Electric motor

Abstract

An interior permanent magnet motor comprises a stator, a rotor body having an outer surface that faces the stator, a set of pockets 104 in the rotor body close to the outer surface of the rotor and a rare earth magnet 105 located within each of the pockets. Each pocket in the rotor is associated with an expansion region below the surface of the pocket that faces away from the axis of rotation of the rotor which accommodates expansion of the magnet when the magnet undergoes a process of hydrogen decrepitation. The expansion region may include voids 107 formed into the rotor body, one of which may be open at an end (see 109, 108) into the pocket into which powdered magnet may flow once decrepitated. The void may comprise a network of passages along which powdered magnet can flow. The expansion region may comprise portions of rotor body that can deform during the expansion process to enlarge the volume of the pocket and to accommodate the expansion of the magnet such that the portions function as leaf springs (Fig.3, 209) supported at one or both ends. The magnets may be press fit within a respective pocket such that the magnets slightly deform the expansion region so that the portion of the rotor body in the expansion region applies a biasing force pressing the magnet onto a face of the pocket.

Description

This invention relates to improvements in electric motors that include rare earth permanent magnets, in particular to an interior permanent magnet motor. 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. On 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. Interior permanent magnet motors. 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 difficult by mechanical means. They are either a tight press fit or in some cases also held in place by adhesive. 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. GB2487656A teaches that NdFeB magnets typically exhibit a volume expansion of 5% during the decrepitation process. 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 and a rare earth magnet located within each of the pockets, in which each pocket in the rotor includes an expansion region below the surface of the pocket that faces away from the axis of rotation of the rotor which accommodates expansion of the magnet when the magnet undergoes a process of hydrogen decrepitation. The expansion region may include one or more voids formed into the rotor body at least one of which is open at an end into the pocket into which powdered magnet may flow once decrepitated. This accommodates the material with little or no deformation of the rotor required because the magnet powder can flow into the void. The void may comprise a network of passages along which powdered magnet can flow. At least one part of the network may terminate at a surface of the magnet. There may be multiple additional voids which may be isolated from other voids, especially to define a deformable region. The voids may comprise a honeycomb of perforations that extend axially along the rotor body in the expansion region, reducing the overall rigidity of that region of the rotor body. The expansion region may comprise portions of rotor body that can bend or shear or otherwise deform during the expansion process to enlarge the volume of the pocket and to accommodate the expansion of the magnet. These portions may function as leaf springs supported at one or both ends. The leaf springs may exert a force upon the magnet in normal to help retain it in a defined position. The rotor body may comprise a stack of plates that are fixed together, each plate being cut away in the expansion region below each pocket to define voids into which adjacent remaining portion of the expansion region can deflect to reduce the overall volume of the expansion region in turn accommodating an expanding magnet. The cut away regions may be stamped or laser cut into each plate during manufacture. having a top surface facing the stator and a bottom surface opposite the top surface, region below the bottom surface of each pocket The voids may comprise a series of elongate slots arranged in parallel to the base of the magnet and to each other to define leaf spring elements between adjacent slots which deflect as the magnet expands, closing up the voids. The elongate slots may be bisected by a transverse slot that extends away from the magnet such that two sets of cantilevered springs are formed that are deflected as the magnet expands. On deflection the central slot will open and provide additional space to accommodate powdered magnet material. Being cantilevered the resistance to deflection is reduced compared with leaves attached at both ends. In addition to or alternative to cut outs, each plate may be provided with one or more shear lines in region below each pocket which weaken that region of the rotor body. These shear lines may be arranged in parallel to the base of the magnet and to each other to define leaf spring elements between adjacent slots which deflect as the magnet expands. Each shear line may comprise a portion where the plate is thinner than the surrounding plate, or may include a set of perforations, or both. In each case the shear line can be easily deformed when subject to the pressure of an expanding magnet by closing up the shear line. A plurality of discrete shear lines may be provided, or a set of adjoining shear lines. In another alternative, voids may be provided in a close packed pattern to define the expansion region, the voids deforming under pressure from the magnets. The pattern may define identifiable rows of voids, but that could be arranged in a wide variety of patterns achieving the effect of making the expansion region relatively easy to deform. The magnets may each be a press fit within a respective pocket. During normal use the magnets may slightly deform the expansion region where provided, so that the portion of the rotor body in the expansion region applies a biasing force pressing the magnet onto the face of the pocket that is perpendicular to the direction of magnetisation. Typically where the magnets are rectangular bars this will be a planar surface of the pocket this closest to the circumferential face of the rotor body and faces towards the axis of rotation of the rotor. An expansion region may be provided below each pocket of the rotor body. 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 in cross section an interior permanent magnet motor in accordance with the present invention; Figure 2 shows a first arrangement of a pocket and an expansion region below a pocket for a first embodiment of the invention; Figure 3 shows a second arrangement of a pocket and an expansion region below a pocket for a second embodiment of the invention; Figure 4 shows a third arrangement of a pocket and an expansion region below a pocket for a third embodiment of the invention; Figure 5 shows an alternative arrangement of a pocket and an expansion region below a pocket for a fourth embodiment of the invention; Figure 6 is a plan view of a shear line formed in a plate of the rotor body and Figure 7 a view in cross section about the line A-A; Figure 8 is a plan view of an alternative shear line formed in a plate of the rotor body and Figures 9 and 10 are views in cross section about the lines B-B and C-C respectively. As shown in Figure 1, an embodiment of a motor 100 in accordance with the present invention comprises a stator 101 and a rotor 102. The stator 101 and rotor 102 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 co-axial. The stator comprises a yoke from which extend a set of inwards facing teeth, one of which is labelled 103. Coils of conductive wire 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 are close to, the outer circumferential face of the rotor separated only by a small air gap. The rotor 102 comprises a rotor body having an outer surface that faces the tips of the teeth of the stator. The body comprises a stack of plates of magnetically conductive material. The sheets are laminated together to form a solid body with many layers. Each layer may be provided with an insulated coating that prevents the flow of electric current along the axial length of the body. In Figure 1 only the upper most plate is visible. Machined into each laminated plate of the rotor body is a set of pockets 104. As shown in Figure 1 there are 8 pockets but there may be more of fewer pockets. Each pocket is formed by cut outs in each plate that are aligned with those of the adjacent plates to form recesses that extend from a free end of the rotor into the rotor body. The pockets are located a short distance below the circumferential surface of the rotor. The person skilled in the art will be aware of other rotor arrangements such as spoke or 'V' shape arrangements of magnets within the rotors. Each pocket receives one or more magnets 105. In this example the magnets are rare earth permanent magnets including NdFeB shaped as rectangular bars. The magnets have a cross section that is generally the same as the cross section of the pocket and are a press fit into the pocket. Below each pocket 104 the rotor body is provided with an expansion region that includes voids or shear lines in each plate that make this region easily deformable in order to allow the magnets to expand as required during recycling of the motor rotor. This is indicted in Figure 1 generally by the shaded area 106. By below we mean that the voids are located so that flux that has passed from the stator across the air gap and into the portion of the magnet nearest the surface of the rotor that then goes generally radially through the magnet will exit into a region containing the voids. Various different arrangements are possible, and the following examples should be considered a non-exhaustive list intended only to illustrate several different principles of construction of the rotor body. First embodiment. As shown in Figure 2 a magnet 105 is fitted into a pocket 104 and an expansion region below the pocket includes a set of cut outs 107. These cut outs define elongate leaves that are easily deformed as the magnet expands to at least partially close up the cut outs. A central cut out is provided that is connected to a large void 109 which can accommodate powdered magnetic material during recycling. Deformation occurs in the direction of the solid arrows due to pressure from the expanding magnet 105. By maintaining steel in close proximity to a large proportion of the inward side of the magnet of Figure 2 the magnetic flux path is minimally affected. In a preferred implementation, the cut outs within the rotor steel would be chosen such that the remaining steel did not magnetically saturate. Thus the deformable expansion region provides both for normal operation of the motor, and easier recycling. Second embodiment As shown in Figure 3, a network of cut outs 207 is provided which extend in parallel with the base of the magnet 105 and connect to a central cut out 208 that extends orthogonally from the base of the magnet. The cut outs define two sets of two cantilevered leaf springs 209, one set each side of the central cut out 208. When the magnet expands these can easily deflect in the direction shown by the solid arrows. In normal use once the magnet is magnetised these springs may more radially outwards to provide improved magnetic contact between the magnet and rotor steel. Third embodiment As shown in Figure 4, a network similar to that of Figure 2 is provided but instead of cut outs the network is formed from shear lines 307 where the laminate plates are weakened. Compared with Figure 2, this embodiment has the advantage of greater magnetic flux carrying capacity, yet maintaining the inwardly deformable structure during the hydrogen decrepitation recycling process. Figure 6 and 7 shows a shear line 307 where a V-shaped cut is made in one side of a plate. Figures 8, 9 and 10 show an alternative shear line 307a formed from perforations that extend through the plate. In each case, the removal of material in the shear lines provides space for the surrounding material of the rotor body to deform, accommodating expansion of the magnet. Fourth embodiment As shown in Figure 5 a network similar to Figure 3 is provided where the shear lines 407 are parallel to the base of the magnet. Otherwise, this is the same as Figure 4 in function with the shear lines providing weaknesses that reduce the force required to deform the motor rotor in the expansion region below the magnet. The applicants have appreciated that an expansion region below each pocket is beneficial when a motor has reached an end of life and is being recycled. In particular, where a process of hydrogen decrepitation is used the magnets will expand as they turn to powdered form. By allowing the pocket to expand and in some arrangements allowing powder to flow into voids connected to the pocket the powder will not become tightly packed into the pocket which would make it difficult to remove the powder as the packed powder may also impede expansion and full hydrogenation of the magnet. Whilst the examples given show a cylindricial rotor with bar magnets embedded below the outer circumferential surface and voids below the pocket, by which we mean between the pocket and the axis or rotation of the rotor, the invention can be applied to other shapes of rotor and other alingments of the bar magnets. In particular it may be applied to a rotor where the magnets are embedded in radial spokes giving the rotor a star shape in cross section. The magnets may be oriented generally at 90 degrees to that shown in Figure 1. In another arrangement the magnets may be radial and embedded in a cylindrical rotor. In another the magnets may be located within v-shaped rather than rectangular pockets, with one magnet in each arm of the v-shape and the point of the v facing the rotor. 5 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 anda rare earth magnet located within each of the pockets,in which each pocket in the rotor is associated with an expansion region below the surface of the pocket that faces away from the axis of rotation of the rotor which accommodates expansion of the magnet when the magnet undergoes a process of hydrogen decrepitation.

2. An interior permanent magnet motor according to claim 1 in which the expansion region includes one or more voids formed into the rotor body at least one of which is open at an end into the pocket into which powdered magnet may flow once decrepitated.

3. An interior permanent magnet motor according to any preceding claim in which the void comprises a network of passages along which powdered magnet can flow.

4. An interior permanent magnet motor according to any preceding claim in which the expansion region includes multiple additional voids which are isolated from other voids to define a deformable region.

5. An interior permanent magnet motor according to claim 4 in which the voids comprise a honeycomb of perforations that extend axially along the rotor body in the expansion region.

6. An interior permanent magnet motor according to any preceding claim in which the expansion region comprises portions of rotor body that can bend or shear or otherwise deform during the expansion process to enlarge the volume of the pocket and to accommodate the expansion of the magnet such that the portions function as leaf springs supported at one or both ends.

7. An interior permanent magnet motor according to any preceding claim in which the rotor body comprises a stack of plates that are fixed together, each plate being cut away in the expansion region below each pocket to define voids into which adjacent remaining portion of the expansion region can deflect to reduce the overall volume of the expansion region in turn accommodating an expanding magnet.

8. An interior permanent magnet motor according to claim 7 in which the cut away regions may be stamped or laser cut into each plate during manufacture.

9. An interior permanent magnet motor according to any preceding claim in which the voids comprise a series of elongate slots arranged in parallel to the base of the magnet and to each other to define leaf spring elements between adjacent slots which deflect as the magnet expands, closing up the elongate slots.

10. An interior permanent magnet motor according to claim 9 in which the elongate slots are bisected by a transverse slot that extends away from the magnet such that two sets of cantilevered springs are formed that are deflected as the magnet expands.

11. An interior permanent magnet motor according to claim 7 or claim 8 in which each plate is provided with one or more shear lines in region below each pocket which weaken that region of the rotor body.

12. An interior permanent magnet motor according to any preceding claim in which the magnets are a press fit within a respective pocket such that the magnets slightly deform the expansion region so that the portion of the rotor body in the expansion region applies a biasing force pressing the magnet onto the face of the pocket that is perpendicular to the direction of magnetisation.

13. A rotor assembly for an interior permanent magnet motor according to any preceding claim.

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