Permanent magnet temperature control to reduce loss

Abstract

Permanent magnet temperature control to reduce loss. An electric drive system includes a permanent magnet motor, a rotor magnet connected to a rotor thereof, a heating source, such as a resistive heating element, connected to the rotor magnet, and an electronic controller. The controller selectively heats the rotor magnet via the heating source during a predetermined low load / high speed mode of operation of the electric drive system. The controller may preemptively cool the rotor magnet in response to a predicted or upcoming high load / low speed mode of operation, such as via a pre-cooled electrical coolant. The heating element may include a positive temperature coefficient heating element disposed between the rotor yoke and the rotor magnet. A method includes detecting a low load / high speed mode of operation and selectively heating a rotor magnet via a heating source during such a mode. A motor vehicle includes wheels and an electric drive system, an electric machine of which powers one or more wheels.

Description

Technical Field

[0001] The present disclosure relates to methods and systems for reducing losses in an electric drive system having a permanent magnet type electric machine (PM machine). Background Art

[0002] Advanced hybrid electric and all-electric / battery electric vehicles include one or more electric traction motors. Each traction motor is energized by a controlled discharge of a high-energy battery pack, such as a lithium-ion propulsion battery pack or a battery pack having another battery chemistry suitable for the application. A PM machine is a particular electrical machine configuration in which physical magnets are mounted to or embedded within the rotor structure of the machine. The permanent magnets of the rotor ("rotor magnets") are made of rare earth materials (such as neodymium or samarium cobalt), of iron-containing materials (such as ferrite), or of other high remanence materials such that the rotor can generate and maintain a strong magnetic field. Such rotor magnets are typically cooled during their operation, for example via the circulation of air or a cooling oil / electrical coolant. This action allows the rotor magnets to support high torque operating modes and provides a high coercivity suitable for mitigating demagnetization of the rotor magnets. Summary of the Invention

[0003] Disclosed herein is an electric drive system having a permanent magnet (PM) machine of the above type, and a related control strategy for selectively heating the permanent magnets of the rotor ("rotor magnets") during a predetermined low load operation mode. For example, the electric drive system can be used as part of a motor vehicle, where the PM machine is configured as a traction motor, e.g., mounted on a hybrid electric vehicle or an all-electric / battery electric vehicle, or as a traction motor for a train, a ship, an airplane, or another electrically driven mobile platform, or as part of a stationary power plant. For the sake of illustration consistency only, the electric drive system is described hereinafter as being mounted on a motor vehicle for use. In such an embodiment, the load driven by the PM machine can include one or more wheels, without limiting the present teachings to such an example.

[0004] An electric drive system according to one embodiment includes a PM machine having a rotor, where one or more rotor magnets are attached to the rotor. A heating source is connected to the one or more rotor magnets. An electronic controller in communication with the PM machine and the heating source is programmed to selectively heat the one or more rotor magnets via the heating source during a predetermined low load / high speed operation mode of the electric drive system.

[0005] The electronic controller can be programmed to predict an upcoming high load / low speed operation mode of the PM machine and, in response to the upcoming high load / low speed operation mode, preemptively request cooling of the rotor magnets via a cooling source.

[0006] In PM motors of different configurations, the rotor magnets can be made of rare earth materials or iron-containing materials (such as ferrite).

[0007] The heating source in one or more configurations includes a resistive heating element. For example, the resistive heating element can include a positive temperature coefficient (PTC) heating element disposed between the rotor yoke and the rotor magnets of the rotor.

[0008] The heating source can optionally include a supply of preheated electric coolant. The rotor can include a rotor shaft that defines an axial fluid passage therein, and the axial fluid passage is configured to direct the preheated electric coolant through the rotor shaft to heat the rotor magnets.

[0009] In one or more embodiments of the present teachings, the inverter circuit of the electric drive system is connected to the PM motor. In such an embodiment, the electronic controller can be configured to command pulse width modulation (PWM) harmonics via the inverter circuit that is part of the heating source to thereby generate eddy currents within the rotor magnets. This action occurs at a level suitable for heating the rotor magnets.

[0010] The present disclosure also includes a method of selectively heating the PM motor of the above electric drive system. An embodiment of the method includes detecting, via an electronic controller, a predetermined low load / high speed operation mode of the electric drive system, and then selectively heating the rotor magnets via the heating source using the electronic controller during the predetermined low load / high speed operation mode. The method can include predicting, via the electronic controller, an upcoming high load / low speed operation mode of the electric drive system, and in response to the upcoming high load / low speed operation mode, pre-cooling the rotor magnets using the electronic controller via a cooling source.

[0011] Also disclosed herein is a motor vehicle, a representative configuration of which includes a body, a plurality of wheels connected to the body, and an electric drive system. The electric drive system can include a PM motor having a rotor connected to one or more wheels, a plurality of rotor magnets connected to the rotor, a PTC heating element disposed between the rotor yoke and the rotor magnets, and an electronic controller. The electronic controller is configured to selectively heat the rotor magnets via the PTC heating element during a predetermined low load / high speed operation mode of the electric drive system.

[0012] The electronic controller can predict an upcoming high load / low speed operation mode of the motor vehicle and pre-cool the rotor magnets using pre-cooled electric coolant, for example, by requesting pre-cooled electric coolant from an on-vehicle coolant supply in response to the upcoming high load / low speed operation mode. This action can include commanding the pre-cooled electric coolant to circulate through the PM motor.

[0013] As described above, the rotor may include a rotor shaft defining an axial fluid passage. Such a passage may be configured to direct heated electrical coolant and / or precooled electrical coolant to heat and / or pre-cool the rotor magnets, respectively.

[0014] The present invention may also include the following technical solutions:

[0015] 1. An electric drive system, comprising:

[0016] A permanent magnet (PM) motor having a rotor;

[0017] One or more rotor magnets connected to the rotor;

[0018] A heat source connected to the one or more rotor magnets; and

[0019] An electronic controller in communication with the PM motor and the heat source, the electronic controller being programmed to selectively heat the one or more rotor magnets via the heat source during a predetermined low load / high speed operating mode of the electric drive system.

[0020] 2. The electric drive system according to technical solution 1, wherein the electronic controller is programmed to predict an upcoming high load / low speed operating mode and preemptively request cooling of the rotor magnets in response to the upcoming high load / low speed operating mode.

[0021] 3. The electric drive system according to technical solution 2, wherein the electronic controller is configured to preemptively request cooling of the rotor magnets by requesting circulation of precooled electrical coolant around or through the rotor magnets.

[0022] 4. The electric drive system according to technical solution 2, wherein the rotor magnets are made of rare earth materials, and wherein the electronic controller is configured to preemptively request cooling of the rotor magnets made of rare earth materials to thereby prevent demagnetization of the rotor magnets.

[0023] 5. The electric drive system according to technical solution 1, wherein the rotor magnets are made of ferrite, such that the rotor magnets include ferrite magnets.

[0024] 6. The electric drive system according to technical solution 1, wherein the rotor includes a rotor yoke, and wherein the heat source includes a positive temperature coefficient (PTC) heating element disposed between the rotor yoke and the rotor magnets.

[0025] 7. The electric drive system according to technical solution 1, wherein the heat source includes a supply of preheated electrical coolant.

[0026] 8. The electric drive system according to aspect 7, wherein the rotor includes a rotor shaft, and an axial fluid passage is defined therein, and the axial fluid passage is configured to guide preheated electric coolant through the rotor shaft.

[0027] 9. The electric drive system according to aspect 1, wherein the rotor is in fluid communication with pre-cooled electric coolant, and the electronic controller is configured to request the pre-cooled electric coolant to circulate around and / or through the rotor magnets to selectively cool the rotor magnets during the terminal stage of a predetermined low-load / high-speed operation mode.

[0028] 10. The electric drive system according to aspect 1, further comprising:

[0029] An inverter circuit connected to the PM motor, wherein the electronic controller is configured to command pulse-width modulation (PWM) harmonics via the inverter circuit as part of a heating source to thereby generate eddy currents in the rotor magnets at a level suitable for heating the rotor magnets.

[0030] 11. A method for selectively heating a permanent magnet (PM) motor of an electric drive system, the PM motor having a plurality of rotor magnets connected to a rotor, the method comprising:

[0031] Detecting, via an electronic controller, a predetermined low-load / high-speed operation mode of the electric drive system; and

[0032] During the predetermined low-load / high-speed operation mode, selectively heating the rotor magnets via a heating source using the electronic controller.

[0033] 12. The method according to aspect 11, further comprising:

[0034] Predicting, via the electronic controller, an upcoming high-load / low-speed operation mode of the electric drive system; and

[0035] In response to the upcoming high-load / low-speed operation mode, pre-cooling the rotor magnets using the electronic controller via a cooling source.

[0036] 13. The method according to aspect 11, wherein the heating source includes a resistive heating element, and selectively heating the rotor magnets via the heating source includes activating the resistive heating element.

[0037] 14. The method according to aspect 13, wherein the rotor includes a rotor yoke, and activating the resistive heating element includes activating a positive temperature coefficient (PTC) heating element disposed between the rotor yoke and the rotor magnets.

[0038] 15. The method according to aspect 11, wherein the heating source includes a supply of preheated electrical coolant, and wherein selectively heating the rotor magnet via the heating source includes circulating the preheated electrical coolant around or through the rotor magnet.

[0039] 16. The method according to aspect 15, wherein the rotor includes a rotor shaft defining an axial fluid passage, and wherein selectively heating the rotor magnet via the heating source includes circulating the preheated electrical coolant through the axial fluid passage.

[0040] 17. The method according to aspect 11, wherein the heating source includes an inverter circuit connected to the PM motor, and the method further includes:

[0041] Using an electronic controller to command pulse width modulation harmonics via the inverter circuit to thereby generate eddy currents in the rotor magnet; and

[0042] Heating the rotor magnet using the eddy currents.

[0043] 18. A motor vehicle, comprising:

[0044] A body;

[0045] A plurality of wheels connected to the body; and

[0046] An electric drive system connected to the body, the electric drive system including:

[0047] A permanent magnet (PM) motor having a rotor connected to one or more wheels, the rotor including a rotor yoke;

[0048] A plurality of rotor magnets connected to or integrated with the rotor;

[0049] A positive temperature coefficient heating element disposed between the rotor yoke and the rotor magnet, and

[0050] An electronic controller in communication with the PM motor and the positive temperature coefficient heating element, the electronic controller being configured to selectively heat the rotor magnet via the positive temperature coefficient heating element during a predetermined low load / high speed operation mode of the electric drive system.

[0051] 19. The motor vehicle according to aspect 18, further including a supply of precooled electrical coolant, wherein the electronic controller is configured to predict an upcoming high load / low speed operation mode of the motor vehicle and, in response to the upcoming high load / low speed operation mode, preemptively cool the rotor magnet using the precooled electrical coolant, which includes commanding the precooled electrical coolant to circulate through the PM motor.

[0052] 20. The motor vehicle according to aspect 19, wherein the rotor includes a rotor shaft defining an axial fluid passage configured to direct heated electric coolant and / or precooled electric coolant to heat and / or pre-cool the rotor magnets, respectively.

[0053] The above features and advantages of the present teachings, as well as other features and advantages, will be readily apparent from the following detailed description of some of the best modes and other embodiments for carrying out the present teachings, when taken in conjunction with the accompanying drawings, as defined in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 Illustrated is an electrical system of a representative motor vehicle having a permanent magnet (PM) machine, the rotor magnet temperature of which is regulated during a low load "cruise" mode, as elaborated herein.

[0055] Figure 2 is a time plot of output torque that illustrates Figure 1 different heating zones of a representative low load operating mode of a motor vehicle.

[0056] Figure 3 Illustrated is a simplified heating circuit for use with a Figure 1 PM machine.

[0057] Figure 4 is a side view illustration of a rotor shaft of a PM machine that can be used in accordance with one aspect of the present disclosure for Figure 1 a motor vehicle.

[0058] Figure 5 is an output torque versus output speed plot that illustrates different temperature control zones, where such zones are used as part of this control strategy.

[0059] Figure 6 is a flowchart describing Figure 1 an embodiment of a method for regulating the temperature of the rotor magnets of the PM machine illustrated in

[0060] Modifications may be made to the present disclosure or it may be embodied in alternative forms, with representative embodiments shown in the drawings and described in detail hereinafter. The inventive aspects of the present disclosure are not limited to the disclosed embodiments. On the contrary, the present disclosure is intended to cover alternatives falling within the scope of the disclosure as defined by the appended claims. DETAILED DESCRIPTION

[0061] Referring to the drawings, in which like reference numerals correspond to like or similar components throughout the several views, an electric drive system 10 is shown in Figure 1Shown in the figure. The electric drive system 10 includes one or more rotating electric motors in the form of permanent magnet (PM) motors 12, such as electric traction motors for use in motor vehicles 18, or alternatively electric traction motors for use in trains 18A or other rail vehicles, ships 18B or other surface or underwater vessels, or aircraft 18C or other fixed-wing or rotor-equipped aerial vehicles. For the sake of clarity and consistency of illustration, the electric drive system 10 is described herein as part of a motor vehicle 18 without limiting the application of the electric drive system 10 to such a host system, or to mobile systems in general.

[0062] The present solution selectively heats a set of rotor magnets 14 of the PM motor 12 during pre-determined low-load operating conditions, where the rotor magnets 14 are connected or integrated with the rotor 12R of the PM motor 12. During such conditions, the PM motor 12 operates at a high steady-state output speed or near a high steady-state output speed, i.e., with little acceleration and relatively low output torque. Consistent with an exemplary vehicle use case, such conditions can be consistent with "cruise mode", which is typically experienced by a user of a motor vehicle 18 when traveling along a stretch of road at a desired steady speed. Heating the rotor magnets 14 during cruise mode reduces the magnetic flux density in the rotor 12R of the PM motor 12. Thus, as a specific benefit of this strategy, core losses are reduced.

[0063] The method described herein can be used in applications where the rotor magnets 14 are made of rare earth materials such as neodymium iron boron (NdFeB) or samarium cobalt (SmCo). In other embodiments, the rotor magnets 14 can be made of ferrite as a ceramic magnet, or of other iron-containing materials. Figure 1 Applications within the electric drive system 10 can utilize either configuration.

[0064] Due to the slow dynamics of rotor cooling, rotor magnets 14 having a rare earth material configuration can be used in applications where the load on the PM motor 12 is largely predictable, such as when a motor vehicle 28 or other host system exhibits repeatable or restricted / highly controlled route options. Aspects of this strategy also include cooling the rotor magnets 14 after such heating to help avoid demagnetization, where this action occurs in anticipation of the next peak load. Thus, in some embodiments, the load anticipation may be a control factor. In other embodiments, the rotor magnets 14 can be made of an iron-containing material (such as ferrite) to prevent or avoid such demagnetization problems, which are typically associated with rare earth materials. Thus, the specific material composition of the rotor magnets 14 can vary within the scope of the present disclosure.

[0065] Figure 1The PM motor 12 also includes a wound stator 12S that is circumscribed around the rotor 12R, i.e., as a radial flux configuration, although axial flux implementations are also contemplated within the scope of the present disclosure. The rotor shaft 120 of the PM motor 12 is connected to the driven load. In Figure 1 an exemplary vehicle use case, the driven load can include one or more wheels 20 that are disposed / connected relative to / with respect to the vehicle body 21.

[0066] In other embodiments, the PM motor 12 can include multiple motors that are similarly configured, e.g., for independently or jointly driving one or more wheels 20. Thus, the use of a single PM motor 12 herein is not intended to limit the present teachings to a single-motor configuration of the electric drive system 10. Each PM motor 12 is connected to a direct current (DC) voltage bus 15 having positive (+) and negative (-) bus rails. When the PM motor 12 is configured as an alternating current (AC) rotating motor as shown, one or more phase windings 17 of the stator 12S are connected to the AC side of an inverter circuit 16. The PM motor 12 is connected to the DC voltage bus 15 via the inverter circuit 16. As with embodiments using multiple PM motors 12, in one or more embodiments, the present disclosure can be extended to multi-inverter topologies. Thus, the use of "a" or "an" when referring to components of the electric drive system 10 is intended to cover "one or more" unless otherwise specified.

[0067] As contemplated herein, Figure 1 the electric drive system 10 illustrated in also includes an electrochemical battery pack 13 that is connected across the positive and negative bus rails (+, -) of the DC voltage bus 15. The battery pack 13 - configured in this instance as a high-voltage traction battery pack 13, e.g., a 300 - 1000V lithium-ion or lithium-metal configuration suitable for powering the drive mode of a motor vehicle 18 - is operable to output a DC voltage (VDC) to the DC voltage bus 15. The DC voltage (VDC) is converted into an AC voltage (VAC) suitable for exciting the stator 12S of the PM motor 12.

[0068] When the stator 12S is excited in this manner, the rotation of the rotor 12R thus produces an output torque (T O ). The output torque (T O ) is then directed via the rotor shaft 120 that is coupled to or integrally formed with the rotor 12R. The rotation of the rotor shaft 120 is ultimately transmitted directly or via an intervening drive shaft (not shown) to one or more wheels 20. Thus, the motor vehicle 18 is electrically propelled along a road surface in an electric or hybrid-electric drive mode. Vehicles of alternative configurations (i.e., trains 18A, 18B, and 18C) can be similarly propelled over or through their respective media, i.e., along tracks, over / through water bodies, and through air, respectively.

[0069] Still referring to Figure 1 , the electric drive system 10 further includes an electronic control system (“electronic controller”) (C) 50, which is programmed in software and equipped with hardware, i.e., “configured” to perform the various monitoring and heating / cooling control processes described below with reference to Figures 2 - 6 . Like the PM motor 12 and the inverter circuit 16, for clarity and simplicity of illustration, the electronic controller 50 is depicted as a single device. In a practical embodiment, the electronic controller 50 may be implemented as a distributed control system, i.e., a system in which multiple processing nodes communicate with each other from different locations within the electric drive system 10, e.g., as separate motor control processors, transceiver nodes, and the like. Thus, in various embodiments, the electronic controller 50 may be implemented as one or more computer devices.

[0070] In response to receiving an input signal (CC I ) from the sensor suite 11, the electronic controller 50 is configured to execute method 100 to selectively heat the rotor magnet 14. The sensor suite 11 may include one or more physical sensors or computing units that together provide the output torque (T O ) and motor output speed (N O ) of the electric traction motor 12, as well as the rotor magnet temperature (T RM ) of the rotor magnet 14. Such values may be calculated or measured and reported by the sensor suite 11 of Figure 1 .

[0071] Method 100 (one embodiment of which is described below with reference to Figure 6 ) may be implemented as one or more algorithms or instruction sets, where the electronic controller 50 ultimately transmits an output signal (CC O ) to the PM motor 12 for the purpose of regulating the temperature of the rotor magnet 14. In some embodiments, the electronic controller 50 may also selectively and preemptively cool the rotor magnet 14 via an associated thermal management system 19, as schematically illustrated in Figure 1 , such as via the supply of a coolant using pumps, heat exchangers, valves, etc.

[0072] In addition to the heating control signal described below, the output signal (CC O)It may also include an on / off state command or a pulse width modulation (PWM) signal for controlling the conduction state of each power switch 160 of the inverter circuit 16. Such a PWM signal may also be used in one or more embodiments to generate eddy currents within the rotor 12R to provide or supplement the heating of the rotor magnet 14. As appreciated in the art, the power switch 160 may be embodied in various ways as an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a bipolar junction transistor (BJT), etc. The computer-readable code or instructions for implementing the method 100 may be executed by one or more processors 52 and stored in a tangible non-transitory portion of the memory 54, where the memory 54 embodies at least one computer-readable storage medium, such as a magnetic or optical medium, a CD-ROM, and / or a solid-state / semiconductor memory (e.g., various types of RAM or ROM).

[0073] The terms "controller" and related terms (such as control module, module, control, control unit, processor, and similar terms) refer to one or various combinations of a (one or more) application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a (one or more) electronic circuit, a (one or more) central processing unit (e.g., a (one or more) microprocessor), and a (one or more) associated non-transitory memory component in the form of a memory and storage device (read-only, programmable read-only, random access, hard drive, etc.). The non-transitory components of the memory 54 are those components capable of storing machine-readable instructions in the form of one or more software or firmware programs or routines, a (one or more) combinational logic circuit, a (one or more) input / output circuit and device, signal conditioning, and buffer circuits, and other components, which can be accessed by the (one or more) processors 52 to provide the described functions.

[0074] (One or more) input / output circuits and devices include analog / digital converters and related devices for monitoring inputs from sensors, where such inputs are monitored at a preset sampling frequency or in response to a triggering event. Software, firmware, programs, instructions, control routines, code, algorithms, and similar terms refer to a set of instructions executable by the controller, which includes calibration and look-up tables. Each controller executes a (one or more) control routine to provide the desired function. The routines may be executed at regular intervals, e.g., at intervals of approximately 50 - 100 microseconds (ms) during ongoing operations. Alternatively, the routines may be executed in response to the occurrence of a triggering event.

[0075] Now referring to Figure 2 , an exemplary low-load operation mode for the motor vehicle 18 is illustrated via the timing diagram 22 Figure 1Selective heating of the rotor magnet 14 described above. The time graph 22 depicts Figure 1 Representative cruise modes (CM) of the motor vehicle 18 of Figure 1 with different heating regions (REG-1:H, REG-2:H) over time (t), where time (t) is expressed in minutes (min). As used herein, "low load" refers to the period during which the output torque (T O ) decreases from a relatively high torque level (T MAX ) to a relatively low and sustained torque level (T MIN ) over a period of time (Δt), and the output torque (T O ) is shown on the vertical axis in Newton meters (Nm).

[0076] For example, at time t 0 starting, arrow 24 indicates the decrease in the output torque (T O ), where Figure 1 the steady-state operation of the PM motor 12 of Figure 1 starts at time t 1 . Arrow 25 indicates the steady-state operation such as the cruise mode. At time t 3 , the output torque (T O ) may suddenly increase (arrow 26), for example due to an increased torque request from the operator of the motor vehicle 18 of Figure 1 or an autonomously generated torque request, where at approximately t 4 it reaches a relatively high torque level again.

[0077] Figure 1 Selective heating of the rotor magnet 14 in Figure 1 can occur in the first region (REG 1:H) between time t 0 and t 2 . At time t 2 , the electronic controller 50 can anticipate exiting the cruise mode (arrow 25). For example, the electronic controller 50 can process Figure 1 the input signal (CC I ) of Figure 1 , which includes the current rotational speed and the user-generated or autonomously generated torque request for the PM motor(s) 12, to estimate an increase in the output torque (T O ) and whether an exit from the cruise mode (arrow 25) is imminent. Figure 1 The electronic controller 50 of Figure 1 can also use forward-looking information, such as road slope data or terrain data, or past driving history, to determine whether such an increase in the output torque (T O ) may be required, such as when traveling towards a steep slope.

[0078] ​​The electronic controller 50 may incorporate other factors into the pre-emptive cooling determination, such as but not limited to considering past driving profiles or the nature of the driving route. As appreciated in the art, Figure 1 Some representative host systems, including the train 18A, the aircraft 18C, and certain use cases of the possible ship 18B or the motor vehicle 18 (such as autonomous taxis), may travel on well-defined or closed / confined routes. Thus, the torque / speed trajectory of such host systems can be highly predictable and repeatable. Therefore, at a given point in time, the electronic controller 50 may be able to determine with high confidence that the host system is about to exit the low-load operation mode, i.e., the cruise mode of the motor vehicle 18 in a typical use case. When this occurs, Figure 1 The electronic controller 50 illustrated in 2 may stop heating the rotor magnet 14, which is the start of the second region (REG 2:C). This action may be consistent with an optional pre-cooling of the rotor magnet 14, such as by circulating a suitable electrical coolant within the associated thermal management system 19 schematically illustrated in Figure 1 .

[0079] Referring to Figure 3 , in one or more embodiments using the heating circuit 27, according to the present disclosure, the rotor magnet 14 is selectively heated by operating the Figure 1 electronic controller 50. Here, the battery pack 13 may be selectively connected to the resistive heating element 30, which is embedded within or connected to the rotor magnet 14 of the rotor 12R. For example, the resistive heating element 30 may be disposed between the rotor yoke 12Y and the rotor magnet 14 of the PM motor 12 (see Figure 1 ). The electronic controller 50 may command the switch 29 to open or close, for example, using the output signal (CC O ) to selectively connect the battery pack 13 to or disconnect it from the resistive heating element 30, respectively. Alternatively, a secondary battery (not shown), i.e., a battery other than the battery pack 13, may be used as the DC power source for energizing the resistive heating element 30. Thus, the use of the battery pack 13 for this purpose is exemplary and not limiting.

[0080] In one possible configuration, the resistive heating element 30 may be a positive temperature coefficient (PTC) heating element 300. In such an embodiment, the PTC heating element 300 may be composed of a ceramic material suitable for the application to provide a rapid heating response in a highly predictable and controllable efficient manner by the electronic controller 50. Such a solution may enable a uniform heat distribution to enter the rotor magnet 14. In other embodiments, the resistive heating element 30 may include one or more wires passing through or in close proximity to the rotor magnet 14.

[0081] The electronic controller 50 can also be configured to command the inverter circuit 16 to generate PWM harmonics to selectively generate one or more eddy currents within the rotor magnet 14. In such an instance, the inverter circuit 16 can act as part of the heating source 30S, where the electronic controller 50 thus at least partially uses the eddy currents to heat the rotor magnet 14. Thus, as contemplated herein, the heating source 30S can include the resistive heating element 30, the inverter circuit 16, and other possible heating devices, such as the circulation of preheated coolant arriving at / passing through the rotor magnet 14 (arrow CC H ).

[0082] For optional cooling of the rotor magnet 14, upon request of the electronic controller 50, pre-cooled electrical coolant can be circulated through and / or around the rotor magnet 14 (arrow CC C ), i.e., coolant at a temperature lower than that of the rotor magnet 14, to extract heat from the rotor magnet 14. The resulting heated coolant (arrow CC HX ) is then discharged from the rotor magnet 14 and conveyed to a downstream heat exchanger (not shown) before being possibly recirculated back to the rotor magnet 14 as pre-cooled electrical coolant (arrow CC C ).

[0083] Now referring Figure 4 , in yet another method, the electronic controller 50 can be configured to preemptively request cooling of the rotor magnet 14 by requesting the circulation of pre-cooled electrical coolant (arrow CC C ) around or through the rotor magnet. The pre-cooled electrical coolant (arrow CC C ) can circulate around the rotor magnet 14, circulate near the rotor magnet 14, circulate across the rotor magnet 14, and / or circulate through the rotor magnet 14, e.g., through an axial fluid passage 33 defined by the inner diameter wall 330 of the rotor shaft 120. In a simplified embodiment, Figure 1 the rotor shaft 120 of the PM motor 12 of C can include a shaft body 32 having a cylindrical end piece 34 attached to or integrally formed with the shaft body 32. The pre-cooled electrical coolant (CC

[0084] ) can circulate through the axial fluid passage 33 to help cool the rotor shaft 120 and thus the rotor magnet 14 connected to it or disposed within it. C The flow of the pre-cooled electrical coolant (CC HX ) through the shaft body 32 in this way extracts heat from the rotor magnet 14. The heated coolant (CC Figure 4The configured rotor magnet 14, the heated coolant (CC H ) can be directed through the axial fluid passage 33 to heat the rotor magnet 14. As in the Figure 3 embodiment, this can be accomplished using the flow of pre-cooled electrical coolant (CC C ) through the axial fluid passage 33 either alone or in combination with the heating element 30 and / or with other heat sources 30S.

[0085] As illustrated by the torque-speed diagram 40 in Figure 5 , the output torque (T Figure 1 ) in Newton meters (Nm) and the output speed (N O ) in revolutions per minute (RPM) of the PM motor 12 of O are shown on the vertical and horizontal axes, respectively. In an exemplary event, such as at the start of a startup or another rapid acceleration event, the output torque (T O ) from the electric traction motor 12 is at a relatively high torque level (T 0 ) at a low speed N MAX ), such as approximately 0 RPM or another minimum angular velocity (N MIN ). As the speed of the PM motor 12 increases to an intermediate speed N 1 , the output torque (T O ) can remain at a relatively high torque level (T MAX ) for a period of time, as indicated by the relatively flat locus line 41 between the nominal speeds N 0 and N 1 . At the speed N 1 , the operator of the motor vehicle 18 or another host system having the PM motor 12 can begin to enter a low-load operation mode, such as the cruise mode described above. The speed begins to decrease from the speed N 1 , as indicated by the locus curve 42, and decays via the locus curve 44 until the maximum speed at the speed N 3 , where the maximum speed (N MAX ) corresponds to a low load, i.e., the minimum torque level (T O ) of the output torque (T MAX ).

[0086] The torque-speed diagram 40 is divided into nominal first, second, and third heating regions R1, R2, and R3. The first heating region R1, starting from the speed N 0 and continuing just beyond the speed N 1 , corresponds to a relatively low PM temperature region, i.e., where Figure 1The rotor magnet 14 of Figure 1 maintains a region below a calibrated or predetermined temperature lower limit. The first heating region R1 may be separated from the second heating region along a boundary line 43. The second heating region R2 may in turn be separated from the third heating region R3 along a boundary line 45, where the boundary lines 43 and 45 are predetermined values that may be recorded in the memory 54 of the electronic controller 50 shown in

[0087] In the second heating region R2, which corresponds to Figure 1 the relatively light load of the PM motor 12 of Figure 1 at a lower angular velocity, the electronic controller 50 may start heating the rotor magnet 14 of Figure 1 . Among other potential benefits, such heating can in particular help to reduce cogging torque and vibration and improve efficiency. As the speed of the PM motor 12 increases and enters the third heating region R3, MAX the rotor magnet 14 of

[0088] may be heated to a lesser extent than in the second heating region R2 to extend the flux weakening capability. Before re-entering the first heating region R1 from the second heating region R2, i.e., before achieving a relatively high torque level (T Figure 1 ), the pre-cooling time depends on the cooling capacity of the rotor magnet 14 and the timing of the next peak load. In some applications, it may be possible for the electronic controller 50 to predict the peak load, and this will be particularly valuable when the rotor magnet 14 is made of rare earth materials, for example by minimizing the possibility of demagnetization or preventing demagnetization.

[0089] As described above, aspects of the present disclosure may include pre-cooling the rotor magnet 14 in the event of a sudden load change. In the rare earth construction of the rotor magnet 14, there is a higher risk of demagnetization when the load suddenly increases. Thus, when the load is predictable, the thermal management according to the present disclosure may be performed by Figure 1is coordinated by the electronic controller 50 because the thermal dynamics involved in cooling are relatively slow. However, the rotor magnet 14 made of an iron-containing material such as ferrite behaves differently. Like heating a rare-earth magnet, heating such a rotor magnet 14 reduces core loss, but there is a small risk of demagnetization due to sudden load changes. Therefore, the pre-cooling time can be shortened compared to pre-cooling a rare-earth magnet. As a result, for unpredictable as well as predictable loads, ferrite magnets can be used for the rotor magnet 14, while the rare-earth PM can perform optimally for more predictable loads.

[0090] Now referring to Figure 6 , for simplicity of illustration, the method 100 according to a representative embodiment is represented as a series of algorithmic code segments or logic blocks. Each constituent logic block can be executed by the (one or more) processors 52 of the (one or more) electronic controllers 50 to implement the performance of the associated process steps. Figure 1

[0091] Starting from block B102 (“INIT”), the electronic controller 50 can be initiated in response to a set of entry criteria. Such criteria can be specific to the particular host system of the electric drive system 10 shown in Figure 1 . For example, if the electric drive system 10 is used on board a motor vehicle 18, suitable entry conditions can include an on-cycle indicating that the motor vehicle 18 is in a driving state, where the PM motor 12 is energized and ready to generate or has generated an output torque (T O ). The method 100 then proceeds to block B104.

[0092] At block B104 (“T O , N O , T RM ”), the electronic controller 50 can receive, measure, or otherwise determine a set of load parameters. Such load parameters can include the output torque (T O ), the motor speed (N O ) of the PM motor 12, and the rotor magnet temperature (T RM ) of the rotor magnet 14. Such values can be calculated or measured and reported by the sensor suite 11 of Figure 1 . When the load parameters have been determined, the method 100 proceeds to block B105.

[0093] Block B105 (“OPM = CM?”) includes determining whether the current operating mode (OPM) corresponds to Figure 1The predetermined low-load state of the PM motor 12. In a representative case of the motor vehicle 18, the low-load state may include a cruise mode (CM). When the current operating mode corresponds to the predetermined low-load state, the method 100 proceeds to block B106, and alternatively, when the current operating mode does not correspond to the predetermined low-load state, the method 100 returns to block B102.

[0094] At block B106 (“T RM = T CAL ”), the electronic controller 50 may selectively heat Figure 1 the rotor magnet 14 until the rotor magnet temperature (T RM ) reaches a target temperature (T CAL ). The target temperature (T CAL ) may be based on the specific construction material of the rotor magnet 14 for a particular application and on its desired properties based on the magnetization curve associated with such material. In one or more embodiments, a look-up table may be recorded in Figure 1 the memory 54 for access by the processor(s) 52 to utilize Figure 5 the corresponding target temperatures (T CAL ) of the second and third heating regions R2 and R3. Based on the current torque-speed operating point, the electronic controller 50 may extract the corresponding target temperature (T CAL ), and thereafter control the heating element 30 or its various alternative embodiments in a closed loop Figure 3 until the rotor magnet temperature (T RM ) reaches the target temperature (T CAL ). Thereafter, the method 100 proceeds to block B107.

[0095] Block B107 (“EOC?”) includes determining via Figure 1 the electronic controller 50 whether the low-load state of the PM motor 12 has ended. Continuing the exemplary case of the motor vehicle 18 where the low-load state is the cruise mode, block B107 includes determining whether the torque-speed operating point indicates the end or terminal phase of the cruise mode, i.e., EOC. When the low-load state of the PM motor 12 has ended, the method 100 proceeds to block B108. Otherwise, the method 100 alternately repeats blocks B106 and B107 until the low-load state of the PM motor 12 has ended.

[0096] Block B108 (“DEC T RM ”) includes reducing the temperature of the rotor magnet 14. For example, as Figure 5 illustrated, this would require transitioning from the second heating region R2 to the first heating region R1. This may occur by interrupting the heating process and by introducing cooling, e.g., via Figure 3 andFigure 4 of the electric coolant (CC C ) around and / or through the rotor magnet 14 or the rotor shaft 120.

[0097] As described above, when the rotor magnet 14 is made of rare earth materials, anticipatory cooling to resume high-load operation modes (e.g., the start mode or high-acceleration drive mode of the motor vehicle 18) may be beneficial. Similarly, the ability to anticipate the future and accurately determine the end of the mode during low-load operation modes can be achieved in certain highly repeatable drive profiles (e.g., in certain use cases of Figure 1 the exemplary train 18A, ship 18B, or aircraft 18C). Various use cases of the motor vehicle 18 can exhibit similar drive profiles, enabling prediction of the end of the low-load operation mode, e.g., when starting from a standstill or based on current traffic conditions. The method 100 then proceeds to block B110.

[0098] At block B110 (“OPM = PM”), the method 100 includes performing a high-load operation mode. For example, in the exemplary case of the motor vehicle 18, this may require performing a propulsion mode (PM) with higher torque / lower speed characteristics, e.g., Figure 3 of the first or second heating regions R1 or R2. The method 100 then returns to block B102.

[0099] Among other benefits, the method 100 as set forth above enables, in particular, selective heating of Figure 6 the Figure 1 , Figure 3 and Figure 4 rotor magnet 14, thereby controlling the magnetic properties in a load-specific manner. The heating control strategy executed by the electronic controller 50 helps reduce the magnetic flux density in the PM motor 12 and thus reduce the core losses therein during low-load operation modes such as cruise mode. Due to the slow dynamics of cooling, aspects of the method 100 may be particularly useful in applications with predictable loads to prevent demagnetization in the event of a sudden onset of high load. Thus, the present teachings address potential problems of motor losses at relatively light loads and high operating speeds. In view of the foregoing disclosure, those skilled in the art will readily appreciate these and other attendant benefits of the present disclosure.

[0100] For purposes of this detailed embodiment, unless specifically stated otherwise: singular includes plural and vice versa; the words "and" and "or" shall be both conjunctive and disjunctive; the words "any" and "all" shall each mean "any and all"; and the words "comprising", "containing", "including", "having", and the like shall each mean "including but not limited to". In addition, approximating words such as "about", "almost", "substantially", "generally", "approximately", and the like may each be used herein to denote, for example, "at, near, or almost at", or "within 0 - 5% of", or "within acceptable manufacturing tolerances", or any logical combination thereof. Finally, directional adjectives and adverbs such as head, tail, inboard, outboard, starboard, port, vertical, horizontal, up, down, front, rear, left, right, etc. may be relative to a motor vehicle, such as the forward driving direction of the motor vehicle when the vehicle is operatively oriented on a horizontal driving surface.

[0101] The detailed description and the drawings or figures support and describe the present teachings, but the scope of the present teachings is defined only by the claims. Although some best modes and other embodiments for practicing the present teachings have been described in detail, there are various alternative designs and embodiments for practicing the present teachings as defined in the appended claims.

Claims

1. An electric drive system, comprising: a permanent magnet (PM) motor having a rotor; one or more rotor magnets connected to the rotor; a heating source connected to the one or more rotor magnets; and an electronic controller in communication with the PM motor and the heating source, the electronic controller being programmed to selectively heat the one or more rotor magnets via the heating source during a predetermined low load / high speed operation mode of the electric drive system.

2. The electric drive system according to claim 1, wherein the electronic controller is programmed to predict an upcoming high load / low speed operation mode and, in response to the upcoming high load / low speed operation mode, preemptively request cooling of the rotor magnets.

3. The electric drive system according to claim 2, wherein the electronic controller is configured to preemptively request cooling of the rotor magnets by requesting circulation of pre-cooled electric coolant around or through the rotor magnets.

4. The electric drive system according to claim 2, wherein the rotor magnets are composed of rare earth material, and wherein the electronic controller is configured to preemptively request cooling of the rotor magnets composed of rare earth material so as to thereby prevent demagnetization of the rotor magnets.

5. The electric drive system according to claim 1, wherein the rotor magnets are composed of ferrite such that the rotor magnets comprise ferrite magnets.

6. The electric drive system according to claim 1, wherein the rotor comprises a rotor yoke, and wherein the heating source comprises a positive temperature coefficient (PTC) heating element disposed between the rotor yoke and the rotor magnets.

7. The electric drive system according to claim 1, wherein the heating source comprises a supply of pre-heated electric coolant.

8. The electric drive system according to claim 7, wherein the rotor comprises a rotor shaft that defines an axial fluid passage therein, the axial fluid passage being configured to direct the pre-heated electric coolant through the rotor shaft.

9. The electric drive system according to claim 1, wherein the rotor is in fluid communication with pre-cooled electric coolant, and the electronic controller is configured to request circulation of the pre-cooled electric coolant around and / or through the rotor magnets to selectively cool the rotor magnets during a terminal stage of the predetermined low load / high speed operation mode.

10. The electric drive system according to claim 1, further comprising: an inverter circuit connected to the PM motor, wherein the electronic controller is configured to command pulse width modulation (PWM) harmonics via the inverter circuit, which is part of the heating source, so as to thereby generate eddy currents in the rotor magnets at a level suitable for heating the rotor magnets.

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