Method for controlling the operation of an electrical machine

Abstract

Method for controlling the operation of an electrical machine, in particular of a motor vehicle, in a fault condition, comprising the steps: - detecting a current current vector (6) of the electrical machine; - determining a change voltage vector (12) as a function of the detected current current vector (6); - changing the current operating point (15, 15') of the electrical machine to a changed operating point (16, 16') by setting the change voltage vector (12), wherein the power generated by changing the operating point (15, 15') is equal to the, in particular thermal, power loss or less than the, in particular thermal, power loss of the electrical machine; - Executing an active short-circuit condition starting from the changed operating point (16, 16').

Description

The invention relates to a method for controlling an operation of an electric machine, in particular of a motor vehicle, in a fault state.Methods for controlling the operation of electric machines, in particular for motor vehicles, in which operation a fault condition occurs, are fundamentally known from the prior art. For example, it is known that when a fault condition occurs, the electric machine or the drive arrangement comprising it is to be brought into a safe state. For this purpose, for example, the current operation is set and the electric machine is operated in such a way that the intermediate circuit is discharged for the transition to the safe state. In particular, for the transition to the safe state, a so-called "active short-circuit state" ("AKS") is carried out, in which, for example, the switches of the high side or the switches of the low side of the inverter assigned to the electric machine are simultaneously switched. In other words, the phases of the electric machine are short-circuited to transition to the safe state.Depending on the operating state from which the safe state is to be assumed, i.e. in which operating state the fault state occurs, it is possible that, by executing the active short-circuit state, in particular if the electrical energy store has been decoupled or "dropped off" in the fault state, high transient currents are generated which can generate a high counter field in the electrical machine. In particular, if the operating temperature of the electric machine is increased, for example if it was previously operated in continuous operation, the opposing fields at the increased operating temperature can potentially lead to demagnetization of the permanent magnets of the electric machine, in particular of the permanent magnets in the rotor. To prevent this, permanent magnets having rare earth elements are used, which increase demagnetization resistance of the permanent magnets. In other words, the use of the permanent magnets which have the rare earths, in particular terbium and dysprosium, makes it possible to reliably prevent the permanent magnets from being demagnetized even at elevated operating temperature when the comparatively high opposing fields are generated.In the prior art, however, there is a tendency to avoid the use of rare earths to the greatest possible extent. This means that, when the rare earths are dispensed with, demagnetization of the permanent magnets of the electric machine can potentially occur at elevated operating temperatures as a result of the generation of the opposing fields, which can lead to damage or destruction of the electric machine, in particular of its permanent magnets. In other words, when reducing or eliminating the described rare earths, there is a higher probability of demagnetizing the permanent magnets at higher magnet temperatures when the opposing fields are present.The object of the invention is to specify a method, improved in comparison therewith, for controlling the operation of an electric machine in a fault state, in which method, in particular, the use of rare earths can be at least reduced without risking demagnetization of the permanent magnets.The object is achieved by a method having the features of claim 1. Advantageous embodiments are the subject matter of the dependent claims.As described, the invention relates to a method for controlling an operation of an electric machine, in particular an electric machine of a motor vehicle, in a fault state. The electric machine can be part of an electric drive arrangement of the motor vehicle, i.e. the motor vehicle can be driven by the electric machine as a drive device. The electric machine is assigned the usual components in the drive arrangement, for example an inverter, an electrical energy store and the like. The control of the operation of the electric machine can be carried out in particular by the inverter in that the inverter outputs corresponding electric voltages in order to set currents in the electric machine. As described, the electric machine is to be transferred to a safe state in the event of a fault state, in particular by carrying out an active short-circuit state.The invention is based on the finding that the method comprises the following steps:detecting a current current vector of the electric machine;determining a change voltage vector depending on the detected current current vector;changing the current operating point of the electric machine to a changed operating point by setting the change voltage vector, wherein the power generated by the change of the operating point is equal to the, in particular thermal, power loss or less than the, in particular thermal, power loss of the electric machine;executing an active short-circuit state starting from the changed operating point.Accordingly, it is proposed that the current vector of the electric machine is initially detected. In particular, the position of the current vector is detected in a defined coordinate system, in particular an alpha-beta coordinate system or a d-q coordinate system. A change voltage vector is then determined as a function of the detected current current vector. In this case, the position of the change voltage vector in the described coordinate system can again be determined, in particular with respect to the previously detected current vector. For example, the change voltage vector has a fixed position relative to the detected current current vector.Subsequently, the current operating point of the electric machine is changed in a power-neutral manner into a changed operating point of the electric machine, in which the previously determined change voltage vector is set, wherein the power loss in the electric machine at least balances or exceeds the power generated by the change in the operating point. In a special case, the change of the operating point can therefore be referred to as "power neutral". This means, for example, that the current operating point of the electric machine is changed by setting the change voltage vector into a changed operating point, specifically in such a way that no more than the thermal power loss which occurs in the process is generated. In other words, the change voltage vector is set in the control of the electric machine and thus brings about a change of the operating point starting from the current operating point into the changed operating point. The change in operating point brings about the avoidance of the transient overcurrents, so that transient currents in the electric machine are prevented from generating a counter-field which could demagnetize the permanent magnets in the electric machine. The change voltage vector is also selected so that it performs the operating point change in a power-free manner.The electric machine or the electric drive arrangement can thus be prepared for the execution of the active short-circuit state by assuming the changed operating point described. The changed operating point can be, for example, as close as possible to a steady state AC current or a steady state AKS current with respect to the d-q current coordinate system. Considering this coordinate system, the transition to the changed operating point does not take place linearly, but follows a curved path in the d-q coordinate system. By changing the operating point to the changed operating point, the active short-circuit state can subsequently be carried out starting from the changed operating point without high transient currents flowing in the electric machine, which can generate comparatively high opposing fields.This allows the electrical machine to be brought into the safe state by carrying out the active short-circuit state in certain fault states even for electrical machines which do not have any rare earths or have a reduced proportion of rare earths compared with electrical machines known from the prior art.As described above, the variation voltage vector is determined based on the detected current current vector. In one embodiment of the method, the change voltage vector can be determined with a defined phase shift, in particular in the range of + / - π / 2, with respect to the detected current vector. As is known, a defined phase shift between the current vector and the voltage vector, in this case the change voltage vector, in this range or by the defined phase shift described causes no power to be generated.Furthermore, it is possible not to set the defined phase shift to exactly + / - π / 2, but rather to deviate from it in a targeted manner. Although this generates slight power, it is compensated for by the losses within the electric machine. This enables an improvement in the change of the operating point starting from the current operating point into the changed operating point. In particular, the slight generation of the power, which is compensated by the losses within the electric machine, can bring about a further approach to the desired operating point, in particular the steady state AC current, or the center point of the spiral in the d-q current coordinate system.Purely by way of example, the defined phase shift between the change voltage vector and the detected current vector can comprise a phase angle of 250° to 270°, in particular 255° to 265°. As already described, the precise phase angle or the precise phase shift can be set depending on the losses prevailing in the electric machine. The higher the losses, the more power can be generated, since this can be compensated by the losses. In other words, the higher the losses in the electric machine, the phase shift can be spaced further from Pi half.The method described can furthermore be further developed to the effect that the change voltage vector is shifted by an additional shift to the detected current vector depending on at least one power loss element. The defined phase shift can thus comprise the additional shift. The power dissipation element can form, for example, an additional element within the electrical arrangement, for example a resistor, a varistor or the like, by means of which electrical power can be consumed or converted into power dissipation. In particular, the power dissipation element can transfer electrical energy into heat and thus emit it to the environment. For example, the power loss element can be arranged parallel to the intermediate circuit in order to selectively consume the power generated by setting the change voltage vector in the form of power loss in the transition to the changed operating state.In a further embodiment of the method, it can be provided that a discharge voltage vector for reducing the voltage, in particular depending on the detected current vector, is set before the change voltage vector is set. Depending on the current operating state, which is present when the fault state occurs or when the fault state is detected, it may be expedient to first reduce the intermediate circuit voltage in the intermediate circuit or to specifically bring about a voltage drop in order to subsequently be able to set the change voltage vector.If the current voltage in the fault state is above a defined voltage limit value, for example a permissible intermediate circuit voltage, the direct setting of the change voltage vector can bring about an additional increase in the voltage. In order to ensure that the change voltage vector can be set without negative effects, by setting the discharge voltage vector, the voltage is first reduced, so that the change voltage vector can subsequently be ensured. The discharge voltage vector can be adjusted in particular such that the voltage vector and the current vector are or run approximately synchronously in order to generate an active power in a targeted manner in order to bring about the voltage drop.In the described embodiment, it can be provided in particular that the discharge voltage vector is set to a static state vector which delimits the vector segment in which the current current vector is detected. As described, an active power is intended to be generated by the discharge voltage vector in a targeted manner for a period of time in order to cause a voltage drop, so that the change voltage vector can subsequently be executed or can be set securely. By assuming the static state vector, it is advantageously possible that the switch positions in the inverter can stand statically for the duration of the application of the discharge voltage vector.In other words, it is not necessary to generate the discharge voltage vector by modulating or changing the switch positions of the switch elements of the inverter, but the discharge voltage vector can be placed specifically on a static state vector or set as such. For this purpose, the current current vector can be detected, as described above. It is thereby possible to identify in which of the vector segments of the hexagon describing the switch positions of the inverter the current current vector lies in the coordinate system. Based thereon, the discharge voltage vector may be set to the static state vector that bounds the current vector segment in which the current current vector was detected. In particular, the discharge voltage vector is set to the static state vector which tracks the current vector or which backward limits the vector segment. The described setting of the discharge voltage vector to the static state vector promotes, in particular, that neither control or regulation nor modulation of the voltage vector is required, but rather the setting of the discharge voltage vector can be carried out particularly quickly and easily, i.e. in particular with little computing time outlay and computing power outlay.Furthermore, the method may provide that an electrical decoupling of an electrical energy store and / or an operating event of the electrical machine and / or of a motor vehicle having the electrical machine is detected as a fault state, in particular as a function of an operating temperature of the electrical machine. As already described at the beginning, the detection of the fault state can form the trigger for the execution of the method. In this case, the method can be executed in particular only when an electrical decoupling of the electrical energy store, from which the electrical machine is fed or into which the electrical machine can feed energy by recuperation, is present in a decoupled manner or the decoupling is present.In this case, it is namely not possible for electrical energy for the operation of the electric machine or the change in the operating state to be able to be drawn from the electrical energy store or for electrical energy to be able to be fed into the electrical energy store during a generator operation of the electric machine. This has the result that, in the event of an operating point change upstream of the AKS circuit, an impermissible voltage increase occurs by another method. A direct AKS circuit (without previous operating point change) would avoid this, but high transient currents arise, which in turn cause high opposing fields. If, in contrast, the electrical energy store is not decoupled, it is also possible to use methods other than the method described herein, since in this case no consideration is required for the generated power when the current operating point changes.Further, the fault condition may be detected as an operating event of the electric machine and / or the motor vehicle having the electric machine. Such fault states can describe accident states or what are known as "crash" states, for example. Specifically, the fault state can include the current operating event of the electric machine, i.e. whether it is operated in a motor operating state or in a generator operating state. As already described, in particular the intermediate circuit can be charged in a generator operating state, namely when the electrical energy store is decoupled.Execution of the method described above may also be expedient in motor operation, since, for example, the regulation of the voltage no longer functions correctly, since, for example, the voltage vector no longer receives a controlled variable. As described above, the discharge voltage vector can be optionally executed in the described cases, but is not necessary in all current operating states or operating events. When detecting the fault state, the operating temperature of the electric machine can also be taken into account in all states. If the operating temperature is below a temperature threshold value, for example, the execution of the method may be dispensed with, since demagnetization may not be possible below the temperature threshold value. Otherwise, for example, if the temperature limit value is exceeded, the method can be carried out to prevent demagnetization of the electric machine in order to change the current operating point to the changed operating point and only then to assume or execute the active short-circuit state.The described method can be further developed to the effect that the discharge voltage vector and / or the change voltage vector are set for a defined time period or variable time period, in particular depending on a current operating point. In the first alternative, a defined time period for which the vectors can be set can be predefined for the discharge voltage vector and / or the change voltage vector. In this case, a first time period or discharge time period for the discharge voltage vector, if it is set, can be predefined. Furthermore, a second time period or change time period can be predefined for the change voltage vector, which can differ from the first time period.According to the second alternative, it can be provided that the defined time period, i.e. the first time period and / or the second time period, can be defined as a function of a current operating point. This means that, for example, the first time period and / or the second time period can be changed depending on the current and / or voltage. Depending on how high the voltage is currently, the discharge voltage vector can be set to be longer or shorter, for example. It is likewise possible to define, depending on the current, for example in the case of high currents, a longer time period for setting the change voltage vector or, in the case of lower currents, a shorter time period for setting the change voltage vector. As already described, after the change voltage vector has been set, namely after the changed operating point has been assumed, the transition to the active short-circuit state is carried out.In addition, the method can provide for the discharge voltage vector and / or the change voltage vector to be statically set or to be tracked, in particular on the basis of a change in the current vector. According to the first described variant, the discharge voltage vector and / or the change voltage vector can be statically set. This means that they are determined and remain unchanged over the duration of execution. In other words, the discharge voltage vector and / or the change voltage vector is statically set once and is not changed until the voltage has been sufficiently reduced by the discharge voltage vector or until the changed operating state has been reached by setting the change voltage vector.In the second alternative, it can be provided that the discharge voltage vector and / or the change voltage vector can be tracked. For example, during the duration of the setting of the discharge voltage vector and / or of the change voltage vector, a change in the current vector can be detected. As already described, the discharge voltage vector and / or the change voltage vector can be determined based on the detected current vector. By detecting the change in the current vector, it is thus possible to determine and set different discharge voltage vectors or change voltage vectors at different times, i.e. they can be tracked together with the current vector, or the discharge voltage vector and / or the change voltage vector can be set so that it or these "rotate" with the current vector.The method can furthermore be further developed to the effect that the discharge voltage vector and / or the change voltage vector are determined cyclically, in particular before the occurrence of the fault state. In other words, the determination can be carried out cyclically across the operation of the electric machine, such that for the current operating state, i.e. the current detected current vector, the corresponding discharge voltage vector and / or the change voltage vector is always already determined. As soon as a fault state occurs or has been detected, the discharge voltage vector determined last and / or the change voltage vector determined last can be set directly without further computing effort or loss of time. This reduces the computing effort and the time associated with the determination process when the fault state occurs or when the fault state is detected. Instead, the already determined vectors can be accessed and they can be set directly.In addition to the described method, the invention relates to a control device for controlling an operation of an electric machine, in particular of a motor vehicle, in a fault state, wherein the control device is designed to record a current current vector of the electric machine and to determine a change voltage vector as a function of the recorded current vector and to change the current operating point of the electric machine into a changed operating point by setting the change voltage vector in such a way that the power generated by the change of the operating point is equal to the, in particular thermal, power loss or less than the, in particular thermal, power loss of the electric machine, and to execute an active short-circuit state starting from the changed operating point. The control device can be designed, for example, as an inverter of the drive arrangement or of the motor vehicle or the control device can comprise such an inverter.The invention also relates to a drive arrangement which comprises an electric machine, an electrical energy store and a control device described above. The invention further relates to a motor vehicle which comprises a drive arrangement described above and / or a control device described above.All advantages, details and features described with respect to the method can be completely transferred to the control device, the drive arrangement and the motor vehicle.The invention is explained below on the basis of exemplary embodiments with reference to the figures. The figures are schematic representations and show: FIG. 1 is a schematic flow diagram of a method for controlling the operation of an electric machine; FIG. 2 is a schematic current diagram; FIG. 3 is a schematic state diagram; and FIG. 4 is a schematic voltage diagram.FIG. 1 shows, by way of example, with reference to blocks 1- 5, the execution of the method described herein for controlling the operation of an electric machine, in particular of a drive device of a motor vehicle. The electric machine can be part of a drive arrangement which comprises, for example, an electrical energy store, an inverter and the electric machine. The inverter may be part of a control device or may form the control device which is configured to control the operation of the electric machine.The method starts, for example, in block 1, in which a fault state of the electric machine is present or detected. In block 1, it is possible in principle to select whether the method described herein is to be carried out. If, for example, there is no fault state, regular operation of the electric machine can be continued. When detecting the fault state, it can be determined, for example, whether the electrical energy store of the drive arrangement is electrically coupled or whether a so-called battery shedding has been carried out, in which the electrical energy store has been electrically decoupled from the drive arrangement, such that there is no longer an electrical connection. In such a case, for example, no acquisition of electrical energy from the electrical energy store is possible on the part of the electrical machine and also no feeding of electrical energy into the electrical energy store is possible.Optionally, further operating events of the motor vehicle, of the electric machine or of the drive arrangement can be detected, for example whether an accident state or "crash" state is present. Optionally, it is also possible to detect in block 1 which operating temperature the electric machine has or whether the current operating temperature is above or below a temperature limit value. The method described herein is carried out in particular when the operating temperature of the electric machine is above the temperature limit value, since the probability of demagnetization of the permanent magnets of the electric machine is then higher. If it is determined in block 1 that an error state is present or is present, which requires execution of the method, the method branches from block 1 to block 2.In one embodiment of the method, blocks 2- 4 may be executed cyclically during the operation of the electric machine or of the drive arrangement, so that the steps for determination or determination, which are described below with reference to blocks 2- 4, are already executed, so that processing of blocks 2- 4 is limited to setting the vectors already determined. It is likewise possible to capture the vectors, in particular with sufficient computing power, only when an error state occurs.In block 2, the current vector 6 or the "current pointer" of the electric machine is detected. For example, in this figure, FIG. 3 is shown in the state diagram which represents the hexagon of the static switching states of the inverter, for example with respect to a B6 bridge, in the form of the vectors v1-v6. These vector segments I-VI are bounded from one another. In principle, any inverter or any control device can be used to actuate the electric machine. The corresponding state diagram can be transferred to such cases and is used only for illustrative purposes in the specific example in the present case.In the exemplary embodiment shown, the current current vector 6 lies, for example, between the static state vectors v 4 and v 5, i.e. in the vector segment IV. The state can, however, be changed as desired, such that the current vector 6 can also lie in any other desired vector segment I-VI or on one of the state vectors v 1-v 6. As described, the current vector 6 can be determined cyclically automatically in the operating state, so that in block 2 the previously determined or detected current vector 6 can be output directly.Optionally, the method can branch from block 2 to block 3, in which a discharge voltage vector 7 can be determined. For example, this can be carried out if the intermediate circuit voltage 9 (cf. FIG. 4 ) in the intermediate circuit is above a defined voltage limit value 10. If the fault state is detected, for example, on the basis of a generator operation of the electric machine or the fault state occurs in a generator operation of the electric machine, so that no current can flow into the electric energy store as a result of the electrical decoupling of the electric energy store, this leads to a charging of the intermediate circuit, so that the intermediate circuit voltage 9 is increased. If this voltage is above the voltage limit value 10, the voltage can first be reduced by the discharge voltage vector 7 in order subsequently to improve or enable the further execution of the method. The voltage limit value in FIG. 4 is, purely by way of example, approximately 850 V. The transition to the time domain 14 takes place after the expiration of the previously defined time period 11.FIG. 3 shows the discharge voltage vector 7. The discharge voltage vector 7 can be set, for example, for a first time period 11 (cf. FIG. 4 ) or discharge time period. If the discharge voltage vector 7 is not to be set, the system can branch directly from block 2 to block 4. The discharge voltage vector 7 is determined as a function of the detected current vector 6. For this purpose, it is sought that the discharge voltage vector 7 runs as parallel as possible to the current vector 6, so that active power can be generated in order to reduce the intermediate circuit voltage 9 in the intermediate circuit. An ideal discharge voltage vector 7' is schematically shown in FIG. 3. Since the ideal discharge voltage vector 7' lies between the static state vectors v4, v5, comparatively complicated actuation is necessary for setting the discharge voltage vector'. In order to simplify the control, the discharge voltage vector 7 is advantageously fixed for the described first time interval 11 to the static state vector v 4, which bounds the vector segment IV in which the current vector 6 is currently located.Purely by way of example, the current vector 6 is currently located in the vector segment IV of the alpha-beta coordinate system which is bounded by the state vectors v4, v5. The discharge voltage vector 7 can therefore be set, by way of example, to the rear static state vector v 4, which bounds the vector segment IV in which the current vector 6 is currently located, to the rear. Alternatively, the discharge voltage vector 7 may also be set to the forward state vector v 5 that forwardly bounds the current vector segment iV. As a result, the control of the inverter can advantageously be kept particularly simple, since no modulation is required, but rather the switching position can be kept constant for the first time interval 11. Alternatively, setting of the discharge voltage vector 7' is also possible, for example if enough computing power or sufficiently rapid activation is possible.As described, the setting of the discharge voltage vector 7 is purely optional and, if it does not require the intermediate circuit voltage 9 in the intermediate circuit, it can also be dispensed with. Purely by way of example, FIG. 4 shows a fault state in which the discharge voltage vector 7 is set. For example, starting from a regular operating state, a fault state is detected at a point in time 8, so that the electrical energy store is decoupled. In the exemplary embodiment shown, the electric machine was operated in generator operation, so that the intermediate circuit voltage 9 increases as a result of decoupling the electric energy store.By way of example, the intermediate circuit voltage 9 exceeds the voltage limit value 10 in this case, with the result that the discharge voltage vector 7 is set, as described. Subsequently, as soon as the first time interval 11 has elapsed, the method can branch from block 3 to block 4. If, as likewise described above, the voltage limit value 10 is not exceeded, for example when the electric machine is operated in a motor mode, the setting of the discharge voltage vector 7 can be dispensed with. For example, the discharge voltage vector 7 is set for the first time period 11. The first time span 11 can be statically predefined or can be defined as a function of the current operating point, for example, of the intermediate circuit voltage. For example, the first time span 11 can be selected such that the intermediate circuit voltage 9 falls below the voltage limit 10 again after the expiration of the first time span 11.In block 4, a change voltage vector 12 is determined based on the current vector 6 detected in block 2. As described, this may be done cyclically before the fault condition occurs. In the exemplary embodiment shown, the change voltage vector 12 is determined with a defined phase shift 13 with respect to the current vector 6. For example, as shown in FIG. 3, the variation voltage vector 12 is shifted by a phase angle in a range of π / 2 from the current vector 6. Alternatively, the phase shift can also be shifted in the opposite direction, i.e. in a range of -π / 2 to the current vector 6. This has the effect that no active power is generated by the setting of the change voltage vector 12 and the current operating point of the electric machine can therefore be transferred to a changed operating point without generating power. In this case, the phase angle or the phase shift 13 can deviate from π / 2 and can be, for example, in a range from 250° to 270°, in particular 255° to 265°.Although a low active power is generated in this case, this is compensated for by the losses within the electric machine, so that the voltage in the intermediate circuit does not increase. The phase shift 13 can be further different from π / 2 the more power loss is generated in the electric machine. For this purpose, at least one power dissipation element, for example a resistor, a varistor or the like, can be provided, with which power can be converted into heat in a targeted manner. The power dissipation element is arranged, for example, parallel to the intermediate circuit. Furthermore, the phase shift 13 can also be defined as an operating point-dependent function, for example as a function of which power loss is currently being generated in the electric machine. If, for example, high currents flow in the electric machine, higher losses are to be expected, so that the phase shift 13 can be selected to be correspondingly greater.FIG. 4 shows that the change voltage vector 12 is set for a second time period 14 or a change time period. As a result, as is shown for example in FIG. 2 for two different current operating points 15, 15', the change of the operating point does not follow a straight line but a curved path. By way of example, a spiral path 17, 17' is shown, which represents a direct AKS circuit. The operating point change achieved by the change voltage vector 12 takes place on a curved path 18 from operating point 15, 15' to operating point 16, 16' (shown in dashed lines) and therefore in a more direct manner compared to the spiral path 17. As a result, the operating point can be approximated to a desired operating point over the second time period 14, or the current operating point 15, 15' can be approximated as close as possible to the desired operating point which represents the center point of the spiral. This means that the current operating point 15, 15' can be changed to a changed operating point 16, 16' which is as close as possible to the center point of the spiral which corresponds to the steady-state ACS flow, wherein starting from current operating points 15, 15' the spirals shown are followed and at the end of the second time period 14 a changed operating point 16, 16' which is drawn in by way of example and which is located on the spiral and is as close as possible to the center point of the spiral is taken.The generated discharge voltage vector 7 and the generated change voltage vector 12 can be generated statically, so that they do not change over the periods 11, 14. It is likewise possible for these to be tracked with the rotating current indicator 6, i.e. to be continuously re-determined and set at different points in time within the time periods 11, 14.If the vectors 7, 12 are statically set, this has the advantage that the computing effort and the control effort can be significantly reduced. If these are continuously tracked, the accuracy of the method can be improved.Starting from the changed operating point 16, 16' reached in block 4, a branch can be made to block 5, in which the active short-circuit state can be executed in order to transfer the electric machine or the electric drive arrangement into the safe state. The active short-circuit state follows the second time period 14, for example in FIG. 4. As can be seen, the setting of the change voltage vector 12 allows the operating point 15, 15' to be changed in a power-neutral manner into the changed operating point 16, 16', with the result that no high transient currents can flow during the transition into the active short-circuit state and consequently also no high opposing fields are produced. The proposed method thus enables a dispensing with or at least a reduction of corresponding rare earths in the electric machine without risking demagnetization in a fault state, in particular a fault state above a temperature limit value with decoupled electric energy store.As described, the method can be carried out on a control device, in particular by means of an inverter. The control device or the inverter and the electric machine can be part of a drive arrangement. The drive arrangement is arranged in particular in a motor vehicle. All the details described with respect to the method can therefore also be transferred to the control device, the drive arrangement and the motor vehicle. All the advantages, details and features shown in the individual exemplary embodiments can be combined with one another as desired, interchanged and transferred to one another.Reference numerals denote reference numerals1-5 Block 6 current vector 7, 7' discharge voltage vector 8 time point 9 intermediate circuit voltage 10 voltage limit value 11 first time period 12 change voltage vector 13 phase shift 14 second time period 15, 15' current operating point 16, 16' changed operating point 17 spiral path 18 curved path

Claims

Method for controlling an operation of an electric machine, in particular of a motor vehicle, in a fault state, comprising the steps of: - detecting a current current vector (6) of the electric machine; - determining a change voltage vector (12) as a function of the detected current current vector (6); - changing the current operating point (15, 15') of the electric machine into a changed operating point (16, 16') by setting the change voltage vector (12), wherein the power generated by the change of the operating point (15, 15') is equal to the, in particular thermal, power loss loss or less than the, in particular thermal, power loss of the electric machine; - carrying out an active short-circuit state on the basis of the changed operating point (16, 16').Method according to Claim 1, characterized in that the change voltage vector (12) is determined with a defined phase shift (13), in particular in the range + / - Pi / 2, with respect to the detected current vector (6).Method according to Claim 2, characterized in that the change voltage vector (12) is displaced as a function of at least one power loss element by an additional displacement with respect to the detected current vector (6).Method according to one of the preceding claims, characterized in that before the change voltage vector (12) is set, a discharge voltage vector (7, 7') is set for reducing the voltage, in particular depending on the detected current vector (6).Method according to claim 4, characterised in that the discharge voltage vector (7, 7') is set to a static state vector (v1-v6) which delimits the vector segment (I-VI) in which the current current vector (6) is detected.Method according to one of the preceding claims, characterized in that an electrical decoupling of an electrical energy store and / or an operating event of the electrical machine and / or of a motor vehicle having the electrical machine is detected as the fault state, in particular as a function of an operating temperature of the electrical machine.Method according to one of the preceding claims, characterized in that the discharge voltage vector (7, 7') and / or the change voltage vector (12) are set for a defined time period (11, 14) or variable time period (11, 14), in particular depending on a current operating point.Method according to one of the preceding claims, characterized in that the discharge voltage vector (7, 7') and / or the change voltage vector (12) is statically set or is tracked, in particular on the basis of a change in the current vector (6).Method according to one of the preceding claims, characterized in that the discharge voltage vector (7, 7') and / or the change voltage vector (12) is determined cyclically, in particular before the occurrence of the fault state.Control device for controlling an operation of an electric machine, in particular of a motor vehicle, in a fault state, wherein the control device is designed to record a current current vector (6) of the electric machine and to determine a change voltage vector (12) as a function of the recorded current current vector (6) and to change the current operating point (15, 15') of the electric machine into a changed operating point (16, 16') by setting the change voltage vector (12) such that the power generated by the change of the operating point (15, 15') is equal to the, in particular thermal, power loss or less than the, in particular thermal, power loss of the electric machine, and to execute an active short-circuit state starting from the changed operating point (16, 16').A drive arrangement comprising an electric machine, an electric energy store and a control device according to the preceding claim.Motor vehicle, comprising a drive arrangement according to the preceding claim and / or a control device according to claim 10.

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