Abstract:
A brushless motor comprises a parts protecting case; a drive shaft rotatable about its axis relative to the parts protecting case; a fixed housing for rotatably holding therein the drive shaft; a stator tightly held by the fixed housing; a shock absorbing unit interposed between the fixed housing and the parts protecting case to suppress transmission of vibration of the fixed housing to the parts protecting case; a yoke connected to the drive shaft to rotate therewith; and a plurality of magnets held by the yoke. The yoke has a vibration damping structure to suppress or at least minimize transmission of vibration of the yoke to the drive shaft.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to electric motors and more particularly to brushless motors suitable for driving a blower used in an automotive air conditioning device. 
     2. Description of the Prior Art 
     In order to clarify the task of the present invention, one conventional brushless motor used for the above-mentioned field will be briefly described. 
     The brushless motor comprises a driving unit, a drive shaft powered by the driving unit and a parts protecting case. The parts protecting case houses therein the driving unit. The drive shaft projects upward from the case. A blower of the air conditioning device is fixed to the drive shaft. The driving unit comprises a yoke fixed to the drive shaft, a plurality of magnets circumferentially arranged and secured to an inner surface of the yoke having S- and N-poles thereof arranged alternately, a fixed housing for rotatably supporting the drive shaft and a stator mounted to the fixed housing. Upon energization of the driving unit, the yoke and thus the drive shaft is rotated at a high speed relative to the fixed housing. 
     However, in the brushless motor having the abovementioned structure, it is very difficult to smoothly rotate the drive shaft because of inevitable vibration of the yoke. If, due to the vibration of the yoke, the drive shaft fails to rotate smoothly, the parts protecting case tends to produce a marked noise caused by resonance. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a brushless motor, which assures a smoothed and noiseless operation. 
     It is another object of the present invention to provide a brushless motor which is simple, compact and economical. 
     According to a first aspect of the present invention, there is provided a brushless motor which comprises a parts protecting case; a drive shaft rotatable about its axis relative to the parts protecting case; a fixed housing for rotatably holding therein the drive shaft; a stator tightly held by the fixed housing; a shock absorbing unit interposed between the fixed housing and the parts protecting case to suppress transmission of vibration of the fixed housing to the parts protecting case; a yoke connected to the drive shaft to rotate therewith; a plurality of magnets held by the yoke; and a vibration damping structure provided by the yoke to suppress transmission of vibration of the yoke to the drive shaft. 
     According to a second aspect of the present invention, there is provided a brushless motor which comprises a parts protecting case; a drive shaft rotatable about its axis relative to the parts protecting case; a fixed housing for rotatable holding herein the drive shaft, the fixed housing having a flange portion; a stator tightly held by the fixed housing; a shock absorbing unit interposed between the flange portion of the fixed housing and an upper side of the parts protecting case to suppress transmission of vibration of the fixed housing to the parts protecting case; a cup-shaped yoke including at least a conical portion, a cylindrical portion and a junction portion through which the conical and cylindrical portions are united, the yoke being connected to the drive shaft to rotate therewith; and a plurality of magnets held by the cylindrical portion of the yoke, wherein the junction portion of the yoke is shaped thin as compared with the conical and cylindrical portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a sectional view of a brushless motor which is a first embodiment of the present invention; 
     FIG. 2 is a partially sectioned side view of the brushless motor of the first embodiment; 
     FIG. 3 is a plan view of a yoke employed in the brushless motor of the first embodiment; 
     FIG. 4 is a sectional view of the yoke; 
     FIG. 5 is an enlarged sectional view of a portion of the yoke where a groove is provided; 
     FIG. 6 is an enlarged sectional view of a portion of another yoke wherein a groove is provided; 
     FIG. 7 is an enlarged sectional view of a portion of still another yoke where a groove is provided; 
     FIG. 8 is a graph showing a noise level or buzzing level produced by the brushless motor of the first embodiment and a conventional brushless motor; 
     FIG. 9 is a view similar to FIG. 2, but showing a brushless motor of a second embodiment of the present invention; 
     FIG. 10 is a plan view of a lower annular elastic member that is to be fixed to a lower side of a fixed housing; 
     FIG. 11 is a plan view of an upper annular elastic member that is to be set on an upper side of the fixed housing; 
     FIG. 12 is a sectional view of the upper annular elastic member; 
     FIG. 13 is a plan view of a fixing plate; 
     FIG. 14 is a sectional view of the fixing plate; 
     FIG. 15 is a sectional view of an essential portion of the brushless motor of the second embodiment, showing one condition of the fixing plate; and 
     FIG. 16 is a view similar to FIG. 15, but showing another condition of the fixing plate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, the present invention will be described with reference to the accompanying drawings. 
     For ease of understanding, the description will be made with the aid of directional terms such as right, left, upper, lower, rightward, etc. However, it is to be noted that such terms are to be understood with respect to the drawing or drawings on which related part and structure are illustrated. 
     Referring to FIGS. 1 to  8 , particularly FIG. 1, there is shown a brushless motor  1 A which is a first embodiment of the present invention. 
     Designated by numeral  3  is a sirocco fan driven by the brushless motor  1 A for producing an air flow in an air flow passage defined in the air conditioner. As shown, the sirocco fan  3  has a center portion bolted to a leading end of a drive shaft  5  of the brushless motor  1 A. 
     As is best seen from FIG. 1, the brushless motor  1 A comprises an electric parts protecting case  7  from which the drive shaft  5  extends upward. The parts protecting case  7  comprises a lower case member  29  and an upper case member  27  which are detachably coupled to receive therein after-mentioned parts. 
     The drive shaft  5  is rotatably held by a fixed housing  11  through upper and lower bearings  9  and  10 . The fixed housing  11  comprises an upwardly extending cylindrical portion  13  that rotatably holds the drive shaft  5  and a laterally extending mounting flange portion  15 . The cylindrical portion  13  projects upward from the parts protecting case  7 , while the flange portion  15  is received in the parts protecting case  7 , as shown. 
     For supporting the fixed housing  11 , the upper case member  27  of the parts protecting case  7  is formed with a plurality of supporting bosses  23  which are integrally connected through bridge portions. The supporting bosses  23  are projected upward from an annular groove (no numeral) formed in the upper case member  27 . Each boss  23  has a threaded bore facing upward. A lower annular elastic member  17  is put in the annular groove having openings (no numerals) thereof received on the bosses  23 . The elastic member  17  is made of rubber or the like. A peripheral edge of the flange portion  15  of the fixed housing  11  is supported on the lower annular elastic member  17 . An upper annular elastic member  19  made of rubber or the like is put on the peripheral edge portion of the flange portion  15  having openings (no numerals) thereof received on the bosses  23 . An annular fixing plate  21  is put on front flat ends of the supporting bosses  23  and secured thereto by means of threaded bolts  25  engaged with the threaded bores of the bosses  23 . With this, the flange portion  15  of the fixed housing  11  is resiliently supported by the parts protecting case  7 . That is, the lower and upper annular elastic members  17  and  19  constitute a so-called shock absorbing unit “SAU-1”. 
     The driving circuit  33  installed in the parts protecting case  7  functions to control electric current fed to an after-mentioned stator  31 . The driving circuit  33  comprises an electric circuit board  35  having a predetermined wiring pattern, a switching element  37  for changing the direction of current fed to the stator  31 , electrolytic capacitors  39  and so on. The switching element  37  is secured to an aluminum heat sink  41  through a bolt. The heat sink  41  is secured to the upper case member  27  of the case  7  and formed with heat radiation fins  41 a that are projected into the outside of the parts protecting case  7 . With the fins  41   a , heat generated by the switching element  37  effectively radiates to surrounding air. 
     The electric circuit board  35  is held, through bolts  45 , by bosses  43  that extend downward from the upper case member  27 . The above-mentioned driving circuit  33  is connected to the stator  31  through a metal joint bar  47 , a metal bus bar  49  and a metal terminal pin  51 . As shown, the metal joint bar  47  and the metal terminal pin  51  pass through the electric circuit board  35  and the metal bus bar  49  is positioned below the electric circuit board  35  with its both ends respectively connected to the bar  47  and the pin  51 . 
     The stator  31  comprises a plurality of core units  61  that are concentrically disposed about and secured to the cylindrical portion  13  of the fixed housing  11 . Each core unit  61  comprises a coil  59  that is wound around a core proper through an insulator  57 . 
     A yoke  53  is shaped like a cup and arranged to surround the stator  31 . The yoke  53  comprises a shaft holding portion  63  secured to the drive shaft  5 , a circular flat portion  65  having an inner end integral with a lower end of the shaft holding portion  63 , a conical portion  69  having an upper end integral with a periphery of the circular flat portion  65  and a larger cylindrical portion  71  having an upper end integral with a lower end of the conical portion  69 . The conical portion  69  is formed with a plurality of equally spaced circular openings  67  (see FIG. 2) through which air is introduced into the interior of the cup-shaped yoke  53 . 
     A plurality of permanent magnets  55  are secured to an inner surface of the larger cylindrical portion  71  of the yoke  53  at equally spaced intervals. These permanent magnets  55  are so arranged that every two magnets  55  located at diametrically opposed positions have their N and S poles facing the same poles. As shown, the group of the permanent magnets  55  is arranged to surround the group of the core units  61  of the stator  31  leaving a small cylindrical space therebetween. 
     As is best seen from FIG. 5, a junction portion  75  between the conical portion  69  and the cylindrical portion  71  is shaped thin as compared with the other portions  69  and  71 , and the thin junction portion  75  is formed with an annular groove  77 . The annular groove  77  has a trapezoidal cross section. Under operation of the motor  1 A, the grooved thin junction portion  75  serves as a vibration damping means  73 . That is, under operation, the grooved thin junction portion  75  serves as a fulcrum of vibration of the cylindrical portion  71 . 
     If desired, the junction portion  75  may comprise a plurality thinner portions leaving thicker portions. Furthermore, if desired, as is seen from FIGS. 6 and 7, the annular groove  77  of the junction portion  75  may have a rectangular cross section or U-shaped cross section. 
     When, in operation, electric current is forced to flow in the stator  31  changing the flowing direction thereof, S-N, S-S and/or N-N relation is continuously established between the core units  61  and the permanent magnets  55  producing attraction and/or repulsion force therebetween. With this, the cup-shaped yoke  53  is forced to rotate about the stator  31 . 
     Under rotation of the yoke  53 , vibration is inevitably produced particularly at the cylindrical portion  71  of the yoke  53 . However, due to the fulcrum function possessed by the grooved thin junction portion  75 , transmission of the vibration of the cylindrical portion  71  to the shaft holding portion  63  is suppressed or at least minimized. That is, the vibration of the cylindrical portion  71  is damped by the grooved thin junction portion  75 . Thus, the drive shaft  5  can rotate smoothly without being affected by the vibration of the cylindrical portion  71 . This reduces or minimizes the possibility of undesired resonance of the parts protecting case  7  which would occur when marked vibration is applied to the case  7  from the drive shaft  5 . 
     FIG. 8 is a graph depicting a noise or buzzing produced by the brushless motor  1 A of the above-mentioned first embodiment. For comparison, the noise or buzzing of a conventional brushless motor is also shown. As is seen from this graph, the brushless motor  1 A of the first embodiment shows a lower noise level throughout the service rotation speed (viz., from 900 rpm to 2700 rpm). 
     Referring to FIGS. 9 to  16 , particularly FIG. 9, there is shown a brushless motor  1 B which is a second embodiment of the present invention. 
     Since the motor  1 B of this second embodiment is similar in construction to the motor  1 A of the above-mentioned first embodiment, only part that is different from that of the first embodiment  1 A will be described in detail in the following. 
     Substantially same parts as those of the first embodiment  1 A are denoted by the same numerals, and detailed explanation of such same parts will be omitted from the following description. 
     In the second embodiment  1 B, a much improved shock absorbing unit “SAU-2” is used for resiliently supporting the fixed housing  11  (see FIG. 1) on the parts protecting case  7 . 
     That is, as is seen from FIG. 10 that is a plan view of the upper case member  27  and FIG. 15 that is a sectional view of the upper case member  27  and its associated parts, the upper side of the upper case member  27  is formed with an annular recess  79 . A plurality of bosses  89  are projected form a bottom surface of the annular recess  79 . In the annular recess  79 , there is received a lower annular elastic member  81  of rubber or the like, that has a plurality of openings  81   a  through which the bosses  89  are projected upward. On the lower annular elastic member  81 , there is put a peripheral portion of the mounting flange portion  15  of the fixed housing  11  that has a plurality of cuts  15   a  through which the bosses  89  are projected upward. On the peripheral portion of the mounting flange portion  15 , there is put an upper annular elastic member  83  of rubber or the like, such as one as shown in FIG. 11, that has a plurality of openings  83   b  through which the bosses  89  are projected upward. On the upper annular elastic member  83 , there is put an annular fixing plate  85 , such as one as shown in FIG. 13, that has a plurality of small openings  85   a  to which heads of the bosses  89  are engaged. As is seen from FIG. 15, by bolts  87 , the annular fixing plate  85  is secured to the heads of the bosses  89 . 
     In the following, the detail of the shock absorbing unit “SAU-2” will be described with reference to FIGS. 15 and 16. 
     As is seen from FIG. 15, the lower annular elastic member  81  is formed with a plurality of upward projections  81   a  on which the mounting flange portion  15  of the fixed housing  11  is seated, and also the upper annular elastic member  83  is formed with a plurality of upward projections  83   a  each having U-shaped cross section and received in a depressed part  91  of the annular fixing plate  85 , as shown. The shape of the upper annular elastic member  83  is well understood from FIGS. 11 and 12. As is seen from FIGS. 13 and 14, each depressed part  91  of the annular fixing plate  85  is provided by bending four portions  91   a  of the plate  85 . 
     Under rotation of the yoke  53 , vibration is inevitably produced that is transmitted to the fixed housing  11 . However, due to presence of the above-mentioned improved shock absorbing unit “SAU-2” between the fixed housing  11  and the parts protecting case  7 , the vibration of the yoke  53  is damped. That is, for damping such vibration, the projections  81   a  and  83   a  of the lower and upper annular elastic members  81  and  83  are resiliently deformed in such a manner as is shown in FIG.  16 . Thus, the drive shaft  5  can rotate smoothly without being affected by the vibration of the yoke  53 . This reduces or reduces the possibility of undesired resonance of the parts protecting case  7 . 
     If desired, in the second embodiment  1 B, the vibration damping means  73  provided on the yoke  53  may be removed. 
     The entire contents of Japanese Patent Application 11-180552 (filed Jun. 25, 1999) are incorporated herein by reference. 
     Although the invention has been described above with reference to certain embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments will occur to those skilled in the art, in light of the above teachings.