Abstract:
A rotor disk for a rotor of a synchronous reluctance machine consists of a disk body material with high magnetic permeability. In order to improve the ability of the rotor disk to stand centrifugal and thermal loads, spokes extending in radial direction between a shaft opening and a disk periphery are provided with spoke openings. These openings extend over a wide distance both in circumferential direction and in radial direction to worsen the heat conducting properties of the spokes and to render the spokes less stiff.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rotor disk for rotor of a synchronous reluctance machine. The heat conduction properties and stiffness of the rotor disk have been modified by providing openings in radial extending spokes to thereby improve the ability of the rotor disk to stand centrifugal and thermal loads. 
     BACKGROUND OF THE INVENTION 
     Referring to  FIG. 1 , a typical rotor  100  of a synchronous reluctance machine consists of a plurality of rotor disks  110  stacked together in axial direction. Each rotor disk  110  comprises essentially a disk body of high magnetic permeability material, and longitudinal flux barriers  120  of low magnetic permeability material. Typically the flux barriers  120  are created by cutting material from the disk body, the low magnetic permeability material thereby being air. The flux barriers  120  are configured to give the rotor disk  110  an anisotropic magnetic structure such that axes of maximum reluctance i.e. q-axes  130 , and axes of minimum reluctance i.e. d-axes  140  are formed. Each pole of the rotor disk  110  typically comprises 3-5 radial distanced longitudinal flux barriers  120  in turns with flux paths  150  of corresponding shape. A radial extending symmetry line of each pole coincides with a q-axis  130 . The rotor disk  110  is mechanically self-sustained in that the flux paths  150  are connected to one another by narrow tangential ribs  160  at a disk periphery  170 , and eventually also with radial bridges  180  at q-axes  130 . 
     Between neighbouring rotor poles there are spokes  190  extending in radial direction between a shaft opening  200  and the disk periphery  170 . Typically, if the spokes  190  are symmetrical, the symmetry axis of each spoke  190  coincides with a d-axis  140 . The spokes  190  are typically solid elements consisting of the disk body material, but they may comprise some holes or openings in different shapes and for different purposes such as for inserting tie bolts or for functioning as a flux barrier  120 . 
     JP 2004-254354 discloses a rotor disk with triangle-shaped openings in the spokes. These openings function as flux barriers. 
     JP 2006-042467 discloses a rotor disk with wide openings in the spokes. The function of the openings is to affect the rigidity of the rotor disc such that it can better stand deformations due to the centrifugal force. 
     JP 2001-136717 discloses a rotor disk with relatively large openings in the spokes. JP2001-136717 does not explain the purpose of the openings, but the explanation is probably related to the mechanical properties of the rotor disk. The width of the openings at the radial inward ends of the same is only slightly greater than the overall width of the openings. 
     U.S. Pat. No. 7,560,846 discloses in FIG. 13 a rotor disk comprising triangular shaped openings in the spokes. The openings are configured to receive a coupling member fixing and orienting the rotor disks together. 
     U.S. Pat. No. 6,300,703 discloses in FIG. 31 a rotor disk with a long and narrow opening in the spokes. U.S. Pat. No. 6,300,703 does not explain the purpose of the openings, but they appear to function as additional flux barriers. 
     When a synchronous reluctance machine operates, iron losses in the rotor  100  cause the rotor  100  to heat up. Although such losses are relatively low in a synchronous reluctance machine, the temperature at the flux paths  150  separated by flux barriers  120  still may become quite high because the generated heat cannot be effectively conducted away. It is only the radial innermost flux paths  150 , i.e. the spokes  190 , that have a large heat conducting area towards the rotor shaft. All the remaining flux paths  150  are connected to the rotor shaft only via narrow ribs  160  or bridges  180  which do not provide an adequate heat conducting capacity for keeping the rotor temperatures down. Therefore, in a conventional synchronous reluctance machine there is a great temperature difference between the spokes  190  and the remaining flux paths  150  during a long-term operation. 
     Great temperature differences between different parts of a rotor disk  110  cause thermal tensions within the same. Together with centrifugal load these tensions result in an excess deformation of the rotor disk  110  which lead to hairline cracks and ultimately destroy the rotor disk  110 . The prior art rotor disks  110  do not provide a satisfactory solution for preventing this from happening.  FIG. 2  shows a simulated detail of a conventional rotor disk  110  in a deformed state under a certain load condition. The deformations in  FIG. 2  are exaggerated for the sake of illustration. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a rotor disk with an improved ability to stand centrifugal and thermal loads, and to provide a corresponding synchronous reluctance machine. 
     These objects are achieved by the present teachings. 
     The invention is based on the realization that by providing rotor disk spokes with openings, and to thereby worsening heat conducting properties of the spokes, tensions caused by temperature differences within the rotor disk are reduced. At the same time the spokes become less stiff and allow greater deformation of the rotor disk without permanent damage to the disk body material. Even if some prior art rotor disks comprise spokes with openings, the dimensions of such openings are too small for remarkably altering the heat conduction properties or stiffness of the spokes. 
     According to a first aspect of the invention, there is provided a rotor disk for a rotor of a synchronous reluctance machine. The rotor disk comprises a disk body material with high magnetic permeability, a plurality of longitudinal flux barriers comprising a material with low magnetic permeability and configured to give the rotor disk an anisotropic magnetic structure such that at least one axis of maximum reluctance i.e. a q-axis, and at least one axis of minimum reluctance i.e. a d-axis are formed, a spoke extending in radial direction between a shaft opening and a disk periphery, and in circumferential direction between two adjacent q-axes, the spoke having a spoke area substantially contoured by the shaft opening, the two adjacent q-axes, two adjacent flux barriers at opposite sides of a d-axis, and the disk periphery. Part of the spoke area is not occupied by the disk body material, the spoke area portion or portions not occupied by the disk body material being distributed in circumferential direction over an angular distance α of at least 25° measured from the rotor disk centre. 
     By not occupying the whole spoke area with the disk body material, and by distributing the non-occupied portion in circumferential direction over a wide angular distance, temperature differences within the rotor disk are effectively reduced and greater deformation of the same is allowed without permanent damage to the disk body material. 
     According to one embodiment of the invention, the spoke area portion or portions not occupied by the disk body material are distributed in circumferential direction over an angular distance α of at least 27°, such as 30°, 33° or 35°. Even more even temperature distribution is achieved and even greater deformations allowed when the non-occupied portion is increased in circumferential direction. 
     According to one embodiment of the invention, the spoke area portion or portions not occupied by the disk body material are distributed in circumferential direction over an angular distance α corresponding to at least 38%, such as 40%, 45% or 50%, of the angular distance between two adjacent q-axes. Depending on the pole number of the rotor it may be reasonable to express the distribution of the non-occupied portion in circumferential direction in terms of a value relative to the pole sector instead of an absolute value. 
     According to one embodiment of the invention, the spoke area portion or portions not occupied by the disk body material are distributed in radial direction over a distance d corresponding to at least 50%, such as 60%, 70% or 80%, of the maximum distance D between the shaft opening and the disk periphery. By distributing the non-occupied portion over a long distance in radial direction, temperature differences within the rotor disk are further reduced and even greater deformation of the same is allowed without permanent damage to the disk body material. 
     According to one embodiment of the invention, the spoke area comprises a continuous portion not occupied by the disk body material, the continuous portion extending in circumferential direction over an angular distance α of at least 25°, such as 27°, 30°, 33° or 35° or, alternatively, the continuous portion extending in circumferential direction over an angular distance α corresponding to at least 38%, such as 40%, 45% or 50%, of the angular distance between two adjacent q-axes or, alternatively, the continuous portion extending in radial direction over a distance d corresponding to at least 50%, such as 60%, 70% or 80%, of the maximum radial distance D between the shaft opening and the disk periphery or, alternatively, the angular distance α over which the continuous portion extends in circumferential direction being at least four times, such as five, seven or ten times, the angular dimension β of the continuous portion in the middle of the same in radial direction. By making the non-occupied portion continuous, temperature differences within the rotor disk are further reduced and even greater deformation of the same is allowed without permanent damage to the disk body material. 
     According to one embodiment of the invention, less than 75%, such as less than 70% or less than 65%, of the spoke area is occupied by the disk body material. Less disk body material leads to further decreased heat conduction and to even less stiff spokes. 
     According to one embodiment of the invention, a radial outermost part of the spoke area portion or portions not occupied by the disk body material has a rounding in shape of an arc of a circle extending over 200° such as over 220°. With such a rounding shape mechanical stresses about this portion are distributed evenly and stress concentration points are prevented. 
     According to one embodiment of the invention, the spoke has a cut-out at the disk periphery. A narrow cut-out at the disk periphery further improves the flexibility of the spoke at the radial outermost portion of the same. 
     According to one embodiment of the invention, the spoke area portion or portions not occupied by the disk body material is filled with air. Cutting out disk body material is a simple way of providing the non-occupied portions and leads to air-filled spoke openings. 
     According to one embodiment of the invention, the spoke area does not comprise but a single continuous portion not occupied by the disk body material. Manufacturing of one continuous portion appears simpler than manufacturing of a plurality of portions. 
     According to one embodiment of the invention, the shaft opening comprises a key hole on a q-axis. By directing the key holes towards q-axes the heat conduction path between two adjacent spokes is divided into two narrow necks which further decreases the heat conduction area towards the shaft. 
     According to a second aspect of the invention, there is provided a synchronous reluctance machine comprising a rotor disk according to the description hereinbefore. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in greater detail with reference to the accompanying drawings, wherein 
         FIG. 1  shows a conventional rotor of a synchronous reluctance machine, 
         FIG. 2  shows a simulated detail of a conventional rotor disk in a deformed state under different loads, 
         FIG. 3  shows a rotor disk according to one embodiment of the invention, and 
         FIGS. 4   a - 4   f  show preferred embodiments of spokes for rotor disks according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 3 , a rotor disk  110  with four poles is shown. The rotor disk  110  is symmetrical, each pole comprising four flux paths  150  separated by three flux barriers  120 . The flux paths  150  are connected to one another by narrow tangential ribs  160  at the disk periphery  170 , and some of them are additionally connected with radial bridges  180  crossing the flux barriers  120  in the middle. The disk periphery  170  may have a circular shape, but the shape can also be slightly non-circular and/or it may comprise recesses or cut-outs. About the rotor disk centre  300  there is a shaft opening  200  with four key holes  210 . A rotor shaft (not shown) is inserted into the shaft opening  200  in a complete rotor  100 . The disk body is made of electrical steel, and the flux barriers  120  consist of air-filled openings formed by cutting out the disk body material. Each pole has an axis of maximum reluctance i.e. a q-axis  130  which coincides with a radial extending symmetry axis of each pole. Between each pair of adjacent poles there is an axis of minimum reluctance i.e. a d-axis  140 . 
     At each d-axis  140  there is a spoke  190  extending in radial direction between the shaft opening  200  and the disk periphery  170 . In circumferential direction the spokes  190  extend between two adjacent q-axes  130 . Each spoke  190  thereby has a spoke area contoured by (or defined by) the shaft opening  200  (including two key holes  210 ), two adjacent q-axes  130 , two adjacent flux barriers  120  at opposite sides of a d-axis  140 , and the disk periphery  170 . When defining the contours of the spoke areas, the tangential ribs  160  at the disk periphery  170  should be omitted as well as any rounding or chamfering of the flux barrier shape close to the disk periphery  170 . Also any bridges  180  crossing the flux barriers  120  should be omitted. Instead, the contours should be considered to follow the flux barrier edges and natural extensions of the same when considering the overall shape of the whole flux barriers  120 . 
     Some preferred embodiments of spokes  190  for rotor disks  110  according to the invention are shown in  FIGS. 4   a - 4   f . The spoke areas are not completely occupied by the disk body material but they are provided with spoke openings  220  in different shapes.  FIGS. 4   a - 4   c  show three different spokes  190 , each with a single continuous spoke opening  220  in shapes of a funnel  230 , the Eiffel Tower  240  and a letter T  250 , respectively.  FIG. 4   d  shows a spoke  190  with a plurality of round openings  260  together forming a shape of a funnel.  FIG. 4   e  shows a spoke  190  with an arc-shaped narrow opening  270  close to the shaft opening  200 , and with a narrow cut-out  280  at the disk periphery  170 . Finally,  FIG. 4   e  shows a spoke  190  with two symmetrical openings  310 . 
     The spoke openings  220  cause the heat conduction path between the radial outermost parts of the spokes  190  and the rotor shaft to increase in length and to decrease in cross section compared with solid spokes  190 . The heat conducting properties of the spokes  190  are thereby worsened, and as a result the radial outermost parts of the spokes  190  become warmer during operation of the machine. This brings the spoke temperature closer to the already high temperature of the remaining flux paths  150 , and tensions caused by temperature differences within the rotor disk  110  are reduced. It is to be noted that in contrast to induction machines (increased copper losses) and permanent magnet machines (weakened magnetic field), the increased rotor temperature does not have any negative effect on performance in a synchronous reluctance machine. 
     The spoke openings  220  also render the spokes  190  less stiff and allow greater deformation of the same without permanent damage to the rotor disk material. This has significance when centrifugal forces in a rotating rotor  100  tend to deform the rotor disks  110  by drawing the flux paths  150  outwards. The radial outermost parts of the spoke openings  220  may be provided with a rounding  290  in a shape of an arc of an almost full circle according to  FIGS. 4   b  and  4   c . Such a rounding  290  distributes mechanical stresses about this portion evenly over the whole length of the rounding  290  and prevents stress concentration points from occurring. The narrow cut-out  280  at the disk periphery  170  according to  FIG. 4   e  further improves the flexibility of the spoke  190  at the radial outermost portion of the same. 
     Looking closer at the spoke  190  of  FIG. 4   a , the spoke opening  220  extends in circumferential direction over an angular distance α of about 45° measured from the rotor disk centre  300 . This corresponds to about 50% of the angular distance (90°) between two adjacent q-axes  130 . In radial direction the spoke opening  220  extends over a distance d corresponding to about 82% of the maximum distance D between the shaft opening  200  and the disk periphery  170 . The angular distance α (45°) over which the spoke opening  220  extends in circumferential direction is about 16 times the angular dimension β (2,85°) of the spoke opening  220  in the middle of the same in radial direction. Only about 63% of the spoke area is occupied by the disk body material while the rest is occupied by the spoke opening  220 . 
     The key holes  210  are directed towards q-axes  130  in order to divide the heat conduction path between two adjacent spokes  190  into two narrow necks instead of one broader one. Such a division further improves the mechanical and thermal properties of the spokes  190 . 
     The invention is not limited to the embodiments shown above, but the person skilled in the art may, of course, modify them in a plurality of ways within the scope of the invention as defined by the claims. Thus, for example, the invention is not limited to rotor disks  110  with spoke openings  220  filled with air but the spoke openings  220  may be filled with any thermally isolating material. Also, the pole number of the rotor is not limited to four, but the pole number may be two, six, eight, or even greater. Finally, the invention is not limited to machines using solely reluctance component for creating torque. Indeed, such machine may additionally comprise other means, for example permanents magnets, for creating torque.