Abstract:
A kitchen appliance has a base, a driven mechanism rotatable relative to the base, and an electric motor with a stator and a rotor for driving the driven mechanism. The stator comprises a stator core with two primary stator poles with stator windings wound there on and two auxiliary stator poles. The stator windings are configured in such a way that primary magnetic poles with the same polarity are formed at the primary poles and induced magnetic poles with the same polarity which is opposite to the polarity of the primary magnetic poles are formed at the auxiliary poles when the motor is electrified.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201010181698.3 filed in The People&#39;s Republic of China on May 21, 2010. 
     FIELD OF THE INVENTION 
     This invention relates to a kitchen appliance driven by an electric motor. 
     BACKGROUND OF THE INVENTION 
     Currently there are various kitchen appliances using electric motors. Some kitchen appliances desire the electric motor to be smaller without reducing its power. One such kitchen appliance is the food processor. 
     Conventionally, the food processor uses a two-pole universal motor. The stator of the motor has a stator core which includes a yoke with a rectangular configuration and two salient poles which extend inwardly from two opposing sides of the yoke and have stator windings wound thereon. When the stator windings are electrified, two magnetic poles with opposite polarity are formed at the salient poles and two magnetic circuits are formed. Each magnetic circuit passes through two stator poles, the rotor and a side of the yoke so that it has a relatively long path. 
     The present invention aims to provide a new kitchen appliance, especially a food processor which is permitted to be smaller and cheaper. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect thereof, the present invention provides a kitchen appliance comprising: a base; a driven mechanism rotatable relative to the base; and an electric motor with a stator and a rotor for driving the driven mechanism; wherein the stator comprises a stator core having a yoke which comprises at least two first sections, each with a primary pole extending there from and at least two second sections, each with an auxiliary pole extending there from, the at least two primary poles and the at least two auxiliary poles are alternately arranged in the circumferential direction of the stator core, and the at least two primary poles have stator windings wound thereon; and wherein the stator windings are configured in such a way that primary magnetic poles with the same polarity are formed at the primary poles and induced magnetic poles with the same polarity which is opposite to the polarity of the primary magnetic poles are formed at the auxiliary poles when the motor is electrified. 
     Preferably, the ratio of an outer diameter D of the rotor to a minimum outer dimension Y of the stator is greater than 7:10. 
     Preferably, the first sections are narrower than the second sections. 
     Preferably, there are no stator windings wound about the auxiliary poles. 
     Alternatively, the auxiliary poles have stator windings wound thereon, and the stator windings wound on the auxiliary poles have less turns than the stator windings wound on the primary poles. 
     Preferably, each of the primary poles and auxiliary poles comprises a neck extending radially inwardly from the yoke and a pole shoe extending circumferentially from the neck and having a pole face. 
     Preferably, the distance between the first sections and the pole faces of the primary poles are greater than the distance between the second sections and the pole faces of the auxiliary poles. 
     Preferably, the necks of the primary poles are narrower than the necks of the auxiliary poles. 
     Preferably, the motor is a single phase series motor. 
     According to a second aspect, the present invention provides a kitchen appliance comprising: a base; a driven mechanism rotatable relative to the base; and an electric motor with a stator and a rotor for driving the driven mechanism; wherein the stator comprises a stator core with two pairs of stator poles and stator windings wound about at least one pair of stator poles, and two pairs of magnetic poles with opposite polarity are alternatively formed at the two pairs of stator poles when the motor is electrified. 
     Preferably, the stator core having a yoke which comprises two first sections, each with a primary pole extended there from and two second sections each with an auxiliary pole extended there from, the two primary poles and the two auxiliary poles are alternately arranged in the circumferential direction of the stator core, and the primary poles have stator windings wound thereon; and wherein the stator windings are configured in such a way that primary magnetic poles with the same polarity are formed at the primary poles and induced magnetic poles with the same polarity which is opposite to the polarity of the primary magnetic poles are formed at the auxiliary poles when the motor is electrified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment 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 labeled 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. 
         FIG. 1  is a food processor according to a preferred embodiment of the present invention; 
         FIG. 2  is a schematic sectional view of an electric motor being part of the food processor of  FIG. 1 ; and 
         FIG. 3  is a magnetic path diagram for the motor of  FIG. 2 ; 
         FIG. 4  is a schematic sectional view of an electric motor according to a second embodiment of the present invention; and 
         FIG. 5  is a schematic sectional view of an electric motor according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A food processor according to a preferred embodiment of the present invention is shown in  FIG. 1 . The food processor  80  comprises a base  82 , a container  84  fixedly or removably seated on the base  82  for containing food, an electric motor (not shown in  FIG. 1 ), a working piece  86  for processing food in the container  84  and a coupling  88  for connecting the working piece  86  to the motor. Preferably, the working piece  86  is removably connected to the coupling  88 . 
     The electric motor is fixed within the base  110 . Preferably the motor is a four-pole single phase series motor.  FIG. 2  is a schematic view of a cross section of the motor, with the rotor windings omitted. The motor comprises a rotor  20  and a stator  40  magnetically coupled to the rotor  20 . 
     The rotor  20  comprises a shaft  22 , a rotor core  24  fixed on the shaft  22 , and a commutator (not shown) fixed on the shaft  22  adjacent the rotor core  24 . The rotor core  24  is formed by stacking laminations in the axial direction of the rotor. The rotor core  24  has a plurality of teeth  26  and rotor windings are wound about the teeth  26 . A winding slot  28  is formed between each two adjacent teeth  26 . The commutator has a plurality of segments. The rotor windings are wound in the winding slots  28  of the rotor core  24  and connected to the segments of the commutator. 
     The stator  40  comprises a stator core  42  and stator windings  44 . The stator core  42  is formed by stacking laminations in the axial direction of the motor and comprises a yoke with two first sections  46  and two second sections  48 . The first sections  46  and second sections  48  are alternately arranged in the circumferential direction of the stator core  42 . Two primary poles  50  and two auxiliary poles  52  extend inwardly from the first sections  46  and the second sections  48  respectively. The stator windings  44  are wound about the primary poles  50 , and no windings are wound on the auxiliary poles  52 . The motor also has brushes fixed with respect to the stator  40  to make sliding contact with the commutator to transfer electricity to the rotor windings via the commutator. The stator core  42  is symmetrical. The primary poles  50  and auxiliary poles  52  are salient poles and each comprises a neck  54  extending radially inwardly from the yoke and a pole shoe having a pole face  56  extending circumferentially from the neck  54 . Each pole face  56  forms a continuous arced surface facing the rotor  20 . Together, the pole faces form an intermittent cylindrical wall. 
     In the circumferential direction, the necks of the primary poles  50  are narrower than the necks of the auxiliary poles  52 . In the radial direction, the first sections  46  are narrower than the second sections  48 , and the distance between the first sections  46  and the pole faces  56  of the primary poles  50  are greater than the distance between the second sections  48  and the pole faces  56  of the auxiliary poles  52 . Thus, a larger space is formed between the first sections  46  and the primary poles  50  to accommodate the stator windings  44 . It should be understood that the circumferential direction and the radial direction described above are not only for round or cylindrical structures, other shapes such as square and oval are also included within the scope of the present invention. 
     When the stator windings  44  are electrified, two primary magnetic poles with the same polarity are formed at the two primary poles  50  by the stator windings  44  and two induced magnetic poles with the same polarity which is opposite to the polarity of the primary magnetic poles are formed at the two auxiliary poles  52 . That is, four magnetic poles and four magnetic paths are formed, as shown in  FIG. 3 . Each magnetic path passes through a primary pole  50 , the stator yoke, an auxiliary pole  52 , the air gap between the auxiliary pole  52  and the rotor  20 , the rotor  20  and the air gap between the primary pole  50  and the rotor  20 . Therefore the flux path is shorter than a 2 pole motor. 
     In the embodiment, the rotor  20  has an outer diameter D, the stator  40  has a minimum outer dimension Y, and the ratio of the outer diameter D to the minimum outer dimension Y is greater than 7:10. The outer diameter of the rotor  20  means the outer diameter of the rotor core  24 . The minimum outer dimension of the stator  40  means the distance between two points of intersection between a straight line extending through the center of the stator  40  and the periphery of the stator core  42 . 
     The outer diameter D of the rotor  20  is slightly less than the diameter of a circle defined by the arced surfaces of the pole faces of the poles  50  and  52 , and the ratio of the diameter of the circle to the minimum outer diameter of the stator  40  is also greater than 7:10. It should be understandable that the width of the gap between the primary poles  50  and the rotor core  42  may be equal to or different from the width o the gap between the auxiliary poles  52  and the rotor core  42 . When the two distances are different, the arced surfaces of the pole faces of the primary poles  50  and the arced surfaces of the poles faces of the auxiliary poles  52  define two circles with different diameters. In this configuration, the diameter of the circle means the diameter of the smaller circle. 
     In the preferred embodiment, the auxiliary poles  52  project inwardly from the inner side of the second sections  48  so as to be formed as salient poles. Alternatively, the auxiliary poles  52  may be formed as non-salient poles which are sunk relative to the inner side of the second sections  48 . When the auxiliary poles  52  are salient poles, they may also have stator windings wound thereon, and the stator windings wound on the auxiliary poles  52  have less turns than the stator windings  44  wound on the primary poles  50 . 
     On the condition of having the same output, the four-pole motor in the present invention has reduced stator windings and a smaller stator core than the two-pole motor of the conventional food processor. Therefore, the four-pole motor may be smaller and consume less material, which allows the food processor to be smaller, cheaper and lighter. Besides, the four-pole motor has less copper losses and shorter magnetic circuits, so the efficiency of the motor is improved and the energy consumed by the food processor is reduced. 
     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. 
     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. 
     For example, as an alternative, the container may be rotated by the motor while the coupling and the working piece are fixed relative to the base. Alternately, the food processor may be a handheld mixer without a container. 
     Furthermore, the stator of the motor in the present invention may have three primary poles with stator windings wound thereon, at which three field magnetic poles are formed, and three auxiliary poles without stator windings, at which three induced magnetic poles are formed, as shown in  FIG. 4 . Alternatively, the stator may have four primary poles and four auxiliary poles as shown in  FIG. 5 .