Abstract:
In a method for optimizing a space vector pulse width modulation, a voltage is connected to a load, by combining discrete switching states of a plurality of switches to control the load, the load being switched to zero potential by two of the switching states. In the case the maximum degree of control is increased, during a pulse width modulation period, at least one of the switching states, that switches to zero potential, is omitted.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for optimizing a space vector pulse width modulation, in which a voltage is connected to a load by combining discrete switching states of a plurality of switches to control the load, the load being switched to zero potential by two of the switching states, and relates to a device for carrying out the method. 
         [0003]    2. Description of the Related Art 
         [0004]    “Space vector pulse width modulation” SVPWM is often used to operate polyphase machines, which, on the one hand makes possible a relatively good degree of control, and on the other hand is relatively simple to implement. In particular, the control signals for the three half-bridges that are required for operating a polyphase machine are generated in this context. The signals between the three half-bridges are phase-shifted by 120°. The three half-bridges include six switches having 8 admissible discrete switching states, which emit periodic signals. Six of the switching states are active, i.e. they lead to a voltage. Two switching states are designated as null vectors and do not conduct any voltage to the load. 
         [0005]    The maximum degree of control of the inverter, which is controlled by the SVPMW method, is limited by a nonideal hardware. In order for the curve of the switched voltage to be able to follow the control signal, in the case of real power switches, a minimum distance in time t Min  is required between two switching processes of the same switch, whereby the maximum degree of modulation A Max  is reduced to 
         [0000]        A   Max =( T   PWM −2 *t   Min )/ T   PWM .  (1)
 
         [0006]    A null pointer that is to be present for a very short time, that is less than t Min , can no longer be set. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a method for optimizing the space vector pulse width modulation, in which the maximum degree of modulation is increased. 
         [0008]    One essential aspect of the present invention is that one of the two switching states, that switch to zero potential, in the form of the null pointer is omitted. This has the advantage that one switch is not switched, whereby one switch-on process and one switch-off process are omitted. With that, the maximum degree of modulation A MaxNew  is increased 
         [0000]        A   Max New =( T   PWM   −t   Min )/ T   PWM .  (2)
 
         [0009]    The switching state that switches to zero potential is advantageously omitted in the middle of the PWM period. The switching state that switches to zero potential is extended at the beginning and at the end of each PWM period by that time which the switching state switching to zero potential (null pointer) would have required in the middle of the PWM period. 
         [0010]    The minimum interval between two switching processes of the same switch is determined by the switch-on process and the subsequent switch-off process or vice versa, which, in turn, is specified by the real hardware used and its switching speeds, since the switching speed cannot be set to be infinitesimally short. 
         [0011]    In a device for optimizing a space vector pulse width modulation, in which a voltage is connected to a load by combining discrete switching states of a plurality of switches to control the load, the load being switched to zero potential by two of the switching states, with the aid of which the maximum degree of modulation is to be increased, means being available which suppress at least one of the switching states that switches to zero potential, during the pulse width modulation period. 
         [0012]    The times are first calculated according to space vector pulse width modulation SVPWM and then, either at all times or only upon the exceeding of the maximum degree of modulation achievable up to now, the null pointer in the middle of a PWM period is omitted and its time duration is added to the zero pointer at the beginning and at the end of the PWM period. The maximum achievable degree of modulation increases in both cases. In the first case, the switching losses are reduced by one-third. By contrast, it is ensured in the second case that the response in the modulation range, that was achievable up to now, remains unchanged. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a 3-phase inverter for controlling a polyphase machine. 
           [0014]      FIG. 2  shows a table showing switching states of the switches used in the inverter according to  FIG. 1 . 
           [0015]      FIG. 3  shows control signals for switches T 1 , T 2  and T 3  according to the related art. 
           [0016]      FIG. 4  shows control signals for switches T 1 , T 2  and T 3  according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  shows a polyphase machines  1  which is connected to a control unit  2 , which controls torque m or rotational speed n of the polyphase machines  1 . In addition, polyphase machines  1  is connected to a 3-phase inverter  3  as a power section, which has three half-bridges T 1 , T 4 ; T 2 , T 5  and T 3 , T 6 . Each half-bridge is connected to one phase of polyphase machines  1 . Thus, phase  1  of polyphase machine  1  is connected to half-bridge T 1 , T 4  of inverter  3 . Phase  2  of polyphase machine  1  is at half-bridge T 2 , T 4 , while phase  3  leads to half-bridge T 3 , T 6 . 
         [0018]    For the six switches T 1 , T 2 , T 3 , T 4 , T 5  and T 6  developed as power semiconductors there are eight admissible states, of which 6 are active states, which means that at these switching states a voltage is present at the individual phases of polyphase machines  1 . The exact discrete switching states and the periodic voltage signals, present respectively at phases  1  to  3 , may be inferred from the table of  FIG. 2 . The voltages at phases  1  to  3  are phase-shifted by 120°. 
         [0019]    In the table, the “space vectors” are also given that are used according to the “space vector pulse width modulation”. Besides the six active states, two null states  0  and  7  are also shown, which are designated as null vectors, and which supply no voltage to phases  1  to  3  of polyphase machines  1 . 
         [0020]    Now, in order to be able to set a desired voltage phasor, during a PWM period, an active state is set for a time T k  and an adjacent active state is set for time T k+1 , so that between these two states only one switch has to be switched on and one switch has to be switched off. A null vector is set for the remaining time. In  FIG. 3 , in exemplary fashion, the control signals are shown for the three power semiconductors T 1 , T 2  and T 3  that are positioned at the top in the bridge branches. The control signals for lower three power semiconductors T 4 , T 5  and T 6  come about by inverting the signals shown. 
         [0021]    Frequently, an interlocking time has to be maintained between the control signals, which is intended to prevent that, during the switching process, based on finite switching times, two of the switches, developed as power semiconductors, conduct briefly. Therefore, after the switching off of the upper power semiconductor, the lower power semiconductor is first switched on, delayed by the interlocking time, and similarly, after the switching off of the lower semiconductor, the upper one is switched on, delayed. 
         [0022]    Times T 0 , T k  and T k+1  are calculated by control unit  2 , using the method of the space vector pulse width modulation (SVPWM method), T 0  standing for the control of a null pointer. 
         [0023]    According to the related art, whose PWM period is shown in  FIG. 3 , power semiconductor T 1  sets a null pointer for the time period of 2 (T 0 /4). Power semiconductor T 2  is activated for a time period of T k+1 +T 0 /2, while power semiconductor T 3  is switched to active for time period T k +T k+1 +T 0 /2. At the beginning and at the end of the PWM period, a null pointer is switched for the time period of T 0 /4 in each case. 
         [0024]    Consequently, according to the related art, a maximum degree of control of 96% comes about, calculated according to equation (1), for a PWM period duration of 100 μs and a minimum interval in time of 2 μs between two switching processes. 
         [0025]    As may be seen in  FIG. 4 , the null pointer now drops out in the middle of the PWM period. In one embodiment, this happens only when the desired degree of modulation exceeds the degree of modulation achievable by using the method up to now. For this, the null pointer is optionally extended either at all times or only upon the exceeding of the maximum degree of modulation up to now, at the beginning and at the end of each PWM period. This leads to modified control signals. Thus, one may do without a control of power semiconductor T 1 . The time period for the control signal for power semiconductor T 2  now amounts to T k+1 , while the time period for power semiconductor T 3  amounts to T k+1 +T k  in this embodiment. The control using the null pointer at the beginning and at the end of the PWM period amounts to T 0 /2 in each case, and is thus twice as large as in the example as in  FIG. 3 . That being the case, maximum degree of modification A MaxNew  increases according to equation (2), since power semiconductor T 1  was not activated, and a switching on and switching off sequence is omitted. The operating range that could not be set up to now is halved thereby. 
         [0026]    Thus a maximum degree of modification A MaxNew  of 98% comes about for a PWM period duration of 100 μs and a minimum interval in time of 2 μs between two switching processes, that is, the range that cannot be set is cut in half compared to the method according to the related art.