Power conversion device, refrigeration cycle device, and motor system

Abstract

This power conversion device (100) supplies power to a three-phase motor (11) that drives a compressor (10) for compressing a refrigerant. The power conversion device (100) includes: a three-phase inverter (102) having at least six semiconductor switches; and an inverter control unit (102a). The three-phase inverter (102) has a U-phase leg (102U), a V-phase leg (102V), and a W-phase leg (102W). When preheating the refrigerant, the inverter control unit (102a) selects two legs from among the U-phase leg (102U), the V-phase leg (102V), and the W-phase leg (102W). The inverter control unit (102a) carries out control to turn a positive-electrode-side semiconductor switch of a first leg and a negative-electrode-side semiconductor switch of a second leg on and off at the same timings, carries out control to turn a negative-electrode-side semiconductor switch of the first leg and a positive-electrode-side semiconductor switch of the second leg on and off at the same timings, and keeps a positive-electrode-side semiconductor switch and a negative-electrode-side semiconductor switch of a third leg turned off.

Description

Power conversion device, refrigeration cycle device, and motor system

[0001] The present disclosure relates to a power conversion device, a refrigeration cycle device, and a motor system.

[0002] To extend the life of a refrigeration cycle device, it is necessary to preheat the refrigerant to prevent it from liquefying inside the compressor. One method for preheating the refrigerant involves wrapping an external heater around the compressor, but this increases the cost and size of the system. Another method involves preheating the refrigerant by passing DC or AC current through the windings of a three-phase motor inside the compressor using the switching operation of a three-phase inverter used during normal operation, thereby generating copper loss in the windings or iron loss in the core. This method reduces the cost and size of the system (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2000-297967

[0004] However, the conventional technology has a problem in that common mode noise is large.

[0005] An object of the present disclosure is to reduce common mode noise.

[0006] a U-phase leg corresponding to the U-phase of the three-phase motor; a positive-side U-phase switch which is a positive-side semiconductor switch among the at least six semiconductor switches connected to a positive side of a DC power source, and a negative-side U-phase switch which is a negative-side semiconductor switch among the at least six semiconductor switches connected to a negative side of the DC power source; a positive-side V-phase switch which is a positive-side semiconductor switch among the at least six semiconductor switches connected to the positive side of the DC power source, and a negative-side V-phase switch which is a negative-side semiconductor switch among the at least six semiconductor switches connected to the negative side of the DC power source; and a V-phase leg corresponding to the V-phase of the three-phase motor; and a W-phase leg corresponding to the W phase of the three-phase motor, the W-phase leg including a positive W-phase switch that is a positive side semiconductor switch of the at least six semiconductor switches that is connected to the positive side of the DC power supply, and a negative W-phase switch that is connected in series with the positive W-phase switch and is a negative side semiconductor switch of the at least six semiconductor switches that is connected to the negative side of the DC power supply, wherein when the refrigerant is preheated, the inverter control unit selects two legs from the U-phase leg, the V-phase leg, and the W-phase leg, and performs on / off control of the positive side semiconductor switch of a first leg of the selected two legs and the negative side semiconductor switch of a second leg of the selected two legs at the same time, performs on / off control of the negative side semiconductor switch of the first leg and the positive side semiconductor switch of the second leg at the same time, and maintains off the positive side semiconductor switch and the negative side semiconductor switch of a third leg other than the two selected legs. A refrigeration cycle device according to an aspect of the present disclosure includes the power conversion device and a compressor.A motor system according to one aspect of the present disclosure includes the power conversion device and the three-phase motor.

[0007] According to the present disclosure, it is possible to provide a power conversion device capable of reducing common-mode noise.

[0008] 1 is a diagram showing an example of a refrigeration cycle apparatus 1 having a power conversion apparatus according to a first embodiment. FIG. 1 is a diagram showing an example of a power supply unit and a three-phase inverter. FIG. 2 is a diagram showing an example of an inverter control unit during a refrigerant compression operation. FIG. 3 is a diagram showing an example of the operation of the inverter control unit during refrigerant preheating control. FIG. 4 is a diagram showing an example of the operation (waveform) of each component when current is applied to the U phase and the V phase of the three phases of a three-phase motor. FIG. 5 is a diagram showing an example of the operation (waveform) of each component in a second embodiment. FIG. 6 is a diagram showing an example of the operation (waveform) of each component in a third embodiment. FIG. 7 is a diagram showing an example of the operation (waveform) of each component in a fourth embodiment. FIG. 8 is a diagram showing an example of a current waveform when an AC component is superimposed on a triangular AC current. FIG. 9 is a diagram showing an example of switching between two legs selected by the inverter control unit. FIG. 10 is a diagram showing an example of a current conduction pattern when burst heating is performed. FIG. 11 is a diagram showing an example of a current waveform during burst heating. FIG. 12 is a flowchart showing an example of the operation of the inverter control unit during preheating control in a power converter according to an eighth embodiment. FIG. 13 is a diagram showing an example of a change in switching frequency in variable control of the switching frequency.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same reference numerals indicate the same or corresponding parts. <Embodiment 1> Hereinafter, a power conversion device 100 according to embodiment 1 will be described with reference to the drawings. FIG. 1 is a diagram showing an example of a refrigeration cycle device 1 having a power conversion device 100 according to embodiment 1. The refrigeration cycle device 1 has the power conversion device 100 and a compressor 10. The refrigeration cycle device 1 is, for example, an air conditioner (Air To Air: ATA), a refrigeration / freezer, or an air conditioning / heating / hot water supply device (Air To Water: ATW).

[0010] The power conversion device 100 in Fig. 1 includes a power supply unit 101 and a three-phase inverter 102. The power conversion device 100 is electrically connected to a compressor 10. In the example shown in Fig. 1, the three-phase inverter 102 is electrically connected to the compressor 10. The power conversion device 100 supplies electric power to a three-phase motor 11 provided inside the compressor 10.

[0011] The power conversion device 100 (specifically, a three-phase inverter 102) is electrically connected to a three-phase motor 11. The motor system 2 shown in FIG.

[0012] The compressor 10 has a three-phase motor 11 and a compression mechanism 12. The compressor 10 compresses the refrigerant used in the heat pump cycle. The three-phase motor 11 included in the compressor 10 drives the compression mechanism 12 of the compressor 10 during the refrigerant compression operation. The three-phase inverter 102 applies a voltage of a desired frequency and amplitude to the three-phase motor 11.

[0013] The power supply unit 101 and the three-phase inverter 102 are connected by a power line PL1, and the three-phase inverter 102 and the three-phase motor 11 are connected by a power line PL2. One end of the power supply unit 101 is connected to a ground line GL via a power supply ground line PGL. Ground parasitic capacitance SC exists between the three-phase inverter 102 and the ground line GL, and between the three-phase motor 11 and the ground line GL.

[0014] 2 is a diagram illustrating an example of a power supply unit 101 and a three-phase inverter 102. The power supply unit 101 has a diode rectifier 101a and a smoothing capacitor 101b. The power supply unit 101 is electrically connected to a three-phase AC system power supply 103. In the example shown in FIG. 2, the diode rectifier 101a is electrically connected to the three-phase AC system power supply 103.

[0015] The AC voltage of the three-phase AC system power supply 103 is rectified by a diode rectifier and converted into a DC voltage. A smoothing capacitor 101b reduces ripple components in the DC voltage. The DC voltage output by the power supply unit 101 is supplied to a three-phase inverter 102 via a power line PL1. The three-phase AC system power supply 103 may be, for example, a DC (such as a storage battery), an AC (such as a generator or converter), a single-phase (two-wire or three-wire), or a three-phase (three-wire or four-wire). The power supply unit 101 may include, for example, a power factor correction circuit using a semiconductor switch, a step-up chopper circuit, a step-down chopper circuit, or an active filter circuit. Alternatively, the power supply unit 101 may be a combination of multiple circuits. The DC voltage output by the power supply unit 101 may be highly pulsating.

[0016] The three-phase inverter 102 includes at least six semiconductor switches and an inverter control unit 102a. In the example shown in Fig. 2, the three-phase inverter 102 includes semiconductor switches UP, VP, WP, UN, VN, and WN. Each of the semiconductor switches UP, VP, WP, UN, VN, and WN includes an anti-parallel diode. The semiconductor switches UP, VP, WP, UN, VN, and WN are electrically connected to the three-phase motor 11.

[0017] The semiconductor switches UP, VP, WP, UN, VN, and WN include a U-phase leg 102U, a V-phase leg 102V, and a W-phase leg 102W. The U-phase leg 102U corresponds to the U-phase of the three-phase motor 11. The V-phase leg 102V corresponds to the V-phase of the three-phase motor 11. The W-phase leg 102W corresponds to the W-phase of the three-phase motor 11. The U-phase may be referred to as the "first phase," the V-phase may be referred to as the "second phase," and the W-phase may be referred to as the "third phase." The U-phase leg 102U may be referred to as the "first leg," the V-phase leg 102V may be referred to as the "second leg," and the W-phase leg 102W may be referred to as the "third leg."

[0018] <U-phase leg 102U> The U-phase leg 102U has a semiconductor switch UP which is a positive-side semiconductor switch connected to the positive side of a DC power supply (e.g., power supply unit 101), and a semiconductor switch UN which is a negative-side semiconductor switch connected to the negative side of the DC power supply (e.g., power supply unit 101). The semiconductor switch UN is connected in series with the semiconductor switch UP. The semiconductor switch UP is also referred to as the "positive-side U-phase switch UP." The semiconductor switch UN is also referred to as the "negative-side U-phase switch UN." The positive-side semiconductor switch is also referred to as the "positive-side switch," and the negative-side semiconductor switch is also referred to as the "negative-side semiconductor switch."

[0019] <V-Phase Leg 102V> The V-phase leg 102V has a semiconductor switch VP which is a positive-side semiconductor switch connected to the positive side of a DC power supply (e.g., power supply unit 101), and a semiconductor switch VN which is a negative-side semiconductor switch connected to the negative side of the DC power supply (e.g., power supply unit 101). The semiconductor switch VN is connected in series with the semiconductor switch VP. The semiconductor switch VP is also referred to as the "positive-side V-phase switch VP." The semiconductor switch VN is also referred to as the "negative-side V-phase switch VN." The positive-side semiconductor switch is also referred to as the "positive-side switch," and the negative-side semiconductor switch is also referred to as the "negative-side semiconductor switch."

[0020] <W-phase leg 102W> The W-phase leg 102W has a semiconductor switch WP, which is a positive-side semiconductor switch connected to the positive side of a DC power supply (e.g., power supply unit 101), and a semiconductor switch WN, which is a negative-side semiconductor switch connected to the negative side of the DC power supply (e.g., power supply unit 101). The semiconductor switch WN is connected in series with the semiconductor switch WP. The semiconductor switch WP is also referred to as the "positive-side W-phase switch WP." The semiconductor switch WN is also referred to as the "negative-side W-phase switch WN." The positive-side semiconductor switch is also referred to as the "positive-side switch," and the negative-side semiconductor switch is also referred to as the "negative-side semiconductor switch."

[0021] The three legs (i.e., U-phase leg 102U, V-phase leg 102V, and W-phase leg 102W) are connected in parallel. Each leg is connected to output terminals U, V, and W corresponding to the U-phase, V-phase, and W-phase, respectively, and is connected to power line PL2 via the output terminals U, V, and W.

[0022] The semiconductor switches UP, VP, WP, UN, VN, and WN are formed using self-extinguishing semiconductor elements such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors).The semiconductor switches UP, VP, WP, UN, VN, and WN may be formed by connecting a plurality of semiconductor switches in parallel or in series.

[0023] The inverter control unit 102a is connected to the semiconductor switches UP, VP, WP, UN, VN, and WN, respectively. The inverter control unit 102a controls the semiconductor switches UP, VP, WP, UN, VN, and WN. The three-phase inverter 102 applies a desired voltage to the three-phase inverter 102 by switching the semiconductor switches UP, VP, WP, UN, VN, and WN on and off based on the control signal output from the inverter control unit 102a.

[0024] The three-phase inverter 102 has at least two operating modes. The operating modes include a refrigerant compression operation, which is a normal operation, and a refrigerant preheating control, which is an operation for preheating the refrigerant. The refrigerant preheating control is also called preheating control. The inverter control unit 102a can switch the operating mode of the three-phase inverter 102 according to predetermined conditions.

[0025] In addition to controlling the rotational drive of the three-phase motor 11 in accordance with the refrigerant compression operation, the three-phase inverter 102 can also control the preheating of the refrigerant by passing a DC or AC current through the windings of the three-phase motor 11 to prevent the refrigerant from liquefying inside the compressor 10, thereby using winding copper loss and core iron loss. By performing refrigerant preheating control, it is possible to eliminate the need for a heater that has conventionally been externally attached to the compressor 10.

[0026] 3 is a diagram illustrating an example of an inverter control unit 102a during refrigerant compression. The inverter control unit 102a includes, for example, voltage commands Vu_ref, Vv_ref, and Vw_ref for each phase, a carrier 102b (for compression), a comparator 102c for determining whether the voltages are large or small, an AND circuit 102d, a NOR circuit 102e, and a Td delay unit 102f. The comparator 102c, the AND circuit 102d, the NOR circuit 102e, and the Td delay unit 102f constitute a circuit for providing a dead time during which both the positive and negative semiconductor switches of each leg are off to prevent a leg short circuit between the positive and negative semiconductor switches of each leg. The Td delay unit 102f delays any dead time to be inserted by the time Td.

[0027] In the refrigerant compression operation, for example, a comparator compares sinusoidal voltage commands Vu_ref, Vv_ref, and Vw_ref, each phase shifted by 120 degrees, with a triangular carrier waveform to generate a base current for the on and off signals of the semiconductor switches. This current is then passed through an AND circuit 102d, a NOR circuit 102e, and a Td delay unit 102f to insert dead time and generate the final on and off signals for the semiconductor switches. This applies a desired three-phase AC voltage to the three-phase motor 11, causing the three-phase motor 11 to rotate and compress the refrigerant.

[0028] The dead time can be set to any value as long as it is equal to or longer than the minimum time required to prevent a leg short circuit. In order to reduce distortion in the output voltage or current, it is desirable to set the dead time to a short value.

[0029] In refrigerant preheating control, compared to refrigerant compression operation control, the switching frequencies of the semiconductor switches UP, VP, WP, UN, VN, and WN are often set higher from the viewpoint of heating performance or noise, and common-mode noise in particular can be a problem. Therefore, this application discloses a control method for reducing common-mode noise in the preheating control of the power conversion device 100. Common-mode noise is defined as the current that flows in the ground line GL due to the switching operation of the semiconductor switches.

[0030] <Refrigerant Preheating Control> The refrigerant preheating control of the three-phase inverter 102 will be described in detail below. FIG. 4 is a diagram showing an example of the operation of the inverter control unit 102a during refrigerant preheating control. The three-phase motor 11 is, for example, a Y-connection motor, and the motor winding impedances of each phase are Zu, Zv, and Zw. However, the three-phase motor 11 may also be a delta-winding motor, and the multiple windings may be connected in series or parallel, or a dual three-phase motor may be used. The number of poles may be any number.

[0031] Preheating control involves selecting two of the three phases and energizing the selected two phases for a fixed period of time. Figure 4 shows an example in which the U and V phases are selected. In this case, the current Iout for preheating the refrigerant flows only through the U and V phases due to the line output voltage Vout, which is the difference between the U-phase voltage Vu and the V-phase voltage Vv of the power conversion device 100, and ideally does not flow through the W phase. Furthermore, the semiconductor switches of the power conversion device 100 are configured such that only the U-phase and V-phase semiconductor switches UP, UN, VP, and VN perform on-off switching operations. The W-phase semiconductor switches WP and WN are both off. In Figure 4, the voltage of each phase is defined as positive when viewed from the negative side line of the DC voltage toward the corresponding phase output terminal. The output current is defined as positive when flowing from the U-phase output terminal to the V-phase output terminal via the U-phase and V-phase impedances. The voltage of the DC power supply is Vdc.

[0032] In preheating control, the positive-side semiconductor switch of the first phase and the negative-side semiconductor switch of the second phase, and the negative-side semiconductor switch of the first phase and the positive-side semiconductor switch of the second phase are diagonally paired, and the on / off patterns of the selected diagonal pairs of semiconductor switches for the two phases are set to be the same. When current is applied to the U-phase and V-phase windings shown in Figure 3, semiconductor switches UP and VN are a diagonally paired switch, and semiconductor switches UN and VP are also a diagonally paired switch, and semiconductor switches UP and VN have the same switching pattern, and semiconductor switches UN and VP have the same switching pattern.

[0033] That is, when preheating the refrigerant, the inverter control unit 102a selects two legs from the U-phase leg 102U, the V-phase leg 102V, and the W-phase leg 102W, controls the switching of the two selected legs, and turns off the positive side semiconductor switch and the negative side semiconductor switch of one leg other than the two selected legs.

[0034] Specifically, when preheating the refrigerant, the inverter control unit 102a selects two of the U-phase leg 102U, the V-phase leg 102V, and the W-phase leg 102W, controls the on / off of the positive pole side semiconductor switch of the first leg of the two selected legs and the negative pole side semiconductor switch of the second leg of the two selected legs at the same timing, controls the on / off of the negative pole side semiconductor switch of the first leg and the positive pole side semiconductor switch of the second leg at the same timing, and maintains the positive pole side semiconductor switch and the negative pole side semiconductor switch of the third leg other than the two selected legs off.

[0035] In the present embodiment, inverter control unit 102a selects, for example, U-phase leg 102U and V-phase leg 102V from U-phase leg 102U, V-phase leg 102V, and W-phase leg 102W, and controls switching of selected U-phase leg 102U and V-phase leg 102V. Specifically, inverter control unit 102a controls the on / off of positive-side U-phase switch UP and negative-side V-phase switch VN at the same timing, controls the on / off of negative-side U-phase switch UN and positive-side V-phase switch VP at the same timing, and keeps the positive-side W-phase switch WP and negative-side W-phase switch WN of W-phase leg 102W off.

[0036] The switching frequencies of the two-phase semiconductor switches selected by inverter control unit 102a are, for example, 9 kHz or higher. In this embodiment, the switching frequencies of the semiconductor switches in U-phase leg 102U and V-phase leg 102V are 9 kHz or higher. That is, the switching frequencies of positive U-phase switch UP, negative U-phase switch UN, positive V-phase switch VP, and negative V-phase switch VN are each 9 kHz or higher. In this case, heating efficiency is improved due to increased iron loss, and heating time can be shortened.

[0037] FIG. 5 is a diagram showing an example of the inverter control unit 102a during refrigerant preheating control. Two carriers 102b, rotated 180 degrees, are shown as an example of the carriers 102b used to generate the switching patterns of each semiconductor switch. However, for example, only one carrier 102b may be used. In this case, any signal logic or circuit that generates a switching pattern similar to that shown in FIG. 4 may be used. Within the scope of the present application, one or more carriers 102b may be used to generate the switching pattern, and the signal logic of the switch may be appropriately switched for each switch.

[0038] The duty is the ratio of the on-time of each semiconductor switch to one control period (e.g., t1 to t7) during which the inverter output current Iout alternates between positive and negative in FIG. 6 (described later). In this embodiment, the duty of the positive-side U-phase switch UP and the negative-side V-phase switch VN is indicated as "DUTY1," and the duty of the positive-side V-phase switch VP and the negative-side U-phase switch UN is indicated as "DUTY2." Each semiconductor switch may be set to one duty or three or more duties. Alternatively, a mechanism for switching between multiple duties may be used.

[0039] Fig. 6 is a diagram showing an example of the operation (waveform) of each component when current is applied to the U-phase and V-phase of the three phases of the three-phase motor 11. In Fig. 6, the drive signal for each semiconductor switch output by the inverter control unit 102a is "1" when the semiconductor switch is on, and "0" when the semiconductor switch is off. In Fig. 6, the positive-side U-phase switch UP and the negative-side V-phase switch VN, and the negative-side U-phase switch UN and the positive-side V-phase switch VP have the same on / off switching pattern. The positive-side W-phase switch WP and the negative-side W-phase switch WN are both off, so they are omitted from Fig. 6.

[0040] For example, the ON time of the semiconductor switch is set to prevent the refrigerant from liquefying, thereby providing the desired amount of heat depending on the impedance of various loads.

[0041] In this embodiment, the waveform of the current output from the three-phase inverter 102 (i.e., the inverter output current Iout) is a discontinuous triangle, which allows a desired amount of heating to be obtained.

[0042] Figure 6 shows an example where DUTY1 = DUTY2 = 0.15, which outputs a discontinuous AC current in a triangular shape. The definition of DUTY and the DUTY that can be specified are not limited to this. The actual current waveform is not a perfect triangle because the impedance of the three-phase motor winding is a combination of inductance, resistance, and capacitance components.

[0043] The operation of the output current Iout shown in Figure 6 will be explained below. Between time t1 and time t2, only the positive U-phase switch UP and the negative V-phase switch VN are turned on. As a result, the output voltage Vout between the U-phase output terminals as seen from the V-phase becomes positive. Therefore, the output current Iout increases.

[0044] Between time t2 and time t3, all semiconductor switches are turned off. At this time, the inductance component of the motor windings causes the output current Iout to continue flowing. This causes the anti-parallel diodes VP and UN to conduct. As a result, the output voltage Vout becomes negative and the output current Iout decreases.

[0045] Between time t3 and time t4, all the semiconductor switches continue to be off. When the output current Iout reaches zero at time t3, the anti-parallel diode becomes non-conductive and zero current continues.

[0046] These operations from time t1 to time t4 result in a positive current being output. On the other hand, for the negative current, the negative U-phase switch UN and the positive V-phase switch VP are switched. The current operation is the same as for the positive current.

[0047] Between time t4 and time t5, only the negative U-phase switch UN and the positive V-phase switch VP are turned on. As a result, the output voltage Vout between the U-phase output terminals as seen from the V-phase becomes negative. As a result, the output current Iout decreases.

[0048] Between time t5 and time t6, all semiconductor switches are turned off. At this time, the inductance component of the motor windings causes the output current Iout to continue flowing. This causes the anti-parallel diodes of the negative V-phase switch VN and the positive U-phase switch UP to conduct. As a result, the output voltage Vout becomes positive, and the output current Iout increases.

[0049] Between time t6 and time t7, all the semiconductor switches continue to be off. When the output current Iout reaches zero at time t6, the anti-parallel diode becomes non-conductive and zero current continues.

[0050] The specified duty is determined according to the winding impedance of each motor so as to achieve the desired heating power. In other words, the power is controlled by the ON time of the semiconductor switch.

[0051] In particular, when the motor winding impedance is low, the heating power tends to be excessive. In this case, it is effective to adjust the heating power by turning off all semiconductor switches within one control period as shown in Figure 4 and providing a zero current section. The time during which all semiconductor switches are turned off is set to a predetermined time that is longer than the dead time during refrigerant compression operation, rather than preheating control. In other words, the time during which all semiconductor switches are off in one period of the output current Iout output from the three-phase inverter 102 is longer than the dead time during refrigerant compression operation. This allows for an appropriate power to suppress refrigerant liquefaction, thereby shortening the heating time and saving energy.

[0052] On the other hand, the off time of the semiconductor switches determined from the desired power may be set to the dead time during normal refrigerant compression operation. Therefore, the time during which all semiconductor switches are off in one cycle of the output current Iout output from the three-phase inverter 102 may be equal to or longer than the dead time during refrigerant compression operation. Even in this case, by setting the power appropriate for suppressing liquefaction of the refrigerant, it is possible to shorten the heating time or save energy.

[0053] Through the series of control operations described above, a current is passed from the power conversion device 100 to the windings of the three-phase motor 11 to preheat the refrigerant.

[0054] The desired heating power is determined, for example, from the amount of power and heating time required to suppress liquefaction of the refrigerant. Other options include the maximum power that will not burn out the motor, or 50 W or less that does not require an application for high-frequency equipment in Japan. In this embodiment, the power output from the three-phase inverter 102 is 50 W or less. This has the advantage that high-frequency equipment is not required.

[0055] The mechanism by which common mode noise is reduced by preheating control of the power conversion device 100 will be described below.

[0056] In the preheating control of the power conversion apparatus 100, as shown in FIG. 6 , two semiconductor switches constituting a diagonal pair are set to the same switching pattern. By setting the two semiconductor switches of the diagonal pair to the same duty cycle and comparing them with two carriers having a 180-degree phase difference, the output voltages of each phase are inverted. Therefore, in the waveform example shown in FIG. 6 , the U-phase and V-phase output voltages Vu and Vv are inverted. As a result, a preheating current Iout flows through the U-phase and V-phase windings due to the line voltage Vout between the U-phase and V-phase. Meanwhile, because the output voltages Vu and Vv of each phase are inverted, an inverted ground current also flows through the ground capacitance of each phase. This cancels out the current flowing through the ground wire GL.

[0057] According to the first embodiment, it is possible to reduce common-mode noise generated by the preheating control of the power conversion device 100. The control of the three-phase inverter 102 is not essentially limited to refrigerant preheating control. For example, it is applicable to the control of three-phase inverters or converters of induction heating devices, solar power generation, electric vehicles, battery storage systems, etc. In this case, it is also possible to reduce common-mode noise.

[0058] <Second Embodiment> A power conversion device 100 according to a second embodiment will be described below with reference to the drawings. FIG. 7 is a diagram showing an example of the operation (waveform) of each component in the second embodiment. In this embodiment, the waveform of the current output from the three-phase inverter 102 (i.e., the inverter output current Iout) is a continuous triangular waveform. The second embodiment differs from the first embodiment in that DUTY1 = DUTY2 = 0.25. The output current waveform Iout does not have a section where the current is continuously zero, and is a continuous triangular AC current. A triangular AC current is also called a triangular waveform signal.

[0059] When the impedance of a three-phase motor is high, it is necessary to increase DUTY1 and DUTY2 and increase the current value to achieve the desired amount of heating. When DUTY1 = DUTY2 = less than 0.25, the current is discontinuous, but when DUTY1 = DUTY2 = 0.25 or greater, the current becomes continuous in a triangular pattern. When DUTY1 = DUTY2 = 0.25, compared to when DUTY1 = DUTY2 = less than 0.25, there is a control feature in that the diagonal pair of switches outputting opposite polarity voltages are turned on at the same time as the anti-parallel diodes become non-conductive.

[0060] According to the second embodiment, by outputting a continuous triangular AC current, it is possible to increase the heating power compared to the discontinuous triangular AC current waveform of the first embodiment.

[0061] <Third Embodiment> A power conversion device 100 according to a third embodiment will now be described with reference to the drawings. FIG. 8 is a diagram illustrating an example of the operation (waveform) of each component in the third embodiment. The example illustrated in FIG. 8 illustrates an example of the waveform of each component that outputs a continuous AC current with a triangular shape different from that in the second embodiment. This is the case when DUTY1 = DUTY2 = 0.5 in FIGS. 6 and 7 of the first and second embodiments. When DUTY1 = DUTY2 = 0.5, it is necessary to insert a dead time in which all semiconductor switches are turned off to prevent short-circuiting of the upper and lower arms of the positive and negative semiconductor switches. Therefore, in practice, the time obtained by subtracting the dead time from the time when DUTY1 = DUTY2 = 0.5 is the actual on time of each semiconductor switch.

[0062] As with DUTY1 = DUTY2 = 0.25 in Figure 7, the output current waveform Iout has no zero-current interval and is a continuous triangular AC current. Therefore, the heating power is roughly the same when DUTY1 = DUTY2 = 0.25 in Figure 7 and when DUTY1 = DUTY2 = 0.5 in Figure 8. The difference between these is the current state when the semiconductor switch is on. When DUTY1 = DUTY2 = 0.25 or less, the output current is zero when the semiconductor switch is on. On the other hand, when DUTY1 = DUTY2 = 0.5, it is not zero. Furthermore, when a MOSFET is used as the semiconductor switch, its own anti-parallel diode is conductive when the semiconductor switch is turned on, and when the semiconductor switch is turned on, the reverse conduction path usually switches from the anti-parallel diode to the channel. When an IGBT is used as the semiconductor, the channel cannot be reverse-conductive even when the semiconductor switch is turned on, so its own anti-parallel diode is conductive. A feature of this device is that when the polarity of the output current switches, the semiconductor switch that was on becomes forward conductive.

[0063] The duty can also be selected within a range greater than 0.25 and less than 0.5. Within this duty range, the current is a continuous triangular AC current, and the operation is the same as for the above-mentioned duty = 0.5.

[0064] According to the third embodiment, by outputting a continuous triangular AC current, it is possible to increase the heating power compared to the discontinuous triangular AC current waveform of the first embodiment.

[0065] <Fourth Embodiment> A power conversion device 100 according to a fourth embodiment will be described below with reference to the drawings. Fig. 9 is a diagram showing an example of the operation (waveform) of each component in the fourth embodiment. The example shown in Fig. 9 shows an example of the waveforms of each component, including a transient state, when a DC current is superimposed on a triangular AC current. In the power conversion device 100 according to the fourth embodiment, a DC component is superimposed on the AC current output from the three-phase inverter 102.

[0066] Specifically, for DUTY1 of the positive side U-phase switch UP and the negative side V-phase switch VN, which corresponds to a positive output voltage, and DUTY2 of the negative side U-phase switch UN and the positive side V-phase switch VP, which corresponds to a negative output voltage, the positive voltage DUTY1 is made larger than the negative voltage DUTY2, thereby superimposing a positive DC current.

[0067] 9, in the steady state, DUTY1 and DUTY2 are 0.5. In reality, due to the resistance component of the motor windings, DUTY1 is slightly larger than 0.5 and DUTY2 is slightly smaller than 0.5.

[0068] The control means for superimposing the DC current includes feedback control, which senses the output current Iout to obtain a predetermined DC current component, and open loop control, which does not sense the output current Iout but obtains a predetermined DC current component with fixed DUTY1 and DUTY2. The predetermined DC component is determined from the predetermined heating power, as in the first embodiment.

[0069] For example, of two legs selected from U-phase leg 102U, V-phase leg 102V, and W-phase leg 102W, the on-time of the positive-side semiconductor switch of the first leg and the negative-side semiconductor switch of the second leg are made different from the on-time of the negative-side semiconductor switch of the first leg and the positive-side semiconductor switch of the second leg, and a DC component is superimposed on the AC current output from three-phase inverter 102 by feedback control or open-loop control.

[0070] Although dead time is omitted in Fig. 9, it is necessary to insert dead time in practice. Although omitted in Fig. 9, by making the negative voltage DUTY2 larger than the positive voltage DUTY1, it is possible to superimpose a negative DC current, and the same effect can be obtained.

[0071] According to the fourth embodiment, the heating power provided by the DC current can be added to the heating power provided by the triangular AC current in the first to third embodiments. This is effective as a means for increasing the heating power when the motor winding impedance is high. That is, the fourth embodiment can increase the heating power.

[0072] Fifth Embodiment A power conversion device 100 according to a fifth embodiment will be described below with reference to the drawings. Fig. 10 is a diagram showing an example of a current waveform when an AC component is superimposed on a triangular AC current. In the power conversion device 100 according to the fifth embodiment, an AC component is superimposed on the AC current output from the three-phase inverter 102. In the fifth embodiment, a dead time is inserted, similar to the fourth embodiment (e.g., Fig. 9).

[0073] Specifically, the ratio of the positive voltage DUTY1 to the negative voltage DUTY2 in the fourth embodiment is changed over time to superimpose an AC component on the triangular AC current. The frequency of the AC to be superimposed can be selected from a wide range, from low to high frequencies. For example, it may be a low frequency of 20 Hz or less, which is the lower limit of the audible frequency, or a high frequency of 10 kHz or more, which is close to the switching frequency. The selectable frequencies are not limited to the above examples.

[0074] As a means for superimposing an AC current component, there is a feedback control in which the output current Iout is sensed to obtain a predetermined AC current component. There is also an open-loop control in which the output current Iout is not sensed but is instead set to fixed DUTY1 and DUTY2 to obtain a predetermined AC current component. As in the first embodiment, this predetermined AC current component is determined from a predetermined heating power.

[0075] For example, by changing the on-time of the positive-side semiconductor switch of the first leg and the negative-side semiconductor switch of the second leg of two legs selected from the U-phase leg 102U, the V-phase leg 102V, and the W-phase leg 102W, and the on-time of the negative-side semiconductor switch of the first leg and the positive-side semiconductor switch of the second leg over time, an AC component is superimposed on the AC current output from the three-phase inverter 102 by feedback control or open-loop control.

[0076] According to the fifth embodiment, the heating power can be increased by superimposing an AC component on the AC current.

[0077] Sixth Embodiment A power conversion device according to a sixth embodiment will be described below with reference to the drawings. FIG. 11 is a diagram showing an example of switching between two legs selected by the inverter control unit 102a. In the sixth embodiment, the two phases to be energized are switched sequentially. That is, in the sixth embodiment, when preheating the refrigerant, the two legs selected by the inverter control unit 102a are switched sequentially by the inverter control unit 102a.

[0078] 11 , "UV" indicates the U-phase leg 102U and the V-phase leg 102V (i.e., the U-phase and the V-phase), "VW" indicates the V-phase leg 102V and the W-phase leg 102W (i.e., the V-phase and the W-phase), and "WU" indicates the W-phase leg 102W and the U-phase leg 102U (i.e., the W-phase and the U-phase). That is, in the example shown in FIG. 11 , the two legs selected by the inverter control unit 102a are switched in the following order: a set of the U-phase leg 102U and the V-phase leg 102V (also referred to as a first group), a set of the V-phase leg 102V and the W-phase leg 102W (also referred to as a second group), and a set of the W-phase leg 102W and the U-phase leg 102U (also referred to as a third group).

[0079] The order of the phases can be selected in any order, for example, WU phase, VW phase, UV phase. It is also not necessary to heat all of the UV, VW, and WU phases; for example, only the UV and VW phases can be used. The energized phases at the start and end of the preheating operation can also be selected in any order. The switching of each phase does not have to be continuous; there may be a pause between all phases. The energization time for each phase can be as short as one control cycle, or it can be in units of a few milliseconds, a few seconds, a few minutes, or a few hours.

[0080] According to the sixth embodiment, uneven heating of each phase can be reduced.

[0081] Seventh Embodiment A power conversion device according to a seventh embodiment will be described below with reference to the drawings. FIG. 12 is a diagram showing an example of a current conduction pattern when burst heating is performed. In the seventh embodiment, a constant current conduction time and a constant non-current conduction time are set for the two selected phases. For example, the sum of the switching operation time and non-switching operation time of each semiconductor switch of the two legs selected by the inverter control unit 102a is longer than the time of one control cycle in which the output voltage of the current output from the three-phase inverter 102 alternates between positive and negative. FIG. 12 shows an example in which the UV phase is selected. The current conduction time and non-current conduction time may be an extremely short time equivalent to one control cycle, or may be on the order of several milliseconds, several seconds, several minutes, or several tens of minutes.

[0082] FIG. 13 shows an example of a current waveform during burst heating. FIG. 13 shows a case where a continuous triangular AC current is applied. The current waveform may be a discontinuous triangular current, a DC superimposed current, a low-frequency current, or an AC superimposed current. Alternatively, various forms are possible, such as a combination of these current waveforms. The energized and deenergized periods do not have to be the same.

[0083] According to the seventh embodiment, by providing a current-on period and a current-off period, it is possible to realize a desired heating amount when averaged over time. Furthermore, by using the burst method, the frequency of the current rising or falling from zero current is reduced, thereby reducing noise in the high frequency band.

[0084] <Eighth Embodiment> A power conversion device 100 according to an eighth embodiment will be described below with reference to the drawings. Fig. 14 is a flowchart showing an example of the operation of the inverter control unit 102a during preheating control in the power conversion device 100 according to the eighth embodiment. In the eighth embodiment, preheating is started or ended based on a detected amount correlated with the amount of liquefaction of the refrigerant.

[0085] First, the inverter control unit 102a acquires a detected quantity correlated with the amount of liquefaction of the refrigerant (step S1). This detected quantity is, for example, the refrigerant temperature detected by a temperature sensor or the stop time and operation time of the on / off control of the selected two legs.

[0086] The inverter control unit 102a determines whether the detected amount satisfies the criterion for starting preheating control (step S2). If the detected amount satisfies the criterion for starting preheating control, the inverter control unit 102a starts preheating control (step S3). If the detected amount does not satisfy the criterion for starting preheating control, the inverter control unit 102a acquires the detected amount again. The inverter control unit 102a continues to acquire the detected amount and waits until the criterion for starting preheating control is satisfied.

[0087] When preheating control is performed, the detected amount is acquired in the same manner as in step S1 (step S4). As in step S1, the detected amount is, for example, a temperature sensor, and the stop time and operation time of preheating control.

[0088] The inverter control unit 102a determines whether the detected amount satisfies the criterion for terminating preheating control (step S5). If the detected amount satisfies the criterion for terminating preheating control, the inverter control unit 102a terminates preheating control. If the detected amount does not satisfy the criterion for terminating preheating control, the inverter control unit 102a performs preheating control again and acquires the detected amount. The inverter control unit 102a continues to acquire the detected amount and continue preheating until the criterion for terminating preheating control is satisfied.

[0089] According to the eighth embodiment, a predetermined amount of heating can be achieved. Also, energy saving can be achieved by controlling preheating only at the timing when the refrigerant needs to be in a liquefied state.

[0090] Ninth Embodiment A power conversion device 100 according to a ninth embodiment will be described below with reference to the drawings. FIG. 15 is a diagram illustrating an example of a change in the switching frequency during variable control of the switching frequency. While the first to eighth embodiments illustrate an example in which the switching frequency is fixed, the ninth embodiment dynamically changes the switching frequency. That is, the inverter control unit 102a changes the switching frequencies of the semiconductor switches of the two-phase legs selected by the inverter control unit 102a during operation of the three-phase inverter 102. For example, the inverter control unit 102a changes the switching frequencies of the positive-side U-phase switch UP, the negative-side U-phase switch UN, the positive-side V-phase switch VP, and the negative-side V-phase switch VN during operation of the three-phase inverter 102.

[0091] 15 shows an example in which the switching frequency is changed in a triangular shape. Other examples include a sine wave shape and a rectangular shape with a step change. The upper limit, lower limit, and median value of the switching frequency are also dynamically changed.

[0092] According to the ninth embodiment, the noise spectrum can be spread by changing the switching frequency, and the quasi-peak or average noise can be reduced.

[0093] The features of the above-described embodiments can be combined with each other.

[0094] Various aspects of the present disclosure are summarized below as appendices. (Supplementary Note 1) A power conversion device that supplies power to a three-phase motor that drives a compressor that compresses a refrigerant, comprising: a three-phase inverter having at least six semiconductor switches electrically connected to the three-phase motor; and an inverter control unit that controls the at least six semiconductor switches, wherein the three-phase inverter includes: a positive U-phase switch that is a positive side semiconductor switch of the at least six semiconductor switches that is connected to a positive side of a DC power supply; and a negative U-phase switch that is a negative side semiconductor switch of the at least six semiconductor switches that is connected in series with the positive U-phase switch and is connected to the negative side of the DC power supply, a U-phase leg that corresponds to the U-phase of the three-phase motor; a positive V-phase switch that is a positive side semiconductor switch of the at least six semiconductor switches that is connected to the positive side of the DC power supply; and a negative V-phase switch that is a negative side semiconductor switch of the at least six semiconductor switches that is connected in series with the positive V-phase switch and is connected to the negative side of the DC power supply, a V-phase leg that corresponds to the V-phase of the three-phase motor; and a W-phase leg including a positive W-phase switch which is a positive side semiconductor switch among the at least six semiconductor switches connected to the positive side of the DC power supply, and a negative W-phase switch which is a negative side semiconductor switch among the at least six semiconductor switches which is connected in series with the positive W-phase switch and is connected to the negative side of the DC power supply, the W-phase leg corresponding to the W phase of the three-phase motor, wherein when the refrigerant is preheated, the inverter control unit selects two legs from the U-phase leg, the V-phase leg, and the W-phase leg, and performs on / off control of the positive side semiconductor switch of a first leg of the selected two legs and the negative side semiconductor switch of a second leg of the selected two legs at the same time, and performs on / off control of the negative side semiconductor switch of the first leg and the positive side semiconductor switch of the second leg at the same time, and maintains off the positive side semiconductor switch and the negative side semiconductor switch of a third leg other than the two selected legs. A power conversion device characterized by:(Supplementary Note 2) The power conversion device according to Supplementary Note 1, wherein an on-time of the semiconductor switches is set so as to suppress liquefaction of the refrigerant. (Supplementary Note 3) The power conversion device according to Supplementary Note 1 or 2, wherein a waveform of a current output from the three-phase inverter is a discontinuous triangle. (Supplementary Note 4) The power conversion device according to Supplementary Note 1 or 2, wherein a waveform of a current output from the three-phase inverter is a continuous triangle. (Supplementary Note 5) The power conversion device according to Supplementary Note 1 or 2, wherein a DC component is superimposed on the AC current output from the three-phase inverter. (Supplementary Note 6) The power conversion device according to Supplementary Note 1 or 2, wherein an AC component is superimposed on the AC current output from the three-phase inverter. (Supplementary Note 7) The power conversion device according to Supplementary Note 1 or 2, wherein a time during which all of the semiconductor switches are off in one period of the current output from the three-phase inverter is equal to or longer than a dead time during a refrigerant compression operation. (Supplementary Note 8) The power conversion device according to Supplementary Note 1 or 2, characterized in that a time during which all of the semiconductor switches are off in one cycle of the current output from the three-phase inverter is longer than a dead time during a refrigerant compression operation. (Supplementary Note 9) The power conversion device according to Supplementary Note 5, characterized in that the on-time of the positive electrode side semiconductor switch of the first leg and the negative electrode side semiconductor switch of the second leg is made different from the on-time of the negative electrode side semiconductor switch of the first leg and the positive electrode side semiconductor switch of the second leg, and the DC component is superimposed on the AC current output from the three-phase inverter by feedback control or open loop control. (Supplementary Note 10) The power conversion device according to Supplementary Note 6, wherein an on-time of the positive pole side semiconductor switch of the first leg and the negative pole side semiconductor switch of the second leg, and an on-time of the negative pole side semiconductor switch of the first leg and the positive pole side semiconductor switch of the second leg are changed over time, and the AC component is superimposed on the AC current output from the three-phase inverter by feedback control or open loop control.(Supplementary Note 11) The power conversion device according to Supplementary Note 1 or 2, wherein the two selected legs are switched sequentially by the inverter control unit. (Supplementary Note 12) The power conversion device according to Supplementary Note 1 or 2, wherein the sum of the switching operation time and the non-switching operation time of each semiconductor switch of the two selected legs is longer than the time of one control cycle in which the output voltage of the current output from the three-phase inverter alternates between positive and negative. (Supplementary Note 13) The power conversion device according to Supplementary Note 1 or 2, wherein each of the switching frequencies of the positive-side U-phase switch, the negative-side U-phase switch, the positive-side V-phase switch, and the negative-side V-phase switch is 9 kHz or higher. (Supplementary Note 14) The power conversion device according to Supplementary Note 1 or 2, wherein the power output from the three-phase inverter is 50 W or less. (Supplementary Note 15) The power conversion device according to Supplementary Note 1 or 2, wherein the preheating is started or ended based on a detected amount correlated with the amount of liquefaction of the refrigerant. (Supplementary Note 16) The power conversion device according to Supplementary Note 15, wherein the detected quantity is the temperature of the refrigerant detected by a temperature sensor, or a stop time and an operation time of the on / off control of the selected two legs. (Supplementary Note 17) The power conversion device according to Supplementary Note 1 or 2, wherein the inverter control unit changes the switching frequencies of the positive U-phase switch, the negative U-phase switch, the positive V-phase switch, and the negative V-phase switch during operation of the three-phase inverter. (Supplementary Note 18) A refrigeration cycle device comprising: the power conversion device according to Supplementary Note 1 or 2; and a compressor. (Supplementary Note 19) A motor system comprising: the power conversion device according to Supplementary Note 1 or 2; and the three-phase motor.

[0095] REFRIGERATION CYCLE DEVICE, 2 MOTOR SYSTEM, 10 COMPRESSOR, 11 THREE-PHASE MOTOR, 100 POWER CONVERSION DEVICE, 101 POWER SUPPLY UNIT, 102 THREE-PHASE INVERTER, 102a INVERTER CONTROL UNIT, 102U U-PHASE LEG, 102V V-PHASE LEG, 102W W-PHASE LEG, UP, VP, WP, UN, VN, WN SEMICONDUCTOR SWITCHES.

Claims

1. A power conversion device that supplies power to a three-phase motor that drives a compressor for compressing a refrigerant, the power conversion device comprising: a three-phase inverter having at least six semiconductor switches electrically connected to the three-phase motor; and an inverter control unit that controls the at least six semiconductor switches, wherein the three-phase inverter includes a positive U-phase switch that is a positive-side semiconductor switch among the at least six semiconductor switches connected to the positive electrode side of a DC power supply, and a negative U-phase switch that is a negative-side semiconductor switch among the at least six semiconductor switches connected in series to the positive U-phase switch and connected to the negative electrode side of the DC power supply, and a U-phase leg corresponding to the U-phase of the three-phase motor; a positive V-phase switch that is a positive-side semiconductor switch among the at least six semiconductor switches connected to the positive electrode side of the DC power supply, and a negative V-phase switch that is a negative-side semiconductor switch among the at least six semiconductor switches connected in series to the positive V-phase switch and connected to the negative electrode side of the DC power supply, and a V-phase leg corresponding to the V-phase of the three-phase motor; a positive W-phase switch that is a positive-side semiconductor switch among the at least six semiconductor switches connected to the positive electrode side of the DC power supply, and a negative W-phase switch that is a negative-side semiconductor switch among the at least six semiconductor switches connected in series to the positive W-phase switch and connected to the negative electrode side of the DC power supply, and a W-phase leg corresponding to the W-phase of the three-phase motor; when preheating the refrigerant, the inverter control unit selects two legs out of the U-phase leg, the V-phase leg, and the W-phase leg, and performs on / off control of the positive-side semiconductor switch of the first leg among the two selected legs and the negative-side semiconductor switch of the second leg among the two selected legs at the same timing, and performs on / off control of the negative-side semiconductor switch of the first leg and the positive-side semiconductor switch of the second leg at the same timing, and maintains the positive-side semiconductor switch and the negative-side semiconductor switch of a third leg other than the two selected legs in the off state. A power conversion device characterized by the above.

2. The on-time of the semiconductor switch is set so as to suppress liquefaction of the refrigerant. The power conversion device according to claim 1, characterized in that.

3. The waveform of the current output from the three-phase inverter is a discontinuous triangle. The power conversion device according to claim 1 or 2, characterized in that.

4. The waveform of the current output from the three-phase inverter is a continuous triangle. The power conversion device according to claim 1 or 2, characterized in that.

5. A DC component is superimposed on the alternating current output from the three-phase inverter. The power conversion device according to claim 1 or 2, characterized in that.

6. An AC component is superimposed on the alternating current output from the three-phase inverter. The power conversion device according to claim 1 or 2, characterized in that.

7. The time during which all the semiconductor switches are turned off in one cycle of the current output from the three-phase inverter is equal to or longer than the dead time during the refrigerant compression operation. The power conversion device according to claim 1 or 2, characterized in that.

8. The time during which all the semiconductor switches are turned off in one cycle of the current output from the three-phase inverter is longer than the dead time during the refrigerant compression operation. The power conversion device according to claim 1 or 2, characterized in that.

9. The on-time of the positive-side semiconductor switch of the first leg and the negative-side semiconductor switch of the second leg, and the on-time of the negative-side semiconductor switch of the first leg and the positive-side semiconductor switch of the second leg are made different from each other, and a DC component is superimposed on the alternating current output from the three-phase inverter by feedback control or open-loop control. The power conversion device according to claim 5, characterized in that.

10. The on-time of the positive electrode side semiconductor switch of the first leg and the on-time of the negative electrode side semiconductor switch of the second leg, and the on-time of the negative electrode side semiconductor switch of the first leg and the on-time of the positive electrode side semiconductor switch of the second leg are changed with time, and the AC component is superimposed on the AC current output from the three-phase inverter by feedback control or open-loop control. The power conversion device according to claim 6, characterized in that.

11. The two legs to be selected are sequentially switched by the inverter control unit. The power conversion device according to claim 1 or 2, characterized in that.

12. The sum of the switching operation time and the non-switching operation time of each semiconductor switch of the two selected legs is longer than the time of one control cycle in which the output voltage of the current output from the three-phase inverter repeats positive and negative. The power conversion device according to claim 1 or 2, characterized in that.

13. The switching frequencies of the positive electrode side U-phase switch, the negative electrode side U-phase switch, the positive electrode side V-phase switch, and the negative electrode side V-phase switch are 9 kHz or more. The power conversion device according to claim 1 or 2, characterized in that.

14. The power output from the three-phase inverter is 50 W or less. The power conversion device according to claim 1 or 2, characterized in that.

15. Starting or ending the preheating based on a detected amount correlated with the amount of liquefaction of the refrigerant. The power conversion device according to claim 1 or 2, characterized in that.

16. The detected amount is the temperature of the refrigerant detected by a temperature sensor, or the stop time and operation time of the on-off control of the two selected legs. The power conversion device according to claim 15, characterized in that.

17. The inverter control unit changes the switching frequencies of the positive electrode side U-phase switch, the negative electrode side U-phase switch, the positive electrode side V-phase switch, and the negative electrode side V-phase switch during the operation of the three-phase inverter. The power conversion device according to claim 1 or 2, characterized in that.

18. A refrigeration cycle apparatus comprising the power conversion device according to claim 1 or 2 and a compressor.

19. A motor system comprising the power conversion device according to claim 1 or 2 and the three-phase motor.

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