Patent Publication Number: US-2021188100-A1

Title: Drive control device and drive device for railroad cars

Description:
TECHNICAL FIELD 
     The present disclosure relates to a drive control device and a railway vehicle driving apparatus. 
     BACKGROUND ART 
     Some types of vehicle driving apparatuses for driving a railway vehicle employ a generator and an electric motor. Patent Literature 1 discloses an example of such a vehicle driving apparatus. The vehicle driving apparatus disclosed in Patent Literature 1 includes an internal combustion engine, an induction generator driven by the internal combustion engine, a converter, an inverter that drives an induction electric motor, and a power storage device. Since the internal combustion engine cannot perform self-starting, this vehicle driving apparatus, when starting the internal combustion engine, uses the converter to convert direct current (DC) power supplied from the power storage device into alternating current (AC) power and to supply the AC power to the induction generator. This causes the induction generator attached to the internal combustion engine to operate as an electric motor to apply a torque from the induction generator, thereby causing the internal combustion engine to start. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2014-91504 
     SUMMARY OF INVENTION 
     Technical Problem 
     To start the internal combustion engine in the manner as described above, the power storage is required to store sufficient power. Thus the vehicle driving apparatus disclosed in Patent Literature 1, after the internal combustion engine is started, charges the power storage device using AC power that is output from the induction generator driven by the internal combustion engine, in order to enable the next starting of the internal combustion engine. Specifically, the converter included in the vehicle driving apparatus converts the AC power output from the induction generator into DC power suitable for charging of the power storage device and supplies the DC power to the power storage device. When the charging of the power storage device ends, the vehicle driving apparatus converts the AC power output from the induction generator into DC power suitable for driving of the induction electric motor by the inverter and supplies the DC power to the inverter. An output voltage of the converter when supplying power to the power storage device is lower than an output voltage of the converter when supplying power to the inverter. Thus, according to the vehicle driving apparatus, power is not supplied to the inverter during a period in which power is supplied to the power storage device. In other words, even after the internal combustion engine is started, the railway vehicle cannot run during a period in which the power storage device is being charged. This causes a problem in that the railway vehicle cannot run immediately after the internal combustion engine is started. 
     The present disclosure is made in view of the above-described circumstances, and an objective of the present disclosure is to provide a drive control device and a railway vehicle driving apparatus that enable running of a railway vehicle after an internal combustion engine is started and during charging of a power storage device. 
     Solution to Problem 
     To achieve the aforementioned objective, a drive control device according to the present disclosure is a drive control device for controlling a railway vehicle driving apparatus for driving a railway vehicle using, as a motive power source, an internal combustion engine, and includes a main converter, a first inverter, a step-down circuit, and a converter controller. The main converter converts alternating current (AC) power supplied from a generator to a primary terminal thereof into direct current (DC) power and outputs the DC power from a secondary terminal thereof, the internal combustion engine driving the generator to generate and output the AC power, or converts DC power supplied to the secondary terminal into AC power and supplies the AC power to the generator. The first inverter converts the DC power output from the secondary terminal of the main converter into AC power and outputs the AC power to an electric motor. The step-down circuit steps down a voltage of the DC power output from the secondary terminal of the main converter and supplies the stepped-down DC power to a power storage device. The converter controller controls power conversion performed by the main converter. The main converter, when the converter controller acquires a start command providing instruction for starting of the internal combustion engine, converts DC power supplied from the power storage device into AC power and supplies the AC power to the generator. The main converter, after the internal combustion engine is started, converts the AC power output by the generator into DC power and supplies the DC power to the first inverter and the step-down circuit. 
     Advantageous Effects of Invention 
     According to the present disclosure, the step-down circuit steps down an output voltage of the main converter and supplies power to the power storage device with the stepped down output voltage. Thus, the output voltage of the main converter can be set, after the internal combustion engine is started, at a voltage that is higher than a voltage suitable for charging of the power storage device and is suitable for driving of the electric motor by the first inverter. This enables driving of the electric motor while charging the power storage device with a stepped down output voltage of the main converter, and thus enables running of the railway vehicle after the internal combustion engine is started and during charging of the power storage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating configuration of a railway vehicle driving apparatus according to Embodiment 1 of the present disclosure; 
         FIG. 2  is a timing chart illustrating an operation of processing of starting an internal combustion engine that is performed by the railway vehicle driving apparatus according to Embodiment 1; 
         FIG. 3  illustrates an example flow of an electric current in the railway vehicle driving apparatus according to Embodiment 1; 
         FIG. 4  illustrates an example flow of an electric current in the railway vehicle driving apparatus according to Embodiment 1; 
         FIG. 5  illustrates an example flow of an electric current in the railway vehicle driving apparatus according to Embodiment 1; 
         FIG. 6  is a block diagram illustrating configuration of a railway vehicle driving apparatus according to Embodiment 2 of the present disclosure; 
         FIG. 7  is a block diagram illustrating configuration of a railway vehicle driving apparatus according to Embodiment 3 of the present disclosure; 
         FIG. 8  is a block diagram illustrating configuration of a railway vehicle driving apparatus according to Embodiment 4 of the present disclosure; 
         FIG. 9  is a timing chart illustrating an operation of processing of starting an internal combustion engine that is performed by the railway vehicle driving apparatus according to Embodiment 4; 
         FIG. 10  illustrates an example flow of an electric current in the railway vehicle driving apparatus according to Embodiment 4; 
         FIG. 11  illustrates an example flow of an electric current in the railway vehicle driving apparatus according to Embodiment 4; and 
         FIG. 12  illustrates an example flow of an electric current in the railway vehicle driving apparatus according to Embodiment 4. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a railway vehicle driving apparatus according to embodiments of the present disclosure is described in detail with reference to the drawings. In the drawings, the same or similar components are denoted by the same reference signs. 
     Embodiment 1 
     As described in Embodiment 1 below, a railway vehicle driving apparatus  1  (hereinafter referred to as “the driving apparatus”) includes: an internal combustion engine  2  that serves as a motive power source; an internal-combustion-engine controller  3  that controls the internal combustion engine  2 ; a speed sensor  4  that detects an engine speed of the internal combustion engine  2 ; a generator  11  that rotates by being driven by the internal combustion engine  2  to output alternating current (AC) power or rotates by receiving AC power to rotate the internal combustion engine  2 ; a power storage device  16  that serves as an electric power source in starting the internal combustion engine  2 ; an electric motor  15  that generates railway-vehicle motive power; and a drive control device  10  that controls various elements included in the railway vehicle driving apparatus  1 . 
     Further, a start command signal S 1  is supplied to the driving apparatus  1  from a starting switch arranged in a non-illustrated driver cab, and an operation command signal S 2  is supplied to the driving apparatus  1  from a master controller arranged in the driver cab. The start command signal S 1  is a signal that provides an instruction for starting of the internal combustion engine  2 . The start command signal S 1  is set at a low (L) level when the internal combustion engine  2  is to be stopped, and the start command signal S 1  is set at a high (H) level when the internal combustion engine  2  is to be started. The operation command signal S 2  is a signal that indicates one of a power running notch that provides instruction for an acceleration for a railway vehicle and a brake notch that provides instruction for a deceleration for the railway vehicle. The driving apparatus  1 , when the start command signal S 1  is set at the H level, starts the internal combustion engine  2 . The internal combustion engine  2 , after being started, is controlled by the internal-combustion-engine controller  3 . 
     The internal combustion engine  2  includes a diesel engine, a gasoline engine, or the like. The internal combustion engine  2  includes an output shaft that is connected to an input shaft of the generator  11  and drives the generator  11  to generate electricity. Since the internal combustion engine  2  cannot perform self-starting, the generator  11  operates as an electric motor and rotates to start the internal combustion engine  2 . The internal-combustion-engine controller  3 , after the internal combustion engine  2  is started, controls the engine speed of the internal combustion engine  2 . 
     The operation command signal S 2  is supplied to the internal-combustion-engine controller  3 . The internal-combustion-engine controller  3  calculates a target engine speed of the internal combustion engine  2  corresponding to the operation command signal S 2  and controls the internal combustion engine  2  to bring an actual engine speed of the internal combustion engine  2  acquired from the speed sensor  4  close to the target engine speed. 
     The speed sensor  4  includes a pulse generator (PG) attached to the internal combustion engine  2  and outputs a signal indicating the engine speed of the internal combustion engine  2  obtained from a pulse signal output by the PG. 
     The generator  11  is connected to the internal combustion engine  2 , is driven by the internal combustion engine  2  to generate AC power, and supplies the AC power to the drive control device  10 . Further, the generator  11 , when causing the internal combustion engine  2  to start, rotates by receiving AC power from the drive control device  10 , thereby starting the internal combustion engine  2 . 
     The electric motor  15  includes a three-phase induction motor and rotates by being driven by AC power output from a first inverter  14 . The electric motor  15  is connected to an axle via, for example, a coupling, in order to transmit a torque to the axle. 
     The power storage device  16  includes a secondary battery having multiple battery cells and stores power for driving of the generator  11  to start the internal combustion engine  2 . 
     The drive control device  10  controls (i) an operation of starting the internal combustion engine  2  by driving the generator  11  using the power stored in the power storage device  16  and (ii) an operation of, after the internal combustion engine  2  is started, using the power generated by the generator  11 , rotating the electric motor  15  to generate the railway-vehicle motive power while storing power in the power storage device  16 . 
     The drive control device  10  includes: a main converter  12  that converts AC power supplied to primary terminals thereof into direct current (DC) power and outputs the DC power from secondary terminals thereof, or converts DC power supplied to the secondary terminals into AC power and supplies the AC power to the generator  11 ; a first inverter  14  that converts the DC power output from the secondary terminals of the main converter  12  into AC power and outputs the AC power; a filter capacitor  13  for smoothening that is arranged in a circuit between the main converter  12  and the first inverter  14 ; a speed sensor  24  for obtaining a rotational speed of the electric motor  15 ; a step-down circuit  17  that steps down an output voltage of the main converter  12  and supplies power to the power storage device  16  with the stepped-down output voltage; a converter controller  18  that controls the power conversion performed by the main converter  12 ; an inverter controller  19  that controls the first inverter  14 ; and a step-down circuit controller  20  that controls the step-down circuit  17 . 
     The primary terminals of the main converter  12  are connected to the generator  11 , and the secondary terminals of the main converter  12  are connected to the first inverter  14 . The main converter  12  operates in accordance with control by the converter controller  18 . The main converter  12 , when causing the internal combustion engine  2  to start, converts DC power that is supplied from the filter capacitor  13  charged with power supplied from the power storage device  16  to the secondary terminals into AC power, and supplies the AC power from the primary terminals to the generator  11  to rotate the generator  11 . The rotation of the generator  11  causes rotation of the internal combustion engine  2 , thereby starting the internal combustion engine  2 . Further, the main converter  12 , after the internal combustion engine  2  is started, in accordance with control by the converter controller  18 , converts AC power supplied from the generator  11  to the primary terminals into DC power and supplies the DC power from the secondary terminals to the first inverter  14  and the power storage device  16 . 
     The first inverter  14 , in accordance with control by the inverter controller  19 , converts the DC power output from the secondary terminals of the main converter  12  into AC power and outputs the AC power to the electric motor  15 . The electric motor  15  rotates by being driven by the AC power output by the first inverter  14 . 
     The speed sensor  24  includes a PG arranged in the electric motor  15  and outputs a signal indicating the rotational speed of the electric motor  15  that is obtained from a pulse signal output by the PG. 
     The step-down circuit  17  is arranged in a circuit between the secondary terminals of the main converter  12  and the power storage device  16 , and when the output voltage of the main converter  12  is higher than a charging voltage of the power storage device  16 , steps down the output voltage of the main converter  12  and applies the stepped-down voltage to the power storage device  16 . The step-down circuit  17  includes contactors Q 1 , Q 2 , and Q 3  that are connected in series, a voltage dividing resistor R 1  connected in parallel to the contactor Q 1 , and a voltage dividing resistor R 2  connected in parallel to the contactor Q 2 . The voltage dividing resistor R 1  preferably has a resistance value that is sufficiently smaller than a resistance value of the voltage dividing resistor R 2 . Specifically, the resistance value of the voltage dividing resistor R 1  is preferably several ohms (a), and the resistance value of the voltage dividing resistor R 2  is preferably several hundred Q. The step-down circuit controller  20  controls conductivity/non-conductivity of the contactors Q 1 , Q 2 , and Q 3 . Different combinations of conductivity/non-conductivity of the contactors Q 1 , Q 2 , and Q 3  result in different resistance values of the step-down circuit  17 . 
     The start command signal S 1  and the operation command signal S 2  are supplied to the converter controller  18 . The converter controller  18  operates in accordance with the start command signal S 1  and the operation command signal S 2  and controls on/off timings of multiple switching elements included in the main converter  12 , thereby causing the main converter  12  to operate as a DC-AC converter to convert DC power supplied from the power storage device  16  into AC power, or as an AC-DC converter to convert the AC power supplied from the generator  11  into DC power. The converter controller  18  controls the on/off timings of the multiple switching elements included in the main converter  12  by sending switching control signals S 3  to the multiple switching elements. 
     Specifically, when the start command signal S 1  is set at the L level and the operation command signal S 2  indicates a braking command, the converter controller  18  stops the main converter  12 . 
     Further, when the start command signal S 1  is set at the H level and a voltage of the filter capacitor  13  that is detected by a voltage detector  22  reaches a threshold voltage EFC 1  suitable for starting of the internal combustion engine  2 , the converter controller  18  controls the main converter  12  to cause the main converter  12  to convert DC power supplied from the power storage device  16  into AC power and to supply the AC power to the generator  11 . In this case, the converter controller  18  calculates, based on a current detected by a current detector  21  and flowing from the main converter  12  to the generator  11 , an actual torque of the generator  11  that operates as an electric motor. Then the converter controller  18  controls on/off operation of the multiple switching elements included in the main converter  12  such that the actual torque of the generator  11  approaches a target torque suitable for starting of the internal combustion engine  2 . The converter controller  18  holds in advance the target torque suitable for starting of the internal combustion engine  2 . 
     The converter controller  18 , after causing the main converter  12  to start supplying of AC power to the generator  11  and then upon determination based on an output signal of the speed sensor  4  that the engine speed of the internal combustion engine  2  reaches a reference engine speed Th 1  that enables independent rotation of the internal combustion engine  2 , determines that the internal combustion engine  2  is started. The converter controller  18  holds in advance a value of the reference engine speed Th 1 . The current detector  21  connected to the primary terminals of the main converter  12  detects a phase current for each of a U-phase, V-phase and W-phase that flows through a circuit between the generator  11  and the main converter  12 . The converter controller  18 , after the internal combustion engine  2  is started, calculates an output voltage of the generator  11  based on (i) the engine speed of the internal combustion engine  2  that is acquired from the speed sensor  4  and (ii) the current values that are acquired from the current detector  21 . 
     The converter controller  18 , when the operation command signal S 2  indicates a power running notch after the internal combustion engine  2  is started, controls on/off timings of the multiple switching elements included in the main converter  12  based on the output voltage of the generator  11  and a target voltage corresponding to the power running notch indicated by the operation command signal S 2 , in order to bring the output voltage of the main converter  12  close to the target voltage. 
     The operation command signal S 2  is supplied to the inverter controller  19 . The inverter controller  19  calculates a target torque of the electric motor  15  based on (i) the power running notch indicated by the operation command signal S 2  and (ii) the rotational speed of the electric motor  15  that is acquired from the speed sensor  24 . Further, the inverter controller  19  calculates an actual torque of the electric motor  15  based on current values acquired from a current detector  23 . The current detector  23  detects a phase current for each of U-phase, V-phase and W-phase that flows from the first inverter  14  to the electric motor  15 . The inverter controller  19 , in order to bring the actual torque of the electric motor  15  close to the target torque, controls multiple switching elements included in the first inverter  14 . The inverter controller  19  controls on/off timings of the multiple switching elements included in the first inverter  14  by sending switching control signals S 4  to the multiple switching elements. 
     The step-down circuit controller  20 , as described later, opens and closes the contactors Q 1 , Q 2 , and Q 3  based on the start command signal S 1 , the engine speed of the internal combustion engine  2  that is acquired from the speed sensor  4 , and a voltage value acquired from the voltage detector  22 , thereby changing a resistance value of a circuit between the secondary terminals of the main converter  12  and the power storage device  16 . 
     Next, an operation of the driving apparatus  1  having the above-described configuration is described with reference to the timing chart illustrated in  FIG. 2 . 
     During a period in which the railway vehicle is stopped, the start command signal S 1  is at the L level and the operation command signal S 2  indicates a brake notch Bl, as illustrated in (A) and (B) of  FIG. 2 . Hereinafter, a timing at which the start command signal S 1  changes from the L level to the H level is referred to as the “time T 1 ”. 
     As illustrated in (C), (D), and (E) of  FIG. 2 , the step-down circuit controller  20 , in response to the start command signal S 1  and the operation command signal S 2 , keeps all of the contactors Q 1 , Q 2 , and Q 3  open until the time T 1 . Then in response to a change at the time T 1  in the start command signal S 1  from the L level to the H level, the step-down circuit controller  20 , while keeping the contactor Q 1  open, closes the contactors Q 2  and Q 3  to charge the filter capacitor  13  using the power storage device  16  as an electric power source. Upon closing of the contactors Q 2  and Q 3 , current flows from the power storage device  16  to the filter capacitor  13  through the contactors Q 3  and Q 2  and the voltage dividing resistor R 1 , as illustrated in  FIG. 3  using a solid arrow. The flow of current to the filter capacitor  13  via the voltage dividing resistor R 1  prevents an inrush current from flowing to the filter capacitor  13 . 
     As current flows from the power storage device  16  to the filler capacitor  13 , an amount of power stored in the secondary battery included in the power storage device  16  gradually decreases from a maximum power amount W 2  as illustrated in (H) of  FIG. 2 , and a both-ends voltage that is a voltage between both ends of the filter capacitor  13  gradually increases from a voltage EFC 0  as illustrated in (F) of  FIG. 2 . 
     The step-down circuit controller  20  monitors the both-ends voltage of the filter capacitor  13  using an output signal from the voltage detector  22  and detects, at a time T 2 , reach of the both-ends voltage EFC to the threshold voltage EFC 1 . Then the step-down circuit controller  20  closes the contactor Q 1  as illustrated in (C) of  FIG. 2 , in order to start the internal combustion engine  2  using the power storage device  16  as an electric power source. 
     The converter controller  18 , in response to (i) the start command signal S 1  at the H level, (ii) the operation command signal S 2  indicating the brake notch Bl, and (iii) the output signal from the voltage detector  22  indicating that the both-ends voltage EFC of the filter capacitor  13  reaches the threshold voltage EFC 1 , starts controlling on/off operation of the multiple switching elements included in the main converter  12 , thereby causing the main converter  12  to convert DC power supplied from the power storage device  16  into AC power and to supply the AC power to the generator  11 . This leads to flowing of current from the power storage device  16  to the main converter  12  through the contactors Q 3 , Q 2 , and Q 1 , as illustrated in  FIG. 4  using a solid arrow. 
     More specifically, the converter controller  18 , based on the current flowing from the main converter  12  to the generator  11  and detected by the current detector  21 , calculates the actual torque of the generator  11  that operates as an electric motor. Then the converter controller  18  controls on/off timings of the multiple switching elements included in the main converter  12 , in order to bring the actual torque close to the target torque suitable for starting of the internal combustion engine  2 . The target torque suitable for starting of the internal combustion engine  2  is determined based on characteristics of the internal combustion engine  2 . The converter controller  18  performs the above-described control, thereby causing the main converter  12  to convert DC power supplied from the power storage device  16  to the secondary terminals into AC power and to supply the AC power from the primary terminals to the generator  11 . This allows the generator  11  to operate as an electric motor and to rotate the internal combustion engine  2  and thus causes, as illustrated in (G) of  FIG. 2 , gradual increase in the engine speed of the internal combustion engine  2  on and after the time T 2 . 
     A timing at which the engine speed of the internal combustion engine  2  reaches the reference engine speed Th 1  that enables independent rotation of the internal combustion engine  2  is referred to as a “time T 3 ”. In other words, the internal combustion engine  2  is started and starts independent rotation at the time T 3 . 
     The converter controller  18 , when detecting based on the output signal from the speed sensor  4  that the engine speed of the internal combustion engine  2  reaches the reference engine speed Th 1 , controls on/off operation of the multiple switching elements included in the main converter  12 , thereby causing the main converter  12  to convert AC power that the generator  11  driven by the internal combustion engine  2  supplies to the primary terminals into DC power and to output the DC power from the secondary terminals. As illustrated in (F) of  FIG. 2 , the converter controller  18  controls conduction ratios of the multiple switching elements included in the main converter  12  such that the main converter  12  outputs the DC power with the threshold voltage EFC 1 . 
     The step-down circuit controller  20 , when detecting based on the output signal from the speed sensor  4  that the engine speed of the internal combustion engine  2  reaches the reference engine speed Th 1 , opens the contactors Q 1  and Q 2  while keeping the contactor Q 3  closed, as illustrated in (C), (D), and (E) of  FIG. 2 , in order to, on and after a time T 4  described later, (i) step down, using the step-down circuit  17 , the voltage that is output by the main converter  12  from the secondary terminals and is higher than a voltage suitable for charging of the power storage device  16  and (ii) supply power to the power storage device  16  with the stepped-down voltage. 
     Thereafter, the power running notch is input from the master controller, and thus the operation command signal S 2  indicates a power running notch N 1 . This timing is referred to as a time T 4 . On and after the time T 4 , the internal-combustion-engine controller  3  controls the internal combustion engine  2  to bring the engine speed of the internal combustion engine  2  close to an engine speed corresponding to the power running notch N 1 , thereby increasing the engine speed of the internal combustion engine  2  as illustrated in (G) of  FIG. 2 . In accordance with the increase in the engine speed of the internal combustion engine  2 , a rotational speed of the generator  11  and the output voltage of the generator  11  increase. 
     Then the converter controller  18 , in response to the operation command signal S 2  indicating the power running notch N 1 , starts performing on/off control operation for the multiple switching elements included in the main converter  12 , in order to bring the output voltage of the main converter  12  close to a fixed voltage corresponding to the power running notch N 1 , for example, to 600V. Specifically, the converter controller  18  calculates the output voltage of the of the generator  11  based on (i) the engine speed of the internal combustion engine  2  that is acquired from the speed sensor  4  and (ii) the current values that are acquired from the current detector  21 . Then the converter controller  18 , based on the output voltage of the generator  11  and a target voltage corresponding to the power running notch indicated by the operation command signal S 2 , controls the conduction ratios of the multiple switching elements included in the main converter  12 , in order to bring the output voltage of the main converter  12  close to the target voltage. 
     As illustrated in (G) of  FIG. 2 , the output voltage of the main converter  12 , that is, the voltage of the filter capacitor  13 , increases on and after the time T 4 . As a result, current flows from the main converter  12  to the power storage device  16  through the voltage dividing resistors R 1  and R 2  and the contactor Q 3  to charge the power storage device  16 , as illustrated in  FIG. 5  using a solid arrow. Thus, on and after the time T 4 , the amount of power stored in the power storage device  16  gradually increases from a power amount W 1  to the maximum power amount W 2 . The flow of current through the voltage dividing resistors R 1  and R 2  leads to applying of a voltage to the power storage device  16  at, for example, about 300V. In other words, the step-down circuit  17  steps down the voltage that is output by the main converter  12  and is higher than the voltage suitable for charging of the power storage device  16 , and the power storage device  16  is charged with the voltage suitable for charging of the power storage device  16 . 
     Furthermore, the inverter controller  19  calculates the target torque of the electric motor  15  based on the power running notch N 1  and the rotational speed of the electric motor  15  that is acquired from the speed sensor  24 . Further, the inverter controller  19  calculates the actual torque of the electric motor  15  based on current values that are detected for the currents flowing in the phases of the electric motor  15  and are acquired from the current detector  23 . Further, the inverter controller  19 , in order to bring the actual torque close to the target torque, controls on/off operation of the multiple switching elements included in the first inverter  14 . Thus, the electric motor  15  is driven in response to the operation command signal S 2  on and after the time T 4 , to generate the railway-vehicle motive power. This enables running of the railway vehicle while charging the power storage device  16 . 
     As described above, the drive control device  10  according to Embodiment 1 enables, while charging the power storage device  16  after the internal combustion engine  2  is started, driving of the electric motor  15 . 
     Embodiment 2 
     In Embodiment 1, the first inverter  14  that drives the electric motor  15  is connected to the secondary terminals of the main converter  12 . However, a device other than the electric motor  15  that is to be supplied power may be supplied power by connecting another inverter to the secondary terminals of the main converter  12  and supplying power from the other inverter to the device. As illustrated in the example of  FIG. 6 , a drive control device  10  according to Embodiment 2 includes, in addition to the elements included in the drive control device  10  according to Embodiment 1, a second inverter  31  connected to the secondary terminals of the main converter  12  and an inverter controller  32  that controls the second inverter  31 . The second inverter  31  converts DC power supplied from the main converter  12  to primary terminals thereof into AC power and supplies, from secondary terminals thereof, the AC power to a load device  33  installed in the railway vehicle. The load device  33  may be any electronic devices installed in the railway vehicle, such as an air conditioner, a lightning device, and a blower. The inverter controller  32  controls on/off operation of multiple switching elements included in the second inverter  31 . An output voltage of the second inverter  31  may be a value different from that of the output voltage of the secondary terminals of the first inverter  14 . 
     The inverter controller  32  calculates output power of the second inverter  31  based on current values acquired from a current detector  34 . Then the inverter controller  32  controls on/off operation of the multiple switching elements included in the second inverter  31 , in order to bring the output power of the second inverter  31  close to a target voltage suitable for power consumption by the load device  33 . The inverter controller  32  controls on/off timings of the multiple switching elements included in the second inverter  31  by sending switching control signals S 5  to the multiple switching elements. The current detector  34  detects a phase current for each of the U-phase, V-phase and W-phase that flows from the second inverter  31  to the load device  33 . 
     The contactors Q 1 , Q 2 , and Q 3  are closed and opened at timings similar to those in Embodiment 1. The converter controller  18 , by performing control similar to that in Embodiment 1, causes the first inverter  14  to drive the electric motor  15  and causes the second inverter  31  to run the load device  33 , after the internal combustion engine  2  is started and during charging of the power storage device  16 . 
     As described above, the drive control device  10  according to Embodiment 2 enables, while charging the power storage device  16  after the internal combustion engine  2  is started, driving of the electric motor  15  and running of the load device  33 . 
     Embodiment 3 
     The step-down circuit  17  may include any circuit that can step down the voltage of the DC power output by the main converter  12  and can supply the stepped-down DC power to the power storage device  16 . As illustrated in the example of  FIG. 7 , the step-down circuit  17  included in a driving apparatus  1  according to Embodiment 3 includes a diode D 1  as a substitute for the contactor Q 2  of the step-down circuits  17  included in the driving apparatuses  1  according to Embodiment 1 and 2. The anode of the diode D 1  is connected to the power storage device  16  via the contactor Q 3 . The cathode of the diode D 1  is connected to the secondary terminals of the main converter  12  via the contactor Q 1 . The voltage dividing resistor R 2  is connected in parallel to the diode D 1 . 
     The contactors Q 1  and Q 3  are closed and opened at timings similar to those in Embodiment 1. Upon closing of the contactor Q 3  in response to the start command signal S 1  at the H level, current flows from the power storage device  16  to the main converter  12  through the contactor Q 3 , the diode D 1 , and the voltage dividing resistor R 1 . Further, upon closing of the contactor Q 1  in response to the output signal from the voltage detector  22  indicating that the both-ends voltage EFC of the filter capacitor  13  reaches the threshold voltage EFC 1 , current flows from the power storage device  16  to the main converter  12  through the contactor Q 3 , the diode D 1 , and the contactor Q 1 . Then, after the internal combustion engine  2  is started and the contactor Q 1  is opened, current flows from the main converter  12  to the power storage device  16  through the voltage dividing resistors R 1  and R 2  and the contactor Q 3 , thereby charging the power storage device  16 . The converter controller  18 , by performing control similar to that in Embodiment 1, causes the first inverter  14  to drive the electric motor  15 , after the internal combustion engine  2  is started and during charging of the power storage device  16 . 
     As described above, the drive control device  10  according to Embodiment 3 enables driving by the first inverter  14  of the electric motor  15  while achieving power supply to the power storage device  16  with a stepped-down voltage that is obtained by stepping down the voltage of the DC power output by the main converter  12  using the step-down circuit  17  having simple configuration by inclusion of the diode D 1  and the voltage dividing resistor R 2 . That is to say, running of the railway vehicle can be achieved after the internal combustion engine  2  is started and during charging of the power storage device  16 . Employment of the diode D 1  as a substitute for the contactor Q 2  enables simplification in configuration of the step-down circuit  17 . 
     Embodiment 4 
     The step-down circuit  17  may include any circuits that can step down the voltage of the DC power output by the main converter  12  and can supply the stepped-down DC power to the power storage device  16 . As illustrated in the example of  FIG. 8 , the step-down circuit  17  included in a driving apparatus  1  according to Embodiment 4 does not include the voltage dividing resistor R 2  included in the step-down circuits  17  according to Embodiments 1 and 2 and includes a contactor Q 4  and a DC-DC converter  35 . The drive control device  10  further includes a step-down circuit controller  36  that controls power conversion performed by the DC-DC converter  35 . Specifically, the step-down circuit controller  36  controls multiple switching elements included in the DC-DC converter  35 . The step-down circuit controller  36 , after the internal combustion engine  2  is started, controls on/off operation of the multiple switching elements included in the DC-DC converter  35  to bring the output voltage of the DC-DC converter  35  that is acquired from a voltage detector  37  close to the target voltage suitable for charging of the power storage device  16 , in response to the operation command signal S 2  indicating the power running notch. The step-down circuit controller  36  controls the on/off operation of the multiple switching elements included in the DC-DC converter  35  by sending switching control signals S 6  to the multiple switching elements. The step-down circuit controller  36  controls the on/off operation of the multiple switching elements included in the DC-DC converter  35  as described above, thereby causing the DC-DC converter  35  to step down the voltage of the DC power output by the main converter  12  and to supply the stepped-down DC power to the power storage device  16 . 
     An operation of the driving apparatus  1  having the above-described configuration is described with reference to the timing chart illustrated in  FIG. 9 . Similarly to Embodiment 1, during a period in which the railway vehicle is stopped, the start command signal S 1  is at the L level and the operation command signal S 2  indicates the brake notch B  1 , as illustrated in (A) and (B) of  FIG. 9 . Hereinafter, a timing at which the start command signal S 1  changes from the L level to the H level is referred to as the “time T 1 ”. 
     As illustrated in (C), (D), (E), and (I) of  FIG. 9 , the step-down circuit controller  20 , in response to the start command signal S 1  and the operation command signal S 2 , keeps all of the contactors Q 1 , Q 2 , Q 3 , and Q 4  open until the time T 1 . Then in response to a change at the time T 1  in the start command signal S 1  from the L level to the H level, the step-down circuit controller  20 , while keeping the contactors Q 1  and Q 4  open, closes the contactors Q 2  and Q 3  to charge the filter capacitor  13  using the power storage device  16  as an electric power source. Upon closing of the contactors Q 2  and Q 3 , current flows from the power storage device  16  to the filter capacitor  13  through the contactors Q 3  and Q 2  and the voltage dividing resistor R 1 , as illustrated in  FIG. 10  using a solid arrow. Flow of current to the filter capacitor  13  via the voltage dividing resistor R 1  prevents an inrush current from flowing to the filter capacitor  13 . 
     As current flows from the power storage device  16  to the filler capacitor  13 , the amount of power stored in the secondary battery included in the power storage device  16  gradually decreases from a maximum power amount W 2  as illustrated in (H) of  FIG. 9 , and the both-ends voltage of the filter capacitor  13  gradually increases from a voltage EFC 0  as illustrated in (F) of  FIG. 9 . 
     The step-down circuit controller  20  monitors the both-ends voltage of the filter capacitor  13  using the output signal from the voltage detector  22  and detects, at a time T 2 , reach of the both-ends voltage EFC to the threshold voltage EFC 1 . Then the step-down circuit controller  20  closes the contactor Q 1  while keeping the contactor Q 4  open as illustrated in (C) of  FIG. 9 , in order to start the internal combustion engine  2  using the power storage device  16  as an electric power source. 
     Similarly to Embodiment 1, the converter controller  18 , in response to (i) the start command signal S 1  at the H level, (ii) the operation command signal S 2  indicating the brake notch B  1 , and (iii) the output signal from the voltage detector  22  indicating that the both-ends voltage EFC of the filter capacitor  13  reaches the threshold voltage EFC 1 , starts controlling on/off operation of the multiple switching elements included in the main converter  12 , thereby causing the main converter  12  to convert DC power supplied from the power storage device  16  into AC power and to supply the AC power to the generator  11 . This leads to flow of current from the power storage device  16  to the main converter  12  through the contactors Q 3 , Q 2 , and Q 1 , as illustrated in  FIG. 11  using a solid arrow. 
     The converter controller  18  performs the above-described control, thereby causing the main converter  12  to convert DC power supplied from the power storage device  16  to the secondary terminals into AC power and to supply the AC power from the primary terminals to the generator  11 . This allows the generator  11  to operate as an electric motor and to rotate the internal combustion engine  2  and thus causes, as illustrated in (G) of  FIG. 9 , gradual increase in the engine speed of the internal combustion engine  2  on and after the time T 2 . 
     A timing at which the engine speed of the internal combustion engine  2  reaches the reference engine speed Th 1  is referred to as a “time T 3 ”. In other words, the internal combustion engine  2  is started and starts independent rotation at the time T 3 . 
     The converter controller  18 , when detecting based on the output signal from the speed sensor  4  that the engine speed of the internal combustion engine  2  reaches the reference engine speed Th 1 , controls on/off operation of the multiple switching elements included in the main converter  12 , thereby causing the main converter  12  to convert AC power that the generator  11  driven by the internal combustion engine  2  supplies to the primary terminals into DC power and to output the DC power from the secondary terminals. As illustrated in (F) of  FIG. 9 , the converter controller  18  controls conduction ratios of the multiple switching elements included in the main converter  12  such that the main converter  12  outputs the DC power with the threshold voltage EFC 1 . 
     The step-down circuit controller  20 , when detecting based on the output signal from the speed sensor  4  that the engine speed of the internal combustion engine  2  reaches the reference engine speed Th 1 , opens the contactors Q 1  and Q 2  and closes the contactor Q 4  while keeping the contactor Q 3  closed, as illustrated in (C), (D), (E), and (I) of  FIG. 9 , in order to, on and after a time T 4  described later, (i) step down, using the step-down circuit  17 , the voltage that is output by the main converter  12  from the secondary terminals and is higher than a voltage suitable for charging of the power storage device  16  and (ii) supply power to the power storage device  16  with the stepped-down voltage. The step-down circuit controller  36  controls on/off operation of the switching elements included in the DC-DC converter  35 , thereby charging the power storage device  16  with the output voltage of the DC-DC converter  35 . 
     Thereafter, the power running notch is input from the master controller, and thus the operation command signal S 2  indicates the power running notch N 1 . This timing is referred to as a time T 4 . On and after the time T 4 , the internal-combustion-engine controller  3  controls the internal combustion engine  2  to bring the engine speed of the internal combustion engine  2  close to an engine speed corresponding to the power running notch N 1 , thereby increasing the engine speed of the internal combustion engine  2  as illustrated in (G) of  FIG. 9 . In accordance with the increase in the engine speed of the internal combustion engine  2 , a rotational speed of the generator  11  and the output voltage of the generator  11  increase. 
     Then the converter controller  18 , in response to the operation command signal S 2  indicating the power running notch N 1 , starts performing on/off control operation for the multiple switching elements included in the main converter  12 , in order to bring the output voltage of the main converter  12  close to a fixed voltage corresponding to the power running notch N 1 . 
     The step-down circuit controller  36 , on and after the time T 4 , controls on/off operation of the multiple switching elements included in the DC-DC converter  35 . As a result, the DC-DC converter  35  steps down the voltage of the DC power output by the main converter  12  and supplies the stepped-down DC power to the power storage device  16 . Then the step-down circuit controller  36  controls conduction ratios of the multiple switching elements included in the DC-DC converter  35 , in order to bring the output voltage of the DC-DC converter  35  acquired from the voltage detector  37  close to the target voltage suitable for charging of the power storage device  16 . 
     On and after the time T 4 , the output voltage of the main converter  12  that is, the voltage of the filter capacitor  13 , increases as illustrated in (G) of  FIG. 9 , and the step-down circuit controller  36  controls the multiple switching elements included in the DC-DC converter  35 . As a result, current flows from the main converter  12  to the power storage device  16  through the contactor Q 4 , the DC-DC converter  35 , and the contactor Q 3  to charge the power storage device  16 , as illustrated in  FIG. 12  using a solid arrow. Thus, on and after the time T 4 , the amount of power stored in the power storage device  16  gradually increases from a power amount W 1  to the maximum power amount W 2 . Processing by the DC-DC converter  35  leads to applying of a voltage to the power storage device  16  at, for example, about 300V. In other words, the DC-DC converter  35  steps down the voltage that is output by the main converter  12  and is higher than the voltage suitable for charging of the power storage device  16 , and the power storage device  16  is charged with the voltage suitable for charging of the power storage device  16 . 
     Furthermore, the inverter controller  19  calculates the target torque of the electric motor  15  based on the power running notch N 1  and the rotational speed of the electric motor  15  that is acquired from the speed sensor  24 . Further, the inverter controller  19  calculates the actual torque of the electric motor  15  based on current values that are detected for the currents flowing in the phases of the electric motor  15  and are acquired from the current detector  23 . Further, the inverter controller  19 , in order to bring the actual torque close to the target torque, controls on/off operation of the multiple switching elements included in the first inverter  14 . Thus, the electric motor  15  is driven in response to the operation command signal S 2  on and after the time T 4 , to generate the railway-vehicle motive power. This enables running of the railway vehicle while charging the power storage device  16 . 
     As described above, the drive control device  10  according to Embodiment 4 enables driving of the electric motor  15  after the internal combustion engine  2  is started. 
     Embodiments of the present disclosure are not limited to the aforementioned embodiments. Any of the embodiments described above may be combined. For example, the drive control devices  10  according to Embodiments 3 and 4 may further include the second inverter  31 . Further, the drive control device  10  may further include another inverter that is connected to the secondary terminals of the main converter via a step-up circuit or a step-down circuit. 
     Circuit configurations of the driving apparatus  1  and the drive control device  10  are not limited to the above-described examples, and any circuit configurations may be employed. For example, the step-down circuit  17  may include a variable resistor. Setting a resistance value of the variable resistor after starting of the internal combustion engine  2  sufficiently larger than a resistance value of the variable resistor before starting of the internal combustion engine  2  enables stepping down of the voltage of the DC power output by the main converter  12  and supplying of the stepped-down DC power to the power storage device  16 . Further, any step-down circuit can be arranged as the step-down circuit  17 . For example, a switching regulator may be arranged. Moreover, the resistance values of the voltage dividing resistors R 1  and R 2  in the aforementioned embodiments are examples, and these values may be appropriately determined based on the output voltage of the main converter  12  and characteristics of the power storage device  16 . 
     The control performed by the converter controller  18  is not limited to the above-described example. For example, by feedback of the output current of the main converter  12 , the converter controller  18  may adjust the multiple switching elements included in the main converter  12 . Further, the control performed by the inverter controller  19  is not limited to the above-described example. The speed sensor  24  may be omitted from the driving apparatus  1 , and the inverter controller  19  may acquire the rotational speed of the electric motor  15  from an automatic train control (ATC) device. Moreover, the inverter controller  19  may perform sensorless vector control by estimating the rotational speed of the electric motor  15 . 
     The timings at which the step-down circuit controller  20  closes and opens the contactors Q 1 , Q 2 , and Q 3  are not limited to the above-described examples. For example, the step-down circuit controller  20  may open the contactor Q 3  when the power storage device  16  is sufficiently charged after the time T 4 . In this case, determination as to whether the storage device  16  is sufficiently charged may be made based on an estimated value of an amount of power stored in the power storage device  16  that is estimated using charging/discharging current, a terminal voltage of the secondary battery included in the power storage device  16 , a temperature of the secondary battery, or the like. Further, the step-down circuit controller  20  included in the driving apparatus  1  according to Embodiment 4 may close the contactor Q 4  when the operation command signal S 2  indicates the power running notch. In other words, the step-down circuit controller  20  may close the contactor Q 4  at the time T 4  of  FIG. 9 . 
     Although examples are describe above of detecting a phase current for each of the U-phase, V-phase and W-phase using the current detectors  21 ,  23 , and  24 , phase currents of at least two phases among the U-phase, V-phase and W-phase may be detected. 
     The foregoing describes some example embodiments for explanatory purposes. Although the foregoing discussion has presented specific embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. This detailed description, therefore, is not to be taken in a limiting sense, and the scope of the invention is defined only by the included claims, along with the full range of equivalents to which such claims are entitled. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Railway vehicle driving apparatus 
               2  Internal combustion engine 
               3  Internal-combustion-engine controller 
               4 ,  24  Speed sensor 
               10  Drive control device 
               11  Generator 
               12  Main converter 
               13  Filter capacitor 
               14  First inverter 
               15  Electric motor 
               16  Power storage device 
               17  Step-down circuit 
               18  Converter controller 
               19 ,  32  Inverter controller 
               20 ,  36  Step-down circuit controller 
               21 ,  23 ,  34  Current detector 
               22 ,  37  Voltage detector 
               31  Second inverter 
               33  Load device 
               35  DC-DC converter 
             D 1  Diode 
             Q 1 , Q 2 , Q 3 , Q 4  Contactor 
             R 1 , R 2  Voltage dividing resistor