Patent Publication Number: US-2007114870-A1

Title: Induction motor capable of utilizing magnetic fluxes of end-turns of a stator to increase torque of a rotor

Description:
FIELD OF THE INVENTION  
      The present invention relates to an induction motor making effective use of end-turns; and, more particularly, to an induction motor capable of utilizing magnetic fluxes of end-turns of a stator to increase a torque of a rotor.  
     BACKGROUND OF THE INVENTION  
      In general, an electric motor, which converts an electric energy to a mechanical energy to thereby generate a rotation force, has been extensively used in household electronic products and industrial equipments. The electric motor is largely divided into an alternating current (AC) motor and a direct current (DC) motor.  
      As one kind of the AC motor, there is known an induction motor in which an electric current is induced in a secondary winding by electromagnetic induction of a primary winding of a coil connected to a power supply and a rotational torque is obtained by interaction between the current induced in the secondary winding and rotating magnetic fields.  
      A conventional induction motor will now be described with reference to  FIG. 1 .  
       FIG. 1  is a cross-sectional view of the conventional induction motor. As shown, the conventional induction motor  10  includes a stator  11  fixedly secured to a housing  14 , a rotor  12  rotatably provided at the inner side of the stator  11  with a gap left therebetween, and a shaft  13  press-fitted to a center portion of the rotor  12  for rotation therewith.  
      The stator  11  includes coils  11   a  supplied with an alternating current for creating rotating magnetic fields and stator cores  11   b  made of a magnetic material, magnetic fluxes generated by the rotating magnetic fields of the coils  11   a  flowing through the stator cores  11   b.    
      Each of the stator cores  11   b  is formed by stacking a multiple number of identically shaped silicon steel plates in an axial direction. A plural number of radial slots (not shown) are formed at intervals along an inner circumferential surface of each of the stator cores  11   b . The coils  11   a  are wound in the slots by using a winding method such as distributed winding, concentrated winding, coaxial winding or the like.  
      The rotor  12  includes rotor conductors  12   a  for generating a torque through interaction between a current induced by the coils  11   a  and magnetic fluxes, and a rotor core  12   b  made of a magnetic material through which the magnetic fluxes flow. The rotor conductors  12   a  are attached to the rotor core  12   b.    
      The rotor conductors  12   a  are made of high conductivity metal, such as aluminum and copper, or a magnet.  
      The rotor core  12   b  is formed by stacking a multiple number of identically shaped silicon steel plates in an axial direction. A plural number of radial slots (not shown) are formed at intervals on an outer peripheral surface or at an inner side of the rotor core  12   b . As similar to the coils  11   a , the rotor conductors  12   a  are fitted in the slots in parallel with the axial direction.  
      The rotor core  12   b  is provided at its opposite ends with end rings  12   c  that interconnect the rotor conductors  12   a  fitted inside the rotor core  12   b  to form a circuit.  
      The end rings  12   c  are usually made of aluminum which allows the end rings  12   c  to be integrally formed with the rotor conductors  12   a  by a diecasting method in case of the rotor conductors  12   a  being metal.  
      The shaft  13  is inserted through and fixed to the rotor core  12   b , and the shaft  13  is rotatably supported through bearings  14   b  on shaft seats  14   a  formed at opposite sides of the housing  14 .  
      The operation of the conventional induction motor  10  will now be described. If an alternating current is applied to the coils  11   a , magnetic fields are created in a direction perpendicular to a motor axis and rotating magnetic fluxes are generated through the stator cores  11   b . The rotating magnetic fluxes are interlinked with the rotor conductors  12   a  of the rotor  12  through a gap between the stator core  11   b  and the corresponding rotor conductor  12   a , thereby inducing an electric current in the rotor conductors  12   a . At this time, the electric current induced in the rotor conductors  12   a  cooperates with the magnetic fluxes to generate a torque in the rotor  12  according to Fleming&#39;s left hand rule.  
      In the conventional induction motor  10 , end-turns  11   c  are formed at opposite ends of each of the stator cores  11   b . The end-turns  11   c  form a circuit by interconnecting the coils  11   a  wound in the respective slots of the stator cores  11   b . In such an induction motor  10 , a multiple number of poles need to be formed on the stator  11  in order to create the rotating magnetic fields. To this end, the coils  11   a  are not wound through the neighboring two slots of the stator cores  11   b  but wound through two slots arranged distant from each other with one or more other slots disposed therebetween. For this reason, use of the end-turns  11   c  is unavoidable, while the length thereof may vary depending on the method of winding the coils  11   a.    
      In the induction motor  10 , however, the end-turns  11   c  occupy a substantial length of the coils  11   a  wound on the stator  11 , despite the fact that the magnetic fluxes created by the end-turns  11   c  cannot serve as an effective magnetic flux contributing to the torque of the rotor  12 . Accordingly, the end-turns  11   c  are of no use in improving efficiency of the induction motor  10  but merely increase a copper loss, i.e., an intrinsic resistance, of the coils  11   a.    
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide an induction motor capable of making effective use of end-turns.  
      In accordance with an aspect of the present invention, there is provided an induction motor including a stator having stator cores and coils wound on the stator cores to leave end-turns, and a rotor rotatably provided at an inner side of the stator with a gap left between the stator and the rotor, wherein: the rotor has a rotor core provided at an axial end with an end-turn utilizing portion extending radially outwardly in a confronting relationship with the end-turns of the stator with a gap left therebetween, so that magnetic fluxes generated in the end-turns flow through the end-turn utilizing portion of the rotor core.  
      Preferably, the rotor core is made of compressed soft-magnetic powder. Further, the rotor core may be also provided at the other axial end with the end-turn utilizing portion.  
      In accordance with an aspect of the present invention, there is provided an induction motor including a stator having stator cores and coils wound on the stator cores to leave end-turns, and a rotor rotatably provided at an inner side of the stator with a gap left between the stator and the rotor, wherein: each of the stator cores is provided at an axial end with an end-turn utilizing portion axially extending therefrom in a confronting relationship with one of the end-turns; and a rotor has a rotor core axially extended to face the end-turn utilizing portions of the stator with a gap left therebetween, so that the magnetic fluxes generated in the end-turns are transferred through the end-turn utilizing portions to the rotor core.  
      Preferably, the stator cores are made of compressed soft-magnetic powder. Further, each of the rotor cores may be also provided at the other axial end with the end-turn utilizing portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments, given in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a cross-sectional view of a conventional induction motor;  
       FIG. 2  is a cross-sectional view of an induction motor in accordance with one embodiment of the present invention, which makes effective use of end-turns;  
       FIG. 3  is a perspective view depicting a rotor of the induction motor illustrated in  FIG. 2 ;  
       FIG. 4  is a cross-sectional view of an induction motor in accordance with another embodiment of the present invention, which makes effective use of end-turns; and  
       FIG. 5  is a cross-sectional view taken along the line V-V in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.  
       FIG. 2  is a cross-sectional view of an induction motor  100  in accordance with a first embodiment of the present invention, which makes effective use of end-turns of a stator to increase torque of a rotor. As illustrated, the induction motor  100  in accordance with the first embodiment includes a housing  140 , a stator  110  fixedly secured to the inside of the housing  140 , and a rotor  120  press-fitted onto a shaft  130  rotatably supported on shaft seats  141  of the housing  140  through bearings  142 , the rotor  120  being disposed at the inner side of the stator  110  with a gap left between the stator  110  and the rotor  120  and the rotor  120  and the shaft  130  being rotatable together. The rotor  120  has a rotor core  122  made of compressed soft magnetic powder and the rotor core  122  is provided at an axial end with an end-turn utilizing portion  123  protruded to confront end-turns of the stator  110 . Although the end-turn utilizing portion  123  is formed at one axial end of the rotor core  122  in this embodiment, it should be noted that it may be formed at both axial end of the rotor core  122 .  
      The stator  110  includes coils  111  supplied with an alternating current for creating rotating magnetic fields and stator cores  112  made of a magnetic material, magnetic fluxes generated by the rotating magnetic fields of the coils  111  flowing through the stator cores  112 . Each of the coils  111  is wound in a plurality of slots (not shown) radially formed along an inner circumferential surface of each of the stator cores  112 , thus leaving end-turns  113  at opposite axial ends of each of the stator cores  112 .  
      The rotor  120  includes rotor conductors  121  for generating a torque through interaction between an electric current induced by the coils  111  and magnetic fluxes, and a rotor core  122  made of a magnetic material through which the magnetic fluxes flow.  
      The rotor conductors  121  are made of high conductivity metal, such as aluminum and copper, or a magnet. As similar to the coils  111 , the rotor conductors  121  are fitted in slots of the rotor core  122  in parallel with an axial direction. The rotor conductors  121  are attached to an outer peripheral surface of the rotor core  122 . Alternatively, the rotor conductors  121  may be disposed in the rotor core  122 .  
      As shown in  FIGS. 2 and 3 , the rotor core  122  extends radially outwardly at one axial end, e.g., the top end, toward the end-turns  113  of the stator  110 , thus providing the end-turn utilizing portion  123  that is axially offset with respect to the stator cores  112  and confronts the end-turns  113  with a gap left therebetween. The rotor core  122  is provided with a plurality of slots into which the rotor conductors  121  are fitted and a shaft-receiving hole  124  through which the shaft  130  is inserted for fixation thereto.  
      With such arrangements, magnetic fluxes of the end-turns  113  flow through the end-turn utilizing portion  123  to induce an electric current in the rotor conductors  121 .  
      The rotor core  122  is molded by compressing soft magnetic powder in such a manner as to form the slots (not shown), the shaft-receiving hole  124  and the end-turn utilizing portion  123 . The iron-based particles of the soft magnetic powder are coated with an insulating material for the purpose of electrical insulation.  
      In order to compressively mold the soft magnetic powder into the rotor core  122 , a molding cavity corresponding in shape to the rotor core  122  is provided within a compression-molding machine, and the soft magnetic powder is filled in the molding cavity and then compressed by a punch or the like to form the rotor core  122  having the slots (not shown), the shaft-receiving hole  124  and the end-turn utilizing portion  123 . In this process, a lubricant and/or a binder may be added to the soft magnetic powder.  
      By the process of compression-molding the rotor core  122 , the soft magnetic powder is made to form a soft magnetic composite (“SMC”) having a three-dimensional shape. As opposed to a conventional rotor core made of silicon steel plates, the rotor core  122  thus molded provides an increased flexibility of design, thus making it possible to form the end-turn utilizing portion  123  with ease.  
      Hereinafter, there will be described the operation of the induction motor having the afore-mentioned structure.  
      If an alternating current is applied to the coils  111 , rotating magnetic fluxes are formed in the stator cores  112  and interlinked with the rotor conductors  121  though the gap therebetween, thereby inducing an electric current in the rotor conductors  121 . The electric current induced in the rotor conductors  121  cooperates with the magnetic fluxes to generate a torque of the rotor  120 . At this time, the magnetic fluxes formed in the end-turns  113  are allowed to flow through the gap and through the end-turn utilizing portion  123  to induce an electric current in the rotor conductors  121 . This improves efficiency of the induction motor compared with a conventional induction motor as shown in  FIG. 1  wherein the magnetic fluxes of the end-turns are not utilized.  
      Thus, in accordance with the present embodiment, the magnetic fluxes generated in the end-turns  113  are effectively utilized to increase the torque of the rotor  120 , thereby improving efficiency of the induction motor  100 .  
      As described above, in the first embodiment of the present invention, since the induction motor  100  includes the rotor  120  having the end-turn utilizing portion  123  confronting the end-turns  113  of the stator  110 , magnetic fluxes generated in the end-turns  113  flow through the end-turn utilizing portion  123  to be used in inducing an electric current in the rotor conductor  121 . Accordingly, magnetic fluxes generated in the end-turns  113  of the stator  110  can be effectively utilized to increase the torque of the rotor  120 , thereby enhancing efficiency of the induction motor  100 .  
      Hereinafter, a second embodiment of the present invention will be described with reference to  FIGS. 4 and 5 .  
       FIG. 4  is a cross-sectional view of an induction motor  200  in accordance with the second embodiment of the present invention, which makes effective use of end-turns of a stator to increase torque of a rotor. As illustrated, the induction motor  200  in accordance with the second embodiment includes a housing  240 , a stator  210  fixedly secured to the inside of the housing  240 , the stator  210  having stator cores  212  made of compressed soft magnetic powder, each of the stator  212  cores  212  provided at opposite axial ends with end-turn utilizing portions  214 , a rotor  220  press-fitted onto a shaft  230  rotatably supported on shaft seats  241  of the housing  240  through bearings  242 , the rotor  220  being disposed at the inner side of the stator  210  with a gap left between the stator  210  and the rotor  220 , the rotor  220  and the shaft  230  being rotatable together. The rotor  220  has a rotor core  222  axially extended such that the opposite ends of the rotor core  222  are disposed to face the end-turn utilizing portions  214  of the stator  210  with the gap left therebetween.  
      The stator  210  includes coils  211  supplied with an alternating current for creating rotating magnetic fields and stator cores  212  made of a magnetic material, magnetic fluxes generated by the rotating magnetic fields flowing through the stator cores  212 .  
      Each of the stator cores  212  has a plurality of slots (not shown) radially formed along an inner circumferential surface thereof. The coils  211  are wound in the slots in such a manner as to leave end-turns  213  at axial opposite ends of the stator cores  212 . The end-turn utilizing portions  214  of each of the stator cores  212  extend in an axial direction in such a manner as to adjoin the corresponding end-turns  213 .  
      In the illustrated embodiment, the end-turn utilizing portions  214  are formed at both axial ends of each of the stator cores  212  in an effort to maximize efficiency of the induction motor  200 . Alternatively, a single end-turn utilizing portion may be formed only at one axial end of each of the stator cores  212 .  
      Referring to  FIG. 4 , the end-turn utilizing portions  214  are integrally formed with each of the stator cores  212  and are protruded such that they are disposed between the end-turns  213  and the axial opposite ends of the rotor core  222 , with gaps left between the end-turn utilizing portions  214  and the end-turns  213 .  
      The magnetic fluxes generated in the end-turns  213  are transferred through the end-turn utilizing portions  214  to the rotor  220  to thereby induce an electric current in rotor conductors  221 , which will be set forth later.  
      The stator cores  212  are molded by compressing soft magnetic powder to have the coil winding slots (not shown) and the end-turn utilizing portions  214 . The iron-based particles of the soft magnetic powder are coated with an insulating material for the purpose of electrical insulation.  
      In order to compressively mold the soft magnetic powder into the stator cores  212 , a molding cavity corresponding in shape to the stator cores  212  is provided within a compression-molding machine. The soft magnetic powder is filled in the molding cavity and then compressed by a punch or the like to form the stator cores  212  having the slots (not shown) and the end-turn utilizing portions  214 . In this process, a lubricant and/or a binder may be added to the soft magnetic powder.  
      By the process of compression-molding the stator cores  212 , the soft magnetic powder is made to form a soft magnetic composite having a three-dimensional shape. As opposed to a conventional stator cores made of identically shaped silicon steel plates, the stator cores  212  thus molded provides an increased flexibility of design, thus making it possible to form the end-turn utilizing portions  214  with ease.  
      The rotor  220  includes rotor conductors  221  for generating a torque through interaction between an electric current induced by the coils  211  and magnetic fluxes, and the rotor core  222  made of a magnetic material through which the magnetic fluxes flow.  
      The rotor conductors  221  are made of high conductivity metal, such as aluminum and copper, or a magnet. As similar to the coils  211 , the rotor conductors  221  are fitted in slots of the rotor core  222  in parallel with an axial direction. The rotor conductors  221  are attached to an outer peripheral surface of the rotor core  222 . Alternatively, the rotor conductors  221  may be disposed in the rotor core  222 .  
      The rotor core  222  is axially extended such that the opposite ends thereof face the end-turn utilizing portions  214  with the gap left therebetween; and, therefore, the magnetic fluxes can be transferred from the end-turn utilizing portions  214  to the rotor core  222 .  
      In the present embodiment, the rotor core  222  is formed by stacking a plurality of silicon steel plates. Alternatively, the rotor core  222  may be formed by compression-molding soft magnetic powder as similar to the stator cores  212 . The rotor core  222  is provided with a plurality of slots into which the rotor conductors  221  are fitted and a shaft-receiving hole  224  through which the shaft  230  is inserted for fixation thereto. The rotor  220  is provided at its opposite ends with end rings  223  that interconnect the rotor conductors  221  to form a circuit.  
      There will now be described the operation of the induction motor having the afore-mentioned structure in accordance with the second embodiment.  
      If an alternating current is applied to the coils  211 , rotating magnetic fluxes are formed in the stator cores  212  and interlinked with the rotor conductors  221  though the gap, thereby inducing an electric current in the rotor conductors  221 . The electric current induced in the rotor conductors  221  cooperates with the magnetic fluxes to generate a torque. At this time, the magnetic fluxes formed in the end-turns  213  are transferred to the rotor core  222  through the gaps and the end-turn utilizing portions  214  to thereby induce an electric current in the rotor conductors  221 . This improves efficiency of the induction motor compared with a conventional induction motor as shown in  FIG. 1  wherein the magnetic fluxes of the end-turns are not utilized.  
      Thus, in accordance with the present embodiment, the magnetic fluxes generated in the end-turns  213  are effectively utilized to increase the torque of the rotor  220 , thereby improving efficiency of the induction motor  200 .  
      As described above, in the induction motor  200  of the second embodiment, the stator  210  has the end-turn utilizing portions  214  and the rotor  220  has the axially opposite ends extending to face the respective end-turn utilizing portions  214 , so that the magnetic fluxes generated in the end-turns  213  of the stator  210  can be transferred through the end-turn utilizing portions  214  to the rotor  220  to induce an electric current in the rotor conductors  221 . Accordingly, the magnetic fluxes generated in the end-turns  213  of the stator  210  can be effectively utilized to increase the torque of the rotor  220 , thereby enhancing efficiency of the induction motor  200 .  
      While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.