Method for fast de-excitation of rotor of externally excited synchronous machine for motor vehicle, externally excited synchronous machine for motor vehicle, and motor vehicle

Abstract

The invention relates to a method for the rapid de-excitation of a rotor (4) of an externally excited synchronous machine (2) of a motor vehicle (1), the motor vehicle (1) or the synchronous machine (2) comprising an excitation circuit (10) having at least one transistor (11, 12) for controlling the rotor current of the synchronous machine (2),-when the motor vehicle (1) or the synchronous machine (2) is in normal operation, the motor vehicle (1) or the synchronous machine (2) is de-excited by the excitation circuit (10). The at least one transistor (11, 12) is operated in a switching mode, in which the at least one transistor implements an electronic switch,-the at least one transistor (11, 12) is operated in a resistive mode, in which the at least one transistor implements a resistance, during an emergency operation of the motor vehicle (1) or the synchronous machine (2), in which the at least one transistor (11, 12) is operated in the resistive mode, in which the at least one transistor implements a resistance for a rapid de-excitation of the rotor (4), the rotor current is at least partially reduced by means of at least one transistor (11, 12) implementing a resistance.

Description

Technical Field

[0001] The invention relates to a method for rapidly de-exciting a rotor of an externally excited synchronous motor / separately excited synchronous motor for a motor vehicle, wherein the motor vehicle or the synchronous motor comprises an excitation circuit having at least one transistor for controlling the rotor current of the synchronous motor. In addition, the invention relates to an externally excited synchronous motor for a motor vehicle, comprising a rotor and an excitation circuit module, which realizes an excitation circuit having at least one transistor for controlling the rotor current of the synchronous motor. In addition, the invention relates to a motor vehicle. Background Art

[0002] An externally excited synchronous motor is an electric motor that is used, in particular, as a drive device for a motor vehicle configured as an electric vehicle or a hybrid vehicle. In contrast to a permanent magnet synchronous motor in which a permanent magnet is installed in or at the rotor, the rotor of an externally excited synchronous motor includes a winding composed of conductor wires that can be energized by a rotor current. The rotor current, which can also be referred to as an excitation current, induces a magnetic field of the rotor, which interacts with the magnetic field present on the stator side of the synchronous motor, thereby converting electrical energy into kinetic energy or vice versa. The voltage required for this is provided from the high-voltage battery of the motor vehicle, wherein the corresponding direct current is applied to the rotor through a loop ring. In order to control the operation of the synchronous motor, a power module, in particular an excitation circuit module for the rotor, is provided, wherein the power module has a semiconductor structural element, such as a transistor. Examples of corresponding synchronous motors are known from US2013 / 0 193903A1, DE 10 2016 219 770 A1 or DE 10 2021 211 472 A1.

[0003] When operating an externally excited synchronous machine, situations can be envisaged in which rapid deexcitation of the rotor is desired or required, i.e., the rotor current is reduced as quickly as possible and the rotor magnetic field is thus weakened. An example of such a situation is an accident in which the rotor must be deexcited as quickly as possible, i.e., the energy must be discharged and the electric drive torque must be switched off. Summary of the invention

[0004] The object of the present invention is to provide an improved approach with regard to the rapid deexcitation of the rotor of an externally excited synchronous machine.

[0005] According to the invention, in a method of the type described at the outset, this object is achieved in that, during normal operation of the motor vehicle or the synchronous machine, at least one transistor is operated in a switching mode in which the at least one transistor implements an electronic switch, and during emergency operation of the motor vehicle or the synchronous machine, at least one transistor is operated in a resistance mode in which the at least one transistor implements a resistance, wherein, for rapid deexcitation of the rotor, the rotor current is at least partially reduced by means of the at least one transistor implementing a resistance.

[0006] The transistor is a component of an excitation circuit, which is used for switching processes related to the control and energization of the rotor during normal operation of a motor vehicle or a synchronous motor. In the case of normal operation or switching mode, the transistor can be brought into two switching states / switching states by a control voltage, which can also be referred to as a gate voltage, i.e., a conducting state or an on state and a closed state or an off state. In the conducting state, the transistor is low-ohmic for the rotor current to be controlled and has a value close to 0 ohm. In the closed state, the transistor is high-ohmic for the rotor current to be controlled. Preferably, all switching states associated with at least one transistor set in normal operation can be realized by means of the conducting and closed states. In particular, normal operation is understood as a situation in which the synchronous motor is used to tow the motor vehicle or is used in the scope of energy recovery or regeneration. In general, normal operation involves an accident-free situation of a motor vehicle or a synchronous motor.

[0007] The invention is based in particular on the idea of ​​using the transistor for the purpose of deexciting the rotor as quickly as possible during emergency operation, or rather of alienating the use of the transistor by bringing the transistor into an atypical operating range in which the transistor is a preferably ohmic resistor. In this case, the transistor therefore has a resistance value between the value of a transistor that implements a closed switch and the value of a transistor that implements an open switch. During emergency operation, the rotor current or the excitation current flows through the transistor that forms the resistor, thereby consuming the rotor current accordingly. In this case, the rotor current is usually additionally reduced by other current-carrying components that respectively implement the resistor, for example by the winding of the rotor.

[0008] Preferably, the resistance value can be controlled, for example, by means of a control voltage, so that the control voltage can be adjusted in a situation-specific manner, in particular to achieve deexcitation as quickly as possible. In particular, it can be provided that at least one transistor in resistance mode is operated in a linear mode, in which the transistor realizes a variable resistance. Thus, depending on the control voltage or gate voltage, the transistor can be operated in a cut-off region, in which the off state is realized, and in a conduction region, in which the on state is realized. In the cut-off region, the control voltage is low, wherein, in the case of a collector-emitter voltage or a source-drain voltage occurring at the transistor, the resulting rotor current is approximately zero, which is a collector-emitter current or a source-drain current in the transistor. In the conduction region, the control voltage is high enough so that the rotor current can flow through the transistor. Between the cut-off region and the conduction region there is a linear region, which may be narrow, in which the resistance decreases substantially linearly with increasing control voltage, so that in this region the rotor current increases substantially linearly with increasing control voltage. Thus, in emergency operation, the control voltage can be purposefully brought into this linear region. Furthermore, the control voltage can be purposefully adjusted within this linear region to a value such that the resulting resistance is as favorable or suitable as possible for the current situation.

[0009] In the method according to the invention, it can be provided that during emergency operation, the excitation circuit is brought into a switching state in such a way that the rotor current in the freewheeling circuit (Freilauf) containing at least one transistor implementing a resistor is reduced. In this embodiment, the excitation circuit is switched in such a way that a closed current loop is obtained in which the rotor current circulates. The high-voltage battery of the motor vehicle is not contained in this current loop, so that the rotor current runs only through the inductance of the winding of the rotor. Since the current circulating in this current loop flows through the transistor implementing the resistor, this current flow is correspondingly blocked or reduced.

[0010] During emergency operation, the excitation circuit can be brought alternately into a variety of different switching states, in which the rotor current is reduced by different transistors that respectively implement resistors. Therefore, it can be provided that the excitation circuit has a plurality of transistors, some of which are also used as switches during emergency operation, so that the remaining transistors that are used as resistors can be energized by the rotor current. For example, it is conceivable that in each of these possible switching states, one of the transistors is used as a resistor energized by the rotor current, while the switch position of the remaining one or more transistors is such that the transistor that implements the resistor is energized. This approach is advantageous because the operation of the transistor as a resistor causes it to heat up, wherein alternating the operation of the transistor as a resistor results in uniform heat dissipation on different transistors. This simplifies the cooling that may be required and prolongs the service life of these components.

[0011] Preferably, an asymmetrical full-bridge circuit with a plurality of transistors is used as the excitation circuit. Preferably, an insulated gate bipolar transistor, a so-called IGBT, or a metal oxide semiconductor field effect transistor, a so-called MOSFET, is used as the at least one transistor. Different power semiconductor technologies are conceivable, for example Si-IGBT and / or Si-MOSFET and / or silicon carbide MOSFET.

[0012] According to a possible specific implementation, a full-bridge circuit with two transistors is used as an excitation circuit, wherein, during emergency operation, the excitation circuit is alternately brought into two different switching states, wherein, in one of the two switching states, the rotor current is reduced in the high-side freewheeling circuit / high-side freewheeling circuit with the aid of one of the transistors, and wherein, in the other of the two switching states, the rotor current is reduced in the low-side freewheeling circuit / low-side freewheeling circuit with the aid of the other of the transistors.

[0013] For normal operation, it can be provided that during normal operation the field circuit is brought into a switching state such that a rotor current is built up. This corresponds to starting the synchronous machine or to a situation in which a drive torque for a motor vehicle is generated by means of the synchronous machine.

[0014] During normal operation, the field circuit can be brought into a switching state such that electrical energy is fed back from the synchronous machine to the high-voltage battery and / or the intermediate circuit capacitor of the motor vehicle. In this case, the kinetic energy of the motor vehicle or the rotor is converted or regenerated into electrical energy, which is again stored in the high-voltage battery and / or the intermediate circuit capacitor.

[0015] Furthermore, it is conceivable that during normal operation the field circuit is brought into a switching state such that the rotor current is reduced, in particular in the freewheeling circuit. This is a planned deexcitation of the rotor which is slower than the rapid deexcitation described above.

[0016] According to the invention, the object is also achieved in an externally excited synchronous machine of the type mentioned at the outset in that the externally excited synchronous machine has a control device which is set up to apply a control voltage to the control connection, in particular the gate connection, of at least one transistor in such a way that during normal operation of the synchronous machine the at least one transistor can be operated in a switching mode in which the at least one transistor implements an electronic switch and during emergency operation of the synchronous machine the at least one transistor can be operated in a resistance mode in which the at least one transistor implements a resistor, wherein the rotor current can be at least partially reduced for rapid deexcitation of the rotor by means of the at least one transistor implementing a resistor. The synchronous machine according to the invention can be multiphase, in particular three-phase. All advantages, features and aspects explained in conjunction with the method according to the invention can likewise be transferred to the externally excited synchronous machine according to the invention and vice versa.

[0017] In the externally excited synchronous motor according to the present invention, it can be provided that the high-voltage battery is electrically connected to the rotor via an excitation circuit module. Alternatively or additionally, it is conceivable that the high-voltage battery is electrically connected to the stator of the synchronous motor via a main converter module that implements the main converter. The excitation circuit module can have a power level of 15 kW. The main converter module can be implemented as a B6 bridge circuit. The excitation circuit module and / or the main converter module can be referred to as a power module. The corresponding module can have a circuit board having semiconductor components connected thereto, in particular soldered thereto. The rotor current or the excitation current can be supplied to the rotor via electrical sliding contacts.

[0018] According to the invention, it can be provided that the field circuit module and / or the main converter module are coupled to at least one intermediate circuit capacitor. An intermediate circuit capacitor, which can also be referred to as an intermediate circuit capacitor, is a capacitor arranged in an intermediate circuit of a power electronics system. With the aid of the intermediate circuit capacitor, electrical energy can be stored temporarily, in particular during fast transient processes. It is therefore preferred that the field circuit module and the main converter module are physically connected to the at least one intermediate circuit capacitor and are fed by the intermediate circuit capacitor. The at least one intermediate circuit capacitor is in turn connected to a high-voltage battery.

[0019] It is particularly preferred that the synchronous machine according to the invention has at least one cooling module, which is thermally coupled to the field circuit module, so that at least one transistor for reducing the rotor current can be cooled by at least one cooling module during emergency operation. As already mentioned above, the consumption of electrical energy by the transistor leads to the generation of heat. The cooling module can be configured as a cooling body, in particular a cooling plate, at which the field circuit module and possible main converter module are arranged. The cooling effect achieved by means of the cooling module can be caused by air cooling. Therefore, the cooling module can have corresponding heat sinks such as cooling ribs or cooling fins, through which heat can be output from the cooling module to the ambient air. Additionally or alternatively, the cooling effect can be caused by a cooling liquid, in particular a circulating cooling liquid, wherein the cooling module can have a cooling channel through which the cooling liquid flows. Therefore, the cooling module is integrated or can be integrated into a cooling cycle, in particular a motor vehicle. A cooling liquid, in particular water or a water-glycol mixture circulates in the cooling cycle. A cooling element, such as, for example, a heat exchanger can be integrated into the cooling cycle. A refrigerator can be realized by means of the cooling cycle.

[0020] Furthermore, the object of the present invention is achieved by a motor vehicle, which according to the present invention comprises an externally excited synchronous motor according to the above description and a high-voltage battery for supplying power to the externally excited synchronous motor. All advantages, features and aspects explained in conjunction with the method according to the present invention and the externally excited synchronous motor according to the present invention can be transferred to the motor vehicle according to the present invention in the same manner, and vice versa. The motor vehicle according to the present invention can also be referred to as an electric vehicle or a hybrid vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Further advantages and details of the invention emerge from the exemplary embodiments described below and with reference to the drawings.

[0022] Schematically:

[0023] Figure 1 A schematic diagram of a motor vehicle according to the invention according to an embodiment is shown, the motor vehicle comprising an externally excited synchronous machine according to the invention according to an embodiment, wherein in the motor vehicle, a method according to the invention according to an embodiment is performed,

[0024] Figure 2 Shows Figure 1 Circuit diagram of a power electronics system of a motor vehicle,

[0025] Figure 3 Shows that there is Figure 2 The coordinate system of the characteristic curves of the transistors of the power electronic system, and

[0026] Figure 4-7 Shows the different switching states Figure 2Circuit diagram of the excitation circuit of a power electronic system. DETAILED DESCRIPTION

[0027] Figure 1 A motor vehicle 1 according to the invention is shown according to one exemplary embodiment, which comprises an externally excited synchronous machine 2 according to the invention according to one exemplary embodiment. The method according to the invention according to one exemplary embodiment is explained below with reference to the motor vehicle 1 and the synchronous machine 2 .

[0028] The motor vehicle 1 includes a high-voltage battery 3, which is configured to supply current to the synchronous motor 2 and can also be referred to as a high-voltage energy storage device. The synchronous motor 2 is an electric motor having a rotor 4 and a stator 5, which is configured to traction the motor vehicle. In addition, the synchronous motor 2 includes an excitation circuit module 6 and a main converter module 7, both of which are configured to control and energize the windings of the rotor 4 and the stator 5. The control device 8 of the synchronous motor 2 is configured to control these power modules. Although in the illustrated embodiment, the modules 6, 7 and the control device 8 are components of the synchronous motor 2, that is, the synchronous motor 2 forms a structural unit together with these components, they can also be arranged in a distributed manner on the motor vehicle 1.

[0029] Below, reference Figure 2 The details related to modules 6 and 7 are explained, wherein the figure shows a circuit diagram of a power electronics system of a motor vehicle 1, which includes a high-voltage battery 3 and modules 6 and 7. Modules 6 and 7 each include a printed circuit board or circuit board, on which the electronic components are soldered. The main converter module 7 realizes the main converter 9 and connects the stator 5 to the high-voltage battery 3. The main converter 9 is a B6 bridge circuit, so that the synchronous machine 2 is correspondingly three-phase.

[0030] The excitation circuit module 6 implements an excitation circuit 10 for controlling the rotor current or excitation current of the synchronous motor 2. This means that the rotor 4 is connected to the high-voltage battery 3 via the excitation circuit module 6 or via the excitation circuit 10. The excitation circuit 10 has two transistors 11 and 12 and implements an asymmetric full-bridge circuit. By way of example, the transistors 11 and 12 are insulated gate bipolar transistors, abbreviated to IGBTs, but may also be metal oxide semiconductor field effect transistors, abbreviated to MOSFETs. In order to control the transistors 11 and 12, a control device 8 is provided, which generates a corresponding control voltage or gate voltage. Figure 2 In the diagram, the control device 8 is only schematically shown, wherein for the sake of clarity, the lines between the control device 8 and the modules 6 , 7 are not shown.

[0031] In this power generation system, a plurality of intermediate circuit capacitors 13 are incorporated. Specifically, both the field circuit 10 and the main converter 9 are coupled to the intermediate circuit capacitor(s) 13 .

[0032] In order to better understand the method according to the present invention, now refer to Figure 3 , which shows a characteristic curve 14 for each of the two transistors 11, 12. The abscissa 15 shows the value of the control voltage at the gate of the respective transistor 11, 12, which can be generated by means of the control device 8. The ordinate 16 shows the value of the resulting current through the respective transistor 11, 12, i.e. the collector-emitter current or source-drain current at the collector-emitter voltage or source-drain voltage at the respective transistor 11, 12.

[0033] It can be seen that the transistors 11, 12 each have different operating regions, which have different characteristics in relation to the characteristic curve 14. The transistors 11, 12 can be operated in a cut-off region 17, which realizes the off state, and in a conduction region 18, which realizes the on state. In the regions 17, 18, the transistors 11, 12 realize a switch for the collector-emitter current or the source-drain current, which can be switched on and off or opened and closed by a corresponding control voltage. If the control voltage is low and the transistors 11, 12 are therefore operated in the cut-off region 17, the resulting rotor current is close to zero. In this case, a high-ohmic resistance is present in relation to the rotor current and the switch realized by the respective transistors 11, 12 is therefore open. At sufficiently high values ​​of the control voltage, the transistors are operated in the conduction region 18, in which the rotor current can flow through the respective transistors 11, 12. In this case, a low-ohmic resistance is present in relation to the rotor current, which is also realized by the respective transistors 11, 12. Between the regions 17 , 18 there is a narrow linear region 19 in which the resistance realized by means of the respective transistor 11 , 12 decreases substantially linearly with increasing control voltage, so that in the linear region 19 the rotor current increases substantially linearly with increasing control voltage.

[0034] The following references Figures 4 to 7 , which respectively show the excitation circuit 10 in different switching states. The motor vehicle 1 or the synchronous machine 2 can operate in normal operation and in emergency operation. Figures 4 to 7 The switching state shown in FIG. 1 can exist in normal operation, wherein Figure 6 and Figure 7 The switching states shown in can additionally be set in the case of emergency operation. In the case of emergency operation, for example due to an accident, the rotor 4 is quickly de-excited. In addition, Figures 4 to 7 2 and 3 show the ohmic resistance 20 and the inductance 21 realized by the windings of the rotor 4. In these figures, the current flow is shown by means of lines indicated by bold lines and corresponding arrows.

[0035] Figure 4The switching state of the excitation circuit 10 is shown in normal operation, in which a rotor current is established. In this state, the control voltage present at the gates of the two transistors 11, 12, which is generated by means of the control device 8, causes the two transistors 11, 12 to be in the conductive region 18. The transistors 11, 12 thus each implement a closed switch. In this state, an excitation field is established or maintained in the rotor 4, or a drive torque is generated for the motor vehicle 1.

[0036] Figure 5 The following switching state of the excitation circuit 10 is shown in the case of normal operation, in which current is fed back into the high-voltage battery 3 or the intermediate circuit capacitor 13. In this state, the control voltage generated by means of the control device 8, which is present at the gate of the two transistors 11, 12, is such that the two transistors 11, 12 are in the cut-off region 17. Therefore, the transistors 11, 12 each realize an open switch. Specifically, in this state, a deceleration torque is generated for the motor vehicle 1, and the motor vehicle is accordingly in the regeneration mode.

[0037] Figure 6 A switching state of the excitation circuit 10 is shown, which is first explained based on normal operation. In this state, the control voltage generated by the control device 8 at the gates of the transistors 11, 12 is such that the transistor 11 is in the conductive region 18 and the transistor 12 is in the blocked region 17. In this switching state, the rotor current driven by the inductance 21 circulates through the winding of the rotor 4 and the transistor 11 within the scope of the high-side freewheeling loop.

[0038] Figure 7 A switching state of the excitation circuit 10 is shown, which is also explained first based on normal operation. In this state, the control voltage generated by the control device 8 at the gates of the transistors 11, 12 is such that the transistor 11 is in the blocking region 17 and the transistor 12 is in the conducting region 18. In this switching state, the rotor current driven by the inductance 21 circulates through the winding of the rotor 4 and the transistor 12 within the scope of the low-side freewheeling loop.

[0039] exist Figure 6 and Figure 7 In the switching state shown in , the winding of the rotor 4 is disconnected from the high-voltage battery 3 and the intermediate circuit capacitor 13, so that the rotor current is consumed by the resistor 20. In other words, the electrical energy stored in the rotor current is converted into heat energy. The rotor 4 is de-excited. However, this requires some time, which in certain situations, such as in the event of an accident, lasts too long. Against this background, the present invention proposes a solution for quickly de-exciting the rotor 4 as quickly as possible. This so-called emergency operation is Figure 6 and Figure 7This is achieved in the case of the switching states shown, which are explained below.

[0040] refer to Figure 6 , the control voltage applied to the transistor 11 is generated in such a way that the transistor is no longer in the conductive region 18, but instead in the linear region 19. Accordingly, the transistor 11 through which the rotor current flows no longer merely realizes a closed switch, but instead realizes an ohmic resistance. Therefore, the electrical energy is not consumed solely by means of the resistor 20, but also by means of the transistor 11, which also realizes a resistance. Therefore, the deexcitation of the rotor 4 is correspondingly carried out more quickly than in normal operation. The control voltage can be adjusted, for example, according to the characteristic curve 14, so that the resistance realized by means of the transistor 11 has an ideal value specific to the situation.

[0041] Similarly, in Figure 7 In the case of the switching state shown in FIG. 1 , the control voltage applied to transistor 12 is generated so that this transistor is also in the linear region 19. Therefore, in this case, Figure 7 In the state shown in , not only the resistor 20 but also the transistor 12 which also realizes a resistor is used for demagnetizing the rotor 4. Here, too, the realized resistance can be adjusted in a targeted manner in a case-specific manner.

[0042] The optional aspects related to the emergency operation are explained below. For this purpose, it is provided, for example, that the excitation circuit 10 is alternately switched from the control voltage generated by the control device 8 by means of a corresponding periodic change. Figure 6 The switch state shown in the figure changes to Figure 7 . In these different switching states, the rotor current is reduced alternately via the transistors 11, 12. The resistive heating that occurs during the consumption of the corresponding electrical energy is therefore distributed to the two transistors 11, 12 and is therefore reduced for the individual transistors 11, 12.

[0043] In order to dissipate heat from the transistors 11, 12, in particular with reference again to Figure 1 In the case of the motor vehicle 1, a cooling module 22 is provided which is thermally coupled to the excitation circuit module 6. The cooling module 22 as a cooling plate is incorporated into a cooling circuit 23 of the motor vehicle 1, wherein a coolant, such as water, circulates in the cooling circuit 23 having a heat exchanger 24. The cooling module has cooling channels through which the coolant flows. In addition, the cooling module 22 has cooling ribs or cooling fins in order to output heat to the ambient air.

Claims

1. A method for rapidly deexciting a rotor (4) of an externally excited synchronous motor (2) of a motor vehicle (1), characterized in that: A motor vehicle (1) or a synchronous machine (2) comprises an excitation circuit (10) for controlling a rotor current of the synchronous machine (2), the excitation circuit having at least one transistor (11, 12). - during normal operation of the motor vehicle (1) or the synchronous machine (2), operating the at least one transistor (11, 12) in a switching mode in which the at least one transistor implements an electronic switch, - During emergency operation of the motor vehicle (1) or the synchronous machine (2), the at least one transistor (11, 12) is operated in a resistance mode, in which the at least one transistor implements resistance, wherein the rotor current is at least partially reduced by means of the at least one transistor (11, 12) implementing resistance for rapid deexcitation of the rotor (4).

2. The method according to claim 1, characterized in that The at least one transistor (11, 12) in the resistance mode is operated in a linear mode in which the at least one transistor realizes a variable resistance.

3. The method according to claim 1 or 2, characterized in that: During emergency operation, the excitation circuit (10) is placed in a switching state such that the rotor current is reduced in the freewheeling circuit of the at least one transistor (11, 12) incorporating a control resistor.

4. The method according to any one of the preceding claims, characterized in that During emergency operation, the field circuit (10) is placed alternately in a plurality of different switching states, in which the rotor current is reduced by means of different transistors (11, 12) which each implement a resistance.

5. The method according to any one of the preceding claims, characterized in that An asymmetrical full-bridge circuit having a plurality of, in particular two, transistors (11, 12) is used as the excitation circuit (10), and / or an insulated gate bipolar transistor or a metal oxide semiconductor field effect transistor is used as the at least one transistor (11, 12).

6. A method according to any one of the preceding claims, wherein: A full bridge circuit having two transistors (11, 12) is used as an excitation circuit (10), The invention is characterized in that, during emergency operation, the excitation circuit (10) is alternately placed in two different switching states, wherein in one of the switching states, the rotor current is reduced in the high-side freewheeling circuit by means of one of the transistors (11, 12), and in the other of the switching states, the rotor current is reduced in the low-side freewheeling circuit by means of the other of the transistors (11, 12).

7. The method according to any one of the preceding claims, characterized in that During normal operation, the excitation circuit (10) is in a switching state in which a rotor current is built up, either so that electrical energy is fed back from the synchronous machine (2) to a high-voltage battery (3) and / or an intermediate circuit capacitor (13) of the motor vehicle (1), or the rotor current is reduced, in particular in a freewheeling circuit.

8. An externally excited synchronous motor for a motor vehicle (1), the externally excited synchronous motor comprising a rotor (4), an excitation circuit module (6) and a control device (8), the excitation circuit module realizing an excitation circuit (10) having at least one transistor (11, 12) for controlling the rotor current of the synchronous motor (2), the control device being configured to apply a control voltage to a control connection terminal of the at least one transistor (11, 12) so that - during normal operation of the synchronous machine (2), the at least one transistor (11, 12) can be operated in a switching mode in which the at least one transistor implements an electronic switch, - during emergency operation of the synchronous machine (2), the at least one transistor (11, 12) can be operated in a resistance mode, in which the at least one transistor implements a resistance, wherein For rapid deexcitation of the rotor (4), the rotor current can be at least partially reduced by means of the at least one transistor (11, 12) implementing a resistor.

9. The externally excited synchronous motor according to claim 8, characterized in that: The high-voltage battery (3) is electrically connected to the rotor (4) via an excitation circuit module (6), and / or the high-voltage battery (3) is electrically connected to the stator (5) of the synchronous motor (2) via a main converter module (7) that implements a main converter (9).

10. The externally excited synchronous motor according to claim 8 or 9, characterized in that: The field circuit module (6) and / or the main converter module (7) are coupled to at least one intermediate circuit capacitor (13).

11. The externally excited synchronous motor according to any one of claims 8 to 10, characterized in that: At least one cooling module (22) is provided, which is in particular incorporated or can be incorporated into a cooling circuit (23), and is thermally coupled to the field circuit module (6), so that during emergency operation, the at least one transistor (11, 12) for reducing the rotor current can be cooled by the at least one cooling module (22).

12. A motor vehicle, characterized in that: The motor vehicle comprises an externally excited synchronous motor (2) according to any one of claims 8 to 11, and the motor vehicle further comprises a high-voltage battery (3) for supplying power to the externally excited synchronous motor (2).

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