Rotor assembly method and jig

Abstract

A permanent magnet rotor assembly method comprising: stacking laminations (300, Fig. 5) on a jig 200; applying adhesive to the laminations; placing the stack and jig in a curing chamber; curing the ad

Description

TECHNICAL FIELD The present disclosure relates to rotor assembly. Aspects of the invention relate to a method of assembling a rotor of a permanent magnet electric motor, and a jig for assembling a rotor of a permanent magnet electric motor. BACKGROUND Permanent magnet electric motors can be used to provide traction for electric vehicles. The skilled person will be familiar with permanent magnet electric motors, examples of which are described in an article by Christoper Jaszczolt entitled: ‘Understanding Permanent Magnet Motors’ January 31,2017; Control Engineering. In use, permanent magnets in the rotor generate a magnetic field that interacts with stator windings and induces torque in the rotor. SUMMARY OF THE INVENTION Rotors for a permanent magnet electric motor comprise a plurality of laminations arranged in a stack, and permanent magnets which are embedded into the lamination stack. A number of methods exists for assembling rotors, however it is desired to find a more precise and robust method for manufacturing rotors for permanent magnet electric motors in order to improve manufacturing processes. Aspects and embodiments of the invention provide a method and an apparatus for assembling a rotor of a permanent magnet electric motor, a permanent magnet electric motor, and a vehicle, as claimed in the appended claims. According to an aspect herein permanent magnets are fixed to laminations of a permanent magnet electric motor using an adhesive and then cured. According to an aspect herein there is provided a method of assembling a rotorfor a permanent magnet electric motor, the method comprising assembling first and second rotor laminations in a stack on a jig, depositing an adhesive on the first and / or second rotor laminations, placing the jig and the rotor laminations in a curing chamber, and curing the adhesive at a temperature of at least 100°C for at least 1 hour. The method described herein advantageously uses adhesive to hold the stack in place. In embodiments herein, both the jig and the first and / or second rotor laminations can be placed into a curing chamber, thus simplifying the assembly process. In some embodiments, the step of depositing an adhesive comprises placing a plurality of permanent magnets into apertures in the first and second laminations; and depositing the adhesive around the apertures of the first and / or second rotor laminations, to adhere the magnets to the first and / or second rotor laminations. In this way, the permanent magnets can be held in place and affixed to the first and / or second rotor laminations in an efficient manner. Placing the plurality of permanent magnets into apertures in the first and second laminations and depositing the adhesive around the apertures occurs when the laminations are assembled on the jig. In some embodiments, the step of depositing an adhesive comprises depositing the adhesive between the first and second rotor laminations. Thus, the first and second rotor laminations can be affixed together according to embodiments herein. In some embodiments, the method may further comprise placing a first washer between a first side of the stack and the jig, and placing a second washer between a second side of the stack and the jig. The adhesive is highly adherence to metal and thus, the washers act as a barrier between the stack and the jig, preventing the adhesive from causing the stack to adhere to the jig. The first and second washers may comprise PTFE. PTFE washers help keep the lamination stacks in place and also allow curing. Furthermore PTFE does not adhere to metal. The method may further comprise aligning location features on the first and / or second laminations with a corresponding location feature of the jig, such that the first and second laminations are substantially aligned on the jig. Aligning the first and / or second rotor laminations allows for the apertures on the first and second laminations to be aligned, and thus able to receive the permanent magnets. The method may further comprise applying a compressive force to the stack. Applying a compressive force encourages the adhesive to form a stronger bond during curing. The step of assembling the first and second rotor laminations may comprise positioning each of the rotor laminations on a central boss of the jig. According to another aspect of the invention, there is provided a jig for assembling a rotorof permanent electric motor. The jig comprises a base member which supports a central boss and a top member configured to receive a portion of the central boss, wherein the jig comprises of one or more materials capable of withstanding a temperature of 140°C for 2 hours. The ability of the jig to withstand such temperatures means that the jig can be placed in the curing chamber alongside the rotor laminations, without losing its integrity or structure. The central boss may comprise a threaded portion and a rotor lamination guide portion. The top member may comprise a hole configured to receive the threaded portion in use, and the rotor lamination guide portion may be configured to receive a plurality of rotor laminations in use. Optionally, the jig may comprise a fixture member having threads configured to engage the threaded portion of the central boss, wherein the hole in the top member comprises a through hole, and wherein the threaded portion of the central boss is configured pass through the hole in the top member to engage the fixture member in use. The rotor lamination guide portion of the central boss may comprise a groove configured to receive a location feature of the rotor laminations in use. The groove provides a means of ensuring all of the rotor laminations are aligned on the jig. The jig may comprise a location pin, wherein the base member, the first washer, and the rotor lamination guide portion of the central boss each comprise a hole for receiving the location pin. The location pin ensures all of the jig components are aligned prior to being placed into the curing chamber, thus helping the jig maintain its structure. The jig may comprise first and second washers, wherein the first washer is configured to be placed adjacent to the base member in use, and the second washer is configured to be placed adjacent to the top member in use. The first and second washers prevent the lamination stack from adhering to the jig after depositing the adhesive on the rotor laminations. The first and second washer may comprise PTFE. 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 flow chart of a method of assembling a rotor for a permanent magnet electric motor according to an embodiment of the invention; Figure 2 is a schematic diagram showing a jig for assembling a rotor of a permanent magnet electric motor in accordance with an embodiment of the invention; Figure 3a is a schematic diagram of the base member and central boss of the jig of Figure 2, shown in more detail; Figure 3b is a schematic diagram of the top member of the jig; Figures 4a to 4d show in more detail, respectively a fixture member, a first washer, a second washer and a location pin of the jig; Figure 5 is a schematic diagram of a plurality of laminations assembled on the jig of Figure 2; and Figure 6 shows a vehicle comprising a rotor as output by the methods of manufacture described herein. DETAILED DESCRIPTION There is a need for providing improved methods of assembling rotors for permanent magnet electric motors, in order to improve reliability and scalability in manufacturing processes. As mentioned above, assembly of a rotor comprises arranging a plurality of rotor laminations into a stack, the rotor laminations comprising thin circular sheets of metal having a plurality of apertures. Permanent magnets are then embedded into the lamination stack in the bores created by aligned apertures of the stacked sheets. In embodiments herein, it has been recognised that the manufacturing process can be improved by using an adhesive to fix the permanent magnets into the lamination apertures, and then curing the adhesive in a curing chamber. This provides a simpler, more reliable assembly process. Fixing the magnets using an adhesive is challenging due to the high temperatures required to cure the adhesive, and because desirable adhesives to use are highly adherent to metal, and thus must be applied precisely. The present disclosure addresses the above-described challenges by providing a method of assembling a rotor for a permanent magnet electric motor, where the laminations are assembled on a stack on a jig. Adhesive is deposited on the laminations, and washers on the jig act as a barrier, preventing the laminations from adhering to the jig. The jig and laminations are placed in a curing chamber to cure the adhesive and fix the laminations and magnets in place. The jig consists of one or more materials suitable for withstanding high temperatures over a prolonged period of time, and thus the jig maintains its integrity during the curing process. In this way the present disclosure provides an improved method for assembling a rotor for a permanent magnet electric motor, that is both reliable and scalable. Figure 1 is a flowchart of a method 100 of assembling a rotor for a permanent magnet electric motor according to an embodiment of the present invention. Briefly, the method 100 comprises in a first step 102 assembling first and second rotor laminations in a stack on a jig. In a second step 104 the method comprises depositing an adhesive on the first and / or second rotor laminations. In a third step 106 the method comprises placing the jig and the rotor laminations in a curing chamber, and in a fourth step 108 the method comprises curing the adhesive at a temperature of at least 100°C for at least 1 hour. In more detail, the skilled person will be familiar with permanent magnet electric motors and rotors in permanent magnet electric motors. A rotor of a permanent magnet electric motor comprises a plurality of laminations. A lamination is a flat sheet of metal. Laminations are often circular, or approximately circular, in shape with a hole at the centre for receiving a rod. In a rotor, a plurality of laminations are arranged on the rod, to form a stack. The laminations of the plurality of laminations have apertures configured to form a bore when aligned in the stack to receive the permanent magnets. When the laminations are assembled in a stack, the apertures are aligned along the stack, and the magnets are inserted into the apertures. As illustrated in Figure 1, at Step 102 of the method 100, first and second rotor laminations are assembled in a stack on a jig. While first and second laminations are specified, the skilled person will appreciate that this is an example, and that any number of laminations may be assembled in the stack. The jig is used to hold the rotor laminations in place during the assembly process. An example jig is illustrated below in Figures 2, 3a, 3b, 4a, 4b, 4c, 4d and 5, although it will be appreciated that this is merely an example and that other jig designs may equally be used. The step of assembling first and second laminations may comprise aligning location features on the first and second laminations with a corresponding location feature of the jig (for example, aligning a lug on the lamination with a groove on the jig, discussed later with reference to Figure 3a), such that the first and second laminations are substantially aligned on the jig. The first and second laminations are aligned so that the apertures in the laminations are aligned along the stack, and thus able to receive the permanent magnets. In some embodiments the jig comprises a central boss, and in such embodiments, the step of assembling 102 the rotor laminations comprises positioning the rotor laminations on the central boss. The central boss is discussed in greater detail with reference to Figures 2 and 3. The method comprises depositing (e.g. applying), at Step 104, an adhesive on the first and / or second rotor lamination. In some embodiments an epoxy-based adhesive is used, but it will be appreciated that any adhesive may be used that is suitable for adhering metals in the manner described herein. In some embodiments, the adhesive is used to adhere the magnets to the rotor laminations. As such, depositing the adhesive may comprise placing a plurality of permanent magnets into apertures in the first and second laminations, and depositing the adhesive around the apertures of the rotor laminations to adhere the magnets to the rotor laminations. In other embodiments, the adhesive may be used to adhere the first and second laminations together. As such, depositing the adhesive may comprise depositing the adhesive between the first and second rotor laminations. Returning to Figure 1, the method then comprises placing, at Step 106, the jig and the rotor laminations in a curing chamber and curing, at Step 108, the adhesive at a temperature of at least 100°C for at least one hour. Thus, the jig may generally be comprised of (or consist of) one or more materials capable of withstanding such temperatures. The above parameters may generally be taken as lower limits. In some embodiments, the material can withstand a temperature of 140°C for 2 hours. Any suitable material may be used, for example steel. The step of curing the adhesive hardens the adhesive, thus fixing the rotor laminations with respect to each other, and / or fixing the permanent magnets into the apertures of the rotor laminations. Accordingly, the rotor laminations and the permanent magnets can be fixed in a single step. In some embodiments, the method further comprises placing a first washer between a first side of the stack and the jig, and placing a second washer between a second side of the stack and the jig. As such, a washer (which is not a component part of the jig) can be placed at either end of the lamination stack to act as a barrier between the lamination stack and the jig. In this way, adhesive deposited on the rotor laminations can be prevented from leaking and making contact with the jig, and so the rotor laminations can be prevented from adhering to the jig during curing. In some embodiments, the first and second washers comprise PTFE. PTFE is advantageous as it can withstand very high temperatures (often over 300 degrees Celsius) and furthermore, it does not bond to epoxy-based adhesives, making it a good material to prevent or at least inhibit leakage of the adhesive from the stack, and without bonding to the stack itself, or melting during the curing process. In some embodiments of the present disclosure, the method may further comprise applying a compressive force to the stack. Applying a compressive force to the stack may be performed prior to placing the jig in the curing chamber to cause formation of a stronger bond between the laminations and / or between the magnets and the laminations during curing. Applying a compressive force prior to placing the jig and laminations in the curing chamber may also ensure that the alignment of the rotor laminations along the stack is maintained during curing. Accordingly, the method of assembling a rotor for a permanent magnet electric motor according to an embodiment of the present invention provides a scalable, reliable manufacturing method for permanent magnet electric rotors. The jig and the rotor laminations are both placed in the curing chamber, and the laminations and the permanent magnets are fixed in a single step, thus providing an efficient assembly method. Additionally, the step of placing the washers between the lamination stack and the jig ensure that the rotor laminations do not adhere to the jig during curing. Figure 2 is a schematic diagram showing a jig 200 for assembling a rotor of a permanent magnet electric motor. The jig 200 may be used with the method described with reference to Figure 1. As illustrated, the jig 200 comprises a base member 202, which supports a central boss 204, and a top member 206 configured to receive a portion of the central boss 204. The jig may generally comprise materials capable of withstanding temperatures greater than 100 degrees Celsius for 1 hour. In some embodiments, the jig consists of one or more materials capable of withstanding a temperature of 140°C for 2 hours. This material property enables the jig to maintain its structure when placed into a curing chamber, as discussed with reference to Figure 1. In some embodiments, the one or more materials may be steel, but any suitable material may be used. As illustrated in Figure 2, the jig 200 further comprises a fixture member 214 and a location pin 220, both of which will be discussed in detail below. The base member 202, central boss 204, and top member 206 are illustrated in greater detail in Figures 3a and 3b. Figure 3a shows the base member 202, comprising a cylindrical base and a threaded portion 204a received in the base. The central boss 204 comprises a lamination guide portion 204b configured to receive and locate a plurality of rotor laminations. The lamination guide portion 204b further comprises a central hole 204c for receiving the threaded portion 204a. In use, the threaded portion 204a extends through the central hole of the lamination guide portion 204b. The rotor lamination guide portion 204b is configured to receive the plurality of rotor laminations such that a lamination stack is assembled on the jig 200. The rotor laminations are supported by the base member 202, and surround the rotor lamination guide portion 204b. In some embodiments the lamination guide portion 204b comprises a groove 210. The groove 210 is configured to receive a corresponding location feature of the lamination, for example a lug (not shown) on the inner edge of the rotor lamination. In the embodiment shown in Figure 3a, the lamination guide portion comprises two grooves 210 on an outer surface, however any suitable number of grooves may be included. The groove 210 is used to align the laminations along the stack on the jig 200. For example, the groves 210 can be used to slot the laminations onto the lamination guide portion 204b and hold them in place, thus preventing rotation of the laminations about the lamination guide portion 204. The top member 206 is shown in Figure 3b, and comprises a hole 212 configured to receive the threaded portion 204a in use. The hole for receiving the threaded portion may be in the centre of the top member 206. The base member 202, central boss 204, and top member may further comprise holes 202a, 204d and 206b respectively for receiving a location pin 220, the location pin prevents rotation of the respective parts of the jig. Further components of the jig 200 are illustrated in Figures 4a to 4d. The jig 200 may further comprise a fixture member 214, shown in Figure 4a. The fixture member 214 may have threads configured to engage the threaded portion 204a of the central boss 204. As mentioned previously the top member 206 comprises a hole 212. The hole 212 may be a through hole, wherein the threaded portion 204a of the central boss 204 is configured to pass through the hole 212 in the top member 206 to engage the fixture member 214 in use. The fixture member 214 may be, for example, a nut, which is configured to be screwed on to the threaded portion 204a of the central boss 204 adjacent to the top member 206. The fixture member 214 provides support to the jig 200, which helps to maintain alignment of the various jig components. The jig further comprises two washers 216, 218, which are illustrated in Figures 4b and 4c respectively. The first washer 216 is configured to be placed adjacent to the base member 202 in use, and the second washer 218 is configured to be placed adjacent to the top member 206 in use (as shown in Figure 2). Accordingly, when a plurality of laminations are assembled into a stack on the jig 200, the first washer 216 and the second washer 218 act as a barrier between the lamination stack and the jig. The presence of this barrier is advantageous since the adhesive deposited on the rotor lamination is highly adherent to metal, and the washers prevent the lamination stack adhering to the jig 200. In some embodiments, the washers are PTFE, however any suitable material may be used. In some embodiments, the jig comprises a location pin 220, and the base member 202, the top member 206, the rotor lamination guide portion 204b of the central boss 204, and the second washer 218 each comprise a hole (202a, 204d, 206b and 218b respectively) for receiving the location pin 220. The hole for receiving the location pin 220 is shown in Figures 3a,3b, and 4c for each of the base member 202, the lamination guide portion 204b, the top member 206, and the second washer 218. In some embodiments, the hole is off-centre. For the lamination guide portion 204b, the second washer 218 and the top member 206, the off-centre hole is a through hole, however in the base member 202, the off-centre hole 202a may extend only partially through the base member 202, for example 50%. In use, the location pin 220 extends through the off-centre holes in the top member 206, second washer 218, lamination guide portion 204b, and base member 202. In some embodiments the jig comprises the location pin 220 to ensure the components of the jig remain aligned during the rotor assembly process, and to provide extra support to the jig 200. In the embodiment shown in Fig 4b, the first washer 216 (adjacent to the base member 202) does not include an off-centre hole. In Fig. 4c and 5, the diameter of the inner hole of the washer 216 and the position of the location pin relative to the jig 200 is such that the location pin 220 does not interact with the first washer 216: the location pin 220 extends through the central hole of the first washer 216. It will be appreciated however this is merely an example, and that other configurations are equally possible, for example, in other embodiments, the first washer 216 also comprises a hole for receiving the location pin 220. An example showing a plurality of rotor laminations 300 assembled in a stack on the jig 200 according to an embodiment of the present disclosure is shown in Figure 5. As discussed above with reference to Figure 1, once assembled on the jig 300, adhesive is deposited on the rotor laminations 300, and the jig 200 and the rotor laminations are placed into a curing chamber to cure the adhesive and fix the laminations and permanent magnets in place. Once the adhesive is hardened, the jig 200 and rotor laminations 300 are removed from the curing chamber, and the lamination stack is removed from the jig (the washers act as a barrier, preventing the adhesive from making contact with the jig, thus preventing the rotor stack from adhering to the jig). The assembled rotor (the lamination stack and permanent magnets) can subsequently be used in a permanent magnet electric motor. As noted above, permanent magnet electric motors produced using the method above may be used in electric vehicles 600 such as that illustrated in Figure 6. In such examples, permanent magnet electric motors can be used to provide traction. 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 method of assembling a rotor for a permanent magnet electric motor, the method comprising;assembling first and second rotor laminations in a stack on a jig;depositing an adhesive on the first and / or second rotor laminations;placing the jig and the rotor laminations in a curing chamber; andcuring the adhesive at a temperature of at least 100°C for at least 1 hour.

2. The method of claim 1, wherein the step of depositing an adhesive comprises:placing a plurality of permanent magnets into apertures in the first and second laminations; and depositing the adhesive around the apertures of the first and / or second rotor laminations, to adhere the magnets to the first and second rotor laminations.

3. The method of claim 1 or 2, wherein the step of depositing an adhesive comprises: depositing the adhesive between the first and second rotor laminations.

4. The method of any one of claims 1 to 3, comprising placing a first washer between a first side of the stack and the jig, and placing a second washer between a second side of the stack and the jig.

5. The method of claim 4, wherein the first and second washers comprise PTFE.

6. The method of any preceding claim, comprising aligning location features on the first and / or second laminations with a corresponding location feature of the jig, such that the first and second laminations are substantially aligned on the jig.

7. The method of any preceding claim, comprising applying a compressive force to the stack.8 The method of any preceding claim, wherein the step of assembling the first and second rotorlaminations comprises positioning each of the rotor laminations on a central boss of the jig.

9. A jig for assembling a rotor of permanent magnet electric motor: the jig comprising:a base member which supports a central boss; anda top member configured to receive a portion of the central boss,wherein the jig comprises materials capable of withstanding a temperature of 140°C for 2 hours.

10. The jig of claim 9, wherein the central boss comprises a threaded portion and a rotor lamination guideportion, and wherein the top member comprises a hole configured to receive the threaded portion in use, and wherein the rotor lamination guide portion is configured to receive a plurality of rotor laminations in use.

11. The jig of claim 10, comprising a fixture member having threads configured to engage the threaded portion of the central boss, wherein the hole in the top member comprises a through hole, and wherein thethreaded portion of the central boss is configured pass through the hole in the top member to engage the fixture member in use.

12. The jig of any one of claims 9 to 11, wherein the rotor lamination guide portion of the central boss comprises a groove configured to receive a location feature of the rotor laminations in use.

13. The jig of any one of claims 9 to 12, comprising first and second washers, wherein the first washer is configured to be placed adjacent to the base member in use, and the second washer is configured to be placed adjacent to the top member in use.

14. The jig of any one of claims 9 to 13, comprising a location pin, wherein the base member, the second washer, and the rotor lamination guide portion of the central boss each comprise a hole for receiving the location pin.

15. The jig of claim 14, wherein the first and second washer comprise PTFE.11

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