Method and device for the secured operation of an electrical machine

Abstract

The invention relates to a device for operating an electrical machine using an inverter. The device is configured to determine, for a first cycle, an operating switching pattern for the operation of the switching group within the first cycle, which is designed to bring about a desired operating point of the electrical machine, and to determine a sequence of transition switching patterns for a sequence of successive cycles, which is designed to transfer the electrical machine from an actual operating point to a safe state.

Description

[0001] Method and device for the safe operation of an electrical

[0002] machine

[0003] The invention relates to a method and a corresponding device that enable gentle, safe operation of an electrical machine.

[0004] An electrically powered vehicle comprises an electric drive motor powered by electrical energy from an electrical energy storage device. The alternating current for operating the drive motor can be generated using an inverter.

[0005] In the event of a safety-relevant event, e.g., a vehicle accident, it may be necessary to transfer the electrical machine to a safe state in which no critical voltages are present at the phase connections of the electrical machine. Transferring the electrical machine to the safe state may involve a relatively high load on components, particularly the switching elements, of the inverter and / or on components, particularly the magnets, of the electrical machine, which makes it necessary to design the components of the inverter and / or the electrical machine for a correspondingly high load.

[0006] This document deals with the technical task of enabling a particularly efficient and reliable design of an inverter and / or an electrical (drive) machine.

[0007] The problem is solved by each of the independent claims. Advantageous embodiments are described, among other things, in the dependent claims. It should be noted that additional features of a patent claim dependent on an independent patent claim can form a separate invention, independent of the combination of all features of the independent patent claim, without the features of the independent patent claim or only in combination with a subset of the features of the independent patent claim, which invention can be made the subject of an independent claim, a divisional application, or a subsequent application. This applies equally to technical teachings described in the description, which can form an invention independent of the features of the independent patent claims.

[0008] According to one aspect, a device for operating an electrical machine by means of an inverter is described. The electrical machine can be designed to drive a motor vehicle. The inverter is designed to generate at least one corresponding phase voltage for operating the electrical machine using at least one switching group with one or more switching elements. A switching group can comprise a half-bridge with a high-side switching element and a low-side switching element. The individual switching elements can each comprise a transistor, e.g. a MOSFET or an IGBT. The electrical machine typically comprises three phases (or more). The inverter can comprise three switching groups for the corresponding three phases of the electrical machine, each of which is designed to generate a phase voltage for a corresponding phase of the electrical machine.

[0009] The device is configured to determine, for a first cycle (from a sequence of consecutive first cycles), an operating switching pattern for the operation of the switching group within the first cycle, wherein the operating switching pattern is designed to effect a target operating point of the electrical machine. The operating switching pattern can be determined using a control system (e.g., a current control system) that is configured to acquire sensor data relating to the actual operating point of the electrical machine and, based thereon, to determine the operating switching pattern. The operating switching pattern can be used for normal operation of the electrical machine.

[0010] The device is configured to determine, for the first cycle and in addition to the operating switching pattern, a sequence of transition switching patterns for a sequence of (directly) consecutive cycles, which is designed to transition the electric machine into a safe state starting from the actual operating point of the electric machine. The sequence of (directly) consecutive cycles can begin with the respective first cycle or directly after the respective first cycle.

[0011] The safe state may include an active short circuit (ACS) of several (especially all) phases of the electrical machine. Alternatively or additionally, the safe state may include

[0012] • a phase voltage that is lower than a predefined voltage threshold (while the actual and / or target operating point of the electrical machine has a phase voltage that is higher than the voltage threshold, e.g. higher than the voltage threshold by a factor of 2 or more); and / or

[0013] • a mechanical power of the electrical machine that is less than a predefined power threshold (while the actual and / or target operating point of the electrical machine has a mechanical power that is greater than the power threshold, e.g. a factor of 2 or more greater than the power threshold).

[0014] Thus, in addition to the operating switching pattern, a sequence of transition switching patterns can be determined which can be used, if necessary (i.e., in the event of a fault, or generally when there is a requirement to transition to the safe state), to transfer the electrical machine to the safe state. The sequence of transition switching patterns can be determined (in advance) using a model, in particular a mathematical model, of the electrical machine. The sequence of cycles can, for example, comprise between 5 and 15 cycles. The individual cycles can, for example, each have a duration of 200 ps or less, in particular 100 ps or less.

[0015] By proactively determining a sequence of transition switching patterns, an efficient and reliable transition to the safe state can be achieved when needed, particularly without causing (significant) overshoots in one or more phase currents. This enables an efficient design of the inverter and / or the electrical machine. The transition to the safe state can also be achieved if the control system for operating the electrical machine has failed completely or partially due to the fault.

[0016] The individual switching patterns for a cycle may each comprise one or more pulses, each of which indicates the time (within the cycle) and / or the duration (within the cycle) for the closing of a switching element (in particular the high-side switching element or the low-side switching element) of the respective switching group during the cycle.

[0017] The device can be configured to determine an operating switching pattern and a sequence of transition switching patterns for the respective first cycle and for all three (or more) switching groups of the inverter. Corresponding switching patterns of different phases can be identical, except for a phase shift (e.g., of 120° each).

[0018] The device can be configured to check (for the respective first cycle) whether there is a request to transfer the electrical machine to the safe state. In particular, it can be checked whether or not there is a fault in the electrical machine. The inverter can be operated in the first cycle according to the determined operating switching pattern (without using the sequence of transition switching patterns) in order to bring about the target operating point of the electrical machine if it is determined that there is no request to transfer the electrical machine to the safe state (in particular that there is no fault in the electrical machine). On the other hand, the inverter can be operated in the sequence of consecutive cycles (beginning with the respective first cycle) according to the determined sequence of transition switching patterns (if applicable).without using the operating switching pattern) to transfer the electrical machine (starting from the actual operating state) to the safe state when it is determined that there is a requirement to transfer the electrical machine to the safe state (in particular that there is a fault in the electrical machine and / or that the transition to the safe state is requested (e.g. in response to an accident and / or in the event of a data communication failure)).

[0019] Thus, the determined operating switching pattern can be used selectively (to continue the normal operation of the electrical machine), or alternatively the sequence of transition switching patterns can be used (to terminate the normal operation of the electrical machine in the event of a fault and to transfer the electrical machine to the safe state).

[0020] The device can be configured to determine, for a sequence of consecutive first cycles, an operating switching pattern for the operation of the switching group within the respective first cycle, and to determine a sequence of transition switching patterns for the sequence of consecutive cycles starting from the respective first cycle (in particular for the sequence beginning with the respective first cycle). The sequence of transition switching patterns determined for the respective first cycle can be stored in a (secure) memory unit. Alternatively, the sequence of transition switching patterns determined for the respective preceding first cycle can be deleted from the memory unit (and overwritten with the currently determined sequence of transition switching patterns).

[0021] Thus, the most recently determined sequence of transition switching patterns (if necessary, only) can be stored in a memory unit, making it available for transferring the electrical machine to the safe state when needed. The memory unit can be separate from the device, if necessary, to ensure that the electrical machine can be transferred to the safe state even if the device is compromised. This can ensure particularly safe operation of the electrical machine.

[0022] The device can be configured to determine the sequence of transition switching patterns for the sequence of consecutive cycles such that the sequence of transition switching patterns ends in a (safe) state in which (all) high-side switching elements of the three (in particular all) switching groups of the inverter or (alternatively) (all) low-side switching elements of the three (in particular all) switching groups of the inverter are permanently switched on (i.e., closed). For example, (possibly only) the last transition switching pattern of the sequence of transition switching patterns can cause the high-side switching elements or the low-side switching elements to remain permanently closed. One or more other states of the inverter can be effected by the one or more preceding transition switching patterns (in particular to limit the phase currents). In this way, the AKS can be effected in a particularly reliable and gentle manner.

[0023] The device can be configured to determine the sequence of transition switching patterns such that during the entire transition to the safe state,

[0024] • the phase currents of one or more phases of the electrical machine do not exceed a certain, predefined, current threshold value; the current threshold value may correspond to the maximum value of the individual phase currents during (normal) operation of the electrical machine; and / or

[0025] • the sequence of transition switching patterns comprises a plurality of different transition switching patterns (e.g. two or more, or three or more, or four or more different transition switching patterns).

[0026] According to another aspect, an inverter is described which comprises the device described in this document.

[0027] According to a further aspect, a (road) motor vehicle (in particular a passenger car or a truck or a bus or a motorcycle) is described which comprises the device described in this document.

[0028] According to a further aspect, a method for operating an electric machine (in particular a drive motor of a motor vehicle) using an inverter is described. The method comprises, for a first cycle (from a sequence of consecutive first cycles), determining an operating switching pattern for the operation of the switching group within the first cycle, which is designed to bring about a desired operating point of the electric machine, and determining a sequence of transition switching patterns for a sequence of consecutive cycles, which is designed to transfer the electric machine from an actual operating point to a safe state.

[0029] According to another aspect, a software (SW) program is described. The SW program can be configured to be executed on a processor (e.g., on a vehicle control unit) and thereby to carry out the method described in this document.

[0030] According to a further aspect, a storage medium is described. The storage medium can comprise a software program configured to be executed on a processor and thereby to carry out the method described in this document.

[0031] It should be noted that the methods, devices, and systems described in this document can be used both alone and in combination with other methods, devices, and systems described in this document. Furthermore, any aspects of the methods, devices, and systems described in this document can be combined in a variety of ways. In particular, the features of the claims can be combined in a variety of ways. Furthermore, features listed in parentheses are to be understood as optional features.

[0032] The invention will be described in more detail below using exemplary embodiments.

[0033] Figure 1a shows exemplary components of a vehicle with an electric drive motor; Figure 1b shows an exemplary inverter or converter for an electric motor of a vehicle;

[0034] Figure 2a shows an exemplary control circuit for operating an electrical machine;

[0035] Figure 2b shows an exemplary sequence of switching patterns for operating an inverter; and

[0036] Figure 3 is a flowchart of an exemplary method for determining switching patterns for the operation of an inverter.

[0037] As stated at the outset, the present document is concerned with enabling a particularly efficient and reliable design of an inverter and / or an electric machine. In this context, Fig. 1a shows exemplary components of a vehicle 140 having an electric machine 103 for driving the vehicle 140. The vehicle 140 can be a vehicle 140 with a hybrid drive or a vehicle 140 with a fuel cell stack (fuel cell electric vehicle, FCEV). The electric machine 103 is coupled to one or more wheels 141 of the vehicle 140 in order to drive the one or more wheels 141 and thus the vehicle 140. The electric machine 103 is operated with electrical energy from an electrical, in particular an electrochemical, energy storage device 130. The energy storage device 130 can be designed to provide a direct current with a specific direct voltage.

[0038] The vehicle 140 has an inverter 100 configured to generate phase voltages for the different phases of the electric machine 103 based on the DC voltage from the energy storage device 130. The inverter 100 can be operated by a (control) device 101.

[0039] Fig. 1b shows an exemplary inverter 100 configured to

[0040] Basis of a vehicle electrical system voltage UDC 110 (e.g. the DC voltage of the

[0041] Energy storage device 130) to generate phase voltages 111 (i.e., alternating voltages) for the coils of the stator of the electric machine 103. The inverter 100 can have an intermediate circuit with an intermediate circuit capacitor 105, to which the vehicle electrical system voltage UDC 110 is applied.

[0042] The inverter 100 comprises a plurality of switches or switching elements 102, 104, which in the illustrated example are each arranged in a half-bridge (generally in a switching group) for each phase 121, 122, 123 (e.g., for three phases). The switching elements 102, 104 are controlled by the (control) device 101 to generate the phase voltages 111 for the electrical machine 103. The individual phase voltages 111 and / or the resulting phase currents 112 can be provided to the electrical machine 103 via corresponding phase lines.

[0043] In order to operate the electric machine 103 at a specific operating point, e.g., at a specific speed and / or with a specific torque, a control loop 200 (or a control system) can be used, as shown by way of example in Fig. 2a, which can be used, for example, to effect specific target properties of the phase currents 112. For example, a specific target value 202 can be specified for the phase currents 112. The target value 202 can be specified, for example, in a dq coordinate system or in an aß coordinate system. The coordinate transformations are not shown in Fig. 2a for the sake of simplicity.

[0044] Based on the control error 203 (which results from the setpoint 202 and the actual value of the phase currents 112), a control value 204 can be determined using a controller 201. The control value 204 can, for example, correspond to a specific voltage vector in the dq coordinate system or in the aß coordinate system. A switching pattern for operating the inverter 100 can be determined from the voltage vector using a modulation method. The inverter 100 can then be operated with the determined switching pattern to generate the phase voltages 111.

[0045] Using the control loop 200, operating switching patterns for the operation of the inverter 100 can be determined for a temporal sequence of modulation periods (generally cycles) that follow one another at a specific repetition rate or frequency (e.g., 10 kHz or more). The inverter 100 can then be operated in the sequence of modulation periods according to the respectively determined operating switching pattern in order to set the desired operating point of the electric machine 103 corresponding to the desired value 202.

[0046] A switching pattern can each have a sequence of one or more switching pulses, wherein a switching pulse indicates the time and duration at which a switching element 102, 104 of a switching group of the inverter 100 is closed in order to couple the respective phase to the high-side voltage (e.g. +UDC / 2) or to the low-side voltage (e.g. -UDC / 2), and thus to generate a (possibly sinusoidal) phase voltage 111.

[0047] It may be necessary to transfer the electric machine 103 to a safe state in the event of a fault, e.g., an accident involving the vehicle 140. The safe state may exist when the electric machine 103 only generates a relatively small torque (less than a torque threshold) and the external electrical terminals of the electric machine 103 do not have an excessively high voltage (which is thus less than a voltage threshold).

[0048] In particular, in the case of a permanently excited electrical machine (PSM) 103, the inverter 100 can suddenly short-circuit all phase connections of the electrical machine 103 in order to transfer the electrical machine 103 to the safe state. This can be achieved by closing all high-side switching elements 102 or by closing all low-side switching elements 104. This measure can be referred to as an active short circuit (ACS). The ACS can be set suddenly and / or permanently by the inverter 100. This means that the (possibly rotating) electrical machine 103 cannot generate any electrical voltage at the phase connections and, as a result, does not generate any significant electrical power and thus does not generate any significant mechanical conduction or torque.

[0049] The phase current 112 that occurs after the transient events have subsided is typically smaller than the typical continuous phase current of the electric machine 103. On the other hand, the sudden switching in the AKS can lead to a transient increase in the phase current 112 (e.g., up to 2 to 4 times the maximum permissible phase current). This short-term current can determine the design and thus the cost of the inverter 100. Furthermore, this transient current can determine the design and thus the cost of the exciting magnets of the electric machine 103.

[0050] If the transition to the AKS occurs in a controlled manner rather than abruptly, the transient increase in phase current 112 (beyond the maximum permissible operating current) can be avoided. The controlled transition from the actual operating state of the electrical machine 103 to the safe AKS state can be achieved by gradually adjusting the operating point of the electrical machine 103 over a sequence of modulation periods. The voltage vectors and the corresponding switching patterns for the sequence of modulation periods can be determined using the control loop 200.

[0051] Transition to the safe state, in particular to the AKS state, occurs primarily in the event of a fault in which it cannot be guaranteed that all components (e.g., current sensors, voltage sensors, computing resources, rotor position sensors, etc.) are available for the operation of the control loop 200. A safe and / or reliable transition to the safe state should therefore be achieved with as few components of the control loop 200 as possible, in particular without any of them.

[0052] The transition to the safe state, in particular to the AKS state, can be predictively calculated, e.g., cyclically, approximately in each modulation period or in each cycle, while the control system 200 is still in a known fault-free state, and the required temporal sequence of switching operations of the inverter 100 can be stored in a protected and / or independent part of the (control) device 101. This protected part of the (control) device 101 can be activated in the event of a fault and can then autonomously and independently execute the predicted temporal sequence of switching operations for the inverter 100, which results in the electrical machine 103 being transferred to the safe state.

[0053] The sequence of switching operations can be calculated by setting, in particular regulating, the state of the electrical machine 103 to a terminal voltage of 0V in several preceding modulation periods or cycles on a machine model of the electrical machine 103, based on the respective current actual operating state of the electrical machine 103. The switching operations (or pulse patterns or switching patterns) of the inverter 100 required for this purpose are determined and stored. This advance calculation can follow for the cycle for which the regular pulse or switching pattern for the operation of the electrical machine 103 is also calculated.

[0054] Fig. 2b shows the operating switching pattern 221 determined for the first (e.g., current) cycle (i.e., modulation period) 211, which is used to set a specific target operating point of the electrical machine 103. The operating switching pattern 221 can be determined using the control loop 200. Furthermore, for a sequence of cycles 212 (which optionally directly follows the first cycle 211 or starts with the first cycle 211), a corresponding sequence of transition switching patterns 222 can be determined, which can be used to transfer the electrical machine 103 to the safe state, in particular to the AKS state, in the event of a fault. The sequence of transition switching patterns 222 can be determined using a mathematical model of the electrical machine 103.

[0055] A corresponding procedure can be followed for temporally successive first cycles 211. In each case, an operating switching pattern 221 (for the desired and / or normal operation of the electric machine 103) and a sequence of transition switching patterns 222 (for a possible transition to the safe state) can be determined.

[0056] By precalculating a sequence of transition switching patterns 222, a transient short-circuit current can be reliably and functionally safely avoided during the transition to the safe state. The transient short-circuit current is thus no longer relevant to the design of the inverter 100 and / or the electric machine 103, which leads to a cost reduction of the inverter 100 (smaller semiconductor area) and / or the electric machine 103 (lower requirement for rare earths).

[0057] Alternatively, the electric machine 103 can be efficiently designed for increased torque density.

[0058] As explained above, a mathematical model of the electric machine 103 can be used to determine the sequence of transition switching patterns 222. An exemplary model (for a permanent magnet synchronous machine) in the dq coordinate system includes the following equations: where U d and U q the phase voltages 111 and i d and i q the phase currents 112 of the electrical machine 103 in the dq coordinate system. L d and L q are inductances of the electrical machine 103, and r s is an ohmic resistance of the stator of the electrical machine 103. m is the (electrical) speed of the electrical machine 103. pM is the magnetic flux of the permanent magnets of the electrical machine 103, p is the number of pole pairs, and M dis the torque produced by the electric machine 103.

[0059] For the safe state, the absence of voltage of the electrical machine 103 can be specified, ie U d = 0 and U q = 0. Assuming r s = 0 we get (approximately):

[0060] From i q = 0 also follows that the torque M d = 0.

[0061] In the case of a controlled transition to the safe state, the above-mentioned values ​​for the d- and q-current can be specified as setpoints 202. The control circuit 200 then gradually, in a sequence of cycles 211, causes the safe state (with U d = 0 and U q = 0) is reached.

[0062] During the predicted transition to the safe state, the mathematical model of the electric machine 103 can be used to determine step by step (for the sequence of cycles 212) the actual state of the electric machine 103 in the respective cycle 212, based thereon to determine the voltage vector for the inverter 100 and based thereon to determine the transition switching pattern 222 for the respective cycle 212.

[0063] As a final state, all low-side switching elements 104 or all high-side switching elements 102 can be closed in order to keep the electrical machine 103 in the safe state.

[0064] Fig. 3 shows a flowchart of a (possibly computer-implemented) method 300 for operating an electrical machine 103 (in particular a synchronous machine) by means of an inverter 100. The electrical machine 103 can be designed to drive a motor vehicle 140. The inverter 100 is designed to generate at least one corresponding phase voltage 111 for operating the electrical machine 103 using at least one switching group with one or more switching elements 102, 104. Typically, the inverter 100 has at least or exactly three switching groups for generating corresponding phase voltages 111 for the different phases of the electrical machine 103.

[0065] The method 300 can be repeated for a sequence of consecutive (first) cycles 211. The method 300 comprises, for a first cycle 211 from the sequence of first cycles 211, determining 301 an operating switching pattern 221 for the operation of the switching group within the first cycle 211, wherein the operating switching pattern 221 is directed to effect a target operating point of the electric machine 103. The target operating point can be defined, for example, by a specific torque and / or by a specific speed of the electric machine 103. The torque and / or the speed are (significantly) greater than a torque threshold or a speed threshold (which apply to the safe state of the electric machine 103). Thus, an operating switching pattern 221 can be determined that is directed to continuing to operate the electric machine 103 in normal operation.

[0066] The method 300 further includes (for the first cycle 211) determining 302 a sequence of transition switching patterns 222 for a sequence of consecutive cycles 212 (e.g., for two or more, or three or more, or four or more cycles 212), which is designed to transition the electric machine 103 from the respective actual operating point into the safe state (at which the torque is less than the torque threshold). The sequence of transition switching patterns 222 can be determined in advance and can optionally only be used (optionally used instead of the operating switching pattern 211) if a fault in the electric machine 103 is detected for the first cycle 211.

[0067] The present invention is not limited to the embodiments shown. In particular, it should be noted that the description and figures are intended only to illustrate the principle of the proposed methods, devices, and systems by way of example.

Claims

Claims 1) Device (101) for operating an electrical machine (103) by means of an inverter (100); wherein the inverter (100) is designed to generate at least one corresponding phase voltage (111) for the operation of the electrical machine (103) using at least one switching group with one or more switching elements (102, 104); wherein the device (101) is set up for a first cycle (211), - to determine an operating switching pattern (221) for the operation of the switching group within the first cycle (211), which is designed to bring about a desired operating point of the electrical machine (103); and - to determine a sequence of transition switching patterns (222) for a sequence of consecutive cycles (212) which is designed to transfer the electrical machine (103) from an actual operating point into a safe state. 2) Device (101) according to claim 1, wherein the device (101) is arranged - to determine the operating switching pattern (221) using a control system (200) which is designed to acquire sensor data relating to the actual operating point of the electric machine (103) and to determine the operating switching pattern (221) based thereon; and / or - to determine the sequence of transition switching patterns (222) based on a model, in particular a mathematical model, of the electrical machine (103). 3) Device (101) according to one of the preceding claims, wherein the device (101) is arranged - to check whether there is a requirement to transfer the electrical machine (103) to the safe state; - to operate the inverter (100) in the first cycle (211) according to the determined operating switching pattern (221) in order to effect the desired operating point of the electrical machine (103) when it is determined that there is no requirement to transfer the electrical machine (103) to the safe state; and - to operate the inverter (100) in the sequence of consecutive cycles (212) according to the determined sequence of transition switching patterns (222) in order to transfer the electrical machine (103) to the safe state when it is determined that there is a request to transfer the electrical machine (103) to the safe state. 4) Device (101) according to one of the preceding claims, wherein the device (101) is arranged for a sequence of successive first cycles (211) respectively, - to determine an operating switching pattern (221) for the operation of the switching group within the respective first cycle (211); and - to determine a sequence of transition switching patterns (222) for a sequence of successive cycles (212) starting from the respective first cycle (211). 5) Device (101) according to claim 4, wherein the device (101) is arranged - storing the sequence of transition switching patterns (222) determined for the respective first cycle (211) in a memory unit; and - the sequence of transition switching patterns (222) determined for the respective previous first cycle (211) from the memory unit to delete. 6) Device (101) according to one of the preceding claims, wherein a switching pattern (211, 222) for a cycle (211, 212) comprises one or more pulses each indicating a time and / or a duration for the closing of a switching element (102, 104) of the switching group during the cycle (211, 212). 7) Device (101) according to one of the preceding claims, wherein the safe state comprises - an active short circuit of several phases of the electrical machine (103); - a phase voltage (111) which is less than a predefined voltage threshold; and / or - a mechanical power of the electric machine (103) which is less than a predefined power threshold value. 8) Device (101) according to one of the preceding claims, wherein - the electrical machine (103) comprises three phases; - the inverter (100) comprises three switching groups for the corresponding three phases of the electrical machine (103), each of which is designed to generate a phase voltage (111) for a corresponding phase of the electrical machine (103); and - the device (101) is configured to determine an operating switching pattern (221) and a sequence of transition switching patterns (222) for the first cycle (211) and for the three switching groups of the inverter (100). 9) Device (101) according to claim 8, wherein the device (101) is arranged to determine the sequence of transition switching patterns (222) for the sequence of consecutive cycles (212) such that the Sequence of transition switching patterns (222) ends in a state in which high-side switching elements (102) of the three switching groups of the inverter (100) or low-side switching elements (104) of the three switching groups of the inverter (100) are permanently switched on. 10) Device (101) according to one of the preceding claims, wherein - a cycle (211, 212) has a duration of 200 ps or less, in particular 100 ps or less; and / or - the sequence of cycles (212) comprises between 5 and 15 cycles (212). 11) Method (300) for operating an electrical machine (103) by means of an inverter (100); wherein the inverter (100) is designed to generate at least one corresponding phase voltage (111) for the operation of the electrical machine (103) using at least one switching group with one or more switching elements (102, 104); wherein the method (300) comprises, for a first cycle (211), - determining (301) an operating switching pattern (221) for the operation of the switching group within the first cycle (211), which is designed to effect a desired operating point of the electric machine (103); and - determining (302) a sequence of transition switching patterns (222) for a sequence of consecutive cycles (212) which is designed to transfer the electrical machine (103) from an actual operating point into a safe state.

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