WO2012084031A2

WO2012084031A2 - Rotor with incremental poles

Info

Publication number: WO2012084031A2
Application number: PCT/EP2010/070507
Authority: WO
Inventors: Robert Chin; Juhani Mantere
Original assignee: Abb Research Ltd
Priority date: 2010-12-22
Filing date: 2010-12-22
Publication date: 2012-06-28
Also published as: WO2012084031A3

Abstract

A rotor 40 for an electrical machine comprises a pole 10 for creating a magnetic field for a stator 30 to interact with. The pole 10 is configured such that the magnetic field's magnitude decreases in increments 110, 120 in a circumferential direction from the middle of the pole 10 towards an edge 20 of the same. The decreasing magnetic field' s magnitude contributes to decreasing a cogging torque, and the incremental realization renders the pole manufacturing cheaper compared with poles 10 configured to have a continuously decreasing magnetic field's magnitude.

Description

Rotor with incremental poles

TECHNICAL FIELD

The present invention relates to reducing cogging of an electrical machine by adjusting magnetic properties of rotor poles.

BACKGROUND ART

In a permanent magnet (PM) electrical machine, a rotor with magnet poles exhibits torque ripples caused by a cyclic torque called cogging. Cogging occurs when magnetic fields created by the poles interact with the different permeances of stator tooth and stator slots, respectively, when the rotor is rotating. It is known to address the cogging problem by shaping the magnets such that a magnetic field' s magnitude changes smoothly when a pole moves over a stator tooth edge. Referring to figure 1, a conventionally shaped pole 10 may be continuously sloped towards the pole edges 20 such that an air gap height between the pole 10 and a certain circumferential position of the stator 30 changes smoothly when the rotor 40 rotates. Such pole shape is known e.g. from JP1234038 and WO2010109056. Moreover, JP2005287265 discloses an electrical machine wherein torque ripples have been reduced by appropriately shaping salient poles between magnets attached to a rotor core. Finally, US20090261676 discloses a rotor wherein magnetic poles are mounted

together from elementary elements.

Magnets shaped according to documents JP1234038,

WO2010109056 and US20090261676 are more expensive compared with magnets comprising only simple shapes, such as a rectangular cuboid shape. In the case of the two first mentioned documents it takes more machining actions to provide the sloped surfaces, and in the case of the last document the whole rotor is machined after fixing the magnets to a rotor hub in order to shape the pole surfaces to correspond to the rotor hub radius. It is therefore desirable to provide a rotor with poles in simple shapes, and preferably comprising planar surface shapes only.

Although the magnets according to JP2005287265 have a simple shape, the cogging problem is not addressed in a

satisfactory manner as the change in the magnetic field' s magnitude remains sharp. In addition to the high

manufacturing costs, the poles according to US20090261676 too are affected by the cogging problem resulting from a sharp change in the magnetic field's magnitude.

SUMMARY OF THE INVENTION

One object of the invention is to provide a rotor which addresses a cogging problem of an electrical machine in a cheap manner.

A further object of the invention is to provide an improved electrical machine.

These objects are achieved by the rotor according to

appended claim 1, and the electrical machine according to appended claim 15.

The invention is based on the realization that a rotor pole whose magnetic field' s magnitude changes in increments is cheaper to manufacture and may still have a satisfactory cogging-reducing effect compared with a rotor pole with a constantly changing magnetic field's magnitude.

According to a first aspect of the invention, there is provided a rotor for an electrical machine, the rotor comprising a pole for creating a magnetic field for a stator to interact with. The pole is configured such that the magnetic field' s magnitude decreases in increments in a circumferential direction from the middle of the pole towards an edge of the same.

According to one embodiment of the invention, the increments are symmetrical about a radial middle line of the pole. A symmetrical pole is cheap to manufacture because symmetry leads to a reduced number of differing pole parts.

According to one embodiment of the invention, the increments are arranged at equal distances between a radial middle line of the pole and each edge of the same. By arranging the increments at equal distances the number of differing pole parts may be further reduced.

According to one embodiment of the invention, the number of increments is one or two. The number of increments needs to be kept small in order to achieve advantage with regard to manufacturing costs. The fewer number of increments the cheaper pole.

According to one embodiment of the invention, the pole comprises a plurality of magnets mounted adjacent to each other in a circumferential direction of the pole. Although the principle of the present invention can be applied to a rotor with electromagnetic poles, a preferred embodiment of the invention comprises permanent magnet poles because of the simplicity of such construction.

According to one embodiment of the invention, the magnets have a parallelepiped shape. Such a shape is cheap to manufacture, and a magnet with such a shape is easy to put together from a plurality of equally shaped magnet elements.

According to one embodiment of the invention, the

parallelepiped shape is a rectangular cuboid. Such a shape is even more simple and cheap to manufacture.

According to one embodiment of the invention, the upper surfaces of the magnets are planar, and the magnets are directed such that a normal to the upper surface of each magnet crosses a rotational axis of the rotor. With such a construction an almost constant air gap height can be achieved with magnets comprising only planar surfaces. According to one embodiment of the invention, a magnetic material comprised in a magnet closer to the middle of the pole creates a stronger magnetic field than a magnetic material comprised in a magnet closer to an edge of the pole. By selecting the magnetic materials appropriately, the increments in the magnetic field's magnitude can be achieved when keeping the pole height constant.

According to one embodiment of the invention, a magnet closer to an edge of the pole has a higher coercivity than a magnet closer to the middle of the pole. Because the magnets closer to an edge of the pole are more exposed to de^¬ magnetization, providing only these parts of the pole with a more expensive high-coercivity material contributes to achieving a cheap pole.

According to one embodiment of the invention, a magnet closer to an edge of the pole has a higher temperature grade than a magnet closer to the middle of the pole. Because the magnets closer to an edge of the pole are more exposed to overheating, providing only these parts of the pole with a more expensive high-temperature material contributes to achieving a cheap pole.

According to one embodiment of the invention, the pole has a constant height. With such a construction both the pole and the hub get a simple shape and are cheap to manufacture.

According to one embodiment of the invention, the pole's height decreases in circumferential direction in increments from the middle of the pole towards an edge of the same. With such a construction the increments in the magnetic field' s magnitude can be achieved when all the magnets are made of same material.

According to one embodiment of the invention, there is a plurality of poles, and the spaces between adjacent poles in circumferential direction comprise non-magnetic or ferro^¬ magnetic material such that the rotor has a smooth outer periphery shape, such as a circular shape. A rotor with a smooth outer periphery shape is more silent than a rotor with sharp edges on the outer periphery. According to a second aspect of the invention, there is provided an electrical machine comprising a rotor according to any of the embodiments described hereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail with reference to the accompanying drawings, wherein figure 1 shows a rotor with conventional poles, figure 2 shows one embodiment of the invention comprising poles with magnets in different heights, figure 3 shows one embodiment of the invention with poles comprising magnets in different materials, figure 4 shows several embodiments of the invention with poles comprising magnets in different shapes, and figure 5 shows two pole shapes according to two different embodiments of the invention in a coordinate system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to figure 2, a rotor 40 of a PM machine comprises a hub 50 and a plurality of poles 10 attached to the hub 50. The hub 50 has a circular outer periphery about a rotational axis 60 of the rotor 40, and it is attached to a rotor shaft 70. Each pole 10 includes a set of five magnets 80, 90, 100 arranged in a row in circumferential direction of the hub 50, all the five magnets 80, 90, 100 consisting of same magnetic material. The magnets 80, 90, 100 have three different heights 210, 220, 230, the first magnets 80 in the middle of each pole 10 having greatest height 210, the second magnets 90 having second greatest height 220, and the third magnets 100 having smallest height 230. The pole height thereby decreases in circumferential direction in two increments 110, 120 when moving from the middle of a pole 10 towards an edge 20 of the same. In a complete PM machine the rotor 40 is assembled within a stator 30. As a result from the decreasing pole height, each pole's 10 maximal distance from the rotational axis 60 decreases, and an air gap height 200 between each pole 10 and the stator 30 increases, respectively, when moving from the middle of a pole 10 towards an edge 20 of the same.

The magnets 80, 90, 100 create a magnetic field for the stator 30 to interact with. Resulting partly from the decreased height 220, 230 of the second and the third magnets 90, 100, and partly from the correspondingly

increased air gap height 200, the magnetic field's

magnitude, when measured e.g. at a distance of the stator 30, decreases in increments 110, 120 in a circumferential direction from the middle of a pole 10 towards an edge 20 of the same.

Figure 2a shows one pole 10 in a planar illustration of the rotor's 40 outer periphery. The first magnet 80 has a first width 130 which is two times the second width 140 of the remaining magnets 90, 100. The pole 10 is thereby

symmetrical about a radial middle line 150 of the same.

According to figure 2b, the upper and lower surfaces 160, 170 of the magnets 80, 90, 100 are not planar but they both have a shape of an arc of a circle with a centre of radius at the rotational axis 60, the lower surface 170 additionally having a radius equal to the radius of the hub's 50 outer periphery. The angular width of the magnets 80, 90, 100 is constant, and the side surfaces of the same are planar. In other words, the magnets 80, 90, 100

according to figure 2 have a cross section shape which corresponds to a subtraction of two sector areas with coincident centres and central angles, but different radii.

Referring to figure 3, according to one embodiment of the invention each pole 10 includes a set of five magnets 80, 90, 100 with equal height 240. The magnets 80, 90, 100 consist of three different magnetic materials, the first magnets 80 in the middle of each pole 10 consisting of a first magnetic material, the second magnets 90 consisting of a second magnetic material, and the third magnets 100 consisting of a third magnetic material. Moreover, the spaces 180 between adjacent poles 10 in circumferential direction comprise non-magnetic material 190 in height equal with the height 240 of the magnets 80, 90, 100. The rotor 40 thereby has a circular outer periphery. The first magnetic material creates a strongest magnetic field, the second magnetic material creates a second strongest magnetic field, and the third magnetic material creates a weakest magnetic field. Consequently, the magnetic field's magnitude

decreases in increments 110, 120 in a circumferential direction from the middle of each pole 10 towards an edge 20 of the same. The cross section shape of the magnets 80, 90, 100 corresponds to that of the embodiment according to figure 2. As explained before, the embodiments according to figure 2 and figure 3 comprise magnets 80, 90, 100 with the upper and lower surfaces 160, 170 including a shape of an arc of a circle. As such a shape is expensive to manufacture,

alternative magnet shapes in combination with alternative hub shapes are proposed in figure 4. According to figure 4a the hub 50 has a shape of a regular polygon, and the lower surfaces 170 of the magnets 80, 90, 100 lie in a straight row on a side of the polygon. The magnets 80, 90, 100 have a shape of a rectangular cuboid which corresponds to a

rectangular cross section shape. This combination of hub and magnet shapes results to an air gap height 200 which is non- constant over the width 130, 140 of each magnet 80, 90, 100. Starting from the middle of the pole 10 and moving in a circumferential direction towards an edge 20 of the same, the air gap height 200 first continuously decreases until the first increment 120 where the air gap rapidly increases. Likewise, the air gap height 200 decreases when moving over the second and the third magnets 90, 100 towards an edge 20 of the pole 10, between the second and the third magnets 90, 100 there being a second increment 120 during which the air gap again rapidly increases. Although the polygon in figure 4 is illustrated as a hexagon, in a real rotor 40 the polygon may have any appropriate number of sides, often much greater than six. In figure 5 a pole's 10 shape has been expressed by showing the pole's 10 maximal distance d_r from the rotational axis 60 as a function of circumferential distance d_c from the middle of the pole 10. In other words, the vertical axis of the coordinate system in figure 5 represents the distance d_r of the points on the pole's 10 upper surface 160 from the rotational axis 60, and the horizontal axis represents a circumferential distance d_c from the middle of the pole 10. The distance r_h represents the radius of the hub 50, while distances I and II represent the first and the second increments 110, 120, respectively. The first curve 260 in solid line corresponds to the embodiment according to figure 2, and the second curve 270 in broken line corresponds to the embodiment according to figure 4a. For the sake of simplicity, the circumferential distance d_c from the middle of the pole 10 is considered to be an angular distance in the case of the embodiment according to figure 2, and a linear distance in the case of the embodiment according to figure 4. The second curve 270 in figure 5 may therefore not exactly correspond to the embodiment of figure 4, and figure 5 should therefore be considered as a schematic illustration only .

In practice, in the case of large machines where the radius of the rotor 40 is great and the circumferential dimension of the magnets 80, 90, 100 is relatively small, whether the upper and lower surfaces 160, 170 are curved or planar has a small effect on the air gap height 200. Conclusively, the larger the machine the smaller the negative effect of using magnets 80, 90, 100 in rectangular cuboid shape. Moreover, even when using magnets 80, 90, 100 in rectangular cuboid shape only, the air gap height 200 can be further adjusted by increasing the number of increments 110, 120.

Theoretically, an infinite increase of the number of

increments 110, 120 enables an arbitrary air gap height adjustment, but in practice the number of increments 110, 120 needs to be very small in order to achieve an advantage with regard to manufacturing costs. Consequently, if a satisfactory magnetic character cannot be achieved with a small number of increments 110, 120, it is probably cheaper to give the pole 10 a continuous form than to increase the number of increments 110, 120. It is therefore to be

emphasized that as far as a magnetic character of a pole 10 is concerned, any pole comprising an incrementally changing pole height can be replaced with a pole 10 having a

continuously and smoothly changing height. At the same time it is to be noted that "an increment" in the context of the present invention does not necessarily mean a non-continuous change, but is also to be understood to mean a rapid change in comparison with an overall pole shape.

Referring to figure 4b, the hub 50 can be shaped

appropriately to adjust the positions of the individual magnets 80, 90, 100. For example, by shaping the hub 50 to comprise the desired increments 110, 120, it is possible to achieve the same effect of incrementally decreasing magnetic field's magnitude with fewer magnet sizes. Preferably, the pole 10 is put together by using magnet elements 250 in one size only. According to figure 4b the first magnet 80 consists of two elements 250 arranged side by side, and the second and the third magnets 90, 100 consist of one element 250 each, all the elements 250 being of equal size. In order to further adjust the magnetic field's magnitude, two or more elements 250 may be arranged on top of each other.

Referring to figure 4c, the hub 50 can be shaped

appropriately to receive the individual magnets 80, 90, 100 and to embed the pole 10. The resulting rotor 40 has a smooth periphery and the air gap height 200 is constant provided that the upper surfaces 160 of the magnets 80, 90, 100 are shaped as an arc of a circle with a centre of radius at the rotational axis 60, and a radius equal to the radius of the hub's 50 outer periphery. Such embodiment is shown in figure 4c with a solid line illustrating the upper surface 160 of the magnets 80, 90, 100. Alternatively, the upper surface 160 of the magnets 80, 90, 100 may be planar as illustrated with the broken line.

Figure 4d and 4e show two further embodiments with magnets 80, 90, 100 having planar lower surfaces 170 and arched upper surfaces 160. In the embodiment of figure 4e the increments 110, 120 are created by making the magnets 80, 90, 100 of different magnetic materials. According to the embodiment illustrated with a solid line, the upper surfaces 160 of the magnets 80, 90, 100 are shaped as an arc of a circle, and the spaces 180 between two adjacent poles 10 are additionally completed with a non-magnetic material 190 such that the outer periphery of the rotor 40 becomes circular. Alternatively, the upper surfaces 160 of the magnets 80, 90, 100 may be planar as illustrated with the broken line. Also any other of the embodiments according to figures 2 and 4 may comprise magnets 80, 90, 100 consisting of different magnetic materials. In such a case the magnitude of the magnetic field (measured at a distance of the stator 30) created by each of the magnets 80, 90, 100 does not only depend on the size and position, but also on the material of each respective magnet 80, 90, 100.

Figure 4f shows an embodiment where the magnets 80, 90, 100 have a trapezoidal cross section shape, rectangles on the left side of the figure representing special types of trapezoids. Consequently, the upper and lower surfaces 160, 170 of the magnets 80, 90, 100 are parallel and planar. The air gap height 200 has been rendered almost constant by shaping the hub 50 appropriately. The magnets 80, 90, 100 are directed such that a normal to the upper and lower surfaces 160, 170 of each magnet 80, 90, 100 crosses the rotational axis 60 of the rotor 40. In the case of the rectangular cross sections on the left side of the figure it is the symmetry axes of the rectangles that meet at the rotational axis 60 of the rotor 40.

When a PM machine is operated, a movement over stator teeth causes the pole edges 20 to experience flux concentration at instances when only part of a pole 10 faces a stator tooth. The flux concentration in its turn may cause de- magnetization and overheating of the pole edges 20.

Therefore, it may be advantageous to provide the magnets closer to an edge 20 of a pole 10 with a higher coercivity and with a higher temperature grade than the magnets closer to the middle of the pole 10. The invention is not limited to the embodiments shown above, but the person skilled in the art may, of course, modify them in a plurality of ways within the scope of the

invention as defined by the claims.

Claims

1. A rotor (40) for an electrical machine, the rotor (40) comprising a pole (10) for creating a magnetic field for a stator (30) to interact with,

characterized in that the pole (10) is configured such that the magnetic field' s magnitude decreases in

increments (110, 120) in a circumferential direction from the middle of the pole (10) towards an edge (20) of the same.

2. A rotor (40) according to claim 1, wherein the

increments (110, 120) are symmetrical about a radial middle line (150) of the pole (10) .

3. A rotor (40) according to any of the preceding claims, wherein the increments (110, 120) are arranged at equal distances between a radial middle line (150) of the pole

(10) and each edge (20) of the same.

4. A rotor (40) according to any of the preceding claims, wherein the number of increments (110, 120) is one or two .

5. A rotor (40) according to any of the preceding claims, wherein the pole (10) comprises a plurality of magnets (80, 90, 100) mounted adjacent to each other in a circumferential direction of the pole (10) .

6. A rotor (40) according to claim 5, wherein the magnets (80, 90, 100) have a parallelepiped shape.

7. A rotor (40) according to claim 6, wherein the

parallelepiped shape is a rectangular cuboid.

8. A rotor (40) according to any of claims 5 to 7, wherein the upper surfaces (160) of the magnets (80, 90, 100) are planar, and the magnets (80, 90, 100) are directed such that a normal to the upper surface (160) of each magnet (80, 90, 100) crosses a rotational axis (60) of the rotor (40) .

9. A rotor (40) according to any of claims 5 to 8, wherein a magnetic material comprised in a magnet closer to the middle of the pole (10) creates a stronger magnetic field than a magnetic material comprised in a magnet closer to an edge (20) of the pole (10) .

10. A rotor (40) according to any of claims 5 to 9, wherein a magnet closer to an edge (20) of the pole (10) has a higher coercivity than a magnet closer to the middle of the pole (10) .

11. A rotor (40) according to any of claims 5 to 10, wherein a magnet closer to an edge (20) of the pole (10) has a higher temperature grade than a magnet closer to the middle of the pole (10) .

12. A rotor (40) according to any of the preceding claims, wherein the pole (10) has a constant height (240) .

13. A rotor (40) according to any of claims 1-11, wherein the pole's (10) height decreases in circumferential direction in increments (110, 120) from the middle of the pole (10) towards an edge (20) of the same.

14. A rotor (40) according to any of the preceding claims, wherein there is a plurality of poles (10), and the spaces (180) between adjacent poles (10) in

circumferential direction comprise non-magnetic or ferro-magnetic material (190) such that the rotor (40) has a smooth outer periphery shape, such as a circular shape .

15. An electrical machine comprising a rotor (40) according to any of the preceding claims.