Method for de-excitation of a rotor of an electric machine, control device, electric machine and motor vehicle

Abstract

Method for de-exciting rotor windings (15) of a rotor (3) of an electric machine (2) for a motor vehicle (1), wherein the electric machine (2) comprises a stator (4) and the rotor (3) rotatably mounted relative to the stator (4), with the rotor windings (15) for generating a rotor magnetic field, wherein an active rectifier (19) comprising at least one field-effect transistor (23) controllable by means of a control voltage is provided on the side of the rotor (3), wherein the rectifier (19) electrically connects a voltage source provided on the side of the rotor (3) to the rotor windings (15) and an alternating voltage provided by the voltage source can be converted into a direct voltage by means of the rectifier (19), wherein the field-effect transistor (23) or at least one of the field-effect transistors (23) is brought into an operating state by means of the control voltage for de-exciting the rotor windings (15).in which the respective field-effect transistor (23) forms an ohmic resistance (33), wherein energy stored in the rotor windings (15) causes an electric current to flow through the at least one field-effect transistor (23) forming the ohmic resistance (33), such that at least part of this energy is converted into heat energy.

Description

[0001] The present invention relates to a method for de-exciting rotor windings of a rotor of an electric machine for a motor vehicle, wherein the electric machine comprises a stator and the rotor mounted rotatably with respect to the stator, with the rotor windings for generating a rotor magnetic field.

[0002] Electric machines are frequently used as traction motors in motor vehicles, which can be purely electric or hybrid vehicles. Electric machines comprise a stator and a rotor rotatably mounted relative to the stator. Both the stator and rotor have windings made of conductive wire and, optionally, permanent magnets. Electromagnetic interactions between the magnetic fields generated by the windings and, if applicable, the permanent magnets, produce a driving or braking torque. If the rotor has no windings but only permanent magnets, it is called a permanent magnet synchronous machine. If the rotor has no permanent magnets but only windings, referred to as rotor windings, it is called a separately excited synchronous machine.Externally excited synchronous machines offer additional degrees of freedom in the control and design of the electrical machine.

[0003] One problem associated with separately excited synchronous machines is that in certain cases, particularly in the event of a fault, high electrical currents or voltages are induced, especially when a high magnetic field is currently being generated by the rotor windings. These could damage electrical components or semiconductor components. As a countermeasure, the rotor windings are de-excited, meaning the magnetic fields present in the rotor windings, and thus the energy stored in them, are dissipated in a targeted and rapid manner. This is often achieved by dissipating the energy stored in the rotor windings, specifically by converting it into heat energy. Corresponding concepts are known, for example, from DE 10 2009 040 394 A1, DE 10 2022 121 516 A1, and US 2013 / 0 193 903 A1.

[0004] The present invention aims to provide an advantageous concept for the de-excitation of rotor windings of a rotor of an electrical machine, in particular with regard to a feasibility that is as simple and cost-effective as possible.

[0005] According to the invention, the problem is solved in a method of the type mentioned above by providing an active rectifier on the rotor comprising at least one field-effect transistor controllable by means of a control voltage, wherein the rectifier electrically connects a voltage source provided on the rotor to the rotor windings and an alternating voltage provided by the voltage source can be converted into a direct voltage by means of the rectifier, wherein the field-effect transistor or at least one of the field-effect transistors is brought into an operating state by means of the control voltage in order to de-excite the rotor windings, in which the respective field-effect transistor forms an ohmic resistance, wherein energy stored in the rotor windings causes an electric current to flow through the at least one field-effect transistor forming the ohmic resistance.so that at least some of this energy is converted into heat energy.

[0006] The invention is based in particular on the idea that, in addition to its primary function of rectifying an electrical voltage, the rectifier is assigned a further function: namely, to act as a means of converting energy stored in the rotor windings when de-excitation, especially rapid de-excitation, of the rotor windings is required. De-excitation is understood as a process in which the energy present in the rotor windings, which is present or stored within the electromagnetic field generated by the rotor windings, is extracted from the rotor windings. Within the scope of the present invention, this energy is dissipated, i.e., converted into heat energy.Since a component that is typically already present, namely the rectifier, is used for this purpose, no further components specifically designed for this purpose are required, which contributes to simplicity, minimal manufacturing effort and the lowest possible overall weight of the electrical machine.

[0007] The rotor is rotatably mounted relative to the stator, which typically has stator windings to generate a stator magnetic field. For this purpose, a rotor shaft can be supported by suitable bearings, such as ball or roller bearings. Preferably, the rotor and stator are arranged within a housing of the electric machine, with the stator preferably being fixed in position relative to the housing. The housing is, in a given assembly state, rigidly connected to the body of the vehicle.

[0008] The rotor windings and / or the stator windings each have at least one electrically conductive wire wound around, for example, rotor or stator teeth. Each winding acts as a field coil which, when energized, generates a magnetic field, i.e., the rotor or stator magnetic field.

[0009] A rectifier is understood to be, in particular, an electrical component comprising at least one field-effect transistor, typically several field-effect transistors. Thus, the rectifier, or at least the at least one field-effect transistor, can be controlled by means of control signals, whereby the control signals can be directed to control the operation of the rectifier, for example, to perform a rectification and, optionally, an inverter operation. In contrast to active rectifiers, passive rectifiers typically only have semiconductor diodes, so that an alternating voltage applied to an input of the rectifier is always converted into a direct voltage appearing at an output of the rectifier, regardless of control signals. However, according to the invention, an active rectifier is provided.

[0010] Regarding the field-effect transistor, which can also be called a FET transistor, it is conceivable that the field-effect transistor, or at least one of the field-effect transistors, is a metal-oxide-semiconductor field-effect transistor. In this case, the field-effect transistor can also be called a MOSFET transistor. It is conceivable that the control signals are applied to the respective field-effect transistor as a gate-source voltage, whereby a current flow is regulated with respect to a source-drain voltage depending on the gate-source voltage. This source-drain voltage, assuming the field-effect transistor exhibits ohmic resistance, is accompanied by power loss that causes the dissipation of the electrical energy stored in the rotor windings. The corresponding operating state of the field-effect transistor can also be described as a linear operating state.In this state, the field-effect transistor has a constant resistance and behaves like a voltage-controlled resistor, whose resistance value is determined by the gate-source voltage.

[0011] Regarding the metal-oxide-semiconductor field-effect transistor (MEFT), several operating states can be selectively induced depending on the control signals or the gate-source voltage. For example, at specific gate-source voltage values, the MEFT acts as a switch for the source-drain current flow, with these operating states being selectively set by the control signals during rectification. Another operating state, the occurrence of which depends on the gate-source voltage, is also known as the linear operating state, in which the MEFT acts as a resistive load. The linear operating state occurs when the gate-source voltage exceeds a threshold voltage, which is defined as the voltage at which a source-drain current first flows.Furthermore, a linear operating state can exist if the source-drain voltage is lower than the gate-source voltage. In normal operation of an electric machine, the linear operating state is usually avoided because it carries the risk of the metal-oxide-semiconductor field-effect transistor burning out. However, within the scope of the present invention, the control signals are specifically generated such that the metal-oxide-semiconductor field-effect transistor assumes precisely this operating state during the de-excitation of the stator windings, whereby the resulting electrical properties of the metal-oxide-semiconductor field-effect transistor are specifically used to dissipate the energy present from the rotor windings.

[0012] Regarding the rectifier, it is preferably provided that it comprises at least one circuit board, in particular having electrically conductive conductor tracks, and at least one field-effect transistor arranged thereon. The circuit board is understood to be a printed circuit board on which the semiconductor components are arranged. The circuit board can be made of a plastic. The semiconductor components can be attached to the circuit board by means of solder and / or press-fit connections. The circuit board preferably has a planar, in particular flat, extent which, to achieve the most effective possible thermal connection, can extend parallel to an outer surface of the cooling section that supports the circuit board.

[0013] It is conceivable that the field-effect transistor, or at least one of the field-effect transistors, is switched to an operating state by means of the control voltage to de-excite the rotor windings. In this state, the respective field-effect transistor exhibits an ohmic resistance with a predetermined value. In this embodiment, the control voltage is not simply generated in such a way that the field-effect transistor is operated in a linear state at any arbitrary point. Instead, it is selectively controlled so that the field-effect transistor exhibits a specifically predetermined resistance value. This resistance value can be chosen such that, on the one hand, the energy stored in the rotor windings is dissipated with sufficient efficiency, while on the other hand, the resulting excessive heating of the field-effect transistor could not lead to damage or burnout.

[0014] It is particularly preferred that when the respective field-effect transistor is switched to the operating state in which it exhibits a resistance of a predetermined value, the temperature dependence of this resistance is taken into account. Thus, in the linear operating state, the resistance value implemented by the field-effect transistor depends not only on the gate-source voltage but also on the current temperature of the transistor. This temperature dependence therefore provides an additional control basis for generating the control voltage. Metal-oxide-semiconductor field-effect transistors behave in such a way in the linear operating range that, at a constant gate-source voltage, the resistance decreases with increasing temperature, which could lead to the transistor burning out.

[0015] Specifically, the temperature dependence can be taken into account by determining the current temperature of the respective field-effect transistor. In this embodiment, a concrete result regarding the determination of the field-effect transistor's temperature thus constitutes a control basis, allowing the actual conditions regarding the temperature of the field-effect transistor to be incorporated into the corresponding control system as realistically as possible. In this embodiment, burnout can be prevented by reducing the gate-source voltage when the field-effect transistor heats up, in order to counteract or compensate for a temperature-induced decrease in resistance.

[0016] The current temperature can be determined using measurement technology. For example, a temperature sensor can be installed near the rectifier or the field-effect transistor to generate measured values ​​of the current temperature. Alternatively, the current temperature can be determined using a model and / or a lookup table. Compared to measuring the temperature directly, this approach saves on hardware components. Using the model, the relationships between physical and electrical quantities for the field-effect transistor can be determined based on its specific structural configuration. The results can also be saved and used as a lookup table.

[0017] During the de-energizing of the rotor windings by means of the field-effect transistor, excessive heat can be generated in its vicinity, necessitating targeted heat dissipation. To counteract this, it is conceivable that the field-effect transistor, or at least one of the field-effect transistors, is arranged on or within a cooling section of the rotor that forms a heat sink. According to this embodiment, thermal coupling between the heat sink and the field-effect transistor causes or enhances the dissipation of thermal energy from the field-effect transistor. For this purpose, the field-effect transistor is preferably in direct contact with the heat sink, preferably in a contact connection, apart from a thermally conductive medium optionally provided for mounting the field-effect transistor.

[0018] Particularly preferred in a possible further development is the provision that the cooling section has at least one cooling channel through which a cooling fluid flows. The cooling channel is understood to be a cavity or hollow space in the rotor designed to guide the cooling fluid. The cooling channel is preferably elongated, with the cooling fluid flowing along its longitudinal direction. The rotor, which is preferably made of a metal, has an outer surface on which the field-effect transistor can be positioned. The cooling channel preferably runs directly below the area of ​​this outer surface in which the rectifier or the field-effect transistor is located. The cooling fluid is preferably a coolant, such as water or oil.

[0019] Particularly preferably, the cooling channel, or at least one of the cooling channels, has at least one channel wall bounding the cooling channel with a deflection structure that deflects, and in particular turbulences, the fluid flowing along the channel wall. The deflection structure can be any geometric shape on the surface of the channel wall that deviates from a smooth or flat structure. The deflection structure can have protrusions arranged on the inside of the cooling channel that project from or extend away from the channel wall. The deflection structure can, in particular, be provided only in a section of the cooling channel that is located directly below the rectifier. The deflection structure causes any laminar flow of the cooling fluid present in the cooling channel to be converted into a turbulent flow, thus making the heat transfer to the cooling fluid more effective.The deflection structure can have at least one cooling fin and / or at least one cooling rib. A cooling fin or cooling rib is understood to be, in particular, a rib-like, elongated structure whose longitudinal direction extends along the channel wall. The longitudinal direction can be perpendicular or oblique with respect to the flow direction of the cooling fluid, thereby increasing the deflection effect on the cooling fluid accordingly. The deflection structure can have several cooling fins or cooling ribs arranged sequentially with respect to the flow direction.

[0020] The rectifier circuit board can be attached to the cooling section. A thermal interface material can be arranged between the circuit board and the cooling section. A thermal interface material is defined as a material with a sufficiently high thermal conductivity coefficient to ensure the most efficient heat transfer possible from the rectifier to the cooling section. The thermal conductivity coefficient can be at least 10 W / (m K). The thermal interface material is preferably a thermally conductive adhesive and thus, in addition to achieving the most efficient heat transfer possible, also serves as a fastening medium by which the rectifier is attached to the cooling section.

[0021] Alternatively, or in addition, the circuit board can have a metal core. Due to the typically high thermal conductivity of metals, heat transfer through the circuit board, which occurs during heat transfer from the at least one field-effect transistor to the cooling fluid, is even more efficient. This is particularly advantageous when the circuit board's support structure, apart from the metal core and any conductive traces, is made of plastic. The metal core can be made of aluminum and / or copper. The metal core is covered by a layer of plastic, especially on the side facing the field-effect transistor.The same generally applies to the direction towards the cooling section, whereby the metal core can also be exposed in this respect, so that there can be direct contact between the metal core and the cooling section, apart from any heat conducting medium.

[0022] Preferably, the electric machine has an inductive rotary transformer comprising at least one rotor-side field coil, which forms the voltage source, and at least one stator-side field coil, which is located on the stator side. Electrical energy can be inductively transferred from the stator-side field coil to the rotor-side field coil. The inductive rotary transformer enables contactless and therefore wear-free power transfer from the stator to the rotor. The rotor-side field coil and the stator-side field coil move past each other during the rotation of the rotor. Magnetic fields generated by the stator-side field coil cause a voltage to appear on the rotor-side field coil by means of electromagnetic induction. The rotor-side field coil thus acts as the voltage source, with the generated voltage being an alternating voltage.This is converted by the rectifier into the DC voltage required for operation or excitation of the stator windings. Preferably, several field coils are arranged concentrically around a rotational axis of the rotor and thus along the circumferential direction.

[0023] Although the invention provides for de-excitation of the rotor windings by means of the field-effect transistor implementing the ohmic resistance, an alternative approach is conceivable as a further possible option, particularly under certain operating conditions of the electric machine or the motor vehicle. Instead of de-excitation of the rotor windings by means of the rectifier, a DC voltage present at the rotor windings can be converted into an AC voltage, whereby the energy stored in the rotor windings is inductively transferred from the rotor-side field coil to the stator-side field coil. For this purpose, the rectifier is brought into a state by means of appropriately generated control voltages in which the DC voltage present at the rotor windings is converted into an AC voltage, which in turn is applied to the rotor-side field coil. Accordingly, power is dissipated to the stator-side field coil.Within this embodiment, a bidirectional transmission of power or energy is therefore possible via the rotary transformer and the rectifier.

[0024] It is conceivable that compliance with a dissipation condition is verified, whereby the dissipation condition is only fulfilled or fulfillable if at least one safety indicator is present that indicates the existence of an operating state of the electric machine and / or the motor vehicle in which the transfer of energy from the rotor-side field coil to the stator-side field coil is detrimental. If the dissipation condition is not fulfilled, the rotor windings can be de-excited by transferring the energy stored in the rotor windings from the rotor-side field coil to the stator-side field coil. If the dissipation condition is fulfilled, the energy stored in the rotor windings can be converted into heat energy by the at least one field-effect transistor forming the ohmic resistance.The presence of safety information may imply that, in the event of energy transfer from the rotor-side field coil to the stator-side field coil, damage to a component of the electric machine and / or the vehicle could occur. The safety information may also indicate that, in the event of this transfer, a hazard to a user could arise, for example, from exposed components carrying an electrical charge, which could be the case, in particular, in an accident.

[0025] The safety information, or at least one of the safety information notices, may indicate that the state of charge of an electrical energy storage device of the motor vehicle, which provides the electrical energy intended for operating the electric motor, and, if applicable, of an intermediate circuit capacitor of the electric motor, exceeds a predetermined limit. For example, the energy transferred from the rotor-side field coil to the stator-side field coil is typically used to charge the electrical energy storage device, which is not possible if it has a correspondingly high state of charge. The predetermined limit can be chosen such that it is exceeded if the electrical energy storage device is fully charged or has no further free storage capacity sufficient to absorb the transferred energy.The relevant safety information can be obtained from information about the current charge status of the electrical energy storage system, which is available within the vehicle's control system. Alternatively, or in addition, the charge status can be measured. It is also conceivable that the safety information, or at least one of the safety information messages, indicates that a fault condition exists in a drive unit of the vehicle. This applies particularly to the previously mentioned case where the vehicle has been involved in an accident. In this case, the safety information can be generated, for example, if a control signal triggering an airbag in the vehicle is present.

[0026] Furthermore, the present invention relates to a control device, in particular for a motor vehicle or an electric machine. According to the invention, the problem is solved in such a control device by providing it with a computer-readable storage medium on which executable instructions are stored. When executed by a processing unit of the control device, these instructions cause the control device to perform at least one of the steps of the method described above. In particular, the control device is configured to generate and output control signals for carrying out this method. These control signals can be directly the control voltage that is output to the at least one field-effect transistor. Alternatively, the control voltage to be output to the field-effect transistor can be generated from the control signals.Moreover, the control device is preferably configured to process the relevant information and, in particular, to verify compliance with the dissipation condition. Furthermore, the control device is preferably configured to generate and output control commands or the control voltage provided during the rectification process carried out by means of the rectifier. All advantages, features, and aspects explained in connection with the method according to the invention are equally transferable to the control device according to the invention, and vice versa.

[0027] In particular, the control device is located at a position different from the rotor. The control device can be located on the stator side or on the side of the vehicle. Accordingly, a transmission of the control commands or the control voltage from a stationary section, especially the stator, to the rotor is required. For this purpose, the electric machine can have an inductive communication rotary transformer comprising at least one rotor-side communication coil and at least one stator-side communication coil, wherein the control commands or the control voltage generated by the control device, which are used for the control operation of the rectifier, can be inductively transmitted from the stator-side communication coil to the rotor-side communication coil.The aspects explained above in connection with the inductive rotary transformer apply in principle equally and analogously to the communication rotary transformer.

[0028] Furthermore, the present invention relates to an electric machine. According to the invention, the problem is solved in such a machine by the fact that it comprises a control device, in particular according to the preceding descriptions, a stator, and a rotor with rotor windings for generating a rotor magnetic field, the rotor being rotatably mounted with respect to the stator. An active rectifier comprising at least one field-effect transistor controllable by means of a control voltage generated by the control device is provided on the rotor side. The rectifier electrically connects a voltage source on the rotor side to the rotor windings, and the rectifier enables the conversion of an alternating voltage provided by the voltage source into a direct voltage.wherein the field-effect transistor, or at least one of the field-effect transistors, can be brought into an operating state by means of the control voltage for de-exciting the rotor windings, in which the respective field-effect transistor forms an ohmic resistance, wherein energy stored in the rotor windings causes an electric current to flow through the at least one field-effect transistor forming the ohmic resistance, such that at least part of this energy is converted into heat energy. All advantages, features, and aspects explained in connection with the method and control device according to the invention are equally transferable to the electrical machine according to the invention, and vice versa.

[0029] Preferably, the electric machine is connectable to a vehicle's drivetrain, whereby, relative to the state connected to the drivetrain, a traction torque can be generated by the electric machine and transmitted to the vehicle's wheels via the drivetrain. For example, an open end of a rotor shaft extending along the axis of rotation can be provided, particularly one protruding from the housing of the electric machine. This end, as well as a component of the drivetrain, can each have a connecting element, such as a connecting flange, by means of which a mechanical connection, particularly a rotationally fixed one, can be established between the shaft and the drivetrain. The drivetrain generally comprises all components by which a mechanical coupling between the electric machine and the wheels can be established.The drivetrain can therefore include drive shafts and / or transmissions, in particular gearboxes and / or differentials and / or clutches.

[0030] Finally, the present invention relates to a motor vehicle comprising an electric machine forming a traction motor, in particular according to the preceding descriptions, and a control device, in particular according to the above descriptions relating thereto, wherein the electric machine comprises a stator and a rotor mounted rotatably with respect to the stator, with rotor windings for generating a rotor magnetic field, wherein an active rectifier comprising at least one field-effect transistor controllable by means of a control voltage generable by the control device is provided on the side of the rotor, wherein the rectifier electrically connects a voltage source provided on the side of the rotor to the rotor windings and an alternating voltage provided by the voltage source can be converted into a direct voltage by means of the rectifier.wherein the field-effect transistor, or at least one of the field-effect transistors, can be brought into an operating state by means of the control voltage for de-exciting the rotor windings, in which the respective field-effect transistor forms an ohmic resistance, wherein energy stored in the rotor windings causes an electric current to flow through the at least one field-effect transistor forming the ohmic resistance, such that at least part of this energy is converted into heat energy. All advantages, features, and aspects explained in connection with the inventive method, the inventive control device, and the inventive electrical machine are equally transferable to the inventive motor vehicle and vice versa.

[0031] Preferably, the motor vehicle according to the invention comprises an electrical energy storage device in which energy usable for the traction of the motor vehicle can be stored. This energy is in the form of electrical energy and is converted into kinetic energy by means of the electric machine. Conversely, the operation of the electric machine in a recuperation mode is conceivable, in which the kinetic energy of the motor vehicle is converted into electrical energy, which is stored in the energy storage device. A direct current voltage can typically be provided by means of the energy storage device, which can in particular be a lithium-ion battery. However, an alternating current voltage is required for the operation of the electric machine, so a power electronics unit can be provided on the electric machine side by means of which the direct current voltage provided by the energy storage device can be converted into an alternating current voltage.The control device mentioned above, or another control device, may be configured to generate and output control signals directed to the operation of the power electronics unit.

[0032] Preferably, the motor vehicle according to the invention comprises a cooling system through which a coolant fluid can be conveyed, wherein the cooling section forming a heat sink, which has already been explained above, is integrated into the cooling system. The cooling system can thus be designed to form a cooling circuit in which the coolant fluid can be conveyed by means of a conveying medium. In this embodiment, the coolant fluid circulates from the conveying medium to the cooling section and back again, and is thus recirculated. The conveying medium can be a coolant pump. A cooling device for cooling the coolant fluid, such as a heat exchanger, can be integrated into the cooling system.

[0033] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the figures. These show schematically: Fig. 1 A schematic diagram of a motor vehicle according to the invention, shown from the side, in an exemplary embodiment, comprising an electric machine according to an exemplary embodiment and a control device according to an exemplary embodiment, Fig. 2 a highly schematic diagram of the electric machine of the motor vehicle of the Fig. 1, Fig. 3 a flowchart of a method according to the invention in an exemplary embodiment, which is based on the one in the Fig. 1 motor vehicle shown and the one in the Fig. The electric machine shown in section 2 is explained. Fig. 4 an excerpt of the schematic diagram of the Fig. 2, which shows a rectifier during the de-excitation of a rotor of the electric machine, and Fig. 5 a coordinate system to illustrate the relationship between the value of a control voltage and the value of an ohmic resistance, which in a linear operating state of field-effect transistors of the electrical machine of the Fig. 2 occurs at different temperature values.

[0034] Fig. Figure 1 shows a motor vehicle 1 according to an embodiment of the invention, comprising an electric machine 2 according to an embodiment of the invention. The electric machine 2 comprises a rotor 3 and a stator 4. The electric machine 2 is an internal rotor designed as a separately excited synchronous machine. Thus, the rotor 3 is arranged in a region of the electric machine 2 that is radially further inward than a region in which the stator 4 is arranged. A rotor shaft 5 of the rotor 3 is rotatably mounted on a housing 6 of the electric machine 2, for example by means of a ball or roller bearing.

[0035] The electric machine 2 is configured to operate in a drive mode in which electrical energy stored in an electrical energy storage device 7 of the vehicle 1 is converted into kinetic energy of the vehicle 1. A generated drive torque, which is used to propel the vehicle 1, can be transmitted from the electric machine 2 to a drivetrain 8 of the vehicle 1. The drive torque can only be transmitted to the rear wheels, but can also be transmitted to the front wheels additionally or alternatively. The electric machine 2 can also be operated in a recuperation mode in which kinetic energy of the vehicle 1 is converted into electrical energy by means of the electric machine 2, which can be used, for example, to recharge the electrical energy storage device 7.

[0036] The following introduces definitions of relevant spatial directions with reference to the electric machine 2. The rotor shaft 5 is rotatably mounted about an axis of rotation 9, which extends along a longitudinal direction 10 of the electric machine 2. A radial direction 11 extends perpendicular to the longitudinal direction 10. A circumferential direction 12 is perpendicular to the radial direction 11. This means that a point rotating about the axis of rotation 9 moves along the circumferential direction 12.

[0037] Details regarding electric machine 2 are explained below. Fig. Figure 2 shows a highly schematic view of the electric machine 2, illustrating in particular the distribution of the components of the electric machine 2 between the rotor 3 and the stator 4. A power electronics unit 13 is provided on the stator 4, by means of which a DC voltage supplied by the electrical energy storage device 7 can be converted into an AC voltage. For this purpose, a control device 14 according to the invention is provided in one exemplary embodiment, which is configured to generate control signals or a corresponding control voltage for the operation of the power electronics unit 13 and output them to it. Although the control device 14 is shown here as a component of the electric machine 2, it can also be provided outside the electric machine 2 and thus instead be a component of the motor vehicle 1.The alternating voltage generated by the power electronics unit 13 is used to power stator windings of the stator 4 (not shown in detail in the figures) and rotor windings 15 of the rotor 3, of which in . Fig. 2, which is only schematically indicated, can be energized. The stator windings and rotor windings 15, each consisting of a conductor wire, generate magnetic fields due to the electric current, i.e., the rotor magnetic field and a stator magnetic field, which interact with each other in the course of generating the drive or recuperation torque.

[0038] Details regarding the transfer of electrical energy, or rather power, from the energy storage device 7 to the stator windings 15 are explained below. First, the direct current voltage provided by the energy storage device 7 is converted into an alternating current voltage by the power electronics unit 13. This is transferred to a stator-side field coil 16 located on the stator 4, via which inductive energy transfer occurs to a rotor-side field coil 17 located on the rotor 3. The field coils 16 and 17 thus form an inductive rotary transformer 18, enabling contactless energy transfer from the stator 4, or stationary section of the electric machine 2, to its rotor 3.

[0039] The rotor-side field coil 17 realizes a voltage source present on the side of the rotor 3, from which an alternating electrical voltage is present.

[0040] This is supplied to a rectifier 19 of the rotor 3, which converts this alternating voltage into a direct voltage. The rectifier 19 thus connects the rotor-side field coil 17 to the rotor windings 15, so that the voltage supplied via the rotor-side field coil 17 and converted into a direct voltage by the rectifier 19 is supplied to the rotor windings 15 to generate the rotor magnetic field. The active rectifier 19 comprises several, namely eight, field-effect transistors 23, of which in the Fig. 2 For the sake of clarity, only one is designated with the corresponding reference numeral, and these are metal-oxide-semiconductor field-effect transistors. The rectification of the alternating voltage supplied by the rotor-side field coil 17 is carried out depending on the control voltages generated by the control unit 14, which are each applied to one of the field-effect transistors 23.

[0041] In addition to generating the control signals intended for the power electronics unit 13, the control device 14 is also configured to generate the aforementioned control signals or control voltages, by means of which the operation of the active rectifier 19 is controlled, and to output them to the active rectifier 19 via an inductive communication rotary transformer 20. The communication rotary transformer 20 comprises a stator-side communication coil 21 located on the side of the stator 4 and a rotor-side communication coil 22 located on the side of the rotor 3, wherein the control signals intended for the operation of the active rectifier 19 are inductively transmitted from the stator-side communication coil 21 to the rotor-side communication coil 22 and subsequently to the field-effect transistors 23. The operating principle of the communication rotary transformer 20 is fundamentally the same as that of the rotary transformer 18.

[0042] The following describes normal operation of the rectifier 19, in which the alternating voltage present at the rotor-side field coil 17 is converted into a direct voltage by means of the rectifier 19. The rectifier 19 comprises eight field-effect transistors 23, which are arranged in the Fig. The schematic diagram shown in Figure 2 is arranged in four rows and two columns. In normal operation, in which the rotor windings 15 are excited, the control voltages are generated such that the four field-effect transistors 23 shown in the second and third rows are permanently switched on, whereas the remaining four field-effect transistors 23 are operated in switching mode.

[0043] The following refers to the Fig. 3 Reference is made to a flowchart of the inventive method according to an exemplary embodiment, which is based on the motor vehicle 1 of the Fig. 1 and the electric machine 2 of the Fig. 2 is explained. The procedure comprises steps 24-27. The processing and evaluation steps carried out during the execution of the procedure, as well as the generation of control voltages, are performed by the control unit 14. For this purpose, the control unit 14 includes a computer-readable storage medium 28 on which executable instructions 29 are stored. When these instructions are executed by means of a processing unit 30 of the control unit 14, they cause the control unit 14 to carry out the steps provided for in the context of the present procedure.

[0044] Regarding the first step 24, the initial situation is assumed to be one in which the rotor windings 15 are to be de-energized. In step 24, the control device 14 checks a dissipation condition, the fulfillment of which depends on the presence of safety information 31. Safety information 31 is only present under certain circumstances.

[0045] Safety Information 31 is present, for example, when the state of charge of the electrical energy storage device 7, and / or, if applicable, of an intermediate circuit capacitor (not shown) of the electric machine, exceeds a predefined limit. The predefined limit is chosen such that it is exceeded when the electrical energy storage device 7 is fully charged. The corresponding Safety Information 31 is determined based on information about the current state of charge of the energy storage device 7 available from the vehicle control system; alternatively, or by means of a measurement, the state of charge can also be determined. Furthermore, Safety Information 31 is present when a fault condition exists in a drive unit, such as the power electronics unit 13, of the motor vehicle 1. This can occur, for example, after an accident.In this case, safety information 31 is generated, for example, when a control signal triggering an airbag of motor vehicle 1 is present.

[0046] If safety information 31 or several safety information messages 31 are present, then the dissipation condition is met; otherwise, the procedure continues in the next step 25. If the dissipation condition is not met, the rotor windings 15 are de-energized as explained below. The relevant factor here is that the rectifier 19 can not only convert the alternating voltage present at the rotor-side field coil 17 into a direct voltage present at the rotor windings 15, but also vice versa, so that bidirectional energy transfer is thus enabled by means of the rotary transformer 18.During the de-excitation process in step 25, the control voltages are generated by the control unit 14 and output to the field-effect transistors 23 in such a way that the DC voltage present at the rotor windings 15 is converted into an AC voltage, which in turn causes an inductive energy transfer from the rotor-side field coil 17 to the stator-side field coil 16. With regard to the... Fig. 2. In this process, the four field-effect transistors 23 shown in the first and fourth lines are switched on, with the remaining four field-effect transistors 23 operating in a clocking mode.

[0047] The following describes the case in which the dissipation condition was met during step 24, i.e., in which at least one piece of safety information 31 is available. In this case, the procedure continues with steps 26 and 27, with step 26 being performed cyclically in parallel with step 27. In step 26, temperature information 32 relating to the current temperature of the field-effect transistor 23 is determined. For this purpose, a temperature sensor, not shown in detail in the figures, is provided in the area of ​​the rectifier 19. The temperature information 32 is determined using a model and a lookup table, which is stored, for example, by the control unit 14, as part of a data aggregation process.Using the model, dependencies of physical and electrical quantities for the field-effect transistors 23 are determined according to their specific structural configurations, and these results are also used in the lookup table.

[0048] In step 27, the control device 14 generates the control voltages such that the field-effect transistors 23 are switched to a linear operating state in which each of the field-effect transistors 23 forms an ohmic resistance 33. This state is determined by the Fig. Figure 4 indicates the rectifier 19, where the field-effect transistors 23 are indicated as voltage-dependent controllable resistors. Fig. For clarity, only one of the resistors 33 shown is labeled with a corresponding reference symbol in Figure 4. The value of each resistor 33 depends on the respective control voltage. Because the field-effect transistors 23 each form a resistance 33, the power dissipation occurring in the resistors 33 causes them to heat up, which ultimately leads to the de-energization of the rotor coils 15. The resulting current flow through the field-effect transistors 23 and resistors 33 is shown in Fig. 4 indicated by a dashed line.

[0049] The control voltages are generated by the control unit 14 such that the respective field-effect transistor 23 forms an ohmic resistance 33 with a specifically predetermined resistance value. The temperature information 32 mentioned earlier is taken into account for this purpose. Thus, it shows Fig. Figure 5 shows a coordinate system 34 relating to the relationship between the resistance value of the ohmic resistor 33 in milliohms, plotted along the ordinate axis 35, and the value of the respective control voltage in volts, which in this case forms a gate-source voltage and is plotted along the abscissa axis 36. Several curves are shown, each corresponding to a temperature value of the field-effect transistor 23. Thus, the line comprising unfilled circles represents a temperature of 150 °C, the line comprising filled circles a temperature of 125 °C, the line comprising star symbols a temperature of 100 °C, the line comprising square symbols a temperature of 75 °C, the line comprising triangle symbols a temperature of 50 °C, and the line comprising diamond symbols a temperature of 25 °C.The resistance value clearly decreases with increasing control voltage at a constant temperature. In the linear operating state, which is represented by the left, steep sections of the curve in the... Fig. As shown in the curves 5, there is an extremely strong temperature dependence on the resistance value at a constant gate-source voltage. The resistance values ​​decrease with increasing temperature, which in turn leads to a higher current flow through the field-effect transistor 23, which in turn causes a further increase in temperature. Without countermeasures, this vicious cycle could end with the field-effect transistor 23 burning out and thus being destroyed. To counteract this, the temperature information 32 is continuously and cyclically acquired in step 26, whereby the control voltage is reduced accordingly in the event of increasing temperature in order to increase the resistance value and thus reduce the current flow and consequently the temperature rise.

[0050] Due to the dissipation of the energy present in the rotor windings 15 by means of the ohmic resistors 33, heat is generated in the rectifier 19. The same principle applies to the normal operation of the rectifier 19. This heat generation necessitates targeted cooling in the rectifier 19, the details of which are explained below. The motor vehicle 1 includes a cooling system (not shown in detail in the figures) that forms a cooling circuit in which a cooling fluid circulates.

[0051] To cool the cooling fluid, the cooling system also includes a cooling device, specifically a heat exchanger.

[0052] The rectifier 19 is arranged on a cooling section 37 of the rotor 3, which forms a heat sink and is integrated into the cooling system 23 and through which the cooling fluid flows. The cooling section 37 is located in the Fig.3 is indicated by dashed lines. The cooling fluid flows from the conveying medium into cooling channels of the cooling section 37 (not shown in detail in the figures) and from there to the cooling device and then back to the conveying medium. The cooling channels running through the cooling section 37 are located directly below the rectifier 19. To enable the most efficient heat transfer from the rectifier 19 to the cooling fluid, the thickness of the rotor 3 material remaining between the cooling channels and the rectifier 19, which in this case is a metal, is as small as possible and preferably only a few millimeters.

[0053] In the cooling channels, deflection structures are provided on a channel wall that delimits the cooling channel. These structures deflect and swirl the cooling fluid flowing through the cooling channel, resulting in more efficient heat transfer from the rectifier 19 to the cooling fluid. The deflection structure comprises several rib-like, elongated structures arranged sequentially perpendicular to the flow direction; these structures can also be referred to as cooling fins or cooling ribs. QUOTES INCLUDED IN THE DESCRIPTION

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

[0000] DE 10 2009 040 394 A1

[0003] DE 10 2022 121 516 A1

[0003] US 2013 / 0 193 903 A1

[0003]

Claims

[1] Method for de-exciting rotor windings (15) of a rotor (3) of an electric machine (2) for a motor vehicle (1), wherein the electric machine (2) comprises a stator (4) and the rotor (3) rotatably mounted with respect to the stator (4) with the rotor windings (15) for generating a rotor magnetic field, wherein an active rectifier (19) comprising at least one field-effect transistor (23) controllable by means of a control voltage is provided on the side of the rotor (3), wherein the rectifier (19) electrically connects a voltage source provided on the side of the rotor (3) with the rotor windings (15) and an alternating voltage provided by the voltage source can be converted into a direct voltage by means of the rectifier (19), wherein the field-effect transistor (23) or at least one of the field-effect transistors (23) is brought into an operating state by means of the control voltage for de-exciting the rotor windings (15),in which the respective field-effect transistor (23) forms an ohmic resistance (33), wherein energy stored in the rotor windings (15) causes an electric current to flow through the at least one field-effect transistor (23) forming the ohmic resistance (33), such that at least part of this energy is converted into heat energy. [2] Method according to claim 1, characterized by , that the field-effect transistor (23) or at least one of the field-effect transistors (23) is a metal-oxide-semiconductor field-effect transistor. [3] Method according to claim 2, characterized by , that the field-effect transistor (23) or at least one of the field-effect transistors (23) is brought into the operating state by means of the control voltage to de-excitate the rotor windings (15), in which the respective field-effect transistor (23) forms the ohmic resistance (33) with a predetermined resistance value. [4] Method according to claim 3, characterized by, that when the respective field-effect transistor (23) is put into the operating state in which this field-effect transistor (23) forms the ohmic resistance (33) with a predetermined resistance value, a temperature dependence of the resistance value is taken into account. [5] Method according to claim 4, characterized by , that the consideration of the temperature dependence is carried out by determining a current temperature of the respective field-effect transistor (23), wherein the determination of the current temperature is carried out in particular by measurement and / or on the basis of a model and / or on the basis of a lookup table. [6] Method according to any of the preceding claims, characterized by , that the field-effect transistor (23) or at least one of the field-effect transistors (23) is arranged on or in a cooling section (37) of the rotor (3) forming a heat sink. [7] Method according to claim 6, characterized by, that the cooling section (37) has at least one cooling channel through which a cooling fluid can flow. [8] Method according to any of the preceding claims, characterized by , that the electric machine (2) has an inductive rotary transformer (18) comprising at least one rotor-side field coil (17) located on the side of the rotor (3) and forming the voltage source, and at least one stator-side field coil (16) located on the side of the stator (4), wherein electrical energy is inductively transferred from the stator-side field coil (16) to the rotor-side field coil (17). [9] Method according to claim 8, characterized by, that as an alternative to de-excitation of the rotor windings (15) by means of the rectifier (19) a DC voltage present on the rotor windings (15) is converted into an AC voltage, whereby the energy stored in the rotor windings (15) is inductively transferred from the rotor-side field coil (17) to the stator-side field coil (16). [10] Method according to claim 9, characterized by, that the fulfillment of a dissipation condition is checked, wherein the dissipation condition is only fulfilled or fulfillable if at least one safety information (31) is available which indicates the existence of an operating state of the electric machine (2) and / or the motor vehicle (1) in which the transfer of energy from the rotor-side field coil to the stator-side field coil is disadvantageous, wherein the de-excitation of the rotor windings (15) is carried out, in the case of non-fulfillment of the dissipation condition, by transferring the energy stored in the rotor windings (15) from the rotor-side field coil to the stator-side field coil, and in the case of fulfillment of the dissipation condition, by converting the energy stored in the rotor windings (15) into heat energy by the at least one field-effect transistor (23) forming the ohmic resistance (33). [11] Method according to claim 10, characterized by, that the safety information (31) or at least one of the safety information (31) indicates that a charge state of an electrical energy storage device (7) of the motor vehicle (1), which provides the electrical energy intended for the operation of the electric machine (2), exceeds a specified limit. [12] Method according to claim 10 or 11, characterized by , that the safety information (31) or at least one of the safety information (31) indicates that a fault condition exists on the part of a drive unit of the motor vehicle (1). [13] Control device (14) with a computer-readable storage medium (28) on which executable instructions (29) are stored which, when executed by means of a processing device (30) of the control device (14), cause it to perform at least one of the steps of the method according to one of the preceding claims. [14] Electric machine (2) comprising a control device (14), in particular according to claim 13, a stator (4) and a rotor (3) rotatably mounted with respect to the stator (4) with rotor windings (15) for generating a rotor magnetic field, wherein an active rectifier (19) comprising at least one field-effect transistor (23) controllable by means of a control voltage generable by the control device (14) is provided on the side of the rotor (3), wherein the rectifier (19) electrically connects a voltage source present on the side of the rotor (3) with the rotor windings (15) and an alternating voltage provided by the voltage source can be converted into a direct voltage by means of the rectifier (19), wherein the field-effect transistor (23) or at least one of the field-effect transistors (23) can be brought into an operating state by means of the control voltage to de-energize the rotor windings (15),in which the respective field-effect transistor (23) forms an ohmic resistance (33), wherein energy stored in the rotor windings (15) causes an electric current to flow through the at least one field-effect transistor (23) forming the ohmic resistance (33), such that at least part of this energy is converted into heat energy. [15] Motor vehicle (1) comprising an electric machine (2) forming a traction motor, in particular according to claim 14, and a control device (14), in particular according to claim 13, wherein the electric machine (2) comprises a stator (4) and a rotor (3) rotatably mounted with respect to the stator (4) with rotor windings (15) for generating a rotor magnetic field, wherein an active rectifier (19) comprising at least one field-effect transistor (23) controllable by means of a control voltage generable by the control device (14) is provided on the side of the rotor (3), wherein the rectifier (19) electrically connects a voltage source present on the side of the rotor (3) with the rotor windings (15) and an alternating voltage provided by the voltage source can be converted into a direct voltage by means of the rectifier (19),wherein the field-effect transistor (23) or at least one of the field-effect transistors (23) can be brought into an operating state by means of the control voltage for the purpose of de-exciting the rotor windings (15), in which the respective field-effect transistor (23) forms an ohmic resistance (33), wherein an energy stored in the rotor windings (15) causes an electric current to flow through the at least one field-effect transistor (23) forming the ohmic resistance (33), such that at least a part of this energy is converted into heat energy.

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