Patent Publication Number: US-2023135835-A1

Title: Magnetic component part for a rotor assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP Application No.21206343.2, having a filing date of Nov. 4, 2021, the entire contents of which are hereby incorporated by reference. 
     FIELD OF TECHNOLOGY 
     The following relates to the technical field of electromechanical transducers having a rotor which comprises permanent magnets. In particular, the following relates to a magnetic component part for a rotor assembly of an electromechanical transducer. Further, the following relates to a rotor assembly, to an electromechanical transducer and to a wind turbine, which are all equipped with at least one of such magnetic component parts. Furthermore, the following relates to a method for manufacturing a rotor assembly of an electromechanical transducer, wherein the rotor assembly comprises at least one of such magnetic component parts. 
     BACKGROUND 
     Electromechanical transducers are machines, which convert electrical energy into mechanical energy or vice versa. An electric motor is a widely used electromechanical transducer that converts electrical energy into mechanical energy using magnetic field linkage. An electric generator is an electromechanical transducer that converts mechanical energy into electrical energy also using a magnetic field linkage. 
     An electromechanical transducer comprises a stator and a rotor. The stator is an assembly, which represents the stationary part of an electromechanical transducer. The rotor is an assembly, which represents the moving part of an electromechanical transducer. 
     In order to realize a magnetic field linkage permanent magnets may be used in particular for a rotor of an electromechanical transducer. In recent years, especially since the introduction of rare-earth magnetic materials, permanent magnet (PM) electromechanical transducers have become popular since they eliminate the need for commutators and brushes, which are commonly used with conventional Direct Current (DC) electromechanical transducer. The absence of an external electrical rotor excitation eliminates losses on the rotor and makes permanent magnet electromechanical transducers more efficient. Further, the brushless design of a PM electromechanical transducer allows conductor coils to be located exclusively in the stationary stator. In this respect it is mentioned that non-PM electromechanical transducers, which are equipped with commutators and brushes, are susceptible to significantly higher maintenance costs. 
     PM electromechanical transducers are also known for their durability, controllability, and absence of electrical sparking. Thanks to their advantages the PM electromechanical transducers are widely used in many applications such as electric vehicles (electromechanical transducer is a motor) or in power generation systems (electromechanical transducer is a generator) such as for instance a wind turbine. 
     One technical problem of PM electromechanical transducers is cogging torque which is due to permeability variation between the rotor and stator, i.e., slotting. Cogging torque is an undesired effect that contributes to an output ripple (also called torque ripple), to vibrations, and to noise in an electromechanical transducer. 
     It is known that skewing of the rotor magnets can reduce or theoretically eliminate cogging torque in permanent magnet generators. For instance, US 6,867,524 B2 discloses a permanent magnet motor comprising a rotor having at least three segments. Each of the three segments is formed sequentially adjacent and aligned along an axis of the rotor. Each segment has at least one pair of permanent magnets disposed at a substantially equal interval in a peripheral direction of the rotor. First and second segments are skewed relative to each other by a first angular displacement, and the first and third segments are skewed relative to each other by a second angular displacement. The first and second angular displacements are selected to cause a net sum of torque ripple produced by each of the segments to be substantially equal to zero during an operation of the motor. 
     CN 102868246 B discloses a large-capacity low-speed permanent magnet wind power generator with a stator winding, a permanent magnet rotor and a motor core, wherein the stator winding adopts a plurality of sets of three-phase windings and a concentrated integer slot arrangement. The electrical angle between the corresponding phases in each set of three-phase windings is 60°/n, where n is the number of sets of three sets of three-phase windings, 4≥n≥2. The number of turns of each phase of the stator winding is increased to reduce the axial length of the motor core. 
     Rotor or Stator skewing is a technique widely used in the world of electrical machines to remove the cogging torque and torque ripple. These ripples are caused by the interaction between the permanent magnets of the rotor and the stator teeth of a permanent magnet machine, as well as back-emf distortion and saturation under load conditions. 
     While skewing has advantages like passive reduction of cogging torque and torque ripples, passive reduction of vibrations and passive reduction of noise there is also the drawbacks of passive reduction of torque. This leads to lower power, lower efficiency and lower annual energy production (AEP). 
     The reduction of torque due to skewing can only be solved by removing the skewing. As the goal in most applications is to reduce the vibrations and noise, the reduction of torque due to skewing has to be accepted as it passively reduces the torque ripples. 
     Therefore, there may be a need for improving skewing of an electromechanical transducer. 
     SUMMARY 
     According to a first aspect of embodiments of the invention there is provided an electromechanical transducer comprising a stator assembly and a rotor assembly. The electromechanical transducer comprises a stator assembly and a rotor assembly comprising a rotor shaft having a longitudinal axis, a mounting structure connected to the rotor shaft, and at least one magnetic component part comprising at least one permanent magnet, wherein a skew angle between the permanent magnet and the stator assembly has a value of 60% to 92% of a cogging torque period, or for an integral machine, wherein the skew angle between the permanent magnet and the stator assembly has a value of 35° to 55° electrical degrees. 
     The described electromechanical transducer is based on the idea that rather than compensating the cogging torque completely by setting the skew angle equal to the tooth pitch angle, the skew angle is optimized between zero and the tooth pitch angle to satisfy the level of noise and vibration and increase the torque at the same time. 
     The described electromechanical transducer may have the advantage of increasing the generator torque, the increase can be potentially in the range of 2%. The increase may vary according to different types of magnetic component parts or different types of wind turbines. 
     The term “cogging torque period” equals to a period of the cogging torque. Such period is 60° for integral slot machines but it could be different for fractional slot machines, i.e. according to the pole and slot combination. 
     It has been found that the optimized skew angle lies in a range of 35° to 55° (for an integral slot machine) or has a value of 60% to 92% of a cogging torque period or a maximum skew angle of 60 electrical degrees for an integral machine. In this range, the compensation of the cogging torque is only slightly reduced while the torque is increased. The skew angle can be defined between the permanent magnet and the lamination of the stator assembly. 
     The relation of the skew angle can be measured either in electrical degrees or in mechanical degrees. The measurement in electrical degrees includes a comparison of electromagnetic field lines of the stator like e.g., the tooth pitch angle and of the rotor. The mechanical angle is the electrical angle divided by the pole pair number. 
     The optimized skew angle can be determined relative to a tooth or a central axis of the magnetic component part. On the other hand, the optimized skew angle can be defined as a certain percentage of a maximal or full skew angle. 
     The orientation of a tooth of a stator assembly is defined as an angle between an edge of the tooth and the rotational axis. The edge of the tooth is an edge neighboring the interspace between two teeth. 
     It is mentioned that the mounting structure may comprise any mechanical fastening means which allow for a mechanical connection with a support structure of the rotor assembly. Thereby, the fastening means of the mounting structure and fastening means of the support structure may be complementary with respect to each other. Specifically, the fastening means of the mounting structure and/or the fastening means of the support structure may comprise a screw, a bolt, a nut, an inside or an outside thread, a clamping element, a split pin or any other element which allows for a mechanical fastening between the mounting structure and the support structure. 
     It is pointed out that the mounting structure may comprise any contour which extends from the base element and/or which is formed as a recess within the base element. Specifically, the mounting structure may be a protrusion and/or a recess. 
     According to a further embodiment of the invention the skew angle is relative to a central axis of the magnetic component part. Such definition of the skew angle is easy to calculate and to measure. 
     According to a further embodiment of the invention the skew angle is between 40° to 45° or the skew angle has a value of 75% to 80% of the cogging torque period. This may provide the advantage of higher torque (e.g., plus 3% or more), same or lower torque ripple, also lower airgap force so less excitation for noise and vibration. In addition, an improvement in module rattling may be reached. 
     According to a further embodiment of the invention the skew angle is 45° or the skew angle has a value of 75% of the cogging torque period. This may provide the advantage that the increase of cogging torque is kept at a minimum while the torque may be 2% higher. 
     According to a further embodiment of the invention the skew angle is relative to the longitudinal axis of the rotor shaft. Such definition of the skew angle is easy to calculate and to measure. 
     According to a further embodiment of the invention the mounting structure is arranged with an angle relative to the longitudinal axis of the rotor shaft which is equal to the skew angle. This may provide the advantage that only the rotor machining angle needs to be modified. 
     According to a further embodiment of the invention the skew angle has a value smaller than the maximal or full skew angle and is maximized to the manufacturing tolerances and assembly imperfections and operation deformations. Such design could maximize the cogging torque and torque ripple cancelation according to the manufacturing tolerances and assembly imperfections and operation deformations. 
     According to an embodiment of the invention the electromechanical transducer is a generator. 
     According to a further aspect of embodiments of the invention there is provided a wind turbine for generating electrical power. The provided wind turbine comprises tower, a rotor, which is arranged at a top portion of the tower, and which comprises at least one blade, and an electromechanical transducer as described above, wherein the electromechanical transducer is mechanically or directly coupled with the rotor, i.e. direct-drive. 
     According to a further aspect of embodiments of the invention there is provided a method for manufacturing a rotor assembly of an electromechanical transducer. The provided method comprises mounting at least one magnetic component part as described above to a mounting structure of the rotor assembly, wherein a skew angle between the permanent magnet and the stator assembly has a value of 60% to 92% of a cogging torque period, or for an integral slot machine, wherein the skew angle between the permanent magnet and the stator assembly has a value of 35° to 55° electrical degrees. 
     Also, the described rotor assembly manufacturing method is based on the idea that by using exclusively the magnetic component parts as described above the mechanical skew angle is optimized between zero and the tooth pitch angle to satisfy the level of noise and vibration and increase the torque at the same time. 
     The aspects defined above and further aspects of embodiments of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. Embodiments of the invention will be described in more detail hereinafter with reference to examples of embodiment but to which embodiments of the invention are not limited. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG.  1    shows a wind turbine according to an embodiment of the present invention; 
         FIG.  2 A  shows a top view example of a known magnetic component part in a linear machine with a rotor magnet skew angle set to zero; 
         FIG.  2 B  shows a top view example of a known magnetic component part in a linear machine with a rotor magnet skew angle set to maximal or full; 
         FIG.  3    shows a top view example of a magnetic component part in a linear machine with an optimized rotor magnet skew angle according to an embodiment of the present invention, wherein the magnetic component part is mounted to a support structure of a rotor assembly as shown in  FIG.  1   ; 
         FIG.  4    shows a diagram of torque versus a skew angle ratio of a maximal or full skew angle; 
         FIG.  5    shows a diagram of torque change versus an absolute skew angle; and 
         FIG.  6    shows a diagram of ripple versus an absolute skew angle. 
     
    
    
     DETAILED DESCRIPTION 
     The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs. 
       FIG.  1    shows a wind turbine  100  according to an embodiment of the invention. The wind turbine  100  comprises a tower  120 , which is mounted on a non-depicted fundament. On top of the tower  120  there is arranged a nacelle  122 . In between the tower  120  and the nacelle  122  there is provided a yaw angle adjustment device  121 , which is capable of rotating the nacelle  122  around a non-depicted vertical axis, which is aligned with the longitudinal extension of the tower  120 . By controlling the yaw angle adjustment device  121  in an appropriate manner it can be made sure, that during a normal operation of the wind turbine  100  the nacelle  122  is always properly aligned with the current wind direction. However, the yaw angle adjustment device  121  can also be used to adjust the yaw angle to a position, wherein the nacelle  122  is intentionally not perfectly aligned with the current wind direction. 
     The wind turbine  100  further comprises a rotor  110  having three blades  114 . In the perspective of  FIG.  1    only two blades  114  are visible. The rotor  110  is rotatable around a rotational axis  110   a . The blades  114 , which are mounted at a hub  112 , extend radially with respect to the rotational axis  110   a . 
     In between the hub  112  and a blade  114  there is respectively provided a blade adjustment device  116  in order to adjust the blade pitch angle of each blade  114  by rotating the respective blade  114  around a non-depicted axis being aligned substantially parallel with the longitudinal extension of the blade  114 . By controlling the blade adjustment device  116  the blade pitch angle of the respective blade  114  can be adjusted in such a manner that at least when the wind is not so strong a maximum wind power can be retrieved from the available wind power. However, the blade pitch angle can also be intentionally adjusted to a position, in which only a reduced wind power can be captured. 
     As can be seen from  FIG.  1   , the rotor  110  is directly coupled with a shaft  125 , which is coupled in a known manner to an electromechanical transducer  140 . The electromechanical transducer is a generator  140 . 
     Accordingly, the turbine is a direct drive type, in which the hub is directly connected to the generator  140 , i.e., no gearbox is present. 
     Further, a brake  126  is provided in order to stop the operation of the wind turbine  100  or to reduce the rotational speed of the rotor  110  for instance (a) in case of an emergency, (b) in case of too strong wind conditions, which might harm the wind turbine  100 , and/or (c) in case of an intentional saving of the consumed fatigue life time and/or the fatigue life time consumption rate of at least one structural component of the wind turbine  100 . 
     The wind turbine  100  further comprises a control system  153  for operating the wind turbine  100  in a highly efficient manner. Apart from controlling for instance the yaw angle adjustment device  121  the depicted control system  153  is also used for adjusting the blade pitch angle of the rotor blades  114  in an optimized manner. 
     In accordance with basic principles of electrical engineering the generator  140  comprises a stator assembly  145  and a rotor assembly  150 . The generator  140  may include an external rotor  150  which is arranged outside the stator  145 . 
     The stator assembly  145  comprises a plurality of coils for generating electrical current in response to a time alternating magnetic flux. The rotor assembly comprises a plurality of permanent magnets, which are arranged in rows being aligned with a longitudinal axis of the rotor assembly  150 . As will be described below in detail, the permanent magnets are skewed with an optimized skew angle to satisfy the level of noise and vibration and increase the torque at the same time when the generator  140  is in operation. 
       FIG.  2 A  shows in a top view a known electromechanical transducer or generator  140 , here, by example as a linear machine. The generator  140  includes the stator assembly  145  and the rotor assembly  150 . 
     The stator assembly  145  includes at least on stator iron  200  and stator windings or stator coils  202  arranged between teeth of the stator iron  200 . The rotor assembly  150  includes a magnetic component part  204  with at least one permanent magnet  206 . 
     The magnetic component part  204  and the permanent magnets  206 , respectively are arranged with a skew angle α set to zero, i.e. machine with no skew. The skew angle α can be defined as an angle between an edge  208  of the permanent magnets  206  and the rotational axis  110   a  or the axial axis of the stator. The edge  208  is an edge of the permanent magnet  206  which runs parallel or almost parallel to the rotational axis  110   a . At least, the edge  208  has a smaller angle towards the rotational axis  110   a  then a neighboring edge of the respective same permanent magnet  206 . 
     In the example shown on  FIG.  2 A , the skew angle is zero. Thus, the generator  140  or the rotor magnets  206  are unskewed. In other words, the permanent magnets  206  are aligned with the rotational axis  110   a . 
       FIG.  2 B  shows in a top view a further known electromechanical transducer or generator  140 , here, by example as a linear machine. The generator  140  includes the stator assembly  145  and the rotor assembly  150 . 
     The generator  140  shown in  FIG.  2 B  is similar to the generator  140  shown in  FIG.  2 A . However, the skew angle α is different. 
     According to the example shown in  FIG.  2 B  full skewing is implemented. Thus, the skew angle α max  of the rotor magnet  206  is set to be equal to a tooth pitch angle of the stator assembly  145  (or equal to cogging torque period). This skew angle α max  is named as a maximal or full skew angle as it allows the best or full cancellation of cogging torque or torque ripples. 
     Accordingly, the edge  208  of the magnetic component part  204  and the permanent magnets  206 , respectively is arranged with the maximal skew angle α max  relative to the rotational axis  110   a . The skew angle α max  of the rotor magnet  206  may for example be 60°. 
       FIG.  3    shows in a top view an electromechanical transducer or generator  140  according to an embodiment of the invention. Here, the generator  140  is exemplary depicted as a linear machine. The generator  140  includes the stator assembly  145  and the rotor assembly  150 . 
     In contrast to the examples shown in  FIGS.  2 A and  2 B , the embodiment of  FIG.  3    employs an optimized skew angle α opt  which lies between the skew angle α of zero as shown in  FIG.  2 A  and the maximum skew angle α max  as shown in  FIG.  2 B . The optimized skew angle α opt  does not include the skew angle α of zero and the maximum skew angle α max . Thus, the optimized skew angle α opt  lies in an interval between the two skew angles or limits which do not belong to the interval. In other words, the skew angle α opt  of the rotor magnet  206  is set to be smaller than the maximum skew angle α max  and larger than the skew angle α of zero. 
     This optimized skew angle α opt  is named as an optimized skew angle as it allows to satisfy the level of noise and vibration and to increase the torque at the same time. 
     In addition, the optimal skew angle can also be defined as being smaller than manufacturing tolerances. This could also lead to good cogging torque cancellation as tolerances and imperfections are accounted for. 
     According to the embodiment of the invention, the edge  208  of the magnetic component part  204  and the permanent magnets  206 , respectively is arranged with the optimized skew angle α opt  relative to the rotational axis  110   a . Below, ranges of the optimized skew angle α opt  are given. 
     For integral slot machine, the optimized skew angle α opt  of the rotor magnet  206  may for example be 35° to 55°, desirably 40° to 45° and more desirably 45° electrical degrees. 
     According to another definition, the optimized skew angle α opt  of the rotor magnet  206  may have a value of 60% to 92%, desirably of 75% to 80% and most desirably of 75% of a maximum skew angle α max  of for example 60° electrical for integral slot machines or is the electrical period of cogging torque of an integral slot machine. 
       FIG.  4    shows a diagram of torque versus a skew angle ratio of a maximum skew angle. 
     The torque per unit is depicted as a function of the skew angle per unit. A skew angle of 1 equals to the full or maximum skew angle α max . 
     According to the described technology, the skew angle is reduced below the maximum skew angle α max . It can be seen that the reduction of the skew angle can increase the torque by about 1% to about 5%. 
       FIG.  5    shows a diagram of torque change versus an absolute skew angle. 
     The torque in percent is depicted as a function of the absolute skew angle in electrical degrees. A skew angle of 60° equals to the full or maximum skew angle α max . 
     According to the described technology, the skew angle is reduced below the maximum skew angle α max . It can be seen that the reduction of the skew angle can increase the torque by about 1% to about 5%. The increase is valid for all load situations. For a skew angle of about 40° to 59° degrees the torque increase is almost the same for all load situations. 
       FIG.  6    shows a diagram of ripple versus an absolute skew angle. 
     The ripple in percent is depicted as a function of the absolute skew angle in electrical degrees. A skew angle of 60° equals to the full or maximum skew angle α max . 
     According to the described technology, the skew angle is reduced below the maximum skew angle α max . It has been found that decreasing or reducing the skew angle is not increasing ripple for all load situations. Further, for certain values or ranges of a decreased or optimized skew angle ripple is not increasing or is even decreasing. 
     For an optimized skew angle α opt  of the rotor magnet  206  from 59° to about 40° ripple is decreasing for 100% load. For an optimized skew angle α opt  of the rotor magnet  206  from 59° to about 50° ripple is decreasing for 75% load. For loads of 50% and 25% ripple is not increasing for an optimized skew angle α opt  of the rotor magnet  206  from 59° to about 50°. 
     According to the described technology, the findings of the diagrams as shown in  FIG.  6    and in  FIG.  4    and/or 5 are taken together to achieve an optimized skew angle α opt  of 35° to 50°, desirably of 40° to 45° and most desirably of 45°. 
     This has the advantage that the torque can be increased by about 1% to about 5% while cogging torque and torque ripples are almost kept the same or can even be decreased. 
     Although the present invention has been disclosed in the form of embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.