Inverter control device and method

Abstract

The present invention relates to an inverter control device and method. In a method of one embodiment of the present invention, the rotation speed of a motor is estimated; when the DC terminal voltage of an inverter providing an output voltage to the motor is below a specified level, a slip frequency instruction is determined using the energy of a DC terminal capacitor of the inverter and a DC terminal energy instruction; and a frequency instruction determined by adding the rotation speed of the motor and the slip frequency instruction is provided to the inverter.

Description

Technical Field

[0001] The invention relates to an inverter control device and method. Background Art

[0002] Generally, an inverter is a reverse conversion device that converts direct current (DC) into alternating current (AC). An inverter used in industry is defined as a series of devices that receive power from a commercial power supply and change the voltage and frequency to supply it to the motor, thereby controlling the motor speed to be used efficiently. This inverter is controlled by variable voltage variable frequency (VVVF), and can change the voltage and frequency input to the motor according to the pulse width modulation (PWM) output.

[0003] Figure 1 This is a diagram of the structure of a common inverter.

[0004] Generally, the inverter 100 receives three-phase AC power, the rectifier 110 rectifies the received AC power, and the smoother 120 smoothes and stores the DC voltage rectified by the rectifier 110. The inverter 130 outputs the DC voltage stored in the DC link capacitor as the smoother 120 as an AC voltage having a predetermined voltage and frequency according to a PWM control signal, and supplies the AC voltage to the motor.

[0005] Generally, the rectifier 110 is composed of a diode as a passive element. Since such a diode cannot be turned on and off and cannot control the DC terminal voltage stored in the smoothing unit 120, the DC terminal voltage of the smoothing unit 120 depends on the AC input voltage. In addition, since the power flows unidirectionally from the AC power supply of the power grid to the DC terminal voltage of the inverter 100, regenerative operation is not possible. Therefore, it is difficult for the existing inverter 100 to actively operate according to the power supply and load status.

[0006] As described above, the inverter 100 using the rectifier unit 110 composed of diodes cannot stably maintain the DC terminal voltage due to a voltage drop or power failure of the AC power supply, so there is a problem that the inverter cannot operate continuously. When the inverter is difficult to operate continuously, it takes a long time to restart the inverter 100. Summary of the invention

[0007] Problem that the invention aims to solve

[0008] An object of the present invention is to provide an inverter control device and method thereof that enable an inverter to be continuously operated even when power supply to the inverter is abnormal.

[0009] Means used to solve problems

[0010] In order to solve the technical problems mentioned above, an inverter control device of one embodiment of the present invention may include: a first calculation unit, which estimates the rotational speed of a motor; a first determination unit, which determines a slip frequency instruction by using the energy of a DC-terminal capacitor of the inverter and a DC-terminal energy instruction when the DC-terminal voltage of an inverter that provides an output voltage to the motor is below a specified level; and a generation unit, which provides a frequency instruction to the inverter that is determined by adding the rotational speed of the motor and the slip frequency instruction.

[0011] In an embodiment of the present invention, the first estimating unit may estimate the rotation speed of the motor by using an output voltage and an output current of an inverter and a nominal value of the motor.

[0012] In one embodiment of the present invention, the first estimating unit may include: a second determining unit, which determines the output power using the output voltage and output current of the inverter; a third determining unit, which determines the load torque using the output power and the output frequency of the inverter; a second estimating unit, which estimates the slip frequency using the load torque, the rated slip frequency of the inverter, and the rated torque; and a third estimating unit, which estimates the speed of the motor using the difference between the slip frequency estimated by the second estimating unit and the output frequency of the inverter.

[0013] In an embodiment of the present invention, the first estimating unit may further include a low pass filter (LPF) configured to perform low pass filtering on the slip frequency estimated by the second estimating unit.

[0014] In one embodiment of the present invention, the first determination unit may include: a fourth determination unit, which determines the required energy based on the energy of the DC-end capacitor of the inverter and the DC-end energy instruction; a fifth determination unit, which determines the power instruction based on the required energy; a sixth determination unit, which determines the torque instruction using the power instruction and the speed of the motor; and a seventh determination unit, which determines the slip frequency instruction from the torque instruction using a torque constant.

[0015] In one embodiment of the present invention, the fifth determining unit may include a ratio controller.

[0016] In one embodiment of the present invention, the torque constant may be determined by using the rated torque of the motor and the rated slip frequency of the motor.

[0017] In an embodiment of the present invention, the generating unit may determine a voltage command from the frequency command and provide the voltage command to the inverter.

[0018] The inverter control device according to an embodiment of the present invention may further include a control unit, and when the DC terminal voltage of the inverter is below a predetermined level, the control unit provides an action flag to the first determination unit.

[0019] In addition, for the technical purpose as described above, an inverter control method of one embodiment of the present invention may include: a step of estimating the rotational speed of a motor; a step of determining a slip frequency command using the energy of a DC-terminal capacitor of the inverter and a DC-terminal energy command when a DC-terminal voltage of an inverter that provides an output voltage to the motor is below a specified level; and a step of providing a frequency command to the inverter that is determined by adding the rotational speed of the motor and the slip frequency command.

[0020] In one embodiment of the present invention, the estimating step may include: a step of determining the output power using the output voltage and output current of the inverter; a step of determining the load torque using the output power and the output frequency of the inverter; a step of estimating the slip frequency using the load torque, the rated slip frequency of the inverter and the rated torque; and a step of estimating the speed of the motor using the difference between the estimated slip frequency and the output frequency of the inverter.

[0021] In one embodiment of the present invention, the determining step may include: a step of determining the required energy based on the energy of the DC-end capacitor of the inverter and the DC-end energy instruction; a step of determining the power instruction based on the required energy; a step of determining the torque instruction using the power instruction and the speed of the motor; and a step of determining the slip frequency instruction from the torque instruction using a torque constant.

[0022] Effects of the Invention

[0023] The present invention as described above has the effect of enabling the inverter to operate continuously by performing a regenerative operation of converting the mechanical energy of the motor into the electrical energy of the inverter in the event of an input power failure, thereby maintaining a constant DC terminal voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram of the structure of a common inverter.

[0025] Figure 2 This is a diagram for explaining the structure of a conventional inverter control system.

[0026] Figure 3 yes Figure 2 Detailed diagram of the inverter control unit.

[0027] Figure 4 This is a curve used to illustrate the relationship between the inverter operating frequency and the inverter output voltage.

[0028] Figure 5 FIG. 1 is a diagram showing a structure of an inverter system according to an embodiment of the present invention.

[0029] Figure 6 yes Figure 5 A detailed structural diagram of an embodiment of a speed estimation unit.

[0030] Figure 7 yes Figure 5 A detailed structural diagram of an embodiment of a slip frequency generating unit.

[0031] Figure 8 It is used to illustrate Figure 5 The structure diagram of the control relationship between the motor and the slip frequency generating unit.

[0032] Fig. 9 FIG. 1 is an exemplary diagram for illustrating an inverter control method according to an embodiment of the present invention.

[0033] Fig.10 FIG. 1 is an exemplary diagram for explaining the voltage-current relationship that changes due to the control operation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0034] In order to fully understand the structure and effect of the present invention, the preferred embodiment of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below, but can be implemented in various forms and can be variously changed. However, the description of this embodiment is intended to provide a complete disclosure of the present invention and fully disclose the scope of the present invention to those of ordinary skill in the art to which the present invention belongs. In the accompanying drawings, for ease of explanation, the size of the constituent elements is enlarged, and the proportion of the constituent elements may be exaggerated or reduced.

[0035] The terms "first", "second", etc. can be used to describe various constituent elements, but these constituent elements should not be limited to the above terms. The above terms can only be used to distinguish one constituent element from another constituent element. For example, without departing from the scope of the present invention, the "first constituent element" can be named as the "second constituent element", and similarly, the "second constituent element" can also be named as the "first constituent element". In addition, unless the context clearly stipulates otherwise, the expression of the singular includes the representation of the plural. Unless otherwise defined, the terms used in the embodiments of the present invention can be interpreted as the meanings known to those skilled in the art.

[0036] Below, refer to Figures 2 to 4 For an explanation of the existing inverter control system, refer to Figures 5 to 10 An inverter control device and method according to an embodiment of the present invention is described.

[0037] Figure 2 This is a diagram for explaining the structure of a conventional inverter control system. Figure 3 yes Figure 2 Detailed configuration diagram of the inverter control unit 200. If the user inputs the command frequency ω ref , the inverter control unit 200 synthesizes a command voltage corresponding to the command frequency and outputs it to the inverter unit 130.

[0038] exist Figure 3 In the embodiment of the present invention, the inverter control unit 200 includes a voltage determination unit 210 , an integration unit 220 , a phase determination unit 230 , and an output unit 240 .

[0039] The voltage determination unit 210 can determine the inverter operation frequency ω v / f Determine the output voltage V V / f The inverter operating frequency ω v / f Indicates from 0 to the command frequency ω ref The voltage up to this point. Figure 4 This is a curve used to illustrate the relationship between the inverter operating frequency and the inverter output voltage.

[0040] When the inverter 100 is initially started, the operating frequency of the inverter is v / f It starts to increase from 0, so a small voltage is output at the initial startup, and as the frequency increases, a voltage proportional to the frequency is output. If the output frequency reaches the command frequency ω as the target frequency ref , the frequency will not increase further and the machine will run at a constant speed.

[0041] That is, the voltage determination unit 210 determines the voltage according to Figure 4 The relationship between the inverter operating frequency ω v / f Determine the output voltage V V / f size.

[0042] The integrator 220 calculates the operating frequency ω v / f Integrate to determine the phase of the output voltage θ V / f The trigonometric function output unit 230 outputs a three-phase AC sine wave. The output unit 240 multiplies the AC sine wave and the output voltage to output a command voltage V as_ref 、V bs_ref 、V cs_ref .

[0043] That is, the inverter control unit 200 synthesizes three-phase pulse width modulation (PWM) voltages corresponding to the three-phase command voltages and outputs a final command voltage V abc_PWM .exist Figure 2In the inverter 100, the inverter unit 130 outputs a three-phase AC output voltage V from the DC terminal voltage of the smoothing unit 120. an 、V bn 、V cn The induction motor 400 is supplied with power, and the three-phase AC output voltage depends on the on / off state of the switch of the inverter 130. In the inverter 130, each phase is composed of two switches connected in series, and each phase operates independently of each other to generate an output voltage. At this time, the output voltages of each phase are controlled to have a phase difference of 120 degrees.

[0044] In the conventional inverter control system, the inverter 100 receives AC power from the three-phase power supply 300 as an energy source, and converts the AC power into DC power through the rectifier 110. At this time, the rectifier 110 is composed of a diode, so the current always flows in one direction.

[0045] The smoothing unit 120 is composed of a capacitor or a battery, etc., and maintains a predetermined voltage. The three-phase switch of the inverter unit 130 converts a DC voltage into an AC voltage, and controls the output voltage of the inverter 100 by turning on / off the switch.

[0046] The motor 400 functions as a load of the three-phase inverter 100. The inverter control unit 200 controls the switching state of the inverter unit 130 so that the motor 400 rotates at the same speed as the command frequency.

[0047] In the conventional inverter control system as described above, since the rectifier 110 is composed of a diode as a passive element, it is difficult to control the DC terminal voltage, and when the three-phase power supply is abnormally operated, such as a momentary power failure or voltage drop, the inverter 100 cannot continue to operate and cannot control the DC terminal voltage to be constant, so there is a problem that a failure of the inverter 100 occurs. In addition, in this case, it takes a considerable amount of time to restart the inverter 100 depending on the situation, so there is a problem that a large loss occurs.

[0048] Therefore, a voltage ride-through operation that can continue to operate regardless of the power supply state is required.

[0049] The present invention is proposed to solve the above-mentioned problems and provides an inverter control device that receives electric energy of an inverter from kinetic energy of a motor through regenerative operation, thereby continuously executing the operation of the inverter and the motor even when power supply is abnormal.

[0050] That is, the control device of the present invention uses the kinetic energy of load inertia to maintain the DC power supply of the inverter when the power is off, and performs regenerative operation by adjusting the operating frequency appropriately. Therefore, the greater the inertia and the smaller the load, the more stable the DC power supply can be maintained for a long time.

[0051] Figure 5 FIG. 1 is a diagram showing a structure of an inverter system according to an embodiment of the present invention.

[0052] As shown in the figure, the inverter control device according to an embodiment of the present invention may include a command voltage generating unit 10 for supplying a command voltage to the inverter 1 , a speed estimating unit 20 for estimating the speed of the motor 2 , a slip frequency generating unit 30 , and a control unit 40 .

[0053] The control unit 40 may check the DC terminal voltage of the inverter 1 and, when the voltage is equal to or lower than a predetermined level, transmit a flag (Falg) to the slip frequency generating unit 30 to start the operation of the slip frequency generating unit 30 .

[0054] The speed estimation unit 20 can estimate the speed of the rotor of the electric motor 2 . Figure 6 yes Figure 5 Detailed configuration diagram of an embodiment of the speed estimation unit 20. Since open loop control is performed in the case of general voltage / frequency operation, it is difficult to use the speed information obtained by a speed measuring device such as an encoder. Therefore, the speed estimation unit 20 of an embodiment of the present invention estimates the speed of the motor 2 by using the available inverter output voltage and current and the nominal value of the motor 2.

[0055] As shown in the figure, the speed estimation unit 20 of the present invention may include an output power determination unit 21 , a load torque determination unit 22 , a slip frequency estimation unit 23 , a speed determination unit 24 and a low pass filter (LPF) 25 .

[0056] The output power determination unit 21 may consider the output voltage V of the inverter 1 V / f The output power of inverter 1 is determined by the torque component output current of inverter 1, that is, the synchronous coordinate system q-axis current Iqse and the number of poles of each instruction reference.

[0057] In addition, the load torque determination unit 22 can use the output power and the output frequency ω of the inverter 1 v / f To determine the load torque.

[0058] The load torque outputted by the output power determination unit 21 and the load torque determination unit 22 is expressed by the following mathematical formula.

[0059] Mathematical formula 1

[0060]

[0061] The slip frequency estimation unit 23 can estimate the slip frequency using the load torque, the rated slip frequency and the rated torque of the inverter 1. The slip frequency estimated by the slip frequency estimation unit 24 is expressed by the following equation.

[0062] Mathematical formula 2

[0063]

[0064] The speed determination unit 24 can estimate the speed of the motor 1 using the difference between the estimated slip frequency and the output frequency of the inverter 1. The estimated speed of the motor 1 is expressed by the following mathematical formula.

[0065] Mathematical formula 3

[0066]

[0067] At this time, the LPF 25 may be disposed between the output terminal of the slip frequency estimation unit 23 and the input terminal of the speed determination unit 24 to perform low-pass filtering, and the filtering band may be determined in consideration of the stability of the system.

[0068] exist Figure 5 In the embodiment, the slip frequency generating unit 30 can compare the DC terminal voltage command V* with the DC terminal voltage command V* under the control of the control unit 40 when the DC terminal voltage is below the predetermined level. dc and the actual DC terminal voltage V dc To determine the slip frequency for keeping the DC terminal voltage constant. Generally, the slip frequency is the difference between the inverter output frequency and the motor speed. In one embodiment of the present invention, when the DC terminal voltage reaches below a specified level, the motor speed can be made greater than the inverter output frequency by determining the slip frequency as a negative number.

[0069] Figure 7 yes Figure 5 A detailed structural diagram of an embodiment of a slip frequency generating unit.

[0070] As shown in the figure, the slip frequency generating unit 30 according to an embodiment of the present invention may include an energy determining unit 31 , a power command determining unit 32 , a torque command determining unit 33 and a slip frequency command generating unit 34 , and may perform regeneration operation using slip frequency.

[0071] The slip frequency can be calculated from the difference between the electric energy corresponding to the DC terminal voltage command and the electric energy corresponding to the actual DC terminal voltage.

[0072] If the energy of the DC terminal capacitor is referred to as E, the DC terminal energy command is referred to as E*, and the energy of the capacitor required for the regenerative operation of one embodiment of the present invention is referred to as E cap, the energy of the DC terminal capacitor and the energy of the capacitor required for regenerative operation are respectively expressed as the following mathematical formulas. The energy determination unit 31 can determine the energy of the capacitor required for regenerative operation through mathematical formula 5.

[0073] Mathematical formula 4

[0074]

[0075] Mathematical formula 5

[0076]

[0077] At this time, in the above mathematical formula, C cap is the capacitance of the DC terminal capacitor, V* dc It is the DC terminal voltage command.

[0078] On the other hand, the power of the DC terminal capacitor is as follows.

[0079] Mathematical formula 6

[0080]

[0081] The power command determination unit 32 may receive the energy of the capacitor required for the regenerative operation and determine the power command P* cap For example, the power command determination unit 32 may be a proportional control gain K p Proportional controller.

[0082] Generally, a proportional controller measures the output of a controlled object and calculates an error by comparing it with a desired reference value or a set value, and uses the error value to calculate a control value required for control, thereby performing a control action proportional to the magnitude of the error value in the current state. However, this is only an example, and it is self-evident to those skilled in the art that the power instruction determination unit 32 can be composed of other controllers.

[0083] The torque command determination unit 33 may determine the torque command using the speed estimated by the speed estimation unit 20 .

[0084] The relationship between the rated torque, speed and force of the motor is as shown in the following mathematical formula.

[0085] Mathematical formula 7

[0086]

[0087] Here, T rated is the rated torque of motor 2, pole is the number of poles, P rated is the rated power of motor 2, ω ratedis the rated speed of the motor 2. Therefore, the torque command determination unit 33 can determine the torque command using the above equation 7.

[0088] The slip frequency command determination unit 34 may use the torque constant of the motor 2 To calculate the slip frequency. The relationship between torque and slip frequency is as follows.

[0089] Mathematical formula 8

[0090] K t =ω sl_rated / T rated

[0091] That is, if the above relationship is used, the slip frequency command determination unit 34 can determine the slip frequency command using the torque command and the torque constant. rated is the rated torque of motor 2, ω sl_rated is the rated slip frequency of motor 2.

[0092] exist Figure 5 In the above process, the slip frequency command outputted from the slip frequency generating unit 30 is added to the estimated speed of the motor 2 outputted from the speed estimating unit 20 , and the resultant is inputted to the command voltage generating unit 10 .

[0093] The command voltage generating unit 10 is used to control the output voltage of the inverter 1 , and can determine the output voltage and the operating frequency of the inverter 1 from the command frequency and provide a composite voltage to the inverter 1 .

[0094] The command voltage generating unit 10 performs the Figure 2 The inverter control unit 200 has the same function as the inverter control unit 200, because the operation of the command voltage generating unit 10 and the operation of the inverter 1 are similar to those of the reference Figure 2 The descriptions are the same, so detailed descriptions thereof are omitted.

[0095] Figure 8 It is used to illustrate Figure 5 7 and 8.

[0096] exist Figure 8 The transfer function in the control structure is as follows.

[0097] Mathematical formula 9

[0098]

[0099] At this time, K p is the gain of the proportional controller of the power command determination unit 32, and are the calculated motor speed and torque constants respectively. At this time, Depend on Figure 5 The speed estimation unit 20 determines the speed of the vehicle.

[0100] In equation 9, if the speed and torque constants are estimated accurately, they can be expressed as in equation 10.

[0101] Mathematical formula 10

[0102]

[0103] That is, the transfer function of the slip frequency generating unit 30 can be expressed in the form of a first-order low-pass filter as shown in Equation 10, and the bandwidth can be selected by the gain of the ratio controller as the power command determining unit 32 .

[0104] The control unit 40 of one embodiment of the present invention continuously checks the DC terminal voltage, and provides a flag to enable the slip frequency generating unit 30 when the DC terminal voltage is below a predetermined level, thereby enabling the slip frequency generating unit 30 to be turned on. Figure 8 The control shown is to back up the kinetic energy of the motor 2.

[0105] Fig. 9 is an example diagram for illustrating an inverter control method according to an embodiment of the present invention. Fig.10 This is an exemplary diagram for explaining the voltage-current relationship of a control operation according to an embodiment of the present invention.

[0106] As shown in the figure, in the control method of one embodiment of the present invention, the control unit 40 can continuously confirm the DC terminal voltage of the inverter 1 (S90), and can confirm whether the DC terminal voltage of the inverter 1 is below a specified level (S91). In the case where the input power fails, the DC terminal voltage decreases due to the influence of the input power.

[0107] The level of the DC terminal voltage at which the control unit 40 transmits the operation flag to the slip frequency generating unit 30 in the present invention can be determined by the setting of the operator.

[0108] When the control unit 40 determines that the DC terminal voltage of the inverter 1 is below a predetermined level, the control unit 40 may transmit an operation flag to turn on the operation of the slip frequency generating unit 30. Fig.10 In the embodiment, if the DC terminal voltage becomes less than a predetermined reference level (10A) while the input power is normal and the DC terminal voltage is maintained at a predetermined level, the control unit 40 may transmit an operation flag to the slip frequency generating unit 30 to start the operation of the slip frequency generating unit 30. At this time, as an example, when a 220V commercial power supply is used, the reference level (10A) may be 200V.

[0109] Even in this case, the speed estimation unit 20 can estimate the rotation speed of the rotor of the electric motor 2 by continuing the operation.

[0110] After that, when the speed estimation unit 20 estimates the speed of the motor 2 ( S93 ), the slip frequency generation unit 30 may generate a slip frequency command using the energy of the DC terminal voltage.

[0111] Thereafter, a frequency command may be generated by adding the slip frequency to the estimated speed ( S95 ), and the command voltage generating unit 10 may generate an output voltage of the inverter corresponding to the frequency command and supply the output voltage to the inverter 1 ( S96 ).

[0112] Reference Fig.10 , when the DC terminal voltage is less than the reference level (10A), the frequency command is generated by adding the slip frequency command to the estimated speed of the motor 2. At this time, the inverter output frequency runs at a speed lower than the motor speed, and the magnitude of the output voltage can be determined by the relationship between voltage and frequency. That is, the output frequency of the inverter 1 can be output at a speed lower than the speed of the motor 2, and the DC terminal voltage of the inverter 1 can be kept constant.

[0113] Afterwards, if the DC terminal voltage returns to the specified value T recovery (S97), the control unit 40 can control to transmit an action flag to the slip frequency generating unit 30 to turn off the slip frequency generating unit 30, so that the normal operation is achieved (S98). Fig.10 , the control unit 40 can control the inverter output frequency to increase when the power is restored to restore to the speed before the failure.

[0114] As described above, according to the present invention, when an input power failure occurs, a regenerative operation is performed to convert the mechanical energy of the motor into the electrical energy of the inverter, thereby maintaining a constant DC terminal voltage and allowing the inverter to continue to operate without stopping.

[0115] It should be understood that the embodiments of the present invention described above are merely exemplary, and those skilled in the art can implement various variations and embodiments within the scope of equivalents based on these embodiments. Therefore, the true technical protection scope of the present invention should be determined by the attached claims.

Claims

1. An inverter control device, wherein: include: A first estimation unit estimates the rotation speed of the motor; a first determination unit that determines a slip frequency command using energy of a DC terminal capacitor of the inverter and a DC terminal energy command when a DC terminal voltage of an inverter that provides an output voltage to the motor is below a predetermined level; as well as a generating unit that provides the inverter with a frequency command determined by adding the rotation speed of the motor and the slip frequency command; The first determining unit includes: a fourth determining unit, which determines the required energy according to the energy of the DC terminal capacitor of the inverter and the DC terminal energy instruction; a fifth determining unit, configured to determine a power instruction according to the required energy; a sixth determination unit that determines a torque command using the power command and the rotation speed of the motor; and The seventh determination unit determines the slip frequency command using a torque constant from the torque command.

2. The inverter control device according to claim 1, wherein: The first estimation unit estimates the rotation speed of the motor using an output voltage and an output current of the inverter and a nominal value of the motor.

3. The inverter control device according to claim 1, wherein: The first estimation unit comprises: a second determining unit, which determines the output power by using the output voltage and output current of the inverter; a third determining unit, which determines a load torque by using the output power and an output frequency of the inverter; a second estimating unit that estimates a slip frequency using the load torque, a rated slip frequency of the inverter, and a rated torque; and The third estimation unit estimates the rotation speed of the electric motor using a difference between the slip frequency estimated by the second estimation unit and the output frequency of the inverter.

4. The inverter control device according to claim 3, wherein: The first estimating unit further includes a low-pass filter that performs low-pass filtering of the slip frequency estimated by the second estimating unit.

5. The inverter control device according to claim 1, wherein: The fifth determination unit includes a ratio controller.

6. The inverter control device according to claim 1, wherein: The torque constant is determined using a rated torque of the motor and a rated slip frequency of the motor.

7. The inverter control device according to claim 1, wherein: The generating unit determines a voltage command from the frequency command and provides the voltage command to the inverter.

8. The inverter control device according to claim 1, wherein: The system further includes a control unit configured to provide an action flag to the first determination unit when the DC terminal voltage of the inverter is below a predetermined level.

9. An inverter control method, wherein: include: Steps for estimating the speed of the motor; When a DC terminal voltage of an inverter providing an output voltage to the motor is below a predetermined level, determining a slip frequency command by using energy of a DC terminal capacitor of the inverter and a DC terminal energy command; as well as a step of providing the inverter with a frequency command determined by adding the rotation speed of the motor and the slip frequency command; The step of determining the slip frequency command comprises: The step of determining the required energy according to the energy of the DC terminal capacitor of the inverter and the DC terminal energy instruction; The step of determining a power instruction according to the required energy; The step of determining a torque command using the power command and the speed of the motor; and The step of determining the slip frequency command from the torque command using a torque constant.

10. The inverter control method according to claim 9, wherein: The step of estimating the rotation speed of the motor comprises: The step of determining the output power by using the output voltage and output current of the inverter; The step of determining the load torque by using the output power and the output frequency of the inverter; The step of calculating the slip frequency by using the load torque, the rated slip frequency of the inverter and the rated torque; and A step of estimating the rotation speed of the electric motor by using the difference between the estimated slip frequency and the output frequency of the inverter.

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