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 an automobile or within an automobile and a method for operating such an electric drive system.

Background Art

[0002] An inverter is an electrical device that converts a DC voltage into an AC voltage. Such inverters are used, for example, in modern automobiles, solar power generation (solar inverters), components of frequency converters, and many other applications for generating an appropriate AC voltage from a DC voltage. These inverters and their application fields are generally known in a wide range of circuit variations, and it is not necessary to explain them in detail regarding their circuit design and operating modes.

[0003] In modern automobiles, electric drive systems are increasingly being used for reasons such as sustainability and CO2 emissions avoidance. Such drive systems include one or more electric machines powered by a polyphase AC voltage, such as a synchronous machine or an asynchronous machine. So-called two-level inverters (abbreviated as 2L inverters) are widely used to generate an AC voltage. In a two-level inverter, an AC voltage having 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. An example of such a two-level inverter is disclosed, for example, in the paper "Power Electronic Architectures for Electric Vehicle" by H.v. Hoeck, published in 2010 by the IEEE in "Emobility - Electrical Power Train".

[0005] In addition to the two-level inverter topology described above, three-level or multi-level inverter topologies also exist, which can generate three-level or multi-level voltage levels. Examples of multi-level inverter topologies are described, for example, in Patent Document US10903758B2 or Patent Document US2017 / 0185130A1.

[0006] The advantages of multiple voltage levels include less harmonics, slower voltage changes in phase output, less electromagnetic radiation (EME), and, most importantly, the ability to handle higher voltages. For these reasons, inverters with three or more levels are now primarily used in high-voltage applications. Power engineering applications such as solar inverters or wind turbines are established use cases for such three-level or multi-level inverter topologies. Electric vehicles (e.g., 400V voltage) do not see voltages higher than this. On the other hand, in solar power generation, voltages of 1kV or higher are common, and in other renewable energy sources such as wind energy, the voltage is even higher.

[0007] However, as stated 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 consensus is that the advantages of the aforementioned three-level or multi-level inverters are not sufficient to justify their use in electric drive systems for electric vehicles. For all these reasons, three-level or multi-level inverter topologies are not used in electric vehicles today. [Overview of the project]

[0008] The present invention aims to provide a three-level or multi-level inverter suitable for efficient use in automotive electric drive systems. In particular, the present invention further aims to optionally improve the efficiency of an electric drive system equipped with a double rotor made of a magnetic flux propagation material consisting of a solid material, and / or enable a better cost / profit ratio compared to a two-level inverter.

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

[0010] therefore, — An electric drive system for or within a vehicle is provided, comprising: at least one polyphase electromachine having a double rotor, the double rotor being composed of a magnetic flux propagation material made of a solid material; a three-level or multi-level inverter circuit for driving the electromachine, a controllable three-level or multi-level inverter designed to be coupled to the electromachine at the output side and to supply an AC voltage to the electromachine; 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 affecting the overall efficiency of the electric drive system.

[0011] - Furthermore, the present invention provides an operating method for an electric drive system, wherein the synchronous machine can be operated in a 3-level or multi-level operating mode or a 2-level operating mode by a controllable 3-level or multi-level inverter, depending on the overall efficiency of the electric drive system.

[0012] This invention is based on the recognition that, currently, two-level inverters are substantially used in automotive electric drive systems. Three-level or multi-level inverters are currently found primarily in non-automotive (high-voltage) applications. The additional advantages associated with the use of three-level or multi-level inverters have not yet been justified in automotive applications due to the associated additional costs.

[0013] This invention solves this problem by using a combination of a special inverter circuit and the adaptation of the entire drive system, thereby increasing overall profits without increasing associated costs.

[0014] The fundamental concept of this invention is the use of a novel, controllable three-level or multi-level inverter capable of operating in three-level or multi-level operation (hereinafter referred to as 3L operation) and two-level operation (hereinafter referred to as 2L operation). A dedicated operating mode setting device sets each operating mode by appropriately activating the inverter's power switch. The operating mode is set according to the overall efficiency of the entire drive system, and not simply based on the electromechanism and / or inverter used. For overall efficiency, firstly, the phase current of the electromechanism can be used. Furthermore, in addition to the detected phase current of the electromechanism, other characteristics of the electromechanism and inverter that affect overall efficiency can also be taken into consideration in the overall efficiency.

[0015] The idea behind the present invention is to reduce losses, primarily at low loads, by operating the inverter at 3L. In this case, the inverter losses hardly increase or even decrease at all operating points. Therefore, the overall efficiency of the drive system, i.e., the inverter and electrical machinery, is significantly improved, especially when used in electric vehicles.

[0016] The core idea of ​​this invention lies in the use of certain electromechanisms equipped with a double rotor made of a solid rotor material, i.e., a solid structure. In particular, such electric motors are associated with high losses. This invention solves the problem of high losses in double rotors made of solid materials in known electromechanisms. The fundamental finding here is that electromechanisms with double rotors made of solid materials have high losses in the rotor. In terms of design, losses in electromechanisms cannot be reduced, or can only be reduced slightly. Reducing losses by increasing the frequency in 2L operation has little effect and increases losses in the inverter, thereby affecting the overall efficiency.

[0017] The fundamental mechanism for reducing losses in the solid material of a double rotor is based on the fact that the amplitude of certain magnetic flux densities in the solid material of the double rotor that do not contribute to torque generation should be reduced. This portion, defined by harmonics in the magnetic flux density, is approximately directly proportional to the change in THD-induced losses in terms of the square of its amplitude. Therefore, changing the switching frequency of the inverter is ineffective because it results in a linearly proportional change in losses.

[0018] Reducing losses in solid materials significantly reduces the overall losses in electrical machinery and contributes greatly to its economical use. Therefore, a consequent finding that is part of this invention is that losses in electrical machinery can be effectively reduced by an inverter circuit that reduces only the amplitude of harmonics in magnetic flux density.

[0019] To achieve this, the following measures and aspects were considered in the selection of the inverter configuration and operating mode:

[0020] The functions of the 2L inverter are replaced with those of a dedicated, controllable 3L inverter, reducing harmonics in the inverter's phase output. This reduces harmonics in magnetic flux density and stator current. No frequency changes are necessary for this purpose.

[0021] In 2L operation, increasing the switching frequency can reduce losses, but this also significantly increases the inverter's switching losses and does not significantly improve overall efficiency; therefore, increasing the switching frequency is not implemented. Increasing the switching frequency may actively support loss optimization, but this is not an essential aspect.

[0022] The 3L inverter used provides three voltage levels (3L) and is preferably (though not strictly necessary) three-phase. Three voltage levels and three phases allow for relatively high cost efficiency. However, this system can be extended 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 present invention results in fewer harmonics, significantly reducing power losses in the electrical machinery. Switching losses in the 3L inverter are also relatively reduced, but conduction losses increase.

[0024] In both electromechanical and 3L inverters, the general loss mechanism changes depending on the load. In 3L operation, mechanical losses are significantly reduced due to fewer harmonics. Harmonic losses are dominant at low currents. At high currents, the dominant loss mechanism changes, with resistive conduction losses or copper losses becoming dominant, and harmonic-induced losses tending to be secondary or relatively small. Switching losses in inverters are reduced in 3L inverters compared to 2L inverters (by about 50%). At low loads (currents), these switching losses are dominant, but at high currents, conduction losses become dominant, making 2L operation more efficient. These findings lead to the idea according to the present invention to use a 3L inverter for low loads and a 2L inverter for high loads. This operation is made possible by the controllable 3-level or multi-level inverter according to the present invention.

[0025] Overall, the advantages of 2L operation can be combined with the advantages of 3L operation, and in particular in the case of an electromechanical machine with a double rotor motor, the overall efficiency of the electric drive system can be significantly improved compared to known electric drive systems.

[0026] It is important that the operating mode setting device does not necessarily have to perform a sudden switch from 2L operation to 3L operation or vice versa. On the contrary, such a switch is feasible if it is carried out continuously, for example instead of a fading occurring from an inner power switch to an outer power switch. This fading can be implemented, for example, taking into account the average current values of different power switches, such that the operating time or the time for which each power switch is turned on is taken into account. Additionally or alternatively, it is also conceivable that the power switches are switched in a specified order and / or slowly.

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

[0028] Advantageous embodiments and developments are apparent from the description with reference to the other dependent claims and the figures of the drawings. According to a preferred aspect of the invention, the operating mode setting device has an evaluation device. The evaluation device is designed to optimize the overall efficiency of the electric drive system based on the phase current and optionally at least one other characteristic of the electric drive system.

[0029] Typically, but not necessarily, the overall efficiency is numerically calculated by an evaluation circuit. Additionally or alternatively, the overall efficiency may be determined based on a predetermined 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 numerically calculated, for example, in a so-called offline operation, before the electric drive system is operated. This can be achieved with relatively few computer resources and is particularly preferred when a large number of characteristics are considered in the numerical prediction of the optimal overall efficiency. Furthermore, in offline operation, more time is available for calculations. Thus, in so-called real-time operation, very dynamic decisions for each operating mode (2L operation or 3L operation) are also conceivable and possible, for example, via a look-up table. For example, for these purposes, a trained artificial neural network learned based on previous characteristics and characteristic curves can be used.

[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 considering the phase currents and optionally at least one other characteristic. The optimization of the overall efficiency can be performed analytically and / or, for example, via a pre-generated appropriate look-up table.

[0031] The operating mode used in each case (e.g., 2L operation or 3L operation) is, for example, a characteristic of the electromechanical machine that affects the overall efficiency. Other characteristics can be seen in the specific configuration of the rotor of the electromechanical machine, for example, that the rotor is a double rotor and / or that the double rotor is made of a magnetic flux propagation material consisting 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, through which a first measuring device can be electromechanically coupled. The first measuring device is designed to detect phase current.

[0034] According to a preferred exemplary embodiment, the inverter includes a t-type neutral point clamp (TNPC) inverter architecture. These offer several advantages over multi-level active neutral point clamp (ANPC) inverter topologies. In contrast to the ANPC topology, conduction losses are lower because a maximum of three switches conduct in series instead of four. Switching losses are similarly lower because the output voltage waveform is the same, but at high switching frequencies (e.g., greater than 10 kHz), the total chip area required for the TNPC topology is smaller than that of the two-level topology. Similar to ANPC, hybrid inverter topologies can be constructed for TNPC to further improve efficiency and / or optimize manufacturing costs. For example, different switching techniques can be used at zero potential or intermediate bridge branching for this purpose. In particular, losses can be significantly reduced by using gallium nitride (GaN) in the case of TNPC inverters composed solely of insulated-gate bipolar transistors (IGBTs). Hybrid TNPC inverter topologies can also be used for motor control in electric vehicles, although this is not commonly seen in practice.

[0035] TNPC-based 3L inverters can operate in two operating modes to improve system efficiency. In the case of a 3L TNPC inverter, the zero-potential (intermediate) bridge branch can be switched off to operate in 2L mode and switched on to change to 3L mode. Switching between the two operating modes is done to improve system efficiency. To this end, the load is measured in the control and adjustment logic, and the system switches between 2L and 3L mode using predetermined optimization characteristics.

[0036] Additionally or alternatively, TNPC-based 3L inverters may be designed asymmetrically to reduce inverter costs. Asymmetrical design refers to a zero-potential (intermediate) bridge branch having a lower energizing capacity than the outer bridge branch. This is because, to optimize overall efficiency, the zero-potential bridge branch is not used under high load conditions. The outer bridge branch is designed for peak currents, while the zero-potential bridge branch is designed for low or continuous currents.

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

[0038] Preferably, the operating mode switching device is designed to control the inverter to activate the first and second driver stages in 3-level or multi-level operation, depending on the overall efficiency, and to deactivate at least one driver stage, preferably the inner second driver stage, in 2-level operation.

[0039] Typically, but not always required, the first driver stage has at least one bridge circuit, in particular a half-bridge circuit, whose center tap forms the output load terminal of the inverter circuit. Thus, each bridge circuit 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 supply 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 potential or reference potential is applied) and designed to supply a second voltage level to the load output. Semiconductor-based power switches can be manufactured from a variety of semiconductor materials of any choice. 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 always necessary, the second driver stage includes at least one third power switch, the load path of which is 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 supply a third voltage level to the load output that is midway between the first and second voltage levels.

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

[0042] In a first preferred variation of the homogeneous inverter topology, the semiconductor switch is designed as a GaN power switch, e.g., a GaN MOSFET. In a particularly preferred second variation, the semiconductor switch is designed as a SiC power switch, specifically a SiC MOSFET. Furthermore, IGBT-based power switches, e.g., silicon-based IGBTs with Si or SiC diodes, are also feasible.

[0043] In a particularly preferred, so-called hybrid inverter topology, at least two different switch types and / or at least two different semiconductor technologies are provided for the inverter's semiconductor switches, i.e., the semiconductor switches of the first driver stage and / or the semiconductor switches of the second driver stage. In a hybrid inverter topology, not all power switches within the inverter use the same semiconductor material. In particular, the power switches of the zero-potential bridge branch, i.e., the power switches of the second driver stage, use a different technology (a different switch type) than the external switches of the first driver stage. As a result, switching losses and conduction losses are reduced, and efficiency is improved. Furthermore, there are cost advantages. It is especially recommended to optimize the power switches of the zero-potential bridge branch (second driver stage) to make switching losses and reverse recovery losses as low as possible. This is because the zero-potential bridge branch (second driver stage) operates at low currents, and its low reverse recovery loss also reduces the switching losses of the external switches. Hybrid designs are particularly recommended when the inverter is asymmetric. The lower the energizing capacity of the zero-potential bridge branch (second driver stage), the lower the additional cost of switching the loss-optimized switches.

[0044] In a particularly preferred first modification, the semiconductor switch of the first driver stage is designed as an IGBT (Si or SiC) with a freewheeling diode. In this case, the semiconductor switch of the second driver stage can preferably be designed as a SiC power switch, in particular as a SiC MOSFET.

[0045] Similarly, in a second preferred modification, the semiconductor switch of the first driver stage is designed as a SiC MOSFET. In this case, the semiconductor switch of the second driver stage can be designed as a GaN-based MOSFET.

[0046] In a preferred third modification, the semiconductor switch of the first driver stage is designed as an IGBT with a freewheeling diode. In this case, the semiconductor switch of the second driver stage can be designed as a GaN power switch, in particular as a GaN MOSFET.

[0047] In a particularly preferred exemplary embodiment, the flux-propagating material of the rotor is made of iron or an iron alloy. An electric rotating magnetic field machine, here preferably a synchronous machine with a double rotor, can be designed so that the flux-propagating material in the rotor is made of a solid structure, i.e., a solid material. This is because, in the view of an idealized synchronous machine, there is no periodic relative motion between the directional vector of the rotating magnetic field generated by the stator windings and the double rotor. Therefore, the magnetic flux density at the operating point is constant, and no iron loss occurs in the material. In such a permanent magnet exciter, where the magnets are mounted on the rotor surface, the distance between the inductor grooves and the flux-propagating material, thus ensured, allows the use of solid materials without increasing additional losses.

[0048] Similarly, according to a particularly preferred exemplary embodiment, the electromachine comprises a stator having an inductor, the inductor being designed to propagate magnetic flux mainly in the radial direction, and in particular to avoid tangential magnetic flux propagation. Thus, the stator has a so-called "yoke-less" design, which avoids circumferential magnetic flux propagation in particular. This eliminates the need for a magnetic return path for the inductor, reducing weight and iron losses.

[0049] According to one embodiment, the stator inductor has a radial yoke thickness of less than 30%, preferably less than 20%, and particularly preferably less than 10% of the total radial thickness of the inductor. In a so-called "yoke-less" design, the mechanical connection of the inductor teeth is provided in this manner, but this is electromagnetically unnecessary, and there is no functionally relevant magnetic flux. Thus, the term "yoke-less" refers to the electromagnetic design of the inductor.

[0050] Similarly, according to a 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 synchronous machines utilizing a three-level or multi-level inverter topology substantially improve the overall efficiency of the drive system.

[0051] Similarly, according to a particularly preferred exemplary embodiment, the electromechanism is designed as an electrically operable wheel hub motor for an automobile. The wheel hub motor is an electromechanism that is directly mounted to the wheel, particularly the hub of the vehicle, and simultaneously supports the wheel hub. Part of the hub motor transmits the torque it generates directly to the driving wheel, causing the wheel to rotate. In the case of an electric wheel hub motor, both internal and external rotor motors are conceivable. The main advantage of such an electric wheel hub motor is that, compared to a drive concept with a central motor, it eliminates the need for a conventional driveline, which includes components required in each case depending on the specifications (such as a transmission, cardan shaft, differential gear, and drive shaft). The elimination of their transmission losses can also potentially increase the overall efficiency of the drive system. Efficient regeneration, i.e., the recovery of electrical energy when the vehicle brakes, can also be implemented in an electric wheel hub motor.

[0052] The embodiments and developments described above can be combined with each other as desired, if useful. Further possible embodiments, other developments, and practices of the present invention include combinations of features of the invention that are described or described above with respect to the exemplary embodiments, which are not expressly mentioned. In particular, those skilled in the art can also add individual embodiments as improvements or additions to each of the basic forms of the invention. [Brief explanation of the drawing]

[0053] The present invention will be described in more detail below with reference to exemplary embodiments shown in the schematic diagrams. [Figure 1]This is a block diagram showing an electric drive system according to the present invention. [Figure 2] Figure 1 is a schematic cross-sectional view of an example of an electric machine of the electric drive system according to the present invention. [Figure 3] This is a block diagram of a three-level or multi-level inverter circuit for an electric drive system according to the present invention, corresponding to Figure 1. [Figure 4] This is a circuit diagram of a particularly preferred embodiment of the inverter circuit according to the present invention. [Figure 5] This is a flowchart illustrating a method for operating the electric drive system according to the present invention.

[0054] The attached drawings are intended to provide a further understanding of embodiments of the present invention. These drawings illustrate embodiments and, together with this specification, are helpful in illustrating the principles and concepts of the present invention. Many other embodiments and advantages mentioned will be apparent from the drawings. The elements of the drawings are not necessarily shown to scale relative to one another.

[0055] In the drawings, elements, features, and components that are similar and functionally identical, as well as elements, features, and components that operate in the same manner, are denoted by the same reference numeral unless otherwise specified. [Modes for carrying out the invention]

[0056] Figure 1 is a block diagram of an electric drive system for an automobile according to the present invention. Here, the electric drive system designated by reference numeral 10 is preferably (but not necessarily) provided for use in an automobile. The drive system 10 comprises at least one polyphase electromechanism 11 and an inverter circuit 12.

[0057] The electrical machine 11 is connected to the inverter circuit 12 that drives the electrical machine 11 on the input side. According to the present invention, the inverter circuit 12 is designed as a 3-level or multi-level inverter circuit 12. The inverter circuit 12 comprises an inverter 13 and an operating mode setting device 14.

[0058] The inverter 13 is coupled to the electromachine 11 via its load output 15 and to the supply voltage source 18 via power terminals 16 and 17. The inverter 13 is designed to convert the DC voltage VDC received on the supply side to an AC voltage VAC. The inverter 13 is designed as a multiphase inverter 13, and the number of phases of the inverter 13 usually corresponds to the number of phases of the electromachine 11. The electromachine 11 is driven by the phase current supplied from the inverter 13 at the load output 15.

[0059] According to the present invention, the operating mode of the inverter circuit 12 can be set via an operating mode setting device 14 coupled to the input side to the electromechanical unit 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 operates in both two-level operation and three-level or multi-level operation, and occurs, for example, when transitioning from one operating mode to the next. The structure of the operating mode setting device 14 and its operating modes will be described in further detail below with reference to Figures 3 to 6.

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

[0061] It is equally preferable if the electromechanism 11 of the electric drive system 10 is an electrically operated wheel hub motor for an automobile. However, other applications are also feasible and advantageous.

[0062] Figure 2 is a schematic cross-sectional view showing an example of an electric machine of the electric drive system according to the present invention shown in Figure 1.

[0063] The electromachine 11 is preferably designed as a synchronous machine 11. One aspect that is important to the present invention, but not essential, is a situation in which the synchronous machine or electromachine 11 comprises a double rotor 20, and further comprises a magnetic flux propagation material made of a solid material. A cross-section of the double rotor synchronous machine 11 is shown in Figure 2. The double rotor machine 20 comprises an outer rotor 21 and an inner rotor 22. A stator 23 is positioned between the two rotors 21, 22 in a manner known to itself. The stator 23 is preferably a yokeless stator 23, although it is not necessarily required.

[0064] The outer rotor 21 and 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 as a solid, full-volume structure.

[0065] In the illustrated example, two magnets 24 and 25 with opposing poles are positioned between the outer rotor 21 and the stator 23, on the inner surface of the outer rotor 21 within the outer air gap 26. It is also feasible and advantageous for the magnets 24 and 25 to be embedded in pocket-like recesses in the outer rotor 21 that are specially provided for this purpose. However, it would also be feasible for the magnets 24 and 25 to be spaced away from the outer rotor 21, i.e., not directly positioned on its inner surface. The magnetic field lines 27 between the north and south poles of the opposing magnets 24 and 25 pass through the core material of the outer rotor 21 in this case.

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

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

[0068] Figure 3 is a block diagram of a three-level or multi-level inverter circuit for the electric drive system according to the present invention shown in Figure 1.

[0069] As already explained with reference to the drawings, the inverter circuit 12 includes two power terminals 16 and 17, a load output 15, a 3-level or multi-level inverter 13, and an operating mode setting device 14.

[0070] For example, a first supply potential V11, which is a positive supply potential, can be connected to the first power terminal 16. For example, a second supply potential V12, which is a negative supply potential or reference potential, can be connected to the second power terminal 17. As a result, a supply DC voltage VDC = V11 - V12 exists between power terminals 16 and 17.

[0071] A multiphase load current I1 flows through the load output 15, and each phase of the electromachine 11 that can be connected via the load output 15 operates.

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

[0073] The inverter 13 has a first driver stage 40 and at least one second driver stage 41. The second driver stage 41 is designed to supply an output load current to the load output 15 that is smaller than the output load current supplied by the first driver stage 40.

[0074] The operating mode setting device 14 serves the purpose of setting and controlling the operating mode of the inverter 13, and consequently, the operating mode of the entire inverter circuit 12. In particular, the inverter 13 is designed to operate in either a first operating mode in 3-level or multi-level operation, or a second operating mode in 2-level operation. It is also conceivable that it has at least a third operating mode, including a hybrid form of 2-level operation and 3-level or multi-level operation. The third operating mode is particularly feasible and useful when transitioning from the first operating mode to the second operating mode, and vice versa.

[0075] The operating mode setting device 14 controls the operating mode of the inverter 13 used according to the overall efficiency of the entire electric drive system 10. Thus, the overall efficiency is a function of the detected phase current of the electromachine 11 and is also a function of at least one other characteristic of the electric drive system that optionally affects the overall efficiency.

[0076] To set each 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 device 45.

[0077] The evaluation device 42 is designed to optimize the overall efficiency of the electric drive system 10 based on the phase current and optionally at least one other characteristic. This can be done, for example, in situ, i.e., during the operation of the electric drive system 10. However, preferably, relatively computationally intensive calculations are performed in advance, for example, by appropriate calculations (e.g., numerical or analytical) and / or using a predetermined characteristic field. For example, numerical efficiency calculations for 2L and 3L operation, and mapping of functions having a determined output, are performed in advance, i.e., offline. Selecting a better efficiency by switching, and applying lookup tables to determine the efficiency, can be done more or less dynamically during operation, but not limited to this.

[0078] For optimization, the evaluation device 42 includes an optimization module 46. The optimization module 46 first calculates the overall efficiency. The overall efficiency is then optimized analytically, or via a lookup table, for example, via an optimization function, taking into account the phase current 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 electromechanism 11 via the sensor input 47 in order to detect the phase current of the electromechanism 11.

[0080] The actual control of the inverter is performed by a control device 45 specially provided for this purpose. The control device 45 sets each operating mode of the inverter 13, namely whether the inverter 13 operates in 3-level operation, multi-level operation, or 2-level operation. For example, the control device 45 can control the inverter 13 so that both driver stages 40 and 41 are activated when operating in 3-level or multi-level operation, and the second driver stage 40 is deactivated when operating in 2-level operation.

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

[0082] A DC supply voltage VDC is supplied at power terminals 16 and 17, with a supply potential V11 = VDC / 2 applicable to the first power terminal 16, and a supply potential V12 = -VDC / 2 applicable to the second power terminal 17. It is also possible to configure the second power terminal 17 to have 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 terminal 16.

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

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

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

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

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

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

[0089] Alternatively (not shown in Figure 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 Figure 4), power switches T7-T12 can be designed as IGBTs with freewheeling diodes, and power switches T1-T6 can be designed as GaN power switches, particularly GaN-MOSFETs.

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

[0092] Figure 5 shows a flowchart of a method for operating the electric drive system according to the present invention. The electric drive system is, for example, the drive system shown in Figure 1, and has a synchronous machine with a double rotor. The double rotor is made of a magnetic flux propagation material made of a solid material.

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

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

[0095] A hybrid configuration of 3-level or multi-level operation and 2-level operation is also feasible. Such a hybrid configuration is advantageous, for example, during transitions from 3-level or multi-level operation to 2-level operation, in order to avoid abrupt switching. The latter may involve losses, resulting in reduced efficiency.

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

[0097] 10 Electric drive system 11. Electrical machinery, synchronous machines 12 (3-level or multi-level) inverter circuits 13 (3-level or multi-level) inverters 14. Operating 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 Status 24,25 Magnets with counter poles (outer rotor) 26 (Outer) air gap 27 (Outer) Magnetic Field Lines 28,29 Magnets with counter poles (of the inner rotor) 30 (inside) air gap 31 (Inner) Magnetic Field Lines 40. First (outer) driver stage 41. Second (inner) driver stage 42 Evaluation device 43 1st measuring device 45 Control device 46 Optimization Modules 47 Sensor Input 50 intermediate circuit 51, 52 Intermediate circuit capacitors 53a~53c Half-bridge circuit 54a~54c Center tap 55 Center tap I1 (polyphase) load current S1, S2 Processing Steps S11, S13 Substep S21, S22 Substep T1-T3 are the first power switches and high-side switches for the half-bridge circuit. T4~T6 Second power switch and low-side switch for the half-bridge circuit T7~T12 Power Switch VAC (Output Side) AC Voltage VDC (Input) DC Voltage V11 (positive) supply potential V12 (negative) supply potential, reference potential

Claims

1. An electric drive system for driving an automobile, A multiphase electromachine comprising a double rotor, the double rotor being composed of a magnetic flux propagation material made of a solid material, A three-level or multi-level inverter circuit for driving the electrical machine, - A controllable three-level or multi-level inverter designed to be coupled to the electromachine on the output side and to supply an AC voltage to the electromachine, - An electric drive system comprising an operating mode setting device designed to selectively operate the inverter in either three-level operation, multi-level operation, or two-level operation in response to at least one phase current affecting the overall efficiency of the electric drive system, wherein the operating mode setting device is configured to optimize the overall efficiency, and an evaluation device provided for calculating the overall efficiency analytically or based on a predetermined characteristic field.

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

3. The inverter circuit further, - Two power supply terminals that can be coupled to the first and second supply potentials of the voltage source, - The inverter circuit has load output terminals for each phase of the electrical machine, and the inverter circuit has a load output that is coupled to the electrical machine, The electric drive system according to 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. The electric drive system according to claim 1 or 2, wherein the operating mode setting device is further designed to operate the inverter in accordance with the detected phase current of the electric machine.

5. The electric drive system according to claim 1, wherein the evaluation device first calculates the overall efficiency and / or then optimizes the overall efficiency numerically, analytically, or via a lookup table using an optimization function.

6. The electric drive system according to claim 1 or 2, 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, and the first measuring device is designed to detect the phase current of the electric machine.

7. The electric drive system according to claim 1 or claim 2, wherein the inverter includes a t-type neutral point clamp inverter architecture.

8. The electric drive system according to claim 1 or 2, wherein the inverter is coupled to the electric machine via a load output and comprises a first driver stage and at least one second driver stage, the second driver stage being designed to supply to the load output an output load current smaller than the output load current supplied by the first driver stage.

9. The electric drive system according to claim 8, comprising an operating mode switching device, a control device designed to control the inverter such that in three-level or multi-level operation the first driver stage and the second driver stage are activated, and in two-level operation at least one of the driver stages is deactivated.

10. The electric drive system according to claim 8, wherein the first driver stage comprises at least one bridge circuit, in particular a half-bridge circuit whose center tap forms the output load terminal of the inverter circuit, each of the bridge circuits comprises at least one first power switch connected to a first power terminal and designed to provide a first voltage level to the load output, and each of the bridge circuits further comprises at least one second power switch connected to a second power terminal and designed to provide a second voltage level to the load output.

11. The electric drive system according to claim 10, wherein the second driver stage has at least one third power switch designed to supply a third voltage level between the first voltage level and the second voltage level to the load output, with the load path connected in series between the intermediate circuit and the center tap of the first driver stage.

12. The electric drive system according to claim 1 or 2, wherein all power switches of the inverter are designed as semiconductor switches of the same switch type and / or the same semiconductor technology.

13. The electric drive system according to claim 12, wherein the semiconductor switch is designed as a GaN power switch and / or a SiC power switch, in particular as a SiC-MOSFET.

14. The electric drive system according to claim 8, wherein at least two different switch types and / or at least two different semiconductor technologies are provided for the semiconductor switches of the inverter.

15. The electric drive system according to claim 14, wherein the semiconductor switch of the first driver stage is designed as an IGBT with a freewheeling diode, and the semiconductor switch of the second driver stage is designed as a SiC power switch, in particular as a SiC-MOSFET.

16. The electric drive system according to claim 14, wherein the semiconductor switch of the first driver stage is designed as a SiC-MOSFET, and the semiconductor switch of the second driver stage is designed as a GaN-MOSFET.

17. The electric drive system according to claim 14, wherein the semiconductor switch of the first driver stage is designed as an IGBT with a freewheeling diode, and the semiconductor switch of the second driver stage is designed as a GaN power switch, in particular as a GaN-MOSFET.

18. The electric drive system according to claim 1 or claim 2, wherein the magnetic flux propagation material of the rotor is made of iron or an iron alloy.

19. The electric drive system according to claim 1 or 2, wherein the electric machine has a stator equipped with an inductor, and the inductor is designed to propagate magnetic flux mainly in the radial direction, and in particular to avoid the propagation of magnetic flux in the tangential direction.

20. The electric drive system according to claim 19, wherein the inductor of the stator has a radial yoke thickness of less than 30%, preferably less than 20%, and particularly preferably less than 10% of the total radial thickness of the inductor.

21. The electric drive system according to claim 1 or claim 2, wherein the electric machine is a wheel hub motor for an electric vehicle.

22. The method for operating an electric drive system according to claim 2, wherein the synchronous machine is capable of operating in either a three-level or multi-level operating mode or a two-level operating mode by a controllable three-level or multi-level inverter, depending on the overall efficiency of the electric drive system.

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