Patent Publication Number: US-2023143105-A1

Title: Motor drive device that calculates insulation resistance value of motor

Description:
TECHNICAL FIELD 
     The present invention relates to a motor drive device for calculating the insulation resistance value of a motor. 
     BACKGROUND ART 
     In a motor drive device that controls driving of a motor in a machine tool, a forging machine, an injection molding machine, an industrial machine, or various robots, a converter unit (rectifying circuit) converts AC power input from an AC power supply into DC power and outputs the DC power to a DC link, and an inverter unit further converts the DC voltage in the DC link into AC power and supplies the AC power as power for driving the motor. The “DC link” refers to a circuit portion that electrically connects the DC output side of the converter unit and the DC input side of the inverter unit to each other, and is also called, e.g., a “DC link portion,” a “direct-current link,” a “direct-current link portion,” or a “direct-current intermediate circuit.” 
     A motor failure may occur upon insulation deterioration of the motor due to any factor. The insulation deterioration of the motor is detected by measuring an insulation resistance value representing the resistance value between the winding of the motor and the ground. 
     A motor drive device, for example, is known that includes a rectifying circuit that rectifies an AC voltage supplied from an AC power supply via a first switch into a DC voltage, a power supply unit that uses a capacitor to smooth the DC voltage rectified by the rectifying circuit, an inverter unit that converts the DC voltage smoothed by the power supply unit into an AC voltage by a switching operation of a semiconductor switching element and drives a motor, a current detection unit that measures a current value flowing through a resistor having one end connected to a coil of the motor and the other end connected to one terminal of the capacitor, a voltage detection unit that measures a voltage value between two ends of the capacitor, a second switch that grounds the other terminal of the capacitor, and an insulation resistance detection unit that detects an insulation resistance value of the motor representing a resistance between the coil of the motor and the ground, using two sets of current values and voltage values of the current value and the voltage value measured when an operation of the motor is stopped, the first switch is turned off, and the second switch is set in two states of OFF and ON (see, e.g., PTL 1). 
     An insulation deterioration detection device for a motor for detecting occurrence of insulation deterioration of a field winding in an active AC motor, for example, is known to include a plurality of detection elements that individually detect an electromagnetic wave when partial discharge has occurred in the field winding of each phase of the AC motor, a ground wire that collectively grounds signals output from the plurality of detection elements, a detection means for extracting a partial discharge signal from the ground wire, a determination means for determining whether the partial discharge falls outside a permissible range, based on a signal output from the detection means, and a warning means for outputting a warning when the determination means determines that the partial discharge falls outside the permissible range (see, e.g., PTL 2). 
     CITATIONS LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Unexamined Patent Publication (Kokai) No. 2015-129704 
         [PTL 2] Japanese Unexamined Patent Publication (Kokai) No. 2002-311080 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Conventionally, the insulation resistance value of a motor may be preferably measured upon cut-off between an AC power supply and a motor drive device that drives the motor. However, it is troublesome and time-consuming to perform a cut-off operation and a connection operation between the motor drive device and the AC power supply for every measurement of the insulation resistance value of the motor, leading to poor efficiency. It is also conceivable to interpose a circuit breaker between the motor drive device and the AC power supply, but this method is disadvantageous in terms of costs. Therefore, demand has arisen for a technique that can measure the insulation resistance value of a motor at low cost without disconnecting a motor drive device from an AC power supply. 
     Solution to Problem 
     According to one aspect of the present disclosure, a motor drive device includes a converter unit configured to convert alternating-current power input from an alternating-current power supply into direct-current power and output the direct-current power to a direct-current link, a direct-current link capacitor provided in the direct-current link, an inverter unit configured to convert the direct-current power in the direct-current link into alternating-current power for driving a motor and output the alternating-current power, two voltage dividing resistors connected in series with each other and interposed between a positive potential line and a negative potential line forming the direct-current link, the voltage dividing resistors having a connection point between the voltage dividing resistors connected to a winding of the motor, a measurement resistor connected between the connection point between the voltage dividing resistors and one of the positive potential line and the negative potential line of the direct-current link, a resistance voltage measurement unit configured to measure a resistance voltage defined as a potential difference between two ends of the measurement resistor, a direct-current component extraction unit configured to extract a direct-current component from the resistance voltage measured by the resistance voltage measurement unit, an alternating-current component extraction unit configured to extract an alternating-current component from the resistance voltage measured by the resistance voltage measurement unit, a direct-current link potential measurement unit configured to measure a positive potential in the positive potential line of the direct-current link and a negative potential in the negative potential line of the direct-current link, and an insulation resistance value calculation unit configured to calculate an insulation resistance value of the motor, based on the direct-current component extracted by the direct-current component extraction unit, the alternating-current component extracted by the alternating-current component extraction unit, and the positive potential and the negative potential measured by the direct-current link potential measurement unit. 
     Advantageous Effects of Invention 
     According to one aspect of the present disclosure, it is possible to measure the insulation resistance value of a motor at low cost without disconnecting a motor drive device from an AC power supply. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a circuit diagram illustrating a motor drive device (first mode) according to one embodiment of the present disclosure. 
         FIG.  2    is a circuit diagram illustrating a modification of the motor drive device (first mode) according to the embodiment of the present disclosure. 
         FIG.  3    is a circuit diagram (part  1 ) illustrating an equivalent circuit used to derive equation (1) in calculation processing by an insulation resistance value calculation unit according to the first mode. 
         FIG.  4    is a circuit diagram (part  2 ) illustrating another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  5    is a circuit diagram (part  3 ) illustrating still another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  6    is a circuit diagram (part  4 ) illustrating still another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  7    is a circuit diagram (part  5 ) illustrating still another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  8    is a circuit diagram (part  6 ) illustrating still another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  9    is a circuit diagram (part  7 ) illustrating still another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  10    is a circuit diagram (part  8 ) illustrating still another equivalent circuit used to derive equation (1) in the calculation processing by the insulation resistance value calculation unit according to the first mode. 
         FIG.  11    is a circuit diagram illustrating a motor drive device (second mode) according to another embodiment of the present disclosure. 
         FIG.  12    is a circuit diagram (part  1 ) illustrating an equivalent circuit used to derive equation (11) in calculation processing by an insulation resistance value calculation unit according to the second mode. 
         FIG.  13    is a circuit diagram (part  2 ) illustrating another equivalent circuit used to derive equation (11) in the calculation processing by the insulation resistance value calculation unit according to the second mode. 
         FIG.  14    is a circuit diagram (part  3 ) illustrating still another equivalent circuit used to derive equation (11) in the calculation processing by the insulation resistance value calculation unit according to the second mode. 
         FIG.  15    is a circuit diagram (part  4 ) illustrating still another equivalent circuit used to derive equation (11) in the calculation processing by the insulation resistance value calculation unit according to the second mode. 
         FIG.  16    is a circuit diagram (part  5 ) illustrating still another equivalent circuit used to derive equation (11) in the calculation processing by the insulation resistance value calculation unit according to the second mode. 
         FIG.  17    is a circuit diagram (part  6 ) illustrating still another equivalent circuit used to derive equation (11) in the calculation processing by the insulation resistance value calculation unit according to the second mode. 
         FIG.  18    is a circuit diagram (part  7 ) illustrating still another equivalent circuit used to derive equation (11) in the calculation processing by the insulation resistance value calculation unit according to the second mode. 
         FIG.  19    is a flowchart illustrating the operation sequence of the motor drive device according to any embodiment of the present disclosure. 
         FIG.  20    is a graph for explaining simulation results for the positive potential and the negative potential of a DC link when an AC power supply and the motor drive device according to any embodiment of the present disclosure are connected to each other by delta connection. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A motor drive device for calculating the insulation resistance value of a motor will be described below with reference to the drawings. In the drawings, the same or similar reference numerals denote the same or similar members. To facilitate understanding, these drawings use different scales, as appropriate. Further, the modes illustrated in the drawings are merely examples for carrying out the present disclosure, which is not limited to the modes illustrated in the drawings. 
       FIG.  1    is a circuit diagram illustrating a motor drive device (first mode) according to one embodiment of the present disclosure. Although details will be described later, one end of a measurement resistor  14  is connected to a negative potential line or a positive potential line of a DC link. The mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link will be referred to as a first mode hereinafter, and the mode in which one end of the measurement resistor  14  is connected to the positive potential line of the DC link will be referred to as a second mode hereinafter. The first mode and the second mode are different from each other only in terms of whether one end of the measurement resistor  14  is connected to the negative potential line or the positive potential line of the DC link. 
     The case where a motor  3  is controlled by a motor drive device  1  connected to an AC power supply  2  will be taken as an example herein. In this embodiment, the type of motor  3  is not particularly limited to any specific type, and may be implemented as, e.g., an induction motor or a synchronous motor. The numbers of phases of the AC power supply  2  and the motor  3  do not particularly limit this embodiment, and a three- or single-phase configuration, for example, may be used. In the example illustrated in  FIG.  1   , each of the AC power supply  2  and the motor  3  has a three-phase configuration. Examples of the AC power supply  2  may include a 400-V three-phase AC power supply, a 200-V three-phase AC power supply, a 600-V three-phase AC power supply, and a 100-V single-phase AC power supply. Machines equipped with motors  3  include, e.g., a machine tool, a robot, a forging machine, an injection molding machine, an industrial machine, various electrical appliances, an electric train, an automobile, and an aircraft. 
     The motor drive device  1  according to the embodiment of the present disclosure includes a converter unit  11 , a DC link capacitor  4 , an inverter unit  12 , two voltage dividing resistors  13 - 1  and  13 - 2 , a measurement resistor  14 , a resistance voltage measurement unit  15 , a DC component extraction unit  16 , an AC component extraction unit  17 , a DC link potential measurement unit  18 , and an insulation resistance value calculation unit  19 , as illustrated in  FIG.  1   . 
     The converter unit  11  converts AC power input from the AC power supply  2  into DC power and outputs the DC power to a DC link located on the DC output side. The converter unit  11  is embodied as a three-phase bridge circuit when three-phase AC power is supplied from the AC power supply  2 , and is embodied as a single-phase bridge circuit when single-phase AC power is supplied from the AC power supply  2 . In the example illustrated in  FIG.  1   , since the AC power supply  2  is designed as a three-phase AC power supply, the converter unit  11  is embodied as a three-phase bridge circuit. Examples of the converter unit  11  may include a diode rectifier, a 120-degree conduction rectifier, and a PWM switching control rectifier. When, for example, the converter unit  11  serves as a 120-degree conduction rectifier and a PWM switching control rectifier, it is embodied as a bridge circuit of switching elements and diodes connected in antiparallel with the switching elements, and performs bidirectional AC/DC power conversion by ON/OFF control of each switching element in accordance with a drive command received from a host controller (not illustrated). In this case, examples of the switching elements may include FETs, IGBTs, thyristors, GTOs (Gate Turn-OFF thyristors), and transistors, but other types of semiconductor elements may be used. 
     The inverter unit  12  converts the DC power in the DC link into AC power for driving the motor  3  and outputs the AC power. The inverter unit  12  is formed by a bridge circuit of switching elements and diodes connected in antiparallel with the switching elements. The inverter unit  12  is embodied as a three-phase bridge circuit when the motor  3  is designed as a three-phase AC motor, and is embodied as a single-phase bridge circuit when the motor  3  is designed as a single-phase AC motor. In the example illustrated in  FIG.  1   , since the motor  3  is designed as a three-phase AC motor, the inverter unit  12  is embodied as a three-phase bridge circuit. The inverter unit  12  is controlled in its power conversion operation by, e.g., the PWM switching control scheme. In other words, the inverter unit  12  converts the DC power in the DC link into AC power for driving the motor  3  and outputs the AC power to the motor  3  in response to a PWM switching command from a host controller (not illustrated), and converts the AC power regenerated by the motor  3  into DC power and returns the DC power to the DC link in motor regeneration. 
     As in the general motor drive device, the inverter unit  12  is controlled in its power conversion operation by a host controller (not illustrated). In other words, the host controller generates a switching command for controlling the speed, the torque, or the rotor position of the motor  3 , based on. e.g., the speed (speed feedback) of the motor  3 , the current (current feedback) flowing through the winding of the motor  3 , a predetermined torque command, and a motion program for the motor  3 . The power conversion operation by the inverter unit  12  is controlled based on the PWM switching command generated by the host controller. 
     The DC link capacitor  4  is provided in the DC link that connects the DC output side of the converter unit  11  and the DC input side of the inverter unit  12  to each other. The DC link capacitor  4  has a function for suppressing pulsation of the DC output of the converter unit  11 , and a function for storing DC power used to generate AC power by the inverter unit  12 . Examples of the DC link capacitor  4  may include an electrolytic capacitor and a film capacitor. 
     The two voltage dividing resistors  13 - 1  and  13 - 2  connected in series with each other are interposed between the positive potential line and the negative potential line forming the DC link. A DC link voltage defined as the potential difference between the positive potential in the positive potential line of the DC link and the negative potential in the negative potential line of the DC link is divided by the voltage dividing resistors  13 - 1  and  13 - 2 . The connection point between the voltage dividing resistors  13 - 1  and  13 - 2  is connected to the winding (its input terminal) of the motor  3 . In the example illustrated in  FIG.  1   , the voltage dividing resistors  13 - 1  and  13 - 2  are respectively formed by two DC link resistors connected in series with each other between the positive potential line and the negative potential line of the DC link. The DC link resistors are implemented as general resistors. The resistance value of the voltage dividing resistor  13 - 1  and that of the voltage dividing resistor  13 - 2  are preferably almost equal to each other, and these resistance values are defined as R in the example illustrated in  FIG.  1   . However, the resistance value of the voltage dividing resistor  13 - 1  and that of the voltage dividing resistor  13 - 2 , may take different values. It suffices to measure the resistance values R of the respective voltage dividing resistors  13 - 1  and  13 - 2  in advance, or use values specified in specification tables for the voltage dividing resistors  13 - 1  and  13 - 2  as these resistance values R. 
     The measurement resistor  14  is connected between the connection point between the voltage dividing resistors  13 - 1  and  13 - 2  and the positive potential line or the negative potential line of the DC link.  FIG.  1    illustrates the first mode in which the measurement resistor  14  is connected between the connection point between the voltage dividing resistors  13 - 1  and  13 - 2  and the negative potential line of the DC link. The measurement resistor  14  is implemented as a general resistor. The resistance value of the measurement resistor  14  is defined as R in . It suffices to measure the resistance value R in  of the measurement resistor  14  in advance, or use a value specified in a specification table for the measurement resistor  14  as this resistance value R in . 
     The resistance voltage measurement unit  15  measures a resistance voltage defined as the potential difference between the two ends of the measurement resistor  14  in the state in which the power conversion operation by the inverter unit  12  is kept stopped (i.e., the state in which all the switching elements in the inverter unit  12  are kept OFF). A signal associated with the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  is sent to the DC component extraction unit  16  and the AC component extraction unit  17 . 
     The DC component extraction unit  16  extracts a DC component from the resistance voltage measured by the resistance voltage measurement unit  15 . The DC component extraction unit  16  is implemented as, e.g., a low-pass filter that removes an AC component from the signal output from the resistance voltage measurement unit  15  and outputs a DC component. A signal associated with the DC component of the resistance voltage extracted by the DC component extraction unit  16  is sent to the insulation resistance value calculation unit  19 . 
     The AC component extraction unit  17  extracts an AC component from the resistance voltage measured by the resistance voltage measurement unit  15 . The AC component extraction unit  17  is implemented as, e.g., a high-pass filter that removes a DC component from the signal output from the resistance voltage measurement unit  15  and outputs an AC component. A signal associated with the AC component of the resistance voltage extracted by the AC component extraction unit  17  is sent to the insulation resistance value calculation unit  19 . 
     The DC link potential measurement unit  18  measures the positive potential in the positive potential line of the DC link and the negative potential in the negative potential line of the DC link in the state in which the power conversion operation by the inverter unit  12  is kept stopped (i.e., the state in which all the switching elements in the inverter unit  12  are kept OFF). A signal associated with the positive potential and the negative potential of the DC link measured by the DC link potential measurement unit  18  is sent to the insulation resistance value calculation unit  19 . 
     The insulation resistance value calculation unit  19  calculates an insulation resistance value R m  defined as the resistance value of an insulation resistor  20  provided between the winding of the motor  3  and the ground, based on the DC component of the resistance voltage extracted by the DC component extraction unit  16 , the AC component of the resistance voltage extracted by the AC component extraction unit  17 , and the positive potential and the negative potential of the DC link measured by the DC link potential measurement unit  18 . The calculation processing by the insulation resistance value calculation unit  19  will be described in detail later. 
     In the embodiment described above with reference to  FIG.  1   , the voltage dividing resistors  13 - 1  and  13 - 2  are respectively formed by two DC link resistors connected in series with each other between the positive potential line and the negative potential line of the DC link. As a modification to this embodiment. OFF resistors for the switching elements in the bridge circuit within the inverter unit  12  may be exploited as the voltage dividing resistors  13 - 1  and  13 - 2 .  FIG.  2    is a circuit diagram illustrating a modification of the motor drive device (first mode) according to the embodiment of the present disclosure. When switching elements respectively provided in an upper arm and a lower arm in the same phase forming the bridge circuit in the inverter unit  12  are turned off, OFF resistors having a predetermined resistance value occur. Since the OFF resistors for the switching elements of the upper arm and the lower arm in the same phase have a function for dividing the DC link voltage, the OFF resistors for the switching elements of the upper arm and the lower arm in the same phase can be used as the voltage dividing resistors  13 - 1  and  13 - 2 . In this modification, the resistance values of the OFF resistors for the switching elements are defined as R. It suffices to calculate the resistance values R of the OFF resistors for the switching elements, based on the measurement results of the voltages applied to the switching elements in the OFF state. 
     The calculation processing by the insulation resistance value calculation unit  19  will be described below, separately for the first mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link, and the second mode in which one end of the measurement resistor  14  is connected to the positive potential line of the DC link. 
     The calculation processing by the insulation resistance value calculation unit  19  according to the first mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link will be described first. 
     As described above,  FIGS.  1  and  2    illustrate the first mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link, i.e., the measurement resistor  14  is connected between the connection point between the voltage dividing resistors  13 - 1  and  13 - 2  and the negative potential line of the DC link. 
     In the first mode illustrated in  FIGS.  1  and  2   , the insulation resistance value calculation unit  19  calculates the insulation resistance value R m  of the motor  3  in accordance with equation (1). In equation (1), R in  is the resistance value of the measurement resistor  14 , V p  is the value of the positive potential of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped, and V n  is the value of the negative potential of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped. V inL  is the value of the DC component of the resistance voltage, when the power conversion operation by the inverter unit  12  is kept stopped, extracted by the DC component extraction unit  16  (low-pass filter), and V inH  is the value of the AC component of the resistance voltage, when the power conversion operation by the inverter unit  12  is kept stopped, extracted by the AC component extraction unit  17  (high-pass filter). The value V com  is given by (V p +V n )/2, and the value V dif  is given by (V p −V n )/2. 
     
       
         
           
             
               
                 
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     The process of derivation of equation (1) used in the calculation processing by the insulation resistance value calculation unit  19  according to the first mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link will be described herein.  FIGS.  3  to  10    are circuit diagrams illustrating equivalent circuits used to derive equation (1) in calculation processing by an insulation resistance value calculation unit according to the first mode. 
     The calculation of the insulation resistance value R m  by the insulation resistance value calculation unit  19  uses the data measured by the resistance voltage measurement unit  15  and the DC link potential measurement unit  18  in the state in which the power conversion operation by the inverter unit  12  is kept stopped. In the state in which the power conversion operation by the inverter unit  12  is kept stopped, no circuits associated with the inverter unit  12  appear in  FIGS.  3  to  10   . 
     The positive potential V p  in the positive potential line of the DC link illustrated in  FIGS.  1  and  2    is represented as an equivalent power supply  31  in  FIG.  3   . Similarly, the negative potential V n  in the negative potential line of the DC link illustrated in  FIGS.  1  and  2    is represented as an equivalent power supply  32  in  FIG.  3   . In this case, the voltage dividing resistors  13 - 1  and  13 - 2 , the measurement resistor  14 , and the insulation resistor  20  of the motor  3  illustrated in  FIGS.  1  and  2    are connected to the equivalent power supplies  31  and  32  in a configuration as illustrated in  FIG.  3   . The resistance value of each of the voltage dividing resistors  13 - 1  and  13 - 2  is defined as R, the resistance value of the measurement resistor  14  is defined as R in , and the resistance value of the insulation resistor  20  of the motor  3  is defined as R m . The value of the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  is defined as V in . 
     In the equivalent circuit illustrated in  FIG.  3   , since the equivalent power supply  32  is connected to one end of the voltage dividing resistor  13 - 2  and one end of the measurement resistor  14 , the equivalent power supply  32  can be divided into two equivalent power supplies  32  and  33  having the same voltage value V n , as illustrated in  FIG.  4   . In other words, the equivalent power supply  32  is connected to one end of the voltage dividing resistor  13 - 2 , and the equivalent power supply  33  is connected to one end of the measurement resistor  14 , as illustrated in  FIG.  4   . 
     In the equivalent circuit illustrated in  FIG.  4   , when Thevenin&#39;s theorem is applied to a circuit portion T1, an equivalent circuit as illustrated in  FIG.  5    is obtained. Referring to  FIG.  5   , the voltage value V 1  of an equivalent power supply  34  is given by equation (2), and the resistance value of an equivalent resistor  21  is given by R/2. 
     
       
         
           
             
               
                 
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     In the equivalent circuit illustrated in  FIG.  5   , when Thevenin&#39;s theorem is applied to a circuit portion T2, an equivalent circuit as illustrated in  FIG.  6    is obtained. Referring to  FIG.  6   , the voltage value V 2  of an equivalent power supply  35  is given by equation (3), and the resistance value R 1  of an equivalent resistor  22  is given by equation (4). 
     
       
         
           
             
               
                 
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     In the equivalent circuit illustrated in  FIG.  6   , since the equivalent power supplies  33  and  35  are connected in series with each other via the equivalent resistor  22  and the measurement resistor  14 , when the equivalent power supplies  33  and  35  are combined into one equivalent power supply, an equivalent power supply  36  as illustrated in  FIG.  7    is obtained. Referring to  FIG.  7   , the voltage value V 3  of the equivalent power supply  36  is given by equation (5). 
     
       
         
           
             
               
                 
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     In the equivalent circuit illustrated in  FIG.  7   , when the voltage value V 3  of the equivalent power supply  36  is separated into a differential component and a common-mode component, equivalent power supplies  37  and  38  as illustrated in  FIG.  8    are obtained. Referring to  FIG.  8   , the voltage value V 4  of the equivalent power supply  37  represents a differential component, which is given by equation (6). The voltage value V 5  of the equivalent power supply  38  represents a common-mode component, which is given by equation (7). 
     
       
         
           
             
               
                 
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                   7 
                   ) 
                 
               
             
           
         
       
     
     The equivalent circuit illustrated in  FIG.  8    can be divided into an equivalent circuit for a DC component illustrated in  FIG.  9    and an equivalent circuit for an AC component illustrated in  FIG.  10   . 
     In the equivalent circuit of the DC component, the equivalent resistor  22  and the measurement resistor  14  are connected to the equivalent power supply  37  containing the differential component V 4 , as illustrated in  FIG.  9   . The voltage component V inL  applied to the measurement resistor  14  is calculated using Ohm&#39;s law, as equation (8). In equation (8), V com  is substituted for (V p +V n )/2 representing a signal component containing no DC component, and V dif  is substituted for (V p −V n )/2 representing a signal component containing a DC component. The voltage component V inL  corresponds to a DC component obtained by removing, by the DC component extraction unit  16  (low-pass filter), an AC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     8 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     inL 
                   
                   = 
                   
                     
                       
                         
                           R 
                           in 
                         
                         ( 
                         
                           R 
                           + 
                           
                             2 
                             ⁢ 
                             
                               R 
                               in 
                             
                           
                         
                         ) 
                       
                       
                         
                           
                             R 
                             m 
                           
                           ⁢ 
                           R 
                         
                         + 
                         
                           
                             R 
                             in 
                           
                           ( 
                           
                             R 
                             + 
                             
                               2 
                               ⁢ 
                               
                                 R 
                                 m 
                               
                             
                           
                           ) 
                         
                       
                     
                     ⁢ 
                     
                       V 
                       dif 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In the equivalent circuit of the AC component, the equivalent resistor  22  and the measurement resistor  14  are connected to the equivalent power supply  38  containing the common-mode component V 5 , as illustrated in  FIG.  10   . The voltage component V inH  applied to the measurement resistor  14  is calculated using Ohm&#39;s law, as equation (9). In equation (9), V com  is substituted for (V p +V n )/2 representing a signal component containing no DC component, and V dif  is substituted for (V p −V n )/2 representing a signal component containing a DC component. The voltage component V inH  corresponds to an AC component obtained by removing, by the AC component extraction unit  17  (high-pass filter), a DC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     9 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     inH 
                   
                   = 
                   
                     
                       
                         - 
                         
                           RR 
                           in 
                         
                       
                       
                         
                           
                             R 
                             m 
                           
                           ⁢ 
                           R 
                         
                         + 
                         
                           
                             R 
                             in 
                           
                           ( 
                           
                             R 
                             + 
                             
                               2 
                               ⁢ 
                               
                                 R 
                                 m 
                               
                             
                           
                           ) 
                         
                       
                     
                     ⁢ 
                     
                       V 
                       com 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Solving simultaneous equations defined by equations (8) and (9) for the insulation resistance value R m  yields equation (1) for calculating the insulation resistance value R m . In the first mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link, the insulation resistance value calculation unit  19  calculates the insulation resistance value R m  of the motor  3  in accordance with equation (1). In equation (1), the value V com  is expressed as (V p +V n )/2, and the value V dif  is expressed as (V p −V n )/2. V p  is the positive potential in the positive potential line of the IC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped, V n  is the negative potential in the negative potential line of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped. The DC component V inL  is obtained by removing, by the DC component extraction unit  16  (low-pass filter), an AC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  when the power conversion operation by the inverter unit  12  is kept stopped. The AC component V inH  is obtained by removing, by the AC component extraction unit  17  (high-pass filter), a DC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  when the power conversion operation by the inverter unit  12  is kept stopped. It suffices to measure the resistance value R in  of the measurement resistor  14  in advance, or use a value specified in a specification table as this resistance value R in . The resistance value R of each of the voltage dividing resistors  13 - 1  and  13 - 2  can be calculated by solving simultaneous equations defined by equations (8) and (9). In other words, “calculating the insulation resistance value R m  of the motor  3  in accordance with equation (1)” means “calculating the insulation resistance value R m  of the motor  3 , based on the DC component V inL  of the resistance voltage extracted by the DC component extraction unit  16 , the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17 , and the positive potential V p  and the negative potential V n  of the DC link measured by the DC link potential measurement unit  18 .” 
     In the first mode, the value V com  and the AC component V inH , and the value V dif  and the DC component V inL  may be measured after conversion into a direct current through a rectifying circuit. In this case, for example, the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17  is rectified, the absolute value of the rectified component is obtained, and the insulation resistance value R m  of the motor  3  is calculated in accordance with equation (10) based on this value. Alternatively, for example, the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17  is rectified, and the absolute value of the rectified component is obtained, while the DC component V inL  of the resistance voltage extracted by the DC component extraction unit  16  is rectified, and the absolute value of the rectified component is obtained, and the insulation resistance value R m  of the motor  3  is calculated in accordance with equation (10) based on these values. Since the absolute values of the AC component V inH  in equation (9) and the DC component V inL  in equation (8) are taken, some signs in equation (10) are different from those in equation (1). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     10 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     R 
                     m 
                   
                   = 
                   
                     
                       
                         
                           V 
                           com 
                         
                         ( 
                         
                           
                             V 
                             dif 
                           
                           + 
                           
                             V 
                             inL 
                           
                         
                         ) 
                       
                       
                         
                           V 
                           inH 
                         
                         ⁢ 
                         
                           V 
                           dif 
                         
                       
                     
                     ⁢ 
                     
                       R 
                       in 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The calculation processing by the insulation resistance value calculation unit  19  according to the second mode in which one end of the measurement resistor  14  is connected to the positive potential line of the DC link will be described below. 
       FIG.  11    is a circuit diagram illustrating a motor drive device (second mode) according to another embodiment of the present disclosure. In the second mode, the measurement resistor  14  is connected between the connection point between the voltage dividing resistors  13 - 1  and  13 - 2  and the positive potential line of the DC link. Since circuit components other than those constituting this configuration are the same as the circuit components illustrated in  FIG.  1   , the same reference numerals denote the same circuit components, and a detailed description thereof will not be given. As in the first mode, in the second mode, OFF resistors for the switching elements in the bridge circuit within the inverter unit  12  may be utilized as the voltage dividing resistors  13 - 1  and  13 - 2 . 
     In the second mode illustrated in  FIG.  11   , the insulation resistance value calculation unit  19  calculates the insulation resistance value R m  of the motor  3  in accordance with equation (11). In equation (11). R in  is the resistance value of the measurement resistor  14 , V p  is the value of the positive potential of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped, and V n  is the value of the negative potential of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped. V inL  is the value of the DC component of the resistance voltage, when the power conversion operation by the inverter unit  12  is kept stopped, extracted by the DC component extraction unit  16  (low-pass filter), and V inH  is the value of the AC component of the resistance voltage, when the power conversion operation by the inverter unit  12  is kept stopped, extracted by the AC component extraction unit  17  (high-pass filter). The value V com  is given by (V p +V n )/2, and the value V dif  is given by (V p −V n )/2. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     11 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     R 
                     m 
                   
                   = 
                   
                     
                       - 
                       
                         
                           
                             V 
                             com 
                           
                           ( 
                           
                             
                               V 
                               dif 
                             
                             + 
                             
                               V 
                               inL 
                             
                           
                           ) 
                         
                         
                           
                             V 
                             inH 
                           
                           ⁢ 
                           
                             V 
                             dif 
                           
                         
                       
                     
                     ⁢ 
                     
                       R 
                       in 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The process of derivation of equation (11) used in the calculation processing by the insulation resistance value calculation unit  19  according to the second mode in which one end of the measurement resistor  14  is connected to the positive potential line of the DC link will be described herein.  FIGS.  12  to  18    are circuit diagrams illustrating equivalent circuits used to derive equation (11) in calculation processing by an insulation resistance value calculation unit according to the second mode. 
     The calculation of the insulation resistance value R m  by the insulation resistance value calculation unit  19  uses the data measured by the resistance voltage measurement unit  15  and the DC link potential measurement unit  18  in the state in which the power conversion operation by the inverter unit  12  is kept stopped. In the state in which the power conversion operation by the inverter unit  12  is kept stopped, no circuits associated with the inverter unit  12  appear in  FIGS.  12  to  18   . 
     The positive potential V p  in the positive potential line of the DC link illustrated in  FIG.  11    is represented as an equivalent power supply  51  in  FIG.  12   . Similarly, the negative potential V n  in the negative potential line of the DC link illustrated in  FIG.  3    is represented as an equivalent power supply  52  in  FIG.  12   . In this case, the voltage dividing resistors  13 - 1  and  13 - 2 , the measurement resistor  14 , and the insulation resistor  20  of the motor  3  illustrated in  FIG.  11    are connected to the equivalent power supplies  51  and  52  in a configuration as illustrated in  FIG.  12   . The resistance value of each of the voltage dividing resistors  13 - 1  and  13 - 2  is defined as R, the resistance value of the measurement resistor  14  is defined as R in , and the resistance value of the insulation resistor  20  of the motor  3  is defined as R m . The value of the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  is defined as V in . 
     In the equivalent circuit illustrated in  FIG.  12   , when Thevenin&#39;s theorem is applied to the equivalent power supplies  51  and  52  and the voltage dividing resistors  13 - 1  and  13 - 2 , an equivalent circuit as illustrated in  FIG.  13    is obtained. Referring to  FIG.  5   , the voltage value V 6  of an equivalent power supply  54  is given by equation (12), and the resistance value of an equivalent resistor  41  is given by R/2. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     12 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     6 
                   
                   = 
                   
                     
                       
                         V 
                         p 
                       
                       + 
                       
                         V 
                         n 
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     In the equivalent circuit illustrated in  FIG.  13   , when Thevenin&#39;s theorem is applied to a circuit portion T3, an equivalent circuit as illustrated in  FIG.  14    is obtained. Referring to  FIG.  14   , the voltage value V 7  of an equivalent power supply  55  is given by equation (13), and the resistance value R 2  of an equivalent resistor  42  is given by equation (14). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     13 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                     
                   
                     
                       
                         V 
                       
                       7 
                     
                     = 
                     
                       
                         
                           R 
                           m 
                         
                         ( 
                         
                           
                             V 
                             p 
                           
                           + 
                           
                             V 
                             n 
                           
                         
                         ) 
                       
                       
                         R 
                         + 
                         
                           2 
                           ⁢ 
                           
                             R 
                             m 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     14 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     R 
                     2 
                   
                   = 
                   
                     
                       RR 
                       m 
                     
                     
                       R 
                       + 
                       
                         2 
                         ⁢ 
                         
                           R 
                           m 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     In the equivalent circuit illustrated in  FIG.  14   , since an equivalent power supply  53  and the equivalent power supply  55  are connected in series with each other via the equivalent resistor  42  and the measurement resistor  14 , when the equivalent power supplies  53  and  55  are combined into one equivalent power supply, an equivalent power supply  56  as illustrated in  FIG.  15    is obtained. Referring to  FIG.  15   , the voltage value V 8  of the equivalent power supply  56  is given by equation (15). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     15 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     8 
                   
                   = 
                   
                     
                       
                         
                           - 
                           
                             ( 
                             
                               R 
                               + 
                               
                                 R 
                                 m 
                               
                             
                             ) 
                           
                         
                         ⁢ 
                         
                           V 
                           p 
                         
                       
                       + 
                       
                         
                           R 
                           m 
                         
                         ⁢ 
                         
                           V 
                           n 
                         
                       
                     
                     
                       R 
                       + 
                       
                         2 
                         ⁢ 
                         
                           R 
                           m 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     In the equivalent circuit illustrated in  FIG.  15   , when the voltage value V 8  of the equivalent power supply  56  is separated into a differential component and a common-mode component, equivalent power supplies  57  and  58  as illustrated in  FIG.  16    are obtained. Referring to  FIG.  16   , the voltage value V 9  of the equivalent power supply  57  represents a differential component, which is given by equation (16). The voltage value V 10  of the equivalent power supply  58  represents a common-mode component, which is given by equation (17). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     16 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     9 
                   
                   = 
                   
                     - 
                     
                       
                         
                           V 
                           p 
                         
                         - 
                         
                           V 
                           n 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     17 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     10 
                   
                   = 
                   
                     
                       - 
                       
                         R 
                         ⁡ 
                         ( 
                         
                           
                             V 
                             p 
                           
                           + 
                           
                             V 
                             n 
                           
                         
                         ) 
                       
                     
                     
                       2 
                       ⁢ 
                       
                         ( 
                         
                           R 
                           + 
                           
                             2 
                             ⁢ 
                             
                               R 
                               m 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     The equivalent circuit illustrated in  FIG.  16    can be divided into an equivalent circuit for a DC component illustrated in  FIG.  17    and an equivalent circuit for an AC component illustrated in  FIG.  18   . 
     In the equivalent circuit of the DC component, the equivalent resistor  42  and the measurement resistor  14  are connected to the equivalent power supply  57  containing the differential component V 9 , as illustrated in  FIG.  17   . The voltage component V inL  applied to the measurement resistor  14  is calculated using Ohm&#39;s law, as equation (18). In equation (18), V com  is substituted for (V p +V B )/2 representing a signal component containing no DC component, and V dif  is substituted for (V p −V n )/2 representing a signal component containing a DC component. The voltage component V inL  corresponds to a DC component obtained by removing, by the DC component extraction unit  16  (low-pass filter), an AC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     18 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     inL 
                   
                   = 
                   
                     
                       
                         - 
                         
                           
                             R 
                             in 
                           
                           ( 
                           
                             R 
                             + 
                             
                               2 
                               ⁢ 
                               
                                 R 
                                 in 
                               
                             
                           
                           ) 
                         
                       
                       
                         
                           
                             R 
                             m 
                           
                           ⁢ 
                           R 
                         
                         + 
                         
                           
                             R 
                             in 
                           
                           ( 
                           
                             R 
                             + 
                             
                               2 
                               ⁢ 
                               
                                 R 
                                 m 
                               
                             
                           
                           ) 
                         
                       
                     
                     ⁢ 
                     
                       V 
                       dif 
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
     In the equivalent circuit of the AC component, the equivalent resistor  42  and the measurement resistor  14  are connected to the equivalent power supply  58  containing the common-mode component V 10 , as illustrated in  FIG.  18   . The voltage component V inH  applied to the measurement resistor  14  is calculated using Ohm&#39;s law, as equation (19). In equation (19), V com  is substituted for (V p +V n )/2 representing a signal component containing no DC component, and V dif  is substituted for (V p −V n )/2 representing a signal component containing a DC component. The voltage component V inH  corresponds to an AC component obtained by removing, by the AC component extraction unit  17  (high-pass filter), a DC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     19 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     V 
                     inH 
                   
                   = 
                   
                     
                       
                         - 
                         
                           RR 
                           in 
                         
                       
                       
                         
                           
                             R 
                             m 
                           
                           ⁢ 
                           R 
                         
                         + 
                         
                           
                             R 
                             in 
                           
                           ( 
                           
                             R 
                             + 
                             
                               2 
                               ⁢ 
                               
                                 R 
                                 m 
                               
                             
                           
                           ) 
                         
                       
                     
                     ⁢ 
                     
                       V 
                       com 
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     Solving simultaneous equations defined by equations (18) and (19) for the insulation resistance value R m  yields equation (11) for calculating the insulation resistance value R m . In the second mode in which one end of the measurement resistor  14  is connected to the positive potential line of the DC link, the insulation resistance value calculation unit  19  calculates the insulation resistance value R m  of the motor  3  in accordance with equation (11). In equation (11), the value V com  is expressed as (V p +V n )/2, and the value V dif  is expressed as (V p −V n )/2. V p  is the positive potential in the positive potential line of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped. V n  is the negative potential in the negative potential line of the DC link measured by the DC link potential measurement unit  18  when the power conversion operation by the inverter unit  12  is kept stopped. The DC component V inL  is obtained by removing, by the DC component extraction unit  16  (low-pass filter), an AC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  when the power conversion operation by the inverter unit  12  is kept stopped. The AC component V inH  is obtained by removing, by the AC component extraction unit  17  (high-pass filter), a DC component from the resistance voltage of the measurement resistor  14  measured by the resistance voltage measurement unit  15  when the power conversion operation by the inverter unit  12  is kept stopped. It suffices to measure the resistance value R in  of the measurement resistor  14  in advance, or use a value specified in a specification table as this resistance value R in . The resistance value R of each of the voltage dividing resistors  13 - 1  and  13 - 2  can be calculated by solving simultaneous equations defined by equations (18) and (19). In other words, “calculating the insulation resistance value R m  of the motor  3  in accordance with equation (11)” means “calculating the insulation resistance value R m  of the motor  3 , based on the DC component V inL  of the resistance voltage extracted by the DC component extraction unit  16 , the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17 , and the positive potential V p  and the negative potential V n  of the DC link measured by the DC link potential measurement unit  18 .” 
     In the second mode, the value V com  and the AC component V inH , and the value V dif  and the DC component V inL  may be measured after conversion into a direct current through a rectifying circuit. In this case, for example, the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17  is rectified, the absolute value of the rectified component is obtained, the AC component V inH  of the resistance voltage extracted in accordance with equation (20) based on this value is rectified, and the absolute value of the rectified component is obtained, while the DC component V inL  of the resistance voltage extracted by the DC component extraction unit  16  is rectified, and the absolute value of the rectified component is obtained, and the insulation resistance value R m  of the motor  3  is calculated in accordance with equation (20) based on these values. Since the absolute values of the AC component V inH  in equation (19) and the DC component V inL  in equation (18) are taken, some signs in equation (20) are different from those in equation (11). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     20 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   
                     R 
                     m 
                   
                   = 
                   
                     
                       
                         
                           V 
                           com 
                         
                         ( 
                         
                           
                             V 
                             dif 
                           
                           + 
                           
                             V 
                             inL 
                           
                         
                         ) 
                       
                       
                         
                           V 
                           inH 
                         
                         ⁢ 
                         
                           V 
                           dif 
                         
                       
                     
                     ⁢ 
                     
                       R 
                       in 
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
           
         
       
     
     As described above, in both the first mode and the second mode, the measurement processing by the resistance voltage measurement unit  15  and the measurement processing by the IX link potential measurement unit  18  are performed in the state in which the power conversion operation by the inverter unit  12  is kept stopped (i.e., the state in which all the switching elements in the inverter unit  12  are kept OFF), and the insulation resistance value calculation unit  19  calculates the insulation resistance value R m , based on the data measured at this time. In other words, according to one embodiment of the present disclosure, the insulation resistance value R m  of the motor  3  can be measured by operating the resistance voltage measurement unit  15 , the DC component extraction unit  16 , the AC component extraction unit  17 , the DC link potential measurement unit  18 , and the insulation resistance value calculation unit  19  in the state in which the power conversion operation by the inverter unit  12  is kept stopped. Hence, unlike the conventional techniques, the motor drive device  1  may not be preferably disconnected from the AC power supply  2  in measuring the insulation resistance value R m  of the motor  3 . In addition, no circuit breaker may be preferably interposed between the motor drive device  1  and the AC power supply  2  to measure the insulation resistance value R m  of the motor  3 , resulting in low cost. Even when no circuit breaker is interposed between the motor drive device  1  and the AC power supply  2 , the insulation resistance value R m  of the motor  3  can be measured. 
     The insulation resistance value R m  calculated by the insulation resistance value calculation unit  19  may be displayed on, e.g., a display unit (not illustrated). With this operation, the operator can quickly and easily know the insulation resistance value R m  of the motor  3 . 
     A determination unit (not illustrated) that performs insulation deterioration determination of the motor  3  by comparing the insulation resistance value R m  calculated by the insulation resistance value calculation unit  19  with a threshold serving as a criterion for determining insulation deterioration may be provided, and information indicating that “insulation deterioration has occurred in the motor  3 ” may be displayed on the display unit when the determination unit determines that the insulation resistance value R m  has exceeded the threshold. Examples of the display unit may include an independent display device, a display device attached to the motor drive device  1 , and display devices attached to a personal computer and a portable terminal. Alternatively, the operator may be notified that “insulation deterioration has occurred in the motor  3 ” by an acoustic device that emits a sound like that produced by, e.g., a voice, a loudspeaker, a buzzer, or a chime, in addition to or instead of such a display unit. With this operation, the operator can quickly and easily understand that insulation deterioration has occurred in the motor  3 . Hence, the operator can easily carry out a remedial measure such as replacement or repairing of the motor  3 . 
     The DC component extraction unit  16 , the AC component extraction unit  17 , the insulation resistance value calculation unit  19 , and the host controller may be implemented as a combination of an analog circuit and an arithmetic processing unit, or may be implemented solely as an arithmetic processing unit, or may be implemented solely as an analog circuit. When, for example, the DC component extraction unit  16 , the AC component extraction unit  17 , the insulation resistance value calculation unit  19 , and the host controller are constructed in software program form, the function of each of the DC component extraction unit  16 , the AC component extraction unit  17 , the insulation resistance value calculation unit  19 , and the host controller can be implemented by running the arithmetic processing unit in accordance with the software program. Alternatively, the DC component extraction unit  16 , the AC component extraction unit  17 , the insulation resistance value calculation unit  19 , and the host controller may be implemented as a semiconductor integrated circuit in which a software program for implementing the function of each unit is written. As another alternative, the DC component extraction unit  16 , the AC component extraction unit  17 , the insulation resistance value calculation unit  19 , and the host controller may be implemented as a recording medium in which a software program for implementing the function of each unit is written. The DC component extraction unit  16 , the AC component extraction unit  17 , the insulation resistance value calculation unit  19 , and the host controller, for example, may be provided in a numerical controller for a machine tool, or may be provided in a robot controller that controls a robot. 
     The resistance voltage measurement unit  15  and the DC link potential measurement unit  18  may be implemented as a combination of an analog circuit and an arithmetic processing unit, or may be implemented solely as an arithmetic processing unit, or may be implemented solely as an analog circuit. As the resistance voltage measurement unit  15  and the DC link potential measurement unit  18 , measurement units generally provided in the motor drive device  1  may be exploited. However, calculation of the insulation resistance value R m  by the insulation resistance value calculation unit  19 , uses the data measured by the resistance voltage measurement unit  15  and the DC link potential measurement unit  18  in the state in which the power conversion operation by the inverter unit  12  is kept stopped. 
       FIG.  19    is a flowchart illustrating the operation sequence of the motor drive device according to any embodiment of the present disclosure. The flowchart illustrated in  FIG.  19    is applicable to the motor drive devices  1  in both the first mode and the second mode. 
     In measuring an insulation resistance value R m  of the motor  3 , first, in step S 101 , a host controller (not illustrated) turns off all the switching elements in the inverter unit  12  to stop the power conversion operation by the inverter unit  12 . 
     In step S 102 , the resistance voltage measurement unit  15  measures a resistance voltage V in  defined as the potential difference between the two ends of the measurement resistor  14 . A signal associated with the resistance voltage V in  of the measurement resistor  14  measured by the resistance voltage measurement unit  15  is sent to the DC component extraction unit  16  and the AC component extraction unit  17 . 
     In step S 103 , the DC component extraction unit  16  extracts a DC component V inL  from the resistance voltage V in  measured by the resistance voltage measurement unit  15 . A signal associated with the DC component V inL  of the resistance voltage extracted by the DC component extraction unit  16  is sent to the insulation resistance value calculation unit  19 . 
     In step S 104 , the AC component extraction unit  17  extracts an AC component V inH  from the resistance voltage V in  measured by the resistance voltage measurement unit  15 . A signal associated with the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17  is sent to the insulation resistance value calculation unit  19 . 
     The process in step S 103  and the process in step S 104  may be performed in reverse order. 
     In step S 105 , the DC link potential measurement unit  18  measures a positive potential V p  in the positive potential line of the DC link and a negative potential V n  in the negative potential line of the DC link. A signal associated with the positive potential V p  and the negative potential V n  of the DC link measured by the DC link potential measurement unit  18  is sent to the insulation resistance value calculation unit  19 . 
     The series of processes in steps S 102  to S 104  and the process in step S 105  may be performed in reverse order. 
     In step S 106 , the insulation resistance value calculation unit  19  calculates an insulation resistance value R m  of the motor  3 , based on the DC component V inL  of the resistance voltage extracted by the DC component extraction unit  16 , the AC component V inH  of the resistance voltage extracted by the AC component extraction unit  17 , and the positive potential V p  and the negative potential V n  of the DC link measured by the DC link potential measurement unit  18 . In the first mode in which one end of the measurement resistor  14  is connected to the negative potential line of the DC link, the insulation resistance value calculation unit  19  calculates the insulation resistance value R m  of the motor  3  in accordance with equation (1). In the second mode in which one end of the measurement resistor  14  is connected to the positive potential line of the DC link, the insulation resistance value calculation unit  19  calculates the insulation resistance value R m  of the motor  3  in accordance with equation (11). 
     After step S 106 , for example, the insulation resistance value R m  calculated by the insulation resistance value calculation unit  19  may be displayed on a display unit (not illustrated). As another example, a determination unit (not illustrated) that performs insulation deterioration determination of the motor  3  by comparing the insulation resistance value R m  calculated by the insulation resistance value calculation unit  19  with a threshold serving as a criterion for determining insulation deterioration may be provided, and information indicating that “insulation deterioration has occurred in the motor  3 ” may be displayed on the display unit when the determination unit determines that the insulation resistance value R m  has exceeded the threshold. As still another example, the operator may be notified that “insulation deterioration has occurred in the motor  3 ” by an acoustic device that emits a sound like that produced by, e.g., a voice, a loudspeaker, a buzzer, or a chime, in addition to or instead of such a display unit. 
       FIG.  20    is a graph for explaining simulation results for the positive potential and the negative potential of a DC link when an AC power supply and the motor drive device according to any embodiment of the present disclosure are connected to each other by delta connection. As presented in equations (1) and (11), the value V com  expressed as (V p +V n )/2, and the value V dif  expressed as (V p −V n )/2 are used to calculate the insulation resistance value R m .  FIG.  20    reveals that the difference “V p −V n ” between the positive potential V p  and the negative potential V n  stays constant and therefore functions as a DC component, and the sum “V p +V n ” of the positive potential V p  and the negative potential V n  is AC waveform and therefore functions as an AC component. 
     REFERENCE SIGNS LIST 
     
         
           1  Motor drive device 
           2  AC power supply 
           3  Motor 
           4  DC link capacitor 
           11  Converter unit 
           12  Inverter unit 
           13 - 1 ,  13 - 2  Voltage dividing resistor 
           14  Measurement resistor 
           15  Resistance voltage measurement unit 
           16  DC component extraction unit 
           17  AC component extraction unit 
           18  DC link potential measurement unit 
           19  Insulation resistance value calculation unit 
           20  Insulation resistor for motor 
           21 ,  22  Equivalent resistor 
           31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 ,  38  Equivalent power supply 
           41 ,  42  Equivalent resistor 
           51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58  Equivalent power supply