Abstract:
Several embodiments of rotors for rotating electrical machines configured to increase the stiffness without increasing the weight and to promote the flow of coolant across the rotor. This is achieved by utilizing stiffening ribs and flow openings in an intermediate portion of the rotor that interconnects the cylindrical portion that carries the permanent magnets and the hub section.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates to a rotor for a permanent magnet type rotary electrical machine having permanent magnets adapted to cooperate with the coils formed on armatures of a stator.  
           [0002]    In both motors and generators such as those used in vehicles such as motorcycles, a rotary machine has been known in which permanent magnets fixed to a rotor are rotated around a stator. In such a machine not only does the rotor rotate at high speed, but also the permanent magnets are subjected to repeated attractive and repulsive forces between the magnets and the coils of the stator. This can and does cause generation of vibration and noise. Because the quality and strength of permanent magnets gas improved and permanent magnets with very large magnetic flux density are now used, the attractive and repulsive forces are increased further.  
           [0003]    To prevent or reduce the generation of vibration and noise, a high rigidity of the rotor is desirable. Therefore, rotors of large thickness has been used and the whole rotor has been made of a casting with substantial thickness.  
           [0004]    Thus although the noise and vibration are reduced the rotor has not only a large weight, but also a large moment of inertia. When such a generator is used in a vehicle, it causes not only an increase in vehicle weight, but also a drop in the acceleration and deceleration of the engine. Because electric power consumption has increased especially in the newer model vehicles the load of the generator is increased. The resulting generator tends to be larger in size and to have a larger capacity. This still further amplifies the weight and moment of inertia.  
           [0005]    It is, therefore, a principle object of this invention to provide a rotor for a rotating electrical machine with increased rigidity without increased weight and moment of inertia to prevent generation of vibration and noise.  
           [0006]    The machines of this type can generate substantial heat and this heat must be dissipated. This can be done by using fans or other auxiliary cooling structures. This brings with it further problems of weight and inertia, not to mention added cost.  
           [0007]    Therefore it is a further object of this invention to provide a rotor for a rotating electrical machine that is capable of effecting improved cooling property as well as its size reduction and capacity increase.  
         SUMMARY OF INVENTION  
         [0008]    A first feature of the invention is adapted to be embodied in a rotor for a rotating electrical machine comprised of a rim portion carrying a plurality of spaced permanent magnets, a hub portion adapted to be affixed to a rotatable shaft, and an interconnecting portion for interconnecting the rim and hub portions. A plurality of openings are defined by the interconnecting portion for reducing the weight and rotational inertia of said rotor without significantly reducing its strength.  
           [0009]    As a further feature of the rotor set forth in the preceding paragraph, the openings may be formed to create a fan effect for cooling the machine.  
           [0010]    A further feature of the invention is embodied in a rotor for a rotating electrical machine comprised of a rim portion carrying a plurality of spaced permanent magnets, a hub portion adapted to be affixed to a rotatable shaft, and an interconnecting portion for interconnecting the rim and hub portions. In connection with this feature, a plurality of reinforcing ribs are formed in the interconnecting portion. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIG. 1 is a cross sectional view of a rotating electrical machine constructed in accordance with a first embodiment of the invention.  
         [0012]    [0012]FIG. 2 is a cross sectional view, in part similar to FIG. 1 but showing only the rotor used in the generator.  
         [0013]    [0013]FIG. 3 is an end elevational view of the rotor.  
         [0014]    [0014]FIG. 4 is an end elevational view of the opposite side of the rotor.  
         [0015]    [0015]FIG. 5 is a sectional view taken along line  5 - 5  of FIG. 4.  
         [0016]    [0016]FIG. 6 is a graph of coil temperature in relation to rotational speed comparing this embodiment with the prior art.  
         [0017]    [0017]FIG. 7 is a cross sectional view, in part similar to FIG. 2, but of a rotor of another embodiment.  
         [0018]    [0018]FIG. 8 is a cross sectional view, in part similar to FIG. 2, but of a rotor of still another embodiment.  
         [0019]    [0019]FIG. 9 is a left side view of the embodiment of FIG. 8 FIG. 10 is a right side view of the embodiment of FIG. 8.  
         [0020]    [0020]FIG. 11 is a cross sectional view taken along the line  11 - 11  of FIG. 10. 
     
    
     DETAILED DESCRIPTION  
       [0021]    Referring first to the embodiment of FIGS.  1 - 5 , a rotating electrical machine such as a generator is indicated generally by the reference numeral  21 . Although described as a generator, it will be readily apparent to those skilled in the art, the invention can be used equally with an electric motor.  
         [0022]    The generator  21  is incorporated in an engine, shown only partially, for a motorcycle or the like, and is disposed and driven at one end of the engine crankshaft  22  (FIG. 1). In such an arrangement, the generator  21  is enclosed in a generator housing section  23  formed between a crankcase  24  of the engine and a generator cover  25  mounted to the crankcase  24 .  
         [0023]    One end of the crankshaft  22  protrudes from the crankcase  24  into the generator housing section  23 . A stator  26  is fixedly supported on the crankcase  24  in surrounding relation to the protruding portion of the crankshaft  22 .  
         [0024]    The stator  26  is formed of radial magnetic pole teeth  27  of a stator core, and coils  28  wound thereon. In the case of a three-phase AC generator, the number of magnetic pole teeth  27  is a multiple of three, and for example, 9, 12, 15 and 18 poles are provided. The winding directions of the coils  28  of each phase are arranged to be normal or reverse corresponding to the magnetic poles of permanent magnets  29  of a rotor, indicated generally by the reference numeral  31  and to be described later, to which these coils  28  face so that voltages induced by the coils  28  of the same phase have the same polarity.  
         [0025]    The rotor  31  is comprised of a hub section  32  fitted fixedly on the crankshaft  22 , a cylindrical section  33  facing close to the outside circumference of the magnetic pole teeth of the stator  26 , and a plate-like spoke section  34  for connecting the hub section  32  and the cylindrical section  26 . In this embodiment, the hub section  32 , spoke section  34  and cylindrical section  33  of the rotor  31  are integrally formed from a material like steel by cold forging or hot forging.  
         [0026]    The hub section  32  is nonrotatably key-fitted on a tapered portion formed at one end of the crankshaft  22 , and fixed axially to the crankshaft  22  by a bolt  35 .  
         [0027]    The permanent magnets  29  are bonded on the inside circumferential surface of the cylindrical section  33  and a small clearance is provided between the permanent magnets  29  and the outside circumference of the stator  26 . The permanent magnets  29  are magnetized such that they have polarities changing circumferentially at regular intervals, that is, into twelve or sixteen poles.  
         [0028]    The permanent magnets  29  used here are preferably neodymium-iron-boron magnets with high flux density. The magnets  29  have a very large flux density, so that their thickness can be decreased, which is suited for size reduction and weight saving of the generator  21 . Between the permanent magnets  29  and the spoke section  34  is mounted a spacer  36  made of non-magnetic material.  
         [0029]    For stiffening purposes, a number of radial ribs  37  are formed on the inside surface of the spoke section  34 , that is, on a surface on the stator  26  side. These ribs  37  extend from the hub section  32  to the cylindrical section  33 . The ends  37   a  of the ribs  37  near the cylindrical section  33  are each bent in an arc in the rotational direction R (FIGS. 3 and 4) of the rotor  31 . The spoke section  34  is also formed with a number of windows  38  that extend therethrough in the direction of a rotation axis B of the rotor in a position close to but not interfering with the ribs  37  and particularly their curved portions  37   a.    
         [0030]    Groove-like slant faces  39  are formed on the surface of the spoke section  34  opposite to that on which the ribs  37  are provide. These groove-like slant faces  39  are formed at one side of the windows in the circumferential direction, configured such that they begin at the opening edges (leading side) of the windows  38  and their depths are gradually shallower in the rotational direction (R) of the rotor  26 .  
         [0031]    The generator  21  is air-cooled or liquid-cooled. In the case of liquid cooling, the whole generator is immersed in cooling oil. That is, cooling oil, for example engine lubricating oil, is circulated in the generator housing section  23 . The cooling oil is preferably cooled down by an oil cooler or the like (not shown).  
         [0032]    As is clear from FIGS. 3, 4, this embodiment has twelve ribs  37  and twelve windows  38 , so that the number of magnetic poles of the magnets  29  fixed to the cylindrical section  33  is preferably twelve. However, it should be understood that the numbers of ribs  37  and windows  38  are not limited to that, but they may be changed depending on the number of magnetic poles of the permanent magnets or the like.  
         [0033]    In this generator  21 , rotation of the crankshaft  22  causes the rotor  31  to rotate in the direction (R). As a result of the rotation of the rotor  31 , magnetic field produced by the permanent magnets  29  fixed to the cylindrical section  33  is rotated, which changes the number of magnetic fluxes passing through coils of the stator  26 , resulting in induction of voltages in the coils. The permanent magnets  29  are subjected alternately to attractive and repulsive forces every time they move past different magnetic pole teeth while facing thereto. These forces form a vibration source to the cylindrical section  33 , which could cause vibration of the entire rotor  31 .  
         [0034]    However since the cylindrical section  33  and the spoke section  34  of the rotor  31  are united in one body and the spoke section  34  is formed with the ribs  37  the rotor is stiffened. Thus, sufficiently large rigidity is effected for the united body of the cylindrical section  33  and the spoke section  34 , so that vibration of the united body of the cylindrical section  33  and the spoke section  34  is restricted, decreasing noise associated with the vibration. This is also accomplished without unnecessarily increasing the total weight.  
         [0035]    In addition, the formation and shape of the windows  38  causes cooling air or cooling oil in the generator housing  23  to be circulated inside the rotor  31 , accelerating the cooling of the stator  26 . Therefore, size reduction and capacity increase of the generator  21  is effected. In this case, the cooling air or the cooling oil which has entered the rotor  31 , strikes against the ribs  37  is dispersed into surrounding areas. Thus, the ribs  37  have the function of enhancing cooling effects.  
         [0036]    Because the opening edges of the windows on the opposite side from the ribs are formed slant faces  39  in the rotational direction (A) of the rotor, cooling air (oil) flows efficiently into the rotor  31  along the slant faces  39  as guides. Thus, improvement of cooling performance is further effected. Further, the ribs  37  are bent in the rotational direction (R) of the rotor at the ends on the cylindrical side, so that cooling air (oil) which has entered at the windows  38  is introduced radially inwardly of the rotor along the bent sections  37 A as guides. Therefore, cooling air (oil) is possibly introduced successfully also to the rotor  31  near the hub section 32 , enabling improvement of the cooling property as well.  
         [0037]    [0037]FIG. 6 illustrates the stator coil temperature, in comparison, of the case where ribs ( 37 ), windows ( 38 ) and slant faces ( 39 ) are provided according to this embodiment (cooling type, dash lines) and the case where no ribs, windows nor slant faces are provided (standard type, dash lines). In this case, measurement of the stator coil temperature is made on the generator  21  under the same load.  
         [0038]    According to the result of the measurements, it can be seen that maximum temperature of the coil and ΔT max  (rise of the coil temperature relative to the temperature of the mounting seat surface of the coil) are both considerably lower in the cooling type according to this invention than in the standard type.  
         [0039]    [0039]FIG. 7 illustrates a rotor of another embodiment. A rotor indicated generally at  31   a  is arranged such that a hub section  32   a  is separated from the united body of a cylindrical section  33   a  and a spoke section  34   a , and both members are coupled with rivets  51 . In this embodiment, the rotor  31   a  is divided into two members ( 32   a  and the united body of  33   a  and  34   a ) without decreasing the effect of this invention, thereby facilitating forming of each member. In FIG. 7, like parts as shown in FIG. 2 are designated by like reference numerals and further description is believed to be unnecessary to permit those skilled in the art to utilize this embodiment.  
         [0040]    Referring now to FIGS.  8 - 11 , a rotor constructed in accordance with another embodiment is indicated generally at  31   b . The rotor  31   b  is similar to that of FIGS.  1 - 5  in that a hub section  32   b , a cylindrical section  33   b  and a spoke section  34   b  are molded integrally. The spoke section  34   b  is formed with approximately fan-shaped twelve windows  38   b  at regular intervals in the circumferential direction.  
         [0041]    As a result, between adjacent windows  38   b  are formed straight sections  61  extending approximately in the radial direction. These straight sections  61  act as stiffening ribs. On one edges of the straight sections  61  on the side in the leading side of the rotational direction of the rotor are formed, by chamfering, slant faces  62  in the longitudinal direction of the straight sections  61 . That is, the slant faces  62  face to the outside of the rotor  31   b.    
         [0042]    In this embodiment, with the rotor  31   b  rotating in the direction of R, cooling air or cooling oil flows smoothly and efficiently into the rotor  31   b  along the slant faces  62  as guides. In FIG. 11, dash lines show the flow of the cooling air (oil). Therefore, cooling properties of the rotor  31   b  and the stator (not shown) enclosed in the rotor  31   b  is improved.  
         [0043]    Thus it can be seen that in the disclosed embodiments ribs of the rotor extend approximately radially. Therefore, rigidity can be increased without need of increasing the weight and moment of inertia of the rotor, so that vibration of the stator is restricted as well as generation of noise. Also openings formed between the ribs permit coolant to flow from one side of the rotor to the other for cooling. Of course, the embodiments described are only preferred embodiments of the invention and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.