DESCRIPTION Field [0001] The present invention relates to an AC motor drive system. Background [0002] As a configuration example of AC motor drive systems, there has been an AC motor drive system in which an inverter that converts DC power into AC power having a voltage value and a frequency different from those of a system power supply to drive an AC motor and a charging/discharging circuit for charging and discharging a power storage device, which stores and discharges the DC power, are connected in parallel, via a smoothing capacitor, to a DC bus on the output side of a converter that converts AC power from the system power supply into DC power. [0003] As an example of such an AC motor drive system, for example, Patent Literature 1 discloses a technology for an AC motor drive system that uses, when regenerative power regenerated from an AC motor via an inverter charges a power storage device via a charging/discharging circuit, a predetermined regeneration-time-current-command-value-integral-component initial value for proportional integral control (PI control) of a charging-current-command-value generating unit in the charging/discharging circuit to cope with regenerative power having a steep regeneration initial value. Citation List Patent Literature [0004] Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-239252 Summary Technical Problem [0005-] However, according to the conventional technology, the regeneration-time-current-command-value-integral-component initial value is set to a value close to the allowable current value of the reactor in the charging/discharging circuit. Therefore, charging to the power storage device is started using, as an initial value of the charging current command value at regeneration start time, a charging current of a maximum amount of the AC motor drive system irrespective of the magnitude (the quantity) of the regenerative power. Therefore, when the actual regenerative power is smaller than the maximum regenerative power scheduled by the AC motor drive system, to supplement the regenerative power from the AC motor, the power storage device is charged also by using electric power supplied from the system power supply via the converter. Therefore, there is a problem in that, even during a regenerative operation, the converter performs an operation at the power running time and consumes electric power. [0006] Moreover, according to the conventional technology, every time electric power on the output side of the converter exceeds a predetermined regeneration-time-power compensation threshold, the regeneration-time-current-command-value-integral-component initial value is set in the PI control unit of the charging-current-command-value generating unit. Therefore, there is a problem in that the charging current command value becomes discontinuous and the electric current flowing in the power storage device and the reactor of the charging/discharging circuit greatly changes, thereby reducing the life of the power storage device and the elements of the charging/discharging circuit. [0007] The present invention has been devised in view of the above and it is an object of the present invention to obtain an AC motor drive system capable of generating a charging current command value for a power storage device that copes with steep regenerative power generation and that is in accordance with the magnitude of regenerative power. Solution to Problem [0008] In order to solve the above problems and achieve the object, as aspect of the present invention is an AC motor drive system including: a converter that supplies DC power; an inverter that converts the DC power into AC power; a DC bus that connects the converter and the inverter; an AC motor driven by the AC power; a DC-voltage-value detecting unit that detects a DC voltage value on an output side of the converter; a power storage device that is charged with the DC power from the DC bus and discharges the charged DC power to the DC bus; a charging/discharging circuit connected to the DC bus in parallel with the inverter and connected between the DC bus and the power storage device, the charging/discharging circuit causing the power storage device to be charged and discharge; a charging/discharging-current-value detecting unit that detects a charging/discharging current value of the power storage device; and a charging/discharging control unit that outputs a control signal for controlling the inverter on a basis of the DC voltage value and the charging/discharging current value, wherein when regenerative power from the AC motor via the inverter exceeds a predetermined power threshold, the charging/discharging control unit causes the power storage device to be charged such that the DC voltage value becomes a voltage threshold corresponding to the power threshold and causes a charging current at a start time of charging to the power storage device to start from a charging current value that is based on a DC bus voltage value of the DC bus. Advantageous Effects of Invention [0009] The AC motor drive system according to the present invention has an effect in that it is possible to obtain an AC motor drive system capable of generating a charging current command value for a power storage device that copes with steep regenerative power generation and that is in accordance with the magnitude of regenerative power. Brief Description of Drawings [0010] FIG. 1 is a block diagram illustrating an entire AC motor drive system according to a first embodiment. FIG. 2 is a block diagram illustrating a charging/discharging control unit in the AC motor drive system according to the first embodiment. FIG. 3 is a diagram illustrating temporal changes of electric power P, a DC bus voltage value Vdc, and a regeneration-time-power compensating operation flag Fa in the AC motor drive system according to the first embodiment. FIG. 4 is a diagram schematically illustrating a waveform of a DC bus voltage value Vdc(t) of a DC bus when electric power Pcnv(t) is a negative value in the AC motor drive system according to the first embodiment. FIG. 5 is a diagram illustrating a relation between a power value |Pcnv(t)| and the DC bus voltage value Vdc in the AC motor drive system according to the first embodiment. FIG. 6 is a block diagram illustrating a charging-current-command-value generating unit in a regeneration-time control unit in the AC motor drive system according to the first embodiment. FIG. 7 is a block diagram illustrating a regeneration-time-cur rent-command-value-integral-component generating unit in the charging-current-command-value generating unit in the AC motor drive system according to the first embodiment. FIG. 8 is a block diagram illustrating a regeneration-time- cur rent-command-value-differential -component generating unit in the charging-current-command-value generating unit in the AC motor drive system according to the first embodiment. FIG. 9 is a block diagram illustrating a DC-bus-side-charging-current-command-value output unit in the AC motor drive system according to the first embodiment. FIG. 10 is a diagram illustrating temporal changes of the electric power P and the DC bus voltage Vdc in the AC motor drive system according to the first embodiment. FIG. 11 is a diagram illustrating a configuration example of a regeneration-time-current-command-value-integral-component-initial-value generating unit in the AC motor drive system according to the first embodiment. FIG. 12 is a diagram illustrating temporal changes of regenerative power Pload(t), a DC-bus-side-charging-current command value Hi*, and a regeneration-time-current-command-value differential component value lid* in the AC motor drive system according to the first embodiment. FIG. 13 is a block diagram illustrating an entire AC motor drive system according to a second embodiment. FIG. 14 is a diagram illustrating a relation between the DC bus voltage value Vdc and a regenerative power |Pcnv(t)| of a converter when a capacitance value of a smoothing capacitor is fixed and an AC motor varies an AC voltage value Vac in a regenerative operation in the AC motor drive system according to the second embodiment. FIG. 15 is a block diagram illustrating a charging/discharging control unit in the AC motor drive system according to the second embodiment. Description of Embodiments [0011] Embodiments of an AC motor drive system according to the present invention are explained in detail below with reference to the drawings. Note that the present invention is not limited by the embodiments. [0012] Note that, in this specification, units of physical quantities are clearly described. However, the physical quantities are not limited to the units. An operator |A| represents an absolute value (a positive number) of A. [0013] First Embodiment. FIG. 1 is a block diagram illustrating an entire first embodiment of the AC motor drive system according to the present invention. [0014] An AC motor drive system 1 illustrated in FIG. 1 includes a charging/discharging control unit 2, a converter 11, a smoothing capacitor 13, an inverter 14, a charging/discharging circuit 15, an AC motor 16, a power storage device 17, a DC-voltage-value detecting unit 18, and a charging/discharging-current-value detecting unit 19. [0015] AC power is supplied to the AC motor drive system 1 illustrated in FIG. 1 from a system power supply 10 such as a transformer substation or a transformer facility in a factory via wires R, S, and T. [0016] The converter 11 converts AC power from the system power supply 10 into DC power. The converted DC power is output to a DC bus 12 from the converter 11. Note that the DC bus 12 includes a high-potential-side DC bus 12a and a low-potential-side DC bus 12b. [0017] The smoothing capacitor 13 is disposed in one or a plurality of places among the output portion of the converter 11, a portion on the DC bus 12, the input portion of the inverter 14 explained below, and a portion on the DC bus 12 side of the charging/discharging circuit 15 explained below. The smoothing capacitor 13 smoothes DC power between the high-potential-side DC bus 12a and the low-potential-side DC bus 12b. The capacitance of the smoothing capacitor 13 is represented as C [F]. [0018] The DC power smoothed by the smoothing capacitor 13 is output to the inverter 14 and the charging/discharging circuit 15 via the DC bus 12. The inverter 14 and the charging/discharging circuit 15 are connected to the DC bus 12 in parallel. [0019] The inverter 14 converts DC power into AC power and drives the AC motor 16. The voltage value and the frequency of the AC power output from the inverter 14 are different from the voltage value and the frequency of the AC power supplied from the system power supply 10. [0020] The charging/discharging circuit 15 is a circuit that stores DC power flowing in the DC bus 12 in the power storage device 17 and discharges electric power stored in the power storage device 17 to the DC bus 12. As the charging/discharging circuit 15, a current reversible chopper circuit can be exemplified. When the charging/discharging circuit 15 is the current reversible chopper circuit, electric power flowing in the DC bus 12 is stored as a charging current to the power storage device 17. Conversely, the electric power stored in the power storage device 17 is discharged as a discharging current to the DC bus 12. Note that, in the following explanation, when an electric current flowing to the power storage device 17 is represented without distinguishing between, the charging current and the discharging current, the electric current is described as charging/discharging current. [0021] In the charging/discharging circuit 15, the current reversible chopper circuit is controlled by a control signal from the charging/discharging control unit 2 and the charging/discharging circuit 15 controls the amount of charging/discharging current. A DC bus voltage value Vdc of the DC bus 12 detected by the DC-voltage-value detecting unit 18 and a charging/discharging current value Ic detected by the charging/discharging-current-value detecting unit 19 are input to the charging/discharging control unit 2 as observation values. The charging/discharging control unit 2 outputs a control signal to the charging/discharging circuit 15. [0022] As the converter 11, a resistance regeneration-type converter in which a resistance regeneration circuit is added to a three-phase full-wave rectifier circuit or a power supply regeneration-type converter in which switching elements are respectively connected in anti-parallel with diodes from which a three-phase full-wave rectifier circuit is configured and an AC reactor is inserted in series on the input side can be exemplified. [0023] First, an explanation will be given of a case when the converter 11 is the resistance regeneration-type converter. In the resistance regeneration-type converter, when the AC motor 16 decelerates or stops and regenerative power is generated, the regenerative power is stored in the smoothing capacitor 13 via the inverter 14 and increases the voltage value of the DC bus 12. When the voltage value of the DC bus 12 increases to a voltage higher than a predetermined short-circuit start voltage value, the resistance regeneration circuit short-circuits the high-potential-side DC bus 12a and the low-potential-side DC bus 12b via the resistor in the resistance regeneration-type converter and converts the energy stored in the smoothing capacitor 13 into heat in the resistor. Thereafter, electric charges stored in the smoothing capacitor 13 as a result of the short circuit are discharged. Therefore, when the voltage value of the DC bus 12 decreases to a voltage lower than a predetermined short-circuit end voltage value, the high-potential-side DC bus 12a and the low-potential-side DC bus 12b short-circuited by the resistance regeneration circuit are disconnected. When the converter 11 is the resistance regeneration-type converter, the converter 11 repeats such an operation to consume the regenerative power. [0024] Next, an explanation will be given of a case when the converter 11 is the power supply regeneration-type converter. In the power supply regeneration-type converter, when the voltage value of the DC bus 12 increases to a voltage higher than a predetermined regeneration start voltage value due to the regenerative power, the switching elements in the power supply regeneration-type converter become a conduction state for a predetermined period by a control circuit in the power supply regeneration-type converter according to the phase of the waveform of the system power supply 10. Electric charges stored in the smoothing capacitor 13 are regenerated to the system power supply 10 via the AC reactor in the power supply regeneration-type converter. The regenerative operation to the system power supply 10 is continued until the voltage value of the DC bus 12 decreases to a voltage lower than the predetermined regeneration end voltage value. The regenerative power generated by the AC motor 16 is regenerated to the system power supply 10 by the regenerative operation. [0025] FIG. 2 is a block diagram illustrating the charging/discharging control unit 2 in the AC motor drive system 1. The charging/discharging control unit 2 illustrated in FIG. 2 includes a power-running-time control unit 21, a regeneration-time control unit 3, a current-command-value integrating unit 22, and a control-signal generating unit 23. [0026] The power-running-time control unit 21 receives, as an input, the DC bus voltage value Vdc (detected by the DC-voltage-value detecting unit 18) of the DC bus 12, the voltage of which has dropped due to the power running operation of the AC motor 16, and outputs a power-storage-device-side discharging current command value lb*, which is a command value for controlling a discharging current for discharging from the power storage device 17, and a power-running-time-power compensating operation flag Fb for determining a period during which the power storage device 17 is caused to discharge. [0027] The regeneration-time control unit 3 receives, as an input, the DC bus voltage value Vdc (detected by the DC-voltage-value detecting unit 18) of the DC bus 12, the voltage of which has risen due to the regenerative operation of the AC motor 16 and outputs a power-storage-device-side charging current command value la*, which is a command value for controlling a charging current for charging the power storage device 17, and a regeneration- time-power compensating operation flag Fa for determining a period during which the power storage device 17 is charged. [0028] The current-command-value integrating unit 22 generates an integrated charging/discharging current command value Ic*, which is a command value of a charging/discharging current of the power storage device 17, by using the power-storage-device-side charging current command value Ia* and the power-storage-device-side discharging current command value Ib*. [0029] The control-signal generating unit 23 reduces the difference between the integrated charging/discharging current command value Ic* and the charging/discharging current value Ic to be finally eliminated by using the integrated charging/discharging current command value Ic* from the current-command-value integrating unit 22 and the charging/discharging current value Ic of the power storage device 17 from the charging/discharging-current-value detecting unit 19. The control-signal generating unit 23 generates a control signal for controlling the charging/discharging circuit 15 in a period of the power-running-time-power compensating operation flag Fb from the power-running-time control unit 21 or the regeneration-time-power compensating operation flag Fa from the regeneration-time control unit 3. [0030] FIGS. 3(a) to 3(c) are diagrams illustrating temporal changes of the electric power P, the DC bus voltage value Vdc, and the regeneration-time-power compensating operation flag Fa. In FIG. 3(a), a temporal change of regenerative power Pload(t) regenerated from the AC motor 16 via the inverter 14 is indicated by a thick line. One of the functions of the AC motor drive system 1 illustrated in FIG. 1 is to charge, with respect to the regenerative power Pload(t), the power storage device 17 with electric power indicated on the vertical axis in a portion indicated by a lattice pattern in FIG. 3(a), i.e., charging power |Pc(t)|, to thereby suppress electric power regenerated in the converter 11 such that it does not exceed a power threshold PthA illustrated in FIG. 3(a) so as to limit the peak of the electric power converted into heat and consumed by the converter 11 or the electric power regenerated to the system power supply 10. [0031] The regenerative power Pload(t) indicated by the thick line in FIG. 3(a) is a schematic example of a waveform generated when the AC motor 16 stops or performs a quick deceleration operation. In FIG. 3(a), power running power of the AC motor 16 is represented by a positive number and regenerative power is represented by a negative number. Charging power and a charging current to the power storage device 17 are represented by positive numbers and discharging power and a discharging current are represented by negative numbers. [0032] Electric power Pcnv(t) in a portion indicated by hatching in FIG. 3(a) is defined by the following Formula (1) . [0033] Pcnv(t)=Pload(t)-Pc(t) (1) [0034] The electric power Pcnv(t) represents electric power on the DC bus 12 side of the converter 11. When the electric power Pcnv(t) is a positive number value, this indicates that the converter 11 converts electric power and outputs the electric power from the system power supply 10 to the DC bus 12 by a power value |Pcnv(t)|. Conversely, when the electric power Pcnv(t) is a negative number value, this indicates that the converter 11 converts electric power into heat and consumes the electric power from the DC bus 12 by the power value |Pcnv(t)| or regenerates the electric power to the system power supply 10. [0035] When the electric power Pcnv(t) is a negative value and the converter 11 is the resistance regeneration-type converter, as explained above, while the DC bus voltage value Vdc(t) of the DC bus 12 fluctuates between the short-circuit start voltage value and the short-circuit end voltage value, the electric power Pcnv(t) is consumed in the resistor in the converter 11. [0036] When the electric power Pcnv(t) is a negative value and the converter 11 is the power supply regeneration-type converter, as explained above, while the DC bus voltage value Vdc(t) of the DC bus 12 fluctuates between the regeneration start voltage value and the regeneration end voltage value, the electric power Pcnv(t) is regenerated to the system power supply 10 via the AC reactor in the converter 11. [0037] FIG. 4 is a diagram schematically illustrating a waveform of the DC bus voltage value Vdc(t) of the DC bus 12 when the electric power Pcnv(t) is a negative value. FIG. 4(a) illustrates a waveform when the power value |Pcnv(t)| is relatively large. FIG. 4(b) illustrates a waveform when the power value |Pcnv(t)| is relatively small. In FIG. 4(a) and FIG. 4(b), a DC bus voltage value Vdc indicated by a thick broken line is a time average value of the DC bus voltage value Vdc(t). For example, the DC bus voltage value Vdc can be obtained by causing the DC bus voltage value Vdc(t) to pass through a low pass filter (LPF). The DC-voltage-value detecting unit 18 detects the DC bus voltage value Vdc(t). [0038] When FIG. 4(a) and FIG. 4(b) are compared, the DC bus voltage value Vdc, which is the time average value, is high when the power value |Pcnv(t)| is relatively large. The DC bus voltage value Vdc, which is the time average value, is low when the power value |Pcnv(t)| is relatively small. The waveform of the DC bus voltage value Vdc(t) is formed by charging of the electric power Pcnv(t) to the smoothing capacitor 13 and discharging from the smoothing capacitor 13 to the converter 11. Therefore, the DC bus voltage value Vdc depends on not only the power value |Pcnv(t)| but also the capacitance value C of the smoothing capacitor 13. [0039] A transfer function of the low pass filter is combined with a transfer function of a charging-current-command-value generating unit 4 explained below. Therefore, in characteristics after the combination, attention should be paid to the stability of the AC motor drive system 1. In general, as the transfer function of the low pass filter, a'lower-order characteristic is preferable to ensure a degree of freedom of the transfer function of the charging-current-command-value generating unit 4. If a desired DC bus voltage value Vdc can be obtained by a primary low pass filter, it is preferable to adopt the primary low pass filter. [0040] Note that, in FIG. 4(a) and FIG. 4(b), the DC bus voltage value Vdc(t) falls within a range between the short-circuit start voltage value (or the regeneration start voltage value) and the short-circuit end voltage value (or the regeneration end voltage value). However, it is noted that, in the actual operation, the DC bus voltage value Vdc(t) is sometimes outside the range according to the limitation on the operation speed and the temporal relation with the phase of the system power supply 10. [0041] FIG. 3 referred to above and FIG. 10 referred to below illustrate that the DC bus voltage value Vdc during the regenerative operation increases such that it becomes larger than the DC bus voltage value before the regenerative operation. However, as it is evident from the above explanation, the DC bus voltage value Vdc during the regenerative operation is determined on the basis of the correlation between the short-circuit start voltage value (or the regeneration start voltage value) and the short-circuit end voltage value (or the regeneration end voltage value). That is, when the short-circuit start voltage value (or the regeneration start voltage value) is slightly higher than the DC voltage value before the regenerative operation and, on the other hand, the short-circuit end voltage value (or the regeneration end voltage value) is substantially lower than the DC bus voltage value before the regenerative operation, the DC bus voltage value Vd during the regenerative operation decreases such that it becomes smaller than the DC bus voltage value before the regenerative operation. [0042] FIG. 5 is a diagram illustrating a relation between the power value |Pcnv(t)| and the DC bus voltage value Vdc. As explained above, the relation between the power value |Pcnv(t)| and the DC bus voltage value Vdc when the capacitance value of the smoothing capacitor 13 is C is indicated by a thick solid line illustrated in FIG. 5(a). Similarly, the relations when the capacitance value of the smoothing capacitor 13 is C1 and C2 are indicated by broken lines illustrated in FIG. 5(a). [0043] In general, in FIG. 5(a), the relation of CKC Imax) [0075] The second switching unit 464 outputs, in response to the inputs of the output value LI1i output from the second limiter 463 and the regeneration-time-current-command-value integral component initial value Iinit output from the regeneration-time-current-command-value-integral-component-initial-value generating unit 42, a selection result Hi*, which is a value obtained by carrying out selection indicated by the following Formula (7), by using the regeneration-time-power-compensating-operation start signal Sa, which is the output of the regeneration-time- power-compensating-operation control unit 5. [0076] [0077] The selection result I1i* is output to the first delay unit 465 and the DC-bus-side-charging-current-command-value output unit 48. [0078] The first delay unit 465 delays the input value by one unit of a control time interval and outputs the input value. The result obtained by delaying the selection result I1i*, which is the output value of the second switching unit 4 64, by one unit of the control time interval by the first delay unit 4 65, is an output value ZI1i*. The processing represented by the above Formula (5) is executed by the first two-input adder 462, whereby an integral function for the multiplication value Ki-I1pp output from the second multiplier 461 is realized. That is, the selection result Hi* output from the second switching unit 464 is a regeneration-time-current-command-value integral component value. [0079] The regeneration-time-current-command-value-integral-component generating unit 46 illustrated in FIG. 7 includes the configuration explained above. Therefore, a regeneration-time-current-command-value integral component value I1i* retains a value of 0 before regeneration-time-power-compensating-operation start time by the first switching unit 44. An integral operation is started from the regeneration-time-current-command-value integral component initial value Iinit at the regeneration-time-power-compensating-operation start time by the second switching unit 4 64. The maximum of the regeneration-time-current-command-value integral component value I1i* is prevented from exceeding the current limit value Imax by the second limiter 463. Note that the constant Ki, which is the integral gain, is a value including a factor due to a control time interval. [0080] FIG. 8 is a block diagram illustrating the regeneration-time-current-command-value-differential-component generating unit 47 in the charging-current-command-value generating unit 4. The regeneration-time-current-command-value-differential-component generating unit 47 illustrated in FIG. 8 includes a second delay circuit 471, a second subtractor 472, a third multiplier 473, and a third limiter 474. The multiplication value Kp-ErrA, which is output from the first multiplier 43 and is input to the regeneration-time-current-command-value-differential-component generating unit 47, is input to the second delay unit 471 and the minuend terminal of the second subtractor 472. [0081] The second delay unit 471 delays the input by one unit of a control time interval and outputs the input. The result obtained by delaying the multiplication value Kp-ErrA, which is output from the first multiplier 43, by one unit of the control time interval by the second delay unit 471 is output as an output value ZKpEr. The output value ZKpEr of the second delay unit 471 is input to the subtrahend terminal of the second subtractor 472. [0082] The second subtractor 472 outputs a subtraction value DifKpEr defined by the following Formula (8) to the third multiplier 473. [0083] DifKpEr=Kp-ErrA-ZKpEr (8) [0084] The third multiplier 473 generates a multiplication value Ildp obtained by multiplying the subtraction value DifKpEr by a predetermined constant Kd, which is a differential gain, and outputs the multiplication value I1dp to the third limiter 474. [0085] The third limiter 474 performs processing represented by the following Formula (9) on the multiplication value Ildp on the basis of the value 0 and the current limit value Imax and outputs the multiplication value Ildp to the DC-bus-side-charging-current-command-value output unit 48. [0086] f 0 (in the case of Ildp < 0) l1d* < Ilpp (in the case of 0 < Ildp < Imax) (9) ( Imax (in the case of Ildp > Imax) [0087] The processing represented by Formula (8) is carried out by the second subtractor 472, whereby a differential function for the multiplication value Kp-ErrA output from the first multiplier 43 is realized. Therefore, the output of the third limiter 474 becomes the regeneration-time-current-command-value differential component value lid*. [0088] The regeneration-time-current-command-value-differential-component generating unit 47 includes the configuration explained above. Therefore, the maximum of the regeneration-time-current-command-value differential component value l1d* is prevented from exceeding the current limit value Imax by the third limiter 474. Note that the constant Kd, which is the differential gain, is a value including a factor due to a control time interval. [0089] FIG. 9 is a block diagram illustrating the DC-bus-side-charging-current-command-value output unit 48. The DC-bus-side-charging-current-command-value output unit 48 illustrated in FIG. 9 includes a three-input adder 481, a fourth limiter 482, and a third switching unit 483. [0090] The three-input adder 481 outputs, to the fourth limiter 482, a sum l1c* of the regeneration-time-current-command-value proportional component value I1p* output from the first limiter 45, the regeneration-time-current-command-value integral component value I1i* output from the regeneration-time-current-command-value-integral-component generating unit 46, and the regeneration-time-current-command-value differential component value l1d* output from the regeneration-time-current-command-value-differential-component generating unit 47. [0091] The fourth limiter 482 outputs an output value LIlc*. The output value LIlc* is 0 when the sum He* is a negative value, is the current limit value Imax when the sum I1c* exceeds the current limit value Imax in the AC motor drive system 1, and is a value the same as the input value when the sum I1c* is a positive value and is the current limit value Imax or less. The output value LIlc* output from the fourth limiter 482 can be represented by the following Formula (10). [0092] [0093] The third switching unit 483 generates, by using the regeneration-time-power compensating operation flag Fa, a DC-bus-side-charging-current command value I1* defined by the following Formula (11) and outputs the DC-bus-side-charging-current command value I1*. [0094] [0095] The third switching unit 483 outputs the output value LIlc* as the DC-bus-side-charging-current command value I1* in a period during which the regeneration-time-power compensating operation flag Fa indicates valid and outputs 0 in the other periods. The DC-bus-side-charging-current command value I1* of the third switching unit 483 is output to the charging-current-command-value converting unit 7. [0096] A charging current value Is to the power storage device 17 at the regenerative operation initial time can be represented by the following Formula (12-1) by using maximum regenerative power Pmax at the regenerative operation initial time illustrated in FIG. 3(a). [0097] Is-VthA=|Pmax|-|PthA| (12-1) [0098] As explained above, the relation illustrated in FIG. 5(b) is present between the DC bus voltage value Vdc of the DC bus 12 and the regenerative power. The relation when the capacitance value of the smoothing capacitor 13 is C, i.e., indicated by a thick solid line in FIG. 5(b) is represented by a function fc(Vdc). When the DC bus voltage value assumed to be the maximum regeneration power Pmax in the function fc(Vdc) is defined as a maximum DC bus voltage value Vmax, the relation of the following Formula (12-2) holds. [0099] |Pmax|=fc(Vmax) (12-2) [0100] Formula (12-1) can be transformed into the following Formula (12-3) according to the above Formula (12-2). [0101] Is=(l/VthA)fc(Vmax)-|PthA|/VthA (12-3) [0102] In the formula, 1/VthA and -|PthA|/VthA are respectively constants, values of which are known in advance. Therefore, when these values are respectively defined by the following Formulas (12-4) and (12-5), the above Formula (12-3) can be represented by the following Formula (12-6). [0103] a=l/VthA (12-4) [0104] b=-|PthAI/VthA (12-5) [0105] Is=a.fc(Vmax)+b (12-6) [0106] However, in the AC motor drive system 1, a peak of the regenerative power is suppressed by a charging operation to the power storage device 17. Therefore, even if the DC bus voltage value Vdc, which is the output of the DC-voltage-value detecting unit 18, is observed, the value of the maximum DC bus voltage value Vmax cannot be obtained. Therefore, the maximum DC bus voltage value Vmax is estimated from the observable DC bus voltage value Vdc. Pmaxl, Pmax2, Vmaxl, and Vmax2, for which the relations of the following Formula (12-7) and Formula (12-8) hold from the above Formula (12-2), are respectively defined. However, it is assumed that Formula (12-9) holds between Pmaxl and Pmax2. [0107] IPmaxl|=fc(Vmaxl) (12-7) [0108] IPmax2|=fc(Vmaxl) (12-8) [0109] Pmaxl>Pmax2 (12-9) [0110] FIGS. 10(a-l) to 10(b-2) are diagrams illustrating temporal changes of the electric power P and the DC bus voltage Vdc. As indicated by a broken line in FIG. 10(a), even in a change at regenerative operation start time of steep regenerative power generated, for example, when the AC motor 16 suddenly stops, a delay occurs in the actual regenerative power as indicated by a thick solid line in FIG. 10(a) because of a factor such as impedance or inductance of the inverter 14 or the DC bus 12. The rate of change of the actual regenerative power immediately after the regenerative operation start is steeper as the maximum regenerative power Pmax is larger. That is, a change in regenerative power in one unit of a control time interval immediately after the regenerative operation start indicated by AtO in FIG. 10(a-l) and FIG. 10(a-2) is larger when the maximum regenerative power Pmax is Pmaxl than when the maximum regenerative power Pmax is Pmax2. APmaxl in FIG. 10(a-l) is larger than APmax2 in FIG. 10(a-2). [0111] Accordingly, a change in the DC bus voltage value Vdc in one unit of the control time interval immediately after the regenerative operation start indicated by AtO in FIGS. 10(b-l) and 10(b-2) is also larger when the maximum regenerative power Pmax is Pmaxl than when the maximum regenerative power Pmax is Pmax2. ∆Vdcl in FIG. 10(b-l) is larger than AVdc2 in FIG. 10(a-2). [0112] Therefore, a unique relation is present between the maximum DC-bus voltage value Vmax and the change AVdc of the DC bus voltage value Vdc in one unit of the control time interval. This relation is defined by a function g(AVdc) indicated by the following Formula (13). [0113] Vmax=g(AVdc) (13) [0114] When the above Formula (13) is substituted in Formula (12-6), the following Formula (14) is obtained. A function of generating the charging current value Is represented by the following Formula (14) is a function of the regeneration-time-current-command-value-integral-component-initial-value generating unit 42. [0115] Is=a.fc(g(∆Vdc) )+b (14) [0116] However, the regeneration-time-current-command-value-integral—component—initial—value generating unit 42 operates not only at the regenerative operation start time but also at entire operation time of the AC motor drive system 1. Therefore, the left side of the above Formula (14) is preferably the regeneration-time-current-command-value integral component initial value Iinit, which is a candidate value of a regeneration-time-current-command-value integral component initial value, as indicated by the following Expression (15) rather than the charging current value Is at the regeneration operation initial time. The regeneration-time-current-command-value integral component initial value Iinit changes to a regeneration-time-current-command-value integral component initial value at the time when the regeneration-time-power-compensation start signal Sa becomes valid in the second switching unit 464 in the regeneration-time-current-command-value-integral-component generating unit 46. [0117] Iinit=a-fc(g(AVdc))+b (15) [0118] FIGS. 11(a) to 11(c) are block diagrams illustrating configuration examples of the regeneration-time-cur rent-command- value-integral -component- initial -value generating unit 42. FIG. 11(a) illustrates a block diagram of a regeneration-time-current-command-value-integral-component-initial-value generating unit 42a. The regeneration-time-current-command-value-integral-component-initial-value generating unit 42a includes a third subtractor 421, a third delay unit 422, a AVdc/Vmax conversion unit 423, a Vmax/|Pmax| conversion unit 424, a fourth multiplier 425, a constant-b storing unit 426, and a second two-input adder 427. The DC bus voltage value Vdc, which is the output of the DC-voltage-value detecting unit 18, is input to the minuend terminal of the third subtractor 421 and the third delay unit 422. [0119] The third delay unit 422 delays the input by one unit of the control time interval and outputs the input. The result obtained by delaying the DC bus voltage value Vdc by one unit of the control time interval by the third delay unit 422 is an output value ZVdc. The output value ZVdc of the third delay unit 422 is input to the subtrahend terminal of the third subtractor 421. [0120] The third subtractor 421 generates a value AVdc obtained by subtracting ZVdc from Vdc and outputs the value AVdc. AVdc is input to the AVdc/Vmax conversion unit 423. The AVdc/Vmax conversion unit 423 realizes the correspondence relation indicated by the above Formula (13), for example, through reading of an LUT or calculation by using an approximation formula and outputs the estimation value of the maximum DC bus voltage value Vmax. The maximum DC bus voltage value Vmax, which is the output of the AVdc/Vmax conversion unit 423, is input to the Vmax/|Pmax| conversion unit 424. [0121] The Vmax/|Pmax| conversion unit 424 realizes the correspondence relation indicated by the above Formula (12-2), for example, through reading of an LUT or calculation by using an approximation formula and outputs an absolute value |Pmax| of maximum regenerative power. The absolute value |Pmax| of the maximum regenerative power, which is the output of the Vmax/|Pmax| conversion unit 424, is input to the fourth multiplier 425. [0122] The fourth multiplier 425 multiplies the input absolute value |Pmax| of the maximum regenerative power by a constant "a" indicated by the above Formula (12-4) and outputs the obtained value. The output value is input to one input end of the second two-input adder 427. A constant "b" is input to the other input end of the second two-input adder 427 from the constant-b storing unit 426 that stores the constant "b" indicated by the above Formula (12-5) . [0123] The second two-input adder 427 sums the output of the fourth multiplier 425 and the output of the constant-b storing unit 426 and outputs the regeneration-time-current-command-value integral component initial value Iinit indicated by the above Formula (15) to the second switching unit 464 (FIG. 7) in the regeneration-time-current-command-value-integral-component generating unit 46. [0124] FIG. 11(b) illustrates a block diagram of a regeneration-time-current-command-value-integral-component-initial-value generating unit 42b. The regeneration-time-current -command-value -integral -component -initial -value generating unit 42b has a configuration in which the AVdc/Vmax conversion unit 423 and the Vmax/|Pmax| conversion unit 424 illustrated in FIG. 11(a) are integrated and that is realized by a Avdc/|Pmax| conversion unit 428 that realizes the correspondence relation from AVdc to |Pmax|, which is a complex function fc(g(AVdc)), for example, through reading of an LUT or calculation by using an approximation formula and outputs |Pmax|, which is the absolute value of the maximum regeneration power Pmax. [0125] FIG. 11(c) illustrates a block diagram of a regeneration-time-current-command-value-integral-component-initial-value generating unit 42c. The regeneration-time-cur rent-command-value-integral -component-initial -value generating unit 42c has a configuration in which the AVdc/Vmax conversion unit 423 illustrated in FIG. 11(a), the Vmax/|Pmax| conversion unit 424 illustrated in FIG. 11(a), the fourth multiplier 425 illustrated in FIGS. 11(a) and 11(b), the constant-b storing unit 426 illustrated in FIGS. 11(a) and 11(b), and the second two-input adder 427 illustrated in FIG. 11(a) and 11(b) are integrated and that is realized by a AVdc/Iinit conversion unit 429 that collectively realizes the correspondence relation of the above Formula (15), for example, through reading of an LUT or calculation by using an approximation formula and outputs the regeneration-time-current-command-value integral component initial value Iinit from AVdc in FIGS. 11 (a) and 11(b) . [0126] The charging-current-command-value generating unit 4 is configured as explained above. Therefore, it is possible to calculate the DC-bus-side-charging-current command value I1*, which is a charging current command value from the smoothing capacitor 13, i.e., a DC-bus-side-charging-current command value by adopting a value based on the DC bus voltage value Vdc and a regenerative operation start time differential value as an integral component initial value of proportional integral and differential control (PID control) and using the voltage threshold VthA during regeneration as a command value and using an observation value as the DC bus voltage value Vdc. [0127] By subjecting the charging-current-command-value generating unit 4 to the PID control and introducing an integral component initial value, for generation of steep regenerative power from the AC motor 16, it is possible to calculate a DC-bus-side-charging-current command value with high responsiveness according to the magnitude of the regenerative power. [0128] The DC-bus-side-charging-current command value I1*, which is the output of the charging-current-command-value generating unit 4, is generated by the DC bus voltage value Vdc o f the DC bus 12 and the voltage threshold VthA, which is a command value for the DC bus 12. Therefore, the DC-bus-side-charging-current command value I1* is a current command value on the DC bus 12 side of the charging/discharging circuit 15. On the other hand, for the generation of the control signal, which is the output of the charging/discharging control unit 2, the charging/discharging current value Ic, which is the output of the charging/discharging-current detecting unit 19, is used as the observation value. Therefore, a command value for the charging/discharging current value Ic needs to be a current command value on the power storage device 17 side of the charging/discharging circuit 15. [0129] If a loss of the charging/discharging circuit 15 is regarded as small and neglected and a voltage value across both ends of the power storage device 17 is represented as Vcap, the relation of the following Formula (16-1) holds between the DC-bus-side-charging-current command value I1* of the charging/discharging circuit 15 and a power-storage-device-side-charging-current command value Ia*. [0130] Il*.Vdc=Ia*.Vcap (16-1) [0131] During regenerative power compensation, the DC bus voltage value Vdc of the above Formula (16-1) is controlled to the voltage threshold VthA during regeneration. Therefore, the above Formula (16-1) changes to the following Formula (16-2). [0132] Ia*=(VthA/Vcap)-Il* (16-2) [0133] In the above Formula (16-2), it is necessary to always observe the both-end voltage value Vcap of the power storage device 17 and execute a division. To omit a detecting unit for the both-end voltage value Vcap of the power storage device 17 and omit the division with complicated calculation, the both-end voltage value Vcap of the power storage device 17 is substituted by a predetermined substitute both-end voltage value Vcfix. When the substitute both-end voltage value Vcfix is used, the above Formula (16-2) changes to the following Formula (16-3) . [0134] Ia*=(VthA/Vcfix)-Il* (16-3) [0135] The substitute both-end voltage value Vcfix is not particularly limited. However, for example, it is satisfactory if a minimum that the both-end voltage value Vcap of the power storage device 17 can take is used. When the substitute both-end voltage value Vcfix is set as the minimum of the both-end voltage value Vcap, the power-storage-device-side-charging-current command value la* is a value larger than an original value thereof. However, the power-storage-device-side-charging-current command value la* sufficiently functions as a power-storage-device-side-charging-current command value according to a loss of the charging/discharging circuit 15 and a feedback function of the PID control of the charging-current-command-value generating unit 4. [0136] Therefore, the charging-current-command-value converting unit 7 in the regeneration-time control unit 3 includes, in the charging-current-command-value converting unit 7, a substitute-both-end-voltage-value storing unit that stores an inverse 1/Vcfix of the predetermined substitute both-end voltage value Vcfix. The charging-current-command-value converting unit 7 calculates a product of three values, i.e., the inverse, the DC-bus-side-charging-current command value I1* input from the charging-current-command-value generating unit 4, and the regeneration-time-voltage threshold VthA input from the regeneration-time-power/voltage conversion unit 6 (the above Formula (16-3)) and generates the power-storage-device-side-charging-current command value la*. The power-storage-device-side-charging-current command value la*, which is the output of the charging-current-command-value converting unit 7, is output to the current-command-value integrating unit 22. [0137] The AC motor drive system in the present embodiment explained above includes a converter that supplies DC power; an inverter that converts the DC power into AC power; a DC bus that connects the converter and the inverter; an AC motor driven by the AC power; a DC-voltage-value detecting unit that detects a DC voltage value on an output side of the converter; a power storage device that is charged with the DC power from the DC bus and discharges the charged DC power to the DC bus; a charging/discharging circuit connected to the DC bus in parallel with the inverter and connected between the DC bus and the power storage device, the charging/discharging circuit causing the power storage device to be charged and discharge; a charging/discharging-current-value detecting unit that detects a charging/discharging current value of the power storage device; and a charging/discharging control unit that outputs a control signal for controlling the inverter on a basis of the DC voltage value and the charging/discharging current value. When regenerative power from the AC motor via the inverter exceeds a predetermined power threshold, the charging/discharging control unit causes the power storage device to be charged such that the DC voltage value becomes a voltage threshold corresponding to the power threshold and causes a charging current at a start time of charging to the power storage device to start from a charging current value that is based on a DC bus voltage value of the DC bus. [0138] Moreover, it is satisfactory if the charging current value at a start time of charging to the power storage device is based on an amount of change of the DC voltage value at a start time of charging. [0139] Further, it is satisfactory if the charging/discharging control unit includes an integral control unit, a proportional integral control unit, or a proportional integral and differential control unit corresponding to the DC voltage value and the voltage threshold, and at a start time of charging to the power storage device, the charging/discharging control unit sets an integral component in the integral control unit, the proportional integral control unit, or the proportional integral and differential control unit to a value corresponding to the DC voltage value at the start time of charging. [0140] The AC motor drive system in the present embodiment has effects explained below. Note that FIGS. 12(a) to 12(c) are respectively diagrams illustrating temporal changes of the regenerative power Pload(t), the DC-bus-side-charging current command value Hi*, and the regeneration-time-current-command-value differential component value lid*. [0141] First, according to introduction of the regeneration-time-current-command-value integral component initial value Iinit, at generation start time of steep regenerative power, whereas the regeneration-time-current-command-value integral component value I1i* having a delayed response as indicated by a broken line in FIG. 12(b) has to be generated in the conventional configuration, in the AC motor drive system in the present embodiment, it is possible to obtain the regeneration-time-current-command-value integral component value I1i* with a quick response as indicated by a solid line in FIG. 12(b) and thus it is possible to obtain a control signal with high responsiveness. [0142] Second, the regeneration-time-current-command-value integral component initial value Unit is generated as a value corresponding to the DC-bus-side-charging-current command value I1* at the regenerative operation start time. Therefore, because the use of a regeneration-time-current-command-value-integral-component initial value that is an unnecessarily large value can be prevented, it is possible to prevent unnecessary power supply from the system power supply at the regenerative operation start time. [0143] Third, because the regeneration-time-current-command- value-integral -component-initial -value generating unit 42 is always operating, it is possible to prevent a large change from occurring in the regeneration-time-current-command-value integral component value Hi* even if the regeneration-time-power-compensating-operation start signal Sa becomes valid at time other than immediately after the regenerative operation start time and the regeneration-time-current-command-value integral component initial value Unit is replaced by the regeneration-time-current-command-value integral component value Hi* and thus it is possible to obtain a control signal with high continuity. Therefore, it is possible to extend the life of the power storage device 17 and the reactor element in the charging/discharging circuit 15. [0144] Fourth, in the generation of the regeneration-time-current-command-value integral component initial value Unit of the regeneration-time-current-command-value-integral-component-initial-value generating unit 42, only the DC bus voltage value Vdc is used as the observation value. Thus, the current-value detecting unit of the DC bus 12 in which a large current flows is unnecessary. Therefore, it is possible to reduce the costs of the AC motor drive system, save resources by reducing the capacitance and eliminating the attachment member, and avoid a risk of uncontrollability due to the magnetic flux saturation of the current-value detecting unit. [0145] Fifth, the configuration is such that the input to the regeneration-time-current-command-value-differential-component generating unit 47 is performed not via the first switching unit 44. Accordingly, for the generation of the regeneration-time-current-command-value differential component value I1d*, it is unnecessary to wait for the generation of the regeneration-time-power compensating operation flag Fa of the regeneration-time-power-compensating-operation control unit 5. Thus, it is possible to generate the regeneration-time-current-command-value differential component value I1d* immediately after the start of generation of electric power during regeneration. Therefore, it is possible to generate an effective control signal immediately after the start of a regeneration compensating operation. [0146] Note that the contribution of the regeneration-time-current-command-value differential component value l1d* (FIG. 12(c)) to the DC-bus-side-charging-current command value I1*(a thick solid line in FIG. 12(b)) is limited and small. Therefore, with a configuration in which the regeneration-time-current-command-value-differential-component generating unit 47 is excluded from the charging-current-command-value generating unit 4, it is possible to obtain the AC motor drive system that has the first to fourth effects described above. However, the three-input adder 481 in the DC-bus-side-charging-current-command-value output unit 48 in this case is replaced by a two-input adder. [0147] Further, within a range in which a steady error is allowed, even when both the first limiter 45 and the regeneration-time-current-command-value-differential-component generating unit 47 are omitted from the charging-current-command-value generating unit 4, it is still possible to obtain the AC motor drive system that has the first to fourth effects described above. However, in this case, the three-input adder 481 in the DC-bus-side-charging-current-command-value output unit 48 is also omitted. [0148] Note that, in FIG. 1, it is assumed that the charging/discharging circuit 15 is a single-phase chopper. Therefore, FIG. 1 illustrates a case where there is only one charging/discharging-current-value detecting unit 19. For the purpose of suppressing a ripple of a charging/discharging current of the power storage device 17, it is also possible to configure the charging/discharging circuit 15 from a multiple-phase, i.e., n-phase chopper (n is an integer equal to or larger than 2). When the charging/discharging circuit 15 is configured from an n-phase chopper, it is possible to reduce the ripple of the charging/discharging current of the power storage device 17 to 1/n. Accordingly, because heat generation of the power storage device 17 can be suppressed, it is possible to extend the life of the power storage device 17. When the charging/discharging circuit 15 is configured from an n-phase chopper, m charging/discharging-current detecting units (m is an integer equal to or larger than 1 and equal to or smaller than n) are mounted, m charging/discharging current values are input to the control-signal generating unit 23 in the charging/discharging control unit 2, and a charging/discharging current Ic of the power storage device 17 is calculated and used. [0149] By configuring the charging/discharging circuit 15 from the n-phase chopper, it is possible to suppress a charging/discharging current per phase; therefore, a response of a charging/discharging current to a control signal, which is an output of the charging/discharging control unit 2, becomes quick. Therefore, a response of a charging current to a control signal at the regenerative operation start time is improved compared with a response in the case of the single-phase chopper. [0150] Note that, in the configuration illustrated in FIG. 1, the AC motor drive system can further include an auxiliary-charge control unit that generates a control signal for actuating the charging/discharging circuit 15 to charge and discharge the desired electric power to and from the power storage device 17 in a period during which the AC motor 16 carries out neither a power running operation nor a regenerative operation and when electric power during the power running operation of the AC motor 16 or electric power during the regenerative operation is smaller than a predetermined threshold. Conversely, when it is unnecessary to suppress supplied power from the converter 11 during power running, a configuration may be such that the power-running-time control unit 21 and the current-command-value integrating unit 22 explained in the present embodiment are not present. [0151] Note that, in the present embodiment, the form is explained in which the charging/discharging control unit 2 is configured by a combination of various kinds of hardware, However, the present invention is not limited to this form. That is, a part or all of the components in the charging/discharging control unit 2 may be realized by software by which the components can be replaced. [0152] Second Embodiment. FIG. 13 is a block diagram illustrating an entire second embodiment of the AC motor drive system according to the present invention. An AC motor drive system la illustrated in FIG. 13 includes a charging/discharging control unit 2a, the converter 11, the smoothing capacitor 13, the inverter 14, the charging/discharging circuit 15, the AC motor 16, the power storage device 17, the DC-voltage-value detecting unit 18, the charging/discharging-current-value detecting unit 19, and an AC-voltage-value detecting unit 8. That is, the AC motor drive system la illustrated in FIG. 13 is different from the AC motor drive system 1 illustrated in FIG. 1 in that the AC motor drive system la includes the AC-voltage-value detecting unit 8. [0153] The AC-voltage-value detecting unit 8 detects an AC voltage value Vac, which is a voltage value between system power supply lines connected to the system power supply 10 side of the converter 11 and outputs the AC voltage value Vac to the charging/discharging control unit 2a. Note that, in the present embodiment, the same names and the reference numerals and signs are used for units same as or equivalent to the units in the first embodiment. Explanation of the units is omitted. [0154] The AC voltage value Vac in the system power supply input to the converter 11 is different depending on the length of a wire from the system power supply 10 to the converter 11. When a plurality of AC motor drive systems are connected to the same system power supply, the AC voltage value Vac input to the converter 11 of one AC motor drive system fluctuates according to the operation states (busyness) of the other AC motor drive systems. When the AC voltage value Vac in the converter 11 fluctuates, the voltage value Vdc of the DC bus 12, which is the output of the converter 11, also fluctuates. [0155] Even if the AC voltage value Vac of the converter 11 fluctuates, the AC motor drive system la in the present embodiment can suppress regenerative power regenerated via the converter 11 to the predetermined power threshold PthA during regeneration. [0156] FIG. 14 is a diagram illustrating a relation between the DC bus voltage value Vdc and regenerative power |Pcnv(t)| of the converter 11 in the regenerative operation of the AC motor 16 when the capacitance value of the smoothing capacitor 13 is fixed at C and the AC voltage value Vac fluctuates. In FIG. 14, Vacl