EP2550720A2

EP2550720A2 - An electrical machine

Info

Publication number: EP2550720A2
Application number: EP11715256A
Authority: EP
Inventors: Sakari SEPPÄNEN
Original assignee: Switch Drive Systems Oy
Current assignee: Switch Drive Systems Oy
Priority date: 2010-03-23
Filing date: 2011-03-14
Publication date: 2013-01-30
Also published as: WO2011117466A3; FI20105288A0; WO2011117466A2; CN103155357A; CN103155357B

Abstract

An air-gap diameter of a rotor of an electrical machine is arranged to have a con- cave axial profile or a bore diameter of a stator is arranged to have a convex axial profile so that the air-gap (107) of the electrical machine is substantially constant in the axial direction (z), even though the stator core is bent (130) by magnetic forces caused by a magnetic flux corresponding to the nominal use of the electrical machine. It is also possible to have both the concave rotor air-gap diameter profile and the convex stator bore diameter profile. Because of the concave rotor airgap diameter profile and/or convex stator bore diameter profile, a support structure (105) which is needed in large diameter electrical machines for supporting a stator core assembled of tangential segments can be allowed to be more flexible and thus savings in the weight of the support structure can be achieved.

Description

An electrical machine Field of the invention

The invention relates generally to rotating electrical machines. More particularly, the invention relates to a rotor of an electrical machine and to a stator of an elec- trical machine.

Background

One of the challenges in designing large diameter electrical machines, such as for example directly driven wind generators, is related to stiffness of a stator structure. In the electrical machines of the kind mentioned above, the stator structure typical- ly comprises a stator core assembled of laminated tangential segments and a support structure arranged to mechanically support the segmented stator core. Magnetic fields in the air-gap between a rotor and the stator core produce forces acting to the air-gap surface of the stator core and thus tending to bend the stator core and, as a consequence, to change the air-gap. When designing an electrical machine of the kind mentioned above, the effect of the magnetic forces to the air- gap can be taken into account by providing the electrical machine with a so large air-gap that the changes of the air-gap can be tolerated and/or by making the support structure so stiff that the changes of the air-gap remain sufficiently small. Especially in permanent magnet machines, and also in induction machines, the air- gap should be sufficiently small in order to achieve optimal operation. In conjunction with a large diameter electrical machine, in which the air-gap diameter can be even more than 2000 mm, it may be especially challenging to keep the air-gap within allowed limits, because the nominal width of the air-gap is small in relation to other dimensions of the electrical machine. Hence, the dimensioning of the sup- port structure that supports the stator core is mainly dictated by the stiffness requirements related to the need to keep the air-gap within the allowed limits instead of the requirements related to fatigue resistance. The support structure dimensioned mainly on the basis of the above-mentioned stiffness requirements is significantly heavier than a corresponding support structure that is dimensioned mainly on the basis of the requirements related to the fatigue resistance. Summary

In accordance with the first aspect of the invention, there is provided a new rotor for an electrical machine. The rotor according to the invention comprises:

- a shaft, and

- an electromagnetically active part surrounding the shaft and having an axial length L between its first end and its second end, the active part facing in a radial direction towards a stator core when the rotor is being used as a part of the electrical machine, wherein a diameter of a smallest circle capable of encircling the active part at an axial distance z from the first end of the active part towards the second end of the active part is D_R(z) which is a positive concave function of z on the range from 0 to L and a minimum value of which is:

DR(Z_c) = min{D_R(z = 0), D_R(z = L)} - B, where the parameter B is at least 0.25 mm and z_c at which D_R(z) reaches its mini- mum value is preferably at the middle of the axial length of the active part, i.e. z_c = L/2, but it is, however, allowable that z_c somewhat deviates from L/2 so that z_c belongs to a range from l_/2 - L/20 to L/2 + L720.

As the diameter of the smallest circle capable of encircling the electromagnetically active part is decreasing towards the middle region of the axial length of the active part, the stator core can be allowed to be bent by magnetic forces. With proper shaping of the concave function D_R(z) that represents the axial profile of the air- gap diameter of the rotor, it is possible to achieve a situation in which the width of the air-gap has a substantially same value at different axial distances from the first end of the active part when the stator core is bent by the magnetic forces. As the stator core can be allowed to be bent, a support structure which is needed e.g. in large diameter electrical machines for supporting a stator core assembled of tangential segments can be allowed to be more flexible and thus savings in the weight and price of the support structure can be achieved. In other words, the above-mentioned support structure does not have to be dimensioned mainly on the basis of the stiffness requirements as is the case in conjunction with corres- ponding electrical machines according to the prior art described earlier in this document.

In accordance with the second aspect of the invention, there is provided a new stator for an electrical machine. The stator according to the invention comprises: - a stator core having an axial length L between its first end and its second end,

- windings partly locating in slots of the stator core, and

- a support structure arranged to mechanically support the stator core, wherein a diameter of a largest circle capable of being encircled by the bore of the stator core at an axial distance z from the first end of the stator core towards the second end of the stator core is D_s(z) which is a positive convex function of z on the range from 0 to L and a maximum value of which is:

Ds(z_c) = max{Ds(z = 0), D_s(z = L)} + B, where the parameter B is at least 0.25 mm and z_c at which D_s(z) reaches its max- imum value is preferably at the middle of the axial length of the stator core, i.e. z_c = L/2, but it is, however, allowable that z_c somewhat deviates from L/2 so that z_c belongs to a range from l_/2 - L/20 to L/2 + L/20.

Therefore, the stator according to the invention has a bore that is pre-shaped in order to allow the stator core to be bent by magnetic forces. In accordance with the third aspect of the invention, there is provided a new electrical machine. The electrical machine comprises a rotor according to the invention and/or a stator according to the invention, wherein the shape of D_R(z) representing the axial profile of the air-gap diameter of the rotor and/or the shape of D_s(z) representing the axial profile of the air-gap diameter of the stator is/are selected in such a manner that the width of the air-gap between the rotor and the stator is substantially constant on the axial range from z = 0 to z = L when the stator core is bent by magnetic forces caused by a magnetic flux corresponding to the nominal use of the electrical machine. 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 explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 a shows a schematic side view of a rotor according to an embodiment of the invention, figure 1 b shows a schematic section view of a stator than can be used together with the rotor illustrated in figure 1 a, figure 1 c illustrates a situation in which the rotor illustrated in figure 1 a has been put together with the stator illustrated in figure 1 b and magnetic forces are acting on the rotor and the stator, figure 1 d shows a rotor according to an embodiment of the invention seen along the axial direction, figure 2a shows an axial profile of the air-gap diameter of a rotor according to an embodiment of the invention, figure 2b shows an axial profile of the air-gap diameter of a rotor according to another embodiment of the invention, figure 3a shows a schematic section view of a stator according to an embodiment of the invention, figure 3b shows a schematic side view of a rotor than can be used together with the stator illustrated in figure 3a, figure 3c illustrates a situation in which the stator illustrated in figure 3a has been put together with the rotor illustrated in figure 3b and magnetic forces are acting on the rotor and the stator, figure 4a shows an axial profile of the air-gap diameter of a stator according to an embodiment of the invention, and figure 4b shows an axial profile of the air-gap diameter of a stator according to another embodiment of the invention.

Description of the embodiments

Figure 1 a shows a schematic side view of a rotor according to an embodiment of the invention. The rotor comprises a shaft 101 and an electromagnetically active part 102. The active part surrounds the shaft and has an axial length L between its first end 1 10 and its second end 1 1 1 . The z co-ordinate shown in figure 1 a indicates the axial distance from the first end 1 10 of the active part towards the second end 1 1 1 of the active part. The active part 102 faces, in the radial direction, towards a stator core when the rotor is being used as a part of an electrical machine. As illustrated in figure 1 a, the active part has such a shape that a diameter DR(Z) of a smallest circle capable of encircling the active part is a positive concave function of z. A point z_c at which D_R(z) reaches its minimum value is preferably at the middle of the axial length of the active part, i.e. z_c = L/2, but it is, however, allowable that z_c somewhat deviates from L/2 for example so that z_c belongs to a range from L/2 - L/20 to L/2 + L/20. The shape of the function D_R(z) represents the axial profile of the air-gap diameter of the rotor.

Figure 1 b shows a schematic section view of a stator than can be used together with the rotor illustrated in figure 1 a. A dot-and-dash line 140 in figure 1 b is the axis of the rotational symmetry. The stator comprises a stator core 103 that may have been assembled of tangential laminated segments, windings 1 04 partly locating in slots of the stator core, and a support structure 1 05 arranged to mechanically support the stator core. The support structure 1 05 is connected to end shields 106. Figure 1 c illustrates a situation in which the rotor illustrated in figure 1 a has been put together with the stator illustrated in figure 1 b and magnetic forces are acting on the rotor and the stator. The stator core 1 03 and the support structure 1 05 are bent by the magnetic forces as illustrated with an arrow 1 30 shown in figure 1 c. As can be seen from figure 1 c, the width of the air-gap 1 07 between the rotor and the stator is substantially constant on the axial range from z = 0 to z = L, because the shape of the axial profile of the air-gap diameter of the rotor is selected to be commensurate with the bending of the stator core 1 03 and the support structure 105. As the stator core can be allowed to be bent, the support structure can be allowed to be more flexible and thus savings in the weight and price of the support structure can be achieved. In figures 1 a and 1 c, the concavity of the axial profile of the air-gap diameter of the rotor and also the bending of the stator core and the support structure are strongly exaggerated for the sake of illustrative purposes. In practical applications, a suitable degree of the concavity of the axial profile of the air-gap diameter of the rotor is such that the minimum value of D_R(z), see figure 1 a, is:

DR(Z_c) = min{D_R(z = 0), D_R(z = L)} - B, (1 ) where the parameter B is at least 0.25 mm. In rotors according to certain particular embodiments of the invention the parameter B is at least 0.5 mm. In rotors according to certain particular embodiments of the invention the parameter B is at least 2 mm. For example, in a rotor having D_R(z = 0) = D_R(z = L) > 2000 mm and L > 1 000 mm, D_R(z_c) is less than or equal to 1 999.75 mm.

The air-gap diameter D_R(z = 0) at the first end 1 1 0 of the active part 1 02 is typically the same as the air-gap diameter D_R(z = L) at the second end 1 1 0 of the active part, i.e. D_R(z = 0) = D_R(z = L), but the principle illustrated above is also applicable in conjunction with conical rotors in which D_R(z = 0) does not equal D_R(z = L). The above-presented equation (1 ) is valid for both conical and non-conical rotors. It should be noted that the cross section of a rotor according to an embodiment of the invention does not have to be a circle. This is illustrated in figure 1 d which shows a rotor according to an embodiment of the invention seen along the axial direction, i.e. along the negative z-direction of the co-ordinate system 190. The rotor comprises a shaft 101 and an active part surrounding the shaft. The active part comprises a center part 102a attached to the shaft and permanent magnet modules 102b, 102c, 102d, and 102e attached to the center part. The permanent magnets modules include permanent magnets each of which having a radial direction of magnetization. A dashed-line circle 108a represents the smallest circle ca- pable of encircling the active part at an end of the active part and a dashed-line circle 108b represents the smallest circle capable of encircling the active part at a location z_c at which the axial profile of the air-gap diameter of the rotor reaches its minimum value.

A rotor according to an embodiment of the invention comprises excitation windings each of which having a radial magnetic axis. The rotor can be either a salient pole rotor or a cylindrical rotor.

Figure 2a shows an axial profile of an air-gap diameter of a rotor according to an embodiment of the invention. A curve 220 indicates the diameter of the smallest circle capable of encircling the active part of the rotor at different axial distances z from a first end of the active part. The curve 220 represents a smooth, concave form of the axial profile of the air-gap diameter. If the curve 220 represents a function that is differentiable at least twice with respect to z, the concavity is manifested by the fact that the second derivative d²D_R(z)/dz² is non-negative, i.e. d²D_R(z)/dz² > 0. It should be, however, noted that the axial profile of the air-gap diameter does not necessarily have to be smooth. Figure 2b shows an axial profile of an air-gap diameter of a rotor according to another embodiment of the invention. A curve 221 indicates the diameter of the smallest circle capable of encircling the active part of the rotor at different axial distances z from a first end of the active part. In this case, the axial profile of the air-gap diameter is not smooth but it has stepwise changes. The stepwise changes are, however, allowable if they are sufficiently small.

An electrical machine according to an embodiment of the invention comprises a rotor, wherein the shape of D_R(z) is selected in such a manner that the air-gap 107 shown in figure 1 c between the rotor and the stator of the electrical machine is substantially constant on the axial range from z = 0 to z = L when the stator core 103 is bent by magnetic forces caused by a magnetic flux corresponding to the nominal use of the electrical machine.

Figure 3a shows a schematic section view of a stator according to an embodiment of the invention. A dot-and-dash line 340 is the axis of the rotational symmetry. The stator comprises a stator core 303 that may have been assembled of tangential laminated segments. The stator core has an axial length L between its first end 312 and its second end 313. The Stator comprises windings 304 partly locating in slots of the stator core. The Stator comprises a support structure 305 arranged to mechanically support the stator core. The support structure 305 is connected to end shields 306.

The z co-ordinate shown in figure 3a indicates the axial distance from the first end 312 of the stator core towards the second end 313 of the stator core. The stator core 303 faces, in the radial direction, towards an electromagnetically active part of a rotor when the stator is being used as a part of an electrical machine. As illustrated in figure 3a, the bore of the stator core has such a shape that a diameter Ds(z) of a largest circle capable of being encircled by the bore of the stator core is a positive convex function of z. A point z_c at which D_s(z) reaches its maximum value is preferably at the middle of the axial length of the stator core, i.e. z_c = L/2, but it is, however, allowable that z_c somewhat deviates from L/2 for example so that z_c belongs to a range from L/2 - L/20 to L/2 + L/20. The shape of the function D_s(z) represents the axial profile of the air-gap diameter of the stator.

Figure 3b shows a schematic section view of a rotor than can be used together with the stator illustrated in figure 3a. The rotor comprises a shaft 301 and an elec- tromagnetically active part 302 surrounding the shaft. Figure 3c illustrates a situation in which the stator illustrated in figure 3a has been put together with the rotor illustrated in figure 3b and magnetic forces are acting on the rotor and the stator. The stator core 303 and the support structure 305 are bent by the magnetic forces as illustrated with an arrow 330 shown in figure 3c. As can be seen from figure 3c, the width of the air-gap 307 between the rotor and the stator is substantially con- stant on the axial range from z = 0 to z = L, because the shape of the axial profile of the air-gap diameter of the stator is selected to be commensurate with the bending of the stator core 303 and the support structure 305. As the stator core can be allowed to be bent, the support structure can be allowed to be more flexible and thus savings in the weight and price of the support structure can be achieved. In figures 3a and 3c, the convexity of the axial profile of the air-gap diameter of the stator and also the bending of the stator core and the support structure are strongly exaggerated for the sake of illustrative purposes. In practical applications, a suitable degree of the convexity of the axial profile of the air-gap diameter of the stator is such that the maximum value of D_s(z), see figure 3a, is: Ds(Zc) = max{Ds(z = 0), D_s(z = L)} + B, (2) where the parameter B is at least 0.25 mm. In stators according to certain particular embodiments of the invention, the parameter B is at least 0.5 mm. In stators according to certain particular embodiments of the invention, the parameter B is at least 2 mm. For example, in a stator having D_s(z = 0) = D_R(z = L) > 2000 mm and L > 1000 mm, D_s(z_c) is greater than or equal to D_s(z = 0) + 0.25 mm.

The air-gap diameter D_s(z = 0) at the first end 312 of the stator core 303 is typically the same as the air-gap diameter D_s(z = L) at the second end 313 of the stator core, i.e. D_s(z = 0) = D_s(z = L), but the principle illustrated above is also applicable in conjunction with stators having a conical bore in which D_s(z = 0) does not equal D_s(z = L). The above-presented equation (2) is valid for both conical and non- conical bores of a stator core.

Figure 4a shows an axial profile of an air-gap diameter of a stator according to an embodiment of the invention. A curve 422 indicates the diameter of the largest circle capable of being encircled by the bore of the stator core at different axial dis- tances z from a first end of the stator core. The curve 422 represents a smooth, convex form of the axial profile of the air-gap diameter. If the curve 422 represents a function that is differentiable at least twice with respect to z, the convexity is manifested by the fact that the second derivative d²D_R(z)/dz² is non-positive, i.e. d²D_s(z)/dz² < 0.

It should be, however, noted that the axial profile of the air-gap diameter does not necessarily have to be smooth. Figure 4b shows an axial profile of an air-gap diameter of a stator according to another embodiment of the invention. A curve 423 indicates the diameter of the largest circle capable of being encircled by the bore of the stator core at different axial distances z from a first end of the stator core. In this case, the axial profile of the air-gap diameter is not smooth but it has stepwise changes. The stepwise changes are, however, allowable if they are sufficiently small.

An electrical machine according to an embodiment of the invention comprises a stator, wherein the shape of D_s(z) is selected in such a manner that the air-gap 307 shown in figure 3c between the rotor of the electrical machine and the stator is substantially constant on the axial range from z = 0 to z = L when the stator core 303 is bent by magnetic forces caused by a magnetic flux corresponding to the nominal use of the electrical machine.

It is also possible to provide an electrical machine with both a stator having a convex axial profile of the air-gap diameter and a rotor having a concave axial profile of the air-gap diameter, wherein the axial profiles of the air-gap diameters of the stator and the rotor are selected in such a manner that the width of the air-gap between the rotor and the stator is substantially constant on its axial range when the stator core is bent by magnetic forces caused by a magnetic flux corresponding to the nominal use of the electrical machine. 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.

Claims

What is claimed is:

1 . An electrical machine comprising a rotor and a stator, the rotor comprising :

- a shaft (1 01 ), and

- an electromagnetically active part (1 02) having an axial length L between its first end (1 1 0) and its second end (1 1 1 ) and surrounding the shaft on the whole axial range between its first and second ends, the electromagnetically active part facing in a radial direction towards a stator core when the rotor is being used as a part of the electrical machine, wherein a diameter of a smallest circle capable of encircling the electromagnetical- ly active part at an axial distance z from the first end (1 1 0) of the electromagnetically active part towards the second end (1 1 1 ) of the electromagnetically active part is D_R(z) which is a positive concave function of z on the range from z = 0 to z = L and a minimum value of which is:

DR(ZC) = min{D_R(z = 0), D_R(z = L)} - B, where B is at least 0.25 mm and z_c at which D_R(z) reaches its minimum value belongs to a range from L/2 - L/20 to L/2 + L/20, characterized in that the concavity of DR(Z) is capable of compensating bending of a stator core (103) caused by magnetic forces caused by a magnetic flux corresponding to a nominal use of the electrical machine so that the width of an air-gap (107) between the rotor and the stator is substantially constant on the range from z = 0 to z = L when the stator core is bent by the magnetic forces.

2. An electrical machine according to claim 1 , wherein B is at least 0.5 mm.

3. An electrical machine according to claim 1 , wherein B is at least 2.0 mm.

4. An electrical machine according to any of the preceding claims, wherein D_R(z = 0) and D_R(z = L) are greater than or equal to 2 meters and L is greater than equal to 1 meter.

5. An electrical machine according to any of the preceding claims, wherein D_R(z = 0) is substantially same as D_R(z = L).

6. An electrical machine according to any of the preceding claims, comprising permanent magnets each of which having a radial direction of magnetization.

7. An electrical machine according to any of the preceding claims, comprising excitation windings each of which having a radial magnetic axis.

8. An electrical machine comprising a rotor and a stator, the stator comprising:

- a stator core (303) having an axial length L between its first end (312) and its second end (313), - windings (304) partly locating in slots of the stator core, and

- a support structure (305) arranged to mechanically support the stator core, wherein a diameter of a largest circle capable of being encircled by a bore of the stator core at an axial distance z from the first end (312) of the stator core towards the second end (313) of the stator core is D_s(z) which is a positive convex function of z on the range from z = 0 to z = L and a maximum value of which is:

Ds(z_c) = max{Ds(z = 0), D_s(z = L)} + B, where B is at least 0.25 mm and z_c at which D_s(z) reaches its maximum value belongs to a range from L/2 - L/20 to L/2 + L/20, characterized in that the convexity of Ds(z) is capable of compensating bending of the stator core (303) caused by magnetic forces caused by a magnetic flux corresponding to a nominal use of the electrical machine so that the width of an air-gap (307) between the rotor and the stator is substantially constant on the range from z = 0 to z = L when the stator core is bent by the magnetic forces.

9. An electrical machine according to claim 8, wherein B is at least 0.5 mm.

10. An electrical machine according to claim 8, wherein B is at least 2.0 mm.

1 1 . An electrical machine according to any of claims 8-10, wherein D_s(z = 0) and D_s(z = L) are greater than or equal to 2 meters and L is greater than equal to 1 meter.

12. An electrical machine according to any of claims 8-1 1 , wherein D_s(z = 0) is substantially same as D_s(z = L).

13. An electrical machine according to any of claims 8-12, wherein the stator core (303) has been assembled of tangential laminated segments.