JP7666901B2 - Power conversion device and control method thereof - Google Patents

Description

The present invention relates to a power conversion device and a control method thereof.

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 equipped with multiple switching elements converts AC power using the voltage across the capacitor and outputs the converted power to a motor, a regenerative power consumption unit disposed between the capacitor and the inverter consumes regenerative power from the motor, and a switching drive unit outputs a switching control signal to the 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).

US Patent Application Publication No. 2015/0354880

If a regenerative power consumption unit is used to consume regenerative power from the motor to suppress the rise in DC link voltage, a new circuit must be added to suppress the rise in DC link voltage.

The objective of the present invention is to suppress the rise in DC link voltage without adding new circuits.

To this end, 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 a correction means that corrects the gain used in the PFC control circuit according to the degree of fluctuation in the input AC voltage.

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 finding the difference between the rectified voltage generated by the rectifier circuit and an ideal rectified voltage.

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 to 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 a correction means that corrects the duty of a PWM (Pulse Width Modulation) set by the PFC control circuit according to the degree of fluctuation in the input AC voltage.

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 determining the difference between the rectified voltage generated by the rectifier circuit and an ideal rectified voltage.

The present invention further provides a method for controlling 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 method including the steps of detecting the degree of fluctuation in the input AC voltage and correcting the gain used in the PFC control circuit according to the detected degree of fluctuation.

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 the degree of fluctuation in the input AC voltage and correcting the PWM duty set by the PFC control circuit according to the detected degree of fluctuation.

The present invention makes it possible to suppress the rise in DC link voltage without adding new circuits.

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; 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. 11 is a graph showing waveforms of a rectified voltage and a DC link voltage before a countermeasure is taken when the AC power supply is an inferior power supply. FIG. 4 is a diagram showing a gain change regulation voltage and a protection voltage on the graph of FIG. 3 . 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. FIG. 2 is a block diagram showing an example of a functional configuration of a control device for realizing the A system. 11 is a flowchart showing an example of the operation of the control device for realizing the A method. 1 is a graph showing a comparison between an actual rectified voltage and an ideal rectified voltage. 4 is a graph showing a difference between an actual rectified voltage and an ideal rectified voltage. 13 is a graph showing a duty obtained by converting a difference after a low pass filter. FIG. 11 is a block diagram showing an example of a functional configuration of a control device for realizing the B system. 10 is a flowchart showing an example of the operation of a control device for realizing the B method. 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.

In recent years, the need for energy-saving products has increased due to stricter environmental regulations in various countries around the world. In the air conditioner market, the demand for energy-saving products is expanding, particularly in emerging countries, and the rate of inverter use is increasing. The challenge in converting home appliances to inverters is to reduce size, weight, and cost. Current systems are vector-controlled variable transmissions with a commonly used AC/DC/AC configuration. The circuit configuration of this system requires the use of electrolytic capacitors in the DC link section, which prevents the system from being made smaller and lighter.

By the way, one method of achieving compactness and weight reduction is the capacitorless inverter, which replaces electrolytic capacitors with small-capacity film capacitors. Electrolytic capacitors have a short lifespan, but capacitorless inverters are not restricted by this and have the advantage of a longer product lifespan.

However, capacitor-less inverters have drawbacks due to the pulsation of the DC link section. These drawbacks include the inability to ensure the required operating range, and concerns about noise, vibration, and deterioration of efficiency. In particular, they are susceptible to the effects of power sources, such as the effects of system impedance, instantaneous voltage fluctuations, and poor power supplies.

One idea to eliminate these drawbacks is to use a system that adds a boost function to a capacitorless inverter to suppress pulsation in the DC link and increase the voltage utilization rate.

Figure 1 is a diagram showing an example of the configuration of a motor drive device 1 to which this embodiment is applied. As shown in the figure, the motor drive device 1 is connected to an AC power source 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 source 2 is a single-phase power source, but may be a multi-phase power source. The motor 3 is, for example, a permanent magnet embedded synchronous motor.

The rectifier circuit 10 rectifies the single-phase AC (input AC voltage) supplied from the AC power source 2 into 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 connection point a, and the diodes D13 and D14 are connected in series at connection point b. These are then connected in parallel between connection point c and connection point d. The diodes D11 to D14 are connected so that a current flows in the direction from connection point d to connection point c.

The rectifier circuit 10 also 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").

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

One terminal of inductor L1 is connected to connection point c, and the other terminal is connected to connection point e.

One terminal of the switching element SW1 is connected to the connection point e, and the other terminal is connected to the 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 the gate.

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

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").

The boost circuit 20 may also be of an interleaved type.

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

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 the DC link voltage (boosted voltage) Vdc. In other words, the DC link voltage Vdc is the voltage of 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.

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

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

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

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 using, for example, a one-shunt method.

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.

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

The calculator 52 calculates the difference between a predetermined target DC link voltage (hereinafter referred to as the "target DC link voltage") Vdc ^* and the DC link voltage Vdc output by the LPF 51, and outputs the result.

The PI control unit 53 uses PI control to calculate the current value A for bringing the difference output by the calculator 52 closer to zero, 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".

The phase detector 54 detects the phase θ of the AC power supply 2 and outputs the absolute value |cosθ| of the cosine wave of the detected phase θ.

The multiplier 55 multiplies the current value A output by the PI control unit 53 by the absolute value |cosθ| of the cosine wave output by the phase detector 54, and outputs the result A × |cosθ|.

The calculator 56 sets the multiplication result A×|cosθ| output by the multiplier 55 as the 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.

The PI control unit 57 uses PI control to calculate the duty for bringing the difference output by the calculator 56 closer to zero, 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".

The PWM control unit 58 controls the timing for switching the switching element SW1 on/off according to the duty output by the PI control unit 57.

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.

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

The angle estimation unit 72 estimates the angle of the motor 3 based on the actual angular velocity ωrm of the motor 3 estimated by the speed estimation unit 71, and outputs the result.

The three-phase to two-phase conversion unit 73 generates and outputs the actual d-axis current (hereinafter referred to as the "actual d-axis current") Id and the 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.

The calculator 74 calculates the difference between a predetermined target angular velocity ωrm ^* of the motor 3 and the actual angular velocity ωrm of the motor 3 output by the velocity estimation unit 71, and outputs the result.

The PI control unit 75 uses PI control to calculate the current value B for bringing the difference output by the calculator 74 closer to zero, and outputs the result.

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

The calculator 81 sets the multiplication result B × |cos θ| output by the multiplier 76 as a target q-axis current (hereinafter referred to as the "target q-axis current") Iq ^* , calculates the difference between this target q-axis current Iq ^* and the actual q-axis current Iq output by the three-phase to two-phase conversion unit 73, and outputs the result.

The PI control unit 82 uses PI control to calculate 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.

The calculator 83 calculates the difference between 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 converter 73, and outputs the result.

The PI control unit 84 uses PI control to calculate 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.

The two-phase to three-phase conversion unit 85 calculates the 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.

The PWM control unit 86 generates and outputs a signal that controls the timing of switching on/off 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.

The configuration of the motor drive device 1 described above is common, and its operation is also common, so a detailed explanation of its operation will be omitted.

The above describes how pulsation in the DC link unit 30 can be suppressed by using a system that adds a boost function to a capacitorless inverter, but this alone cannot completely suppress pulsation in the DC link unit 30.

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 a poor power supply.

Figure 2 is a graph showing the waveform of the input voltage from the AC power source 2 when the AC power source 2 is a poor power source. In this graph, voltage fluctuations occur in the input voltage, and the maximum value of the input voltage is about 410 V.

In the motor drive device 1 in FIG. 1, a small-capacity film capacitor is used as the capacitor C1, so if a voltage fluctuation occurs in the AC power supply 2, the DC link voltage may jump up. In addition, the boost control by the boost circuit 20 may 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.

Figure 3 is a graph showing the waveforms of the rectified voltage and DC link voltage before countermeasures are taken when 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 410 V or higher, in this graph, the maximum value of the DC link voltage is approximately 423 V, so an overvoltage error occurs and operation stops.

However, for the motor drive device 1, continuing operation is beneficial for the customer. Therefore, in this embodiment, we propose a method for suppressing the rise in DC link voltage.

The first method is to change the gains of the PI control units 53 and 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 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.

The second method is to correct the duty set by the PWM control unit 58 in the PFC control circuit 50 by feedforward in accordance with the degree of voltage fluctuation when a voltage fluctuation of the AC power supply 2 is detected. Here too, the degree of 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.

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

First, let me explain method A.

Method A is a method in which, when the DC link voltage becomes equal to or exceeds 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 less than the protection voltage V2.

Figure 4 shows the gain change regulation voltage V1 and the protection voltage V2 on the graph of Figure 3. When the DC link voltage becomes equal to or greater than the gain change regulation voltage V1 shown in the figure, the gain is changed and the DC link voltage is controlled so that it does not exceed the protection voltage V2.

The first method for calculating the amount of distortion of the AC power supply 2 is to perform FFT (Fast Fourier Transform) on the rectified voltage to calculate the amount of voltage distortion for each frequency of the AC power supply 2, and use this for gain adjustment. The second method is to calculate the magnitude of the difference between the actual rectified voltage and the ideal rectified voltage (the amount of distortion for each frequency), and use this for gain adjustment.

Figure 5 is a graph that is referred to when implementing a method of using the amount of distortion in the voltage 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. Normally, the ACR gain is made stronger to suppress harmonic current so as to satisfy harmonic regulations. If 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.

Figure 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 calculation unit 92, and a gain change unit 93.

The rectified voltage receiving unit 91 receives the rectified voltage from the voltage detection unit 11 (see Figure 1).

The voltage fluctuation 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 distortion amount for each frequency of the AC power source 2 is calculated 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 calculating the difference between the actual rectified voltage and the ideal rectified voltage. In this embodiment, the voltage fluctuation 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 calculating the difference between the rectified voltage generated by the rectifier circuit and the ideal rectified voltage.

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.

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 calculation unit 92, and the gain change unit 93 are realized by the CPU (not shown) reading a program stored in the ROM (not shown) into the RAM (not shown) and executing it.

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

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

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

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

Next, we will explain method B.

Method B is a method for suppressing fluctuations in the waveform of the DC link voltage by feedforward correcting the duty in the PWM control unit 58 of the PFC control circuit 50 according to the amount of distortion in the voltage of the AC power source 2.

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

Figure 8 is a graph showing a comparison between the actual rectified voltage and the ideal rectified voltage. In the figure, the thin line shows the waveform of the actual rectified voltage, and the thick line shows the waveform of the ideal rectified voltage.

In method B, the difference between the actual rectified voltage and the ideal rectified voltage is then calculated.

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 difference waveform after extracting only the low-frequency components from the difference waveform with a low-pass filter.

In method B, the low-pass difference is then converted to 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 can be performed by, for example, calculating what duty a given voltage difference is converted to, assuming that any voltage from 0V to any voltage equal to or greater than the maximum value of the low-pass difference is converted to a duty of 0% to 100%.

Figure 10 is a graph showing the duty thus obtained. If the duty is not corrected, when the actual rectified voltage is greater than the ideal rectified voltage, the DC link voltage rises compared to the ideal state, and when the actual rectified voltage is less than the ideal rectified voltage, the DC link voltage falls compared to the ideal state. Therefore, when correcting the duty, as shown in the figure, if the actual rectified voltage is greater than the ideal rectified voltage, an operation is performed to reduce the duty, and if the actual rectified voltage is less than the ideal rectified voltage, an operation is performed to increase the duty. 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.

Figure 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.

The rectified voltage receiving unit 96 receives the rectified voltage from the voltage detection unit 11 (see Figure 1).

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 by the rectified voltage receiving unit 96. Specifically, the voltage fluctuation 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 calculation unit 97 is provided as an example of a calculation means for calculating the amount of distortion in the input AC voltage by calculating the difference between the rectified voltage generated by the rectifier circuit and the ideal rectified voltage.

The duty change 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 change unit 98 determines the duty according to the amount of fluctuation in the voltage of the AC power supply 2. Then, the duty change unit 98 instructs the PWM control unit 58 to change to the determined duty. In this embodiment, the duty change 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.

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 the CPU (not shown) reading a program stored in the ROM (not shown) into the RAM (not shown) and executing it.

Figure 12 is a flowchart showing an example of the operation of the control device 95.

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).

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

Finally, the duty change unit 98 changes the duty set by the PWM control unit 58 in accordance with the amount of fluctuation in the voltage of the AC power supply 2 calculated in step 952 (step 953).

Figure 13 is a graph showing the waveforms of the rectified voltage and DC link voltage after measures are taken using methods A and B when 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 410 V or higher, in this graph the maximum value of the DC link voltage is approximately 402 V, so operation can continue.

In this manner, in this embodiment, the control device 90 changes the gain used in the PFC control circuit 50 according to the amount of distortion in the voltage of the AC power source 2, or the control device 95 changes the duty set in the PFC control circuit 50 according to the amount of distortion in the voltage of the AC power source 2. This makes it possible to suppress the rise in the DC link voltage without adding a new circuit.

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 (6)

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;
and 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 . 2. The power conversion device according to claim 1, wherein the correction means weakens an ACR gain used in the PFC control circuit when the distortion amount of the input AC voltage is large. 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. 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. 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;
and increasing an AVR gain used in the PFC control circuit when the detected amount of distortion is large . 6. The method for controlling a power conversion device according to 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.