Abstract:
A brushless direct current (BLDC) motor has a 3-phase winding  20  and six stator teeth  14, 15  with alternate stator teeth  14  being wound and the remaining stator teeth  15  being left unwound. The winding  20  has three legs, one for each phase and each leg has one coil  22  wound about one of the stator teeth  14 . Each leg has a first end A,B,C, arranged to receive electrical power and a second end X,Y,Z, which is connected to the second end of the other legs to form a star connection  24 . Selected stator teeth have grooves in a face thereof dividing those teeth into a plurality of stator poles. The motor may be used to drive a fuel pump for an internal combustion engine, typically for a vehicle.

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. 200810141852.7 filed in The People&#39;s Republic of China on Sep. 3, 2008. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a brushless motor and in particular, to a fuel pump having a brushless motor, especially a brushless direct current (BLDC) motor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fuel pumps are used in motor vehicles to transfer liquid fuel, typically gasoline or diesel from a fuel tank to an internal combustion engine. The pump is driven by a small DC motor and to minimize fuel leakage through bearing seals etc, the fuel passes through the interior of the motor. This works very well even with motors having commutators, with the fuel cooling the motor and eliminating sparking between the brushes and the commutator. However, with the advent of high alcohol fuels, chemical reactions between the commutator and the fuel has become problematic leading to the use of graphite commutators and renewed interest in brushless motors to drive the fuel pumps. There are many advantages of brushless motors, especially in automobile applications, such as longer life by eliminating the use of brushes and a commutator. Typically, the brushless motor may be a single phase motor or a three phase motor. Traditionally, these motor have been termed as brushless direct current motors or BLDC motors for short as often the input power to the motor controller is DC power, typically from a battery or rectified AC supply. However, recently the term brushless AC motor or BLAC motor has been coined. This motor is a special type of BLDC motor in which the controller sends power to the motor in the form of a sinusoidal wave instead of a pulse or square wave. However, many people still use the term BLDC to include both types of motors as the difference is in the type of controller. That said, certain modifications are usually made to make the brushless motor more efficient with one or the other type of controller. For the sake of simplicity, we will refer to both types of brushless motors by the generic term BLDC motor or simply as a brushless motor. 
         [0004]    For existing three phase, 4-pole BLDC motors available in the marketplace, including outer rotor and inner rotor models, 6 slots is its most popular and simplest stator lamination structure in the low power applications.  FIG. 1  illustrates a prior art schematic winding diagram for a 3-phase BLDC motor. The stator  12  has a stator core  13  with six stator poles  14 , referred to as slots. The rotor  16  has four magnetic poles  18  formed by four permanent magnets fixed to the outer surface of a rotor core  17  (in known manner). The winding  20  forms a coil  22  about each stator pole  14 . The winding  20  is a 3-phase star winding, meaning that the winding  20  has three legs or phases, with one end (A,B,C) of each leg being connected to the stator terminals (one for each phase) and the other end (X,Y,Z) of each leg being connected together at point  24  to form a star connection. Hence the motor is referred to as a three phase, four pole, six slot BLDC motor. In this geometry there are six coils, two coils for each phase. Thus each leg has two coils  22  electrically connected in series. This is difficult to wind in small diameter motors. 
         [0005]    The main problem with the existing stator geometry is that the winding configuration is complicated for small diameter, lower power applications, such as the automotive fuel pump, water pump and air pump, etc. In these applications, the stator inner diameter is very small, just around 20 mm, therefore it is difficult to assemble more coils, especially for mass production. 
         [0006]    Another problem with existing products is the high cogging torque, which creates noise and vibration. This has restricted the use of BLDC motors in many fields which need low noise and low vibration. In order to solve this problem, one of the most effective methods is the 4 pole, 9 slot configuration, as shown in  FIG. 2 . From this figure we can see that the winding becomes even more complicated with three coils per phase and a lower efficiency caused by longer winding end-turns. 
         [0007]    Another problem with existing products is that it is not suitable for overmold technology in the fuel pump application. As diverse fuels will be used in the future, such as alcohol containing fuels, etc, to avoid oxidation of the magnetic wire, overmold is desired. However, this results in the stator becoming a solid body, i.e., there is no space for fuel to pass through, except through the air gap between the rotor and the stator. However, to maintain motor efficiency, the air gap is very small with the result that the fuel flow is insufficient. 
       SUMMARY OF THE INVENTION 
       [0008]    Hence there is a desire for a brushless direct current (BLDC) motor which has a simple winding structure with low cogging torque. 
         [0009]    This is achieved in the present invention by using a multiple phase stator (i.e. 2, 3 or more phases) with only a single coil for each phase. 
         [0010]    Accordingly, in one aspect thereof, the present invention provides a brushless direct current motor comprising: a rotor having 2*m permanent magnet rotor poles, where m is an integer; and a stator having 2*n stator teeth and an n-phase stator winding with each leg of the stator winding having a single coil wound about a respective one of the stator teeth, where n is an integer greater than 1, wherein the stator teeth form wound teeth having a coil there on and unwound teeth having no coil, wherein the wound teeth are alternately spaced with the unwound teeth, and each wound tooth or each unwound tooth or each stator tooth, has at least one axially extending groove formed in a face thereof dividing each said tooth into a plurality of stator poles. 
         [0011]    Preferably, each wound tooth forms two stator poles and each unwound tooth forms one stator pole. 
         [0012]    Alternatively, each wound tooth forms three stator poles. 
         [0013]    Preferably, each unwound tooth forms two stator poles. 
         [0014]    Preferably, the stator has a core formed from a stack of laminations and the core has a radially outer surface having a number of recesses extending axially of the core. 
         [0015]    Preferably, the recesses are circumferentially aligned with selected stator teeth. 
         [0016]    Preferably, there are n recesses, each aligned with a respective one of the unwound teeth. 
         [0017]    Preferably, the stator core has a stator ring and a number of separately formed stator plugs fixed thereto, each plug forming a wound tooth. 
         [0018]    Preferably, the stator winding of the motor is encased in a resin material. 
         [0019]    Preferably, the stator of the motor is overmolded with a plastics material. 
         [0020]    According to a second aspect thereof, the present invention provides a fuel pump for an internal combustion engine, incorporating a motor as described above. 
         [0021]    Preferably, the fuel pump has a housing accommodating a pump and the motor, the motor being arranged to drive the pump to cause fuel to flow through the housing, and at least one passage for the flow of fuel is formed between the stator and the housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    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 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. 
           [0023]      FIG. 1  is a wiring diagram for the stator of a prior art 3-phase, 6 slot, 4 pole BLDC motor; 
           [0024]      FIG. 2  is a wiring diagram for the stator of a prior art 3-phase, 9 slot, 4 pole BLDC motor; 
           [0025]      FIG. 3  is a wiring diagram for the stator of a 3-phase, 9 slot, 4 pole BLDC motor according to a first embodiment of the present invention; 
           [0026]      FIG. 4  is a diagram showing a comparison of cogging torque between a typical prior art 3-phase, 6 slot, 4 pole BLDC motor and a size comparable 3-phase, 9 slot, 4 pole BLDC motor according to an embodiment of the present invention; 
           [0027]      FIG. 5  is a wiring diagram, similar to  FIG. 3 , for a modified stator according to a second embodiment of the present invention; 
           [0028]      FIG. 6  is a perspective view of the stator of  FIG. 5 ; 
           [0029]      FIG. 7  is a sectional view of a fuel pump, incorporating the stator of  FIG. 6 ; 
           [0030]      FIG. 8  is a perspective view of an assembled stator core for a 9 slot stator according to another embodiment of the present invention; 
           [0031]      FIG. 9  is a perspective view of a stator ring, being a part of the stator core of  FIG. 8 ; 
           [0032]      FIG. 10  is a perspective view of a tooth, being a part of the stator core of  FIG. 8 ; 
           [0033]      FIG. 11  is a wiring diagram similar to  FIG. 5 , for a modified stator winding according to another embodiment of the present invention; and 
           [0034]      FIG. 12  is a wiring diagram similar to  FIG. 11 , for a modified stator according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]      FIG. 3  is a schematic winding diagram for a 3-phase BLDC motor for a first preferred embodiment. The stator  12  has a stator core  13  with six teeth  14 ,  15  forming the stator poles as will be described later. The winding  20  has only three coils  22  formed about alternate teeth  14 . The winding  20  is a 3-phase star winding having three legs, one leg for each phase, with one end A,B,C, of each leg being connected to the stator terminals (one for each phase) and the other end X,Y,Z, of each leg being connected together at 24 to form a star connection. Thus each leg has only one coil  22 . This is easier to wind especially in small diameter motors. However, the wound teeth  14  have a larger circumferential extent than the unwound teeth  15  and have a deep groove  26  in the pole face which extends axially for the length of the tooth  14  and radially outwardly into the tooth, dividing the pole face into two, preferably equal, portions. The groove  26  has the effect of dividing the tooth  14  into two stator poles and forming a dummy slot. Thus the stator effectively has 9 slots or 9 stator poles. The grooves  26  are referred to as dummy slots as no coils are would into the dummy slots. Thus the stator has a simple winding, more simple than the winding of the motor of  FIG. 1  or  FIG. 2 . 
         [0036]      FIG. 4  shows a comparison between the cogging torque of the motor of  FIG. 3  and a comparably sized motor according to  FIG. 1 . Curve  56  represents the cogging torque for the prior art motor and curve  37  represents the cogging torque of the motor of the preferred embodiment. As can be seen the cogging torque has been reduced from about 14 mNm to about 2.5 mNm, a reduction of about 80%, and a decrease in the angular displacement per cycle from 30° to 200, which has the effect of further smoothing the rotation of the rotor. This confirms that the stator is operating as a 9 slot motor. 
         [0037]      FIG. 5  is a schematic winding diagram for a 3-phase BLDC motor for a second preferred embodiment. The wiring diagram also shows the shape of the stator core. The stator  12  has a similar construction to that of  FIG. 3  with the exception that the radially outer surface of the stator core  13  has a number of axially extending recesses  28 . Recesses  28  form passages, between the stator core  13  and the housing  42  in which the stator core  13  is fixed. In a fuel pump application, fuel can flow can flow through these passages, greatly reducing the resistance of the fuel path through the motor and thus reducing the energy required to pump the fuel through the motor. The recesses  28  are shown aligned with the non-wound teeth  15 . This is thought to have no negative impact on the magnetic circuit of the stator while allowing maximum space for the coils  22  formed on the wound teeth  14 . 
         [0038]    The completed stator  12  and rotor  16  set is illustrated in  FIG. 6 . After plating the stator core  13  to increase resistance to corrosion, the stator  12  is over molded with a plastics material or resin material  30 , preferably by an insert molding operation. The pole faces and the radially outer surface of the stator core  13  is not covered with the over mould material. This ensures a good transfer of magnetic flux between the pole faces of the stator and the rotor and also allows a good fit with the motor housing in which the stator core is preferably a press fit. The laminations of the stator core  13  may be held together by welding. This is preferably done by welding together a small nub  32  formed for this purpose in a cut-out  33  in the outer surface of the stator core aligned with the wound teeth  14 , shown in  FIG. 5 . During over molding, this cut-out  33  is filled with mould material to protect the weld. This over mould material forms the strip  31 , which can be seen in  FIG. 6  on the outer surface of the stator core connecting the ends of the stator. 
         [0039]    The windings may be connected to stator terminals for connection to a controller or directly to motor terminals and where used the terminals would also have exposed parts (not shown) not covered by the over molding for making further electrical connections. The rotor core  17  is also shown as being over molded to protect the rotor core from the fuel. The over mould material  30  also helps the efficiency of the fuel pump by making a smooth path for the flow of the fuel and by smoothing the outer surface of the rotor to reduce windage, the resistance created by rotating a non-smooth surface or body having salient features. 
         [0040]      FIG. 7  illustrates a complete fuel pump  40  in sectional view. The fuel pump has a housing  42  of cylindrical form with two open ends which are sealed by end caps  44 ,  45  which connect the fuel pump to the fuel lines. The housing accommodates a pump section  46  and a motor section  50 . The pump section  46  includes an impeller  47  arranged to be rotated by the motor within a volute  48  to draw fuel into the pump section from a fuel inlet  49  in the first end cap  44  and force the fuel through the motor section  50  and out a fuel outlet  51  in the second end cap  45 . The motor section  50  houses the motor which includes the stator  12  which is pressed into the housing  42 , and the rotor  16  with the rotor core  17  located within the stator  12  and a rotor shaft  19  which is journalled in bearings in the pump volute  48  at one end and in the second end cap  45  at the other end. The second end cap is shown being of two parts, a first part sealing the housing and forming the fuel outlet and also the connector for the electrical power to operate the motor, and a second part supporting the bearing for the rotor shaft. The motor section  50  may include an electronics module to accommodate the electronics for operating the BLDC motor; however, in this embodiment the electronics module is either not shown or not mounted inside the fuel pump. 
         [0041]    The fuel flow path of the pump is in through the inlet  49  in the first end cap  44 , into the pump volute  48 , where it is pumped out by the impeller  47  into the interior of the housing  42 , passed the motor by passing through the gap between the rotor core  17  and the stator core  13  or through the passages  52  formed between the recesses  28  in the stator core  13  and the housing  42 , into the second end cap  45  and out of the pump though the outlet  51  of the second end cap  45 , as illustrated in  FIG. 7  by block arrows  60 . 
         [0042]    Although the structure of the stator of  FIG. 3  provides a much simplified winding, the winding slots can become very full making winding, especially automated winding, difficult or impossible. With this in mind, another embodiment is illustrated in  FIGS. 8 to 10 .  FIG. 8  is a perspective view of the stator core  13 , which is formed of four parts, a stator ring  70  as shown in  FIG. 9  and three stator plugs  71 , as shown in  FIG. 10 . 
         [0043]    The stator ring  70  forms the outer surface of the stator core  13  and is the flux return path. The stator ring  70  also forms the unwound teeth  15 . The stator recesses  28  are formed in the radially outer surface at locations corresponding to the unwound teeth  15 . Between the unwound teeth on the radially inner surface of the stator ring  70  are seats  72 . Each seat has two edges  74  and an axially extending ridge  76 . 
         [0044]    The stator plugs  71  form the wound teeth  14 . Each plug  71  has the wound tooth  14  and a foot  73 . The tooth  14  has a pole face with an axially extending groove  26 , dividing the tooth into two poles. The foot  73  has edges  75  forms a flange like circumferential extension to the radially outer side of the plug, opposite the pole face, and forms with the pole face a winding channel for a coil of the winding. The foot  73  has an axially extending trough  77 . 
         [0045]    During assembly, each plug  71  is wound with a coil of the stator winding and then assembled to the stator ring  70 . Each plug  71  is fitted to the stator ring  70  by locating the foot  73  in the seat  72  with the ridge  76  and the trough  77  keying the two parts together with the seat edge  74  and the foot edge  75  assisting to locate the parts. Optionally, the foot may be a tight fit in the seat to temporarily hold the parts together before final joining, preferably by laser welding or similar. Alternatively, the plug can be form locked to the ring. The ring  70  and the plugs  71  are formed by stamping and stacking laminations of steel, especially electrical steel. 
         [0046]    One advantage of this construction is that the winding channel or slots can be filled to a very high fill percentage giving more choices to the motor designer and allowing the possibility of reducing the overall size of the outer diameter of the motor. Another advantage is that the winding gap, i.e. the width of the slot between adjacent pole faces through which the wire must pass in conventional winding methods, can be smaller than the minimum necessary for automated winding methods in conventional stators which allows for further reduction in cogging torque. 
         [0047]    Thus the present invention provides a novel construction for a BLDC motor which has embodiments particularly suited to use in a fuel pump. Embodiments of the BLDC motor can achieve a high winding efficiency of up to 0.98. The BLDC motor of the present invention has a simplified stator winding process for a low power BLDC motor. For the fuel pump, the provision of fuel passages and the reduction in cogging torque is considered an advantage. Certain embodiments are ideally suited to mass production, even for motors having a small diameter. 
         [0048]    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. 
         [0049]    For example, a 4-pole, surface mounted PM (permanent magnet) BLDC rotor is shown, but the invention is also suitable for other variations, such as 6-poles, 8-poles, . . . , 2*m pole (where m is an integer), and inset PM geometry. The invention covers BLDC motors with just with one coil for each phase and is applicable to multi-phase motors, i.e. to motors having 2, 3, 4 or more phases. Therefore this invention is suitable not only for fuel pump applications, but also for other applications. 
         [0050]    While the preferred embodiments show and describe the axially extending grooves being formed only in the wound teeth of the stator, the unwound teeth may have axial grooves, as well as or instead of the wound teeth, thus forming multiple stator poles in the same way as described for the wound teeth. It is convenient to think of the stator teeth as being divided into two sets, the set of wound teeth and the set of unwound teeth. It is possible for all of the stator teeth to have axially extending grooves to form multiple stator poles and it is possible for each of the unwound teeth to form a different number of stator poles to that of each of the wound teeth. However, it is desired for motor symmetry that each tooth of the same set forms the same number of stator poles. The preferred embodiment is for each tooth of the set of wound teeth to form two stator poles, while each tooth of the set of unwound teeth forms a single stator pole.  FIG. 12  illustrates an embodiment in which there are three wound teeth  14  and three unwound teeth  15 , with each wound tooth  14  divided into three stator poles by two axially extending grooves  26  and each unwound tooth  15  divided into two stator poles by one axially extending groove  26 . Hence the motor is referred to as a three phase, four pole, fifteen slot BLDC motor. 
         [0051]    While the preferred embodiment shows the stator winding as being connected in Star, in which one end of each coil or phase winding is connected together to form a common point, the stator winding could be connected in Delta. Indeed, a Delta winding configuration does offer some advantages by simplifying the winding connections as shown in  FIG. 11 . In the Delta configuration of a three phase winding, each phase winding is connected to the other two phase windings. Thus, during winding the wire is connected to a first stator terminal A, wrapped about a first stator tooth to form the first phase winding, connected to a second stator terminal B, wrapped about a second stator tooth to form the second phase winding, connected to a third stator terminal C, wound about a third stator tooth to form the third phase winding and finally connected back to the first stator terminal A. The wire is only cut after being connected to the first stator terminal for the second time, simplifying the winding by eliminating the common Star connection point. 
         [0052]    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.