Electric machine and methods for its operation

Abstract

The invention relates to an electric machine (1) with a first rotor (R1) which has a plurality of electrical coils (2) and a second rotor (R2) which has a plurality of permanent magnets (3), wherein a stator (5) is arranged between the first and the second rotor (R1, R2), and wherein the rotors (R1, R2) are designed as disks and the electric machine (1) is designed as a disk rotor. Furthermore, the invention relates to a method for operating the electric machine (1).

Description

[0001] The invention relates to an electric machine having the features of the preamble of claim 1 and a method for its operation.

[0002] A hybrid electric machine with axial flux magnets is known from the prior art, as described in DE 101 41 872 A1. The hybrid electric machine has a permanent magnet rotor field in addition to an electrically excited rotor field. The generator rotor has a first claw segment and a second claw segment mounted at opposite ends of a rotor shaft. A third middle claw segment is mounted on the rotor shaft between the first and second claw segments. A wound field coil is mounted on the rotor shaft between the first and third claw segments. An axial flux permanent magnet is mounted on the rotor shaft between the second claw segment and the third claw segment.

[0003] The invention is based on the object of specifying an electric machine that is improved compared to the prior art and a method for operating an electric machine that is improved compared to the prior art.

[0004] The object is achieved according to the invention by an electric machine having the features of claim 1 and a method for its operation having the features of claim 2.

[0005] Advantageous embodiments of the invention are the subject of the subclaims.

[0006] An electric machine according to the invention has a first rotor and a second rotor. The first rotor has a plurality of electrical coils. The second rotor has a plurality of permanent magnets. A stator is arranged between the first and second rotors. The rotors are designed, in particular, as disks. The electric machine is designed, in particular, as a disk rotor. The two rotors are coupled to one another in a rotationally fixed manner, in particular by means of a common rotor shaft, i.e., the two rotors and the rotor shaft rotate together and thus together form a double rotor.

[0007] The electric machine is intended in particular for use as an electric drive machine for driving a vehicle, in particular an electric vehicle or hybrid vehicle.

[0008] The electric machine according to the invention is designed as a hybrid-excited axial flux electric machine, ie as an electric machine in the form of an axial flux machine, which, however, is not only permanently excited, but is additionally also separately excited by means of an excitation current or, depending on the operating mode, is separately excitable.

[0009] It therefore differs from a conventional axial flux electric machine, which has two permanent magnet-excited rotors. When the term "conventional axial flux electric machine" is used in this application, it refers to an axial flux electric machine that has two rotors, each of which is designed identically to the second rotor of the electric machine described here. This means that in this conventional axial flux electric machine, instead of the first rotor, there is a rotor that is designed identically to the second rotor of the hybrid-excited electric machine described here, but can be arranged on the rotor shaft, for example, rotated by 180° about a virtual axis running perpendicular to the rotor shaft.Furthermore, this conventional axial flux electric machine does not have any other components that are required for current excitation of the current-excited first rotor, which is not present in this conventional axial flux machine.

[0010] The permanent magnets used for both rotors in conventional axial flux electric machines result in high costs. Furthermore, the permanent magnets are very sensitive to temperature and vibration. If the permanent magnets are damaged or demagnetized, the conventional axial flux electric machine will no longer function. Furthermore, the operation of the conventional axial flux electric machine at high speeds is very difficult because both rotors are always fully magnetized and therefore permanently excited.

[0011] The electric machine described here, designed as a hybrid-excited axial flux electric machine, can overcome these disadvantages. Compared to the conventional axial flux electric machine, it is lower in cost, exhibits better efficiency in the field weakening range, enables partial excitation reduction of the first rotor in the event of faults, and enables fault-tolerant operation in the event of demagnetization of the second rotor. Compared to a current-excited synchronous machine, it exhibits higher power density and better efficiency in the constant force range and enables fault-tolerant operation in the event of excitation faults.

[0012] In a method according to the invention for operating the electric machine, a dual operating mode is provided in which the first rotor is supplied with electrical energy, in particular with an excitation current, and a singular operating mode is provided in which the electrical energy supply to the first rotor, in particular the excitation current, is switched off. A strongly field-weakening operating mode is provided in which a current flow of the excitation current is generated in the first rotor in the opposite direction to that in the dual operating mode. In the strongly field-weakening operating mode, a magnetic field of the current-excited first rotor counteracts a magnetic field of the permanent magnet-excited second rotor.

[0013] In dual operating mode, the magnetic fields of the two rotors are aligned so that the magnetic field of the first rotor amplifies the magnetic field of the second rotor. This is achieved by matching the direction of the excitation current.

[0014] The dual operating mode is specifically divided into a normal operating mode in which the magnetic field of the first rotor and the magnetic field of the second rotor are equally strong, a field-weakening operating mode in which the magnetic field of the first rotor is weaker than the magnetic field of the second rotor, and a field-strengthening operating mode in which the magnetic field of the first rotor is stronger than the magnetic field of the second rotor.

[0015] For example, in the dual operating mode, at speeds above a, in particular predetermined, base speed, the system switches to the field-weakening operating mode, in which, in particular, a change in the energy supply of the first rotor, in particular a change in the current intensity of the excitation current, takes place, so that the magnetic field of the first rotor is weaker than the magnetic field of the second rotor.

[0016] The base speed, for example, is 30% of a maximum speed. This maximum speed is, in particular, a maximum speed of the conventional axial flux electric machine according to the above definition.

[0017] For example, the dual operating mode switches to the singular operating mode when a requested torque falls below a limit torque, especially for low speeds.

[0018] The limit torque is in particular 10% of a maximum torque, wherein this maximum torque is in particular a maximum torque of the conventional axial flux electric machine according to the above definition.

[0019] For example, above a specified limit speed, the system switches to the strongly field-weakening operating mode.

[0020] For example, above a requested maximum torque, the system switches to the field-strengthening operating mode, whereby this maximum torque is in particular the maximum torque of the conventional axial flux electric machine according to the above definition.

[0021] Switching between the operating modes is carried out in particular by changing a current intensity and / or direction of the excitation current for the first rotor, whereby the current intensity can be reduced to zero amperes, in particular for the normal operating mode.

[0022] Embodiments of the invention are explained in more detail below with reference to drawings.

[0023] Showing: Fig. 1 schematically shows an embodiment of an electric machine, Fig. 2 schematically shows a first rotor of the electric machine, Fig. 3 schematically shows a second rotor of the electric machine, Fig. 4 schematically shows a magnetic flux in the electric machine during a dual operating mode, Fig. 5 schematically shows a magnetic flux in the electric machine during a singular operating mode, Fig. 6 schematically shows a magnetic flux in the electric machine during a strong field weakening operating mode, and Fig. 7 schematically shows a torque-speed diagram with the operating modes of the electric machine and a curve of the torque and speed of the electric machine in comparison to a curve of the torque and speed of an exclusively permanent-magnet axial flux electric machine.

[0024] Corresponding parts are provided with the same reference numerals in all figures.

[0025] Fig. Figure 1 shows a schematic representation of an embodiment of an electric machine 1. This electric machine 1 is designed as a hybrid-excited axial flux electric machine, i.e., as an electric machine in the form of an axial flux machine that is not only permanently excited, but also separately excited by an excitation current or, depending on the operating mode, can be separately excited. Possible operating modes are explained in more detail below.

[0026] The electric machine 1 has a first rotor R1 with a plurality of electric coils 2 and a second rotor R2 with a plurality of permanent magnets 3.

[0027] The first rotor R1 is in Fig. 2. The second rotor R2 is shown in Fig. 3 is shown in more detail.

[0028] The two rotors R1, R2 are rotationally fixedly coupled to one another by means of a common rotor shaft 4 and thus together form a double rotor. A stator 5 is arranged between the two rotors R1, R2. In the example shown, the rotor shaft 4 is rotatably mounted relative to the stator 5 by means of rolling bearings 6, in particular ball bearings. The two rotors R1, R2 are each designed as a disk. The electric machine 1 is thus designed in particular as a disk rotor. The coils 2 are arranged on or in the disk of the first rotor R1, in particular evenly distributed around a circumference of the disk. The permanent magnets 3 are arranged on or in the disk of the second rotor R2, in particular evenly distributed around a circumference of the disk.

[0029] Since the electric machine 1 is designed as an axial flux electric machine, the permanent magnets 3 are arranged such that they generate a magnetic field and thus a magnetic flux F in the axial direction, i.e. parallel to the rotor shaft 4. Likewise, the electrical coils 2 are aligned such that, when the excitation current flows through them, they generate a magnetic field and thus a magnetic flux F in the axial direction, i.e. parallel to the rotor shaft 4.

[0030] Depending on the current flow to the coils 2, i.e. depending on the direction of the excitation current flow, the magnetic field of the coils 2 and the magnetic field of the permanent magnets 3 can be aligned in the same direction, so that the magnetic field of the permanent magnets 3 is amplified by the magnetic field of the coils 2, or the magnetic field of the coils 2 can be aligned opposite to the magnetic field of the permanent magnets 3, so that the magnetic field of the permanent magnets 3 is weakened by the magnetic field of the coils 2. The excitation current can also be switched off, so that the electric machine 1 is then only permanently excited. By adjusting the excitation current, i.e. in particular its strength and direction, the operating modes described in more detail below are possible.

[0031] The operating modes are as in Fig. 7, in particular, a dual operating mode DM, a singular operating mode SM, and a strongly field-weakening operating mode SSM are provided. The dual operating mode DM can, for example, be subdivided into a normal operating mode NM, a field-weakening operating mode FSM, and a field-strengthening operating mode FVM. The terms "field-strengthening" and "field-weakening" refer to the resulting total magnetic field of the two rotors R1, R2. Furthermore, a fault operating mode, also referred to as a fault-tolerant operating mode, can be provided.

[0032] The permanent magnets 3 ensure in particular a minimum magnetic flux F in the axial direction through the electric machine 1.

[0033] The excitation current can be supplied to the coils 2, for example, inductively, for example by means of at least one rotating transformer, or it can, for example, be supplied conductively, in particular by means of sliding contacts, as in the embodiment shown.

[0034] In the example shown, these sliding contacts are formed by brushes 7, in particular carbon brushes, and slip rings 8 fastened to the rotor shaft 4, i.e., connected in a rotationally fixed manner. The respective slip ring 8 is arranged on an outer side of the first rotor R1. The slip rings 8 are connected to the coils 2 via winding tunnels 11 in the disk of the first rotor R1. Connecting lines from the slip rings 8 to the coils 2 are routed through these winding tunnels 11. The respective brush 7 is pressed against the respective slip ring 8, in particular by means of a spring element 9, in order to ensure a continuous flow of direct current through the coils 2.

[0035] The coils 2 are connected via the slip rings 8 and brushes 7, in particular, to an external electrical power supply for providing the excitation current, i.e., to its positive terminal A+ and negative terminal A-. In the example shown, this is done via a bridge circuit 10 with four switches S1 to S4, designed in particular as semiconductor switches.

[0036] The coils 2 can be coupled to a half-bridge or H-bridge of the bridge circuit 10 for current control. If the first and fourth switches S1, S4 are closed and the second and third switches S2, S3 remain open, a positive current flow of the excitation current occurs, i.e., in one direction. If the second and third switches S2, S3 are closed and the first and fourth switches S1, S4 remain open, a negative current flow of the excitation current occurs, i.e., in the opposite direction.

[0037] This change in the direction of the excitation current changes the direction of the magnetic flux F of the magnetic field generated by the coils 2, making it either parallel or opposite to the magnetic flux F of the magnetic field generated by the permanent magnets 3, thus either amplifying or weakening this magnetic field. The overall magnetic field and its magnetic flux F are thus either stronger or weaker than the magnetic field of the permanent magnets 3 and their magnetic flux F. The strength of the excitation current can also change the strength of the magnetic field generated by the coils 2 and its magnetic flux F, and thus the amplifying or weakening effect.

[0038] In previously known axial flux electric machines, the permanent magnets 3 are located on both rotor disks. Therefore, the magnetic flux density generated by the dual rotor is constant and symmetrical. In the electric machine 1 described here, which is designed as a hybrid-excited axial flux electric machine, the magnetic flux density in the axial direction can be partially controlled by the excitation current in the coils 2. Therefore, the flux density generated by the two rotors R1, R2 is asymmetrical. Depending on the control of the excitation current, the electric machine 1 can thus be operated in various operating modes, as already mentioned above. These operating modes are explained in more detail below.

[0039] In dual operating mode DM, a positive direct current flows as excitation current through the coils 2 and builds up a magnetic field in the current-excited first rotor R1. The magnetic fluxes of the two rotors R1, R2 are connected and closed with the magnetic field of the stator 5 in the axial direction, as shown in Fig. 4. Thus, both rotors R1, R2 can generate a torque T and the electric machine 1 can generate the maximum torque T in this dual operating mode DM emax and deliver maximum performance.

[0040] The orientation of the respective magnetic field is in Fig. 4 and also in the Fig. 5 and Fig. 6 can be identified by its north pole N and south pole S.

[0041] Depending on the flux density of the current-excited first rotor R1, three different submodes can be defined: the normal operating mode NM, the field-strengthening operating mode FVM and the field-weakening operating mode FSM.

[0042] In the normal operating mode NM, the current-excited first rotor R1 generates the same magnetic flux density as the permanent magnet-excited second rotor R2. Thus, in this normal operating mode NM, both rotors R1, R2 provide the same torque T as in a conventional axial flux electric machine that has two permanent magnet-excited rotors, each of which is designed identically to the second rotor R2 of the electric machine 1 described here. This means that in this conventional axial flux electric machine, instead of the first rotor R1, there is a rotor that is designed identically to the second rotor R2 of the hybrid-excited electric machine 1 described here, but can be arranged on the rotor shaft 4, for example, rotated by 180° about a virtual axis running perpendicular to the rotor shaft 4.Furthermore, this conventional axial flux electric machine also lacks the additional components required for current excitation of the current-excited first rotor R1, which is not present in this conventional axial flux electric machine. When the term "conventional axial flux electric machine" is used in this application, it refers to the axial flux electric machine according to this definition.

[0043] In the field-strengthening operating mode (FVM), the current-excited first rotor R1 generates a higher magnetic flux density than the permanent magnet-excited second rotor R2. This is achieved, in particular, by a higher excitation current than in the normal operating mode (NM). As a result, the electric machine 1 can generate a higher peak torque than a conventional axial flux electric machine, especially in the low speed range.

[0044] In the field-weakening operating mode FSM, the current-excited first rotor R1 generates a lower magnetic flux density than the permanent magnet-excited second rotor R2. This is achieved in particular by a lower current intensity of the excitation current than in the normal operating mode NM. In this way, the flux density F in the electric machine 1 can be actively reduced in order to minimize the iron losses in the electric machine 1. Especially in the high-speed range, field weakening can be easily achieved by reducing the excitation current to zero amperes. This avoids the high d-current for field weakening in the stator 5 and reduces the power loss in the stator 5. In this way, better efficiency is achieved at medium and high speeds n.

[0045] In the singular operating mode SM, the current excitation, i.e., the excitation current, is switched off. The first rotor R1 is thus completely currentless. The interlocking of the coils 2 reduces the magnetic resistance and increases the d-inductance. The magnetic flux F, generated by the permanent magnet-excited second rotor R2, flows passively through the first rotor R1, as shown in Fig. 5. The first rotor R1 behaves like a reluctance rotor in this case.

[0046] Due to the absence of current excitation of the first rotor R1, the air gap flux density is reduced and a certain field weakening is achieved. This is advantageous in the high-speed range, where field weakening is necessary. Furthermore, the torque ripple is reduced because the first rotor R1 operates passively. This results in better efficiency at high speeds n and also at low speeds n with light load.

[0047] Furthermore, the fault operating mode, also referred to as the fault-tolerant operating mode, is possible. Since two rotors R1, R2 are excited independently of one another, the robustness of the electric machine 1 is improved and this operating mode is made possible. If one of the rotors R1, R2, or more precisely its excitation, fails, the electric machine 1 can still operate and reliably provide the torque T. Different reactions can be taken depending on where the fault occurs. If a fault occurs in the excitation circuit for the first rotor R1, for example a failure of one or more brushes 7, this fault operating mode, i.e. the fault-tolerant operating mode, corresponds to the singular operating mode SM described above. No excitation current flows and the electric machine 1 only operates with the permanent magnet excited second rotor R2.

[0048] If the permanent magnets 3 are slightly demagnetized, the lack of flux density in the air gap can be compensated for by increasing the excitation current. In this case, the excitation current is increased accordingly. If the permanent magnets 3 are completely demagnetized, the electric machine 1 continues to operate only with the current-excited first rotor R1 and can thus continue to reliably provide a torque T.

[0049] Another operating mode available is the strong field-weakening SSM operating mode. In the conventional axial flux electric machine with permanent magnet-excited rotors, the field weakening is achieved by injecting a negative d-current into the stator 5. However, the electric machine 1 described here, designed as a hybrid-excited axial flux electric machine, can achieve strong field weakening by injecting a negative excitation current. This is only possible if the coils 2 are controlled by an H-bridge, as shown in Fig. 1. If the second and third switches S2, S3 are activated, ie closed, a negative excitation current can flow through the coils 2, which generates a magnetic field in the opposite direction to the magnetic field generated by the permanent magnets 3 in the second rotor R2, as shown in Fig. 6. In this case, the remaining flux in the air gap is reduced, and strong field weakening is achieved. This operating mode, i.e., the strong field weakening operating mode SSM, can operate the electric machine 1 at a higher speed n than the maximum speed n possible with the exclusively permanent magnet excited conventional axial flux electric machine with two rotor disks each equipped with permanent magnets 3. max and ensure safe failure without active short circuit.

[0050] The electric machine 1 described here offers the possibility of switching between the different operating modes depending on the operating point of the electric machine 1.

[0051] In Fig. Figure 7 shows a comparison between the conventional axial flux electric machine with permanent magnet excitation and two rotor disks each equipped with permanent magnets 3 and the electric machine 1 described here, which is designed as a hybrid-excited axial flux electric machine. A typical torque-speed curve K1 of the conventional axial flux electric machine with permanent magnet excitation is shown. Where T nmax a maximum torque of the conventional axial flux electric machine, n base a base speed of the conventional axial flux electric machine and n max the maximum speed of the conventional axial flux electric machine. Normally, ie in the case of the exclusively permanent magnet excited conventional axial flux electric machine with two rotor disks each equipped with permanent magnets, the basic speed is n base about 30% of the maximum speed n max. The speed range up to the base speed n base is the range of constant torque T. The speed range from the base speed n base is the area of ​​field weakening.

[0052] Also shown is an available torque-speed curve K2 of the electric machine 1 described here, which is designed as a hybrid-excited axial flux electric machine, in the singular operating mode SM. Here, T smax a maximum torque in the singular operating mode SM and is 67% or about 67% of the maximum torque T nmax of the conventional axial flux electric machine. By reducing the magnetic flux density in the air gap, the electric machine 1 is able to generate more torque T in the field weakening range and a higher speed n than the maximum speed n max the conventional axial flux electric machine.

[0053] The strong field-weakening operating mode SSM of electric machine 1 can further increase the maximum speed into the overspeed range.

[0054] Also shown is an extended torque-speed curve K3 of electric machine 1 in the field-strengthening operating mode FVM. Where T emax a maximum torque in this field-strengthening operating mode FVM. The flux density in the air gap is increased by feeding more excitation current into the coils 2. With the same stator currents, the electric machine 1, designed as a hybrid-excited axial flux electric machine, can produce approximately 10% more torque T than the maximum torque T nmax the conventional axial flux electric machine.

[0055] Due to the multiple operating modes and the enabled switching between the operating modes of the electric machine 1 designed as a hybrid-excited axial flux electric machine, the performance of the electric machine 1 is improved and the torque-speed curve is extended to cover more operating points.

[0056] The preferred operating range of the different operating modes is also in Fig. 7. The dual operating mode DM is particularly active in the range of low speed n and high torque T. Mainly depending on the current operating point, switching occurs between the normal operating mode NM, the field-strengthening operating mode FVM, and the field-weakening operating mode FSM in order to meet the requirements for torque T and efficiency.

[0057] In the range of a given low limit torque T s , which is 10% or about 10% of the maximum torque T nmaxof the conventional axial flux electric machine, and a high speed n, the singular operating mode SM is activated in order to achieve a higher torque T and better efficiency. The singular operating mode SM is used in a torque-speed range between the torque-speed curve of conventional axial flux electric machines K1 and the torque-speed curve K2 of the electric machine 1 described here, which is designed as a hybrid-excited axial flux electric machine.

[0058] In the very high speed range, ie especially above a given high limit speed n s , which is 85% or about 85% of the maximum speed n max of the conventional axial flux electric machine, and in the overspeed range the strongly field-weakening operating mode SSM is activated so that the controllability of the electric machine 1 can be ensured in this speed range.

[0059] Depending on the machine parameters, the ranges of the various operating modes may vary slightly. The limits of the various operating modes are also not absolutely fixed. Some factors, such as temperature and aging, can influence the selection of the operating mode and thus change the limits of the operating modes.

[0060] The electric machine 1 described here is intended in particular for use as an electric drive machine for driving a vehicle, in particular an electric vehicle or hybrid vehicle. Typical applications for the operating mode switching of such an electric machine 1 used as an electric drive machine in a vehicle are described below. If the vehicle starts off on a country road, accelerates up to a cruising speed and then continues at this cruising speed, ie at a medium speed and a low requested torque T, the electric machine 1 starts in the normal operating mode NM and then, in particular when the requested torque T exceeds the limit torque T s falls below, switches to the singular operating mode SM in order to achieve good efficiency.

[0061] For overtaking maneuvers in the city or on country roads, for example, electric motor 1 is switched from the singular operating mode SM to the dual operating mode DM, i.e., the first rotor R1 is excited by the excitation current. In particular, depending on the speed n and the requested torque T, the switchover occurs, for example, to the normal operating mode NM, to the field-strengthening operating mode FVM, or to the field-weakening operating mode FSM of this dual operating mode DM. After just a few milliseconds, the excited first rotor R1 can deliver the additional torque T for the overtaking maneuver.

[0062] At a race start, ie especially when starting with maximum possible acceleration, the vehicle can start in the field-strengthening operating mode FVM, so that the maximum torque T emax the electric machine 1 can be released immediately.

[0063] During motorway journeys where field weakening is required, the electric machine 1 can be switched between the field-weakening operating mode FSM and the singular operating mode SM depending on the speed and torque requirements in order to achieve an optimization of the efficiency.

[0064] In the overspeed range, especially from the limit speed n s , the controllability of the electric machine 1 can be ensured by switching to the strong field weakening operating mode SSM.

[0065] In the event of a fault, for example in the event of an excitation failure of the first rotor R1 or in the event of demagnetization of the permanent magnets 3 of the second rotor R2, a plausible constant torque T can still be generated by the other rotor R2, R1 which is still excited, so that the vehicle can still be driven to a workshop for repairs, for example.

[0066] Even if the active short circuit in the inverter is not available, for example due to open phase or thermal overload of the semiconductor switches in the traction inverter, the strong field weakening operating mode SSM can protect the entire system from electrical breakdown and other damage due to the high induced voltage by the magnetic field. List of reference symbols 1 electric machine 2 coils 3 permanent magnet 4 Rotor shaft 5 Stator 6 rolling bearings 7 Brush 8 Slip ring 9 Spring element 10 Bridge circuit 11 winding tunnels A+ positive connection A- negative terminal DM dual operating mode F magnetic flux FSM field-weakening operating mode FVM field-strengthening operating mode n speed N North Pole NM normal operating mode K1 Torque-speed curve conventional axial flux electric machine K2 torque-speed curve singular operating mode K3 torque-speed curve field-strengthening operating mode R1 first rotor R2 second rotor S South Pole S1 first switch S2 second switch S3 third switch S4 fourth switch SM singular operating mode SSM strong field weakening operating mode n base Base speed of conventional axial flux electric machine n max Maximum speed of conventional axial flux electric machine n s Limit speed T torque T emax maximum torque in field-strengthening operating mode T nmax maximum torque conventional axial flux electric machine T s Limit torque Tsmax maximum torque in singular operating mode QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 101 41 872 A1

[0002]

Claims

[1] Electric machine (1) with a first rotor (R1) which has a plurality of electrical coils (2), and a second rotor (R2) which has a plurality of permanent magnets (3), wherein a stator (5) is arranged between the first and the second rotor (R1, R2), and wherein the rotors (R1, R2) are designed as discs and the electric machine (1) is designed as a disc rotor. [2] Method for operating an electric machine (1) according to claim 1, characterized by , that - a dual operating mode (DM) is provided in which the first rotor (R1) is supplied with electrical energy, - a singular operating mode (SM) is provided in which the electrical energy supply of the first rotor (R1) is switched off, and - a strong field weakening operating mode (SSM) is provided, in which a current flow opposite to that in the dual operating mode (DM) is generated in the first rotor (R1). [3] Method according to claim 2, characterized by that the dual operating mode (DM) is divided into a normal operating mode (NM), in which a magnetic field of the first rotor (R1) and a magnetic field of the second rotor (R2) are equally strong, a field-weakening operating mode (FSM), in which the magnetic field of the first rotor (R1) is weaker than the magnetic field of the second rotor (R2), and a field-strengthening operating mode (FVM), in which the magnetic field of the first rotor (R1) is stronger than the magnetic field of the second rotor (R2). [4] Method according to claim 2 or 3, characterized by that in the dual operating mode (DM) at speeds (n) above a base speed (n base) is switched to the field-weakening operating mode (FSM), in which a change in the energy supply of the first rotor (R1) takes place, so that the magnetic field of the first rotor (R1) is weaker than the magnetic field of the second rotor (R2). [5] Method according to one of claims 2 to 4, characterized by that the base speed (n base ) 30% of a maximum speed (n max ) amounts. [6] Method according to one of claims 2 to 5, characterized by that the dual operating mode (DM) is switched to the singular operating mode (SM) when a requested torque (T) for low speeds (n) is below a limit torque (T s ) falls. [7] Method according to claim 6, characterized by that the limit torque (T s ) 10% of a maximum torque (T nmax ) amounts. [8] Method according to one of claims 2 to 7, characterized by that above a given limit speed (n s) is switched to the strong field weakening operating mode (SSM). [9] Method according to one of claims 2 to 8, characterized by that above a requested maximum torque (T nmax ) is switched to the field strengthening operating mode (FVM). [10] Method according to one of claims 2 to 9, characterized by that switching between the operating modes is carried out by changing a current intensity and / or direction of an excitation current for the first rotor (R1).

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