Abstract:
The invention relates to a method for controlling an inverter ( 10 ) using space-vector pulse width modulation, in particular to control an electric machine, the inverter being equipped with a plurality of controllable switches (S) and being designed to provide a polyphase electric current (IU, IV, IV), in particular to supply polyphase electric current (IU, IV, IV) to the electric machine ( 14 ). In said method, a desired current space vector (I 1 *) having a desired phase angle (alpha 1 ) and a desired amplitude (I 1 ) is predefined, and the inverter ( 10 ) is controlled in such a way that a plurality of different successive switching states (V 1 -V 7 ) is established for the switches (S) in order to provide the electric current (IU, IV, IV) in the form of a current space vector (I 2 *), the inverter ( 10 ) being controlled in such a way that the current space vector (I 2 *) is provided at a phase angle (alpha 2 ) which differs from the desired phase angle (alpha 1 ), the difference (delta_beta) of the phase angle (alpha 2 ) from the desired phase angle (alpha 1 ) being limited according to a rotational speed (f) of the desired current space-vector (I 1 *).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for controlling the inverter by means of space vector modulation, in particular for controlling an electrical machine, wherein the inverter has a plurality of controllable switches and is designed to provide a polyphase electric current, in particular in order to supply the electrical machine with electric current in a polyphase fashion, wherein a nominal current space vector having a nominal phase angle and a nominal amplitude is predefined, wherein the inverter is controlled such that a plurality of different consecutive switching states of the switches are set in order to provide the electric current in the form of a current space vector. 
         [0002]    The present invention also relates to a device for controlling an inverter, in particular for controlling an electrical machine, wherein the inverter has a plurality of controllable switches which are connected to provide a polyphase electric current as a function of a nominal current space vector having a nominal phase angle and a nominal amplitude, in particular to energize the electrical machine in a polyphase fashion, having a control device which is designed to control the inverter such that the switches of the inverter assume a plurality of consecutive different switching states in order to provide the electric current in the form of a current space vector. 
         [0003]    Finally, the present invention relates to a motor vehicle drivetrain having at least one electrical machine for providing drive power, an inverter for controlling the electrical machine and having a device for controlling the inverter of type described above. 
         [0004]    Various control methods are known in the technical field of three-phase loads in general and of three-phase electrical machines in particular. In this case, the method of space vector modulation is currently usually preferable for controlling the three-phase load. In the case of this control method, a space vector is formed by consecutive setting of eight basic voltage phasors. In order to provide the string voltage, the basic voltage phasors are switched in a pulse-width-modulated manner, with the result that a corresponding control voltage is generated. 
         [0005]    In the case of the known control methods, the electrical loads are controlled by means of an inverter with power semiconductor switches. The eight consecutive basic voltage phasors for generating the voltage space vector are set by virtue of particular power semiconductor switches of the inverter being alternately switched on and off. In the case of very low rotation speeds of the space vector or, provided the three-phase load is an electrical machine, in the case of low speeds of the controlled electrical machines, individual ones of the power semiconductor switches are switched very often or for a very long time and are thus thermally loaded with an electric current which flows for a very long time or very often. Therefore, the power semiconductor switches must be configured for very long switch-on times and for very large currents, which results in the inverter being generally elaborate in terms of technology. 
         [0006]    In order to respond to an overload, in particular a thermal overload, of the power semiconductor switches, it is proposed in WO 2010/000548 A2, for example, to omit one of two switching states which switch so as to be free of voltage in particular pulse-width-modulation periods in order to reduce the switching losses of the power semiconductor switches. 
         [0007]    Since the loading, in particular thermal loading, of individual power semiconductor switches of the inverter is dependent on a phase angle of the provided current space vector or individual ones of the power semiconductor switches of the inverter are differently loaded for particular phase angles of the provided current space vector, it is proposed in DE 10393516 T1, for example, to use a particular zero vector in particular angular regions of the provided current space vector in order to reduce the switching losses of the power semiconductor switches. 
         [0008]    It is disadvantageous here that, in different control situations and in the case of different rotation speeds of the current space vector, individual ones of the power semiconductor switches are loaded more heavily and, therefore the inverter is loaded unevenly in particular situations and in other situations in which the power semiconductor switches are loaded substantially evenly, the efficiency of the current use is reduced. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention therefore provides a method for controlling an inverter by means of space vector modulation of the type mentioned at the outset, wherein the inverter is controlled such that the current space vector is provided with a phase angle which differs from the nominal phase angle and wherein a deviation of the phase angle from the nominal phase angle is limited as a function of a rotation speed of the nominal current space vector. 
         [0010]    Furthermore, the invention therefore provides a device for controlling an inverter of the type mentioned at the outset, wherein the control device is designed to control the inverter such that the current space vector is provided with a phase angle which is different from the nominal phase angle and a deviation of the phase angle from the nominal phase angle is limited as a function of a rotation speed of the nominal current space vector. 
         [0011]    Finally, the invention provides a motor vehicle drivetrain having at least one electrical machine for providing drive power, an inverter for controlling the electrical machine and having a device for controlling the inverter of the type described above. 
         [0012]    Owing to the fact that the current space vector is provided with a phase angle which is different from the nominal phase angle, in certain situations in which one of the controllable switches is overloaded, in particular thermally overloaded, or threatens to become overloaded, it is possible to relieve, by a current space vector with a phase angle being set which relieves an overloaded controllable switch and loads another controllable switch more heavily. If the rotation speed of the current space vector increases and exceeds a particular value, the controllable switches are loaded evenly in each case by the high rotation frequencies and the temperature carriers of the controllable switches, with the result that relieving a particular switch at the cost of another switch being loaded would be rather disadvantageous. Therefore, it is possible by means of the present method on the basis of the control situation and as a function of the rotation speed of the current space vector to control the inverter in an optimum manner in order to correspondingly load the inverter evenly in any control situation. Thus, the controllable switches can be configured in general for lower loading values, as a result of which the inverter can be manufactured in general in a less elaborate and more cost-effective way. Furthermore, the service life of the inverter is lengthened in general owing to the more even loading of the switches. 
         [0013]    The deviation is preferably reduced with increasing rotation speed. 
         [0014]    As a result, the inverter can balance large loading of individual switches at very low rotation speeds of the nominal current space vector, as a result of which a critical loading phase for the inverter can be balanced. 
         [0015]    It is further preferred if the deviation is set as a function of the nominal phase angle of the current space vector. 
         [0016]    As a result of this, an optimum current space vector can be set individually for particular critical nominal phase angles, which can balance particular loading states of individual controllable switches. 
         [0017]    It is further preferred if the deviation is varied within a deviation range and wherein the deviation range is reduced with increasing rotation speed of the nominal current space vector. 
         [0018]    As a result, the total loading of the electrical machine and the inverter, which is heavier on the whole owing to the deviation, can be adapted to the need to relieve individual controllable switches. 
         [0019]    It is further generally preferred if, below a predefined rotation speed, the phase angle is set as a function of a predefined nominal loading value of the switches. 
         [0020]    As a result of this, a particular loading distribution of the inverter can be set in the case of critical rotation speeds of the current space vector. 
         [0021]    In this case, it is particularly preferred if the phase angle and an amplitude of the current space vector are set such that a power output is identical to a power output corresponding to the nominal phase angle and the nominal amplitude of the nominal current space vector. 
         [0022]    As a result, the deviation of the phase angle can be set without the control of the controlled load being impaired. 
         [0023]    It is further preferred if the maximum deviation of the current space vector from the nominal current space vector is 30 degrees. 
         [0024]    As a result, the total loading of the inverter can be limited since the amplitude of the current space vector increases with a larger deviation of the phase angle from the nominal phase angle and other controllable switches of the inverter are excessively overloaded. 
         [0025]    It is further preferred if the deviation range is reduced linearly between a first predefined rotation speed and a second predefined rotation speed of the nominal current space vector. 
         [0026]    As a result of this, it is possible to vary, in a transition phase, between different control methods with simple means in terms of control technology, wherein synergies of both control methods can be used. 
         [0027]    It is further preferred if an electrical machine is controlled by means of the inverter and the nominal current phasor is determined as a function of the rotor angle of the electrical machine. 
         [0028]    As a result, the nominal current space vector can be determined with a low expenditure in terms of control technology. 
         [0029]    In this case, it is particularly preferred if the phase angle and the amplitude of the current space vector are determined below a predefined rotation speed such that a torque output by the electrical machine is identical to the torque which is output by means of the nominal current space vector. 
         [0030]    As a result, the electrical machine can also be controlled in the case of a phase angle which deviates from the nominal phase angle according to a predefined nominal value, wherein the variation of the phase angle does not impair the control of the electrical machine. 
         [0031]    As a result, by means of the present invention, the controlled load can be controlled in an optimum manner in different control situations, in particular in the case of different rotation frequencies of the current space vector, with the result that the inverter is evenly loaded and the current use by the controlled load is optimum. 
         [0032]    It goes without saying that features, properties and advantages of the method according to the invention also correspondingly pertain to or are applicable to the device according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  schematically shows an inverter for controlling an electrical load; 
           [0034]      FIG. 2  shows a complex phasor diagram to explain the space vector modulation method for controlling the inverter of an electrical load; 
           [0035]      FIG. 3  schematically shows a profile of three string voltages for setting different voltage space vectors; 
           [0036]      FIG. 4  schematically shows a sequence for determining a nominal loading value of the switches or the freewheeling diodes of the inverter; 
           [0037]      FIG. 5  schematically shows a detailed sequence for determining a nominal loading value on the basis of temperature measurements or estimates; 
           [0038]      FIG. 6  schematically shows a complex phasor diagram for setting a current space vector; 
           [0039]      FIG. 7  shows a complex phasor diagram for setting an alternative current space vector; 
           [0040]      FIG. 8  shows a complex phasor diagram for setting a current space vector with a phase angle of zero degrees; 
           [0041]      FIG. 9  shows the deviation of a phase angle of the current space vector from the nominal phase angle as a function of the nominal current space vector; and 
           [0042]      FIG. 10  shows a profile of the maximum deviation of the phase angle from the nominal phase angle as a function of the rotation frequency of the nominal current space vector. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]      FIG. 1  schematically illustrates an inverter for controlling an electrical load, in particular an electrical machine, which inverter is generally denoted by  10 . 
         [0044]    The inverter  10  is connected to a DC voltage source  12  and is used to energize the electrical load  14 , which in this case is designed as an electrical machine  14 , in a three-phase fashion. The inverter has three half-bridges which are connected in parallel with the DC voltage source  12  and have in each case two controllable switches S. Between the switches S, a half-bridge tap  16  is formed in each case, which half-bridge taps are each connected to a phase conductor of the phases U, V, W of the electrical machine  14 . 
         [0045]    In each case, a freewheeling diode D which enables a flow of current in the opposite direction is connected in parallel with the switches S. 
         [0046]    In  FIG. 1 , the switches S are denoted by SHA, SLA, SHB, SLB, SHC, SLC corresponding to the phase U, V, W which they provide and corresponding to the assignment to a high potential of the DC voltage source  12  or to a low potential of the DC voltage source  12 . Correspondingly, the freewheeling diodes are denoted by DHA, DLA, DHB, DLB, DHC, DLC. 
         [0047]    By alternating opening and closing of the switches S, in each case a control voltage is applied between the phase conductors U, V, W, with the result that in each case a phase current IU, IV, IW which drives the electrical machine  14  is correspondingly set. The inverter  10  is preferably designed using semiconductor switches. The switches of the inverter are alternately opened and closed by means of a schematically illustrated control unit  18  in order to provide the phase voltages with a particular profile and to provide a voltage space vector and to correspondingly energize the electrical machine  14  with the phase currents IU, IV, IW. In this case, the voltage phasor is provided by the inverter  10 , whereupon the current space vector is correspondingly set as a function of the controlled load. 
         [0048]      FIG. 2  illustrates a complex phasor diagram to explain the space vector modulation for controlling the three-phase current load  14  or the electrical machine  14 , which complex phasor diagram is generally denoted by  20 . 
         [0049]    The phasor diagram  20  illustrates a voltage phasor V* with a control angle alpha of the electrical machine  14 . The phasor diagram  20  also illustrates six basic voltage phasors V 1 , V 2 , V 3 , V 4 , V 5 , V 6  which arise when one or two of the switches S of the inverter  10  are closed and the electrical machine is correspondingly controlled. In order to set the voltage phasor V* with maximum length, which has the control angle alpha between the basic voltage phasors V 1  and V 2  in this example, said voltage phasor V* is realized by alternate control of the inverter  10  corresponding to the basic voltage phasor V 1  and the basic voltage phasor V 2 . The two basic voltage phasors V 1 , V 2  are alternately set with a predefined switching frequency, with the result that the voltage phasor V* with a phase angle of 30° arises in the case of even switch-on times of the basic voltage phasors V 1 , V 2 . If a voltage phasor V* with a larger control angle alpha must be set, the switch-on time of the basic voltage phasor V 2  is correspondingly increased and the switch-on time of the basic voltage phasor V 1  is reduced. Thus, by clocked control of the switches S of the inverter  10 , the voltage space vector V* can be realized with any control angle alpha. 
         [0050]    If the voltage phasor V*, as in the case illustrated in  FIG. 2 , is to be set with a lower magnitude (smaller length) than the basic voltage space vectors V 1 , V 2 , a zero voltage phasor V 0 , V 7  is correspondingly set, in the case of which the switches SHA, SHB, SHC on the upper side or SLA, SLB, SLC on the lower side of the inverter  10  are opened. The respective other ones of the switches S are correspondingly closed. Correspondingly, the voltage phasor V* can be realized by a combination of the basic voltage space vectors V 1  and V 2  and one of the zero voltage phasors V 0 , V 7 . 
         [0051]    A current space vector I* is set as a function of the voltage space vector V*. The current space vector I* has an amplitude and a phase angle which are set as a function of the controlled electrical load  14 . The phase angle of the current space vector I* can be in phase with the phase angle a of the voltage space vector V* or can have a phase shift. 
         [0052]    In order to energize the electrical load  14  or the electrical machine  14 , the voltage space vector V* is provided by the different basic voltage space vectors V 1 -V 6  and the zero voltage space vectors V 0 , V 7  being set consecutively in quick succession. As a result, the different switches S and the different freewheeling diodes D of the inverter  10  are evenly loaded, in particular evenly loaded in phase, in the case of a voltage space vector V* which rotates in a correspondingly rapid manner. If the rotation frequency of the voltage space vector V* is very low or zero, for example in the case of low speeds of the electrical machine  14 , the corresponding switches S and the freewheeling diodes D of the inverter  10  of a phase U, V, W are loaded over a long period of time, with the result that the corresponding switches S and the freewheeling diodes D can be overloaded and the switches S and the freewheeling diodes D of the inverter  10  are generally loaded unevenly, in particular out of phase. In order to prevent overloading of individual ones of the switches S and the freewheeling diodes D, measures must be taken to distribute the loading to different ones of the switches S and the freewheeling diodes D. 
         [0053]      FIG. 3  illustrates profiles of the phase voltages of the three phases U, V, W within a pulse-width-modulation period T, in order to set the basic voltage space vectors V 0 , V 1 , V 2 , V 7  consecutively. Within the pulse-width-modulation period T, a switch-on time t 0 , t 1 , t 2 , t 7  of the individual basic voltage space vectors V 0 , V 1 , V 2 , V 7  can be varied in order to be able to precisely set the voltage space vector V*. 
         [0054]      FIG. 4  illustrates in principle the determination of a nominal loading value m, which is generally denoted by  30 . By means of the nominal loading value m, the basic goal is to load the switches SHA, SHB, SHC, which are assigned to a high voltage potential of the voltage source  12 , and the switches SLA, SLB, SLC, which are assigned to a low voltage potential of the voltage source  12 , evenly or as evenly as possible. In this case, the switches SHA, SHB, SHC, which are assigned to the high voltage potential of the voltage source  12 , are denoted as upper switches SH below and the switches SLA, SLB, SLC, which are assigned to the low voltage potential of the voltage source  12 , are denoted as lower switches SL below. 
         [0055]    The magnitude V of the voltage space vector V*, the phase angle alpha_V of the voltage space vector V*, the magnitude I of the current space vector I* and the phase angle alpha_I of the current space vector I* are used as input variables. 
         [0056]    Firstly, one of the upper switches SH or one of the upper freewheeling diodes DH is selected, which switch or freewheeling diode has the maximum losses of the upper side for the voltage space vector V* to be set. For this switch SH or this freewheeling diode DH, the maximum possible losses P_Hmax are theoretically determined for the voltage space vector V* to be set for the case in which only V 7  is used as zero voltage phasor. Furthermore, the minimum possible losses P_Hmin of said switch SH or said freewheeling diode DH are theoretically determined for the voltage space vector V* to be set for the case in which only V 0  is used as zero voltage phasor, as is shown at  32 . At  34 , the lower switch SL or the lower freewheeling diode DL is correspondingly selected, which has the maximum losses of the lower switch SL or the lower freewheeling diodes DL for the voltage space vector V* to be set. For this switch SL or this freewheeling diode DL, the maximum possible losses P_Lmax and the minimum possible losses P_Lmin are determined for the voltage space vector V* to be set for the case in which only V 0  or V 7  is used as zero voltage phasor. From said loss values, a new loading value m is calculated at  36  and, what is more, using the formula: 
         [0000]    
       
         
           
             m 
             = 
             
               
                 
                   P 
                   
                     L 
                      
                     
                         
                     
                      
                     max 
                   
                 
                 - 
                 
                   P 
                   
                     H 
                      
                     
                         
                     
                      
                     min 
                   
                 
               
               
                 
                   P 
                   
                     H 
                      
                     
                         
                     
                      
                     max 
                   
                 
                 - 
                 
                   P 
                   
                     H 
                      
                     
                         
                     
                      
                     min 
                   
                 
                 + 
                 
                   P 
                   
                     L 
                      
                     
                         
                     
                      
                     max 
                   
                 
                 - 
                 
                   P 
                   
                     L 
                      
                     
                         
                     
                      
                     min 
                   
                 
               
             
           
         
       
     
         [0057]    The loading value m determined in this way distributes the thermal loading of the inverter  10  on the upper and lower side such that the losses on the upper side are identical to the losses on the lower side. At  38 , the switch-on times tO-t 7  are calculated in order to set the selected loading value m and to correspondingly load the switches S and the freewheeling diodes D more evenly. 
         [0058]    Since the freewheeling diodes D and the switches S have different loading limits, the losses P D , P S  of the freewheeling diodes D and the switches S must be adapted to one another or factorized in order to be able to be compared with one another. Therefore, a comparison power loss P DV  is determined for the freewheeling diodes D, and, what is more, using the formula: 
         [0000]    
       
      
       P 
       DV 
       =c*P 
       D  
      
     
         [0059]    Wherein P DV  is the comparison power loss of the freewheeling diodes P D  is the freewheeling diode losses and the factor c is a constant. In a particular embodiment, the factor c can also be a function of the power loss P D  of the freewheeling diodes D. 
         [0060]    Furthermore, it also becomes clear that the losses P D , P S  of the switches S and the freewheeling diodes D are exclusively a function of the magnitude V of the voltage space vector V*, the phase angle alpha_V, the magnitude I of the current space vector I* and the phase angle alpha_I. 
         [0061]    In an alternative embodiment of the method  30 , instead of the power losses P, the electric current I in the respective component S, D and/or the square of the electric current I 2  in the respective component S, D is used in order to determine the nominal loading value m. 
         [0062]      FIG. 5  illustrates a method in order to determine the loading value m on the basis of an estimated or measured temperature T D , T S  of the switches S and/or of the freewheeling diodes D and to calculate a new nominal loading value m. In  FIG. 5 , the method is generally denoted by  40 . 
         [0063]    In the case of the method  40 , the nominal loading value m is determined during operation as a function of the temperatures of the switches S or the freewheeling diodes D. In general, the temperatures T D , T S  of the switches S and the freewheeling diodes D are used as input variables. At  42 , the most heavily loaded upper switch SH, the most heavily loaded upper freewheeling diode DH, the most heavily loaded lower switch SL and the most heavily loaded lower freewheeling diode DL are calculated by means of the temperatures T D , T S . In other words, the respective component which has the highest temperature is calculated. From said temperatures, at  44  and  46 , the maximum temperature T_H of the upper switches and/or the upper freewheeling diodes is calculated or the maximum temperature T_L of the lower side is calculated from the losses of the lower side. In this case, the temperature T D  of the freewheeling diodes D is factorized in order to be able to compare the temperatures of the switches and the freewheeling diodes D, as is shown at  48 . In order to be able to compare the temperatures of the switches S and the freewheeling diodes D, a comparison temperature of the freewheeling diodes is determined, using the formula: 
         [0000]    
       
      
       T 
       DV 
       =c*T 
       D  
      
     
         [0064]    wherein T DV  is the comparison temperature, T D  is the temperature of the freewheeling diodes D and the factor c is a constant. In a particular embodiment, the factor c can also be a function of the power loss P D  of the freewheeling diodes D. At a summing point  50 , the difference dT between the maximum temperature T_H of the upper side and the maximum temperature T_L of the lower side is calculated. At  52 , an amended nominal loading value m is determined as a function of the temperature difference dT in order to correspondingly compensate the temperature difference dT. If the temperature difference dt&gt;0, the nominal loading value m, Ism is reduced and if the temperature difference dT&lt;0, the nominal loading value m, Ism is increased. As a function of the nominal loading value m, Ism determined in this way, new switch-on times t 0 -t 7  are determined at  54  for the following pulse-width-modulation period T. As a function of the new pulse-width-modulation period T, amended temperatures T D , T S  of the switches S and the freewheeling diodes D are calculated, as is shown at  56 , and are provided as new input variables for the method  40 , as is indicated by the feedback  58 . As a result of this, on the basis of the measured or estimated temperature of the switches S and/or the freewheeling diodes D, a new nominal loading value m can be determined for each pulse-width-modulation period T in order to more evenly load the corresponding switches S and freewheeling diodes D according to the new nominal loading value m, Ism. Owing to the comparison of the temperature of the components of the upper side and of the lower side, and owing to the adaptation of the loading value m, the components of the upper side can be more evenly loaded relative to the components of the lower side. 
         [0065]    In an alternative embodiment of the method  40 , instead of the temperatures of the components S, D, power losses are used to determine the nominal loading value m, Ism, which power losses are calculated or determined by integration of the power loss of the respective component S, D or by integration of the electric current I in the respective component S, D and/or by integration of the square of the electric current I 2  in the respective component S, D over a predefined period of time. 
         [0066]    In another embodiment of the method  40 , instead of the temperatures of the components S, D, the electrical losses P or the electric current I in the respective component S, D and/or the square of the electric current I 2  in the respective component S, D are used to determine the nominal loading value m, Ism, which electrical losses or electric currents are in each case filtered by means of a low-pass filter. 
         [0067]      FIG. 6  schematically illustrates a complex phasor diagram of the current space vector I 1 *. The current space vector I 1 * has a magnitude I 1  and a phase angle alphal . If the inverter  10 , which sets the current space vector I 1 *, is used to control the electrical machine  14 , the electrical machine  14  generates a torque M. In the complex phasor diagram in  FIG. 6 , the individual phases U, V, W are illustrated at an angle of 120 degrees with respect to one another. A projection of the current space vector I 1 * onto the corresponding phases U, V, W corresponds in this case to the current which is set in the associated switch S. By means of said projection, which is indicated by the dashed lines, the loading of the individual switches S or freewheeling diodes D can thus be directly read off. In the illustrated example from  FIG. 6 , the switch SHA is thus loaded most heavily by the phase U, the switch SHC being loaded less by the phase W, while the switch SHA and the switch SHB are loaded very lightly by the phase V. 
         [0068]      FIG. 6  illustrates the provided torque M of the connected electrical machine  14  as a curve, which at the same time represents a curve of constant torque M. The torque M output by the electrical machine  14  is a function of an angle theta by which the current phasor I* runs ahead of the electrical rotor angle of the electrical machine  14  and the amplitude I of the current space vector I 1 *: M=f(theta, I). It can be seen from this that the torque M which is output by the electrical machine  14  is constant, provided the current space vector I 1 * follows the line of constant torque M illustrated in  FIG. 6 . 
         [0069]    The current space vector I 1 * is set such that it runs ahead of an electrical rotor angle of the electrical machine  14  in order to provide the torque M by means of the electrical machine  14 . The current space vector I 1 * runs ahead of the electrical rotor position of the electrical machine  14  by an angle theta. This becomes clear through the formula: 
         [0000]      alpha_ I =alpha_ R +theta, 
         [0070]    wherein alpha_I is the phase angle of the current space vector I 1 *, alpha_R is the electrical angle of the rotor of the electrical machine  14  and theta is the difference angle. 
         [0071]    The difference angle theta is usually between 90 degrees and 180 degrees during operation of the motor. The current space vector I 1 * is set such that the inverter  10  and the electrical machine  14  have an optimum efficiency for the electrical rotor angle alpha_R. 
         [0072]    A variation of the phase angle alpha_I of the current space vector is schematically illustrated in a complex phasor diagram in  FIG. 7 . 
         [0073]    In the complex phasor diagram illustrated in  FIG. 7 , the nominal current space vector I 1 * with the phase angle alphal and the magnitude I 1  is illustrated and the current space vector I 2 * with the phase angle alpha 2  and the magnitude I 2 . The nominal current space vector I 1 * is in this case the current space vector at which the inverter  10  and the electrical machine  14  have an optimum efficiency. Both current space vectors I 1 *, I 2 * output the same torque M since they run on the line of even torque M. The nominal current space vector I 1 * is identical to the current phasor I 1 * from  FIG. 6 . The current space vector  12 * has a phase angle alpha 2  which is greater than the phase angle alpha 1  of the nominal current space vector I 1 *. The difference between the phase angles alpha 1  and alpha 2  is denoted as delta_beta in  FIG. 7 . delta_beta can have different values as a function of the phase angle alphal and fluctuate at most between +30° and −30°. By means of the projection, illustrated in  FIG. 7 , of the current space vector I 2 * onto the corresponding phase axes of the phases U, V, W, it becomes clear that the current in the phase U, that is to say in the switch SHA, is reduced with respect to I 1 * and the current in the phase W, that is to say in the switch SHC and the freewheeling diode DLC, is increased. In total, owing to the larger magnitude of the current space vector I 2 *, the current loading is greater than in the case of the nominal current space vector I 1 *; however, by virtue of this measure, as can be seen in  FIG. 7 , the loading of the most heavily loaded switch SHA and the freewheeling diode DLA can be reduced. As a result, peak loading of the most heavily loaded switches S and also the most heavily loaded freewheeling diodes D can be reduced and the loading can be distributed to other switches S or freewheeling diodes D. As a result, the inverter  10  can be loaded more evenly in phase. Since the current phasor I 2 * follows the line of even torque M, an identical torque M is provided by the electrical machine  14 , with the result that this measure does not represent a restriction for the user of the electrical machine  14  and, for example, no stuttering or drop occurs in the torque M. By virtue of the setting of the current space vector I 2 *, which deviates from the nominal current space vector I 1 *, the losses in the individual phases U, V, W can be distributed and thus overloading of individual components of individual phases can be avoided. In other words, a more even loading of the phases U, V, W can thus be achieved. 
         [0074]    As a result, by providing an alternative current space vector I 2 * with a phase angle alpha 2  deviating from the nominal phase angle alpha 1 , a reduction of the most often loaded switch SHA and the freewheeling diode DLA or the most heavily loaded phase U can thus be achieved and thus the inverter  10  can be generally more evenly loaded. 
         [0075]    If the freewheeling diodes D can be heavily loaded, delta_beta can also be set with a negative value in order to relieve individual ones of the switches S. In the control situation illustrated in  FIG. 7 , the switch SHA is firstly relieved by selection of the zero voltage phasor V 0  and thus the freewheeling diode DLA is more heavily loaded. As a result, the switches SLB, SLC are also more heavily loaded. For the phase angle alpha_ 1 , the freewheeling diode DLA is then loaded most heavily, the switch SLC is loaded least heavily and the switch SLB is loaded very lightly. In this situation, by means of a phase angle alpha_which is smaller than alpha_ 1 , that is to say with a negative deviation angle delta beta, the freewheeling diode DLA can be loaded more heavily, as a result of which the switch SLC is relieved, however, and the switch SLB is more heavily loaded. Thus, the loading of the switches SLB and SLC can be more evenly distributed. However, this takes place at the cost of a heavier loading of the freewheeling diode DLA. 
         [0076]    In other words, the loading is firstly displaced from an upper switch SH to a lower freewheeling diode DL and, what is more, by selection of a suitable temporal distribution of the zero voltage phasors V 0 , V 7  and then the loading is distributed to the phases U, V, W in the case of the zero voltage phasor V 0 , V 7  by setting the deviation angle delta_beta. Thus, in general, the loading of the switches S and the freewheeling diodes D can be set more evenly. 
         [0077]    Preferably, the method  40  from  FIG. 5  is combined with the setting of the alternative current space vector I 2 * from  FIG. 7 . In this case, for example before controlling the inverter  10 , that is to say in the control device  18 , for example, an optimized nominal loading value m and, at the same time, on the basis of the nominal current space vector I 1 * an optimized current space vector I 2 * are determined according to the method  30  also. These values are stored in a characteristic map and the electrical machine  14  is controlled on the basis of the values of the characteristic map. In other words, the nominal loading value m and the current space vector I 2 * are determined offline and the electrical machine is correspondingly controlled. 
         [0078]    As an alternative to this, the optimized current space vector I 2 * can be removed from the characteristic map and the nominal loading value m can be determined during operation of the electrical machine  14  according to the method  30  or  40  on the basis of measured values or estimated values and can be correspondingly optimized in a continual manner. In other words, the nominal loading value m is determined online and adapted. 
         [0079]      FIG. 8  schematically illustrates a complex phasor diagram according to  FIGS. 6 and 7  for a nominal current space vector I 1 * of zero degrees.  FIG. 8  also illustrates the line of even torque M. The line of even torque M has a curvature such that it is not possible to relieve the phase U or to relieve the corresponding switch SHA by providing a current space vector I 2 * with a deviating phase angle. By contrast, by means of a greater or smaller phase angle alpha 2 , the loading of the switch SHA would remain the same or even increase and, furthermore, a further switch of the phase W or the phase V would additionally be loaded. Corresponding profiles of the line of even torque arise for phase angles according to the axes of the phases U, V, W, that is to say for the angles 0 degrees, 60 degrees, 120 degrees, 180 degrees, 240 degrees, 300 degrees, etc. Control by means of a deviating phase angle alpha 2  does not make sense for these phase angles alphal . Control by means of the deviating phase angle alpha 2  for phase angles alphal in the region of 30 degrees, 90 degrees, 150 degrees, etc. is particularly sensible. 
         [0080]    In order to reduce the expenditure in terms of control technology, it is possible, at particular nominal phase angles alphal, to store data relating to the current space vectors I 2 * in a characteristic map. The characteristic map can also take into account that the phase angles of the current space vector I 1 * and the phase angles of the voltage space vector V 1 * can deviate from one another at particular frequencies. 
         [0081]      FIG. 9  schematically illustrates the phase angle deviation delta_beta of the current space vector I 2 * from the nominal current space vector I 1 * as a function of the nominal phase angle alpha 1 . 
         [0082]    The deviation delta_beta is set differently as a function of the nominal phase angle alpha 1  and fluctuates between −15° and +15°. As has already been mentioned above, a phase angle alpha 2  which deviates from the nominal phase angle alphal does not make sense for particular nominal phase angles alphal since relief of a switch S or a freewheeling diode D is not the aim here; however, another switch S or another freewheeling diode is loaded more heavily. For this reason, the deviation delta_beta for this nominal phase angle alphal according to  FIG. 9  is equal to 0, whereas, for other nominal phase angles alphal, such as −150°, −90°, −60°, +60°, +90°, +150°, a deviation delta_beta makes sense in order to relieve the most heavily loaded controllable switch S. For this reason, the deviation delta_beta has a zigzag profile as a function of the nominal phase angle alphal, as is illustrated in  FIG. 9 . According to the invention, the deviation delta_beta is restricted, as indicated by the dashed lines at +6° and −6°, which form the deviation limits delta beta max and delta beta min. If it were possible for a deviation delta_beta for the corresponding nominal phase angles alphal to be greater than the thus defined deviation limit delta beta max and delta_beta_min, the deviation delta_beta is set according to the deviation limit delta_beta_max, delta_beta_min. As a result, the total loading of the inverter  10  and the power loss of the inverter  10  and of the electrical machine  14  can be reduced and the efficiency of the inverter  10  and of the electrical machine  14  can be increased. 
         [0083]      FIG. 10  schematically illustrates the maximum deviation delta_beta_max as a function of a rotation frequency f of the nominal current space vector I 1 *. In this case, the maximum deviation delta_beta_max is constant for low frequencies which are smaller than a first predefined rotation frequency f 1 . If the rotation frequency f of the nominal current space vector I 1 * exceeds the first predefined rotation frequency fl, the maximum deviation delta_beta_max is reduced as a function of or with increasing rotation frequency f. In the case of a second predefined rotation frequency f 2  of the nominal current space vector I 1 *, the maximum deviation delta_beta is reduced to almost 0. From the second predefined rotation frequency f 2 , the rotation frequency f is so high that the controllable switches S are loaded over such a short period of time that the thermal loading of the switches S of the inverter  10  is evenly distributed and a deviation delta_beta of the phase angle alpha 2  from the nominal phase angle alphal would not effect any relief of one of the switches S, rather the total loading of the inverter  10  would increase. Therefore, for rotation frequencies f which are greater than the second predefined rotation frequency f 2 , 0 is set. Between the first predefined rotation frequency f 1  and the second predefined rotation frequency f 2 , the maximum deviation delta_beta is linearly reduced as a function of the rotation frequency f. As a result, it is possible for a transition between the control with deviating phase angle and without deviating phase angle, that is to say between the frequency range for f is less than f 1  and the frequency range for f is greater than f 2 , to be simpler in terms of control technology. Furthermore, as a result of this, it is possible for dynamic effects to be reduced in the case of increasing rotation frequency f, which dynamic effects are avoided by abrupt switchover between the control with deviation delta_beta and without deviation delta_beta. 
         [0084]    It goes without saying that the maximum deviation delta_beta_max, which is illustrated in  FIG. 10 , is to be considered as a magnitude and applies both to the upper limit delta_beta_max and the lower limit delta beta min. 
         [0085]    The present method is preferably used for controlling electrical machines, wherein the maximum possible deviation delta_beta_max is dependent on the type of the electrical machine. The maximum deviation can be up to 30°.