Power conversion device and heat pump device

Abstract

This power conversion device comprises: a rectifier circuit for rectifying an AC voltage supplied from an AC power supply; a first capacitor connected to the DC side of the rectifier circuit; a first power converter connected in parallel with the first capacitor and having phase output terminals to each of which one end of a winding of a motor in which each phase winding is independent is connected; a second power converter having phase output terminals to each of which the other end of the winding of the motor is connected; a second capacitor connected to the DC side of the second power converter; a reactor that is connected by branching from wiring for connecting the AC power supply and the rectifier circuit; a third power converter that is connected between the reactor and the second capacitor and suppresses harmonics; a relay that is inserted between the AC power supply and the third power converter; and a control unit that drives the first and second inverters to charge the second capacitor before starting the activation of the motor and turns on the relay after the charging. The capacitance of the second capacitor is set to be greater than the capacitance of the first capacitor.

Description

Power conversion equipment and heat pump equipment

[0001] An embodiment of the present invention relates to a power conversion device that drives a motor with an open winding structure, and a heat pump apparatus that includes the power conversion device.

[0002] Conventionally, power conversion devices have been known in which the large-capacity DC smoothing capacitor has been replaced with a small-capacity capacitor in order to reduce the size of passive components and extend the device's lifespan. For example, Patent Document 1 discloses a configuration in which a regenerative absorption circuit consisting of a diode, a resistor, and a capacitor connected in series is connected in parallel to a small-capacity smoothing capacitor in the DC section of a power conversion device connected to a single-phase AC power source, so that the maximum pulsating voltage is more than twice the minimum value. In this configuration, the capacitance of the DC section capacitor can be reduced, but the capacitance of the regenerative energy absorption capacitor must be increased to a certain extent to absorb the regenerative energy. If the capacitor capacitance is increased, components may be damaged by inrush current when the power is turned on, so a separate inrush current prevention circuit is required, which poses challenges to system miniaturization and cost reduction.

[0003] Furthermore, Patent Document 1 describes that a single-phase input power conversion device equipped with a small-capacity film capacitor can reduce harmonics in the input current and improve the power factor. However, in a three-phase input power conversion device, current flows only from the maximum phase to the minimum phase of the three-phase AC, so the input current cannot be controlled to a sine wave simply by switching in the power conversion circuit for driving the motor.

[0004] On the other hand, to achieve larger device capacity and faster motor speeds, a dual inverter system is known, in which an open-winding motor with independent windings for each phase is driven by two inverters, as shown in Patent Document 2, for example. Even with this dual inverter system, a small-capacity capacitor can be used in the DC section, but ultimately, a separate inrush current prevention circuit is required for the regenerative energy absorption capacitor, which requires a certain amount of capacity. Furthermore, when connected to a multi-phase AC such as a three-phase AC, the input current of the device cannot be controlled to a sine wave.

[0005] In response to this, Patent Document 3 discloses a system in which a matrix converter is applied to the inverter on the primary side.

[0006] Patent No. 4391768 Patent No. 7218131 Patent No. 5531238

[0007] The configuration of Patent Document 3 makes it possible to control the three-phase input current to a sine wave, but the semiconductor devices for the matrix converter, the choke coils used in the AC filter, and other components must be selected with a rating equal to or greater than the rated capacity of the power conversion device, which creates issues such as an increase in the size and cost of the entire power conversion device.

[0008] Therefore, we provide a power conversion device that does not require a separate inrush current prevention circuit in a dual inverter system, is highly efficient, and can suppress increases in the size and cost of the entire device, and a heat pump device equipped with this power conversion device.

[0009] a first power converter including a rectifier circuit that rectifies an AC voltage supplied from an AC power supply; a first capacitor connected to a DC side of the rectifier circuit; and a diode and a semiconductor switch connected in parallel to the first capacitor, the first power converter having one end of a winding of a motor, each phase of which is independent, connected to each phase output terminal; a second power converter including a diode and a semiconductor switch and having the other end of the motor winding connected to each phase output terminal; a second capacitor connected to the DC side of the second power converter; a reactor branched from a wiring connecting the AC power supply and the rectifier circuit and connected to the third power converter; and a third power converter that suppresses harmonics and is configured by a diode and a semiconductor switch connected between the reactor and the second capacitor; a relay inserted between the AC power supply and the third power converter; and a control unit that drives the first and second inverters to charge the second capacitor before starting the motor and turns on the relay after charging, the capacitance of the second capacitor being set larger than the capacitance of the first capacitor.

[0010] A heat pump device according to an embodiment includes the power conversion device according to the embodiment and the motor, and drives a compressor using the motor.

[0011] FIG. 1 is a diagram showing the configuration of a power conversion device in a first embodiment. FIG. 2 is a diagram showing the configuration of an air conditioner. FIG. 3 is a flowchart showing the processing contents from power-on to starting the motor. FIG. 4 is a timing chart showing current and voltage waveforms corresponding to the processing contents shown in FIG. 3. FIG. 5 is a diagram showing the configuration of a power conversion device in a second embodiment. FIG. 6 is a flowchart showing the processing contents of a regenerative absorption operation in a third embodiment. FIG. 7 is a diagram showing the switching states of each inverter when performing a regenerative absorption operation. FIG. 8 is a timing chart showing current and voltage waveforms corresponding to the processing contents shown in FIG. 6. FIG. 9 is a diagram showing the switching states of each inverter when performing a regenerative absorption operation in a fourth embodiment. FIG. 10 is a diagram showing the configuration of a power conversion device in a fifth embodiment.

[0012] First Embodiment As shown in Fig. 1, a power conversion device 41 of this embodiment drives a motor 10. The motor 10 may be a three-phase permanent magnet synchronous motor or an induction machine, but in this embodiment, it is a permanent magnet synchronous motor. The motor 10 is a so-called open winding motor, and its three-phase windings are not connected to each other, with both terminals being open. In other words, the motor 10 has six winding terminals Ua, Va, Wa, Ub, Vb, and Wb.

[0013] The motor 10 is driven by a dual inverter system using a first inverter 5 and a second inverter 9. The first inverter 5 and the second inverter 9 have the same circuit configuration and are three-phase inverters having three sets of two series-connected switching elements, with the intermediate connection points of each series-connected switching element serving as three output terminals. The phase output terminals of the first inverter 5 are connected to winding terminals Ua, Va, and Wa of the motor 10, respectively, and the phase output terminals of the second inverter 9 are connected to winding terminals Ub, Vb, and Wb of the motor 10, respectively.

[0014] A three-phase AC power supply 1, which serves as the power source for the power conversion device 41, is connected to a rectifier circuit 3 via a three-phase reactor 2 of the power conversion device 41. The rectifier circuit 3 is configured by connecting six diodes in a three-phase bridge configuration. A first capacitor 4, which is a small-capacity film capacitor, and a first inverter 5 are connected to the output terminals of the rectifier circuit 3.

[0015] The second inverter 9 is connected in parallel with both ends of the DC side of the second capacitor 8, which is, for example, an electrolytic capacitor, and the power converter 7. The high-voltage side terminal and the low-voltage side terminal of the first inverter 5, i.e., both ends of the first capacitor 4, are not connected to the second inverter 9, and the second inverter 9 is independent. In other words, the output terminals of the first inverter 5 and the second inverter 9 are connected only via independent windings for each phase of the motor 10.

[0016] Each phase output terminal of the power converter 7 is connected to the three-phase AC power source 1 via a three-phase reactor 6 and a relay 16. The first inverter 5, the second inverter 9, and the power converter 7 are each configured by three-phase bridge-connected semiconductor switching elements, such as IGBTs, with flywheel diodes connected in parallel, and correspond to the first to third power converters, respectively. The relay 16 may be connected either between the three-phase AC power source 1 and the three-phase reactor 6 or between the three-phase reactor 6 and the power converter 7. The relay 16 is a single three-phase relay that does not have a current suppression element or the like connected in parallel and simultaneously opens and closes the U, V, and W phase power lines. On the other hand, no relay or inrush current prevention circuit is provided between the three-phase power source 1 and the rectifier circuit 3. That is, the relay 16 is inserted into the power line between the three-phase AC power source 1 and the rectifier circuit 2, branching to the three-phase reactor 6 and leading to the power converter 7.

[0017] Current sensors 11U and 11V are arranged on the U and V phases of the power supply line connecting the three-phase AC power supply 1 and the three-phase reactor 2. Current sensors 12U and 12V are arranged on the U and V phases of the power supply line connecting the three-phase AC power supply 1 and the three-phase reactor 6. Voltage sensors 13 and 14 detect the terminal voltages of the first capacitor 4 and the second capacitor 8, respectively. Current sensors 15U, 15V, and 15W are arranged between each phase output terminal of the first inverter 5 and the winding terminals Ua, Va, and Wa of the motor 10.

[0018] The detection signals output by the sensors 11 to 15 are input to a control unit 20. The control unit 20 is configured with a microcomputer or the like, and controls the switching of the IGBTs that make up the first inverter 5, the second inverter 9, and the power converter 7 based on the detection signals from the sensors 11 to 15. The control unit 20 also controls the opening and closing of the relay 16. The components in the above configuration, excluding the motor 10, constitute a power conversion device 41.

[0019] The capacitance of the second capacitor 8 is set to be larger than the capacitance of the first capacitor 4. Although a film capacitor is used as the first capacitor 4 and an electrolytic capacitor is used as the second capacitor 8, there is no need to be limited to this as long as the above-mentioned capacitance conditions are met. For example, both capacitors may be electrolytic capacitors or film capacitors.

[0020] The first capacitor 4 only needs to have a capacity large enough to cut off high-frequency components generated by switching between the first inverter 5 and the second inverter 9, and large enough so that the current that charges the first capacitor 4 when the power is turned on does not exceed the withstand voltage of the rectifier circuit 3, more specifically, the withstand voltage of the diodes that constitute the rectifier circuit 3. This capacity is generally about several tens of μF. This allows the three-phase reactor 2 to be made smaller.

[0021] By setting the first capacitor 4 to such a small capacity, the inrush current to the first capacitor 4 generated when power is applied to the power conversion device 41 is kept at an acceptable level, eliminating the need for an inrush current prevention circuit. An inrush current prevention circuit typically consists of a three-phase or single-phase relay for large currents inserted in series with the power line and a PTC thermistor connected in parallel with the relay. Before power is applied, the relay is open. After power is applied, only a limited current flows through the PTC thermistor to gradually charge the capacitor. Once the capacitor voltage rises and large inrush current ceases to flow, the relay is closed. This prevents excessive inrush current from flowing into the rectifier circuit, etc., thereby protecting the device.

[0022] On the other hand, the capacitance of the second capacitor 8 may be determined based on the compensation capacitance of the power converter 7, the output power of the second inverter 9, the ripple current flowing into the second capacitor 8, and the like. This capacitance is generally on the order of several hundred to several thousand μF. Because the second capacitor 8 has a relatively large capacitance, there is a possibility that an inrush current will flow through the rectifier circuit portion of the power converter 7 when the power is turned on. Therefore, the generation of such an inrush current is prevented by the circuit and control described below, thereby eliminating the need for an inrush current prevention circuit for the second capacitor 8 as well. As a result, an inrush current prevention circuit is not required for the power conversion device 41 as a whole.

[0023] FIG. 2 shows the configuration of an air conditioner, which is a heat pump device to which a power converter 41 is applied. Heat pump devices to which the power converter 41 is applied include, in addition to air conditioners, hot water generators such as water heaters and hot and cold water generators such as chillers. The air conditioner 21 comprises refrigerant piping and signal communication lines connecting an indoor unit 24 and an outdoor unit 35. The indoor unit 24, which is installed indoors, houses an indoor heat exchanger 27 and an indoor fan 30 inside. Meanwhile, the outdoor unit 35 is located outdoors and houses devices such as the control unit 20, compressor 22, outdoor heat exchanger 29, four-way valve 26, pressure reducing device 28, outdoor fan 31, and outdoor fan motor 53.

[0024] The compressor 22 is configured by housing a compression section 23 and a motor 10 in the same iron hermetic container 25, and the rotor shaft of the motor 10 is connected to the compression section 23. The compressor 22, four-way valve 26, indoor heat exchanger 27, pressure reducing device 28, and outdoor heat exchanger 29 are connected to form a closed loop by pipes that serve as refrigerant passages. The compressor 22 is, for example, a single-cylinder rotary compressor, but is not limited to this and a multi-cylinder rotary compressor, scroll compressor, or reciprocating compressor may also be used.

[0025] During heating, the four-way valve 26 is in the state shown by the solid line, and the high-temperature refrigerant compressed by the compression section 23 of the compressor 22 is supplied from the four-way valve 26 to the indoor heat exchanger 27, where it condenses and releases heat into the room, heating the room. It is then decompressed by the pressure reducing device 28, becomes colder, and flows to the outdoor heat exchanger 29, where it absorbs heat from the outside air and evaporates, returning to the compressor 22.

[0026] On the other hand, during cooling, the four-way valve 26 is switched to the state shown by the dashed line. Therefore, the high-temperature refrigerant compressed by the compression section 23 of the compressor 22 is supplied from the four-way valve 26 to the outdoor heat exchanger 29, where it condenses by releasing heat to the outdoors. It is then decompressed by the pressure reducing device 28, becomes low temperature, and flows to the indoor heat exchanger 27, where the refrigerant evaporates by absorbing heat from the indoor air, cooling the room and returning to the compressor 22. Air is then blown to the indoor and outdoor heat exchangers 27, 29 by an indoor fan 30 and an outdoor fan 31, respectively, so that the blown air efficiently exchanges heat between the indoor air and the outdoor air in each heat exchanger 27, 29.

[0027] Next, the operation of this embodiment will be described. A control unit 20 is provided to control the entire power conversion device 41. The control unit 20 operates or stops the compressor 22, i.e., the first motor 10, based on instructions, for example, from an indoor control unit (not shown) on the indoor unit 24 side. While the compressor 22 is operating, the control unit 20 performs vector calculations using the current values ​​detected by current sensors 15U, 15V, and 15W and the terminal voltages of the first capacitor 4 and the second capacitor 8 detected by voltage sensors 13 and 14. Based on the calculation results, the control unit 20 coordinates the operation of the switching elements of the first inverter 5 and the second inverter 9, passing a desired current through each winding of the first motor 10 to drive the first motor 10 at a variable speed.

[0028] Furthermore, while motor 10 is in operation, control unit 20 operates power converter 7 as an active filter circuit to suppress and reduce harmonics flowing in the power supply lines. Control unit 20 extracts harmonic currents from the currents flowing in each phase of reactor 2 detected by current sensors 11U and 11V, and controls the operation of each switching element of power converter 7 so that the reactor currents of each phase detected by current sensors 12U and 12V, taking into account the terminal voltage of second capacitor 8, become correction currents that cancel out the harmonic currents flowing in the U, V, and W phases. This active filter circuit operation of power converter 7 suppresses power supply harmonics, making it possible to make the current flowing from rectifier circuit 3 to AC power supply 1 closer to a sine wave.

[0029] The second capacitor 8 has a certain amount of capacity because it is used as a power source for generating a correction current when the power converter 7 functions as an active filter circuit. Operating the power converter 7 as an active filter circuit requires a power supply with a high enough voltage to compensate for harmonics. Therefore, it is desirable to maintain the terminal voltage of the second capacitor 8, which serves as the power source during active filter circuit operation, at a predetermined value or higher. Therefore, the control unit 20 may control the on / off of the switching elements of the power converter 7 to maintain the terminal voltage of the second capacitor 8 at a constant value or higher while the active filter circuit is operating. Furthermore, if the amount of harmonic current generated by the motor 10 is very low, such as when the load on the motor 10 is low, the control unit 20 may detect this and stop the power converter 7, thereby halting the operation of the active filter circuit and reducing the switching loss of the switching elements of the power converter 7.

[0030] Configuring the power conversion device 41 as described above improves the controllability of a motor using a dual inverter consisting of the first inverter 5 and the second inverter 9 and the motor 10 with an open winding configuration. The power converter 7 is operated as an active filter circuit by controlling switching so that the current flowing from the rectifier circuit 3 to the AC power supply 1 approaches a sine wave, thereby suppressing power supply harmonics. In other words, because voltage is applied to the windings of the motor 10 by the two inverters 5 and 9, the operating range of the motor 10 can be expanded. Furthermore, because the voltage applied to the windings of the motor 10 is multilevel, iron loss generated in the motor 10 can be reduced.

[0031] Furthermore, by devising a method for driving the two inverters 5 and 9 using the control unit 20, it is possible to control the power applied to the motor 10 to a constant level and expand the operating range in the high-speed region by injecting reactive power. Also, by connecting the negative side of the DC section of the power converter 7 with the negative side of the DC section of the second inverter 9, the reference voltage of these circuits becomes common. This makes it possible to reduce the number of insulating points in the drive power supply and prevent increases in circuit size and cost.

[0032] 3 and 4, a description will be given of control that eliminates the need for an inrush current prevention circuit when the power conversion device 41 is turned on. The power conversion device 41 may be turned on when the wiring is connected during installation, when a breaker (not shown) provided between the three-phase AC power supply 1 and the power conversion device 41 is opened and then turned on, or when the three-phase AC power supply 1 is restored from a power outage. In the initial state of the power conversion device 41, when the three-phase AC power supply 1 is not connected, i.e., when the power is not turned on, the relay 16 is off.

[0033] When AC power is applied to the power conversion device 41 from the three-phase AC power source 1, the first capacitor 4 is charged via the rectifier circuit 3 (S1). When the terminal voltage of the first capacitor 4 exceeds a threshold (YES in S2), the control unit 20 starts switching the first and second inverters 5 and 9 (S3). Here, a switching pattern is used to excite the motor 10 with DC. DC excitation is generally used to position the rotor before starting the motor 10, and is a type of current flow that does not create a rotating magnetic field by passing current only through specific phases of the motor windings 10. This DC excitation current pattern does not cause the motor 10 to rotate. The PWM duty of the switching pattern for DC excitation is adjusted in advance to prevent large currents from flowing into the second capacitor 8. As a result, the second capacitor 8 is gradually charged via the first and second inverters 5 and 9 (S4), preventing temporary excessive currents from flowing through the rectifier circuit 3.

[0034] Next, when the terminal voltage of the second capacitor 8 exceeds the threshold value (YES in S5), the control unit 20 turns on the relay 16 (S6). At this point, the voltage across the second capacitor 8 has already risen to a sufficiently high voltage, so even if the relay 16 is turned on, a large current will not flow through the rectifier of the power converter 7. Then, drive control of the motor 10 is started (S7).

[0035] The threshold value of the terminal voltage of the second capacitor 8 in step S5 may be set to a value roughly equivalent to the value obtained by rectifying the AC power supply voltage, but may also be set after confirming in advance, for example, a threshold level at which an excessively large inrush current does not flow into the second capacitor 8 even when the power is turned on. Note that the charging of the capacitor in steps S1 and S4 occurs naturally and is not actively controlled by the control unit 20, but is shown in the flowchart to make the explanation of the operation easier to understand.

[0036] As described above, if the relay 16 is turned off when the power is turned on, only the first capacitor 4, which has a relatively small capacity, is charged. Because the first capacitor 4 has a small capacity, the inrush current generated during charging does not exceed the withstand voltage of the elements constituting the rectifier circuit 3 and does not become a large current that would destroy the elements. Once the first capacitor 4 has been charged to a certain extent, the second capacitor 8 is gradually charged via the open-winding motor 10 and the first capacitor 4 by DC excitation drive of the first and second inverters 5 and 9. At this time, the switching pattern of the first and second inverters 5 and 9 is controlled so that the charging current of the second capacitor 8 does not become excessive.

[0037] Thereafter, by turning on relay 16 at the timing when the voltage across second capacitor 8 becomes approximately equal to the value obtained by rectifying the AC power supply voltage, the relatively large-capacity second capacitor 8 can be shifted to steady-state operation without an excessively large charging current flowing from the power converter 7. Therefore, according to this embodiment, a safe shift from power-on to steady-state operation can be achieved without providing a protection circuit such as an inrush current prevention circuit.

[0038] Furthermore, since the relay 16 is provided in the middle of the wiring that supplies power only to the third power converter 7, which operates as an active filter circuit, and the second capacitor 8, only a small current flows through it compared to the current that flows through the reactor 2 that drives the motor with the first and second inverters 5 and 9. Therefore, a small relay with a low rated current value can be used, which contributes to the miniaturization of the device.

[0039] Second Embodiment Hereinafter, the same components as those in the first embodiment will be denoted by the same reference numerals and their description will be omitted, and only the differences will be described. In the drawings of the power conversion devices of the following embodiments, the sensors 11 to 15 and the control unit 20 will not be shown. In a power conversion device 42 of the second embodiment shown in FIG. 5 , a third capacitor 17 is connected to the DC section of a power converter 7 that operates as an active filter circuit. The negative terminal of the second capacitor 8 and the negative terminal of the third capacitor 17 are commonly connected. The anode of a diode 18 is connected to the positive terminal of the second capacitor 8, and the cathode is connected between the positive terminals of the third capacitor 17.

[0040] According to the second embodiment configured as described above, by connecting the negative side of the DC section of the power converter 7 and the negative side of the DC section of the second inverter 9 and inserting the diode 18 between the positive terminals of the second capacitor 8 and the third capacitor 17, it becomes possible to absorb regenerative energy generated when the motor 10 is stopped also by the third capacitor 17. Therefore, the capacitances of the first and second capacitors 4 and 8 can be reduced.

[0041] Third Embodiment The third embodiment shows an example of a regenerative absorption operation in the power conversion device 41 or 42. Fig. 6 shows an operation flow during regenerative absorption, and Fig. 7 shows only elements that are energized in the first inverter 5 and the second inverter 9 when performing the regenerative absorption operation. During normal operation (S11), if the voltage across the first capacitor 4 or the like exceeds a predetermined threshold (YES in S12), the control unit 20 determines that regeneration by the motor 10 has occurred, and performs the regenerative absorption operation shown in Fig. 5 (S13).

[0042] At this time, the power converter 7 is switched to control the voltage across C2 (the second capacitor 8) (S14). That is, the power converter 7 is operated as a power regeneration PWM rectifier. That is, the power converter 7 is switched to return the regenerative power to the three-phase AC power supply 1 so that the voltage across the second capacitor 8, which absorbs the regenerative power of the motor 10, is a predetermined voltage lower than the rated voltage of the element. During the regenerative absorption operation, the first and second inverters 5 and 9 are not driving the motor 10, so no harmonics are generated. Therefore, the power converter 7 does not need to suppress harmonics, and it is possible to stop the active filter circuit operation of the power converter 7 and operate it as a power regeneration PWM rectifier instead. Then, as long as the voltage across the second capacitor 8 does not fall below the threshold for a set time or more (NO in S15), the process returns to step S13 and the regenerative absorption operation continues. When the voltage across the second capacitor 8 falls below the threshold for a set time or more (YES), the regenerative absorption operation and the switching control of the power converter 7 are stopped (S16).

[0043] In the regenerative power absorption operation in step S13, all IGBTs on the upper arm side of the first inverter 5 are turned on, and all IGBTs on the lower arm side are turned off, thereby forming a neutral point for the motor 10. At this time, no power flows from the motor 10 to the first capacitor 4. On the other hand, for the second inverter 9, all IGBTs are turned off, making the configuration equivalent to that of the rectifier circuit 3. As a result, power flows from the motor 10 to the second capacitor 8. In this way, by performing the regenerative power absorption operation during regeneration by the motor 10, regenerative power can be absorbed only by the second capacitor 8, which has a relatively large capacity. By setting the capacitance of the second capacitor 8 to approximately several hundred to several thousand μF, it is possible to prevent overvoltage from occurring even during regeneration.

[0044] FIG. 8 shows the current and voltage waveforms of each part when the flow shown in FIG. 6 is executed. Note that although the current waveforms are three-phase, there is no need to distinguish between the three phases here, so the waveforms are not shown separately. In the figure, the "load current" is the current flowing through the three-phase reactor 2, and the "input current" is the current input from the three-phase AC power source 1 to the three-phase reactors 2 and 6. The "power converter current" is the current flowing through the power converter 7, i.e., the current flowing through the three-phase reactor 6. The "second capacitor voltage" is the terminal voltage of the second capacitor 8, and the "switching voltage" is the collector-emitter voltage of the IGBT that constitutes the power converter 7.

[0045] In Figure 8, regeneration of the motor 10 occurs, and the regenerative absorption operation begins when the voltage of the first capacitor 4 (not shown) rises to a threshold value. When the regenerative absorption operation begins, the voltage across the second capacitor 8 rises temporarily. However, because the power converter 7 operates in parallel as a power regenerative PWM rectifier, the regenerative current flowing into the second capacitor 8 is regenerated to the three-phase AC power supply 1, preventing an extremely large rise. After that, when regeneration of the motor 10 ends, the power converter 7 operates as a power regenerative PWM rectifier, causing the voltage across the second capacitor 8 to decrease. When the time during which the voltage across the second capacitor 8 falls below the threshold value exceeds a set time, the switching of the power converter 7 stops, and the regenerative absorption and power regeneration operation ends. As a result, the switching voltage is fixed at Vdc, which is a DC voltage obtained by full-wave rectifying and smoothing the AC current of the three-phase AC power supply 1. The load current, input current, and power converter current all become "0," and the regenerative absorption operation ends. When the power converter 7 is operating as a power regenerative PWM rectifier, a sinusoidal current synchronized with the voltage of the three-phase AC power supply 1 flows as the input current.

[0046] Furthermore, in determining whether the regenerative absorbing operation has ended in step S15, a "YES" determination may be made when the voltage of the second capacitor 8 shown in FIG. 8 has stabilized. Furthermore, because the actual time for which regenerative power is generated is short, the end of the regenerative absorbing operation may be determined based on the elapsed time from the start of the operation. The threshold value in step S15 does not necessarily have to be set to the same value as the threshold value in step S13.

[0047] (Fourth embodiment) The fourth embodiment also shows an example of the regenerative absorption operation in the power conversion device 41 or 42. Like FIG. 6, FIG. 9 shows only the elements that are energized in the first inverter 5 and the second inverter 9 when the regenerative absorption operation is performed. What differs from the third embodiment is that in the first inverter 5, all of the IGBTs on the upper arm side are turned off and all of the IGBTs on the lower arm side are turned on. In this case, since the IGBTs on the lower arm side are turned on for a long period of time, this is effective when the drive power supply for the second inverter 9 is configured as a bootstrap circuit.

[0048] Fifth Embodiment A power conversion device 43 of the fifth embodiment shown in Fig. 10 has a configuration in which the three-phase reactor 2 is eliminated from the power conversion device 41, and a reactor 44 is inserted on the positive side between the rectifier circuit 3 and the first capacitor 8. By inserting the reactor 44 in the DC section in this way, the number of reactors used can be reduced compared to when the reactor is inserted in the AC section. Note that the reactor inserted in the DC section may be only in the high-potential side line as shown in Fig. 10, or may be distributed at both ends of the high-potential side line and the low-potential side line. In this distributed arrangement, the inductance of each reactor can be approximately half that when only one reactor is inserted.

[0049] (Other Embodiments) The semiconductor switch is not limited to an IGBT, but may be, for example, a power MOSFET. The AC power supply may be single-phase. When setting the capacitance relationship between the first and second capacitors C1 and C2 to (C1 < C2), the first capacitor 4 does not necessarily have to be a film capacitor and the second capacitor 8 an electrolytic capacitor. The second capacitor may be a supercapacitor such as a storage battery or an electrolytic double-layer capacitor. In step S3 of FIG. 3 , before closing the relay 16, the first and second inverters 5 and 9 output a switching pattern that DC excites the motor 10 to charge the second capacitor 8. However, the current flow pattern is not limited to a DC excitation switching pattern and may be any switching pattern that can gradually charge the second capacitor 8 without rotating the motor 10 using the first and second inverters 5 and 9.

[0050] Heat pump equipment is not limited to air conditioners. Furthermore, the power conversion device may be applied to equipment other than heat pump equipment. While several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments may be embodied in various other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit and scope of the invention. These embodiments and their modifications are within the scope and spirit of the invention, and are also within the scope of the inventions and their equivalents as defined in the accompanying claims.

[0051] In the drawings, 1 denotes a three-phase AC power supply, 2 denotes a three-phase reactor, 3 denotes a rectifier circuit, 4 denotes a first capacitor, 5 denotes a first inverter (first power converter), 6 denotes a three-phase reactor, 7 denotes a power converter (third power converter), 8 denotes a second capacitor, 9 denotes a second inverter (second power converter), 10 denotes a motor, 16 denotes a relay, 20 denotes a control unit, 21 denotes an air conditioner, 22 denotes a compressor, 31 denotes a fan, 41 to 43 denote power conversion devices, and 44 denotes a reactor.

Claims

1. A power conversion device comprising: a rectifier circuit which rectifies an AC voltage supplied from an AC power source; a first capacitor connected to a DC side of the rectifier circuit; a first power converter comprising a diode and a semiconductor switch connected in parallel to the first capacitor, and having one end of a winding of a motor having independent phase windings connected to each phase output terminal; a second power converter comprising a diode and a semiconductor switch, and having the other end of the motor winding connected to each phase output terminal; a second capacitor connected to the DC side of the second power converter; a reactor connected by branching off from a wiring connecting the AC power source and the rectifier circuit; a third power converter comprising a diode and a semiconductor switch connected between the reactor and the second capacitor, and suppressing harmonics; a relay inserted between the AC power source and the third power converter; and a control unit which drives the first and second inverters to charge the second capacitor before starting up the motor, and turns on the relay after charging, wherein the capacitance of the second capacitor is set to be larger than the capacitance of the first capacitor.

2. The power conversion device according to claim 1, wherein the first capacitor is a film capacitor and the second capacitor is an electrolytic capacitor.

3. A power conversion device as claimed in claim 1, wherein the first capacitor has a capacity sufficient to cut out high frequency components generated by switching between the first power converter and the second power converter, and a capacity sufficient to ensure that the current charging the first capacitor when the power is turned on does not exceed the capacity of the rectifier circuit 3.

4. The power conversion device according to claim 1, wherein the control unit operates the third power converter as an active filter circuit.

5. The power conversion device according to claim 1, further comprising a third capacitor connected to a DC section of the third power converter, a negative terminal of the second capacitor and a negative terminal of the third capacitor being commonly connected, and a diode having an anode connected to the positive terminal side of the second capacitor and a cathode connected to the positive terminal side of the third capacitor.

6. A power conversion device as described in claim 1, wherein the control unit, when power is regenerated from the motor, performs a regenerative absorption operation by turning off all of the semiconductor switches of the second power converter and turning on or off the semiconductor switches of the first power converter to short-circuit one of the winding ends of the motor and cause the second capacitor to absorb the regenerative power.

7. The power conversion device according to claim 6, wherein the control unit performs the regenerative absorption operation when it detects that the voltage across the first capacitor has exceeded a predetermined threshold value.

8. The power conversion device according to claim 6, wherein said control unit, when performing said regenerative absorption operation, causes said third power converter to operate as a power regenerative PWM rectifier that regenerates power to said AC power supply.

9. The power conversion device according to claim 1, further comprising a reactor inserted between said AC power supply and said rectifier circuit.

10. The power conversion device according to claim 1, further comprising a reactor inserted between said rectifier circuit and said first power converter.

11. A heat pump device comprising: the power conversion device according to any one of claims 1 to 10; and a motor, wherein the motor drives a compressor.

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