Patent Publication Number: US-2013229082-A1

Title: Permanent magnet motor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201210055497.8 filed in The People&#39;s Republic of China on Mar. 5, 2012. 
     FIELD OF THE INVENTION 
     The present invention relates to permanent magnet motors, and in particular, to a permanent magnet motor applied in an air multiplier. 
     BACKGROUND OF THE INVENTION 
     There are many applications for permanent magnet motors, such as bladeless fans, where there is a desire for smaller, lighter and more powerful motors, so that the end product is lighter or better. What is desired in these motors is a high power density, that is, a higher power output per unit volume occupied by the motor. In order to meet the above requirements, such motors usually employ rare earth permanent magnets. However, as the price of the rare earth material increase, the cost of the motor also increases. In addition, as these motors are often used in house appliances, there is a desire for the motor to be quiet, that is, to generate low levels of mechanical noise. One prior art permanent magnet motor which has been used in a bladeless fan used rare earth magnets in a six pole and nine slot configuration. The noise of this kind of motor is less than satisfactory. Therefore, reducing the cost of the motor while maintaining the performance, and at the same time reducing the noise of the motor have gradually become a concern. 
     Thus there is a desire for a permanent magnet motor that has low cost, good performance and generates low levels of noise. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect thereof, the present invention provides a permanent magnet motor comprising: a stator comprising an stator core having twelve teeth and coils wound around the teeth; and a rotor surrounded by the stator and comprising a shaft and a magnet core group secured to the shaft, wherein the magnet core group comprises eight rotor core segments and eight ferrite permanent magnets, each magnet being sandwiched between adjacent rotor core segments and polarized in the circumferential direction of the rotor, and adjacent rotor core segments have opposite polarities. 
     Preferably, each tooth is inclined relative to the axial direction of the motor. 
     Preferably, a curvature of a radially outer surface of each rotor core segment facing the stator is greater than that of a surface of each tooth facing the rotor. 
     Preferably, the magnet core group comprises two clamping plates at two axial ends thereof and a number of connecting rods connecting the two clamping plates, and each rotor core segment comprises an axial through hole through which the corresponding connecting rod passes to engage the corresponding rotor core segment. 
     Preferably, each rotor core segment comprises two lugs extending from two circumferentially opposite sides of a radially outer portion thereof, and one surface of each permanent magnet facing away from the shaft abuts two adjacent lugs. 
     Preferably, the magnet core group further comprises a sleeve secured to the shaft, each rotor core segment is secured to the sleeve, and the sleeve is made of a material with high magnetic reluctance. 
     Preferably, the sleeve is octahedron-shaped and comprises eight dovetail grooves at the corners thereof, extending in the axial direction, and a radially inner end portion of each rotor core segment is dovetail shaped and is form locked to the corresponding dovetail groove in the sleeve. 
     Preferably, the rotor comprises two magnet core groups secured to the shaft, and the two magnet core groups are offset from each other circumferentially. 
     Preferably, the two adjacent magnet core groups are offset from each other by 7.5 degrees. 
    
    
     
       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  shows a permanent magnet motor according to the preferred embodiment of the present invention; 
         FIG. 2  shows a stator of the permanent magnet motor of  FIG. 1 , without coils; 
         FIG. 3  is a cross sectional view of a rotor of the permanent magnet motor of  FIG. 1 ; 
         FIG. 4  is a partially exploded view of the rotor of  FIG. 3 ; 
         FIG. 5  shows a rotor of a permanent magnet motor according to another embodiment of the present invention; and 
         FIG. 6  shows a stator of a permanent magnet motor according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a permanent magnet motor  10  according to a first preferred embodiment of the present invention. The motor is shown without end caps to reveal the rotor and stator arrangement. The motor includes a stator  20  and a rotor  40  received in the stator  20 . Coils  24  forming the stator winding are shown wound about teeth of the stator. The stator is shown more clearly in  FIG. 2 , although the coils have been omitted to show the construction of the stator core more clearly. 
     The stator  20  includes a stator core  22  comprising an annular yoke  26  and twelve teeth  28  extending radially inwards from an inner surface of the yoke  26 . That is to say, there are twelve slots  30  formed by the teeth  28 . Each tooth  28  also extends in the axial direction of the stator  20  and includes a surface  32  that faces the rotor  40 , that is faces towards the axis (not shown) of the motor. The surface  32  of each tooth  28  is curved in the circumferential direction of the stator  20 . Each coil  24  ( FIG. 1 ) is wound around a corresponding tooth  28 . 
     Referring to  FIGS. 3 and 4 , the rotor  40  includes a shaft  42  and a magnet core group  44  secured to the shaft  42 . The magnet core group  44  includes a sleeve  46 , eight rotor core segments  48 , eight permanent magnets  50  made of ferrite magnet, eight connecting rods  52 , and two clamping plates  54  located at the axial ends of the magnet core group. 
     The sleeve  46  is octahedron-shaped and is made of high magnetic reluctance material such as plastics or aluminum. The sleeve  46  defines a shaft hole  56  at a middle portion thereof for fixedly receiving the shaft  42  and eight dovetail grooves  58  at the corners thereof. The shaft hole  56  and the dovetail grooves  58  extend in the axial direction of the sleeve  46 . The sleeve  46  can be over molded or interference fit on the shaft  42 . 
     Each rotor core segment  48  is substantially fan-shaped, including a radially inner portion  60  and a radially outer portion. The radially inner portion  60  is narrower than the radially outer portion. The radially inner portion  60  is dovetail-shaped and is received in a corresponding dovetail groove  58  of the sleeve  46 , thereby securing the rotor core segment  48  to the sleeve  46 . A surface  62  of the radially outer portion that faces away from the inner portion  60  is curved in the circumferential direction of the rotor  40  and the curvature thereof is greater than that of the surface  32  of the tooth  28 . That is, the radius of curvature of the radially outer surface of the rotor core segments is less than the nominal radius of the rotor, measured at a circumferential center of the outer surface of a rotor core segment. This feature reduces the cogging torque and lowers the audible or mechanical noise of the rotor  40 . Each rotor core segment  48  further defines an axial through hole  64  at the middle portion thereof. Two lugs  66  extend respectively from opposite sides of the radial outer portion in the circumference direction thereof. Each rotor core segment  48  is made of a magnetically conductive material such as iron or steel and is preferably made by stacking together a plurality of stamped laminations of electrical steel sheet. 
     Each permanent magnet  50  is a rectangular prism. Each permanent magnet  50  is secured between two adjacent rotor core segments  48 , abutting against two adjacent lugs  66  and the sleeve  46  and sandwiched between the two clamping plates  54 . Each permanent magnet  50  contacts the side surfaces of corresponding rotor core segments  48 . Each permanent magnet  50  is polarized along a direction parallel to the short side thereof, that is, substantially in the circumferential direction of the rotor  40 . At the same time, adjacent permanent magnets  50  have opposite polarities. 
     Each connecting rod  52  passes through the through hole  64  in corresponding rotor core segment  48  to engage with the rotor core segment  48 . The clamping plates  54  are respectively arranged at the axial ends of each magnet  50  and are respectively connected to the connecting rods  52  for example by a tight fit or glue. It should be understood that the connecting rods  52  and the clamping plates  54  can be integrally formed, for example by injection molding. 
     As such, the magnetic flux of adjacent permanent magnets  50  are concentrated in the rotor core segment  48  there between, thereby the eight rotor core segments  48  form eight magnetic poles by magnet flux concentration. Since the sleeve  46  is made of high magnetic reluctance material, the flux between two adjacent permanent magnets  50  is prevented from passing through the sleeve  46 , which reduces the magnetic flux leakage. In this way, the rotor has dense magnet flux and low cost, as ferrite magnet is cheaper than rare earth magnets such as NdFeB magnet. Also, the motor has eight poles and twelve slots. Under such an arrangement, the motor of the first preferred embodiment of the present invention concentrates the magnetic flux effectively so as to improve the power density and efficiency of the motor. 
     Additionally, during testing, it was found that the noise generated by the permanent magnet motor  10  having eight poles and twelve slots described above is about 30 Db, which is significantly less than the noise of 49 Db generated by a permanent magnet motor of the same structure but having six poles and nine slots or the noise of 55 Db generated by a permanent magnet motor of the same structure but having eight poles and nine slots. Therefore, the permanent magnet motor  10  of the first preferred embodiment can meet the requirements of low noise, high power density, and high efficiency. 
     In a second preferred embodiment based on the first embodiment above, two magnet core groups  204  can be employed by the rotor  41 , as shown in  FIG. 5 , to increase the output power of the motor. The two magnet core groups  44  may be offset from each other to reduce the cogging torque. In this embodiment, the two magnet core groups  44  are offset from each other by 7.5 degrees. During testing, it was found that the cogging torque of a motor made according to this second preferred embodiment was reduced to one fifth of that of a motor made according to the first embodiment. 
     In a third embodiment based on the first embodiment above, the teeth  28  of the stator  21  are inclined relative to the shaft, as shown in  FIG. 6 , to reduce the cogging torque. That is, the teeth are skewed by slightly rotating one or more laminations of the stator core as they are being stacked together. 
     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.