Rotor arrangement for a permanent magnet excited electrical machine

Abstract

Rotor arrangement (4), in particular rotor, for an electrical machine (1), comprising: - at least one permanent magnet (7); and - at least two pole shoes (42), each held on a holding area (44) and between which the permanent magnet (7) is arranged, wherein, transversely to the arrangement direction of the pole shoes, the permanent magnet (7) has a first section (71) close to the holding area and a second section (72) remote from the holding area, which each have different thicknesses in the arrangement direction of the pole shoes (42), wherein the second section (72) of the permanent magnet (7) has two mutually adjacent magnetization regions (81, 82) in the arrangement direction of the pole shoes, the respective magnetization direction of which is oriented obliquely to an edge of the pole shoes (42) opposite the holding region (44), wherein the magnetization regions (81, 82) in the second section (72) of the permanent magnet (7) are formed by separate, block- or cuboid-shaped permanent magnets (7), and the permanent magnet (7) in the first section (71) near the holding region is also formed as a separate block- or cuboid-shaped permanent magnet (7), wherein the first section (71) of the permanent magnet (7) has a rectilinear magnetization which runs in the arrangement direction of the pole shoes (42), wherein directions P of the magnetizations of the magnetization regions (81, 82) run obliquely in the direction of the end of the pole piece facing the air gap, against which the respective magnetization region (81, 82) rests.

Description

Technical FieldThe present invention relates to a permanent magnet-excited electric machine, in particular to a rotor arrangement for such an electric machine.Prior ArtKnown from the prior art are rotor arrangements which have pole shoes which provide an excitation magnetic field generated by permanent magnets in the direction of an air gap to a stator arrangement. In a variant for arranging the permanent magnets in the rotor arrangement, the permanent magnets are arranged with their pole direction parallel to the direction of movement of the rotor, so that the pole shoes of the rotor arrangement are located between the permanent magnets. In the pole shoes, the magnetic field emitted by the permanent magnets is deflected in the direction of the air gap.Such types of rotor arrangements are nowadays used in many permanent magnet excited electric machines, for example in steering assistance and in cooling devices. In this case, substantially rectangular permanent magnets are used which are simple to produce and with little outlay, with magnetization anisotropy and a magnetization direction in the same direction thereto being oriented in the direction of the movement of the rotor. Such rotors are shown, for example, in DE 699 236 A, U.S. Pat. No. 1,996,946 A, FR 2 476 930 A1, DD 2 22 745 A1, U.S. Pat. No. 6,323,572 B1, JP H03-36 939 A. A rotor having oblique magnetic field lines is shown in JP 2009-254 143 A.In rotary electric machines, such an arrangement of the permanent magnets is referred to as a spoke arrangement. The pole direction of the permanent magnets extends in the tangential direction.An advantage of such rotor arrangements is that an air gap flux density can be achieved by suitable dimensioning, which is greater than the residual flux density of the permanent magnets used. Since the air gap flux density decisively determines the torque density of the electric machine, efficient electric machines with relatively high torque densities can thereby be constructed.This is achieved by selecting the number of permanent magnets in the rotor such that their radial width is greater than half a tangential thickness of the pole shoes at the air gap between the permanent magnets. This arrangement concentrates the magnetic flux in the air gap to produce an air gap flux density that is higher than the residual flux density of the permanent magnets.The above rotor assemblies are often constructed in combination with permanent magnets of simple, inexpensive magnetic material, particularly a magnetic material without rare earth compounds and with relatively low residual flux density, such as permanent magnets of sixth generation sintered ferrite materials.Permanent magnets made of sintered ferrite materials moreover have a relatively low coercive field strength, so that there is the risk of demagnetization in the case of a sufficiently high field strength in the case of an opposing magnetic field. To compensate for this relatively low coercivity, the permanent magnets are made relatively thick in the direction of their material anisotropy and magnetization to make them more resistant to demagnetization by the stator field of the electric machine.Furthermore, in the rotor arrangement described above, two air gaps are provided in the flux path of a permanent magnet. In order nevertheless to achieve a high air gap flux density and a high resistance to demagnetization, the permanent magnets arranged in this way are therefore frequently designed twice as thick (in the polarity direction) as permanent magnets made of ferrite material with their pole direction parallel to the air gap magnetic field, which, due to their design, have only one air gap in their respective flux path.In particular in the case of rotary electric machines with a specific rotor diameter and a predefined number of poles, in the case of a spoke arrangement, each increase in the thickness of the permanent magnets in the tangential direction (polarity direction) leads to a reduction in their maximum possible radial width. This limitation of the radial width of the permanent magnets limits the flux provided by the permanent magnets and thereby reduces the air gap flux density and the torque density of the electric machine. Also, any increase in the thickness of the permanent magnets leads to a reduction in the angular coverage of the individual pole shoes at the air gap of the electric machine and may also lead to this angle being smaller than its optimum.It is an object of the present invention to provide a rotor arrangement in which the risk or tendency of demagnetization of the permanent magnets by action of the stator magnetic field is reduced.Disclosure of the InventionThis object is achieved by the rotor arrangement according to claim 1 and by the electric machine according to the subordinate claim.Further advantageous embodiments of the present invention are specified in the dependent claims.According to a first aspect, a rotor arrangement for an electric machine is provided, comprising:at least one permanent magnet; andat least two pole shoes, each of which is held on a holding region and between which the permanent magnet is arranged;wherein transversely to the arrangement direction of the pole shoes the permanent magnets each have a first section close to the holding region and a second section remote from the holding region, which sections have different thicknesses in the arrangement direction of the pole shoes, wherein the second section of the permanent magnet in the arrangement direction of the pole shoes each has two adjacent magnetization regions, the respective magnetization direction of which is directed obliquely to an edge of the pole shoes opposite the holding region.One concept of the above rotor arrangement is to provide the permanent magnet with a magnetization direction deviating from the arrangement direction at least in a demagnetization region in which demagnetization of the permanent magnet can occur by action of a stator magnetic field of a stator arrangement. Furthermore, the permanent magnets are provided with a plurality of sections running perpendicular to the arrangement direction of the pole shoes and having different thicknesses.In this way, it is achieved that the magnetization of the permanent magnet no longer completely opposes the demagnetizing stator magnetic field, but only proportionally opposes it. Since the magnetization direction of the permanent magnet in the demagnetization region deviates from the direction parallel to the opposing stator magnetic field or runs transversely or obliquely thereto, a correspondingly reduced demagnetization is effected. This makes it possible that in the demagnetization region in which irreversible demagnetization may occur at allowable stator magnetic fields, the effect of the demagnetization effect is attenuated. In particular, for this purpose, the permanent magnet is constructed in such a way that a directional change of the magnetization is provided in the demagnetization region or the demagnetization regions.Furthermore, the resistance of the section of the permanent magnet close to the air gap (remote from the axis of rotation) is increased by its greater tangential thickness in the second section.Furthermore, the magnetization directions of the magnetization regions can have an angle to the arrangement direction of the pole shoes that is between 30 and 45°.In particular, the magnetization regions in the second section can be formed by separate, in particular block- or cuboidal, permanent magnets.According to one embodiment, the first section of the permanent magnet can abut the holding region with a side face.In particular, the thickness, i.e. the tangential width of the first section, can be less than the thickness, i.e. the tangential width of the second section.It can be provided that the ratio of the thickness of the first section of the permanent magnet to the thickness of the second section of the permanent magnet is between 0.6 cos α and 0.9 cos α, wherein α corresponds to the angle between the arrangement direction of the pole shoes and the course of the magnetization in the second section of the permanent magnet.Furthermore, the first section of the permanent magnet can have a rectilinear magnetization which runs in the arrangement direction of the pole shoes.The first portion and the second portion may be formed directly adjacent to each other.According to a further aspect, an electric machine is provided having a stator arrangement for providing a stator magnetic field and the above rotor arrangement, wherein the pole shoes of the rotor arrangement are separated from the stator arrangement by an air gap, wherein at least the second section of the permanent magnet is provided in a region in which, in an operating state of the electric machine, the direction of the stator magnetic field is opposite to the magnetization direction of the first section.Brief Description of the DrawingsPreferred embodiments of the present invention are explained in more detail below with reference to the attached drawings. The following are shown: FIG. 1 shows a schematic cross-sectional illustration through an electric machine having a rotor arrangement in which the permanent magnets are arranged in a spoke arrangement; and FIG. 2 is a fragmentary cross-sectional view of the electric machine of FIG. 1 illustrating the region in which demagnetization of the permanent magnet may occur.DESCRIPTION OF EMBODIMENTSFIG. 1 shows a schematic cross-sectional illustration of a conventional rotary, permanent magnet-excited electric machine 1 having an internal rotor. The electric machine 1 has a cylindrical stator 2. The stator 2 comprises a cylindrical magnetic stator return region 21, from which stator teeth 22 arranged equidistantly in the circumferential direction project radially inwardly and define with their inwardly directed ends a likewise cylindrical inner recess 3. The stator teeth 22 are provided at their radially inner ends with tooth tips 23 which have a circular arc-shaped, concave outer contour. The stator teeth 22 are provided with stator coils (not shown), by means of which a stator magnetic field can be generated when energized.In the inner recess 3, a rotor 4, which is likewise cylindrical, (rotor arrangement of the electric machine 1) is arranged rotatably on a shaft 5 as a rotor arrangement. The rotor 4 has a rotor body 41 which is assembled with pole shoes 42 which are connected via narrow webs 43 (in the radial direction) to a holding region 44 surrounding a shaft 5.The webs 43 are preferably dimensioned such that they have sufficient mechanical stability to hold the outwardly widening pole shoes 42 against radially acting centrifugal forces and transverse forces acting in the tangential direction during acceleration or deceleration of the motor. Furthermore, the webs 43 are dimensioned such that they have a cross section which is sufficiently small to keep the magnetic flux caused by the permanent magnets 7 through the holding region 44 as low as possible. In particular, it is necessary that the portion of the magnetic flux of the permanent magnets 7 that flows through the webs 43 is sufficient to keep the magnetic flux density in the webs 43 in saturation.Pockets 6 are arranged between the pole shoes 42 and permanent magnets 7 are accommodated therein. In order to delimit the pockets 6 radially outwards, the pole shoes 42 have protrusions 45 which overlap the permanent magnets 7 arranged in the pockets 6 in the tangential direction and hold them reliably in the pockets 6, even against the centrifugal forces acting during rotation of the rotor 4. The pockets 6 preferably have a width in the tangential direction in order to accommodate corresponding permanent magnets 7, so that these rest with their pole faces against side faces of the pole shoes 42. The opposite ends of the protrusions 45 are spaced apart by exposing a corresponding side surface of the permanent magnet 7 so as to form a pole gap between the pole shoes 42.An electric machine 1 of this type is operated, for example, by electric motor operation in that the stator coils are energized in such a way that a circulating stator magnetic field is generated which interacts with the excitation magnetic field which is produced by the permanent magnets 7 via the pole shoes 42 and thereby exerts a torque on the rotor 4.The permanent magnets 7 are usually manufactured by pressing ferrite powder material and then sintered, with magnetization anisotropy being imparted to the ferrite material during pressing by application of a magnetic field, and the sintered material is then magnetized in the direction of anisotropy. For reasons of simpler production, the permanent magnets 7 are provided in the form of blocks, preferably in the form of a square, with their pole direction in the tangential direction, i.e. perpendicular to the boundary surfaces between the pole shoes 42 and the pockets 6.The pockets 6 of the rotor 4 are configured to receive a permanent magnet 7 having two sections 61, 62 in order to receive the permanent magnet 7 having a corresponding first section 71 and a corresponding second section 72. A first section 71 of the permanent magnet 7 is arranged around the shaft 5 near the holding region 44 away from the air gap. A second portion 72 of the permanent magnet 7 is arranged at a radially outer position of the rotor 4 close to the air gap between the rotor 4 and the stator arrangement 2. In an embodiment, the first portion 71 of the permanent magnet 7 may be immediately adjacent to the holding portion 44 with a side surface other than a pole surface.The first section 71 of the permanent magnet 7 is preferably rectangular or block-shaped, so that it can be produced in a particularly simple manner. In particular, it can be provided that the first section 71 of the permanent magnet 7 is formed separately and, combined with the second section 72, forms the multipart permanent magnet 7. The permanent magnets 7 are inserted into the pockets 6 with the same pole direction in the tangential direction.The second section 72 has a plurality of magnetization regions 81, 82, which are arranged on one another in the tangential direction, i.e. in the arrangement direction of the pole shoes and the magnetization directions of which are in each case different. The magnetization regions 81, 82 are preferably rectangular parallelepipedal or block-shaped and can preferably be formed by separate permanent magnets 7.The tangential thickness d i of the first section 71 of the permanent magnet 7 is smaller than the tangential thickness d o of the second section 72 of the permanent magnet 7. the sections 71, 72 of the permanent magnet 7 abut against one another with a side surface which preferably runs transversely to the radial direction and which does not correspond to a pole surface, so that a stepped shape of the permanent magnet 7 is formed in the cross section transversely to the axial direction of the rotor 4.FIG. 2 shows a representation of a detail through a rotor arrangement according to a further embodiment, in which the first section 71 of the permanent magnet 7 has a radial width w i and the second section 72 of the permanent magnet 7 has a radial width w o. The radial width ratio w o / ( w o+ w i) is preferably between 0.3 and 0.5, i.e. the first section 71 of the permanent magnet 7 can have a significantly greater radial width than the second section 72.Furthermore, in the embodiment shown, the second section 72 of the permanent magnet 7 has a tangential thickness d o and the first section 71 of the permanent magnet 7 has a tangential thickness d i wherein the thickness d o is significantly greater than the thickness d i.By providing a plurality of sections 71, 72 of the permanent magnet 7 with different tangential thicknesses, it is furthermore possible to achieve an increase in the fundamental wave of the air gap field of an electric machine constructed with such a rotor arrangement, as compared with the prior art.During operation of an electric machine equipped with the rotor arrangement, the permanent magnet 7 can be loaded considerably by the stator magnetic field acting on the permanent magnet 7. In a region of the permanent magnet 7, a partially irreversible demagnetization by the stator magnetic field may occur. The risk of demagnetization exists in particular in the region of the permanent magnet 7 on the side facing away from the direction of movement of the rotor arrangement, which is closest to the air gap of the electric machine 1, and extends over approximately one fifth to half the radial width of the permanent magnet 7 and over one tenth to one third of the tangential thickness thereof. The demagnetization occurs as soon as the magnetic field strength of the stator magnetic field is sufficient to achieve a field strength with an amplitude equal to the coercive field strength of the permanent magnet 7 in corresponding regions of the permanent magnet 7.In order to avoid the demagnetization in the demagnetization region or to reduce the tendency towards this, a magnetization of the permanent magnet 7 can now be provided, as is represented by the arrows P in FIG. 2.In the first section 71 of the permanent magnet 7, i.e. a section facing away from the air gap, a conventional rectilinear magnetization is provided in a thickness direction (tangential direction or direction parallel to the arrangement direction of the pole shoes 42) between the pole surfaces facing the pole shoes 42.However, the second portion 72 of the permanent magnet 7 facing the air gap can be provided with two magnetization regions 81, 82. In contrast to previous arrangements, the directions P of the magnetizations and preferably also the directions of the anisotropies of the magnetization regions 81, 82 run obliquely in the direction of the end of the pole shoe facing the air gap, against which end the respective magnetization region 81, 82 bears. Thus, the direction of magnetization encloses an angle α with the tangential direction (arrangement direction of the pole shoes 42) in each magnetization region 81, 82. The angle α is preferably between 30 and 45°. In this case, a component of the magnetization running in the arrangement direction of the pole shoes 42 is directed in the same direction and the component of the magnetization running in the radial direction (transversely to the arrangement direction of the pole shoes 42) is directed in the opposite direction to one another.By obliquely arranging the magnetization direction in the magnetization regions 81, 82, the effective thickness of the magnetization regions is increased to d o / cos α. Due to the oblique configuration of the magnetization, a substantially rectilinear demagnetizing stator magnetic field, which impinges substantially perpendicularly on the pole face of the permanent magnet 7 facing the pole shoes 42, intersects the magnetization lines or the magnetization direction of the magnetization regions of the second section at an angle that is different from 0° or 180°.The increased thickness of the second portion 72 compared to the thickness of the first portion 71 can further reduce the tendency to demagnetization. Preferably, the ratio of the thickness of the first portion 71 to the thickness of the second portion 72 is between 0.6 cos α and 0.9 cos α.

Claims

Rotor arrangement (4), in particular rotor, for an electric machine (1), comprising: - at least one permanent magnet (7); and - at least two pole shoes (42), which are each held on a holding region (44) and between which the permanent magnet (7) is arranged, wherein transversely to the arrangement direction of the pole shoes the permanent magnet (7) has a first section (71) close to the holding region and a second section (72) remote from the holding region, which each have different thicknesses in the arrangement direction of the pole shoes (42), wherein the second section (72) of the permanent magnet (7) has in each case two mutually adjacent magnetization regions (81, 82) in the arrangement direction of the pole shoes, the respective magnetization direction of which is aligned obliquely with an edge of the pole shoes (42) opposite the holding region (44), wherein the magnetization regions (81, 82) in the second section (72) of the permanent magnet (7) are protected by separate magnetization regions (81, 82) located in the direction of the permanent magnet (7), The permanent magnet (7) may be formed as a block or rectangular permanent magnet, and the permanent magnet (7) may also be formed as a separate block or rectangular permanent magnet (7) in the first section (71) close to the holding region, wherein the first section (71) of the permanent magnet (7) has a rectilinear magnetization which extends in the arrangement direction of the pole shoes (42), wherein directions P of the magnetizations of the magnetization regions (81, 82) extend obliquely in the direction of the end of the pole shoe facing the air gap, on which the respective magnetization region (81, 82) abuts.The rotor arrangement (4) according to claim 1, wherein the magnetization directions of the magnetization regions have an angle to the arrangement direction of the pole shoes (42) that is between 30° and 60° - preferably between 30° and 45°.The rotor arrangement (4) according to claim 1 or 2, wherein the first section (71) of the permanent magnet (7) is formed separately and, assembled with the second section (72), forms the multi-part permanent magnet (7).The rotor arrangement (4) according to any one of claims 1 to 3, wherein the first portion (71) of the permanent magnet (7) abuts the holding region (44) with a side surface.The rotor assembly (4) according to any one of claims 1 to 4, wherein the thickness of the first portion (71) of the permanent magnet (7) is less than the thickness of the second portion (72) of the permanent magnet (7).The rotor arrangement (4) according to claim 5, wherein the ratio of the thickness of the first section (71) of the permanent magnet (7) to the thickness of the second section (72) of the permanent magnet (7) is between 0.6 cos α and 0.9 cos α, wherein α corresponds to the angle between the arrangement direction of the pole shoes (42) and the course of the magnetization in the second section (72) of the permanent magnet (7).The rotor assembly (4) according to any one of claims 1 to 6, wherein the portions (71) of the permanent magnets (7) are alternately magnetized in the opposite circumferential direction.The rotor arrangement (4) according to any one of claims 1 to 7, wherein the first portion (71) and the second portion (72) of the permanent magnet (7) are formed directly adjacent to each other.Rotor arrangement (4) according to one of Claims 1 to 8, wherein the pole shoes (42) have extensions in the circumferential direction on their radially outer edge, which extensions are designed as fixing elements for the permanent magnets (7), wherein the pole shoes (42) are designed in a layered manner together with the holding region (44) from individual sheet metal laminations.Electric machine (1) having a stator arrangement (2) for providing a stator magnetic field and a rotor arrangement (4) according to one of the preceding claims, wherein the pole shoes (42) of the rotor arrangement (4) are separated from the stator arrangement (2) by an air gap (3), wherein the second section (72) of the permanent magnet (7) is provided in a region in which, in an operating state of the electric machine (1), the direction of the stator magnetic field is opposite to the magnetization direction of the first section (71) of the permanent magnet (7).

Images