Electric drive system and method

Abstract

The invention relates to an electric drive system for or in a motor vehicle, comprising at least one multi-phase electric machine with a double rotor, the double rotor being made of a magnetic flux-propagating material made of a solid material, a three-level or multi-level inverter circuit for driving the electric machine, a controllable three-level or multi-level inverter coupled to the electric machine on the output side and designed to supply the electric machine with an alternating voltage, and an operating mode setting device designed to selectively operate the inverter in three-level operation, multi-level operation or two-level operation depending on at least one parameter influencing the overall efficiency of the electric drive system.

Description

[Technical field]

[0001] The present invention relates to an electric drive system for or in a motor vehicle and to a method of operating such an electric drive system. [Background technology]

[0002] An inverter is an electrical device that converts a direct current voltage into an alternating current voltage. Such inverters are used, for example, in modern automobiles, in photovoltaics (solar inverters), as components of frequency converters, and in many other applications to generate a suitable alternating current voltage from a direct current voltage. These inverters and their fields of application are generally known in a wide range of circuit variations, and it is not necessary to describe in detail their circuit design and mode of operation.

[0003] Electric drive systems are increasingly being used in modern vehicles, for reasons of sustainability and avoiding CO2 emissions. Such drive systems contain one or more electric machines, e.g. synchronous or asynchronous machines, which are fed by a polyphase AC voltage. So-called two-level inverters (2L inverters for short) are widely used to generate the AC voltage. In a two-level inverter, an AC voltage with two voltage levels is generated from the DC voltage of a DC voltage source.

[0004] Two-level inverters are more established than other inverter topologies, especially in the field of drive inverters for electric vehicles. Currently, two-level inverters mainly use IGBT switching elements. Examples of such two-level inverters are disclosed, for example, in the paper "Power Electronic Architectures for Electric Vehicle" by Hv Hoeck, published in "Emobility-Electrical Power Train" by IEEE in 2010.

[0005] In addition to the two-level inverter topologies mentioned above, there are also three-level or multilevel inverter topologies, which can generate three-level or multilevel voltage levels. Examples of multilevel inverter topologies are described, for example, in patent US10903758B2 or patent US2017 / 0185130A1.

[0006] The advantages of multiple voltage levels are fewer harmonics, slower voltage changes at the phase outputs, less electromagnetic radiation (EME) and, most importantly, the ability to handle higher voltages. For these reasons, such three-level or higher inverters are currently used mainly for high-voltage applications. Power engineering applications such as solar inverters or wind turbines are an established field of use for such three-level or multilevel inverter topologies. Higher voltages are not found in electric vehicles (for example, voltages of 400V). On the other hand, in photovoltaics, voltages of 1kV and above are common, and in other renewable energies such as wind energy, the voltages are significantly higher.

[0007] However, as described in the paper "Experimental Validation of Design Concepts for Future EV-Traction Inverters" by Andreas Bubert et al., 2018 IEEE Transportation Electrification Conference and Expo (ITEC), pp. 795-802, the general opinion is that the advantages of three-level or multilevel inverters mentioned above are not sufficient to justify their use in the electric drives of electric vehicles. For all these reasons, three-level or multilevel inverter topologies are not used today in electric vehicles. Summary of the Invention

[0008] The present invention aims to provide a three-level or multilevel inverter suitable for efficient use in electric drive systems of motor vehicles, in particular to improve the efficiency of electric drive systems, optionally with a double rotor made from a flux carrying material of solid material, and / or to allow a better cost / benefit ratio compared to two-level inverters.

[0009] According to the invention, this object is achieved by an electric drive system having the features of claim 1 and / or by a method having the features of claim 24.

[0010] therefore, - an electric drive system for or in a motor vehicle is provided, comprising at least one polyphase electric machine with a double rotor, the double rotor being made of a magnetic flux propagating material made of a solid material, a three-level or multilevel inverter circuit for driving the electric machine, a controllable three-level or multilevel inverter coupled to the electric machine on the output side and designed to supply an alternating voltage to the electric machine, and an operating mode setting device designed to selectively operate the inverter in three-level operation, in multilevel operation or in two-level operation depending on at least one parameter influencing the overall efficiency of the electric drive system.

[0011] There is also provided a method for operating an electric drive system according to the invention, in which the synchronous machine can be operated either in a three-level or multilevel operating mode or in a two-level operating mode by means of a controllable three-level or multilevel inverter, depending on the overall efficiency of the electric drive system.

[0012] The invention is based on the realization that currently, in automotive electric drives, essentially two-level inverters are used. Three-level or multi-level inverters are currently found mainly in non-automotive (high voltage) applications. The additional benefits associated with the use of three-level or multi-level inverters have not yet justified the associated additional costs in automotive applications.

[0013] The present invention solves this problem through the use of a specialized inverter circuit in combination with adaptation of the entire drive system, thereby increasing overall benefits without increasing the associated costs.

[0014] The idea behind the present invention is the use of a novel controllable three-level or multilevel inverter capable of operating in three-level or multilevel operation (hereinafter referred to as 3L operation) and two-level operation (hereinafter referred to as 2L operation). A dedicated operation mode setting device sets the respective operation mode by appropriately operating the power switches of the inverter. The operation mode is set according to the overall efficiency of the entire drive system, and is not simply set based on the electric machine and / or inverter used. For the overall efficiency, the phase current of the electric machine can be used first. Furthermore, in addition to the detected phase current of the electric machine, other characteristics of the electric machine and the inverter that affect the overall efficiency can also be taken into account in the overall efficiency.

[0015] The idea according to the invention is to reduce the losses mainly at low loads by operating the inverter at 3L in this case. In this case, the losses of the inverter are hardly increased or even reduced at all operating points. The overall efficiency of the drive system, i.e. inverter and electric machine, is therefore significantly improved, especially when used in electric drive vehicles.

[0016] The core idea of ​​the present invention is the use of a specific electric machine with a double rotor made of solid rotor material, i.e. solid structure. In particular, such electric motors are associated with high losses. The present invention solves the problem of high losses in the double rotor made of solid material in known electric machines. The underlying discovery here is that electric machines with double rotors made of solid material have high losses in the rotors. By design, the losses in the electric machine cannot be reduced or can only be reduced to a small extent. Reducing the losses by increasing the frequency in 2L operation has little effect and increases the losses in the inverter, thus affecting the overall efficiency.

[0017] The basic mechanism of reducing losses in the solid material of the double rotor is based on the fact that one should reduce the amplitude of the specific magnetic flux density in the solid material of the double rotor that does not contribute to torque generation. This part, defined by harmonics in the magnetic flux density, is almost directly proportional to the change in THD-induced losses in the square of its amplitude. Therefore, a change in the switching frequency of the inverter is not effective, since it would result in an indirectly proportional linear change in losses.

[0018] Reducing the losses in solid materials reduces the overall losses of the electric machine and contributes greatly to its economical use. Thus, the resulting finding that is part of the present invention is that losses in an electric machine can be effectively reduced by an inverter circuit that reduces only the amplitude of the harmonics in the magnetic flux density.

[0019] To achieve this, the following measures and aspects were taken into account in the selection of the inverter configuration and operation mode:

[0020] The function of the 2L inverter is replaced by that of a dedicated controllable 3L inverter, reducing the harmonics in the inverter phase output. This reduces the harmonics in the flux density and the stator currents. No frequency change is required for this.

[0021] Increasing the switching frequency in 2L operation also reduces losses, but it also significantly increases the inverter switching losses and does not significantly improve overall efficiency, so increasing the switching frequency is not implemented. Although increasing the switching frequency may positively support loss optimization, it is not an essential aspect.

[0022] The 3L inverter used provides three voltage levels (3L) and is preferably (but not necessarily) three phases. Three voltage levels and three phases allow for greater cost efficiencies. However, the system can be expanded to any number of phases and any number of voltage levels, provided all phases are of the same design.

[0023] In contrast to known 2L inverters, the operation of the 3L inverter according to the invention results in significantly reduced power losses in the electric machine due to fewer harmonics. The switching losses of the 3L inverter are also relatively reduced, but the conduction losses are increased.

[0024] In both electric machines and 3L inverters, the prevailing loss mechanisms change with the load. In 3L operation, mechanical losses are significantly reduced due to the low harmonics. Harmonic losses dominate at low currents. At high currents, the dominant loss mechanisms change and resistive conduction losses or copper losses dominate, while harmonic induced losses tend to be secondary or relatively small. Switching losses in the inverter are reduced (by about 50%) in 3L inverters compared to 2L inverters. At low loads (currents), these switching losses dominate, but at high currents, conduction losses dominate and 2L operation is more efficient. These findings have led to the idea according to the invention of using a 3L inverter at low loads and a 2L inverter at high loads. The controllable three-level or multilevel inverter according to the invention makes this operation possible.

[0025] Overall, the advantages of 2L operation can be combined with those of 3L operation, particularly in the case of electric machines with double rotor motors, resulting in a significant increase in the overall efficiency of the electric drive system compared to known electric drive systems.

[0026] It is essential that the operating mode setting device does not necessarily have to switch abruptly from 2L to 3L operation or vice versa. On the contrary, such switching can be realized if it is performed continuously, instead of, for example, fading from the inner power switch to the outer power switch. This fading can be performed, for example, in such a way that the operating time or the time that each power switch is turned on is taken into account, taking into account the average current values ​​of the different power switches. Additionally or alternatively, it is also conceivable that the power switches are switched in a specified sequence and / or slowly.

[0027] For example, the operating mode setting device, including the evaluation device, the control device and / or the measuring device, can be designed as a program controlled device, for example a microprocessor or a microcontroller, but it would also be feasible to provide logic circuits such as FPGAs, PLDs, etc. for this function.

[0028] Advantageous embodiments and developments are evident from the further dependent claims and the description with reference to the drawing figures. According to a preferred embodiment of the invention, the operating mode setting device comprises an evaluation device, which is designed to optimize an overall efficiency of the electric drive system based on the phase currents and, optionally, on at least one other characteristic of the electric drive system.

[0029] Typically, but not necessarily, the overall efficiency is calculated numerically by an evaluation circuit. Additionally or alternatively, the overall efficiency may be determined based on a predefined family of characteristics, for example, mapped to a look-up table. The determination of the overall efficiency may be calculated or determined during operation or, for example, in advance. Preferably, the optimal, i.e. most efficient operating strategy is calculated, for example numerically, in so-called offline operation before the electric drive system is put into operation. This can be achieved with relatively few computer resources and is particularly preferred when a large number of characteristics are taken into account in the numerical prediction of the optimal overall efficiency. Furthermore, in offline operation, more time is available for calculation. Thus, in so-called real-time operation, a very dynamic determination of the respective operating mode (2L or 3L operation) can also be envisaged and possible, for example, via a look-up table. For example, for these purposes, a trained artificial network can be used, which is trained based on previous characteristics and characteristic curves, etc.

[0030] According to a preferred embodiment, the evaluation device has an optimization module designed to first determine the overall efficiency. Alternatively or additionally, the overall efficiency can then be optimized via an optimization function, taking into account the phase currents and optionally at least one other characteristic. The optimization of the overall efficiency can be performed analytically and / or via suitable look-up tables, e.g. pre-generated.

[0031] The operating mode used in each case (e.g. 2L or 3L operation) is a property of the electric machine that affects, for example, the overall efficiency. Other properties can be found in the specific configuration of the rotor of the electric machine, for example that the rotor is a double rotor and / or that the double rotor is made of a flux-carrying material made of a solid material.

[0032] According to a preferred exemplary embodiment, the operating mode setting device comprises a measuring device.

[0033] The measuring device has at least one sensor input via which a first measuring device can be coupled to the electric machine, the first measuring device being designed to detect a phase current.

[0034] According to a preferred exemplary embodiment, the inverter comprises a t-type neutral-point clamped (TNPC) inverter architecture. These have various advantages over multilevel active neutral-point clamped (ANPC) inverter topologies. In contrast to the ANPC topology, up to three switches conduct in series instead of four, resulting in lower conduction losses. The output voltage waveform is the same, so switching losses are similarly lower, but at higher switching frequencies (e.g., greater than 10 kHz), the total chip area required for the TNPC topology is smaller compared to the two-level topology. As with ANPC, hybrid inverter topologies can also be built for TNPC to further improve efficiency and / or optimize manufacturing costs. For example, different switch technologies can be used in the zero potential or mid-bridge branch for this purpose. In particular, in the case of TNPC inverters composed only of insulated gate bipolar transistors (IGBTs), losses can be significantly reduced by using gallium nitride (GaN). Hybrid TNPC inverter topologies can also be used for motor control in electric vehicles, but are rarely seen in practice.

[0035] TNPC based 3L inverters can be operated in two operating modes to increase system efficiency. In case of 3L TNPC inverters, the zero potential (middle) bridge branch can be switched off to operate in 2L operation and can be switched on to change to 3L operation. Switching between the two operating modes is done to increase system efficiency. For that purpose, in the control and regulation logic the load is measured and switched between 2L and 3L operation using pre-determined optimization characteristics.

[0036] Additionally or alternatively, the TNPC-based 3L inverter may be designed asymmetrically to reduce the cost of the inverter. The asymmetry refers to the current carrying capacity of the zero potential (middle) bridge branch being lower than the current carrying capacity of the outer bridge branch. This is because the zero potential bridge branch is not used at high loads to optimize the overall efficiency. The outer bridge branch is designed for peak currents and the zero potential bridge branch is designed for small or continuous currents.

[0037] According to one embodiment of the present invention, an inverter comprises a first driver stage and at least one second driver stage designed to provide a smaller output load current to a load output than the output load current provided by the first driver stage.

[0038] Preferably, the operation mode switching device is designed to control the inverter to activate the first and second driver stages in three-level or multi-level operation and to deactivate at least one driver stage, preferably the inner second driver stage, in two-level operation depending on the overall efficiency.

[0039] Typically, but not necessarily, the first driver stage comprises at least one bridge circuit, in particular a half-bridge circuit, the center tap of which forms the output load terminal of the inverter circuit. Each bridge circuit thereby comprises at least one first (semiconductor) power switch connected to a first power supply terminal (e.g., to which a positive power supply potential is applied) and designed to provide a first voltage level to the load output. Each bridge circuit further comprises at least one second (semiconductor) power switch connected to a second power supply terminal (e.g., to which a negative power supply or reference potential is applied) and designed to provide a second voltage level to the load output. Semiconductor-based power switches can be manufactured from a variety of semiconductor materials that can be selected at will. Commonly used materials are Si (silicon) for IGBTs and MOSFETs, SiC (silicon carbide) for MOSFETs, and GaN (gallium nitride) for MOSFETs.

[0040] Typically, but not necessarily, the second driver stage includes at least one third power switch having a load path connected in series between the intermediate circuit and the center tap of the first driver circuit. The power switch of the second driver stage is designed to provide a third voltage level to the load output that is intermediate the first and second voltage levels.

[0041] In the case of a preferred so-called homogeneous inverter topology, all power switches of the inverter, i.e. the power switches of the first and / or second driver stages, are designed as semiconductor switches of the same switch type and / or of the same semiconductor technology, e.g. bipolar transistors, field effect transistors (MOSFETs, JFETs, etc.), thyristors, IGBTs, etc. The term semiconductor technology includes the semiconductor technology used as a manufacturing basis for the power switches, such as Si, SiC, GaAs or GaN technologies.

[0042] In a first preferred variant of the homogeneous inverter topology, the semiconductor switches are designed as GaN power switches, for example GaN MOSFETs. In a particularly preferred second variant, the semiconductor switches are designed as SiC power switches, in particular SiC MOSFETs. Furthermore, IGBT-based power switches, for example silicon-based IGBTs with Si or SiC diodes, are likewise feasible.

[0043] In the case of a so-called hybrid inverter topology, which is particularly preferred, at least two different switch types and / or at least two different semiconductor technologies are provided for the semiconductor switches of the inverter, i.e. for the semiconductor switches of the first driver stage and / or for the semiconductor switches of the second driver stage. In a hybrid inverter topology, not all power switches in the inverter use the same semiconductor material. In particular, for the power switches of the zero potential bridge branch, i.e. for the power switches of the second driver stage, a different technology (different switch type) is used than for the external switches of the first driver stage. As a result, switching and conduction losses are reduced and efficiency is increased. Furthermore, there are cost advantages. It is particularly recommended to optimize the power switches of the zero potential bridge branch (second driver stage) so that their switching and reverse recovery losses are as low as possible. This is because the zero potential bridge branch (second driver stage) operates at low currents and, due to the low reverse recovery losses, the switching losses of the external switches can also be reduced. A hybrid design is particularly recommended if the inverter is asymmetric. The lower the current carrying capacity of the zero potential bridge branch (second driver stage), the lower the additional costs of switching the loss-optimized switches.

[0044] In a first particularly preferred variant, the semiconductor switches of the first driver stage are designed as IGBTs (Si or SiC) with freewheeling diodes. In this case, the semiconductor switches of the second driver stage can preferably be designed as SiC power switches, in particular SiC MOSFETs.

[0045] In a second variant, which is also preferred, the semiconductor switches of the first driver stage are designed as SiC MOSFETs, while the semiconductor switches of the second driver stage can be designed as GaN-based MOSFETs.

[0046] In a third preferred variant, the semiconductor switches of the first driver stage are designed as IGBTs with freewheeling diodes. In this case, the semiconductor switches of the second driver stage can be designed as GaN power switches, in particular GaN MOSFETs.

[0047] According to a particularly preferred exemplary embodiment, the flux-carrying material of the rotor consists of iron or an iron alloy. Electric rotating field machines, here preferably synchronous machines with a double rotor, can be designed in such a way that the flux-carrying material in the rotor is of solid structure, i.e. made of solid material. The reason is that in the idealized synchronous machine view, there is no periodic relative motion between the direction vector of the rotating magnetic field generated by the stator windings and the double rotor. The flux density at the operating point is therefore constant and no iron losses occur in the material. In such permanent magnet excitation machines, where the magnets are attached to the rotor surface, the distance between the inductor grooves and the flux-carrying material ensured thereby allows the use of solid materials without increasing additional losses.

[0048] According to an exemplary embodiment which is also particularly preferred, the electric machine comprises a stator with an inductor, which is designed to propagate magnetic flux mainly in the radial direction and in particular to avoid tangential magnetic flux propagation, hence the so-called "yokeless" design of the stator, which in particular avoids magnetic flux propagation in the circumferential direction, which eliminates the need for a magnetic return path for the inductor, thus reducing weight and iron losses.

[0049] According to one embodiment, the inductor of the stator has a radial yoke thickness of less than 30%, preferably less than 20%, particularly preferably less than 10% of the total radial thickness of the inductor. In the so-called "yokeless" design, a mechanical connection of the inductor teeth is provided in this way, which is not electromagnetically necessary and no functionally relevant magnetic flux exists. The term "yokeless" therefore refers to the electromagnetic design of the inductor.

[0050] According to an equally particularly preferred exemplary embodiment, the synchronous machine is a three-phase synchronous machine. In this case, the inverter circuit is preferably designed as at least a three-phase inverter. It is also a finding of the present invention that a synchronous machine utilizing a three-level or multilevel inverter topology substantially improves the overall efficiency of the drive system.

[0051] According to an exemplary embodiment which is likewise particularly preferred, the electric machine is designed as an electrically operable wheel hub motor of a motor vehicle. A wheel hub motor is an electric machine which is attached directly to a wheel, in particular to the hub of a vehicle, and at the same time supports the wheel hub. A part of the hub motor directly transmits the generated torque to the driving wheel, causing it to rotate. In the case of an electric wheel hub motor, both internal rotor motors and external rotor motors are conceivable. The main advantage of such an electric wheel hub motor, in comparison with a drive concept with a central motor, is that a conventional driveline with the components required in each case depending on the specification (transmission, cardan shaft, differential gear, drive shaft, etc.) is no longer necessary. Their transmission losses are also eliminated, which has the potential to increase the efficiency of the entire drive system. An efficient recuperation, i.e. the recovery of electrical energy when the vehicle is braking, can also be implemented in an electric wheel hub motor.

[0052] The above-mentioned embodiments and developments can be combined with one another as desired, if useful. Further possible embodiments, other developments and implementations of the invention also include combinations not explicitly mentioned of the features of the invention described above or below with respect to the illustrated embodiments. In particular, in this respect, the skilled person will be able to add individual aspects as improvements or additions to the respective basic forms of the invention. [Brief description of the drawings]

[0053] The invention will now be explained in more detail with reference to exemplary embodiments illustrated in the schematic diagrams of the drawings. [Figure 1]1 is a block diagram showing an electric drive system according to the present invention; [Diagram 2] FIG. 2 is a schematic cross-sectional view of an example of an electric machine of the electric drive system according to the present invention shown in FIG. [Diagram 3] FIG. 2 is a block diagram of a three-level or multilevel inverter circuit of an electric drive system according to the present invention, corresponding to FIG. 1; [Figure 4] FIG. 1 is a circuit diagram of a particularly preferred embodiment of an inverter circuit according to the present invention. [Diagram 5] FIG. 4 is a flow diagram of a method of operating an electric drive system according to the present invention.

[0054] The accompanying drawings are intended to provide a further understanding of embodiments of the present invention. These drawings illustrate embodiments and, together with the specification, serve to explain the principles and concepts of the present invention. Other embodiments and many of the stated advantages will be apparent from a review of the drawings. Elements of the drawings are not necessarily shown to scale relative to each other.

[0055] In the drawing figures, similar and functionally identical elements, features and components, and elements, features and components that act in the same manner, are each labeled with the same reference numerals unless otherwise stated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] FIG. 1 is a block diagram of an electric drive system for a vehicle according to the present invention. An electric drive system, designated herein by reference numeral 10, is preferably (though not necessarily) provided for use in a motor vehicle. The drive system 10 comprises at least one polyphase electric machine 11 and an inverter circuit 12.

[0057] The electric machine 11 is connected on the input side to an inverter circuit 12 which drives the electric machine 11 . According to the invention, the inverter circuit 12 is designed as a three-level or multi-level inverter circuit 12. The inverter circuit 12 comprises an inverter 13 and an operation mode setting device 14.

[0058] The inverter 13 is coupled to the electric machine 11 via its load output 15 and to a supply voltage source 18 via power supply terminals 16, 17. The inverter 13 is designed to convert a direct current voltage VDC received at the supply side into an alternating current voltage VAC. The inverter 13 is designed as a multi-phase inverter 13, the number of phases of the inverter 13 typically corresponding to the number of phases of the electric machine 11. The electric machine 11 is driven by phase currents provided by the inverter 13 at the load output 15.

[0059] According to the invention, the operating mode of the inverter circuit 12 can be set via an operating mode setting device 14, which is in particular coupled to the input side to the electric machine 11. In particular, the operating mode setting device 14 can be used to set whether the inverter 13 operates in two-level operation, three-level operation, multi-level operation or mixed operation. Mixed operation is an operating mode in which the inverter is operated both in two-level operation and in three-level or multi-level operation, which occurs for example when transitioning from one operating mode to the next. The structure and the operating mode of the operating mode setting device 14 are explained in more detail below with reference to the following figures 3 to 6.

[0060] The electric machine 11 is preferably, but not necessarily, a three-phase synchronous machine 11. In this case, the inverter circuit 12 preferably comprises a three-phase inverter 13.

[0061] It is likewise preferred if the electric machine 11 of the electric drive system 10 is an electrically operable wheel hub motor for a motor vehicle, however, other applications are also feasible and advantageous.

[0062] FIG. 2 is a schematic cross-sectional view of an example of an electric machine of the electric drive system according to the present invention shown in FIG.

[0063] The electric machine 11 is preferably designed as a synchronous machine 11. One aspect, which is important for the invention but is not essential, is the situation where the synchronous machine or electric machine 11 comprises a double rotor 20, which is furthermore constituted by a flux-transmitting material made of a solid material. A cross section of the double-rotor synchronous machine 11 is shown in FIG. 2. The double-rotor machine 20 comprises an outer rotor 21 and an inner rotor 22. A stator 23 is arranged between the two rotors 21, 22 in a manner known per se. The stator 23 is preferably, but not necessarily, a yokeless stator 23.

[0064] The outer rotor 21 and the inner rotor 22 are preferably not laminated and are made of solid material. The inner rotor 22 is cylindrical. However, it would also be feasible to design the inner rotor 22 to be solid and full volume.

[0065] In the example shown, two magnets 24, 25 with opposite poles are arranged on the inner surface of the outer rotor 21 in the outer air gap 26 between the outer rotor 21 and the stator 23. It is also feasible and advantageous for the magnets 24, 25 to be embedded in pocket-like recesses in the outer rotor 21 specially provided for this purpose. However, it would also be feasible for the magnets 24, 25 to be arranged at a distance from the outer rotor 21, i.e. not directly on its inner surface. The magnetic field lines 27 between the north and south poles of the magnets 24, 25 with opposite poles run here through the core material of the outer rotor 21.

[0066] In the illustrated example, two magnets 28, 29 with opposite poles are arranged on the inner surface of the inner rotor 22 in the inner air gap 30, similarly between the inner rotor 22 and the stator 23. Again, the magnets 28, 29 may be embedded in corresponding pockets in the inner rotor 22 or may be spaced apart from the inner rotor 22. The magnetic field lines 31 between the north and south poles of the magnets 28, 29 with opposite poles now pass through the core material of the inner rotor 22.

[0067] The magnetic flux carrying material of the outer rotor 21 and / or the inner rotor 22 preferably consists of solid iron or a corresponding solid iron alloy.

[0068] FIG. 3 is a block diagram of a three-level or multilevel inverter circuit for the electric drive system according to the present invention shown in FIG.

[0069] As already explained with reference to the drawings, the inverter circuit 12 comprises two power supply terminals 16, 17, a load output 15, a three-level or multilevel inverter 13, and an operation mode setting device .

[0070] A first supply potential V11, for example a positive supply potential, can be connected to the first power supply terminal 16. A second supply potential V12, for example a negative supply potential or a reference potential, can be connected to the second power supply terminal 17. A supply DC voltage VDC=V11-V12 is then present between the power supply terminals 16, 17.

[0071] A multi-phase load current I1 is made to flow at the load output 15, causing each phase of the electric machine 11 connectable via the load output 15 to operate.

[0072] A controllable three-level or multilevel inverter 13 is arranged between power supply terminals 16, 17 and a load output 15. The inverter 13 is designed to convert a direct current voltage VDC received at the supply side into an alternating current voltage VAC in order to supply a multiphase load current I1 to the load output.

[0073] The inverter 13 comprises a first driver stage 40 and at least one second driver stage 41. The second driver stage 41 is designed to provide an output load current to the load output 15 that is less than the output load current provided by the first driver stage 40.

[0074] The operation mode setting device 14 serves the purpose of setting and controlling the operation mode of the inverter 13 and therefore of the entire inverter circuit 12. In particular, the inverter 13 is designed to operate either in a first operation mode in three-level or multi-level operation or in a second operation mode in two-level operation. It is also conceivable to have at least a third operation mode including a mixed form of two-level operation and three-level or multi-level operation. The third operation mode is particularly feasible and useful when transitioning from the first operation mode to the second operation mode and vice versa.

[0075] The operating mode setting device 14 controls the operating mode of the inverter 13 to be used depending on the overall efficiency of the entire electric drive system 10. The overall efficiency is thereby a function of the detected phase currents of the electric machine 11 and optionally also of at least one other characteristic of the electric drive system that affects the overall efficiency.

[0076] To set the respective operating mode to be used, the operating mode setting device 14 comprises at least one of the following devices: - evaluation device 42, - measuring device 43, - control unit 45.

[0077] The evaluation device 42 is designed to optimize the overall efficiency of the electric drive system 10 based on the phase currents and optionally at least one other characteristic. This can be done, for example, in situ, i.e. during operation of the electric drive system 10. However, preferably, the relatively computationally intensive calculations are performed in advance, for example by suitable calculations (for example numerically or analytically) and / or with a predefined characteristic field. For example, the numerical efficiency calculations of 2L and 3L operation and the mapping of the functions with the determined output are performed in advance, i.e. offline. The selection of the better efficiency by switching and the application of look-up tables for determining the efficiency can be performed, but is not limited to, more or less dynamically during operation.

[0078] For the optimization, the evaluation device 42 comprises an optimization module 46, which first calculates the overall efficiency, which is then optimized analytically or via a look-up table, for example via an optimization function, taking into account the phase currents and optionally at least one characteristic.

[0079] The operating mode setting device 14 further comprises a first measuring device 43. The first measuring device 43 comprises a sensor input 47. In this case, the operating mode setting device 14 can be coupled to the electric machine 11 via the sensor input 47 in order to detect the phase currents of the electric machine 11.

[0080] The actual control of the inverter is performed by a controller 45 specially provided for this purpose. The controller 45 sets the respective operating mode of the inverter 13, i.e. whether the inverter 13 operates in three-level, multi-level or two-level operation. For example, the controller 45 can control the inverter 13 to activate both driver stages 40, 41 during three-level or multi-level operation and to deactivate the second driver stage 40 during two-level operation.

[0081] FIG. 4 is a circuit diagram of a particularly preferred embodiment of an inverter circuit according to the present invention.

[0082] A supply DC voltage VDC is provided at the power supply terminals 16, 17, a supply potential V11=VDC / 2 is applicable at the first power supply terminal 16 and a supply potential V12=-VDC / 2 is applicable at the second power supply terminal 17. It would also be possible for the second power supply terminal 17 to be provided with a reference potential, for example the potential of the reference ground GND. In this case the supply potential V11=VDC can be connected to the first power supply terminal 16.

[0083] An intermediate circuit 50 consisting of two intermediate circuit capacitors 51, 52 connected in series is connected to the input side of the inverter 13. The intermediate circuit 50 functions as an energy storage section.

[0084] The inverter 13 shown in FIG. 4 includes a t-type neutral point clamped inverter architecture.

[0085] For this purpose, the first outer driver stage in the case of the illustrated three-phase inverter comprises three half-bridge circuits 53a-53c, each of which is also connected on the load side between the power supply terminals 16, 17 with respect to its load path. The respective center taps 54a-54c of the half-bridge circuits 53a-53c form the output load terminals 15a-15c, respectively, of the inverter 13. Each of the half-bridge circuits 53a-53c comprises first controllable power switches T1, T2, T3 designed as high-side switches. These first power switches T1, T2, T3 are connected to the first power supply terminal 16. The first power switches T1, T2, T3 are designed to supply a first voltage level to the load output 15. Furthermore, each of the half-bridge circuits 53a-53c comprises second controllable power switches T4, T5, T6 designed as low-side switches. These second power switches T4, T5, T6 are connected to a second power supply terminal 17. The second power switches T4, T5, T6 are designed to supply the load output 15 with a second voltage level.

[0086] The second inner driver stage 41 is connected between the center tap 55 of the intermediate circuit and the output load terminals 15a-15c, and thus between the center taps 54a-54c of the half-bridge circuits 53a-53c, respectively. In the illustrated example, the second driver stage 41 is composed of three circuit branches 56a-56c, respectively. Each of the circuit branches 56a-56c consists of a series circuit of two controllable power switches T7 / T8, T9 / T10, T11 / T12 arranged anti-parallel to its load path. The controllable power switches T7 / T8, T9 / T10, T11 / T12 are designed to provide a third voltage level at the load outputs 15a-15c, which is between the first and second voltage levels.

[0087] To actuate the respective controllable power switches, the control device 45 comprises a first control unit 45a and a second control unit 45b. The first control unit 45a is designed to actuate the power switches T1 to T6 of the first driver stage 40. The second control unit 45b is designed to actuate the power switches T7 to T12 of the second driver stage 41.

[0088] In the exemplary embodiment of Fig. 4, the inverter 13 is of the hybrid type. In this case, the power switches of the inverter 13 are not made of the same semiconductor technology and / or the same switch type. In particular, in the illustrated example, the power switches T1 to T6 are designed as Si-IGBTs with Si freewheeling diodes. The power switches T7 to T12 are designed as SiC-MOSFETs.

[0089] Alternatively (not shown in FIG. 4), the power switches T7-T12 can be designed as SiC MOSFETs and the power switches T1-T6 as GaN MOSFETs.

[0090] Alternatively (also not shown in FIG. 4), the power switches T7-T12 can be designed as IGBTs with freewheeling diodes and the power switches T1-T6 can be designed as GaN power switches, in particular GaN-MOSFETs.

[0091] Alternatively (also not shown in FIG. 4 ), in a so-called homogeneous inverter topology, all power switches T1-T12 of inverter 13 may be of the same switch type, e.g. designed as GaN power switches, SiC power switches such as SiC-MOSFETs, and / or manufactured with the same semiconductor technology.

[0092] Figure 5 shows a flow chart of a method for operating an electric drive system according to the invention, for example a drive system according to figure 1, which comprises a synchronous machine with a double rotor, the double rotor being made of a flux-carrying material made of a solid material.

[0093] In a first step S1, the overall efficiency of the electric drive system is determined, for example offline. For this purpose, first the phase currents of the electric machine of the electric drive system are detected (S11). Furthermore, optionally at least one other characteristic that influences the overall efficiency is determined (S13).

[0094] From all this information, in a second step S2 a synchronous machine is put into operation. For this purpose a controllable three-level or multilevel inverter circuit is used. The controllable three-level or multilevel inverter of the inverter circuit is operated either in a three-level or multilevel operating mode S21 or in a two-level operating mode S22 depending on the overall efficiency of the electric drive system and the parameters and optionally characteristics influencing it.

[0095] Mixed forms of three-level or multi-level operation and two-level operation are also possible and advantageous, for example during the transition from three-level or multi-level operation to two-level operation, in order to avoid abrupt switching, which may involve losses and result in reduced efficiency.

[0096] Although the present invention has been fully described above with reference to the preferred embodiment, the present invention is not limited thereto and various modifications are possible. [Explanation of symbols]

[0097] 10 Electric Drive System 11 Electrical machines, synchronous machines 12 (3-level or multi-level) inverter circuits 13 (3-level or multi-level) inverters 14 Operation mode setting device 15 Load Output 15a~15c Output load terminals 16,17 Power terminal 18 Supply voltage source 20 Double rotor, double rotor machine 21 Outer rotor 22 Inner rotor 23 Stator 24,25 (Outer rotor) magnets with opposite poles 26 (Outer) Air Gap 27 (Outer) Magnetic Field Lines 28,29 (inner rotor) magnets with opposite poles 30 (inner) air gap 31 (inner) magnetic field lines 40 1st (outer) driver stage 41 Second (inner) driver stage 42 Evaluation Equipment 43 1st measuring device 45 Control device 46 Optimization Module 47 Sensor Input 50 intermediate circuit 51,52 Intermediate circuit capacitor 53a~53c Half-bridge circuit 54a~54c Center tap 55 Center Tap I1 (polyphase) load current S1, S2 processing steps S11, S13 substeps S21, S22 substeps T1~T3 First power switch of the half-bridge circuit, high-side switch T4~T6 Second power switch of the half-bridge circuit, low-side switch T7~T12 Power switch VAC (output side) AC voltage VDC (input side) DC voltage V11 (positive) supply potential V12 (negative) supply potential, reference potential

Claims

1. 1. An electric drive system for or in a motor vehicle, comprising: At least one multi-phase electric machine having a double rotor, the double rotor being constructed of a magnetic flux carrying material made of a solid material; A three-level or multilevel inverter circuit for driving the electric machine. a controllable three-level or multilevel inverter coupled at its output to said electric machine and designed to supply said electric machine with an alternating voltage; and an operating mode setting device designed to selectively operate the inverter in three-level operation, multi-level operation or two-level operation depending on at least one parameter that influences the overall efficiency of the electric drive system.

2. 2. The electric drive system according to claim 1, wherein the electric machine is designed as a synchronous machine, in particular a three-phase synchronous motor driven by a three-phase inverter.

3. The inverter circuit further comprises: two power supply terminals coupleable to first and second supply potentials of a voltage source; a load output having load output terminals for each phase of the electric machine, the inverter circuit comprising a load output coupled to the electric machine; 3. The electric drive system of claim 1 or 2, wherein the inverter circuit is designed to convert a DC voltage received on the supply side into an AC voltage for driving the electric machine connected to the load output.

4. 4. The electric drive system of claim 1, wherein the operation mode setting device is further configured to operate the inverter in response to detected phase currents of the electric machine.

5. 5. An electric drive system according to claim 1, wherein the operation mode setting device comprises an evaluation device designed to optimise the overall efficiency based on the phase currents and optionally at least one other characteristic.

6. 6. The electric drive system according to claim 5, wherein the evaluation device is provided for calculating the overall efficiency, e.g. for determining the overall efficiency analytically and / or on the basis of a predefined characteristic field.

7. 7. The electric drive system of claim 5 or claim 6, wherein the evaluation device comprises an optimization module designed to first calculate the overall efficiency and / or to secondly optimize the overall efficiency numerically, analytically or via a look-up table using an optimization function.

8. 8. An electric drive system according to claim 1, wherein the operating mode setting device has a first measuring device having a sensor input and is connectable to the electric machine via the first measuring device, the first measuring device being designed to detect the phase currents of the electric machine.

9. The electric drive system of claim 1 , wherein the inverter comprises a t-type neutral point clamped inverter architecture.

10. 10. An electric drive system as claimed in any one of claims 1 to 9, wherein the inverter comprises a first driver stage and at least one second driver stage designed to supply an output load current to the load output that is less than the output load current supplied by the first driver stage.

11. 11. The electric drive system of claim 10, wherein the operation mode switching device comprises a controller designed to control the inverter to activate the first driver stage and the second driver stage in three-level or multi-level operation and to deactivate at least one of the driver stages in two-level operation.

12. 12. An electric drive system according to claim 10 or claim 11, wherein the first driver stage comprises at least one bridge circuit, in particular a half-bridge circuit, the centre tap of which forms the output load terminal of the inverter circuit, each bridge circuit comprising at least one first power switch connected to a first power supply terminal and designed to provide a first voltage level at the load output, and each bridge circuit further comprising at least one second power switch connected to a second power supply terminal and designed to provide a second voltage level at the load output.

13. 13. An electric drive system according to claim 10, wherein the second driver stage comprises at least one third power switch having a load path connected in series between an intermediate circuit and the centre tap of the first driver circuit and designed to supply a third voltage level to the load output, the third voltage level being between the first and second voltage levels.

14. 14. An electric drive system according to any one of claims 1 to 13, wherein all power switches of the inverter are designed as semiconductor switches of the same switch type and / or the same semiconductor technology.

15. 15. The electric drive system according to claim 14, wherein the semiconductor switches are designed as GaN and / or SiC power switches, in particular as SiC-MOSFETs.

16. 14. An electric drive system according to any one of claims 1 to 13, wherein at least two different switch types and / or at least two different semiconductor technologies are provided for the semiconductor switches of the inverter.

17. 17. The electric drive system according to claim 16, wherein the semiconductor switches of the first driver stage are designed as IGBTs with freewheeling diodes and the semiconductor switches of the second driver stage are designed as SiC power switches, in particular SiC MOSFETs.

18. 17. The electric drive system of claim 16, wherein the semiconductor switches of the first driver stage are designed as SiC-MOSFETs and the semiconductor switches of the second driver stage are designed as GaN-MOSFETs.

19. 17. The electric drive system according to claim 16, wherein the semiconductor switches of the first driver stage are designed as IGBTs with freewheeling diodes and the semiconductor switches of the second driver stage are designed as GaN power switches, in particular GaN-MOSFETs.

20. 20. An electric drive system according to any preceding claim, wherein the magnetic flux carrying material of the rotor comprises iron or an iron alloy.

21. 21. An electric drive system according to claim 1, wherein the electric machine comprises a stator with an inductor, the inductor being designed to propagate mainly radial magnetic flux and in particular to avoid propagating tangential magnetic flux.

22. 22. An electric drive system according to claim 21, wherein the inductor of the stator has a radial yoke thickness of less than 30%, preferably less than 20%, particularly preferably less than 10% of the total radial thickness of the inductor.

23. 23. An electric drive system according to any one of claims 1 to 22, wherein the electric machine is a wheel hub motor for an electric vehicle.

24. 24. A method of operating an electric drive system according to any one of claims 1 to 23, wherein the synchronous machine can be operated in a three-level or multilevel operating mode or in a two-level operating mode by a controllable three-level or multilevel inverter depending on the overall efficiency of the electric drive system.

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