BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for operating an electrical machine in a short-circuit operation mode, in particular for application in a motor vehicle. 
         [0002]    Furthermore, the present invention relates to a device for operating an electrical machine in a short-circuit operation mode, in particular for use in a motor vehicle. 
         [0003]    Finally, the present invention relates to a motor-vehicle drive train having at least one electrical machine in a short-circuit operation mode and having a device for operating the electrical machine of the type mentioned above. 
         [0004]    A method such as this and a device such as this are known from the laid-open specification DE 10 2006 003 254 A1. 
         [0005]    It is generally known in the field of motor-vehicle drive-train technology to use an electrical machine as sole drive or together with a drive motor of another type (hybrid drive). Hybrid drives typically consist of a combination of an internal combustion engine and at least one electrical machine and the associated energy stores in the form of a fuel tank and a battery. Different types of hybrid drives exist, wherein a distinction is made between two basic structures, more particularly between series and parallel hybrid drives. A combination of both structures is also designated a power-split hybrid drive. 
         [0006]    The series hybrid drive is characterized by a series circuit of energy converters. This requires for example two electrical machines and one internal combustion engine. One of the electrical machines is operated as a generator; the other electrical machine is operated as a motor. The internal combustion engine itself is not connected to a drive train of the motor vehicle. It charges up the battery via the electrical machine operated as a generator and/or directly provides the required electrical energy to the electrical machine operated as a motor. The power which is necessary for the locomotion of the motor vehicle is therefore finally transferred to the drive train from the electrical machine operated as a motor. 
         [0007]    The parallel hybrid drive is characterized by both an internal combustion engine and an electrical machine being able to transmit the mechanical powers thereof to a drive train. By means of a mechanical coupling of the two machines and the drive train, the powers can be added together. The possibility of said power addition enables a relatively small dimensioning of the two machines, without disadvantages in terms of driving performance arising for the motor vehicle. 
         [0008]    There are different ways of implementing parallel hybrid drives. One possibility is to connect the electrical machine directly to a crankshaft of the internal combustion engine (crankshaft starter generator) or to couple the electrical machine with the internal combustion engine by means of a belt drive. The two drive sources can therefore be used both together and individually for the locomotion of the motor vehicle. The electrical machine can also selectively be operated as a generator or as a motor. 
         [0009]    The power-split hybrid drive is characterized by a combination of the principles which have already been illustrated. The use of a power-split transmission (planetary transmission) makes it possible to transmit a portion of the power of the internal combustion engine directly, that is to say mechanically, to the drive train while the remaining portion of the power is converted into electrical energy by means of a generator. Said electrical energy can in turn be stored in a battery or be transmitted directly to an electrical machine arranged downstream of the transmission. In the case of the power-split hybrid drive, both the electrical machine and the internal combustion engine can be used for the locomotion of the motor vehicle. 
         [0010]    A power electronics unit typically serves to control electrical machines in a motor vehicle and in particular in a hybrid vehicle. Said system includes an inverter, which converts a DC voltage or a direct current from the (high-voltage) battery on board the motor vehicle into an alternating current. The power electronics unit conducts a high voltage of typically 60 volts. 
         [0011]    If a fault occurs in the electrical machine, the machine is transferred into a safe state. The safe state for the electrical machine is typically the active short-circuit operation mode. 
         [0012]    Document DE 10 2006 003 254 A1, mentioned at the outset, describes an electrical machine having a power electronics unit in a short-circuit operation mode. To handle a fault, the electrical machine is first of all switched into a freewheeling mode and then into a short-circuit operation mode. 
         [0013]    In the case of the active short-circuit operation mode, motor windings of the electrical machine are shorted together by means of the power electronics unit. The short-circuit current which occurs owing to the short-circuit operation mode results in the electrical machine receiving a relatively small torque and therefore only slightly influences a drive train. However, if the electrical machine is erroneously put into a freewheeling mode instead of the active short-circuit operation mode, the torque (braking torque) received by the electrical machine increases. Said increased torque can, in particular at high rotational speeds, lead to the drive train being braked to the effect that a destabilization of the vehicle may result. In addition, a battery of the high-voltage system can be unintentionally charged as a result and damaged as a result. 
       SUMMARY OF THE INVENTION 
       [0014]    It is therefore an object of the invention to ensure that the electrical machine is operated in short-circuit operation mode in the event of a fault. 
         [0015]    The object is achieved by means of a method of the type mentioned at the outset, with the steps of: 
         [0016]    detecting at least a first and a second phase current of the electrical machine, 
         [0017]    comparing the detected phase currents with in each case at least one reference value and 
         [0018]    outputting an error message if at least one of the phase currents does not match the respective reference value. 
         [0019]    The above object is also achieved by means of a device for operating an electrical machine of the type mentioned at the outset, having a control and evaluation unit, which is designed to detect at least a first and a second phase current of the electrical machine, to compare the detected phase currents with in each case at least one reference value and to output an error message if at least one of the phase currents does not match the respective reference values. 
         [0020]    Finally, the present invention provides a motor-vehicle drive train having at least one electrical machine in a short-circuit operation mode and having a device for operating the electrical machine of the type described above. 
         [0021]    A short-circuit operation mode is understood here to mean the operation of the electrical machine in which electrical coils of a rotor of the electrical machine are shorted together. 
         [0022]    The invention is based on the concept that phase currents of the electrical machine are monitored if said machine is operated in the short-circuit operation mode. On the basis of previously known reference values, it can then be determined whether the detected phase currents correspond to a short-circuit operation mode or whether a deviation and therefore another operating mode of the electrical machine is present. If another operating mode is present, for example a freewheeling mode, then the error message is output. The freewheeling mode is present when the coils of the rotor are completely electrically separated from one another. 
         [0023]    In this case, the error message can be an electrical signal, for example, which is suitable for further processing in onboard electronics of the motor vehicle. The onboard electronics can then in turn carry out suitable countermeasures on the basis of the error message, in order to ensure a safe operation of the motor vehicle. In addition, it is conceivable that the error message contains information relating to the type of unintentional operating mode of the electrical machine. On the basis of this information, the onboard electronics can then select a particularly suitable countermeasure. 
         [0024]    According to a preferred embodiment of the invention, a third phase current is calculated as a function of the first and second phase currents. 
         [0025]    In this embodiment, all of the phase currents of the electrical machine are detected. It is particularly preferable for the first and second phase currents to be measured directly. On the basis of said two phase currents, the third phase current can be evaluated using electrotechnical basic equations of the electrical machine. However, it is also conceivable to measure all three phase currents directly. An advantage of calculating the third phase current on the basis of the first two phase currents is that a voltage measurement sensor and a voltage measurement line can be eliminated. Therefore, the invention can be implemented particularly economically. 
         [0026]    According to another embodiment, reference-value ranges are used as reference values. 
         [0027]    In this embodiment, the reference values consist of reference-value ranges. Said ranges form intervals within which the detected phase currents must lie. One advantage of this is that slight changes in the phase currents, which could otherwise lead to an erroneous output of the error message, are automatically compensated. This therefore advantageously results in the method being very robust, in particular in relation to slight or temporary deviations, for instance measurement noise or incorrect measurements. 
         [0028]    According to another embodiment, the reference values are determined as a function of characteristic diagrams and detected parameters. 
         [0029]    In this embodiment, the reference values are dynamically adapted to the corresponding state of the electrical machine. For this purpose, characteristic diagrams are used which are calculated in advance. Said characteristics diagrams can be deposited in a data store which can be read by the control and evaluation unit. The corresponding characteristic diagrams can then be present as a mathematical function with two variables which correspond to the parameters. Alternatively or in addition, it is conceivable that a multidimensional characteristic diagram is produced which contains predetermined data which can be read and/or interpolated as a function of the parameter values. 
         [0030]    The characteristic diagrams are preferably adapted to the specific configuration of the corresponding motor vehicle. They are therefore optimally configured for comparison with the detected phase currents. It is particularly preferable for the characteristic diagrams to contain reference-value ranges. As a result, an optimum adaptation to the corresponding motor vehicle from using the characteristic diagrams and the robustness from using the reference-value ranges are combined in this way. Suitable parameters include state parameters of the electrical machine, which permit conclusions to be drawn about the operating state and respective period of operation of the electrical machine. 
         [0031]    According to another preferred embodiment, a rotational speed of a rotor of the electrical machine and a temperature of the electrical machine are used as parameters. 
         [0032]    In this embodiment, the rotational speed of the rotor of the electrical machine and the temperature of the electrical machine are detected and used to evaluate the characteristic diagrams. Said parameters are particularly well-suited for detecting the operating state of the electrical machine. An additional advantage which arises is that said parameters are typically already detected for other purposes in motor vehicles with the electrical machine and so the two parameters are easily available. 
         [0033]    According to another preferred embodiment, a rotor position of the rotor is detected and the rotational speed of the rotor is determined as a function of the rotor position. 
         [0034]    In this embodiment, the rotational speed is indirectly determined by means of the rotor position of the rotor. The rotor position can in turn be calculated on the basis of a change in angle of an incremental position encoder. The use of an incremental position encoder has the advantage that, by means of the measurement values thereof, both the rotational speed and the current position of the rotor can be determined. Consequently, only one sensor (the incremental position encoder) is required in order to detect a plurality of state values of the electrical machine. Here, it is advantageous that previously existing systems are often equipped with an incremental position encoder and so these can additionally be used to implement the invention in a very economical manner. 
         [0035]    According to another embodiment, the temperature of the electrical machine is determined by means of a temperature-monitoring means. 
         [0036]    In this embodiment, the temperature is evaluated by means of a temperature-monitoring means. The temperature-monitoring means is preferably formed by a mathematical model of the electrical machine. Detected measurement values or other determined state values are fed to said model. The mathematical model, the temperature-monitoring means, calculates the temperature of the electrical machine by means of said values fed to said model. Alternatively, it is conceivable that the temperature is measured directly in the electrical machine. It is advantageous when using the temperature-monitoring means that previously detected values, for instance the rotational speed of the rotor, the power consumption thereof and the period of operation can be used to determine the temperature. Said previously determined values are typically made available to control and evaluation units in motor vehicles. Therefore, previously available information in a motor vehicle can also be accessed here. By using the temperature-monitoring means, a temperature sensor on the electrical machine, a corresponding signal line and an associated evaluation unit can therefore be eliminated. This leads to another particularly economical refinement of the invention. 
         [0037]    According to another embodiment, the phase currents are transformed into a fixed-rotor coordinate system by means of a mathematical transformation, in particular by means of a Park transformation, as a function of the rotor position, and the reference values are determined in that same coordinate system. 
         [0038]    In this embodiment, the comparison between the detected phase currents and the reference values takes place in a fixed-rotor coordinate system. For this purpose, the detected phase currents are mathematically transformed by means of the transformation into the fixed-rotor coordinate system. At the same time, it is provided that the reference values are determined in the same fixed-rotor coordinate system. This has the advantage that the reference values can be compared to the phase currents in the fixed-rotor coordinate system directly without further transformation. Moreover, the transformation into the fixed-rotor coordinate system has the advantage that a short-circuit current, which is dependent on the rotational speed of the rotor and the temperature of the electrical machine, is established in said coordinate system but is subsequently constant. A particularly simple monitoring of the phase currents by means of the given reference values results from this. In particular, it is provided that, if reference-value ranges and/or characteristic diagrams are used, these are likewise determined in the fixed-rotor coordinate system. 
         [0039]    According to another embodiment, a rotational speed of a motor-vehicle drive train, which interacts with the electrical machine, is reduced depending on the error message. 
         [0040]    In this embodiment, a countermeasure is initiated if an error message is present. The countermeasure is intended to prevent a destabilization of the vehicle. For this purpose, the rotational speed of the motor-vehicle drive train is reduced depending on the error message. By way of example, this can take place via onboard electronics or a motor control unit, to which the error message is fed. The rotational speed is then preferably reduced until no more safety-relevant braking torque which could lead to a destabilization of the vehicle is generated by the electrical machine. 
         [0041]    In another embodiment, a warning is output to a user depending on the error message. 
         [0042]    In this embodiment, a warning is output to the user, for example the driver of the motor vehicle. Said user is informed of a possible danger by means of this warning and can therefore manually initiate appropriate countermeasures. In this case, it is advantageous for the user to be informed of a risk of an accident owing to a destabilization of the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1  schematically shows a motor-vehicle drive train in a parallel-hybrid construction in a short-circuit operation mode; and 
           [0044]      FIG. 2  shows a schematic block diagram of a control and evaluation unit of the invention, and a high-voltage system with an electrical machine in the motor-vehicle drive train. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    In  FIG. 1 , a motor-vehicle drive train in a parallel-hybrid construction is designated in its entirety with reference number  10 . For the sake of completeness, it is pointed out here that the invention is not limited to a motor-vehicle drive train in a parallel-hybrid construction, but rather can be used in any vehicle having an electrical machine. The motor-vehicle drive train  10  has an internal combustion engine  12  which is connected to a fuel tank  16  via a fuel line  14 . The internal combustion engine  12  has a crankshaft  18  which is connected to a clutch  20 . The clutch  20  can connect the crankshaft  18  to a shaft  22 . An electrical machine  24  is arranged on the shaft  22 . The electrical machine  24  can be operated as a motor and, as a result, output a torque to the shaft  22 . It can also be operated as a generator and consequently receive a torque from the shaft  22  (braking torque). On the output side, the shaft  22  is connected to another clutch  26 . The clutch  26  can connect the shaft  22  to a gear shaft  28 . In addition, the gear shaft  28  is operatively connected to a manual transmission  30 , which converts an absorbed torque. The manual transmission  30  is also operatively connected on the output side to a differential gear  34  via an output shaft  32 . The differential gear  34  in turn forms an operative connection between the output shaft  32  and side axles  36  and  38 . The side axle  36  is also connected to a drive wheel  40  and the side axle  38  is connected to another drive wheel  42 . 
         [0046]    The electrical machine  24  is electrically connected to a power electronics unit  44  by means of a three-phase line  46 . In addition, the power electronics unit  44  is connected to a high-voltage battery  50  via an electrical line  48 . The power electronics unit  44  controls the electrical machine  24 . Depending on a switching state of the power electronics unit  44 , the electrical machine  24  is operated as a generator or motor. In generator operating mode, an alternating current is fed from the electrical machine  24  to the power electronics unit  44  via the three-phase line  46 . Said AC voltage is converted into a DC voltage in the power electronics unit  44 . The DC voltage is then conducted via the line  48  to the high-voltage battery  50  and so the latter is charged. In the motor operating mode of the electrical machine  24 , the electrical DC voltage is drawn from the high-voltage battery  50  via the line  48  and so the power electronics unit  44  can convert said DC voltage into an AC voltage in order to drive the electrical machine  24  via the lines  46  as a result. 
         [0047]    A signal path  52 , which originates from the line  46 , runs to a control and evaluation unit  54 . Measurement values of phase currents present in the line  46  are transmitted via the signal path  52 . A signal path  56 , which originates from the electrical machine  24 , runs to the control and evaluation unit  54 . A change in rotational angle of a rotor of the electrical machine  24  is transmitted along said signal path  56  to the control and evaluation unit  54 . The control and evaluation unit  54  evaluates the information transmitted thereto in a manner according to the invention and so it outputs an error message  58  if necessary. The control and evaluation unit  44  is described in detail with reference to  FIG. 2 . 
         [0048]    It is conceivable for the control and evaluation unit  54  to receive information from the power electronics unit  44  if the latter is intended to establish a short-circuit operation mode. The diagnosis can then be performed, as a result of which erroneous error messages are avoided. 
         [0049]    The control and evaluation unit  54  is schematically illustrated as a block diagram in  FIG. 2 . A high-voltage system  60  is also schematically illustrated. The high-voltage system  60  consists of the high-voltage battery  50 , the power electronics unit  44 , the lines  46  and  48  and the electrical machine  24 . The two lines  48 , which originate from the high-voltage battery  50 , run to the power electronics unit  44 . Here, the two lines  48  conduct the DC voltage, which is illustrated as intermediate-circuit voltage  62 . The power electronics unit  44  converts between the DC voltage in the lines  48  and the AC voltage in the lines  46 . For this purpose, the power electronics unit  44  is connected to the electrical machine  24  in a three-phase configuration via the three lines  46 . To put it more precisely, the lines  46  are electrically connected to coils  64  of a rotor  63 . 
         [0050]    The active short-circuit operation mode is established by the power electronics unit  44  in such a way that the lines  46  are electrically connected to one another and so the coils  64  are shorted together. Phase-current sensors  65  are arranged within the lines  46 , said sensors metrologically detecting a first phase current I u  and a second phase current I v  in the lines  46 . Detected measurement values are then transmitted from the phase-current sensors  65  to a block  66  of the control and evaluation unit  54  via the signal paths  52 . 
         [0051]    The phase-current sensor  65 ′ is an optional phase-current sensor. For this reason, the signal path  52 ′ is illustrated as interrupted. The optional phase-current sensor  65 ′ detects the third phase current I w . It then transmits the corresponding measurement value to the block  66  via the signal path  52 ′. In alternative embodiments, said third phase current I w  is determined within the control and evaluation unit  54  on the basis of electrotechnical basic equations for the electrical machine  24  as a function of the measurement values for the first phase current I u  and the second phase current I v  from the signal paths  52 . 
         [0052]    The electrical machine  24  is also operatively connected to an incremental position encoder  70  via a shaft  68 . The incremental position encoder  70  produces a measurement value as a function of the rotation of the electrical machine  24 . Said measurement value describes a change in rotational angle η m  of the rotor  63  of the electrical machine  24 . Said change in rotational angle η m  is transmitted to the block  72  of the control and evaluation unit  54  via the signal path  54 . The block  72  calculates the rotor position η el  of the rotor  63  as a function of the change in rotational angle η m  from the incremental position encoder  70 . The rotor position η el  is transmitted from the block  72  to the block  66  via a signal path  74 . 
         [0053]    The change in rotational angle η m  is also transmitted to a block  76  via the signal path  54 . The block  76  calculates a rotational angle speed T m  as a function of the change in rotational angle η m . This preferably occurs by differentiating the change in rotational angle η m . The rotational angle speed T m  is transmitted to a block  80  via a signal path  78 . In block  80 , a rotational speed N of the rotor  63  is calculated as a function of the rotational angle speed T m . This preferably occurs by multiplying the rotational angle speed T m  by the factor  60  and dividing the product by the divisor 2π. The rotational speed N resulting therefrom is transmitted to a first comparator  84  and to a second comparator  86  via a signal path  82 . 
         [0054]    In block  66 , the three phase currents I u , I v  and I w  are transformed into a fixed-rotor coordinate system by means of a mathematical transformation, in particular a Park transformation, as a function of the rotor position η el . A first actual phase current I q     —     act  and a second actual phase current I d     —     act  result therefrom. 
         [0055]    This can be expressed mathematically as follows: 
         [0000]    
       
         
           
             
               [ 
               
                 
                   
                     
                       I 
                       d_act 
                     
                   
                 
                 
                   
                     
                       I 
                       q_act 
                     
                   
                 
               
               ] 
             
             = 
             
               T 
                
               
                 [ 
                 
                   
                     
                       
                         I 
                         u 
                       
                     
                   
                   
                     
                       
                         I 
                         v 
                       
                     
                   
                   
                     
                       
                         I 
                         w 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
         [0056]    Here, T represents the transformation matrix. The first actual phase current I q     —     act  is then transmitted to the comparator  84  via a signal path  88 . The second actual phase current I d     —     act  is transmitted to the comparator  86  via another signal path  90 . 
         [0057]    Another block  92  contains a default value Tolerance_I q  for an interval magnitude of a reference range in respect of the actual phase current I q     —     act . Said default value Tolerance_I q  is transmitted from the block  92  to the comparator  84  via a signal path  94 . Correspondingly, a block  96  is provided, which contains a default value Tolerance_I d  for an interval magnitude of a reference range in respect of the actual phase current I d     —     act . Said default value Tolerance_I d  is transmitted from the block  96  to the comparator  86  via a signal path  98 . 
         [0058]    The temperature η RTemp  of the electrical machine  24  is also determined. As illustrated in  FIG. 2 , this can take place directly via a signal path  99 , wherein the temperature η RTemp  is then directly measured in the electrical machine  24 . The measurement value for the temperature η RTemp  is then transmitted to the comparators  84  and  86  via the signal path  99 . 
         [0059]    In an alternative embodiment, it is conceivable that a temperature-monitoring means  100  is used to determine the temperature η RTemp  of the electrical machine  24 . The temperature-monitoring means  100  is shown here as an alternative and is therefore illustrated as interrupted. In the event that the temperature-monitoring means  100  is used, the temperature η RTemp  of the electrical machine  24  is determined inside the temperature-monitoring means  100  itself on the basis of state values of the electrical machine  24 , which are supplied to the temperature-monitoring means. The value of the temperature η RTemp  is then directly transmitted to the comparators  84  and  86  via a signal path  102 . In the event that the temperature-monitoring means  100  is used, no direct signal path  99  exists between the electrical machine  24  and the comparators  84  and  86 . In other words, the part of the signal path  99 , illustrated in  FIG. 2 , between the line  102  and the electrical machine  24  is then not present. 
         [0060]    The comparator  84  has a first characteristic diagram 
         [0000]        I   q     —     ASC   =KF   1 ( N, η   RTemp ) 
         [0061]    as a function of the rotational speed N and the temperature η RTemp  of the electrical machine  24 . The characteristic diagram KF 1  contains reference values I q     —     ASC  for the actual phase current I q     —     act . 
         [0062]    The comparator  86  has a second characteristic diagram 
         [0000]        I   d ASC   =KF   2 ( N, η   RTemp ) 
         [0063]    as a function of the rotational speed N and the temperature η RTemp  of the electrical machine  24 . The characteristic diagram KF 2  contains reference values I d     —     ASC  for the actual phase current I d     —     act . 
         [0064]    A Cartesian coordinate system with an ordinate  104  and an abscissa  106  is illustrated within the comparator  84 . The ordinate  104  relates to the reference value I q     —     ASC , that is to say the nominal phase current in amperes. The abscissa  106  relates to the rotational speed N in units of 1/min. A curve  108  is illustrated within the Cartesian coordinate system, which curve illustrates the reference value I q     —     ASC  as a function of the rotational speed N. For reasons of clarity, only the reference value for a single particular temperature η RTemp  is shown. 
         [0065]    In addition, a reference-value range  110  is illustrated in the coordinate system, within which range the actual phase current I q     —     act  must lie when the electrical machine  24  is operated in the short-circuit operation mode and has the appropriate rotational speed N. Based on the two arrowheads of the reference-value range  110 , two further curves run approximately parallel to the curve  108 . Said two curves illustrate the profile of the reference-value range  110  as a function of the rotational speed N. 
         [0066]    The comparator  84  can therefore determine the correct reference value I q     —     ASC  as a function of the detected rotational speed N, the temperature η RTemp  and the characteristic diagram KF 1 , by transferring the parameters to the characteristic diagram KF 1 . 
         [0067]    The comparator  84  can then determine the reference-value range  110  as a function of the reference value I q     —     ASC  and the default value Tolerance_I q . For this purpose, the absolute value of the default value Tolerance_I q  is halved and subtracted from the reference value I q ASC  for a lower limit of the reference-value range  110 . For an upper limit of the reference-value range  110 , the halved absolute value is added to the reference value I q     —     ASC . 
         [0068]    The comparator  84  can now check whether the actual phase current I q     —     act  lies within the reference range  110 , wherein the reference-value range  110  is then used as reference value. Depending on the result, the comparator  84  produces a fault indicator in the form of an error bit B_Error_ASC_Iq. Said error bit is set to the value “true” in the event of a fault. In all other cases, the error bit is set to the value “false”. 
         [0069]    This can be expressed mathematically as follows: 
         [0000]    
       
         
           
             
               B_Error 
                
               _ASC 
                
               _Iq 
             
             := 
             
               { 
               
                 
                   
                     true 
                   
                   
                     ∀ 
                   
                   
                     
                       
                          
                         
                           
                             I 
                             
                               q 
                                
                               _ 
                                
                               act 
                             
                           
                           - 
                           
                             I 
                             
                               q 
                                
                               _ASC 
                             
                           
                         
                          
                       
                       &gt; 
                       
                         Tolerance_Iq 
                         2 
                       
                     
                   
                 
                 
                   
                     false 
                   
                   
                     ∀ 
                   
                   
                     
                       
                          
                         
                           
                             I 
                             
                               q 
                                
                               _ 
                                
                               act 
                             
                           
                           - 
                           
                             I 
                             q_ASC 
                           
                         
                          
                       
                       &gt; 
                       
                         Tolerance_Iq 
                         2 
                       
                     
                   
                 
               
             
           
         
       
     
         [0070]    A Cartesian coordinate system with an ordinate  112  and an abscissa  114  is likewise illustrated within the comparator  86 . The ordinate  112  relates to the reference value I d     —     ASC , that is to say the nominal phase current in amperes. The abscissa  114  relates to the rotational speed N in units of 1/min. A curve  116  is illustrated within the Cartesian coordinate system, which curve illustrates the reference value I d     —     ASC  as a function of the rotational speed N. For reasons of clarity, only the reference value for a single particular temperature η RTemp  is shown. 
         [0071]    In addition, a reference-value range  118  is illustrated in the coordinate system, within which range the actual phase current I d     —     act  must lie when the electrical machine  24  is operated in the short-circuit operation mode and has the appropriate rotational speed N. Based on the two arrowheads of the reference-value range  110 , two further curves run approximately parallel to the curve  116 . Said two curves illustrate the profile of the reference-value range  118  as a function of the rotational speed N. 
         [0072]    The comparator  86  can therefore determine the correct reference value I d ASC  as a function of the detected rotational speed N, the temperature η RTemp  and the characteristic diagram KF 2 , by transferring the parameters to the characteristic diagram KF 2 . 
         [0073]    The comparator  86  can then determine the reference-value range  118  as a function of the reference value I d     —     ASC  and the default value Tolerance_I d . For this purpose, the absolute value of the default value Tolerance_I d  is halved and subtracted from the reference value I d     —     ASC  for a lower limit of the reference-value range  118 . For an upper limit of the reference-value range  118 , the halved absolute value is added to the reference value I d     —     ASC . 
         [0074]    The comparator  86  can now check whether the actual phase current I d     —     act  lies within the reference range  118 , wherein the reference-value range  118  is then used as reference value. Depending on the result, the comparator  86  produces a fault indicator in the form of an error bit B_Error_ASC_Id. Said error bit is set to the value “true” in the event of a fault. In all other cases, the error bit is set to the value “false”. 
         [0075]    This can be expressed mathematically as follows: 
         [0000]    
       
         
           
             
               B_Error 
                
               _ASC 
                
               _Id 
             
             := 
             
               { 
               
                 
                   
                     true 
                   
                   
                     ∀ 
                   
                   
                     
                       
                          
                         
                           
                             I 
                             
                               d 
                                
                               _ 
                                
                               act 
                             
                           
                           - 
                           
                             I 
                             
                               d 
                                
                               _ASC 
                             
                           
                         
                          
                       
                       &gt; 
                       
                         Tolerance_Id 
                         2 
                       
                     
                   
                 
                 
                   
                     false 
                   
                   
                     ∀ 
                   
                   
                     
                       
                          
                         
                           
                             I 
                             
                               d 
                                
                               _ 
                                
                               act 
                             
                           
                           - 
                           
                             I 
                             d_ASC 
                           
                         
                          
                       
                       &gt; 
                       
                         Tolerance_Id 
                         2 
                       
                     
                   
                 
               
             
           
         
       
     
         [0076]    The fault indicators B_Error_ASC_Id and B_Error_ASC_Iq are then transmitted to a block  124  via signal paths  120  and  122 . The block  124  is a logical OR operation. Said block  124  produces the error signal  58  if one of the fault indicators B_Error_ASC_Id or B_Error_ASC_Iq from the signal paths  120  or  122  has the value “true”. 
         [0077]    The error signal  58  can then be transmitted to onboard electronics for further processing. The onboard electronics are not shown here. Owing to the fault message, the onboard electronics then ensure there is a reduction in the rotational speed within the motor-vehicle drive train  10 . This can take place, for example, by means of a reduction in rotational speed in the internal combustion engine  12 . 
         [0078]    Alternatively or in addition, a warning message is output to a user on the basis of the error message  58 . Said user can then take precautions so that a destabilization of the vehicle does not occur.
A method for operating an electric machine ( 24) in short-circuit operation, in particular for use in a motor vehicle, characterized by the steps of: measuring at least a first and a second phase current of the electric machine ( 24), comparing the measured phase currents with at least one reference value each, and outputting an error message ( 58) if at least one of the phase currents does not agree with the respective reference value ( 110, 118).