Patent Publication Number: US-9887645-B2

Title: Motor drive apparatus having function of suppressing temporal variation of regenerative current

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a motor drive apparatus, and in particular to a motor drive apparatus having the function of suppressing the temporal variation of regenerative current. 
     2. Description of the Related Art 
     During deceleration of a motor, the motor acts to provide regenerative braking. Power regeneration is a known method of handling power when regenerative power is large (for example, refer to Japanese Unexamined Patent Publication No. H06-062584). A converter used for power regeneration is shown in FIG. 1. The converter  1001  used for power regeneration comprises power devices Tr 1  to Tr 6  as transistors each of which is connected in parallel with one of diodes D 1  to D 6 . Power generated by a motor is fed to charge a smoothing capacitor (not shown) provided between terminals  1003  and  1004 , and the power stored on the smoothing capacitor is inversely converted by the converter  1001  and returned to a power supply  1002 . Known methods for controlling the power regeneration converter  1001  include a PWM control method and a 120-degree conduction method. In the PWM control method, regenerative current can be made to have a sinusoidal waveform, and the amount of its temporal variation can be controlled. However, the switching noise increases with the PWM operation, and the cost increases as a result of adding a filter or the like. Accordingly, the 120-degree conduction method is generally preferred for use. 
     The 120-degree conduction method is a method that detects the phase of the supply voltage and that returns the regenerative power to the power supply only during the 120-degree period in which the potential difference of the supply voltage is large.  FIGS. 2A and 2B  show a temporal variation of power phase, timing charts illustrating the ON/OFF operations of the power devices Tr 1  to Tr 6 , and temporal variations of R-phase, S-phase, and T-phase currents when the 120-degree conduction method is used.  FIG. 2A  shows the case when the difference between DC link voltage and supply voltage is large and  FIG. 2B  shows the case when the difference between DC link voltage and supply voltage is small. 
     As shown in  FIG. 2A , when the difference between DC link voltage and supply voltage is large, periods (indicated by hatching) are provided during each of which switching is performed at a given frequency to control the current so as not to exceed a given peak current. In this case, di/dt (the amount of temporal variation of current) is calculated in accordance with the following equation (1), and inrush current is calculated by multiplying di/dt with time. 
     
       
         
           
             
               
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                 d 
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             = 
             
               
                 ( 
                 
                   
                     V 
                     DC 
                   
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                       V 
                       IN 
                     
                     
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               L 
             
           
         
       
     
     where V DC  is the DC link voltage, V IN  is the supply voltage, and L is the inductance of AC reactor and power supply. 
     In the conventional art, when the difference between the DC link voltage and the supply voltage (DC link voltage-supply voltage) is large, inrush current occurs during power regeneration, as in the R-phase current at time t 1  shown in  FIG. 2A . Accordingly, if the power devices are to be protected in the conventional art regenerative method, di/dt must be suppressed by increasing the inductance L of the AC reactor. In the case of the PWM control method, the occurrence of inrush current can be suppressed without increasing the inductance of the AC reactor, but there in turn is the problem that the temperature of the power devices increases. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a motor drive apparatus wherein provisions are made to suppress the occurrence of inrush current as well as the temperature rise of driving devices, while at the same time, achieving a reduction in cost by reducing the size of the AC reactor. 
     According to one embodiment of the present invention, there is provided a motor drive apparatus comprising: a converter configured to convert DC power into AC power and returns the AC power to an AC power supply; a 120-degree conduction regenerative power control unit configured to control the converter using a 120-degree conduction method so that the power recovered through the converter is returned to the AC power supply; a PWM regenerative power control unit configured to control the converter using a PWM control method so that the power recovered through the converter is returned to the AC power supply; an input voltage detection unit configured to detect a supply voltage being supplied from the AC power supply to the converter; a DC link voltage detection unit configured to detect a DC link voltage which is an output voltage of the converter; and a regenerative method switching unit configured to, during power regeneration, perform switching between the 120-degree conduction method and the PWM control method in accordance with a given criterion, based on a voltage value detected by the DC link voltage detection unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present invention will become more apparent from the description of the preferred embodiments as set forth below with reference to the accompanying drawings, wherein: 
         FIG. 1  is a diagram showing the configuration of a converter according to the conventional art; 
         FIG. 2A  is a diagram showing a temporal variation of power phase, timing charts illustrating the ON/OFF operations of power devices, and temporal variations of R-phase, S-phase, and T-phase currents when the difference between DC link voltage and supply voltage is large in a converter that uses a 120-degree conduction method according to the conventional art; 
         FIG. 2B  is a diagram showing a temporal variation of power phase, timing charts illustrating the ON/OFF operations of power devices, and temporal variations of R-phase, S-phase, and T-phase currents when the difference between DC link voltage and supply voltage is small in the converter that uses the 120-degree conduction method according to the conventional art; 
         FIG. 3  is a diagram showing the configuration of a motor drive apparatus according to an embodiment of the present invention; 
         FIG. 4  is a flowchart for explaining the sequence of operations performed by the motor drive apparatus according to the embodiment of the present invention; 
         FIG. 5A  is a diagram showing a temporal variation of power phase, timing charts illustrating the ON/OFF operations of power devices, and temporal variations of R-phase, S-phase, and T-phase currents when the difference between DC link voltage and supply voltage is large in the motor drive apparatus according to the embodiment of the present invention; 
         FIG. 5B  is a diagram showing a temporal variation of power phase, timing charts illustrating the ON/OFF operations of power devices, and temporal variations of R-phase, S-phase, and T-phase currents when the difference between DC link voltage and supply voltage is small in the motor drive apparatus according to the embodiment of the present invention; 
         FIG. 6A  is a diagram showing a temporal variation of phase voltage; 
         FIG. 6B  is a diagram showing a temporal variation of phase current according to the conventional art; 
         FIG. 6C  is a diagram showing a temporal variation of phase current in the motor drive apparatus according to the embodiment of the present invention; 
         FIG. 7A  is a diagram showing a temporal variation of phase current according to the conventional art; and 
         FIG. 7B  is a diagram showing a temporal variation of phase current in the motor drive apparatus according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Motor drive apparatus according to the present invention will be described below with reference to the drawings. 
       FIG. 3  shows the configuration of a motor drive apparatus according to an embodiment of the present invention. The motor drive apparatus  101  according to the embodiment of the present invention includes a converter  1 , a 120-degree conduction regenerative power control unit  2 , a PWM regenerative power control unit  3 , an input voltage detection unit  4 , a DC link voltage detection unit  5 , and a regenerative method switching unit  6 . 
     The converter  1  comprises power devices Tr 1  to Tr 6  and diodes D 1  to D 6 , and converts DC power stored on a smoothing capacitor  9  into AC power and returns the thus recovered AC power to an AC power supply  20 . The ON/OFF operations of the power devices Tr 1  to Tr 6  are controlled by signals applied from a power device commanding unit  7  to the gates of the respective power devices Tr 1  to Tr 6 . An AC reactor (ACR)  21  is provided between the converter  1  and the AC power supply  20 . 
     The 120-degree conduction regenerative power control unit  2  controls the converter  1  using a 120-degree conduction method so that the regenerative power recovered through the converter  1  is returned to the AC power supply  20 . More specifically, the 120-degree conduction regenerative power control unit  2  sends a command for performing power regeneration using the 120-degree conduction method to the power device commanding unit  7 . 
     The PWM regenerative power control unit  3  controls the converter  1  using a PWM control method so that the regenerative power recovered through the converter  1  is returned to the AC power supply  20 . More specifically, the PWM regenerative power control unit  3  sends a command for performing power regeneration using the PWM control method to the power device commanding unit  7 . 
     The input voltage detection unit  4  detects the supply voltage being supplied from the AC power supply  20  to the converter  1 . The value of the supply voltage detected by the input voltage detection unit  4  is sent to a drive apparatus control unit  8 . 
     The DC link voltage detection unit  5  detects the DC link voltage which is an output voltage of the converter  1 . The smoothing capacitor  9  is provided at the output of the converter  1 . The DC link voltage is the voltage appearing between the terminals N and P of the smoothing capacitor  9 . The value of the DC link voltage detected by the DC link voltage detection unit  5  is sent to the drive apparatus control unit  8 . 
     The drive apparatus control unit  8  calculates the difference between the acquired DC link voltage and supply voltage, and determines whether or not the calculated difference is equal to or larger than a predetermined value stored in advance. The result of the determination is sent to the regenerative method switching unit  6 . 
     During power regeneration, the regenerative method switching unit  6  performs switching between the 120-degree conduction method and the PWM control method in accordance with a given criterion, based on the voltage value detected by the DC link voltage detection unit  5 . The regenerative method switching unit  6  may perform switching to select the PWM control method when the difference between the DC link voltage and the supply voltage is equal to or larger than the predetermined value and to select the 120-degree conduction method when the difference between the DC link voltage and the supply voltage is smaller than the predetermined value. 
     An input current detection unit  10  for detecting the current being supplied from the AC power supply  20  to the converter  1  is also provided. The input current detection unit  10  also detects the current that flows when the motor drive apparatus  101  performs regenerative operation. For example, in  FIG. 3 , when the power devices Tr 1  and Tr 4  are ON, and the other power devices are OFF, the current I 1  flowing from the converter  1  to the AC power supply  20  and the current I 2  flowing from the AC power supply  20  to the converter  1  can be detected. Next, the sequence of operations performed by the motor drive apparatus according to the embodiment of the present invention will be described with reference to the flowchart shown in  FIG. 4 . First, in step S 101 , the motor drive apparatus  101  initiates regenerative operation. 
     Next, in step S 102 , the input voltage detection unit  4  detects the supply voltage being supplied from the AC power supply  20  to the converter  1 . 
     In step S 103 , the DC link voltage detection unit  5  detects the DC link voltage. 
     In step S 104 , the drive apparatus control unit  8  calculates the difference between the DC link voltage and the supply voltage. 
     In step S 105 , the drive apparatus control unit  8  determines whether or not the difference between the DC link voltage and the supply voltage is equal to or larger than the predetermined value. 
     When the difference between the DC link voltage and the supply voltage is equal to or larger than the predetermined value, then in step S 106  the regenerative method switching unit  6  selects the PWM regenerative power control unit  3  so that the power regeneration is performed using the PWM control method. 
       FIG. 5A  shows a temporal variation of power phase, timing charts illustrating the ON/OFF operations of the power devices, and temporal variations of the R-phase, S-phase, and T-phase currents when the difference between the DC link voltage and the supply voltage is equal to or larger than the predetermined value. The switching control for the power devices Tr 1  to Tr 6  shown in  FIG. 5A  will be described. In  FIG. 5A , reference numeral  51  is the supply voltage detected, and  52  is the PWM command (one example). Further,  53  is the PWM carrier wave (carrier) as a triangular wave for generating the PWM signal. In the power regeneration using the PWM control method, a comparison is made between the PWM command  52  and the PWM carrier  53 , and when the PWM voltage command  52  is larger, the power device Tr 1  is turned off, and the power device Tr 2  is turned on. Conversely, when the PWM voltage command  52  is smaller than the PWM carrier  53 , the power device Tr 1  is turned on, and the power device Tr 2  is turned off. The power devices Tr 3  and Tr 4  and the power devices Tr 5  and Tr 6  are controlled in a similar manner. 
     By controlling the power devices Tr 1  to Tr 6  as described above, the current flowing in each of the R phase, S phase, and T phase can be made to have a sinusoidal waveform, as shown in  FIG. 5A . The occurrence of inrush current can be suppressed by performing the power regeneration using the PWM control method, as will be described later. 
     On the other hand, when the difference between the DC link voltage and the supply voltage is smaller than the predetermined value, then in step S 107  the regenerative method switching unit  6  selects the 120-degree conduction regenerative power control unit  2  so that the power regeneration is performed using the 120-degree conduction method. 
       FIG. 5B  shows a temporal variation of power phase, timing charts illustrating the ON/OFF operations of the power devices, and temporal variations of the R-phase, S-phase, and T-phase currents when the difference between the DC link voltage and the supply voltage is smaller than the predetermined value. In this case, the temperature rise of the power devices which would occur if the PWM control method were used can be suppressed by performing the power regeneration using the 120-degree conduction method. 
     As has been described above, according to the motor drive apparatus of the present invention, since the occurrence of inrush current can be suppressed by performing the power regeneration using the PWM control method, the size of the AC reactor can be reduced. Further, when the inrush current is relatively small, the temperature rise of the power devices is suppressed by performing the power regeneration using the 120-degree conduction method. 
     In the above embodiment, the temperature rise may be suppressed by making provisions so that the switching of the regenerative method based on the difference between the DC link voltage and the supply voltage is performed only during the early stage of the regeneration when the potential difference is large. 
     Next, a description will be given of how the slope of the current can be suppressed in accordance with the motor drive apparatus according to the embodiment of the present invention.  FIG. 6A  shows the temporal variation of the phase voltage.  FIG. 6B  shows the temporal variation of the phase current according to the conventional art.  FIG. 6C  shows the temporal variation of the phase current in the motor drive apparatus according to the embodiment of the present invention. The variation of the R-phase current at time t 5  in  FIGS. 6B and 6C  is shown in enlarged form in  FIGS. 7A and 7B , respectively. In the conventional art, the slope of the current depends on the inductance L of the reactor and the difference voltage between the DC link voltage and the supply voltage. As a result, the peak current has had to be suppressed so that the current that increases during the delay time of the current detection circuit does not exceed the power device rated value. 
     On the other hand, in the motor drive apparatus according to the embodiment of the present invention, the slope of the current is controlled by the inductance L of the reactor, the difference voltage between the DC link voltage and the supply voltage, and the switching frequency. In the present invention, since the slope of the inrush current can be controlled as desired by the switching frequency, the current that increases during the delay time of the current detection circuit can be reduced to a value sufficiently smaller than the power device rated value. In this way, since the occurrence of inrush current can be suppressed according to the motor drive apparatus of the present invention, the inductance L can be reduced, and thus the size of the AC reactor can be reduced. 
     Further, as shown in  FIG. 6C , in the present invention, control is performed by the switching frequency of the R-phase current at times t 1 , t 3 , and t 5 , but not performed at times t 2  and t 4 . That is, the control based on the switching frequency of the phase current is not performed at every timing, but the switch method is switched at desired timing according to the voltage phase. For example, the occurrence of inrush current can be suppressed by performing the power regeneration using the PWM control method only when the difference between the DC link voltage and the supply voltage is equal to or larger than the predetermined value; on the other hand, when the difference between the DC link voltage and the supply voltage is smaller than the predetermined value, the temperature rise of the power devices can be suppressed by performing the power regeneration using the 120-degree conduction method. 
     According to the motor drive apparatus in the embodiment of the present invention, not only can the occurrence of inrush current and the temperature rise of the driving devices be suppressed, but the cost can also be reduced by reducing the size of the AC reactor.