Abstract:
A rotor ( 1 ) for an induction torque motor according to the present invention comprises an axial body ( 1   a ) and a cylindrical body ( 1   b ) which is coupled to the axial body ( 1   a ) coaxially with the core of the axial body ( 1   a ) and is formed with a soft magnetic material wherein the specific electric resistivity is 0.4 [[mu]ohmsm] or less and the specific magnetic permeability is 1000 to 50000. An induction torque motor (TM) according to the present invention comprises the rotor ( 1 ) for an induction torque motor. Therefore, the rotor ( 1 ) for an induction torque motor and the induction torque motor (TM) according to the present invention can further improve performance.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rotor for an induction torque motor and, in particular, to a rotor for an induction torque motor capable of increasing the performance (the efficiency) of the induction torque motor. The present invention further relates to an induction torque motor including the rotor for an induction torque motor. 
       BACKGROUND ART 
       [0002]    Motors that convert electrical energy into mechanical energy (electric motors or electric machinery) are used for a variety of purposes. In General, motors include a rotor that has a shaft and that rotates and a stator that interacts with the rotor. The rotor rotates due to a magnetic field that varies with rotation (a rotating magnetic field). The motors are generally classified into a synchronous motor and an induction motor (an asynchronous motor). 
         [0003]    In synchronous motors, a rotor is attracted by a rotating magnetic field generated by a supplied alternating electric current and is driven into rotation. Thus, the synchronous motor rotates at a synchronous speed. Examples of a synchronous motor include a permanent magnet synchronous motor (a PM motor) that uses a permanent magnet as a rotor and a reluctance synchronous motor that uses, as a rotor, a material having a low reluctance (a low magnetic resistance) and temporarily functioning as a magnet (such as iron). 
         [0004]    In the induction motors, an induction current is generated in a rotor, which is an electric conductor, due to a rotating magnetic field generated by a stator. Thus, rotation torque corresponding to slip is obtained. Examples of an induction motor include a single-phase induction motor that obtains rotation torque using a predetermined method at startup time and obtains the rotation torque using single-phase alternating current power after startup and a three-phase induction motor, such as a squirrel-cage rotor type three phase induction motor or a wound rotor type three phase induction motor, that obtains the rotation torque using three-phase alternating current power. 
         [0005]    In addition, if motors are classified according to the torque characteristics, torque motors are one of the classes. Torque motors can provide angular displacement or rotation torque of the output shaft in accordance with the level of a supplied electric signal. For example, if a voltage is applied to a torque motor, the torque motor provides angular displacement that is proportional to the applied voltage. Alternatively, if, for example, an electric current is applied to a torque motor, the torque motor provides angular displacement that is proportional to the level of the applied electric current. Accordingly, torque motors are used in a control system that requires an accurate rotation stop position of the output shaft and an accurate rotation torque of the output shaft in response to an electric signal. Thus, torque motors are used in a variety of technical fields. Examples of torque motor include a torque motor having a configuration that is the same as the above-described PM synchronous motor (a synchronous torque motor described in, for example, PTL 1) and a torque motor having a configuration that is the same as the above-described induction motor (an induction torque motor described in, for example, NPL 1). 
         [0006]    The torque motor described in PTL 1 includes a magnetic circuit formed from a solenoid and a pair of stators laterally extending from either end of the solenoid and a rotor that is disposed between circular arc portions each formed by one of the stators and that has a cylindrical magnet core. Upon receiving a magnetic force from the pair of stators excited by the solenoid, the rotor rotates. 
         [0007]    In general, motors are expected to be compact or power-efficient. Accordingly, the performance (the efficiency) represented by a value obtained by dividing the output torque by the input power needs to be increased. 
       CITATION LIST 
     Patent Literature 
       [0008]    PTL 1: Japanese Unexamined Patent Application Publication No. 2006-204012 
       Non Patent Literature 
       [0009]    NPL 1: FUJII ELECTRIC WORKS Co., Ltd., [Online], searched on Sep. 12, 2011, available in the Internet at &lt;URL: http://www.few.co.jp/products/p01.html&gt; and &lt;URL: http://www.few.co.jp/pdf/torque.pdf&gt; 
       SUMMARY OF INVENTION 
       [0010]    Accordingly, the present invention is provided. It is an object of the present invention to provide a rotor for an induction torque motor capable of further increasing the performance of the induction torque motor and an induction torque motor including the rotor for an induction torque. 
         [0011]    According to the present invention, a rotor for an induction torque motor includes a shaft member and a cylindrical member that is concentrically connected to the shaft member and that is formed of a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000. In addition, an induction torque motor according to the present invention includes the rotor for an induction torque motor. Thus, according to the present invention, the performance of each of the rotor for an induction torque motor and the induction torque motor can be improved. 
         [0012]    Further objects, features, and advantages of the present invention will become apparent from the following description of embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of the structure of an induction torque motor according to an embodiment of the present invention. 
           [0014]      FIG. 2  is a perspective view of the structure of a rotor of the induction torque motor. 
           [0015]      FIG. 3  illustrates a relationship between the speed and the torque and a relationship between the speed and an electric current. 
           [0016]      FIG. 4  illustrates a ratio of torque to loss (F/P) under a rotor stopped condition when the drive frequency is 60 [Hz] and a parameter a is 1000. 
           [0017]      FIG. 5  illustrates the ratio of torque to loss (F/P) when the drive frequency is 60 [Hz] and the parameter a is 1500. 
           [0018]      FIG. 6  illustrates the ratio of torque to loss (F/P) when the drive frequency is 60 [Hz] and the parameter a is 2500. 
           [0019]      FIG. 7  illustrates the ratio of torque to loss (F/P) when the drive frequency is 1 [kHz] and the parameter a is 500. 
           [0020]      FIG. 8  illustrates the ratio of torque to loss (F/P) when the drive frequency is 1 [kHz] and the parameter a is 2500. 
           [0021]      FIG. 9  illustrates the ratio of torque to loss (F/P) when the drive frequency is 1 [kHz] and the parameter a is 6000. 
           [0022]      FIGS. 10A and 10B  illustrate the result of analysis obtained for the induction torque motor when the drive frequency is 1 [kHz]. 
           [0023]      FIGS. 11A and 11B  illustrate the result of analysis obtained for the induction torque motor when the drive frequency is 60 [Hz]. 
           [0024]      FIGS. 12A ,  12 B,  12 C, and  12 D illustrate the results of analysis obtained for the induction torque motor when the depth is 2 mm and the width is 0.5 mm and, thus, the aspect ratio(=depth/width) is 4 and if a copper end ring is provided. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    An embodiment of the present invention is described below with reference to the accompanying drawings. Note that in the drawings, the configurations indicated by the same reference numeral are the same configuration and, therefore, description of the configuration is not repeated as needed. In addition, in the present specification, to collectively refer to such configurations, a reference numeral without a suffix is used. In contrast, if the configurations are individually identified, a reference numeral with a suffix is used. 
         [0026]      FIG. 1  is a cross-sectional view of the structure of an induction torque motor according to an embodiment.  FIG. 2  is a perspective view of the structure of a rotor of the induction torque motor according to the embodiment.  FIG. 3  illustrates a relationship between the speed and the torque and a relationship between the speed and an electric current. The abscissa of  FIG. 3  represents the speed (a slip  5 ). The ordinate of  FIG. 3  represents the torque or the current. Note that if the slip S=0, the induction torque motor is at a synchronous speed. In contrast, if the slip S=1, the induction torque motor is at rest. 
         [0027]    As illustrated in  FIG. 1 , an induction torque motor TM according to the present embodiment includes a rotor  1  that has an output shaft and that is rotatable and a stator  2  that interacts with the rotor  1 . The induction torque motor TM rotates the rotor  1  by a magnetic field that rotationally varies (a rotating magnetic field). The induction torque motor TM has a configuration that is substantially the same as an induction motor. In addition, in the example illustrated in  FIG. 1 , the rotor  1  and the stator  2  are accommodated in a frame (a chassis)  3 . 
         [0028]    More specifically, the frame  3  includes a bottomed cylindrical casing  3   a  and a plate-like bracket  3   b  that is attached to the open end (the ceiling end) of the casing  3   a  and that closes the open end of the casing  3   a.  In addition, the rotor  1  and the stator  2  are coaxially disposed in the casing  3   a.  The stator  2  is fixed to the inner peripheral surface of the casing  3   a,  and the rotor  1  is disposed in a rotatable manner relative to the stator  2 . 
         [0029]    The stator  2  is substantially cylindrical in shape. For example, the stator  2  is formed by stacking a plurality of magnetic steel sheets. The cylindrical stator  2  includes an inner surface having a plurality of ridges (convex portions) each protruding inwardly in the radial direction and extending in the axis direction. The ridges are arranged at predetermined intervals in the circumferential direction. The stator  2  further includes a plurality of winding wires formed by winding a conductor wire coated by an insulator, such as an insulated covered copper wire, around each of the plurality of convex portions. Such winding wires and the convex portions form the magnetic poles. Thus, a predetermined number of the magnetic pole are formed (the number of magnetic poles is predetermined). In addition, as described above, the outer peripheral surface of the stator  2  is fixed to the inner peripheral surface of the casing  3   a.    
         [0030]    The rotor  1  includes a shaft member  1   a  having a shape of a rod (a bar or a columnar member) that serves as an output shaft and a cylindrical member  1   b  connected to the shaft member  1   a  so as to be concentric (coaxial) with the shaft center of the shaft member  1   a.  According to the present embodiment, the shaft center  1   a  is inserted into a through-hole formed so as to penetrate the shaft center of the cylindrical member  1   b  in the axis direction. The shaft center  1   a  is fixed to the cylindrical member  1   b  in the substantially middle region of the cylindrical member  1   b.    
         [0031]    In addition, one end of the shaft member  1   a  (the left end in  FIG. 1 ) is rotatably supported by a bearing  4  attached to the casing  3   a.  The other end of the shaft member  1   a  (the right end in  FIG. 1 ) is rotatably supported by a bearing  5  attached to the bracket  3   b.  To transfer the output to the outside of the frame  3 , the shaft member  1   a  is disposed so as to pass through the bracket  3   b  and extend to the outside. In addition, as described above, the rotor  1  is disposed in the cylinder formed by the stator  2  so as to be coaxial with the stator  2  with a predetermined spacing between the outer peripheral surface of the rotor  1  and the inner peripheral surface of the stator  2  (a spacing in the radial direction). 
         [0032]    Furthermore, the cylindrical member  1   b  of the rotor  1  is made of a soft magnetic material having a specific electric resistivity ρ lower than or equal to 0.4 [μΩ·m] and a relative permeability μ higher than or equal to 1000 and lower than or equal to 50000 (in the range of 1000 to 50000) (i.e., ρ≦0.4 [μΩ·m], and 50000≧μ≧1000). Examples of such a soft magnetic material include extra low carbon Cold Heading wire (ELCH2) (an iron series soft magnetic material available from Kobe Steel, Ltd.), ELCH2S, LCA-1, and S10C. It is desirable that the soft magnetic material used for the cylindrical member  1   b  of the rotor  1  be the iron series soft magnetic material “ELCH2”. The chemical composition of ELCH2, expressed in mass %, is 0.005% carbon (C), 0.004% silicon (Si), and 0.26% manganese (Mn). ELCH2 has a high flux density and a low magnetic coercive force. In addition, ELCH2 has a reduced specific electric resistivity lower than that of low carbon steel (C=0.1%) by 30%, which is substantially the same as the specific electric resistivity of industrial nickel Ni. 
         [0033]    Note that while the example illustrated in  FIG. 1  has been described with reference to the shaft member  1   a  separated from the cylindrical member  1   b,  the shaft member  1   a  and the cylindrical member  1   b  may be integrated with each other. 
         [0034]    In addition, according to the present embodiment, as illustrated in  FIG. 2 , the cylindrical member  1   b  further includes a plurality of concave groove portions  11  ( 11 - 1 ,  11 - 2 ,  11 - 3 , . . . ) that are formed in the peripheral surface of the cylindrical member  1   b  and that extend parallel to the axis direction of the cylindrical member  1   b.  It is desirable that the depth of the concave groove portions  11  be greater than or equal to the skin depth δ(sf 0 ) and the aspect ratio (the depth/the width) be greater than or equal to 2. 
         [0035]    Note that let s be the slip of the induction torque motor TM. Then, the skin depth δ(sf 0 ) can be expressed as: 
         [0000]      δ( sf   0 )=ρ/π· s·f   0 ·μ) 1/2  
 
         [0000]    where f 0  [Hz] represents the drive frequency of the induction torque motor TM, μ represents the relative permeability of the cylindrical member  1   b  of the rotor  1 , and ρ [μΩ·m] represents the specific electric resistivity of the cylindrical member  1   b  of the rotor  1 . 
         [0036]    In addition, according to the present embodiment, as illustrated in  FIG. 2 , the cylindrical member  1   b  further includes a pair of circular ring members  12 - 1  and  12 - 2  made of a conductive material having a specific electric resistivity lower than or equal to 0.02 [μΩ·m]. The ring members  12 - 1  and  12 - 2  are disposed along the circumferential edge at either end of the cylindrical member  1   b  in the axis direction. 
         [0037]    Furthermore, as illustrated by a dashed line in  FIG. 2 , to efficiently cool the shaft member  1   a,  the shaft member  1   a  may further has a through-hole  13  formed so as to pass through the shaft center of the shaft member  1   a  in the axis direction. By allowing coolant, such as gas (e.g., air) or liquid (e.g., water), to pass through the through-hole  13 , the rotor  1  and, thus, the induction torque motor TM can be cooled. Note that in such a case, a through-hole is formed in the bottom portion of the casing  3   a  to circulate the coolant. 
         [0038]    As can be seen from a dashed line indicating a torque TQg of a general-purpose motor in  FIG. 3 , if a widely used squirrel-cage rotor type induction motor is driven with a constant voltage and a constant frequency, the torque slightly drops (decreases) from the startup torque level and, thereafter, rises (increases) with increasing speed from the stationary state. The torque reaches its peak at about 80% of the synchronous speed (the maximum torque) and, thereafter, the torque drops (decreases). Subsequently, the speed becomes constant at a point at which the load torque is the same as the torque of the motor. In addition, as can be seen from an alternate long and short dash line indicating an electric current ECg of a general-purpose motor in  FIG. 3 , the electric current is substantially constant at the startup level from the stationary state to a point at which the slip S reaches about 0.5. Thereafter, the electric current ECg gradually drops (decreases) from the startup level with increasing speed. The electric current ECg flows even in a no-load state (the slip S=0). 
         [0039]    In contrast, in the induction torque motor TM, as the electric current is gradually decreased from the startup current level, as can be seen from a dashed line indicating an electric current EC of the torque motor in  FIG. 3 , the torque linearly drops in proportion to the current level, as can be seen from an alternate long and two short dashes line indicating a torque TQa of a torque motor A in  FIG. 3 . Alternatively, the torque drops in a convex curve in a predetermined relationship with the current level, as can be seen from a solid line indicating a torque TQb of a torque motor B in  FIG. 3 . As described above, the rotation torque of the induction torque motor TM can be obtained in accordance with the supplied current level. 
         [0040]    In addition, since, in the induction torque motor TM according to the present embodiment, the cylindrical member  1   b  of the rotor  1  is made of a soft magnetic material having a specific electric resistivity ρ lower than or equal to 0.4 [μΩ·m] and a relative permeability μ higher than or equal to 1000 and lower than or equal to 50000, the performance (the efficiency) can be increased more. The increase in the performance is described in more detail below. 
         [0041]      FIG. 4  illustrates the ratio of torque to loss (F/P) when the drive frequency is 60 [Hz] under a rotor stopped condition and a parameter a is 1000.  FIG. 5  illustrates the ratio of torque to loss (F/P) when the drive frequency is 60 [Hz] and the parameter a is 1500.  FIG. 6  illustrates the ratio of torque to loss (F/P) when the drive frequency is 60 [Hz] and the parameter a is 2500.  FIG. 7  illustrates the ratio of torque to loss (F/P) when the drive frequency is 1 [kHz] and the parameter a is 500.  FIG. 8  illustrates the ratio of torque to loss (F/P) when the drive frequency is 1 [kHz] and the parameter a is 2500.  FIG. 9  illustrates the ratio of torque to loss (F/P) when the drive frequency is 1 [kHz] and the parameter a is 6000. In  FIGS. 4 to 9 , the abscissa represents the specific electric resistivity ρ expressed in and the ordinate represents the non-dimensional [−] relative permeability μ. Note that in  FIGS. 4 to 9 , the ratio of torque to loss (F/P) for the induction torque motor TM including a roller having a structure without the concave groove portions and the circular ring members is illustrated.  FIG. 10  illustrates the result of analysis obtained when the drive frequency is 1 [kHz].  FIG. 10A  illustrates the result of analysis of the torque with respect to a rotational angle, and  FIG. 10B  illustrates the Joule loss with respect to a rotational angle. In  FIG. 10A , the abscissa represents the rotational angle expressed in degree (°), and the ordinate represents the torque expressed in Nm. In  FIG. 10B , the abscissa represents the rotational angle expressed in degree (°), and the ordinate represents the Joule loss expressed in W.  FIG. 11  illustrates the result of analysis obtained when the drive frequency is 60 [Hz].  FIG. 11A  illustrates the result of analysis of the torque with respect to a mechanical angle (an electrical angle×1/2).  FIG. 11B  illustrates the rotor Joule loss with respect to a rotational angle (an electrical angle ×1/2). In  FIG. 11A , the abscissa represents the mechanical degree expressed in degree (°), and the ordinate represents the torque expressed in Nm. In  FIG. 11B , the abscissa represents the rotational angle expressed in degree (°), and the ordinate represents the rotor Joule loss expressed in W. 
         [0042]    In  FIGS. 4 to 9 , each of the solid lines is the curved line of equal performance. The curved line of equal performance is formed from the points that provide the same performance (the efficiency) in the specific electric resistivity ρ−relative permeability μ space. In  FIGS. 4 to 9 , the line of equal performance is given by the ratio of torque to loss (T/P). 
         [0043]    Let i eddy (x) be the eddy current density in the cylindrical member  1   b  of the rotor  1  of the above-described induction torque motor TM, and let B(x) be the magnetic flux density in the cylindrical member  1   b.  Then, the following equations can be obtained: 
         [0000]      i eddy ( x )=( I   0 /δ)*exp(−( x /δ))   (1)
 
         [0000]        B ( x )= B   0 ( x )·exp(−( x/ δ))   (2)
 
         [0000]    where x represents the coordinate system directed toward the shaft center in the radial direction when the outer peripheral surface of the cylindrical member  1   b  is defined as “0”, I 0  represents the supplied external drive current, B 0 (x) represents the magnetic flux density obtained when an external drive current I 0  is supplied, and δ represents the above-described skin depth (δ(sf 0 )=(ρ/π·s·f 0 ·μ) 1/2 ). 
         [0044]    In addition, for the eddy current density i eddy (x) and the magnetic flux density B(x), the following equations can be obtained: 
         [0000]      ∫ i   eddy ( x ) dx=I   0    (3), and
 
         [0000]        B   0   =B   0 (μ)= L (μ)· I   0 ∝(1/( 1+a/ μ))· I   0    (4).
 
         [0000]    Note that equation (3) indicates that the eddy current density is normalized by the external drive current I 0 . In addition, the integral range of equation (3) is from (x=0 to ∞ (infinity)), the expression “A∞B” in equation (4) indicates that A is proportional to B, μ represents the relative permeability of the cylindrical member  1   b  of the rotor  1 , L [H] represents the inductance, and a represents a parameter regarding the distance G [m] between the outer peripheral surface of the cylindrical member  1   b  of the rotor  1  and the inner peripheral surface of the stator  2  (the spacing in the radial direction). The parameter a and the distance G have a seesaw relationship. If the distance G is small, the parameter a is relatively large. In contrast, if the distance G is large, the parameter a is relatively small. That is, the parameter a is inversely proportional to the distance G (a∞1/G). Note that the parameter a is effected by the structure and the magnetic property of the stator  2  in addition to the distance G. The parameter a is used as the index indicating the quality of a motor magnetic circuit. 
         [0045]    A Lorentz force f L (x) acting on a microvolume in the rotor  1  is given as follows: 
         [0000]        f   L ( x )= i   eddy ( x )× B ( x )   (5)
 
         [0000]    Torque T can be obtained by integrating equation (5) over the entire area of the rotor  1  as follows: 
         [0000]    
       
         
           
             
               
                 
                   T 
                   = 
                   
                     
                       ∫ 
                       
                         
                           
                             i 
                             eddy 
                           
                            
                           
                             ( 
                             x 
                             ) 
                           
                         
                         × 
                         
                           B 
                            
                           
                             ( 
                             x 
                             ) 
                           
                         
                          
                         
                            
                           x 
                         
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             ( 
                             
                               
                                 
                                   I 
                                   0 
                                 
                                 · 
                                 
                                   
                                     B 
                                     0 
                                   
                                    
                                   
                                     ( 
                                     μ 
                                     ) 
                                   
                                 
                               
                                
                               
                                 / 
                               
                                
                               δ 
                             
                             ) 
                           
                           ) 
                         
                         · 
                         
                           ∫ 
                           
                             
                               exp 
                                
                               
                                 ( 
                                 
                                   
                                     - 
                                     2 
                                   
                                   · 
                                   
                                     ( 
                                     
                                       x 
                                        
                                       
                                         / 
                                       
                                        
                                       δ 
                                     
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                              
                             
                                
                               x 
                             
                           
                         
                       
                       = 
                       
                         
                           ( 
                           
                             
                               
                                 I 
                                 0 
                               
                               · 
                               
                                 
                                   B 
                                   0 
                                 
                                  
                                 
                                   ( 
                                   μ 
                                   ) 
                                 
                               
                             
                              
                             
                               / 
                             
                              
                             2 
                           
                           ) 
                         
                         = 
                         
                           
                             ( 
                             
                               
                                 
                                   L 
                                    
                                   
                                     ( 
                                     μ 
                                     ) 
                                   
                                 
                                 · 
                                 
                                   I 
                                   0 
                                   2 
                                 
                               
                                
                               
                                 / 
                               
                                
                               2 
                             
                             ) 
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Note that each of the integral ranges of equation (6) is (from X=0 to ∞ (infinity)). 
         [0046]    Like the torque T, an eddy current loss P in the rotor  1  can be obtained as follows: 
         [0000]        P=∫ρ·i   eddy   2 ( x ) dx =(ρ/δ)·( I   0   2 /2)   (7)
 
         [0000]    Note that each of the integral ranges of equation (7) is from (x=0 to ∞ (infinity)). 
         [0047]    Accordingly, from equations (6) and (7), the torque per “loss/slip” of the induction torque motor is given as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                      
                     
                       / 
                     
                      
                     
                       ( 
                       
                         P 
                          
                         
                           / 
                         
                          
                         
                           sf 
                           0 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         L 
                          
                         
                           ( 
                           μ 
                           ) 
                         
                       
                       · 
                       
                         ( 
                         
                           
                             δ 
                              
                             
                               ( 
                               
                                 sf 
                                 0 
                               
                               ) 
                             
                           
                            
                           
                             / 
                           
                            
                           ρ 
                         
                         ) 
                       
                       · 
                       
                         sf 
                         0 
                       
                     
                     = 
                     
                       
                         ( 
                         
                           1 
                            
                           
                             / 
                           
                            
                           
                             ( 
                             
                               1 
                               + 
                               
                                 a 
                                  
                                 
                                   / 
                                 
                                  
                                 μ 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                       · 
                       
                         ( 
                         
                           
                             δ 
                              
                             
                               ( 
                               
                                 sf 
                                 0 
                               
                               ) 
                             
                           
                            
                           
                             / 
                           
                            
                           ρ 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0048]    The results of simulation using the equation under a variety of conditions (the results of numerical analysis) are illustrated in  FIGS. 4 to 9 . In  FIGS. 4 to 6 , the drive frequency is set to a commercial frequency of 60 Hz.  FIG. 4  illustrates the results when the parameter a is 1000 (a=1000).  FIG. 4  illustrates the results when the parameter a is 1000 (a=1000).  FIG. 5  illustrates the results when the parameter a is 1500 (a=1500).  FIG. 6  illustrates the results when the parameter a is 2500 (a=2500). In addition, in  FIGS. 7 to 9 , the drive frequency is 1 kHz.  FIG. 7  illustrates the results when the parameter a is 500 (a=500).  FIG. 8  illustrates the results when the parameter a is 2500 (a=2500).  FIG. 9  illustrates the results when the parameter a is 6000 (a=6000). The symbols in  FIGS. 4 to 9  indicate the soft magnetic materials having a chemical composition described below. The points in  FIGS. 4 to 9  indicate the simulation results when the rotor  1  is made of such soft magnetic materials. In addition, the curved lines in  FIGS. 4 to 9  represent curved lines of equal performance indicated by equation (8). The order of magnitude of the value indicated by the curved line increases as the thickness of the curved line increases. 
         [0049]    The chemical composition (mass %) of each of the soft magnetic materials is illustrated in the following tables 1 to 3. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 &lt;ULTRALOW CARBON SERIES&gt; (mass %) 
               
             
          
           
               
                 MATERIAL 
                 C 
                 Si 
                 Mn 
                 ADDED (INCREASED) ELEMENT 
               
               
                   
               
             
          
           
               
                 ELCH2 
                 0.005 
                 0.004 
                 0.25 
                 — 
               
               
                 ELCH2S 
                 0.005 
                 0.004 
                 0.26 
                 S 
               
               
                 EL-1 
                 0.003 
                 0.70 
                 0.28 
                 — 
               
               
                 EL-2 
                 0.005 
                 2.00 
                 0.27 
                 Cr 
               
               
                 EL-3 
                 0.006 
                 2.49 
                 0.27 
                 Al 
               
               
                 EL-4 
                 0.021 
                 2.46 
                 0.49 
                 Cu, Cr, Al 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 &lt;SUS SERIES&gt; (mass %) 
               
             
          
           
               
                   
                   
                   
                   
                 ADDED 
               
               
                   
                   
                   
                   
                 (INCREASED) 
               
               
                 MATERIAL 
                 C 
                 Si 
                 Mn 
                 ELEMENT 
               
               
                   
               
             
          
           
               
                 SUS430 
                 0.04 
                 0.25 
                 0.55 
                 Ni, Cr 
               
               
                 ELECTROMAGNETIC 
                 0.010 
                 1.0 
                 0.20 
                 Cu, Ni, Cr, Mo, Al 
               
               
                 SUS-1 
               
               
                 ELECTROMAGNETIC 
                 0.007 
                 3.0 
                 0.31 
                 Cr, Al, Ti, Ca 
               
               
                 SUS-2 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 &lt;LOW CARBON SERIES&gt; (mass %) 
               
             
          
           
               
                 MATERIAL 
                 C 
                 Si 
                 Mn 
                 ADDED (INCREASED) ELEMENT 
               
               
                   
               
               
                 S10C 
                 0.11 
                 0.17 
                 0.43 
                 — 
               
               
                 S15C 
                 0.15 
                 0.16 
                 0.42 
                 — 
               
               
                 S20C 
                 0.20 
                 0.15 
                 0.42 
                 — 
               
               
                 S25C 
                 0.26 
                 0.18 
                 0.42 
                 — 
               
               
                 S35C 
                 0.36 
                 0.22 
                 0.72 
                 — 
               
               
                 S45C 
                 0.47 
                 0.18 
                 0.73 
                 — 
               
               
                 LCA-1 
                 0.05 
                 0.03 
                 0.30 
                 Al 
               
               
                 LCA-2 
                 0.15 
                 0.02 
                 0.34 
                 Al, Cr, B 
               
               
                   
               
             
          
         
       
     
         [0050]    Note that the simulation result for S20C is obtained using the specific electric resistivity and the magnetic permeability when magnetic annealing is not performed. In addition, SUS430 is ferrite based. 
         [0051]    As can be seen from  FIGS. 4 to 9 , in the specific electric resistivity ρ−relative permeability μ space, the performance (the efficiency) increases toward the upper left. That is, the performance (the efficiency) increases with decreasing specific electric resistivity ρ and with increasing relative permeability. In addition, as can be seen from  FIGS. 4 to 6  which illustrate the results when the parameter a is 1000, 1500, and 2500 and, in particular, as can be seen from  FIG. 5 , the soft magnetic materials having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000 are present in substantially the same performance curved line. Accordingly, from the simulation result, it is desirable that the rotor of an induction torque motor be made of a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000. The simulation result illustrated in  FIGS. 4 to 9  indicates that such a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000 is ELCH2, ELCH2S, LCA-1, LCA-2, S10C, S15C, or S25C. It is more desirable that the specific electric resistivity be lower than or equal to 0.3 [μΩ·m], and it is still more desirable that the specific electric resistivity be lower than or equal to 0.2 [μΩ·m]. In addition, it is more desirable that the relative permeability be higher than or equal to 2000 and lower than or equal to 20000, and it is still more desirable that the relative permeability be higher than or equal to 3000 and lower than or equal to 10000. 
         [0052]    In addition, as can be seen from a comparison of  FIG. 7  and  FIG. 9 , the impact of a difference between the properties of the soft magnetic materials on the performance depends on the parameter a. If the parameter a is relatively small, the distance (the spacing in the radial direction) G between the outer peripheral surface of the cylindrical member  1   b  of the rotor  1  and the inner peripheral surface of the stator  2  is large and, thus, the property of the soft magnetic material that forms the cylindrical member  1   b  does not significantly influence the performance. In contrast, if the parameter a is relatively large, the distance (the spacing in the radial direction) G between the outer peripheral surface of the cylindrical member  1   b  of the rotor  1  and the inner peripheral surface of the stator  2  is small and, thus, the property of the soft magnetic material that forms the cylindrical member  1   b  significantly influence the performance. Accordingly, as illustrated in  FIGS. 4 to 6 , it is desirable that the parameter a be at least in the range of 1000 a 2500. Furthermore, as can be seen from  FIG. 9 , if the cylindrical member  1   b  is made of ELCH2, the performance is maximized. Thus, it is more desirable that the soft magnetic material used to form the cylindrical member  1   b  of the rotor  1  be an iron soft magnetic material (i.e., ELCH2). 
         [0053]      FIGS. 10 and 11  illustrate the simulation results when the drive frequency are 1 kHz and 60 kHz, respectively. In  FIG. 10 , the gap (the distance) between the stator  2  and the rotor  1  is 0.15 mm and a=1500. In  FIG. 11 , the gap (the distance) between the stator  2  and the rotor  1  is 0.1 mm and a=2500. As can be seen from  FIGS. 10 and 11 , a ratio obtained by dividing the maximum torque by the rotor Joule loss (referred to as “ratio of torque to loss”) is the highest for ELCH2 and is the second highest for S10C. Accordingly, it is desirable that the magnetic material have a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000. In addition, if the drive frequency of 1 kHz, which is higher than 60 Hz, improves the ratio of torque to loss more. 
         [0054]    In addition, according to the present embodiment, the cylindrical member  1   b  of the rotor  1  of the induction torque motor TM has a plurality of concave groove portions  11  that are formed in the peripheral surface of the cylindrical member  1   b  and that extend parallel to the axis direction of the cylindrical member  1   b.  By including the plurality of concave groove portions  11  in this manner, the rotor  1  of the induction torque motor TM according to the present embodiment can regulate an area in which an eddy current flows. As a result, a squirrel-cage property can be provided to the cylindrical member  1   b.  Accordingly, unlike the torque property of a rotor without the concave groove portions  11  such that the torque decreases substantially in proportion to a decrease in the eddy current (an increase in speed) (refer to the torque property of the torque motor A illustrated in  FIG. 3 ), a torque property such that the torque gently decreases with decreasing eddy current (increasing speed) in a range from the starting point to a point at which the current (the speed) reaches a predetermined value and the torque relatively rapidly decreases with decreasing eddy current (increasing speed) in a range in which the eddy current (the speed) exceeds the predetermined value (refer to the torque property of the torque motor B illustrated in  FIG. 3 ) can be provided to an induction torque motor including the rotor  1 . That is, when the induction torque motor TM is achieved, the rotor  1  of the induction torque motor TM having such a configuration can maintain the torque at a substantially constant level even when a few slip occurs. 
         [0055]    In addition, in the rotor  1  of the induction torque motor TM according to the present embodiment, the depth of the concave groove portions  11  is greater than or equal to the skin depth δ(sf 0 ), and the aspect ratio (the depth/the width) is greater than or equal to 2. 
         [0056]      FIG. 12  illustrates the results of analysis when the depth is 2 mm and the width is 0.5 mm and, thus, the aspect ratio(=depth/width) is 4 and when a copper end ring is provided.  FIG. 12A  illustrates the result of analysis of the torque with respect to the rotational angle for S10C.  FIG. 12B  illustrates the Joule loss with respect to the rotational angle for S10C.  FIG. 12C  illustrates the result of analysis of the torque with respect to the rotational angle for ELCH2.  FIG. 12D  illustrates the Joule loss with respect to the rotational angle for ELCH2. In  FIGS. 12A and 12C , the abscissa represents the rotational angle expressed in degree (°), and the ordinate represents the torque expressed in Nm. In  FIGS. 12B and 12D , the abscissa represents the rotational angle expressed in degree (°), and the ordinate represents the Joule loss expressed in W. The size of the above-described copper end ring, which is an example of the circular ring member 12, is 14.9 mm in external radius (the same as the rotor external radius), is 12.0 mm in internal radius, and is 3.0 mm in height. 
         [0057]    As indicated by the result in  FIG. 12 , by providing the concave groove portions  11  having an aspect ratio greater than or equal to 2, the rotor  1  of the induction torque motor TM according to the present embodiment can reliably regulate an area in which an eddy current flows. 
         [0058]    In addition, in the rotor  1  of the induction torque motor TM according to the present embodiment, the cylindrical member  1   b  includes the pair of circular ring members  12 - 1  and  12 - 2  formed along the circumferential edge at either end of the cylindrical member  1   b  in the axis direction. The circular ring members  12 - 1  and  12 - 2  are made of a conductive material having a specific electric resistivity lower than or equal to 0.02 [μΩ·m]. Accordingly, by further including the circular ring members  12  made of a relatively good conductor, the rotor  1  of the induction torque motor TM according to the present embodiment can reliably maintain a flow path of the eddy current at either end of the rotor  1  and, thus, the eddy current is enabled to efficiently flow. As a result, an excellent performance can be provided. 
         [0059]    In addition, the above-described magnetic property, such as magnetic permeability, can be obtained by producing a ring-shaped sample having, for example, an external radius of 18 mm×an internal radius of 10 mm using a wire material of each of the compositions, performing magnetic annealing (or intentionally eliminating the need for magnetic annealing to reduce the number of steps), winding a magnetic field applying coil and a magnetic flux detecting coil around the sample, and measuring the B-H curved line using an automatic magnetization measuring apparatus. 
         [0060]    The present specification describes the variety of techniques mentioned above. The primary technologies among the above-described technologies are summarized below. 
         [0061]    According to an embodiment, the rotor for an induction torque motor is used as a rotor of an induction torque motor. The rotor includes a shaft member and a cylindrical member that is connected to the shaft member so as to be concentric with the shaft center of the shaft member and that is made of a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000. 
         [0062]    The results of simulation analysis for the induction torque motor indicates that a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000 (in the range of 1000 to 50000) is located in the best section of a curved line of equal performance, unlike the other soft magnetic materials. As used herein, the term “curved line of equal performance” refers to a line formed by a set of points that provide the same performance in the specific electric resistivity−relative permeability space. Accordingly, since such a rotor for an induction torque motor includes a cylindrical member made of a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩm] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000, the performance of the induction torque motor including the rotor for an induction torque motor can be more improved than that of an induction torque motor having a rotor including a cylindrical member made of another soft magnetic material. 
         [0063]    In addition, according to another embodiment, in the above-described rotor for an induction torque motor, let T [N·m] be the torque of the induction torque motor, let P [W/kg] be the eddy current loss of the rotor, let s be the slip of the induction torque motor, let f 0  [Hz] be the drive frequency of the induction torque motor, let ρ [μΩ·m] be the specific electric resistivity of the soft magnetic material, let μ be the relative permeability of the soft magnetic material, and let δ(sf 0 ) [m] be the skin depth of the cylindrical member at a drive frequency of f 0 . Then, when the torque T per “loss P/slip sf 0 ” is expressed as follows: 
         [0000]        T/ ( P/sf   0 )∝1/(1 +a/ μ)·(δ( sf   0 )/ρ)   (C1)
 
         [0000]    where the symbol ∝ in expression “A∝B” indicates that A is proportional to B. At that time, the parameter a in equation C1 is in the range of 1000≦a≦2500. 
         [0064]    In such a rotor for an induction torque motor, since the parameter a in equation C1 is in the range 1000≦a≦2500, an induction torque motor including the rotor for an induction torque motor can more reliably reflect the property of the soft magnetic material that constitutes the cylindrical member to the torque property and, thus, the performance of the induction torque motor can be more reliably improved. 
         [0065]    In addition, according to still another embodiment, in the above-described rotors for an induction torque motor, the cylindrical member further has a plurality of concave groove portions that are formed in a peripheral surface of the cylindrical member and that extend parallel to the axis direction of the cylindrical member. 
         [0066]    By including the plurality of concave groove portions, such a rotor for an induction torque motor can regulate an area in which an eddy current flows. As a result, a squirrel-cage property can be provided to the cylindrical member. Accordingly, unlike the torque property of a rotor without the concave groove portions such that the torque decreases substantially in proportion to a decrease in the eddy current (an increase in speed), the rotor for an induction torque motor can provide the following torque property, that is, a torque property such that the torque gently decreases with decreasing eddy current (increasing speed) in a range from the starting point to a point at which the eddy current (the speed) reaches a predetermined value and the torque relatively rapidly decreases with decreasing eddy current (increasing speed) in a range in which the eddy current (the speed) exceeds the predetermined value. That is, when an induction torque motor is achieved, such a configuration can maintain the torque at a substantially constant level even when a few slip occurs. 
         [0067]    In addition, according to yet still another embodiment, in the above-described rotors for an induction torque motor, the depth of the concave groove portions is greater than or equal to the skin depth δ(sf 0 ), and the aspect ratio (the depth/the width) is greater than or equal to 2. 
         [0068]    Since such a rotor for an induction torque motor has the concave groove portions having such a shape, an area in which an eddy current flows can be more reliably regulated. 
         [0069]    In addition, according to yet still another embodiment, the above-described rotor for an induction torque motor further includes the pair of circular ring members made of a conductive material having a specific electric resistivity lower than or equal to 0.02 [μΩ·m]. The ring members are disposed along the circumferential edges at either end of the cylindrical member in the axis direction. 
         [0070]    Since such a rotor for an induction torque motor further includes the circular ring members formed of a relatively good conductor, a flow path of the eddy current can be more reliably provided at either end of the rotor for an induction torque motor and, thus, the eddy current is allowed to more efficiently flow. As a result, an excellent performance can be provided. 
         [0071]    In addition, according to yet still another embodiment, an induction torque motor includes a stator and a rotor. The rotor is any one of the above-described rotors for an induction torque motor. 
         [0072]    Since such an induction torque motor includes a rotor having a cylindrical member made of a soft magnetic material having a specific electric resistivity lower than or equal to 0.4 [μΩ·m] and a relative permeability higher than or equal to 1000 and lower than or equal to 50000, the performance of an induction torque motor including the rotor for an induction torque motor can be more improved than that of an induction torque motor having a rotor including a cylindrical member made of another soft magnetic material. 
         [0073]    This application claims the benefit of Japanese Patent Application No. 2012-46018, filed Mar. 2, 2012, which is hereby incorporated by reference herein in its entirety. 
         [0074]    Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom. 
       INDUSTRIAL APPLICABILITY 
       [0075]    According to the present invention, a rotor for an induction torque motor and an induction torque motor using the rotor for an induction torque motor can be provided.