Electric motor

Abstract

An interior permanent magnet motor comprising: a rotor body (102, Fig. 1) having a set of pockets (104, Fig. 1), or magnet insertion holes; a magnet (105, Fig. 2) within each pocket; and pocket liners (106, Fig. 2) which fit in a space between the magnet and the pocket, partially filling that space and mechanically locating the magnet. The liner, perhaps U-shaped, may create voids into which the magnet can expand when subject to Hydrogen decrepitation, perhaps by holding the magnet away from some pocket walls by means of spacers (109, Fig. 2 or 407, Fig. 10). The spacers may also be resilient and bias the magnet against one wall of the pocket. The magnet may be in direct contact with two opposing walls by means of protrusions 104a extending through the liner. The liner may have a locating feature to engage with a groove and / or recess in the pocket. There may be gas channels (111, Fig. 2) between the magnet and the outside of the rotor. The liner and magnet may be inserted into the rotor together or separately, and the liner may include a temporary sheet of low friction material withdrawn after assembly.

Description

This invention relates to improvements in electric motors that include permanent magnets and in particular interior permanent magnet motors. It is known to provide electric motors that include permanent magnets which generate a magnetic field that in use will interact with a corresponding magnetic field generated by electrical current flowing through motor windings. Various configurations are known, and each has specific advantages and disadvantages. One type of motor is the interior permanent magnet motor, sometimes known as a buried magnet motor. These motors comprise a rotor that has a set of magnets located in pockets formed into the rotor below the outer surface of the rotor. These are usually arranged in a pattern of alternating North and South poles facing the outer surface. The stator is provided with a set of teeth around which coils of conductive wire are wound. By applying suitable patterns of current to the coils the rotor can generate a controlled amount of torque and rotate at a controlled speed. To prevent noise in operation of the motor the magnets are a close press fit within the pockets, so that the magnets and pockets have a complimentary shape. This also ensures that there are no air gaps between the magnets and the surrounding pocket which would disrupt the flow of magnetic field between the rotor magnets and the magnetic field generated by the stator. One difficulty presented by an interior permanent magnet motor compared with a motor where the magnets are mounted on the surface is that it become difficult to repair or recycle the motor. Because the rare earth metals used in permanent magnets are a limited resource it is beneficial to recycle that material but to separate the magnets from the rotor is very difficult. They are either a tight press fit or in some cases also held in place by adhesive, or both. GB2487656A filed by the University of Birmingham describes a process for recycling magnets using hydrogen. The magnets are exposed to hydrogen gas, whilst in situ within an assembly, such that hydrogen decrepitation occurs. The decrepitation process causes the selected rare earth magnets to disintegrate into a particulate form which may be attracted to a magnet such that it can be removed from the remaining assembly. This process may be used to remove faulty or damaged rare earth magnets from assembly arrangements for various devices such as motor or generator arrangements. The recovered rare earth particulate may be reused to form new rare earth magnets. The applicant has appreciated that this process is challenging where the magnets are embedded in a rotor of the motor as is the case with an interior permanent magnet motor. Because they are tightly packed into pockets or held by adhesive very little area is exposed to any hydrogen flow, and where the magnets are decrepitated, they expand and become even more closely packed into the pockets. According to a first aspect the invention provides an interior permanent magnet motor comprising: a stator, a rotor body having an outer surface that faces the stator, a set of pockets in the rotor body close to the outer surface of the rotor, the pocket having a top face, a bottom face that is spaced from and opposes the top face and is closer to the axis of rotation of the rotor than the top face is to the axis of rotation, and a pair of opposed side walls that connect the top face to the bottom face, and a magnet located within each of the pockets that is undersized relative to the pocket, in which the motor further comprises: a pocket liner which fits within the pocket in a space between the magnet and the pocket, the pocket liner partially filling that space between the magnet and the pocket and mechanically locating the magnet within the pocket. The pocket liner most preferably defines voids into which the magnet can expand when subject to a process of decrepitation. The pocket liner may include a plurality of spacer elements having rounded edges that engage the magnet. The provision of spacer elements having rounded edges provides a ’lead in’, which aids assembly of the magnets into the pocket liners and prevents localised scraping of the magnets surface. These spacer elements can also be arranged in a way that can provide higher localised contact pressure (improving retention) and would potentially allow better gas flow around the magnet for recycling. This may also assist in removal of the magnets from the rotor at the end of the life of the rotor. The pocket liner may include at least one outward facing locating feature that locks the liner in position by engaging with a corresponding recess provided in a side wall of the pocket. This may be resilient allowing it to deform on removal or insertion of the magnet and liner into the pocket. The rotor body may include at least one groove that extends from an edge of the rotor pocket down into the pocket and which acts as a guide for the at least one locating feature towards and into the recess during assembly. The depth of the groove may be less than the depth of the recess for engaging the locking feature with the locking feature or a part of the pocket liner that supports the locking part being resiliently deformable to snap into the recess when in the correct position. The pocket liner may comprise at least one inward facing spacer element that holds a side of the magnet clear of a side wall of the pocket liner. Preferably there are at least two spacer elements, one for each side of the magnet. These may each have a rounded tip where they engage the magnets. The pocket liner may include a resilient spacer element that is located between the bottom face of the pocket and the magnet and applies a biasing force onto the magnet to press the magnet into contact with the top face of the pocket, i.e., to bias it with a radial component away from the axis of rotation of the rotor. The pocket may include at least one channel formed into the base into which a locating portion of the pocket liner is received. The pocket liner, pocket and magnet may be shaped such that a central portion of a face of the magnet is in direct contact with the base of the pocket at the same time that the magnet is in contact with the top face of the pocket. The base of the pocket liner may therefore include at least one cut out through which the base of the pocket projects or the bottom of the magnet, or both, to allow them to make direct contact. Alternatively, the base of the magnet and the bottom surface of the pocket may be indirectly in contact through the pocket liner. Two or more of the pocket liner, the magnet and the pocket walls may define a least one channel that extends from a surface of the magnet and terminates at an outer surface of the rotor body. This channel or channels permit hydrogen used in recycling of the magnets to contact the magnets. The channels may be located between the pocket and the outer surface of the pocket liner that faces away from the magnet, or between the inner surface of the pocket liner and the magnet. The channel or channels may be located at the edges of the magnet or in close proximity to the edges with no channel located in a central region of the magnet walls or base. By central region we mean a zone that extends over at most 25 percent of the surface centred on a centre line of the surface. By close to the edge, we mean that the distance from the edge of the magnet to each edge of the channel is less than 25 percent of the width of the face of the magnet. The pocket liner my comprise a U-shaped body that has two opposed sides connected by base portion, the sides walls and base portion supporting the spacer elements. The body may comprise a sheet of material, preferably steel that is folded to form the required u-shape. This may include a number of cut outs that extend through the u-shaped body and also a number of protrusions that project from the main plane of the body to form locating features or spacer elements. The U-shaped body may contain features at either end that once the U-shaped body has been pushed into the pocket may be folded in to position to secure the U-shaped body in place, or to prevent the magnet from being inadvertently removed from the assembly. The spacer elements on the side walls may project outward from the side walls away from the magnet to contact the pocket or may project inwards from the side walls to contact the magnet. The walls of the u- shaped body may include openings that extend through the bracket to expose a surface of the magnets. One or more of the spacer elements that are located at the sides of the magnet may be resilient. This allows the pocket liner to hold the magnet securely in place without adhesive. If preferred, one or more separate channels may be defined which can receive adhesive without the adhesive spreading into other channels or recesses that are provided for expansion of the magnet. The rotor body may comprise a solid rotor, or a laminated rotor formed from a stack of rotor plates. The pocket liner may comprise a resilient material, such as a spring steel. It may comprise the same material that is used for the construction of the rotor body or may be a different material. The pocket liner may comprise a magnetic material. The magnets may comprise a range of different compositions. One possible suitable magnet material is NdFeB which is suitable of use in a hydrogen based decrepitation process for removal of the magnets Other material include but are not limited to Strontium Cobalt SmCo and Strontimum Ferrite SmFe. The magnets may be generally rectangular in cross section and the pockets may also be generally rectangular in cross section but oversized relative to the magnet. According to a second aspect the invention provides a method of assembling a motor according to the first aspect comprising: providing the rotor body; inserting a pocket liner into a pocket in the body; and pressing a magnet into the pocket guided by the pocket liner. The method may comprise inserting all the pocket liners before inserting any magnets or the liner and magnets may be inserted simultaneously. The method may comprise applying a sheet of low friction material into the pocket liner prior to inserting the magnet, pressing the magnet into the pocket sliding on the sheet of material, and removing the sheet of material once the magnet is at least partially inserted into the pocket. In an alternative the method may comprise the steps of: providing the rotor body; inserting a magnet into each pocket liner outside of a pocket in the body; and pressing the pocket liner and the magnet into the pocket. The two alternate methods may each additionally comprise applying a sheet of low friction material into the pocket liner prior to inserting the magnet, pressing the magnet into the pocket sliding on the sheet of material, and removing the sheet of material once the magnet is at least partially inserted into the pocket. This prevents scratches being formed as the magnets are inserted. The risk of scratches is greatly reduced where the pocket liner has spacer elements that have rounded ends that engage with the magnets. According to a further aspect the invention provides a rotor assembly for an interior permanent magnet motor of the first aspect of the invention. There will now be described by way of example only two embodiments of the present in invention with reference to and as illustrated in the accompanying drawings of which: Figure 1 shows an interior permanent magnet motor in accordance with the present invention; Figure 2 shows a first arrangement of a pocket liner in cross section located in the rotor body; Figure 3 shows a second arrangement of a pocket liner in cross section located in the rotor body; Figure 4 shows the pocket liner of the first embodiment in perspective in isolation from the rest of the motor; Figure 5 shows the magnet, pocket liner and pocket of the first embodiment in an exploded view; Figure 6 shows the pocket liner of the second embodiment in perspective in isolation from the rest of the motor; Figure 7 shows the magnet, pocket liner and pocket of the second embodiment in an exploded view; Figure 8 shows an alternative arrangement in which the pocket liner is spaced from the sides of the magnets by spacer elements, and Figure 9 shows a still further arrangement of a pocket liner and pocket; Figure 10 shows a modification to the pocket liner of Figure 2 that includes internally facing spacer elements; and Figures 11 and 12 shows the pocket liner of the first embodiment in perspective in isolation from the rest of the motor with additional features for enclosing the ends of the magnet to prevent accidental release when subject to axial load. As shown in Figure 1 , an embodiment 100 of a motor in accordance with the present invention comprises a stator 101 and a rotor 102. The stator and rotor are each generally cylindrical with a rotational axis of symmetry that extends into the page in Figure 1. The rotor is located inside the stator and the axes of both are aligned. The stator comprises a yoke from which extend a set of inwards facing teeth 103. Coils of conductive wire (not shown) are wrapped around each tooth and are connected together to form a set of phases, typically three phases. The tips of the teeth face, and a close to, the outer circumferential face of the rotor. The rotor comprises a rotor body having an outer surface that faces the tips of the teeth of the stator. The body comprises a stack of sheets of magnetically conductive material. The sheets are laminated together to form a solid body. Machined into the rotor body is a set of closed sided pockets 104. As shown in Figure 1 there are 8 pockets but there may be more of fewer pockets. Each pocket 104 is formed by cut outs in each plate that are aligned to form recesses that extend from a free end of the rotor into the rotor body. The pockets 104 are located a short distance below a continuous and generally smooth cylindrical circumferential surface of the rotor. In this example the rotor is cylindrical but it is within the scope of this invention to provide a rotor that does not have a circumferential surface. The invention could be applied to a spoke shaped rotor where the magnets extend radially from the centre within each spoke or where the rotors form a series of V shaped portions that each contain two magnets. They are closed in that they have a top and bottom surface connected by sides and are open to the outside of the rotor only at the axial ends of the rotor body. These ends may be capped off if desired to cover and protect the magnets. Each pocket 104 receives a magnet 105. In this example the magnets are rare earth permanent magnets including NdFeB. The magnets have a cross section that is generally the same as the cross section of the pocket but are undersized. To hold the magnets in place each pocket also receives a pocket liner 106. The configuration of this pocket liner, and the shape of the pocket, is different for the first embodiment and for a second embodiment and they are shown in more detail in Figures 2 to 5 of the drawings. The function of the pocket liner is the same in each case- to locate the magnets whilst defining channels which enable the magnet to expand when being decrepitated during recycling. The overall shape of the pocket liner is the same for each embodiment. The liner 106 comprises a u-shaped body formed by pressing of a flat sheet of metal, although other forms can be used. The body surrounds the sides and base of the magnet when fitted into the pocket. The body in these examples does not cover the ends of the magnets. Figures 11 and 12 show alternative designs for a liner 206,306 in which foldable tabs 207,307 are provided that retain the magnets in the slot and prevent them being removed axially until the tabs are folded out of the way. The body has a base that face the bottom of the magnet and two side walls that each face a respective side face of the magnet. Within the side walls and base are cut outs that define recesses that permit the flow of hydrogen gas during recycling of the rotor and magnets. These recesses connect to channels that can support an inflow of hydrogen gas when the motor rotor is to be recycled. As will be described resilient spacer elements are provided to apply a bias force to the magnet and optional locking elements that engage in recesses in the rotor body to lock the pocket liner in place may be provided. First embodiment. As shown in Figures 2, Figure 4 and Figure 5, the pocket liner 106 comprises a u-shaped body having a generally planar base 107 and two side walls 108 that project orthogonally from opposed edges of the base. The side walls each have two outward facing resilient spacer elements 109 pressed from them and a through hole. The base has two cut outs 110 that are located midway along each free edge to provide an H-shape or I-shape base portion. Six holes are formed into the base, and two pairs of outward facing resilient spacer elements 109 are provided. These are best seen in Figures 4 and 5. The magnet 105 is a bar magnet having a regular rectangular cross section that is a close fit within the u-shaped pocket liner 106. The pocket itself 104 has protrusions 104a in a bottom surface that are spaced apart such that the central bar of the H-shaped base will be located snugly between them when the pocket liner and magnet are correctly positioned in the pocket. These can be seen in Figure 5. The resilient spacers hold the walls of the pocket liner 106 clear of the side walls of the pocket to define channels 111. Gas in these channels can pass through the hole in each side wall to come into contact with the sides of the magnets. The base of the magnet 105 contacts the base of the pocket where the two protrusions are present, and the upper face of the magnet is pushed into contact with the top of the pocket by the resilient spacer elements on the base. In use the pocket liner 106 is first inserted into a pocket 104, the magnet 105 is then pushed into the pocket until the base snaps into place between the two raised protrusions on the base of the pocket. These hold the pocket liner in place. The magnet 105 may be retained by the resilient biasing force generated by the spacer elements. Where tabs are provided on the axial ends of the liner they can be folded into place to provide additional retention or security of the magnet to the pocket liner. An alternative pocket liner 406 is shown in Figure 10 which alternatively includes a set of resilient spacer elements 407 in place of or in addition to the spacer elements shown in Figure 2. These hold the side walls and base of the pocket liner clear of the magnet. This may be beneficial in providing greater surface area of the magnet that can be exposed to hydrogen when recycling. Providing rounded tips to these spacer elements also helps reduce potentially damage through scratching to the magnet as it is pressed into the pocket liner during assembly. Second embodiment As shown in Figure 3, Figure 6 and Figure 7, an alternative pocket liner 506 and the magnet are the same as that of the first embodiment except for the base of the pocket liner and the bottom surface of the pocket. The base of the pocket liner 506 has a central square opening 507 to provide an O-shaped base, and the bottom face of the pocket is provided with a single square protrusion 508 that is complimentary to the opening in the pocket liner. This retains the pocket liner in place in the pocket. With the first and second embodiments the resilient spacer elements that extend out from the sides of the pocket liner may locate within recesses in the side walls of the pocket to lock the pocket liner in position. This may replace or be in addition to the protrusion on the base of the pocket. Third embodiment Figure 8 is a cross sectional view similar to Figure 2 of an alternative arrangement of a pocket liner 606 in which the resilient spacers 607 extend inward towards the magnet rather than outward. In this arrangement the side walls of the pocket liner contact the pocket wall and not the magnet. Channels for gas are provided between the pocket liner and the magnet. Outwardly directed locking elements are formed on the base of the pocket liner that are located in grooves formed in the base of the pocket. Fourth embodiment Figure 9 shows a still further modified arrangement of a pocket liner 706 in which the magnet and side walls of the pocket liner are the same as the embodiment of Figure 8. This differs in that the base has a cut out similar to the ones of the first and second embodiments to provide direct contact between the base of the magnet and the pocket. Regions of the pocket base towards the edges of the magnet are cut away to provide additional channels for gas to flow along. The provision of the pocket liner provides significant benefits in the ease with which the motor can be recycled at the end of its life. The rotor and stator can be easily split apart using mechanical processing. The magnets can then easily be removed from the pockets using a process known as hydrogen decrepitation such as described in GB2387656A. The channels provided between at least two of the pocket, the pocket liner and the magnet allow the sides of the magnets and base to be exposed directly to hydrogen and also define voids which provide room for the magnet to expand into as it turns to powder. Once the magnet has been turned to powder it can easily be extracted and separated from the other parts of the motor. By use of a liner that has the same material type as the rotor it does not need to be mechanically extracted but it is possible to do so if desired allowing the rotor body, and even the pocket liners, to be re-used to form a new motor rotor with fresh magnets. In some arrangements the shape of the liners may magnets to be simply mechanically pushed out of the rotor pockets intact, potentially allowing for the magnets to be re-used in other applications following inspection and if necessary re-coating for corrosion protection. This enhances sustainability of the products through requiring minimal energy to extract magnets and to be able to reemploy them. Acknowledgement The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 821114

Claims

1. An interior permanent magnet motor comprising: a stator,a rotor body having an outer surface that faces the stator,a set of pockets in the rotor body close to the outer surface of the rotor,the pocket having a top face, a bottom face that is spaced from and opposes the top face and is closer to the axis of rotation of the rotor than the top face is to the axis of rotation, and a pair of opposed side walls that connect the top face to the bottom face, anda magnet located within each of the pockets that is undersized relative to the pocket, in which the motor further comprises:a pocket liner which fits within the pocket in a space between the magnet and the pocket, the pocket liner partially filling that space between the magnet and the pocket and mechanically locating the magnet within the pocket.

2. An interior permanent magnet motor according to claim 1 in which the pocket liner defines voids into which the magnet can expand when subject to a process of decrepitation.

3. An interior permanent magnet motor according to claim 1 or claim 2 in which the pocket liner includes a plurality of spacer elements having rounded edges that engage the magnet.

4. An interior permanent magnet motor according to any preceding claim in which the pocket liner includes at least one outward facing locating feature that locks the liner in position by engaging with a corresponding recess provided in a side wall of the pocket.

5. An interior permanent magnet motor according to claim 4 in which the rotor body includes at least one groove that extends from an edge of the rotor pocket down into the pocket and which acts as a guide for the at least one locating feature towards and into the recess during assembly.

6. An interior permanent magnet motor according to any preceding claim in which the pocket liner comprises at least one inward facing spacer element that holds a side of the magnet clear of a side wall of the pocket liner.

7. An interior permanent magnet motor according to any preceding claim in which the pocket liner includes a resilient spacer element that is located between the bottom face of the pocket and the magnet and applies a biasing force onto the magnet to press the magnet into contact with the top face of the pocket.

8. An interior permanent magnet motor according to any preceding claim in wich the pocket liner, pocket and magnet are shaped such that a central portion of a face of the magnet is in direct contact with the base of the pocket at the same time that the magnet is in contact with the top face of the pocket.

9. An interior permanent magnet motor according to any preceding claim in wich two or more of the pocket liner, the magnet and the pocket walls define a least one channel that extends from a surface of the magnet and terminates at an outer surface of the rotor body.

10. An interior permanent magnet motor according to any preceding claim in which the pocket liner comprises a U-shaped body that has two opposed sides connected by base portion, the sides walls and base portion supporting the spacer elements.

11. A method of assembling a motor according to the first aspect comprising: providing the rotor body;inserting a pocket liner into a pocket in the body; andpressing a magnet into the pocket guided by the pocket liner.

12. The method of claim 11 comprising inserting all the pocket liners before inserting any magnets or the liner and magnets may be inserted simultaneously.

13. The method of claim 11 or claim 12 comprising applying a sheet of low friction material into the pocket liner prior to inserting the magnet, pressing the magnet into the pocket sliding on the sheet of material, and removing the sheet of material once the magnet is at least partially inserted into the pocket.

14. The method of claim 11 comprising the steps of:providing the rotor body;inserting a magnet into each pocket liner outside of a pocket in the body; and pressing the pocket liner and the magnet into the pocket.

15. A rotor assembly for an interior permanent magnet motor according to any one of claims 10 to 10.

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