Patent Publication Number: US-2022234450-A1

Title: Three-Phase AC Motor Drive Device, Rail Vehicle Equipped With Same, and Three-Phase AC Motor Drive Method

Description:
TECHNICAL FIELD 
     The present invention relates to a three-phase AC motor drive device, a rail vehicle equipped with the same, and a three-phase AC motor drive method. 
     BACKGROUND ART 
     For a drive device for driving a motor for a rail vehicle, a three-phase AC motor is widely used. In recent years, the application of a permanent magnet synchronous motor (hereinafter abbreviated as “PMSM” in some cases) as a three-phase AC motor has been promoted for the purpose of downsizing a drive system for a rail vehicle and improving the efficiency of the drive system. 
     When the PMSM rotates, an induced voltage occurs between terminals of the motor due to a magnetic flux of a permanent magnet. Therefore, when a rail vehicle is continuously operated in a state in which a short circuit failure or the like occurs in an inverter device for driving the PMSM, a short circuit current continuously flows in the inverter device due to the induced voltage of the PMSM and a braking force occurs in the PMSM. 
     When one inverter device is in such a state, acceleration performance of the rail vehicle may decrease and the short circuit accident current may continuously flow to cause burnout of a device or the like. Therefore, a failure of one inverter device prevents the rail vehicle from being normally operated. In the drive device for driving the PMSM, motor cutout contacts (hereinafter abbreviated as “MCOKs”) for electrically disconnecting the PMSM from the inverter device at the time of a failure of the inverter device may be provided. 
     In addition, an inverter device for driving a three-phase AC motor such as a PMSM inputs, to a control device, phase current information of a current detector that detects phase currents of three phases and inter-line voltage information of a voltage detector that detects a voltage between the three phases. The phase current information and the inter-line voltage information are used for control calculation for driving the three-phase AC motor by the inverter device and are used for protective detection to stop the inverter device or the like so as to ensure safety and prevent a failure of the device. As an example of this configuration, Patent Literature 1 and Patent Literature 2 disclose examples of circuit configurations for disconnecting a PMSM from an inverter device at the time of a failure of the device. Techniques relating to them are described at the end as Comparative Example 1 illustrated in  FIG. 9  and Comparative Example 2 illustrated in  FIG. 10 . 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-Open No. 2011-78254 
         Patent Literature 2: Japanese Patent Application Laid-Open No. 2013-192300 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The circuit configurations described in Patent Literature 1 and Patent Literature 2 have, for example, the following two problems. 
     First, when a short circuit failure occurs in a voltage detector (alternating current potential transformer: hereinafter abbreviated as “ACPT” in some cases) between phases, and a motor cutout contactor (MCOK) is in a released state (that is, an opened state), there is a problem that a current detector cannot detect an inter-phase short circuit current of the voltage detector. On the other hand, even when an inverter device and a motor, particularly a PMSM, are disconnected from each other, there is a demand for an ammeter attached or built on the inverter device side to detect a regenerative current generated by the motor. 
     Second, for a rail vehicle, the inverter device is installed under a floor of the vehicle in many cases. In such a case, there is a problem that it is desirable that the current detector and the ACPT be installed on the rear side (that is, at a position close to the motor) with respect to the MCOK as viewed from the inverter device. There are the following two reasons, for example. One is that the current detector and the ACPT may not be able to be installed in the inverter device due to a dimensional limit on a drive device such as the inverter device. The other one is that it is desirable that the current detector and the ACPT be installed on the rear side of the MCOK in a case in which the MCOK is retrofitted in the inverter device or the like from the perspective of the installation position of the current detector in the existing inverter device. 
     The foregoing problems may occur even when, for example, compatibility of a drive system using an induction motor as a three-phase AC motor with a drive system using a PMSM is ensured to save labor and achieve simplification. For actual rail vehicles, replacement of existing induction motors with PMSMs is sequentially promoted. 
     Solution to Problems 
     To solve the foregoing problems, according to the present invention, a three-phase AC motor drive device that drives a load includes an inverter device that includes a plurality of current controllers each having a combination of a current control element configured to conduct or block a current flowing toward one direction and a rectifying element connected to the current control element in parallel and configured to conduct a current flowing toward, and is configured to convert direct-current power supplied from a power supply into three-phase alternating-current power and drive the load; a motor cutout contact configured to electrically connect or disconnect the inverter device to or from the load; a voltage detector configured to detect a voltage between three phases and having terminals connected to circuits of at least two of the phases; and a current detector configured to detect phase currents of the three phases. In a connection from the inverter device to the load, the inverter device, the motor cutout contact, the voltage detector, the current detector, and the load are arranged in this order. 
     Advantageous Effects of Invention 
     In the phase-three AC motor drive device according to the present invention, when a short circuit failure occurs in the voltage detector, the motor cutout contact is released. Releasing the motor cutout contact electrically disconnects the inverter device from the AC motor. Even in this state, the current detector can detect a short circuit current flowing in a path between the load and the voltage detector. 
     The case in which the motor cutout contact is released corresponds to a case in which the load of the three-phase AC motor drive device used in a rail vehicle is a permanent magnet synchronous motor and a failure such as a short circuit failure of the voltage detector or a reduction in output of the inverter device occurs. In such a case, a driver or the like may take optimal security measures to eliminate a regenerative braking action of the permanent magnet synchronous motor and continuously operate the vehicle with other remaining power. 
     In addition, when the load is changed from an induction motor to a permanent magnet synchronous motor, basic design may be made such that the motor cutout contact required to avoid a regenerative braking action caused at the time of the foregoing failure, and the voltage detector are disposed in the immediate vicinity of the inverter device in this order. To secure a space for the motor cutout contact, the current detector is disposed in the immediate vicinity of the load while avoiding the space. In other words, even in a case in which the motor cutout contact is not required in the inverter device at the initial design stage, when the space is secured, and the motor cutout contact needs to be retrofitted, there is no trouble. That is, it is easy to secure the installation configuration of the inverter device and compatibility of device arrangement, regardless of whether the motor cutout contact is present. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram illustrating an example of a three-phase AC motor drive device (present device) according to an embodiment of the present invention. 
         FIG. 2  is a configuration diagram illustrating a modification of a connection form of a voltage detector. 
         FIG. 3A  is a configuration diagram illustrating an example of a configuration of an inverter unit. 
         FIG. 3B  is a configuration diagram illustrating another example of the configuration of the inverter unit. 
         FIG. 4A  is a configuration diagram illustrating a first comparative example of arrangement of an inverter unit. 
         FIG. 4B  is a configuration diagram illustrating a second comparative example (comparative device  61   b ) of the arrangement of the inverter unit. 
         FIG. 4C  is a configuration diagram illustrating arrangement of an inverter unit (present device  61   c ) according to Example 1. 
         FIG. 5  is a configuration diagram of a present device according to Example 2. 
         FIG. 6  is a configuration diagram of a present device according to Example 3. 
         FIG. 7  is a configuration diagram of a present device according to Example 4. 
         FIG. 8  is a configuration diagram of a present device according to Example 5. 
         FIG. 9  is a configuration diagram of a three-phase AC motor drive device (comparative device  71 ) according to Comparative Example 1. 
         FIG. 10  is a configuration diagram of a comparative device  72 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     As an embodiment of the present invention, Example 1 is described below with reference to drawings. A three-phase AC motor drive device according to the embodiment of the present invention is abbreviated as a present device, while a three-phase AC motor drive device according to a comparative example is abbreviated as a comparative device. 
     Example 1 
       FIG. 1  is a configuration diagram illustrating an example of a present device  61  according to Example 1. An inverter device  1  is constituted by a current control element that can conduct or block a current flowing from a high-pressure side to a low-pressure side, and a diode that can conduct a current flowing toward a direction opposite to the one direction. 
     As the current control element, a power semiconductor element such as an insulated gate bipolar transistor (IGBT) or a power metal oxide semiconductor field effect transistor (MOSFET) is used in general. 
     Silicon is used as a material of these power semiconductor elements in many cases. However, in recent years, power semiconductor elements in which SiC (silicon carbide) or GaN (gallium nitride) is used have been increasing in number to contribute to reductions in system losses. Therefore, SiC or GaN may be used in the current control element used in the inverter device  1  according to the present invention. 
     The inverter device  1  converts direct-current power output from a direct-current power supply not illustrated into three-phase alternating-current power and drives an AC motor  2 . The direct-current power supply that inputs power to the inverter device  1  is a direct-current power supply input unit for the inverter device. A smoothing capacitor  3  is connected to the inverter device  1  in parallel. The inverter device  1  is connected to the higher-level direct-current power supply via the smoothing capacitor  3 . The AC motor  2  is described as a load in some cases. 
     As the AC motor  2 , an induction motor, a PMSM_ 2 ′, or the like is used. Although  FIG. 1  illustrates that the inverter device  1  is configured to drive the single AC motor  2 , the inverter device  1  may be configured to drive a plurality of AC motors  2 . In some drawings, the PMSM_ 2 ′ is indicated by a reference sign  2 ′ to distinguish them. 
     ACPTs_ 11   a  to  11   c  that detect current values of phases and MCOKs_A_ 4   a  to  4   c  are provided on the alternating-current output side of the inverter device  1 . The MCOKs_A_ 4   a  to  4   c  are collectively referred to as MCOKs_A_ 4 , while the ACPTs_ 11   a  to  11   c  are collectively referred to as ACPTs_ 11 . A control device  31  acquires detection output of the ACPTs_ 11  and controls a switching operation of each current control element of the inverter device  1  based on the detection output such that the AC motor  2  outputs desired torque. 
     When current values detected by the ACPTs_ 11  are abnormally high or the like, the inverter device  1  may be stopped or the MCOKs_A_ 4  may be released to electrically disconnect the inverter device  1  from the AC motor  2  for device protection and safety. 
     In accordance with a release instruction output from the control device  31 , the MCOKs_A_ 4  connect or disconnect main circuit contacts that electrically connect the three phases of the inverter device  1  and the AC motor  2 . A configuration for turning on and releasing for each phase or a configuration for turning on and releasing in coordination with the plurality of phases may be provided. 
     In addition, the ACPTs_ 11  may be only the two ACPTs_ 11   a  and  11   c  disposed in two of the phases, for example. That is, it is not necessary to detect currents of all the three phases. A configuration for detecting currents in two of the three phases and calculating a current of the remaining one phase while assuming that three-phase currents are in equilibrium may be provided. 
     An ACPT_ 21   a  is provided between phases (U and V phases in the example of  FIG. 1 ) on the alternating-current output side of the inverter device  1  and on the AC motor  2  side and detects a voltage between the phases, that is, a voltage between terminals of the AC motor. When the control device  31  or the like acquires a detection signal of the ACPT_ 21   a  and the AC motor  2  is the PMSM_ 2 ′ or the like, the control device  31  or the like controls output of the inverter device  1  after confirming the voltage between the terminals, thereby being able to start an operation as an inverter in a stable manner. 
     In this case, the inverter device  1 , the AC motor  2 , the MCOKs_A_ 4 , the current detectors  11 , and the ACPT_ 21   a  are connected in the order of the inverter device  1 , the MCOKs_A_ 4 , the ACPT_ 21   a , the current detectors  11   a  to  11   c , and the AC motor  2 . 
     The present device  61  has the foregoing connection configuration. Therefore, even when the MCOKs_A_ 4  are released to disconnect the inverter device  1  from the AC motor  2 , and the ACPT_ 21   a  has a short circuit failure (short circuit between the U phase and the V phase in  FIG. 1 ), the current detectors  11   a  and  11   b  can detect a short circuit current flowing in a path extending from the AC motor  2  to the ACPT_ 21   a.    
     In addition, even when not only the short circuit failure of the ACPT_ 21   a  occurs but also a short circuit occurs between phases due to burnout of the AC motor  2  or a wiring coil or insulation deterioration in the present device  61 , a driver or the like can detect the short circuits. The present device  61  is, for example, suitable to be used as a drive device for a rail vehicle or the like. In the case of the use, when such a short circuit current is detected, and all the MCOKs_A_ 4  are released, information indicating that the short circuit current flows can be given to a higher-level device such as a vehicle cab. When the information is given to the vehicle cab, the driver or the like can take security measures based on the information. Specifically, it is also possible to stop operating a corresponding vehicle and prevent a device failure from spreading. 
       FIG. 2  is a configuration diagram illustrating a present device  62  according to a modification in which the connection form of the ACPT_ 21   a  is different from the present device  61  illustrated in  FIG. 1 .  FIG. 1  illustrates the example in which the ACPT_ 21   a  is connected at a single position between the U phase and the V phase. However, as illustrated in  FIG. 2  (not illustrating the control device  31 ), a configuration in which the voltage detector (ACPT_ 21   a ) is connected between the U phase and the V phase and a voltage detector (ACPT_ 21   b ) is connected between the V phase and the W phase, that is, a configuration in which voltage detectors are connected at two or more positions between the phases, may be provided. 
       FIG. 3  are configuration diagrams exemplifying different inverter units  6  to explain the present device  61  according to Example 1.  FIG. 3( a )  illustrates that only the current detectors  11  are interposed (connected) in the phases, while  FIG. 3( b )  illustrates that the current detectors  11  and the MCOKs are interposed in the phases and the ACPT_ 21   a  is disposed between the phases. In addition,  FIG. 3( a )  illustrates an example of a unit configuration of a general drive device for a rail vehicle, and the smoothing capacitor  3 , the inverter device  1 , and the current detectors  11  are disposed in the inverter unit  6  by a housing or the like. Since a three-phase induction motor is used as the AC motor  2  in a conventional drive device for a rail vehicle, the inverter unit  6  is constituted mainly in a portion surrounded by a dotted-line frame illustrated in  FIG. 3( a ) . Note that “interposed” indicates “connected to a circuit”. 
     In recent years, there has been a rail vehicle in which the PMSM_ 2 ′ is used as the AC motor  2  to downsize the foregoing drive system and improve the efficiency of the drive system. In this case, as illustrated in  FIG. 3( b ) , the smoothing capacitor  3 , the inverter device  1 , the current detector  11   c , and the ACPT_ 21   a  are provided in the inverter unit  6 . 
     In particular, while the dimensions and shape of the housing for the inverter unit  6  are maintained and compatibility with a vehicle using an induction motor as a conventional AC motor  2  is maintained, the AC motor  2  may be replaced with the PMSM_ 2 ′. For example, to update the drive device for the rail vehicle or the like, an additional MCOK_A_ 4  and an additional ACPT_ 21   a  are provided as built-in devices in the inverter unit  6 . In this case, the following problem with installation occurs. 
       FIG. 4  are schematic diagrams of the arrangement of inverter units in rail vehicles.  FIG. 4( a )  illustrates a comparative example,  FIG. 4( b )  illustrates a comparative device  61   b  obtained by adding an MCOK to (a), and  FIG. 4( c )  illustrates installation simplified with the present device  61  illustrated in  FIGS. 1, 2, and 3 ( b ).  FIG. 4( a )  illustrates an example of an overview of installation of the inverter unit  6  in the rail vehicle and illustrates an example of the arrangement of devices in the conventional inverter unit  6 . 
     As illustrated in a lower part of  FIG. 4( a ) , in the conventional rail vehicle, a current detector  11  may be arranged near a terminal for external output in the inverter unit  6  due to a limit on a device implementation space within the inverter unit  6 . As described above, when the AC motor  2  is replaced with the PMSM_ 2 ′, an additional MCOK_ 4  and an additional current detector  21   a  are provided. In this case, when the order in which devices are connected is the same as the conventional order illustrated in  FIG. 10 , the devices are connected in the order of the inverter device  1 , the current detectors  11 , and the MCOKs_ 4 . As a result, as indicated by the comparative device  61   b  illustrated in  FIG. 4( b ) , the installation within the inverter unit  6  becomes complex. 
     On the other hand, when the inverter device  1 , the MCOKs_ 4 , and the current detectors  11  are connected in this order, like the present device  61  according to Example 1 illustrated in  FIG. 1 , the installation within the inverter unit  6  can be simplified as indicated with the present device  61   c  illustrated in  FIG. 4( c ) . Therefore, it is possible to simplify the replacement of the AC motor  2  with the PMSM_ 2 ′ while the dimensions and shape of the housing for the inverter unit  6  and compatibility with a vehicle using an induction motor as a conventional AC motor  2  are maintained, and an effect of saving labor is exhibited. 
     Example 2 
     Next, Example 2 according to the present invention is described with reference to  FIG. 5 .  FIG. 5  is a configuration diagram of a present device  62  according to Example 2. The present device  62  according to Example 2 illustrated in  FIG. 5  is different from the present device  61  according to Example 1 in that one connection terminal of the ACPT_ 21   a  is connected between the inverter device  1  and the MCOK_A- 4   a  in a U phase circuit. 
     With this configuration, the present device  62  according to Example 2 illustrated in  FIG. 5  can block the foregoing short circuit current. Regarding this, even in a configuration in which a single MCOK_A is interposed in each phase in the present device  62 , when the ACPT_ 21   a  has a short circuit failure, a short circuit current flows between the ACPT_ 21   a  and the AC motor  2 . The short circuit current is detected by the current detectors  11   a  and  11   b  and input to the control device  31 . 
     When the short circuit current is input to the control device  31 , for example, a driver can manually take security measures at an electric vehicle cab or the control device  31  can automatically take security measures. That is, when this short circuit current is equal to or larger than a predetermined current value, the control device  31  outputs a release instruction to the MCOKs_A_ 4   a  to  4   c  (particularly,  4   a  and  4   b  in the case of  FIG. 5 ). Therefore, the MCOKs_A_ 4   a  to  4   c  (particularly,  4   a  and  4   b  in the case of  FIG. 5 ) can be released to block the short circuit current. 
       FIG. 5  only illustrates the example of the configuration of the present device  62 . That is, the one connection point (terminal) of the voltage detector  21  is connected between the inverter device  1  and the MCOK_A_ 4  in the U phase circuit. In addition, the other connection terminal (terminal) of the ACPT_ 21   a  is connected between the MCOK_ 4 _ 4   b  and the current detector  11   b  in a V phase circuit. 
     In the example of the configuration, the connection relationship between the U phase and the V phase may be reversed. That is, one connection point of the ACPT_ 21   a  connected to two of the three phases may be connected between the inverter device  1  and the MCOK_A and the other connection point may be connected to the MCOK_A and the current detector. 
     Example 3 
     Next, Example 3 of the present invention is described with reference to  FIG. 6 .  FIG. 6  is a configuration diagram of a present device  63  according to Example 2. The present device  63  according to Example 3 illustrated in  FIG. 6  is different from the present device  62  according to Example 2 illustrated in  FIG. 5  in that a first MCOK_A_ 4   b  and a second MCOK_B_ 5   b  are provided to form a series two-stage switch only in the V phase. 
     The second MCOK_B_ 5   b  is interposed between a connection point of the ACPT_ 21   a  and the current detector  11   b . That is, the second MCOK_B_ 5   b  is connected such that the connection point of the ACPT_ 21   a  on the V phase side matches a connection point between the first MCOK_A_ 4   b  and the second MCOK_B_ 5   b.    
     In the example, the voltage detector  21   b  is disposed between the V phase and the W phase in the present device  63  according to Example 3 illustrated in  FIG. 6 . In the present device  63 , the voltage detector  21   b  is connected such that a connection point of the voltage detector  21   b  on the V phase side matches the connection point between the first MCOK_A_ 4   b  and the second MCOK_B_ 5   b.    
     The present device  63  can block a ground fault circuit (that is, a ground fault current). In the present device  63 , for example, when the ACPT_ 21   a  has a ground fault on the side on which the ACPT_ 21   a  is connected to the V phase, the current detector  11   b  detects a ground fault current and inputs the ground fault current to the control device  31 . 
     When the ground fault current detected by the control device  31  is equal to or larger than a predetermined value, the control device  31  outputs a release instruction to the second MCOK_B_ 5   b . Therefore, the second MCOK_B_ 5   b  is released to block the ground fault circuit (that is, the ground fault current). 
     Example 4 
     Next, Example 4 of the present invention is described with reference to  FIG. 7 .  FIG. 7  is a configuration diagram of a present device  64  according to Example 3. The present device  64  according to Example 4 illustrated in  FIG. 7  is different from the present device  61  according to Example 1 in that second MCOKs_B_ 5   a  to  5   c  are interposed between the current detectors  11   a  to  11   c  and the AC motor  2  to form series two-stage switches in all three U, V, and W phases. 
     In a rail vehicle or the like provided with the present device  64  according to Example 4 illustrated in  FIG. 7 , an effect of continuously operating the vehicle is reliably obtained by blocking the foregoing short circuit current. A description will be given while making comparison with the present devices  61  and  62  according to Example 1 illustrated in  FIGS. 1, 2, and 3 ( b ). In the present devices  61  and  62 , when the ACPT_ 21   a  has a short circuit failure, a short circuit current flows between the ACPT_ 21   a  and the AC motor  2  and is detected by the current detectors  11   a  and  11   b  and input to the control device  31 . 
     When the foregoing short circuit current can be blocked, the rail vehicle or the like provided with the present device  64  can be continuously operated. That is, since a plurality of circuits of the same type constitutes drive devices or the like for the rail vehicle, only a failed drive device is disconnected and the vehicle can be continuously operated using the other remaining drive device. 
     The control device  31  outputs a release instruction to the second MCOKs_B_ 5   a  to  5   c . In the present device  64  illustrated in  FIG. 7 , particularly, the second MCOKs_B_ 5   a  and  5   b  are released to have a high effect of blocking a short circuit current caused by a short circuit failure of the ACPT_ 21   a.    
     In the present device  64  illustrated in  FIG. 7 , the measures taken at the time of the failure of the ACPT_ 21   a  or the like can prevent the short circuit current caused by regenerative power by the ACPT_ 21   a  and the AC motor  2  (particularly, in the case in which the AC motor is the PMSM_ 2 ′) from continuously flowing to generate a braking force in the AC motor  2 . 
     Since the two MCOKs are provided in each of the phases as illustrated in  FIG. 7 , the first MCOKs_A_ 4  are turned on (closed) after turning on of the second MCOKs_B_ 5  provided on the AC motor  2  (PMSM_ 2 ′) side at the time of the activation of the inverter device  1 . 
     More specifically, the second MCOKs_B_ 5  are turned on first, the ACPT_ 21   a  detects an inter-phase voltage (voltage between terminals) of the AC motor  2  to estimate the position and speed of a rotor of the motor. 
     After that, in a state in which the inverter device  1  is activated by the control device  31  based on information of the estimated position and the estimated speed, the first MCOKs_A are turned on. This can prevent an eddy current and an operation such as torque vibration and can start an operation as an inverter in a stable manner. 
     Example 5 
       FIG. 8  is a configuration diagram of a present device  65  according to Example 5. The present device  65  according to Example 5 illustrated in  FIG. 8  is different from the present device  64  according to Example 4 illustrated in  FIG. 7  in the following features. That is, in the present device  65 , the current detectors  11   a  to  11   c  are provided on the AC motor  2 ′ side with respect to the second MCOKs_B_ 5   a  to  5   c.    
     In addition, in the present device  65 , one (U phase side in  FIG. 8 ) of two phases connected to the ACPT_ 21   a  is connected between the first MCOK_A_ 4   a  and the second MCOK_B_ 5   a . In addition, in the present device  65 , the other phase (V phase in  FIG. 8 ) is connected between the second MCOK_B_ 5   b  and the current detector  11   b.    
     The present device  65  having the foregoing configuration can start an operation as an inverter in a stable manner, like the present device  64  according to Example 4 illustrated in  FIG. 7 , so that security is improved. That is, the present device  65  can prevent an eddy current and an operation such as torque vibration due to the order of turning on of the first MCOKs_A_ 4   a  to  4   c  and the second MCOKs_B_ 5   a  to  5   c.    
     As a result, the operation as the inverter can be started in a stable manner. In addition, since the current detectors  11   a  to  11   c  are disposed in the immediate vicinity of the AC motor  2  ( 2 ′), the present device  65  can detect a short circuit current caused by a short circuit failure on the AC motor side so that security is improved. 
     A defective state in which the permanent magnetic synchronous motor (PMSM_ 2 ′) performs a regenerative operation at the time of a failure in each of a comparative device  71  illustrated in  FIG. 9  and a comparative device  72  illustrated in  FIG. 10  is described below. While the AC motor  2  of each of the present devices  61  to  65  illustrated in  FIGS. 1 to 8  is an induction motor or the PMSM_ 2 ′, a motor of each of the comparative devices  71  and  72  illustrated in  FIGS. 9 and 10  is limited to the PMSM_ 2 ′ and is indicated by a reference sign  2 ′ to distinguish them. 
     Comparative Example 1 
       FIG. 9  is a configuration diagram of a three-phase AC motor drive device (comparative device  71 ) according to Comparative Example 1. In the comparative device  71  illustrated as Comparative Example 1 in  FIG. 9 , current detectors  11   a  and  11   b  and MCOKs_A_ 4   a  to  4   c  are interposed in different phases on connection lines between an inverter device  1  and the PMSM_ 2 ′. In addition, in the comparative device  71 , an ACPT_ 21   a  is disposed between phases. 
     Regarding the order in which the devices are connected, in a region extending from the inverter device  1  to the PMSM_ 2 ′, the current detectors  11   a  and  11   b  are interposed and are closer to the inverter device  1  than the MCOKs_A_ 4   a  to  4   c  are and farther from the PMSM_ 2 ′ than the MCOKs_A_ 4   a  to  4   c  are. Therefore, in a state in which the MCOKs_A_ 4   a  to  4   c  are released, a regenerative current of the PMSM_ 2 ′ cannot be measured. 
     The ACPT_ 21   a  is disposed closer to the PMSM_ 2 ′ than the MCOKs_A_ 4   a  to  4   c  are. Therefore, in a state in which the MCOKs_A_ 4   a  to  4   c  are released, a regenerative voltage of the PMSM_ 2 ′ can be measured. However, when the ACPT_ 21   a  has a short circuit failure, it is difficult to disconnect a portion of the failure only by performing an operation from a vehicle cab. As a result, a driver may need to give up continuously operating a rail vehicle with other normal power. 
     Comparative Example 2 
       FIG. 10  is a configuration diagram of the comparative device  72 .  FIG. 10  exemplifies that the comparative device  72  illustrated as Comparative Example 2 is used for a rail vehicle as a general direct-current electric vehicle. When the comparative device  72  is a direct-current electric vehicle, the comparative device  72  has a configuration in which one is connected to a direct-current train line and the other is connected to a portion that is a wheel or the like and contacts the ground. For the direct-current train line, a portion of the smoothing capacitor  3  on the direct-current power supply higher-level side is connected to a smoothing reactor  51  via a pantograph  52  that is a power collector. 
     In addition, a configuration for obtaining direct-current power by rectifying alternating-current power by an alternating-current overhead contact line or a configuration for obtaining direct-current power by a third rail method is known. Furthermore, a configuration for obtaining alternating-current power by non-contact power transmission and converting the alternating-current power to direct-current power by a rectifier or the like is used. 
     In the comparative device  72  illustrated in  FIG. 10 , current detectors  11   a  and  11   b  and MCOKs_A_ 4   a  and  4   c  are interposed in different phases on connection lines between an inverter device  1  and the PMSM_ 2 ′, and an ACPT_ 21   a  is disposed between phases. 
     The comparative device  72  illustrated in  FIG. 10  and the comparative device  71  illustrated in  FIG. 9  have a common configuration in which the current detectors  11   a  and  11   b  are disposed between the inverter device  1  and the MCOKs_A_ 4   a  and  4   c  or the ACPT_ 21   a . Therefore, the comparative devices  71  and  72  have a feature in which the current detectors  11   a  and  11   b  cannot measure a regenerative current of the PMSM_ 2 ′ in a state in which the MCOKs_A_ 4   a  to  4   c  are released. 
     On the other hand, the comparative device  72  illustrated in  FIG. 10  and the comparative device  71  illustrated in  FIG. 9  are different in the following feature. That is, the comparative device  72  has a circuit configuration in which only the one MCOK_A_ 4   b  among the three MCOKs_A_ 4   a  to  4   c  is interposed between the ACPT_ 21   a  and the PMSM_ 2 ′. In addition, the current detectors  11   a  and  11   b  are disposed closer to the inverter device  1  than the MCOK_A_ 4   b  is and farther from the PMSM_ 2 ′ than the MCOK_A_ 4   b  is. 
     With the foregoing circuit configuration, the comparative device  72  illustrated in  FIG. 10  cannot measure a regenerative voltage and a regenerative current of the PMSM_ 2 ′ in a state in which the MCOKs_A_ 4   a  to  4   c  are released. However, when the ACPT_ 21   a  has a short circuit failure, it is possible to disconnect a portion of the failure only by performing an operation from the vehicle cab. As a result, it is possible to continuously operate the rail vehicle with other normal power. 
     The present devices  61  to  65  can be summarized as follows. The present devices  61  and  62  according to Example 1 illustrated in  FIGS. 1, 2, and 3 ( b ) are representative examples. 
     [1] Each of the present devices  61  and  62  is a three-phase AC motor drive device that uses the inverter device  1  to drive the three-phase AC motor  2  as a load. The inverter device  1  used in this device has a plurality of current controllers and converts direct-current power supplied from a power supply into alternating-current power for the three U, V, and W phases to drive the load. 
     Each of the current controllers is a combination of a current control element and a rectifying element. The current control element conducts or blocks a current flowing toward one direction. The rectifying element is connected to the current control element in parallel and conducts a current flowing toward a direction opposite to the one direction. 
     In each of the present devices  61  and  62 , the motor cutout contacts represented by the MCOKs_A_ 4  are connected between the inverter device  1  and the load and switch whether to electrically connect or disconnect the inverter device  1  to or from the load. 
     In addition, three-phase current power generated by the inverter device  1  for the U, V, and W phases is supplied to the load. On the other hand, the terminals of the current detector ACPT_ 21   a  having the pair of terminals are connected to at least two phases, for example, the U phase and the V phase. To detect a voltage between the U, V, and W phases, it is sufficient if the single ACPT_ 21   a  illustrated in  FIG. 1  is provided or if the two ACPTs_ 21   a  and  21   b  illustrated in  FIG. 2  are provided. 
     In addition, the current detectors  11  that detect three-phase currents to be supplied from the inverter device  1  to the load are connected to the U, V, and W phases, respectively. The circuit configuration from the inverter device  1  to the load is as follows. That is, the inverter device  1 , the load, the MCOKs_A_ 4 , the current detectors  11 , and the ACPT_ 21   a  are connected such that the inverter device  1  is connected in the immediate vicinity of the MCOKs_A_ 4 , then to the ACPT_ 21   a , then to the current detectors  11 , and then to the load. 
     In each of the present devices  61  and  62  having the foregoing connection configuration, when the ACPT_ 21   a  has a short circuit failure (for example, a short circuit between the U and V phases illustrated in  FIG. 1 ), the MCOKs_A_ 4  are released. This release of the MCOKs_A_ 4  electrically disconnects the inverter device  1  from the AC motor  2 . Even in this state, the current detectors  11   a  and  11   b  can detect a short circuit current flowing in a path between the AC motor  2  and the ACPT_ 21   a . This information can be given to the vehicle cab or the like. As a result, a driver or the like can easily take optimal security measures. 
     The case in which the MCOKs_A_ 4  are released corresponds to a case in which the load of the present device  61  or  62  is the PMSM_ 2 ′, the present device  61  or  62  is used in, for example, a rail vehicle, a failure such as a short circuit failure of the ACPT_ 21   a  or a reduction in output of the inverter device  1  occurs. In such a case, a driver or the like may take optimal security measures to eliminate a regenerative braking action of the PMSM_ 2 ′ and continuously operate the vehicle with other remaining power. 
     The following convenience is obtained by the configurations of the present devices  61  and  62 . That is, when the load at the initial design stage is an induction motor, and an MCOK is not required due to a low regenerative braking action, basic design may be made such that an MCOK required due to replacement of the induction motor used as the load with the PMSM_ 2 ′ when it is not provided in the inverter device  2 , and the ACPT_ 21   a  are disposed in the immediate vicinity of the inverter device  1  in this order. 
     That is, to secure a space for the MCOKs and the ACPT_ 21   a , the current detectors  11  are disposed in the immediate vicinity of the load while avoiding the space. In other words, even in the case in which the MCOKs and the ACPT_ 21   s  are not required in the inverter device  1  at the initial design stage, when the space is secured and the MCOKs and the ACPT_ 21   a  needs to be retrofitted, there is no trouble. That is, it is easy to secure the installation configuration of the inverter device  1  and compatibility of device arrangement, regardless of whether the MCOKs and the ACPT_ 21   a  are present. 
     [2] In each of the present devices  61  and  62  according to Example 1 illustrated in  FIGS. 1, 2, and 3 ( b ), the MCOKs_A_ 4  are preferably connected between connection points between the inverter device  1  and the ACPT_ 21   a . The advantages of the MCOKs_A_ 4  being disposed in the immediately vicinity of the inverter device  1  are described above. 
     [3] A connection form of the circuits of the two phases to which the ACPT_ 21  is connected in the present device  62  according to Example 2 illustrated in  FIG. 5  is as follows. In the circuit of one (for example, the U phase) of the two phases, the MCOK_A_ 4   a  is connected between the connection point of the ACPT_ 21   a  and the current detector  11   a . In addition, in the circuit of the other phase (for example, the V phase), the MCOK_A_ 4   b  is connected between the inverter device  1  and the connection point of the ACPT_ 21   a.    
     Regarding the detection of an inter-phase voltage, even when the ACPT_ 21  fails, a state in which the current detectors  11   a  to  11   c  can detect current values of the phases is maintained, and thus the control device  31  illustrated in  FIG. 5  easily take corresponding security measures. 
     [4] In the present device  63  according to Example 3 illustrated in  FIG. 6 , the MCOK_A_ 4   b  is connected as a first motor cutout contact in the V phase between the inverter device  1  and the connection point of the ACPT_ 21   a . In the V phase, the MCOK_B_ 5   b  is connected as a second motor cutout contact between the connection point of the ACPT_ 21   a  and the current detector  11   b.    
     According to the control device  31  illustrated in  FIG. 6 , for example, even when a V-phase ground fault accident occurs due to a ground fault of the ACPT_ 21   a , a state in which the current detector  11   b  can detect a V-phase ground fault current is maintained, and thus it is possible to easily take corresponding security measures. 
     [5] The present device  64  according to Example 4 illustrated in  FIG. 7  includes the MCOKs_B_ 5  as second motor cutout contacts between the current detectors  11  and the load. According to this, the MCOKs_B_ 5  as the second motor cutout contacts form series two-stage switches with the MCOKs_A_ 4  as the first motor cutout contacts. 
     Even when a regenerative braking state occurs in, for example, a vehicle having the PMSM_ 2 ′ due to a short circuit failure of the ACPT_ 21   a , the control device  31  illustrated in  FIG. 7  maintains a state in which the current detectors  11   a  and  11   b  can detect a regenerative current. In this case, when the control device  31  releases the MCOKs_B_ 5 , it is easy to take corresponding security measures. 
     In addition, when only the second MCOKs_B_ 5  provided on the PMSM_ 2 ′ side are turned on (closed) at the time of the activation of the inverter device  1  and the ACPT_ 21   a  detects an inter-phase voltage (voltage between terminals) of the PMSM_ 2 , the position and speed of the rotor of the motor are estimated. After that, the first MCOKs_A are turned on based on information of the estimated position and the estimated speed in a state in which the inverter device  1  is activated by the control device  31 . This can prevent an eddy current and an operation such as torque vibration and start an operation as an inverter in a stable manner. 
     [6] The present device  65  according to Example 5 illustrated in  FIG. 8  includes the series two-stage switches constituted by the first MCOKs_A_ 4  and the second MCOKs_B_ 5  between the inverter device  1  and the current detectors  11   a  to  11   c . In addition, the circuit of the U phase of the two phases connected to the current detector ACPT_ 21   a  is connected between the first MCOK_A_ 4   a  and the second MCOK_B_ 5   a . Furthermore, the circuit of the other V phase is connected between the second MCOK_B_ 5   b  and the current detector  11   b.    
     The present device  65  having the foregoing configuration can start the operation as the inverter at a higher level in a stable manner so that security is improved. That is, the present device  65  can prevent an eddy current and an operation such as torque vibration due to the order of turning on of the first MCOKs_A_ 4   a  to  4   c  and the second MCOKs_B_ 5   a  to  5   c . In addition, since the current detectors  11   a  to  11   c  are disposed in the immediate vicinity of the AC motor  2  in the present device  65 , the present device  65  can detect a short circuit current caused by a short circuit failure on the AC motor side so that security is improved. 
     LIST OF REFERENCE SIGNS 
     Inverter device,  2  Three-phase AC motor (as load),  2 ′ Permanent magnet synchronous motor (PMSM as load),  4 ,  4   a  to  4   c  First motor cutout contact (MCOK_A),  5 ,  5   a  to  5   c  Second motor cutout contact (MCOK_B),  11 ,  11   a  to  11   c  Current detector,  21   a ,  21   b  Voltage detector (ACPT),  31  Control device,  61  to  65  Three-phase AC motor drive device (present device)