Abstract:
An electric motor has a housing, a rotor rotatably installed in the housing, n permanent magnets with the same polarity fixed at an inner surface of the housing. The rotor is a wound armature having a plurality of teeth extending in a radial direction. The housing includes n inner projecting parts, the permanent magnets and the inner projecting parts being alternately arranged in a circumferential direction about the rotor. The magnets generate 2n magnetic circuits via the housing and the rotor, n being an integer greater than one. The size of at least a portion of a first air gap formed between the permanent magnets and the rotor is greater than the size of at least a portion of a second air gap formed between the inner projecting parts and the rotor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 200810141634.3 filed in The People&#39;s Republic of China on Jul. 18, 2008, and Patent Application No. 200810181479.8 filed in The People&#39;s Republic of China on Nov. 10, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of electric motors, and in particular to a permanent magnet DC motor. 
     BACKGROUND OF THE INVENTION 
     The permanent magnet DC motor has been widely used in fields such as office automatization, household appliance, and automobile electric parts, for it has the advantages of simple structure, reliable electromagnetic properties, and simple maintenance. 
     Referring to  FIG. 8 , a traditional permanent magnet DC motor usually uses four permanent magnets  101 ˜ 104 , fixed symmetrically inside the iron shell or housing of the motor stator to produce four field poles, thus, the number of permanent magnets equals the number of field poles and each magnetic circuit is formed by two magnets in series. Viewed from outside the stator, the orientation of adjacent magnets is opposite. However, this traditional rotor configuration will not be suitable for low power motors with a small size. When using high remanence rare earth magnets, thin permanent magnets can satisfy the magnetic requirement of the low power motor. However, thin magnets, especially magnets having a wall thickness of less than 2 mm, are very delicate and are easy to be damaged during manufacture and assembly. To thicken the magnets in order to avoid being damaged in assembly will result in low utilization and wastage of the magnet material. As there are two pieces of magnet in series in each equivalent magnetic circuit, and the magnetic field is so strong, the stator housing will be saturated. In other words, the magnet material is wasted. Furthermore, adjacent magnets with opposite magnetic poles will complicate the process of assembling the magnets into the housing. 
     U.S. Pat. No. 4,296,343 discloses an electric motor using two magnets and two inwardly projecting pole pieces of the housing to create four field poles. However, the size of the air gaps between the magnets and the armature is equal to the size of the air gaps formed between the inwardly projecting pole pieces and the armature. The resulting field distribution between the magnet poles and the housing poles is uneven. 
     SUMMARY OF THE INVENTION 
     To solve the above mentioned problems, the present invention provides a motor which includes: a housing; a rotor rotatably mounted in the housing; and n permanent magnets with the same polarity fixed to an inner surface of the housing. The housing includes n inner projecting parts. The permanent magnets and the inner projecting parts are alternately arranged in a circumferential direction about the rotor. The magnets generate 2n magnetic circuits via the housing and the rotor, n being an integer greater than one. The size of at least a portion of first air gaps formed between the permanent magnets and the rotor is greater than the size of at least a portion of second air gaps formed between the inner projecting parts and the rotor. This results in imbalance of Back-EMF between the magnet field poles and the induced field poles being reduced. 
     Preferably, the first air gaps have a constant width and the second air gaps have a constant width. 
     Alternatively, the first air gaps have a constant width, and the second air gaps have a varying width, the size of the first air gaps being greater than the minimum size of the second air gaps. 
     Alternatively, the first air gaps have a varying width, and the second air gaps have a constant width, the maximum size of the first air gaps being greater than the size of the second air gaps. 
     Alternatively, the first air gaps have a varying width, and the second air gaps have a varying width, the maximum size of the first air gaps being greater than the maximum size of the second air gaps. 
     Preferably, the size of the first air gaps formed between the permanent magnets and the rotor is in the range of 0.4 mm to 0.8 mm, and the size of the second air gaps formed between the inner projecting parts and the rotor is in the range of 0.4 mm to 0.6 mm. 
     More preferably, the size of the first air gaps formed between the permanent magnets and the rotor is in the range of 0.4 mm to 0.7 mm, and the size of the second air gaps formed between the inner projecting parts and the rotor is in the range of 0.4 mm to 0.46 mm. 
     Preferably, the motor is a permanent magnet DC motor. 
     Preferably, n is equal to two, and the two permanent magnets form two magnet field poles at the two permanent magnets and two induced field poles at the two inner projecting parts respectively. 
     Preferably, the circumferential width of the permanent magnets is greater than the circumferential width of the inner projecting part, measured in radians. 
     Preferably, inner surfaces of the permanent magnets face the rotor, outer surfaces of the permanent magnets are fixed to an inner surface of the housing, and the inner surface and the outer surface of each permanent magnet are non-concentric. 
     Preferably, the thickness of a middle part of each permanent magnet is greater than the thickness of the circumferential end parts thereof, such that the air gap between the middle part and the rotor is smaller than the air gap between the end parts and the rotor. 
     Preferably, the thickness of the housing is uniform, and the inner projecting parts of the housing are formed by stamping or drawing inwards the corresponding parts of the housing in a radial direction of the housing. 
     Preferably, the rotor is a wound armature having a plurality of radially extending teeth about which an armature winding is wound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. 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 sectional view of a motor according to a first embodiment of the present invention; 
         FIG. 2  is a magnetic flux distribution and field density diagram of the motor of  FIG. 1 ; 
         FIG. 3  shows two Back-EMF waves respectively for one coil of the armature of the motor of  FIG. 1  and one coil of the armature of a prior art motor; 
         FIG. 4  is FFT of the Back-EMF waves of  FIG. 3 ; 
         FIG. 5  is a sectional view, similar to  FIG. 1  of a motor according to a second embodiment; 
         FIG. 6  is a sectional view, similar to  FIG. 1  of a motor according to a third embodiment; 
         FIG. 7  is a sectional view, similar to  FIG. 1  of a motor according to a fourth embodiment; and 
         FIG. 8  is a sectional view of a conventional motor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The technical problem to be solved, the technical solution and the beneficial effects of the present invention are best understood from the following detailed description of preferred embodiments with reference to the accompanying figures. It is to be understood that, the specific embodiments described here are merely examples to explain the invention and are not intended to limit the present invention. 
       FIG. 1  and  FIG. 2  show a DC motor according to a preferred first embodiment of the present invention having a housing  20 , a rotor  40  rotatably mounted in the housing  20 , a pair of permanent magnets  60  diametrically fixed on an inner surface of the housing  20  and located between the housing  20  and the rotor  40 . 
     The housing  20  is made of a magnetically conductive material and has a substantially cylindrical shape. Two parts of the housing  20  between the magnets  60  in the circumferential direction project inwards in the radial direction to form two inner projecting parts  22 . The inner projecting parts  22  may be formed by stamping or drawing inwards the corresponding parts of the housing  20 . 
     The rotor  40  is fixed on a shaft  44 . The rotor  40  is a wound armature comprising a plurality of laminations stacked along the axial direction of the motor. The rotor  40  comprises a plurality of teeth  42  extending in the radial direction of the motor. Windings (not shown) are wound on the teeth  42 . 
     Preferably, the magnets  60  are made of rare earth materials. The inner surface of the permanent magnets  60  face the teeth  42  of the rotor  40 , and the outer surfaces thereof are fixed to the inner surface of the housing  20 . The two permanent magnets  60  and the two inner projecting parts  22  are alternately disposed about the circumference of the rotor  40 . Along the radial direction of the rotor, the air gaps formed between the inner surface of the magnets  60  and the outer surface of the rotor  40  are defined as first air gaps  30 , and the air gaps formed between the inner surface of the inner projecting parts  22  of the housing  20  and the outer surface of the rotor  40  are defined as second air gaps  32 . 
     The two magnets  60  form four magnetic field poles, for example, two magnet field poles N at the two permanent magnets  60  and two induced field poles S at the two inner projecting parts  22 . Each magnet field pole N and each induced field poles S cooperatively form a magnetic circuit passing through the rotor  40 . Thus, there is only one piece of permanent magnet in series in each magnetic circuit which results in making full use of the magnets and the saturation problem of the magnetic circuit is avoided or at least greatly reduced. 
     In order to reduce imbalance in the Back-EMF between the magnet field poles and the induced field poles, the size of at least one portion of the first air gap  30  is greater than the size of at least one portion of the second air gap  32 . Please note, the size of the air gap in the present invention means the width of the air gap measured in the radial direction of the rotor. Also, the air gaps may be constant or varying. By constant we mean that the width of the air gap measured in the radial direction is constant. By varying, we mean that the width of the air gap varies in the circumferential direction, preferably, but not essentially, in a smooth manner. 
     In the first embodiment of  FIG. 1 , the first air gaps  30  and the second air gaps  32  are varying air gaps.  FIG. 5  illustrates a second embodiment, similar to the first embodiment except that the first air gaps  30  and the second air gaps  32  are constant air gaps.  FIG. 6  illustrates a third embodiment, similar to the first embodiment except that the first air gaps  30  are constant air gaps and the second air gaps  32  are varying air gaps.  FIG. 7  illustrates a fourth embodiment, similar to the first embodiment except that the first air gaps  30  are varying air gaps and the second air gaps  32  are constant air gaps. 
     Specifically, when the air gaps  30  are constant and the air gaps  32  are constant, the size of the air gaps  30  should be greater than the size of the air gaps  32 . When the air gaps  30  are constant and the air gaps  32  are varying, the size of the air gaps  30  should be greater than the minimum size of the air gaps  32 . Preferably, the size of the air gaps  30  is greater than or equal to the maximum size of the air gaps  32 . When the air gaps  30  are varying and the air gaps  32  are constant, the maximum size of the air gaps  30  should be greater than the size of the air gaps  32 . When both the air gaps  30  and the air gaps  32  are varying, the maximum size of the air gaps  30  should be greater than the minimum size of the air gaps  32 . Preferably, the maximum size of the air gaps  30  is greater than the maximum size of the air gaps  32 . For example, the size of the air gap  30  is in the range of 0.4 mm to 0.8 mm, and the size of the air gap  32  is in the range of 0.4 mm to 0.6 mm. In one preferred embodiment, the size of the air gap  30  is in the range of 0.4 mm to 0.7 mm, and the size of the air gap  32  is in the range of 0.4 mm to 0.46 mm, i.e., the maximum size of the air gaps  30  is greater than the maximum size of the air gaps  32  and the minimum size of the air gaps  30  is equal to the minimum size of the air gaps  32 . 
     Referring back to the first embodiment as shown in  FIG. 1 , preferably the inner surface and the outer surface of the magnets  60  are non-concentric arcs. The middle part of the permanent magnet  60  is thicker than the two circumferential end parts such that the air gap between the middle part and the armature  40  is smaller than the air gap between the end parts and the armature  40 . Preferably, the thickness of the magnet  60  is gradually reduced from the middle part to the end parts such that the air gap between the magnet  60  and the armature  40  becomes gradually wider from the middle part to the end parts. 
     Preferably, in order to further reduce imbalance of Back-EMF, the angle α subtended by each magnet  60  is greater than the angle β subtended by each inner projecting part  22 . That is, the circumferential extent or width of the magnet poles is preferably greater than that of the induced poles. 
       FIG. 3  shows two Back-EMF waves respectively for one coil of the armature of a motor according to the first embodiment of  FIG. 1 , in which the size of at least one portion of the air gap  30  is greater than the size of at least one portion of the air gap  32  and one coil of the armature of a prior art motor according to U.S. Pat. No. 4,296,343, in which the size of the air gaps  30  and  32  are constant and equal to each other.  FIG. 4  is a FFT (Fast Fourier Transform) of the Back-EMF waves of  FIG. 3 . 
     Referring to  FIG. 3 , the dashed line in  FIG. 3  represents the wave for one coil of the armature of the prior art motor and the solid line represents the wave for one coil of the armature of the first embodiment. It can be seen that the imbalance of Back-EMF in the motor of the first embodiment is reduced compared with the prior art motor, which may be confirmed by viewing the FFT of  FIG. 4 , which shows that the even order harmonic components in the FFT of the Back-EMF waves corresponding to the motor of the first embodiment is less compared with the prior art motor. In  FIG. 4  the checkered columns represent the prior art motor while the hatched columns represent the motor of the first embodiment. 
     In the above embodiments of the present invention, by using half the number of permanent magnet pieces with the same polarizations it is capable of achieving the same number of field poles as compared with the conventional motor, which simplifies the stator assembly process. Furthermore, material utilization ratio of the rare earth magnets is increased and the magnets can be made with a comparatively greater thickness, which reduces the possibility of the magnets being broken during production and assembly of the magnets into the housing. As the orientation of the magnetic field of each magnet is the same, the process of assembling and magnetizing the magnets is simplified. The size of at least one portion of the air gap  30  is greater than the size of at least one portion of the air gap  32 , which results in a reduction of the imbalance of Back-EMF between the magnet field poles and the induced field poles. The width of the magnets being greater than the width of the inner projecting part, measured in radians, is also beneficial to reduce imbalance of the Back-EMF between the magnet field poles and the induced field poles. 
     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. 
     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.