Patent Publication Number: US-2018054099-A1

Title: Rotor of an electrical rotating machine with permanent magnets

Description:
The invention relates to a rotor of an electrical rotating machine with permanent magnets providing improved magnetic performance. 
     In a commonly known way, the electrical rotating machine comprises a stator and a rotor integral with a shaft. The rotor can be integral with a driving and/or driven shaft and can belong to an electrical rotating machine in the form of an alternator, electric motor or reversible machine able to function in the two modes. 
     The stator is mounted in a casing configured to rotationally support the shaft for example via bearings. The stator comprises a body made from a laminated core of thin laminae forming a crown, the internal face of which is equipped with slots open towards the interior to receive phase windings. In a winding of the distributed wave type, windings are obtained for example from a continuous wire coated with enamel or from conductive elements in the form of pins held together by welding. Alternatively, in a winding of the “concentric” type, phase windings consist of closed coils which are wound around the teeth of the stator. Protection between the laminated core and the winding wire is assured either by a paper-type insulator or by a plastic moulding or by means of an insert. Windings are poly-phase windings joined by star or delta connections, the outputs of which are connected to an electronic control unit. 
     In addition, the rotor comprises a body formed by a laminated core kept in the form of pack by means of a suitable attachment system, such as rivets axially passing right through the rotor or with staples or even with buttons. The rotor comprises poles formed by permanent magnets accommodated in slots arranged in the rotor body. 
     Electrical rotating machines coupled with an electric turbo-compressor shaft are well-known (“electric supercharger” in English). This turbo-compressor at least partly enables the loss of power of the internal combustion engine with reduced cubic capacity used on many motor vehicles to be compensated, in order to decrease their consumption and their emission of polluting particulates (principle known as “downsizing” in English). For this purpose, the electric turbo-compressor comprises a compressor turbine. The compressor is disposed on the air-intake conduit upstream or downstream from the internal combustion engine to enable the intake air to be compressed so that the cylinders of the internal combustion engine are filled to the maximum. 
     The electrical machine is activated to drive the turbine of the compressor in order to minimize the coupling response time, particularly during the transitional acceleration stages, or in the automatic restarting phase of the internal combustion engine after its deactivation (“stop and start” operation in English). 
     The object of the invention is to improve the magnetic performances of this type of electrical machine which is very compact and the speed of which can reach 70000 rpm, particularly 60000 to 80000 rpm. 
     For this purpose, the object of the invention is a rotor of an electrical rotating machine particularly of an electrical machine able to rotate at a speed of about 60000 to 80000 rpm, comprising:
         a rotor body and   a set of permanent magnets,
 
characterized in that a ratio between a space occupied by the set of permanent magnets and a space defined by said rotor body is higher than 30%, for example higher than 45%, preferably higher than 50%.
       

     This arrangement allows the machine equipped with a compact rotor to provide high specific power while reducing the inertia of the rotor. 
     According to an embodiment, the rotor body is made of metal and the space defined by the rotor body is equal to the volume of metal of said body. 
     According to an embodiment, said permanent magnets are made of rare earth. 
     According to an embodiment, an external diameter of said rotor ranges between 20 mm and 50 mm, particularly between 24 mm and 30 mm, for example between 20 mm and 35 mm. 
     This type of rotor is particularly suitable for high speeds, particularly about 60000 to 80000 rpm. 
     According to an embodiment, said external diameter of said rotor is about 26 mm. 
     According to an embodiment, the rotor comprises four poles. 
     According to an embodiment, a ratio between a volume of air in said rotor body and a space of the set of permanent magnets is higher than 10%. This enables the inertia of the rotor to be minimized and hence the acceleration performance of the electrical machine to be improved. 
     Preferably, said ratio is approximately 20%. 
     According to an embodiment, said rotor body comprises a plurality of slots each accommodating at least one magnet of the set of permanent magnets. 
     According to an embodiment, each slot axially passes right through said rotor. 
     According to an embodiment, each slot is defined on its external periphery by a polar wall. 
     According to an embodiment, said polar wall comprises an inner face in contact with a permanent magnet. 
     According to an embodiment, said inner face is flat. 
     Alternatively, said inner face is curved. 
     According to an embodiment, two adjacent slots are separated by an arm pertaining to said rotor body. 
     According to an embodiment, each arm is connected to a polar wall via a bridge. 
     Preferably, a ratio between a minimum thickness of a bridge measured in a radial direction and the radius of the rotor ranges between 6% and 15%, particularly between 8% and 10%. 
     According to an embodiment, a minimum thickness of a bridge measured in a radial direction is strictly lower than a minimum thickness of a corresponding polar wall measured in a radial direction. 
     Preferably, the thickness of the bridges measured in a radial direction is higher than or equal to 1.2 mm, for example substantially equal to 1.2 mm. 
     Preferably, the thickness of the bridges measured in a radial direction is lower than or equal to 1.5 mm. 
     According to an embodiment, a minimum thickness of a bridge measured in a radial direction is strictly lower than a measured minimum thickness of an arm in an orthoradial direction. 
     Preferably, the thickness of the arm measured in an orthoradial direction is higher or equal to 1.5 mm, for example substantially equal to 1.5 mm. 
     Preferably, the thickness of the arm measured in an orthoradial direction is lower than or equal to 3.5 mm. 
     According to an embodiment, a ratio between a minimum thickness of a bridge and a minimum thickness of an arm ranges between 30% and 80%. This enables a good compromise between the magnetic flux of the machine and the mechanical resistance of the rotor to be obtained. 
     According to an embodiment, an angular opening of each permanent magnet is at least equal to 30°. 
     According to an embodiment, said permanent magnets have radial magnetization. 
     According to an embodiment, said rotor body consists of a laminated core or is a solid block. 
     According to an embodiment, each slot has an angular opening strictly higher than 30°, particularly strictly higher than 40°. 
     According to an embodiment, each permanent magnet is substantially in the form of a rectangular parallelepiped. 
     According to an embodiment, each permanent magnet is substantially in the form of a tile or has a combined form with a flat face on one side and a curved face on the other. 
     According to an embodiment, the rotor body has an external periphery having a cylindrical face substantially in the form of that of a cylinder. 
     Such a rotor enables the inductance (Lq) in the axis passing between the permanent magnets to be increased. This enables a reluctance torque which contributes to the production of engine torque at high-speed to be obtained. This is particularly suitable for electrical machines rotating at high-speed, namely at speeds of at least 60000 rpm. 
     Another object of the invention is an electrical rotating machine rotor comprising:
         a rotor body and   a set of permanent magnets,
 
the rotor body comprising a plurality of slots each accommodating at least one magnet of the set of permanent magnets, each slot being defined on its external periphery by a polar wall, two adjacent slots being separated by an arm pertaining to said rotor body, each arm being connected to a polar wall via a bridge, a ratio between a minimum thickness of a bridge measured in a radial direction and the radius of the rotor ranging between 6% and 15%, particularly between 8% and 10%.
       

     According to an embodiment, the external diameter of the rotor is about 26 mm. 
     All or some of the features mentioned previously again apply to this further aspect of the invention. 
     Another object of the invention is an electrical rotating machine comprising a coiled stator and a rotor as previously defined. 
     According to an embodiment, said electrical rotating machine has a response time of about 250 ms in order to change from 5000 to 70000 rpm. 
     According to an embodiment, the operating voltage is 12V and the current in permanent mode is about 150 Amps. 
     According to an embodiment, an external diameter of the stator ranges between 35 mm and 80 mm, particularly between 45 mm and 55 mm, for example between 48 mm and 52 mm. 
     The final object of the invention is a rotor of an electrical rotating machine, particularly of an electrical machine, able to rotate at a speed of about 60000 to 80000 rpm, comprising:
         a rotor body and   a set of permanent magnets,
 
characterized in that, in a plane orthogonal to the axis of the rotor, a ratio between a surface defined by the set of permanent magnets, divided by a surface defined by said rotor body, is higher than 30%, for example higher than 45%, and preferably higher than 50%.
       

     All or some of the features mentioned previously again apply to this further aspect of the invention. 
    
    
     
       The invention will be understood better on reading the description below and on examining the figures which accompany it. These figures are only given on a purely illustrative, but by no means restrictive, basis of the invention. 
         FIG. 1  is a sectional view of a turbo-compressor comprising an electrical rotating machine according to the present invention; 
         FIG. 2  shows a perspective view of the rotor for the electrical rotating machine according to the present invention; 
         FIG. 3  is a sectional view of the rotor of the electrical rotating machine according to the present invention; 
         FIG. 4  is a perspective view of a permanent magnet intended to be inserted inside a slot of the rotor according to the present invention; 
         FIG. 5  shows a partial sectional view illustrating an alternative embodiment of the rotor of the electrical machine according to the present invention. 
     
    
    
     Identical, similar or analogous elements keep the same reference symbol from one figure to the next. 
       FIG. 1  shows a turbo-compressor  1  comprising a turbine  2  provided with vanes  3  able to take in, via an inlet  4 , uncompressed air resulting from a source of air (not illustrated) and to expel compressed air via outlet  5  after passing through a volute with the reference symbol  6 . Outlet  5  could be connected to an intake manifold (not illustrated) located upstream or downstream from the internal combustion engine so that the cylinders of the internal combustion engine are filled to the maximum. In this case, the intake of the air is performed in an axial direction, i.e. along the axis of turbine  2 , and compression is performed in a radial direction perpendicular to the axis of turbine  2 . 
     Alternatively, air intake is radial while compression is axial. 
     Alternatively, air intake and compression are performed in the same direction relative to the axis of the turbine (axial or radial). 
     For this purpose, turbine  2  is driven by an electrical machine  7  mounted inside casing  8 . This electrical machine  7  comprises a stator  9 , which could be poly-phase, surrounding a rotor  10  with the presence of an air-gap. This stator  9  is mounted in casing  8  configured to rotationally support a shaft  19  via bearings  20 . Shaft  19  is fixed in rotation with turbine  2  as well as with rotor  10 . Stator  9  is preferably mounted in casing  8  by shrink-fitting. 
     In order to minimize the inertia of turbine  2  at the time of a request for acceleration by the driver, electrical machine  7  has a short response time ranging between 100 ms and 600 ms, particularly between 200 ms and 400 ms, for example about 250 ms in order to change from 5000 to 70000 rpm. Preferably, the operating voltage is 12 V and the current in permanent mode is about 150 A. Preferably, electrical machine  7  is able to provide a current peak, i.e. a current supplied over a continuous duration of less than 3 seconds, ranging between 150 A and 300 A, particularly between 180 A and 220 A. Alternatively, electrical machine  7  is able to function in alternator mode or is an electrical machine of the reversible type. 
     More precisely, stator  9  comprises a body made from a laminated core of thin laminae forming a crown, the internal face of which is equipped with slots open towards the interior to receive phase windings. In a winding of the distributed wave type, the windings are obtained for example from a continuous wire coated with enamel or from conductive elements in the form of pins joined together by welding. Alternatively, in a winding of the “concentric” type, phase windings consist of closed coils which are wound around the teeth of the stator. Protection between the laminated core and the winding wire is assured either by a paper-type insulator or by plastic mouldings or by means of an insert. These windings are poly-phase windings joined by star or delta connections, the outputs of which are connected to an inverter. 
     Rotor  10  of rotational axis X shown in more detail on  FIG. 2  has permanent magnets. Rotor  10  comprises a body  11  formed here by a laminated core extending in a radial plane perpendicular to axis X in order to reduce any eddy currents. This body  11  is made of ferromagnetic material. The laminae are retained by fixing means, for example rivets, axially passing right through the laminated core to form an easy-to-handle and transportable unit. 
     Alternatively, the laminae are joined together by means of staples or buttons. Body  11  can be fixed in rotation to the shaft of the electrical rotating machine in various ways, for example by hafting the fluted shaft with force inside central opening  12  of rotor  10 , or using a key device. Alternatively, rotor body  11  could be cast from solid ferromagnetic material. 
     Rotor body  11  has an internal periphery  15  defining central cylindrical opening  12  having an internal diameter D 1  for example of about 10 mm, an external periphery  16  defined by a cylindrical face having an external diameter D 2  for example of about 26 mm, but more generally able to range between 20 mm and 50 mm, particularly between 24 mm and 30 mm, as well as by two axial end-faces  17 ,  18  of annular form extending in a radial plane between internal periphery  15  and external periphery  16 . A space V of rotor body  11  is defined by internal periphery  15 , external periphery  16  and the two axial end-faces  17 ,  18 . In other words, space V of rotor body  11  is that defined by the laminated core of the rotor body. In addition, an external diameter of the stator ranges between 35 mm and 80 mm, particularly between 45 mm and 55 mm, and preferably for example between 48 mm and 52 mm. 
     Rotor  10  comprises a plurality of slots  21  in each of which a permanent magnet  22  is accommodated. In order to optimize the magnetic performance of the machine, the ratio between the space occupied by the set of permanent magnets  22  and space V defined by rotor body  11  is higher than 30%, preferably higher than 50%. 
     More precisely, each slot  21  axially passes right through body  11 , i.e. from one axial end-face  17 ,  18  to the other. Two adjacent slots  21  are separated by an arm  25  emerging from a core  26  of rotor  10 , so that an alternation of slots  21  and arms  25  exists whenever a circumference of rotor  10  is followed. Rotor body  11  also comprises polar walls  31  each located between two adjacent arms  25 . Each polar wall  31  extends between an inner face  36  in contact with a permanent magnet  22  and the external periphery of rotor  10 . Moreover, each arm  25  is connected to a corresponding polar wall  31  via a bridge  32 . 
     Thus, as  FIG. 3  shows, slots  21  are each defined by two faces  35  of two adjacent arms  20  facing one another, an inner flat face  36  of a polar wall  31  extending in an orthoradial direction, a flat face  37  arranged in core  26  parallel with face  36  and inner faces  38  of two bridges  32 . The junctions between faces  35  and  38  could be rounded in order to facilitate machining. 
     In the exemplary embodiment, a minimum thickness L 1  of a bridge  32  measured in a radial direction relative to axis X is strictly lower than a minimum thickness L 2  of a corresponding polar wall  31  measured in a radial direction relative to axis X. 
     In addition, the minimum thickness L 1  of a bridge  32  is strictly lower than a minimum thickness L 3  of an arm  25  measured in an orthoradial direction relative to axis X. A ratio between a minimum thickness L 1  of a bridge  32  measured in a radial direction and the external radius (D 2 /2) of the rotor ranges between 6% and 15%, particularly between 8% and 10%. 
     Preferably, a ratio between the minimum thickness L 1  of a bridge  32  and the minimum thickness L 3  of an arm  25  ranges between 30% and 80%. This enables a good compromise between the magnetic flux of the machine and the mechanical resistance of magnets  22  to be obtained inside slots  21  of rotor  10 . 
     In the example considered, the L 1  thickness of bridges  32  is approximately equal to 1.2 mm and the L 3  thickness of arms  25  is approximately equal to 1.5 mm. In all cases, the minimum thickness L 1  of the bridges is lower than or equal to 1.5 mm and the minimum thickness L 3  of the arms is lower than or equal to 3.5 mm. 
     It should be noted that “minimum” thickness L 1 -L 3  of an element is understood to mean the smallest thickness measured in the given direction (radial or orthoradial direction) corresponding to the smallest dimension of the smallest section of the element, the thickness of which is to be measured. 
     Alternatively, the thicknesses L 1 , L 2  of bridge  32  and polar wall  31  are equal and substantially constant, while having a value higher than or equal to 1.2 mm. 
     In this case, as quite visible on  FIG. 4 , magnets  22  have a rectangular parallelepiped form, the angles of which are slightly chamfered. Magnets  22  hence present a rectangular cross section which is substantially constant. 
     Magnets  22  have radial magnetization, i.e. the two faces  41 ,  42  parallel in relation to each other having an orthoradial orientation are magnetized so as to be able to generate a magnetic flux in a radial direction M relative to axis X. Among these parallel faces  41 ,  42 , inner face  41  located on the side of the axis of rotor  10  and outer face  42  located on the side of the external periphery of rotor  10  are evident. 
     As quite visible on  FIGS. 3 and 5  where letters N and S correspond to the north and south poles respectively, magnets  22  located in two consecutive slots  21  have alternate polarities. Thus, from one slot  21  to the other, inner faces  41  of magnets  22  supported against flat face  37  arranged in core  26  have alternate polarity and outer faces  42  of magnets  22  in contact with inner face  36  of the corresponding polar wall  31  have alternate polarity. 
     Inner  41  and outer  42  faces of each magnet  22  are level in this case. Alternatively, as illustrated on  FIG. 5 , outer face  42  of each magnet  22  is curved, while inner face  41  of magnet  22  is flat, or vice versa. Inner face  36  of polar wall  31  then has a corresponding curved form. Hence the retention of magnet  22  inside a slot  21  is improved. Alternatively, the two side faces  41  and  42  are curved in the same direction (see dotted line  50 ), so that magnet  22  generally has a tile shape. 
     In addition, magnets  22  do not fill slots  21  completely, so that there are two empty spaces  45  on both sides of magnet  22 . The volume of air defined by all spaces  45  of rotor  10  enables the inertia of rotor  10  to be reduced. In order to minimize this inertia in an optimum way and hence improve the acceleration performance of the electrical rotating machine, the ratio between the volume of air in rotor body  11  and a volume of the set of permanent magnets  22  is higher than 10%. Preferably the ratio is approximately 20%. 
     For this purpose, angular opening α 1  of a slot  21  is higher than the angular opening α 2  of a corresponding permanent magnet  22 . The angular opening α 1 , α 2  of a given element (slot  21  or magnet  22 ) is defined by the angle formed by two planes each passing through axis X and through one of the ends of said element. In an exemplary embodiment, angular opening α 1  of each slot  21  is strictly higher than 40°, while the angular opening α 2  of a magnet  22  is at least 30°. In a particular exemplary embodiment, angular opening α 1  of each slot  21  is about 73°, while the angular opening α 2  of a magnet  22  is about 67°. 
     Magnets  22  are preferably made of rare earth in order to maximize the magnetic power of the machine. Alternatively however, they could be made of ferrite according to application and the required power of the electrical machine. Alternatively, magnets  22  can be made of different materials to reduce the costs. For example, a rare earth magnet and a less powerful but less expensive ferrite magnet can be used alternately in the slots. Certain slots  21  could also be left empty according to the required power of the electrical machine. For example, two diametrically opposite slots  21  can be empty. The number of slots  21  here is equal to four, just as the number of associated magnets  22 . It is however possible to increase the number of slots  21  and magnets  22  according to application. 
     In addition, a single permanent magnet  22  is inserted inside each slot  21 . Alternatively, several magnets  22  stacked over one another inside the same slot  21  could be used. For example two permanent magnets  22  stacked axially or orthoradially over one another, which, as the case may be, can be made from different materials, could be used. 
     Rotor  10  could also comprise, inside each slot  21 , a spring-type mounting element for the magnets or pin made from a magnetic material which is more flexible than magnets  22 . These mounting elements permit easier insertion of magnets  22  in slots  21  which is performed by making magnets  22  slide parallel with axis X of rotor  10  and guarantee the mechanical mounting of the magnets. Alternatively, the magnets can be held in the slot by an adhesive. 
     Rotor body  11  can also comprise two retention plates (not illustrated) arranged on both sides of rotor  10  on its axial end faces. These retention plates ensure magnets  22  are axially held inside slots  21  and also serve to balance the rotor. The flanges are made of non-magnetic material, for example aluminium. 
     Of course, the above description was only given by way of example and does not restrict the scope of the invention from which there would be no departure if the various elements were replaced by any other equivalents.