Patent Publication Number: US-2023150683-A1

Title: Electric machine

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is based upon and claims the benefit of priority from British Patent Application No. GB 2116498.3, filed on Nov. 16, 2021, the entire contents of which are herein incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure concerns an electric machine suitable for an aircraft propulsion system, and a propulsion system comprising the electric machine. 
     Description of Related Art 
     Electric machines can take the form of either generators, which convert mechanical torque to electrical energy, or motors, which convert electrical energy to mechanical torque. In either case, it is desirable to provide electric machines having a high power density, i.e. a high power per unit mass or volume. Such high power densities are particularly useful for applications such as aircraft propulsion systems, where both high power and low mass are desirable. 
     Such high power densities can result in cooling challenges. In particular, it can be difficult to provide sufficient coolant flow at a sufficiently low temperature to maintain low electric machine temperatures during use, where the electric machine has a physically small volume. Such problems are exacerbated where the coolant comprises a gaseous coolant such as air, which has a low heat transfer coefficient. 
     SUMMARY 
     The present disclosure seeks to provide an electric machine having improved cooling. 
     According to a first aspect there is provided an electric machine comprising: 
     a stator winding located within a stator housing;
 
the stator housing comprising an inlet chamber configured to communicate with a supply of coolant, and an outlet;
 
the stator housing comprising a baffle configured to separate the stator winding and outlet from the inlet chamber; wherein
 
the baffle comprises at least one aperture therethrough configured to supply an impingement flow of coolant to the stator winding.
 
     Advantageously, for a given coolant flow and inlet temperature, the effectiveness of the coolant in reducing stator winding temperatures is increased relative to conventional flood cooling, since the apertures within the baffle direct coolant at higher velocity toward the stator windings, which increases cooling effectiveness. 
     The baffle may be provided adjacent an end winding of the stator. The baffle may at least partly surround generally opposite sides of the end winding. The baffle may comprise at least one aperture adjacent each side of the end winding. 
     The stator winding may comprise first and second end windings at opposite sides of the stator housing. 
     The baffle may be provided adjacent the first end winding, and the outlet may be provided adjacent the second end winding. The stator winding may define a channel through which coolant is configured to flow from the baffle to the outlet. 
     Alternatively, a first outlet may be provided adjacent a first end winding separated from a first inlet by a first baffle, and a second outlet may be provided at a second end winding separated from a second inlet by a second baffle. The second baffle may comprise an end plate of the stator housing. Advantageously, interference between the baffle and winding terminations is avoided. 
     The first inlet may be configured to provide a lower coolant flow rate in use relative to the second inlet. The first outlet may be configured to pass a higher coolant flow rate in use relative to the second outlet. The winding may define a channel configured to communicate between the second inlet and first outlet. Consequently, coolant may flow in use from both the first and second inlets and through the stator winding to the first outlet, thereby preventing hot-spots in the winding. 
     The or each aperture in the or each baffle may comprise a nozzle configured to accelerate flow through the aperture. 
     According to a second aspect of the invention there is provided an aircraft propulsion system comprising an electric machine in accordance with the first aspect. 
     The electric machine may comprise one or both of a generator and a motor. 
     The propulsion system may comprise an internal combustion engine, and the generator may be configured to be driven by the internal combustion engine. 
     Alternatively or additionally, the propulsion system may comprise a propulsor, and the motor may be configured to drive the propulsor. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG.  1    is a plan view of an aircraft comprising an electric propulsion system; 
         FIG.  2    is a cross-sectional side view of the propulsion system of the aircraft of  FIG.  1   ; 
         FIG.  3    is a cross-sectional view of an electric machine in the form of a motor for use in the propulsion system of  FIG.  2   ; 
         FIG.  4    is a cross-sectional side view of part of a stator and stator housing of the electric machine of  FIG.  3   ; 
         FIG.  5    is a perspective side view of a baffle of the electric machine of  FIG.  3   ; 
         FIG.  6    is a cross-sectional side view of part of a first alternative stator for the electric machine of  FIG.  3   ; and 
         FIG.  7    is a cross-sectional side view of part of a second alternative stator for the electric machine of  FIG.  3   . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG.  1   , an aircraft  1  is shown. The aircraft is of conventional configuration, having a fuselage  2 , wings  3 , tail  4  and a pair of propulsion systems  5 . One of the propulsion systems  5  is shown in figure detail in  FIG.  2   . Each propulsion system is provided with electrical energy from an energy storage system and/or a generator  20 , which may be driven by a prime mover such as a gas turbine engine  22 . 
       FIG.  2    shows the propulsion system  5  schematically. The propulsion system  5   
     comprises an electrical machine in the form of an electric motor  28  configured to drive a propulsor in the form of a ducted fan  12  via a fan shaft  14 . The motor  28  is fed with cooling fluid such as air or liquid coolant via cooling air duct  16 , which is provided with pressurised cooling flow from the fan  12 . The fan  12  is housed within a nacelle  24 . 
     The motor  28  is of a conventional type, comprising a permanent magnet electric machine, though other types of electric machines could utilise the disclosed cooling arrangement, such as induction machines and brushed machines. 
       FIG.  3    shows the motor  28  in more detail. The motor  28  comprises a motor housing  30  which houses motor components. The motor housing  30  provides structural support for motor components, and is mounted to structural components of the propulsion system. Additionally, the housing  30  is generally fluid tight, save for openings where coolant ducts and electrical connections are provided. 
     The motor comprises a rotor  32  which is coupled to the fan shaft  14 , and is surrounded by a stator  34 , provided radially outward of the rotor  32 . The rotor  32  is configured to rotate about an axis X. The stator  34  comprises electrical windings  36 , which can be energised to produce a rotating magnetic field. The electrical windings  36  comprise end windings  37 , which are provided at axially forward and aft ends of the stator winding  36 . This rotating magnetic field interacts with a magnetic field of the rotor  29 , to cause rotation when acting as a motor. The windings are wound round a stator core  38 , which typically comprises a plurality of laminations made of steel or similar ferromagnetic material. 
     The motor housing  30  forms a stator housing, which contains the stator  34 , including the windings  36  and core  38 . A drive plate  31  is provided at an axial end, through which an axle (not shown) extends, for providing motive power. The stator housing  30  is penetrated by a coolant inlet duct  40 , and a coolant outlet duct  42 . The coolant inlet duct  40  communicates with an inlet chamber in the form of a coolant manifold  44  provided within the stator housing  30 . The coolant inlet manifold  48  comprises a space defined by the inner walls of the stator housing  30  and by a baffle  50 . The baffle  50  is positioned between the stator end windings  37  and the housing  30 , and thereby controls fluid flow between the coolant inlet manifold  48  and stator end windings  37 . 
     Part of the stator  34  is shown in further detail in  FIG.  4   . The baffle  50  is shown in further detail in  FIG.  5   , and is in the form of a ring or toroid which extends around the end windings  37  of the stator. The baffle  50  has a curved profile, which is curved to approximately match the curvature of the end windings  37 , is spaced from the end windings  37  in use, and has an open end at an axially extending inward side to accept the end windings  37 . Referring again to  FIG.  4   , seals  53 ,  55  are provided to seal between the baffle  50  and housing  30 . The baffle  50  further comprises a plurality of apertures  52 , which extend through the baffle  50  between the inlet manifold  44  and the stator end windings  37 . The baffle apertures are distributed about the whole circumference of the baffle ring, and various apertures preferably extend radially inwardly and outwardly, as well as axially, in a direction generally toward the end windings  37 . In some cases, the apertures  37  may be shaped to define a convergent nozzle, with the apertures converging in the direction of the end windings  37 . In one embodiment, the baffle  50  is formed by Additive Layer Manufacture (ALM) such as Direct Layer Deposition (DLD) to allow for complex nozzle geometries, which may direct coolant flow in desired directions at desired velocities and flow rates. A further plurality of apertures  54  are provided within the stator windings  36 . The stator core  38  comprises a hollow portion, through which coolant can flow. Similarly, at the other axial end, further apertures  56  are provided in the stator windings  36 , which lead to an outlet manifold  58 , which in turn communicates with the coolant outlet  42 . A sleeve  33  is provided at each end, to confine the coolant within the stator, and provides a seal between the stator and rotor. 
     A coolant passageway is therefore defined by the inlet  40 , baffle apertures  52 , end winding apertures  52 , stator core  38 , aperture  56 , outlet manifold  58  and outlet  42 . In use therefore, coolant fluid (in this case air, though liquid coolants such as water and glycol could also be used) flows through the inlet  40 , baffle apertures  52 , end winding apertures  52 , stator core  38 , aperture  56 , outlet manifold  58  and out the outlet  42 , as shown by the arrows in  FIG.  4   , picking up heat and cooling the stator windings  36  and stator core  38  in use. 
     It has however been found that particular attention needs to be paid to cooling of the stator end windings  37 . These experience high heat loads and temperatures in use, which can lead to failure. The present arrangement provides improved cooling of this area in particular. 
     As shown by the arrows in  FIG.  4   , coolant entering the inlet manifold is held back by the baffle  50 . The apertures  52  in the baffle  50  act to restrict the flow of coolant, whilst also accelerating the coolant and acting as nozzles, thereby directing coolant to impinge upon the stator end windings. 
     This direct impingement flow toward the stator end windings  37  has been found to greatly increase cooling effectiveness. Computer Fluid Dynamics (CFD) simulations were performed on a simulated motor. In the simulation, 5 litres/minute of coolant was supplied, which resulted in end winding temperatures of 108° C. In comparison simulations, with the same geometry, coolant temperatures and cooling flows, but with the baffle removed, hot spots of 131° C. were found at the end windings. Consequently, the impingement cooling provided by the baffle provided a 23° C. reduction in end winding temperatures. By raising the coolant flow rate to 19 litres/minute, a maximum winding temperature of 97° C. could be achieved—a 34° C. reduction. As will be understood, for various applications, the maximum number of holes and the diameter of the holes could be adjusted to provide optimum cooling. Without wishing to be limited by theory, this increased coolant effectiveness is believed to result from the increased local flow velocity at the end winding surface caused by the apertures acting as nozzles, resulting in an increased heat transfer coefficient. Additionally, the high-velocity coolant jets create a thing boundary layer of coolant over the windings, thus increasing the surface area in contact with the high-velocity coolant, and reducing hot-spots. 
       FIG.  6    shows a first alternative cooling arrangement for an electric machine in the form of a motor  128 . The motor  128  is similar to the motor  28 , having a rotor  132  and stator  134  comprising a core  138  and stator winding  136  provided within a housing  130 . However, the arrangement of cooling inlets, outlets and baffles differs from that of the motor  28 . 
     In the motor  128 , first and second inlets  140   a ,  140   b  are provided, which communicate respectively with first and second outlets  142   a ,  142   b . Each inlet  140   a ,  140   b  is provided at a respective axial end of the stator housing  130 , and feeds into a respective inlet manifold  148   a ,  148   b . Each inlet  140   a ,  140   b  is separated from its respective outlet  142   a ,  142   b  by a respective baffle  150   a ,  150   b . The baffles  150   a ,  150   b  are similar to the baffle  50  of the first embodiment, having apertures (not shown) extending therethrough, to provide fluid communication between the respective inlets  140   a ,  140   b  and outlets  142   a ,  142   b . Each baffle  150   a ,  150   b  is positioned adjacent a respective end winding  137   a ,  137   b  of the stator winding  136 . 
     Consequently, in use, coolant (such as air or a liquid coolant) flows from respective inlets  140   a ,  140   b  into respective inlet manifolds  148   a ,  148   b , through the apertures provided in the respective baffles  150   a ,  150   b , whereupon the coolant flow impinges on the end windings  137   a ,  137   b . Spent coolant is then directed out of respective outlets  142   a ,  142   b.    
     Consequently, each end winding is actively cooled by impingement cooling, while the stator core  138  is not actively cooled by the coolant. Such a design may be appropriate where room can be provided for a coolant system at each end, and where temperatures of the uncooled stator core are acceptable in use. 
     In order to prevent leakage between the two ends of the stator, full vacuum impregnation of the stator winding may be utilised, to ensure that no fluid passage is provided through the central portion of the stator winding. 
       FIG.  7    shows a second alternative cooling arrangement for an electric machine in the form of a motor  228 . 
     The motor  228  is again similar to the motors  28 ,  128 , having a rotor  332  and stator  334  comprising a core  238  and stator winding  236  provided within a housing  230 . The housing  230  comprises a drive plate  231  through which an axle extends. However, the arrangement of cooling inlets, outlets and baffles differs from that of the motors  28 ,  128 . 
     In the motor  228 , first and second inlets  240   a ,  240   b  are provided, which communicate respectively with first and second outlets  242   a ,  242   b  via respective baffles  250   a ,  250   b . Each inlet  240   a ,  240   b  is provided at a respective axial end of the stator housing  230 , and feeds into a respective inlet manifold  248   a ,  248   b . The first inlet  240   a  is separated from its outlet  242   a  by a baffle  250   a . The baffle  150   a  is similar to the baffles  50 ,  150  of the first and second embodiments, having apertures (not shown) extending therethrough, to provide fluid communication between the inlets  240   a , and outlet  242   a . The baffle  150   a  is positioned adjacent a first end winding  237   a , of the stator winding  236 , and extends around radially inner and outer sides of the end winding  237   a.    
     The second end winding  237   b  is provided with a baffle  250   b  having a different form. The baffle  250   b  forms part of the housing, and is provided at the end of the motor opposite the drive plate  231 . Again, apertures  252  are provided through the baffle  250   b , which extend in a generally axial direction, toward the end winding  237   b . Consequently, impingement cooling is provided in this location, though the impingement may be less effective, since coolant is provided in an axial direction only. As such, either reduced cooling effectiveness is achieved, or increased coolant flow is required. 
     In one embodiment, flow through the stator core  238  may be provided by providing a second inlet  240   b  and first outlet  242   a  having a larger diameter relative to the first inlet  240   a  and second outlet  242   b . Coolant flow may be permitted through channels (not shown) in the stator core  238 , resulting in the airflow shown in  FIG.  7   . 
     Such an arrangement allows for increased space in the stator housing for winding terminations on the non-drive end of the stator housing  230 . 
     Consequently, in use, coolant (such as air or a liquid coolant) flows from respective inlets  140   a ,  140   b  into respective inlet manifolds  148   a ,  148   b , through the apertures provided in the respective baffles  150   a ,  150   b , whereupon the coolant flow impinges on the end windings  137   a ,  137   b . Spent coolant is then directed out of respective outlets  142   a ,  142   b.    
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. For example, the electric machine may comprise a generator, such as the generator  20  driven by the gas turbine engine  22  of a propulsion system. The electric machine may be utilised in non-aerospace applications, or for non-propulsive purposes in an aircraft. Examples include fuel and hydraulic pumps. 
     Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.