Patent Publication Number: US-2023142019-A1

Title: Aircraft electric motor

Description:
STATEMENT OF FEDERAL SUPPORT 
     This invention was made with government support under Contract No. DE-AR0001351 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     The present disclosure relates to electric motors, and more particularly, to electric motor assemblies with high efficiency and power density with a light weight for aircraft applications. 
     Traditional electric motors may include a stator and a rotor, with electrical motor windings in the stator that, when energized, drive rotation of the rotor about a central axis. Heat is generated in the motor windings, which are located in slots in the stator. The windings are separated from the exterior of the motor by layers of insulation and laminated steel, which makes up the stator. These contributors to internal thermal resistance limit the allowable heat generation and thus the allowable electrical current in the windings. The energy density of an electric motor is typically limited by heat dissipation from the motor windings of the stator. The requirement to be met is a maximum hot spot temperature in the motor windings that is not to be exceeded. Conventional motor thermal management includes natural convection from large fins on the outside of a motor jacket, or liquid cooling in the motor jacket. Both of these solutions undesirably add volume and/or weight to the motor, due to the addition of, at least, the jacket. 
     BRIEF DESCRIPTION 
     According to some embodiments of the present disclosure, aircraft electric motors are provided. The aircraft electric motors include a motor unit having a rotor and a stator, wherein the stator includes a plurality of windings and cooling channels arranged to provide cooling to the plurality of windings, a drive unit configured to drive operation of the motor unit, and a cooling system. The cooling system includes at least one cooling channel integrally formed within at least one winding, wherein the at least one cooling channel comprises an inlet to receive a two-phase cooling fluid and an outlet configured to discharge the two-phase cooling fluid, wherein a cross-sectional area of the at least one cooling channel at the inlet is less than a cross-sectional area of the at least one cooling channel at the outlet. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one winding has a first end and a second end, wherein the inlet is formed in the first end and the outlet is formed in the second end. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one winding has a first end and a second end, wherein the inlet is formed in the first end and the outlet is formed in the first end. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one cooling channel comprises a plurality of cooling channels formed within the at least one winding. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one winding has a first end and a second end, wherein the inlet of each of the plurality of cooling channels is formed in the first end and the outlet of each of the plurality of cooling channels is formed in the second end. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one winding has a first end and a second end, wherein the inlet of each of the plurality of cooling channels is formed in the first end and the outlet of each of the plurality of cooling channels is formed in the first end. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one winding comprises a turn proximate the second end of the winding. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one cooling channel comprises a single inlet and a plurality of outlets, wherein the single inlet is fluidly connected to each of the plurality of outlets. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the least one cooling channel comprises an inlet section defined by a single passage and a branching section defined by a plurality of passages, wherein the single passage of the inlet section is fluidly coupled to each of the plurality of passages of the branching section. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one cooling channel comprises at least one surface feature arranged to reduce a cross-sectional area of the at least one cooling channel proximate the inlet and has a decreasing impact on the cross-sectional area of the at least one cooling channel in a direction toward the outlet. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one surface feature comprises at least one rib or blade extending from the inlet toward the outlet along the at least one cooling channel. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the at least one surface feature comprises a surface coating applied to a surface of the at least one cooling channel. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the cross-sectional area of the inlet of the at least one cooling channel is 50% or less than the cross-sectional area of the outlet. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the cross-sectional area of the outlet of the at least one cooling channel is at least twice the cross-sectional area of the inlet. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the windings are arranged in a U-shape configuration. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the motor unit comprises rotor having U-shaped magnets arranged about the windings of the stator. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the cooling system further includes a header and a heat exchanger configured to supply cooling fluid into the at least one cooling channel. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include a pump configured to pump the cooling fluid from the header into the at least one cooling channel. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the rotor and stator are arranged in an annular configuration. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the two-phase cooling fluid is one of a hydrofluorocarbon (HFC), a hydrofluro-olefin (HFO), or a hydrofluoroether (HFE). 
     In accordance with some embodiments, aircraft electric motors are provided. The aircraft electric motors include a motor unit having a rotor and a stator, wherein the stator includes a plurality of windings and cooling channels arranged to provide cooling to the plurality of windings, a means for driving operation of the motor unit, and a cooling system. The cooling system includes at least one cooling channel integrally formed within at least one winding, wherein the at least one cooling channel comprises an inlet to receive a two-phase cooling fluid and an outlet configured to discharge the two-phase cooling fluid, wherein the at least one cooling channel defines a means for accommodating a change in phase of the two-phase fluid as it passes through the at least one cooling channel. 
     In addition to one or more of the features described herein, or as an alternative, further embodiments of the aircraft electric motors may include that the means for driving operation of the motor unit comprises at least one power module system, and the means for accommodating a change in phase of the two-phase fluid comprises a cross-sectional area of the at least one cooling channel at the inlet being less than a cross-sectional area of the at least one cooling channel at the outlet. 
     The foregoing features and elements may be executed or utilized in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, that the following description and drawings are intended to be illustrative and explanatory in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiments. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG.  1 A  is a partial view of an embodiment of electric motor; 
         FIG.  1 B  is a cross-sectional view of an embodiment of a stator core of the electric motor of  FIG.  1 A ; 
         FIG.  2 A  is a schematic illustration of an aircraft electric motor in accordance with an embodiment of the present disclosure; 
         FIG.  2 B  is a side elevation view of the aircraft electric motor of  FIG.  2 A ; 
         FIG.  2 C  is a partial cut-away illustration of the aircraft electric motor of  FIG.  2 A ; 
         FIG.  2 D  is a separated-component illustration of the aircraft electric motor of  FIG.  2 A ; 
         FIG.  3 A  is a schematic illustration of a rotor and stator of an aircraft electric motor in accordance with an embodiment of the present disclosure; 
         FIG.  3 B  is a schematic illustration of the rotor and stator of  FIG.  3 A  as arranged within a rotor sleeve in accordance with an embodiment of the present disclosure; 
         FIG.  4 A  is a schematic illustration of a portion of an aircraft electric motor system in accordance with an embodiment of the present disclosure; 
         FIG.  4 B  is a schematic illustration of a first end of the portion shown in  FIG.  4 A ; 
         FIG.  4 C  is a schematic illustration of a second end of the portion shown in  FIG.  4 A ; 
         FIG.  5 A  is a schematic illustration of a portion of an aircraft electric motor system in accordance with an embodiment of the present disclosure; 
         FIG.  5 B  is a schematic illustration of a first end of the portion shown in  FIG.  5 A ; 
         FIG.  5 C  is a schematic illustration of a second end of the portion shown in  FIG.  5 A ; 
         FIG.  6 A  is a schematic illustration of a portion of an aircraft electric motor system in accordance with an embodiment of the present disclosure; 
         FIG.  6 B  is a schematic illustration of a first end of the portion shown in  FIG.  6 A ; 
         FIG.  6 C  is a schematic illustration of a first end of the portion shown in  FIG.  6 A ; 
         FIG.  7 A  is a schematic illustration of a portion of an aircraft electric motor system in accordance with an embodiment of the present disclosure; 
         FIG.  7 B  is a schematic illustration of a first end of the portion shown in  FIG.  7 A ; and 
         FIG.  8    is a schematic view of a power system of an aircraft that may employ embodiments of the present disclosure. 
     
    
    
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, that the following description and drawings are intended to be illustrative and explanatory in nature and non-limiting. 
     DETAILED DESCRIPTION 
     Referring to  FIGS.  1 A- 1 B , schematic illustrations of an electric motor  100  that may incorporate embodiments of the present disclosure are shown.  FIG.  1 A  illustrates a cross-sectional view of the electric motor  100  and  FIG.  1 B  illustrates a cross-sectional view of a stator core of the electric motor  100 . The electric motor  100  includes a rotor  102  configured to rotate about a rotation axis  104 . A stator  106  is located radially outboard of the rotor  102  relative to the rotation axis  104 , with a radial air gap  108  located between the rotor  102  and the stator  106 . As illustrated, the rotor  102  may be mounted on a shaft  110  which may impart rotational movement to the rotor  102  or may be driven by rotation of the rotor  102 , as will be appreciated by those of skill in the art. The rotor  102  and the shaft  110  may be fixed together such that the rotor  102  and the shaft  110  rotate about the rotation axis  104  together as one piece. 
     The stator  106  includes a stator core  112  in which a plurality of electrically conductive stator windings  114  are disposed. In some embodiments, such as shown in  FIG.  1 A , the stator core  112  is formed from a plurality of axially stacked laminations  116 , which are stacked along the rotation axis  104 . In some embodiments, the laminations  116  are formed from a steel material, but one skilled in the art will readily appreciate that other materials may be utilized. The stator windings  114 , as shown, include core segments  118  extending through the stator core  112  and end turn segments  120  extending from each axial stator end  122  of the stator core  112  and connecting circumferentially adjacent core segments  118 . When the stator windings  114  are energized via an electrical current therethrough, the resulting field drives rotation of the rotor  102  about the rotation axis  104 . Although  FIG.  1 A  illustrates the stator core  112  arranged radially inward from the stator windings  114 , it will be appreciated that other configurations are possible without departing from the scope of the present disclosure. For example, in some embodiments, the stator structure may be arranged radially inward from a rotating rotor structure. 
       FIG.  1 B  is an axial cross-sectional view of the stator core  112 . Each lamination  116  of the stator core  112  includes a radially outer rim  124  with a plurality of stator teeth  126  extending radially inwardly from the outer rim  124  toward the rotation axis  104 . Each of the stator teeth  126  terminate at a tooth tip  128 , which, together with a rotor outer surface  130  (shown in  FIG.  1 A ) of the rotor  102 , may define the radial air gap  108 . Circumferentially adjacent stator teeth  126  define an axially-extending tooth gap  132  therebetween. Further, in some embodiments, a plurality of stator fins  134  extend radially outwardly from the outer rim  124 . 
     Electric motors, as shown in  FIGS.  1 A- 1 B  may require cooling due to high density configurations, various operational parameters, or for other reasons. For example, high-power-density aviation-class electric motors and drives may require advanced cooling technologies to ensure proper operation of the motors/drives. These machines are generally thermally limited at high power ratings and their performance can be improved by mitigating thermal limitations. To maintain desired temperatures, a thermal management system (TMS) is integrated into the system, which provides cooling to components of the system. 
     Onboard an aircraft, power requirements, and thus thermal management system (TMS) loads, are substantially higher during takeoff. Sizing of the TMS for takeoff conditions (i.e., maximum loads) results in a TMS having a high weight to accommodate such loads. This results in greater weight and lower power density during cruise conditions which do not generate such loads, and thus does not require a high cooling capacity TMS. Balancing weight constraints and thermal load capacities is important for such aviation applications. 
     In view of such considerations, improved aviation electric motors are provided herein. The aviation electric motors or aircraft electric motors, described herein, incorporate lightweight materials and compact design to reduce weight, improve thermal efficiencies, improve power efficiencies, and improve power density. 
     Turning now to  FIGS.  2 A- 2 D , schematic illustrations of an aircraft electric motor  200  in accordance with an embodiment of the present disclosure are shown.  FIG.  2 A  is an isometric illustration of the aircraft electric motor  200 ,  FIG.  2 B  is a side elevation view of the aircraft electric motor  200 ,  FIG.  2 C  is a partial cut-away view illustrating internal components of the aircraft electric motor  200 , and  FIG.  2 D  is a schematic illustration of components of the aircraft electric motor  200  as separated from each other. The aircraft electric motor  200  includes a motor housing  202 , a cooling system  204 , a first power module system  206 , and a second power module system  208 . 
     The motor housing  202  houses a stator  210  and a rotor  212 , with the rotor  212  configured to be rotatable about the stator  210 . In this illustrative embodiment, the rotor  212  includes a U-shaped magnet  214  arranged within a similarly shaped U-shaped rotor sleeve  216 . The rotor sleeve  216  is operably connected to a hub  218 . The hub  218  is fixedly attached to a first shaft  220 . The first shaft  220  is operably connected to a second shaft  222 . In some configurations, the first shaft  220  may be a high speed shaft and may be referred to as an input shaft. In such configurations, the second shaft  222  may be a low speed shaft and may be referred to as an output shaft. The connection between the first shaft  220  and the second shaft  222  may be by a gear assembly  224 , as described herein. 
     The cooling system  204  is configured to provide cooling to the components of the aircraft electric motor  200 . The cooling system  204 , as shown in  FIG.  2 D , includes a heat exchanger  226  and a header  228 . The heat exchanger  226  and the header  228  may form a closed-loop cooling system that may provide air-cooling to a working fluid at the heat exchanger  226 . The header  228  may be, in some configurations, a two-phase di-electric cooling header. A cooled working fluid may be pumped from the heat exchanger  226  into the header  228  using a pump  229  and distributed into embedded cooling channels  230  that are arranged within the stator  210 . As the aircraft electric motor  200  is operated, heat is generated and picked up by the working fluid within the embedded cooling channels  230 . This heated working fluid is then passed through the header  228  back to the heat exchanger  226  to be cooled, such as by air cooling. Although described as air-cooling, other cooling processes may be employed without departing from the scope of the present disclosure. 
     As shown, the heat exchanger  226  of the cooling system  204  may be a circular structure that is arranged about the motor housing  202 . This configuration and arrangement allows for improved compactness of the system, which may be advantageous for aircraft applications. The rotor sleeve  216  with the magnets  214 , the stator  210 , and the gear assembly  224  fit together (although moveable relative to each other) within the motor housing  202 , providing for a compact (low volume/size) design. 
     As noted above, the rotor sleeve  216  may be operably coupled to a first shaft  220  by the hub  218 . The first shaft  220  may be operably coupled to a first gear element  232  and the second shaft  222  may be operably coupled to a second gear element  234 . The first and second gear elements  232 ,  234  may form the gear assembly  224 . The first and second gear elements  232 ,  234  are arranged to transfer rotational movement from the first shaft  220 , which is driven in rotation by the hub  218  and the rotor sleeve  216  of the rotor  212 , to the second shaft  222 . In some embodiments, the first shaft  220  may be operably connected to a sun gear as the first gear element  232  that engages with a plurality of planetary gears and drives rotation of the second gear element  234  which may be operably connected to the second shaft  222 . In some embodiments, the second shaft  222  may be connected to a fan or other component to be rotated by the aircraft electric motor  200 . 
     The aircraft electric motor  200  includes the first power module system  206  and the second power module system  208 . The first and second power module systems  206 ,  208  can include capacitors and other electronics, including, but not limited to, printed circuit boards (PCBs) that may enable control and operation of the aircraft electric motor  200 . As such, the first and second power module systems  206 ,  208  may form, at least, a portion of a drive unit of the aircraft electric motor  200 . The profile of the aircraft electric motor  200  of the present disclosure presents a low profile or compact arrangement that reduces the volume of the entire power system, which in turn can provide for improved weight reductions. In some embodiments, the first and second power module systems  206 ,  208  may be electrically connected to the stator  210  to cause an electric current therein. As the electric current will induce an electromagnetic field which will cause the rotor  212  to rotate. 
     Referring now to  FIGS.  3 A- 3 B , schematic illustrations of a portion of an aircraft electric motor  300  in accordance with an embodiment of the present disclosure is shown.  FIGS.  3 A- 3 B  illustrate a portion of a rotor  302  and a stator  304  of the aircraft electric motor  300 .  FIG.  3 A  illustrates the rotor  302  and the stator  304  and  FIG.  3 B  illustrates these components arranged within a rotor sleeve  306 . 
     The rotor  302  is formed of a plurality of U-shaped magnets  308 . In some configurations, the plurality of magnets  308  can be arranged with alternating polarity in a circular or annular structure. Arranged within the “U” of the U-shaped magnets  308  is the stator  304 . The stator  304  is formed of a plurality of windings  310 . In this configuration, the windings  310  are arranged with a header  312 . The header  312  may be part of a cooling system, such as that shown and described above. The header  312  can be configured to cycle a working fluid through cooling channels  314  for cooling of the windings  310 , as shown in  FIG.  3 B . As shown in  FIG.  3 B , the cooling channels  314  may include a flow restrictor  315  arranged at an inlet side (or an outlet side) of the cooling channel  314 . The flow restrictor  315  may be used to throttle the flow of a cooling fluid to provide efficient cooling within the cooling channels  314 . 
     The windings  310  may be wrapped about a support structure  316 . The support structure  316 , in some embodiments and as shown in  FIG.  3 B , may include a laminate portion  318  and a magnetic portion  320 . In some such embodiments, the laminate portion  318  may be formed from cobalt steel laminate and the magnetic portion  320  may be formed from a soft magnetic composite. The laminate portion  318  may be provided to capture in-plane flux from outer and inner rotor. The magnetic portion  320  may be provided to capture end rotor flux and may take a shape/filler in a gap through the end turns of the coil. The windings  308  include end connections  322  and may be electrically connected to one or more power module systems of the aircraft electric motor, such as shown above. 
     As shown in  FIG.  3 B , the magnets  306  are U-shaped and arranged within the rotor sleeve  306 . The rotor sleeve  306  is a substantially U-shaped sleeve that is sized and shaped to receive the U-shaped magnets  308 . In this illustrative configuration, the rotor sleeve  306  can include an inner sleeve  324 . The inner sleeve  324  may be configured to provide support to a portion of the magnets  308 . It will be appreciated that there is no direct contact between the windings  310  and the magnets  308 . This lack of contact enables free rotation of the rotor  302  relative to the stator  304  during operation. 
     High-power-density aviation-class electric motor and drives, as described above, may require advanced cooling technologies. These machines are generally thermally limited at high power ratings and their performance can be improved by mitigating thermal limitations. Two-phase cooling is a highly efficient approach for cooling the heat generating components. In accordance with embodiments of the present disclosure, a cooling refrigerant is configured to boil and evaporate within embedded micro-channels in the electric components such as the winding. 
     In accordance with embodiments of the present disclosure, the channel structure of the cooling channels within windings of an aircraft electric motor are enhanced with structures that enhance heat transfer. As the fluid flows along the channel path, the pressure increases due to phase change from liquid to gas. In accordance with embodiments of the present disclosure, to accommodate the increase in pressure along the flow path, the cooling channels increase in area along the flow path. This effect can be achieved, in accordance with embodiments of the present disclosure, by increasing the diameter of the channel along a diverging flow path, modifying inner surface features to increase effective flow area, or by configuring the channels in a branching format. In accordance with embodiments of the present disclosure, the cooling channels can be of any size, particularly millimeter sized channels and microchannels may be implemented in some embodiments. Combined with an in-slot motor cooling configuration, the flow channels and motor windings can be co-located inside of motor slots, thus enhancing cooling of the systems. 
     Turning now to  FIGS.  4 A- 4 C , schematic illustrations of a portion of an aircraft electric motor  400  in accordance with an embodiment of the present disclosure are shown. The illustration of  FIGS.  4 A- 4 C  is of a winding  402  that may be part of a stator, similar to that shown and described above. In accordance with embodiments of the present disclosure, the winding  402  includes an embedded or internal cooling channel  404 . The winding  402  extends from first end  406  to a second end  408 . The cooling channel  404  extends from an inlet  410  formed in the first end  406  of the winding  402  to an outlet  412  formed in the second end  408  of the winding  402 . 
     The cooling channel  404 , in this embodiment, is not a uniform diameter passage through the winding  402 , but rather has an increasing diameter from the inlet  410  to the outlet  412 . In this embodiment, the transition is smooth and continuous from the inlet  410  to the outlet  412 . In some non-limiting embodiments, the inlet  410  may have a cross-sectional (circular) area that is 50% or less of the cross-sectional (circular) area of the outlet  412 . Stated another way, in some non-limiting embodiments, the cross-sectional area at the outlet  412  is at least twice the cross-sectional area at the inlet  410 . 
     Turning now to  FIGS.  5 A- 5 C , schematic illustrations of a portion of an aircraft electric motor  500  in accordance with an embodiment of the present disclosure are shown. The illustration of  FIGS.  5 A- 5 C  is of a winding  502  that may be part of a stator, similar to that shown and described above. In accordance with embodiments of the present disclosure, the winding  502  includes an embedded or internal cooling channel  504 . The winding  502  extends from a first end  506  to a second end  508 . The cooling channel  504  extends from an inlet  510  formed in the first end  506  of the winding  502  to a plurality of outlets  512  formed in the second end  508  of the winding  502 . 
     The cooling channel  504 , in this embodiment, is formed of an inlet section  514  that extends from the inlet  510  toward the second end  508  of the winding  502 . The cooling channel  504  separates from the inlet section  514  into a branching section  516 . In some embodiments, the inlet section  514  may not have any depth or length and may only be defined by the inlet  410  formed in the first end  506  of the winding  502  and the branching of the cooling channel  504  may begin immediately. In other embodiments, the inlet section  514  may extend for a length into the winding  502  prior to separating into the branching section  516 . As shown, the inlet  510  and the outlets  512  may be substantially similar in cross-sectional diameter. However, as a result of the branching of the cooling channel  504  and the multiple outlets  512 , the exit or outlet total cross-sectional area is greater than that of the inlet  510 . In some non-limiting embodiments, the inlet  510  may have a cross-sectional (circular) area that is  50 % or less of the total cross-sectional area of the outlets  512  (aggregated). Stated another way, in some non-limiting embodiments, the total cross-sectional area at the outlets  512  is at least twice the cross-sectional area at the inlet  510 . 
     It will be appreciated that the smooth increase in diameter configuration described with respect to  FIGS.  4 A- 4 C  can be implemented with the branching configuration of  FIGS.  5 A- 5 C . For example, each branch of the branching section my include increasing diameter passages. 
     Turning now to  FIGS.  6 A- 6 C , schematic illustrations of a portion of an aircraft electric motor  600  in accordance with an embodiment of the present disclosure are shown. The illustration of  FIGS.  6 A- 6 C  is of a winding  602  that may be part of a stator, similar to that shown and described above. In accordance with embodiments of the present disclosure, the winding  602  includes an embedded or internal cooling channel  604 . The winding  602  extends from a first end  606  to a second end  608 . The cooling channel  604  extends from an inlet  610  formed in the first end  606  of the winding  602  to an outlet  612  formed in the second end  608  of the winding  602 . 
     The cooling channel  604 , in this embodiment, is formed of a substantially uniform diameter passage (e.g., a diameter of the inlet  610  is substantially the same as a diameter of the outlet  612 ). However, to achieve an increased cross-sectional area, similar to that described above, the cooling channel  604  includes one or more surface features  614  on the interior surface of the passage walls. The surface features  614  are structures or material distributed along the axial length of the cooling channel  604  that are configured to restrict flow and/or reduce a cross-sectional area of the cooling channel. As shown in the embodiment of  FIGS.  6 A- 6 C , the surface features  614  are formed as axial ribs or blades that extend from the inlet  610  to the outlet  612 . In a flow direction from the inlet  610  to the outlet  612 , the surface features  614  reduce the amount of cross-sectional area they occupy within the cooling channel  604 . In this illustrative embodiment, the surface features  614  extend the full length of the cooling channel  604  (i.e., from the inlet  610  to the outlet  612 ). In other embodiments, the surface features may terminate at a point upstream of the outlet, and thus may not be present at all at the outlet. 
     In the embodiment of  FIGS.  6 A- 6 C , the surface features  614  are configured as ribs, rails, protrusions, blades, or similar structure. However, such surface features, in accordance with embodiments of the present disclosure are not so limited in structure. For example, the surface features that impact the cross-sectional flow area through a cooling channel may be formed as fins (axial and concentric), fins (rectangular, triangular, rounded, pin fins), turbulators, dimples, coatings having area reducing properties (e.g., thickness, roughness, texture, etc.). 
     The surface features  614  cause the cross-sectional area of the cooling channel  604  to increase from the inlet  610  to the outlet  612 . In some non-limiting embodiments, the inlet  610  may have an unobstructed cross-sectional area that is 50% or less than an unobstructed cross-sectional area at the outlet  612 . Stated another way, in some non-limiting embodiments, the total cross-sectional flow area at the outlet  612  is at least twice the total cross-sectional flow area at the inlet  610 . It will be appreciated that the surface features described with respect to  FIGS.  6 A- 6 C  may be incorporated into the other configurations described herein (e.g.,  FIGS.  4 A- 4 C,  5 A- 5 C , etc.). 
     Turning now to  FIGS.  7 A- 7 B , schematic illustrations of a portion of an aircraft electric motor  700  in accordance with an embodiment of the present disclosure are shown. The illustration of  FIGS.  7 A- 7 B  is of a winding  702  that may be part of a stator, similar to that shown and described above. In accordance with embodiments of the present disclosure, the winding  702  includes a plurality of embedded or internal cooling channels  704 . The winding  702  extends from a first end  706  to a second end  708 . Each cooling channel  704  extends from an inlet  710  formed in the first end  706  of the winding  702  to an outlet  712  also formed in the first end  708  of the winding  702 . That is, the inlet  710  and the outlet  712  are formed in the same end (first end  706 ) of the winding  702 . 
     The cooling channels  704  are similar to the cooling channels  404  described with respect to  FIGS.  4 A- 4 C  in that the cooling channels  704  have a relatively small diameter inlet  710  and a larger diameter outlet  712 . However, because the inlet  710  and the outlet  712  are formed in the same end (first end  706 ) of the winding  702 , each cooling channel  704  includes a turn  714 . In this embodiment, the turn  714  is proximate the second end  708  of the winding  702 . 
     Similar to the cooling channel  404 , in this embodiment, the cooling channels  704  are not uniform diameter passages through the winding  702 , but rather have increasing diameters from the inlet  710  to the outlet  712 . In this embodiment, the transition is smooth and continuous from the inlet  710  to the outlet  712  (inclusive of the turn  714 ). In some non-limiting embodiments, the inlet  710  may have a cross-sectional (circular) area that is  50 % or less of the cross-sectional (circular) area of the outlet  712 . Stated another way, in some non-limiting embodiments, in some non-limiting embodiments, the cross-sectional area at the outlet  712  is at least twice the cross-sectional area at the inlet  710 . 
     It will be appreciated that the multiple cooling channel configuration of  FIGS.  7 A- 7 B  may be arranged to include features or configurations shown and described above (e.g., surface features, branching channels, etc.). Furthermore, the multi-channel configuration of  FIGS.  7 A- 7 B  may be implemented with the other configurations, such that, for example, in  FIGS.  4 A- 4 C , multiple cooling channels  404  may be formed in a single winding  402  that extend from an inlet at the first end  406  to an outlet at the second end  408 . Similarly, in the branching configuration, multiple inlets may be arranged to each branch to outlets that are formed in the first or second end of the winding (e.g., having a turn if the outlet is formed at the same end as the inlet). 
     Referring now to  FIG.  8   , a power system  800  of an aircraft  802  is shown. The power system  800  includes one or more engines  804 , one or more electric motors  806 , a power bus electrically connecting the various power sources  804 ,  806 , and a plurality of electrical devices  810  that may be powered by the engines  804  and/or motors  806 . The power system  800  includes a power distribution system  812  that distributes power  814  through power lines or cables  816 . The electric motors  806  of the aircraft  802  may be configured similar to the aircraft electric motors shown and described above. 
     Advantageously, embodiments of the present disclosure provide for improved electric motors for aircraft and aviation applications. The aircraft electric motors of the present disclosure have improved cooling channels that may improve cooling while eliminating or at least reducing the challenges with cooling windings of aircraft electric motors. For example, advantageously, embodiments of the present disclosure may enable improved flow (e.g., flow stability) of a two-phase cooling fluid that passes through the cooling channels. As a two-phase cooling fluid passes from the relatively narrow inlet end of the cooling channels it may be a substantially liquid fluid. However, as heat is picked up along the cooling channel from the material of the winding, the fluid may transition to a gas and thus require a greater volume to ensure continuous and unimpeded flow, and thus the cross-sectional area of the cooling channels will be increased to accommodate the change in phase of the two-phase fluid as it passes through the cooling channel. As described herein, the cooling fluids may be a saturated refrigerant (e.g., dielectric refrigerants including, but not limited to, hydrofluorocarbons (HFC), hydrofluro-olefins (HFO), and/or hydrofluoroethers (HFE)). 
     The terms “about” and “substantially” are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” or “substantially” can include a range of ±8% or 5%, or 2% of a given value. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.