Patent Publication Number: US-2020295628-A1

Title: Electric Machine Having A Cooling Device

Description:
PRIORITY CLAIM 
     This is a U.S. national stage of application No. PCT/EP2017/050192, filed on Jan. 5, 2017. Priority is claimed on the following application: Country: Germany, Application No.: 10 2016 201 870.1, filed: Feb. 8, 2016; the content of which is/are incorporated herein in its entirety by reference 
     The present invention is directed to an electric machine having a cooling device. 
    
    
     FIELD OF THE INVENTION 
     An electric machine of the type mentioned above is already known from US2002/0135245, wherein winding heads of a stator winding which protrude axially over a stator lamination stack are overmolded with a thermally conductive plastic and are in heat exchange contact with a cooling jacket of a fluid cooling device via the potting. In this way, the heat losses occurring in the winding head can be dissipated together with the heat losses occurring in the stator lamination stack via a common cooling system, and the power of the electric machine can be increased. 
     However, a problem consists in that when heat is dissipated radially from the winding heads to a cooling device arranged radially outwardly of the latter a transfer of heat must take place through a radial potting medium layer which, due to geometrical factors, is relatively heavy and has a comparatively poor thermal conductivity, which can lead to an unwanted buildup of heat when the electric machine is in operation and to a rise in operating temperature and reduced efficiency. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to enable a further increase in power of the electric machine in a given constructional size. The invention aims to further increase the power density of an electric machine by undertaking measures to improve dissipation of power losses. 
     According to the present invention, it is provided that the winding heads which are potted with a thermally conductive plastic are cooled via the outer circumferential surface and additionally also via the inner circumferential surface of the winding heads so that extraction of heat losses can be quantitatively increased. 
     For purposes of heat extraction in the case of an electric inrunner machine, the outer circumferential surface of a winding head is in thermally conductive contact with an outer stator support in a known manner, while the inner circumferential surface of a winding head can be wetted with a cooling fluid via a first fluid cooling device, accordingly allowing heat to be extracted directly from the interior of the electric machine. This cooling via the inner circumferential surfaces of the winding heads is particularly effective in an inrunner motor because a radial dimension of the potting compound extending from the surface to be cooled to the winding is comparatively small. Therefore, the risk of heat buildup is negligible compared to an outer circumferential surface of the winding heads. Heat is likewise extracted from the axial front sides of the winding heads through the action of the first fluid cooling device. 
     The inventive electric machine is further provided with a first fluid conducting element which is secured to the winding head and formed in such a way that a cooling fluid introduced into the interior during operation of the electric machine is substantially impeded from penetrating into the air gap located between the rotor and the stator. This means that although there is free fluid present in the interior in the area of the winding heads, this fluid does not penetrate into the air gap of the electric machine and generate unwanted drag losses and power losses. At the same time, a cooling fluid in the form of oil is also protected against unwanted thermal loading occurring in the air gap which would result in a destruction of the chemical chain structure of the oil and, therefore, in an unwanted premature degradation of the oil. 
     Thus based on the present invention, a noticeable increase in power of the electric machine can be achieved. The first fluid cooling device can accordingly also be constructed in particular as an oil cooling device, and this can be utilized at the same time to lubricate the bearings of the rotor shaft as will be discussed at greater length later. 
     According to an advantageous configuration, the fluid conducting element can be disk-shaped or pot-shaped in particular and can have a dividing wall area which is secured to an inner circumferential surface of a winding head so as to be substantially tight against fluid by a radially outer fastening portion, possibly including a sealing element, this dividing wall area being constructed so as to be closed with the exception of a central through-opening for the rotor shaft. In this way, the interior of the electric machine is again divided into a substantially dry rotor space and into wet spaces axially adjacent thereto for the winding head located at the front side of the stator. 
     The first fluid conducting element can advantageously have an axial stop cooperating with a winding head. An axial stop of this kind secures the axial position of a fluid conducting element in the direction of the rotor and prevents the fluid conducting element from being pulled into the area of the rotor and destroyed. As has already been mentioned, the first fluid conducting element can be constructed on the whole in a substantially pot-shaped manner so that the dividing wall area forms a pot base and the axial stop is formed at a cylindrical portion connected to the base. The axial stop can be formed in particular as a one-sided radial collar which contacts a potted body formed of a potting compound and the winding head. Further, the cylindrical wall area can be arranged or can extend at a radial distance from the winding heads and can have in circumferential direction a plurality of recesses, particularly recesses having the largest possible surface area, for passage of a cooling fluid. The first fluid conducting element can be secured in position additionally, for example, through a catch connection and by an adhesive. 
     According to a further embodiment, the first fluid conducting element can also extend at the front side axially beyond a winding head and can be axially supported by an axial supporting surface at a housing element, e.g., a bearing endshield. Supporting surfaces acting axially at both sides are advantageously provided at a fluid conducting element of this kind such that the fluid conducting element can be axially embedded or clamped in when installed and can accordingly be captively secured to, or relative to, the stator. In this case, further retaining means can be omitted. 
     Further advantageously, the first fluid conducting element can have in the area of the central through-opening for the rotor shaft a fluid repelling surface which opens toward a front side of the electric machine and through which a fluid impinging on it is repelled in direction of the front side. In particular, the fluid repelling surface can be conical or spherical so that a fluid flow directed toward the rotor is reflected back into the axial region of the winding heads. 
     To prevent whirling and a development of heat induced by it, it is further suggested to produce the first fluid conducting element from a non-ferromagnetic material. This fluid conducting element can preferably be made of a heat-resistant plastic, particularly from a thermoplastic or thermosetting plastic material. 
     According to a preferred embodiment of the invention, the cooling fluid can be supplied via the rotor shaft, for which purpose a fluid inlet channel is formed in this rotor shaft and is fluidically connected with an area of the interior, i.e., a wet space, mentioned above, facing a front side of the stator by at least one first fluid outlet opening. In an advantageous manner, a plurality of fluid outlet openings are provided so as to be distributed around the circumference of the rotor shaft and also at both areas of the winding heads located at the front side of the stator. 
     An improved separation of the rotor space from fluid is achieved in that a second fluid conducting element which, together with the first fluid conducting element, forms a labyrinth seal for the cooling fluid is formed in the area of the first fluid outlet opening. Recesses or contours are further provided in this axial area at the rotor shaft which cause the fluid to be slung back into the wet space from the rotor space while the machine is running, for which purpose the above-mentioned structures lie opposite one another radial to the fluid repelling surface of the first fluid conducting element. 
     As has already been mentioned, the first fluid cooling device can be utilized for cooling the winding heads and simultaneously lubricating the rotor bearings in that the rotor shaft has in the area of the second fluid conducting element at least one second fluid outlet opening which is arranged axially adjacent to a rotor bearing. In order to achieve a fluid flow directed to a rotor bearing, the second fluid conducting element can extend in direction of the rotor bearing and axially overlap the second fluid outlet opening at a radial distance therefrom. In this way, a fluid flow exiting from the second fluid outlet opening is supplied directly to an adjacent rotor bearing through the second fluid conducting element. 
     In a particularly advantageous manner, the first fluid cooling device is formed as an oil circuit which is completed by a coolant pump and a heat exchanger. While lubricant or coolant is supplied via the rotor shaft, a fluid outlet channel is formed for discharging the coolant, this fluid outlet channel being formed at the bottom geodesically relative to the stator with respect to a normal operating position of the electric machine. This fluid outlet channel is fluidically connected to the interior of the electric machine at least by one fluid inlet opening. 
     As has already been stated, some of the heat losses occurring in the winding heads are guided off to the stator support via the outer circumferential surface of the winding heads. In this respect, it may be advantageous in order to further improve heat dissipation if the electric machine has a second fluid cooling device with a fluid cooling jacket formed at the stator. This second fluid cooling device can be constructed as a water cooling device or as an oil cooling device, and the fluid cooling jacket has a first wall element and a second wall element which are formed so as to be substantially cylindrical, spaced apart from one another radially and sealed relative to one another. 
     The first wall element can advantageously comprise the stator support, and the second wall element can advantageously be formed as a housing of the electric machine. The second fluid cooling device accordingly serves to remove heat losses imposed via the stator lamination stack and, at the same time, the heat losses occurring in the winding heads. To this end, the fluid cooling jacket can advantageously extend axially entirely or at least partially along the winding heads at the stator which are potted with potting compound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in the following by way of example with reference to the accompanying figures, in which: 
         FIG. 1  is a schematic view of an electric machine in longitudinal section; 
         FIGS. 2A , B is an enlarged sectional view of the electric machine from  FIG. 1  in the region of the winding heads of the stator with first fluid conducting elements arranged therein; 
         FIGS. 3A , B is a further view of an electric machine in the region of the winding heads of a stator with first fluid conducting elements arranged therein in an alternative embodiment form. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     Like subject matter, functional units or comparable components are designated by like reference characters throughout the figures. Further, summarizing reference characters are used for components and objects which occur several times in an embodiment example or in a diagram but which are collectively described with respect to one or several features. Components or objects which are designated by like or summarizing reference characters may be implemented alike but also differently with respect to individual, several or all features such as, e.g., the dimensioning, insofar as the description does not implicitly or explicitly indicate otherwise. Identical subject matter, functional units and comparable components in various embodiment examples are not described repeatedly so as to avoid repetition, and only the differences between the embodiment examples are described. 
       FIG. 1  shows an electric machine  10  which is formed as an asynchronous motor and provided in a powertrain of a motor vehicle for transmitting a drive torque to vehicle wheels. To this end, the electric machine  10  comprises a stator  12  arranged in a housing  64  and a rotor  36  which is rotatably mounted therein and arranged on a rotor shaft  34  via which power can be tapped for driving the vehicle. 
     The stator  12  comprises a cylindrical stator support  14  with a stator lamination stack  16  secured to the latter. This stator lamination stack  16  is constructed in a known manner with a yoke and with stator teeth which are directed radially inward and which carry a stator winding  18  with winding heads  20  protruding axially over the stator lamination stack  16 . The stator winding  18  is connected to a plurality of external connection lines  74  by a power connection unit  76  inside a switchbox  72  arranged at the housing  64 , and electrical power can be impressed into the stator winding  18  by an energy storage, not shown, through the external connection lines  74 . 
     As can be seen in  FIGS. 1 ;  2 A, B, the stator winding  18  extends axially on both sides beyond the stator lamination stack  16  and forms winding heads  20  in these areas. These winding heads  20  are potted with a thermally conductive potting compound  22 , particularly a thermally conductive plastic, so that an outer circumferential surface  24  and an inner circumferential surface  26  are formed axially on both sides at the stator  12 . The outer circumferential surface  24  is in thermally conducting contact with the stator support  14  which is formed as a first wall element  60  of a fluid cooling jacket  58 . The fluid cooling jacket  58  further comprises a second wall element  62 , constructed in this instance as the housing  64  of the electric machine  10 , which is likewise formed substantially cylindrically, is spaced apart radially from the first wall element  60  and sealed relative to the latter by sealing elements  66 . A helical fluid cooling channel  63  in which a coolant circulating, for example, as oil or water is guided between the electric machine  10  and a heat exchanger, not shown, extends between the wall elements  60 ,  62 . Accordingly, as a whole, the above-described arrangement provides a fluid cooling device  56 . 
     In a known manner, the rotor  36  is formed as a squirrel cage rotor and is rotatably mounted by the rotor shaft  34  in a cylindrical interior space  30  formed by the stator  12  accompanied by formation of a radial air gap  32 . The rotor shaft  34  is supported by two rotor bearings  52   a, b  which are constructed as rolling element bearings and which are secured on the one hand in a bearing endshield  68   a  formed as a housing base and, on the other hand, in a bearing endshield  68   b . A portion of the rotor shaft  34  exiting axially from the housing base or bearing endshield  68   a  can be connected to further components of a vehicle powertrain via a toothing  34   a  provided on it. The bearing endshield  68   b  is closed on the axially opposite side by a housing cover  70 . 
     In addition to fluid cooling device  56 , the electric machine  10  has a further fluid cooling device  38 , in particular with an oil as cooling fluid, with which the inner circumferential surfaces  26  of the winding heads  20  and at least partially also the end faces  27   a, b  thereof can be wetted. For this purpose, a fluid inlet channel  46  having a plurality of first fluid outlet openings  46   a  ( FIGS. 2 a, b   ) at both axial positions of the winding heads  20  is formed inside the rotor shaft  34 . Accordingly, these fluid outlet openings  46   a  are fluidically connected with those areas of the interior space  30  which face the front sides  42 ,  44  of the electric machine  10 . In order to prevent fluid from entering the axial area of the rotor  36 , first fluid conducting elements  40  are provided in the present instance at both winding head positions and are secured, respectively, to a potted winding head  20  and formed in such a way that a fluid introduced into the interior space  30  is substantially prevented from penetrating into the air gap  32  between rotor  36  and stator  12  during operation of the electric machine  10 . 
     In particular, a first fluid conducting element  40  of this type has a substantially closed dividing wall area  40   a  which is secured in a substantially fluid-tight manner to the inner circumferential surface  26  of a winding head  20  by a radially outer fastening portion  40   b  accompanied by a sealing element  40   e . This dividing wall area  40   a  is constructed so as to be closed with the exception of a central through-opening  40   c  for the rotor shaft  34 . 
     As can be seen in  FIGS. 1 ;  2 A, B, the fluid conducting element  40  is constructed in a substantially pot-shaped manner, and the dividing wall area  40   a  forms a base. A cylindrical portion  40   f  extends from this dividing wall area  40   a  in direction of the front side  44  and contacts the potting compound of the winding head  20  at the front side by an annular collar  40   h  which protrudes radially outward. The cylindrical portion  40   f  is guided at a radial distance to the inner circumferential surface  26  of a winding head  20  and has a plurality of large-area recesses  40   i  which are distributed along the circumference and by which the cooling fluid can pass through to the winding heads  20 . Radially inwardly in the area of the through-opening  40   c , a first fluid conducting element  40  forms a fluid repelling surface  40   g  which opens toward a front side  42 ,  44  of the electric machine  10  and through which a fluid impinging on it is repelled in direction of the front side  42 ,  44  and is kept away from the rotor space. It can further be seen that the rotor shaft  34  has in the area of the first fluid outlet opening  46   a  a second fluid conducting element  48  which, together with the first fluid conducting element  40 , forms a labyrinth seal  50  for the cooling fluid. A second fluid conducting element  48  is constructed as a sleeve which is fitted on the rotor shaft  34  and forms radially opposite the first fluid conducting element  40  in the area of the fluid repelling surface  40   g  an annular collar  48   a  which protrudes radially outward so that a labyrinth seal  50  is formed from elements  40   g  and  48   a  and substantially prevents cooling fluid from entering the area of the rotor  36  during rotation of the rotor  36 . The first fluid conducting element  40  is produced in the present instance from a non-ferromagnetic material, particularly from a heat-resistant plastic, while the second fluid conducting element  48  can be a plastic element or a metal element, for example, a sheet-metal sleeve. 
     Second fluid openings  46   b  are provided at the rotor shaft  34  in the area of the second fluid conducting element  48  for lubrication of the rotor bearings  52   a, b . These fluid outlet openings  46   b  are arranged axially adjacent to the rotor bearings  52   a, b  and are overlapped by a conducting portion  48   b  of the second fluid conducting element  48 . In other words, the second fluid conducting element  48  extends in direction of a rotor bearing  52   a, b  so as to overlap the second fluid outlet openings  46   b  at a radial distance therefrom. Accordingly, a fluid flow exiting from the second fluid outlet openings  46   b  can be selectively directed to the rotor bearings  52   a, b  through the second fluid conducting elements  48 . 
     To guide off the fluid located in the interior space  30 , the fluid cooling device  38  has a fluid outlet channel  54  ( FIG. 1 ) which is formed at the bottom geodesically with respect to the stator  12  in a normal operating position of the electric machine  10  and which is fluidically connected to the interior space  30  by a fluid inlet opening  54   a . A maximum level Pmax. for the cooling fluid is indicated in  FIG. 1  and is set radially between air gap  32  and fluid inlet opening  54  in this operating position. Referring again to fluid cooling device  56 , it is further shown that the fluid cooling jacket  58  at stator  12  extends in axial direction almost completely over the winding heads  20  which are potted with potting compound  22 . 
     For production of the electric machine, the stator  12  with the stator lamination stack  16  and stator winding  18  can be produced first. Winding heads  20  protrude axially at both sides over the stator lamination stack  16 . In a further step, this pre-built unit is inserted into the cylindrical stator support  14 , whereupon the winding heads  20  can be potted with a potting compound  22 . The unit produced in this way can then be inserted into the housing  64 , a first fluid conducting element  40  being secured to the winding heads already on the bearing endshield  68   a  formed by the housing base. The rotor  36  can now be inserted with the second fluid conducting elements  48 , and the rotor shaft  34  is guided through the rotor bearing  52   a  on the aforementioned side  42  of bearing endshield  68   a . Subsequently, the first fluid conducting element  40  is likewise secured to the winding head  20  on the free axial or front side  44 . After arranging the power connection unit  76 , this front side can also be closed through the bearing endshield  68   b  and the housing cover  70 . 
       FIGS. 3A , B show an embodiment of an electric machine  10  as an alternative to the electric machine  10  described above. The preceding description of the figures is referred to for the basic construction of this alternative embodiment. In this embodiment, the first fluid conducting element  40  extends on the front side axially beyond a winding head  20  and is axially supported by a supporting surface  40   k  directly or indirectly at the housing  64 , particularly at the bearing endshield  68   b  ( FIG. 3 a   ) or at the housing base  64   a  ( FIG. 3 b   ). Supporting surfaces acting axially at both sides are advantageously provided at a fluid conducting element  40  of this kind so that the fluid conducting element  40  is axially embedded or clamped in when installed in the interior space  30  and is accordingly captively secured to the stator or relative to the stator. A plurality of cutouts  40   i  extending axially beyond the end of the winding heads  20  in direction of front sides  42 ,  44  of stator  12  are in turn provided in circumferential direction at the first fluid conducting elements  40 , which are pot-shaped or bucket-shaped in this instance, so that a cooling fluid exiting radially from the rotor shaft  34  can pass through to the winding heads  20 . Accordingly, the cooling fluid can reach the inner circumferential surfaces  26  and the end faces  27   a, b  of the winding heads  20  and cool them. The second fluid conducting elements  48  are identical to those shown in  FIGS. 1, 2A , B seen from the first fluid outlet openings  46   a  in direction of the rotor. 
     On the other hand, for supplying coolant or lubricant to the rotor bearings  52   a, b , a stationary conducting element  78  which is shaped as an annular cap is provided at the bearing endshields  68   a, b  and axially overlaps the second fluid outlet openings  46   b  at a radial distance therefrom. 
     Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.