E-machine system with rotor shaft

Abstract

An e-machine (electric machine) comprising a stator, and a rotor disposed at least partially within the stator. The rotor comprises a rotor shaft 200 having a longitudinal axis, where the rotor shaft

Description

TECHNICAL FIELD

[0001] The technical field generally relates to an e-machine, and, more particularly, relates to an e-machine having a rotor with an interior channel therethrough, through which interior channel a liquid coolant may be passed. BACKGROUND

[0002] The use of e-machines, such as traction motors, is increasingly prevalent in various industries, such as the automotive industry. Electric vehicles, for example, rely upon traction motors within an electric vehicle powertrain (EVPT) as a source of wheel propulsion. Traction motors are also used in other vehicle applications, such as in electric assist units for e-turbochargers.

[0003] Traction motors transform electrical energy into mechanical energy. Typically, a conventional traction motor comprises a stator that includes windings through which an electrical current can be passed, and a rotor including a rotor shaft which is disposed within the stator. When the windings of the stator receive an electrical current, for example via a battery or other such power source, a magnetic field is generated within the stator. This magnetic field interacts with a magnetic field associated with the rotor, for example a magnetic field which exists due to magnets included in the rotor. The interaction of these magnetic fields causes the rotor to rotate within the stator. This rotation is used to apply torque to mechanical components, such as vehicle wheels, the rotating assembly of an e-turbocharger, or other mechanical components.

[0004] Traction motors used in automotive applications typically require a high power density to perform their intended function, which normally causes them to generate a significant amount of heat. In addition, automotive traction motors are typically exposed to high operating temperatures. For example, traction motors used in e-turbochargers are typically located close to a high-temperature exhaust outlet. Extended exposure to such large amounts of heat can reduce the performance and life expectancy of the traction motor. Traction motors used in EVPTs may also be exposed to or generate large amounts of heat, with similar negative effects on the performance and life expectancy of the traction motor.

[0005] It is therefore desirable to provide improved techniques for cooling an e machine, such as a traction motor, in order to improve the performance, power density and life expectancy of the e-machine. However, providing such cooling remains challenging. There may be detrimental increases in costs, part count, device complexity, size, bulkiness, and / or weight of the e-machine if these cooling features are included.

[0006] There is therefore a need for an e-machine system, such as a traction motor, that includes effective cooling features whilst still being compact and low-weight. There is also a need for such an e-machine to have reduced manufacturing complexity and costs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. SUMMARY

[0007] According to a first aspect, there is provided an e-machine comprising: a stator; and a rotor disposed at least partially within the stator, the rotor comprising: a rotor shaft having a longitudinal axis, wherein the rotor shaft comprises an inlet, an outlet, and an interior channel fluidly connecting the inlet to the outlet so as to allow for liquid coolant to flow from the inlet to the outlet, wherein the rotor shaft is formed from a first material having a first density; and an interior sleeve disposed within the interior channel, the interior sleeve comprising a passage through which liquid coolant may flow, wherein the interior sleeve is formed from a second material having a second density, the second density being less than the first density.

[0008] According to embodiments, the interior sleeve is cylindrical and is attached to a radially inner surface of the interior channel.

[0009] According to embodiments, the first material has a first thermal conductivity, and wherein the second material has a second thermal conductivity that is greater than the first thermal conductivity.

[0010] According to embodiments, the e-machine is a traction motor.

[0011] According to embodiments, the first material comprises steel.

[0012] According to embodiments, the second material comprises aluminum or an aluminum alloy.

[0013] According to a second aspect, there is provided an electric vehicle powertrain comprising an e-machine as described above.

[0014] According to a third aspect, there is provided an electrical assist unit for a turbocharger, the electrical assist unit comprising an e-machine as described above.

[0015] According to a fourth aspect, there is provided a method for forming a rotor shaft for an e-machine. The method includes: forming at least two sub-assemblies of the rotor shaft from a first material having a first density; attaching an interior sleeve to at least one of the at least two sub-assemblies, the interior sleeve having a passage through which liquid coolant may flow and being formed from a second material having a second density, that is less than the first density. The method further includes connecting the at least two sub-assemblies together to assemble the rotor shaft, wherein the assembled rotor shaft comprises an inlet, an outlet, and an interior channel fluidly connecting the inlet to the outlet, with the interior sleeve disposed within the interior channel to allow for liquid coolant to flow from the inlet, through the passage of the interior' sleeve, and to the outlet.

[0016] According to embodiments, the step of attaching the interior sleeve to the at least one of the at least two sub-assemblies comprises press-fitting the interior sleeve to the at least one of the at least two sub-assemblies.

[0017] According to embodiments, the step of connecting the at least two subassemblies together to form the rotor shaft comprises welding the at least two subassemblies together, preferably by laser welding.

[0018] According to embodiments, the first material has a first thermal conductivity, and wherein the second material has a second thermal conductivity that is greater than the first thermal conductivity.

[0019] According to embodiments, the first material comprises steel.

[0020] According to embodiments, the second material comprises aluminum or an aluminum alloy. BRIEF DESCRIPTION OF DRAWINGS

[0021] The various embodiments will hereinafter be described in conjunction with the following drawings, wherein like numerals denote like elements, and wherein:

[0022] FIG. 1 is a schematic illustration of a rotor shaft not in accordance with embodiments of the present disclosure;

[0023] FIG. 2A is a diagram of a rotor shaft not in accordance with embodiments of the present disclosure and having an interior channel with a first diameter located therethrough;

[0024] FIG. 2B is a diagram of a rotor shaft not in accordance with embodiments of the present disclosure and having an interior channel with a second diameter therethrough;

[0025] FIG. 3 is a diagram of a rotor shaft in accordance with embodiments of the present disclosure;

[0026] FIG. 4A is a schematic illustration of a rotor shaft in accordance with embodiments of the present disclosure;

[0027] FIG. 4B is a schematic illustration of the rotor shaft of FIG. 4A positioned within a stator;

[0028] FIG. 5 is a diagram of a turbocharger system;

[0029] FIG. 6 is a diagram of an electric vehicle powertrain; and

[0030] FIG. 7 is a flowchar t illustrating a method of forming a rotor shaft in accordance with embodiments of the present disclosure. DETAILED DESCRIPTION

[0031] The following detailed description is merely exemplary in nature and is not intended to limit the application and uses of embodiments of the present disclosure. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding introduction, brief summary or the following detailed description. Embodiments of the present disclosure may be described herein in terms of functional and / or logical block components and various method steps. In addition, those skilled in the art will appreciate that embodiments of the present disclosure may be practiced in conjunction with any number of systems, and that the systems described herein is merely exemplary embodiments of the present disclosure.

[0032] For the sake of brevity, conventional techniques related to certain functions of e-machines (and the individual operating components of e-machines) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and / or physical couplings between the various elements. It should be noted that alternative or additional functional relationships or physical connections may be present in an embodiment of the present disclosure.

[0033] As stated above, effective cooling of an e-machine, such as a traction motor, allows for improved electromagnetic performance and an increased life expectancy of the e-machine. Exposure to excessive temperatures can deteriorate motor windings, can cause partial demagnetization of magnets used in the motor, and can increase the electrical resistivity of the motor windings which leads to a further increase in the heat generated by the windings. Thermal deformation or thermal cracking of components of the e-machine is also a possibility.

[0034] There are multiple different techniques for cooling an e-machine such as a traction motor. One technique is to use air cooling. Air cooling methods typically employ the use of cooling fins to channel one or more air flows over and around the e-machine in order to extract heat.

[0035] Another type of cooling technique is water cooling, such as using a cooling water jacket disposed around the housing of a stator of the e-machine.

[0036] However, the present inventors recognized that the use of cooling water jackets is not optimal, since the rotor of the e-machine is disposed relatively far from this exterior cooling water jacket, thereby limiting the effectiveness of this water cooling j ackers in cooling the rotor of the e-machine. Other methods of stator cooling, such as forming cooling channels within the stator itself, can negatively interfere with the magnetic field produced by the stator. Furthermore, air cooling can be ineffective at removing a sufficient amount of heat in certain applications.

[0037] A more efficient technique for cooling the rotor of the traction motor is to flow a coolant such that the coolant is directly in contact with the rotor. An example system is shown in FIG. 1, in which liquid coolant, for example an oil coolant, enters into the rotor shaft 1 via an inlet 10, flows through an interior channel 20 of the rotor shaft 1, and flows out of the rotor shaft 1 via an outlet 30. In this manner, liquid coolant may more effectively remove heat from the rotor shaft 1 as compared to the other cooling techniques described above.

[0038] Whilst the exemplary rotor shaft 1 shown in FIG. 1 shows the inlet 10 being disposed at an opposite end of the rotor shaft 1 as compared to the outlet 30, with both of the inlet 10 and the outlet 30 being located on a longitudinal axis of the rotor shaft 1, it will be appreciated that other locations for the inlet 10 and the outlet 30 are possible. For example, the inlet 10 and the outlet 30 may be concentrically located at one end of the rotor shaft, such that the liquid coolant flows into and out of the rotor shaft 1 from the same end of the rotor shaft 1 in a circulating manner.

[0039] Whilst the thermal effects of internally flowing a liquid coolant within a rotor shaft is relatively straightforward to conceptualize when the rotor is not rotating or is rotating at relatively low rotational speeds, the inventors have found that there are additional effects associated with this particular rotor cooling technique that occur when the rotor shaft 1 is rotating at the high rotational speeds required for typical automotive applications. These additional effects are explained below.

[0040] Referring to FIG. 2A, a diagram of a rotor shaft 1 with an interior channel 20 disposed therethrough is shown, the interior channel 20 being cylindrical in shape and having a diameter DI. The thickness of the material of the rotor shaft 1 disposed between the interior channel 20 and a magnet 60 of the rotor is denoted Tl. As can be seen from this diagram, liquid coolant, such as oil, flows through the inlet 10, flows through the interior channel 20, and flows out of the outlet 30.

[0041] If the diameter DI of the interior channel 20 is relatively small, the rotor shaft 1 will have a relatively large mass. In other words, as DI decreases, Tl increases for a given overall diameter of the rotor shaft 1.

[0042] A large rotor shaft mass is undesirable for several reasons. A large mass of the rotor shaft 1 will reduce the ability of the liquid coolant to remove heat from the rotor shaft, since there is relatively less coolant that can flow through the interior channel 20 as compared to the relatively larger amount of material of the rotor shaft 1. Additionally, a larger mass of the rotor shaft 1 will cause the rotor shaft 1 to have a larger inertia, thereby reducing the transient response of the e-machine and increasing the time required for the e-machine to output a given demanded torque.

[0043] In order to overcome these issues, it is possible to increase the diameter of the interior channel 20. Turning to FIG. 2B, a diagram of a rotor shaft 1 with an interior channel 20 disposed therethrough is shown, the interior channel 20 being cylindrical in shape and having a diameter D2, which diameter is larger than the diameter DI shown in FIG. 2A. The larger diameter' D2 of the interior channel 20 in FIG. 2B reduces the overall mass of the rotor shaft 1, which now has a thickness of material T2, smaller than Tl, that is disposed between the interior channel 20 and the magnets 60. This smaller thickness of material reduces the inertia of the rotor shaft 1 and consequently improves the transient response of the e-machine. Furthermore, the smaller thickness T2 of the remaining material of the rotor shaft 1 can be cooled more effectively by the relatively larger amount of coolant that can flow through the larger' interior channel 20.

[0044] Additionally, the present inventors have found that, with a larger diameter of interior channel, additional advantageous cooling effects arise when the rotor shaft 1 is rotated.

[0045] In particular, when the rotor shaft 1 is rotated, colder, denser liquid that is closer to the longitudinal axis of the rotor shaft 1 is forced radially outwards towards the hotter material of the rotor shaft 1 due to centrifugal force, thereby improving the cooling effectiveness of the liquid coolant flowing through the interior channel 20.

[0046] Given the above advantageous effects, it would first appear that increasing the diameter of the interior channel 20 would be a solution to the above described problem of improving the cooling of an e-machine whilst also decreasing the weight of the e-machine.

[0047] However, when testing such a cooling technique at the high rotational speeds required by certain automotive applications, the present inventors found that a larger diameter interior channel 20 presented some problems at these higher rotational speeds, such as rotational speeds up to around 35,000 revolutions per minute (RPM).

[0048] In particular, at these high rotational speeds, due to mis-match in the velocity of liquid coolant close to the radially outer ends of the interior channel 20 and the interior channel 20 itself, frictional windage losses caused by the relative movement between the liquid coolant and the interior channel 20 increase. In addition, high rotational speeds of the rotor shaft 1 introduce additional flow effects into the liquid coolant, causing large amounts of turbulence and other non-uniform flow effects in the liquid coolant. The non-uniformity of flow results in a reduction in the interaction of the liquid coolant with walls of the interior channel 20, thereby reducing the heat transfer between the rotor shaft 1 and the liquid coolant.

[0049] These disadvantageous effects associated with high rotational speeds are not as pronounced when the diameter of the interior channel 20 is smaller.

[0050] After investigative efforts, the present inventors found that these disadvantageous effects on the heat transfer between the liquid coolant in the interior channel 20 and the rotor shaft 1 caused by the high rotational speeds required by certain automotive applications, such as the speeds associated with electric assist units for e-turbochargers or for EVPT traction motors, outweighed the advantageous effects on the heat transfer caused by rotating the rotor shaft at lower rotational speeds.

[0051] In order to retain the advantageous effects associated with a smaller diameter of the interior channel 20 of reducing windage losses and increasing the uniformity of flow of the liquid coolant, and also to retain the advantageous effects associated with a larger diameter interior channel of reducing the overall mass of the rotor shaft to improve transient response times of the e-machine, the following arrangement was conceived.

[0052] Turning to FIG. 3, a diagram of a rotor shaft 200 in accordance with embodiments of the present invention is shown. The rotor shaft 200 includes an inlet 210, an interior channel 220, and an outlet 230. An interior sleeve 240 is disposed within and is attached to the interior channel 220. The interior sleeve 240 is formed from a material that is of lower density than the material of the rotor shaft 200. In exemplary embodiments, the interior sleeve 240 is formed from a non-magnetic material. Additionally, in exemplary embodiments, the interior sleeve 240 is formed from a material that has a higher thermal conductivity as compared to the thermal conductivity of the material of the rotor shaft 200. For example, if the rotor shaft 200 was formed from a steel alloy, a less dense material with a higher thermal conductivity, such as aluminum or an aluminum alloy, may be used as the material for the interior sleeve 240. Other materials that may be used for the interior sleeve include copper or copper' alloys.

[0053] The inclusion of an interior sleeve 240 made from a material which is less dense than the material of the rotor shaft 200 is advantageous. In particular, the use of a lower density interior sleeve 240 allows for a reduction in the overall mass of the rotor shaft 200 whilst still allowing for a retention of the smaller diameter interior channel 220 through which liquid coolant may flow, thereby reducing the windage losses and the reduced heat transfer caused by rotating a rotor shaft with a larger diameter bore at the high rotational speeds associated with some automotive applications. The reduced mass of the rotor shaft 200 reduces the inertia of the rotor shaft 200, thereby improving the transient response of an e-machine including this rotor shaft 200. Additionally, if an interior sleeve 240 is formed from a material with increased thermal conductivity as compared to the material of the rotor shaft 200, heat transfer between the rotor- shaft and the liquid coolant is improved. For example, as can be seen in FIG. 3, the diameter of an interior channel 220 having an initial diameter D2 can be effectively decreased in size to a smaller diameter D3 through the use of the interior sleeve 240 disposed within the interior channel.

[0054] In exemplary embodiments, the interior sleeve 240 may be formed from a material that has improved wettability as compared to the material of the rotor shaft 200. For example, if the rotor shaft 200 was formed from steel, the interior sleeve may be formed from aluminum or an aluminum alloy having a greater wettability for the liquid coolant than the steel. This improved wettability causes lower thermal gradients to arise at t he boundary between the liquid coolant and the rotor shaft 200.

[0055] In some embodiments, the use of an interior sleeve 240 allows for about a 20% to 30% mass reduction of the rotor shaft 200 as compared to a rotor shaft having a similar size diameter interior channel but not including an interior sleeve 240.

[0056] It is noted that the inclusion of an interior sleeve 240 does increase the stress imposed on the rotor shaft 200 at high rotational speeds. However, computational modelling carried out by the inventors showed that the increase in the maximum stress experienced by the rotor shaft caused by the inclusion of an interior sleeve was well within the compressive yield stress limits of the rotor shaft 200.

[0057] A schematic of the rotor shaft 200 in accordance with the present invention is shown in FIG. 4A. As can be seen in FIG. 4A, the rotor shaft 200 includes an interior channel 220 through which liquid coolant may flow. The liquid coolant enters into the interior channel 220 via an inlet 210 and exits from the interior channel 220 via an outlet 230. An interior sleeve 240 is disposed within the interior channel 220, such that the liquid coolant flows through the interior sleeve 240. As described above, the interior sleeve 240 is formed from a material that is less dense than the material forming the rotor shaft 200.

[0058] FIG. 4B shows the rotor shaft 200 in accordance with exemplary embodiments disposed within a rotor core 300 including magnets. As can be seen from FIG. 4B, liquid coolant flowing through the interior channel 220 and through the interior sleeve 240 is brought closer to and may more effectively cool the rotor shaft 200 as compared to a hypothetical cooling water jacket (not shown) disposed which would be disposed around an exterior surface of a stator within which the rotor core 300 is disposed.

[0059] The rotor shaft 200 and rotor core 300 may be incorporated, as part of a traction motor, into an automobile for various applications, for example for supplying torque to wheels of the automobile via an electric vehicle powertrain or for supplying torque in an e-turbocharger.

[0060] FIG. 5 shows an exemplary system 500 including a turbocharger 520 and an internal combustion engine 110. The turbocharger 520 includes a combustion gas inlet 134, a shaft 122, a compressor 124, a turbine 126, and an electric assist unit 540 associated with the compressor 124. The e-machine with the rotor shaft 200 according to embodiments of the present invention may be used in the electric assist unit 540. The electric assist unit 540 is electrically connected to a power source 550. In some embodiments, the power source 550 comprises a battery, such as a 48V battery. In some embodiments, the power source 550 is operably connected to a generator 560. In some embodiments, the generator 560 may be powered by the internal combustion engine 110. In alternative embodiments, the generator 560 may be powered by a separate power source (not shown). In the exemplary system 500, the electrical assist unit 540, at least in part, applies torque to the compressor 124. Using the electric assist unit 540 to apply torque to the compressor 124 allows for a reduction in the phenomenon known as turbocharger lag and also improves the dynamic response of the turbocharger.

[0061] FIG. 6 shows an exemplary vehicle 600 including wheels 610. The wheels 610 are connected via shafts 620, which may include a differential 630. Torque is provided to the wheels 610 via an electric powertrain which is operably coupled to the wheels 610. The electric powertrain includes an e-machine 640 that includes a rotor shaft 200 according to embodiments of the present invention. The electric powertrain further includes a power source 650 operably coupled to the e-machine 640 for providing electrical energy to the e-machine 640. Method for forming the rotor shaft

[0062] When forming the rotor shaft 200, it is desirable to ensure that the interface between a radially outer surface of the interior sleeve 240 and the radially inner surface of the interior channel 220 does not overly decrease the heat transfer between the interior sleeve and the interior channel. It has been found by the present inventors that forming the rotor shaft 200 in two or more sub-assemblies, and then press-fitting the interior sleeve 240 into at least one of these sub-assemblies before connecting the subassemblies together, for example by a welding process, such as laser-welding, to form the rotor shaft results in an acceptable thermal interface between the interior sleeve 240 and the interior channel 220.

[0063] Turning to FIG. 7, a method 700 for forming a rotor shaft is shown. At step 710, at least two sub-assemblies of the rotor shaft are formed from a first material having a first density. In some embodiments, the at least two sub-assemblies of the rotor shaft are formed from a steel alloy.

[0064] At step 720, an interior sleeve is attached to at least one of the at least two subassemblies of the rotor shaft. In some embodiments, the interior sleeve is attached to the at least one of the at least two sub-assembles of the rotor shaft by press-fitting. Various other options exist for connecting the interior sleeve to the at least one of the at least two sub-assembles of the rotor shaft, which options include: interference fitting, shrink fitting, adhesive bonding and frictional welding, amongst others. In some embodiments, the interior sleeve is formed from aluminum or an aluminum alloy.

[0065] At step 730, the at least two sub-assemblies of the rotor shaft are connected together. In some embodiments, the at least two sub-assemblies of the rotor shaft are welded together using laser welding. In other embodiments, the at least two subassemblies of the rotor shaft are connected together via frictional welding. Various other options exist for connecting the at least two sub-assemblies include, which options 5 include: brazing, adhesive bonding, ultrasonic welding, plasma arc welding, amongst others.

[0066] While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments 10 are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without 15 departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.

Claims

What is claimed is:

1. An e-machine comprising:a stator; anda rotor disposed at least partially within the stator, the rotor comprising:a rotor- shaft having a longitudinal axis, wherein the rotor shaft comprises an inlet, an outlet, and an interior channel fluidly connecting the inlet to the outlet so as to allow for liquid coolant to flow from the inlet to the outlet, wherein the rotor shaft is formed from a first material having a first density; andan interior sleeve disposed within the interior channel, the interior sleeve comprising a passage through which liquid coolant may flow, wherein the interior sleeve is formed from a second material having a second density, the second density being less than the first density.

2. The e-machine of claim 1, wherein the interior sleeve is cylindrical and is attached to a radially inner surface of the interior channel.

3. The e-machine of claim 1 or claim 2, wherein the first material has a firstthermal conductivity, and wherein the second material has a second thermal conductivity that is greater than the first thermal conductivity.

4. The e-machine of any preceding claim, wherein the e-machine is a traction motor.

5. The e-machine of any preceding claim, wherein the first material comprises steel.

6. The e-machine of any preceding claim, wherein the second material comprises aluminum or an aluminum alloy.

7. An electric assist unit for a turbocharger, the electric assist unit comprising an e-machine according to any preceding claim.

8. An electric vehicle powertrain comprising an e-machine according to any ofclaims 1 to 7.

9. A method for forming a rotor shaft for an e-machine, the method comprising:forming at least two sub-assemblies of the rotor shaft from a first material having a first density;attaching an interior sleeve to at least one of the at least two sub-assemblies, the interior sleeve having a passage through which liquid coolant may flow, the interior sleeve being formed from a second material having a second density, the second density being less than the first density; andconnecting the at least two sub-assemblies together to assemble the rotor shaft, wherein the assembled rotor shaft comprises an inlet, an outlet, and an interior channel fluidly connecting the inlet to the outlet, with the interior sleeve disposed within the interior channel to allow for liquid coolant to flow from the inlet, through the passage of the interior sleeve, and to the outlete.

10. The method of claim 9, wherein the step of attaching the interior sleeve to the at least one of the at least two sub-assemblies comprises press-fitting the interior sleeve to the at least one of the at least two sub-assemblies.

11. The method of claim 9 or claim 10, wherein the step of connecting the at least two sub-assemblies together to form the rotor shaft comprises welding the at least two subassemblies together.

12. The method of claim 11, wherein the step of connecting the at least two subassemblies together to form the rotor shaft comprises laser-welding the at least two subassemblies together.

13. The method of any of claims 9 to 12, wherein the first material has a first thermal conductivity, and wherein the second material has a second thermal conductivity that is greater than the first thermal conductivity14. The method of any of claims 9 to 13, wherein the first material comprises steel.

15. The method of any of claims 9 to 14, wherein the second material comprises aluminum or an aluminum alloy.15

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