Patent Description:
Electric machines are used in a wide variety of settings, including industrial, commercial, and consumer applications. In any setting, an electric machine may generate a considerable amount of heat through a number of different pathways. For example, heat is typically generated in an electric machine through electric resistance in electric current flowing through a rotor and/or stator, hysteresis losses due to changing magnetic fields, and resistive heating due to the eddy currents generated by magnetic fields. Additionally, friction in an electric machine's moving components also typically generate heat, requiring lubrication.

The heat generated by an electric machine may contribute to inefficiencies, malfunction, and failures if not properly managed. Electric machines typically have some form of cooling system to manage heat loads. Some cooling systems seek to distribute a coolant close to the heat source by providing cooling mechanisms that are inserted inside a rotor shaft or that surround a stator. These mechanisms, however, add weight and complexity, cause windage losses, and introduce the potential for rubbing with the rotor. Additionally, these mechanisms may themselves accumulate heat that impedes the performance of the cooling system and consequently the performance of the electric machine.

High performance applications require electric machines with a high power density, as the marketplace demands electric machines with larger power outputs and yet smaller machine sizes. The combination of larger power outputs and smaller machine sizes found in high power density electric machines gives rise to very demanding cooling requirements. Some cooling systems provide uneven cooling across various portions of a rotor, which can lead to rotor and/or rotor shaft warping of due to local temperature differences, as well as increased potential for rubbing between the rotor and stator. Additionally, if an electric machine is not sufficiently cooled, the temperature of the rotor, stator, or other components can exceed temperature limitations, leading to critical failures.

Accordingly, there exists a need for electric machines with improved cooling and lubrication distribution systems, and for systems and methods of cooling and lubricating electric machines.

<CIT>describes a lubricating device for a rotating electrical machine. <CIT> describes the cooling structure of a motor. <CIT> describes an electrically operated coolant pump. <CIT> describes a vapor trap and regulator for superconductive turbogenerators.

Claim <NUM> defines an electric machine. Claim <NUM> defines a method of cooling an electric machine. In the following, any method and/or apparatus referred to as embodiments but nevertheless do not fall within the scope of the appended claims are to be understood as examples helpful in understanding the invention. Aspects and advantages will be set forth in part in the following description, or may be obvious from the description, or may be learned through practicing the presently disclosed subject matter.

In one aspect, the present disclose embraces electric machines. An exemplary electric machine includes a stator, a rotor, and a rotor shaft operably coupled to the rotor. The rotor shaft includes a hollow region configured to relieve a coolant. The hollow region is defined by an inner surface that has a slope that increases from a first inner diameter at a first end to a second inner diameter at a second end. A force that includes a centrifugal force generated when rotating the rotor shaft at an operating rate of rotation causes the coolant to flow across the inner surface of the rotor shaft from the first end to the second end at a velocity depending at least in part on the slope of the inner surface of the rotor shaft.

In another aspect, the present disclosure embraces a method of cooling an electric machine. An exemplary method includes injecting a coolant into a first end of a hollow region of a rotor shaft of an electric machine in which the rotor shaft includes an inner surface which has a slope that increases from a first inner diameter at a first end to a second inner diameter at a second end. The slope may include a frustoconical or sloped profile and/or a stepped profile. The exemplary method further includes rotating the rotor shaft at an operational rate of rotation, with the rotating generating a force that includes centrifugal force acting upon the coolant in the rotor shaft. The force causes the coolant to flow across the inner surface of the rotor shaft from the first end to the second end. The exemplary method further includes transferring heat from the rotor shaft to the coolant flowing across the inner surface of the rotor shaft.

These and other features, aspects and advantages will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and, together with the description, serve to explain certain principles of the presently disclosed subject matter.

A full and enabling disclosure, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figures, in which:.

Reference now will be made in detail to exemplary embodiments of the presently disclosed subject matter, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation and should not be interpreted as limiting the present disclosure. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

It is understood that terms "upstream" and "downstream" refer to the relative direction with respect to fluid flow in a fluid pathway. It is also understood that terms such as "top", "bottom", "outward", "inward", and the like are words of convenience and are not to be construed as limiting terms. The terms "a" and "an" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Here and throughout the specification and claims, range limitations are combined and interchanged, and such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

The present disclosure generally provides electric machines with improved rotor coolant and lubrication distribution systems, and improved methods of cooling and lubricating an electric machine. The presently disclosed rotor coolant and lubrication distribution systems include one or more of a rotor shaft with a sloped inner surface that utilizes centrifugal force to cause coolant to flow across an inner surface of the rotor shaft, an impeller mounted on a rotor shaft to distribute coolant to an inner surface of the rotor shaft, and/or a rotor cooling conduit integrally formed within the body of a rotor shaft. These features, among others, may be included in an electric machine individually or in combination.

An electric machine configured according to the present disclosure may exhibit an improved power density through improved cooling efficiency, and/or through weight savings from the elimination of cooling mechanisms or other componentry found in previous electric machines for distributing a coolant. The presently disclosed rotor coolant and lubrication systems may provide substantially uniform cooling across a longitudinal length of a rotor shaft, thereby providing a substantially uniform temperature distribution and/or a substantially uniform width of the air gap across a longitudinal length of the rotor shaft. Such substantially uniform temperature distribution and/or substantially uniform width of the air gap reduce the potential for rotor warping and rubbing, among other things. The width of the air gap may be ascertained using any desired method known in the art. Additionally, the presently disclosed electric machines may exhibit reduced windage losses, simplified assembly and maintenance, and reduced potential for downtime caused by damaged or worn componentry. In some embodiments the presently disclosed rotor coolant and lubrication distribution systems may allow for more compact, lighter-weight electric machines, further improving power density and operating efficiency.

An electric machine may function as an electric motor and/or an electric generator. An electric motor converts electrical energy into mechanical energy. An electric generator converts mechanical energy into electrical energy. Some examples where an electric machine may be utilized include aircraft, marine vessels, motor vehicles, power generation facilities, manufacturing facilities, industrial machinery, and the like. In the context of an aircraft, an electric machine may be used to supply power to a turbomachine engine, such as a turbofan engine in an aircraft. The power from the electric machine may be used to start the turbomachine engine, or to provide propulsive thrust to the aircraft, including commercial, military, or civilian aircraft, as well as unmanned aircraft such as unmanned aerial vehicles, electric rotorcraft, drones, and the like. In the context of a generator, an electric machine may be used to supply electrical power to auxiliary systems, including auxiliary systems in an aircraft. In some embodiments, and electric machine may function as both an electric motor and as a generator during different operating states. For example, an electric machine may function as an electric motor to start an aircraft engine, and as a generator to supply electric power to systems in the aircraft.

It will be appreciated that an electric machine may be used in numerus other settings, and it is intended that the presently disclosed rotor coolant and lubrication distribution systems may be implemented in an electric machine in any setting.

Various embodiments of the present disclosure will now be described in greater detail. Referring to <FIG>, an exemplary electric machine <NUM> is shown. The electric machine <NUM> includes a stator assembly <NUM> and a rotor assembly <NUM>. The rotor assembly <NUM> includes a rotor core <NUM> and a rotor shaft <NUM> operably coupled to the rotor core <NUM>. In some embodiments, for example, the rotor core <NUM> may be operably coupled to the rotor shaft <NUM> via an interference fit, a shrink fit, a press fit, or any other suitable fit known in the art. In some embodiments, the rotor core <NUM> may include a plurality of laminations (not shown) stacked annularly along the longitudinal axis A of the rotor shaft <NUM>. A plurality of magnets (e.g. permanent magnets) may be secured in channels formed within the laminations, and end-caps <NUM> may be provided to secure the laminations and magnets as stacked.

The rotor shaft <NUM> may be coupled to a housing assembly <NUM> with one or more bearing assemblies <NUM>. A plurality of coolant seals <NUM> may provide a seal between the rotor assembly <NUM> and the housing assembly <NUM>. During operation of the electric machine <NUM>, the rotor assembly <NUM> rotates about a longitudinal axis A under force provided by a magnetic field generated by an electric power source (in the case of an electric motor) or under force provided by a mechanical power source (in the case of a generator).

The rotor shaft <NUM> includes a hollow region <NUM> defined by an inner surface <NUM>. At least a portion of the inner surface <NUM> may have a sloped inner surface <NUM> with a slope θ that extends across at least a portion of the longitudinal length of the rotor shaft <NUM> from a first inner diameter D<NUM> at a first end <NUM> of the rotor shaft <NUM> to a second inner diameter D<NUM> at a second end <NUM> of the rotor shaft <NUM>. The sloped inner surface <NUM> may encompass all or part of the hollow region <NUM> of the rotor shaft <NUM>. The inner surface <NUM> may include one or more of a number of configurations, non-exhaustive examples of which are described with respect to <FIG>.

The electric machine includes a cooling conduit <NUM> that defines a pathway for circulating a coolant <NUM>. In some embodiments, as shown in <FIG>, the cooling conduit <NUM> delivers coolant <NUM> to a nozzle <NUM>, and the nozzle <NUM> injects the coolant <NUM> into an impeller <NUM>. As shown, the impeller <NUM> receives a radial stream of coolant <NUM> from the nozzle <NUM>; however the impeller <NUM> also may be configured to receive an axial stream of coolant <NUM>, as well as a stream of coolant <NUM> with any other orientation. The impeller <NUM> directs the stream of coolant <NUM> into the first end <NUM> of the rotor shaft <NUM>. For example, as shown in <FIG>, the impeller <NUM> may direct the radial stream of coolant axially into the rotor shaft <NUM>. The coolant <NUM> may take the form of a film <NUM> on the inner surface <NUM> of the rotor shaft <NUM>. The coolant <NUM> flows across the inner surface <NUM> of the rotor shaft <NUM> at least in part under centrifugal force generated by rotation of the rotor shaft <NUM>. A film <NUM> of coolant <NUM> may cover all or a portion of the inner surface <NUM> of the rotor shaft <NUM>. The thickness of the film <NUM> may depend on one or more of the slope θ of the inner surface of the rotor shaft <NUM>, the flow rate of coolant <NUM>, the rotating speed of the rotor shaft <NUM>, and/or the viscosity of the coolant. Additionally, in some embodiments the thickness of the film <NUM> may depend on the presence or absence of surface features on the rotor shaft <NUM>, and the configuration thereof. Such surface features may include coolant dams, ridges, grooves, bumps, dimples, and the like, non-exhaustive examples of which are as described with reference to <FIG>.

The flow of coolant <NUM> across the inner surface <NUM> of the rotor shaft <NUM> includes an axial component, and may also include a radial component. Surface features may be provided to augment the flow of coolant <NUM> and/or the thickness of the film <NUM> of coolant <NUM>. For example, surface features may alter the direction of coolant flow, and/or induce turbulence in the coolant. Coolant <NUM> discharging from the second end <NUM> of the rotor shaft <NUM> may accumulate in a sump area <NUM>. Coolant <NUM> accumulating in the sump area <NUM> may be circulated through a coolant circulation system as described with respect to <FIG>. The coolant circulation system includes the cooling conduit <NUM> and the inner surface <NUM> of the rotor shaft <NUM>. In some embodiments, the cooling conduit <NUM> may define a pathway from the second end <NUM> of the rotor shaft <NUM> through or around at least a portion of the housing assembly <NUM> and back to the first end <NUM> of the rotor shaft <NUM>. Alternatively, or in addition, the cooling conduit <NUM> may define a pathway to one or more components of a coolant circulation system as described herein with reference to <FIG>.

With the rotor shaft <NUM> rotating at an operating rate of rotation, a force that includes a centrifugal force caused by rotation of the rotor shaft <NUM> acts upon the coolant <NUM>. The centrifugal force overcomes the force of gravity as the rate of rotation increases, causing the coolant <NUM> to cling to the inner surface <NUM> of the rotor shaft <NUM>, forming a film of coolant <NUM> that covers at least a portion of the inner surface <NUM> of the rotor shaft <NUM>. The force acting upon the coolant <NUM> may additionally include other vector components, including a slope vector corresponding to the slope θ of the inner surface <NUM> of the rotor shaft <NUM>. The force causes the coolant <NUM> to flow across the inner surface <NUM> of the rotor shaft <NUM>. The thickness of the film <NUM> and/or the velocity of the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> may depend at least in part on the slope θ of the inner surface <NUM> of the rotor shaft <NUM> and/or the rotating speed of the rotor shaft <NUM>.

The slope θ of the inner surface <NUM> of the rotor shaft <NUM> may range from <NUM>° to <NUM>° relative to a longitudinal axis A of the rotor shaft <NUM>. A slope of <NUM>° corresponds to a cylindrical profile of the inner surface <NUM> of the rotor shaft <NUM>. A slope greater than <NUM>° and less than <NUM>° corresponds to a frustoconical or sloped profile of the inner surface <NUM> of the rotor shaft <NUM>. A slope of <NUM>° corresponds to a perpendicular stepped profile transitioning from one region to another of the inner surface <NUM> of the rotor shaft <NUM>. A sloped inner surface <NUM> of the rotor shaft <NUM> may have a slope θ that ranges from greater than <NUM>° to less than <NUM>°. For example, the slope θ may range from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, or such as from <NUM>° to <NUM>°. The slope θ may be at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, or such as at least <NUM>°. The slope θ may be less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, or such as less than <NUM>°.

In some embodiments, the thickness of the film <NUM> of coolant <NUM> may be between <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils, such as from <NUM> to <NUM> mils. The thickness of the film may be at least <NUM> mil, such as at least <NUM> mils, such as at least <NUM> mils, such as at least <NUM> mils, such as at least <NUM> mils, such as at least <NUM> mils, such as at least <NUM> mils. The thickness of the film may be less than <NUM> mils, such as less than <NUM> mils, such as less than <NUM> mils, such as less than <NUM> mils, such as less than <NUM> mils, such as less than <NUM> mils.

The inner surface of the rotor shaft has a thermally conductive relationship with at least a portion of the rotor shaft, and at least a portion of the rotor shaft has a thermally conductive relationship with at least a portion of the rotor core <NUM>. During operation, heat energy Q generated by the electric machine <NUM> transfers to the coolant flowing across the inner surface <NUM> of the rotor shaft <NUM>. For example, heat energy Q transfers from the rotor shaft <NUM> and/or the rotor core <NUM> to the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> by thermal conduction. The coolant <NUM> exits the rotor shaft <NUM> having been heated by the thermally conductive relationship with the rotor shaft <NUM>. The coolant <NUM> exiting the rotor shaft <NUM> may flow into a sump area <NUM> (<FIG>) and/or into a cooling conduit <NUM> as described below with respect to <FIG>.

The electric machine <NUM> may utilize any desired coolant, such as cooling oil or other fluids. Exemplary coolants include deionized water, propylene glycol, ethylene glycol, polyalkylene glycol, betaine, and oils (e.g., jet oils, hydrocarbons, mineral oil, castor oil, silicone oils, fluorocarbon oils, transformer oils).

The electric machine <NUM> includes an air gap <NUM> defined by an annular space between the outer surface of the rotor core <NUM> and the inner surface of the stator <NUM>. The air gap <NUM> includes a width component, spanning between the outer surface of the rotor core <NUM> and the inner surface of the stator <NUM>, which may vary from time-to-time and from region-to-region, for example, with changes in the temperature of the rotor assembly <NUM> (i.e., the rotor core <NUM> and/or the rotor shaft <NUM>) and/or changes in the temperature of the stator <NUM>. The temperature of the rotor assembly <NUM> and/or of the stator <NUM> may be measured using one or more rotor temperature sensors. For example, as shown, an electric machine <NUM> may include a first rotor temperature sensor <NUM> and a second rotor temperature sensor <NUM> each selectively located at a position so as to measure a local temperature of the rotor assembly <NUM> (e.g., of the rotor core <NUM> and/or of the rotor shaft <NUM>). The first rotor temperature sensor <NUM> may be selectively located to measure a first local temperature T<NUM> at a first region R<NUM> of the rotor assembly <NUM> (e.g., of the rotor core <NUM> and/or of the rotor shaft <NUM>) and the second rotor temperature sensor <NUM> may be selectively located to measure a second local temperature T<NUM> at a second region R<NUM> of the rotor assembly <NUM> (e.g., of the rotor core <NUM> and/or of the rotor shaft <NUM>).

An electric machine <NUM> may additionally include a third stator temperature sensor <NUM> and a fourth stator temperature sensor <NUM> each selectively located at a position so as to measure a local temperature of the stator <NUM>. The third stator temperature sensor <NUM> may be selectively located to measure a third local temperature T<NUM> at a third region R<NUM> of the stator <NUM> and the fourth stator temperature sensor <NUM> may be selectively located to measure a fourth local temperature T<NUM> at a fourth region R<NUM> of the stator <NUM>. Any desired temperature sensor may be used, including a thermistor (e.g., a positive temperature coefficient thermistor) or a noncontact temperature sensor (e.g., an infrared temperature sensor).

In some embodiments, the slope θ of the inner surface <NUM> of the rotor shaft <NUM> may be selected to provide a substantially uniform temperature and/or a substantially uniform width of the air gap <NUM> across a longitudinal length of the rotor shaft <NUM> and/or rotor core <NUM> (e.g., as between the first region R<NUM> and the second region R<NUM>). During operation, the temperature T of the rotor shaft <NUM> and/or rotor core <NUM> depends at least in part on the quantity of heat energy Q transferred from the inner surface <NUM> of the rotor shaft to the coolant <NUM>. Likewise, the first temperature T<NUM> of the first region R<NUM> of the rotor shaft <NUM> and/or rotor core <NUM> depends at least in part on a first quantity of heat energy Q<NUM> transferred from the inner surface <NUM> of the rotor shaft <NUM> to the coolant <NUM> across a first annular surface area S<NUM> corresponding to the first region R<NUM>. The second temperature T<NUM> of a second region R<NUM> of the rotor shaft <NUM> and/or rotor core <NUM> depends at least in part on a second quantity of heat energy Q<NUM> transferred from the inner surface <NUM> of the rotor shaft to the coolant <NUM> across a second annular surface area S<NUM> corresponding to the second region R<NUM>. A difference between the first temperature T<NUM> of the first region R<NUM> and the second temperature T<NUM> of the second region R<NUM> may lead to a difference in the width of the air gap <NUM> as between the first region R<NUM> and the second region R<NUM> due to differences in thermal expansion. Such a difference in temperature and/or air gap <NUM> width may lead to a warped rotor shaft <NUM> and/or rotor core <NUM>, rubbing between the rotor core <NUM> and the stator <NUM>, windage losses, and other inefficiencies, malfunction, or failures.

Such a difference between the first temperature T<NUM> of a first region R<NUM> and the second temperature T<NUM> of the second region R<NUM> (and/or a difference in air gap <NUM> width as between the first region R<NUM> and the second region R<NUM>) may arise from a difference in the quantity of heat energy Q generated in or transferring to the respective regions R<NUM> and R<NUM>. Additionally, or in the alternative, such a difference between the temperature T<NUM> of a first region R<NUM> and the temperature T<NUM> of the second region R<NUM> (and/or a difference in air gap <NUM> width as between the first region R<NUM> and the second region R<NUM>) may arise from a difference between in the quantity of heat energy Q transferred from the respective regions R<NUM> and R<NUM>. However, in some embodiments, the slope θ of the inner surface <NUM> of the rotor shaft <NUM> may be selected so as to at least partially offset an expected change in the rate of heat transfer per unit area q due to an increasing temperature of the coolant <NUM> across the longitudinal length of the rotor shaft <NUM> with a proportional change in the annular surface area of the inner surface of the rotor shaft <NUM>. Under certain operating conditions, the offset obtained by the selected slope θ may provide for a substantially uniform quantity of heat energy Q transferring from the inner surface <NUM> of the rotor shaft <NUM> to the coolant <NUM> across a given length of the longitudinal axis of the rotor shaft <NUM> as between the first annular surface area S<NUM> and the second annular surface area S<NUM>.

For purposes of the present disclosure, such a substantially uniform quantity of heat energy Q transferring will be said to exist under the following condition: Area_S<NUM>/Area_S<NUM> = q_Si/q_S<NUM>, where Area_S<NUM> is the area of the first annular surface area S<NUM> and Area_S<NUM> is the area of the second annular surface area S<NUM> for the given length of the longitudinal axis of the rotor shaft <NUM>, and q_S<NUM> is the rate of heat transfer q for the first annular surface area S<NUM>, and q_S<NUM> is the rate of heat transfer q for the second annular surface area S<NUM>. The rates of heat transfer may be ascertained from Fourier's law using known thermal conductivity values and known temperature gradients between the inner surface <NUM> of the rotor shaft <NUM> and the coolant <NUM> at various points along the longitudinal axis of the rotor shaft, which temperature gradients may be ascertained using any desired method known in the art.

The rate of heat transfer per unit area q from the inner surface <NUM> of the rotor shaft <NUM> to the coolant <NUM> is inversely proportional to the temperature difference between the coolant <NUM> and the inner surface <NUM> of the rotor shaft <NUM>. The temperature of the coolant <NUM> increases as the coolant <NUM> flows across the inner surface <NUM> of the rotor shaft <NUM> and heat energy Q transfers to the coolant <NUM>, thereby decreasing the rate of heat transfer per unit area q. As such, during operation of the electric machine <NUM>, the first annular surface area S<NUM> may exhibit a first rate of heat transfer q<NUM> and the second annular surface area S<NUM> may exhibit a second rate of heat transfer q<NUM> as heat energy Q transfers to the coolant <NUM>, such that the first rate of heat transfer q<NUM> exceeds the second rate of heat transfer q<NUM>. However, the sloped inner surface of the rotor shaft <NUM> provides an increasingly larger annular surface area for a given region of the rotor shaft. An increase in the temperature of the coolant <NUM> and corresponding change to the rate of heat transfer per unit area q across the longitudinal length of the rotor shaft <NUM> may be at least partly offset by selectively providing a sloped inner surface <NUM> of the rotor shaft having a selected slope θ. Similarly, a difference between the quantity of heat energy Q generated in or transferring to the respective regions R<NUM> and R<NUM> may be at least partially offset by selectively providing a sloped inner surface <NUM> of the rotor shaft having a selected slope θ. In some embodiments, the width of the air gap <NUM> as between the first region R<NUM> and the second region R<NUM> may be at least partially equalized by selectively providing a sloped inner surface <NUM> of the rotor shaft having a selected slope θ.

In some embodiments, the slope θ of the inner surface <NUM> of the rotor shaft <NUM> may be selected so as to maintain a substantially uniform temperature as between the temperature T<NUM> of the first region R<NUM> and the temperature T<NUM> of the second region R<NUM>. For example, the slope θ of the inner surface <NUM> of the rotor shaft <NUM> may be selected so as to selectively increase the annular surface area from a first annular surface area S<NUM> corresponding to a first region R<NUM> to a second annular surface area S<NUM> corresponding to a second region R<NUM>. Further in addition or in the alternative, the slope θ of the inner surface <NUM> of the rotor shaft <NUM> may be selected so as to maintain a substantially uniform width of the air gap <NUM> as between the first region R<NUM> and the second region R<NUM>. The selected slope θ may be uniform or variable. The slope θ may be selected at least in part to offset an increasing temperature of the coolant <NUM> across a longitudinal length of the rotor shaft and/or to at least partially offset a difference between the amounts of heat energy Q generated in and/or transferring to respective regions of the rotor shaft.

Now referring to <FIG>, various exemplary configurations of a rotor shaft <NUM> will be discussed. The examples are provided by way of explanation and should not be interpreted as limiting the present disclosure. As shown, an exemplary rotor shaft <NUM> includes a hollow region <NUM> defined by an inner surface <NUM>. The hollow region <NUM> of the rotor shaft <NUM> extends across at least a portion of the longitudinal length of the rotor shaft <NUM>, including up to the entire longitudinal length of the rotor shaft <NUM>. The hollow region <NUM> of the rotor shaft <NUM> may include one or more sloped inner surfaces <NUM>. A sloped inner surface <NUM> of the rotor shaft <NUM> may include up to the entire longitudinal length of the hollow region <NUM>. The sloped inner surface <NUM> extends across at least a portion of the longitudinal length of the rotor shaft from a first inner diameter D<NUM> at a first end <NUM> of the rotor shaft <NUM> to a second inner diameter D<NUM> at a second end <NUM> of the rotor shaft <NUM>. The difference between the first inner diameter D<NUM> and the second inner diameter D<NUM> define the slope θ of the inner surface <NUM>. The inner surface <NUM> may have a slope θ with any desired profile, including a frustoconical or sloped profile and/or a stepped profile.

The slope θ of the inner surface <NUM> may be linear or nonlinear. For example, the slope θ of the inner surface <NUM> may remain constant across at least a portion of the longitudinal length of the hollow region <NUM>, or the slope θ may exhibit an increasing or decreasing rate of change across at least a portion of the longitudinal length of the hollow region <NUM>.

The rotor shaft <NUM> shown in <FIG> has a sloped inner surface <NUM> with a frustoconical or sloped profile <NUM> and a linear slope θ. The slope θ remains linear along the longitudinal length of the rotor shaft <NUM> from an upstream point X<NUM> to a downstream point X<NUM>.

The rotor shaft shown in <FIG> has a sloped inner surface <NUM> with a stepped cylindrical profile <NUM>. The stepped cylindrical profile includes a plurality of cylindrical regions adjoined by a step transitioning from one cylindrical region to the next. The step may be perpendicular or transverse (i.e., nonorthogonal) to the longitudinal axis. As shown in <FIG>, the stepped cylindrical profile includes five cylindrical regions (i.e., a first cylindrical region <NUM>, a second cylindrical region <NUM>, third cylindrical region <NUM>, fourth cylindrical region <NUM>, fifth cylindrical region <NUM>) and four steps (i.e., a first step <NUM>, a second step <NUM>, a third step <NUM>, a fourth step <NUM>); however, any number of cylindrical regions and/or steps may be provided. A rotor shaft with a stepped cylindrical profile <NUM> may have a linear slope θ and/or a non-linear slope θ. As shown in <FIG>, a first stepped cylindrical profile <NUM> has first linear slope θ along a longitudinal length of the rotor shaft <NUM> from an upstream point X<NUM> to a downstream point X<NUM>. A second stepped cylindrical profile <NUM> has a second linear slope θ along a longitudinal length of the rotor shaft <NUM> from an upstream point X<NUM> to a downstream point X<NUM>. The first stepped cylindrical profile <NUM> and the second stepped cylindrical profile <NUM> together form a third stepped cylindrical profile <NUM>, which has a non-linear slope θ along a longitudinal length of the rotor shaft <NUM> from an upstream point X<NUM> to a downstream point X<NUM>.

The rotor shafts <NUM> shown in <FIG> and <FIG> each have a sloped inner surface <NUM> with a frustoconical or sloped profile and a non-linear slope θ. As shown in <FIG>, a sloped inner surface <NUM> may include a first slope θ<NUM> at an upstream point X<NUM> and a second slope θ<NUM> at a downstream point X<NUM> on the longitudinal length of the rotor shaft. The first slope θ<NUM> may exceed the second slope θ<NUM>, such that the non-linear slope θ decreases across a longitudinal length of the rotor shaft. As shown in <FIG>, the sloped inner surface <NUM> may include a third slope θ<NUM> at an upstream point X<NUM> and a fourth slope θ<NUM> at a downstream point X<NUM> on the longitudinal length of the rotor shaft. The fourth slope θ<NUM> may exceed the third slope θ<NUM> exceeds, such that the non-linear slope θ increases across a longitudinal length of the rotor shaft.

In some embodiments, the hollow region <NUM> of the rotor shaft <NUM> may include a combination of cylindrical regions and sloped regions. A cylindrical region may transition to a sloped region, and/or a sloped region may transition to a cylindrical region. The transition may be a step transition and/or a gradual transition. For example, as shown in <FIG>, the hollow region <NUM> of the rotor shaft <NUM> may include a cylindrical region <NUM> which gradually transitions to a sloped region <NUM>. Alternatively, or in addition, a rotor shaft <NUM> may include a sloped region that gradually transitions to a cylindrical region. As shown in <FIG>, the hollow region <NUM> of the rotor shaft <NUM> may include a plurality of cylindrical regions adjoined by a sloped region. Any number of cylindrical regions and/or sloped regions may be provided. For example, <FIG> shows three cylindrical regions (i.e., a first cylindrical region <NUM>, a second cylindrical region <NUM>, and a third cylindrical region <NUM>), and two sloped regions (i.e., a first sloped region <NUM>, and a second sloped region <NUM>). The sloped regions may have a linear and/or non-liner slope, and the slope θ of one sloped region may differ from the slope θ of another sloped region. For example, the first sloped region <NUM> has a first slope θ<NUM> at a first point X<NUM> on the longitudinal axis and the second sloped region <NUM> has a second slope θ<NUM> at a point X<NUM> on the longitudinal axis.

Now referring to <FIG>, the inner surface <NUM> of the rotor shaft <NUM> or a portion thereof may include surface features, such as ridges, grooves, bumps, dimples, and the like, or combinations thereof. While such surface features are described generally with reference to the inner surface <NUM> of the rotor shaft <NUM>, such surface features may be additionally or alternatively included on all or at least a portion of a rotor cooling conduit <NUM>, as discussed below with respect to <FIG>. In some embodiments, surface features may be configured to augment the flow of coolant <NUM> across the inner surface of the rotor shaft <NUM>. Additionally, or in the alternative, surface features may augment the rate of heat transfer from the inner surface <NUM> of the rotor shaft <NUM> to the coolant <NUM>. For example, various types of surface features may prevent or induce pooling, prevent or induce turbulence, provide a uniform or varied film <NUM> thickness, and/or advance or inhibit the flow of coolant <NUM> along the inner surface <NUM> of the rotor shaft <NUM>.

As shown in <FIG>, the inner surface of the rotor shaft <NUM> may include one or more ridges <NUM> and/or grooves <NUM>. The ridges <NUM> may have any desired height and/or any desired width, and the grooves <NUM> may have any desired depth and/or any desired width. The ridges <NUM> and/or grooves <NUM> may have any desired angular orientation relative to the longitudinal axis A of the rotor shaft <NUM>. The angular orientation of the grooves <NUM> and/or ridges <NUM> may be perpendicular, parallel, transverse (i.e., nonorthogonal), or a combination thereof, relative to the longitudinal axis A. As shown in <FIG>, the ridges <NUM> and/or grooves <NUM> may be oriented perpendicular to the longitudinal axis A, forming a plurality of annular ridges <NUM> and/or annular grooves <NUM>. Additionally, or in the alternative, as shown in <FIG>, the ridges <NUM> and/or grooves <NUM> may have a spiral orientation relative to the longitudinal axis A. The spiral orientation may have a forward pitch or a reverse pitch relative to the direction of rotation of the rotor shaft <NUM>. A forward pitch typically advances coolant <NUM> along the inner surface <NUM> of the rotor shaft <NUM>, whereas a reverse pitch typically impedes advancement of coolant <NUM> along the inner surface <NUM> of the rotor shaft <NUM>.

As shown in <FIG>, the inner surface <NUM> of a rotor shaft <NUM> may include an array of intersecting ridges <NUM> (<FIG>), an array of intersecting grooves <NUM> (<FIG>), and/or an array of intersecting ridges <NUM> and grooves <NUM> (<FIG>).

As shown in <FIG>, the inner surface <NUM> of a rotor shaft <NUM> may include an array of dimples <NUM> (<FIG>), an array of bumps <NUM> (<FIG>), and/or an array of dimples and bumps <NUM> (<FIG>).

In some embodiments, surface features, such as grooves, ridges, bumps, dimples, and the like, or combinations thereof, on the inner surface <NUM> of the rotor shaft <NUM> may induce turbulence in the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM>. Additionally, or in the alternative, such surface features may augment the direction in which coolant <NUM> flows across the inner surface of rotor shaft <NUM>. Such turbulence or augmented direction of flow may enhance heat transfer between the inner surface <NUM> of the rotor shaft <NUM> and the coolant <NUM>. Additionally, such surface features add surface area to the inner surface <NUM> of the rotor shaft <NUM>, which also may augment heat transfer between the inner surface <NUM> of the rotor shaft <NUM> and the coolant <NUM>.

In some embodiments, surface features may be selectively provided, omitted, shaped, and/or configured at a first annular surface area S<NUM> and/or a second annular surface area S<NUM> so as to augment temperature and/or heat transfer. For example, surface features may be selectively provided, omitted, shaped, and/or configured so as to provide a substantially uniform temperature and/or width of the air gap <NUM> across a longitudinal length of the rotor shaft <NUM> and/or rotor core <NUM> (e.g., as between the first region R<NUM> and the second region R<NUM>). For example, surface features may be selectively provided, omitted, shaped, and/or configured so as to at least partially offset an expected change in the rate of heat transfer per unit area q due to an increasing temperature of the coolant <NUM> across the longitudinal length of the rotor shaft <NUM> with a proportional change in the annular surface area of the inner surface of the rotor shaft <NUM>. Under certain operating conditions, the offset obtained by the selected surface features may provide for a substantially uniform quantity of heat energy Q from the inner surface <NUM> of the rotor shaft <NUM> to the coolant <NUM> across a given length of the longitudinal axis of the rotor shaft <NUM> as between the first annular surface area S<NUM> and the second annular surface area S<NUM>.

Surface features may provide an increasingly larger annular surface area for a given region of the rotor shaft <NUM>. An increase in the temperature of the coolant <NUM> and corresponding change to the rate of heat transfer per unit area q across the longitudinal length of the rotor shaft <NUM> may be at least partly offset, and/or the width of the air gap <NUM> may be at least partially equalized, by selectively providing, omitting, shaping, and/or configuring surface features, as between respective regions R<NUM> and R<NUM> of the inner surface <NUM> of the rotor shaft <NUM>. Similarly, a difference between the quantity of heat energy Q generated in or transferring to the respective regions R<NUM> and R<NUM> may be at least partially offset by selectively providing, omitting, shaping, and/or configuring surface features as between respective regions R<NUM> and R<NUM> of the inner surface <NUM> of the rotor shaft <NUM>.

In some embodiments, surface features may be selectively provided, omitted, shaped, and/or configured so as to maintain a substantially uniform temperature as between the temperature T<NUM> of the first region R<NUM> and the temperature T<NUM> of the second region R<NUM>. For example, surface features may be selectively provided, omitted, shaped, and/or configured so as to selectively increase the annular surface area from a first annular surface area S<NUM> corresponding to a first region R<NUM> to a second annular surface area S<NUM> corresponding to a second region R<NUM>. Further in addition or in the alternative, surface features may be selectively provided, omitted, shaped, and/or configured so as to maintain a substantially uniform width of the air gap <NUM> as between the first region R<NUM> and the second region R<NUM>. The provision, omission, shape, and or configuration of surface features may be uniform or variable. The surface features may be selectively provided, omitted, shaped, and/or configured at least in part to offset an increasing coolant <NUM> temperature across a longitudinal length of the rotor shaft and/or to at least partially offset a difference between the quantity of heat energy Q generated in or transferring to respective regions of the rotor shaft <NUM>.

Now referring to <FIG>, and <FIG>, in some embodiments an electric machine <NUM> may include an impeller <NUM> operably coupled to the rotor shaft <NUM>. The impeller <NUM> includes an annular body <NUM> configured to be operably coupled to the rotor shaft <NUM>, for example, by one or more of a pressed fit, a channel and groove, a retaining pin, a set screw, or the like, thereby allowing the impeller <NUM> to rotate in conjunction with the rotor shaft <NUM>. The annular body has one or more impeller blades <NUM> and a corresponding one or more impeller channels <NUM>, which scoop and/or push coolant <NUM> axially with respect to the rotor shaft <NUM>. The coolant <NUM> may be supplied to the impeller <NUM>, for example, by one or more nozzles <NUM>, which may be configured to inject the coolant <NUM> radially towards the impeller <NUM>. The one or more nozzles <NUM> may be located radially adjacent to the impeller, and may inject a stream of coolant <NUM> towards the impeller at any desired angle or orientation.

As shown, a plurality of impeller blades <NUM> and corresponding impeller channels <NUM> are provided. The impeller channels <NUM> transition from a radial orientation to an axial orientation, redirecting the coolant <NUM> axially as indicated by arrows <NUM>. The plurality of impeller channels <NUM> may be unidirectional (<FIG>) or bidirectional (<FIG>). A unidirectional plurality of impeller channels <NUM> direct a stream of coolant <NUM> in one direction axially along a rotor shaft <NUM>, for example, allowing the coolant <NUM> to flow through a plurality of coolant supply holes <NUM> and into the hollow region <NUM> of the rotor shaft <NUM>. A bidirectional plurality of impeller channels <NUM> direct a stream of coolant in both directions axially along a rotor shaft <NUM>. In the case of a bidirectional plurality of impeller channels <NUM> (<FIG>), a first portion of coolant <NUM> flows in a first axial direction as indicated by arrow <NUM>, and a second portion of coolant <NUM> flows in a second axial direction as indicated by arrow <NUM> (<FIG>). The first axial flow of coolant <NUM> indicated by arrow <NUM> passes through a plurality of coolant supply holes <NUM> and into the hollow region <NUM> of the rotor shaft <NUM>. The second axial flow of coolant <NUM> indicated by arrow <NUM> passes through a bearing cooling or lubrication channel <NUM> or the like, configured to cool or lubricate one or more bearing assemblies <NUM>. Coolant <NUM> passing through the bearing cooling or lubrication channel <NUM> may be returned to the cooling conduit <NUM> and/or to a coolant reservoir (<FIG>).

Now referring to <FIG>, in some embodiments, the inner surface <NUM> of the rotor shaft <NUM> may include one or more annular coolant dams traversing the perimeter of the inner surface <NUM>. An annular coolant dam may be located at the first end <NUM> of the rotor shaft <NUM>, at the second end <NUM> of the rotor shaft <NUM>, and/or at an intermediate point between the first end <NUM> and the second end <NUM> of the rotor shaft <NUM>. <FIG> shows a first annular coolant dam <NUM>, which may be located at the first end of the rotor shaft <NUM> and a second annular coolant dam <NUM>, which may be located at the second end of the rotor shaft <NUM>. An annular coolant dam may include a rubber, elastomeric, or thermoplastic material formed into the shape of an annulus sized to be fitted into an annular groove <NUM> formed on the inner surface of the rotor shaft <NUM>. Alternatively or in addition, an annular coolant dam may be formed as an integral part of the rotor shaft <NUM>. As shown, an annular groove <NUM> receives and/or retains the first annular coolant dam <NUM>, and the second annular coolant dam <NUM> is formed as an integral part of the rotor shaft <NUM>. The first annular coolant dam may be located at an interface between the rotor shaft <NUM> and a drive shaft (not shown).

An annular coolant dam (e.g., the first annular coolant dam <NUM>) may be configured to scoop or push coolant <NUM> axially along the inner surface <NUM> of the rotor shaft <NUM>. For example, as shown, one or more nozzles <NUM> may inject a stream of coolant <NUM> such that the stream of coolant passes across the first annular coolant dam <NUM> and into the hollow region <NUM> of the rotor shaft <NUM>. An annular coolant dam (e.g., the first annular coolant dam <NUM>) may be utilized in addition or as an alternative to an impeller <NUM> for providing an axial flow of coolant <NUM> to the inner surface <NUM> of the rotor shaft <NUM>. A annular coolant dam (e.g., the first annular coolant dam <NUM>) may accumulate a head of coolant <NUM>, causing the coolant <NUM> to flow across the inner surface <NUM> of the rotor shaft <NUM>. Accordingly, a force acting upon the coolant <NUM> and causing the coolant <NUM> to flow across the inner surface <NUM> of the rotor shaft <NUM> may include a longitudinal force induced by an annular coolant dam (e.g., the first annular coolant dam <NUM>).

An annular coolant dam (e.g., the second annular coolant dam <NUM>) may provide a longitudinal force inverse to the longitudinal direction of coolant flow, increasing the thickness of the coolant <NUM> (e.g., the film <NUM>) flowing across the inner surface <NUM> of the rotor shaft <NUM>. For example, such thickness may depend at least in part on the height of the second annular coolant dam <NUM>. As coolant <NUM> accumulates along the inner surface <NUM> of the rotor shaft <NUM>, the thickness of the coolant <NUM> increases and exceeds the height of the annular coolant dam (e.g., the second annular coolant dam <NUM>), thereby allowing the coolant <NUM> to flow past the annular coolant dam (e.g., the second annular coolant dam <NUM>).

As shown in <FIG>, coolant <NUM> may be discharged from the hollow region <NUM> of the rotor shaft <NUM> through an open end of the rotor shaft <NUM> (<FIG>), and/or through one or more axial coolant discharge holes <NUM> (<FIG>), and/or through one or more radial coolant discharge holes <NUM> (<FIG>). It will be appreciated that the coolant discharge holes <NUM>, <NUM> may also be angled, and need not be limited to axial or radial orientations. Additionally, it will be appreciated that a rotor shaft <NUM> may be provided with an annular coolant dam <NUM>, one or more axial coolant discharge holes <NUM>, and/or one or more radial coolant discharge holes <NUM>, alone or in combination with one another. Coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> may flow longitudinally past the annular coolant dam <NUM> and/or through the one or more axial coolant discharge holes <NUM> or radial discharge holes <NUM>, and out of the hollow region <NUM> of the rotor shaft <NUM>. Coolant flowing out of the hollow region <NUM> of the rotor shaft <NUM> may flow into a sump area <NUM> (<FIG>) and/or into a cooling conduit <NUM> as described below with respect to <FIG>.

Now referring to <FIG>, in some embodiments, an electric machine <NUM> may include a rotor cooling conduit <NUM> integrally formed within the body of the rotor shaft <NUM>. The rotor cooling conduit <NUM> defines a pathway for circulating the coolant <NUM> through the body of the rotor shaft <NUM>. During operation, coolant <NUM> circulates through the rotor cooling conduit <NUM>, and heat energy Q transfers from the rotor shaft <NUM> and/or the rotor core <NUM> to the coolant <NUM> flowing through the rotor cooling conduit <NUM> by thermal conduction. The coolant <NUM> exits the rotor cooling conduit <NUM> having been heated by the thermally conductive relationship with the rotor shaft <NUM>. Coolant flowing out of the rotor cooling conduit <NUM> may flow into a sump area <NUM> (<FIG>) and/or into a cooling conduit <NUM> as described below with respect to <FIG>.

As shown, the rotor shaft <NUM> may have a hollow region <NUM> defined by an inner surface <NUM>. However, a rotor cooling conduit <NUM> also may be integrally formed within the body of a rotor shaft <NUM> that does not include such a hollow region <NUM>. Accordingly, a rotor shaft <NUM> may be cooled with coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM>, with coolant <NUM> flowing through a rotor cooling conduit <NUM> integrally formed within the body of the rotor shaft <NUM>, or with coolant flowing both across the inner surface <NUM> of the rotor shaft <NUM> and through the rotor cooling conduit <NUM>. When the rotor shaft <NUM> includes a hollow region <NUM>, the inner surface <NUM> of the rotor shaft <NUM> may have a cylindrical profile as shown in <FIG>, a frustoconical or sloped profile as shown in <FIG>, or a combination of cylindrical regions and frustoconical or sloped regions. A rotor cooling conduit <NUM> may be provided as an alternative or in addition to a hollow region <NUM> defined by an inner surface <NUM> of the rotor shaft <NUM>. Accordingly, a flow of coolant <NUM> may be supplied to a rotor cooling conduit <NUM>, to a hollow region <NUM> of the rotor shaft <NUM>, or both. Additionally, any of the various surface features described herein or combinations thereof may be provided within a rotor cooling conduit <NUM>.

A rotor cooling conduit <NUM> may have any desired angular orientation relative to the longitudinal axis A of the rotor shaft <NUM>. The rotor cooling conduit <NUM> may include one or more channels, tubes, pathways, inter-connected or interlaced unit cells, or the like that traverse the rotor shaft <NUM> in a perpendicular, parallel, and/or a transverse (i.e., nonorthogonal) orientation relative to the longitudinal axis. As shown, the rotor cooling conduit <NUM> may traverse the rotor shaft <NUM> with a spiral orientation relative to the longitudinal axis. As shown in <FIG>, a rotor cooling conduit <NUM> may form a spiral with a relatively constant inner diameter and/or outer diameter. Alternatively, as shown in <FIG>, a rotor cooling conduit <NUM> may form a spiral with a sloped inner diameter (ID) and/or outer diameter (OD). At least a portion of the rotor cooling conduit <NUM> may have a slope θC that extends across at least a portion of the longitudinal length of the rotor shaft <NUM> from a first diameter (e.g., ID and/or OD) D<NUM> at a first end <NUM> of the rotor shaft <NUM> to a second inner diameter (e.g., ID and/or OD) D<NUM> at a second end <NUM> of the rotor shaft <NUM>.

With the rotor shaft <NUM> rotating at an operating rate of rotation, a force that includes a centrifugal force caused by rotation of the rotor shaft <NUM> acts upon the coolant <NUM>. The centrifugal force overcomes the force of gravity as the rate of rotation increases, causing the coolant <NUM> to flow through the rotor cooling conduit <NUM>, at least partially filling the rotor cooling conduit <NUM>. The force acting upon the coolant <NUM> may additionally include other force components, including a slope component corresponding to the slope θC of the rotor cooling conduit <NUM>. The force causes the coolant <NUM> to flow across the inner surface <NUM> of the rotor cooling conduit <NUM>. The thickness of the film <NUM> and/or the velocity of the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> may depend at least in part on the slope θC of the rotor cooling conduit <NUM> and/or the rotating speed of the rotor shaft <NUM>.

The slope θC of the rotor cooling conduit <NUM> may range from <NUM>° to <NUM>° relative to a longitudinal axis A of the rotor shaft <NUM>. A slope of <NUM>° corresponds to a cylindrical profile of the rotor cooling conduit <NUM>. A slope greater than <NUM>° and less than <NUM>° corresponds to a frustoconical or sloped profile of the rotor cooling conduit <NUM>. A slope of <NUM>° corresponds to a perpendicular stepped profile transitioning from one region to another of the rotor cooling conduit <NUM>. A rotor cooling conduit <NUM> may have a slope θC that ranges from greater than <NUM>° and less than <NUM>°. For example, the slope θC may range from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, such as from <NUM>° to <NUM>°, or such as from <NUM>° to <NUM>°. The slope θC may be at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, such as at least <NUM>°, or such as at least <NUM>°. The slope θC may be less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, such as less than <NUM>°, or such as less than <NUM>°.

In some embodiments, the slope θC of the rotor cooling conduit <NUM>, the surface area thereof, and/or the presence or omission of surface features thereon, may be selected to provide a uniform temperature and/or a uniform width of the air gap <NUM> across a longitudinal length of the rotor shaft <NUM> and/or rotor core <NUM> (e.g., as between the first region R<NUM> and the second region R<NUM>). For example, the slope θC of the rotor cooling conduit <NUM>, the surface area S thereof, and/or the presence or omission of surface features thereon, may be selected so as to at least partially offset an expected change in the rate of heat transfer per unit area q due to an increasing temperature of the coolant <NUM> across the longitudinal length of the rotor shaft <NUM> with a proportional change in the annular surface area of the inner surface of the rotor shaft <NUM>. Under certain operating conditions, the offset obtained by the selected slope θC of the rotor cooling conduit <NUM>, the surface area S thereof, and/or the presence or omission of surface features thereon may provide for a substantially uniform quantity of heat energy Q from the inner surface <NUM> of the rotor cooling conduit <NUM> to the coolant <NUM> across a given length of the longitudinal axis of the rotor shaft <NUM> as between the first annular surface area S<NUM> and the second annular surface area S<NUM> of the rotor cooling conduit <NUM>.

The slope θC of the rotor cooling conduit <NUM> may provide an increasingly larger annular surface area for a given region of the rotor shaft. An increase in the temperature of the coolant <NUM> and corresponding change to the rate of heat transfer per unit area q across the longitudinal length of the rotor shaft <NUM> may be at least partly offset, and/or the width of the air gap <NUM> may be at least partially equalized, by selectively providing a rotor cooling conduit <NUM> having a selected slope θC, a selected surface area S, and or a selected inclusion or omission of surface features, as between respective regions R<NUM> and R<NUM> of the rotor cooling conduit <NUM>. Similarly, a difference between the quantity of heat energy Q generated in or transferring to the respective regions R<NUM> and R<NUM> may be at least partially offset by selectively providing a rotor cooling conduit <NUM> having a selected slope θC, a selected surface area S, and or a selected inclusion or omission of surface features.

In some embodiments, the slope θC, the surface area S, and/or the inclusion or omission of surface features, as between respective regions R<NUM> and R<NUM> of the rotor cooling conduit <NUM>, may be selected so as to maintain a substantially uniform temperature as between the temperature T<NUM> of the first region R<NUM> and the temperature T<NUM> of the second region R<NUM>. For example, the slope θC, the surface area S, and/or the inclusion or omission of surface features may be selected so as to selectively increase the annular surface area from a first annular surface area S<NUM> corresponding to a first region R<NUM> to a second annular surface area S<NUM> corresponding to a second region R<NUM>. Further in addition or in the alternative, the slope θC, the surface area S, and/or the inclusion or omission of surface features may be selected so as to maintain a substantially uniform width of the air gap <NUM> as between the first region R<NUM> and the second region R<NUM>. The selected slope θC may be uniform or variable. The slope θC, surface area S, and/or the inclusion or omission of surface features, may be selected at least in part to offset an increasing coolant <NUM> temperature across a longitudinal length of the rotor shaft and/or to at least partially offset a difference between the amounts of heat energy Q generated in and/or transferring to respective regions of the rotor shaft <NUM>.

The present disclosure additionally embraces systems for cooling an electric machine <NUM>. <FIG> shows an exemplary system <NUM> for cooling an electric machine <NUM>. The exemplary system <NUM> includes a coolant pathway <NUM> through the electric machine <NUM>, a heat exchanger <NUM>, a coolant reservoir <NUM>, and a coolant pump <NUM>. The coolant pathway <NUM> through the electric machine <NUM> is defined at least in part by the inner surface <NUM> of the rotor shaft <NUM> and/or a rotor cooling conduit <NUM>. The exemplary system <NUM> further includes a cooling conduit <NUM>, which defines a pathway for circulating a coolant <NUM> through the system <NUM>. In some embodiments, at least a portion of the cooling conduit <NUM> may define a pathway through or around at least a portion of the housing assembly <NUM> of the electric machine <NUM>. Additionally, or in the alternative, at least a portion of the cooling conduit <NUM> may define a pathway that runs external from the housing assembly <NUM>.

As shown in <FIG>, the cooling conduit <NUM> defines a pathway to the first end <NUM> of the rotor shaft <NUM>. The cooling conduit <NUM> may have any desired configuration, including a cooling jacket surrounding at least a portion of the electric motor, and/or one or more internal channels, tubes, pathways, inter-connected or interlaced unit cells, or the like within the housing assembly <NUM> or otherwise surrounding at least a portion of the electric machine <NUM>. During operation, coolant is pumped from the coolant reservoir <NUM> by the coolant pump <NUM> into the cooling conduit <NUM>, which directs coolant <NUM> to the first end <NUM> of the rotor shaft <NUM>. The coolant <NUM> may be directed across the inner surface <NUM> of the rotor shaft <NUM> and/or through a rotor cooling conduit <NUM>, thereby transferring heat energy Q from the rotor assembly <NUM> (i.e., the rotor core <NUM> and/or the rotor shaft <NUM>) to the coolant <NUM>. The coolant <NUM> exits the second end of the rotor shaft <NUM>, having been heated by the thermally conductive relationship with the inner surface <NUM> of the rotor shaft <NUM> and/or the inner surface of the rotor cooling conduit <NUM>.

Coolant <NUM> discharging from the second end of the rotor shaft <NUM> flows through the cooling conduit <NUM> to the heat exchanger <NUM>. The heat exchanger <NUM> defines at least a portion of the cooling conduit <NUM>, providing a cooling surface <NUM> that has a thermally conductive relationship with coolant <NUM> flowing therethrough. The cooling surface <NUM> is configured to transfer heat energy Q from the coolant <NUM> to a heat sink fluid <NUM>. The heat exchanger <NUM> may have any desired configuration suitable to transfer heat energy Q from the coolant <NUM> to the heat sink fluid <NUM>. Suitable heat exchangers include shell and tube, plate and shell, and plate fin configurations, and the like. The heat exchanger <NUM> may be an external component or integrally formed within at least a portion of the housing assembly <NUM>. The heat sink fluid <NUM> may be any desired fluid, including a liquid or a gas. Coolant passing through the heat exchanger <NUM> flows to the coolant reservoir <NUM>, however in some embodiments a coolant reservoir <NUM> need not be provided. For example, the cooling conduit <NUM> may itself define a cooling reservoir <NUM>. The coolant reservoir <NUM> may be an integral portion of the electric machine, or an external component.

In some embodiments, the heat sink fluid <NUM> may include an airflow that passes over a surface of the heat exchanger <NUM> and/or a surface of the cooling conduit <NUM>. In some embodiments, the heat exchanger <NUM> may be an air-cooled oil cooler. Alternatively, or in addition, the heat exchanger <NUM> may include a surface of the housing assembly <NUM> across which the airflow passes. In some embodiments such as when the electric machine has been installed on an aircraft, the housing assembly <NUM> may take the form of a nacelle, and the heat sink fluid <NUM> may be airflow which passes over a surface of the nacelle.

The present disclosure additionally embraces methods of cooling an electric machine. Exemplary methods may include or utilize any of the features or embodiments described herein, including any combination thereof. The following exemplary methods and features thereof are provided by way of example and are not to be interpreted as limiting the present disclosure.

<FIG> shows an exemplary method <NUM> of cooling an electric machine. The exemplary method <NUM> includes injecting <NUM> a coolant <NUM> into a first end <NUM> of a hollow region <NUM> of a rotor shaft <NUM> of an electric machine <NUM>. The electric machine <NUM> may be configured in accordance with the present disclosure, including any selection or combination of embodiments described herein. In the exemplary method <NUM>, the electric machine includes a stator <NUM> and a rotor assembly <NUM>. The rotor assembly <NUM> includes a rotor core <NUM> and a rotor shaft <NUM> operably coupled to the rotor core <NUM>. The rotor shaft <NUM> includes an inner surface <NUM> having a slope θ that increases from a first inner diameter D<NUM> at a first end <NUM> to a second inner diameter D<NUM> at a second end <NUM>. The slope θ may include a frustoconical or sloped profile and/or a stepped profile.

The exemplary method <NUM> continues with rotating <NUM> the rotor shaft <NUM> at an operational rate of rotation, generating a force that includes a centrifugal force acting upon the coolant <NUM> in the rotor shaft <NUM>, causing the coolant <NUM> to flow across the inner surface <NUM> of the rotor shaft <NUM> from the first end <NUM> to the second end <NUM>. The coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> may form a film <NUM>. The thickness of the film <NUM> and/or the velocity of the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> depend at least in part on the slope θ of the inner surface <NUM> of the rotor shaft <NUM> and/or the rotating speed of the rotor shaft <NUM>.

The exemplary method <NUM> additionally includes transferring heat <NUM> from the rotor shaft <NUM> to the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM>. The temperature of the coolant <NUM> flowing across the inner surface <NUM> of the rotor shaft <NUM> increases from the heat transferring thereto.

In some embodiments, exemplary methods of cooling an electric machine <NUM> additionally may include discharging coolant from the second end of the rotor shaft <NUM>, and directing the coolant through a heat exchanger <NUM>, and transferring heat from the coolant <NUM> flowing through the heat exchanger <NUM> to a heat sink fluid <NUM>. The temperature of the coolant <NUM> flowing through the heat exchanger <NUM> decreases from the heat transferring therefrom.

In some embodiments, exemplary methods of cooling an electric machine <NUM> may include scooping or pushing a coolant <NUM> through a plurality of coolant supply holes <NUM> and into a hollow region <NUM> of the rotor shaft <NUM> and/or into a rotor cooling conduit <NUM>, with an impeller <NUM> such as described herein with reference to <FIG> and <FIG>.

In some embodiments, exemplary methods of cooling an electric machine <NUM> may include selectively providing a slope θ of the inner surface <NUM> of the rotor shaft <NUM>, so as to at least partially offset an expected change in the rate of heat transfer per unit area q due to an increasing temperature of the coolant <NUM> across the longitudinal length of the rotor shaft <NUM> with a proportional change in the annular surface area of the inner surface of the rotor shaft <NUM>. Under certain operating conditions, the offset obtained by the selected slope θ may provide for a substantially uniform quantity of heat energy Q from the inner surface <NUM> of the rotor shaft <NUM> to the coolant <NUM> across a given length of the longitudinal axis of the rotor shaft <NUM> as between the first annular surface area S<NUM> and the second annular surface area S<NUM> on the inner surface of the rotor shaft <NUM>. Additionally, or in the alternative, exemplary methods may include selectively providing a slope θ of the inner surface <NUM> of the rotor shaft <NUM>, so as to at least in part provide a substantially uniform temperature and/or substantially uniform width of the air gap <NUM> across a longitudinal length of the rotor shaft <NUM> and/or rotor core <NUM> (e.g., as between the first region R<NUM> and the second region R<NUM>).

Exemplary methods may additionally or alternatively include selectively providing a slope θC so as to at least partially offset an expected change in the rate of heat transfer per unit area q due to an increasing temperature of the coolant <NUM> across the longitudinal length of the rotor shaft <NUM> with a proportional change in the annular surface area of the inner surface of the rotor shaft <NUM>. Under certain operating conditions, the offset obtained by the selected slope θ may provide for a substantially uniform quantity of heat energy Q from the inner surface <NUM> of the rotor cooling conduit <NUM> to the coolant <NUM> across a given length of the longitudinal axis of the rotor shaft <NUM> as between the first annular surface area S<NUM> and the second annular surface area S<NUM> of the rotor cooling conduit <NUM>. Further exemplary methods may include selectively providing a slope θC of the rotor cooling conduit <NUM> so as to at least partially provide a substantially uniform temperature and/or substantially uniform width of the air gap <NUM> across a longitudinal length of the rotor shaft <NUM> and/or rotor core <NUM> (e.g., as between the first region R<NUM> and the second region R<NUM>).

Various components of the electric machine <NUM> may be manufactured using any desired technology, including machining, drilling, casting, additive manufacturing, a combination thereof, or any other technique. By way of example, additive manufacturing process may be used to manufacture the rotor assembly <NUM> (e.g., the rotor core <NUM> and/or the rotor shaft <NUM>) and/or the impeller <NUM>. An additive manufacturing process may include any process which involves layer-by-layer construction or additive fabrication (as opposed to material removal as with conventional machining processes). Such processes may also be referred to as "rapid manufacturing processes". Additive manufacturing processes include, but are not limited to: Direct Metal Laser Melting (DMLM), Laser Net Shape Manufacturing (LNSM), electron beam sintering, Selective Laser Sintering (SLS), 3D printing, such as by inkjets and laserjets, Binder Jetting (BJ), Material Jetting (MJ), Photopolymer Jetting (PJ), Sterolithography (SLA), Electron Beam Melting (EBM), Fused Deposition Modeling (FDM), Laser Engineered Net Shaping (LENS), Direct Metal Deposition (DMD), and Hybrid Processes (HP).

Any desired materials may be used to manufacture the components described herein. Exemplary materials include aluminum alloys, steel alloys, nickel alloys (e.g., superalloys), and composites such as ceramic matrix composite (CMC) materials. Exemplary CMC materials may include silicon carbide, silicon, silica, or alumina matrix materials and combinations thereof. Ceramic fibers may be embedded within the matrix, such as oxidation stable reinforcing fibers including monofilaments like sapphire and silicon carbide, yarn including silicon carbide, alumina silicates, and chopped whiskers and fibers, and optionally ceramic particles (e.g., oxides of Si, Al, Zr, Y, and combinations thereof) and inorganic fillers (e.g., pyrophyllite, wollastonite, mica, talc, kyanite, and montmorillonite). As further examples, the CMC materials may also include silicon carbide (SiC) or carbon fiber cloth.

Claim 1:
An electric machine (<NUM>) comprising:
a stator (<NUM>) and a rotor core (<NUM>);
a rotor shaft (<NUM>) operably coupled to the rotor core (<NUM>), the rotor shaft (<NUM>) comprising a hollow region (<NUM>) configured to receive a coolant (<NUM>), the hollow region (<NUM>) defined by an inner surface (<NUM>) having a slope that increases from a first inner diameter at a first end (<NUM>) to a second inner diameter at a second end (<NUM>); and
an impeller (<NUM>) comprising an annular body (<NUM>) mounted on the rotor shaft (<NUM>), the rotor shaft (<NUM>) fitting within an annular space defined by the annular body (<NUM>), the impeller (<NUM>) comprising a plurality of impeller blades (<NUM>) and a corresponding plurality of impeller channels (<NUM>) transitioning from a radial orientation to an axial orientation, the plurality of impeller blades (<NUM>) and the corresponding plurality of impeller channels (<NUM>) configured to scoop or push the coolant (<NUM>) through a plurality of coolant supply holes (<NUM>) in the rotor shaft (<NUM>) into the first end (<NUM>) of the hollow region (<NUM>) of the rotor shaft (<NUM>);
wherein a force comprising a centrifugal force generated when rotating the rotor shaft (<NUM>) at an operating rate of rotation causes the coolant (<NUM>) to flow across the inner surface (<NUM>) of the rotor shaft (<NUM>) from the first end (<NUM>) to the second end (<NUM>) at a velocity depending at least in part on the slope of the inner surface (<NUM>) of the rotor shaft (<NUM>).