WO2012034713A2

WO2012034713A2 - A pole piece for an electric machine and a method for assembling a pole piece

Info

Publication number: WO2012034713A2
Application number: PCT/EP2011/050837
Authority: WO
Inventors: Stefan Maroti; Henrik Stiesdal
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-09-15
Filing date: 2011-01-21
Publication date: 2012-03-22
Also published as: WO2012034713A3

Abstract

A pole piece (1, 11) for an electric machine, in particular for a rotor for a wind turbine, comprising a non-magnetic cover (2) accomodating at least one magnet (4) and a thin base plate (6) closing an aperture of the cover (2), whereby at least two edges of the cover (2) and the thin base plate (6) are connected by a folded and/or rolled joint.

Description

A pole piece for an electric machine and a method for

assembling a pole piece

The invention relates to a pole piece for an electric ma^¬ chine, in particular for a rotor for a wind turbine, compris^¬ ing a non-magnetic cover accommodating at least one magnet and a thin base plate closing an aperture of the cover.

Permanent magnets are being used to an increasing extent in large electrical machines like motors and generators due to the increased efficiency and robustness compared with elec^¬ trical excitation. In particular rare-earth magnets, primar- ily based on NdFeB have turned out to provide a very high en^¬ ergy product and are therefore very useful for compact ma^¬ chines. However, in the practical application some problems still remain unsolved. Magnet materials corrode very easily and need a high degree of protection. The materials are also rather brittle and cannot safely be fixed by bolting alone. Furthermore, the known manufacturing methods comprise a long series of steps, some of which are expensive and involve wastage of costly material. A conventional method of mounting permanent magnets on e . g. a large generator rotor comprises, firstly, extensive surface protection of the individual magnets, secondly, fixing the magnets to a rotor rim by gluing and thirdly wrapping the completed rotor with glued-on magnets with a fibreglass ban- dage .

This traditional method has several drawbacks. The surface protection is expensive and it is difficult to mount the per^¬ manent magnets for a planned lifetime of e. g. twenty years. The magnets cannot be magnetised in situ, and this means that all work is done with magnetised parts, which require special tools and a stringent control of work to avoid hazardous situations. Once mounted on the rotor and covered by the fi- breglass bandage the magnets cannot be removed for re- magnetizing in case of an irreversible demagnetization event.

In order to overcome these problems solutions have been de- veloped whereby magnets are manufactured as complete pole pieces. In a pole piece one or more magnets are fixed by gluing to a steel base plate and are covered with a protec^¬ tive cover. This cover is typically welded or soldered to the base plate to provide a protective enclosure. However, this is considered an expensive solution.

In the next assembling step the magnets are mounted in a pole piece. One or more magnets are fixed by gluing to a steel base plate and are covered with a protective cover. The base plate is typically machined to provide proper attachment in^¬ terface geometry to the generator rotor. In order to ensure that the magnets will not move inside the protective cover if the glue joint to the base plate gives way, the inside of the protective cover is filled with a filling mass. This method more or less eliminates the drawbacks of traditional magnet mounting. Expensive surface protection is not required, the magnets can be magnetized after mounting in the pole pieces, and the pole pieces can be removed for re-magnetizing in case of an irreversible demagnetisation event. Some practical problems remain, however. Firstly the process has many steps, some of which are expensive and involve the removal of expensive magnet material by grinding. Secondly, the long process involves numerous steps that are critical for the quality of the finished product, e. g. gluing. Thirdly, the fixing of the protective cover can be difficult without dam^¬ aging the filling mass.

EP 1 860 755 A2 discloses a magnet retaining arrangement for mounting on a radial outer rim of a rotor. A circumferential array of magnet carriers is affixed to the outer rim of the rotor and has a radially outer surface. A pole piece re^¬ tainer made of non magnetic material is affixed to each mag^¬ net carrier and is formed with an axially extending channel. At least one pole piece made of a magnetic material such as steel is located adjacent to the outer surface of each magnet carrier. Each pole piece retainer can be welded to its asso^¬ ciated magnet carrier, alternatively each pole piece retainer may be adhesively bonded to the magnet carrier.

It is therefore an object of the present invention to provide a pole piece for an electric machine which can be manufac^¬ tured by a simplified process.

According to the present invention this object is achieved in the above defined pole piece in that at least two edges of the cover and the base plate are connected by a folded and/or rolled joint.

According to the invention the connection between cover and base plate is effected simply by folding and/or rolling whereby a joint is formed so that the at least one magnet is surrounded by a closed cover. The manufacturing process of the inventive pole piece is simplified compared to conven^¬ tional manufacturing processes as no welding, soldering or gluing is required. Accordingly the inventive pole piece can be produced faster and at lower costs. According to the invention the rim formed by cover and base plate may be folded and/or rolled in all four circumferential directions. As the base plate is thin it can easily be folded and/or rolled so that a joint is formed. When all four circumferential directions are connected accordingly the at least one magnet is securely embedded.

According to a further development of the invention the folded and/or rolled joint can be post formed to a predeter^¬ mined thickness and/or shape. The joint may be post formed in order to facilitate mounting the pole piece on a rotor rim. In the inventive pole piece it is particularly preferred that the joint is a dovetail joint. In this case the dovetail joint can be inserted into a dovetail roof. As an alterna^¬ tive the joint can be post formed into a rectangular shape for being inserted into a rectangular groove. Other shapes are possible, the joint might e. g. comprise at least one in^¬ clined surface so that an undercut is created.

In the inventive pole piece it may be envisaged that at least one magnet is placed on top of the at least one magnetic base plate. In the installed state the magnetic base plate faces the rotor rim. The magnet, which is positioned on top of the magnetic base plate is on the outer side of the rim. According to the inventive pole piece the at least one mag^¬ netic base plate may be fixed by an adhesive and/or a filling mass. However, the use of glue or a filler is only optional.

In order to firmly fix the magnetic base plate to the cover the magnetic base plate may be fitted with a press-fit.

Preferably the folded and/or rolled joint of the inventive pole piece is airtight so that the at least one magnet is not subjected to corrosion. Consequently no additional corrosion protection of the magnet is required since the airtight en^¬ closure prevents it from being exposed to humidity.

The invention further relates to a rotor for a wind turbine. The inventive rotor comprises a rim with plural pole pieces as described above.

The invention further relates to a method for assembling a pole piece for an electric machine, in particular for a rotor for a wind turbine, comprising the following steps: position- ing at least one magnet in a non-magnetic cover and closing an aperture of the cover by positioning a thin base plate on top of the at least one magnet. The inventive method is characterised by the feature that at least two edges of the cover and the thin base plate are con^¬ nected by a folded and/or rolled joint. Further embodiments of the inventive method are explained in the subclaims.

The invention and its underlaying principle will be better understood when consideration is given to the following de- scription of preferred embodiments.

In the accompanying drawings :

Fig. 1 shows the components of an inventive pole piece;

Fig. 2 shows the pole piece of fig. 1 after assembling;

Fig. 3 shows the pole piece of fig. 2 with a folded joint; Fig. 4 shows the pole piece with post formed joint;

Fig. 5 shows the pole piece after installation; and

Fig. 6 shows another embodiment of a pole piece with a

rolled joint.

Fig. 1 shows the assembling of the components of a pole piece 1. Firstly a non-magnetic cover 2 in the form of a tray is produced. Cover 2 is made out of sheet metal or of coiled material with a thickness of 0,3 to 1 mm, in this embodiment the cover 2 has a thickness of 0,5 mm. Cover 2 is a deep drawn stainless steel tray comprising a cavity 3 for one or multiple non-magnetized magnets or magnet modules. As can be seen in Fig. 1 the outer edge of cover 2 is bent perpendicu- lar twice.

Subsequently at least one magnet 4 is inserted into cover 2. Preferably multiple magnets 4 are disposed in the cover 2 in the form of an array. Magnet 4 is a sintered NeFeB magnet. Magnet 4 is positioned on top of a magnetic base plate 5 and fixed into position by an adhesive. As an alternative fixing may be realized by a filling mass. The magnetic base plate 5 is press-fit into cover 2 so that a firm connection is real^¬ ized .

Finally the aperture of cover 2 is closed by a thin base plate 6. Fig. 2 to 4 show further steps of the assembly of the pole piece 1. As can be seen in Fig. 2 the thin base plate 6 is slightly larger than the cover 2 so that bent edges 7 of the thin base plate 6 face the respective bent edges 8 of cover 2. The thin base plate 6 which is in the installed state used as bottom cover is made out of magnetic steel with a thickness of 0,5 mm. Other embodiments are pos^¬ sible where the thin base plate 6 has a thickness between 0,3 to 1 mm.

Subsequently cover 2 and thin base plate 6 are connected by folding their bent edges 7, 8 as is shown in Fig. 3. At least two edges are folded to form a connection and an air^¬ tight enclosure for the magnets 4, however, in the present embodiment the edges in all four circumferential directions are folded. The bent edges 7, 8 are folded in a rectangular shape.

In the next manufacturing step as depicted in Fig. 4 the folded rim is post formed to a specific rim thickness t to provide guidance in a groove, in which the pole piece 1 is subsequently inserted.

Fig. 5 shows the pole piece after installation in a groove 9 of a rotor yoke 10. The folded rim comprising the bent edges 7, 8 of cover 2 and thin base plate 6 is post formed and "calibrated" to fit groove 9.

Other embodiments of the folded rim are possible where the joint is a dovetail joint. Fig. 6 shows a detail of a sectional view of another embodi^¬ ment of a pole piece 11 with a rolled joint. For like compo^¬ nents the same reference signs are used as in the first em^¬ bodiment. As can be seen in Fig. 6 a narrow gap 12 is pro^¬ vided between magnet 4 and cover 2. Typically this gap 12 has a width of 0,3 to 0,4 mm, preferably 0,35 mm. The mag^¬ netic base plate 5 is press-fit into cover 2, accordingly the magnetic base plate 5 contacts cover 2.

In contrast to the first embodiment the bent edges 13, 14 are rolled in order to form a joint. Rolling the bent edges 13, 14 does not require expensive processing like machining, welding or soldering processes. Appliance of glue as an ad^¬ hesive or a filler material is optional. Accordingly a pole piece can be assembled easily whereby the required corrosion protection of the magnets is achieved. After assembly the pole piece can be shipped for magnetisation.

Claims

1. A pole piece (1, 11) for an electric machine, in par^¬ ticular for a rotor for a wind turbine, comprising a non-magnetic cover (2) accomodating at least one magnet

(4) and a thin base plate (6) closing an aperture of the cover (2), characterised in that at least two edges of the cover (2) and the thin base plate (6) are connected by a folded and/or rolled joint.

2. A pole piece according to claim 1, characterised in that the rim formed by cover (2) and thin base plate (6) is folded and/or rolled in all four circumferential direc^¬ tions .

3. A pole piece according to claim 1 or 2, characterised in that the folded and/or rolled joint is post formed to a predetermined thickness and/or shape.

4. A pole piece according to claim 3, characterised in that the joint is a dovetail joint.

5. A pole piece according to any of the preceding claims, characterised in that the at least one magnet (4) is disposed on top of the at least one magnetic base plate

(5) .

6. A pole piece according to claim 5, characterised in that the at least one magnetic base plate (5) is fixed by an adhesive and/or a filing mass.

7. A pole piece according to claim 5 or 6, characterised in that the magnetic base plate (5) is fitted in the cover (2) with a press-fit.

8. A pole piece according to any of the preceding claims, characterised in that the folded and/or rolled joint is airtight .

9. A rotor for a generator for a wind turbine, comprising a rim with plural pole pieces according to any of claims 1 to 8.

10. Method for assembling a pole piece (1, 11) for an elec^¬ tric machine, in particular for a rotor for a wind turbine, comprising the following steps:

- positioning at least one magnet (4) in a non-magnetic cover (2 ) ;

- closing an aperture of the cover (2) by positioning a thin base (6) plate on top of the at least one magnet (4) ;

characterised in that at least two edges of the cover (2) and the thin base plate (6) are connected by a folded and/or rolled joint.

11. Method according to claim 10, characterised in that the rim formed by cover (2) and base plate (6) is folded and/or rolled in all four circumferential directions.

12. Method according to claim 10 or 11, characterised in

that the folded and/or rolled joint is post formed to a predetermined thickness and/or shape.

13. Method according to any of claims 10 to 12, characterised in that the joint is post formed to a dovetail j oint .

14. Method according to any of claims 10 to 13, character^¬ ised in that the thin base plate (6) is fitted in the cover with a press-fit.