Patent Publication Number: US-2021194403-A1

Title: Motor drive system comprising power network between inverter and motor

Description:
TECHNICAL FIELD 
     This disclosure relates to a motor driving system, and more particularly, a motor driving system having a power network circuit configured with a passive element between an inverter and a motor. 
     BACKGROUND ART 
       FIG. 1  shows an equivalent per-phase circuit with a three-phase structure of a general motor. In the per-phase circuit substituted as shown in  FIG. 1 , an output voltage v 1  and a counter electromotive force voltage v 2  of an inverter are shown as input and output voltages, and the phase impedance is shown as a series combination of an inductor and a resistor. Here, v 2  is the counter electromotive force voltage, and its size is generally proportional to speed. In the case of a voltage-type inverter, the output voltage v 1  of the inverter is limited (Vdc/√3 in the case of PWM, 2Vdc/π in the case of Six-step), so the maximum operating area is limited when considering the inductor, the resistance voltage drop and the counter electromotive force. 
       FIG. 2  shows a torque-speed curve of a conventional motor. Referring to  FIG. 2 , a maximum speed at which a constant torque is maintained as marked in green is referred to as a base speed, and this speed is determined by the limit of the voltage size mentioned above. In other words, if the DC-link voltage increases and thus the maximum value of v 1  increases, the base speed increases. 
       FIGS. 3A and 3B  show conventional methods for raising an output voltage to increase the bases peed.  FIG. 3A  is a boost converter method, and  FIG. 3B  is a z-source inverter method. 
     Referring to  FIG. 3A , in a voltage-type inverter, the output voltage is limited according to the size of a DC-link voltage, so a boost converter is used to increase the DC-link voltage. By using the boost converter connected in series to the inverter, the DC-link voltage may be increased to a range in which the boost converter within a device rating is controllable. Therefore, there is an advantage that the maximum output voltage of the inverter can be improved, but there is a disadvantage that the volume and switching loss increase due to additional inductor and switch. In addition, if the DC-link voltage increases, the voltage stress of a switching element increases, and an insulation interval or the like of the driving circuit should be considered again. Therefore, it is necessary to redesign the driving circuit by element selection and artwork. 
     Referring to  FIG. 3B , a Z-source inverter is used to boost the DC-link voltage with only a passive element without using a boost converter. In this method, by adding a shoot-through time of an inverter leg (the time to turn on both upper and lower switches), the energy transfer process performed by the boost converter is performed by the inverter itself. However, since the inverter output is limited by the shoot-through time, there is a problem that the inverter cannot use the voltage to its maximum. In addition, there is a problem that the driving circuit needs to be redesigned because the voltage stress increases like the boost converter. 
     RELATED LITERATURES 
     Non-Patent Literature 
     Non-patent Literature 1: P. Fang Zheng, “Z-source inverter,” IEEE Transactions on Industry Applications, vol. 39, no. 2, pp. 504-510, 2003 
     DISCLOSURE 
     Technical Problem 
     In order to solve the above problem according to an embodiment of the present disclosure, there is a need for a way to increase a motor speed without requiring an additional switch and increasing a DC-link voltage itself. 
     Technical Solution 
     A motor driving system having a power network between an inverter and a motor comprises an AC motor, an inverter unit configured to apply a voltage to the AC motor, a controller configured to control an output voltage of the inverter unit, and a power network circuit disposed between the inverter unit and the AC motor, wherein the power network circuit may be configured with passive element. 
     In an embodiment, the power network circuit may be configured as a T-type impedance model. 
     In an embodiment, the power network circuit may be configured with at least one of: 
     a capacitor connected in series to the AC motor; a capacitor connected in series to the AC motor and an inductor connected in parallel thereto; an inductor connected in parallel to the AC motor and a capacitor connected in series thereto; a capacitor and an inductor in series to the AC motor and an inductor connected in parallel thereto; a capacitor and an inductor in series to the AC motor and a capacitor connected in parallel thereto; an inductor connected in series to the AC motor and an inductor and a capacitor connected in parallel thereto; and a capacitor connected in series to the AC motor and an inductor and a capacitor connected in parallel thereto. 
     In an embodiment, the power network circuit may further include a mechanical or electric switch, and the mechanical or electric switch may be turned off in a first mode in which the AC motor is operating at a speed less than a predetermined speed and be turned on in a second mode in which the AC motor is operating at a speed equal to or greater than the predetermined speed. 
     In an embodiment, when the power network circuit is configured with a capacitor connected in series to the AC motor, a value of the capacitor may be determined based on an inductance of the AC motor, a magnetic flux density of a permanent magnet of the AC motor and a maximum voltage of the inverter unit. 
     In an embodiment, the power network circuit may be configured with at least one of a capacitor and an inductor, the T-type impedance model may include a first impedance and a second impedance connected in series to the AC motor; and a third impedance extending from a node between the first impedance and the second impedance and connected in parallel to the first impedance and the second impedance, and the first impedance, the second impedance and the third impedance may be expressed as follows. 
     first impedance: jX11-jX12 
     second impedance: jX22-jX12 
     third impedance: jX12 
     In an embodiment, values of the capacitor and the inductor configuring the power network circuit may be determined such that a following formula satisfies 0. 
     
       
         
           
             
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     Here, r m  is an equivalent resistance of the AC motor. 
     In an embodiment, the AC motor may be at least one selected from a surface-mounted permanent magnet motor, an induction motor and an interior permanent magnet synchronous motor. 
     In an embodiment, when a power ratio of the power network circuit is greater than a power ratio of the inverter unit, a value of the impedance configuring the power network circuit may be selected to minimize the sum of a size of the power network circuit and a size of the inverter unit. Here, the size of the power network circuit may be the sum of maximum values of reactive powers of all passive elements included in the power network circuit. In another aspect of the present disclosure, there is provided a method of designing a motor driving system having a power network between an inverter and a motor, the motor driving system including: an AC motor; an inverter unit configured to apply a voltage to the AC motor; a controller configured to control an output voltage of the inverter unit; and a power network circuit disposed between the inverter unit and the AC motor, wherein the power network circuit is configured with a passive element and configured as a T-type impedance model. 
     In an embodiment, when the power network circuit is configured with a capacitor connected in series to the AC motor, a value of the capacitor may be determined based on an inductance of the AC motor, a magnetic flux density of a permanent magnet of the AC motor and a maximum voltage of the inverter unit. 
     In an embodiment, the T-type impedance model may include a first impedance and a second impedance connected in series to the AC motor; and a third impedance extending from a node between the first impedance and the second impedance and connected in parallel to the first impedance and the second impedance, and the first impedance, the second impedance and the third impedance may be expressed as follows. 
     first impedance: jX11-jX12 
     second impedance: jX22-jX12 
     third impedance: jX12 
     In an embodiment, a value of the impedance configuring the power network circuit may be determined such that a following formula satisfies 0. 
     
       
         
           
             
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     Here, r m  may be an equivalent resistance of the AC motor. 
     In an embodiment, a value of the impedance configuring the power network circuit may be selected to minimize the sum of a size of the power network circuit and a size of the inverter unit. Here, the size of the power network circuit may be the sum of maximum values of reactive powers of all passive elements included in the power network circuit. 
     Advantageous Effects 
     In the motor driving system having a power network between an inverter and a motor according to an embodiment of the present disclosure, since a power network circuit configured with a passive element is inserted between the inverter and the motor, the maximum output of the corresponding motor is improved under the same voltage and current limit conditions. The power network circuit may be applied to an induction motor (IM), a surface-mounted permanent magnet synchronous motor (SPM) and an interior permanent magnet synchronous motor (IPM), and improved output has been confirmed. The passive element included in the power network circuit may be configured in various ways, but in this specification, for the sake of simplicity of explanation, the effect will be explained about the case where capacitors are connected in series, the case where two elements of LC or CL are used, and the case where three elements such as LCC are used. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an equivalent per-phase circuit with a three-phase structure of a general motor. 
         FIG. 2  shows a torque-speed curve of a conventional motor. 
         FIGS. 3A and 3B  show conventional methods for raising an output voltage to increase a driving speed. 
         FIG. 4  shows a motor driving system  1000  having a power network between an inverter and a motor according to an embodiment of the present disclosure. 
         FIG. 5  shows a per-phase impedance structure model of a power network circuit  130  according to an embodiment of the present disclosure. 
         FIG. 6  shows a case where an induction motor is used as an AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor connected in series to the AC motor. 
         FIGS. 7A and 7B  show a speed-torque curve ( FIG. 7A ) and a speed-output power curve ( FIG. 7B ) according to a capacitor value of the circuit of  FIG. 6 . 
         FIG. 8  shows a case where a surface-mounted permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor connected in series to the AC motor. 
         FIGS. 9A and 9B  show a speed-torque curve ( FIG. 9A ) and a speed-output power curve ( FIG. 9B ) according to a capacitor value of the circuit of  FIG. 8 . 
         FIG. 10  shows a case where an interior permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor connected in series to the AC motor. 
         FIGS. 11A and 11B  show a speed-torque curve ( FIG. 11A ) and a speed-output power curve ( FIG. 11B ) according to a capacitor value of the circuit of  FIG. 10 . 
         FIG. 12  shows a case where a surface-mounted permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor  131  connected in series to the AC motor and an inductor  132  connected in parallel thereto. 
         FIGS. 13A  and  FIG. 13B  show a speed-torque curve ( FIG. 13A ) and a speed-output power curve ( FIG. 13B ) according to a ratio of the capacitor and the inductor in the circuit of  FIG. 12 . 
         FIG. 14  shows a case where a surface-mounted permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with an inductor  133  connected in series to the AC motor and a capacitor  134  connected in parallel thereto. 
         FIGS. 15A and 15B  show a speed-torque curve ( FIG. 15A ) and a speed-output power curve ( FIG. 15B ) according to a ratio of the capacitor and the inductor in the circuit of  FIG. 14 . 
         FIG. 16  show a speed-torque relationship of the induction motor. 
         FIG. 17  show a per-phase equivalent circuit when a surface-mounted permanent magnet synchronous motor (SPM) is used as the AC motor. 
         FIG. 18  shows the change of ratio of an output current (I2) according to impedance X12 and a current (I12) flowing to a middle impedance X12 of a T-type equivalent model. 
         FIGS. 19A and 19B  show a power network circuit  130  configured with a per-phase CL, LC. 
         FIGS. 20A and 20B  show a capability curve when the power network circuit  130  is configured with an LCC in an embodiment. 
     
    
    
     In the graphs of the drawings, a block dotted line represents a case where the power network circuit  130  is not provided. 
     REFERENCE SIGNS LIST 
       100 : motor driving system 
       110 : AC motor 
       120 : inverter unit 
       130 : power network circuit 
       140 : controller 
       150 : voltage source 
     BEST MODE 
     The following detailed description of the present disclosure refers to the accompanying drawings that illustrate specific embodiments in which the present disclosure may be implemented as an example. These embodiments are described in sufficient detail to enable a person skilled in the art to practice the present disclosure. It should be understood that various embodiments of the present disclosure are different from each other, but need not be mutually exclusive. For example, specific shapes, structures and characteristics described herein implemented in an embodiment may be implemented in other embodiments without departing from the idea and scope of the present disclosure. In addition, it should be understood that locations or arrangement of individual components in each embodiment disclosed herein may be changed without departing from the idea and scope of the present disclosure. Accordingly, the following detailed description is not intended in a limiting sense, and the scope of the present disclosure, if properly described, is limited only by the appended claims, along with all scopes equivalent to those claims. Like reference numerals in the drawings refer to the same or similar functions over several aspects. 
       FIG. 4  shows a motor driving system  100  having a power network between an inverter and a motor according to an embodiment of the present disclosure. The motor driving system  100  including a power network between an inverter and a motor includes an AC motor  110 , an inverter unit  120  for applying a voltage to the AC motor  110 , a controller  140  for controlling an output voltage of the inverter unit  120 , and a power network circuit  130  disposed between the inverter unit  120  and the AC motor  110 . Here, the power network circuit  130  may be configured with a passive element, and preferably may be composed only of a capacitor, an inductor, or a combination thereof. That is, the power network circuit  130  may be a lossless system having only ineffective components. In addition, a voltage source  150  for supplying a DC voltage to the inductor unit  120  may be further included. 
     The controller  140  plays a role of controlling the output voltage of the inverter unit  120 . The controller  100  may include at least one of a current command generator, a weak magnetic flux controller, a current controller, a voltage controller, a step voltage generator, a compensation voltage generator, a PWM unit, and a coordinate converter, but is not limited thereto. The controller  140  needs current and angle information to control the output voltage of the inverter unit. For example, the controller  140  may obtain a current value through a sensor mounted at an output terminal of the motor  110  or recover a phase current from a 3 or 1 shunt resistance of a leg. In addition, the angle information may be read through an additional device such as a Hall sensor or a resolver, or the angle information may be electrically estimated through a sensorless method or the like, but the present disclosure is not limited thereto. 
       FIG. 4  shows a three-phase motor and a three-phase inverter, but the motor driving system  100  of the present disclosure is not limited to the three-phase circuit, but may also be applied to a single-phase or other multi-phase (five-phase, six-phase, seven-phase, or the like) circuit. 
     In addition, the AC motor  110  may be at least one selected from an induction motor (IM), a surface-mounted permanent magnet synchronous motor (SPM), and an interior permanent magnet synchronous motor (IPM), but is not limited thereto, and any type of AC motor is included therein. 
       FIG. 5  shows a per-phase impedance structure model of a power network circuit  130  according to an embodiment of the present disclosure.  FIG. 5  shows a circuit diagram in which the power network circuit  130  is inserted between the inverter unit  120  and the AC motor  120  in an equivalent per-phase circuit of  FIG. 1 . According to an embodiment of the present disclosure, the power network circuit  130  may be configured as a T-type impedance model, and the input and output characteristics of the power network circuit  130  may be determined by appropriately adjusting X11, X12 and X22 values. That is, the power network circuit  130  may be configured by determining the value of the passive element corresponding to X11, X12, and X22. 
     In a specific embodiment, the power network circuit  130  may be configured as follows, but is not limited thereto. 
     1) a capacitor connected in series to the AC motor 
     2) a capacitor connected in series to the AC motor and an inductor connected in parallel thereto 
     3) an inductor connected in parallel to the AC motor and a capacitor connected in series thereto 
     4) a capacitor and an inductor in series to the AC motor and an inductor connected in parallel thereto 
     5) a capacitor and an inductor in series to the AC motor and a capacitor connected in parallel thereto 
     6) an inductor connected in series to the AC motor and an inductor and a capacitor connected in parallel thereto 
     7) a capacitor connected in series to the AC motor and an inductor and a capacitor connected in parallel thereto 
     As in 1) to 7) mentioned above, when connecting a capacitor and an inductor, the values of each capacitor and inductor may be determined in various ways depending on the type of the AC motor. For example, if the power network circuit  130  is configured with capacitors connected in series, X12 becomes 0. 
       FIG. 6  shows a case where an induction motor is used as an AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor connected in series to the AC motor. 
       FIGS. 7A and 7B  show a speed-torque curve ( FIG. 7A ) and a speed-output power curve ( FIG. 7B ) according to a capacitor value of the circuit of  FIG. 6 . Referring to  FIGS. 7A and 7B , it may be understood that the maximum output power at a specific speed is increased compared to the case where there is no capacitor (without cap). 
       FIG. 8  shows a case where a surface-mounted permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor connected in series to the AC motor. 
       FIGS. 9A and 9B  show a speed-torque curve ( FIG. 9A ) and a speed-output power curve ( FIG. 9B ) according to a capacitor value of the circuit of  FIG. 8 . Referring to  FIGS. 9A and 9B , it may be understood that the speed for outputting the maximum power is increased compared to the case where there is no capacitor (without cap). 
       FIG. 10  shows a case where an interior permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor connected in series to the AC motor. 
       FIGS. 11A and 11B  show a speed-torque curve ( FIG. 11A ) and a speed-output power curve ( FIG. 11B ) according to a capacitor value of the circuit of  FIG. 10 . Referring to  FIGS. 11A and 11B , it may be understood that even in this case, the maximum output power at a specific speed is increased compared to the case where there is no capacitor (without cap). 
       FIG. 12  shows a case where a surface-mounted permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with a capacitor  131  connected in series to the AC motor and an inductor  132  connected in parallel thereto. 
       FIGS. 13A  and  FIG. 13B  show a speed-torque curve ( FIG. 13A ) and a speed-output power curve ( FIG. 13B ) according to a current limit value of the inverter unit in the circuit of  FIG. 12 .  FIGS. 13A  and  FIG. 13B  show a capability curve according to the output current at the inverter unit in a state in which the output voltage at the inverter unit  120  is limited and the current input to the AC motor  110  is limited, and LC connected in series and parallel are connected thereto, so it may be understood that the AC motor  110  may show higher output power than the case where there is no LC (a black dotted line). Since optimization is not made in the two degrees of freedom, a large current is needed at the output terminal of the inverter. 
       FIG. 14  shows a case where a surface-mounted permanent magnet synchronous motor is used as the AC motor  110  according to an embodiment of the present disclosure and the power network circuit  130  is configured with an inductor  133  connected in series to the AC motor and a capacitor  134  connected in parallel thereto.  FIGS. 15A and 15B  show a speed-torque curve ( FIG. 15A ) and a speed-output power curve ( FIG. 15B ) according to a current limit value of the inverter unit in the circuit of  FIG. 14 . 
       FIGS. 15A and 15B  show a capability curve according to the output current at the inverter unit in a state where the output voltage at the inverter unit  120  is limited and the current input to the AC motor  110  is limited, and LC connected in series and parallel are applied thereto, so it may be understood that the AC motor  110  may show higher output power than the case where there is no LC (a black dotted line). Since optimization is not made in the two degrees of freedom, a large current is needed at the output terminal of the inverter. 
     Hereinafter, a method of determining values of passive elements configuring the power network circuit  130  will be described. 
     A. Design of a Power Network Circuit Configured with a Capacitor 
     As for the motor driving system  100  of the embodiment of  FIG. 8 , in a surface-mounted permanent magnet synchronous motor (SPM) having a symmetrical structure, Formula 1 may be determined as below. Since the surface-mounted permanent magnet motor has a perfectly symmetrical structure and the rotation frequency is fixed as a synchronous frequency, the capacitor value may be determined simply as in Formula 1. 
     
       
         
           
             
               
                 
                   
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     Here, Ls represents an inductance of the motor, λ represents a magnetic flux density of the permanent magnet, Vmax represents a maximum voltage of the inverter unit, and Is_rated represents a current limit value of the motor. Seeing Formula 1, the capacitor value for obtaining the maximum output ata specific speed ω may be determined. 
     That is, if the power network circuit is configured with a capacitor connected in series to the AC motor, the value of the capacitor may be determined based on the inductance of the AC motor, the magnetic flux density of the permanent magnet of the AC motor, and the maximum voltage of the inverter unit. 
     Meanwhile, in the induction motor (IM), a slip frequency varies according to the current, and the synchronization frequency is changed at a fixed speed. Therefore, the formula for determining Cs in consideration of the above factors becomes complicated. Thus, according to an embodiment of the present disclosure, if the induction motor is used, the value of the capacitor connected in series may be designed by determining the maximum torque in a stop state and the maximum speed for outputting a rated torque as shown in  FIG. 16 . 
     Specifically, in the induction motor (IM), the maximum and minimum values of the capacitor value connected in series may be determined according to Formula 2 below, and in Formula 3, the value of the capacitor may be determined as a value between the maximum and minimum values determined in Formula 2. 
     
       
         
           
             
               
                 
                   
                     
                       
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                     [ 
                     
                       Formula 
                        
                       
                           
                       
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                       2 
                     
                     ] 
                   
                 
               
             
           
         
       
     
     Here, Ls is an inductance of the motor. Rr is a resistance of the rotor and Rs is a resistance of the stator. 
     σ is a leakage coefficient of the motor. ωe_rated is a rated electric speed of the motor. 
     ωr_rated is a rated speed of the motor rotor. Vs_max is a synthesizable voltage limit. 
     ωr_rated is a rated magnetic flux of the motor. Idqs 0 , e 0  is a current for generating the corresponding torque Tmax 0  at 0 speed. 
         C   max ( T   max0 )≤ C   s   ≤C   max (ω r_rated )  [Formula 3]
 
     B. Design of a Power Network Circuit Configured with Two Elements of LC or CL 
     A method of designing a power network circuit configured with the LC element described above when the surface-mounted permanent magnet synchronous motor (SPM) is used as the AC motor  110  will be described.  FIG. 17  shows a per-phase equivalent circuit when the surface-mounted permanent magnet synchronous motor (SPM) is used as the AC motor. 
     As described above, according to an embodiment of the present disclosure, the power network circuit  130  may be configured as a T-type impedance model, and the input and output characteristics of the power network circuit  130  may be determined by appropriately adjusting the X11, X12, and X22 values. 
     For example, referring to  FIG. 5 or 17 , the power network circuit is configured with at least one of a capacitor and an inductor, and the T-type impedance model may be configured with a first impedance and a second impedance connected in series to the AC motor, and a third impedance extending from a node between the first impedance and the second impedance and connected in parallel to the first impedance and the second impedance. Here, the first impedance, the second impedance and the third impedance may be expressed as follows. 
     First impedance: jX11-jX12 
     Second impedance: jX22-jX12 
     Third impedance: jX12 
     For the electromotive force of the surface-mounted permanent magnet synchronous motor (SPM), since a power factor of the phase current is mainly controlled to 1, the counter electromotive force of the motor may be replaced with a resistance. If the output voltage v 1  of the inverter unit and the counter electromotive force voltage v 2  of the surface-mounted permanent magnet synchronous motor (SPM) may be expressed as a per-phase equivalent circuit, as shown in  FIG. 17 . The right impedance (jX22-jX12) of the T-model is a value including the resistance of the motor and the inductance value. 
     In  FIG. 17 , rm of the per-phase equivalent circuit (the equivalent resistance of the motor) may be determined as in Formula 4 according to the output speed and power of the motor. 
     
       
         
           
             
               
                 
                   
                     r 
                     m 
                   
                   = 
                   
                     
                       1 
                       6 
                     
                      
                     
                       
                         
                           ( 
                           
                             
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                               f 
                             
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                     Formula 
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                   ] 
                 
               
             
           
         
       
     
     Here, λf represents a magnetic flux density of the permanent magnet, wr represents the speed, and Pm represents an output power of the motor. 
     In an embodiment, the power factor of the input impedance may be limited to 1 in order to minimize conduction loss caused by ineffective power. Therefore, if the voltage boosting ratio is multiplied by a at the frequency to be designed, the input impedance must be exhibited as 1/α 2  by the energy conservation law. For this, if the input impedance (zin=v 1 /i1) viewing the motor from the inverter unit in  FIG. 17  is calculated, Formula 5 below may be obtained. 
     
       
         
           
             
               
                 
                   
                     Z 
                     in 
                   
                   = 
                   
                     
                       
                         
                           
                             X 
                             12 
                             2 
                           
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                        
                       
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     In order to show the impedance viewed from the inverter as a desired equivalent resistance of the power factor 1 as in the above formula, two degrees of freedom (X12, X22) are used among three impedances (X11, X12, X22). Additional optimization is possible through the remaining 1 degree of freedom. An optimization method using the remaining one degree of freedom may minimize the current flowing through the center impedance (X12). Through this, it is possible to minimize the output current of the inverter by reducing the current that does not contribute to the output power. 
     That is, X11, X12 and X22 may be determined so that the imaginary part becomes 0 in Formula 5. 
       FIG. 18  shows the change of ratio of an output current (I2) according to impedance X12 and a current (I12) flowing to a middle impedance X12 of a T-type equivalent model. A required X12 value may be determined by referring to the graph of  FIG. 18 . 
       FIGS. 19A and 19B  show a power network circuit  130  configured with a per-phase CL, LC.  FIG. 19A  shows a power network circuit  130   a   1  configured with a capacitor connected in series and an inductor connected in parallel, and  FIG. 19B  shows a power network circuit  130   a   2  configured with an inductor connected in series and a capacitor connected in parallel. 
     As such, since the power network circuit is designed so that the imaginary part becomes 0 through Formula 5, the power factor of the inverter becomes 1, thereby minimizing the power loss of the inverter. 
     Meanwhile, in another embodiment, the power network circuit may be designed so that the imaginary part does not become 0. 
     In the power network circuit, minimizing the loss of the inverter by designing the power factor of the inverter to be 1 may not minimize the size (or, power loss) of the entire system. In other words, in terms of minimizing the size (or, power loss) of the entire system, it would be more advantageous to obtain a gain in terms of power loss in other parts of the system (for example, the power network circuit) even though the power loss of the inverter is somewhat damaged. 
     Due to the recent development of semiconductor devices, the inverter loss is decreasing in the latest inverters and its size is also decreasing. In light of this trend, the ratio of the power loss of the power network circuit is likely to be greater than the power loss of the inverter when the latest inverter is used. 
     In the embodiment, if the power ratio of the power network circuit is greater than the power ratio of the inverter, the power network circuit may be configured to minimize the size (or, power loss) of the entire motor driving system. For example, when the sum of the size (or, power loss) of the power network circuit and the size (or, power loss) of the inverter unit is minimized, the size (or, power loss) of the entire system may be minimized. Here, when the power network circuit is configured with passive elements, the size of the power network circuit may be expressed as the sum of maximum values of reactive powers of all passive elements. 
     A user or designer may design the power network circuit by determining characteristic values of passive elements based on the purpose of the power network circuit applied to the motor driving system, the power ratio of the inverter, the power ratio of the power network circuit, and the like. 
     C. Design of a Power Network Circuit Configured with Three Elements 
       FIGS. 20A and 20B  show a capability curve when the power network circuit  130  is configured with an LCC in an embodiment. The capability curves of  FIGS. 20A and 20B  are drawn by limiting the current of the motor  110  to 1 and changing the current limit at the inverter unit. Since the power factor is designed as 1 at the target speed of 2400 r/m, it may be found that the maximum torque and maximum power are output at the corresponding speed. This curve has a similar shape to the series capacitor, but has a higher efficiency than the series capacitor structure because the power factor is designed as 1 at the maximum speed. The capacity curve of an existing motor is as shown by a black dotted line. 
     In addition, in an embodiment of the present disclosure, the power network circuit  130  may further include a mechanical switch or an electric switch. The mechanical or electric switch may be turned off in a first mode in which the AC motor is operating at a speed less than a predetermined speed and turned on in a second mode in which the AC motor is operating at a speed equal to or greater than a predetermined speed. That is, if a speed greater than the predetermined speed is required, the mechanical or electric switch may be controlled to be turned on so that the power network circuit of the present disclosure operates. In this case, compensating the existing mechanical gear system may be configured in an electrical form, which has an advantage of compensating for the disadvantages such as wear, noise and clutch timing of the existing mechanical gear. 
     In the method for designing a motor driving system having a power network between an inverter and a motor according to an embodiment of the present disclosure, the motor driving system may include an AC motor, an inverter unit configured to apply a voltage to the AC motor, a controller configured to control an output voltage of the inverter unit, and a power network circuit disposed between the inverter unit and the AC motor, and the power network circuit may be configured with a passive element and configured as a T-type impedance model. 
     In an embodiment, if the power network circuit is configured with a capacitor connected in series to the AC motor, a value of the capacitor may be determined based on an inductance of the AC motor, a magnetic flux density of a permanent magnet of the AC motor and a maximum voltage of the inverter unit. 
     In an embodiment, the power network circuit may be configured with at least one of a capacitor and inductor, and the T-type impedance model may include a first impedance and a second impedance connected in series to the AC motor, and 
     a third impedance extending from a node between the first impedance and the second impedance and connected in parallel to the first impedance and the second impedance, and the first impedance, the second impedance and the third impedance may expressed as follows. 
     First impedance: jX11-jX12 
     Second impedance: jX22-jX12 
     Third impedance: jX12 
     In this case, values of the capacitor and the inductor configuring the power network circuit may be determined such that a following formula satisfies 0. 
     
       
         
           
             
               
                 X 
                 11 
               
               - 
               
                 
                   
                     X 
                     12 
                     2 
                   
                    
                   
                     X 
                     22 
                   
                 
                 
                   
                     r 
                     m 
                     2 
                   
                   + 
                   
                     X 
                     22 
                     2 
                   
                 
               
             
             , 
           
         
       
     
     Here r m  is an equivalent resistance of the AC motor. 
     Even though the present disclosure is described above based on specific matters such as specific components and limited embodiments and drawings, this is provided only to help a more general understanding of the present disclosure, and the present disclosure is limited to the embodiments. The present disclosure may be changed or modified in various ways by a person with ordinary knowledge in the technical field to which the present disclosure belongs. 
     Therefore, the idea of the present disclosure should not be limited to or determined by the described embodiment, and not only the appended claims but also all modifications or equivalents of the claims are regarded as falling within the scope of the idea of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The motor driving system having a power network between an inverter and a motor according to an embodiment of the present disclosure improves the maximum output of the motor. 
     The power network circuit is configured to improve the output of various motors such as an induction motor (IM), a surface-mounted permanent magnet synchronous motor (SPM) and an interior permanent magnet synchronous motor (IPM), and may be widely used in the power field.