Patent Publication Number: US-2022224199-A1

Title: Electrical machine

Description:
This invention relates to an electrical machine such as a motor or generator and more particularly but not solely to an electrical generator for an unmanned aerial vehicle. 
     Unmanned aerial vehicles or so-called UAVs are lightweight and can travel substantial distances. UAVs generally comprise a small and lightweight internal combustion engine which has the ability to start and stop in flight. In order to achieve this and to provide electrical power to components of the UAV, the internal combustion engine is typically coupled to an electrical generator which charges a battery that can power a starter motor and other electrical components of the UAV. In some cases, the starter motor also acts as the electrical generator. 
     Most failures of electric generators and motors are attributed to overheating issues. Therefore, it is well known that cooling of an electric generator or motor is an essential part of the design of the machine. The components of the machine that produce the most heat are the windings, the electrical steel laminations, other conductors and sometimes magnets (due to eddy current losses). Therefore, maintaining the recommended operational temperatures of components in the prescribed ambient environment is an essential part of the design. 
     Normally, the cooling of an electric machine is carried out via the machine casing or housing. Heat from the components, like the winding mostly escape via conduction and radiation into the casing or housing. At the casing or housing, the heat is dissipated via its outer surface fins or liquid cooling pipes. Heat at the surface of the casing can also be dissipated using an external fan or fan mounted on the shaft of the electric machine. A problem with an external fan mounted on the shaft is that it occupies a large amount of space. This is not a problem where size and weight of the electric machine is irrelevant. However, in applications such as UAVs, where size and weight are crucial, an innovative way of cooling is needed. 
     Thus, in accordance with the present invention as seen from a first aspect, there is provided an electrical machine comprising a stator having a plurality of windings and a rotor having a plurality of permanent magnets arranged to rotate around the windings, wherein the magnets are mounted to an outer portion of a rotor body, the rotor body further having an inner hub portion and a plurality of circumferentially-spaced arms which extend radially from the hub portion to the outer portion of the body, at least one of the arms comprising a vane which causes an axial airflow as the rotor rotates. 
     In use, the axial airflow flows over the windings and helps to prevent overheating without the need for any additional cooling. Since the cooling is conveniently provided by part of the rotor, the size of the machine is no greater than conventional machines. 
     The outer portion of the rotor body may be annular such that it surrounds the stator, the plurality of circumferentially-spaced arms extending radially from the hub portion to one end of the annular outer portion of the body. The rotor body thus forms an enclosure around the outside and one end of the stator. 
     Each arm may comprise a vane to increase the airflow. 
     The vane may extend substantially the full radial length of the arm. 
     Each vane may direct air towards the windings as the rotor is rotated in a normal direction of rotation. 
     The pitch of the or each vane may be adjustable. The or each vane may be removable or replaceable. 
     The stator may comprise a plurality of radially-extending coils, each coil extending around a core of magnetisable material which extends radially from a yoke. 
     The cores and yoke may be formed by laminated magnetisable members of an identical shape. 
     The yoke may be annular, a heatsink being disposed inside the yoke and thermally connected thereto, the or each vane being arranged to cause an airflow over the heatsink. Airflow passageways may extend between the heatsink and the yoke. 
     The heatsink may be annular and the hub portion of the rotor body may extend into the centre of the heatsink, the hub portion being arranged to connect to a rotary shaft which either rotates the rotor (in the case of a generator) or which is rotated by the rotor (in the case of a motor). 
     The heatsink may be annular and the hub portion of the rotor body may extend into a hollow centre of the heatsink. 
     The hub portion of the rotor body may be rotatably mounted to the heatsink by a bearing. The heatsink also serves to dissipate heat from the bearing. 
     Also in accordance with the present invention, as seen from the first aspect, there is provided an assembly comprising an internal combustion engine having a body, a rotary shaft extending from the body and an electrical machine as hereinbefore defined, the stator being fixed relative to the engine body and the rotor being mounted on the shaft. 
     The or each vane may be arranged to direct air towards the internal combustion engine, thereby ensuring that hot air from air from around the internal combustion engine is not directed at the stator. 
     The stator may be mounted between the internal combustion engine and the circumferentially-spaced arms of the hub portion. 
     Since the stator and rotor are respectively mounted to the engine body and the engine shaft, being components of the engine which are spatially fixed relative to each other, it is possible to omit the bearing by mounting the stator to the engine body and by mounting the rotor to the engine shaft. 
     Also in accordance with the present invention, as seen from a second aspect, there is provided a method of mounting an electrical machine to a supporting apparatus having a body and a rotary shaft extending from the body, the method comprising securing a stator of the electrical machine to the body of the apparatus, fitting an alignment tool to the stator and displacing the rotor axially along the tool into engagement with the shaft, securing the rotor to the shaft and removing the tool. 
     The use of the tool ensures that a sufficient gap is provided between the rotor and the stator to ensure free rotational movement of the rotor relative to the stator. 
     The method may initially comprise mounting the rotor on the tool and then engaging the tool with the stator. 
     The method may comprise inserting fingers of the tool through a hub of the stator and engaging the end of the fingers with the stator. 
    
    
     
       Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which: 
         FIG. 1  is a front view of an embodiment of electrical generator in accordance with a first aspect of the present invention; 
         FIG. 2  is a perspective view from the front and the right side of a rotor of the electrical generator of  FIG. 1 ; 
         FIG. 3  is a perspective view from the front and the right side of the electrical generator of  FIG. 1 ; 
         FIG. 4  is a perspective view from the rear and the left side of the electrical generator of  FIG. 1 , when the coil windings are not fitted; 
         FIG. 5  is a right-side view of the electrical generator of  FIG. 1 ; 
         FIG. 6  is a sectional view through an assembly in accordance with the first aspect of the present invention comprising an internal combustion engine and the electrical generator of  FIG. 1  fitted thereto; 
         FIG. 7  is a perspective view from the front and the right side of a rotor of an alternative embodiment of electrical generator in accordance with the first aspect of the present invention; and 
         FIG. 8  is a sectional view illustrating the method in accordance with the second aspect of the invention of mounting the electrical generator of  FIG. 1  to an internal combustion engine. 
     
    
    
     Referring to the  FIGS. 1 to 6  of the drawings, there is shown an embodiment of electrical generator  10  comprising a stator  11  having a laminated stator body  12 . The stator body  12  comprises plurality of circumferentially-spaced finger portions  13  which extend radially outwardly from an annular yoke portion  14 . The stator body  12  is formed of identically-shaped one-piece sheets of ferromagnetic material, which are mounted together in a stack that extends axially of the stator  11 . 
     A coil winding  25  is disposed around each of the fingers  13 . The outer ends of each finger  13  comprise an enlarged head forming respective stator teeth  15 . An annular heatsink  16  is disposed inside the yoke  14  of the laminated stator body  12  and comprises fins  17  which extend radially outwardly and which contact the radially inner face of the yoke  14 . The heatsink  16  also comprises apertured lugs  18  which extend radially outwardly and which are bolted to the axially inner face of the yoke  14  of the laminated stator body  12 . 
     The electrical generator  10  further comprises a rotor  19  having a one-piece rotor body  20  of aluminium. The rotor body  20  comprises an annular central hub portion  21  and a plurality of circumferentially-spaced arms  22  which extend radially from the hub portion  21  to an annular outer portion  23  of the rotor body  20 . The annular outer portion  23  of the rotor body  20  circumferentially surrounds the stator  11  and the plurality of circumferentially-spaced arms  22  connect to the annular outer portion  23  at one axial end thereof, so that the rotor body  20  forms an enclosure around the outside and front face of the stator  11 . A plurality of permanent magnets  26  are disposed around the radially inner face of the annular outer portion  23  of the rotor body  20 . The permanent magnets  26  are circumferentially spaced from each other by a slot  27 . A radial gap of approximately 0.8 mm is provided between the radially outer ends of the stator teeth  15  and the radially inner face of the rotor magnets  26 . 
     In accordance with the first aspect of the present invention, each radially-extending arm  22  of the rotor body  20  comprises a vane  24  which extends substantially the full radial length of the arm  22 . 
     A bearing  28  is disposed between the radially outer surface of the hub portion  21  of the rotor body  20  and the radially inner surface of the annular heatsink  16  of the stator  11 . A boss  29  is disposed inside the hub portion  21  of the rotor body  20  for mounting the rotor  19  to the shaft  30  of an internal combustion engine  31 . The heatsink  16  of the stator  11  is mounted to the front of a body  32  of the internal combustion engine  31 . 
     In use, the internal combustion engine  31  turns the rotor  19  in the counter-clockwise direction, so that the vanes  24  force air rearwardly through the stator  11  towards the internal combustion engine  31 . The airflow cools the coils  25  and also flows between the fins  17  of the heatsink  16  to dissipate heat from the heatsink and the stator body  12 . 
     Referring to  FIG. 7  of the drawings, there is shown the rotor body  35  of an alternative embodiment of electrical generator, which is similar in construction to the rotor body  20  of the electrical generator  10  of  FIGS. 1 to 6 . In this embodiment, separate vane members  37  are fitted to each of the radially-extending arms  36  by means of bolts: this allows the vane members  37  to removed, replaced or changed for vane members having a different vane angle. 
     Since the stator  11  and rotor  19  are respectively mounted to the engine body  31  and the engine shaft  30 , it will be appreciated that it is possible to omit the bearing  28 . Thus, referring to  FIG. 8  of the drawings, the heatsink  16  of the stator  11  is securely bolted to the body  32  of the internal combustion engine. The rotor  19  is then slid axially onto the axially-extending fingers  41  of a cylindrical mounting tool  40 . The distal ends of the fingers  41  are then engaged with the heatsink  16  of the stator  11  and the rotor  19  is then displaced axially along the tool into engagement with the shaft  30 . The rotor  19  is then secured to the shaft  30  and the tool  40  is removed. 
     The use of the tool  40  ensures that a sufficient gap of e.g. 0.8 mm is provided between the rotor  19  and the stator  11  to ensure free rotational movement of the rotor  19  relative to the stator  11 . 
     It will be appreciated that the present invention may be used for motors, generators, combined motors and generators or an electrical machine of any type. The vane profiles either pushes or draws air through the gaps in between the stator components. This happens when the rotor is normally rotating and results in the air being blown on to the components and can be in any direction.