Power conversion device and motor control device

Abstract

This power conversion device (50) comprises: a converter circuit (2) that converts, to a DC current, an AC current input from an AC power supply (1); an inverter circuit (5) that converts the DC current output from the converter circuit (2) into an AC current and supplies the AC current to a load; a first capacitor (3) that is connected between a positive electrode-side bus (31) and a negative electrode-side bus (32) that connect the converter circuit (2) and the inverter circuit (5); a regenerative power absorption circuit (4) that absorbs the regenerative power of a motor (7); a shunt resistor (6) that is installed on the negative electrode-side bus (32); and an overcurrent determination circuit (8) that sends, to the inverter circuit (5), a stop signal for stopping power supply to the motor (7), when the current detected on the basis of a voltage across the shunt resistor (6) exceeds a protection threshold value. The protection threshold value is a current value of a previously set magnitude that is less than 80% of the magnitude of the demagnetization current of the motor (7).

Description

Power conversion device and motor control device

[0001] The present disclosure relates to a power conversion device and a motor control device.

[0002] The motor control device includes a power conversion device that converts power supplied from an AC power supply and outputs it to a load (a motor). The power conversion device includes a converter circuit that rectifies AC current input from the AC power supply and outputs DC current, a smoothing capacitor that smoothes the converter output, and an inverter circuit that converts the DC current output from the converter into AC current and outputs it to the load to drive the load.

[0003] In recent years, efforts have been made to reduce the capacity of smoothing capacitors used in power conversion devices in order to achieve smaller size and higher power factors. Typically, motor control devices using small-capacity smoothing capacitors are equipped with a regenerative power absorption circuit in which a series connection of a diode and a capacitor is connected in parallel to the smoothing capacitor to prevent overvoltage across the smoothing capacitor during regeneration at the load. The voltage across the smoothing capacitor is called the bus voltage.

[0004] Furthermore, the power conversion device is provided with an overcurrent protection circuit to protect the semiconductor elements constituting the inverter circuit and the load. Generally, as in the power generation device disclosed in Patent Document 1, the threshold value for determining an overcurrent is set to 80% or more of the demagnetization current, which is the minimum current at which irreversible demagnetization occurs in the load, in consideration of variations in load characteristics and in order to avoid the occurrence of irreversible demagnetization in the load due to sudden fluctuations in operating conditions.

[0005] Japanese Patent Application Publication No. 10-56799

[0006] However, although the power generating device disclosed in Patent Document 1 can prevent irreversible demagnetization from occurring in the load, 80% or more of the demagnetization current flows through the regenerative power absorption circuit, which requires the capacitance of the capacitor in the regenerative power absorption circuit to be large, resulting in a problem of the regenerative power absorption circuit becoming larger.

[0007] The present disclosure has been made in consideration of the above, and aims to provide a power conversion device that achieves miniaturization of the capacitor in the regenerative power absorption circuit while avoiding the occurrence of irreversible demagnetization in the load.

[0008] In order to solve the above-mentioned problems and achieve the object, a power conversion device according to the present disclosure includes a converter circuit that converts AC current input from an AC power source into DC current, an inverter circuit that converts the DC current output from the converter circuit into AC current and supplies the AC current to a load, a positive bus bar and a negative bus bar that connect the converter circuit and the inverter circuit, a first capacitor connected between the positive bus bar and the negative bus bar, a regenerative power absorption circuit connected in parallel to the first capacitor and that absorbs regenerative power of the load, a shunt resistor installed in the negative bus bar, and an overcurrent determination circuit that sends a stop signal to the inverter circuit to stop power supply to the load when the magnitude of the current detected based on the voltage across the shunt resistor exceeds a protection threshold value. The protection threshold value is a preset current value that is less than 80% of the demagnetizing current of the load.

[0009] The power conversion device according to the present disclosure has the advantage of being able to reduce the size of the capacitor in the regenerative power absorption circuit while avoiding the occurrence of irreversible demagnetization in the load.

[0010] Circuit diagram of a motor control device according to embodiment 1. Diagram showing an example of a hardware configuration realizing a motor control unit provided in a power conversion device according to embodiment 1. Circuit diagram of an overcurrent determination circuit of the power conversion device according to embodiment 1. Diagram showing a simulated waveform of a bus voltage of the power conversion device according to embodiment 1. Diagram showing a simulated waveform of a motor current of the power conversion device according to embodiment 1. Structural diagram schematically showing the configuration of an air conditioner according to embodiment 2. Diagram showing the configuration of an outdoor unit of an air conditioner according to embodiment 2.

[0011] Hereinafter, a power conversion device and a motor control device according to an embodiment will be described in detail with reference to the drawings.

[0012] 1 is a circuit diagram of a motor control device according to embodiment 1. The motor control device 100 includes a power conversion device 50 that converts power input from an AC power supply 1 into DC and then converts it back into AC and outputs it to a motor 7, which is a load, and a motor control unit 40 that controls the rotational speed of the motor 7.

[0013] The power conversion device 50 includes a converter circuit 2 that converts AC current input from an AC power source 1 into DC current, an inverter circuit 5 that converts the DC current output from the converter circuit 2 into AC current, and a positive bus 31 and a negative bus 32 that connect the converter circuit 2 and the inverter circuit 5. The power conversion device 50 also includes a first capacitor 3 that is a smoothing capacitor that smoothes the DC current output from the converter circuit 2, a regenerative power absorption circuit 4 that is installed in parallel with the first capacitor 3, a reactor 14 for power factor improvement, a shunt resistor 6 that detects a motor current i.d.c. that is a current flowing through a motor 7, and an overcurrent determination circuit 8 that determines whether the motor current i.d.c. is an overcurrent. The power conversion device 50 also includes a snubber resistor 12 and a snubber capacitor 13 that convert high-frequency components into heat and absorb them.

[0014] The AC power supply 1 is, for example, a three-phase AC power supply, but may also be a single-phase AC power supply. The motor 7 is, for example, a permanent magnet synchronous motor, and the inductance per phase is set to be 2 mH or more and 6 mH or less.

[0015] The converter circuit 2 includes a plurality of diodes 2a, 2b, 2c, 2d, 2e, and 2f.

[0016] The inverter circuit 5 is configured, for example, as a full-bridge circuit including six switching elements 5a. Specifically, the switching elements 5a are connected in series to form a series body. Three series bodies are connected in parallel to form a full-bridge circuit. A freewheeling diode 5b is connected in anti-parallel to each switching element 5a. Each switching element 5a performs on / off operation independently in accordance with a pulse-width modulation signal input from the motor control unit 40. The on / off operation of each switching element 5a converts direct current into alternating current. Note that, while examples of the switching elements 5a include insulated gate bipolar transistors and metal-oxide semiconductor field-effect transistors, other elements may also be used. The motor 7 is driven by the currents iu, iv, and iw output by the inverter circuit 5.

[0017] The first capacitor 3 is installed between the positive bus bar 31 and the negative bus bar 32. A small-capacity capacitor is used as the first capacitor 3. For example, a film capacitor with a capacitance of 20 μF or more and 30 μF or less is selected as the first capacitor 3.

[0018] The regenerative power absorption circuit 4 includes a diode 9 connected between the positive bus 31 and the positive side of the first capacitor 3, a second capacitor 10 connected in series with the diode 9, and a discharge resistor 11 connected in parallel with the second capacitor 10. The capacitance of the second capacitor 10 is five times or less the capacitance of the first capacitor 3. For example, an electrolytic capacitor with a capacitance of 50 μF or more and 150 μF or less is selected as the second capacitor 10. Furthermore, the second capacitor 10 and the discharge resistor 11 may each be composed of a plurality of elements.

[0019] The reactor 14 is installed on the positive bus 31. The reactor 14 has an inductance of about 300 μH or more and 1 mH or less.

[0020] The shunt resistor 6 is installed on the negative bus 32, and a voltage drop occurs when the motor current i.d.c. flows through it. The amount of voltage drop across the shunt resistor 6 is proportional to the magnitude of the motor current i.d.c. In this way, the voltage across the shunt resistor 6, i.e., the potential difference generated by the shunt resistor 6, indicates the magnitude of the motor current i.d.c., so the motor current i.d.c. can be detected using the voltage across the shunt resistor 6. The overcurrent determination circuit 8 determines whether an overcurrent is flowing through the motor 7 based on the current detected using the voltage across the shunt resistor 6.

[0021] The power conversion device 50 also includes an amplifier circuit 15 installed between the shunt resistor 6 and the overcurrent determination circuit 8. The amplifier circuit 15 amplifies a voltage corresponding to the amount of voltage drop caused by the shunt resistor 6 and transmits the amplified voltage to the overcurrent determination circuit 8.

[0022] The motor control unit 40 has a current detection unit 41 that detects the magnitude of the motor current i.d.c. flowing through the bus, and a pulse width modulation signal generation unit 42 that outputs a pulse width modulation signal to each switching element 5a of the inverter circuit 5 based on the detection result of the current detection unit 41.

[0023] 2 is a diagram showing an example of a hardware configuration that realizes the motor control unit included in the power conversion device according to embodiment 1. The motor control unit 40 is realized by a processing circuit that includes a processor 91 that executes various processes, a memory 92 that is a main memory, and a storage device 93 that stores information.

[0024] The processor 91 may be a computing device such as an arithmetic unit, a microprocessor, a microcomputer, a CPU (Central Processing Unit), or a DSP (Digital Signal Processor). The memory 92 may be a non-volatile or volatile semiconductor memory such as a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory), or an EEPROM (Electrically Erasable Programmable Read Only Memory). The storage device 93 stores a program for executing processing to control the motor 7. The processor 91 reads the program stored in the storage device 93 into the memory 92 and executes it. The processor 91 reads the program stored in the storage device 93 into the memory 92 and executes it, thereby realizing the functions of the motor control unit 40.

[0025] 3 is a circuit diagram of an overcurrent determination circuit of a power conversion device according to embodiment 1. The overcurrent determination circuit 8 includes a low-voltage power supply 17, which is a DC power supply with a lower voltage than the AC power supply 1, a plurality of resistors 19 and 20 connected in series between the low-voltage power supply 17 and ground, a comparator 16 that compares the magnitude of the DC voltage divided by the plurality of resistors 19 and 20 with a voltage corresponding to the voltage drop across the shunt resistor 6, and a low-pass filter 22 composed of a resistor 18 and a capacitor 21. The DC voltage of the low-voltage power supply 17 is divided by the resistors 19 and 20, and a reference potential is input to the comparator 16. Here, assuming that a protection threshold is a preset current value that is less than 80% of the demagnetization current, which is the minimum current at which irreversible demagnetization occurs in the motor 7, the reference potential is set to a voltage corresponding to the voltage drop across the shunt resistor 6 when a current equal to the protection threshold flows through the shunt resistor 6. A voltage corresponding to the voltage drop caused by the shunt resistor 6 passes through a low-pass filter 22 composed of a resistor 18 and a capacitor 21, and is input to the comparator 16. The comparator 16 outputs a high-level or low-level overcurrent determination signal to the inverter circuit 5 depending on whether the voltage corresponding to the voltage drop caused by the shunt resistor 6 is greater than or equal to a reference potential. As described above, the voltage drop caused by the shunt resistor 6 indicates the magnitude of the motor current i.dc, and therefore the comparator 16 can be considered to output a high-level or low-level overcurrent determination signal to the inverter circuit 5 depending on whether the magnitude of the motor current i.dc is greater than or equal to a protection threshold. One of the high-level overcurrent determination signal and the low-level overcurrent determination signal is assigned to a "stop signal," and the other is assigned to a "drive signal."

[0026] Next, the operation of the power conversion device 50 will be described. When a current flows through the motor 7, a voltage drop occurs across the shunt resistor 6 according to the resistance value of the shunt resistor 6 and the magnitude of the motor current i.dc. Therefore, a voltage equivalent to the amount of voltage drop occurring across the shunt resistor 6 is input to the overcurrent determination circuit 8.

[0027] When the magnitude of the motor current i.d.c. is less than a predetermined protection threshold, the overcurrent determination circuit 8 outputs a drive signal to the inverter circuit 5. Upon receiving the drive signal, the inverter circuit 5 performs switching using the switching element 5a based on the pulse-width modulation signal input from the pulse-width modulation signal generator 42, thereby supplying power to the motor 7. On the other hand, when the magnitude of the motor current i.d.c. is equal to or greater than the predetermined protection threshold, the overcurrent determination circuit 8 transmits a stop signal to the inverter circuit 5. Upon receiving the stop signal, the inverter circuit 5 immediately switches its switching element 5a to the off state, thereby stopping the power supply to the motor 7. At this time, the power stored in the motor 7 flows through the freewheeling diode 5b of the inverter circuit 5 to the first capacitor 3 and the second capacitor 10 connected between the positive bus 31 and the negative bus 32. As a result, a voltage rise occurs in the bus voltage Vdc that is determined by the power stored in the motor 7 and the capacitances of the first capacitor 3 and the second capacitor 10.

[0028] FIG. 4 is a diagram showing a simulated waveform of the bus voltage of the power conversion device according to the first embodiment. FIG. 5 is a diagram showing a simulated waveform of the motor current of the power conversion device according to the first embodiment. Note that FIG. 4 also shows, by a dashed line, a simulated waveform of the bus voltage of a power conversion device according to a comparative example of the first embodiment. Also, FIG. 5 also shows, by a dashed line, a simulated waveform of the motor current of a power conversion device according to a comparative example of the first embodiment. The power conversion device according to the comparative example has the same capacitances of the first capacitor 3 and the second capacitor 10 and the same inductance of the motor 7 as the power conversion device according to the first embodiment. However, the protection threshold is set to a current value that is 90% of the demagnetization current, which is the minimum current at which irreversible demagnetization occurs in the motor 7 (the load). Here, the withstand voltage of the first capacitor 3, the inverter circuit 5, and the snubber capacitor 13 is assumed to be 800 V. Note that the dotted lines in FIG. 4 indicate the withstand voltages.

[0029] When a stop signal is output from the overcurrent determination circuit 8 to the inverter circuit 5 and the switching element 5a is turned off, the power stored in the motor 7 flows into the first capacitor 3 and the second capacitor 10 through the freewheeling diode 5b of the inverter circuit 5, causing the bus voltage Vdc to rise. In the power conversion device according to the comparative example, a current value that is 90% of the magnitude of the demagnetizing current is set as the protection threshold, and therefore the power stored in the motor 7 is greater than in the power conversion device according to embodiment 1. For this reason, in the power conversion device according to the comparative example, the time during which the motor current idc, which is the current flowing due to the power stored in the motor 7, flows is longer than in the power conversion device according to embodiment 1, and the increase in the bus voltage Vdc is also greater.

[0030] In the power conversion device according to the comparative example, the bus voltage Vdc exceeds 800 V, and therefore the capacitance of the second capacitor 10 must be increased to prevent overvoltage breakdown of the first capacitor 3, the snubber capacitor 13, and the elements included in the inverter circuit 5. Use of a second capacitor 10 with a large capacitance necessitates an increase in the size of the regenerative power absorption circuit 4 in the power conversion device according to the comparative example. On the other hand, in the power conversion device 50 according to the first embodiment, the bus voltage Vdc rises only to 785 V, thereby suppressing overvoltage breakdown of the first capacitor 3, the snubber capacitor 13, and the elements included in the inverter circuit 5. As described above, the power conversion device 50 according to the first embodiment has less power stored in the motor 7 at the time the motor 7 is stopped due to an overcurrent than the power conversion device according to the comparative example, and therefore can reduce the rise in the bus voltage Vdc due to regenerative power from the motor 7.

[0031] The power conversion device 50 according to the first embodiment can suppress an increase in the bus voltage Vdc even when a capacitor with a small capacity is used for the second capacitor 10, and therefore can achieve a reduction in size of the second capacitor 10 of the regenerative power absorption circuit 4 while avoiding irreversible demagnetization of the motor 7, which is the load. Furthermore, the power conversion device 50 according to the first embodiment can achieve a reduction in cost because a capacitor with a small capacity can be used for the second capacitor 10.

[0032] Embodiment 2 Fig. 6 is a configuration diagram that schematically shows the configuration of an air conditioning apparatus according to embodiment 2. As shown in Fig. 6, an air conditioning apparatus 60 according to embodiment 2 includes an indoor unit 600 installed indoors, an outdoor unit 900 installed outdoors, and refrigerant piping 700 for circulating refrigerant between the indoor unit 600 and the outdoor unit 900. The outdoor unit 900 is capable of communicating with the indoor unit 600 via a communication line (not shown).

[0033] 7 is a diagram showing the configuration of an outdoor unit of an air conditioner according to embodiment 2. An outdoor unit 900 of an air conditioner 60 according to embodiment 2 includes the motor control device 100 according to embodiment 1.

[0034] The outdoor unit 900 includes a compressor 901 incorporating the motor 7 shown in the first embodiment, a four-way valve 902 , an expansion valve 903 , and an outdoor heat exchanger 904 .

[0035] Inside the compressor 901, a compression mechanism 911 that compresses the refrigerant and a motor 7 that operates the compression mechanism 911 are provided.

[0036] The outdoor unit 900 can perform heating or cooling operation by switching the four-way valve 902. The compression mechanism 911 is driven by a motor 7 that is variably controlled in speed.

[0037] The indoor unit 600 includes an indoor heat exchanger 601 .

[0038] During heating operation, as shown by the solid arrow, the refrigerant is pressurized by the compression mechanism 911 and sent out, passes through the four-way valve 902, the indoor heat exchanger 601, the expansion valve 903, the outdoor heat exchanger 904 and the four-way valve 902 and returns to the compression mechanism 911.

[0039] During cooling operation, as shown by the dashed arrow, the refrigerant is pressurized by the compression mechanism 911 and sent out, passes through the four-way valve 902, the outdoor heat exchanger 904, the expansion valve 903, the indoor heat exchanger 601 and the four-way valve 902 and returns to the compression mechanism 911.

[0040] During heating operation, the indoor heat exchanger 601 acts as a condenser to release heat, and the outdoor heat exchanger 904 acts as an evaporator to absorb heat. During cooling operation, the outdoor heat exchanger 904 acts as a condenser to release heat, and the indoor heat exchanger 601 acts as an evaporator to absorb heat. The expansion valve 903 reduces the pressure of the refrigerant to expand it.

[0041] The air conditioning device 60 of embodiment 2 includes a motor control device 100 equipped with the power conversion device 50 of embodiment 1, and therefore can avoid irreversible demagnetization of the motor 7 that operates the compression mechanism 911.

[0042] The configurations shown in the above embodiments are merely examples of the content, and may be combined with other known technologies, or parts of the configurations may be omitted or modified without departing from the spirit of the invention.

[0043] 1 AC power supply, 2 Converter circuit, 2a, 2b, 2c, 2d, 2e, 2f, 9 Diode, 3 First capacitor, 4 Regenerative power absorption circuit, 5 Inverter circuit, 5a Switching element, 5b Freewheeling diode, 6 Shunt resistor, 7 Motor, 8 Overcurrent determination circuit, 10 Second capacitor, 11 Discharge resistor, 12 Snubber resistor, 13 Snubber capacitor, 14 Reactor, 15 Amplification circuit, 16 Comparator, 17 Low-voltage power supply, 18, 19, 20 Resistor, 21 Capacitor, 22 Low-pass filter, 31 Positive bus bar, 32 Negative bus bar, 40 Motor control unit, 41 Current detection unit, 42 Pulse width modulation signal generation unit, 50 Power conversion device, 60 Air conditioning device, 91 Processor, 92 Memory, 93 Storage device, 100 Motor control device, 600 Indoor unit, 601 Indoor heat exchanger, 700 refrigerant piping, 900 outdoor unit, 901 compressor, 902 four-way valve, 903 expansion valve, 904 outdoor heat exchanger, 911 compression mechanism.

Claims

1. A power conversion device comprising: a converter circuit that converts AC current input from an AC power source into DC current; an inverter circuit that converts the DC current output from the converter circuit into AC current and supplies it to a load; a positive bus bar and a negative bus bar that connect the converter circuit and the inverter circuit; a first capacitor connected between the positive bus bar and the negative bus bar; a regenerative power absorption circuit that is connected in parallel to the first capacitor and absorbs regenerative power of the load; a shunt resistor installed on the negative bus bar; and an overcurrent determination circuit that sends a stop signal to the inverter circuit to stop power supply to the load when the magnitude of the current detected based on the voltage across the shunt resistor exceeds a protection threshold, wherein the protection threshold is a current value of a preset magnitude that is less than 80% of the demagnetizing current of the load.

2. The power conversion device according to claim 1, wherein the regenerative power absorption circuit comprises a diode connected between the positive bus and the positive side of the first capacitor, a second capacitor connected in series with the diode, and a discharge resistor connected in parallel with the second capacitor, and the capacitance of the second capacitor is five times or less that of the first capacitor.

3. The power conversion device according to claim 1 or 2, further comprising a reactor for improving the power factor, which is installed on the positive bus bar.

4. The power conversion device according to any one of claims 1 to 3, further comprising an amplifier circuit for amplifying a voltage corresponding to the amount of voltage drop occurring in the shunt resistor.

5. The power conversion device according to any one of claims 1 to 4, wherein the overcurrent determination circuit comprises a low-voltage power supply that is a DC power supply with a lower voltage than the AC power supply, a plurality of resistors connected in series between the DC power supply and ground, and a comparator that compares the voltage divided by the plurality of resistors with a voltage equivalent to the amount of voltage drop occurring in the shunt resistor, and sends the stop signal to the inverter circuit when the voltage equivalent to the amount of voltage drop occurring in the shunt resistor is greater than the voltage divided by the plurality of resistors.

6. A motor control device comprising the power conversion device according to any one of claims 1 to 5, wherein the load is a motor, and the motor control device comprises a motor control unit that controls the motor by adjusting the AC current output from the inverter circuit.

7. The motor control device according to claim 6, wherein the motor is a compressor motor for an air conditioner.

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