Brushless DC motor
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a brushless DC motor, and particularly to reducing core loss caused in a return yoke for obtaining a motor of high speed rotation. 
     There are a variety of systems known for a brushless DC motor. A construction of being manufactured by the assignee of the present application in its own factory is shown in FIG. 5. 
     Namely, in FIG. 5, numeral 1 is a stator housing of a disk shape, a rotary axis 3 is rotatably mounted in bearing portion 2 mounted in a center portion of said stator housing 1. 
     A rotor yoke 4 of a disk shape is fixedly mounted to an end portion of said rotary axis 3 as one body, a magnet member 5 of ring shape is fixedly mounted under said rotor yoke 4 as one body. 
     A return yoke 6 of a ring disk is fixedly mounted on one surfacce of said stator housing 1, said return yoke 6 is composed of an iron plate. 
     Further, a plurality of a driving coils 7 of a triangle shape are mounted on said return yoke 6 as one body. 
     Referring now to the operation of the above noted brushless DC motor: 
     In the condition of FIG. 5, the certain driving signals are applied to said driving coils 7 through an electronic switching means (not shown), said rotor yoke 4 is continuously rotated by a magnetic operation in relation of said magnet member 5, said driving coil 7 and said return yoke 6. 
     With the above described brushless DC motor, there are presented the following various problems. 
     Core loss increases in proportion to a self-multiplication of rotary speed of said rotor yoke 4, whereby it is very difficult to obtain a preferable efficiency and power consumption for a high speed rotation about 2000 RPM such as a spindle motor for floppy disk (FDD) and VTR. 
     Further, according to the increase of said core loss, in case said motor was employed for said FDD and VTR, circuit parts of said FDD and VRT are injured by heat therefrom, whereby a characteristic of said FDD and VTR becomes worse and life thereof becomes lower. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the present invention to eliminate the aforementioned deficiencies of the prior motor and provide a brushless DC motor which reduces return yoke for obtaining high speed rotation thereof. 
     With the burshless DC motor of the present invention, a rotary axis is rotatably mounted to a bearing portion of a stator housing, a rotor yoke is fixedly mounted to an end portion of said rotary axis, a magnet member is mounted to one face of said rotor yoke, a return yoke of ring shape is mounted to one face of said stator housing, a plurality of cavity portions are formed in said return yoke in the radial direction thereof, a driving coil is positioned opposite said rotor yoke. 
     Therefore, core loss caused in said return yoke is reduced by said cavity portions of said return yoke, whereby it is able to obtain high stability at high speed rotation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate a preferred embodiment of the brushless DC motor according to the present invention. 
     FIG. 1 shows a cross-sectional view of a complete motor construction, 
     FIG. 2 shows a flat plan view of a return yoke, 
     FIG. 3 shows a partial flat plan view of a combination construction of said return yoke and driving coil in FIG. 1, 
     FIG. 4 shows a perspective view of another embodiment of said return yoke in FIG. 1, and 
     FIG. 5 shows a cross-sectional view of a brushless DC motor of prior construction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to a preferred embodiment of the present invention accompanying drawings in which: 
     Numeral 1 shows a stator housing of disk shape, a rotary axis 3 is rotatably mounted in bearing portions 2 which are positioned at the axial center of said stator housing. 
     A rotor yoke 4 of disk shape is fixedly mounted on an end portion of said rotary axis 3, a magnet member 5 of ring shape is mounted on one face of said rotor yoke 4 as one body. 
     A return yoke 6 of ring shape is fixedly mounted on one face of said stator housing 1, said return yoke 6 is made of a metal band member 6a composed of one thin iron plate shown in FIG. 2. Said metal band member 6a has an anti-eddy current construction in which a beginning portion 6aA thereof whirls to an end portion thereof. Said metal band member 6a is composed of a single band whereby a plurality of cavity portions of almost ring shape are created in the radial direction of the yoke. 
     Therefore, it is possible to obtain the same electric magnetic operation as a stacked construction that uses a plurality of metal plates. 
     Further, a plurality of driving coils 7 of triangular shape from the viewpoint of flat plan are mounted on said return yoke 6. 
     Referring now to the operation of the brushless DC motor not the above noted construction of the present invention: 
     In the condition of FIG. 1, a driving signal is applied to said driving coils 7 via an electronic switching means of shown, said rotor yoke 4 is continuously rotated by the magnetic cooperation of said magnet 5, driving coil 7 and return yoke 6. In the above noted rotary operation, it is possible to reduce the core loss of said return yoke 6 itself by said cavity portions 8 formed in said return yoke 6, further it is able to reduce core loss caused in high speed rotation thereof. 
     FIG. 4 shows another embodiment of said return yoke 6, said return yoke 6 is composed of metal ring plate 6b of a certain thickness different from said eddy shaped metal band 6a aforementioned in the above noted embodiment, a plurality of cavity portions 8 of arcuate shape are formed therein. 
     Therefore, said return yoke 6 in FIG. 4 is able to reduce core loss in the same way as said laminar metal construction as shown in FIG. 2. 
     Further, it is possible to obtain the same effect by a return yoke construction in which a plurality of metal band rings having different diameters from each other are constructed by an adhesive means as one body so as to have a plurality of cavity portions 8 therebetween. 
     According to the present invention, it is possible to remarkably reduce core loss caused in said return yoke and to obtain high stability of rotation in a high speed rotation of an FDD spindle motor and VTR cylinder motor.