shaft that is between two opposing magnetic poles of respective adjacent permanent magnets. Preferably there may be 4 axial grooves, or there may be 8 axial grooves. However, there other numbers of grooves may be possible. The method may comprise forming one or more circumferential grooves that extend at least partially circumferentially around the outer surface of the shaft. At least one of the one or more circumferential grooves extends fully circumferentially around the outer surface of the shaft.

[0029] When there are two or more circumferential grooves, the circumferential grooves may be separated by a longitudinal gap. When there are three circumferential grooves, each may be separated by a longitudinal gap. Each of the longitudinal gaps may be substantially the same, or they may be different.

[0030] One or more of the circumferential grooves may be located in an area of the shaft that is between two respective adjacent permanent magnets. One or more of the circumferential grooves may be located in an area of the shaft that is between two axial ends of a respective permanent magnet.

[0031] A groove in the outer surface of the shaft may have a depth that is proportional to the diameter of the shaft. The depth of the groove may be between 1% and 20% of the diameter of the shaft, preferably between 5% and 13% of the diameter of the shaft. For example, a groove may be formed between 1mm and 2mm into the outer surface of the shaft, preferably 1.8mm.

[0032] At least one of the one or more grooves may be filled with a different material to the material of the shaft. The material in the at least one of the one or more grooves may be an adhesive.

[0033] We also discuss an electric machine, comprising: a rotor as discussed above; and a stator, comprising a plurality of stator pole teeth arranged circumferentially about an axis of rotation of the machine to define a gap about the axis of rotation of the machine, each stator pole tooth having a coil wound there around to form a stator coil stack, the coil stacks for producing a magnetic field when a current is passed there through to interact with the permanent magnets of the rotor, wherein the rotor is arranged concentrically within the gap.

[0034] The electric machine may be a motor a motor or a generator. The electric machine may be coupled to a turbo machine, a turbo charger, or a turbo charger for a fuel cell. LIST OF FIGURES

[0035] The present invention will now be described, by way of example only, and with reference to the accompanying figures, in which:

[0036] Figure 1 shows a simplified shaft of a rotor;

[0037] Figure 2 shows an axial cut-through view of the rotor also having the permanent magnets mounted to the shaft;

[0038] Figure 3 shows a longitudinal cut-through view of the rotor; and

[0039] Figure 4 shows a cut-through view of a simplified electric machine.

[0040] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] In brief, the present invention provides a rotor for an electric machine that is formed of a shaft onto which are mounted a plurality of permanent magnets. The shaft has one or more axial grooves formed in the radial outer surface of the shaft. The grooves extend longitudinally along a least a partial longitudinal length of the shaft. Such an arrangement advantageously reduces eddy currents that would ordinarily be produced in the shaft of the rotor when used in such an electric machine.

[0042] Figures 1 to 3 show a rotor according to the present invention. Figure 1 shows a simplified shaft of a rotor. Figure 2 shows an axial cut-through view of the rotor also having the permanent magnets mounted to the shaft. Figure shows a longitudinal cut- through view of the rotor.

[0043] With reference to Figure 1 to 3, the shaft 10 may comprise a steel core that has a diameter, an axial length and an axis of rotation 100 that extends along the axial length of the shaft. A plurality of permanent magnets 40 are mounted to the radial outer surface of the shaft 10. In some arrangements, a retaining sleeve 50 is disposed radially outwards of the permanent magnets 40; the retaining sleeve 50 surrounds the magnets and retains the magnets on the surface of the shaft. Whilst the figures show the retaining sleeve 50, the retaining sleeve 50 is not necessary for the invention to work, and the rotor may be provided without a retaining sleeve 50. In practice, the retaining sleeve 50 helps to maintain the mechanical integrity of rotor when the rotor is rotating, particularly at higher rotational speeds.

[0044] In the most fundamental form of the invention, the shaft 10 comprises one or more axial grooves 20 (or slots) formed in the outer surface of the shaft 10. The axial grooves 20 extend longitudinally along at least a partial longitudinal length of the shaft 10.

[0045] The purpose of the axial grooves 20 is to break up the eddy current loops reducing the heat generation during operation and also reducing the opposing magnetic field during magnetisation of the permanent magnets 40. As such, arranging one or more axial grooves 20 within the surface of the rotor shaft reduces magnetic losses due to eddy current losses within the shaft.

[0046] In some arrangements, the one or more axial grooves 20 are formed in the radial outer surface of the shaft 10 at a position on the shaft that is coincident with a region that is between two adjacent permanent magnets 40.

[0047] In some arrangements, the one or more axial grooves 20 may be formed in the radial outer surface of the shaft 10 at a position on the shaft that is coincident with a region that is between the poles of a respective permanent magnet 40. This arrangement may be used as an alternative to the axial groove 20 being coincident with a region that is between two adjacent permanent magnets 40, or in the case when there are plural axial grooves 20, axial grooves 20 may be located at a position on the shaft 10 that is coincident with a region that is between two adjacent permanent magnets 40 and also at a position on the shaft 10 that is coincident with a region that is between the poles of a respective permanent magnet 40.

[0048] Given the above, the number of axial grooves 20 may be determined by the number of magnetic poles and their location on the shaft. In an example arrangement there are permanent magnets 40 mounted to the shaft providing four magnetic poles, where each magnetic pole covers a 90° arc of the external surface of the shaft. The magnetic poles alternate each 90° arc, as such they alternate North / South each 90° arc. In this example arrangement, each of the magnetic poles is provided by a permanent magnet extending over a 45° arc of the external surface of the shaft. It is to be noted that the magnetic poles of the permanent magnets are radially arranged, so each permanent magnets have their magnetic poles aligned radially. As such, two North poles are arranged adjacent each other, and then two South poles are arranged adjacent each other, and so on around the shaft.

[0049] In this example arrangement, the grooves 20 may be located to be coincident with a region that is between two adjacent alternating magnetic poles, there will preferably be 4 axial grooves 20. In an arrangement where the grooves 20 are located coincident with a region that is between each respective pole (for example between adjacent magnetic segments that make up a single magnetic pole in this example arrangement), there will preferably be 4 axial grooves 20. In an arrangement where axial grooves 20 may be located at a position on the shaft 10 that is coincident with a region that is between two adjacent alternating magnetic poles and also at a position on the shaft 10 that is coincident with a region that is between each respective pole (for example between adjacent magnetic segments that make up a single magnetic pole in this example arrangement), there will preferably be 8 axial grooves 20.

[0050] In other arrangements, the rotor shaft 10 may comprise one or more circumferential grooves 30 formed in the radial outer surface of the shaft 10. The circumferential grooves 30 may extend at least partially circumferentially around the radial outer surface of the shaft, or they may extend fully circumferentially around the outer surface of the shaft.

[0051] In the case where there are plural circumferential grooves 30, the circumferential grooves may be separated by a longitudinal gap. When there are plural longitudinal gaps (for example where there are at least three circumferential grooves), each of the gaps may be the same, or they may be different.

[0052] Using the above example arrangement where there are four alternating magnetic poles, each covering a 90° arc of the shaft, and each pole comprising two permanent magnet pieces covering an arc of 45° of the shaft, each magnetic pole may extend only a portion of the longitudinal length of the shaft. Further permanent magnets are provided longitudinally along the shaft, and having the same pole arrangement as the poles described above. There may be two or more longitudinal sections of permanent magnets.

[0053] In such an example. The one or more circumferential grooves 30 may be formed in the radial outer surface of the shaft 10 at positions relative to the magnetic poles or magnet segments. For example, the circumferential grooves may be located between adjacent magnet sections, and the longitudinal gap between respective circumferential grooves 30 is substantially the same as the longitudinal length of the permanent magnet. The circumferential groove may instead be located between the longitudinal length of the permanent magnet. In further examples, circumferential grooves 30 may be provided between adjacent permanent magnets and between the longitudinal length of the permanent magnet.

[0054] As with the above arrangements having axial grooves 20, the purpose of the circumferential grooves 30 is to break up the eddy current loops reducing the heat generation during operation and also reducing the opposing magnetic field during magnetisation of the permanent magnets 40. As such, arranging one or more circumferential grooves 30 within the surface of the rotor shaft reduces magnetic losses due to eddy current losses within the shaft.

[0055] Whilst Figure 1 shows both axial 20 and circumferential 30 grooves in the surface of the shaft, the invention does not need both axial 20 and circumferential 30 grooves formed in the surface of the shaft 10 to work. For example, the shaft may comprise axial grooves 20 only, or the shaft may comprise circumferential grooves 30 only. Or, as shown in Figure 1 , the shaft may comprise both axial 20 grooves and circumferential 30 grooves.

[0056] There is a trade-off to be made with regards to the number and position and orientation of the grooves 20,30 in the surface of the shaft. Whilst a larger number of grooves will improve the performance of the rotor due to the reduction in the eddy currents being formed in the shaft, the grooves will reduce the stiffness of the shaft. As such, a shaft comprising a large number of grooves will have a lower stiffness and thus the mechanical performance (for example in its maximum rotational speed) will be reduced. With regards to the orientation of the grooves, it has been found that the axial grooves have the least effect on the lateral stiffness of the shaft compared to circumferential grooves.

[0057] Experimental tests were performed using a rotor comprising the different arrangement of grooves 20,30. A target magnet temperature was set. The speed of the rotor was then increased until the target magnet temperature was reached. If a loss reduction has been achieved the input power and speed should be higher.

[0058] As can be seen, the arrangement having plural circumferential grooves 30 and plural axial grooves 20 offered the best improvement in performance of the rotor.

[0059] For ease of manufacture the design preferably uses a post magnetisation process, where the magnets are only magnetised just before the shaft is inserted into the motor core. This allows all of the other processes to be carried out on an inert shaft.

[0060] To evaluate the magnetisation process first a pre-magnetised version was assembled and tested. This was found to be 1.37 times better than the baseline. The surface flux density of the shafts was found to be the same indicating that the coercivity of the magnets was not able to be established within the post-magnetisation jig. The most likely cause of this is eddy currents forming on the shaft surface as the magnetising field is ramped up. These eddies currents generated their own magnetic field that resisted the magnetising field resulting is a poor end result. By adding the grooves these eddy currents were broken up and resulted in the magnets being able to achieve a better performance by having the remnance and coercivity fully established as part of the post magnetisation process.

[0061] By comparing final iteration (3 circumferential + 8 axial; 4 axial aligned to magnet gap + 4 axial aligned between magnetic poles) to the pre-magnetised baseline we can see there is an increase of performance of around 9% This is likely caused by the reduction of eddies currents during operation. These can be caused by harmonics in the phase current / stator tooth tip etc. The grooves act to break these currents up resulting in lower l2R losses.

[0062] It has been found that the depth of the groove has the greatest impact on the eddy current generated in the shaft, whereas it has been found that the width of the groove has little to no impact. Again, there is a trade-off when it comes to the depth of the groove since a deeper groove will give a better performance improvement over a shallower groove, however a deeper groove will impact the structural integrity and stiffness of the shaft, limiting the mechanical performance of the rotor.

[0063] In its broadest sense, a groove depth may be chosen to be proportional to the diameter of the shaft 10. Preferably the depth may be between 5% and 13% of the radial diameter of the shaft 10. In a real world scenario, the groove may be formed between 1 mm and 2mm into the outer surface of the shaft 10, preferably in the region of 1.8mm.

[0064] The groove may be filled with a different material to the material of the shaft 10. We will go into this in a little more detail when we discuss the method of manufacturing such a rotor, but this material may be, for example, an adhesive such as the adhesive used to mount the permanent magnets to the shaft.

[0065] As mentioned above, some arrangements use a retention sleeve 50 that is disposed radially outwards of the permanent magnets 40. The retention sleeve 50 surrounds and retains the permanent magnets 40 on the surface of the shaft 10. Preferably the retention sleeve 50 is formed of carbon fibre. The retention sleeve 50 acts to maintain the magnets 40 in place on the shaft. During use, when the rotor is spinning, circumferential forces react with the shaft to eject the magnets from the shaft. Whilst the adhesive in practice is strong enough to retain the magnets in place, the retention sleeve provides additional support. Furthermore, should the bond between one or more of the magnets and the shaft fail, the retention sleeve may act to contain the debris that might otherwise be ejected from the rotor at force, otherwise destroying the motor.

[0066] With regards to the manufacture to such a rotor, in its broadest sense the method may comprise forming one or more axial grooves 20 in the outer surface of the shaft, and then mounting the plurality of permanent magnets 40 to the radial outer surface of the shaft 10.

[0067] Forming the axial grooves 20 in the surface of the shaft 10 may be performed by machining as would be known in the art, or the shaft may be formed with the grooves 20 already in place. As discussed above, the shaft may comprise the axial grooves 20, circumferential grooves 30 or a combination of the axial 20 and circumferential 30 grooves. The number, arrangement and form of the grooves may be as discussed above.

[0068] When mounting the magnets 40 to the surface of the shaft 10, one method utilises first 60 and second 70 end rings. A first end ring 60 is mounted at a first position on the shaft 10, then the permanent magnets 40 are mounted onto the shaft such that a first end of the magnets 40 abuts the first end ring 60. Then a second end ring 70 is mounted on the shaft at a second end of the permanent magnets 40 (opposing the first end of the permanent magnets) such that the second end of the permanent magnets abuts the second end ring 70. The end rings 60,70 are used to correctly located the magnets 40 on the shaft. The magnets 40, and first 60 and second 70 end rings may be mounted on the shaft using an adhesive.

[0069] Once the magnets 40 and end rings 60,70 are in place, the outer surfaces of the magnets and end rings are ground down such that they have the same radial thickness as each other.

[0070] As discussed above, one arrangement uses a retention sleeve 50, which may preferably be comprised of carbon fibre. The manufacturing method may therefore comprise providing the retention sleeve 50 on the outer radial surface of the permanent magnets 40. Once the magnets 40, end rings 60, 70 and retention sleeve 50 (if used) are in place, the rotor may be balanced, that is portions of mass are added or removed from the rotor in order to compensate for any imbalances within the rotor.

[0071] As discussed above, the magnets 40 may be provided to the rotor pre-magnetised, or they may be provided to the rotor in an un-magnetised form. If the magnets 40 are provided in the un-magnetised form, the method of manufacture may also then comprise magnetising the permanent magnets 40 after the magnets are mounted to the shaft. Techniques for magnetising the magnets will be known to the skilled reader. However, the grooves 20,30 provided on the surface of the rotor advantageously improve the magnetisation of the magnets. Large magnetic fields are used to magnetise the magnets, which can cause eddy currents in the shaft in arrangements that do not have grooves in the surface of the shaft. As such, the grooves in the shaft reduces the eddy currents in the shaft and thus improves the magnetisation process.

[0072] Once manufactured, the rotor may be used in an electric machine.

[0073] Figure 4 shows a cross section of a simplified electric machine. The machine comprises a stator 110 that comprises a plurality of stator pole teeth 120 arranged circumferentially about an axis of rotation 100 of the machine to define a gap about the axis of rotation 100 of the machine. Each stator pole tooth having a coil wound there around to form a stator coil stack, the coil stacks for producing a magnetic field when a current is passed there through to interact with the permanent magnets of the rotor. The rotor comprises magnets 40 on a shaft 10 having grooves 20,30 formed therein, and is located concentrically within a gap in the stator.

[0074] The electric machine may be configured and run as a motor, or a generator.

[0075] In either case, the rotor of the machine may be coupled to a variety of devices to drive or be driven by the devices. Some intended applications of such an electric machine could be in the field of turbo machines, and turbocharger, for example turbochargers for fuel cells. However, such an electric machine is not limited to such a field.

[0076] Turbochargers are well known devices for supplying air to an inlet of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine inlet manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings. Instead of driving the turbochargers or turbo machines via exhaust gasses, alternative turbochargers may be driven by an electrical machine such as the electric machine described above.

[0077] No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

CLAIMS:1 . A rotor for an electric machine, the rotor comprising: a shaft (10) having a diameter, an axial length and an axis of rotation (100) extending along the axial length; and a plurality of permanent magnets (40) mounted to a radial outer surface of the shaft (10), wherein the shaft (10) comprises one or more axial grooves (20) formed in the radial outer surface of the shaft (10), the one or more axial grooves (20) comprising one or more grooves extending longitudinally along a least a partial longitudinal length of the shaft (10).

2. A rotor according to claim 1 , wherein at least one of the one or more axial grooves (20) formed in the radial outer surface of the shaft (10) is located in an area of the shaft that is between two respective adjacent permanent magnets (40).

3. A rotor according to claim 1 or 2, wherein at least one of the one or more axial grooves (20) formed in the radial outer surface of the shaft (10) is located in an area of the shaft (10) that is between two opposing magnetic poles of respective adjacent permanent magnets (40).

4. A rotor according to claim 1 , 2 or 3, wherein there are 4 axial grooves (20) , or wherein there are 8 axial grooves (20).

5. A rotor according to any preceding claim, comprising one or more circumferential grooves (30) formed in the radial outer surface of the shaft (10) that extend at least partially circumferentially around the radial outer surface of the shaft.

6. A rotor according to claim 5, where at least one of the one or more circumferential grooves (30) extends fully circumferentially around the outer surface of the shaft (10).

7. A rotor according to claim 5 or 6, wherein, when there are two or more circumferential grooves (30), the circumferential grooves are separated by a longitudinal gap.

8. A rotor according to claim 5, 6 or 7, wherein there are three circumferential grooves (30), each being separated by a longitudinal gap.

9. A rotor according to claim 8, wherein each of the longitudinal gaps are substantially the same, or wherein each of the longitudinal gaps are not the same.

10. A rotor according to any one of claims 5 to 9, wherein one or more circumferential grooves (30) are located in an area of the shaft (10) that is between two respective adjacent permanent magnets (40).

11. A rotor according to any one of claims 5 to 10, wherein one or more circumferential grooves (30) are located in an area of the shaft (10) that is between two axial ends of a respective permanent magnet (40).

12. A rotor according to any preceding claim, wherein a groove (20,30) in the outer surface of the shaft has a depth that is proportional to the diameter of the shaft (10).

13. A rotor according to claim 12, wherein the depth of the groove (20,30) is between 1% and 20% of the diameter of the shaft, preferably between 5% and 13% of the diameter of the shaft (10).

14. A rotor according to any preceding claim, wherein a groove (20,30) is formed between 1mm and 2mm into the outer surface of the shaft (10), preferably 1.8mm.

15. A rotor according to any preceding claim, wherein at least one of the one or more s grooves (20,30) is filled with a different material to the material of the shaft (10).

16. A rotor according to claim 15, wherein the material in the at least one of the one or more grooves (20,30) is an adhesive.

17. A rotor according to any preceding claim, comprising a retention sleeve (50) radially outwards of the permanent magnets (40) for surrounding and retaining the permanent magnets on the surface of the shaft.

18. A rotor according to claim 17, wherein the retention sleeve (50) is formed of carbon fibre.

19. A rotor according to any preceding claim, comprising a first end ring (60) mounted to the outer surface of the shaft (10) adjacent a first axial end of the permanent magnets (40), and a second end ring (70) mounted to the outer surface of the shaft adjacent a second axial end of the permanent magnets (40).

20. A rotor according to claim 19, wherein the first (60) and second (70) end rings have a radial thickness that is substantially the same as the radial thickness of the permanent magnets (40).

21. A rotor according claim 19 or 20, wherein the first (60) and second (70) end rings are adhered to the outer surface of the shaft (10).

22. A rotor according to any preceding claim, wherein the permanent magnets (40) are adhered to the shaft (10).

23. A method of manufacturing a rotor for an electric machine, the rotor comprising a shaft (10) having a diameter, an axial length and an axis of rotation extending along the axial length, and a plurality of permanent magnets (40), the method comprising: forming one or more axial grooves (20) in the outer surface of the shaft and which extends longitudinally along a least a partial longitudinal length of the shaft; and mounting the plurality of permanent magnets (40) to a radial outer surface of the shaft (10).

24. A method according to claim 23, wherein mounting the plurality of magnets (40) to the radial outer surface of the shaft (10) comprises: mounting a first end ring (60) on a first position on the shaft (10); mounting the permanent magnets (40) on the shaft such that a first end of the magnets abuts the first end ring (60); mounting a second end ring (70) at a second end of the permanent magnets (40) opposing the first end of the permanent magnets such that the second end of the permanent magnets abuts the second end ring (70).

25. A method according to claim 24, and further comprising the step of grinding down a radially outer surface of the magnets (40) and the first (60) and second (70) end rings such that they have the same radial thickness as each other.

26. A method according to claim 24 or 25, wherein mounting the first (60) and second (70) end rings comprises adhering the respective first (60) and second (70) end rings to the radial outer surface of the shaft (10).

27. A method according to any one of claims 23 to 26, wherein mounting the permanent magnets (40) on the shaft (10) comprises adhering the permanent magnets (40) to the radial outer surface of the shaft (10).

28. A method according to any one of claims 23 to 27, comprising providing a retention sleeve (50) on an outer radial surface of the permanent magnets (40), the retaining sleeve for retaining the magnets in place on the surface of the shaft (10).

29. A method according to claim 28, wherein the retention sleeve (50) is formed of carbon fibre.

30. A method according to any one of claims 23 to 29, comprising balancing the rotor.

31. A method according to any one of claims 23 to 30, comprising magnetising the permanent magnets (40) after the magnets are mounted to the shaft.

32. A method according to any one of claims 23 to 31 , wherein at least one of the one or more axial grooves (20) formed in the radial outer surface of the shaft (10) is located in an area of the shaft that is between two respective adjacent permanent magnets (40).

33. A method according to any one of claims 23 to 32, wherein at least one of the one or more axial grooves (20) formed in the outer surface of the shaft (10) is located in an area of the shaft that is between two opposing magnetic poles of respective adjacent permanent magnets (40).

34. A method according to any one of claims 23 to 33, wherein there are 4 axial grooves, or wherein there are 8 axial grooves.

35. A method according to any one of claims 23 to 34, comprising forming one or more circumferential grooves (30) that extend at least partially circumferentially around the outer surface of the shaft (10).

36. A method according to claim 35, where at least one of the one or more circumferential grooves (30) extends fully circumferentially around the outer surface of the shaft (10).

37. A method according to claim 35 or 36, wherein, when there are two or more circumferential grooves (30), the circumferential grooves are separated by a longitudinal gap.

38. A method according to claim 35, 36 or 37, wherein there are three circumferential grooves (30), each being separated by a longitudinal gap.

39. A method according to claim 38, wherein each of the longitudinal gaps are substantially the same, or wherein each of the longitudinal gaps are not the same.

40. A method according to any one of claims 35 to 39, wherein one or more circumferential grooves (30) are located in an area of the shaft (10) that is between two respective adjacent permanent magnets (40).

41. A rotor according to any one of claims 35 to 40, wherein one or more circumferential grooves (30) are located in an area of the shaft (10) that is between two axial ends of a respective permanent magnet (40).

42. A method according to any one of claims 23 to 41 , wherein a groove (20,30) in the outer surface of the shaft (10) has a depth that is proportional to the diameter of the shaft (10).

43. A method according to claim 42, wherein the depth of the groove (20,30) is between 1% and 20% of the diameter of the shaft (10), preferably between 5% and 13% of the diameter of the shaft.

44. A method according to any one of claims 23 to 43, wherein a groove (20,30) is formed between 1mm and 2mm into the outer surface of the shaft, preferably 1.8mm.

45. A method according to any one of claims 23 to 44, wherein at least one of the one or more grooves (20,30) is filled with a different material to the material of the shaft.

46. A method according to claim 45, wherein the material in the at least one of the one or more grooves (20,30) is an adhesive.

47. An electric machine, comprising: a rotor according to any one of claims 1 to 22; and a stator, comprising a plurality of stator pole teeth arranged circumferentially about an axis of rotation of the machine to define a gap about the axis of rotation of the machine, each stator pole tooth having a coil wound there around to form a stator coil stack, the coil stacks for producing a magnetic field when a current is passed there through to interact with the permanent magnets of the rotor, wherein the rotor is arranged concentrically within the gap.

48. An electric machine according to claim 47, wherein the electric machine is a motor.

49. An electric machine according to claim 47, wherein the electric machine is a generator.

50. An electric machine according to claim 48 or 49, coupled to a turbo machine, a turbo charger, or a turbo charger for a fuel cell.

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