Patent Description:
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.

European patent application <CIT> discloses a rotary electric machine. A cooling unit includes an outer periphery cooling portion that is disposed along an outer peripheral surface of a coil end and includes a plurality of injection holes for injecting a cooling medium onto the outer peripheral surface 3a, and an end surface cooling portion that is disposed along an axial end surface of the coil end and includes a plurality of injection holes for injecting the cooling medium onto the axial end surface.

International patent application <CIT> relates to a cooling channel for a winding overhang of an electrical machine. In order to conduct a cooling fluid, the cooling channel is designed with at least one inflow and at least one outflow and is annular for provision around the winding overhang. The cooling channel has an axially movable contact pressure part that is situated such that a cooling fluid flows against the contact pressure part, thus generating a contact pressure on the cooling channel.

Japanese patent application <CIT> discloses an outer shell composed of a stator frame and bearing brackets, a rotor that has a stator and a rotary shaft, and at the same time, partition members that separate the upstream side from the downstream one in axial flow fans which are mounted to the rotary shaft for fixing. An inner circumference side coil winding end part projects inside nearly into an arc shape at the end of stator coil winding covered with the partition members, while being located on the downstream side.

Japanese patent application <CIT> discloses an electric rotary machine including brackets, which has a core in a stator frame and are arranged on both sides, a winding constituted of a bottom side winding accommodated in the core and an aperture side winding, a rotating shaft which is arranged in the core and rotatably supported by the brackets, a baffle plate constituted of a base part covering an end portion of the winding, a side part and a fixing part. An axial flow fan is formed of a blade constituted of a hub, which is arranged corresponding to the base part of the baffle plate and is fitted in a rotating shaft and a tip, and a wind tunnel mounted on the frame. A boundary point turning to an end portion of the winding from an aperture side winding, the other end of the base part connected with a slant part and a circular arc part in this order from the side part of the baffle plate, and a rear edge of the front end formed of a cord length constituting the axial flow fan are installed. The boundary point, the other end and rear edge are almost coincidingly arranged in an axial direction position. The circular arc part of the baffle plate and the connection part of the base part are arranged to almost coincide with the substantially central part of the cord length of the blade tip.

Japanese patent publication <CIT> discloses an electric machine comprising a stator having a cooling flow provided by apertures located in a housing. A baffle redirects cooling flow within the housing.

The present disclosure seeks to provide an electric machine having improved cooling.

According to a first aspect there is provided an electric machine as defined in claims <NUM> to <NUM>.

According to a second aspect of the invention there is provided an aircraft propulsion system as defined in claims <NUM> to <NUM>.

With reference to <FIG>, an aircraft <NUM> is shown. The aircraft is of conventional configuration, having a fuselage <NUM>, wings <NUM>, tail <NUM> and a pair of propulsion systems <NUM>. One of the propulsion systems <NUM> is shown in figure detail in <FIG>. Each propulsion system is provided with electrical energy from an energy storage system and / or a generator <NUM>, which may be driven by a prime mover such as a gas turbine engine <NUM>.

<FIG> shows the propulsion system <NUM> schematically. The propulsion system <NUM> comprises an electrical machine in the form of an electric motor <NUM> configured to drive a propulsor in the form of a ducted fan <NUM> via a fan shaft <NUM>. The motor <NUM> is fed with cooling fluid such as air or liquid coolant via cooling air duct <NUM>, which is provided with pressurised cooling flow from the fan <NUM>. The fan <NUM> is housed within a nacelle <NUM>.

The motor <NUM> 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> shows the motor <NUM>, which is not in accordance with the present invention, in more detail. The motor <NUM> comprises a motor housing <NUM> which houses motor components. The motor housing <NUM> provides structural support for motor components, and is mounted to structural components of the propulsion system. Additionally, the housing <NUM> is generally fluid tight, save for openings where coolant ducts and electrical connections are provided.

The motor comprises a rotor <NUM> which is coupled to the fan shaft <NUM>, and is surrounded by a stator <NUM>, provided radially outward of the rotor <NUM>. The rotor <NUM> is configured to rotate about an axis X. The stator <NUM> comprises electrical windings <NUM>, which can be energised to produce a rotating magnetic field. The electrical windings <NUM> comprise end windings <NUM>, which are provided at axially forward and aft ends of the stator winding <NUM>. This rotating magnetic field interacts with a magnetic field of the rotor <NUM>, to cause rotation when acting as a motor. The windings are wound round a stator core <NUM>, which typically comprises a plurality of laminations made of steel or similar ferromagnetic material.

The motor housing <NUM> forms a stator housing, which contains the stator <NUM>, including the windings <NUM> and core <NUM>. A drive plate <NUM> is provided at an axial end, through which an axle (not shown) extends, for providing motive power. The stator housing <NUM> is penetrated by a coolant inlet duct <NUM>, and a coolant outlet duct <NUM>. The coolant inlet duct <NUM> communicates with an inlet chamber in the form of a coolant manifold <NUM> provided within the stator housing <NUM>. The coolant inlet manifold <NUM> comprises a space defined by the inner walls of the stator housing <NUM> and by a baffle <NUM>. The baffle <NUM> is positioned between the stator end windings <NUM> and the housing <NUM>, and thereby controls fluid flow between the coolant inlet manifold <NUM> and stator end windings <NUM>.

Part of the stator <NUM> is shown in further detail in <FIG>. The baffle <NUM> is shown in further detail in <FIG>, and is in the form of a ring or toroid which extends around the end windings <NUM> of the stator. The baffle <NUM> has a curved profile, which is curved to approximately match the curvature of the end windings <NUM>, is spaced from the end windings <NUM> in use, and has an open end at an axially extending inward side to accept the end windings <NUM>. Referring again to <FIG>, seals <NUM>, <NUM> are provided to seal between the baffle <NUM> and housing <NUM>. The baffle <NUM> further comprises a plurality of apertures <NUM>, which extend through the baffle <NUM> between the inlet manifold <NUM> and the stator end windings <NUM>. 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 <NUM>. In some cases, the apertures <NUM> may be shaped to define a convergent nozzle, with the apertures converging in the direction of the end windings <NUM>. In one embodiment, the baffle <NUM> 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 <NUM> are provided within the stator windings <NUM>. The stator core <NUM> comprises a hollow portion, through which coolant can flow. Similarly, at the other axial end, further apertures <NUM> are provided in the stator windings <NUM>, which lead to an outlet manifold <NUM>, which in turn communicates with the coolant outlet <NUM>. A sleeve <NUM> 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 <NUM>, baffle apertures <NUM>, end winding apertures <NUM>, stator core <NUM>, aperture <NUM>, outlet manifold <NUM> and outlet <NUM>. 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 <NUM>, baffle apertures <NUM>, end winding apertures <NUM>, stator core <NUM>, aperture <NUM>, outlet manifold <NUM> and out the outlet <NUM>, as shown by the arrows in <FIG>, picking up heat and cooling the stator windings <NUM> and stator core <NUM> in use.

It has however been found that particular attention needs to be paid to cooling of the stator end windings <NUM>. 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>, coolant entering the inlet manifold is held back by the baffle <NUM>. The apertures <NUM> in the baffle <NUM> 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 <NUM> has been found to greatly increase cooling effectiveness. Computer Fluid Dynamics (CFD) simulations were performed on a simulated motor. In the simulation, <NUM> litres / minute of coolant was supplied, which resulted in end winding temperatures of <NUM>. In comparison simulations, with the same geometry, coolant temperatures and cooling flows, but with the baffle removed, hot spots of <NUM> were found at the end windings. Consequently, the impingement cooling provided by the baffle provided a <NUM> reduction in end winding temperatures. By raising the coolant flow rate to <NUM> litres / minute, a maximum winding temperature of <NUM> could be achieved - a <NUM> 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> shows a first alternative cooling arrangement for an electric machine in the form of a motor <NUM>, which is not in accordance with the present invention. The motor <NUM> is similar to the motor <NUM>, having a rotor <NUM> and stator <NUM> comprising a core <NUM> and stator winding <NUM> provided within a housing <NUM>. However, the arrangement of cooling inlets, outlets and baffles differs from that of the motor <NUM>.

In the motor <NUM>, first and second inlets 140a, 140b are provided, which communicate respectively with first and second outlets 142a, 142b. Each inlet 140a, 140b is provided at a respective axial end of the stator housing <NUM>, and feeds into a respective inlet manifold 148a, 148b. Each inlet 140a, 140b is separated from its respective outlet 142a, 142b by a respective baffle 150a, 150b. The baffles 150a, 150b are similar to the baffle <NUM> of the first embodiment, having apertures (not shown) extending therethrough, to provide fluid communication between the respective inlets 140a, 140b and outlets 142a, 142b. Each baffle 150a, 150b is positioned adjacent a respective end winding 137a, 137b of the stator winding <NUM>.

Consequently, in use, coolant (such as air or a liquid coolant) flows from respective inlets 140a, 140b into respective inlet manifolds 148a, 148b, through the apertures provided in the respective baffles 150a, 150b, whereupon the coolant flow impinges on the end windings 137a, 137b. Spent coolant is then directed out of respective outlets 142a, 142b.

Consequently, each end winding is actively cooled by impingement cooling, while the stator core <NUM> 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> shows a second alternative cooling arrangement for an electric machine in the form of a motor <NUM>, which is in accordance with the present invention.

The motor <NUM> is again similar to the motors <NUM>, <NUM>, having a rotor <NUM> and stator <NUM> comprising a core <NUM> and stator winding <NUM> provided within a housing <NUM>. The housing <NUM> comprises a drive plate <NUM> through which an axle extends. However, the arrangement of cooling inlets, outlets and baffles differs from that of the motors <NUM>, <NUM>.

In the motor <NUM>, first and second inlets 240a, 240b are provided, which communicate respectively with first and second outlets 242a, 242b via respective baffles 250a, 250b. Each inlet 240a, 240b is provided at a respective axial end of the stator housing <NUM>, and feeds into a respective inlet manifold 248a, 248b. The first inlet 240a is separated from its outlet 242a by a baffle 250a. The baffle 150a is similar to the baffles <NUM>, <NUM> of the first and second embodiments, having apertures (not shown) extending therethrough, to provide fluid communication between the inlets 240a, and outlet 242a. The baffle 150a is positioned adjacent a first end winding 237a, of the stator winding <NUM>, and extends around radially inner and outer sides of the end winding 237a.

The second end winding 237b is provided with a baffle 250b having a different form. The baffle 250b forms part of the housing, and is provided at the end of the motor opposite the drive plate <NUM>. Again, apertures <NUM> are provided through the baffle 250b, which extend in a generally axial direction, toward the end winding 237b. 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 <NUM> may be provided by providing a second inlet 240b and first outlet 242a having a larger diameter relative to the first inlet 240a and second outlet 242b. Coolant flow may be permitted through channels (not shown) in the stator core <NUM>, resulting in the airflow shown in <FIG>.

Such an arrangement allows for increased space in the stator housing for winding terminations on the non-drive end of the stator housing <NUM>.

Claim 1:
An electric machine (<NUM>) comprising:
a stator winding (<NUM>) located within a stator housing (<NUM>);
the stator housing comprising an inlet chamber (<NUM>) configured to communicate with a supply of coolant, and an outlet (<NUM>);
the stator housing comprising a baffle (<NUM>) configured to separate the stator winding and outlet from the inlet chamber; wherein
the baffle comprises at least one aperture (<NUM>) therethrough configured to supply an impingement flow of coolant to the stator winding; wherein
a first outlet (142a) is provided adjacent a first end winding (137a) separated from a first inlet (140a) by a first baffle (150a), and a second outlet (142b) is provided at second end winding (137b) separated from a second inlet (104b) by a second baffle (150b); characterized in that
the first inlet is configured to provide a lower coolant flow rate in use relative to the second inlet;
the first outlet is configured to pass a higher coolant flow rate in use relative to the second outlet; and
the winding (<NUM>) defines a channel configured to communicate between the second inlet and first outlet.