Abstract:
An electric motor comprises a stator and a rotor rotatably mounted confronting the stator. The rotor has a rotor core which comprises at least a first tooth, a second tooth and a third tooth which are adjacent. Each of the teeth comprises a rib section radially and outwardly extending from a core and a tooth section located at an outer end of the rib section. The angle between the first tooth and the second tooth is n degrees, wherein when the rotor core is rotated n degrees to move the first tooth towards the second tooth, the rotated second tooth does not coincide with the pre-rotated third tooth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 200810217601.2 filed in The People&#39;s Republic of China on Nov. 21, 2008. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to an electric motor and in particular, to a rotor core of an electric motor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electromagnetic noise is emitted when a motor is operating. One element of electromagnetic noise is harmonic noise. Harmonic noise is mainly caused by the motor&#39;s torque ripple. For example, when a permanent magnet direct current (PMDC) motor is operating, the position of the coils with respect to the stator and the current through the windings varies periodically, which results in a periodic torque ripple.  FIG. 3  is a cross sectional view of a twelve slot rotor core of a conventional motor. In such a conventional motor, harmonic noise comprises the 12th order harmonic noise and its integral multiple level harmonic noise, such as 24th order harmonic noise, 36th order harmonic noise, etc. 
         [0004]    It&#39;s desired to eliminate or at least reduce harmonic noise to make people feel more comfortable, especially in HVAC (heating, ventilation and air-conditioning) applications. One known way to reduce harmonic noise is to provide the rotor poles with grooves running axially along the pole face. These grooves, known as dummy slots as they have no windings laid therein, allow the motor to function as though it has a greater number of poles in terms of harmonic noise but still being easy to manufacture. 
         [0005]    However, the air gap between rotor and the stator is increased due to the dummy slots, and motor performance is decreased. Therefore, there is a desire for an improved motor having a reduced harmonic noise. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, in one aspect thereof, the present invention provides a rotor core for an electric motor, comprising at least a first tooth, a second tooth adjacent the first tooth and a third tooth adjacent the second tooth, each of said teeth comprising a rib section radially and outwardly extending and a tooth section located at outer end of said rib section, the angle between the first tooth and the second tooth being n degrees, wherein when the rotor core is rotated n degrees to move the first tooth towards the second tooth, the rotated second tooth does not coincide with the pre-rotated third tooth. 
         [0007]    Preferably, the angle between the rib section of the first tooth and the rib section of the second tooth is larger or smaller than the angle between the rib section of the second tooth and the rib section of the third tooth. 
         [0008]    Preferably, one angle between two adjacent rib sections of the teeth is equal to a base angle defined by dividing 360° by the number of teeth, and at least one angle between two adjacent rib sections of said teeth is equal to the base angle +Δθ, wherein Δθ is from 1% to 40% of the base angle. 
         [0009]    Preferably, the rotor core comprises twelve teeth, at least one angle between two adjacent rib sections being equal to 30°, at least one angle between two adjacent rib sections being equal to 28°, at least one angle between two adjacent rib sections being equal to 32°. 
         [0010]    Preferably, the circumferential width of the tooth section of the second tooth is larger or smaller than the circumferential width of the tooth section of the third tooth. 
         [0011]    Preferably, winding slots are formed between adjacent teeth and have winding slot openings between adjacent tooth sections, the circumferential width of one winding slot opening being larger than that of another winding slot openings; or the circumferential width of the winding slot openings being the same, and the all the angles between adjacent winding slot openings are not the same. 
         [0012]    Preferably, one angle between two adjacent winding slot openings is equal to a base angle defined by dividing 360° by the number of teeth, and at least one angle between two adjacent winding slot openings is equal to the base angle +Δφ, wherein Δφ is from 1% to 40% of the base angle. 
         [0013]    Preferably, the rotor core comprises twelve teeth and twelve winding slots, at least one angle between two adjacent winding slot openings being equal to 30°, at least one angle between two adjacent winding slot openings being equal to 32°, at least one angle between two adjacent winding slot openings being equal to 28°. 
         [0014]    Preferably, the number of coil turns deposited in the winding slot between said first tooth and said second tooth is larger or smaller than the number of coil turns deposited in the winding slot between said second tooth and said third tooth. 
         [0015]    Preferably, the circumferential width of rib section of said second tooth is larger or smaller than that of rib section of said third tooth. 
         [0016]    Preferably, when the angles between adjacent teeth are not all equal, the circumferential width of each tooth section is the same. 
         [0017]    According to a second aspect, the present invention also provides an electric motor incorporating a rotor core as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Two preferred embodiments of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labelled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0019]      FIG. 1  is a sectional view of a rotor core of a motor in accordance with a first embodiment of the present invention; 
           [0020]      FIG. 2  is sectional view of a rotor core of a motor in accordance with a second embodiment of the present invention; and 
           [0021]      FIG. 3  is a sectional view of a rotor core of a conventional motor; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  illustrates a sectional view of a motor in accordance to one embodiment of the present invention. The rotor core comprises twelve teeth. Each tooth, such as tooth  10 , comprises a rib section  11  which radially and outwardly extends from a ring shape core center and a tooth section  12  which locates at the outward end of the rib section  11  and expands in a circumferential direction. Similarly, tooth  20  comprises a rib section  21  and a tooth section  22 , and tooth  30  comprises a rib section  31  and a tooth section  32 , etc. 
         [0023]    In this embodiment, each angle between adjacent teeth is represented by references θ 1 ˜θ 12  respectively. Each angle is measured from the radial center line of one tooth to the radial center line of the adjacent tooth. For example, angle θ 1  is the angle between tooth  10  and tooth  20  and is measured from the radial center line of the rib section  11  of tooth  10  to the radial center line of the rib section  21  of tooth  20 . Similarly, angle θ 2  is measured from the radial center line of rib section  21  of tooth  20  to radial center line  32  of tooth  30 , etc. 
         [0024]    In this embodiment, there are twelve teeth distributed asymmetrically. The value in degrees of angles θ 1 ˜θ 12  is 30°, 30°+Δθ, 30°Δθ, 30°+Δθ, 30°−Δθ, 30°−Δθ, 30°, 30°−Δθ, 30°+Δθ, 30°+Δθ, 30°Δθ, 30°−Δθ, respectively, wherein  30 ° is a base angle defined by dividing 360° by the number of winding slots or teeth, and Δθ is a variable. It is preferable that the variable Δθ is from 1% to 40% of the base angle. In this embodiment the variable Δθ is equal to 2°. 
         [0025]    As mentioned above, the angle θ 1  is not equal to the angle θ 2 . Therefore, when the rotor rotates 30° (i.e., the value of angle θ 1 ) in a direction which mores tooth  10  towards tooth  20 , rib section  11  of the rotated tooth  10  will coincide with the position of the rib section  21  of the pre-rotated tooth  20 , while rib section  21  of the rotated tooth  20  will not coincide with the position of the rib section  31  of the pre-rotated tooth  30 . In other words, the value of all angles θ 1 ˜θ 2  is not the same, therefore, when the rotor core is rotated through a predetermined angle, the rotated rotor core will not coincide with the pre-rotated rotor core. Original variation regularity of magnetic field is changed due to asymmetry of the rotor core. When the rotor core is rotated 360°, the torque ripple of the motor varies irregularly, which reduces the harmonic noise. As the harmonic noise is reduced by an asymmetric rotor core, the air gap between the rotor core and the stator is not changed. It should be noted that the circumferential extent of the tooth section is the same for all of the teeth. 
         [0026]    In the embodiment described above, the rotor core comprises twelve teeth, and the angles between the rib sections of adjacent teeth are not the same, but the present invention should not be limited to this configuration. Alternatively, the circumferential width of rib sections of the teeth may not be the same, while the angle between adjacent rib sections is the same. Furthermore, the number of turns of coils deposited in winding slots can be different to meet specific requirement. 
       Embodiment Two 
       [0027]      FIG. 2  shows a cross section of a rotor core of a motor in accordance to another embodiment of the present invention. The rotor core comprises twelve teeth, each of which comprises a rib section extending radially and outwardly from a core and a tooth section located at the outer end of the rib section. For example, tooth  10  comprises a rib section  11  and a tooth section  12 , and tooth  20  comprises a rib section  21  and a tooth section  22 , etc. 
         [0028]    In this embodiment, the twelve rib sections are evenly distributed circumferentially about the rotor core. In other words, all the rib sections are arrayed symmetrically. However, the circumferential width of the twelve tooth sections is not the same. For example, the circumferential width of tooth section  22  is smaller than that of adjacent tooth section  32 . Therefore, when tooth  20  is rotated towards tooth  30 , tooth section  22  of the rotated tooth  20  will overlap but not totally coincide with the position of tooth section  32  of the pre-rotated tooth  30 . As is known, a winding slot opening is formed between adjacent tooth sections. Since tooth sections are not distributed symmetrically along a circumferential direction of the rotor core, the winding slot openings are not distributed symmetrically. However, in the preferred embodiment, the circumferential width of all winding slot openings is the same. 
         [0029]    Thus the value of the angles between adjacent winding slot openings is not the same. The angle between two adjacent winding slot openings is measured from the radial center line of one winding slot opening to the radial center line of the adjacent winding slot opening. Referring to  FIG. 2 , the twelve angles formed between adjacent winding slot openings are represented by angles φ 1 ˜φ 12 , respectively, wherein angle φ 1  is the angle between the two winding slots that are separated by tooth  10 , and angle φ 2  is the angle between the two adjacent winding slots that are spaced by tooth  20 , etc. The values in degrees of angles φ 1 ˜φ 12  are 30°−Δφ, 30°−Δφ, 30°, 30°+Δφ, 30°+Δφ, 30°+Δφ, 30°−Δφ, 30°Δφ, 30°, 30°−Δφ, 30°+Δφ and 30°+Δφ, respectively, wherein 30° is a base angle defined by dividing 360° by the number of winding slots, and Δφ is a variable preferably from 1% to 40% of the base angle. In this embodiment, Δφ is equal to 2°. 
         [0030]    In this embodiment, original variation regularity of magnetic field and original torque ripple periodicity is broke by the asymmetrical distribution of winding slot openings, which results in a reduced harmonic component and reduced harmonic noise. Furthermore, air gap between the rotor core and motor stator is not increased. 
         [0031]    In the second embodiment described above, circumferential width of each winding slot opening is the same, but the angles between adjacent winding slot openings is not the same. Alternatively, the angles between adjacent winding slot openings may be the same, but the circumferential width of the winding slot openings is not the same. In the alternative embodiment, winding slot openings are also distributed asymmetrically along a circumferential direction of the rotor core, and the harmonic noise of the motor will be reduced. 
         [0032]    In addition, a motor can be provided having at least two of the following features: circumferential width of tooth sections of teeth being different, circumferential width of rib sections of teeth being different, the angles between adjacent rib sections being different, and the angles between adjacent winding slots being different. 
         [0033]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
         [0034]    Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow.