Abstract:
A disk-type brushless single-phase DC motor comprising a single armature coil attached to a stator yoke of a stator, the armature coil having a closed loop structure. The armature coil has a plurality of uniformly spaced apexes corresponding in number to ½ of the number of poles in a rotor magnet. The armature coil also has sides each connecting neighboring apexes of the armature coil while being radially curved. Cogging generating protrusions are protruded from the stator yoke at positions spaced in a rotation direction of the rotor magnet from respective apexes of the armature coil by a desired angle. The armature coil having a closed loop structure maximizes an effective coil torque generated whereas the cogging generating protrusions generates an optimum cogging torque. Accordingly, it is possible to provide a stable drive performance even when a smaller amount of current is supplied, while achieving an improvement in assembling workability.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a disk-type brushless single-phase DC motor, and more particularly to a disk-type brushless single-phase DC motor including an armature coil attached to a stator yoke of a stator in such a fashion that it faces the lower surface of a rotor magnet having a plurality of alternating N and S poles, the armature coil having a closed loop structure provided with a plurality of uniformly spaced apexes, and cogging generating protrusions of a miniature size protruded from the stator yoke at positions spaced in a rotation direction of the rotor magnet from respective apexes of the armature coil by an angle corresponding to ¼ of an angular width of one pole, thereby being capable of achieving an improvement in drive efficiency. 
     2. Description of the Prior Art 
     Generally, disk-type brushless single-phase DC motors are used in miniature fan motors for simple rotating appliances requiring no precise rotation, for example, office appliances such as computers. 
     Referring to FIG. 8 a disk-type brushless single-phase DC motor is illustrated which includes a housing  100  constituting a lower portion of the motor and serving to support elements of the motor, and a rotor  300  constituting an upper portion of the motor and arranged over the housing  100 . The rotor  300  is rotatably coupled to the housing  100  by means of a shaft  200 . 
     A multipolar rotor magnet  310  is mounted on the lower surface of the rotor  300  within the rotor  300 . The multipolar rotor magnet  310  has a plurality of alternating N and S poles. The upper end of the shaft  200  is fixedly mounted to the central portion of the rotor  300 . The shaft  200  extends downwardly through a bearing housing  110  upwardly protruded from the central portion of the housing  100  in such a fashion that it is rotatably supported by bearings mounted in the bearing housing  110 . The upper end of the bearing housing  110  has a stepped structure in order to fixedly mount a stator  400  thereon. 
     The stator  400  mainly includes a printed circuit board  410 , a stator yoke  420  laid on the printed circuit board  410 , and armature coils  430  attached to the upper surface of the stator yoke  420  by means of an adhesive. 
     The driving of the disk-type brushless single-phase DC motor having the above mentioned configuration is achieved by a rotation of the rotor  300  carried out by an electromagnetic force generated between the armature coils  430  of the stator  400  and the rotor magnet  310 . 
     This will be described in more detail. When single-phase current is supplied to the armature coils  430  via the printed circuit board  410 , an electromagnetic force is generated in accordance with an interaction between the armature coils  430  and the rotor magnet  310 , thereby generating a drive force. By this drive force, the rotor  300  rotates. 
     In this case, a coil torque  600  is generated between the armature coils  430  and the rotor magnet  310  by the electromagnetic force, as shown in FIG.  9 . The coil torque exhibits a maximum value at the middle portion of each pole in the rotor magnet  310  and decreases gradually as the pole extends from the middle portion thereof to each lateral end thereof. The coil torque becomes zero at each lateral end of each pole, thereby causing the rotor  300  to stop. 
     The point, where the coil torque is zero, is called a “dead point”. A cogging generating means is provided for a magnetic start-up at such a dead point. 
     Such a cogging generating means provides a cogging force serving as a load against the coil torque. Such a cogging force is adapted to increase the minimum coil torque while decreasing the maximum coil torque, thereby obtaining a substantially uniform torque. That is, a cogging torque, which has a waveform  700  in FIG. 9, is generated simultaneously with the generation of the coil torque, which has a waveform  600  in FIG. 9, thereby obtaining an ideal resultant torque which has a waveform  800  in FIG.  9 . The cogging torque, which serves as a load against the coil torque, has an output level inversely proportional to the output level of the coil torque, thereby reducing the variation in the resultant torque. As a result, the motor can drive stably. 
     A variety of motors provided with such a cogging means have been proposed in, for example, U.S. Pat. No. 4,620,139, U.S. Pat. No. 4,757,222, and Japanese Patent Publication No. Heisei 7-213041. The cogging means disclosed in the patents generates an appropriate cogging torque serving as a load against a coil torque. In accordance with a combination of the cogging torque and coil torque, an ideal resultant torque is obtained. 
     Meanwhile, the coil torque and cogging torque exhibit a phase difference corresponding to about ¼ of the pole width therebetween. Accordingly, the cogging generating means is arranged at a position where the coil torque is zero during a rotation of the rotor. 
     In U.S. Pat. Nos. 4,620,139 and 4,757,222, as shown in FIG. 10, the cogging generating means comprises iron cores  440  coupled to or fitted to the stator yoke  420  in such a fashion that they are protruded from the stator yoke  420  toward the rotor magnet  310 . Alternatively, the cogging generating means may be provided by cutting out opposite arc-shaped peripheral portions of the stator yoke  420 , as shown in FIG.  11 . In this case, the cogging generating means comprises arc-shaped cutouts  450 . In both cases, however, there is a problem in that it is difficult to determine an accurate position of the cogging generating means because the position of the cogging generating means has an inseparable relation with the attachment position of the armature coil. 
     In both cases, an accurate position for installing the cogging generating means thereon is first determined with respect to each armature coil  430  attached to the stator coil  420 . The coupling or fitting of the iron core  440  to the stator coil  420  is carried out at the determined position. In the case wherein the arc-shaped cutouts are used as the cogging generating means, those cutouts are formed at positions determined as above, respectively. However, the position determination for the cogging generating means is very difficult unless a jig is used. 
     Since a pair of armature coils  430  are practically attached to the stator yoke  420  in such a fashion that they are opposite to each other, a great loss of magnetic force occurs at stator yoke portions where no armature coil is attached, thereby generating a reduced coil torque. As a result, an insufficient drive torque is obtained. This results in a considerable performance degradation. 
     On the other hand, in Japanese Patent Publication No. Heisei 7-213041, the cogging generating means comprises magnetic members  460  as shown in FIG.  12 . Each magnetic member  460  is positioned at an angle θ (0&lt;θ&lt;π, where π is an electrical angle and equal to 180°) from the dead point. In particular, the magnetic members have a screw construction so that they also serve as a fixing means for fixing the printed circuit board  410  and stator yoke  420  to each other. 
     In this case, however, the screw members preferentially have the function for fixing the printed circuit board  410  and stator yoke  420  to each other over the cogging generating function. For this reason, after the printed circuit board  410  and stator yoke  420  are fixed to each other, the screw members may have different gaps with respect to the associated rotor magnets  310 , respectively. As a result, the cogging torque generated by the cogging generating means may vary for different screw members. 
     In other words, a difference in fastening degrees of the screw members result in a variation in the magnetic force generated by the rotor magnets  310 , thereby generating an instable drive torque. 
     In the case of a miniature motor, furthermore, it is impossible to fasten the magnetic members  460  having a very small size unless a specific tool is used. Moreover, it is also impossible to adjust the fastening degree of the magnetic members  460 . Consequently, it is impossible to practically apply such a construction to miniature motors. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the invention is to provide a disk-type brushless single-phase DC motor including an armature coil arranged on a stator yoke and having a shape capable of generating a maximized effective coil torque, and cogging generating protrusions respectively arranged at positions spaced from apexes of the armature coil by a desired angle while being integral with the stator yoke, thereby being capable of stably outputting an ideal drive torque even when a smaller amount of current is supplied. 
     Another object of the invention is to provide a disk-type brushless single-phase DC motor including cogging generating protrusions capable of appropriately coping with a variation in the use purpose of the motor by a simple variation in the structure thereof while generating an optimum torque. 
     Another object of the invention is to provide a disk-type brushless single-phase DC motor capable of generating a stable and sufficient drive torque even when the amount of supply current is minimized. 
     In accordance with the present invention, these objects are accomplished by providing a disk-type brushless single-phase DC motor comprising a single armature coil attached to a stator yoke of a stator, the armature coil having a closed loop structure. 
     The armature coil has a plurality of uniformly spaced apexes corresponding in number to ½ of the number of poles in a rotor magnet. The armature coil also has curved sides each connecting neighboring apexes of the armature coil. Cogging generating protrusions are protruded from the stator yoke at positions spaced in a rotation direction of the rotor magnet from respective apexes of the armature coil by a desired angle. 
     The cogging generating protrusions correspond in number to the apexes of the armature coil and are uniformly spaced from one another. 
     In accordance with the present invention, the armature coil can also be installed at an accurate position by virtue of the cogging generating protrusions protruded from the stator yoke. In accordance with the present invention, the formation of the cogging generating protrusions and the coupling of the armature coil can also be simplified, thereby achieving an improvement in fabrication efficiency. It is also possible to provide a stable drive performance while obtaining an ideal drive torque even when a smaller amount of current is supplied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: 
     FIG. 1 is a sectional view illustrating a disk-type brushless single-phase DC motor according to the present invention; 
     FIG. 2 is an exploded perspective view of the disk-type brushless single-phase DC motor shown in FIG. 1; 
     FIGS.  3   a  to  3   d  are plan views respectively illustrating various embodiments of an armature coil according to the present invention; 
     FIGS.  4   a  and  4   b  are sectional views respectively illustrating different embodiments of cogging generating protrusions according to the present invention; 
     FIGS.  5   a  to  5   c  are diagrams respectively illustrating waveforms of cogging torques according to various shapes of cogging generating protrusions according to the present invention; 
     FIG. 6 is a schematic view illustrating the principle of improving an effective drive torque in accordance with the present invention; 
     FIG. 7 is a view illustrating an armature coil having straight sides in accordance with the present invention; 
     FIG. 8 is a sectional view illustrating a conventional disktype brushless single-phase DC motor; 
     FIG. 9 is a waveform diagram of outputs generated in a conventional disk-type brushless single-phase DC motor; and 
     FIGS. 10 to  12  are views respectively illustrating various examples of conventional cogging generating protrusions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a disk-type brushless single-phase DC motor according to the present invention is illustrated, respectively. As shown in FIGS. 1 and 2, the motor includes a housing  1  constituting a lower portion of the motor, and a rotor  3  constituting an upper portion of the motor and arranged over the housing  1 . The rotor  3  is rotatably coupled at its central portion to the central portion of the housing  1  by means of a shaft  2 . 
     The shaft  2  is fixedly mounted at its upper end to the lower surface of the rotor  3 . The shaft  2  extends downwardly through a hollow bearing holder  11  upwardly protruded from the central portion of the housing  1  in such a fashion that it is rotatably supported by bearings  5  mounted in the bearing holder  11 . 
     The rotor  3  is a rotating member for the motor. A rotor magnet  32  is mounted on the lower surface of the rotor  3  by means of a magnetic yoke  32  attached to the rotor  3 . The rotor magnet  32  has a flat annular shape and is provided with a plurality of alternating N and S poles. The number of poles in the rotor magnet  32  corresponds to 2P, where P is an integer not less than 1. 
     The bearing holder  11 , which has a hollow structure, is upwardly protruded from the central portion of the housing  1 . The upper end of the bearing holder  11  has a reduced diameter as compared to the lower end of the bearing holder  11  so that it has a stepped structure in order to seat a stator  4  thereon. 
     The stator  4  mainly includes a printed circuit board  41 , a stator yoke  42  laid on the printed circuit board  41 , and an armature coil  43  attached to the upper surface of the stator yoke  42 . The printed circuit board  41  of the stator  4  serves to supply single-phase current from an external source to the armature coil  43  via circuits patterned on opposite surfaces thereof. The stator yoke  42  is a conductive flat plate laid on the printed circuit board  41  in such a fashion that it faces the rotor magnet  32 . The armature coil  43 , which is attached to the upper surface of the stator yoke  42 , interacts with the rotor magnet  32 , thereby generating an electromagnetic force. 
     FIGS.  3   a  to  3   d  illustrate various embodiments of the armature coil according to the present invention, respectively. In all cases, the armature coil  43  has a closed loop structure having a plurality of uniformly spaced apexes. A cogging generating protrusion  44  is protruded from the stator yoke  42  at a position spaced from an associated one of the apexes of the armature coil  43  by a desired angle. The cogging generating protrusion  44  serves as a load against a coil torque generated by virtue of an electromagnetic force interacting between the armature coil  43  and rotor magnet  32 . 
     The number of apexes in the armature coil  43  corresponds to ½ of the number of poles in the rotor magnet  32 . The number of cogging generating protrusions  44  each being positioned at an angle from an associated one of the apexes of the armature coil  32  also corresponds to ½ of the number of poles in the rotor magnet  32 . 
     For example, where the rotor magnet  32  has 6 poles, the armature coil  43  has 3 apexes corresponding to ½ of the 6 poles. Where the rotor magnet  32  has 8 poles, the armature coil  43  has 4 apexes. For a rotor magnet having 10 or 12 poles, the armature coil  43  has a closed loop shape having 5 or 6 apexes. 
     The armature coil  43  has sides each connecting neighboring apexes while being radially inwardly curved with a desired radius of curvature. Of course, the armature coil  43  may have straight sides in accordance with the present invention, as shown in FIG.  7 . However, it is more preferable that the armature coil  43  have curved sides in terms of an increase in coil torque. This will be described hereinafter in detail. Each side of the armature coil  43  has a length not more than the pole width of the rotor magnet  32 . 
     Each cogging generating protrusion  44  is arranged at a position spaced from an associated one of the apexes of the armature coil  43  by a desired angle in a rotation direction of the rotor magnet  32  without overlapping with the armature coil  43 . Preferably, the angle of each cogging generating protrusion  44  from the associated apex of the armature coil  43  corresponds to ¼ of the angular width of one pole in the rotor magnet  32 . The angular width of one pole in the rotor magnet  32  correspond. to the value obtained by dividing 360°, namely, the sum of angular widths of all poles in the rotor magnet  32 , by the number of poles. 
     For example, where the rotor magnet  32  has 6 poles, the width of one pole thereof is 60° (360°/6). Accordingly, the angle of each cogging generating protrusion  44  from the associated apex of the armature coil  43  is 15° obtained by dividing the pole width of 60° by 4. That is, each cogging generating protrusion  44  is arranged at a position shifted by an angle of 15° from the associated apex of the armature coil  43  in the rotation direction of the rotor magnet  32 . 
     In the cases wherein the rotor magnet  32  has 8 poles, 10 poles, and 12 poles, the position of each cogging generating protrusion  44  is spaced from the associated apex of the armature coil  43  by angles of 11.25°, 9°, and 7.5°, respectively. 
     As shown in FIGS.  4   a  and  4   b , the cogging generating protrusions  44  may be formed by partially cutting out the stator yoke  42  having a flat plate structure and upwardly bending desired portions of the stator yoke  42  at the edges of the cutouts in such a fashion that the upper end of the bent portions are arranged near the rotor magnet  32 . Alternatively, the cogging generating protrusions  44  may be integrally formed with the stator yoke  42  in such a fashion that they are upwardly protruded from the upper surface of the stator yoke  42 , when the stator yoke  42  is molded. 
     Meanwhile, a dead point, where the coil torque generated between the rotor magnet  32  and armature coil  43  during the rotation of the rotor becomes zero, is always formed at a fixed point spaced from each apex of the armature coil  43  by a constant angle. In this regard, it may be possible to use a single cogging generating protrusion or cogging generating protrusions reduced in number from the number of apexes of the armature coil  43  in accordance with the present invention. In this case, it is possible to expect a cogging generating effect identical or similar to those obtained when a plurality of cogging generating protrusions corresponding in number to the apexes of the armature coil  43 . 
     In particular, the cogging torque generated by each cogging generating protrusion  44  varies in various forms depending on different protruded structures of the cogging generating protrusion  44 , as shown in FIGS.  5   a  to  5   c . Accordingly, it is possible to effectively apply the present invention to a variety of motors having different magnetized structures for the rotor magnet by selecting an appropriate protruded structure of the cogging generating protrusion  44  in accordance with the magnetized structure of the motor to which the present invention is applied. 
     Where the magnetized structure of the rotor magnet  32  varies, the output form of the coil torque varies correspondingly. In order to obtain an ideal resultant torque with respect to such a varied coil torque, it is required to generate a cogging torque having an output form exhibiting the same variation as the output form of the coil torque. In accordance with the present invention, this can be easily achieved by simply varying the protruded structure of the cogging generating protrusion  44 . 
     The height b of the cogging generating protrusion  44  determines the magnitude of the cogging torque whereas the width a of the cogging generating protrusion  44  determines an electrical angle at which a cogging is generated. The electrical angle corresponds to the sum of angles of two poles in the rotor magnet. In particular, the width a at the tip of the cogging generating protrusion  44  determines a variation in peak cogging torque. In accordance with the present invention, therefore, a cogging torque matching with the coil torque generated is generated by appropriately combining together the above mentioned design parameters for the cogging generating protrusion. 
     The output pattern of the cogging torque varies depending on the material of the cogging generating protrusion  44  as well as the size of the cogging generating protrusion  44 , namely, the width a and height b. Accordingly, the size and material of the cogging generating protrusion  44  are appropriately adjusted to output a cogging torque matching with the coil torque generated, thereby generating an ideal resultant torque. 
     It is preferred that the sides of the armature coil  43  connecting neighboring apexes have a curved shape rather than a straight shape, as shown in FIG.  6 . It is more preferable that the inner surface of each curved side of the armature coil  43  extends radially inwardly to the inner peripheral surface of the rotor magnet  32  when viewed in a plan view. Referring to the equation “F=IL×B” with regard to the effective coil torque, established in accordance with Fleming&#39;s left-hand law, it can be understood that an increased effective radial length of the armature coil at an area, where N and S poles overlap with each other, results in an increase in coil torque, thereby reducing the consumption of current. 
     The effective radial length L of the armature coil, where the sides connecting neighboring apexes have a curved shape, is longer than the effective radial length l of the armature coil where the sides have a straight shape. In the case where the sides connecting neighboring apexes have a curved shape, accordingly, there is an advantage in that a higher coil torque can be obtained using a relatively reduced amount of supply current, I, thereby preventing loss of current. 
     Where each cogging generating protrusion  44  provided at the stator yoke  42  is arranged at a position, where it can come into contact with the armature coil  43 , it serves as a guide upon coupling the armature coil  43 . In this case, accordingly, the coupling of the armature coil  43  can be accurately achieved without using any jig. The cogging generating protrusions  44  also serves to prevent the coupled armature coil  43  from moving. Accordingly, a more stable workability is provided when the stator yoke  42  and armature coil  43  are fixed to each other by means of an adhesive. 
     In accordance with the present invention, therefore, it is possible to eliminate use of special tools and associated processes upon assembling elements of the motor because it is unnecessary to use any jig required in conventional cases. 
     Since the armature coil  43  comprises a single coil having a closed loop shape in accordance with the present invention, the fabrication thereof can be simplified. It is also possible to greatly reduce loss of an electromagnetic force generated between the rotor magnet  32  and armature coil  43 , thereby maximizing the torque efficiency of the motor. 
     In accordance with the present invention, the cogging generating protrusions  44  are integrally formed with the stator yoke  42 . Accordingly, the number of processes for forming the cogging generating protrusions  44  is reduced. Furthermore, the variation in the shape and size of the cogging generating protrusions  44  can be more freely achieved. Accordingly, it is very easy to cope with a change in the use purpose of the motor or a variation in the magnetized structure of the rotor magnet. 
     In particular, the formation of the cogging generating protrusions  44  is carried out simultaneously with the fabrication of the stator yoke  42  in accordance with the present invention. Accordingly, there is a great advantage in terms of a mass production. Since the cogging generating protrusions  44  can serve as a guide upon coupling the armature coil  43  to the stator yoke  42 , this coupling can be more accurately achieved. As a result, an enhancement in productivity is obtained. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.