Power convertor and method for controlling power converter circuit

Abstract

This power converter comprises: a power converter circuit that converts direct-current power to alternating-current power to drive an induction motor; and a controller that outputs a gate pulse signal to the power converter circuit to control the power converter circuit. The controller can determine a current command and a magnetic flux command that are used for generating the gate pulse signal, and hold a hold value corresponding to the value of the magnetic flux command immediately before the start of falling of the current command. When interrupting the power converter, the controller determines the current command and the magnetic flux command on the basis of the hold value.

Description

Title of Invention: POWER CONVERTER AND METHOD FOR CONTROLLING POWER CONVERTER CIRCUIT Technical Field

[0001] The present invention relates to a power converter and a method for controlling a power converter circuit. Background Art

[0002] Conventionally, in railway vehicles, a drive system in which a power converter is used to drive an AC at a variable speed has been widely used. In addition, in many railway vehicles, induction motors are used to drive the vehicles, and a system in which a single power converter drives a plurality of induction motors at once is widely and generally adopted.

[0003] During an operation of a railway vehicle, a driving operation is frequently performed in which a power converter is started from a stopped state to accelerate, then the power converter is stopped, the vehicle enters a coasting state, and from this state, the power converter is restarted to accelerate or decelerate. When the railway vehicle enters a coasting state, no voltage is applied to an induction motor, but current continues to flow inside the induction motor for a while after the power converter stops. This current is consumed due to the internal resistance of the induction motor and gradually attenuates, but a magnetic flux continues to be generated until the current disappears, and thus a residual magnetic flux is present inside the induction motor. When the power converter is restarted in a state in which this residual magnetic flux is large, there is a risk of negative effects such as breakage of the power converter due to generation of excessive current and degradation of ride comfort of the vehicle due to generation of excessive torque.

[0004] As a countermeasure for the above-described problems, for example, a technique described in Patent Literature 1 is known. Patent Literature 1 discloses, as a control device for a power converter having a function of sufficiently attenuating a magnetic flux to stop an induction motor, a device which includes a voltage calculation unit that generates a voltage command based on a current command, and a PWM calculation unit that outputs a gate pulse signal for a power converter circuit based on the voltage command, and which attenuates a residual magnetic flux inside the induction motor at the time of stopping the power converter circuit by adding, to an excitation current command, an operation amount calculated according to an amount of change in the excitation current command over time to obtain an interruption time excitation current command during an interruption time from fall of the excitation current command until the excitation current command reaches zero, and setting this interruption time excitation current command as a current command for the voltage calculation unit. Citation List Patent Literature

[0005] Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2017-77079 Summary of Invention Technical Problem

[0006] In the technique described in Patent Literature 1, control in a case that the power converter is interrupted during rise of a magnetic flux command is not considered. Therefore, in such a case, a residual magnetic flux inside the induction motor may not be sufficiently attenuated. Solution to Problem

[0007] According to the present invention, a power converter includes: a power converter circuit that converts direct-current power into alternating-current power and drives an induction motor; and a control device that outputs a gate pulse signal to the power converter circuit and controls the power converter circuit. The control device obtains a current command and a magnetic flux command that are used to generate the gate pulse signal, is capable of holding a retention value according to a value of the magnetic flux command immediately before start of fall of the current command, and obtains the current command and the magnetic flux command based on the retention value when the power converter circuit is to be interrupted. According to the present invention, a method for controlling a power converter circuit that converts direct-current power into alternating-current power and drives an induction motor includes: obtaining a current command and a magnetic flux command and generating a gate pulse signal based on the current command and the magnetic flux command; outputting the gate pulse signal to the power converter circuit; and holding a retention value according to a value of the magnetic flux command immediately before start of fall of the current command, wherein the current command and the magnetic flux command are obtained based on the retention value when the power converter circuit is to be interrupted. Advantageous Effects of Invention

[0008] According to the present invention, even in a case that the power converter is interrupted during rise of a magnetic flux command, a residual magnetic flux inside the induction motor can be sufficiently attenuated. Brief Description of Drawings

[0009] Fig. 1 is a diagram illustrating a configuration of a power converter according to an embodiment of the present invention. Fig. 2 is a functional block diagram illustrating details of a control device according to a first embodiment of the present invention . Fig. 3 is a control block diagram illustrating details of a pattern generator according to the first embodiment of the present invention. Fig. 4 is a diagram illustrating an example of each command and each state amount when interruption control is performed after completion of rise of a d-axis magnetic flux command. Fig. 5 is a diagram illustrating an example of each command and each state amount when interruption control is performed before completion of rise of a d-axis magnetic flux command by a conventional control method. Fig. 6 is a diagram illustrating an example of each command and each state amount when interruption control is performed before completion of rise of a d-axis magnetic flux command in the power converter according to the first embodiment of the present invention. Fig. 7 is a functional block diagram illustrating details of a control device according to a second embodiment of the present invention . Fig. 8 is a control block diagram illustrating details of a pattern generator according to the second embodiment of the present invention. Fig. 9 is a control block diagram illustrating details of a pattern generator according to a third embodiment of the present invention . Fig. 10 is a diagram illustrating each command and each state amount when interruption control is performed before completion of rise of a d-axis magnetic flux command in a power converter according to a third embodiment of the present invention . Description of Embodiments

[0010] Hereinafter, a power converter according to an embodiment of the present invention will be described with reference to the drawings. Although the following embodiments will be described using application of a railway vehicle as an example, the embodiments are also applicable to other applications such as general industrial applications.

[0011] (First Embodiment) Fig. 1 is a diagram illustrating a configuration of a power converter according to an embodiment of the present invention. The power converter illustrated in Fig. 1 is connected to an induction motor 3, and includes a power converter circuit 1 and a control device 2 which controls the power converter circuit 1. The power converter circuit 1 converts direct-current power supplied from an external direct-current power source into alternating-current power and outputs the alternating-current power to the induction motor 3, thereby driving the induction motor 3.

[0012] The power converter circuit 1 includes a U-phase upper arm element 5a, a U-phase lower arm element 5b, a V-phase upper arm element 5c, a W-phase lower arm element 5d, a W-phase upper arm element 5e, and a W-phase lower arm element 5f, which are semiconductor switching elements. The U-phase upper arm element 5a and the U-phase lower arm element 5b, the V-phase upper arm element 5c and the V-phase lower arm element 5d, and the W-phase upper arm element 5e and the W-phase lower arm element 5f are connected in series in the power converter circuit 1 and form U-phase, V-phase, and W-phase upper and lower arm circuits, respectively. Power lines connected to the induction motor 3 are connected between the upper arm elements 5a, 5c, and 5e and the lower arm elements 5b, 5d, and 5f of the upper and lower arm circuits .

[0013] The power converter circuit 1 converts direct-current power supplied from the direct-current power source into three-phase alternating-current power by switching and driving the semiconductor switching elements 5a to 5f according to gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2 output from the control device 2. The three-phase alternating-current power after the conversion is output to the induction motor 3 from the power converter circuit 1 via the power lines of the phases, whereby drive control of the induction motor 3 is performed by the power converter according to the present embodiment.

[0014] The direct-current power supplied from the direct-current power source is smoothed by a smoothing capacitor 4 and input to the power converter circuit 1. A voltage between terminals of the smoothing capacitor 4, that is, a voltage Ecf of the direct-current power input to the power converter circuit 1, is detected by a direct-current voltage sensor 6, and the detected value is input to the control device 2.

[0015] A U-phase current sensor 7a, a V-phase current sensor 7b, and a W-phase current sensor 7c that detect a U-phase current iu, a V-phase current iv, and a W-phase current iw flowing toward the induction motor 3, respectively, are disposed in the power lines of the phases disposed between the power converter circuit 1 and the induction motor 3. The results of detecting the phase currents by the current sensors 7a to 7c are input to the control device 2.

[0016] The control device 2 generates the gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2 based on the detected value of the direct-current voltage Ecf by the direct-current voltage sensor 6 and the detected values of the U-phase current iu, the V-phase current iv, and the W-phase current iw by the current sensors 7a to 7c .

[0017] Fig. 2 is a functional block diagram illustrating details of the control device 2 according to a first embodiment of the present invention. The control device 2 includes functional blocks that are a current command generator 8, a pattern generator 9, a coordinate transformer 10, a rotation speed estimator 11, a frequency command generator 12, a voltage command generator 13, and a pulse command generator 14. The control device 2 includes a microcomputer having, for example, a CPU, a memory, and the like and can implement functions corresponding to the functional blocks illustrated in FIG. 2 by executing a predetermined program using the CPU. Some or all of the functions of the control device 2 may be implemented using a logic circuit such as a field-programmable gate array (FPGA).

[0018] A control command cmd to drive or stop the power converter circuit 1 is input to the control device 2 from an external. The current command generator 8 generates a d-axis current command Idpl and a q-axis current command Iqp according to the control command cmd and outputs the d-axis current command Idpl and the q-axis current command Iqp. For example, when the control command cmd to drive the power converter circuit 1 is input, the current command generator 8 generates the d-axis current command Idpl and the q-axis current command Iqp such that alternating-current power required to drive the induction motor 3 with predetermined torque is output from the power converter circuit 1.

[0019] The pattern generator 9 calculates an excitation current command Idp2 and a d-axis magnetic flux command (pdp based on the d-axis current command Idpl input from the current command generator 8, and outputs the excitation current command Idp2 and the d-axis magnetic flux command (pdp to the voltage command generator 13. The excitation current command Idp2 is a command value for a current in a d-axis direction, which is the direction of a rotating magnetic field of the induction motor 3. During a normal operation of the power converter circuit 1, the excitation current command Idp2 is equal to the d-axis current command Idpl. During the stop of the power converter circuit 1, the excitation current command Idp2 corresponds to a corrected value obtained by correcting the d-axis current command Idpl in order to attenuate a magnetic flux of the induction motor 3. The pattern generator 9 switches methods for calculating the excitation current command Idp2 and the d-axis magnetic flux command cpdp according to a control state of the induction motor 3. The methods for calculating the excitation current command Idp2 and the d-axis magnetic flux command cpdp by the pattern generator 9 will be described later in detail.

[0020] The coordinate transformer 10 obtains a detected d-axis current value Idf and a detected q-axis current value Idf by performing rotational coordinate transformation on the U-phase current iu, the V-phase current iv, and the W-phase current iw detected by the respective current sensors 7a to 7c, and outputs these detected current values to the rotation speed estimator 11. The coordinate transformer 10 obtains the detected d-axis current value Idf and the detected q-axis current value Idf by using the direction of the rotational magnetic field of the induction motor 3 as the d-axis direction and using, as the q-axis direction, a direction in which a current flows to generate torque.

[0021] The rotation speed estimator 11 estimates a rotor angular frequency of the induction motor 3 based on the excitation current command Idp2 and the q-axis current command Iqf input from the pattern generator 9 and the current command generator 8, respectively, and the detected d-axis current value Idf and the detected q-axis current value Idf input from the coordinate transformer 10, and outputs the result of the estimation as an estimated rotor angular frequency value core.

[0022] The d-axis magnetic flux command (pdp obtained by the pattern generator 9, the q-axis current command Iqp generated by the current command generator 8, and the estimated rotor angular frequency value core estimated by the rotation speed estimator 11 are input to the frequency command generator 12. The frequency command generator 12 calculates, based on the input information, an angular frequency of an alternating-current voltage to be applied to the induction motor 3 and outputs the angular frequency as a first-order angular frequency col.

[0023] The direct-current voltage Ecf detected by the direct-current voltage sensor 6, the excitation current command Idp2 and the d-axis magnetic flux command cpdp obtained by the pattern generator 9, the q-axis current command Iqp generated by the current command generator 8, the estimated rotor angular frequency value core obtained by the rotation speed estimator 11, and the first-order angular frequency col obtained by the frequency command generator 12 are input to the voltage command generator 13. The voltage command generator 13 calculates, based on the input information, a modulation rate Vc of the power converter circuit 1 and a voltage command declination 5, and outputs the results of the calculation as a voltage command for an output voltage to the induction motor 3 from the power converter circuit 1.

[0024] signals Sul, Su2, Svl, Sv2, Swl, and Sw2 for the semiconductor switching elements 5a to 5f of the power converter circuit 1 based on the modulation rate Vc and the voltage command declination 6 that are the voltage command generated by the voltage command generator 13, and the first-order angular frequency col obtained by the frequency command generator 12. The gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2 obtained by the pulse command generator 14 are output to the power converter circuit 1 from the control device 2 as described above and are used for drive control of the semiconductor switching elements 5a to 5f of the power converter circuit 1.

[0025] The functional block diagram of Fig. 2 illustrates an example of the power converter device that implements speed sensorless control by obtaining the estimated rotor angular frequency value core of the induction motor 3 by the rotation speed estimator 11. Although the present invention will be described below using this example, the present invention is applicable even in a case where a speed sensor is disposed in place of the rotation speed estimator 11 and control with the speed sensor is performed to detect the rotation speed of the induction motor 3 by this speed sensor. That is, an embodiment described below does not limit the configuration according to the present invention.

[0026] Fig. 3 is a control block diagram illustrating details of the pattern generator 9 according to the first embodiment of the present invention. In the present embodiment, for example, as illustrated in Fig. 3, the pattern generator 9 is configured by combining a switching contact 15, an adder 16, a switching contact 17, a delay element 18, a subtractor 19, a gain 20, an integral element 21, a switching contact 22, a gain 23, a switching contact 24, a delay element 25, a multiplier 26, a divider 27, a gain 28, and a differential element 29.

[0027] As described above, the pattern generator 9 switches the methods for calculating the excitation current command Idp2 and the d-axis magnetic flux command cpdp according to the control state of the induction motor 3. Specifically, the methods for calculating the excitation current command Idp2 and the d-axis magnetic flux command cpdp are switched between a case where the operation of the power converter circuit 1 is controlled such that the power converter circuit 1 outputs alternating-current power to drive the induction motor 3 with predetermined torque (hereinafter referred to as "normal control") and a case where the operation of the power converter circuit 1 is controlled such that output of alternating-current power from the power converter circuit 1 to the induction motor 3 is interrupted (hereinafter referred to as "interruption control"). The switching of the calculation methods can be performed based on the control command cmd input to the control device 2.

[0028] The switching contact 15 performs switching of the excitation current command Idp2 to be output from the pattern generator 9. The pattern generator 9 switches the switching contact 15 so as to output, as the excitation current command Idp2, the d-axis current command Idpl input from the current command generator 8 during the execution of the normal control, and output, as the excitation current command Idp2, output of the adder 16 during the execution of the interruption control. The adder 16 adds output of the differential element 29 to the d-axis current command Idpl. This added value is output as the excitation current command Idp2 in the interruption control from the adder 16 through the switching contact 15, as described above .

[0030] The switching contact 17 switches a d-axis current command retention value Idph. The pattern generator 9 switches the switching contact 17 so as to set, as the d-axis current command retention value Idph, the d-axis current command Idpl output from the current command generator 8 and input to the pattern generator 9 during the execution of the normal control, and set output of the delay element 18 as the d-axis current command retention value Idph during the execution of the interruption control. The delay element 18 delays output of the switching contact 17 and causes the delayed output to be output.

[0031] Due to the operations of the switching contact 17 and the delay element 18, the d-axis current command retention value Idph is sequentially updated according to the value of the d-axis current command Idpl input to the pattern generator 9 from the current command generator 8 during the execution of the normal control. On the other hand, after the switching from the normal control to the interruption control, the value of the d-axis current command Idpl input to the pattern generator 9 from the current command generator 8 is held as the d-axis current command retention value Idph.

[0032] The subtractor 19 subtracts output of the integral element 21 from the d-axis current command Idpl. This subtracted value is output to the gain 20.

[0033] The gain 20 multiplies the output of the subtractor 19 by a preset reciprocal of a second-order time constant T2 of the induction motor 3. This multiplied value is output to the integral element 21.

[0034] The integral element 21 integrates the output of the gain 20. The integrated value is output to the integral element 21, the switching contact 22, and the switching contact 24.

[0035] The switching contact 22 switches a magnetizing current command 10 to be input to the gain 23. The pattern generator 9 switches the switching contact 22 so as to input the output of the integral element 21 as the magnetizing current command 10 to the gain 23 during the execution of the normal control, and input output of the divider 27 as the magnetizing current command 10 to the gain 23.

[0036] The gain 23 multiplies the magnetizing current command 10 input from the switching contact 22 by preset excitation inductance L of the induction motor 3. This multiplied value is output from the pattern generator 9 as the d-axis magnetic flux command cpdp .

[0037] The switching contact 24 switches a magnetizing current command retention value lOh. The pattern generator 9 switches the switching contact 24 so as to set the output of the integral element 21 as the magnetizing current command retention value lOh during the execution of the normal control and set output of the delay element 25 as the magnetizing current command retention value lOh. The delay element 25 delays output of the switching contact 24 and causes the delayed output to be output.

[0038] Due to the operations of the switching contact 24 and the delay element 25, the magnetizing current command retention value lOh is sequentially updated according to the value of the magnetizing current command 10 output from the integral element 21 and input to the gain 23. On the other hand, after the switching from the normal control to the interruption control, the value of the magnetizing current command 10 input to the gain 23 immediately before the switching is held as the magnetizing current command retention value TOh.

[0039] The multiplier 26 multiplies the d-axis current command Idpl by the magnetizing current command retention value lOh. This multiplied value is output to the divider 27.

[0040] The divider 27 divides the output of the multiplier 26 by the d-axis current command retention value Idph. This divided value is output to the switching contact 22 and the gain 28.

[0041] The gain 28 multiplies the output of the divider 27 by the preset second-order time constant T2 of the induction motor 3. This multiplied value is output to the differential element 29.

[0042] The differential element 29 calculates an amount of change in the output of the gain 28 over time. This calculated value is output to the adder 16. Due to the operations of the above-described constituent elements, the pattern generator 9 according to the present embodiment can obtain the excitation current command Idp2 based on the d-axis current command Idpl and the magnetizing current command 10, and generate the d-axis magnetic flux command cpdp based on the magnetizing current command 10 and the excitation inductance L set in advance according to the characteristics of the induction motor 3. Specifically, during the execution of the normal control, it is possible to obtain the excitation current command Idp2 from the d-axis current command Idpl, and obtain the magnetizing current command 10 based on the d-axis current command Idpl and the second-order time constant T2 set in advance according to the characteristics of the induction motor 3. In addition, during the execution of the interruption control, it is possible to obtain the magnetizing current command 10 based on the d-axis current command Idpl, the magnetizing current command retention value lOh, and the d-axis current command retention value Idph, and obtain the excitation current command Idp2 based on the magnetizing current command 10 and the second-order time constant T2.

[0044] Subsequently, an operation of the power converter according to the present embodiment will be described with reference to Figs. 4 to 6 illustrating representative operation examples.

[0045] Fig. 4 is a diagram illustrating an example of each command and each state amount when interruption control is performed after completion of rise of a d-axis magnetic flux command cpdp. Although Fig. 4 illustrates an example of the operation of the power converter in a case where the control device 2 described with reference to Figs. 2 and 3 is used as an application example of the present invention, similar results are obtained by a conventional control method to which the present invention is not applied.

[0046] In a case where a normal command is in an on state, that is during the execution of the normal control, the switching contacts 15, 17, 22, and 24 are each in a normal switching state in the pattern generator 9 as described above. In the example illustrated in Fig. 4, this state continues from the start of rise of the d-axis current command Idpl until the d-axis current command Idpl reaches a constant value and immediately before interruption. On the other hand, in a case where an interruption command is in an on state, that is, during the execution of the interruption control, the switching contacts 15, 17, 22, and 24 are each in a switching state during interruption in the pattern generator 9 as described above. In the example illustrated in Fig. 4, this state continues from fall of the d-axis current command Idpl until the d-axis current command Idpl reaches zero.

[0047] In addition, in the example illustrated in Fig. 4, the d-axis current command Idpl is indicated by a solid line, and the magnetizing current command 10 is indicated by a broken line superimposed on the solid line. During the normal control, the magnetizing current command 10 operates as a first-order lag of the second-order time constant T2 with respect to the d-axis current command Idpl due to the operations of the subtractor 19, the gain 20, the integral element 21, and the switching contact 22 . After the rise of the d-axis current command Idpl ended, the d-axis current command Idpl reaches the constant value, and a sufficient time elapses, the d-axis current command Idpl matches the magnetizing current command 10, as illustrated in Fig. 4. Therefore, in the example illustrated in Fig. 4, during the execution of the interruption control, due to the operations of multiplier 26, the divider 27 and the switching contact 22, the d-axis current command Idpl and the magnetizing current command 10 are the same value.

[0049] On the other hand, the excitation current command Idp2 is the same value as that of the d-axis current command Idpl during the normal control due to the operations of the switching contact 15, the adder 16, the gain 28, and the differential element 29, and is a value obtained by multiplying a change rate (dIO / dt) of the magnetizing current command 10 over time by the second-order time constant T2 and adding the value obtained by the multiplication to the d-axis current command Idpl during the interruption control.

[0050] The d-axis magnetic flux command cpdp is obtained by multiplying the magnetizing current command 10 by the excitation inductance L. In Fig. 4, a steady value of the excitation current command Idp2 is defined as IdpO.

[0051] When the rise of the d-axis current command Idpl is completed and the d-axis current command Idpl reaches the constant value during the normal control, the value of the d-axis current command Idpl matches the excitation current command Idp2 and the magnetizing current command 10. The value of the d-axis magnetic flux command cpdp at this time becomes equal to a value obtained by multiplying the steady value IdpO of the excitation current command Idp2 by the excitation inductance L. When the switching from the normal control to the interruption control is performed in this state, the value of the d-axis magnetic flux command cpdp continuously changes from the above-described multiplied value. As a result, during a period of time from the start of the fall of the d-axis current command Idpl until the d-axis current command Idpl reaches zero, it is possible to reduce a d-axis magnetic flux cpd to zero without undershooting a command value by the d-axis magnetic flux command cpdp . Therefore, the d-axis magnetic flux (pd can be attenuated without remaining when the power converter circuit 1 is to be interrupted.

[0052] Next, a problem in the interruption control in a case that the present invention is not applied will be described below using an operation example shown in Fig. 5. Fig. 5 is a diagram illustrating an example of each command and each state amount when interruption control is performed before completion of rise of a d-axis magnetic flux command cpdp by a conventional control method. Fig. 5 illustrates an example of an operation waveform in which the d-axis current command Idpl falls in a period of time after the start of rise of the d-axis magnetic flux command (pdp until the d-axis magnetic flux command <pdp reaches a constant value, unlike Fig. 4. Each command and each state amount in Fig. 5 that change in a similar manner to those in Fig. 4 will not be described below.

[0053] In a conventional control method, as illustrated in Fig. 4, in a state in which the d-axis magnetic flux command (pdp becomes a constant value after completion of rise of the d-axis magnetic flux command (pdp, in a case that the control transitions from the normal control to the interruption control, the d-axis magnetic flux <pd can be sufficiently attenuated. However, when the control transitions from the normal control to the interruption control during the rise of the d-axis magnetic flux command (pdp, the d-axis magnetic flux command (pdp becomes discontinuous as illustrated in Fig. 5 and an excessively large command value is output as the d-axis magnetic flux command (pdp. As a result, the d-axis magnetic flux (pd crosses zero and undershoots, and the d-axis magnetic flux (pd remains in the induction motor 3 even after the power converter is stopped. When the power converter is restarted in a state in which the d-axis magnetic flux (pd remains in the induction motor 3, the residual d-axis magnetic flux (pd causes an excessive current or torque to occur. To prevent this, even when the control transitions from the normal control to the interruption control during the rise of the d-axis magnetic flux command (pdp, it is required to attenuate the d-axis magnetic flux (pd to zero at a constant change rate while the d-axis magnetic flux command (pdp does not become discontinuous.

[0054] In the power converter according to the present embodiment, means for sufficiently attenuating a magnetic flux of the induction motor 3 when the power converter is to be interrupted is disposed, which solves the above-described issue. Specifically, as described with reference to Fig. 3, in the pattern generator 9 of the control device 2, the d-axis current command Idpl immediately before switching from the normal control to the interruption control is held as the d-axis current command retention value Idph due to the switching contact 17 and the delay element 18, and the magnetizing current command 10 immediately before switching from the normal control to the interruption control is held as the magnetizing current command retention value lOh due to the switching contact 24 and the delay element 25. By dividing a value obtained by multiplying the d-axis current command Idpl by the magnetizing current command retention value lOh by the d-axis current command retention value Idph, the magnetizing current command 10 during the execution of the interruption control is calculated.

[0055] Therefore, even when the control transitions to the interruption control during the rise of the d-axis magnetic flux command (pdp, the d-axis current command Idpl can be corrected by using the ratio of the d-axis current command retention value Idph to the magnetizing current command retention value lOh immediately before the transition. As a result, the magnetizing current command 10 can be a continuous value, and the d-axis magnetic flux command (pdp that continuously decreases can be calculated using this magnetizing current command 10. That is, the d-axis magnetic flux (pd can be attenuated to zero at a constant change rate, and it is possible to prevent the d-axis magnetic flux (pd from remaining in the induction motor 3.

[0056] In addition, in the power converter according to the present embodiment, the d-axis magnetic flux command (pdp is calculated based on the value obtained by correcting the d-axis current command Idpl. Therefore, the d-axis magnetic flux command (pdp can be decreased to zero at a constant change rate from immediately before the interruption during a falling period (d-axis current command falling period Td) of the d-axis current command Idpl.

[0057] Fig. 6 is a diagram illustrating an example of each command and each state amount when interruption control is performed before completion of rise of a d-axis magnetic flux command cpdp in the power converter according to the first embodiment of the present invention. Each command and each state amount in Fig. 6 that change in a similar manner to those in Fig. 4 will not be described below.

[0058] In Fig. 6, the control transitions from the normal control to the interruption control during rise of the d-axis magnetic flux command cpdp, as in Fig. 5. However, as described above, in the power converter according to the present embodiment, the d-axis current command Idpl is corrected by using the ratio of the d-axis current command retention value Idph to the magnetizing current command retention value lOh, and the d-axis magnetic flux command cpdp is obtained by using this corrected value as the magnetizing current command 10. Due to this effect, as illustrated in Fig. 6, it is possible to continuously change the d-axis magnetic flux command cpdp before and after switching from the normal control to the interruption control and attenuate the d-axis magnetic flux cpd to zero at a constant change rate. A falling period of the d-axis magnetic flux command cpdp matches the d-axis current command falling period Td.

[0059] The value of the excitation current command Idp2 during the execution of the interruption control is calculated by multiplying the change rate (dIO / dt) of the magnetizing current command 10 over time by the second-order time constant T2 of the induction motor 3 and adding this multiplied value to the d-axis current command Idpl. Since the magnetizing current command 10 decreases at a constant change rate, the value of the excitation current command Idp2 at this time is a value obtained by translating the d-axis current command Idpl in the negative direction .

[0060] As described above, in the power converter according to the present embodiment, even when the control transitions from the normal control to the interruption control during the rise of the d-axis magnetic flux command cpdp, the actual d-axis magnetic flux cpd of the induction motor 3 can fall at a constant change rate. As a result, the d-axis magnetic flux cpd can be attenuated to zero without undershooting. That is, a remaining amount of the d-axis magnetic flux (pd when the power converter is to be interrupted can be zero. Therefore, it is possible to suppress the occurrence of an excessive current or torque when the power converter is to be restarted.

[0061] According to the first embodiment of the present invention described above, the following effects are obtained.

[0062] (1) The power converter includes the power converter circuit 1 that converts direct-current power into alternating-current power and drives the induction motor 3, and the control device 2 that outputs the gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2 to the power converter circuit 1 and controls the power converter circuit 1. The control device 2 obtains the d-axis current command Idpl, the excitation current command Idp2, and the d-axis magnetic flux command cpdp that are used to generate the gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2, is capable of holding the magnetizing current command retention value lOh and the d-axis current command retention value Idph according to the value of the d-axis magnetic flux command immediately before the start of fall of the d-axis current command Idpl, and obtains the d-axis current command Idpl, the excitation current command Idp2, and the d-axis magnetic flux command cpdp based on these retention values when the power converter circuit 1 is to be interrupted. Therefore, even in a case that the power converter is interrupted during rise of the d-axis magnetic flux command cpdp, a residual magnetic flux inside the induction motor can be sufficiently attenuated.

[0063] (2) The control command cmd to drive or stop the power converter circuit 1 is input to the control device 2. The control device 2 executes, based on the control command cmd, either the normal control in a period before the start of fall of the d-axis current command Idpl or the interruption control in a period after the start of the fall of the d-axis current command Idpl, and is capable of holding a retention value according to the value of the d-axis magnetic flux command cpdp immediately before switching from the normal control to the interruption control. Specifically, the control device 2 includes the current command generator 8 that generates the d-axis current command Idpl and the q-axis current command Iqp according to the control command cmd, the pattern generator 9 that obtains the excitation current command Idp2 and the excitation current command 10 based on the d-axis current command Idpl, and generates the d-axis magnetic flux command cpdp based on the magnetizing current command 10 and the excitation inductance L set according to the characteristics of the induction motor 3, the voltage command generator 13 that generates the voltage command based on the excitation current command Idp2, the d-axis magnetic flux command (pdp, and the q-axis current command Iqp, and the pulse command generator 14 that generates the gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2 based on the voltage command. The pattern generator 9 holds, as the magnetizing current command retention value lOh, the value of the magnetizing current command 10 immediately before the switching from the normal control to the interruption control. Therefore, in a case that the power converter is interrupted during rise of the d-axis magnetic flux command <pdp, it is possible to generate the gate pulse signals Sul, Su2, Svl, Sv2 , Swl, and Sw2 that can sufficiently attenuate a residual magnetic flux in the induction motor 3.

[0064] (3) The pattern generator 9 obtains the excitation current command Idp2 from the d-axis current command Idpl using the switching contact 15, and obtains the magnetizing current command 10 using the subtractor 19, the gain 20, the integral element 21 and the switching contact 22 based on the d-axis current command Idpl and the second-order time constant T2 set according to the characteristics of the induction motor 3. In addition, during the execution of the interruption control, the pattern generator 9 obtains the magnetizing current command 10 using the switching contact 24, the delay element 25, the multiplier 26, and the divider 27 based on the d-axis current command Idpl, the magnetizing current command retention value lOh, and the d-axis current command retention value Idph, and obtains the excitation current command Idp2 using the gain 28, the differential element 29, and the adder 16 based on the magnetizing current command 10 and the second-order time constant T2. Therefore, in each of the normal control and the interruption control, the values of the excitation current command Idp2 and the magnetizing current command 10 required to generate the gate pulse signals Sul, Su2, Svl, Sv2, Swl, and Sw2 can be appropriately obtained.

[0065] (4) The pattern generator 9 continuously holds the magnetizing current command retention value lOh and the d-axis current command retention value Idph after the switching from the normal control to the interruption control. Specifically, the pattern generator 9 holds, as the magnetizing current command retention value lOh and the d-axis current command retention value Idhp, the value of the magnetizing current command 10 and the value of the excitation current command Idp2 immediately before the switching from the normal control to the interruption control by using the switching contact 24 and the delay element 25, and the switching contact 17 and the delay element 18, respectively. In addition, by the multiplier 26 and the divider 27, the pattern generator 9 obtains, as the value of the magnetizing current command 10 during the execution of the interruption control, the value obtained by dividing the product of the d-axis current command Idpl and the magnetizing current command retention value lOh by the d-axis current command retention value Idph. Therefore, even when the control transitions to the interruption control during rise of the d-axis magnetic flux command cpdp, the value of the magnetizing current command 10 that continuously changes can be calculated. As a result, it is possible to attenuate the d-axis magnetic flux <pd to zero at a constant change rate and reliably prevent the d-axis magnetic flux cpd from remaining in the induction motor 3. (Second Embodiment) Next, a second embodiment of the present invention will be described. The present embodiment describes an example in which a method for calculating the magnetizing current command 10 is different from that in the first embodiment. Differences from the first embodiment will be mainly described below.

[0067] A power converter according to the present embodiment has a configuration identical or similar to that of the power converter described in the first embodiment and illustrated in Fig. 1, but a functional configuration of a control device 2 in the power converter according to the present embodiment is different from that in the first embodiment.

[0068] Fig. 7 is a functional block diagram illustrating details of the control device 2 according to a second embodiment of the present invention. The present embodiment is different from the first embodiment described with reference to Fig. 2 in that the pattern generator 9 is replaced with a pattern generator 9A, the d-axis current command Idpl and the d-axis current command falling period Td are output from the current command generator 8 to the pattern generator 9A. This is due to the fact that the d-axis current command falling period Td is used to calculate the magnetizing current command 10 and the d-axis magnetic flux command cpdp in the pattern generator 9A.

[0069] Fig. 8 is a control block diagram illustrating details of the pattern generator 9A according to the second embodiment of the present invention. The pattern generator 9A according to the present embodiment adds input of the d-axis current command falling period Td, and includes a divider 30, a gain 31, and a switching contact 32 in place of the switching contact 17, the delay element 18, the switching contact 22, the multiplier 26, the divider 27, and the differential element 29 included in the pattern generator 9 described in the first embodiment and illustrated in Fig. 3.

[0070] The divider 30 divides the magnetizing current command retention value lOh by the d-axis current command falling period Td. This divided value is output to the gain 31.

[0071] The gain 31 inverts the positive or negative sign of the output of the divider 30 by multiplying the output of the divider 30 by -1 and outputs the inverted value to the gain 28 and the switching contact 32.

[0072] In the pattern generator 9A according to the present embodiment, the differential element 29 is not disposed on the output side of the gain 28. Therefore, the gain 28 multiplies the second-order time constant T2 of the induction motor 3 by the output of the divider 30 that has the positive or negative sign inverted by the gain 31, and outputs the multiplied value to the adder 16.

[0073] The switching contact 32 switches a value input to the integral element 21. The pattern generator 9A switches the switching contact 32 such that the output of the gain 20 is input to the integral element 21 during the execution of the normal control and that the output of the gain 31 is input to the integral element 21 during the execution of the interruption control. Therefore, in the present embodiment, the d-axis magnetic flux command cpdp is obtained by calculating the excitation current command Idp2 and the magnetizing current command 10 during the interruption by using the d-axis current command falling period Td instead of the d-ais current command retention value Idph described in the first embodiment.

[0074] In addition, the pattern generator 9A inverts the sign of a value obtained by dividing the magnetizing current command retention value lOh by the d-axis current command falling period Td by using the gain 31. This processing corresponds to the calculation of the change rate of the magnetizing current command 10 over time by the differential element 29 in the first embodiment. That is, in the present embodiment, the excitation current command Idp2 during the interruption can be obtained without using the differential element 29.

[0075] In addition, the ratio of the d-axis current command falling period Td to the magnetizing current command retention value lOh is integrated by the integral element 21 in the interruption control, and thus the integral element 21 can be shared during the normal operation and the interruption. Therefore, the continuity of the d-axis magnetic flux command (pdp during the transition from the normal control to the interruption control can be reliably guaranteed.

[0076] Although the magnetizing current command 10 that decreases at a constant rate during the interruption is calculated by using the ratio of the d-axis current command falling period Td to the magnetizing current command retention value lOh, the present embodiment does not need to be limited to this configuration. For example, a change rate limiter that causes the magnetizing current command 10 during the interruption to fall at a constant change rate can implement the magnetizing current command 10 that decreases at a constant rate. In this case, the magnetizing current command 10 that decreases at a constant rate can be implemented by setting input to the change rate limiter as a value obtained by subtracting the input to the limiter from zero by the magnetizing current command 10, and limiting output by using the ratio of the d-axis current command falling period Td to the magnetizing current command retention value lOh.

[0077] According to the configuration described in the present embodiment, when the interruption control is executed before completion of rise of the d-axis magnetic flux command, each command and each state amount that are identical or similar to those described with reference to Fig. 6 in the first embodiment can be obtained.

[0078] Therefore, even in the present embodiment, when the control transitions from the normal control to the interruption control during rise of the d-axis magnetic flux command tpdp, the d-axis magnetic flux command (pdp can continuously fall at a constant rate and thus the d-axis magnetic flux cpd can be attenuated to zero without undershooting. That is, a remaining amount of the d-axis magnetic flux cpd when the power converter is to be interrupted can be zero. Therefore, it is possible to suppress the occurrence of an excessive current or torque when the power converter is to be restarted. In addition, in the present embodiment, the differential element 29 described in the first embodiment is not required, and thus, for example, even in a case that the d-axis current command Idpl is affected by disturbance and rapidly changes, the power converter can be stably operated without issuance of the excitation current command Idp2.

[0080] According to the second embodiment of the present invention described above, the pattern generator 9A continuously holds the magnetizing current command retention value lOh after the switching from the normal control to the interruption control. Specifically, the current command generator 8 outputs the d-axis current command falling period Td representing a period of time from the start to end of the fall of the d-axis current command Idpl. The pattern generator 9A uses the switching contact 24 and the delay element 25 to hold, as the magnetizing current command retention value lOh, the value of the magnetizing current command 10 immediately before the switching from the normal control to the interruption control. In addition, the pattern generator 9A uses the divider 30, the gain 31, and the integral element 21 to obtain, as the value of the magnetizing current command 10 during the execution of the interruption control, a value obtained by integrating a value obtained by inverting the sign of a value obtained by dividing the magnetizing current command retention value lOh by the d-axis current command falling period Td. Therefore, even when the control transitions to the interruption control during rise of the d-axis magnetic flux command (pdp, the value of the magnetizing current command 10 that continuously changes can be calculated as in the first embodiment. As a result, it is possible to attenuate the d-axis magnetic flux cpd to zero at a constant change rate and reliably prevent the d-axis magnetic flux cpd from remaining in the induction motor 3.

[0081] (Third Embodiment) Next, a third embodiment of the present invention will be described. The present embodiment is different from the first and second embodiments in a method for holding the magnetizing current command retention value lOh. Differences from the first and second embodiments will be mainly described below.

[0082] Fig. 9 is a control block diagram illustrating details of a pattern generator 9B according to the third embodiment of the present invention. The pattern generator 9B according to the present embodiment includes a switching contact 32, a minimum value limiter 33, and a gain 34 in place of the switching contact 17, the delay element 18, the switching contact 22, the switching contact 24, the delay element 25, the multiplier 26, the divider 27, and the differential element 29 included in the pattern generator 9 described in the first embodiment and illustrated in Fig. 3.

[0083] The minimum value limiter 33 limits output from the subtractor 19 using an upper limit value 0 and outputs the limited output to the gain 34. That is, in a case where the magnitude of the difference of the magnetizing current command 10 from the d-axis current command Idpl obtained by the subtractor 19 is a negative value, the value is output to the gain 34 without a change, and in a case where the magnitude of the difference is a positive value, the upper limit value 0 is output to the gain 34.

[0084] The gain 34 multiplies the output of the minimum value limiter 33 by a reciprocal of a preset first-order time constant To of the induction motor 3. This multiplied value is output from the gain 34 to the switching contact 32 and the gain 28.

[0085] In the pattern generator 9B according to the present embodiment, the differential element 29 is disposed on the output side of the gain 28, as in the pattern generator 9A according to the second embodiment. Therefore, the gain 28 is limited to a range less than or equal to the upper limit value 0 by the minimum value limiter 33, multiplies the second-order time constant T2 of the induction motor 3 by the difference between the d-axis current command Idpl and the magnetizing current command 10 multiplied by the reciprocal of the first-order time constant To by the gain 34, and outputs the multiplied value.

[0086] The switching contact 32 also switches a value input to the integral element 21 as in the second embodiment. The pattern generator 9B switches the switching contact 32 such that the output of the gain 20 is input to the integral element 21 during the execution of the normal control and that the output of the gain 34 is input to the integral element 21 during the execution of the interruption control. Therefore, in the present embodiment, calculating the d-axis magnetic flux command <pdp is obtained by the excitation current command Idp2 and the magnetizing current command 10 during the interruption by using the first-order time constant To, instead command retention value Idph described in of the d-axis current the first embodiment. The pattern generator 9B according to the present embodiment operates as a first-order lag using the second-order time constant T2 as a time constant due to the subtractor 19, the gain 20, and the integral element 21 via the switching contact 32 during the normal operation, as in the first embodiment. On the other hand, the pattern generator 9B operates as a first-order lag with an input limiter using the first-order time constant To as a time constant due to the subtractor 19, the minimum value limiter 33, the gain 34, and the integral element 21 via the switching contact 32 during the interruption. In this case, since it suffices for the time constant during the interruption to be sufficiently shorter than the second-order time constant T2, the time constant during the interruption is not limited to the first-order time constant To of the induction motor 3, and another time constant may be used.

[0088] As described above, the pattern generator 9B according to the present embodiment switches the time constant of the first-order lag element from the second-order time constant T2 to the first-order time constant To when the control is to be switched from the normal control to the interruption control. In this case, since the first-order time constant To is sufficiently shorter than the second-order time constant T2, the falling rate and falling period of the d-axis magnetic flux command cpdp during the interruption substantially match those of the d-axis current command Idpl. In addition, the continuity of the d-axis magnetic flux command (pdp is maintained by the integral element 21 constituting the first-order lag element. Therefore, even when the control transitions from the normal control to the interruption control during rise of the d-axis magnetic flux command cpdp, the d-axis magnetic flux command cpdp can be decreased continuously and at a constant rate.

[0089] Furthermore, the pattern generator 9B according to the present embodiment obtains the excitation current command Idp2 during the interruption by multiplying output of the gain 34 by the second-order time constant T2 and adding the multiplied value to the d-axis current command Idpl by using the gain 28 and the adder 16. Since the output of the gain 34 is equal to the change rate of the magnetizing current command 10 over time, the excitation current command Idp2 required for a continuous operation of the d-axis magnetic flux command cpdp can be calculated by performing the above-described operation.

[0090] When the control is switched from the normal control to the interruption control in a state after the start of rise of the d-axis magnetic flux command cpdp and before the d-axis magnetic flux command cpdp reaches a constant value, the interruption control is started in a state in which the d-axis current command Idpl is greater than the magnetizing current command 10. In this case, in the configuration of the pattern generator 9B illustrated in Fig. 9, if the minimum value limiter 33 is not present, the subtractor 19, the gain 34 and the integral element 21 are configured as the first-order lag element, the output of the subtractor 19 becomes positive regardless of a decrease in the d-axis current command Idpl, and as a result, the magnetizing current command 10 operates largely in the positive direction, which is the opposite direction to the d-axis current command Idpl. In this case, the excitation current command Idp2 and the d-axis magnetic flux command (pdp during the interruption become excessive and cause torque shock or overcurrent. To prevent this the input of the gain 34 is required to be less than or equal to zero. Therefore, in the pattern generator 9B according to the present embodiment, the minimum value limiter 33 that limits the output of the subtractor 19 to zero or a negative value less than zero and causes the limited output to be input to the gain 34 is disposed on the upstream side of the gain 34.

[0091] Fig. 10 is a diagram illustrating each command and each state amount when interruption control is performed before completion of rise of a d-axis magnetic flux command (pdp in a power converter according to the third embodiment of the present invention. Each command and each state amount in Fig. 10 that change in a similar manner to those in Fig. 4 will not be described below.

[0092] In the power converter according to the present embodiment, the pattern generator 9B having the configuration illustrated in Fig. 9 is used, and thus the d-axis magnetic flux command (pdp does not become discontinuous as illustrated in Fig. 10 even when the switching from the normal control to the interruption control is performed during rise of the d-axis magnetic flux command (pdp. In addition, it is possible to suppress an increase in the d-axis magnetic flux command (pdp during the interruption by switching the time constant of the first-order lag element from the second-order time constant T2 to the first-order time constant To.

[0093] In addition, in the interruption control, the d-axis magnetic flux command (pdp is output at a constant value in a period of time when the d-axis current command Idpl is greater than the magnetizing current command 10, and the d-axis magnetic flux command cpdp decreases at a constant change rate in a period of time after the d-axis current command Idpl matches the magnetizing current command 10. Therefore, the d-axis magnetic flux command cpdp can be reduced to zero within the d-axis current command falling period Td.

[0094] As described above, even in the power converter according to the present embodiment, when the control transitions from the normal control to the interruption control during rise of the d-axis magnetic flux command cpdp, the actual d-axis magnetic flux cpd of the induction motor 3 can be continuously decreased at a constant change rate, and thus the d-axis magnetic flux <pd can be attenuated to zero without undershooting. That is, the residual amount of magnetic flux cpd when the power converter is interrupted to stop driving the induction motor 3 can be zero. Therefore, it is possible to suppress the occurrence of an excessive current or torque when the power converter is to be restarted.

[0095] According to the third embodiment of the present invention described above, the pattern generator 9B uses the integral element 21, which is an integrator, to hold the retention value according to the value of the d-axis magnetic command cpdp immediately before the start of the rise of the d-axis current command Idpl. Specifically, the pattern generator 9B obtains the difference between the d-axis current command Idpl and the output of the integral element 21 by the subtractor 19. Then, the pattern generator 9B obtains, as the magnetizing current command 10, the output of the integral element 21 when the product of the difference and the reciprocal of the second-order time constant T2 is input to the integral element 21 during the execution of the normal control, and obtains, as the magnetizing current command 10, the output of the integral element 21 when the product of the difference and the reciprocal of the first-order time constant To shorter than the second-order time constant T2 is input to the integral element 21. Furthermore, the pattern generator 9B includes the minimum value limiter 33 that limits the input of the integral element 21 during the execution of the interruption control to the range less than or equal to 0. Therefore, even when the control transitions to the interruption control during rise of the d-axis magnetic flux command (pdp, the value of the magnetizing current command 10 that continuously changes can be calculated as in the first and second embodiments. As a result, it is possible to attenuate the d-axis magnetic flux cpd to zero at a constant change rate and reliably prevent the d-axis magnetic flux cpd from remaining in the induction motor 3.

[0096] The present invention is not limited to the embodiments and the modifications described above, and can be implemented using any constituent elements without departing from the gist of the present invention. In addition, each of the embodiments and the modifications may be adopted alone, or two or more of the embodiments and the modifications may be adopted in arbitrary combination. That is, in the present invention, the abovedescribed effects can be obtained by arbitrarily combining the features of each of the embodiments.

[0097] The embodiments and the modifications described above are merely examples, and the present invention is not limited to the contents of the embodiments and the modifications as long as the features of the invention are not impaired. Although the various embodiments and modifications are described above, the present invention is not limited to the contents of the embodiments and the modifications. Other aspects considered within the technical spirit of the present invention are also included within the scope of the present invention. List of Reference Signs

[0098] 1: power converter, 2: control device, 3: induction motor, 4: smoothing capacitor, 5a: U-phase upper arm element, 5b: U-phase lower arm element, 5c: V-phase upper arm element, 5d: V- phase lower arm element, 5e: W-phase upper arm element , 5f: W- phase lower arm element, 6: direct-current voltage sensor, 7a: U- phase current sensor, 7b: V-phase current sensor, 7c: W-phase current sensor, 8: current command generator, 9, 9A, 9B: pattern generator, 10: coordinate transformer, 11: rotation speed estimator, 12: frequency command generator, 13: voltage command generator, 14: pulse command generator, 15: switching contact, 16: adder, 17: switching contact, 18: delay element, 19: subtractor , 20: gain, 21: integral element, 22: switching contact, 23: gain, 24: switching contact, 25: delay element, 26: multiplier, 27: divider, 28: gain, 29: differential element, 30: divider, 31: gain, 32: switching contact, 33: minimum value limiter, 34: gain, Ecf: direct-current voltage, Sul: U-phase upper arm gate pulse signal, Su2: U-phase lower arm gate pulse signal, Svl: V-phase upper arm gate pulse signal, Sv2 : V-phase lower arm gate pulse signal, Swl: W-phase lower arm gate pulse signal, Sw2: W-phase lower arm gate pulse signal, iu: U-phase current, iv: V-phase current, iw: W-phase current, cmd: control command, Idpl: d-axis current command, ldp2: excitation current command, iqp: q-axis current command, cpdp: d-axis magnetic flux command, Idf: d-axis current value, Iqf: q-axis current value, 10: magnetizing current command, Idph: d-axis current command retention value, lOh: magnetizing current command retention value, IdpO: steady value of excitation current command, are: rotor angular frequency value, col : first-order angular frequency, Vc: modulation rate, 5: voltage command declination, L: excitation inductance, To: first-order time constant, T2: second-order time constant, Td: d-axis current command falling period

Claims

1. A power converter comprising:a power converter circuit that converts direct-current power into alternating-current power and drives an induction motor; anda control device that outputs a gate pulse signal to the power converter circuit and controls the power converter circuit, whereinthe control device obtains a current command and a magnetic flux command that are used to generate the gate pulse signal, is capable of holding a retention value according to a value of the magnetic flux command immediately before start of fall of the current command, and obtains the current command and the magnetic flux command based on the retention value when the power converter circuit is to be interrupted.

2. The power converter according to claim 1, whereina control command to drive or stop the power converter circuit is input to the control device,the control device executes, based on the control command, either normal control in a period of time before start of the fall of the current command or interruption control in a period of time after the start of the fall of the current command, and is capable of holding the retention value according to a value of the magnetic flux command immediately before switching from the normal control to the interruption control.

3. The power converter according to claim 2, wherein the control device includes: a current command generator that generates a d-axis currentcommand and a q-axis current command according to the control command;a pattern generator that obtains an excitation current command and a magnetizing current command based on the d-axis current command, and generates the magnetic flux command based on the magnetizing current command and excitation inductance set in advance according to characteristics of the induction motor;a voltage command generator that generates a voltage command based on the excitation current command, the magnetic flux command, and the q-axis current command; anda pulse command generator that generates the gate pulse signal based on the current command, andthe pattern generator holds, as the retention value, a value of the magnetizing current command immediately before the switching from the normal control to the interruption control.

4. The power converter according to claim 3, whereinduring the execution of the normal control, the pattern generator obtains the excitation current command from the d-axis current command and obtains the magnetizing current command based on the d-axis current command and a second-order time constant set according to the characteristics of the induction motor, andduring the execution of the interruption control, the pattern generator obtains the magnetizing current command based on the d-axis current command and the retention value and obtains the excitation current command based on the magnetizing current command and the second-order time constant.

5. The power converter according to claim 4, whereinthe pattern generator continuously holds the retention valueafter the switching from the normal control to the interruption control.

6. The power converter according to claim 5, wherein the pattern generator holds the value of the magnetizing current command and a value of the excitation current command immediately before the switching from the normal control to the interruption control as a first retention value and a second retention value, respectively, and obtains, as a value of the magnetizing current command during the execution of the interruption control, a value obtained by dividing a product of the d-axis current command and the first retention value by the second retention value.

7. The power converter according to claim 5 or 6, wherein the current command generator outputs a falling period representing a period of time from the start to end of the fall of the current command, and the pattern generator holds, as the retention value, the value of the magnetizing current command immediately before the switching from the normal control to the interruption control, and obtains, as a value of the magnetizing current command during the execution of the interruption control, a value obtained by integrating a value obtained by inverting a sign of a value obtained by dividing the retention value by the falling period.

8. The power converter according to any one of claims 4 to 7,whereinthe pattern generator holds the retention value using an integrator .

9. The power converter according to claim 8, whereinthe pattern generatorobtains a difference between the d-axis current command and output of the integrator, obtains, as the magnetizing current command, output of the integrator when a product of the difference and a reciprocal of the second-order time constant is input to the integrator, and obtains, as the magnetizing current command, output of the integrator when a product of the difference and a reciprocal of a first-order time constant shorter than the second-order time constant is input to the integrator.

10. The power converter according to claim 9, whereinthe pattern generator includes a limiter that limits input of the integrator during the execution of the interruption control to a range less than or equal to 0.

11. A method for controlling a power converter circuit that converts direct-current power into alternating-current power and drives an induction motor, the method comprising:obtaining a current command and a magnetic flux command and generating a gate pulse signal based on the current command and the magnetic flux command;outputting the gate pulse signal to the power converter circuit; andholding a retention value according to a value of themagnetic flux command immediately before start of fall of the current command, whereinthe current command and the magnetic flux command are obtained based on the retention value when the power converter circuit is to be interrupted.

12. The method for controlling the power converter circuitaccording to claim 11, the method further comprising:receiving input of a control command to drive or stop the power converter circuit; andexecuting, based on the control command, either normal control in a period of time before the start of the fall of the current command or interruption control in a period of time after the start of the fall of the current command, whereinthe retention value according to a value of the magnetic flux command immediately before switching from the normal control to the interruption control is held.

13. The method for controlling the power converter circuit according to claim 12, the method further comprising:generating a d-axis current command and a q-axis current command according to the control command;obtaining an excitation current command and a magnetizing current command based on the d-axis current command;generating the magnetic flux command based on the magnetizing current command and excitation inductance set according to characteristics of the induction motor;generating a voltage command based on the excitation current command, the magnetic flux command, and the q-axis currentcommand;generating the gate pulse signal based on the voltage command; andholding, as the retention value, a value of the magnetizing current command immediately before the switching from the normal control to the interruption control.

14. The method for controlling the power converter circuit according to claim 13, whereinduring the execution of the normal control, the excitation current command is obtained from the d-axis current command, and the magnetizing current command is obtained based on the d-axis current command and a second-order time constant set according to the characteristics of the induction motor, andduring the execution of the interruption control, the excitation current command is obtained based on the d-axis current command and the retention value, and the excitation current command is obtained based on the magnetizing current command and the second-order time constant.

15. The method for controlling the power converter circuit according to claim 14, wherein after the switching from the normal control to the interruption control, the retention value is continuously held.

16. The method for controlling the power converter circuit according to claim 15, the method further comprising: holding a value of the magnetizing current command and a value of the excitation current command immediately before the switching from the normal control to the interruption control as a first retention value and a second retention value,respectively; andobtaining, as a value of the magnetizing current command during the execution of the interruption control, a value obtained by dividing a product of the d-axis current command and the first retention value by the second retention value.

17. The method for controlling the power converter circuit according to claim 15 or 16, the method further comprising: obtaining a falling period representing a period of time from the start to end of the fall of the current command; and obtaining, as a value of the magnetizing current command during the execution of the interruption control, a value obtained by integrating a value of the magnetizing current command at a start time of the fall of the current command with a value obtained by inverting a sign of a value obtained by dividing the retention value by the falling period.

18. The method for controlling the power converter circuit according to any one of claims 14 to 17, wherein the retention value is held using an integrator.

19. The method for controlling the power converter circuit according to claim 18, the method further comprising: obtaining a difference between the d-axis current command and output of the integrator, wherein during the execution of the normal control, output of the integrator when a product of the difference and a reciprocal of the second-order time constant is input to the integrator is obtained as the magnetizing current command, and during the execution of the interruption control, output ofthe integrator when a product of the difference and a reciprocal of a first-order time constant shorter than the second-order time constant is input to the integrator is obtained as the magnetizing current command.

20. The method for controlling the power converter circuit according to claim 19, the method further comprising:limiting input of the integrator during the execution of the interruption control to a range less than or equal to 0.

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