Patent Publication Number: US-2019199147-A1

Title: Stator, and motor comprising same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage application of International Patent Application No. PCT/KR2017/009410, filed Aug. 29, 2017, which claims the benefit under 35 U.S.C. § 119 of Korean Application Nos. 10-2016-0114086, filed Sep. 5, 2016; and 10-2017-0013935, filed Jan. 31, 2017, all of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     Embodiments relate to a stator and a motor including the same. 
     BACKGROUND ART 
     A motor is a device which obtains torque by converting electrical energy into mechanical energy and is generally used for a vehicle, home appliances, industrial apparatuses, and the like. 
     The motor may include a housing, a rotating shaft, a stator disposed on an inner circumferential surface of the housing, a rotor installed on an outer circumferential surface of the rotating shaft, and the like. Here, the stator of the motor causes an electrical interaction with the rotor and induces the rotor to rotate. 
     Particularly, the motor may be used for a device for securing stability in steering of a vehicle. For example, the motor may be used for an auxiliary steering system using separate power. 
     Although auxiliary steering devices using fluid pressure were previously used, recently, electronic power steering systems (EPS) with low power loss and having high accuracy have been used. 
     The EPS system is an apparatus capable of allowing a driver to safely drive a vehicle by securing stability in turning of the vehicle and providing a quick restoring force. The EPS system controls driving of a steering shaft of the vehicle by driving the motor through an electronic control unit (ECU) according to driving conditions sensed by a vehicle speed sensor, a torque angle sensor, a torque sensor, and the like. 
     The motor includes a stator and a rotor. 
     The stator may include a plurality of teeth which form a plurality of slots. The rotor may include a plurality of magnets arranged to face the teeth. Here, adjacent teeth are arranged to be spaced apart and form a slot opening. That is, the slot opening may be formed to inhibit a leakage of flux between the adjacent teeth. 
     Accordingly, although the flux moves through a tooth side having high permeability while the rotor rotates, torque pulsation may occur due to a difference in permeability in a slot opening area. 
     Accordingly, while the rotor rotates, cogging torque may occur due to a difference in permeability between a stator core of a metal material and air in the slot opening which is an empty space. Since the cogging torque is a cause of noise and vibration, a reduction in cogging torque is more important to increase quality of the motor. 
     DISCLOSURE 
     Tecnical Problem 
     The present invention is directed to providing a motor capable of improving quality by reducing cogging torque and torque ripple. 
     Aspects of the embodiment are not limited to the above-stated aspect and other unstated aspects can be clearly understood by those skilled in the art from the following description. 
     Technical Solution 
     One aspect of the present invention provides a stator including a stator core, which includes a plurality of teeth, and a coil wound on each of the plurality of teeth. Here, each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. The shoe includes a plurality of grooves. Also, a width of the groove based on a circumferential direction is 90% to 110% of a width of a slot opening formed between the teeth. 
     An angle formed by a side surface of the body and a side surface of the shoe, which is connected to the side surface of the body, may be 145° to 155°. 
     Another aspect of the present invention provides a stator including a stator core which includes a plurality of teeth and a coil wound on each of the plurality of teeth. Here, each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. Also, an angle formed by a side surface of the body and a side surface of the shoe, which is connected to the side surface of the body, is 145° to 155°. 
     Another aspect of the present invention provides a stator including a stator core, which includes a plurality of teeth, and a coil wound on each of the plurality of teeth. Here, each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. The shoe includes a plurality of semicircular grooves. A center C 1  of a lateral cross section of the groove is disposed to be spaced at a certain angle θ2 apart from one side end point P of the shoe in a circumferential direction. Also, the angle θ2 is 0.45 to 0.55 of an angle θ1 formed by an end point of the shoe, which is adjacent to the end point of the shoe, and the center of the rotating shaft. 
     The angle θ2 may be 0.5 of the angle θ1. 
     The number of the grooves may be two, and the two grooves may be arranged to be symmetrical to each other on the basis of a reference line which passes a center of a width of the shoe based on a circumferential direction and a center of the stator core. 
     The grooves may be arranged along an axial direction of the stator core. 
     Another aspect of the present invention provides a motor including a rotating shaft, a rotor including a hole into which the rotating shaft is inserted, and a stator disposed outside the rotor. Here, the stator includes a stator core including a plurality of teeth and a coil wound on each of the plurality of teeth. Each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. The shoe includes a plurality of grooves. Also, a width of the groove based on a circumferential direction is 90% to 110% of a width of a slot opening formed between the teeth. 
     An angle formed by a side surface of the body and a side surface of the shoe, which is connected to the side surface of the body, may be 145° to 155°. 
     Another aspect of the present invention provides a motor including a rotating shaft, a rotor including a hole into which the rotating shaft is inserted, and a stator disposed outside the rotor. Here, the stator includes a stator core including a plurality of teeth and a coil wound on each of the plurality of teeth. Each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. Also, an angle formed by a side surface of the body and a side surface of the shoe, which is connected to the side surface of the body, is 145° to 155°. 
     A frequency of cogging torque waveforms during unit rotation may be three times a least common multiple of the number of magnets of the rotor and number of teeth. 
     Another aspect of the present invention provides a motor including a rotating shaft, a rotor including a hole into which the rotating shaft is inserted, and a stator disposed outside the rotor. Here, the stator includes a stator core including a plurality of teeth and a coil wound on each of the plurality of teeth. Each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. The shoe includes a plurality of semicircular grooves. A center C 1  of a lateral cross section of the groove is disposed to be spaced at a certain angle θ2 apart from one side end point P of the shoe in a circumferential direction. Also, the angle θ2 is 0.45 to 0.55 of an angle θ1 formed by an end point of the shoe, which is adjacent to the end point of the shoe, and the center of a rotating shaft. 
     A radius R of the groove may be 0.9 to 1.1 of a distance D between the shoe of the stator and the rotor. 
     Another aspect of the present invention provides a motor including a rotating shaft, a rotor including a hole into which the rotating shaft is inserted, and a stator disposed outside the rotor. Here, the stator includes a stator core including a plurality of teeth and a coil wound on each of the plurality of teeth. Each of the plurality of teeth includes a body, on which the coil is wound, and a shoe connected to the body. The shoe includes a plurality of semicircular grooves. Also, a radius R of the groove is 0.9 to 1.1 of a distance D between the shoe of the stator and the rotor. 
     The radius R of the groove may be equal to the distance D between the shoe of the stator and the rotor. 
     The rotor may include a rotor core and magnets arranged on an outer circumferential surface of the rotor core, and the radius R of the groove may be equal to a distance D between the shoe of the stator and the magnet of the rotor. 
     Advantageous Effects 
     A motor according to an embodiment provides an advantageous effect of greatly reducing cogging torque by increasing a main cogging degree using grooves formed on teeth of a stator. 
     Also, the motor may increase motor quality by reducing cogging torque and torque ripple using grooves having a semicircular shape. 
     Here, the quality may be further improved, while maintaining motor performance, by limiting a position of the groove on the basis of a slot opening and limiting a radius of the groove on the basis of an air gap. 
     A variety of advantages and effects of the present invention are not limited to the above description and will be more easily understood in a process of describing detailed embodiments of the present invention. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view of a motor according to an embodiment; 
         FIG. 2  is a view illustrating a stator and a rotor; 
         FIG. 3  is a view illustrating a groove of a tooth; 
         FIG. 4  is a table illustrating a main cogging degree increased by the motor according to the embodiment; 
         FIG. 5  is a view illustrating a width of the groove; 
         FIGS. 6 a    and  FIG. 6 b    are tables illustrating a change in a cogging torque waveform according to the width of the groove; 
         FIG. 7  is a view illustrating an angle between a body and a shoe of the tooth; 
         FIG. 8  is a graph illustrating a change in cogging torque according to an angle between the body and the shoe of the tooth; 
         FIG. 9  is a graph illustrating a change in cogging ripple according to an angle between the body and the shoe of the tooth; 
         FIG. 10  is a graph illustrating a change in a cogging torque waveform according to an angle between the body and the shoe of the tooth; 
         FIG. 11  is a lateral cross-sectional view of the motor taken along a line A-A of  FIG. 1 ; 
         FIG. 12  is a view illustrating an arrangement relationship between a stator core and the rotor of the motor according to the embodiment; 
         FIG. 13  is a view illustrating the stator and the rotor in an area B 1  of  FIG. 12 ; 
         FIG. 14  is a view illustrating the tooth and the grooves of the motor according to the embodiment; 
         FIGS. 15 a  and 15 b    are views illustrating comparison between torques of the motor according to the embodiment and a motor without grooves; 
         FIGS. 16 a  and 16 b    are views illustrating cogging torque and torque ripple according to a radius of the groove and a distance of an air gap formed in the motor according to the embodiment; 
         FIGS. 17 a  and 17 b    are views illustrating a comparison between torques according to a radius of the groove and a distance of an air gap formed in the motor according to the embodiment; 
         FIGS. 18 a  and 18 b    are views illustrating cogging torque and torque ripple according to a position of a center C 1  of the groove formed in the motor according to the embodiment; 
         FIG. 19  is a table illustrating performance of the motor according to the embodiment and a motor without grooves; 
         FIG. 20  is a view illustrating performance of the motor according to the embodiment and a motor without grooves at a room temperature; 
         FIG. 21  is a table illustrating performance of the motor according to the embodiment and a motor with square notches formed therein; and 
         FIG. 22  is a view illustrating performance of the motor according to the embodiment and a motor with square notches formed therein. 
     
    
    
     MODES OF THE INVENTION 
     Although a variety of modifications and several embodiments of the present invention may be made, exemplary embodiments will be illustrated in the drawings and will be described. However, it should be understood that the present invention is not limited to the exemplary embodiments and includes all changes and equivalents or substitutes included in the concept and technical scope of the present invention. 
     The terms including ordinal numbers such as “second,” “first,” and the like may be used for describing a variety of components. However, the components are not limited by the terms. The terms are used only for distinguishing one component from another component. For example, without departing from the scope of the present invention, a second component may be referred to as a first component, and similarly, a first component may be referred to as a second component. The term “and/or” includes any and all combinations of one or a plurality of associated listed items. 
     When it is stated that one component is “connected” or “joined” to another component, it should be understood that the one component may be directly connected or joined to the other component but another component may be present therebetween. On the other hand, when it is described that one component is “directly connected” or “directly joined” to another component, it should be understood that no other component is present therebetween. 
     While the embodiments are described, when any one component is described as being “on or under” another component, the two components may come into direct contact with each other or may come into indirect contact with each other with another component interposed therebetween. Also, the term “on or under” may include not only an upward direction but also a downward direction on the basis of one component. 
     Terms used herein are used merely for describing exemplary embodiments and are not intended to limit the present invention. Singular expressions, unless clearly defined otherwise in context, include plural expressions. Throughout the application, it should be understood that the terms “comprise,” “have,” and the like are used herein to specify the presence of stated features, numbers, steps, operations, elements, components or combinations thereof but do not preclude the presence or addition of one or more other features, numbers, steps, operations, elements, components, or combinations thereof. 
     Unless defined otherwise, the terms used herein including technical or scientific terms have the same meanings as those which are generally understood by one of ordinary skill in the art. Terms such as those defined in commonly used dictionaries should be construed as having meanings equal to contextual meanings of related art and should not be interpreted in an idealized or excessively formal sense unless defined otherwise herein. 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Regardless of drawing signs, equal or corresponding elements will be referred to as like reference numerals and an overlapping description thereof will be omitted. 
       FIG. 1  is a longitudinal cross-sectional view of a motor according to an embodiment. 
     Referring to  FIG. 1 , a motor  1  according to the embodiment may include a housing  100 , a bracket  200 , a stator  300 , a rotor  400 , and a rotating shaft  500 . Here, the bracket  200  may be disposed to cover an open top of the housing  100 . 
     The housing  100  and the bracket  200  may form an outward form of the motor  1 . Here, the housing  100  may have a cylindrical shape with an opening on top. 
     Accordingly, an accommodation space may be formed therein by a combination between the housing  100  and the bracket  200 . Also, in the accommodation space, as shown in  FIG. 1 , the stator  300 , the rotor  400 , the rotating shaft  500 , and the like may be arranged. 
     The housing  100  has a cylindrical shape such that the stator  300  may be disposed on an inner circumferential surface thereof to be supported. A pocket portion, which accommodates a bearing  10  supporting a bottom of the rotating shaft, may be provided on a bottom of the housing  100 . 
     Also, a pocket portion supporting a top of the rotating shaft  500  may be provided in the bracket  200  disposed above the housing  100 . Also, the bracket  200  may include a hole or a groove into which a connector, to which an external cable is connected, is inserted. 
     The stator  300  may be supported by the inner circumferential surface of the housing  100 . Also, the stator  300  is disposed outside the rotor  400 . 
     The stator  300  causes an electrical interaction with the rotor  400  and induces the rotor  400  to rotate. 
     The rotor  400  is disposed inside the stator  300 . The rotor  400  may include a rotor core and a magnet combined with the rotor core. The rotor  400  may have the following forms which are classified according to a method of combining the rotor core with the magnet. 
     The rotor  400  may be embodied such that a magnet is combined with an outer circumferential surface of a rotor core. This type of rotor may include an additional can member  20  combined with the rotor core to inhibit a separation of the magnet and to increase a combinational force therebetween. Otherwise, the magnet and the rotor core may be doubly injected to be integrally formed. 
     The rotor  400  may be embodied such that a magnet is combined with an inside of a rotor core. This type of rotor may include a pocket, into which the magnet is inserted, in the rotor core. 
     Meanwhile, the rotor core may be formed by stacking a plurality of plates having a thin steel plate shape. Otherwise, the rotor core may be embodied to have one cylindrical shape so as to not form a skew angle, and the magnet may be attached to the rotor core so as to not form a skew angle. Meanwhile, the rotor core may be formed by stacking a plurality of pucks (unit core) which form a skew angle. 
     The rotating shaft  500  may be combined with the rotor  400 . When an electromagnetic interaction occurs between the stator  300  and the rotor  400  due to current supply, the rotor  400  rotates, and in connection therewith, the rotating shaft  500  rotates. The rotating shaft  500  may be connected to a steering shaft of a vehicle and may transfer power to the steering shaft. The rotating shaft  500  may be supported by the bearing  10  as shown in  FIG. 1 . 
     A sensing magnet assembly  600  is an apparatus which is combined with the rotating shaft  500  to be in connection with the rotor  400  so as to detect a position of the rotor  400 . 
     The sensing magnet assembly  600  may include a sensing magnet and a sensing plate. The sensing magnet and the sensing plate may be combined with each other to be coaxial. 
     The sensing magnet may include a main magnet disposed in a circumferential direction to be adjacent to a hole forming an inner circumferential surface and a sub magnet formed on an edge. The main magnet may be disposed to be equal to a drive magnet which is inserted into the rotor  400  of the motor. The sub magnet is segmented more than the main magnet and includes a plurality of magnetic poles. Accordingly, it is possible to segment a magnet more, measure a rotation angle, and more easily drive the motor. 
     The sensing plate may be formed as a metallic circular-plate-shape. The sensing magnet may be combined with a top surface of the sensing plate. Also, the sensing plate may be combined with the rotating shaft  500 . Here, a hole, through which the rotating shaft  500  passes, is formed in the sensing plate. 
     A sensor which senses a magnetic force of the sensing magnet may be disposed on a printed circuit board (PCB)  700 . 
     Here, the sensor may be a hall integrated chip (IC). The sensor senses changes in the N pole and the S pole of the main magnet or the sub magnet and generates a sensing signal. Since at least three sensing signals for obtaining information on U, V, and W phases are necessary in the case of a three-phased brushless motor, at least three sensors may be arranged. 
     The PCB  700  may be combined with a bottom surface of the bracket  200  and be disposed on the sensing magnet assembly  600  such that the sensor faces the sensing magnet. 
       FIG. 2  is a view illustrating the stator and the rotor,  FIG. 3  is a view illustrating a groove of a tooth,  FIG. 4  is a table illustrating a main cogging degree increased by the motor according to the embodiment,  FIG. 5  is a view illustrating a width of the groove,  FIGS. 6 a  and 6 b    are tables illustrating a change in a cogging torque waveform according to the width of the groove,  FIG. 7  is a view illustrating an angle between a body and a shoe of the tooth,  FIG. 8  is a graph illustrating a change in cogging torque according to an angle between the body and the shoe of the tooth,  FIG. 9  is a graph illustrating a change in cogging ripple according to an angle between the body and the shoe of the tooth, and  FIG. 10  is a graph illustrating a change in a cogging torque wave according to an angle between the body and the shoe of the tooth. 
     Referring to  FIGS. 2 to 10 , changes in cogging torque and torque ripple caused by the groove and shoe disposed on the tooth of the stator will be described. 
     Referring to  FIGS. 1 and 2 , the stator  300  may include a stator core  310  and a coil  320 . Also, in the stator  300 , an insulator  330  may be disposed between the stator core  310  and the coil  320 . 
     The stator core  310  may be formed by stacking a plurality of plates having a thin steel plate shape. Also, the stator core  310  may be formed by combining and connecting a plurality of divided cores with or to one another. 
     The stator core  310  may include a yoke  311 , a plurality of teeth  312 , and grooves  315  formed in the teeth  312 . Here, the groove  315  may be referred to as a notch. 
     The teeth  312  protruding toward a center C may be arranged on the yoke  311 . Here, when viewed in an axial view (C), the yoke  311  may have an annular shape. The coil  320  is wound on the tooth  312 . The plurality of teeth  312  may be arranged at certain intervals along an inner circumferential surface of the annular yoke. Although twelve teeth  312  are shown in  FIG. 2  totally, the present invention is not limited thereto and a variety of modifications may be made according to the number of poles of magnets  420 . 
     The magnets  420  may be attached to an outer circumferential surface of a rotor core  410 . An end of the tooth  312  is disposed to face the magnet  420 . Here, the rotor  400  may include the rotor core  410  and a plurality of such magnets  420  arranged on the rotor core  410 . 
     Referring to  FIG. 3  which illustrates an area B of  FIG. 2 , the tooth  312  may include a body  313  and a shoe  314 . Here, the body  313  may be referred to as a coil-wound portion. Also, the shoe  314  may be referred to as a protrusion portion. 
     The body  313  is a place where the coil  320  is wound. The shoe  314  is disposed on an end of the body  313 . An end surface of the shoe  314  is disposed to face the magnet  420 . A gap between the adjacent teeth  312  is formed as a space for winding the coil  320 . Here, the winding space S means a slot S. 
     The shoes  314  of the adjacent teeth  312  are arranged to be separated from each other such that a slot opening O is formed. The slot opening O is an inlet of the winding space, into which a nozzle for winding the coil  320  is inserted. 
     The end surface of the shoe  314  may include the grooves  315 . The grooves  315  may be concavely formed on the end surface of the shoe  314 . Although the grooves  315  have an angular shape, the present invention is not limited thereto. Also, the groove  315  may be arranged along an axial direction of the stator core  310 . In other words, the grooves  315  may be arranged lengthwise along a height direction (axial direction) of the stator core  310  from a top end to a bottom end of the stator core  310 . 
     Two such grooves  315  may be arranged. Referring to  FIGS. 2 and 3 , the two grooves  315  may be arranged to be symmetrical to each other on the basis of a reference line L which passes a center of a width of the tooth  312  and the center C of the stator core  310 . The grooves  315  increase a frequency of cogging torque waveforms per unit period by performing a function corresponding to that of the slot opening O, which causes a change in magnetic flux density, so as to greatly reduce the cogging torque. 
     Referring to  FIG. 4 , in the case of a motor including eight poles and twelve slots without the grooves  315 , a main cogging degree corresponds to 24, which is a least common multiple of 8 which is the number of the magnets  420 , and 12, which is the number of the slots. For example, in the case of a motor including 6 poles and 9 slots, a main cogging degree corresponds to 18 which is a least common multiple of 8 which is the number of magnets and 9 which is the number of slots. Here, the main cogging degree means a frequency of cogging torque waveforms per unit rotation (one rotation) of the motor. Here, the frequency refers to the number of repeated cogging torque waveforms which form peaks. Also, the number of slots corresponds to the number of the teeth  312 . 
     In the case of a motor including 8 poles and 12 slots with two grooves  315 , since the number of slots is regarded as being increased from 12 to 36 by the grooves  315 , a main cogging degree increases three times from 24 to 72. Since increasing the main cogging degree three times using the two grooves  315  as described above means increasing the frequency of cogging torque waveforms, cogging torque may be greatly reduced. 
     Referring to  FIGS. 5, 6   a , and  6   b , a width W1of the groove  315  is set within a range of 90% to 110% of a width W2of the slot opening O. Here, the width W1of the groove  315  means a distance from one side end to the other side end of an inlet of the groove  315  on the basis of a circumferential direction of the stator core  310 . Also, the width W2of the slot opening O means a distance from one side end to the other side end of an inlet of the slot opening O on the basis of the circumferential direction of the stator core  310 . 
     As shown in  FIG. 6 a   , when the width W1of the groove  315  deviates from the range of 90% to 110% of the width W2of the slot opening O, a problem occurs in which a component of the stator, that is, a main cogging degree which is the same as the number of poles of the magnets  420 , is included in a cogging torque waveform. 
     However, as shown in  FIG. 6 b   , it may be seen that when the width W1of the groove  315  is within the range of 90% to 110% of the width W2of the slot opening O, only cogging torque waveforms corresponding to a main cogging degree of 72 are detected. 
     Referring to  FIG. 7 , an angle θ formed by the body  313  and the shoe  314  of the tooth  312  may be 145° to 155°. In detail, an angle θ formed by a side surface  313   a  of the body  313  and a side surface  314   a  of the shoe  314  connected to the side surface  313   a  of the body  313  may be 145° to 155°. 
     Referring to  FIG. 8 , it may be seen that cogging torque is greatly reduced when the angle θ formed by the body  313  and the shoe  314  is within a range from 145° to 155°. Simultaneously, referring to  FIG. 9 , it may be seen that torque ripple is low when the angle θ formed by the body  313  and the shoe  314  is within the range from 145° to 155° and greatly increases when the angle θ deviates from the range from 145° to 155°. Particularly, it may be seen that the torque ripple rapidly increases when the angle θ is greater than 155°. 
     Referring to  FIG. 10 , it may be checked that an amplitude of cogging torque waveforms is gradually reduced as the angle θ formed by a side surface  314   a  of the shoe  314  goes farther from 145° toward 155°. 
       FIG. 11  is a lateral cross-sectional view of the motor taken along a line A-A of  FIG. 1 ,  FIG. 12  is a view illustrating an arrangement relationship between the stator core and the rotor of the motor according to the embodiment,  FIG. 13  is a view illustrating the stator and the rotor in an area B 1  of  FIG. 12 ,  FIG. 14  is a view illustrating the tooth and the grooves of the motor according to the embodiment,  FIGS. 15 a  and 15 b    are views illustrating comparison between torques of the motor according to the embodiment and a motor without grooves,  FIGS. 16 a  and 16 b    are views illustrating cogging torque and torque ripple according to a radius of the groove and a distance of an air gap formed in the motor according to the embodiment,  FIGS. 17 a  and 17 b    are views illustrating comparison between torques according to a radius of the groove and a distance of an air gap formed in the motor according to the embodiment,  FIGS. 18 a  and 18 b    are views illustrating cogging torque and torque ripple according to a position of a center C 1  of the groove formed in the motor according to the embodiment,  FIG. 19  is a table illustrating performance of the motor according to the embodiment and a motor without grooves,  FIG. 20  is a view illustrating performance of the motor according to the embodiment and a motor without grooves at a room temperature,  FIG. 21  is a table illustrating performance of the motor according to the embodiment and a motor with a square notch formed therein, and  FIG. 22  is a view illustrating performance of the motor according to the embodiment and a motor with a square notch formed therein. 
     Referring to  FIGS. 11 to 22 , changes in torque, cogging torque and torque ripple caused by an air gap and grooves arranged on the tooth of the stator will be described. 
     Referring to  FIG. 11 , the stator  300  of the motor  1  may include the stator core  310 , the coil  320 , and the insulator  330 . Also, the stator core  310  may include the yoke  311 , the plurality of teeth  312 , and a plurality of the grooves  315  formed in the teeth  312 . Here, nine teeth  312  may be formed on the yoke  311 . Also, corresponding to the teeth  312 , six magnets  240  may be arranged on the rotor core  410 . 
     Also, each of the teeth  312  may include the body  313 , on which the coil  320  is wound, and the shoe  314  formed to extend from the body  313 . 
     The yoke  311  may have a cylindrical shape. 
     The plurality of teeth  312  may be arranged to protrude from the yoke  311  toward the center C. 
     As shown in  FIG. 12 , the teeth  312  may be arranged at certain intervals along an inner circumferential surface of the yoke  311  to protrude toward the center C. Here, the plurality of teeth  312  may be arranged on the inner circumferential surface of the yoke  311  to be spaced at certain intervals apart. 
     Accordingly, a space where the coil  320  is wound may be formed between one tooth  312  and another tooth  312  which are arranged to be adjacent to each other. Here, the space means a slot S. 
     Also, as the shoes  314  are arranged to be spaced apart, an opening portion of the slot S may be formed. Here, the opening portion means a slot opening O. 
     Accordingly, the slot opening O means a space between an end point P of any one shoe  314  and an end point P of another shoe  314  adjacent thereto, and the slot opening O forms a certain angle θ1 on the basis of the center C of the rotating shaft  500 . Here, the angle θ1 may be four degrees on the basis of the center C of the rotating shaft  500 . 
     As shown in  FIG. 13 , the angle θ1 refers to a width W2of the slot opening O, and the width W2means a distance between one side and the other side of the slot opening O. 
     The coil  320  may be wound on the body  313 . Here, the insulator  330  may be disposed on the body  313 . The insulator  330  insulates the body  313  from the coil  320 . 
     Also, the body  313  may be disposed to protrude from the yoke  311  toward the center C. 
     The shoe  314  may be formed to extend from an end part of the body  313 . Also, the shoes  314  may be arranged to face the magnets  420 . 
     Also, the plurality of grooves  315  may be arranged on the shoe  314 . When viewed in an axial view, the groove  315  may have a semicircular shape. As shown in  FIG. 12 , the shoes  314  may be arranged to face the magnets  420  while being spaced at a certain interval apart from the magnets  420 . Accordingly, an air gap G may be formed between an inner surface of the shoe  314  and an outer surface of the magnet  420 . 
     Here, the air gap G may mean a distance between the shoes  314  and the rotor  400 . Preferably, the air gap G may mean a distance between the shoes  314  and the magnets  420 . Here, an inside may mean a direction of being arranged to face the center C on the basis of the center C, and an outside may mean a direction opposite to the inside. 
     The two grooves  315  may be on the shoe  314 . Here, the two grooves  315  may be arranged to be symmetrical to each other on the basis of a reference line L which passes a center of a width of the shoe  314  and the center C of the stator core  310 . 
     The grooves  315  perform a function of reducing a change in static magnetic energy (variation) by performing a function corresponding to the slot openings O which cause a change in magnetic flux density. Accordingly, the grooves  315  greatly reduce cogging torque by increasing a frequency of cogging torque waveforms per unit period. 
       FIGS. 15 a  and 15 b    are views illustrating comparison between torques of the motor according to the embodiment and a motor without grooves.  FIG. 15 a    illustrates pulsation of the motor without grooves, and  FIG. 15 b    illustrates pulsation of the motor according to the embodiment. 
     Referring to  FIGS. 15 a  and 15 b   , since torque pulsation (repeated torque waveforms) with respect to cogging torque and torque ripple may be calculated as a least common multiple of the number of poles (magnets) and the number of slots, in the case of a motor including 6 poles and 9 slots without the groove  315  as shown in  FIG. 15 a   , 18, which is a least common multiple, corresponds to pulsation. Here, the pulsation may mean the number of repeated waveforms which form peaks. 
     Also, in the case of the motor  1  according to the embodiment as shown in  FIG. 15 b   , since the two grooves  315  are formed on each of the shoes  314 ,  54 , which is a least common multiple of 6 poles and 27 slots, may correspond to pulsation. 
     Accordingly, since a frequency of pulsation increases three times, the motor  1  may greatly reduce cogging torque. 
     Meanwhile, the grooves  315  may be formed toward the rotating shaft  500 . As shown in  FIG. 14 , the grooves  315  may be arranged lengthwise along a height direction (axial direction) from a top end to a bottom end of the stator core  310 . Here, the grooves  315  may have a semicylindrical shape. 
     Accordingly, as shown in  FIGS. 12 and 13 , a lateral cross section of the groove  315  may have a semicircular shape. Accordingly, the groove  315  may be formed to have a certain radius R on the basis of the center C 1  of the groove  315 . 
     Accordingly, since a length of a magnetic path is equal in a lateral direction (a radial direction), the semicircular groove  315  is advantageous for reducing cogging torque. Here, the lateral direction may be perpendicular to the axial direction. 
       FIGS. 16 a  and 16 b    are views illustrating cogging torque and torque ripple according to a radius of the groove and a distance of an air gap formed in the motor according to the embodiment.  FIG. 16 a    is a view illustrating cogging torque according the radius of the groove and the distance of the air gap, and  FIG. 16 b    is a view illustrating torque ripple according to the radius of the groove and the distance of the air gap. 
       FIGS. 17 a  and 17 b    are views illustrating comparison between torques according to the radius of the groove and the distance of the air gap formed in the motor according to the embodiment.  FIG. 17 a    is a view illustrating torque of the motor when the radius of the groove is different from the distance of the air gap, and  FIG. 17 b    is a view illustrating torque of the motor when the radius of the groove is from the same as the distance of the air gap. 
     Meanwhile, a radius R of the groove  315  may be formed corresponding to a distance D of the air gap G. Here, the distance D of the air gap G may be a distance between the shoe  314  and the center of the width of the magnet  420 . 
     Meanwhile, the radius R of the groove  315  may be formed within a range of 0.9 to 1.1 in proportion to the distance D of the air gap G. That is, the radius R of the groove  315  may be determined within a range of ±10% of the distance D of the air gap G. Preferably, the radius R of the groove  315  may be formed to be equal to the distance D of the air gap G. 
     As shown in  FIG. 16 a   , cogging torque rapidly decreases when the radius R of the groove  315  is 0.45 mm and is located in a lowest position when the radius R of the groove  315  is 0.5 mm which is equal to the distance D of the air gap G. Then, an increase of the radius R of the groove  315  to 0.55 mm may be seen. 
     That is, cogging torque rapidly decreases when the radius R of the groove  315  is 0.9 in proportion to the distance D of the air gap G and is located in a lowest position when the radius 
     R of the groove  315  is equal to the distance D of the air gap G. Then, an increase of the radius R of the groove  315  to 1.1 in proportion to the distance D of the air gap G may be seen. 
     Accordingly, the cogging torque has a lowest value when the radius R of the groove  315  is equal to the distance D of the air gap G. 
     As shown in  FIG. 16 b   , it may be seen that torque ripple maintains a low value when the radius R of the groove  315  is 0.45 mm to 0.55 mm. That is, it may be seen that the torque ripple rapidly decreases when the radius R of the groove  315  is 0.4 mm and is smoothly maintained when the radius R of the groove  315  is 0.45 mm to 0.55 mm. 
     Here, the torque ripple is located in a lowest position when the radius of the groove  315  is equal to the distance D of the air gap G. That is, the cogging ripple has a lowest value when the radius R of the groove  315  is 0.5 mm, which is equal to the distance D of the air gap G. 
     Accordingly, it may be seen that the torque ripple maintains a low value when the radius R of the groove  315  is 0.9 to 1.1 in proportion to the distance D of the air gap G. That is, it may be seen that the torque ripple rapidly decreases when the radius R of the groove  315  is 0.8 in proportion to the distance D of the air gap and the torque ripple is smoothly maintained when the radius R of the groove  315  is 0.9 to 1.1 in proportion to the distance D of the air gap. 
     Accordingly, the torque ripple has a lowest value when the radius R of the groove  315  is equal to the distance D of the air gap G. 
     Referring to  FIGS. 17 a  and 17 b   , it may be seen that in comparison to a case in which the radius R of the groove  315  is different from the distance D of the air gap G, the torque of the motor  1  is improved 22% when the radius R of the groove  315  is equal to the distance D of the air gap G. 
     That is, cogging torque may be reduced by designing the radius R of the groove  315  to be equal to the distance D of the air gap G. 
     Meanwhile, referring to  FIGS. 12 and 13 , the center C 1  of the groove  315  may be disposed to be spaced at a certain angle θ2 in a circumferential direction apart from one side end point P of the shoe  314 . Here, the angle θ2 means a distance between the one side end point P of the shoe  314  and the center C 1  of the groove  315 . Additionally, the angle θ2 refers to a position of the center C 1  of the groove  315  in relation to the angle θ1 of the slot opening O. Here, for clarity, the angle θ2 may be referred to as a second angle θ2, and the angle θ1 may be referred to as a first angle θ1. 
     The second angle θ2 may be formed within a range of 0.45 to 0.55 in proportion to the first angle θ1. For example, when the first angle θ1 is 4 degrees, the second angle θ2 may be formed within a range of 1.8 degrees to 2.2 degrees. Preferably, the second angle θ2 may be formed to be 2.0 degrees. 
     Accordingly, the center C 1  of the groove  315  may be formed to be 0.45 to 0.55 from the one side end point P in proportion to the first angle θ1. Preferably, the center C 1  of the groove  315  may be formed to be 0.5 from the one side end point P of the shoe  314  in proportion to the first angle θ1. 
       FIGS. 18 a  and 18 b    are views illustrating cogging torque and torque ripple according to the position of the center C 1  of the groove formed in the motor according to the embodiment. Here, the number of the grooves  315  is 2, the radius R of the groove  315  and the distance D of the air gap G are the same as 0.5 mm, and the first angle θ1 of the slot opening O is four degrees. 
     As shown in  FIG. 18 a   , cogging torque increases at an inflection point when a position of the center C 1  of the groove  315  is located at an angle of 2.0 degrees from the one side end point P of the shoe  314 . 
     Accordingly, when the position of the center C 1  of the groove  315  is present within a range of 1.8 degrees to 2.2 degrees from the one side end point P of the shoe  314 , the motor  1  may perform effective performance. Preferably, when the position of the center C 1  of the groove is located at 2.0 degrees from the one side end point P of the shoe  314 , the cogging torque has a lowest value such that the motor  1  may perform optimum performance. 
     As shown in  FIG. 18 b   , a torque ripple value is maintained at about 100 mNm when a position of the center C 1  of the groove  315  is located within a range of 1.8 degrees to 2.5 degrees from the one side end point P of the shoe  314 . 
     Accordingly, in consideration of cogging torque and torque ripple of the motor  1 , a range of the center C 1  of the groove  315  located at 1.8 degrees to 2.2 degrees from the one side end point P of the shoe  314  may be checked as an optimum range. Particularly, when the position of the center C 1  of the groove  315  is located at 2.0 degrees from the one side end point P of the shoe  314 , the cogging torque has a lowest value such that the motor  1  may perform optimum performance. 
     Hereinafter, referring to  FIGS. 19 to 22 , performance of the motor  1  will be described. 
     Referring to  FIGS. 19 and 20 , it may be seen that in comparison to a motor without grooves, when the grooves  315  are included as in the motor  1 , cogging torque is reduced 79% and torque ripple is reduced 39.7%. Also, it may be seen that variation of values other than the cogging torque and the torque ripple are insignificant. 
     Accordingly, in the case of the motor  1 , in comparison with a motor without grooves, quality may be increased by reducing cogging torque and torque ripple while performance is indifferent from that of the motor without grooves. 
     Referring to  FIGS. 21 and 22 , it may be seen that in comparison to a motor with square notches, when the above-described semicircular grooves  315  are included as in the motor  1 , cogging torque is reduced 67.4% and torque ripple is increased 2.5%. Also, it may be seen that variation of values other than the cogging torque and the torque ripple are insignificant. 
     Here, the square notch has a difference between a length of one side and a length of a diagonal line. Particularly, based on a center of the square notch, a difference is present between a length of a side in a semidiameter direction (radial direction) and a diagonal line. 
     That is, since the square notches have a difference in directional lengths of a magnetic path, the semicircular grooves  315  of the motor  1  are more effective in an aspect of cogging torque. 
     Accordingly, in the case of the motor  1 , in comparison with the motor with square notches, quality may be increased by reducing cogging torque while performance is indifferent from that of the motor without a groove. 
     Although an exemplary embodiment of the present invention has been described above, it should be understood by one of ordinary skill in the art that a variety of modifications and a variety of changes may be made without departing from the concept and scope of the present invention which are defined in the following claims. Also, differences related to the modifications and applications will be interpreted as being included in the scope of the present invention defined by the attached claims. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1 : motor,  10 : bearing,  100 : housing,  200 : bracket,  300 : stator,  310 : stator core,  311 : yoke,  312 : tooth,  313 : body,  314 : shoe,  315 : groove,  320 : coil,  400 : rotor,  410 : rotor core,  420 : magnet,  500 : rotating shaft,  600 : sensing magnet assembly,  700 : PCB