JP2011172450A - Inverter device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively provide an inverter device which, when a motor for a compressor is controlled by a so-called sensorless vector control, prevents breakage of piping and contributes also to energy saving. <P>SOLUTION: The inverter device 1 controls the motor 6 for the compressor constituting a refrigerant circuit. This device includes: an inverter main circuit 3 which applies a three-phase PWM system of a three-phase pseudo AC voltage to the motor; a shunt resistor 2 for converting a current flowing through the motor into a voltage; an OP amplifier 31 which amplifies a terminal voltage of the shunt resistor; and a control device 11 which detects the current flowing through the motor from the output of the OP amplifier, estimates a rotor position of the motor based on the detected current, and controls ON/OFF operation of the respective switching elements of the inverter main circuit. In a situation where an output voltage of the OP amplifier, into which the terminal voltage of the shunt resistor is input, is put into a saturation state, the control device changes control coefficient in a direction of decreasing change in the current flowing through the motor. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an inverter device that controls a compressor motor by a sensorless vector method without using a magnetic pole position sensor.

  Conventionally, for example, when a brushless motor (motor) for driving a compressor constituting a refrigerant circuit of a commercial refrigerator is operated by sensorless vector control, a voltage command value, a rotation speed (angular frequency) is calculated from a phase current flowing through the inverter main circuit. ) And a phase, a small and inexpensive shunt resistor is used as a device for detecting this phase current.

  The shunt resistance type inverter device (one shunt type, see Patent Document 1) includes a three-phase PWM (Pulse Width Modulation) type inverter main circuit, and the voltage supplied from the DC power supply unit can be arbitrarily changed. The voltage is converted into a three-phase pseudo-AC voltage having a variable frequency and output, and supplied to an electric motor (for example, a synchronous motor). The inverter main circuit is configured by connecting three arms that are connected in series to a DC power supply unit in the form of a three-phase bridge.

  That is, the inverter main circuit includes a U-phase upper arm switching element, a U-phase lower arm switching element, a V-phase upper arm switching element, a V-phase lower arm switching element, and a W-phase switching element. The upper arm switching element and the W-phase lower arm switching element are provided.

  Each switching element is turned on when the pulse signal input to the gate is at “H” level, and turned off when it is at “L” level. The shunt resistor is connected to a DC bus, and a DC bus current Idc (current flowing through the electric motor) flows through the shunt resistor.

  In this case, the terminal voltage of the shunt resistor is input to the control device via the OP amplifier (amplifier). That is, the DC bus current Idc is converted into a voltage by the shunt resistor, amplified by the OP amplifier, and then taken into the control device.

  The control device constituted by the microcomputer is based on the pulse signals U, Ubar, V, Vbar, W, Wbar output by itself, and the DC bus current Idc (voltage) that flows through the shunt resistor at a predetermined cycle (carrier signal cycle). And the three-phase current flowing through the motor from the detected DC bus current Idc, that is, the U-phase current Iu, the V-phase current Iv, and the W-phase current Iw are estimated. To do.

  Then, the three-phase two-phase coordinate conversion unit of the control device uses the estimated three-phase currents Iu, Iv, and Iw to perform three-phase to two-phase conversion on each phase current Iu, Iv, and Iw, and the q-axis current Iq and D-axis current Id is calculated. Next, the position / velocity estimator of the control device calculates the q-axis current Iq and the d-axis current Id calculated by the three-phase / two-phase coordinate conversion unit, and the q-axis current control unit and the d-axis current control of the control device. The rotation speed of the electric motor and the magnetic pole position of the rotor (rotor) are estimated using the q-axis voltage Vq and the d-axis voltage Vd calculated by the unit.

  Next, the speed control unit of the control device calculates a target q-axis current (the q-axis current is proportional to the torque) by PI control from the rotation speed and the target rotation speed estimated by the position / speed estimator. Next, the q-axis current control unit calculates the q-axis voltage Vq by PI control from the target q-axis current calculated by the speed control unit and the actual q-axis current Iq calculated by the three-phase two-phase coordinate conversion unit. Is calculated.

  On the other hand, the flux weakening control unit of the control device obtains the target d-axis current from the rotation speed (estimated value) estimated by the position / speed estimator and the target q-axis current calculated by the speed control unit. Next, the d-axis current control unit performs d-axis voltage Vd by PI control from the target d-axis current calculated by the magnetic flux weakening control unit and the actual d-axis current Id calculated by the three-phase two-phase coordinate conversion unit. Is calculated.

  Then, the two-phase three-phase coordinate conversion unit of the control device performs two-phase three-phase conversion on the q-axis voltage Vq and the d-axis voltage Vd calculated by the q-axis current control unit and the d-axis current control unit. Voltage command value Vu, V-phase voltage command value Vv, and W-phase voltage command value Vw. The three-phase voltage command values Vu, Vv, Vw are further pulse width modulated, and pulse signals U, Ubar, V, Vbar, W, Wbar for controlling ON / OFF of the switching elements of the arms of the inverter main circuit are respectively obtained. It was generated and output.

JP 2007-312511 A JP 2000-262088 A

  Here, the target rotation speed is determined based on the temperature in the storage compartment of the commercial refrigerator. Then, the speed control unit of the control device, based on the deviation e between the rotational speed estimated by the position / speed estimator and the target rotational speed, reduces (eliminates) the deviation e in a predetermined control coefficient (gain). The target q-axis current is calculated by controlling PI (proportional integration, P (proportional) only, I (integral) only, D (differential) only, PD, PID, and so on). . Similarly, the q-axis current control unit also uses a predetermined control coefficient from the target q-axis current calculated by the speed control unit and the actual q-axis current Iq calculated by the three-phase two-phase coordinate conversion unit. The q-axis voltage Vq is calculated by PI control. When the electric motor to be controlled is an electric motor for a compressor (for example, a rotary compressor) constituting the refrigerant circuit, the discharge valve of the rotary compression unit in the refrigerant discharge process Immediately before opening, the pressure becomes the highest and the load (load applied to the electric motor) increases, and the load significantly decreases in the refrigerant suction process.

  For this reason, the q-axis current Iq and the q-axis voltage Vq that eventually flow according to the target q-axis current calculated by the speed control unit to control the rotation speed to the target rotation speed are large in the discharge process. Thus, the change becomes smaller in the inhalation process. This is shown in FIGS. 3 and 4. FIG.

  At this time, the change in the q-axis current Iq and the q-axis voltage Vq becomes smaller as shown in (b) of FIG. 3 and FIG. As shown in (a), the changes in the q-axis current Iq and the q-axis voltage Vq become large. That is, due to the control coefficient, an excessive torque is generated in the compressor suction process as shown in FIG. 3 and FIG. 4B, so that the vibration generated in the compressor increases and is connected to the compressor. Inconveniences such as breakage of the piping that occurs. There is also a problem that power for generating extra torque is consumed.

  Therefore, by changing the control coefficient, the q-axis current Iq and the q-axis voltage Vq can be drastically reduced in the compressor intake process as shown in FIG. 3 and FIG. Since the problem can be solved, conventionally, control using the control coefficient of (a) is performed instead of the control coefficient of (b).

  However, when the control of the control coefficient (a) is performed, the q-axis current Iq can be drastically decreased at a light load, but the q-axis current Iq is reduced at a heavy load (for example, a discharge process at an overload). The current increases rapidly, and the current flowing through the motor increases rapidly. As a result, the terminal voltage of the shunt resistor also rises, and the output voltage of the OP amplifier reaches the saturation region (showing the input / output voltage of the OP amplifier in FIG. 2), and the DC bus for the control device to detect the rotor position. The current Idc (current flowing through the electric motor) cannot be accurately detected.

  In order to eliminate this state, it is conceivable to reduce the gain (amplification degree) of the OP amplifier. However, this reduces the resolution, and the accuracy of rotor position detection in the control device decreases. Further, an OP amplifier that is compatible with full scale (full scale of DC power supply voltage) may be used, but there is a problem that it is expensive.

  The present invention has been made to solve the conventional technical problem, and when the compressor motor is controlled by so-called sensorless vector control, the breakage of the pipe can be prevented and the inverter can contribute to energy saving. The object is to provide the device at a low cost.

  In order to solve the above-described problems, an inverter device of the present invention controls a compressor motor that constitutes a refrigerant circuit, and two switching elements that perform opposite ON / OFF operations are connected in series to a DC power source. The three arms are connected in a three-phase bridge shape, an inverter main circuit that applies a three-phase PWM method three-phase pseudo-AC voltage to the motor, a shunt resistor for converting the current flowing through the motor into a voltage, The OP amplifier that amplifies the terminal voltage of the shunt resistor, and the current flowing to the motor is detected from the output of the OP amplifier, the rotor position of the motor is estimated based on the detected current, and each switching of the inverter main circuit is performed. Control means for controlling the ON / OFF operation of the element, and the output voltage of the OP amplifier to which the terminal voltage of the shunt resistor is input is saturated. In situations where a state, the control means may change the control factor in a direction to reduce a change in the current flowing through the electric motor.

  The inverter device of the invention of claim 2 is characterized in that, in the above, the control means changes the control coefficient so that the change of the current flowing through the electric motor is reduced stepwise when the output voltage of the OP amplifier is saturated. And

  According to a third aspect of the present invention, there is provided an inverter device according to the first aspect, wherein the control means changes the control coefficient in such a direction that the change of the current flowing through the motor is reduced when the output voltage of the OP amplifier approaches a saturated state. And

  The inverter device according to a fourth aspect of the present invention is characterized in that, in the above, the control means changes the control coefficient so as to reduce the change in the current flowing through the motor stepwise.

  In the inverter device according to the invention of claim 5, in each of the above inventions, the control means changes when the current flowing through the motor is reduced to a predetermined value after changing the control coefficient in a direction to reduce the change in the current flowing through the motor. The control coefficient is returned to the previous value.

  The inverter device of the invention of claim 6 is characterized in that, in the above, the control means returns the control coefficient stepwise.

  According to the present invention, in an inverter device for controlling an electric motor for a compressor constituting a refrigerant circuit, three arms formed by connecting two switching elements that perform opposite ON / OFF operations in series with a DC power source are connected to a three-phase bridge. Inverter main circuit that applies a three-phase PWM method three-phase pseudo AC voltage to the motor, a shunt resistor for converting the current flowing through the motor into a voltage, and an OP that amplifies the terminal voltage of this shunt resistor Control that detects the current flowing to the motor from the amplifier and the output of this OP amplifier, estimates the rotor position of the motor based on the detected current, and controls the ON / OFF operation of each switching element of the inverter main circuit In a situation where the output voltage of the OP amplifier to which the terminal voltage of the shunt resistor is input is saturated, Since the control means changes the control coefficient in a direction to reduce the change in the current flowing to the motor, normally, the control coefficient is changed to suppress the vibration of the compressor and contribute to energy saving, while Amplifies the terminal voltage of the shunt resistor to detect the rotor position and effectively eliminates the problem of saturation of the output voltage of the OP amplifier that is input to the control means, and accurately operates the compressor motor using the sensorless vector method. It will be able to control.

  In this case, if the control means as in the invention of claim 2 changes the control coefficient so that the change in the current flowing through the electric motor is reduced stepwise when the output voltage of the OP amplifier is saturated, The control coefficient can be changed more accurately according to the fluctuation of the load applied to the compressor motor.

  On the other hand, if the control means changes the control coefficient in such a direction that the change in the current flowing through the motor is reduced when the output voltage of the OP amplifier approaches the saturated state as in the invention of claim 3, the compressor motor Even when the load applied to the amplifier suddenly increases, it is possible to avoid the disadvantage that the output voltage of the OP amplifier is saturated.

  Also in this case, if the control means changes the control coefficient so as to reduce the change in the current flowing through the motor in a stepwise manner as in the invention of the fourth aspect, it corresponds to the fluctuation of the load applied to the compressor motor. Thus, the control coefficient can be changed more accurately.

  According to the invention of claim 5, in addition to the above inventions, the control means changes the control coefficient in a direction to reduce the change of the current flowing through the motor, and then the current flowing through the motor decreases to a predetermined value. In this case, since the control coefficient is returned to the value before the change, inconvenience due to the vibration of the compressor caused by excessive torque can be solved without any trouble.

  Also in this case, if the control means returns the control coefficient in a stepwise manner as in the invention of claim 6, the control coefficient can be returned more accurately according to the fluctuation of the load applied to the compressor motor. It will be possible.

It is a circuit block diagram of the inverter apparatus of one Embodiment to which this invention is applied. It is a figure which shows the input-output voltage of OP amplifier of FIG. It is a figure which shows the change of the q-axis current calculated from the direct current bus-line current which flows into the electric motor of FIG. It is a figure which similarly shows the change of q-axis voltage. It is a figure which shows the functional block of the control apparatus of FIG. 1, an inverter main circuit, and an electric motor.

  Hereinafter, embodiments of the present invention will be described in detail. The inverter device 1 of the embodiment is a so-called one shunt type inverter device, and 3 is a three-phase PWM (Pulse Width Modulation) type inverter main circuit. The inverter main circuit 3 converts a voltage supplied from the DC power supply unit 4 into a three-phase pseudo AC voltage having an arbitrary variable voltage and variable frequency, and outputs the converted voltage. In the embodiment, a compressor constituting a refrigerant circuit of a commercial refrigerator Is supplied to an electric motor 6 (for example, a synchronous motor). The inverter main circuit 3 connects three arms formed by connecting two switching elements (7u to 7w, 8u to 8w) that perform opposite ON / OFF operations in series to the DC power supply unit 4 in a three-phase bridge shape. Configured.

  That is, the inverter main circuit 3 includes a U-phase upper arm switching element 7u, a U-phase lower arm switching element 8u, a V-phase upper arm switching element 7v, and a V-phase lower arm switching element. 8v, W-phase upper arm switching element 7w and W-phase lower arm switching element 8w are provided, and each switching element 7u, 8u, 7v, 8v, 7w, 8w flows in the winding of the motor 6. A diode for circulating current is connected in reverse parallel. Note that a MOS-FET is used as the switching element. This MOS-FET can flow a current bidirectionally when it is ON, but an IGBT can also be used as a switching element.

  The switching elements 7u, 8u, 7v, 8v, 7w, and 8w are turned on when the pulse signal input to the gate is at “H” level, and turned off when the pulse signal is at “L” level. The shunt resistor 2 is connected to the DC power supply unit 4 in series with the DC bus of the inverter main circuit 3, and the DC bus current Idc (one shunt current) flows through the shunt resistor 2.

  The terminal voltage of the shunt resistor 2 is amplified by the OP amplifier 31 and then input to the control device (control means) 11. That is, the shunt resistor 2 converts the DC bus current Idc, which is a current flowing through the motor 6, into a voltage. The converted voltage is amplified by the OP amplifier 31 and then taken into the control device 11.

  32 is a negative feedback resistor, 33, 34 and 35 are resistors, and the terminal voltage of the shunt resistor 2 (on the switching element 8w side) is input to the non-inverting input terminal (+) of the OP amplifier 31, and the ground of the shunt resistor 2 The (GND) side (the ground side of the DC power supply unit 4) is input to the inverting input terminal (−). Reference numeral 36 denotes a power source connected to the non-inverting input (+) in order to cope with a negative current that flows during the magnetic flux weakening control. The input / output voltage of the OP amplifier 31 is shown in FIG.

  The control device 11 composed of a microcomputer takes in the output voltage of the OP amplifier 31 at a predetermined cycle (carrier signal cycle) based on the pulse signals U, Ubar, V, Vbar, W, Wbar output by itself. A DC bus current Idc (current flowing to the motor 6) flowing through the shunt resistor 2 is detected from this output voltage, and three-phase currents flowing to the motor 6 from the detected DC bus current Idc, that is, U-phase current Iu, V-phase current Iv. , W phase current Iw is estimated.

  FIG. 5 shows functional blocks of the control device 11. The bus current Idc flowing through the shunt resistor 2 is taken into the current detection unit 21, and the current detection unit 21 obtains three-phase phase currents Iu, Iv, and Iw. That is, when the U-phase upper arm switching element 7u is ON, the V-phase lower arm switching element 8v is ON, and the W-phase lower arm switching element 8w is ON, the U-phase current Iu (sign is negative) is estimated to be the detected DC bus current Idc.

  When the U-phase upper arm switching element 7u is ON, the V-phase upper arm switching element 7v is ON, and the W-phase lower arm switching element 8w is ON, the W-phase current Iw (sign is negative) is estimated to be the detected DC bus current Idc.

  Further, since the sum of the U-phase current Iu, the V-phase current Iv, and the W-phase current Iw becomes zero, the V-phase current Iv is also estimated by Iv = − (Iu + Iw). In this way, the current detection unit 21 obtains Iu, Iv, and Iw.

  Next, in the three-phase / two-phase coordinate conversion unit 22, the phase currents Iu, Iv, and Iw obtained by the current detection unit 21 are three-phase / two-phase converted, and the q-axis current Iq and the d-axis current Id are calculated. The Next, in the position / velocity estimator 23, the q-axis current Iq and the d-axis current Id calculated by the three-phase / two-phase coordinate conversion unit 22, and the q-axis current control unit 24 and the d-axis current control unit 26 are calculated. The rotation speed of the electric motor 6 and the magnetic pole position of the rotor (rotor) are estimated using the q-axis voltage Vq and the d-axis voltage Vd.

  Next, in the speed control unit 27, the target q is calculated by PI control from the rotational speed estimated by the position / speed estimator 22 and the target rotational speed (for example, calculated based on the temperature in the storage compartment of the commercial refrigerator). An axial current (q-axis current is proportional to torque) is calculated. In this case, based on the deviation e between the rotational speed estimated by the position / speed estimator 22 and the target rotational speed, the speed control unit 27 reduces (eliminates) the deviation e in the direction of decreasing (eliminating) the control coefficient ( PID coefficient (gain), which shows changes in Iq and Vq indicated by (a) in FIGS. 3 and 4, and PI (proportional integration. Note that only P (proportional) and I (integration) only , D (differentiation) only, any of PD and PID is allowed. The same applies hereinafter.) The target q-axis current is calculated by control.

  Next, in the q-axis current control unit 24, a predetermined control coefficient is calculated from the target q-axis current calculated by the speed control unit 27 and the actual q-axis current Iq calculated by the three-phase two-phase coordinate conversion unit 22. Similarly, the q-axis voltage Vq is calculated by PI control using (PID coefficient).

  On the other hand, in the flux weakening control unit 28 (control to reduce the induced voltage), the rotational speed (estimated value) estimated by the position / speed estimator 23 and the target q-axis current calculated by the speed control unit 27 are used. From the target d-axis current. Next, in the d-axis current control unit 26, from the target d-axis current calculated by the magnetic flux weakening control unit 28 and the actual d-axis current Id calculated by the three-phase two-phase coordinate conversion unit 22, the PI control is also performed. A d-axis voltage Vd is calculated.

  In the two-phase / three-phase coordinate conversion unit 29, the two-phase / three-phase conversion is performed on the q-axis voltage Vq and the d-axis voltage Vd calculated by the q-axis current control unit 24 and the d-axis current control unit 26, thereby obtaining the U-phase. Voltage command value Vu, V-phase voltage command value Vv, and W-phase voltage command value Vw are calculated. These three-phase voltage command values Vu, Vv, Vw are further pulse width modulated, and pulse signals for ON / OFF control of the switching elements 7u, 8u, 7v, 8v, 7w, 8w of each arm of the inverter main circuit 3, respectively. U, Ubar, V, Vbar, W, Wbar are generated.

  In the inverter main circuit 3, the switching elements 7u, 8u, 7v, 8v, 7w, 8w are ON / OFF controlled by the pulse signals U, Ubar, V, Vbar, W, Wbar, and the operation of the motor 6 is controlled. is there. In this way, the control device 11 performs sensorless vector control of the electric motor 6.

  Here, since the speed control unit 27 of the control device 11 performs the PI control using the above-described control coefficient, the suction of the compressor in which the electric motor 6 has a light load as shown in FIG. 3 and FIG. In the process, the q-axis current Iq and the q-axis voltage Vq can be rapidly decreased. Thereby, the problem of breakage of pipes due to excessive torque and unnecessary power consumption can be solved. Further, in the discharge process in which the load applied to the electric motor 6 increases, the q-axis current Iq and the q-axis voltage Vq can be rapidly increased to cope with load fluctuations quickly.

  However, in the control based on such a control coefficient, the motor 6 is overloaded in the discharge process in an environment where the outside temperature of the commercial refrigerator becomes high, or when the condenser fan for cooling the condenser of the refrigerant circuit is locked. It becomes a state. In this case, since the q-axis current Iq increases rapidly and the DC bus current Idc flowing through the motor 6 also increases rapidly, the terminal voltage of the shunt resistor 2 also increases abruptly and the output voltage of the OP amplifier 31 is in the saturation region ( 4V or more in FIG. 2).

  When the output voltage of the OP amplifier 31 is saturated, the current detection unit 21 of the control device 11 cannot accurately detect the DC bus current Idc, and it is impossible to estimate the phase currents Iu, Iv, and Iw. The device 11 cannot estimate the magnetic pole position of the rotor.

  Therefore, in the embodiment, the control device 11 monitors the output voltage of the OP amplifier 31, and when the output voltage becomes saturated (4 V or more in the embodiment), the control coefficient in the speed control unit 27 is shown in FIGS. In (b), the values are changed to values that change the q-axis current Iq and the q-axis voltage Vq. The degree of this change is determined in advance by experiments.

  Thereby, after the change, the change in the q-axis current Iq is reduced, and the change in the DC bus current Idc flowing through the electric motor 6 is also reduced, so that the output voltage of the OP amplifier 31 is not saturated even in the discharge process of the compressor. The DC bus current Idc can be detected.

  Here, the control coefficient may be changed step by step for each predetermined step. That is, after the predetermined step is reduced, if the output of the OP amplifier 31 still saturates, the predetermined step may be further reduced. Thus, if it reduces in steps, more precise control of saturation prevention becomes possible.

  In the above description, the control coefficient is changed when the output of the OP amplifier 31 is saturated. However, the present invention is not limited to this, and when the output voltage rises to a value just before saturation (for example, 3.8 V in FIG. 2), The control coefficient may be changed. According to such control, it becomes possible to avoid saturation of the output voltage of the OP amplifier 31 even when the load fluctuates suddenly. Furthermore, even in this case, if the value is changed step by step from the previous value every predetermined step, more accurate control is possible.

  Then, the control device 11 does not saturate the output voltage of the OP amplifier 31 even if the value (for example, peak value) of the DC bus current Idc detected by the shunt resistor 2 is returned to a predetermined low value (the original control coefficient). When the value is reduced to the (presumably value), the control coefficient in the speed control unit 27 is changed to the original value (the value indicating the change in (a) in FIGS. 3 and 4) (return). This makes it possible to return to the compressor vibration suppression and energy saving operation thereafter. In this case as well, more accurate return control is possible if the method is changed stepwise for each predetermined step.

  In the embodiment, the control coefficient of the speed control unit 27 is changed. However, the present invention is not limited to this, and the control coefficient in the q-axis current control unit 24, the d-axis current control unit 26, and other control units in FIG. Also good. In the embodiments, the inverter device of the one shunt resistance method has been described. However, the present invention is not limited to this, and the present invention is also effective for a two-shunt and three-shunt method in which a shunt resistor is inserted in each phase arm.

DESCRIPTION OF SYMBOLS 1 Inverter apparatus 2 Shunt resistance 3 Inverter main circuit 4 DC power supply part 6 Electric motor 7u-7w, 8u-8w Switching element 11 Control apparatus 21 Current detection part 27 Speed control part 31 OP amplifier

Claims (6)

In the inverter device for controlling the compressor motor constituting the refrigerant circuit,
Three arms formed by connecting two switching elements that perform opposite ON / OFF operations in series to a DC power source are connected in a three-phase bridge shape, and a three-phase PWM three-phase pseudo AC voltage is applied to the motor. An inverter main circuit;
A shunt resistor for converting the current flowing through the motor into a voltage;
An OP amplifier for amplifying the terminal voltage of the shunt resistor;
Control for detecting the current flowing through the motor from the output of the OP amplifier, estimating the rotor position of the motor based on the detected current, and controlling the ON / OFF operation of each switching element of the inverter main circuit Means and
In a situation where the output voltage of the OP amplifier to which the terminal voltage of the shunt resistor is input is saturated, the control means changes the control coefficient in a direction to reduce the change in the current flowing through the motor. Inverter device.   The said control means changes the said control coefficient so that the change of the electric current which flows into the said motor may be made small gradually when the output voltage of the said OP amplifier will be in a saturated state. Inverter device.   2. The inverter device according to claim 1, wherein when the output voltage of the OP amplifier approaches a saturated state, the control unit changes the control coefficient in a direction to reduce a change in a current flowing through the electric motor. .   The inverter device according to claim 3, wherein the control unit changes the control coefficient so as to reduce a change in the current flowing through the electric motor in a stepwise manner.   The control means returns the control coefficient to the value before the change when the current flowing through the electric motor decreases to a predetermined value after changing the control coefficient in a direction to reduce the change of the electric current flowing through the electric motor. The inverter device according to any one of claims 1 to 4, wherein:   6. The inverter apparatus according to claim 5, wherein the control means returns the control coefficient in a stepwise manner.

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