Field of the invention

The invention relates generally to rotating electrical machines. More particularly, the invention relates to a rotor segment for a rotor of a permanent magnet electrical machine and to a permanent magnet electrical machine.

Background

Large diameter electrical machines, such as for example directly driven wind generators in which the air-gap diameter can be even more than 2000 mm, comprise in many cases a segmented stator and/or a segmented rotor. In typical cases, the rotor segments and/or the stator segments are manufactured in one site and transported to another site at which the electrical machine is assembled of the said segments. A rotor segment of a permanent magnet electrical machine comprises typically a support structure and permanent magnets attached to the support struc- ture. The permanent magnets are arranged to provide magnetic poles on the air- gap surface of the rotor segment. The air-gap surface is the surface of the rotor segment that faces towards the stator of a permanent magnet electrical machine when the rotor segment is being used as a part of the permanent magnet electrical machine. One of the challenges in assembling a permanent magnet rotor of rotor segments of the kind described above is related to fastening adjacent rotor segments together. The fact that the fastening takes place in the vicinity of permanent magnets susceptible to damaging imposes restrictions to means and methods related to the fastening. Another challenge is related to transportation of the rotor segments, because especially in conjunction with constructions having surface mounted permanent magnets, the permanent magnets closest to the flanks of the rotor segments may be susceptible to damaging during transportation.

Summary

In accordance with the first aspect of the invention, there is provided a new permanent magnet electrical machine. In the permanent magnet electrical machine according to the invention, magnetic poles of a permanent magnet rotor comprising rotor segments are positioned in such a manner that a tangential interval between magnetic axes of adjacent magnetic poles belonging to neighboring rotor segments is greater than an average of tangential intervals between magnetic axes of adjacent magnetic poles belonging to a same rotor segment. Therefore, the tangential distances from the flanks of the rotor segments to closest permanent magnets can be increased and thus more room can be provided for arrangements for fastening the rotor segments together. Furthermore, especially in conjunction with constructions having surface mounted permanent magnets, the sus- ceptibility to damages during transportation is decreased since the permanent magnets are farther from the flanks. The above-described positioning of the magnetic poles decreases the magnetic coupling between the permanent magnet rotor and the stator windings of the permanent magnet electrical machine but, on the other hand, cogging torque may also be decreased. In accordance with the second aspect of the invention, there is provided a new rotor segment for a rotor of a permanent magnet electrical machine. A central angle φ between first and second flanks of a rotor segment according to the invention is at most π radians, i.e. 180 degrees, and the rotor segment comprises a support structure and permanent magnets attached to the support structure. The perma- nent magnets are arranged to provide magnetic poles on an air-gap surface of the rotor segment, the air-gap surface facing towards a stator of the permanent magnet electrical machine when the rotor segment is used as a part of the permanent magnet electrical machine. The magnetic poles are positioned on the air-gap surface so that: αι = φ/2ρ + B-|/R _ag , and α ₂ = φ/2ρ + B ₂ /R _ag , where: ai is a central angle between the first flank of the rotor segment and a magnetic axis of the magnetic pole closest to the first flank, α ₂ is a central angle between the second flank of the rotor segment and a magnetic axis of the magnetic pole closest to the second flank, p is the number of the magnetic poles of the rotor segment,

R _ag is the air-gap radius of the rotor segment, and Bi and B ₂ are tangential lengths selected so that a sufficient tangential interval between permanent magnets of neighboring rotor segments is achieved when the rotor segment is used as a part of a permanent magnet electrical machine. Values suitable for Bi and B ₂ are in a case-specific manner dependent on dimensions and a mechanical construction of the rotor segment and/or on the geometry of the sta- tor windings. In practical cases, 5 mm is a lower limit for B and B ₂ . The terms B-|/R _ag and B ₂ /R _ag manifest the general rule that an angle in radians is the length of an arch divided by a radius.

A number of exemplifying embodiments of the invention are described in accompanied dependent claims. Various exemplifying embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.

The verb "to comprise" is used in this document as an open limitation that neither excludes nor requires the existence of unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

Brief description of the figures

The exemplifying embodiments of the invention and their advantages are ex- plained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 shows a cross-section of a permanent magnet electrical machine according to an embodiment of the invention, and figure 2 shows a cross-section of a permanent magnet electrical machine according to another embodiment of the invention.

Description of the embodiments

Figure 1 shows a cross-section of a permanent magnet electrical machine accord- ing to an embodiment of the invention. The permanent magnet electrical machine shown in figure 1 is an outer rotor machine in which a rotor is arranged to surround a stator. The stator comprises a segmented stator core assembled of stator segments 125, 126, 127, and 128. Stator windings are preferably, but not necessarily, arranged in such a manner that each coil of the stator windings occupies stator slots belonging to only one segment of the stator core. In this case, the stator segments can be wound separately of each other and only terminals of the windings of different stator segments have to be connected in an appropriate manner after assembling the stator core. The rotor of the permanent magnet electrical machine shown in figure 1 comprises four rotor segments 101 , 102, 103, and 104 ac- cording to an embodiment of the invention. The rotor segments 101 -104 are similar to each other and, therefore, in the following text only the rotor segment 101 is explained in more details. In figure 1 , the angle φ denotes a central angle between a first flank 107 of the rotor segment 101 and a second flank 108 of the rotor segment 101 . As there are four similar rotor segments, the central angle φ is π/2 ra- dians, i.e. 90 degrees. It should be, however, noted that, depending on a construction and dimensions of an electrical machine, the rotor as well as the stator can be segmented to have any suitable number of segments. For example, in certain cases a rotor may comprise only two rotor segments in which case the central angle φ would be π radians, i.e. 180 degrees, and in some other cases the rotor may comprise three segments or more than four segments.

The rotor segment 101 comprises a support structure 109 which may comprise laminated and/or solid ferromagnetic material. The rotor segment 101 comprises permanent magnets 1 10, 1 1 1 , 1 12, and 1 13 attached to the support structure 109. In the case shown in figure 1 , the permanent magnets are mounted on the surface of the support structure 109. It is, however, also possible to have a support structure having cavities for the permanent magnets. The permanent magnets are ar- ranged to provide magnetic poles on the air-gap surface of the rotor segment. Arrows drawn on the figure elements representing the permanent magnets indicate the direction of magnetization of each permanent magnet. The magnetic poles are positioned in such a manner that a tangential interval between magnetic axes of adjacent magnetic poles belonging to neighboring rotor segments is greater than an average of tangential intervals between magnetic axes of adjacent magnetic poles belonging to a same rotor segment. In figure 1 , the central angle γ^ corresponds to the tangential interval between the magnetic axes 1 14 and 1 16 of the adjacent magnetic poles belonging to the neighboring rotor segments 101 and 104, respectively. Correspondingly, the central angle γ ₂ corresponds to the tangential interval between the magnetic axes 1 15 and 120 of adjacent magnetic poles belonging to the neighboring rotor segments 101 and 102, respectively. As the rotor segments 101 -104 are mutually similar, γ equals γ ₂ . The central angles βι , β ₂ , and β ₃ correspond to the tangential intervals between the magnetic axes of adjacent magnetic poles belonging to the rotor segment 101 . The positioning of the magnetic poles is accomplished by appropriate positioning of the permanent magnets 1 10-1 13. Where a support structure having cavities for the permanent magnets is used, the positioning of the magnetic poles could also be accomplished by appropriate shaping of ferromagnetic parts which constitute paths for magnetic fluxes. The above-described positioning of the magnetic poles increases the tangential distances from the flanks 107 and 108 of the rotor segment 101 to the closest permanent magnets 1 10 and 1 13, respectively, and thus provides more room for arrangements 121 and 122 for fastening the rotor segment 101 to the neighboring rotor segments 104 and 102. The above-described positioning of the magnetic poles decreases the magnetic coupling between the permanent magnet rotor and the stator windings of the permanent magnet electrical machine but, on the other hand, cogging torque may also be decreased.

The above-described property of the permanent magnet electrical machine shown in figure 1 can be obtained by selecting the central angle on shown in figure 1 to be greater than φ/8 = π/16 radians =1 1 .25 degrees, and the central angle a ₂ shown in figure 1 to be greater than φ/8. The denominator 8 comes from the fact that the rotor segment 101 has four magnetic poles. The central angle on is an an- gle between the first flank 107 of the rotor segment and the magnetic axis 1 14 of the magnetic pole closest to the first flank, and the central angle a ₂ is an angle between the second flank 108 of the rotor segment and the magnetic axis 1 15 of the magnetic pole closest to the second flank. This selection of the central angles on and a ₂ can be characterized with the aid of the following equations: a ₂ = φ/8 + B ₂ /R _ag , where R _ag is the air-gap radius of the rotor segments as shown in figure 1 . In conjunction with an outer rotor machine as the one shown in figure 1 , the air-gap ra- dius R _ag is the radius of the greatest circle capable of being encircled by the rotor. Bi and B ₂ are tangential lengths selected so that a sufficient tangential interval between permanent magnets of neighboring rotor segments is achieved. Values suitable for Bi and B ₂ are in a case-specific manner dependent on dimensions and a mechanical construction of the rotor segments and/or on the geometry of the stator windings. In practical cases, 5 mm is a lower limit for Bi and for B ₂ .

In the permanent magnet electrical machine shown in figure 1 , the number of magnetic poles per a rotor segment is four. In a more general case, where the number of magnetic poles per a rotor segment is p, the central angles ai and a ₂ are: en = φ/2ρ + B Rag, and (2) α ₂ = φ/2ρ + B ₂ /R _ag .

In a rotor segment according to an embodiment of the invention, the magnetic axes of the magnetic poles of the rotor segment are pitched, within the rotor segment, at uniform intervals in the tangential direction. In conjunction with the rotor segment 101 shown in figure 1 , this would mean that βι= β ₂ = β ₃ < φ/4. The fact that β-ι , β ₂ , and β ₃ are less than φ/4 is a consequence of the equations (1 ). Each of βι , β ₂ , and β ₃ is thus less than the sum αι + α ₂ = γι = γ ₂ , i.e. the magnetic axes of all the magnetic poles of the whole rotor are not uniformly pitched. In a rotor segment according to an embodiment of the invention, B is substantially equal to B ₂ , i.e. the central angle ai is substantially equal to the central angle a ₂ .

Figure 2 shows a cross-section of a permanent magnet electrical machine according to an embodiment of the invention. The permanent magnet electrical machine shown in figure 2 is an inner rotor machine in which a stator is arranged to surround a rotor. The stator comprises a segmented stator core assembled of stator segments 225, 226, 227, 228, 229 and 230. Stator windings are preferably, but not necessarily, arranged in such a manner that each coil of the stator windings occupies stator slots belonging to only one segment of the stator core. In this case, the stator segments can be wound separately of each other and only terminals of the windings of different stator segments have to be connected in an appropriate manner after assembling the stator core. The rotor of the permanent magnet electrical machine shown in figure 2 comprises six rotor segments 201 , 202, 203, 204, 205, and 206 according to an embodiment of the invention. The rotor segments 201 -206 are fastened to each other and to a shaft 250. In figure 2, the angle φ denotes a central angle between a first flank 207 of the rotor segment 201 and a second flank 208 of the rotor segment 201 . As there are six similar rotor segments, the central angle φ is π/3 radians, i.e. 60 degrees.

Each of the rotor segments 201 -206 comprises a support structure which may comprise laminated and/or solid ferromagnetic material. Each of the rotor segments 201 -206 further comprises permanent magnets embedded to a surface layer 232. Hence, individual permanent magnets are not shown in figure 2. The permanent magnets are arranged to provide magnetic poles on the air-gap surface of the rotor segments. In figure 2, the number of the magnetic poles per a rotor segment is p and the dashed lines 214, 217, 218, 219, 231 , and 215 represent magnetic axes of some of the p magnetic poles of the rotor segment 201 . The dashed line 216 represents a magnetic axis of one of the two outermost magnetic poles of the rotor segment 206. The magnetic poles are positioned in such a manner that a tangential interval between magnetic axes of adjacent magnetic poles belonging to neighboring rotor segments is greater than an average of tangential intervals between magnetic axes of adjacent magnetic poles belonging to a same rotor segment. In figure 2, the central angle γ^ corresponds to the tangential inter- val between the magnetic axes 214 and 216 of the adjacent magnetic poles belonging to the neighboring rotor segments 201 and 206, respectively. The central angles βι , β ₂ , ... , β _Ρ -2, and β _ρ . ₂ correspond to the tangential intervals between the magnetic axes of adjacent magnetic poles belonging to the rotor segment 201 . The above-described positioning of the magnetic poles increases the tangential distances from the flanks of the rotor segments to the closest permanent magnets and thus provides more room for arrangements for fastening each rotor segment to its neighboring rotor segments. The above-described positioning of the magnetic poles decreases the magnetic coupling between the permanent magnet rotor and the stator windings of the permanent magnet electrical machine but, on the other hand, cogging torque may also be decreased.

In a permanent magnet electrical machine according to an embodiment of the invention, the magnetic axes of the magnetic poles are pitched at uniform intervals within each rotor segment and a stator pole pitch is different from the length of the uniform intervals.

In a permanent magnet electrical machine according to an embodiment of the invention, the length of the uniform intervals, i.e. the rotor pole pitch within each rotor segment, is substantially: where P _s is the stator pole pitch, p is the number of magnetic poles of each rotor segment, τ ₅ is a stator slot pitch, and n is an integer greater than or equal to 1 .

For the sake of illustrative purposes, the following example-case is considered below:

- the number of magnetic poles per a rotor segment = p - the stator pole pitch = P _s ,

- the stator slot pitch = τ ₅ , - the additional tangential distances B and B ₂ , see equations (1 ) and (2), at both flanks of a rotor segment is half of the stator slot pitch, i.e. B ₂ =

- the air-gap radius is R _ag . In conjunction with an outer rotor machine as the one shown in figure 1 , the air- gap radius R _ag is the radius of the greatest circle capable of being encircled by the rotor, and in conjunction with an inner rotor machine as the one shown in figure 2, the air-gap radius R _ag is the radius of the smallest circle capable of encircling the rotor. In the above-mentioned example-case, the central angles φ, α-ι , and α ₂ shown in figures 1 and 2 are:

α-ι = a ₂ = φ/2ρ + x _s /(2 x R _ag ).

Therefore: φ/2ρ = P _s /(2 x R _ag ), and i = a ₂ = (1 + VPs) ^χ (φ/2ρ).

If, for example, the stator pole pitch P _s = 150 mm and the stator slot pitch τ ₅ = 50 mm, the central angles on = on = 1 ¹ / ₃ ^χ φ/2ρ. If the magnetic axes of the magnetic poles of each rotor segment are pitched at uniform intervals, the length of the uni- form intervals, i.e. the rotor pole pitch P _r within each rotor segment, is:

(p x P _s - 2 x xs 2)/p = Ps - Ts/p.

If, for example, the stator pole pitch P _s = 150 mm, the stator slot pitch τ ₅ = 50 mm, and number of poles per a rotor segment p = 24, the rotor pole pitch P _r = 147.9 mm. The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the embodiments described above.