A rotor cage for an induction motor

Abstract

A rotor cage (12, figure 1) suitable for an induction motor (180, figure 7), the cage comprising two end rings 18 connected by bars 20 and a rotor core (16, figure 1) comprising a stack of laminations

Description

TECHNICAL FIELD The present disclosure relates to a rotor cage for an induction motor. Aspects of the invention relate to a rotor for an induction motor, to an induction motor, and to methods of manufacture of a rotor cage for an induction motor and a rotor for an induction motor BACKGROUND It is known to provide electric vehicles with induction motors in order to impart movement to the vehicle through electromagnetic induction. Induction motors comprise a stator surrounding a rotor, wherein the rotor undergoes rotary movement as a result of a current being passed through the stator. The rotor typically comprises a rotor cage and a shaft, where rotation of the rotor cage imparts rotation to the shaft through an interference fit between the two. In operation, existing rotors are subject to high stress, which can consequently lead to damage to the rotor through extended use. 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 cage for an induction motor, a rotor for an induction motor, an induction motor, and methods of manufacture of a rotor cage for an induction motor and a rotor of an induction motor, as claimed in the appended claims. According to an aspect of the present invention, there is provided a rotor cage for an induction motor. The rotor cage comprises two end rings and a plurality of bars connecting the two end rings. The rotor cage further comprises a rotor core comprising a plurality of laminations arranged in a stack, each lamination of the plurality of laminations being an annular disc and comprising one or more slots arranged proximate an outer diameter of the respective lamination, each slot configured to receive a corresponding bar of the rotor cage, and a surface arranged at an inner diameter of the respective lamination, the surface being configured to receive a rotor shaft. The plurality of laminations comprises a first set of laminations with a first inner diameter, and a second set of laminations positioned at proximal and distal ends of the stack, where an inner diameter of each lamination of the second set of laminations is larger than the first inner diameter. The rotor cage of the present aspect is advantageous as the provision of two sets of laminations with differently sized inner diameters in a stack arranged such that the set of laminations with the larger inner diameter are at the proximal and distal ends of the stack reduces the level of yielding experienced by the laminations and the bars of the rotor cage when the rotor cage is in use. This in turn reduces the likelihood of the rotor cage being damaged and / or failing when in use. When in use, the rotor cage may be configured such that in use, the rotor cage receives the rotor shaft and the first set of laminations establishes an interference fit with the rotor shaft. Optionally, the inner diameter of each of the second set of laminations is the same. In such cases, the inner diameter of each lamination of the second set of laminations may be 0.9mm larger than the first inner diameter. The second set of laminations may comprise between 6 to 10 laminations at the proximal end of the stack, and between 6 to 10 laminations at the distal end of the stack. Optionally, the second set of laminations may comprise 6 laminations at the proximal end of the stack, and 6 laminations at the distal end of the stack. Such arrangements can advantageously minimise the level experienced by the laminations and the bars of the rotor cage when the rotor cage is in use whilst ensuring that appropriate torque is transferred between the rotor cage and a rotor shaft. Optionally, the plurality of laminations comprises between 600 and 700 laminations. Each lamination of the plurality of laminations may have a thickness of between 0.1mm and 0.35mm. Optionally, one of the second set of laminations at the proximal end may contact one of the two end rings, and one of the second set of laminations at the distal end may contact the other of the two end rings. Further optionally, each lamination of the plurality of laminations may comprise one or more holes located between the inner diameter and the outer diameter. This can advantageously reduce the weight of the rotor cage. According to another aspect of the present invention, there comprises a rotor for an induction motor. The rotor comprises a rotor cage in accordance with the previously described aspect and a rotor shaft, wherein the rotor shaft is positioned within the inner diameter of the plurality of laminations. Optionally, the rotor may be arranged such that the first inner diameter achieves an interference fit with the rotor shaft and wherein the inner diameter of each of the second set of laminations achieves a clearance fit with the rotor shaft. According to another aspect of the present invention, there comprises an induction motor. The induction motor comprises a rotor in accordance with the previously described aspect, and a stator arranged to surround the rotor, and configured to receive electrical current in order to generate rotational movement in the rotor relative to the stator. According to a further aspect of the present invention, there is provided a method of manufacture of a rotor cage in accordance with aspects described above. The method of manufacture comprises punching the first set and second set of laminations from a plurality of lamination sheets. The method further comprises stacking the first set and second set of laminations such that the second set of laminations is positioned at the proximal and distal ends of the stack and the first set of laminations are positioned between them. Subsequently, the first set and second set of laminations are aligned such that the one or more slots proximate the outer diameter of each of the first set and second set of laminations are in alignment to form one or more outer channels. The method further comprises providing a mould at the proximal and distal ends of the stack, each mould being configured to receive molten metal and being shaped in the form of one of the two end rings. Molten metal is then poured into the one or more outer channels and the moulds at the proximal and distal ends of the stack and the method further comprises cooling the molten metal to form the plurality of bars and the two end rings of the rotor cage. Optionally, punching the first set and second set of laminations from the plurality of lamination sheets comprises punching each lamination of the first set of laminations with a smaller inner diameter than the first inner diameter and punching each lamination of the first set and second set of laminations with an outer diameter larger than the outer diameter, and the method of manufacture then further comprises, following the cooling of the molten metal, machining the first and second set of laminations such that the size of the inner diameter of each lamination of the first set of laminations matches the first inner diameter, and the size of the outer diameter of each lamination of the first set and second set of laminations matches the outer diameter. In a further aspect, there is provided a method of manufacture of a rotor. The method comprises firstly manufacturing a rotor cage in accordance with the aspect described above. The method further comprises heating the stack of the rotor cage, cooling a rotor shaft, inserting the rotor shaft through the inner diameters of each of the laminations of the lamination stack, and returning the rotor shaft and the stack of the rotor cage to room temperature. 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 an isometric view of a rotor for an induction motor; Figure 2A shows a front view of a first type of lamination of a rotor in accordance with an embodiment of the invention; Figure 2B shows a front view of a second type of lamination of a rotor in accordance with an embodiment of the invention; Figure 3 shows a cross section of a rotor in accordance with an embodiment of the invention; Figure 4A shows a graph illustrating experimental data relating to mechanical plastic strain levels of a rotor in accordance with an embodiment of the invention; Figure 4E3 shows a further graph illustrating experimental data relating to mechanical plastic strain levels of a rotor in accordance with an embodiment of the invention; Figure 5 shows a flow diagram illustrating a method of manufacture of a rotor cage in accordance with an embodiment of the invention; Figure 6 shows a flow diagram illustrating a method of manufacture of a rotor in accordance with an embodiment of the invention; Figure 7 shows an isometric view of an induction motor in accordance with an embodiment of the invention; and Figure 8 shows a vehicle in accordance with an embodiment of the invention. DETAILED DESCRIPTION A rotor 10 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 1. Figure 1 illustrates an isometric view of a rotor 10 for use in an induction motor, the induction motor being suitable for use in a vehicle. The induction motor comprises the rotor 10 and a stator (not shown) which surrounds the rotor 10. In use, current is passed through the stator which induces rotation in the rotor 10, with the axis of rotation of the rotor 10 being colinear with the longitudinal axis of the stator. The rotor 10 comprises a rotor cage 12 and a rotor shaft 14. The rotor cage 12 is arranged with a central channel extending throughout the length of the rotor cage 12 through which the rotor shaft 14 extends. The rotor shaft 14 is configured to connectively couple with at least a portion of a surface of the rotor cage 12 located adjacent to the central channel. In use, the stator of the induction motor causes rotation of the rotor cage 12, and the connective coupling achieved between the rotor cage 12 and the rotor shaft 14 imparts rotation to the rotor shaft 14. The rotor cage 12 comprises a rotor core 16, two end rings 18 and a plurality of bars 20 which extend between and connect to the two end rings 18. Optionally, the plurality of bars 20 and the two end rings 18 may comprise a metal. In some embodiments, the plurality of bars 20 and the two end rings 18 may comprise aluminium. The rotor core 16 comprises a plurality of laminations 22 which are arranged in a stack, with the two end rings 18 being located adjacent to and in contact with the two laminations 22 of the plurality of laminations 22 positioned at the proximal and distal ends of the stack. It is to be appreciated that for ease of illustration in Figure 1, the lamination stack is shown as a single body and each individual lamination is not separately demarked. Optionally, the laminations 22 may comprise steel. When assembled, the lamination stack forms the central channel through which the rotor shaft 14 extends. To form the central channel, each lamination 22 of the plurality of laminations 22 comprises a central aperture, and the central channel is formed by aligning the central apertures of each of the plurality of laminations 20 when arranging the stack. Consequently, in use the connective coupling between the rotor shaft 14 and the 4 rotor cage 12 is established specifically between surfaces adjacent to the central aperture of one or more laminations 22 of the lamination stack and the rotor shaft 14. Further details regarding the plurality of laminations 22 is provided with reference to Figures 2A and 2E3 below. The lamination stack is further provided with a plurality of outer channels proximate to an outer diameter of the lamination stack, with each of the outer channels being configured to receive a corresponding bar 20 of the plurality of bars 20. To form these outer channels, each lamination 22 of the plurality of laminations 22 is provided with a plurality of slots (with the number of slots being equal to the number of channels and the number of bars 20), and the channels are formed by aligning the slots of each of the plurality of laminations 20 when arranging the stack. In some embodiments, the channels may be arranged to lie substantially parallel to the longitudinal axis of the lamination stack. In other embodiments, the channels may be arranged to lie in a plane between the longitudinal axis and the lateral axis of the lamination stack. Further details regarding the plurality of laminations 22 is provided with reference to Figures 2A and 2B below. When in use, existing rotors are typically exposed to high stresses at the connection region between the plurality of bars 20 and the two end rings 18. Over an extended period of use, this can lead to damage to (and ultimately failure of) the rotor 10. Embodiments of the present invention mitigate against these stresses through adaptation of the lamination stack, and specifically through the introduction of a plurality of forms of lamination 22 within the lamination stack. It has been determined that the high stresses can be caused by interaction between the plurality of bars 20 and the two end rings 18 with the plurality of laminations 22 in the connection region. The high stresses are also exacerbated by the loads imparted by the connective coupling between the rotor shaft 14 and the lamination stack of the rotor cage 12, since this coupling can cause the lamination stack to push against the plurality of bars 20 and the two end rings 18. Referring now to Figures 2A and 2B, there are shown two types of lamination 50, 52 in accordance with an embodiment of the invention. The laminations 50, 52 are configured to be used in a rotor cage 12 of the form illustrated in Figure 1 (i.e. to be arranged to form a lamination stack of a rotor core 16 with functionality as described in embodiments above). In both types of lamination 50, 52 shown in Figures 2A and 2B, the lamination 50, 52 comprises an annular disc. Each lamination 50, 52 may optionally have a thickness of between 0.1 mm to 0.35mm. In some optional embodiments, each lamination 50, 52 may have a thickness of 0.2mm. It is to be appreciated however that the thickness of each lamination 50, 52 may be varied in accordance with the requirements of the rotor 10, wherein adjusting the thickness of each lamination 50, 52 alters the performance of the induction motor in use. Both types of lamination 50, 52 comprise an outer diameter 54 of substantially the same size. Both types of lamination 50, 52 each additionally comprise a plurality of slots 56 arranged proximate to the outer diameter 54 of the lamination 50,52. As described above, the plurality of slots 56 are appropriately shaped and arranged to receive a corresponding bar 20 of the plurality of bars 20 of the rotor cage 12. As such, the number of slots provided for each lamination 50, 52 will be equal to (or greater than) the number of bars 20 in the rotor cage 12. The plurality of slots 56 provided for an individual lamination 50, 52 may be substantially similar in size. It is to be appreciated that in Figures 2A and 2B, the slots 56 are shown as being of a “semi-open” form, in which the slots 56 are shaped to expose the bars 20 at the outer diameter 54 of the lamination 50, 52 rather than enclose them entirely. However, the embodiments described are equally functional where the slots 56 are of a “closed” form (not shown) in which the bars are fully enclosed by the lamination 50, 52 with no exposure at the outer diameter 54. Furthermore, corresponding slots on separate laminations 50, 52 (i.e. slots 56 on separate laminations 50, 52 which are configured to receive the same bar 20 of the rotor cage 12) may be placed at substantially the same circumferential position and radial distance relative to the centre of the corresponding lamination 50, 52. This arrangement enables the laminations 50, 52 to be assembled into a stack such that the plurality of slots 56 of each lamination 50, 52 combine to form the plurality of outer channels of the rotor core 16 as described in embodiments above. It is to be appreciated that the number of slots 56 illustrated in Figures 2A and 2B are provided for illustrative purposes only and that the total number of slots may be adjusted in accordance with the number of bars 20 present in the rotor cage 12. Optionally, the rotor cage 12 includes sixty bars 20, and the laminations 50, 52 include sixty corresponding slots 56. The laminations 50, 52 additionally comprise respective inner diameters 58, 60. The laminations 50, 52 when in use are arranged to receive the rotor shaft 14 at, or proximate to a surface 62, 64 of the respective inner diameters 58, 60 in accordance with embodiments described above. The inner diameter 60 of the lamination 52 of Figure 2B is configured to be larger than the inner diameter 58 of the lamination 50 of Figure 2A. Optionally, the inner diameter 60 of the lamination 52 of Figure 2B may be configured to be 0.9mm larger than the inner diameter 58 of the lamination 50 of Figure 2A. Further optionally, the inner diameter 60 of the lamination 52 of Figure 2B may be configured to be at least 0.9mm larger than the inner diameter 58 of the lamination 50 of Figure 2A. In existing systems, the laminations 22 included in the lamination stack are typically substantially identical. In particular each of the laminations 22 used in existing rotors 10 will typically comprise the same inner diameter. As a result, when the lamination stack receives the rotor shaft 14, each lamination 22 of the lamination stack will achieve a similar connection between a surface of each lamination 22 adjacent to the central aperture. This connection may be an interference fit. In the rotor cage 12 of the present embodiments, the lamination stack includes two or more types of laminations 50, 52, where the different types of laminations have differently sized inner diameters 58, 60 (such as the laminations 50, 52 shown in Figures 2A and 2B). The provision of such laminations results in the different types of lamination 50, 52 achieving a different connection with the rotor shaft 14 when in use. In particular, the laminations 52 with the larger inner diameter 60 (referred to as the second set of laminations 52) have a reduced connection with the rotor shaft 14 when compared to the laminations 50 with the smaller inner diameter 58 (referred to as the first set of laminations 50). In some embodiments, the laminations 50, 52 are arranged such that the first set of laminations 50 achieve an interference fit with the rotor shaft 14 in use. In some embodiments, the laminations 50, 52 are arranged such that the second set of laminations 52 achieve a clearance fit with the rotor shaft 14 when in use. For ease of reference the smaller inner diameter 58 will be referred to as the first inner diameter 58, and the larger inner diameter 60 will be referred to as the second inner diameter 60. The two types of lamination 50, 52 are arranged in the stack such that at least one of the type of lamination 52 with the second inner diameter 60 is placed at each end of the stack (i.e. at proximal 70 and distal 72 ends of the stack), with at least one of the type of lamination 50 with the first inner diameter 58 being positioned between them. Arranging the lamination stack in this manner results in a reduction in the stress experienced at the connection region between the plurality of bars 20 and the two end rings 18 as a result of the lower degree of connection between the laminations 52 with the second inner diameter 60 provided in this region. The laminations 52 at either extreme end of the stack may be configured to be in contact with a respective end ring 18. As noted with reference to Figure 1, the connection established between the laminations 50, 52 and the rotor shaft 14 imparts rotation to the rotor shaft 14 when the rotor 10 is in use. Reducing the connection therefore reduces the amount of rotational torque imparted to the rotor shaft 14. As such, when forming the lamination stack using the two types of lamination 50, 52, it is important to determine a suitable arrangement which reduces the stress experienced at the connection region between the plurality of bars 20 and the two end rings 18 whilst maintaining a desirable level of torque transfer capability. Consequently in some embodiments, the lamination stack may include between 6 and 10 of the type of lamination 52 with the second inner diameter 60 at each of the proximal 70 and distal 72 ends of the stack. In further embodiments, the lamination stack may include exactly 6 of the type of lamination 52 with the second inner diameter 60 at each of the proximal 70 and distal 72 ends of the stack. In each case, the number of laminations 50 with the first inner diameter 58 is provided in order to ensure functionality of the rotor 10. In some optional embodiments, the total number of laminations 50, 52 in the lamination stack is between 600 and 700. In further optional embodiments, the total number of laminations 50, 52 in the lamination stack is between 670. It is to be appreciated that whilst two types of lamination 50, 52 are shown, in optional embodiments there may be provided three or more types of lamination, each with a different size of inner diameter, and each configured to achieve a different connection with the rotor shaft 14 when in use. The three or more types of lamination may be arranged such that the size of the inner diameter of the types of lamination increases when progressing toward the outer edges of the lamination stack (i.e. towards the proximal 70 and distal 72 ends of the stack). In optional embodiments, one of more of the first set of laminations 50 may be provided with one or more holes located between the inner diameter and the outer diameter of the lamination 50. The provision of these holes acts as to lower the weight of the laminations 50 and the lamination stack, and consequently improve the performance of the rotor 10. Referring to Figure 3, there is shown a cross section of a rotor 10 in accordance with an example embodiment of the invention, illustrating the lamination stack, the end rings 18, two bars of the plurality of bars 20 and the rotor shaft 14. Figure 3 illustrates the first 50 and second 52 sets of laminations arranged in a stack in accordance with embodiments described above. For ease of illustration, the lamination stack is shown as a single body and each individual lamination is not separately demarked. The second set of laminations 52 with second inner diameter 60 are shown as being arranged at each end of the stack (i.e. at proximal 70 and distal 72 ends of the stack, proximate to the end rings 18), with at first set of laminations 50 with the first inner diameter 58 being positioned between them. In the embodiment shown in Figure 3, the first inner diameter 58 of the first set of laminations 50 is arranged such that the laminations 50 contact the rotor shaft 14 (achieving an interference fit with the rotor shaft 14), and the second inner diameter 60 of the first set of laminations 52 is arranged such that the laminations 52 do not contact the rotor shaft 14 (achieving a clearance fit). Referring now to Figures 4A and 4B, there are shown graphs of experimental data illustrating the relationship between the number of laminations 52 in a rotor 10 with a second inner diameter 60 (on the x-axis) and the area for a bar of the plurality of bars 20 which undergoes yielding as a result of plastic strain (on the y-axis) where the area is provided in mm2. Figure 4A illustrates the area of the bar 20 which undergoes yielding at the joint region between the bar 20 and the end ring 18, and Figure 4B illustrates the area of the bar 20 which undergoes yielding at the interface region between the first 50 and second 52 lamination types. At each data point, the number of laminations noted in Figures 4A and 4B indicates the number of laminations 52 with the second inner diameter 60 at both of the proximal 70 and distal 72 ends of the lamination stack (i.e. the data point at 2 on the x-axis indicates a total number of four laminations in the lamination stack). The experimental data provided in Figures 4A and 4B indicates that by increasing the number of laminations 52 in the second set of laminations in the lamination stack, the area of the bar 20 exhibiting yielding at the joint I interface region is reduced up to a certain point. Beyond this point, increasing the number of laminations 52 of this type only minimally reduces the area of the bar 20 undergoing yielding (or indeed can increase the area of the bar exhibiting yielding). The experimental data therefore highlights that the inclusion of the two types of lamination 50, 52 can reduces the yielding experienced by the bar 20 at the joint I interface region. Referring now to Figure 5, there is shown a flow diagram illustrating a method of manufacture 100 of the rotor cage 12 in accordance with present embodiments. The method of manufacture 100 begins by punching, at Step 102, the required number of laminations 50, 52 for the rotor 10 from a plurality of lamination sheets. For each lamination 50, 52, a lamination sheet of appropriate shape and thickness (in accordance with embodiments described above) is provided. The lamination sheets may comprise steel. A separate set of punch dies will be provided for each type of lamination 50, 52 required, where each punch die will be appropriately configured to create the laminations 50, 52 of the type required (e.g., with a correctly sized outer diameter and slots 56 for every lamination 50, 52, and with a differently sized inner diameter 58, 60 for each type of lamination 50, 52). The method of manufacture 100 continues by stacking, at Step 104, the punched laminations 50, 52 into a lamination stack. The stack will be arranged in accordance with embodiments described above i.e., such that the second set of laminations 52 are arranged at the proximal 70 and distal 72 ends of the lamination stack, with the first set of laminations 50 positioned between them. The method of manufacture 100 then proceeds by aligning, at Step 106, the laminations 50, 52 to form an aligned stack. This alignment comprises ensuring that the slots 56 of each of the laminations 50, 52 in the stack are appropriately positioned with respect to one another such that they form the plurality of channels configured to receive the plurality of bars 20 of the rotor cage 12, in accordance with embodiments described above. The alignment may be aided by use of an alignment fixture. The method of manufacture 100 continues by pouring, at Step 108, molten metal into the aligned channels of the lamination stack. The metal used is configured to be suitable for use in the rotor cage 12 and may comprise aluminium. The molten metal flows through the channels toward the proximal 70 and distal 72 ends of the lamination stack and into a mould provided at each end of the stack. The mould is appropriately shaped in the form of the two end rings 18. Sufficient molten metal is provided so as to substantially fill each of the channels and each of the moulds. The method of manufacture 100 then proceeds by cooling, at Step 110, the molten metal in order to form the plurality of bars 20 and the two end rings 18. The result is a formed rotor cage 12 in which the plurality of bars 20 and the two end rings 18 are positioned correctly relative to the lamination stack. In some embodiments, when punching each of the laminations 50, 52, the punched inner diameters 58,60 are configured to be smaller than the intended size of the final inner diameter 58, 60. In such embodiments, following the cooling of the molten metal step, the stack is then machined such that the inner diameters 58, 60 are sized to match their intended size. This step may be performed in order to mitigate against errors in the manufacturing process. In some embodiments, the second set of laminations 52 are not machined as the size of the second inner diameter 60 of this type of lamination 52 is already sufficiently large when compared to the first set of laminations 50 to achieve the required reduced connection with the rotor shaft 14. A similar machining approach may be taken in respect of the outer diameters 54. In particular, when punching each of the laminations 50, 52, the punched outer diameters 54 are configured to be larger than the intended size of the final outer diameter 54. In such embodiments, following the cooling of the molten metal step, the stack is then machined such that the outer diameters 54 are sized to match their intended size. Since the punched outer diameters 54 are configured to be larger than the intended size of the final outer diameter 54, the initial punching operation of the laminations 50, 52 creates slots 56 which are all closed in nature. The machining of the outer diameters 54 of the laminations 50, 52 also therefore acts so as to configure the slots to be arranged as desired (i.e. where the slots 56 are designed to be semi-open, the removal of material exposes the bars 20, and where the slots 56 are designed to be closed, material is removed so as to ensure that the bars 20 are not exposed. In optional embodiments, following alignment of the stack and prior to the pouring of the molten metal into the channels, the aligned stack of laminations 50, 52 may be compressed. This may be performed in order to prevent molten metal flowing between the laminations 50, 52. Referring to Figure 6, there is shown a method of manufacture 150 of the rotor 10 in accordance with present embodiments. The method of manufacture 150 follows the manufacture of the rotor cage 12 described with reference to Figure 5. Following the cooling of the molten metal at Step 110 of Figure 5, the present method of manufacture 150 proceeds by cooling the rotor shaft 14 and heating the lamination stack of the rotor cage 12, at Step 152. This has the effect of thermally contracting the rotor shaft 14 and thermally expanding the lamination stack in order to ensure that an appropriate connection is achieved between the rotor shaft 14 and the lamination stack during construction. Once the rotor shaft 14 and lamination stack have been cooled and heated appropriately, the method 150 proceeds by inserting, at Step 154, the rotor shaft 14 through the central channel formed by the lamination stack. The method of manufacture 150 continues by returning, at Step 156, the rotor shaft 14 and the lamination stack to room temperature. As part of this process, the rotor shaft 14 achieves a first level of connection with the laminations 50 with the first inner diameter 58, whilst achieving a second level of connection (lower than the first level of connection) with the laminations 52 with the second inner diameter 60, in accordance with embodiments described above. In some embodiments, the first level of connection may be an interference fit, and the second level of connection may be a clearance fit. In some embodiments, at Step 152, the lamination stack and the bars 20 and two end rings 18 may be heated, but the rotor shaft 14 may be maintained at room temperature. In optional embodiments, the method of manufacture 150 may proceed with Step 154 only (i.e. by simply inserting the rotor shaft 14 through the central channel formed by the lamination stack with no requirement of adjusting the temperature of the rotor shaft and the lamination stack). Figure 7 illustrates an isometric view of an induction motor 180 according to an embodiment of the present invention. The induction motor 180 comprises a rotor 10 in accordance with embodiments of the present invention as described above which is surrounded by a stator 182. Figure 8 illustrates a vehicle 200 according to an embodiment of the present invention. The vehicle 200 comprises an induction motor 180 which comprises a rotor 10 in accordance with embodiments of the present invention as described above. The induction motor 180 provides drive to the mechanical transmission of the vehicle 200. The induction motor 180 may be one of several mechanisms of providing drive to the mechanical transmission of the vehicle 200. 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 cage for an induction motor, the rotor cage comprising:two end rings and a plurality of bars connecting the two end rings;a rotor core comprising a plurality of laminations arranged in a stack, each lamination of the plurality of laminations being an annular disc and comprising:one or more slots arranged proximate an outer diameter of the respective lamination, each slot of the one or slots being configured to receive a corresponding bar of the plurality of bars of the rotor cage; anda surface arranged at an inner diameter of the respective lamination, the surface being configured to receive a rotor shaft,and wherein the plurality of laminations comprises:a first set of laminations with a first inner diameter; anda second set of laminations positioned at proximal and distal ends of the stack, where an inner diameter of each lamination of the second set of laminations is larger than the first inner diameter.

2. The rotor cage of Claim 1, wherein the inner diameter of each lamination of the second set of laminations is the same.

3. The rotor cage of Claim 2, wherein the inner diameter of each lamination of the second set oflaminations is 0.9mm larger than the first inner diameter.

4. The rotor cage of any previous claim, wherein the second set of laminations comprisesbetween 6 to 10 laminations at the proximal end of the stack, and between 6 to 10 laminations at the distal end of the stack.

5. The rotor cage of Claim 4, wherein the second set of laminations comprises 6 laminations atthe proximal end of the stack, and 6 laminations at the distal end of the stack.

6. The rotor cage of any previous claim, wherein the plurality of laminations comprises between 600 and 700 laminations.

7. The rotor cage of any previous claim, wherein each lamination of the plurality of laminationshas a thickness of between 0.1mm and 0.35mm.

8. The rotor cage of any previous claim, wherein one lamination of the second set of laminationsat the proximal end contacts one of the two end rings, and one of the second set of laminations at the distal end contacts the other of the two end rings.

9. The rotor cage of any previous claim, wherein each lamination of the first set of laminationscomprises one or more holes located between the first inner diameter and the outer diameter.

10. A rotor for an induction motor, comprising:the rotor cage of any of Claims 1 to 9;a rotor shaft, wherein the rotor shaft is positioned within the inner diameter of the plurality of laminations.

11. The rotor of Claim 10, wherein the first inner diameter achieves an interference fit with the rotor shaft and wherein the inner diameter of each lamination of the second set of laminations achieves a clearance fit with the rotor shaft.

12. An induction motor, comprising:the rotor of Claim 10 or 11;a stator arranged to surround the rotor, and configured to receive electrical current in order to generate rotational movement in the rotor relative to the stator.

13. A method of manufacture of a rotor cage according to Claim 1, the method comprising: punching the first set and second set of laminations from a plurality of lamination sheets; stacking the first set and second set of laminations such that the second set of laminations is positioned at the proximal and distal ends of the stack and the first set of laminations are positioned between them;aligning the first set and second set of laminations such that the one or more slots proximate the outer diameter of each of the first set and second set of laminations are in alignment to form one or more outer channels;providing a mould at the proximal and distal ends of the stack, each mould being configured to receive molten metal and being shaped in the form of one of the two end rings;pouring molten metal into the one or more outer channels and the moulds at the proximal and distal ends of the stack; andcooling the molten metal to form the plurality of bars and the two end rings of the rotor cage.

14. The method of manufacture of Claim 13, wherein punching the first set and second set of laminations from the plurality of lamination sheets comprises punching each lamination of the first set of laminations with a smaller inner diameter than the first inner diameter and punching each lamination of the first set and second set of laminations with an outer diameter larger than the outer diameter, and the method of manufacture further comprises:following cooling of the molten metal, machining the first and second set of laminations such that the size of the inner diameter of each lamination of the first set of laminations matches the first inner diameter, and the size of the outer diameter of each lamination of the first set and second set of laminations matches the outer diameter.

15. A method of manufacture of a rotor, the method comprising:manufacturing a rotor cage in accordance with the method of manufacture of Claim 13;heating the stack of the rotor cage;cooling a rotor shaft;5 inserting the rotor shaft through the inner diameters of each of the laminations of the laminationstack; andreturning the rotor shaft and the stack of the rotor cage to room temperature.

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