WO2014005799A1

WO2014005799A1 - Permanent magnet rotor and power tool comprising such rotor

Info

Publication number: WO2014005799A1
Application number: PCT/EP2013/061946
Authority: WO
Inventors: JONAS Gustav MILLINGER; Paul Henric Christofer ROSENGREN; Kjell Ola Tobias Syvertsson
Original assignee: Atlas Copco Industrial Technique Ab
Priority date: 2012-07-06
Filing date: 2013-06-11
Publication date: 2014-01-09

Abstract

A rotor (10) comprising a number of sets (A, B, C) of ring-shaped magnets (12,13), each set comprising at least two ring-shaped magnets (12, 13) that are coaxially arranged one outside the other around a shaft (11) so as to a produce a substantially coherent magnetic flux, wherein the number of sets(A, B, C) are located coaxially next to each other.

Description

Permanent magnet rotor and power tool comprising such rotor

The invention relates to a permanent magnet rotor and a power tool comprising such rotor. Specifically, the invention relates to a radially segmented 4-pole motor.

Background

Conventionally, rotor magnets have been produced by gluing segments of homogenous magnets together. The homogenous magnets are preferably formed from anisotropic ferrous pieces, i.e. from magnets that have an apparent main direction of the grains in the material. The anisotropic ferrous pieces are magnetized in the main direction of the grains. It is also possible to use isotropic ferrous pieces, i.e. pieces that have no apparent main direction of the grains. Such magnets will however have a substantially lower fill factor, which due to the weaker magnetic field ultimately results in a lower efficiency per size of the rotor. By using anisotropic ferrous pieces the magnetic fill factor will roughly be doubled compared to using ferrous materials with a random grain direction.

Two different conventional segmented 4-pole rotor magnets are shown in the cross section in figures 3 and 4, respectively. The simplest way of achieving a 4-pole rotor is to assemble four peripheral segments around a shaft 11 as is shown by the 4-pole rotor 20 in figure 3. The four poles in this rotor 20 consist of two mutually opposed magnet segments 21 and 23 that form north- poles N, and two mutually opposed south-poles 22 and 24 that form south-poles S. The magnet segments 21-24 are usually glued together and a cylindrical casing 25 is arranged peripherally outside the magnet segments 21-24 in order to fix them properly. The casing may either be a tube or relatively thin lining or bandage. Normally, several sets of magnet segments 21-24 are arranged next to each other along the shaft 11, such that a rotor of a desired length is achieved. The more peripheral segments that is used the more ideal motor performance will be achieved. A more efficient rotor 30 is shown in cross section in figure 4, which rotor consists of eight segments 31-38, which are arranged next to each other around a shaft 11. Hence, in addition to the four segments that

constitute the North and South poles 33, 37, and 31, 35, respectively, four intermediate segments 32, 34, 36, and 38 that complete the magnetic field are placed in between them. A casing 39 is arranged peripherally outside the magnet segments 31-38 in order to fix them properly. This rotor-type is known to the skilled person as a 4-pole peripherally segmented inverted Hallbach rotor.

A drawback of such a rotor is that the assembly of the plurality of magnet segments on a relatively small shaft results in complex and expensive assembly procedure.

In addition, peripheral segmentation reduces the magnet fill factor, which gives a poor final motor performance. This is both due to the fact that there is glue between the interfaces of the individual segments. Further though, the surfaces of the individual segments are machined to match each other. This machining degrades the magnetic properties of the material in a zone nearby the surface, which thereby reduces the active volume of the magnets. Also, the outer casing reduces the magnetic ratio per weight. Hence, there is a need for a rotor, which is less complex to assemble and which has a high magnetic fill factor which in turn will yield a higher magnetic remanence in the optimal

magnetizing direction.

Summary of the invention An object of the invention is therefore to provide a rotor, which is less complex to assemble than conventional rotors and/or which has a higher magnetic fill factor. Both these objects are achieved by the invention according to the

independent claims .

The invention is based on the notion that it is possible to produce anisotropic ring segments, in which the grains are pre- directed in a direction that corresponds to a desired 4-pole magnetic field. In accordance with the invention a number of ring shaped magnets are coaxially arranged one outside the other so as to a produce a substantially coherent magnet flux.

According to a first aspect the invention relates to a rotor comprising a permanently magnetized magnet. The rotor comprises at least two ring-shaped magnets that are coaxially arranged one outside the other so as to a produce a substantially coherent magnetic flux. The rotor may also comprise three or more ring- shaped magnets that are coaxially arranged one outside the other.

In a preferred embodiment the rotor includes a number of sets of ring-shaped magnets, each set comprising at least two ring- shaped magnets that are coaxially arranged one outside the other so as to a produce a substantially coherent magnetic flux, and wherein the number of sets are located coaxially next to each other .

In one embodiment the rotor is a multi-pole rotor, wherein the coherent magnetic flux of the ring-shaped magnets includes multiple poles, such as e.g. 4, 6 or 8 poles. In one embodiment the sets of ring-shaped magnets are glued together .

The sets of ring-shaped magnets may be tightly arranged around a shaft. Further, the rotor may comprise at two balancing masses, which are arranged at opposite ends of the shaft so as to keep the sets of ring-shaped magnets together. The balancing masses may be kept at place around the shaft by a stop ring at a first end of the shaft and by a nut at an opposite second end.

According to a second aspect the invention relates to a power tool comprising an electric motor which includes a stator and a rotor as described above. The stator may be arranged outside the rotor. It may however also be arranged inside the stator.

According to a third aspect the invention relates to a method of producing a rotor for an electric motor, which method comprises the following steps:

providing at least a first and a second non-magnetized ring magnet, the first ring magnet having an outer diameter that is smaller than the outer diameter of the second ring magnet and that corresponds to the inner diameter of the second ring magnet in a manner that the first ring magnet fits inside the second ring magnet;

placing the first non-magnetized ring magnet inside the second non-magnetized ring magnet and securing them to each other; and

jointly magnetizing the at least two ring magnets so as to form a coherent magnetic field.

The step of magnetizing the at least two ring magnets may be performed by means of four coils so as to produce a four-pole rotor .

The first and second non-magnetized ring magnets may have an anisotropic grain direction and wherein the step of placing the first non-magnetized ring magnet inside the second non- magnetized ring magnet involves coordination of the individual anisotropic grain directions with respect to each other in order to ultimately achieve a coherent magnetic field.

An advantage of the rotor according to the invention with respect to the rotors of the prior art is that it is relatively feasible to assemble and that the magnetic fill factor will be very high. Because of the construction of the rotor, no casing is needed outside the ring magnets, which will reduce the weight of the rotor and increase the magnetic fill ratio of the rotor.

The actual fill ratio will ultimately depend on the production steps. Firstly, it will depend on the formation of a streamlined anisotropic grain direction of the individual ring magnets.

Further it will depend on the correct placing of the ring magnets with respect to each other and with respect to the coils during the magnetization phase.

Preferred embodiments and other advantages of the invention will be apparent from the detailed description and from the dependent claims .

Short description of the drawings

In the following detailed description reference is made to the accompanying drawings, of which:

Fig. 1 shows a perspective view of a power tool according to an embodiment of the invention;

Fig. 2 shows a longitudinal sectional view of a part of the power shown in fig. 1;

Fig. 3 shows a cross sectional view of a rotor according to the prior art with 4 peripherally segmented magnets arranged around a shaft and inside a casing;

Fig. 4 shows a cross sectional view of a rotor according to the prior art with 8 peripherally segmented magnets arranged around a shaft and inside a casing; and

Fig. 5 shows a cross sectional view of a rotor according to the invention with two radially segmented magnets arranged around a shaft . Detailed description of one embodiment of the invention

In fig. 1 a rotor 10 according to a first embodiment of the invention is shown in a part perspective and part sectional view. Figure 2 shows a sectional view of the same rotor 10.

In the shown embodiment the rotor 10 includes a shaft 11, which is surrounded by magnets 12 and 13 that are co-axially arranged outside each other. The magnets are provided in three sets, each including two magnets 12 and 13, which sets are arranged in three adjacent segments A, B and C. The invention does however comprise embodiments in which 1, 2, 3, 4 or more adjacent segments are arranged along the shaft. The shaft 11 includes a stop ring 14 arranged at a first end 11a of the shaft. In the shown embodiment the stop ring 14 is a solid integrated part of the shaft 11. It may however also be a separate part that is fixable to the shaft 11.

A first balancing mass 15 is arranged at the first end 11a to abut said stop ring 14. A second balancing mass 16 is arranged at the opposite second end lib of the shaft 11. This second balancing mass 16 may be kept at place by a nut 17, which is threaded onto threads on the second end lib of the shaft 11. The balancing masses 15 and 16 serve as abutments for the outer magnets 12 and 13, i.e. the magnets in segments A and B, so as to form a solid unit out of the adjacent sets of ring magnets 12 and 13.

The invention is based on the notion that it is possible to produce anisotropic ring segments, in which the grains are pre- directed in a direction that corresponds to a desired 4-pole magnetic field. A limitation of such ring segments is that they may only be produced in a limited thickness. Therefore, in accordance with the invention a number of ring shaped magnets are coaxially arranged one outside the other so as to a produce a substantially coherent magnet flux. In the embodiment shown in figure 5 the rotor 10 comprises two ring-shaped magnets 11 and 12 that are coaxially arranged one outside the other. However, the number of ring-shaped magnets that are coaxially arranged will however depend on how strong magnetic field is needed. The invention is not limited to any number, but comprises

embodiments in which 2, 3, 4, 5 or more ring-shaped magnets are coaxially arranged one outside the other.

Further, as is visible in figures 1 and 2, several sets of coaxial ring-shaped magnets are coaxially arranged in segments A, B and C next to each other. Namely, magnetic losses increase with the axial length of a magnet segment. Therefore, it is desirable to segment the magnets axially. The number of

segmented sets of magnets 12 and 13 depend on the application and on the needed efficiency of the motor.

The magnets 12 and 13 are individually formed and are

subsequently glued together before they are arranged around the shaft 11. It is to be noted that the joined ring segments 12 and

13 may either be magnetized before or after they are arranged around the shaft 11. If they are magnetized before they are arranged around the shaft 11, they may be magnetized segment be segment, and if they are magnetized after they are arranged around the shaft 11 they are preferably magnetized all at the same time.

The magnets are magnetized by means of coils that are located at suitable locations. In a preferred embodiment the rotor is magnetized to a 4-pole magnetic field. This may be done by means of four coils 41-44 that are located to produce the 4-pole magnetic field as is illustrated in figure 5, where the four coils 41-44 are located 90° apart. The rotor may also be magnetized to e.g. a 6 or 8-pole magnetic field, i.e. by means of 6 or 8 coils. In one embodiment it may be magnetized to a two-pole rotor.

Two diagonally opposed coils 42 and 44 are arranged to produce clockwise directed magnetic fields, and two other diagonally opposed coils 41 and 43 are arranged to produce counterclockwise directed magnetic fields. Together these four coils will produce the desired permanent magnetic flux of the rotor 10. It is worth noting that the anisotropic ring segments 12 an 13 both have a grain direction that corresponds to the added magnetic field. Hence, firstly the individual ring segments 12 and 13 are joined in a manner that puts their individual grain direction in conformity with each other. Subsequently, the joined ring segments 12 and 13 are located between the coils 41 44 in a manner that will stream line the grain direction with the magnetic field to be added, in order to maximise the fill factor of the permanent magnetic field to be produced.

The final fill factor is dependent on the production parameters Firstly it is important that the formation of the anisotropic grain directions will correspond to the added magnetic field. Hence, both the production of the individual ring magnets, and the adjoining and magnetization should be optimized in order to obtain an optimally magnetized rotor.

Above, the invention has been described with reference to specific embodiments. The invention is however not limited to either of these embodiments. Instead the scope of the invention is defined by the following claims.

Claims

1. A rotor (10) comprising a permanently magnetized magnet, characterised in that the rotor comprises at least two ring- shaped magnets (12, 13) that are coaxially arranged one outside the other so as to a produce a substantially coherent magnetic flux.

2. The rotor (10) according to claim 1, wherein the rotor includes a number of sets (A, B, C) of ring-shaped magnets (12, 13), each set comprising at least two ring-shaped magnets (12, 13) that are coaxially arranged one outside the other so as to a produce a substantially coherent magnetic flux, and wherein the number of sets are located coaxially next to each other.

3. The rotor (10) according to either of claims 1 or 2, wherein the rotor is a multi-pole rotor, and wherein the coherent magnetic flux of the ring-shaped magnets (12, 13) includes multiple poles.

4. The rotor (10) according to claim 3, wherein the rotor is a 4-pole rotor, and wherein the coherent magnetic flux of the ring-shaped magnets (12, 13) includes 4 poles.

5. The rotor according to either of the preceding claims, wherein the sets of ring-shaped magnets (12, 13) are glued together .

6. The rotor according to claim 1, wherein the rotor

comprises at least three ring-shaped magnets that are coaxially arranged one outside the other so as to a produce a

substantially coherent magnet flux.

7. The rotor according to either of the preceding claims, wherein the sets of ring-shaped magnets are arranged tightly around a shaft (11) .

8. The rotor according to claim 7, wherein the rotor (10) comprises at two balancing masses (15, 16), which are arranged at opposite ends (11a, lib) of the shaft (11) so as to keep the sets of ring-shaped magnets together.

9. A power tool comprising an electric motor which includes a stator and a rotor (10) according to any of the preceding claims .

10. A power tool according to claim 9, wherein the stator is arranged outside the rotor (10)

11. Method of producing a rotor for an electric motor of a power tool, the method being characterised by the following steps:

providing at least a first and a second non-magnetized ring magnet (12 , 13), the first ring magnet having an outer diameter (dl) that is smaller than the outer diameter (d2) of the second ring magnet and that corresponds to the inner diameter of the second ring magnet in a manner that the first ring magnet fits inside the second ring magnet;

- jointly magnetizing the at least two ring magnets so as to form a coherent magnetic field.

12. Method according to claim 11, wherein the step of

magnetizing the at least two ring magnets is performed by means of four coils so as to produce a four-pole rotor.

13. Method according to claim 12, wherein the first and a second non-magnetized ring magnet (12, 13) have an anisotropic grain direction and wherein the step of placing the first non- magnetized ring magnet (12) inside the second non-magnetized ring magnet (13) involves coordination of the individual anisotropic grain directions with respect to each other in order to ultimately achieve a coherent magnetic field.