Patent Publication Number: US-2021175823-A1

Title: Electrical sub-assembly and associated method of operation

Description:
TECHNICAL FIELD 
     The invention relates to an electrical sub-assembly, particularly to an electrical sub-assembly for use as an electric traction drive for a hybrid vehicle or an electric vehicle. 
     Aspects of the invention relate to an electrical sub-assembly, a vehicle, a method and a controller. 
     BACKGROUND 
     Hybrid and electric vehicles typically utilise motors comprising permanent magnets generally incorporating rare earth elements such as neodymium (Nd). Other motor types are often not used for differing technical reasons, including lower torque density and problems with noise, vibration and harshness. However, motors comprising rare earth permanent magnets have drawbacks of their own. In particular, rare earth magnets are very expensive, at least relative to the cost of the other components of an electric motor (rare earth magnets may double or more the cost of a motor). Rare earth magnets also have poor sustainability credentials, due to methods used in their mining and refinement. 
     Moreover, permanent magnet motors will generate magnetic flux whenever a rotor within such a motor is rotating, regardless of whether or not electrical current is applied to the motor. This has potential system reliability and safety implications as magnetically induced voltages may be generated at electrical terminals of the motor or high currents may be generated through motor windings. In addition, such motors will produce iron losses at no load, which are induced by the rotating magnetic field of the rotor, and may produce a fluctuating, non-zero average torque as the motor rotates, a phenomenon often referred to as “cogging”. Techniques employed to address cogging tend to reduce motor efficiencies. It is also often necessary to apply so-called “field weakening” in order to allow high rotor speeds to be achieved. Otherwise, voltage induced across motor windings by rotation of the rotor may exceed the supply voltage driving the motor, thus preventing higher operating speeds. However, field weakening causes increased resistive losses, resulting in lower efficiencies. Rare earth magnets may also be prone to demagnetisation if overheated and are difficult to cool if located in a rotor. As such, control strategies must often be implemented to prevent overheating. 
     Alternatives to permanent magnet motors include induction motors and wound rotor motors. However, these motors may incur losses in their rotors higher than losses in permanent magnetic motors. These relatively higher losses are due to the motors having more windings, i.e. in both the rotor and the stator. 
     It is desirable to develop new electric motor technologies that do not require the use of permanent magnets, in particular rare earth magnets, and/or that improve efficiencies over those of known alternatives. It is as object embodiments of the invention to at least mitigate one or more problems associated with known arrangements. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided an electrical sub-assembly comprising: a first motor; and a second motor, the second motor being a reluctance motor, wherein each of the first and second motors has a respective rotor drivingly connected to the other and each of the first and second motors is operable to generate torque, i.e. a common supply/source of torque. As such, the first and second motors may be independently operable to generate torque, e.g. that is transmittable to a drivetrain, either by the first motor alone or by the first and second motors in combination with one another. 
     In certain embodiments, the first motor may be a reluctance motor, i.e. the electrical sub-assembly may comprise a first reluctance motor; and a second reluctance motor and each of the of the first and second motors may be operable to generate torque either alone or in combination with one another. 
     The electrical sub-assembly may form part of an electric drive including, but not limited to, a traction drive for a hybrid or electric vehicle. The electrical sub-assembly may reduce electric drive costs and/or improve efficiencies, thus increasing the attractiveness and/or viability of electrically driven solutions including, but not limited to, hybrid and electric vehicles as a means of transport. 
     In certain embodiments, each of the respective rotors may be rotatable about a common axis, such arrangements allowing for simple, compact motor configurations. The respective rotors may be drivingly connected to one other by a drive shaft for transmitting the supply of torque generated by the first and second motors extending therebetween. Optionally, the drive shaft may comprise a monolithic link extending between the respective rotors, allowing for simple, compact and/or robust motor configurations. 
     At least one of the respective rotors may be connected to the drive shaft such that angular correspondence between the at least one of the respective rotors and the drive shaft is maintained. Additionally, or alternatively, the mechanical angle of one of the respective rotors may be offset from the mechanical angle of the other of the respective rotors. This may facilitate a reduction in torque ripple and acoustic noise due to torque ripple. At least one of the first and second motors may be one of a switched reluctance motor and a synchronous reluctance motor. The first and second motors may be substantially the same as one another, i.e. built to the same specification. 
     Suitably, the sub-assembly may comprise a controller configured to control operation of at least one of the first and second motors. The controller may be configured to receive an input indicative of speed and/or torque of at least one of the first and second motors. Further, the controller may be configured to control operation of at least one of the first and second motors in dependence on the input. The controller may be further configured to control operation in dependence on the input by use of a function relating the input to efficiencies of at least one of the first and second motors. 
     According to another aspect of the invention, there is provided a hybrid vehicle or an electric vehicle comprising an electrical sub-assembly as described above. 
     According to another aspect of the invention, there is provided a method of operating an electrical sub-assembly comprising: operating either one or both of a first motor and a second motor, the second motor being a reluctance motor and each of the first and second motors having a respective rotor drivingly connected to the other, to generate torque either by the first motor alone or by the first and second motors in combination with one another. The first motor may be a reluctance motor. As such, torque may be generated by the first and second motors either alone or in combination with one another. 
     The method may comprise selectively operating both of the first and second motors to generate torque substantially equally therebetween. Additionally, or alternatively, the method may comprise selectively operating both of the first and second motors to generate torque substantially unequally therebetween. 
     In certain embodiments, the method may comprise selectively operating one of the first and second motors, e.g. the first motor only, to generate torque at increased efficiency compared to operating both of the first and second motors. Additionally, or alternatively, the method may comprise selectively operating both of the first and second motors to generate torque at increased efficiency compared to operating one of the first and second motors. 
     Optionally, the method may comprise: receiving an input indicative of speed and/or torque of at least one of the first and second motors; and selectively operating either one or both of the first and second motors in dependence on the input. Selectively operating in dependence on the input may comprise using a function relating the input to efficiencies of at least one of the first and second motors. 
     According to another aspect of the invention, there is provided a controller for an electrical sub-assembly, the controller configured to: operate at least one of a first motor and a second motor to generate torque either alone or in combination with one another; receive an input indicative of speed and/or torque of at least one of the first and second motors; and control operation of at least one of first and second motors in dependence on the input by use of a function relating the input to efficiencies of at least one of the first and second motors. Either or both of the first and second motors may be a reluctance motor. 
     It should be understood that features of one of the above-described aspects may be combined with features of one or more of the other above-described aspects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which: 
         FIG. 1  is a cross-sectional view of an electrical sub-assembly according to an embodiment of the invention; 
         FIG. 2  is a contour plot showing indicative performance of the electrical sub-assembly of  FIG. 1  when operating only one of two motors; 
         FIG. 3  is a contour plot showing indicative performance of the electrical sub-assembly of  FIGS. 1 and 2  when operating both of the two motors; 
         FIG. 4  is a contour plot showing indicative peak performance of the electrical sub-assembly of  FIGS. 1 to 3  when operating either one or both of the two motors; and 
         FIG. 5  is a schematic view of a hybrid or electric vehicle according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an electrical sub-assembly  10  according to an embodiment of the invention. The sub-assembly  10  has particular application for use as an electric traction drive for a hybrid or electric vehicle  100  (see  FIG. 5 ). As such, the sub-assembly  10  may be part of a powertrain, i.e. the powertrain may comprise the electrical sub-assembly  10  and a drivetrain. As the skilled reader will understand, the drivetrain may comprise one or more features that deliver mechanical power from the sub-assembly  10  to a final drive, e.g. one or more drive wheels. The drivetrain may comprise one or more of a gearbox, a clutch, an axle, a transmission and a torque converter. Other applications are contemplated. 
     The sub-assembly  10  comprises a first reluctance motor  12  and second reluctance motor  14 . As the skilled reader will understand, a reluctance motor is an electric motor that induces non-permanent magnetic poles on a ferromagnetic rotor. The rotor does not require rotor windings and torque is generated by the motor by the phenomenon of magnetic reluctance. Each of the first and second motors  12 ,  14  comprise a respective rotor  16 ,  18  and a respective stator  20 ,  22 . The respective rotors  16 ,  18  are configured to be driving connected to one another. Used herein, drivingly connected is to be understood to mean connected in manner so as to permit the transmission of torque between one or more connected features. As such, rotation of one of the first and second rotors  16 ,  18  may cause rotation of the other. Each of the respective stators  20 ,  22  may have wound field coils for stator windings. 
     In certain embodiments, at least one of the first and second motors  12 ,  14  may be a synchronous reluctance motor (often abbreviated as SynRM). In certain embodiments, at least one of the first and second motors  12 ,  14  may be a switched reluctance motor (often abbreviated as SRM and sometimes referred to as a variable reluctance motor). Reluctance motors have a simplified design compared to more commonly used motors, e.g. induction motors, as electrical power does not need to be supplied to the rotor. Consequently, reluctance motors may be simple to build and inexpensive, at least when compared to more commonly used motors. As the skilled reader will understand, each of the motors  12 ,  14  may be controlled by proprietary three-phase power electronics However, any suitable means of control may be used, e.g. an asymmetric half-bridge converter. 
     The respective rotors  16 ,  18  may be drivingly connected to one another by a drive shaft  24 . The drive shaft  24  is for transmitting torque generated by either of the first and second motors  12 ,  14  to other features, i.e. the drive shaft  24  may form part of the drivetrain. The first and second motors  12 ,  14  may be arranged relative to one another such that the respective rotors  16 ,  18  are rotatable about a common axis  26  (such an arrangement is shown in  FIG. 1 ). Although, in certain embodiments, the first and second motors  12 ,  14  may be angularly offset such that the respective rotors  16 ,  18  are each rotatable about respective axes. To this end, the drive shaft  24  may comprise one or more of a universal joint, a jaw coupling and a rag joint. In certain embodiments, the drive shaft  24  may be, or at least comprise, a monolithic link extending between the respective rotors  16 ,  18 . Used herein, monolithic is to be understood to mean of one-piece construction. Arranging the first and second motors  12 ,  14  such that respective rotors  16 ,  18  are rotatable about the common axis  26  and having the monolithic link extending between the respective rotors  16 ,  18  may be particularly beneficial, as the arrangement is simple to build and robust. In certain embodiments, the respective rotors  16 ,  18  may be drivingly connected to one another by a drive belt. 
     In certain embodiments, to allow for variation in alignment and/or distance between each of the first and second motors  12 ,  14  (and/or between at least one of the first and second motors  12 ,  14  and one or more other features, e.g. a feature of the drivetrain), the drive shaft  24  may be splined, i.e. the drive shaft may have a plurality of ridges or teeth extending around a circumference thereof for meshing with corresponding grooves of at least one of the respective rotors  16 ,  18 . As such, at least one of the respective rotors  16 ,  18  may have a splined hub. The ridges or teeth may maintain angular correspondence between at least one of the respective rotors  16 ,  18  and the drive shaft  24 . Additionally, or alternatively, angular correspondence between at least one of the respective rotors  16 ,  18  and the drive shaft  24  may be maintained by a keyway and key arrangement. In certain embodiments, either or both the respective rotors  16 ,  18  may be fixedly connected to the drive shaft  24  or formed integrally therewith. 
     In use, each of the first and second motors  12 ,  14  may be operated independently of the other, i.e. power may be supplied to either one or both of the first and second motors  12 ,  14 , or more specifically to one or both of the respective stators  20 ,  22 . Thus, torque may be generated by either one or both of the first and second motors  12 ,  14  alone or in combination with one another. As such, when operated simultaneously, each of the motors  12 ,  14  will contribute to the total amount of torque generated. As previously mentioned, as the respective rotors  16 ,  18  are drivingly connected to one another, rotation of one of the respective rotors may cause rotation of the other. Thus, either of the respective rotors  16 ,  18  may rotate regardless of whether power is supplied the respective stator  20 ,  22 . Advantageously, this no-load rotation (i.e. in at least one of the first and second motors  12 ,  14 ) does not generate magnetic flux, as occurs in permanent magnet motors. Thus, when the either of the respective rotors  16 ,  18  is rotating and the corresponding motor is switched off, there may be no losses and no voltage at the motor terminals. This may provide improvements in efficiencies and/or safety. As the skilled reader will understand, this benefit is derived from the one of the first and second motors  12 ,  14  that is subject to no-load rotation being a reluctance motor, i.e. the other of the first and second motors  12 ,  14  may be otherwise provided, e.g. as an induction motor. 
       FIG. 2  shows efficiency against torque (vertical axis) and speed (horizontal axis) of the sub-assembly  10  when operating only one of the first and second motors  12 ,  14 . Efficiency shown is mechanical power output as a proportion of electrical power input. Peak efficiencies may be exhibited in the areas of the plot corresponding to speeds where vehicles operate frequently, which in the described embodiment corresponds to approximately 3500-7500 rpm. Of course, if the sub-assembly  10  is part of a power train, the speed of one or both of the first and second motors  12 ,  14  may be proportional to speed of the vehicle  100 . However, efficiencies may vary depending on the amount of torque required, e.g. to provide acceleration and/or overcome rolling resistance and air resistance. As shown in  FIG. 2 , lower efficiencies may be achieved at lower amounts of torque. 
     Similarly,  FIG. 3  shows efficiency against torque (vertical axis) and speed (horizontal axis) of the sub-assembly  10  when operating both of the first and second motors  12 ,  14  (note the different scale of the vertical axis). Peak efficiencies may be exhibited at similar speeds to those when operating only one of the first and second motors  12 ,  14 . Although, as shown in  FIG. 3 , lower efficiencies may also be achieved at lower amounts of torque. Thus, by selectively operating each of the first and second motors  12 ,  14  alone and in combination, relatively higher efficiencies may be achievable over greater ranges of torque and/or speed. This effect is illustrated by  FIG. 4 , which shows peak efficiency against torque (vertical axis) and speed (horizontal axis) of the sub-assembly  10  when operating either one or both of the first and second motors  12 ,  14 , i.e. the best efficiencies achievable by operating either one or both of the first and second motors  12 ,  14  for a given torque and/or speed. 
     The sub-assembly  10  may comprise a controller  110  (see  FIG. 5 ) for controlling operation of either or both of the first and second motors  12 ,  14 . The controller  110  may control operation of either or both of the first and second motors  12 ,  14  by selecting one or more control parameters. The one or more control parameters may include voltage and current to be supplied to each of the first and second motors  12 ,  14 . Each of the first and second motors  12 ,  14  may be selectively operated by switching each of the first and second motors  12 ,  14  on and off, or by variably powering the first and second motors  12 ,  14 , i.e. by providing a supply of variable current or voltage. In certain embodiments, the sub-assembly  10  may not comprise the controller  110 , instead the controller  110  may be in operable communication with the sub-assembly  10 , e.g. by being provided elsewhere on the vehicle  100 . 
     The controller  110  may be configured to receive an input indicative of a speed of each or both of the first and second motors  12 ,  14  and/or a torque generated by each or both of the first and second motors  12 ,  14 . As the skilled reader will understand, torque of the sub-assembly  10 , or each or of the first and second motors  12 ,  14 , may be determined by a torque sensor, i.e. a torque transducer. Alternatively, torque may be derived from a current supplied to either or both of the first and second motors  12 ,  14 . Current may be determined by a current transducer. Speed may be determined by at least one of an encoder and a resolver. 
     The controller  110  may be configured to control to each of the first and second motors  12 ,  14  in dependence on the speed of each or both of first and second motors  12 ,  14  and/or the torque generated by each or both of the first and second motors  12 ,  14 . The controller  110  may comprise a processor and an electronic memory. The electronic memory may store a function that relates speed and/or torque of the first and second motors  12 ,  14  to the one or more of the control parameters. The function may be derived from efficiency-torque-speed data such as shown in  FIGS. 2 to 4 . 
     Each of the first and second motors  12 ,  14  may be substantially the same as one another, i.e. each of the first and second motors  12 ,  14  may be produced to the same specification and, thus, may each exhibit the same operating and performance characteristics. This may be particularly beneficial as it allows for the electrical sub-assembly  10  to be of modular construction. The electrical sub-assembly  10  may comprise a plurality of motors, i.e. two or more of a third motor, a fourth motor, etc., wherein each motor has a respective rotor drivingly connected to the rotors of the other motors. Each of the plurality of motors, or a subset thereof, may be operable to generate torque either alone or in combination with one another. The electrical sub-assembly  10  may comprise two, three or more, motors each produced to the same specification. However, in certain embodiments, the electrical sub-assembly  10  may comprise two, three or more, motors each produced to one of two or more different specifications. By using different numbers of motors and/or motors produced to different specifications, improved efficiencies may be possible for a range of different torques and/or speeds. 
     Disadvantages sometimes associated with reluctance motors include high torque ripple (i.e. the difference between a maximum and a minimum torque exhibited during one revolution of the rotor), particularly when operated at low speeds, and acoustic noise due to torque ripple. However, these disadvantages may be reduced by offsetting the mechanical angle of one of the respective rotors  16 ,  18  from the other. As the skilled reader will understand, the mechanical angle is the angle of a rotor relative to a stator. Mechanical degrees in a motor refers to the rotation of a rotor, i.e. one revolution of the rotor equals 360 mechanical degrees. The amount of offset will depend on the design of the respective rotors  16 ,  18 , but may be such that one of the respective rotors  16 ,  18  is offset by 360 mechanical degrees divided by twice the number of rotor poles of each of the respective rotors  16 ,  18  from the other, or some other angular displacement as may be advantageous to minimise motor harmonics. 
     All of the features disclosed in this specification (including any accompanying claims and drawings) and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. While it is envisaged that the invention will have particular application in hybrid or electric cars and commercial vehicles, e.g. trucks and vans, other applications are contemplated, including but not limited to hybrid or electric rail locomotives, off-highway and construction vehicles, marine vessels and static applications. 
     Each feature disclosed in this specification (including any accompanying claims and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings) or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.