Motor vehicle with a separately excited synchronous machine and method for the active discharge of a capacitor in a high-voltage network

Abstract

The invention relates to a motor vehicle (1) comprising a separately excited synchronous machine (2), an excitation circuit (4) associated with the separately excited synchronous machine (2) for supplying current to the excitation winding (24), and a power electronics arrangement (7) connected to a high-voltage network (5), in particular comprising an inverter (8) associated with the synchronous machine (2), with at least one high-voltage network-side capacitor, in particular an intermediate circuit capacitor (19) and / or an interference suppression capacitor, wherein the excitation circuit (4) has at least one semiconductor switch (31, 33) in a discharge path (34) connecting the terminals (20) of the high-voltage network (5) and not comprising the excitation winding (24), wherein the motor vehicle (1) further comprises a control device (10) for controlling the excitation circuit (4), which is used for the active discharge of the at least one capacitor by closing the at least one semiconductor switch (31, 33).33) the unloading section (34) is formed.

Description

[0001] The invention relates to a motor vehicle comprising a separately excited synchronous machine, an excitation circuit associated with the separately excited synchronous machine for supplying current to the excitation winding, and a power electronics arrangement connected to a high-voltage network, in particular comprising an inverter associated with the synchronous machine, with at least one high-voltage network-side capacitor, in particular an intermediate circuit capacitor and / or an interference suppression capacitor, wherein the excitation circuit comprises a bridge circuit forming at least one half-bridge with four branches, each connecting a terminal point of the excitation winding to a terminal of the high-voltage network and each branch comprising a freewheeling diode, wherein the excitation circuit comprises at least one semiconductor switch in a discharge path connecting the terminals of the high-voltage network and not comprising the excitation winding.wherein the motor vehicle further comprises a control device for controlling the excitation circuit, which is configured for the active discharge of the at least one capacitor by closing the at least one semiconductor switch of the discharge path, wherein the at least one discharge path comprises two branches connecting the terminals of the high-voltage network with semiconductor switches bridging the respective freewheeling diodes. The invention also relates to a method for the active discharge of the at least one capacitor in such a motor vehicle.

[0002] Hybrid and electric vehicles use electric machines with corresponding power electronics systems or components connected to a vehicle's high-voltage electrical system (hereinafter referred to as the high-voltage network). These power electronics systems can include, for example, drive inverters, high-voltage DC converters, onboard chargers, electric air conditioning compressors, and / or electric heaters connected to the high-voltage network. The operating voltage of the high-voltage network is typically more than 200 V and can range, for example, from 350 V to 860 V.

[0003] Many such high-voltage components, i.e., power electronics assemblies, may have capacitors on the connection side to the high-voltage network, in particular DC link capacitors and / or interference suppression capacitors, for example, X-class interference suppression capacitors. For reasons of operational safety and functional safety, such capacitors are usually equipped with active discharge circuits, which allow the capacitor to be discharged quickly, for example, in the event of a fault.

[0004] Current discharge circuits typically consist of a semiconductor switching element, such as an IGBT or a MOSFET, and a resistive load, i.e., a discharge resistor, through which the energy is converted into heat during active discharge. Active discharge is subject to specific discharge requirements, such as prescribed discharge times and / or prescribed minimum voltage levels.

[0005] Numerous methods for implementing active discharge via a discharge circuit are known in the prior art. The problem here is that the discharge resistor must be very large, and in particular, massive, due to the high DC link voltage (or generally the voltage across the capacitor) and the high discharge power required. This massive design, necessitated by the high thermal load, results in a large, space-consuming discharge resistor that must be installed separately within the vehicle, for example, within a housing containing the power electronics. One suggestion has been to screw cement resistors into such a housing.

[0006] As an alternative to bulky, space-consuming discharge resistors, it has also been suggested to use multiple pulse-resistant SMD thick-film resistors for active discharge. However, these SMD thick-film resistors are extremely expensive and, due to their quantity, require a considerable area on the circuit board. When used for active discharge, they generate a significant amount of heat on the circuit board.

[0007] In summary, the prior art uses discharge circuits for active discharge, implemented with discrete components, for example on conventional printed circuit boards. The components of the discharge circuit are cooled by free convection in ambient air, particularly inside a housing containing the power electronics assembly.

[0008] Regarding functional safety, it must also be ensured that active discharge can occur multiple times in direct succession, meaning that electrical energy can be converted into heat several times consecutively. This places additional high demands on the robustness and lifespan of the discharge circuits. In total, this results in extremely high demands on the thermal design of the discharge circuit, especially on the discharge resistor. It may even be necessary to use multiple discharge resistors. Consequently, a number of disadvantages exist.

[0009] The current state of the art involves high material costs, for example, for discharge resistors, assembly and interconnection technology, the connection of the discharge resistors, the necessary additional temperature measurement for monitoring the discharge resistors, and also due to the need for oversizing the discharge circuit because of poor cooling. Development costs and effort for the discharge circuits are very high, as the discharge resistors must be integrated on a printed circuit board and / or within the housing containing the power electronics assembly. The discharge circuit must be designed with sufficient thermal mass, and additional filters and interference suppression measures for temperature measurement may be necessary. Finally, the discharge circuit and / or the discharge resistors exhibit poor robustness, particularly due to thermal stress.

[0010] DE 10 2014 202 717 B3 discloses a system for determining the capacitance of an intermediate circuit capacitor and a method for controlling an inverter. For capacitance measurement, it is proposed to at least partially discharge the intermediate circuit capacitor, which supplies an inverter with DC voltage, via the inverter's semiconductor switches in linear or active operation, so that the heating of the semiconductor switches can be related to the voltage drop across the intermediate circuit capacitor. A thermal model is used to determine the thermal energy dissipated during the discharge.

[0011] CN 107196546 A discloses an active discharge system for a motor controller. It proposes operating semiconductor switches in a converter, to which a DC link capacitor is associated, in a linear range so that they can provide a discharge resistance.

[0012] US 2019 / 0320549 A1 discloses an inverter module for powering an electric vehicle. The inverter housing assembly can include a circuit board for active discharge, which comprises an active discharge circuit.

[0013] The generic patent DE 10 2020 132 571 B3 relates to a method and a device for the active discharge of a high-voltage intermediate circuit of an electric vehicle. It proposes discharging the high-voltage intermediate circuit of an electric vehicle by short-circuiting a half-bridge of an excitation circuit of the rotor of a separately excited synchronous machine. Alternatively, it can be provided that the high-voltage intermediate circuit is short-circuited via an excitation circuit of the rotor of a separately excited synchronous machine when the stator of the separately excited synchronous machine is not excited.

[0014] DE 10 2018 202 661 A1 relates to a method for discharging an intermediate circuit capacitor that is electrically coupled to at least one half-bridge, wherein the half-bridge has at least two switches connected in series. For discharge, the switches of the half-bridge are activated to close simultaneously. Specifically, the half-bridge can be part of an inverter of an electric machine.

[0015] The invention is therefore based on the objective of providing a cost-, effort- and complexity-saving, easily implementable method for discharging capacitors, in particular intermediate circuit capacitors, in a motor vehicle.

[0016] To solve this problem, the features of claim 1 are provided according to the invention for a motor vehicle of the type mentioned at the outset.

[0017] The invention thus utilizes the fact that, due to the use of a separately excited synchronous machine, an excitation circuit is also present, which can be used to implement active discharge in an integrated manner. For this purpose, the excitation circuit is designed such that a discharge path exists outside the excitation winding, thus bridging the excitation winding. This discharge path, by means of at least one semiconductor switch, establishes an electrically conductive connection between the terminals of the high-voltage network and the excitation circuit as soon as all at least one semiconductor switch is closed. Through this electrically conductive connection provided by the discharge path, all capacitors provided on the high-voltage network side, in particular DC link capacitors and / or interference suppression capacitors, can be discharged.If the control unit receives a signal indicating the need for active discharge, it can activate at least one semiconductor switch in the discharge path to close, thus enabling discharge via the discharge path. Active discharge can be implemented whenever it is expedient with regard to operational safety and functional safety. For example, such a discharge can be performed when work is being carried out on the vehicle, in the event of an accident, and / or in the event of another fault, particularly concerning the high-voltage network and the connected high-voltage components. Corresponding situations and signals are largely known with respect to the discharge circuits used in the prior art.

[0018] This requires only minor modifications to existing topologies, such as adding at least one additional semiconductor switch. Modifying the excitation circuit eliminates the need for the conventional discharge circuit known from the prior art, which was, for example, implemented as part of a converter component. No additional effort is required regarding the design or interconnection technology, as the existing excitation circuit can perform an additional function. The design is optimized, and the overall system size is significantly reduced. Furthermore, the robustness of the active discharge is also improved. As mentioned, the basis for this integration is the use of a separately excited synchronous machine, which requires an excitation circuit for its excitation power.In other words, the "active discharge" circuit section is implemented via the excitation circuit, which can be provided, for example, as an excitation power module. The at least one capacitor, particularly the DC link capacitor, can be actively discharged, for example in the event of a fault, via the discharge path, which can, for instance, form a new phase branch in the excitation circuit. A discrete discharge circuit is therefore no longer necessary.

[0019] It can be particularly advantageous to provide at least part of the discharge resistance by operating at least one of the at least one semiconductor switch in the linear range. It is therefore conceivable not to completely close at least one of the at least one semiconductor switch in the discharge path, but rather to utilize linear operation via the control device, in which this switch exhibits a specific ohmic resistance that can be used as a discharge resistance. By using at least one of the at least one semiconductor switch in the discharge path as a resistor, particularly in the sense of a "current valve," no discrete resistor needs to be provided.

[0020] In the process according to the invention, heat energy is ultimately generated during discharge, particularly in the area of ​​the excitation circuit itself, for example at the at least one semiconductor switch – especially if operated in the linear region. In this regard, an advantageous embodiment of the present invention may provide that the control device is configured for pulse-width modulated control of at least one of the at least one semiconductor switch of the discharge path during active discharge.Pulse-width modulation (PWM) allows for a very rapid switching between an open and a closed state of the controlled semiconductor switch. This makes it possible to adjust the current flow time appropriately, preventing excessive heat generation and / or thermal stress, while still ensuring that at least one capacitor can be discharged quickly enough, particularly to meet the requirements. At least one of the semiconductor switches in the discharge circuit can therefore be "switched on" or "activated."In this way, it is possible to carry out a suitably adapted discharge using pulse width modulation even at high discharge voltages, for example, intermediate circuit voltages, and / or less suitable ambient conditions, for example, with regard to the ambient temperature or the temperature at at least one of the at least one semiconductor switch. While it is of course conceivable, starting from a worst-case scenario, to set a fixed duty cycle for the pulse width modulation, a particularly advantageous embodiment of the invention provides that the control device for adjusting the duty cycle of the pulse width modulation is designed as a function of the magnitude of the discharge voltage and / or of a temperature of at least one of the at least one semiconductor switch in the discharge path, as measured by a measuring unit.In other words, the duty cycle, i.e., the length of the control pulses, of the pulse-width modulation can depend on the magnitude of the DC link voltage to be discharged and / or the semiconductor temperature. The duty cycle can be controlled by the control unit, which may be a central control unit.

[0021] In a particularly advantageous embodiment of the present invention, an existing basic configuration of the excitation circuit can be further developed by slight modification to provide the discharge path. According to the invention, the excitation circuit comprises a bridge circuit forming at least one half-bridge with four branches, each connecting a terminal of the excitation winding to a terminal of the high-voltage network and each branch including a freewheeling diode. The at least one discharge path comprises two branches connecting the terminals of the high-voltage network, with semiconductor switches bridging the respective freewheeling diodes. The use of a half-bridge in the excitation circuit allows for the provision of certain desired additional states of the excitation circuit, in particular a freewheeling and / or an active short circuit.Within the scope of the present invention, it can be particularly advantageous to provide an active short circuit, at least with respect to the excitation winding, during active discharge, which is possible in a particularly simple manner in the embodiment described here. Accordingly, the invention provides that, at least in a short-circuit path formed by one branch of the discharge path and by a further branch between the connection points of the excitation winding, a semiconductor switch bridging this freewheeling diode is also assigned to the freewheeling diode belonging to the further branch, wherein the control device for establishing a safe state during active discharge is also designed to close the semiconductor switch of the further branch to establish an active short-circuit state of the excitation winding.This means that the separately excited synchronous machine can be operated in a safe state, namely an active short-circuit state, during active discharge. An active short circuit of the excitation winding ensures that no energy is fed back into the high-voltage network, particularly the DC link, by the rotating electrical machine during active discharge.

[0022] In this context, a half-bridge architecture can be assumed, utilizing two semiconductor switches. Each switch connects one terminal of the excitation winding to one terminal of the high-voltage network, bridging the freewheeling diode. To then provide the discharge path, and thus implement the present configuration, a semiconductor switch bridging the freewheeling diode must also be provided in one of the remaining branches. This ensures that when all three switches are closed, both an active discharge occurs via the discharge path and an active short circuit of the excitation winding is created. The branch associated with this added semiconductor switch belongs to both the discharge path and the short-circuit path.

[0023] In this context, when pulse-width modulated control is used, it is preferred that the control device for selecting the semiconductor switch not belonging to the short-circuit path is designed for pulse-width modulated control and / or for operation in the linear range. In this way, even outside of the pulse, the active short circuit is not affected by switching operations and / or ohmic resistances used as discharge resistors and thus remains equivalent and permanent throughout the entire discharge process.

[0024] Furthermore, it is particularly advantageous if the separately excited synchronous machine is connected to the high-voltage grid by means of a converter, especially a three-phase one, whereby the control device for establishing a safe state during active discharge is designed, in addition to controlling the converter, to create an active short circuit. In this case, an active short circuit is thus brought about not only via the short-circuit path with respect to the excitation winding, but also with respect to the windings connected via the converter.It should be noted at this point that it is particularly advantageous for the excitation winding to be located on the rotor side, while the stator windings are connected to the high-voltage grid via the converter, so that they can feed in electrical power generated there or the separately excited synchronous machine can be operated with electrical power from the high-voltage grid, especially for powering the motor vehicle.

[0025] In other words, the stator windings are then connected to the high-voltage grid via the converter, preferably operating in three-phase mode. If both the converter and the excitation circuit are controlled accordingly with respect to the short-circuit path, all windings of the separately excited synchronous machine are in an active short circuit and, in particular, cannot feed any further power into the high-voltage grid, which would then have to be dissipated. During active charging, the separately excited synchronous machine is thus in a safe state, ensuring maximum functional safety with maximum integration and robustness. It should also be noted in this context that at least one of the at least one capacitor can be assigned to or belong to the converter as an intermediate circuit capacitor, as is generally known.

[0026] The present invention achieves further significant advantages when the motor vehicle also has a cooling system with a heat sink to which the excitation circuit is thermally connected for heat dissipation. Since the excitation circuit is cooled anyway, heat generated during active discharge can also be dissipated by means of the heat sink, thus keeping the thermal load from the active discharge in the corresponding circuit section, particularly along the discharge path, to a minimum. The heat sink can, in particular, have at least one channel and / or cavity through which a cooling fluid flows, in order to achieve improved cooling. The channel and / or cavity can be connected to a cooling circuit for the cooling fluid, which can be part of the cooling system.

[0027] In this context, it can be particularly advantageous to provide that at least part of the power assembly is thermally connected to the heat sink for heat dissipation, especially as at least one power module comprising a housing. For example, power modules in separate housings can be provided for the different phases of an inverter, which can be mounted on the heat sink, for example, side by side, and thermally connected to it. In addition to the power modules, the excitation circuit is also thermally connected to the heat sink, since it can be implemented, for example, as an excitation power module, which can be mounted on the heat sink in the same way as the inverter's power modules.In this way, an extremely compact, cooling-efficient arrangement is provided which, after the excitation circuit is cooled and the active discharge function is also integrated into the excitation circuit, enables a fast, effective, robust and thermally low-stress active discharge function.

[0028] In summary, the present invention allows for a significant reduction in material costs, as discharge resistors and discrete control semiconductors can be eliminated. In particular, when the excitation circuit is actively cooled by means of the heat sink, the at least one semiconductor switch in the discharge path can be made smaller, since it is connected to the active cooling of the cooling device. A multitude of discharge resistors is no longer necessary. Manufacturing and assembly costs are also significantly reduced, as is the labor involved, since no effort is required with regard to discharge resistors and any other discrete components of the discharge circuit. Especially in the case of the cooling connection, the robustness of the "active discharge" circuit section is increased, with the integration of this function into the excitation circuit alone resulting in increased robustness.In general, eliminating discharge resistors reduces weight and the required installation space. Flexibility is increased.

[0029] In addition to the motor vehicle, the invention also relates to a method for the active discharge of a capacitor in a high-voltage network of a motor vehicle of the type according to the invention, wherein, upon the presence of a signal in the control unit indicating the need for active discharge, the control unit activates at least one semiconductor switch of the discharge path to close it. All embodiments relating to the motor vehicle according to the invention can be transferred analogously to the method according to the invention, with which the aforementioned advantages can therefore also be obtained. In particular, pulse-width modulated control of at least one semiconductor switch, as described, can be carried out, wherein the duty cycle can be set, in particular, depending on the magnitude of the voltage to be discharged and / or the temperature measurement.Furthermore, by operating at least one of the at least one semiconductor switch in the linear range, at least a part of the discharge resistance can be provided.

[0030] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. These show: Fig. 1 a schematic diagram of a motor vehicle according to the invention, Fig. 2 a view of a heat sink with power modules thermally connected to it, Fig. 3 a circuit diagram of a power electronics arrangement comprising a converter, Fig. 4 a circuit diagram of an excitation circuit extended with respect to an active discharge, and Fig. 5 A schematic diagram to explain the active discharge process.

[0031] Fig. Figure 1 shows a schematic diagram of a motor vehicle 1 according to the invention, in this case an electric motor vehicle. The electric motor vehicle 1 comprises, in a drive train not shown in detail, an electric machine, which here is designed as a separately excited synchronous machine 2 and also serves as the electric motor for the electric motor vehicle 1. The separately excited synchronous machine 2 comprises a rotor with an excitation winding, which is not shown in detail here for the sake of clarity, as well as the stator windings of the stator, one of which is provided for each of the three phases. The excitation winding is connected via a high-voltage component 3, which includes an excitation circuit 4, to a high-voltage network 5 of the motor vehicle 1, which has a higher voltage than a low-voltage network of the motor vehicle 1, which is not shown here.The operating voltage of the high-voltage network 5 can, for example, be in a range of over 200 volts, in particular 350 to 860 volts, and it is a direct current network. It is powered by a battery 6.

[0032] The stator windings of the separately excited synchronous machine 2 are connected to the high-voltage network 5 via a power electronics arrangement 7, which includes a converter 8. Naturally, other high-voltage components or network components can also be provided in and connected to the high-voltage network 5, for example, a DC-DC converter located between the low-voltage network and the high-voltage network 5, an onboard charger for the battery 6, an electric air conditioning compressor, and / or an electric heater. In this case, at least the power electronics arrangement 7 with the converter 8 includes an intermediate circuit with an intermediate circuit capacitor. Such energy-storing capacitors, which may also include interference suppression capacitors in addition to intermediate circuit capacitors, can also be provided in other components, ultimately connected in parallel.

[0033] The motor vehicle 1 further comprises a cooling device 9, which also includes, in a more detailed manner, a cooling circuit for a cooling fluid, which at least provides cooling for the inverter 8 and the excitation circuit 4 by means of a common heat sink.

[0034] The operation of the high-voltage network and the high-voltage components connected to it is controlled in this case by a central control unit 10, which is also designed in particular to carry out the method according to the invention.

[0035] Fig. Figure 2 shows, by way of example, a heat sink 11 of the cooling device 9, which in this case has, in a manner not shown in detail, at least one channel and / or at least one cavity through which the cooling fluid flows in the cooling circuit. Attached to the heat sink 11 and thermally connected to it for heat dissipation are, in addition to power modules 12 for each phase, which together form the inverter 8, also an excitation power module 13, in which the excitation circuit 4 is housed in a casing, so that it too can be cooled by means of the heat sink 11.

[0036] The power modules 12 can also have housings enclosing the corresponding power electronics components, in particular including semiconductor switches. Also shown here are the power connections 14 of the power modules 12 to the high-voltage network 5, as well as the power connections 15 to the stator windings for the individual phases and corresponding control connections 16. For the sake of clarity, these are not shown here for the excitation power module 13.

[0037] Fig. Figure 3 schematically shows a circuit diagram of the power electronics arrangement 7 with the converter 8 as a circuit diagram. The converter 8 clearly comprises a B6 bridge circuit with corresponding components for each of the three phases U, V, and W, which are connected to the corresponding stator windings 17 (only indicated here) of the stator of the separately excited synchronous machine 2. Towards the high-voltage network 5, see connections 18. Two DC link capacitors 19 are shown here as an example; however, configurations are also conceivable in which the converter 8 includes only one DC link capacitor 19 or more than two DC link capacitors 19.

[0038] In the event of faults, accidents, and for maintenance purposes, as well as in other cases where operational safety and functional safety must be ensured, the intermediate capacitors 19 must be actively discharged as quickly as possible to minimize residual voltage remaining in the high-voltage network 5. This discharge is carried out via the excitation circuit 4, which is designed accordingly, as shown in the basic circuit diagram of the Fig. 4 shows.

[0039] The excitation circuit 4, located between the terminals 20 for the high-voltage network and the connection points 21 for the excitation winding 24 (indicated here by its inductance 22 and resistance 23, which corresponds to the rotor winding), clearly comprises a bridge circuit 25 in the form of a slightly modified half-bridge. This circuit has four branches 26, 27, 28, and 29, each containing freewheeling diodes 30. To form a half-bridge, semiconductor switches 31 and 32, which can be, for example, MOSFETs or IGBTs, are provided in branch 27 and branch 28 to bridge the freewheeling diodes 30. However, in this case, the corresponding freewheeling diode 30 in branch 29 is also bridged by a semiconductor switch 33, which can likewise be a MOSFET or IGBT.

[0040] In this way, branches 27 and 29 create a delimited discharge path 34 between terminals 20 for the high-voltage network 5. This occurs when semiconductor switches 31 and 33 are closed by control via the control unit 10, allowing low-resistance current to flow between terminals 20, thus discharging the intermediate circuit capacitors 19. The discharge resistance is provided by operating one of the semiconductor switches 31 or 33 in the linear (resistive) range. If semiconductor switch 32 is also closed simultaneously, the excitation winding 24 is short-circuited, so that branches 28 and 29 together form a short-circuit path 35.

[0041] If a signal indicating the need for active discharge is present in the control unit 10, which can also control the semiconductor switches 31, 32, 33 of the excitation circuit 4, it can close the semiconductor switches 31 and 33 to make the discharge path 34 conducting with low resistance, so that, as in Fig. As shown schematically in Figure 5, the charge of the intermediate circuit capacitors 19 (only one capacitor is shown schematically here) can flow away according to arrow 36. The semiconductor switch 33 is permanently closed to maintain the active short circuit of the excitation circuit 24 undisturbed, which is caused by the equally closed semiconductor switch 32. However, the semiconductor switch 31 is, as shown in Figure 5, Fig. 5 indicated, controlled via pulse width modulation, wherein the control unit 10 adjusts the duty cycle depending on the voltage to be discharged and the voltage measured via a Fig.4. The temperature measured at the semiconductor switch 31 (only indicated) is selected such that thermal overload does not occur and active discharge is enabled for many cycles by means of the excitation circuit 4. A discharge resistor is provided by linear operation of one of the semiconductor switches 31, 33, and here also of semiconductor switch 31.

[0042] In parallel with the active short circuit of the excitation winding 24, the stator windings 17 are also switched to an active short circuit state by either closing all lower semiconductor switches of the B6 bridge of the converter 8 (so-called low-side short circuit) or closing all upper semiconductor switches (high-side short circuit). These active short circuits (ACS) of both the stator windings 17 and the excitation winding 24 ensure that no energy is fed back into the high-voltage grid 5 by the rotating separately excited synchronous machine 2 during active discharge. By connecting the excitation circuit 4 to the active cooling system via the heat sink 11, which also cools the converter 8, this cooling remains active even during active discharge, allowing this function to be implemented in a significantly smaller and more cost-effective manner.In addition to integrating the "active discharge" function into the excitation circuit 4, this circuit is simultaneously able, through appropriate control via the control unit 10, to bring about the active short circuit (ACS) for the excitation winding 24, so that a safe state of the separately excited synchronous machine 2 exists and a discharge can take place quickly and easily.

Claims

[1] Motor vehicle (1) comprising a separately excited synchronous machine (2), an excitation circuit (4) associated with the separately excited synchronous machine (2) for supplying current to the excitation winding (24) and a power electronics arrangement (7) connected to a high-voltage network (5), in particular comprising a converter (8) associated with the synchronous machine (2), with at least one high-voltage network-side capacitor, in particular an intermediate circuit capacitor (19) and / or an interference suppression capacitor, wherein the excitation circuit (4) comprises a bridge circuit (25) forming at least one half-bridge with four branches (26, 27, 28, 29) each connecting a connection point (21) of the excitation winding (24) to a connection (20) of the high-voltage network (5), each branch comprising a freewheeling diode (30), wherein the excitation circuit (4) is connected in aThe discharge path (34) not encompassing the excitation winding (24) has at least one semiconductor switch (31, 33), wherein the motor vehicle (1) further comprises a control device (10) controlling the excitation circuit (4), which is designed for the active discharge of the at least one capacitor by closing the at least one semiconductor switch (31, 33) of the discharge path (34), wherein the at least one discharge path (34) comprises two branches (27, 29) connecting the terminals (20) of the high-voltage network (5) with semiconductor switches (31, 33) bridging the respective freewheeling diodes (30), . characterized by, that at least in a short-circuit path (35) formed by a branch (29) of the discharge path (34) and a further branch (28) between the connection points (21) of the excitation winding (24) and also of the freewheeling diode (30) belonging to the further branch (28) a semiconductor switch (32) bridging this is provided, wherein the control device (10) for establishing a safe state during active discharge is also designed to close the semiconductor switch (32) of the further branch (28) to establish an active short-circuit state of the excitation winding (24). [2] Motor vehicle (1) according to claim 1, characterized by, that the control device (10) is designed for at least temporary pulse width modulated control of at least one of the at least one semiconductor switch (31) of the discharge path (34) and / or for operation of at least one of the at least one semiconductor switch (31, 33) of the discharge path (34) in the linear range during active discharge. [3] Motor vehicle (1) according to claim 2, characterized by , that the control device (10) is designed to adjust the duty cycle of the pulse width modulation as a function of the magnitude of the voltage to be discharged and / or of a temperature measured by means of a measuring unit of at least one of the at least one semiconductor switch (31, 33) of the discharge path (34). [4] Motor vehicle (1) according to any of the preceding claims, characterized by, that when pulse width modulated control of one of the semiconductor switches (31, 33) of the discharge section (34) the control device (10) is designed to select the semiconductor switch (31) not belonging to the short-circuit section (35) for pulse width modulated control. [5] Motor vehicle (1) according to any of the preceding claims, characterized by , that the separately excited synchronous machine (2) is connected to the high-voltage network (5) by means of the, in particular three-phase, converter (8), wherein the control device (10) for establishing a safe state during active discharge is designed in addition to controlling the converter (8) for establishing an active short circuit. [6] Motor vehicle (1) according to claim 5, characterized by , that at least one of the at least one capacitor is assigned or belongs to the converter (8) as an intermediate circuit capacitor (19). [7] Motor vehicle (1) according to any of the preceding claims, characterized by , that it further comprises a cooling device (9) with a heat sink (11) to which the excitation circuit (4) is thermally connected for heat dissipation. [8] Motor vehicle (1) according to claim 7, characterized by , that at least part of the power electronics arrangement (7) is thermally connected to the heat sink (11) for heat dissipation, in particular as at least one power module (12) comprising a housing. [9] Method for the active discharge of a capacitor in a high-voltage network (5) of a motor vehicle (1) according to one of the preceding claims, wherein, when a signal indicating the need for an active discharge is present in the control device (10), the control device (10) controls the at least one semiconductor switch (31, 33) of the discharge path (34) to close.

Images