Patent Publication Number: US-2022224203-A1

Title: Electric Machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related and has right of priority to German Patent Application No. 102021200283.8 filed in the German Patent Office on Jan. 14, 2021, which is incorporated by reference in its entirety for all purposes. 
     FIELD OF THE INVENTION 
     The present invention relates generally to an electric machine having a housing, a stator stationarily arranged in the housing, a rotor including a rotor shaft, and a cooling device for the liquid cooling of the electric machine, and to a method for generating an air flow in an air gap between the stator and the rotor of an electric machine. 
     BACKGROUND 
     Electric machines for drive engineering include, due to the principle thereof, a stator and a rotor. The stator generally includes stator plates and a suitable coil winding, by which a magnetic field is generated in order to set the rotor into rotation. Electric machines are generally designed in the shape of a cylinder, wherein the external cylindrical stator surrounds the internal rotor, which is also cylindrical. At the axial ends of the stator, the coil winding forms a winding overhang, in which a large number of current-carrying conductors is routed in a comparatively small volume. Due to the tight arrangement of the current conductors, the winding overhang is usually the hottest area of an electric machine, in particular due to the superposition of the phases of the coil windings in this area. 
     Cooling is provided in order to increase the continuous power of an electric machine. In the case of electric machines for the traction of electric and hybrid vehicles, a jacket cooling, for example, takes place at the stator with the aid of liquid coolants, which are frequently oil- or water-based. Oil-based coolants absorb the heat losses, for example, directly at the current-carrying conductors and carry the heat losses away. Alternatively, a winding overhang cooling takes place in order to reduce the heat losses. 
     In order to improve a winding overhang cooling using oil-based coolants, DE 10 2008 001 622 A1, for example, describes a coolant spraying device for spraying the electric machine with an oil-based, liquid coolant. The cooling takes place at the axial end faces of the stator and at the winding overhangs present there. A proposed coolant circuit includes a coolant reservoir in order to adjust the flow rate of the liquid coolant of the coolant spraying device in such a way that no coolant of a sump pan is blocked to such an extent that the coolant enters the air gap between the stator and the rotor. 
     A winding overhang cooling with oil-based coolants proves to be a suitable measure for dissipating the heat and, thereby, reducing the thermal losses. In this way, it is possible to bring the obtainable continuous power of electric machines close to the peak performance. This means that the installation space of the electric machines can be reduced due to a higher power density and torque density, which is valuable, in particular, with respect to traction machines in vehicles. 
     Nevertheless, it is shown in practical applications that oil penetrates the air gap between the stator and the rotor and, thus, can result in additional, undesired, rotational speed-dependent, fluid dynamic friction between the stator and the rotor. During the operation of the electric machine, the oil must first be transported out of the air gap. This can take place due to the rotation effect of the rotor, although this has the disadvantage that greater losses initially occur during the start-up of the machine. Oil can enter the air gap also during the operation of the electric machine, for example, from the axial ends of the stator, which results in a reduction of the effectiveness of the motor. 
     SUMMARY OF THE INVENTION 
     Example aspects of the present invention provide an improved electric machine, the effectiveness of which is increased and the losses of which are reduced. 
     Example aspects of the present invention relate, in a first example aspect, to an electric machine, preferably as a traction drive in a vehicle, having a housing, a stator stationarily arranged in the housing, a rotor including a rotor shaft, an air gap forming between the rotor and the stator, and a cooling device for the liquid cooling of the electric machine. The rotor shaft has an axial bore in the axial direction, which extends at least partially into an area of the rotor. The rotor has a radially extending air duct, which extends from an inner side contacting the rotor shaft to an outer side facing the air gap. The rotor shaft has a bore, which is aligned with the air duct in such a way that air is flowable from the rotor shaft into the air gap. 
     In one further example aspect, the present invention relates to a system including an electric machine, as described above, and an air supply device for supplying air into the rotor shaft of the electric machine, wherein the air supply device includes a pump in order to pump air into the rotor shaft and generate an air flow through the air duct into the air gap. 
     Further example aspects of the invention relate to a vehicle including an electric machine and to a method for generating an air flow in an air gap between the stator and the rotor of an electric machine. 
     It is understood that the features, which are mentioned above and which will be described in greater detail in the following, are usable not only in the particular combination indicated, but also in other combinations or alone, without departing from the scope of the present invention. 
     An electric machine including a housing is provided, which is suitable, in particular, as a traction machine for an electric vehicle or a hybrid vehicle. The electric machine includes a stator, which is stationarily arranged in the housing, a rotor having a rotor shaft, and a cooling device for the liquid cooling of the electric machine. The stator and the rotor are arranged in such a way that an air gap forms between the stator and the rotor. The rotor shaft is a hollow shaft and has an axial bore in the axial direction, which extends at least partially so far in the axial direction that the axial bore is arranged in an area of the rotor. The axial bore therefore overlaps at least one area of the rotor in the axial direction. According to example aspects of the invention, the rotor has a radially extending air duct, which extends from an inner side of the rotor, which contacts the rotor shaft, to an outer side of the rotor, which faces the air gap. The rotor shaft has a bore, which is aligned with the air duct of the rotor in such a way that air is flowable out of the rotor shaft into the air gap. The bore of the rotor shaft is arranged in the outer wall of the rotor shaft and connects the axial bore of the rotor shaft to the radially extending air duct. 
     The axial bore of the rotor shaft is preferably designed as a blind hole and open toward one end of the rotor shaft, and so air from the end face of the shaft can enter the axial bore of the rotor shaft. 
     The design of the electric machine according to example aspects of the invention makes it possible to generate a flow of air, which flows through the axial bore of the rotor shaft and the bore at the outer wall of the rotor shaft into the air duct. The air flows radially through the air duct, enters the air gap and is distributed there. The air flows away from the air duct toward the outer ends of the electric machine, i.e., toward the end faces of the rotor and of the stator. Due to the arising air flow in the air gap, oil that has penetrated the air gap for the liquid cooling of the electric machine by the cooling device is displaced and transported outward, i.e., toward the two ends of the electric machine. In this way, not only is oil located in the air gap transported outward. In addition, oil for a winding overhang cooling of the electric machine is prevented from penetrating the air gap from the axial end face. The air gap is kept free of oil, at the least largely free of oil in such a way that residues of oil have no significance for the basic operation. Ventilation losses in the air gap are kept low and the effectiveness of the electric machine is increased without dispensing with a liquid cooling of the electric machine with oil. 
     In one example embodiment, the air duct of the rotor is arranged closer to the center of the rotor in the axial direction. Moreover, the radial bore in the rotor shaft is aligned with the air duct. The essentially central arrangement of the air gap in the axial extension of the rotor has the advantage that air flowing into the air gap is more uniformly distributed than in the case of an eccentric arrangement and is guided in approximately identical amounts to the two opposite axial ends of the air gap. 
     In one further example embodiment of the invention, the electric machine has two or more air ducts in the rotor. As a result, it is possible to increase the air volume and, thereby, more quickly clear oil from the air gap between the rotor and the stator and more reliably keep the air gap free of oil. 
     In one further example embodiment of the invention, the number of air ducts in the rotor is even. Therefore, a number of air ducts is provided that is divisible by two (2). In this way, a rotational symmetry can be generated, and so the arrangement of the air ducts has no effect on the uniformity of the rotation of the electric machine. In one further example embodiment of the invention, the two or more air ducts are arranged at the same axial position of the rotor. The air ducts are all situated in the same plane having the same axial distance from one of the ends of the rotor. 
     According to one preferred example embodiment, the air ducts are uniformly distributed over the circumference of the rotor in the same axial position. In this way, the inflow of the air into the air gap is further equalized. In addition, the effect on the rotation of the electric machine is the lowest and results in the lowest imbalance. 
     According to one preferred example embodiment, the air duct in the rotor is formed by a bore. In this way, it can be subsequently introduced into the laminations of the rotor in order to improve an electric machine. 
     Preferably, the bore that forms the air duct has a diameter from one-tenth of a millimeter (0.1 mm) to approximately five millimeters (5 mm), particularly preferably from one millimeter (1 mm) to two millimeters (2 mm). It can be provided that the diameter of the air duct is established as a function of the axial rotor length or of the radial rotor diameter. The number of the air ducts can also be selected as a function of the diameter. It is also possible to make the number and/or the diameter of the air ducts dependent upon a desired air flow volume. 
     One further preferred example embodiment of the invention provides that air from an air supply device is guided axially into the axial bore of the rotor shaft and, after passing through the air duct, is deflected into the air gap between the rotor and the stator. The air supply device can include, for example, a chamber that operates as an air reservoir. Air can then be suctioned into the interior of the rotor shaft designed as a hollow shaft, for example, due to the rotation of the rotor. The air is preferably withdrawn from the air chamber, which is preferably open toward the surroundings. Alternatively, the air supply device can utilize air from the surroundings of the electric machine. 
     The system according to one further example aspect of the invention includes the electric machine described here as well as an air supply device for supplying air into the rotor shaft of the electric machine. The air supply device, as part of the system, is preferably an active device that includes a pump, by which air is pumped into the rotor shaft. In this way, an air flow is generated, which pumps air into the axial bore of the hollow shaft-like rotor shaft. After passing through the bore in the wall of the rotor shaft and flowing through the air duct, the air flows into the air gap. In this way, it is possible to displace oil located in the air gap also while the rotor is at a standstill. The active air supply is not reliant on air being drawn in due to the motion of the rotor. Rather, air is actively injected into the air gap or is allowed to flow into the air gap. As a result, losses can also be reduced already during the start-up of the electric machine. 
     One further example aspect of the invention relates to a vehicle including an electric machine of the type described here. The electric machine preferably acts as a traction machine and drives at least one wheel of the vehicle. 
     The method according to example aspects of the invention for generating an air flow in an air gap between the stator and the rotor of an electric machine with a cooling device for liquid cooling is applied on the above-described electric machine. This machine includes a rotor with a rotor shaft, which has an axial bore, which extends in the area of the rotor. A bore in the outer wall of the rotor shaft is aligned with an air duct, which extends radially in the rotor and extends to an outer side of the rotor facing the air gap. The method includes, as one step, generating an air flow and introducing the air flow into the axial bore of the rotor shaft. One further step is that of guiding the air flow out of the rotor shaft through a bore into the air duct of the rotor. Another step is that of distributing the air flow into the air gap, which is formed between the rotor and the stator. In a further step, the air flows from the air duct to the rotor ends and displaces liquid, which is possibly present, out of the air gap in such a way that the air gap is essentially free of liquid. In the end, the electric machine has an air gap that has no amount of liquid or oil in the air gap that is relevant for the practical application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described and explained in greater detail in the following with reference to a few selected exemplary embodiments in conjunction with the attached drawings, in which: 
         FIG. 1  shows a diagrammatic longitudinal section of an electric machine; 
         FIG. 2  shows a cross-section of the electric machine along a line A-A from  FIG. 1 ; and 
         FIG. 3  shows a system including the electric machine from  FIG. 1  and an air supply device. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to embodiments of the invention, one or more examples of which are shown in the drawings. Each embodiment is provided by way of explanation of the invention, and not as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be combined with another embodiment to yield still another embodiment. It is intended that the present invention include these and other modifications and variations to the embodiments described herein. 
       FIG. 1  shows an electric machine  10  according to example aspects of the invention including a housing  12 , in which a stator  20  is stationarily arranged, and including a rotor  30  with a rotor shaft  32 . The cylindrically designed electric machine  10  is arranged in such a way that the rotor  30  rotates within the stator  20 . 
     In addition, the housing  12  includes a cooling device  14 , which sprays a coolant, preferably an oil-based coolant, onto the axial sides of the stator  20  in order to cool the stator  20 . In the diagrammatic section drawing of the electric machine  10 , the winding overhangs, which are arranged at the axial ends and are sprayed by the cooling device  14 , are not represented. 
     The stator  20  and the rotor  30  are arranged in such a way that an air gap  24  forms between them. Due to the spraying of the stator  20  with coolant, for example, oil, the coolant can penetrate the air gap  24 . 
     The rotor shaft  32  is designed as a hollow shaft  34  and has an axial bore  36 , which extends into the area of the rotor. The axial bore  36  therefore extends, at the least, into a portion of the axial extension of the rotor  30 . The rotor  30  has a radially extending air duct  40 , which extends from the rotor shaft  32  in the direction of the air gap  24 . The air duct  40  is arranged in rotor laminations  38  and can be designed in the form of a bore. The air duct  40  extends from an inner side  42  contacting the rotor shaft  32  to an outer side  44  facing the air gap  24 . The air duct can have, for example, a diameter from half of a millimeter (0.5 mm) to five millimeters (5 mm), preferably from one millimeter (1 mm) to three millimeters (3 mm), highly preferably of two millimeters (2 mm). 
     A bore  46  is arranged in the rotor shaft  32 . The bore  46  is arranged in an outer wall  48  in the area of the axial bore  36 . The bore  46  is aligned with the air duct  40 , and so air flows out of the axial bore  36  into the air duct  40 . 
     The axial bore  36  is arranged in the rotor shaft  32  in such a way that the rotor shaft  32  is open at one of the axial ends of the rotor shaft  32 . Due to the arrangement of the axial bore  36 , the bore  46 , and the air duct  40 , an air flow is made possible from the outside through the rotor shaft  32  and the rotor  30  into the air gap  24  between the stator  20  and the rotor  30 . The air then flows through the air gap  24  in the direction of the axial end faces of the stator  20  and of the rotor  30 . In this way, liquid coolant, such as, for example, oil, which is utilized for spraying the stator  20 , is displaced out of the air gap  24 . 
     In  FIG. 1 , the air duct  40  is centrally arranged in the axial extension of the rotor  30 . In this way, a uniform air flow can take place from the air duct  40  in the direction of the two end faces of the rotor  30 , and so oil that is possibly present in the air gap  24  can be quickly displaced. The air duct  40  does not need to be centrally arranged in the axial direction (Ax) of the rotor  30 . It is advantageous, however, when the air duct  40  is arranged, at the least, farther in the axial center of the rotor  30  than toward the ends. 
     The dimensions of the air duct  40  can depend on the dimensions of the electric machine. The embodiment shown preferably has an axial length (dimension in the axial direction Ax) of fifty millimeters (50 mm) to two hundred millimeters (200 mm). The rotor diameter preferably has values in the range from eighty millimeters (80 mm) to two hundred millimeters (200 mm). The air duct  40  has a diameter of half of a millimeter (0.5) mm to five millimeters (5 mm). 
       FIG. 2  is a section drawing along the line A-A from  FIG. 1  and shows an electric machine  10  with four air ducts  40 , which are uniformly distributed over the circumference of the rotor. Another, preferably higher number of air ducts  40  can also be provided, of course, in order to displace liquid out of the air gap  24  between the stator  20  and the rotor  30 . A non-uniform distribution of the air ducts is also conceivable, for example, in a grouped arrangement. 
       FIG. 3  shows a system  16  including an electric machine  10  and an air supply device  50  for supplying air into the rotor shaft  32  of the electric machine  10 . The air supply device  50  includes a pump  52  in order to pump air into the rotor shaft  32 . The pump  52  is connected to the rotor shaft designed as a hollow shaft  34  via a, for example, tube-like connection  54 . In this way, air can be pumped into the axial bore  36  of the rotor shaft  32 , and so an air flow is generated in the axial bore, which is pumped through the air duct  40  or the multiple air ducts  40  into the air gap  24 . Air can also already flow through an electric machine  10  at rest, and so fluid, in particular oil-based coolant, is displaced out of the air gap  24  between the stator  20  and the rotor  30  before the machine  10  is set into rotation. Since oil has a higher viscosity than air, the losses of the electric machine can be reduced due to the removal of oil from the air gap  24  and, in fact, even during the start-up phase of the machine. Therefore, the effectiveness and the efficiency of the electric machine  10  are increased overall, which is important, in particular, during the use of the electric machine  10  as a traction motor in a vehicle. 
     The invention was comprehensively described and explained with reference to the drawings and the description. The description and the explanation are to be understood as an example and are not to be understood as limiting. The invention is not limited to the disclosed embodiments. Other embodiments or variations result for a person skilled in the art within the scope of the utilization of the present invention and within the scope of a precise analysis of the drawings, the disclosure, and the following claims. 
     In the claims, the words “comprise” and “comprising” do not rule out the presence of further elements or steps. The indefinite article “a” does not rule out the presence of a plurality. A single element or a single unit can carry out the functions of several of the units mentioned in the claims. The mere mention of a few measures in multiple various dependent claims is not to be understood to mean that a combination of these measures cannot also be advantageously utilized. Reference numbers in the claims are not to be understood as limiting. 
     Modifications and variations can be made to the embodiments illustrated or described herein without departing from the scope and spirit of the invention as set forth in the appended claims. In the claims, reference characters corresponding to elements recited in the detailed description and the drawings may be recited. Such reference characters are enclosed within parentheses and are provided as an aid for reference to example embodiments described in the detailed description and the drawings. Such reference characters are provided for convenience only and have no effect on the scope of the claims. In particular, such reference characters are not intended to limit the claims to the particular example embodiments described in the detailed description and the drawings. 
     REFERENCE CHARACTERS 
     
         
           10  electric machine 
           12  housing 
           14  cooling device 
           16  system 
           20  stator 
           24  air gap 
           30  rotor 
           32  rotor shaft 
           34  hollow shaft 
           36  axial bore 
           38  rotor laminations 
           40  air duct 
           42  inner side 
           44  outer side 
           46  bore 
           48  wall 
           50  air supply device 
           52  pump 
           54  connection