Patent Publication Number: US-2021194324-A1

Title: Rotor air cooling system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present disclosure claims the benefit of priority of co-pending European Patent Application No. 19218519.7, filed on Dec. 20, 2019, and entitled “ROTOR AIR COOLING SYSTEM,” the contents of which are incorporated in full by reference herein. 
     TECHNICAL FIELD 
     The present disclosure relates to a rotor air cooling system. 
     BACKGROUND 
     In electric machines, such as electrical motors in electric or hybrid vehicles, thermal capacity of rotor magnets and stator winding affects available output power of the electric machines. The rotor magnets typically have a thermal limit of 150° C. and when temperature increases further, demagnetization occurs which reduce performance or the machine. The stator winding which consists of multiple copper wires is insulated with a material which has a temperature limit of approximately 180° C. and if the temperature increases further, thermal fatigue cracks can occur which can cause shortcuts and terminate the electric machine. 
     The rotor and the stator windings need to be cooled in order to improve performance of the electric machine. This cooling is commonly performed using a liquid medium such as oil of water, resulting in a more complex and expensive cooling mechanism. 
     SUMMARY 
     An objective is to solve, or at least mitigate, this problem and thus provide an improved rotor air cooling system. 
     This objective is attained in an aspect by a rotor air cooling system including an outer housing and an inner housing configured to be inserted into the outer housing. The system further includes a stator attached to an interior side of the inner housing, a main rotor body included in the inner housing, the main rotor body being arranged around a rotor shaft arranged to extend through the inner housing and the outer housing, wherein cooling air axially enters the main rotor body in an inlet end, passes through cavities of the main rotor body, and exits the main rotor body in an outlet end, and a water jacket arranged between the inner housing and the outer housing. Moreover, the system includes a heat exchanger for dissipating heat from the cooling air exiting the main rotor body; wherein the air exiting the main rotor body is radially guided, via an air outlet of the inner housing being aligned with an air outlet of the outer housing, through the heat exchanger before being arranged to be radially guided via an air inlet of the outer housing being aligned with an air inlet of the inner housing to the air inlet of the main rotor body for recirculation of the cooled air through the main rotor body. 
     Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  schematically illustrates a rotor air cooling system in a side sectional view according to an embodiment; 
         FIG. 2  illustrates a perspective exterior view of the air rotor cooling system according to an embodiment; 
         FIG. 3  illustrates a rotor in a perspective view being utilized in the air rotor cooling system according to an embodiment; 
         FIG. 4  illustrates a portion of the rotor of  FIG. 3 ; 
         FIG. 5  illustrates the rotor of  FIG. 3  being arranged with an axial input endplate according to an embodiment; and 
         FIG. 6  illustrates the rotor of FIG.  5  further being arranged with an impeller according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the disclosure are shown. 
     These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. 
       FIG. 1  schematically illustrates a rotor air cooling system  10  in a side sectional view according to an embodiment. 
     The system  10  includes a rotor  11  contained in an inner housing  12 . The inner housing is configured to be inserted into an outer housing  14 . A cover  15  attached to the inner housing  12  ensures that the inner housing  12  and the outer housing  14  become enclosed spaces upon the inner housing  12  being inserted into the outer housing  14 . The rotor  11  includes a rotor shaft  11   a  penetrating the inner and outer housing  12 ,  14  and a main rotor body  11   b , commonly referred to as a rotor stack, being encapsulated in the inner housing  12  and surrounding the shaft  11   a . The rotor shaft  11   a  is coupled to the inner housing  12  and the outer housing  14  via rotor bearings. As will be illustrated in more detail in the following, the main rotor body  11   b  includes axially oriented cavities  11   c  where air will pass through the main rotor body  11   b  as it rotates. The direction of the rotor cooling air flow is indicated by means of arrows in  FIG. 1 . 
     Hence, air will axially enter the rotor stack  11   b  in an inlet end, pass through the cavities  11   c  of the main rotor body  11   b , and then exit the main rotor body  11   b  at an outlet end. In order to avoid the rotor  11  being overheated, the rotor  11  must be cooled, which is typically undertaken using for instance a liquid cooling medium such as oil or water being applied to the rotor as previously discussed. 
     In this embodiment, a water jacket  13  is arranged in the relatively tight space between the outer housing  14  and the inner housing  12 . A first flange  16  and a second flange  17  is arranged to radially protrude from an exterior side of the inner housing  12  and extend around a periphery of the inner housing  12  to abut an interior side of the outer housing  14  (or vice versa), thereby forming a casing surrounding the inner housing  12  where water can be accommodated, thereby creating the water jacket  13 . Cold water will thus be applied in the casing formed by the inner housing  12 , the outer housing  14  and the two radially protruding flanges  16 ,  17 , thereby forming the water jacket  13  which will cool the inner housing  12 . 
     Fixedly arranged to an interior side of the inner housing  12  is also a stator  18  including stator windings  27 . 
     Further, a heat exchanger  19  may in an embodiment be arranged in an enclosed space  20  at an exterior side of the outer housing  14  for dissipating heat from the heated rotor cooling air having passed through the main rotor body  11   b  and exiting at its outlet end. The space  20  may be enclosed with a metal plate acting as a lid  21 . It may be envisaged that the heat exchanger  19  is integrated with the outer housing  14  as will be illustrated in the following. It is envisaged that the system  10  may include a plurality of heat exchangers utilized to cool air circulating in the system. 
     A number of configurations are envisaged for the heat exchanger  19 ; it may be integrated with or attached to the outer housing  14 . Alternatively, the heat exchanger  19  may be part of or attached to the inner housing  12  or even arranged externally from the inner and outer housing and connected to the system  10  via closed air channels. 
     The inner housing  12  includes an air outlet  22  aligned with an air outlet  23  of the outer housing  14  to enable fluid communication between the outlet end of the main rotor body  11   b  and an inlet of the heat exchanger  19 . 
     As further can be seen, the outer housing  14  includes an air inlet  24  where air will enter into the inner housing  12  from an open backend  25  of the inner housing  12  to enable fluid communication between an outlet end of the heat exchanger  19  and an inlet end of the main rotor body  11   b.    
     Thus, with the created fluid communication between the heat exchanger  19  and the interior of the inner housing  12 , the air exiting the main rotor body  11   b  is radially guided (due to direction of air flow through the system  10 ) to pass via the respective air outlet  22 ,  23  of the inner and outer housing  12 ,  14  through the heat exchanger  19  where heat is dissipated from the air before the air is being radially guided via the air inlet  24  of the outer housing  14  and the open backend  25  of the inner housing  12  to the air inlet of the main rotor body  11   b  for recirculation of the cooled air through the cavities  11   c  of the main rotor body  11   b . 
     Again with reference to  FIG. 1 , to conclude, cooled air axially enters the rotor stack  11   b  at its inlet end and passes through the cavities  11   c  of the rotor stack  11   b . Hence, this cooled air cools the rotor  11 , i.e. the rotor shaft  11   a  as well as the rotor stack  11   b . Further advantageous is that the cooled air entering the inner housing  12  also will cool the stator  18  and its windings  27 . 
     Upon exiting at the outlet end of the rotor stack  11   b , the warmed-up air will rise and thus be radially guided in the inner housing  12  towards the air outlets  22 ,  23  of the inner and outer housing  12 ,  14  via which the warmed-up air enters the enclosed space  20  and is cooled off by the heat exchanger  19 . Further, the cold water of the water jacket  13  surrounding the inner housing  12  will cool the circulated air. it is noted that the water jacket  13  servers as a general cooling element for various components of the system, such as the heat exchanger  19 , the stator  18 , the rotor  11 , etc. 
     The air being cooled off passing through the heat exchanger  19  will reach an outlet end of the heat exchanger  19  and enter the inner housing  12  in a radial direction via the air inlet  24  of the outer housing  14  and the open backend  25  of the inner housing  12  before again being recirculated through the cavities  11   c  of the main rotor body  11   b  in order to advantageously cool the rotor  11  as well as the stator  18  and its windings  27 . 
     Advantageously, an effective rotor cooling system is thus attained using air instead of liquid, such as water or oil. This provides for a more inexpensive and non-complex cooling system  10 . Further, the handling is far messier when using liquid instead of air. 
       FIG. 2  illustrates a perspective exterior view of the air rotor cooling system  10  where for illustrative purposes the inner housing  12  has not been inserted int the outer housing  14 . 
     As can be seen, the inner housing  12  includes an air outlet  22 . Further, the first flange  16  and second flange  17  are shown, which protrudes radially from the inner housing  12 —more so than the other three flanges—in order to abut the inner surface of the outer housing to create the water jacket (not shown in  FIG. 2 ) upon the inner housing  12  being inserted into the outer housing  14  as indicated with the arrow. The water jacket will extend axially between the first flange  16  and the second flange  17  and typically surround the inner housing  12  when the system  10  is in operation. The three flanges in between the enclosing flanges  16 ,  17  serves as a spiral-shaped rib  26  extending along a length of the exterior side for guiding the water accommodated in the casing when the inner housing  12  is inserted into the outer housing  14 . 
     When the inner housing  12  is inserted into the outer housing  14 , the cover  15  enclosing the inner housing  12  will further close the outer housing  14 . Moreover, the air outlet  22  of the inner housing  12  will be aligned with the air outlet  23  of the outer housing  14 , while the air inlet  24  of the outer housing  14  will be in fluid communication with the open backend  25  of the inner housing. 
     In the embodiment shown in  FIG. 2 , the lid  21  of  FIG. 1  has been removed to show the heat exchanger  19  being integrated with the outer housing, though which the rotor cooling air passes from the air inlet  23  to the air outlet  24  and is recirculated in the inner housing  12 . During operation, the lid  21  of  FIG. 1  is attached to the outer housing  14  to create the enclosed space for the heat exchanger  19 . As can be seen, the heat exchanger  19  is in this exemplifying embodiment arranged with wave-shaped cooling fins. However, any appropriate cooling fin shape may be envisaged; such as for instance rectangular fins, offset strip fins, triangular fins, etc. 
     The cavities  11   c  extending through the main rotor body  11   b  from the inlet end to the outlet end, via which the rotor cooling air passes, may extend in an axial direction of the rotor  11 , i.e. in parallel with the rotor shaft  11   a . However, in an embodiment, the cavities  11   c  extend slightly helically trough the main rotor body  11   b  in relation to the rotor shaft  11   a.    
       FIG. 3  illustrates a rotor  11  in a perspective view according to such an embodiment. Hence, the main rotor body  11   b  is arranged around the rotor shaft  11   a . The main rotor body  11   b  includes cavities  11   c  where air will pass through the rotor body  11   b  from the inlet side to the outlet side as illustrated with arrows. In this embodiment, the cavities  11   c  are arranged to extend helically from the inlet end of the main rotor body  11   b  to the outlet end of the main rotor body  11   b  with respect with respect to the rotor shaft  11   a.    
     In an embodiment, the helix angle of the helically shaped cavities  11   c  extending through the main rotor body  11   b  is about 5-10°, such as 6-8°. Advantageously, the helically shaped cavities  11   c  results in greater air velocities through the main rotor body  11   b , and thus a higher air circulation velocity in the system, which effectively improves transfer of heat from the rotor and stator in the system. 
     With reference to  FIGS. 3  and  FIG. 4 , in practice, the main rotor body  11   b  is formed by individual circular metal discs  30 - 34  (in practice tens or even hundreds of discs may be utilized depending on the size of the rotor) axially arranged along the rotor shaft  11   a  and abutting a preceding disc to form the main rotor body  11   b , each having through-holes  30   a - 30   b ,  31   a - 31   b ,  32   a - 32   b  distributed around the disc (9 holes in the example of  FIGS. 3 and 6  in the example of  FIG. 4 ) where the number of holes correspond to the number of cavities  11   c  in the main rotor body  11   b . The through-holes are illustrated as having pentagonal shape in the FIG.s. However, the trough-holes may alternatively be circular, triangular, mushroom-shaped, etc., or have any appropriate shape. Further, any appropriate number of through-holes can be utilized depending on the particular implementation. As is understood, the discs are tightly abutted to form a compact set of discs. The through-holes may further be arranged with cooling fins to increase the heat exchange. This may for instance be attained by arranging inner walls of the through-holes with small tooth-shaped cooling fin members. 
     The discs  30 - 34  are thus “stacked” one after another along the rotor shaft  11   a , which explains why the main rotor body  11   b  commonly is referred to as the “rotor stack”. 
     In order to attain axial cavities  11   c  through the main rotor body  11   b , i.e. cavities  11   c  extending in parallel with the rotor shaft  11   a , the discs  30 - 34  are arranged axially one after another along the rotor shaft  11   a  to form the main rotor body  11   b , the holes of one disc  30  being aligned with the holes of a following disc  31  until all discs have been arranged around the shaft. 
     However, as shown in the embodiment of  FIG. 4 , each disc is rotationally offset around the rotor shaft with respect to the preceding disc thereby creating cavities extending helically through the main rotor body. 
     Thus, after having arranged the first disc  30  around the rotor shaft (the shaft not being shown in  FIG. 4 ), the second disc  31  is arranged around the rotor shaft and slightly rotationally offset with respect to the first disc  30 , in this example in a counter-clockwise direction. As a result, the through-holes  31   a ,  31   b  of the second disc  31  is rotationally offset with respect to the corresponding through holes  30   a ,  30   b  of the preceding first disc  30 . Thereafter, the third disc  32  is arranged around the rotor shaft and slightly rotationally offset with respect to the second disc  31  (in the same rotational direction as the previous discs) causing the through-holes  32   a ,  32   b  of the third disc  32  to become rotationally offset with respect in relation to the corresponding through holes  31   a ,  31   b  of the preceding second disc  31 , and so on. 
     Advantageously, this rotational offset of the discs  30 - 34  causes the cavities  11   c  created by the through-holes to extend helically with respect to the rotor shaft  11   a.    
       FIG. 5  illustrates a further embodiment, where an axial input endplate  40  is arranged around the rotor shaft  11   a  at the inlet end of the main rotor body  11   b . Through-holes of axial input endplate  40  is aligned with the through holes at the inlet end of the main rotor body  11   b  forming the cavities  11   c . Hence, the axial input endplate  40  acts as an axial fan for increasing the flow of cooled air through the main rotor body  11   b.    
     The axial input endplate  40  has at least two advantages; firstly, it is used to balance the discs  30 - 34  of the main rotor body. That is, it can be pressed against the inlet end of the main rotor body  11   b  to have the discs  30 - 34  tightly abut each other along the rotor shaft  11   a . Secondly, the axial input endplate  40 —being an axial fan—guides the air having been cooled off by the heat exchanger into the main rotor body  11   b  axially, creating a high pressure zone at the inlet side of the main rotor body  11   b , which will push air into the cavities  11   c.    
       FIG. 6  illustrates a further embodiment, where an impeller  50  is arranged around the rotor shaft  11   a  at an outlet end of the main rotor body  11   b . the blades of the impeller  50  will advantageously facilitate the guiding of the rotor cooling air in a radially direction towards the heat exchanger (not shown in  FIG. 5 ). The impeller  50  distributes the air radially creating a low pressure zone at the outlet side of the main rotor body  11   b , which will draw pull air out of the cavities 11   c . It is noted that the use of the impeller  50  does not require that the axial input endplate  40  also is utilized. Typically, the impeller  50  is in practice arranged around the rotor shaft  11   a  such that the impeller abuts the outlet end of the main rotor body  11   b . 
     The aspects of the present disclosure have mainly been described above with reference to a few embodiments and examples thereof. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the disclosure, as defined by the appended patent claims. 
     Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.