Power conversion device, motor drive device, and equipment for refrigeration cycle applications

Abstract

A power conversion device (1) comprises: a converter (150) that steps up or steps down an input voltage so as to output a DC voltage; and an inverter (310) that converts the DC voltage, which is a converter output voltage output from the converter (150), into an AC voltage and outputs the AC voltage to a motor (314). The converter (150) outputs a DC voltage corresponding to the rotation speed of the motor (314) such that the leakage current during driving of the motor (314) is less than or equal to a specified value.

Description

Power conversion devices, motor drive devices, and refrigeration cycle application equipment

[0001] The present disclosure relates to a power conversion device that performs power conversion, a motor drive device, and a refrigeration cycle application device.

[0002] Conventionally, there are power conversion devices that include a converter and an inverter. Some converters not only convert an input AC voltage to a DC voltage but also boost or lower the voltage while converting the input AC voltage to a DC voltage. For example, Patent Document 1 discloses a technology for a DC brushless motor drive device that controls the output voltage of a converter circuit so that a DC brushless motor rotates at a desired speed. In a configuration such as the DC brushless motor drive device described in Patent Document 1, it is possible to rotate a DC brushless motor at a desired speed by controlling the operation of the inverter circuit or by controlling the operation of both the converter circuit and the inverter circuit.

[0003] Japanese Patent Application Laid-Open No. 2003-143890

[0004] Generally, the more frequently the switching elements in an inverter are switched, the greater the leakage current generated by the power converter, the connected motor, and the like. While small leakage currents have no effect on the human body, if they reach a certain level, they may affect the human body if they come into contact with the power converter, motor, and other components, even if they are enclosed in a housing. Therefore, it is preferable to minimize the leakage current generated by the power converter, the connected motor, and the like. However, while the above-described conventional DC brushless motor drive device can control both the converter circuit and the inverter circuit to rotate the DC brushless motor at a desired rotation speed, it does not take into account the generation of leakage current. As a result, there is a problem in that the DC brushless motor drive device may control the DC brushless motor to a desired rotation speed under conditions that result in large leakage current.

[0005] The present disclosure has been made in view of the above, and has an object to provide a power conversion device that can suppress an increase in leakage current.

[0006] In order to solve the above-mentioned problems and achieve the object, a power conversion device according to the present disclosure includes a converter that steps up or steps down an input voltage to output a DC voltage, and an inverter that converts the DC voltage, which is the converter output voltage from the converter, into an AC voltage and outputs it to a motor. The converter outputs a DC voltage according to the rotation speed of the motor so that the leakage current during operation of the motor is equal to or less than a specified value.

[0007] The power conversion device according to the present disclosure has an effect of being able to suppress an increase in leakage current.

[0008] 3 is a diagram showing an example of a configuration of a power conversion device according to embodiment 1; FIG. 4 is a diagram showing an example of a control signal to a switching element of an inverter in a power conversion device according to embodiment 1 when the modulation factor is lower than that of the example of FIG. 3 at the same frequency as that of the inverter output voltage in the example of FIG. 3; FIG. 5 is a diagram showing an example of a control signal to a switching element of an inverter in a power conversion device according to embodiment 1 when the modulation factor is higher than that of the example of FIG. 2 and is 1 or more at the same frequency as that of the inverter output voltage in the example of FIG. 2; FIG. 6 is a diagram showing a relationship between the rotation speed of a motor and the voltage generated by the motor in a motor drive device according to embodiment 1; FIG. 1 shows an example of an AC voltage waveform and an AC current waveform when a PFC operation is not performed in a power conversion device according to embodiment 2. FIG. 2 shows an example of an AC voltage waveform and an AC current waveform when a converter performs a PFC operation in a power conversion device according to embodiment 2. FIG. 3 shows an example of a configuration of a power conversion device according to embodiment 3. FIG. 4 shows an example of a configuration of a power conversion device according to embodiment 4. FIG. 1 shows an example of a configuration of a power conversion device according to embodiment 5. FIG. 2 shows an example of a configuration of a power conversion device according to embodiment 5. FIG. 3 shows an example of a configuration of a refrigeration cycle application device according to embodiment 6.

[0009] Hereinafter, a power conversion device, a motor drive device, and a refrigeration cycle applied device according to embodiments of the present disclosure will be described in detail with reference to the drawings.

[0010] First Embodiment. Fig. 1 is a diagram illustrating an example of the configuration of a power conversion device 1 according to a first embodiment. The power conversion device 1 is connected to an AC power supply 110 and a motor 314. The power conversion device 1 converts a first AC voltage supplied from the AC power supply 110, such as a commercial power supply, into a second AC voltage having a desired amplitude and phase, and supplies the second AC voltage to the motor 314. In the example of Fig. 1, the AC power supply 110 is a single-phase AC power supply, but may be a three-phase AC power supply. The power conversion device 1 includes a noise filter 120, a converter 150, an inverter 310, a control unit 400, current detection units 502 and 504, and detection resistors 502a and 504a. The power conversion device 1 and the motor 314 constitute a motor drive device 2.

[0011] The noise filter 120 removes noise from the first AC voltage supplied from the AC power supply 110. The configuration of the noise filter 120 may be a general configuration and is not particularly limited. The noise filter 120 outputs the first AC voltage after the noise removal to the converter 150. Note that the noise filter 120 is not an essential component of the power conversion device 1, and therefore the power conversion device 1 may be configured without the noise filter 120.

[0012] The converter 150 is a power converter connected in parallel to both ends of the input terminal of the inverter 310. The converter 150 boosts or lowers the input voltage to output a DC voltage. In this embodiment, the converter 150 is an AC (Alternating Current)-DC (Direct Current) converter, that is, an AC-DC converter. Since the power conversion device 1 is connected to the AC power supply 110, the input voltage of the converter 150 is a noise-removed first AC voltage obtained by removing noise from the first AC voltage supplied from the AC power supply 110 by the noise filter 120. Note that if the power conversion device 1 is configured without the noise filter 120, the input voltage of the converter 150 is the first AC voltage supplied from the AC power supply 110. The converter 150 includes a rectifier circuit 130 and a step-up / step-down circuit 140.

[0013] The rectifier circuit 130 rectifies the noise-removed first AC voltage output from the noise filter 120 and outputs the rectified voltage to the step-up / step-down circuit 140. When the AC power supply 110 is a single-phase AC power supply as shown in FIG. 1 , the rectifier circuit 130 is a bridge circuit configured by four rectifier elements 131.

[0014] The step-up / step-down circuit 140 includes a switching element 141, a freewheeling diode 142, a reactor 143, a diode 144, and a smoothing capacitor 210. The step-up / step-down circuit 140 steps up or steps down the rectified voltage output from the rectifier circuit 130, and outputs the DC voltage to the inverter 310. In the step-up / step-down circuit 140, the switching element 141 is turned on and off under the control of the control unit 400, and the switching element 141, the freewheeling diode 142, the reactor 143, and the diode 144 step up or step down the rectified voltage output from the rectifier circuit 130. The switching element 141 is, for example, an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a bipolar transistor, etc., but is not limited to these. The smoothing capacitor 210 smoothes the rectified voltage that has been boosted or stepped down by the switching element 141, the freewheeling diode 142, the reactor 143, and the diode 144, and outputs a DC voltage to the inverter 310. The smoothing capacitor 210 is, for example, an electrolytic capacitor, a film capacitor, etc.

[0015] The smoothing capacitor 210 may be disposed outside the converter 150. That is, the smoothing capacitor 210 may be connected to a main circuit inside the converter 150, or may be connected to a main circuit outside the converter 150 between the converter 150 and the inverter 310.

[0016] 1, the converter 150 is configured by the rectifier circuit 130 and the step-up / step-down circuit 140, but the configuration of the converter 150 is not limited to the example in Fig. 1. The converter 150 may be configured such that the rectifier circuit 130 and the step-up / step-down circuit 140 are integrated together.

[0017] The inverter 310 is a power converter connected in parallel across the output terminals of the converter 150. The inverter 310 converts a DC voltage, which is the output voltage of the converter 150, into an AC voltage and outputs the AC voltage to the motor 314. The output voltage of the converter 150 is the same as the input voltage of the inverter 310, which corresponds to the bus voltage in FIG. 1 . In the first embodiment, the AC voltage output from the inverter 310 serves as the second AC voltage. The inverter 310 includes six switching elements 311 and six freewheeling diodes 312. The control unit 400 controls the switching elements 311 to turn on and off, converting the DC voltage into a second AC voltage having a desired amplitude and phase, i.e., generating the second AC voltage, and outputting it to the motor 314, which is the load. In the example of FIG. 1 , the second AC voltage is a three-phase AC voltage. The switching elements 311 may be, for example, an IGBT, a MOSFET, a bipolar transistor, or the like, but are not limited thereto. The circuit configuration of the inverter 310 is not particularly limited and may be a full-bridge circuit, a single-phase bridge circuit, a half-bridge circuit, or the like.

[0018] The current detection unit 502 is a first current detection unit that detects the rectified current output from the rectification circuit 130 at a stage subsequent to the rectification circuit 130. The current detection unit 502 is connected to a detection resistor 502a and detects the rectified current by measuring the current flowing through the detection resistor 502a. Although not shown, the current detection unit 502 outputs the detected value of the rectified current, i.e., the current value, to the control unit 400. In the following description, the current detection unit 502 may be simply referred to as a detection unit.

[0019] The current detection unit 504 is a second current detection unit that detects the current flowing through the inverter 310, which is used to estimate the current flowing from the inverter 310 to the motor 314, which is a load, in the upstream stage of the inverter 310. The current detection unit 504 is connected to a detection resistor 504a, and detects the current flowing through the inverter 310 by measuring the current flowing through the detection resistor 504a. Although not shown in the figure, the current detection unit 504 outputs the detection value of the current flowing through the inverter 310, i.e., the current value, to the control unit 400. In the following description, the current detection unit 504 may be simply referred to as a detection unit.

[0020] The control unit 400 acquires detection values ​​from the current detection units 502 and 504. The control unit 400 uses the acquired detection values ​​to control the operation of the converter 150 and the inverter 310. Specifically, the control unit 400 controls the operation of the switching element 141 included in the step-up / step-down circuit 140 of the converter 150 and the switching element 311 included in the inverter 310. That is, the control unit 400 performs control calculations for controlling the on / off of the switching elements 141 and 311, and generates and outputs control signals for controlling the on / off of the switching elements 141 and 311. The power conversion device 1 may further include a current detection unit that is a detection unit installed at a position not shown in FIG. 1 . The power conversion device 1 may also include a voltage detection unit that is a detection unit not shown in FIG. 1 . The control unit 400 may use the detection values ​​from these detection units not shown to control the operation of the switching element 141 included in the step-up / step-down circuit 140 of the converter 150 and the switching element 311 included in the inverter 310. The positions of detectors such as a current detector and a voltage detector (not shown) installed in the power conversion device 1 may be the same as the positions of detectors installed in a general power conversion device, and are not particularly limited.

[0021] The motor 314 is energized and rotated by application of the second AC voltage, i.e., the three-phase AC voltage, from the inverter 310. In this embodiment, the motor 314 is assumed to be an interior permanent magnet synchronous motor.

[0022] Here, in motor drive device 2, when motor 314 is energized, a potential is generated inside motor 314, creating a potential difference with respect to the ground, which causes leakage current to flow from motor 314 via motor 314's stray capacitance to the ground 320. As a result, if the leakage current is large, it may affect anyone who touches the housing even if motor drive device 2 is covered with a housing. Therefore, a method for suppressing an increase in leakage current generated in motor drive device 2, i.e., power conversion device 1, motor 314, etc., will be described.

[0023] In the power conversion device 1, the inverter 310 operates by turning on and off the switching elements 311. Specifically, based on a control signal for each switching element 311 from the control unit 400, the inverter 310 turns on the switching elements 311 when the control signal is Hi and turns off the switching elements 311 when the control signal is Lo. In the inverter 310, switching by the switching elements 311 occurs as the control signal switches between Hi and Lo. It is generally known that the fewer times the switching elements 311 of the inverter 310 are switched, the smaller the leakage current. Therefore, in order to reduce the leakage current generated in the motor drive device 2, it is sufficient to reduce the number of times the switching elements 311 of the inverter 310 are switched.

[0024] In this embodiment, it is assumed that the control signals output from the control unit 400 to each switching element 311 of the inverter 310 are generated using a pulse width modulation (PWM) method. In the PWM control of the inverter 310, the switching pattern of the switching elements 311 is determined by the modulation rate defined by the following equation (1) and the frequency of the output voltage of the inverter 310.

[0025] Modulation rate = Peak value of line voltage when inverter output voltage is sinusoidal ÷ inverter input voltage = Peak value of line voltage when inverter output voltage is sinusoidal ÷ converter output voltage ... (1)

[0026] The frequency of the output voltage of the inverter 310 is uniquely determined by the rotation speed of the motor 314. The modulation factor is determined by the ratio of the output voltage of the inverter 310 to the output voltage of the converter 150, as shown in equation (1). Therefore, the power conversion device 1 can control the modulation factor by changing the output voltage of the converter 150, regardless of the value of the output voltage of the inverter 310. That is, in the power conversion device 1, the converter 150 outputs a DC voltage corresponding to the rotation speed of the motor 314 so that the leakage current during operation of the motor 314 is equal to or less than a specified value. In this embodiment, the converter 150 rectifies a first AC voltage and outputs a DC voltage by increasing or decreasing the rectified voltage. The inverter 310 converts the DC voltage into a second AC voltage and outputs it to the motor 314.

[0027] FIG. 2 shows a control signal when the modulation factor is low at the same frequency of the output voltage of the inverter 310, and FIG. 3 shows a control signal when the modulation factor is high and equal to or greater than 1. FIG. 2 is a diagram showing an example of a control signal to the switching element 311 of the inverter 310 in the power conversion device 1 according to embodiment 1 when the modulation factor is lower than that of the example in FIG. 3 at the same frequency of the output voltage of the inverter 310 as in the example in FIG. 3. FIG. 3 is a diagram showing an example of a control signal to the switching element 311 of the inverter 310 in the power conversion device 1 according to embodiment 1 when the modulation factor is higher than that of the example in FIG. 2 and equal to or greater than 1. As can be seen from FIGS. 2 and 3 , when the modulation factor is equal to or greater than 1, there are periods when the control signal stays high and periods when the control signal stays low. Therefore, when the modulation factor is equal to or greater than 1, the number of times the control signal switches between high and low, i.e., the number of times the switching element 311 of the inverter 310 switches, is reduced. Therefore, if the power conversion device 1 operates the inverter 310 with the modulation factor increased to 1 or more, the number of times the switching element 311 is switched can be reduced, and as a result, the leakage current can also be reduced.

[0028] FIG. 4 is a diagram showing the relationship between the rotation speed of motor 314 and the voltage generated by motor 314 in motor drive device 2 according to embodiment 1. FIG. 5 is a diagram showing the relationship between the rotation speed of motor 314 and the output voltage of converter 150 for suppressing leakage current in motor drive device 2 according to embodiment 1. As described above, motor 314 used in motor drive device 2 is assumed to be an interior permanent magnet synchronous motor. In motor 314, which is an interior permanent magnet synchronous motor, a voltage is generated by the magnets embedded in motor 314 according to the rotation speed of motor 314. In order to energize motor 314 and rotate it, a voltage equal to or greater than the voltage generated by motor 314 must be applied to motor 314 from inverter 310. Therefore, when the rotation speed of motor 314 is low, the output voltage of inverter 310 applied to motor 314 can be small, and when the rotation speed of motor 314 is high, the output voltage of inverter 310 applied to motor 314 must be large.

[0029] As described above, the modulation factor is expressed by equation (1), but because the output voltage of inverter 310 is changed according to the rotation speed of motor 314, the output voltage of inverter 310, which is the number by which the modulation factor is divided, changes according to the rotation speed of motor 314. In order to make the modulation factor greater than or equal to 1 and reduce the number of times that switching element 311 of inverter 310 is switched, it is also necessary to change the output voltage of converter 150, which is the number by which the modulation factor is divided, according to the rotation speed of motor 314.

[0030] In the present embodiment, the power conversion device 1 includes the converter 150 equipped with the step-up / step-down circuit 140, allowing the output voltage of the converter 150 to be freely changed. Therefore, as shown in FIG. 5 , the power conversion device 1 can suppress leakage current by stepping up or down the output voltage of the converter 150 depending on the rotation speed of the motor 314. Leakage current cannot be reduced to zero and will inevitably occur, but as long as it does not exceed a specified value, it will not affect the human body. Therefore, the power conversion device 1 can prevent the leakage current from affecting the human body by setting the output voltage of the step-up / step-down circuit 140 of the converter 150 depending on the rotation speed of the motor 314 so that the leakage current is equal to or less than a specified value. The specified value may be set in advance by the manufacturer of the power conversion device 1 or the motor drive device 2, taking into account the housing of the power conversion device 1 or the motor drive device 2 (not shown), or may be set in advance by the user of the motor drive device 2 before operation, taking into account the environment in which the motor drive device 2 will be installed, etc.

[0031] FIG. 6 is a flowchart showing the operation of the power conversion device 1 according to the first embodiment. The control unit 400 calculates the modulation factor of the power conversion device 1 (step S1). If the modulation factor is equal to or greater than 1 (step S2: Yes), the control unit 400 ends the operation of the flowchart shown in FIG. 6. If the modulation factor is less than 1 (step S2: No), the control unit 400 controls the step-up / step-down circuit 140 of the converter 150 to adjust the output voltage of the converter 150 (step S3), and returns to step S1. The control unit 400 returns to step S1 and performs the same operation as described above. The control unit 400 may perform the operation of the flowchart shown in FIG. 6 continuously or at a specified cycle.

[0032] Next, a description will be given of the hardware configuration of the control unit 400 included in the power conversion device 1. Fig. 7 is a diagram showing an example of a hardware configuration that realizes the control unit 400 included in the power conversion device 1 according to embodiment 1. The control unit 400 is realized by a processor 91 and a memory 92.

[0033] The processor 91 is a CPU (Central Processing Unit, also called a central processing unit, processing unit, arithmetic unit, microprocessor, microcomputer, processor, or DSP (Digital Signal Processor)) or a system LSI (Large Scale Integration). Examples of memory 92 include non-volatile or volatile semiconductor memories such as RAM (Random Access Memory), ROM (Read Only Memory), flash memory, EPROM (Erasable Programmable Read Only Memory), and EEPROM (Electrically Erasable Programmable Read Only Memory). Memory 92 is not limited to these, and may also be a magnetic disk, optical disk, compact disk, minidisc, or DVD (Digital Versatile Disc).

[0034] As described above, according to the present embodiment, in the power conversion device 1, the control unit 400 controls the step-up / step-down circuit 140 of the converter 150 to adjust the output voltage of the converter 150, i.e., the input voltage of the inverter 310, in accordance with the rotation speed of the motor 314. Specifically, the control unit 400 adjusts the output voltage of the converter 150, i.e., the input voltage of the inverter 310, so that the modulation factor of the inverter 310 of the power conversion device 1 is 1 or greater. At this time, the converter 150 increases or decreases the output voltage of the converter 150 in accordance with the rotation speed of the motor 314 so that the leakage current during driving of the motor 314 is equal to or less than a specified value and the modulation factor of the inverter 310 is 1 or greater. In this way, the power conversion device 1 can suppress an increase in the leakage current by reducing the number of switching operations of the switching element 311 of the inverter 310. Because the power conversion device 1 can suppress an increase in the leakage current, it is possible to prevent the leakage current from having an effect on the human body.

[0035] Second Embodiment In the first embodiment, the operation of converter 150 is specified only with respect to the output voltage of converter 150. In the second embodiment, a case will be described in which converter 150 performs a PFC operation, i.e., a power factor correction operation, that controls the waveform of the AC current to have a sinusoidal shape synchronized with the waveform of the AC voltage.

[0036] In the second embodiment, the configuration of the power conversion device 1 is the same as the configuration of the power conversion device 1 in the first embodiment shown in FIG. 1 . Furthermore, in the second embodiment, the connections of the power conversion device 1 are also the same as the connections of the power conversion device 1 in the first embodiment shown in FIG. 1 . FIG. 8 is a diagram illustrating an example of an AC voltage waveform and an AC current waveform when the converter 150 in the power conversion device 1 according to the second embodiment does not perform a PFC operation. FIG. 9 is a diagram illustrating an example of an AC voltage waveform and an AC current waveform when the converter 150 in the power conversion device 1 according to the second embodiment performs a PFC operation. When converting an AC voltage to a DC voltage, if the current is not taken into consideration, the AC current will have a shape that is far from a sine wave, as shown in FIG. 8 , for example, and the power factor will deteriorate. On the other hand, as shown in FIG. 9 , the converter 150 can improve the power factor by adjusting the AC current waveform to a sine wave synchronized with the AC voltage waveform while increasing or decreasing the output voltage through the switching operation of the switching element 141.

[0037] As described above, according to the present embodiment, in the power conversion device 1, the converter 150 performs a power factor correction operation so that the waveform of the AC current of the first AC voltage becomes a sinusoidal waveform synchronized with the waveform of the first AC voltage. This enables the power conversion device 1 to suppress an increase in leakage current and achieve a high power supply power factor.

[0038] Third Embodiment In a third embodiment, a case where the power conversion device 1 is connected to a DC power supply will be described.

[0039] FIG. 10 is a diagram illustrating a configuration example of a power conversion device 1 according to a third embodiment. The power conversion device 1 is connected to a DC power supply 111 and a motor 314. The power conversion device 1 converts a first DC voltage supplied from the DC power supply 111 into an AC voltage having a desired amplitude and phase, and supplies the AC voltage to the motor 314. The power conversion device 1 includes a noise filter 120, a converter 150, an inverter 310, a control unit 400, current detection units 502 and 504, and detection resistors 502a and 504a. The configuration of the power conversion device 1 according to the third embodiment is the same as that of the power conversion device 1 according to the first embodiment shown in FIG. 1 , except that the rectifier circuit 130 is deleted. That is, in the third embodiment, the converter 150 is a DC-DC converter, i.e., a DC-DC converter. In the converter 150, the configuration of the step-up / step-down circuit 140 is the same as that of the step-up / step-down circuit 140 according to the first embodiment shown in FIG. 1 . In the first embodiment, the noise filter 120 is for AC, but in the third embodiment, the noise filter 120 is for DC. The power conversion device 1 and the motor 314 constitute a motor drive device 2.

[0040] In the third embodiment, the operation of the step-up / step-down circuit 140 included in the converter 150 and the control content of the control unit 400 are the same as the operation of the step-up / step-down circuit 140 included in the converter 150 and the control content of the control unit 400 in the first embodiment. In the third embodiment, the DC voltage supplied from the DC power supply 111 is the first DC voltage, which is the input voltage described above, and the DC voltage output from the converter 150 is the second DC voltage. In this case, the converter 150 steps up or steps down the first DC voltage to output the second DC voltage. Furthermore, the inverter 310 converts the second DC voltage into an AC voltage and outputs it to the motor 314.

[0041] As described above, according to the present embodiment, in the power conversion device 1, the control unit 400 controls the step-up / step-down circuit 140 of the converter 150 to adjust the output voltage of the converter 150, i.e., the input voltage of the inverter 310, in accordance with the rotation speed of the motor 314. Specifically, the control unit 400 adjusts the output voltage of the converter 150, i.e., the input voltage of the inverter 310, so that the modulation factor of the inverter 310 of the power conversion device 1 is 1 or greater. At this time, the converter 150 increases or decreases the output voltage of the converter 150 in accordance with the rotation speed of the motor 314 so that the leakage current during driving of the motor 314 is equal to or less than a specified value and the modulation factor of the inverter 310 is 1 or greater. In this way, the power conversion device 1 can suppress an increase in the leakage current by reducing the number of switching operations of the switching element 311 of the inverter 310. Because the power conversion device 1 can suppress an increase in the leakage current, it is possible to prevent the leakage current from having an effect on the human body. Even when the power conversion device 1 is connected to the DC power supply 111, it is possible to obtain the same effects as in the first embodiment when it is connected to the AC power supply 110.

[0042] Fourth Embodiment In a fourth embodiment, a method for supplying control power to the control unit 400 will be described. Note that the description will be made taking as an example the case of the third embodiment in which the DC power supply 111 is connected to the power conversion device 1, but the description can also be applied to the first and second embodiments in which the AC power supply 110 is connected to the power conversion device 1.

[0043] FIG. 11 is a diagram showing an example of the configuration of a power conversion device 1 according to a fourth embodiment. The power conversion device 1 of this embodiment shown in FIG. 11 is obtained by adding a DC-DC converter 410, i.e., a DC-DC converter, to the power conversion device 1 of the third embodiment shown in FIG. 10. In addition, the position of the control unit 400 is changed to a position other than the main circuit where it can receive control power from the DC-DC converter 410. In this way, the DC-DC converter 410 is connected to the main circuit, and the control unit 400 receives control power from the DC-DC converter 410. The power conversion device 1 and the motor 314 form a motor drive device 2.

[0044] Motor drive device 2 requires a control unit 400 for controlling converter 150 and inverter 310. The power supply voltage of the control power supply for operating control unit 400 is often different from the voltage of the main circuit for driving motor 314, and it is common for a DC-DC converter 410 for the control power supply of control unit 400 to be provided separately in parallel with the main circuit. Furthermore, converter 150 equipped with step-up / step-down circuit 140 for the main circuit generally has a smoothing capacitor 210 attached to the output stage.

[0045] In this embodiment, too, power conversion device 1 varies the output voltage of converter 150 in accordance with the rotation speed of motor 314 in order to suppress leakage current. However, if the output voltage of converter 150 drops too much, the output voltage of DC-DC converter 410 also drops and falls below the power supply voltage of the control power supply that can operate control unit 400, making control unit 400 unable to operate. In this case, motor drive device 2 becomes unable to operate. On the other hand, among the components that make up the main circuit of power conversion device 1, smoothing capacitor 210 generally has the lowest withstand voltage. If the output voltage of converter 150 rises beyond the withstand voltage of smoothing capacitor 210, that component, namely smoothing capacitor 210, will be destroyed. In this case, motor drive device 2 also becomes unable to operate.

[0046] Therefore, in the power conversion device 1, the output voltage of the converter 150 is set to be equal to or higher than the operable voltage of the DC-DC converter 410 and lower than the withstand voltage of the smoothing capacitor 210. Furthermore, the output voltage of the converter 150 at which the modulation factor becomes 1 when the motor 314 is operating at the minimum driving rotation speed is set to be equal to or higher than the minimum operating voltage of the DC-DC converter 410, and the output voltage of the converter 150 at which the modulation factor becomes 1 when the motor 314 is operating at the maximum driving rotation speed is set to be lower than the withstand voltage of the smoothing capacitor 210.

[0047] As described above, according to this embodiment, the power conversion device 1 controls the output voltage of the converter 150 within the above-described range when the DC-DC converter 410 is located at a position connected to the main circuit and the control unit 400 is located at a position to which control power is supplied from the DC-DC converter 410. As a result, the power conversion device 1 can suppress an increase in leakage current by reducing the number of times the switching element 311 of the inverter 310 is switched within the range of the output voltage of the converter 150 that can operate the motor drive device 2 without damaging it. Because the power conversion device 1 can suppress an increase in leakage current, it can prevent the effects of the leakage current on the human body.

[0048] Fifth Embodiment In a fifth embodiment, a step-up / step-down circuit 140 included in a converter 150 will be described, which has a different configuration from those described in the first to fourth embodiments.

[0049] Fig. 12 is a first diagram showing a configuration example of a power conversion device 1 according to embodiment 5. Fig. 12 shows an example in which a converter 150 of the power conversion device 1 employs a Cuk converter circuit as a step-up / step-down circuit 140. As shown in Fig. 12, the step-up / step-down circuit 140, which is a Cuk converter circuit, includes a switching element 141, a freewheeling diode 142, a reactor 143, a diode 144, a capacitor 145, a reactor 146, and a smoothing capacitor 210.

[0050] Fig. 13 is a second diagram showing a configuration example of the power conversion device 1 according to embodiment 5. Fig. 13 shows an example in which a converter 150 of the power conversion device 1 employs a SEPIC (Single Ended Primary Inductor Converter) converter circuit as the step-up / step-down circuit 140. As shown in Fig. 13, the step-up / step-down circuit 140, which is a SEPIC converter circuit, includes a switching element 141, a freewheeling diode 142, a reactor 143, a diode 144, a capacitor 145, a reactor 146, and a smoothing capacitor 210.

[0051] 12 and 13 show the case where the power supply connected to the power conversion device 1 is an AC power supply 110, but the configuration is also applicable to the case where the power supply connected to the power conversion device 1 is a DC power supply 111, as shown in FIGS. 10 and 11. The configuration of the step-up / step-down circuit 140 is not limited to the examples in FIGS. 12 and 13. The configuration of the step-up / step-down circuit 140 can also be configured in a manner other than the circuit configurations shown in FIGS. 12 and 13. The power conversion device 1 and the motor 314 form a motor drive device 2.

[0052] Sixth Embodiment. Fig. 14 is a diagram showing a configuration example of a refrigeration cycle-applied apparatus 900 according to a sixth embodiment. The refrigeration cycle-applied apparatus 900 according to the sixth embodiment includes the power conversion device 1 described in the first embodiment. The refrigeration cycle-applied apparatus 900 may also include the power conversion device 1 described in any of the second to fifth embodiments. Hereinafter, an example will be described in which the refrigeration cycle-applied apparatus 900 includes the power conversion device 1 described in the first embodiment. The refrigeration cycle-applied apparatus 900 according to the sixth embodiment can be applied to products equipped with a refrigeration cycle, such as air conditioners, refrigerators, freezers, and heat pump water heaters. In Fig. 14, components having the same functions as those in the first embodiment are denoted by the same reference numerals as those in the first embodiment.

[0053] The refrigeration cycle application equipment 900 includes a compressor 315 incorporating the motor 314 in embodiment 1, a four-way valve 902, an indoor heat exchanger 906, an expansion valve 908, and an outdoor heat exchanger 910, which are attached via refrigerant piping 912.

[0054] Inside the compressor 315, a compression mechanism 904 that compresses the refrigerant and a motor 314 that operates the compression mechanism 904 are provided.

[0055] The refrigeration cycle device 900 can perform heating or cooling operation by switching the four-way valve 902. The compression mechanism 904 is driven by a motor 314 that is variably controlled in speed.

[0056] During heating operation, as shown by the solid arrow, the refrigerant is pressurized by the compression mechanism 904 and sent out, passes through the four-way valve 902, the indoor heat exchanger 906, the expansion valve 908, the outdoor heat exchanger 910 and the four-way valve 902 and returns to the compression mechanism 904.

[0057] During cooling operation, as shown by the dashed arrow, the refrigerant is pressurized by the compression mechanism 904 and sent out, and returns to the compression mechanism 904 through the four-way valve 902, the outdoor heat exchanger 910, the expansion valve 908, the indoor heat exchanger 906 and the four-way valve 902.

[0058] During heating operation, the indoor heat exchanger 906 acts as a condenser to release heat, and the outdoor heat exchanger 910 acts as an evaporator to absorb heat. During cooling operation, the outdoor heat exchanger 910 acts as a condenser to release heat, and the indoor heat exchanger 906 acts as an evaporator to absorb heat. The expansion valve 908 reduces the pressure of the refrigerant to expand it.

[0059] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or different embodiments may be combined with each other. It is also possible to omit or modify parts of the configurations as long as they do not deviate from the gist of the invention.

[0060] 1 Power conversion device, 2 Motor drive device, 91 Processor, 92 Memory, 110 AC power supply, 111 DC power supply, 120 Noise filter, 130 Rectifier circuit, 131 Rectifier element, 140 Step-up / step-down circuit, 141, 311 Switching element, 142, 312 Freewheeling diode, 143, 146 Reactor, 144 Diode, 145 Capacitor, 150 Converter, 210 Smoothing capacitor, 310 Inverter, 314 Motor, 315 Compressor, 320 Earth floating capacitance, 400 Control unit, 410 DC-DC converter, 502, 504 Current detection unit, 502a, 504a Detection resistor, 900 Refrigeration cycle applied equipment, 902 Four-way valve, 904 Compression mechanism, 906 Indoor heat exchanger, 908 Expansion valve, 910 Outdoor heat exchanger, 912 Refrigerant piping.

Claims

1. A power conversion device comprising: a converter that steps up or down an input voltage to output a DC voltage; and an inverter that converts the DC voltage, which is the converter output voltage from the converter, into an AC voltage and outputs it to a motor, wherein the converter outputs the DC voltage according to the rotation speed of the motor so that leakage current during operation of the motor is equal to or less than a specified value.

2. The power conversion device according to claim 1, which is connected to an AC power supply, and the AC voltage supplied from the AC power supply is defined as a first AC voltage that is the input voltage, and the AC voltage output from the inverter to the motor is defined as a second AC voltage, the converter rectifies the first AC voltage and outputs the DC voltage by stepping up or stepping down the rectified voltage, and the inverter converts the DC voltage to the second AC voltage and outputs it to the motor.

3. The power conversion device according to claim 2, wherein the converter performs a power factor correction operation so that the waveform of the AC current of the first AC voltage becomes a sine wave synchronized with the waveform of the first AC voltage.

4. The power conversion device according to claim 1, which is connected to a DC power supply, and which defines a DC voltage supplied from the DC power supply as a first DC voltage that is the input voltage, and defines the DC voltage output from the converter as a second DC voltage, and the converter steps up or steps down the first DC voltage to output the second DC voltage, and the inverter converts the second DC voltage into the AC voltage and outputs it to the motor.

5. A power conversion device according to any one of claims 1 to 4, wherein the converter increases or decreases the converter output voltage in accordance with the number of revolutions of the motor so that leakage current during driving of the motor is equal to or less than a specified value and the modulation factor of the inverter is equal to or greater than 1.

6. A power conversion device according to any one of claims 1 to 5, comprising: a smoothing capacitor connected to a main circuit inside the converter or to the main circuit outside the converter between the converter and the inverter; a DC / DC converter connected to the main circuit; and a control unit that receives control power from the DC / DC converter and controls the operation of the converter and the inverter, wherein the converter output voltage is equal to or greater than the operable voltage of the DC / DC converter and is less than the withstand voltage of the smoothing capacitor.

7. The power conversion device according to claim 6, wherein the converter output voltage at which the modulation factor becomes 1 when the motor is operating at its minimum rotational speed is equal to or greater than the minimum operating voltage of the DC-DC converter, and the converter output voltage at which the modulation factor becomes 1 when the motor is operating at its maximum rotational speed is less than the dielectric strength of the smoothing capacitor.

8. A motor drive device comprising the power conversion device according to any one of claims 1 to 7.

9. A refrigeration cycle device comprising the power conversion device according to any one of claims 1 to 7.

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