Rotor end cap

Abstract

A permanent magnet rotor end cap, having a bore defined by shaft contact surfaces 620 (520, Fig. 5B) and comprising keys 612 (425, Fig. 5B) between undercut regions 610 (510’, Fig. 5B); each of the un

Description

TECHNICAL FIELD The present disclosure relates to a rotor end cap. Aspects of the invention relate to a rotor, to a permanent magnet synchronous motor, and to a vehicle. BACKGROUND It is known to provide rotors for electric motors in which elements of the rotor are retained by an end cap at each end. In particular, for a permanent magnet synchronous motor, end caps are typically provided to retain the main functional elements of the rotor, which may comprise a set of discs (“pucks”) each containing an array of permanent magnets. The end cap is constructed to retain other rotor elements, and it may also comprise keys to locate the rotor on a shaft - these keys are typically located in grooves on the shaft. Such rotor end caps may be challenging to design so that they achieve their desired functionality while maintaining a suitable lifetime so that they do not become a point of failure for the motor. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a rotor end cap, a rotor, a permanent magnet synchronous motor, and a vehicle, as claimed in the appended claims. In one aspect the present invention provides a rotor end cap for a permanent magnet synchronous motor, the rotor end cap having an inner bore defined by a plurality of shaft contact surfaces, the rotor end cap having a plurality of keys extending in to the inner bore, each key having an undercut region extending radially into the rotor end cap to either side thereof, wherein a depth of the undercut region measured radially into the rotor end cap relative to the shaft contact surface is less than a predetermined minimum value. It is found that reducing the depth of the undercut regions in this way limits plastic strain on the rotor end cap and may prevent the yield strength of the rotor end cap material (generally a metal, and typically a steel) from being exceeded. According to an aspect of the present invention there is provided a rotor end cap for a permanent magnet synchronous motor, the end cap having an inner bore defined by a plurality of shaft contact surfaces, wherein: each of the plurality of shaft contact surfaces is a cylindrical segment disposed at a common shaft contact radius from a central axis of the inner bore; the rotor end cap further comprises a plurality of keys wherein each key is disposed between two undercut regions, wherein each key extends into the inner bore towards the central axis; and each of the undercut regions extends radially in to the rotor end cap between one of the plurality of keys and one of the shaft contact surfaces, wherein each of the undercut regions has a first curved region extending from the key and a second curved region extending from the shaft contact surface, wherein a radius of curvature of the first curved region is less than a radius of curvature of the second curved region. With such a geometry for the undercut region, a shallow undercut region depth can be provided in such a way as to minimize regions of high strain in the rotor end cap, increasing the lifetime of this component. In certain embodiments, the radius of curvature of the first curved region is fixed and the radius of curvature of the second curved region varies. In one arrangement, the radius of curvature of the second region may vary from 5.6 to 7.2 times the radius of curvature of the first region. In a specific arrangement, the radius of curvature of the first curved region may be 0.016 times the shaft diameter, which is equivalent to double the radius of curvature of the shaft contact surfaces. For a rotor shaft in a permanent magnet synchronous motor for use in the powertrain of an electric vehicle, this may be 50mm, for example. In alternative embodiments, the radius of curvature of both of the first curved region and the second curved region may vary, but only such that each radius of curvature of the first curved region is still less than each radius of curvature of the second curved region. In embodiments, there is an intermediate land between the first curved region and the second curved region. This intermediate land may be flat, and such that a plane of the intermediate land is orthogonal to a radius of the rotor end cap. This approach allows for an extended length of undercut region to prevent high strain regions from forming in the rotor end cap. In certain embodiments, the undercut region further comprises a third curved region between each second curved region and its proximate shaft contact surface, wherein a radius of curvature of the third curved region has an opposing sense to the radius of curvature of the second curved region. This enables a smooth transition between the undercut region and the proximate shaft contact surface, thus avoiding a region of high strain from being developed. In embodiments, the radius of curvature of this third curved region may be substantially 0.67 times the radius of curvature of the first curved region. In embodiments, the rotor end cap has an inner face and an outer face each substantially orthogonal to the central axis of the inner bore, wherein each undercut region has an undercut surface facing towards the central axis, and wherein there is a chamfer between the undercut region and one or both of the inner face and the outer face of the rotor end cap. The inclusion of such a chamfer may further reduce strain on the rotor end cap. In embodiments, this chamfer may be up to 0.007 times the shaft diameter. In particular embodiments, a length of the second curved region is greater than a length of the first curved region, wherein said lengths are measured tangentially to a radius of the inner bore. This enables an effective strain reducing shape to be formed in the undercut region. In some such embodiments, a length of the undercut region may be greater than a length of the key - in one case, the undercut length is 1.2 times the key length. In embodiments, the undercut regions to either side of a key are symmetrical about that key. In embodiments, each key has a flat region extending from the first curved region of the undercut region, wherein the flat regions to either side of the key are parallel to each other. In embodiments, a length of this flat region is at least 0.01 times the shaft diameter. In embodiments, a depth of the undercut region measured radially into the rotor end cap relative to the shaft contact surface is less than a predetermined minimum value. In embodiments, this may be 0.03 times the shaft diameter. In a further aspect, the present invention provides a rotor for a permanent magnet synchronous motor, the rotor comprising two rotor end caps as described in the preceding aspect, a rotor body comprising a plurality of permanent magnets, and a shaft, wherein the shaft contact surfaces of the rotor end caps abut the shaft to retain the rotor body on the shaft. A rotor designed according to this approach experiences reduced strain compared to a conventionally constructed rotor, and will as a result have an extended lifetime. In an embodiment of such a rotor, the shaft may comprise a plurality of grooves parallel to an axis of the shaft, wherein each of the keys of the rotor end caps locate in a respective groove of the shaft. In a still further aspect, the invention provides a permanent magnet synchronous motor comprising a rotor as described in the preceding aspect. Such motors have extended lifetimes as a result of reduced strain in the rotor. In a yet further aspect, the invention provide an electric vehicle having a powertrain comprising a permanent magnet synchronous motor as described in the preceding aspect. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle in which embodiments of the invention are employed; Figure 2 illustrates schematically an exemplary powertrain for an electric vehicle in which embodiments of the invention may be employed; Figure 3 shows a sectional view of a permanent magnet synchronous motor in which embodiments of the invention may be employed; Figure 4 shows an isometric view of the rotor and shaft of the permanent magnet synchronous motor of Figure 3; Figures 5A and 5B show respectively a rotor end cap according to an earlier design and a rotor end cap according to an embodiment of the invention; Figures 6A and 6B show respectively detailed views of a rotor cap section for the rotor caps of Figures 5A and 5B; Figures 7A and 7B show isometric views of a rotor cap section as shown in Figure 6B, with Figure 7B showing a modification according to a further embodiment of the invention; Figure 8 shows a rotor end cap according to the further embodiment of the invention illustrated in Figure 7B; Figure 9 shows a rotor end cap according to a still further embodiment of the disclosure; Figure 10 shows the rotor end cap of Figure 9 with additional detail; Figure 11 shows a sectional view of the rotor end cap of Figures 9 and 10; and Figure 12 shows steps of a method of manufacturing a rotor according to an embodiment of the disclosure. DETAILED DESCRIPTION A permanent magnet synchronous motor in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 3, and in subsequent Figures. Figures 1 and 2 provide context for use of such an electric motor in the context of an electric vehicle 100. The powertrain 200 of such an electric vehicle 100 is shown in Figure 2. Power is provided by battery 210 and control from electric drive controller 220, in this case to first electric motor 230 and second electric motor 240 (other embodiments may have only one electric motor). Each electric motor 230, 240 drives vehicle wheels through a transmission 235, 245. Two main types of electric motor used for this purpose are induction motors and permanent magnet synchronous motors. An induction motor has a stator with coils, and a rotor typically with bars in a “squirrel cage” design - current in the stator generates a rotating magnetic field which in turn generates current in the squirrel cage that itself generates a magnetic field opposing the driving magnetic field of the stator. That generates torque in the rotor, causing the rotor to rotate. The permanent magnet synchronous motor uses permanent magnets in the rotor, and the result is that the stator and rotor magnetic fields are locked, and therefore synchronous, with the rotor rotating at the same speed as the magnetic field is rotated in the stator. The two types have different benefits (cost, maturity of technology, ability to freewheel for induction motors -efficiency and small size for permanent magnet synchronous motors). The electric motors used may be of the same or different types - in embodiments considered in detail here, one or both of the electric motors 230, 240 is a permanent magnet synchronous motor. A known configuration for electric vehicles is to have a first electric motor 230 which is an induction motor and a second electric motor 240 which is a permanent magnet synchronous motor, as the different properties of the two motor types can be used in a complementary manner. Figure 3 is a sectional view of a permanent magnet synchronous motor 300. The stator 310 comprises a plurality of windings 320 that establish a magnet field that interacts with the field generated by the permanent magnets 340 in a rotor 330. The rotor 330 is mounted on a shaft 350 which provides torque to a transmission (not shown). Figure 4 is an isometric view of the rotor 330 of Figure 3. The rotor 330 is mounted on the shaft 350, and comprises a series of pucks 410 and two end caps 420. Each puck 410 contains a set of permanent magnets 340 - Figure 3 essentially shows a cross section through a single puck 410. The pucks 410 are aligned on the shaft 350 and retained by the two end caps 420. As can be seen from both Figures 3 and 4, the shaft 350 has grooves 355 and the pucks 410 have puck keys 415 that locate in the grooves 355. The end caps 420 have end cap keys 425 that also locate in the grooves 355. The rotor end caps 420 have the role of holding the elements of the rotor 330 in position, including binding the pucks 410 on to the shaft 350. Issues to address in rotor design include avoiding excessive strain, and ensuring proper rotor balance. It is found that these issues can be addressed with appropriate rotor end cap design. The design of the end cap keys 425 is addressed in embodiments of the present invention. Figure 5A illustrates an earlier end cap design, whereas Figure 5B shows an end cap design according to an embodiment of the invention. Figure 5A shows that to either side of each end cap key 425 there is an undercut 510 - this is necessary to form an effective separation between the key 425 and the shaft contact surface 520 which retains the end cap 420 on the shaft 350. The shaft contact surfaces 520 are cylindrical segments that together define an inner bore 520 of the rotor end cap 420. In this earlier design, the undercut 510 is essentially a circular section, forming a substantially semi-circular-cylindrical shape. It is found that reshaping the undercut enables problematic strains in the end cap 420 to be significantly reduced. Figure 5B shows such a reshape, with a shallower undercut 510’ that has a varying radius of curvature that is greater proximate to the shaft contact surface 520 than it is proximate to the key 425. The undercut region of each arrangement is shown in detail in Figure 6A (for the earlier design) and Figure 6B (for a design in accordance with an embodiment of the invention). Figure 6A shows that for the earlier design the key 425 has a flat keyway surface 612 facing into the axis of the rotor, and a flat land 614 parallel to an equivalent land (not shown in Figure 6A) on the other side of the key 425 - the key 425 is symmetric here, that is, the lands 614 on either side of the key 425 are parallel to a radius of the rotor passing through the centre of the key- with the flat lands 614 enabling the key 425 to slot into the groove on the shaft that also has correspondingly flat lands. Between the keyway surface 612 and the horizontal flat land 614 there is a convex curved region 616 allowing for easy engagement of the key 425 with the groove on the shaft - the horizontal flat land 614 is needed to align the end cap to the rotor shaft during assembly. The undercut 610 has an overall depth D, and a constant radius of curvature R throughout the undercut 610 until there is a transition to the shaft contact surface 620. This transition involves first a flat land 624 parallel the flat land 614 of the key, followed by a convex curved region 626 to complete the transition. The design according to an embodiment of the invention shown in Figure 6B preserves parallel symmetry about the key 425 but loses symmetry within the undercut 610’. The key design is essentially as before -there is a keyway surface 612 facing into the shaft, and flat lands 614 to allow the key to locate in the groove on the shaft, and a convex curved region between them. However, the undercut 610 is no longer a circular section - it is shallower, and has a more complex shape in which the radius of curvature of the undercut varies. Extending from the flat land 614 of the key 425 there is a first undercut region 6101, and extending from the shaft contact surface 620 there is a second undercut region 6102. Both these regions are curved and form circular sections, but the radius of curvature of the first undercut region 6101 is significantly less than the radius of curvature of the second undercut region 6102. The length of the first undercut region 6101 is also significantly less than the length of the second undercut region 6102, wherein said lengths are measured tangentially to a radius of the inner bore. In the arrangement shown in Figure 6B, there is also a flat land 6103 - essentially parallel here to the keyway surface 612 - bridging between the first undercut region 6101 and the second undercut region 6102. The transition between the undercut 610’ and the shaft contact surface 620 is also now managed differently, as less accommodation is needed between them - this is a achieved through a smaller convex joining region 636’, where convex means in an opposing sense to the radius of curvature of the second curved region 6102. In the arrangement shown in Figure 6B, the overall depth D of the undercut is reduced - it is preferably less than 0.03*S in total for a shaft diameters - less than 1,5mm if the shaft diameter (as determined, for example, by the radius of curvature of the shaft contact surfaces 620) is 50mm. The distance between the two flat lands 614 (the width of the key 425) is here designated y, which here is substantially 0.08*S. The undercut region 610’ has a length u here slightly greater than the width of the key, preferably substantially 1.2*y. Flat lands 614 are preferably at least 0.01 *S in length in this arrangement to allow effective engagement of the end cap with the rotor shaft groove. The radius of curvature r1 of the first undercut region 6101 is approximately equal to 0.2*y, or 0.016*S - this allows for ease of end cap assembly as well as avoiding excessive strain concentrations. The radius of curvature r2 for the second undercut region 6102 may in this arrangement vary, as given by the following equation: 5.6 * rl <r2 <7.2 * rl This allows for a suitably large radius of curvature while allowing tangency with smaller radius regions. The smaller convex joining region 636’ here has a radius of curvature r3 substantially equal to 0.67*r1. Using this approach, plastic strain in and around the undercut region is significantly reduced. In the arrangement of Figure 6A, plastic strain was found to be above the yield limit of an exemplary material, whereas in the arrangement of Figure 6B, the plastic strain is reduced by almost 75% and is below the yield limit. A further modification to the end cap is shown with respect to Figures 7A, 7B and 8. Figure 7A is an isometric view of the end cap of Figures 5B and 6B in the region of the undercut 610’. It is found that further reduction of strain can be achieved by adding a chamfer 720 between the flat end cap surface 700 and the curved surface 710 of the undercut facing towards the shaft. A chamfer of up to 0.007*S is found to reduce plastic strain, again taking it below the yield limit of the material, in high strain regions (particularly close to the transition between the shaft contact surface 620 and the undercut 610) without compromising significantly engagement between the rotor end cap and the shaft. A still further modification to the end cap is shown with respect to Figures 9 to 12. This shows a rotor end cap 420 as shown in Figure 8 with the addition of balancing holes 910. These balancing holes 910 are blind holes drilled into the outer face of the rotor end cap - these are added to compensate for rotational imbalance inherent in the rotor. It is found that it is better in this context to subtract material rather than to add it, and to introduce as little stress as possible the location and dimensions of individual balancing holes 910 should be controlled. If significant balance is to be offset, this is done by adding more holes while keeping the dimensions of individual holes within such constraints. The nature of this control is shown in more detail in Figures 10 and 11. As shown in Figure 10, the balancing holes 910 are generally located in a load balancing region 920 forming an annular region of the outer face of the rotor end cap, and preferably on a pitch circle 925. Preferably the minimum diameter P of such a pitch circle 925 is 2*S. The maximum width W of each hole is preferably 0.18*S. The internal dimensions of each balancing hole 910 are shown in Figure 11 - the preferred depth H of each hole is 0.162*S. Each balancing hole has a cylindrical bore region 9101 and a conical end region 9102 - the conical end wall makes an angle A to the plane normal to the axis of the cylindrical bore. Preferably this angle A is approximately 30°. The pattern of holes made may vary from endcap to endcap, depending on the determined imbalance. There may be a different number of holes on each endcap, holes on each endcap may have the same or different positions, or holes may be present on one endcap and not on the other. However, where holes are present, they will follow the rules indicated above. Figure 12 shows a method of balancing a rotor using this approach. First of all, the imbalance of the rotor is determined 1210. To address this imbalance, one or more balancing holes are drilled 1220 according to the constraints set out above, with an appropriate number of balancing holes being drilled to correct the imbalance. While particularly suitable for a rotor end cap, this approach could also be taken for rebalancing of a rotor in an induction motor also. A typical induction motor rotor is of squirrel-cage design, with a series of metal bars connecting two conductive end rings - a number of steel laminations are mounted between the end rings and between and around the metal bars. Blind balancing holes can be drilled into the outer faces of the end rings following the same principles. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A rotor end cap for a permanent magnet synchronous motor, the rotor end cap having an inner bore defined by a plurality of shaft contact surfaces; wherein:each of the plurality of shaft contact surfaces is a cylindrical segment disposed at a common shaft contact radius from a central axis of the inner bore;the rotor end cap further comprises a plurality of keys wherein each key is disposed between two undercut regions, wherein each key extends into the inner bore towards the central axis; andeach of the undercut regions extends radially in to the rotor end cap between one of the plurality of keys and one of the shaft contact surfaces, wherein each of the undercut regions has a first curved region extending from the key and a second curved region extending from the shaft contact surface, wherein a radius of curvature of the first curved region is less than a radius of curvature of the second curved region.

2. The rotor end cap of claim 1, wherein the radius of curvature of the first curved region is fixed and the radius of curvature of the second curved region varies within a predetermined range.

3. The rotor end cap of claim 1 or claim 2, wherein there is an intermediate land between the first curved region and the second curved region.

4. The rotor end cap of claim 3, wherein the intermediate land is flat, and a plane of the intermediate land is orthogonal to a radius of the rotor end cap.

5. The rotor end cap of any preceding claim, wherein the undercut region further comprises a third curved region between each second curved region and its proximate shaft contact surface, wherein a radius of curvature of the third curved region has an opposing sense to the radius of curvature of the second curved region.

6. The rotor end cap of any preceding claim, wherein the rotor end cap has an inner face and an outer face each substantially orthogonal to the central axis of the inner bore, wherein each undercut region has an undercut surface facing towards the central axis, and wherein there is a chamfer between the undercut surface of the undercut region and one or both of the inner face and the outer face of the rotor end cap.

7. The rotor end cap of any preceding claim, wherein a length of the second curved region is greater than a length of the first curved region, wherein said lengths are measured tangentially to a radius of the inner bore.

8. The rotor end cap of claim 7, wherein a tangential length of the undercut region, with respect to the bore central axis, is greater than a tangential length of the key.

9. The rotor end cap of any preceding claim, wherein the undercut regions to either side of a key are symmetrical about that key.

10. The rotor end cap of any preceding claim, where each key has a flat region extending from the first curved region of the undercut region, wherein the flat regions to either side of the key are parallel to each other.

11. The rotor end cap of any preceding claim, wherein a depth of the undercut region measured radially 5 into the rotor end cap relative to the shaft contact surface is less than a predetermined minimum value.

12. A rotor for a permanent magnet synchronous motor, the rotor comprising two rotor end caps as claimed in any of claims 1 to 11, a rotor body comprising a plurality of permanent magnets, and a shaft, wherein the shaft contact surfaces of the rotor end caps abut the shaft.

13. The rotor of claim 12, wherein the shaft comprises a plurality of grooves parallel to an axis of the 10 shaft, wherein each of the keys of the rotor end caps locate in a respective groove of the shaft.

14. A permanent magnet synchronous motor comprising a rotor as claimed in claim 12 or claim 13.15, An electric vehicle having a powertrain comprising a permanent magnet synchronous motor asclaimed in claim 14.1510

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