SURFACE-MOUNTABLE PERMANENT MAGNET ELEMENT

FIELD OF THE INVENTION

The invention relates to a permanent magnet element as defined in the preamble of claim 1.

BACKGROUND OF THE INVENTION

Permanent magnet elements are used secured to the rotor of permanent magnet machines, such as elec- trie motors and generators, to provide the required magnetic field. The types of permanent magnet elements used in the permanent magnet machines include both ones embedded in the rotor and ones which are surface- mountable onto the periphery. This disclosure relates to surface-mountable permanent magnet elements. The elements should be magnetable without the risk of breakage and, when magnetized, easily mountable onto the peripheral surface of the rotor in files and/or rows. The permanent magnet element should be protected against corrosion as efficiently as possible by coating and encasing against outside air. The element should hold both tangential and radial mechanical stress, which is why the magnets should be locked, in addition to the magnetic grip, in place as reliably as possible, either mechanically only or mechanically and by gluing.

Previously known from publication WO 2007/116118 Al is a surface-mountable permanent magnet element which is intended to be mounted onto the pe- riphery of the rotor of a permanent magnet machine as the polar element. The permanent magnet element comprises at least one permanent magnet and a protective case enclosing said at least one permanent magnet. The protective case comprises a base plate which comprises a securing member for securing the base plate to the

rotor. The base plate comprises a base surface which can be placed against the periphery of the rotor, a top surface on which said at least one permanent magnet is arranged, and at least one lateral surface which extends substantially perpendicularly between the base surface and the top surface. The lateral surface comprises a recess. The protective case further comprises a housing formed from a metal thin sheet material. The housing comprises a top wall which covers the permanent magnet, and a skirt which extends from the top wall over the lateral wall of the base plate. The skirt comprises a joint member, which in the WO- publication is a projection formed from the ' skirt and pressed inside the recess to secure the housing and the base plate to each other in such manner that the joint is within the outer dimensions of the element, which is preferred so that the elements can be disposed in file and/or row as close to each other as possible. The projection which is pressed inside the recess provides stress resistance which is sufficient in many applications for maintaining the coherence of the element .

For example in very large generator applications, such as wind power generators, in which the permanent magnet elements are mounted onto the periphery of a rotor having the diameter in the order of meters, the radial and tangential stresses applied to the permanent magnet element become quite high, even if the rotating speed was relatively low (in the order of 16 to 20 r/min) . In the course of time, this may weaken the coherence, compactness and corrosion resistance of the parts of the element. The air gap between the stationary stator and the rotating rotor is typically in the order of 1 mm, so it is obvious that the joint between the housing and the base plate cannot be allowed to become loose. In requiring high operational

reliability with high confidence coefficient, the strength of the joint is a critical factor.

OBJECTIVE OF THE INVENTION The objective of the invention is to eliminate the drawbacks referred to above.

Specifically, the objective of the invention is to disclose a permanent magnet element in which the joint between the housing and the base plate has been further improved.

Further, the objective of the invention is to disclose a strong and compact permanent magnet element with good corrosion resistance.

In addition, the objective of the invention is to disclose a permanent magnet element which is simple in construction, affordable and easily assembled.

SUMMARY OF THE INVENTION The permanent magnet element according to the invention is characterized by what has been presented in claim 1.

According to the invention, the recess is a groove which extends substantially over the entire length of the lateral surface of the base plate. The joint member is formed by bending the edge of the skirt of the housing inside the groove over a substantial portion of the total length of the skirt.

The advantage of the invention is that the edge of the skirt of the housing which is bent inside the groove extending substantially over the entire width of the lateral surface and which is almost equal in length locks the housing and the base plate to each other tightly and efficiently in such manner that they are not able to detach from each other during static

and dynamic long-term stress. A further advantage is that the permanent magnet element is strong, compact, simple in construction, affordable, easily assembled and has good corrosion resistance. In one embodiment of the permanent magnet element, the groove is limited by a first planar surface which is disposed at an angle to the lateral surface of the base plate, the first planar surface facing a first direction. The groove is further limited by a second planar surface facing a second direction which is substantially opposite to the first direction. The bending of the skirt inside the groove comprises a first flange which is parallel to the first planar surface and rests against the first planar sur- face.

In one embodiment of the permanent magnet element, the bending of the skirt inside the groove comprises a second flange which is disposed at an angle to the first flange. The second flange is prefera- bly held in compressive stress against the second planar surface. In other words, the free edge of the second flange rests against the second planar surface, further ensuring the strength of the joint. This is preferably provided in such manner that the bending is formed by compressing, i.e. deep drawing, the edge of the skirt inside the groove.

In one embodiment of the permanent magnet element, the first planar surface and the second planar surface are parallel to each other and spaced at a distance from each other.

In one embodiment of the permanent magnet element, the first planar surface and the second planar surface are disposed at a right angle to the lateral surface. In one embodiment of the permanent magnet element, the groove is rectangular in cross-section.

In one embodiment of the permanent magnet element, the first planar surface and the second planar surface are disposed at such angle to the lateral surface that deviates from the right angle. In one embodiment of the permanent magnet element, the groove is a V-groove.

In one embodiment of the permanent magnet element, the groove is in the form of a parallelogram or trapezoid in cross-section. In one embodiment of the permanent magnet element, the groove is a dovetail.

In one embodiment of the permanent magnet element, the point of connection of the second planar surface and the lateral surface comprises a formation inside the groove, such as a ridge, protuberance or the like, fitted to ensure that the bending inside the groove stays in the groove.

In one embodiment of the permanent magnet element, the base plate and the housing are in the form of a parallelepiped.

In one embodiment of the permanent magnet element, the base plate is rectangular, whereby it comprises four lateral surfaces with the groove formed in each one of them. Correspondingly, the skirt of the housing comprises four planar skirt portions wherein the bendings inside the groove are formed.

In one embodiment of the permanent magnet element, the base plate is made from a ferromagnetic material and the housing is made from a non-magnetic material.

LIST OF FIGURES

In the following section, the invention will be described in detail by means of exemplifying em- bodiments, with reference to the accompanying drawing in which

Fig. 1 schematically presents a rotor of a permanent magnet machine with a set of permanent magnet elements according to one embodiment of the invention mounted onto the periphery thereof, Fig. 2 presents the rotor of Fig. 1 as seen from direction II-II of Fig. 1,

Fig. 3 axonometrically presents a permanent magnet element according to one embodiment of the invention as seen from the top, Fig. 4 shows section IV-IV of Fig. 3,

Fig. 5 to 7 present a detail of the permanent magnet element according to one embodiment of the invention and steps of bending the skirt bending into the groove by deep drawing, Fig. 8 to 10 present, correspondingly to Fig.

7, various forms of grooves and bendings in different embodiments of the permanent magnet element according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 and 2 schematically present an example of a permanent magnet electric machine, such as a motor or generator, wherein the periphery of the rotor 2 comprises in four adjacent files parallel to the pe- riphery a large number of surface-mounted permanent magnet elements 1 disposed side by side and close to each other.

Fig. 3 and 4 present one such surface- mountable permanent magnet element 1. It comprises at least one permanent magnet 3 or a number of permanent magnets 3 side by side, and a protective case 4 enclosing the permanent magnet/s 3. The protective case 4 comprises a base plate 5 made from a ferromagnetic material, such as steel, and having on the base a se- curing member 6, shown in Fig. 4, i.e. in this case a closed hole provided with threading, in which a secur-

ing bolt can be threaded to secure the base plate to the rotor 2. The base plate comprises a base surface 7 which can be placed against the periphery of the rotor 2. The base plate 5 further comprises a top surface 8 on which the permanent magnet 3 is arranged. The base plate 5 is a rectangular parallelepiped with four lateral surfaces 9 which extend perpendicularly between the base surface 7 and the top surface 12. As seen in Fig. 3 and 4, straight grooves 10 have been formed on the lateral surfaces 9 and they extend over the entire width of the lateral surface 9.

The protective case further comprises a housing 11 formed from a non-magnetic metal thin sheet material. The housing 11 comprises a top wall 12 which covers the permanent magnet 3. Extending from the top wall 12 over the lateral surfaces 9 of the base plate 5 are skirts 13. The lower edge of the skirt 13 comprises as a joint member 14 a bending 15 of the skirt 13 inside the groove 10 to secure the housing 11 and the base plate 5 to each other.

Fig. 5 to 10 present different groove forms 10 and correspondingly different forms of the bendings 15 of the skirt 13.

In Fig. 5 to 7, the groove 10 is rectangular in cross-section, whereby the upper first planar surface 16 and the lower second planar surface 17 which limit the groove 10 in the vertical direction are parallel to each other and spaced at a distance s from each other and at a right angle to the lateral surface 9 of the base plate. The bending 15 comprises a first flange 18 which is parallel to the first planar surface 16 and rests against the first planar surface, and a second flange 19 which is disposed at an angle to the first flange 18. The bending 15 is formed ac- cording to Fig. 5 to 7 by pressing the skirt 13 close to the lower edge thereof and close to the upper edge

of the groove 10 by a knife-type pressing blade to curve inside the groove 10 so as to form the first flange 18 which rests against the first planar surface 16, and the second flange 19 which is held in compres- sive stress between the first planar surface 16 and the second planar surface 17 against the second planar surface 17. Fig. 7 also shows that at the same time, a formation 20, such as a ridge, protuberance or the like, is formed by a clenching tool inside the groove 10 to the point of connection of the second planar surface 17 and the lateral surface 9 to ensure that the bending 15 stays in the groove 10.

In the embodiment of Fig. 8, the first planar surface 16 and the second planar surface 17 are dis- posed at such angle to the lateral surface 9 which deviates from the right angle, and the groove 10 is V- shaped in cross-section. The first flange 16 forms an obtuse angle to the lateral surface 9 and rests against the upper first planar surface 16. In the embodiment of Fig. 9, the first planar surface 16 and the second planar surface 17 are disposed at such angle to the lateral surface 9 which deviates from the right angle, but the groove 10 is here a dovetail. In the embodiment of Fig. 10, the groove 10 is trapezoidal in cross-section and the first planar surface 16 and the second planar surface 17 which are parallel to each other and spaced at a distance s from each other are disposed at a skew angle to the lateral surface 9.

The grooves 10 according to Fig. 5 to 7, 8 and 10 are easily and quickly made by a rotating flange blade. The dovetail 10 of Fig. 9 has to be made by end milling, which is somewhat slower. The invention is not limited merely to the exemplifying embodiments referred to above; instead,

many variations are possible within the scope of the inventive idea defined by the claims.