Permanent magnet rotor for electric machine

Abstract

A rotor for an electric machine is formed from a shaft on which a plurality of permanent magnets are mounted. The shaft has one or more axial grooves formed in a radially outer surface of the shaft. The groove extends longitudinally along at least a portion of a longitudinal length of the shaft.

Description

Technical Field

[0001] The present invention relates to a rotor for an electric motor and an electric motor including the rotor. Background Technology

[0002] Known electric motors (e.g., brushless motors and generators) include a rotor having permanent magnets mounted to a shaft concentrically arranged within a stator. The stator includes a plurality of stator pole teeth arranged circumferentially around the axis of rotation of the machine, wherein each stator pole tooth has a coil wound around it to form a stator coil stack. Each stator coil stack generates a magnetic field as current travels through it, interacting with the permanent magnets of the rotor, thereby causing the rotor to rotate about its axis.

[0003] In known electric motors, the rotor shaft comprises steel and is sometimes surrounded by an iron core. During operation, the magnetic field generated by the stator coil stack interacts not only with the permanent magnets on the rotor surface (to drive the rotor around its axis) but also with the steel core at the rotor's center. This interaction results in eddy currents being generated within the rotor shaft, causing the shaft to heat up and degrading the machine's performance.

[0004] Therefore, we have recognized the need for an improved rotor for electric motors. Summary of the Invention

[0005] The present invention provides, according to the appended independent claims, a rotor for an electric motor, a method for manufacturing a rotor for an electric motor, and an electric motor.

[0006] Further advantageous embodiments are provided in the appended dependent claims.

[0007] We describe a rotor for an electric motor, the rotor comprising: a shaft having a diameter, an axial length, and a rotation axis extending along the axial length; and a plurality of permanent magnets mounted to a radially outer surface of the shaft, wherein the shaft includes one or more axial grooves formed in the radially outer surface of the shaft, the one or more axial grooves including one or more grooves extending at least partially longitudinally along the longitudinal length of the shaft. This arrangement advantageously reduces eddy currents that would typically be generated in the shaft of the rotor when used in such an electric motor.

[0008] At least one of the one or more axial grooves formed in the radial outer surface of the shaft may be located in the region of the shaft between two adjacent permanent magnets.

[0009] At least one of the one or more axial grooves formed in the radial outer surface of the shaft may be located in the region of the shaft between two opposite magnetic poles of their respective adjacent permanent magnets. For example, four or eight axial grooves may be present. Other configurations are also possible.

[0010] The rotor may further include one or more circumferential grooves formed in the radially outer surface of the shaft, the one or more circumferential grooves extending at least partially circumferentially around the radially outer surface of the shaft. At least one of the one or more circumferential grooves extends completely circumferentially around the outer surface of the shaft.

[0011] When two or more circumferential grooves exist, the circumferential grooves are separated by longitudinal gaps. For example, there are three circumferential grooves, each separated by a longitudinal gap. Each of the longitudinal gaps may be substantially identical, or each of the longitudinal gaps may be different from one another.

[0012] One or more of the circumferential grooves may be located in the region of the shaft between two adjacent permanent magnets. One or more of the circumferential grooves may also be located in the region of the shaft between two axial ends of the respective permanent magnets.

[0013] The groove in the outer surface of the shaft can have a depth proportional to the diameter of the shaft. The depth of the groove can be between 1% and 20% of the diameter of the shaft, preferably between 5% and 13% of the diameter of the shaft. The groove can be formed in the outer surface of the shaft with a depth between 1 mm and 2 mm, preferably 1.8 mm.

[0014] At least one of the one or more grooves may be filled with a material different from the material of the shaft. The material in at least one of the one or more grooves may be an adhesive.

[0015] The rotor may further include a retaining sleeve radially outside the permanent magnet to surround and hold the permanent magnet on the surface of the shaft. The retaining sleeve may be formed of carbon fiber.

[0016] The rotor may further include a first end ring and a second end ring, the first end ring being mounted adjacent to a first axial end of the permanent magnet to the outer surface of the shaft, and the second end ring being mounted adjacent to a second axial end of the permanent magnet to the outer surface of the shaft. The first end ring and the second end ring may have a radial thickness substantially the same as the radial thickness of the permanent magnet. The first end ring and the second end ring may be adhered to the outer surface of the shaft.

[0017] In any of the above, the permanent magnet can be adhered to the shaft.

[0018] We also discuss a method for manufacturing a rotor for an electric motor, the rotor comprising a shaft and a plurality of permanent magnets, the shaft having a diameter, an axial length, and a rotation axis extending along the axial length, the method comprising: forming one or more axial grooves in the outer surface of the shaft, the one or more axial grooves extending longitudinally along at least a portion of the longitudinal length of the shaft; and mounting the plurality of permanent magnets to the radial outer surface of the shaft. This arrangement advantageously reduces eddy currents that would typically be generated in the rotor shaft when used in such an electric motor.

[0019] Mounting the plurality of magnets onto the radial outer surface of the shaft may include: mounting a first end ring at a first position on the shaft; mounting the permanent magnet on the shaft such that a first end of the magnet is adjacent to the first end ring; and mounting a second end ring at a second end of the permanent magnet opposite to the first end of the permanent magnet such that the second end of the permanent magnet is adjacent to the second end ring.

[0020] The method may further include: grinding the radially outer surfaces of the magnet and the first and second end rings such that the magnet and the first and second end rings have the same radial thickness. Installing the first and second end rings may include: adhering the respective first and second end rings to the radially outer surfaces of the shaft.

[0021] Mounting the permanent magnet on the shaft may include: adhering the permanent magnet to the radial outer surface of the shaft.

[0022] The method may further include providing a retaining sleeve on the radially outer surface of the permanent magnet, the retaining sleeve being used to hold the magnet in place on the surface of the shaft. The retaining sleeve may be formed of carbon fiber.

[0023] The method may further include: balancing the rotor.

[0024] The method may further include magnetizing the permanent magnet after the magnet is mounted onto the shaft.

[0025] 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 the region of the shaft between two respective adjacent permanent magnets.

[0026] At least one of the one or more axial grooves formed in the outer surface of the shaft may be located in the region of the shaft between two opposite magnetic poles of their respective adjacent permanent magnets. Preferably, four axial grooves may be present, or eight axial grooves may be present. However, other numbers of grooves are also possible.

[0027] The method may include 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 completely circumferentially around the outer surface of the shaft.

[0028] When two or more circumferential grooves exist, the circumferential grooves can be separated by longitudinal gaps. When three circumferential grooves exist, each circumferential groove can be separated by longitudinal gaps. Each of the longitudinal gaps can be substantially identical, or each of the longitudinal gaps can be different.

[0029] One or more of the circumferential grooves may be located in the region of the shaft between two adjacent permanent magnets. One or more of the circumferential grooves may also be located in the region of the shaft between two axial ends of the respective permanent magnets.

[0030] The groove in the outer surface of the shaft can have a depth proportional to the diameter of the shaft. The depth of the groove can be between 1% and 20% of the diameter of the shaft, preferably between 5% and 13% of the diameter of the shaft. For example, the groove can be formed in the outer surface of the shaft between 1 mm and 2 mm, preferably 1.8 mm.

[0031] At least one of the one or more grooves may be filled with a material different from the material of the shaft. The material in at least one of the one or more grooves may be an adhesive.

[0032] We also discuss an electric motor comprising a rotor and a stator as discussed above, the stator comprising a plurality of stator pole teeth arranged circumferentially around the axis of rotation of the motor to define a gap around the axis of rotation of the motor, each stator pole tooth having a coil wound around the stator pole tooth to form a stator coil stack for generating a magnetic field as current travels through it to interact with the permanent magnet of the rotor, wherein the rotor is concentrically arranged within the gap.

[0033] The motor can be a motor, generator, or engine. The motor can be connected to a turbine, turbocharger, or turbocharger used in fuel cells. Attached Figure Description

[0034] The invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0035] Figure 1 A simplified shaft of the rotor is shown;

[0036] Figure 2 An axial sectional view of a rotor is shown, which also has permanent magnets mounted to the shaft;

[0037] Figure 3 A longitudinal sectional view of the rotor is shown; and

[0038] Figure 4 A simplified cross-sectional view of the motor is shown. Detailed Implementation

[0039] In short, the present invention provides a rotor for an electric motor, the rotor being formed by a shaft on which a plurality of permanent magnets are mounted. The shaft has one or more axial grooves formed in the radially outer surface of the shaft. The grooves extend longitudinally along at least a portion of the longitudinal length of the shaft. This arrangement advantageously reduces eddy currents that would normally be generated in the shaft of the rotor when used in such an electric motor.

[0040] Figures 1 to 3 A rotor according to the present invention is shown. Figure 1 A simplified shaft of the rotor is shown. Figure 2 An axial sectional view of a rotor is shown, which also has permanent magnets mounted to the shaft. Figure 3 A longitudinal sectional view of the rotor is shown.

[0041] refer to Figures 1 to 3 The shaft 10 may include a steel core having a diameter, an axial length, and a rotation axis 100 extending along the axial length of the shaft. A plurality of permanent magnets 40 are mounted to the radially outer surface of the shaft 10. In some arrangements, a retaining sleeve 50 is disposed radially outside the permanent magnets 40; the retaining sleeve 50 surrounds the magnets and holds them on the surface of the shaft. Although the retaining sleeve 50 is shown in the figures, it is not essential for the operation of the invention, and the rotor may be configured without it. In practice, the retaining sleeve 50 helps maintain the mechanical integrity of the rotor as it rotates, particularly at higher rotational speeds.

[0042] In the most basic form of the invention, the shaft 10 includes 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 portion of the longitudinal length of the shaft 10.

[0043] The purpose of the axial grooves 20 is to disrupt the eddy current loop, thereby reducing heat generation during operation and also reducing the reverse magnetic field during the magnetization of the permanent magnet 40. Thus, 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.

[0044] In some arrangements, one or more axial grooves 20 are formed in the radial outer surface of the shaft 10 at locations where they coincide with the area between two adjacent permanent magnets 40.

[0045] In some arrangements, one or more axial grooves 20 may be formed in the radial outer surface of the shaft 10 at locations coinciding with the regions between the magnetic poles of the respective permanent magnets 40. This arrangement can be used as an alternative to coinciding the regions between two adjacent permanent magnets 40, or, in the presence of multiple axial grooves 20, the axial grooves 20 may be located at locations coinciding with the regions between two adjacent permanent magnets 40 on the shaft 10, and may also be located at locations coinciding with the regions between the magnetic poles of the respective permanent magnets 40 on the shaft 10.

[0046] Given the above, the number of axial grooves 20 can be determined by the number of magnetic poles and their positions on the shaft. In the example arrangement, there are permanent magnets 40 mounted to the shaft, providing four magnetic poles, each covering a 90° arc of the outer surface of the shaft. The magnetic poles alternate every 90° arc, thus alternating north / south every 90° arc. In this example arrangement, each magnetic pole is provided by a permanent magnet extending over a 45° arc of the outer surface of the shaft. It should be noted that the magnetic poles of the permanent magnets are arranged radially, thus the magnetic poles of each permanent magnet are radially aligned. Thus, two north magnetic poles are arranged adjacent to each other, and then two south magnetic poles are arranged adjacent to each other, and so on around the shaft.

[0047] In this example arrangement, the groove 20 can be positioned to coincide with the region between two adjacent alternating magnetic poles, and preferably there will be four axial grooves 20. In an arrangement where the groove 20 is positioned to coincide with the region between each respective magnetic pole (e.g., between adjacent magnetic segments constituting a single magnetic pole in this example arrangement), preferably there will be four axial grooves 20. In an arrangement where the axial grooves 20 can be located at positions on the shaft 10 that coincide with the region between two adjacent alternating magnetic poles, and also at positions on the shaft 10 that coincide with the region between each respective magnetic pole (e.g., between adjacent magnetic segments constituting a single magnetic pole in this example arrangement), preferably there will be eight axial grooves 20.

[0048] In other arrangements, the rotor shaft 10 may include one or more circumferential grooves 30 formed in the radial outer surface of the shaft 10. The circumferential grooves 30 may extend circumferentially at least partially around the radial outer surface of the shaft, or the circumferential grooves 30 may extend circumferentially completely around the outer surface of the shaft.

[0049] In the presence of multiple circumferential grooves 30, the circumferential grooves can be separated by longitudinal gaps. When multiple longitudinal gaps exist (e.g., in the case of at least three circumferential grooves), each gap can be identical or each gap can be different.

[0050] Using the example arrangement described above with four alternating magnetic poles, each pole covering a 90° arc of the axis, and each pole comprising two permanent magnet parts covering a 45° arc of the axis, each pole may extend only a portion of the longitudinal length of the axis. Additional permanent magnets are arranged longitudinally along the axis and have the same pole arrangement as the aforementioned poles. Two or more longitudinal segments of permanent magnets may be present.

[0051] In such an example, one or more circumferential grooves 30 may be formed in the radially outer surface of the shaft 10 at a position relative to the magnetic poles or magnetic segments. For example, the circumferential grooves may be located between adjacent magnetic segments, and the longitudinal gap between the respective circumferential grooves 30 is substantially the same as the longitudinal length of the permanent magnet. Alternatively, the circumferential grooves may be located between the longitudinal lengths of the permanent magnets. In another example, the circumferential grooves 30 may be provided between adjacent permanent magnets and between the longitudinal lengths of the permanent magnets.

[0052] Similar to the arrangement described above with axial grooves 20, the purpose of circumferential grooves 30 is to disrupt eddy current loops, thereby reducing heat generation during operation and also reducing the reverse magnetic field during magnetization of the permanent magnet 40. Thus, 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.

[0053] Although Figure 1 Both axial groove 20 and circumferential groove 30 located on the surface of the shaft are shown, but the present invention does not require both axial groove 20 and circumferential groove 30 to be formed on the surface of the shaft 10 for operation. For example, the shaft may include only axial groove 20, or the shaft may include only circumferential groove 30. Or, as Figure 1 As shown, the shaft may include both an axial groove 20 and a circumferential groove 30.

[0054] The number, location, and orientation of the grooves 20 and 30 on the shaft surface need to be balanced. While a larger number of grooves improves rotor performance due to the reduction of eddy currents formed in the shaft, the grooves will reduce the shaft stiffness. Therefore, a shaft with a large number of grooves will have lower stiffness and thus reduce mechanical properties (e.g., in terms of its maximum rotational speed).

[0055] Regarding the orientation of the grooves, it has been found that axial grooves have the least impact on the lateral stiffness of the shaft compared to circumferential grooves.

[0056] Experimental tests were conducted using rotors with different arrangements of grooves 20 and 30. A target magnet temperature was set. The rotor speed was then increased until the target magnet temperature was reached. If loss reduction has been achieved, the input power and speed should be higher.

[0057]

[0058] As can be seen, the arrangement with multiple circumferential grooves 30 and multiple axial grooves 20 provides optimal improvement in rotor performance.

[0059] For ease of manufacturing, the design preferably uses a post-magnetisation process, where the magnet is magnetized only before the shaft is inserted into the motor core. This allows all other processes to be performed on an inert shaft.

[0060] To initially evaluate the magnetization process, a pre-magnetized version was assembled and tested. This was found to be 1.37 times better than the baseline. The surface flux density of the shaft was found to be the same, indicating that the coercivity of the magnet could not be established within the fixture after magnetization. The most likely reason is that eddy currents form on the shaft surface as the magnetization field ramps up. These eddy currents generate their own magnetic fields that resist the magnetization field, resulting in the undesirable end result.

[0061] By adding grooves, these eddy currents are disrupted, and the magnet is able to achieve better performance by having fully established remnance and coercivity as part of the post-magnetization process.

[0062] By comparing the final iteration (3 circumferential grooves + 8 axial grooves; 4 axial grooves aligned with the magnet gap + 4 axial grooves aligned between the magnetic poles) with the baseline before magnetization, we can see that the performance is improved by approximately 9%. This is likely due to a reduction in eddy currents during operation. These are likely caused by harmonics in the phase current / stator tooth tips, etc. The grooves serve to disrupt these currents, resulting in lower Ig. 2 R loss.

[0063] It has been found that the depth of the groove has the greatest impact on the eddy currents generated in the shaft, while the width of the groove has almost no effect. Similarly, there is a trade-off when it comes to the depth of the groove, because deeper grooves will provide better performance improvements than shallower grooves, however, deeper grooves will affect the structural integrity and stiffness of the shaft, thus limiting the mechanical properties of the rotor.

[0064] In its broadest sense, the groove depth can be chosen to be proportional to the diameter of the shaft 10. Preferably, the depth can be between 5% and 13% of the radial diameter of the shaft 10. In real-world scenarios, the groove can be formed in an area between 1 mm and 2 mm, preferably 1.8 mm, on the outer surface of the shaft 10.

[0065] The groove can be filled with a material different from that of the shaft 10. We will describe this in slightly more detail when we discuss the methods of manufacturing such a rotor, but the material can be, for example, an adhesive, such as the adhesive used to mount permanent magnets to the shaft.

[0066] As described above, some arrangements utilize a retaining sleeve 50 positioned radially outside the permanent magnet 40. The retaining sleeve 50 surrounds the permanent magnet 40 and holds it on the surface of the shaft 10. Preferably, the retaining sleeve 50 is formed of carbon fiber. The retaining sleeve 50 serves to hold the magnet 40 in place on the shaft. During use, as the rotor rotates, circumferential forces react with the shaft to dislodge the magnet from the shaft. While the adhesive is strong enough in practice to hold the magnet in place, the retaining sleeve provides additional support. Furthermore, if the connection between one or more magnets and the shaft fails, the retaining sleeve can be used to contain debris that might otherwise be dislodged from the rotor under force, potentially damaging the motor.

[0067] In its broadest sense, the method of manufacturing such a rotor may include forming one or more axial grooves 20 in the outer surface of the shaft and then mounting a plurality of permanent magnets 40 to the radial outer surface of the shaft 10.

[0068] Forming the axial groove 20 in the surface of the shaft 10 can be performed by machining methods known in the art, or the shaft can be formed with the groove 20 already in place. As discussed above, the shaft may include the axial groove 20, the circumferential groove 30, or a combination of the axial groove 20 and the circumferential groove 30. The number, arrangement, and form of the grooves can be as discussed above.

[0069] When mounting the magnet 40 onto the surface of the shaft 10, one method utilizes a first end ring 60 and a second end ring 70. The first end ring 60 is mounted at a first position on the shaft 10, and then the permanent magnet 40 is mounted onto the shaft such that the first end of the magnet 40 abuts against the first end ring 60. Then, the second end ring 70 is mounted onto the shaft at the second end of the permanent magnet 40 (opposite to the first end of the permanent magnet), such that the second end of the permanent magnet abuts against the second end ring 70. End rings 60 and 70 are used to properly position the magnet 40 on the shaft. Adhesive can be used to mount the magnet 40, the first end ring 60, and the second end ring 70 onto the shaft.

[0070] Once the magnet 40 and end rings 60 and 70 are in place, grind the outer surfaces of the magnet and end rings so that they have the same radial thickness.

[0071] As discussed above, one arrangement uses a retaining sleeve 50, which may preferably comprise carbon fiber. Therefore, a method of manufacture may include providing the retaining sleeve 50 on the radially outer surface of the permanent magnet 40.

[0072] Once the magnet 40, end rings 60, 70 and retaining sleeve 50 (if used) are in place, the rotor can be balanced by adding or removing portions of mass from the rotor to compensate for any imbalances within the rotor.

[0073] As discussed above, the magnet 40 can be provided to the rotor before magnetization, or it can be provided to the rotor in an unmagnetized form. If the magnet 40 is provided in an unmagnetized form, the manufacturing method may further include magnetizing the permanent magnet 40 after it has been mounted to the shaft. Techniques for magnetizing the magnet will be known to the skilled reader. However, the grooves 20, 30 provided on the rotor surface advantageously improve the magnetization of the magnet. Using a large magnetic field to magnetize the magnet could induce eddy currents in the shaft in an arrangement where there are no grooves on the shaft surface. Therefore, the grooves in the shaft reduce the eddy currents in the shaft and thereby improve the magnetization process.

[0074] Once manufactured, the rotor can be used in an electric motor.

[0075] Figure 4A simplified cross-section of the motor is shown. The motor includes a stator 110 comprising a plurality of stator pole teeth 120 arranged circumferentially around a rotation axis 100 of the motor to define a gap around the rotation axis 100. Each stator pole tooth has a coil wound around the stator pole tooth to form a stator coil stack for generating a magnetic field as current travels through it to interact with permanent magnets of the rotor. The rotor includes magnets 40 on a shaft 10 having grooves 20, 30 formed therein, and the rotor is concentrically located within the gap in the stator.

[0076] The motor can be configured and operated as a motor or a generator.

[0077] In either case, the rotor of the machine can be coupled to various devices to drive or be driven by them. Some anticipated applications of such motors can be found in the fields of turbines and turbochargers (e.g., turbochargers for fuel cells). However, such motors are not limited to these fields.

[0078] A turbocharger is a well-known device used to supply air to the inlet of an internal combustion engine at a pressure higher than atmospheric pressure (boost pressure). A conventional turbocharger comprises an exhaust-driven turbine impeller mounted on a rotatable shaft within a turbine housing connected downstream of the engine outlet manifold. Rotation of the turbine impeller causes a compressor impeller mounted on the other end of the shaft, located within a compressor housing. The compressor impeller delivers compressed air to the engine intake manifold. Conventionally, the turbocharger shaft is supported by journal bearings and thrust bearings, which include a suitable lubrication system, located within a central bearing housing connecting the turbine and compressor impeller housings. Instead of driving the turbocharger or turbine via exhaust, an alternative turbocharger can be driven by an electric motor, such as the one described above.

[0079] Undoubtedly, those skilled in the art will conceive of many other effective alternatives. It should be understood that the invention is not limited to the described embodiments, but covers modifications that will clearly fall within the scope of the appended claims to those skilled in the art.

Claims

1. A rotor for an electrical 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 radially outer surface of the shaft (10), wherein, the shaft (10) comprises one or more axial grooves (20) formed in the radially outer surface of the shaft (10), the one or more axial grooves (20) extending longitudinally along at least part of the longitudinal length of the shaft (10). At least one of the one or more axial grooves (20) formed in the radially outer surface of the shaft (10) is located in a region of the shaft between two respective adjacent permanent magnets (40).

2. The rotor of claim 1, wherein, At least one of the one or more axial grooves (20) formed in the radially outer surface of the shaft (10) is located in a region of the shaft (10) between two opposite poles of a respective adjacent permanent magnet (40).

3. The rotor of claim 1 or 2, wherein, There are 4 axial grooves (20), or wherein there are 8 axial grooves (20).

4. The rotor of claim 1, 2, or 3, wherein, 5. The rotor of any preceding claim, comprising one or more circumferential grooves (30) formed in the radially outer surface of the shaft (10), the one or more circumferential grooves (30) extending at least partially circumferentially around the radially outer surface of the shaft. At least one of the one or more circumferential grooves (30) extends completely circumferentially around the outer surface of the shaft (10).

6. The rotor of claim 5, wherein, When there are two or more circumferential grooves (30), the circumferential grooves are separated by longitudinal gaps.

7. The rotor of claim 5 or 6, wherein, There are three circumferential grooves (30), each separated by a longitudinal gap.

8. The rotor of claim 5, 6 or 7, wherein, Each of the longitudinal gaps is substantially the same, or wherein each of the longitudinal gaps is not the same.

9. The rotor of claim 8, wherein, One or more circumferential grooves (30) are located in a region of the shaft (10) between two respective adjacent permanent magnets (40).

10. The rotor of any one of claims 5 to 9, wherein, One or more circumferential grooves (30) are located in a region of the shaft (10) between two axial ends of a respective permanent magnet (40).

11. The rotor of any one of claims 5 to 10, wherein, The grooves (20, 30) in the outer surface of the shaft have a depth proportional to the diameter of the shaft (10).

12. The rotor of any of the preceding claims, wherein, The depth of the grooves (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).

13. The rotor of claim 12, wherein, The grooves (20, 30) are formed in the outer surface of the shaft (10) between 1 mm and 2 mm, preferably 1.8 mm.

14. The rotor of any of the preceding claims, wherein, At least one of the one or more grooves (20, 30) is filled with a material different to that of the shaft (10).

15. The rotor of any of the preceding claims, wherein, The material in at least one of the one or more grooves (20, 30) is an adhesive.

16. The rotor of claim 15, wherein, ​ 17. The rotor of any one of the preceding claims, comprising a retaining sleeve (50) radially outward of the permanent magnets (40) for surrounding and retaining the permanent magnets on the surface of the shaft.

18. The rotor of claim 17, wherein, The retaining sleeve (50) is formed of carbon fiber.

19. The rotor of any one of the preceding claims, 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. The rotor of claim 19, wherein, The first end ring (60) and the second end ring (70) have a radial thickness substantially the same as a radial thickness of the permanent magnets (40).

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

22. The rotor of any of the preceding claims, 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) and a plurality of permanent magnets (40), the shaft (10) having a diameter, an axial length, and an axis of rotation extending along the axial length, the method comprising: forming one or more axial grooves (20) in the outer surface of the shaft, and the one or more axial grooves (20) extending longitudinally along at least a portion of the longitudinal length of the shaft; and mounting the plurality of permanent magnets (40) to a radially outer surface of the shaft (10).

24. The method of claim 23, wherein, Mounting the plurality of magnets (40) to the radially outer surface of the shaft (10) comprises: mounting a first end ring (60) on the shaft (10) at a first location; 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) opposite the first end of the permanent magnets such that the second end of the permanent magnets abuts the second end ring (70).

25. The method of claim 24, and further comprising: The step of grinding the radially outer surfaces of the magnets (40) and the first and second end rings (60, 70) such that the magnets (40) and the first and second end rings (60, 70) have a radial thickness that is the same as each other.

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

27. The method of any one of claims 23-26, wherein, Mounting the permanent magnets (40) on the shaft (10) comprises adhering the permanent magnets (40) to the radially outer surface of the shaft (10).

28. The method of any of claims 23 to 27, comprising providing a retaining sleeve (50) on a radially outer surface of the permanent magnet (40), the retaining sleeve for retaining the magnet in place on a surface of the shaft (10).

29. The method of claim 28, wherein, The retaining sleeve (50) is formed from carbon fibre.

30. The method of any of claims 23 to 29, comprising balancing the rotor.

31. The method of any of claims 23 to 30, comprising magnetising the permanent magnet (40) after mounting the magnet to the shaft.

32. The method of any one of claims 23-31, wherein, At least one of the one or more axial grooves (20) formed in the radially outer surface of the shaft (10) is located in a region of the shaft between two respective adjacent permanent magnets (40).

33. The method of any one of claims 23-32, wherein, At least one of the one or more axial grooves (20) formed in the outer surface of the shaft (10) is located in a region of the shaft between two opposite poles of a respective adjacent permanent magnet (40).

34. The method of any one of claims 23-33, wherein, There are 4 axial grooves, or wherein there are 8 axial grooves.

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

36. The method of claim 35, wherein, At least one of the one or more circumferential grooves (30) extends completely circumferentially around the outer surface of the shaft (10).

37. The method of claim 35 or 36, wherein, When there are two or more circumferential grooves (30), the circumferential grooves are separated by longitudinal gaps.

38. The method of claim 35, 36, or 37, wherein, There are three circumferential grooves (30), each separated by a longitudinal gap.

39. The method of claim 38, wherein, Each of the longitudinal gaps is substantially the same, or wherein each of the longitudinal gaps is not the same.

40. The method of any one of claims 35-39, wherein, One or more circumferential grooves (30) are located in a region of the shaft (10) between two respective adjacent permanent magnets (40).

41. The rotor of any one of claims 35-40, wherein, One or more circumferential grooves (30) are located in a region of the shaft (10) between two axial ends of a respective permanent magnet (40).

42. The method of any one of claims 23-41, wherein, The grooves (20, 30) in the outer surface of the shaft (10) have a depth proportional to the diameter of the shaft (10).

43. The method of claim 42, wherein, The depth of the grooves (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. The method of any one of claims 23-43, wherein, The grooves (20, 30) are formed in the outer surface of the shaft between 1 mm and 2 mm, preferably 1.8 mm.

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

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

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

48. The electric machine of claim 47 wherein, The electric machine is a motor.

49. The electric machine of claim 47 wherein, The electric machine is a generator.

50. The electric machine of claim 48 or 49, coupled to a turbomachine, a turbocharger, or a turbocharger for a fuel cell.

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