Power conversion device and control method thereof

Abstract

To provide a power conversion device and a control method thereof, capable of suppressing a rise in DC link voltage without adding a new circuit.SOLUTION: A power conversion device includes: a rectification circuit which rectifies input AC voltage to generate rectified voltage; a booster circuit which boosts the rectified voltage to generate boosted voltage; a capacitor with small capacitance which holds the boosted voltage; an inverter which converts the boosted voltage held in the capacitor with small capacitance into output AC voltage; a PFC (Power Factor Correction) control circuit which controls such that the boosted voltage held by the capacitor with small capacitance approaches a target voltage; and correction means which corrects a gain used in the PFC control circuit depending on the degree of fluctuation of the input AC voltage.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a power conversion device and a control method thereof. [Background technology]

[0002] A motor drive device is known in which a rectifier rectifies input AC power, a boost converter boosts the power rectified by the rectifier, a capacitor stores a pulsating voltage from the boost converter, an inverter having a plurality of switching elements converts AC power using the voltage across the capacitor and outputs the converted power to a motor, a regenerative power consumption unit arranged between the capacitor and the inverter consumes regenerative power from the motor, and a switching drive unit outputs a switching control signal to a switching element of the regenerative power consumption unit and an operation control signal to an inverter control unit when the voltage across the capacitor is equal to or higher than a predetermined voltage (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2015 / 0354880 Summary of the Invention [Problem to be solved by the invention]

[0004] If a configuration is adopted in which the regenerative power consumption unit consumes the regenerative power from the motor to suppress the rise in the DC link voltage, it is necessary to add a new circuit to suppress the rise in the DC link voltage.

[0005] An object of the present invention is to suppress an increase in the DC link voltage without adding a new circuit. [Means for solving the problem]

[0006] With this object in mind, the present invention provides a power conversion device including a rectifier circuit that rectifies an input AC voltage to generate a rectified voltage, a boost circuit that boosts the rectified voltage to generate a boosted voltage, a small-capacity capacitor that holds the boosted voltage, an inverter that converts the boosted voltage held in the small-capacity capacitor into an output AC voltage, a PFC (Power Factor Correction) control circuit that controls the boosted voltage held in the small-capacity capacitor so that it approaches a target voltage, and correction means that corrects a gain used in the PFC control circuit in accordance with the degree of fluctuation in the input AC voltage.

[0007] The correction means may use the distortion amount of the input AC voltage as the degree of fluctuation of the input AC voltage. In this case, the power conversion device may further include a calculation means for calculating the distortion amount of the input AC voltage by performing FFT (Fast Fourier Transform) processing on the rectified voltage generated by the rectifier circuit. Alternatively, the power conversion device may further include a calculation means for calculating the distortion amount of the input AC voltage by determining the difference between the rectified voltage generated by the rectifier circuit and an ideal rectified voltage.

[0008] The present invention also provides a power conversion device including a rectifier circuit that rectifies an input AC voltage to generate a rectified voltage, a boost circuit that boosts the rectified voltage to generate a boosted voltage, a small-capacity capacitor that holds the boosted voltage, an inverter that converts the boosted voltage held in the small-capacity capacitor into an output AC voltage, a PFC control circuit that controls the boosted voltage held in the small-capacity capacitor to approach a target voltage, and correction means that corrects a PWM (Pulse Width Modulation) duty set by the PFC control circuit in accordance with the degree of fluctuation of the input AC voltage.

[0009] The correction means may use a distortion amount of the input AC voltage as a degree of fluctuation of the input AC voltage. In this case, the power conversion device may further include a calculation means for calculating the distortion amount of the input AC voltage by determining a difference between a rectified voltage generated by the rectifier circuit and an ideal rectified voltage.

[0010] Furthermore, the present invention also provides a control method for a power conversion device including a rectifier circuit that rectifies an input AC voltage to generate a rectified voltage, a boost circuit that boosts the rectified voltage to generate a boosted voltage, a small-capacity capacitor that holds the boosted voltage, an inverter that converts the boosted voltage held in the small-capacity capacitor to an output AC voltage, and a PFC control circuit that controls the boosted voltage held in the small-capacity capacitor to approach a target voltage, the control method for a power conversion device including the steps of detecting a degree of fluctuation in the input AC voltage and correcting a gain used in the PFC control circuit in accordance with the detected degree of fluctuation.

[0011] Furthermore, the present invention also provides a control method for a power conversion device including a rectifier circuit that rectifies an input AC voltage to generate a rectified voltage, a boost circuit that boosts the rectified voltage to generate a boosted voltage, a small-capacity capacitor that holds the boosted voltage, an inverter that converts the boosted voltage held in the small-capacity capacitor to an output AC voltage, and a PFC control circuit that controls the boosted voltage held in the small-capacity capacitor to approach a target voltage, the control method for a power conversion device including the steps of detecting a degree of fluctuation in the input AC voltage, and correcting a PWM duty set by the PFC control circuit in accordance with the detected degree of fluctuation. Effect of the Invention

[0012] According to the present invention, it is possible to suppress an increase in the DC link voltage without adding a new circuit. [Brief description of the drawings]

[0013] [Figure 1] 1 is a diagram showing an example of the configuration of a motor drive device to which an embodiment of the present invention is applied; [Diagram 2] 4 is a graph showing a waveform of an input voltage from an AC power source when the AC power source is a poor power source. [Diagram 3] 13 is a graph showing waveforms of a rectified voltage and a DC link voltage before any countermeasure is taken when the AC power supply is an inferior power supply. [Figure 4]FIG. 4 is a diagram showing a gain change regulation voltage and a protection voltage on the graph of FIG. [Diagram 5] 1 is a graph to be referred to when implementing a method of utilizing the amount of distortion of an AC power supply voltage for gain adjustment. [Figure 6] FIG. 2 is a block diagram showing an example of a functional configuration of a control device for realizing the A method. [Figure 7] 10 is a flowchart showing an example of the operation of a control device for realizing method A. [Figure 8] 1 is a graph showing a comparison between an actual rectified voltage and an ideal rectified voltage. [Figure 9] 4 is a graph showing a difference between an actual rectified voltage and an ideal rectified voltage. [Figure 10] 13 is a graph showing a duty obtained by converting a difference after a low pass filter. [Figure 11] FIG. 11 is a block diagram showing an example of the functional configuration of a control device for realizing the B method. [Figure 12] 10 is a flowchart showing an example of the operation of a control device for implementing method B. [Figure 13] 11 is a graph showing waveforms of a rectified voltage and a DC link voltage after measures are taken according to methods A and B in a case where the AC power supply is an inferior power supply. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In recent years, the need for energy-saving products has increased due to the strengthening of environmental regulations in various countries around the world. In the air conditioner market, the demand for energy-saving products is expanding, especially in emerging countries, and the rate of inverter use is increasing. The challenges in converting home appliances to inverters are to reduce size, weight, and cost. The current system is a commonly used variable transmission with vector control in an AC / DC / AC configuration. The circuit configuration of this system requires the use of an electrolytic capacitor in the DC link section, which hinders the system from being made smaller and lighter.

[0015] By the way, one method to achieve size and weight reduction is the capacitorless inverter, which replaces the electrolytic capacitor with a small-capacity film capacitor. Although electrolytic capacitors have a short lifespan, capacitorless inverters are not subject to the limitations of electrolytic capacitors and have the advantage of a longer product lifespan.

[0016] However, the capacitor-less inverter has drawbacks due to the pulsation of the DC link, such as the inability to secure the required operating range, and concerns over noise, vibration, and deterioration of efficiency. In particular, it is susceptible to the effects of power sources such as the effects of system impedance, instantaneous voltage fluctuations, and poor power sources.

[0017] One possible solution to this problem is to use a system that adds a boost function to the capacitor-less inverter to suppress pulsation in the DC link and increase the voltage utilization rate.

[0018] Fig. 1 is a diagram showing a configuration example of a motor drive device 1 to which the present embodiment is applied. As shown in the figure, the motor drive device 1 is connected to an AC power supply 2 and a motor 3, and includes a rectifier circuit 10, a boost circuit 20, a DC link unit 30, an inverter 40, a PFC (Power Factor Correction) control circuit 50, and an inverter control circuit 60. The motor drive device 1 is an example of a power conversion device. The AC power supply 2 is a single-phase power supply, but may be a multi-phase power supply. The motor 3 is, for example, a permanent magnet embedded synchronous motor.

[0019] The rectifier circuit 10 rectifies a single-phase AC (input AC voltage) supplied from an AC power source 2 into a DC. The rectifier circuit 10 includes a diode bridge DB. The diode bridge DB is configured by connecting directly connected diodes D11 and D12 in parallel with directly connected diodes D13 and D14. That is, the diodes D11 and D12 are connected in series at a connection point a, and the diodes D13 and D14 are connected in series at a connection point b. These are then connected in parallel between the connection points c and d. The diodes D11 to D14 are connected so that a current flows in the direction from the connection point d to the connection point c.

[0020] Furthermore, the rectifier circuit 10 includes a voltage detector 11 at the connection point c. The voltage detector 11 detects the output voltage of the rectifier circuit 10 (hereinafter referred to as the "rectified voltage").

[0021] The boost circuit 20 boosts the rectified voltage and includes an inductor L1, a switching element SW1, and a feedback diode Df1.

[0022] An inductor L1 has one terminal connected to a connection point c and the other terminal connected to a connection point e.

[0023] The switching element SW1 has one terminal connected to a connection point e and the other terminal connected to a connection point f. The switching element SW1 is a high-voltage power switching element. For example, a field effect transistor or an insulated gate bipolar transistor (IGBT) can be used for the switching element SW1. The switching element SW1 is a three-terminal element, and the terminal that controls on / off is called a gate.

[0024] The feedback diode Df1 is connected in parallel to the switching element SW1. The feedback diode Df1 is connected in a direction in which a current flows from the connection point f to the connection point e. The direction in which a current flows through the feedback diode Df1 is opposite to the direction in which a current flows through the switching element SW1.

[0025] The boost circuit 20 also includes a current detection unit 21 that detects the output current of the rectifier circuit 10 (hereinafter, referred to as the “rectified current”).

[0026] The boost circuit 20 may be of an interleaved type.

[0027] The DC link unit 30 includes a diode D2 and a capacitor C1. The diode D2 has one terminal (anode) connected to the connection point e and the other terminal (cathode) connected to the connection point g. The capacitor C1 is a small-capacity film capacitor, and has one terminal connected to the connection point g and the other terminal connected to the connection point h.

[0028] The DC link unit 30 also includes a voltage detection unit 31 at connection point g. The voltage detection unit 31 detects the output voltage of the DC link unit 30 (measures the voltage value). Note that connection point h in the DC link unit 30 is a reference potential, and the potential (voltage) of connection point g relative to this reference potential is sometimes referred to as a DC link voltage (boosted voltage) Vdc. In other words, the DC link voltage Vdc is the voltage of the capacitor C1 (the voltage between connection points g and h). The DC link voltage Vdc is then supplied to the inverter 40 as the output voltage of the DC link unit 30.

[0029] The inverter 40 switches the direct current supplied by the DC link unit 30 to generate an alternating current (output AC voltage) for driving the motor 3, which is a load. Here, the inverter 40 generates a three-phase alternating current (U phase, V phase, and W phase).

[0030] The inverter 40 includes switching elements SW21 to SW26 and feedback diodes (no reference numerals) connected in parallel to the switching elements. Three pairs of switching elements, each pair of which is connected in series, are connected in parallel. That is, the switching elements SW21 and SW22 are connected in series, the switching elements SW23 and SW24 are connected in series, and the switching elements SW25 and SW26 are connected in series. These are connected in parallel between the input terminals P and N of the inverter 40. The switching elements SW21 to SW26 and the feedback diodes (no reference numerals) connected in parallel to them are connected in a direction in which a current flows from the input terminal P to the input terminal N. The direction of current flow in the feedback diode is opposite to the direction of current flow in the switching elements SW21 to SW26.

[0031] The connection point of the switching elements SW21 and SW22 is connected to the motor 3 as a U-phase, the connection point of the switching elements SW23 and SW24 is connected to the motor 3 as a V-phase, and the connection point of the switching elements SW25 and SW26 is connected to the motor 3 as a W-phase.

[0032] The inverter 40 also includes a current detection unit 41 that detects three-phase AC (U phase, V phase, and W phase). The current detection unit 41 may detect the current by, for example, a one-shunt method.

[0033] The PFC control circuit 50 includes an LPF (Low-pass filter) 51, a calculator 52, a PI (Proportional Integral) control unit 53, a phase detector 54, a multiplier 55, a calculator 56, a PI control unit 57, and a PWM (Pulse Width Modulation) control unit 58.

[0034] The LPF 51 blocks high-frequency components of the DC link voltage Vdc detected by the voltage detection unit 31 and passes low-frequency components.

[0035] The calculator 52 calculates a predetermined target DC link voltage (hereinafter referred to as the "target DC link voltage") Vdc *The differential between this and the DC link voltage Vdc output by the LPF 51 is calculated, and the result is output.

[0036] The PI control unit 53 calculates a current value A for making the difference output by the calculator 52 approach zero by PI control, and outputs the result. Hereinafter, the PI control unit 53 is referred to as "AVR", and the gain used in the PI control in this AVR is referred to as "AVR gain".

[0037] The phase detector 54 detects the phase θ of the AC power supply 2, and outputs the absolute value |cos θ| of the cosine wave of the detected phase θ.

[0038] Multiplier 55 multiplies the current value A output by PI control unit 53 by the absolute value |cos θ| of the cosine wave output by phase detector 54, and outputs the result A×|cos θ|.

[0039] The calculator 56 sets the multiplication result A×|cosθ| output by the multiplier 55 as a target rectified current, calculates the difference between this target rectified current and the rectified current detected by the current detection unit 21, and outputs the result.

[0040] The PI control unit 57 calculates a duty for bringing the difference output by the calculator 56 closer to zero by PI control, and outputs the result. Hereinafter, the PI control unit 57 is referred to as "ACR", and the gain used in the PI control in this ACR is referred to as "ACR gain".

[0041] The PWM control unit 58 controls the timing of switching the switching element SW1 on / off in accordance with the duty output by the PI control unit 57.

[0042] The inverter control circuit 60 includes a speed estimation unit 71, an angle estimation unit 72, a three-phase to two-phase conversion unit 73, a calculator 74, a PI control unit 75, and a multiplier 76. The inverter control circuit 60 also includes a calculator 81, a PI control unit 82, a calculator 83, a PI control unit 84, a two-phase to three-phase conversion unit 85, and a PWM control unit 86.

[0043] The speed estimation unit 71 estimates the actual angular speed ωrm of the motor 3 and outputs the result. Here, the actual angular speed ωrm of the motor 3 may be estimated using a known method.

[0044] The angle estimator 72 estimates the angle of the motor 3 based on the actual angular velocity ωrm of the motor 3 estimated by the velocity estimator 71, and outputs the result.

[0045] The three-phase to two-phase conversion unit 73 generates and outputs an actual d-axis current (hereinafter referred to as the "actual d-axis current") Id and an actual q-axis current (hereinafter referred to as the "actual q-axis current") Iq based on the three-phase AC (U phase, V phase, and W phase) detected by the current detection unit 41 and the angle of the motor 3 output by the angle estimation unit 72.

[0046] The calculator 74 calculates a target angular velocity ωrm of the motor 3 that is determined in advance. * and the actual angular velocity ωrm of the motor 3 output by the speed estimation unit 71, and outputs the result.

[0047] The PI control unit 75 calculates a current value B by PI control for bringing the difference output by the calculator 74 closer to zero, and outputs the result.

[0048] The multiplier 76 multiplies the current value B output by the PI control unit 75 by the absolute value |cos θ| of the cosine wave output by the phase detector 54, and outputs the result B×|cos θ|.

[0049] The calculator 81 converts the multiplication result B×|cosθ| output by the multiplier 76 into a target q-axis current (hereinafter referred to as the "target q-axis current") Iq * Then, the target q-axis current Iq * and the actual q-axis current Iq output by the three-phase to two-phase conversion unit 73 is calculated, and the result is output.

[0050] The PI control unit 82 calculates, by PI control, the q-axis voltage to be applied to the motor 3 in order to bring the difference output by the calculator 81 closer to zero, and outputs the result.

[0051] The calculator 83 calculates a predetermined target d-axis current (hereinafter referred to as the "target d-axis current") Id * and the actual d-axis current Id output by the three-phase to two-phase conversion unit 73, and outputs the result.

[0052] The PI control unit 84 calculates, by PI control, the d-axis voltage to be applied to the motor 3 in order to bring the difference output by the calculator 83 closer to zero, and outputs the result.

[0053] The two-phase to three-phase conversion unit 85 calculates three-phase (U phase, V phase, and W phase) voltages based on the q-axis voltage applied to the motor 3 output by the PI control unit 82, the d-axis voltage applied to the motor 3 output by the PI control unit 84, and the angle of the motor 3 output by the angle estimation unit 72, and outputs the result.

[0054] The PWM control unit 86 generates and outputs a signal that controls the timing of switching on / off of the switching elements SW21 to SW26 so that the three-phase (U phase, V phase, and W phase) voltage output by the two-phase to three-phase conversion unit 85 can be applied to the motor 3.

[0055] Incidentally, the configuration of the motor drive device 1 described above is a general one, and the operation thereof is also a general one, so a detailed description of the operation will be omitted.

[0056] Although the suppression of pulsation in the DC link unit 30 by using a system in which a boost function is added to a capacitor-less inverter has been described above, pulsation in the DC link unit 30 cannot be completely suppressed by this alone.

[0057] Therefore, here, we consider measures to be taken when voltage fluctuations occur in the AC power supply 2. Voltage fluctuations in the AC power supply 2 include, for example, instantaneous voltage fluctuations and voltage fluctuations when the AC power supply 2 is an inferior power supply.

[0058] 2 is a graph showing the waveform of the input voltage from the AC power source 2 when the AC power source 2 is an inferior power source. In this graph, voltage fluctuation occurs in the input voltage, and the maximum value of the input voltage is about 410V.

[0059] 1 uses a small-capacity film capacitor as capacitor C1, so there is a possibility that the DC link voltage will jump up if a voltage fluctuation occurs in the AC power supply 2. In addition, there is a possibility that the boost control by the boost circuit 20 will have an adverse effect and increase the jump. If the DC link voltage jumps up, it is necessary to stop operation to protect the elements.

[0060] Figure 3 is a graph showing the waveforms of the rectified voltage and DC link voltage before countermeasures are taken when the AC power source 2 is an inferior power source. In the figure, the thin line indicates the rectified voltage, and the thick line indicates the DC link voltage. For example, if an overvoltage error occurs and operation is stopped when the DC link voltage continues to be 410V or higher, in this graph, the maximum value of the DC link voltage is about 423V, so an overvoltage error occurs and operation is stopped.

[0061] However, for the motor drive device 1, continued operation is beneficial to the customer. Therefore, in this embodiment, a method of suppressing an increase in the DC link voltage is proposed.

[0062] The first method is a method for changing the gains of the PI control units 53, 57 in the PFC control circuit 50 according to the degree of voltage fluctuation when a voltage fluctuation of the AC power supply 2 is detected. Here, the degree of the voltage fluctuation of the AC power supply 2 may be, for example, the amount of distortion of the voltage of the AC power supply 2.

[0063] The second method is a method in which, when a voltage fluctuation of the AC power supply 2 is detected, the duty set by the PWM control unit 58 in the PFC control circuit 50 is corrected by feedforward according to the degree of the voltage fluctuation. In this case as well, the degree of the voltage fluctuation of the AC power supply 2 may be, for example, the amount of distortion of the voltage of the AC power supply 2.

[0064] Below, we will refer to the first method as "Method A" and the second method as "Method B," and explain their specific details.

[0065] First, the method A will be described.

[0066] Method A is a method in which, when the DC link voltage becomes equal to or higher than the gain change specified voltage V1, the gain is changed within a specified range of the distortion amount of the voltage of the AC power source 2, and the DC link voltage is suppressed to equal to or lower than the protection voltage V2.

[0067] Fig. 4 shows the gain change regulation voltage V1 and the protection voltage V2 on the graph of Fig. 3. When the DC link voltage becomes equal to or higher than the gain change regulation voltage V1 shown in the figure, the gain is changed and the DC link voltage is controlled so as not to exceed the protection voltage V2.

[0068] Here, as a first method for calculating the distortion amount of the AC power supply 2, there is a method in which the rectified voltage is subjected to FFT (Fast Fourier Transform) processing to calculate the distortion amount of the voltage for each frequency of the AC power supply 2, and the result is used for gain adjustment. As a second method, there is a method in which the magnitude of the difference between the actual rectified voltage and the ideal rectified voltage (the distortion amount for each frequency) is calculated, and the result is used for gain adjustment.

[0069] FIG. 5 is a graph that is referred to when implementing a method of using the amount of voltage distortion of the AC power source 2 for gain adjustment. As can be seen from this graph, there is a trade-off between the ACR gain and the AVR gain. Usually, the ACR gain is made stronger to suppress harmonic current so as to satisfy harmonic regulations. When the AVR gain is made stronger, the ACR output becomes relatively weaker and harmonic current increases, so it is generally not set in this way. However, in regions with poor power sources (countries with poor power supply conditions where the power source voltage is distorted), if the power source voltage is distorted, there is no need to satisfy harmonic regulations, and it is sufficient to continue operation. Therefore, when the power source voltage is distorted, the AVR gain is controlled to be stronger, and when the power source voltage is not distorted, the AVR gain is controlled to be weaker.

[0070] 6 is a block diagram showing an example of the functional configuration of a control device 90 for implementing this method A. As shown in the figure, the control device 90 includes a rectified voltage receiving unit 91, a voltage fluctuation amount calculating unit 92, and a gain changing unit 93.

[0071] The rectified voltage receiving unit 91 receives the rectified voltage from the voltage detecting unit 11 (see FIG. 1).

[0072] The voltage fluctuation amount calculation unit 92 calculates the amount of fluctuation in the voltage of the AC power source 2 based on the rectified voltage received by the rectified voltage receiving unit 91. Specifically, the voltage fluctuation amount calculation unit 92 calculates the amount of distortion of the voltage for each frequency of the AC power source 2 by performing FFT processing on the rectified voltage. Alternatively, the distortion amount for each frequency of the AC power source 2 may be calculated by obtaining a difference between the actual rectified voltage and the ideal rectified voltage. In this embodiment, the voltage fluctuation amount calculation unit 92 is provided as an example of a calculation means for calculating the amount of distortion of the input AC voltage by performing FFT processing on the rectified voltage generated by the rectifier circuit, and as an example of a calculation means for calculating the amount of distortion of the input AC voltage by obtaining a difference between the rectified voltage generated by the rectifier circuit and the ideal rectified voltage.

[0073] The gain change unit 93 changes the AVR gain used in the PI control unit 53 (see FIG. 1) and the ACR gain used in the PI control unit 57 (see FIG. 1) according to the amount of fluctuation in the voltage of the AC power supply 2 calculated by the voltage fluctuation amount calculation unit 92. Specifically, the gain change unit 93 determines the AVR gain and the ACR gain according to the amount of fluctuation in the voltage of the AC power supply 2. Then, the gain change unit 93 instructs the PI control unit 53 to change to the determined AVR gain and instructs the PI control unit 57 to change to the determined ACR gain. In this embodiment, the gain change unit 93 is provided as an example of a correction means for correcting the gain used in the PFC control circuit according to the degree of fluctuation in the input AC voltage.

[0074] The control device 90 is included in the motor drive device 1 and may be realized by, for example, a microcomputer. In this case, the rectified voltage receiving unit 91, the voltage fluctuation amount calculating unit 92, and the gain changing unit 93 are realized by a CPU (not shown) reading a program stored in a ROM (not shown) into a RAM (not shown) and executing it.

[0075] Fig. 7 is a flowchart showing an example of the operation of the control device 90. This example of the operation may be executed when the control device 90 receives a DC link voltage from the voltage detection unit 31 (see Fig. 1) and the DC link voltage becomes equal to or higher than the gain change specified voltage V1.

[0076] As shown in the figure, in the control device 90, first, the rectified voltage receiving unit 91 receives the rectified voltage from the voltage detecting unit 11 (see FIG. 1) (step 901).

[0077] Next, the voltage fluctuation calculation unit 92 calculates the amount of fluctuation in the voltage of the AC power supply 2 based on the rectified voltage received in step 901 (step 902).

[0078] Finally, the gain change unit 93 changes the AVR gain used in the PI control unit 53 and the ACR gain used in the PI control unit 57 according to the amount of fluctuation in the voltage of the AC power supply 2 calculated in step 902 (step 903).

[0079] Next, the method B will be described.

[0080] Method B is a method for suppressing fluctuations in the waveform of the DC link voltage by correcting the duty in the PWM control unit 58 of the PFC control circuit 50 by feedforward in accordance with the amount of distortion in the voltage of the AC power supply 2.

[0081] In method B, first, the actual rectified voltage is compared with the ideal rectified voltage.

[0082] 8 is a graph showing a comparison between the actual rectified voltage and the ideal rectified voltage, in which the thin line indicates the waveform of the actual rectified voltage and the thick line indicates the waveform of the ideal rectified voltage.

[0083] In method B, next, the difference between the actual rectified voltage and the ideal rectified voltage is calculated.

[0084] Figure 9 is a graph showing the difference between the actual rectified voltage and the ideal rectified voltage. In the figure, the thin line shows the difference waveform obtained by subtracting the ideal rectified voltage waveform from the actual rectified voltage waveform, and the thick line shows the low-pass differential waveform after only the low-frequency components are extracted from the difference waveform with a low-pass filter.

[0085] In method B, the low-pass difference is then converted into a corresponding duty, which is subtracted from the basic duty in the PWM control unit 58. Here, the conversion from the low-pass difference to a duty may be performed by, for example, calculating what duty a given voltage difference is converted into, assuming that any voltage from 0V to an arbitrary voltage equal to or greater than the maximum value of the low-pass difference is converted into a duty of 0% to 100%.

[0086] Fig. 10 is a graph showing the duty thus obtained. When the duty is not corrected, if the actual rectified voltage is larger than the ideal rectified voltage, the DC link voltage increases compared to the ideal state, and if the actual rectified voltage is smaller than the ideal rectified voltage, the DC link voltage decreases compared to the ideal state. Therefore, when the duty is corrected, as shown in the figure, if the actual rectified voltage is larger than the ideal rectified voltage, an operation to reduce the duty is performed, and if the actual rectified voltage is smaller than the ideal rectified voltage, an operation to increase the duty is performed. In other words, the low-frequency components of the voltage difference obtained by subtracting the ideal rectified voltage from the actual rectified voltage are removed, and the duty obtained by converting this is subtracted from the basic duty.

[0087] 11 is a block diagram showing an example of the functional configuration of a control device 95 for implementing this method B. As shown in the figure, the control device 95 includes a rectified voltage receiving unit 96, a voltage fluctuation amount calculating unit 97, and a duty changing unit 98.

[0088] The rectified voltage receiving unit 96 receives the rectified voltage from the voltage detecting unit 11 (see FIG. 1).

[0089] The voltage fluctuation amount calculation unit 97 calculates the amount of fluctuation in the voltage of the AC power supply 2 based on the rectified voltage received by the rectified voltage receiving unit 96. Specifically, the voltage fluctuation amount calculation unit 97 calculates the amount of distortion for each frequency of the AC power supply 2 by calculating the difference between the actual rectified voltage and the ideal rectified voltage. In this embodiment, the voltage fluctuation amount calculation unit 97 is provided as an example of a calculation means that calculates the amount of distortion of the input AC voltage by calculating the difference between the rectified voltage generated by the rectifier circuit and the ideal rectified voltage.

[0090] The duty changing unit 98 changes the duty set by the PWM control unit 58 (see FIG. 1) according to the amount of fluctuation in the voltage of the AC power supply 2 calculated by the voltage fluctuation amount calculation unit 97. Specifically, the duty is determined according to the amount of fluctuation in the voltage of the AC power supply 2. Then, the duty changing unit 98 instructs the PWM control unit 58 to change to the determined duty. In this embodiment, the duty changing unit 98 is provided as an example of a correction means for correcting the PWM duty set by the PFC control circuit according to the degree of fluctuation in the input AC voltage.

[0091] The control device 95 is included in the motor drive device 1 and may be realized by, for example, a microcomputer. In this case, the rectified voltage receiving unit 96, the voltage fluctuation calculation unit 97, and the duty change unit 98 are realized by a CPU (not shown) reading a program stored in a ROM (not shown) into a RAM (not shown) and executing it.

[0092] FIG. 12 is a flow chart showing an example of the operation of the control device 95.

[0093] As shown in the figure, in the control device 95, first, the rectified voltage receiving unit 96 receives the rectified voltage from the voltage detection unit 11 (see FIG. 1) (step 951).

[0094] Next, the voltage fluctuation calculation unit 97 calculates the amount of fluctuation in the voltage of the AC power supply 2 based on the rectified voltage received in step 951 (step 952).

[0095] Finally, the duty changer 98 changes the duty set by the PWM controller 58 in accordance with the amount of fluctuation in the voltage of the AC power supply 2 calculated in step 952 (step 953).

[0096] Fig. 13 is a graph showing waveforms of the rectified voltage and DC link voltage after measures are taken according to methods A and B when the AC power source 2 is an inferior power source. In the figure, the thin line indicates the rectified voltage, and the thick line indicates the DC link voltage. For example, if an overvoltage error occurs and operation stops when the DC link voltage continues to be 410V or higher, the maximum value of the DC link voltage in this graph is approximately 402V, so operation can continue.

[0097] As described above, in this embodiment, the control device 90 changes the gain used in the PFC control circuit 50 in accordance with the amount of distortion in the voltage of the AC power supply 2, or the control device 95 changes the duty set in the PFC control circuit 50 in accordance with the amount of distortion in the voltage of the AC power supply 2. This makes it possible to suppress an increase in the DC link voltage without adding a new circuit. [Explanation of symbols]

[0098] 1...motor drive device, 10...rectifier circuit, 11...voltage detection unit, 20...booster circuit, 30...DC link unit, 40...inverter, 50...PFC control circuit, 53, 57...PI control unit, 58...PWM control unit, 60...inverter control circuit, 90, 95...control device, 91, 96...rectified voltage receiving unit, 92, 97...voltage fluctuation calculation unit, 93...gain change unit, 98...duty change unit

Claims

1. a rectifier circuit that rectifies an input AC voltage to generate a rectified voltage; a boost circuit that boosts the rectified voltage to generate a boosted voltage; a small-capacity capacitor for holding the boosted voltage; an inverter that converts the boosted voltage held in the small-capacity capacitor into an output AC voltage; a PFC (Power Factor Correction) control circuit that controls the boosted voltage held in the small-capacity capacitor so as to approach a target voltage; a correction means for increasing an AVR gain used in the PFC control circuit when a distortion amount of the input AC voltage is large; A power conversion device comprising:

2. The power conversion device as described in Claim 1, characterized in that the correction means weakens the ACR gain used in the PFC control circuit when the distortion amount of the input AC voltage is large.

3. 2. The power conversion device according to claim 1, further comprising a calculation means for calculating the amount of distortion of the input AC voltage by performing FFT (Fast Fourier Transform) processing on the rectified voltage generated by the rectifier circuit.

4. 2. The power conversion device according to claim 1, further comprising a calculation means for calculating the distortion amount of the input AC voltage by determining a difference between a rectified voltage generated by the rectifier circuit and an ideal rectified voltage.

5. A control method for a power conversion device including a rectifier circuit that rectifies an input AC voltage to generate a rectified voltage, a boost circuit that boosts the rectified voltage to generate a boosted voltage, a small-capacity capacitor that holds the boosted voltage, an inverter that converts the boosted voltage held in the small-capacity capacitor into an output AC voltage, and a PFC control circuit that controls the boosted voltage held in the small-capacity capacitor to approach a target voltage, detecting a distortion amount of the input AC voltage; increasing an AVR gain used in the PFC control circuit when the detected distortion amount is large; A method for controlling a power conversion device comprising:

6. The method for controlling a power conversion device as described in claim 5, further comprising the step of weakening an ACR gain used in the PFC control circuit when the detected amount of distortion is large.

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