Abstract:
A direct-drive brushless DC motorization apparatus comprises an outer rotor with poles constructed with segments of permanent magnet material alternatively magnetized north and south. The outer rotor is adapted to be part of a wheel and rotating with the wheel about an axis thereof. A stator core of ferromagnetic material is spaced inwardly of said rotor to define a clearance gap with the rotor such that the rotor is rotatable about the stator core. The stator core has forty-two slots and defines teeth therebetween. A three-phase winding with coils of insulated wire is wound around the teeth of the stator core. The three-phase winding is divided in two sets of consecutive teeth for each of the three phases, with each of the two sets of a same phase being diametrically opposed in the stator core.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority of U.S. Provisional Patent Application No. 61/366,956, incorporated herein by reference. 
     
    
     FIELD OF THE APPLICATION 
       [0002]    The present application relates to the motorization of electric vehicles using permanent-magnet synchronous machine such as brushless DC machines, for electric vehicles such as bicycles, rolling chairs, scooters, tricycles, golf carts, trolleys and small utility vehicles, or the like. 
       BACKGROUND OF THE ART 
       [0003]    In lightweight vehicle applications, it is often desired to produce high starting torque and to ensure variable assistance, whereby electric machines are well suited therefor as opposed to other types of motors or engines. The use of a brushless direct-current (DC) machine is particularly well adapted to these kinds of applications. Some brushless DC machines use a permanent magnet external rotor with three phases of driving coils on the stator core. According to one type of machine, the position of the rotor is sensed with sensors (e.g., Hall effect sensors) and the associated drive electronics. The coils are activated by the drive electronics, based on the detection signals from either the sensors or from the back electromotive force (EMF). 
         [0004]    Brushless DC machines are relatively simple in construction, and cost-effective in maintenance in view of their brushless nature. However, there remains a need to produce increasingly efficient brushless DC machines, in a cost-efficient manner. For instance, the weight of the machine components must be minimized, while not affecting the structural integrity of the machine. 
       SUMMARY OF THE APPLICATION 
       [0005]    It is therefore an aim of the present disclosure to provide a new brushless DC machine for vehicles. 
         [0006]    It is a further aim of the present disclosure to have a brushless DC machine that is relatively lightweight. 
         [0007]    Therefore, in accordance with the present application, there is provided a direct-drive brushless DC motorization apparatus comprising: an outer rotor with poles constructed with segments of permanent magnet material alternatively magnetized north and south, the outer rotor adapted to be part of a wheel and rotating with the wheel about an axis thereof; a stator core of ferromagnetic material spaced inwardly of said rotor and defining a clearance gap with the rotor such that the rotor is rotatable about the stator core, said stator core having forty-two slots and defining teeth between said slots; and a three-phase winding with coils of insulated wire being wound around the teeth of the stator core, the three-phase winding being divided in two sets of consecutive teeth for each of the three phases, with each of the two sets of a same phase being diametrically opposed in the stator core. 
         [0008]    Further in accordance with the present application, the three-phase winding are divided into two sets of seven consecutive teeth for each of the three phases. 
         [0009]    Still further in accordance with the present application, each said phase of the three-phase winding is divided into sets of six and eight consecutive teeth. 
         [0010]    Still further in accordance with the present application, the outer rotor comprises forty poles. 
         [0011]    Still further in accordance with the present application, the outer rotor comprises forty-four poles. 
         [0012]    Still further in accordance with the present application, the stator core has a hub supporting a stator yoke, wherein the hub comprises structural elements extending diametrically between centers of each said set of the same phase. 
         [0013]    Still further in accordance with the present application, the hub has a six-pointed star shape, with an interconnection between points of the star shape and the stator yoke being in alignment with a center of each said set of teeth. 
         [0014]    Still further in accordance with the present application, each said set comprises seven consecutive teeth. 
         [0015]    Still further in accordance with the present application, the stator is fixed to an axle of the wheel. 
         [0016]    Still further in accordance with the present application, the rotor is adapted to be operatively connected to a freewheel of a vehicle to rotate therewith in one rotational orientation. 
         [0017]    Still further in accordance with the present application, the rotor is adapted to be connected to spokes of a wheel, with the spokes projecting radially from a casing of the rotor. 
         [0018]    Still further in accordance with the present application, each said phase comprises 14 teeth. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic diagram of a brushless DC machine system including a brushless DC machine, a power electronics system and current control system; 
           [0020]      FIG. 2  is a side elevation view of a brushless DC machine in accordance with the present disclosure, with a housing removed to show an interior thereof; 
           [0021]      FIG. 3  is a schematic diagram of phase interconnections for the brushless DC machine of  FIG. 2 ; and 
           [0022]      FIG. 4  is a perspective view of a stator core of the brushless DC machine of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Referring to the drawings and more particularly to  FIG. 1 , there is illustrated a brushless DC machine system of the type used in lightweight vehicles, operable in a motor mode and in a generation mode. In one embodiment, the brushless DC machine system is used as a power assistance unit in a bicycle, to add power to the pedaling effort of the rider. Other uses are considered as well, for instance with the use of the brushless DC machine system as the sole motorization unit of vehicles as opposed to being a power assistance unit. The vehicles may include rolling chairs, scooters, tricycles, golf carts, trolleys and small utility vehicles, or the like. 
         [0024]    According to one embodiment, the brushless DC machine system includes a 3-phase permanent magnet brushless DC machine  10 , a power supply circuit  11 , a rotor position detector  12  (e.g., with Hall effect sensors), a current measurement system  13  and/or a current regulation system which is comprised of a current control circuit  14  fed by the current measuring circuit  13  and a torque reference or current reference circuit  16 . The current control circuit  14  is connected to the power supply circuit  11  to control the torque of the machine  10 . 
         [0025]    It is pointed that the machine  10  is identified as being a 3-phase permanent magnet brushless DC machine. This generic identification is deemed to also include permanent magnet synchronous machines, for instance operated with sinusoidal waveform current, or the like. Moreover, reference to a brushless DC motor also covers the use of the machine or motor  10  in a motorization mode and in a generator mode. 
         [0026]    The afore-described system is a conventional operating system for a brushless DC motor, and is one among numerous other systems that may be used to operate the brushless DC machine  10 . However, the system of  FIG. 1  can be used for motor or generator operation with energy recuperation on a battery  15  during braking periods. The afore-described system may be operated with a rectangular waveform current in the winding phases, for cost efficient operation. In such a case, the motor torque is controlled by a simple current regulation and the phase voltage is chopped with a pulse width modulation technique (PWM). Other operation modes are considered as well, such as an operation with sinusoidal waveform current. 
         [0027]    Referring to  FIG. 2 , there is illustrated the brushless DC machine  10 , with a housing and other parts removed to show an interior structure of the machine  10 . The brushless DC machine  10  of  FIG. 2  may be used in a direct-drive configuration, with a rotor of the machine  10  being integral with a wheel of the vehicle powered by the machine  10 . In the direct-drive configuration, the machine  10  has a stator core  20  that is secured to an axle of the wheel of a vehicle, or is generally immovable with respect to an axis of the wheel. The yoke of the stator core  20  has forty-two slots  21 , separated by teeth  22 , typically made of iron (i.e., ferromagnetic material). Although not shown, coils of insulated wire are wound about at least some of the teeth  22 , in accordance with a phase interconnection described below. 
         [0028]    A cylindrical outer rotor  30  is mounted about the stator core  20 , and is separated from the stator core  20  by a suitable clearance gap. The rotor  30  is supported for instance by the casing of the machine  10  (which itself rotates about the axle), and is rotatable with respect to the axis of the wheel. A remainder of the wheel projects radially from the rotor  30  and supports a tire. In an embodiment, spokes of the wheel project radially or peripherally from an outer surface of the rotor  30 . The rotor  30  is constituted of segments of permanent magnets  31  mounted on the rotor inner surface and alternatively magnetized north and south. In  FIG. 2 , there is illustrated forty-four of the permanent magnets  31 , although forty magnets may be used as well with the forty-two slots  21  of the stator core  20 . 
         [0029]    The high number of poles reduces the iron volume. By increasing the number of poles, the flux per pole during operation is reduced as compared with a machine producing a similar power output with a lesser amount of poles. Accordingly, as the sectional dimensions of teeth are proportional to the flux, the sectional dimensions for a forty-two pole machine are smaller than the sectional dimensions for the teeth of a machine with fewer poles, for a similar power output. There results a lower weight for the forty-two pole machine when compared to machines having a fewer amount of poles for a similar power output. 
         [0030]    Referring concurrently to  FIGS. 2-3 , an example of a phase interconnection of the machine  10  is shown. The teeth  22  are regrouped in two continuous sets of teeth per phase, as shown by sets A, B, and C. According to one embodiment, each set comprises seven consecutive teeth  22 . However, other arrangements of sets may also be used, for instance phases each consisting of a set of six and a set of eight consecutive teeth  22 . It is also considered to have other phase configurations, for instance with two sets of six consecutive teeth  22 , two sets of seven consecutive teeth  22 , and two sets of eight consecutive teeth  22 , as an example. Any appropriate number of consecutive teeth per set for a total of six sets may be used. By the arrangement of six sets of teeth with two sets per phase, it is observed that the two sets of a same phase are diametrically opposed in the stator core  20 , as shown by lines A-A, B-B, and C-C. In the embodiment featuring seven consecutive teeth per set, the centers of the sets of a same phase are diametrically opposed. Accordingly, the magnetic forces to which are exposed the sets of teeth  22  operated in a same phase oppose each other and minimize their effect on the center of the stator core  20 . In the illustrated embodiment of  FIG. 3 , a neutral line is also interconnected to the phases. With the 3-phase interconnection described above, the above-referred phase interconnections and components of the system of  FIG. 1  may be off-the-shelf products. 
         [0031]    Referring to  FIG. 4 , the stator core  20  is shown with the coils  40  of insulated wire wound on the teeth  22 . There are two coils per slot  21 , although other suitable configurations may be used as well in the machine  10 . Adjacent coils  40  of a same set are wound in opposite directions, as shown for instance by A 1  and A 2 . 
         [0032]    The interconnection of phases and the coil winding may be any other appropriate alternative. For instance, there may be used a single coil per slot  21 . Also, as an alternative to the interconnection shown in  FIG. 3 , other interconnections could also be used, such as a delta interconnection. 
         [0033]    Still referring to  FIG. 4 , a hub of the stator core  20  is illustrated at  50 . In the embodiment of  FIG. 4 , the hub  50  is shaped as a six-pointed star, with a cylindrical housing portion centrally located in the six-pointed star. The star hub  50  is oriented so as to have the points in central alignment with each set of teeth. Accordingly, when the teeth  22  of any given set (i.e., for phase A, B or C) are subjected to forces resulting from the magnetic fields, the central position of the points of the star hub  50  allows the uniform distribution of the load on the center of the hub  50 . It is considered to use other types and shapes of hubs as well. 
         [0034]    The brushless DC machine structure described in  FIGS. 2-4  is well suited to be used in a direct-drive configuration for lightweight vehicles, such as bicycles. More specifically, the 42-pole arrangement is relatively lightweight compared to machines with similar power output but with fewer poles. The 42-pole arrangement on the other hand has greater diameter than machines with fewer poles, whereby the resulting machine is well suited to be wheel-mounted, as bicycle wheels commonly have large diameters (e.g., 26 inch-29 inch, 650 mm-700 mm). In the direct-drive configuration on a bicycle, the rotor may be operatively connected to a freewheel, such that pedaling actuation is transmitted to the rotor via the cassette on the freewheel. On the other hand, in the absence of a pedaling input, the freewheel allows idling of the cassette while the machine  10  may actuate the wheel. As an alternative, the direct-drive configuration may be used for the front wheel of a bicycle. 
         [0035]    The brushless DC motor structure described in  FIGS. 2-4  is also well suited for applications including electric vehicles, such as bicycles, rolling chairs, scooters, tricycles, golf cars, trolleys and small utility vehicles, or the like. The brushless machine  10  has a relatively high number of teeth  21  in the stator core  20 . However, in the above-referred applications, the stator cores  20  typically have diameters ranging between 150 mm and 350 mm, for example. In such ranges, the teeth  21  have a suitable thickness in terms of structural integrity.