Motor drive device and air conditioner

Abstract

A motor drive device (1) comprises: a rectification circuit (10); a smoothing capacitor (12) that smoothens an output voltage outputted from the rectification circuit (10); an inverter (13) that has a plurality of switching elements (Q1, Q2, Q3, Q4, Q5, Q6), that converts an inputted DC voltage into an AC voltage, and that outputs the AC voltage to windings (Lu, Lv, Lw) of a motor (14); and a control unit (15) that has a current detection unit (15d) for detecting the magnitude of output current outputted from the rectification circuit (10) and a drive unit (15a) that outputs a signal to the switching elements (Q1, Q2, Q3, Q4, Q5, Q6) to cause the same to perform a switching operation. The smoothing capacitor (12) is a film capacitor, and sets the current to be inputted to the inverter (13) greater when the output current is greater during restrained energization in which current is passed through the windings (Lu, Lv, Lw) while rotation of the motor (14) is stopped.

Description

[Technical field]

[0001] The present disclosure relates to a motor drive device that drives a motor and an air conditioner including the same. [Background technology]

[0002] In air conditioners, in order to operate them efficiently, a method of driving the compressor motor at a desired frequency and current by providing an inverter is widely used. The circuit configuration for driving the motor generally includes a filter circuit that suppresses the noise influence on the commercial grid, a rectifier circuit that converts AC power to DC power, a power factor correction circuit that suppresses harmonics and boosts the voltage to a constant level, a smoothing capacitor that smoothes the voltage to a constant level, and an inverter circuit.

[0003] Furthermore, in an air conditioner, when operation is stopped under conditions of low ambient temperature, the refrigerant gas liquefies and returns to the compressor, where the refrigerant dissolves, causing a phenomenon known as "settling." Patent Document 1 discloses a countermeasure against settling, which involves preheating control that warms the compressor by passing electricity through the motor windings without rotating the compressor motor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2001-286183 A Summary of the Invention [Problem to be solved by the invention]

[0005] Electrolytic capacitors are generally used as smoothing capacitors, but electrolytic capacitors have a shorter life span than other electronic components, which is a factor that determines the life span of motor drive devices. Using a film capacitor instead of an electrolytic capacitor is expected to extend the life span, but when compared at the same volume, the capacitance is smaller than that of an electrolytic capacitor. In particular, during preheating operation, the power factor correction circuit is stopped and only the inverter is operated for the purpose of reducing standby power, so the power factor correction circuit cannot smooth the voltage. For this reason, if a film capacitor with the same volume as an electrolytic capacitor is used as a smoothing capacitor, the small capacitance will not smooth the voltage, resulting in large fluctuations, and the fluctuations in the current output from the inverter to the motor will also be large.

[0006] The present disclosure has been made in consideration of the above, and has an object to provide a motor drive device that is capable of suppressing fluctuations in the current output from an inverter to a motor and achieving a long life. [Means for solving the problem]

[0007] In order to solve the above problems and achieve the object, the motor drive device according to the present disclosure includes a rectifier circuit that converts an AC voltage input from an AC power source into a DC voltage, a smoothing capacitor that smoothes the output voltage output from the rectifier circuit, an inverter having a plurality of switching elements that converts the input DC voltage into an AC voltage and outputs the AC voltage to a winding of a motor, and a control unit having a current detection unit that detects the magnitude of the output current output from the rectifier circuit and a drive unit that outputs a signal to the plurality of switching elements to perform a switching operation. The smoothing capacitor is a film capacitor. During a constrained current supply in which current is supplied to the winding while the motor rotation is stopped, the motor drive device increases the current input to the inverter as the output current increases. Effect of the Invention

[0008] The motor drive device according to the present disclosure has the advantage of being able to suppress fluctuations in the current output from the inverter to the motor and to achieve a longer life. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view of an air conditioner according to a first embodiment. [Diagram 2] Circuit diagram of a motor drive device according to a first embodiment [Diagram 3] FIG. 4 is a waveform diagram when a switching element of an inverter of the motor drive device according to the first embodiment is operated; [Figure 4] Circuit diagram of a motor drive device according to a comparative example of the first embodiment. [Diagram 5] FIG. 11 is a waveform diagram when a switching element of an inverter of a motor drive device according to a comparative example of the first embodiment is operated; [Figure 6] FIG. 11 is a waveform diagram when the motor drive device according to the first embodiment performs a switching operation with the on-duty of the switching element kept constant. [Figure 7] A diagram showing the characteristics of the maximum restrained current that can be output to the motor versus the capacitance of the smoothing capacitor. [Figure 8] Circuit diagram of a motor drive device according to a second embodiment [Figure 9] 11 is a flowchart showing the flow of operations of the motor drive device according to the second embodiment. [Figure 10] FIG. 1 is a diagram showing an example of a hardware configuration of a calculation unit of a motor drive device according to a first embodiment and a second embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A motor drive device and an air conditioner according to an embodiment will be described in detail below with reference to the drawings.

[0011] Embodiment 1 Fig. 1 is a perspective view of an air conditioner according to embodiment 1. The air conditioner 100 includes an outdoor unit 5 and an indoor unit 7. The outdoor unit 5 and the indoor unit 7 are connected by piping 6. The outdoor unit 5 includes a motor drive device 1, a compressor 2, a blower 3, and a heat exchanger 4. The motor drive device 1 is connected to the compressor 2, the blower 3, and an AC power source 8. Note that electrical wiring connecting the compressor 2, the blower 3, and the AC power source 8 to the motor drive device 1 is omitted from Fig. 1.

[0012] 2 is a circuit diagram of the motor drive device according to embodiment 1. The motor drive device 1 includes a noise filter 9, a rectifier circuit 10, a DC conversion circuit 11, a smoothing capacitor 12, an inverter 13, a control unit 15, and a current sensor 17. The input side of the noise filter 9 is connected to an AC power source 8. The output side of the inverter 13 is connected to a motor 14. A film capacitor is used as the smoothing capacitor 12.

[0013] The noise filter 9 is a low-pass filter including a coil Lin and a capacitor Cin, and prevents noise generated in the motor drive device 1 from affecting the AC power supply 8.

[0014] The rectifier circuit 10 is connected between the noise filter 9 and the DC conversion circuit 11. The rectifier circuit 10 rectifies the AC voltage input from the AC power supply 8 and outputs a DC voltage. The rectifier circuit 10 may be configured as a full-wave rectifier circuit using four diodes, or may be configured as a synchronous rectifier circuit using a switching element such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor). The rectifier circuit 10 may also be configured as a half-wave rectifier circuit using only one diode. In this case, the number of diodes through which the current input from the AC power supply 8 passes can be halved, thereby reducing the loss generated in the rectifier circuit 10.

[0015] In addition, by connecting full-wave rectifier circuits using four diodes in parallel and reducing the current flowing through each diode, the loss generated by each diode can be reduced. Rectifier circuit 10 in which full-wave rectifier circuits using four diodes are connected in parallel can suppress the temperature rise of the diodes even if the heat sink for dissipating heat from the diode bridge is small.

[0016] The DC converter circuit 11 is connected between the rectifier circuit 10 and the smoothing capacitor 12. The DC converter circuit 11 can be configured with a boost chopper circuit using a plurality of switching elements and a reactor. The DC converter circuit 11 can also be configured as an interleaved converter in which two or more boost chopper circuits are connected in parallel. The interleaved converter in which two or more boost chopper circuits are connected in parallel can reduce the current flowing through each switching element and reactor, and therefore can suppress the increase in size of the components. The interleaved converter in which two or more boost chopper circuits are connected in parallel can reduce the current flowing through each switching element and diode, and therefore can reduce the losses that occur. The interleaved converter in which two or more boost chopper circuits are connected in parallel can suppress the temperature rise of the switching elements even if the heat sink for dissipating heat from the switching elements is small.

[0017] The DC conversion circuit 11 functions as a power factor improvement circuit that boosts the DC voltage output by the rectifier circuit 10 and outputs it to the smoothing capacitor 12. However, in the motor drive device 1 according to the first embodiment, in the operation of the restricted current supply, the switching operation of the DC conversion circuit 11 is stopped in order to suppress the occurrence of loss that occurs in the components used in the DC conversion circuit 11 and to reduce power consumption when the air conditioning is stopped.

[0018] Although the DC conversion circuit 11 has been described here as an interleaved converter using a boost chopper circuit, the circuit configuration of the DC conversion circuit 11 is not limited to this. For example, the DC conversion circuit 11 can also be configured as a step-up / step-down chopper circuit, a flyback circuit, a flyforward circuit, a SEPIC (Single Ended Primary Inductor Converter), a Zeta converter, or a Cuk converter.

[0019] The smoothing capacitor 12 smoothes the voltage output by the DC conversion circuit 11. The DC conversion circuit 11 controls the voltage output from the rectifier circuit 10 to a constant voltage, but the voltage output from the rectifier circuit 10 pulsates at twice the frequency of the AC power source 8. Therefore, the voltage output from the DC conversion circuit 11 also pulsates at twice the frequency of the AC power source 8. The smoothing capacitor 12 smoothes the voltage pulsating at twice the frequency of the AC power source 8. This makes it possible to reduce the pulsation of the current output to the motor 14 by the inverter 13 connected in the downstream stage at twice the frequency of the AC power source 8.

[0020] When the current output from the inverter 13 to the motor 14 pulsates, the motor 14 vibrates, and the piping 6 attached to the outdoor unit 5 and the lead wires and soldered parts of the electronic components constituting the motor drive device 1 deteriorate due to the vibration, shortening the life of the air conditioner 100, and therefore it is necessary to add vibration countermeasure parts, which increases costs. In addition, when the current output to the motor 14 pulsates, vibration noise is generated, which reduces the quality of the air conditioner 100. By using the smoothing capacitor 12 to smooth the voltage pulsating at twice the frequency of the AC power source 8, it is possible to reduce the pulsation at twice the frequency of the AC power source 8 of the current output from the inverter 13 connected in the subsequent stage to the motor 14. By suppressing the pulsation of the current output from the inverter 13 to the motor 14, it is possible to suppress the vibration of the motor 14 and the deterioration due to the vibration of the piping 6 attached to the outdoor unit 5 and the lead wires and soldered parts of the electronic components constituting the motor drive device 1. This makes it possible to reduce the number of vibration countermeasure parts and achieve cost reduction. Furthermore, since vibration noise can be suppressed, the quietness of the air conditioner 100 can be improved.

[0021] The inverter 13 is connected between the smoothing capacitor 12 and the motor 14. The inverter 13 converts the DC voltage output by the DC conversion circuit 11 into AC of an arbitrary frequency and current value, and outputs it to the motor 14.

[0022] The inverter 13 can be configured with a bridge circuit using a plurality of switching elements. In particular, when the motor 14 is a three-phase motor, a full bridge circuit may be used for the inverter 13. In the first embodiment, the inverter 13 is a full bridge circuit and is configured with switching elements Q1, Q2, Q3, Q4, Q5, and Q6. The switching elements Q1, Q2, Q3, Q4, Q5, and Q6 are configured with an IGBT (Insulated Gate Bipolar Transistor) and an anti-parallel free wheel diode. The collector terminal of the switching element Q1 is connected to one end of the smoothing capacitor 12, and the emitter terminal of the switching element Q1 is connected to the collector terminal of the switching element Q2 and the motor 14. The emitter terminal of the switching element Q2 is connected to the other end of the smoothing capacitor 12. The switching elements Q1 and Q2 may be called legs. The leg of switching element Q3 and switching element Q4 and the leg of switching element Q5 and switching element Q6 are connected in the same manner as the leg of switching element Q1 and switching element Q2.

[0023] The multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be semiconductor elements such as IGBT and MOSFET made of silicon, which are commonly used, as well as semiconductor elements made of silicon carbide. Since the semiconductor elements made of silicon carbide have smaller conduction loss than the semiconductor elements made of silicon and can perform high-speed switching operations, the switching loss can be reduced and the power consumption of the air conditioner 100 can be reduced. In addition, the heat generated by the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be reduced, so that the heat dissipation members such as heat sinks for dissipating the heat of the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be made smaller and the cost of the heat dissipation members can be reduced. In addition, the temperature rise of electronic components arranged near the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be suppressed, and reliability can be improved.

[0024] In addition, semiconductor elements made of gallium nitride can be used for the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6. An example of a semiconductor element made of gallium nitride is a HEMT (High Electron Mobility Transistor). HEMTs have smaller conduction loss than MOSFETs made of silicon, and are capable of high-speed switching operations, so that switching loss can be reduced and the power consumption of the air conditioner 100 can be reduced. In addition, heat generation from the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can also be reduced, so that heat dissipation members such as heat sinks for dissipating heat from the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be made smaller and the cost of the heat dissipation members can be reduced. In addition, the temperature rise of electronic components arranged near the multiple switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be suppressed, and reliability can be improved.

[0025] The inverter 13 may have a circuit configuration such as a half-bridge inverter or a single-transistor voltage resonant circuit.

[0026] The control unit 15 includes a drive unit 15a, a calculation unit 15b, and a current detection unit 15d.

[0027] The current detection unit 15d detects the current output from the rectifier circuit 10 and transmits the detection result to the calculation unit 15b. The current sensor 17 is a means for detecting the current output from the rectifier circuit 10, and may be a current sensor using a Hall element or a shunt resistor.

[0028] Using the current value that is the detection result transmitted from the current detection unit 15d, the calculation unit 15b outputs to the drive unit 15a a signal for controlling the on / off of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 according to the magnitude of the current output from the rectifier circuit 10. More specifically, the current input to the inverter 13 is increased as the current output from the rectifier circuit 10 increases, and conversely, the current input to the inverter 13 is decreased as the current output from the rectifier circuit 10 decreases.

[0029] The driving unit 15a converts the signal sent from the calculation unit 15b into a voltage of a magnitude capable of controlling the on / off of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, and causes the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 to perform switching operations. The signal sent from the calculation unit 15b is, for example, a voltage of 3.3V or 5V, and the voltage capable of controlling the on / off of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is, for example, 15V or 18V.

[0030] The motor 14 includes a winding Lu, a winding Lw, and a winding Lv. The windings Lu, Lw, and Lv are star-connected.

[0031] Fig. 3 is a waveform diagram when the switching elements of the inverter of the motor drive device according to embodiment 1 are operated. In Fig. 3, current I1 is a current output from DC conversion circuit 11 to smoothing capacitor 12, current I2 is a current input from smoothing capacitor 12 to inverter 13, and current I3 is a current that charges and discharges smoothing capacitor 12. Furthermore, voltage Vdc is a voltage applied across smoothing capacitor 12.

[0032] In the motor drive device 1 according to the first embodiment, the current output to the motor 14, i.e., the magnitude of the current I2 input to the inverter 13, is varied according to the magnitude of the current output from the rectifier circuit 10. In other words, the larger the current I1 output from the DC conversion circuit 11 to the smoothing capacitor 12, the smaller the current I2 input to the inverter 13 is made. As a result, the waveform of the current I2 has a triangular wave shape with a constant amplitude and an average value I2Ave1 for each carrier period that varies periodically, and is output to the windings Lu, Lv, and Lw of the motor 14.

[0033] Specifically, the calculation unit 15b transmits a signal to the drive unit 15a so as to increase the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 as the current output from the rectifier circuit 10 increases, and decrease the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 as the current output from the rectifier circuit 10 decreases. This changes the average value I2Ave1 per carrier period of the current I2 input to the inverter 13, and changes the current I3 that charges and discharges the smoothing capacitor 12.

[0034] As a result of the above-mentioned operation, during a period when the current output from the rectifier circuit 10 is large, the average value I2Ave1 per carrier period of the current I2 input to the inverter 13 becomes greater than zero, and conversely, during a period when the current output from the rectifier circuit 10 is small, the average value I2Ave1 per carrier period of the current I2 input to the inverter 13 becomes less than zero.

[0035] Since the smoothing capacitor 12 is charged and discharged by the voltage of the full-wave rectified waveform output by the rectifier circuit 10, the voltage across the smoothing capacitor 12 drops during a period when the voltage output by the rectifier circuit 10 is low. Due to the above-mentioned operation, during a period when the current I1 output from the rectifier circuit 10 is small, i.e., during a period when the voltage output by the rectifier circuit 10 is low, the current I2 input to the inverter 13 becomes less than zero, and the current I3 flows from the inverter 13 to the smoothing capacitor 12, charging the smoothing capacitor 12. This makes it possible to suppress fluctuations in the voltage across the smoothing capacitor 12.

[0036] Next, a method for preheating the motor 14 will be described. In the first preheating method, the windings Lu and Lv of the motor 14 are short-circuited by turning on the switching elements Q1 and Q4 and turning off the switching elements Q2 and Q3, and the current flowing from the winding Lu to the winding Lv increases. After that, the switching elements Q1 and Q4 are turned off and the switching elements Q2 and Q3 are turned on, and the current flowing from the winding Lv to the winding Lu increases. As a result, a current flows through the windings Lu and Lv, generating a loss, and thereby warming up the motor 14.

[0037] In this case, the heat generated by the motor 14 is concentrated on the windings Lu and Lv, and the motor 14 cannot be heated uniformly. In addition, since the loss generated by the inverter 13 is concentrated on the switching elements Q1, Q2, Q3, and Q4, the heat generated by the switching elements Q1, Q2, Q3, and Q4 becomes larger than the heat generated by the switching elements Q5 and Q6, and the switching elements Q1, Q2, Q3, and Q4 are more likely to break down than the switching elements Q5 and Q6. Therefore, whether the triangular wave current is passed through the windings Lu and Lv, the windings Lv and Lw, or the windings Lw and Lu is changed at predetermined time intervals. This makes it possible to prevent the heat generated by the motor 14 from being concentrated near any of the windings Lu, Lv, and Lw, and to heat the motor more uniformly. Furthermore, since the heat generation of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be made uniform, it is possible to prevent certain switching elements Q1, Q2, Q3, Q4, Q5, and Q6 from becoming more susceptible to failure.

[0038] In the second preheating method, the windings Lu, Lv, and Lw of the motor 14 are short-circuited by turning on the switching elements Q1, Q4, and Q6 and turning off the switching elements Q2, Q3, and Q5, and the current flowing from the winding Lu to the winding Lv and the current flowing from the winding Lu to the winding Lw increase. After that, the switching elements Q1, Q4, and Q6 are turned off and the switching elements Q2, Q3, and Q5 are turned on, and the current flowing from the winding Lv to the winding Lu and the current flowing from the winding Lw to the winding Lu increase. As a result, a triangular wave current flows through the windings Lu, Lv, and Lw, generating losses and warming up the motor 14.

[0039] By performing such control, the motor 14 can be heated more uniformly compared to the first preheating method. However, even in the second method, the heat generated by the winding Lu is greater than the heat generated by the windings Lv and Lw. In addition, as for the loss generated in the inverter 13, the loss in the switching elements Q1 and Q2 is greater than the loss in the switching elements Q3, Q4, Q5, and Q6, so that the heat generated by the switching elements Q1 and Q2 is greater than the heat generated by the switching elements Q3, Q4, Q5, and Q6, and the switching elements Q1 and Q2 are more likely to break down than the switching elements Q3, Q4, Q5, and Q6. Therefore, the direction in which the triangular wave current flows is changed at predetermined time intervals between from the winding Lu to the windings Lv and Lw, from the winding Lv to the windings Lw and Lu, or from the winding Lw to the windings Lu and Lv. In this case, the heat generated by the motor 14 can be prevented from concentrating near any one of the windings Lu, Lv, and Lw, and the motor can be heated more uniformly. Also, the heat generated by the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 can be made uniform, which prevents a particular switching element Q1, Q2, Q3, Q4, Q5, or Q6 from being easily damaged.

[0040] In the first preheating method and the second preheating method, the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 are switched to make the current flowing through the windings Lu, Lv, and Lw of the motor 14 triangular, thereby limiting the magnitude of the current flowing through the motor 14. This makes it possible to reduce the rated current of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6, thereby preventing the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 from becoming large, and thus reducing the cost of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0041] In the first and second preheating methods, the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 are switched to make the current flowing through the windings Lu, Lv, and Lw of the motor 14 triangular, thereby passing an AC current through the motor 14. Therefore, in the first and second preheating methods, iron loss can be generated in a magnetic core (not shown) constituting the motor 14. Therefore, in the first and second preheating methods, the motor 14 can be heated more uniformly than if losses were generated only in the windings Lu, Lv, and Lw, and localized high temperatures can be suppressed. Therefore, for example, the heat resistance of the insulators used for the windings Lu, Lv, and Lw can be reduced, and the cost of the insulators can be reduced. Or, the thickness of the insulators used for the windings Lu, Lv, and Lw can be reduced, and the motor 14 can be made smaller.

[0042] 4 is a circuit diagram of a motor drive device according to a comparative example of embodiment 1. The motor drive device according to the comparative example does not include current detection unit 15d and current sensor 17. Furthermore, motor drive device 1 according to the comparative example uses an electrolytic capacitor for smoothing capacitor 12. The rest is the same as the motor drive device according to embodiment 1.

[0043] Fig. 5 is a waveform diagram when the switching elements of the inverter of the motor drive device according to the comparative example of embodiment 1 are operated. In Fig. 5, current I1 is a current output from DC conversion circuit 11 to smoothing capacitor 12, current I2 is a current input from smoothing capacitor 12 to inverter 13, and current I3 is a current that charges and discharges smoothing capacitor 12. Furthermore, voltage Vdc is a voltage applied across smoothing capacitor 12.

[0044] In the motor drive device 1 according to the comparative example, the calculation unit 15b transmits a signal that changes the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 from off to on or from on to off in a predetermined cycle. For example, a rectangular wave on-off signal can be used, or a ramp waveform for counting the on-off period can be used. In the motor drive device 1 according to the comparative example, the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 in the switching operation is constant. The switching elements Q1, Q2, Q3, Q4, Q5, and Q6 repeat the on-off operation in a predetermined cycle. As a result, the waveform of the current I2 becomes a triangular wave with a constant amplitude, and is output to the windings Lu, Lv, and Lw of the motor 14.

[0045] Since the DC converter circuit 11 boosts the current output from the rectifier circuit 10, the waveform of the current I1 output from the DC converter circuit 11 to the smoothing capacitor 12 has the same shape as the waveform output from the rectifier circuit 10. Furthermore, since the amplitude of the waveform of the current I2 is constant, the fluctuation amount Δipk2 of the current I2 is the same as the amplitude of the current I2. Furthermore, the average value I2Ave2 of the current I2 is a constant value.

[0046] The method of preheating the motor 14 is the same as that of the motor drive device 1 according to the first embodiment, and a first preheating method in which switching elements Q1 and Q4 are turned on and switching elements Q2 and Q3 are turned off, or a second preheating method in which switching elements Q1, Q4, and Q6 are turned on and switching elements Q2, Q3, and Q5 are turned off, and then switching elements Q1, Q4, and Q6 are turned off and switching elements Q2, Q3, and Q5 are turned on can be applied.

[0047] The electrolytic capacitor used as smoothing capacitor 12 in the motor drive device 1 of the comparative example can have a larger capacitance than a film capacitor of a comparable volume, and therefore can suppress fluctuations in the voltage across smoothing capacitor 12 even during periods when losses are occurring in motor 14 due to restricted current flow.

[0048] On the other hand, motor drive device 1 according to embodiment 1 uses a film capacitor for smoothing capacitor 12. Since film capacitors have a longer life than electrolytic capacitors, the life of motor drive device 1 can be extended by using a film capacitor for smoothing capacitor 12. However, when a film capacitor is used for smoothing capacitor 12, the capacitance is smaller than when an electrolytic capacitor of the same volume is used. For this reason, the following problems arise when motor drive device 1 according to embodiment 1 is caused to perform switching operations with the on-duty of the switching elements kept constant.

[0049] Fig. 6 is a waveform diagram when the motor drive device according to the first embodiment is caused to perform a switching operation with the on-duty of the switching element kept constant. That is, Fig. 6 is a waveform diagram when the motor drive device 1 using a film capacitor as the smoothing capacitor 12 is caused to perform a switching operation similar to that of the motor drive device 1 according to the comparative example. In Fig. 6, current I1 is a current output from the DC conversion circuit 11 to the smoothing capacitor 12, current I2 is a current input from the smoothing capacitor 12 to the inverter 13, and current I3 is a current that charges and discharges the smoothing capacitor 12. Moreover, voltage Vdc is a voltage applied across the smoothing capacitor 12.

[0050] Since the motor drive device 1 according to the first embodiment uses a film capacitor with a small capacitance as the smoothing capacitor 12, when the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is kept constant to perform a switching operation as in the motor drive device 1 according to the comparative example, the current I1 output from the DC conversion circuit 11 to the smoothing capacitor 12 has a waveform having a shape of successive half sine waves. Furthermore, when the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is kept constant to perform a switching operation, a constant current I3 flows through the smoothing capacitor 12 regardless of the magnitude of the voltage output by the rectifier circuit 10.

[0051] In the motor drive device 1 according to the first embodiment, during a period in which loss occurs in the motor 14 due to the constrained energization, the fluctuation of the voltage across the smoothing capacitor 12 increases more than in the motor drive device 1 according to the comparative example using an electrolytic capacitor. Accordingly, the fluctuation of the voltage input to the inverter 13 increases more. Therefore, when the motor drive device 1 according to the first embodiment performs a switching operation with the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 kept constant, the waveform of the current I2 becomes a triangular wave whose amplitude increases and decreases periodically. Therefore, the average value I2Ave3 of the current I2 becomes a waveform with a shape of successive half waves of a sine wave like the current I1, and increases and decreases periodically. Since the amplitude of the triangular wave of the current I2 increases and decreases periodically, the fluctuation amount Δipk3 of the current I2 becomes larger than the fluctuation amount Δipk2 of the current I2 in the motor drive device 1 according to the comparative example. Therefore, if the motor drive device 1 according to embodiment 1 is caused to perform switching operations with the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 kept constant, the maximum current flowing through the inverter 13 will increase, resulting in an increase in noise generated by the motor drive device 1. The increase in noise can be suppressed by increasing the inductance of the coil Lin used in the noise filter 9 or by increasing the capacitance of the capacitor Cin, but this will increase the cost of the components and the size of the components, resulting in an increase in the size of the motor drive device 1.

[0052] In this way, when the motor drive device 1 according to the first embodiment performs switching operations with the on-duties of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 kept constant, noise generated by the motor drive device 1 increases. For this reason, the motor drive device 1 according to the first embodiment suppresses the fluctuation in the voltage across the smoothing capacitor 12, thereby suppressing an increase in the current I2 flowing through the inverter 13. In other words, the amount of fluctuation Δipk1 of the current I2 flowing through the inverter 13 is smaller than the amount of fluctuation Δipk3 of the current I2 when the switching operations are performed with the on-duties of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 kept constant. Therefore, the motor drive device 1 according to the first embodiment increases the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 as the current output from the rectifier circuit 10 increases, and decreases the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 as the current output from the rectifier circuit 10 decreases, thereby suppressing an increase in noise generated by the motor drive device 1. As a result, the motor drive device 1 according to the first embodiment can suppress an increase in the cost of the noise filter 9 and an increase in the size of the components.

[0053] Furthermore, in order to increase the power output to the motor 14, it is necessary to increase the current output to the motor 14. Increasing the current output to the motor 14 can be achieved by increasing the voltage input to the inverter 13 or by increasing the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6.

[0054] When a film capacitor is used for smoothing capacitor 12, the small capacitance results in a waveform close to the full-wave rectified waveform output by rectifier circuit 10, and the average voltage Vpn3 input to inverter 13 when switching operations are performed with the on-duty of switching elements Q1, Q2, Q3, Q4, Q5, and Q6 kept constant is lower than the average voltage Vpn2 input to inverter 13 in motor drive device 1 of the comparative example in which an electrolytic capacitor with a large capacitance is used.

[0055] For this reason, when a film capacitor is used for smoothing capacitor 12, if the on-duty of switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is kept constant during switching operation, the power that can be output to motor 14 during restricted current application will be low.

[0056] On the other hand, when control is performed such that the larger the current output from the rectifier circuit 10 as shown in FIG. 3, the higher the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is made, and the smaller the current output from the rectifier circuit 10, the lower the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is made, the average voltage Vpn1 input to the inverter 13 is greater than the average voltage Vpn3 input to the inverter 13 when switching operation is performed with the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 shown in FIG. 6 kept constant. Therefore, the motor drive device 1 of embodiment 1 controls the on-duty of switching elements Q1, Q2, Q3, Q4, Q5, and Q6 to be higher as the current output from rectifier circuit 10 increases, and controls the on-duty of switching elements Q1, Q2, Q3, Q4, Q5, and Q6 to be lower as the current output from rectifier circuit 10 decreases, thereby preventing a decrease in the power that can be output to motor 14 during restrained current flow.

[0057] The motor drive device of embodiment 1 charges smoothing capacitor 12 from inverter 13 during periods when the voltage output by rectifier circuit 10 is low, thereby suppressing voltage fluctuations, thereby outputting greater power to motor 14 during restricted current flow and suppressing compressor 2 from cooling down.

[0058] FIG. 7 is a diagram showing the characteristics of the maximum constrained energization power that can be output to the motor with respect to the capacitance of the smoothing capacitor. The solid line in FIG. 7 shows the characteristics of the maximum constrained energization power when the motor drive device 1 according to the first embodiment is controlled so that the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is increased as the current output from the rectifier circuit 10 increases, and the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is decreased as the current output from the rectifier circuit 10 decreases. The dashed line in FIG. 7 shows the characteristics of the maximum constrained energization power when the motor drive device 1 according to the first embodiment is controlled so that the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is kept constant. In a region where the capacitance of the smoothing capacitor 12 is large, the voltage fluctuation of the smoothing capacitor 12 is small, so the fluctuation of the power that can be output to the motor 14 is large with respect to the change in the capacitance. On the other hand, in a region where the capacitance of the smoothing capacitor 12 is small, the voltage fluctuation of the smoothing capacitor 12 is large, so that the fluctuation of the power that can be output to the motor 14 increases with respect to the change in the capacitance, and the smaller the capacitance of the smoothing capacitor 12, the greater the decrease in the power that can be output to the motor 14. That is, when the motor drive device 1 according to the first embodiment is caused to perform control to keep the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 constant, the maximum constrained current power decreases in a region where the capacitance of the smoothing capacitor 12 is small. However, when the control by the motor drive device 1 according to the first embodiment is performed, it is possible to suppress a decrease in the average voltage of the smoothing capacitor 12 even if the capacitance changes, so that a large amount of power can be output to the motor 14 compared to a case where the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is kept constant and a switching operation is performed.In other words, if the motor drive device 1 of embodiment 1 is controlled so that the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is increased as the current output from the rectifier circuit 10 increases, and the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 is decreased as the current output from the rectifier circuit 10 decreases, the decrease in the maximum constrained current power in the region where the capacitance of the smoothing capacitor 12 is small can be suppressed.

[0059] As described above, in the motor drive device 1 according to the first embodiment, the inverter 13 changes the magnitude of the current output to the motor 14 in accordance with the magnitude of the current output by the rectifier circuit 10, so that even if a film capacitor with a small capacitance is used for the smoothing capacitor 12, the fluctuation of the voltage across the smoothing capacitor 12 can be suppressed. Therefore, compared to a case where control for changing the magnitude of the current output to the motor 14 in accordance with the magnitude of the current output by the rectifier circuit 10 is not performed, the fluctuation of the current I2 flowing through the inverter 13 can be suppressed, which has the effect of suppressing an increase in the component size and the increase in cost of the noise filter 9. Similarly, by suppressing the fluctuation of the voltage across the smoothing capacitor 12, the average voltage can be suppressed from decreasing, so that even if a film capacitor with a small capacitance is used for the smoothing capacitor 12, the effect of suppressing a decrease in the power that can be output to the motor 14 can be suppressed.

[0060] Embodiment 2 8 is a circuit diagram of a motor drive device according to embodiment 2. Motor drive device 1 according to embodiment 2 differs from motor drive device 1 according to embodiment 1 in that it has a voltage detection unit 15c that detects the voltage across smoothing capacitor 12. Other configurations are similar to those of motor drive device 1 according to embodiment 1. Motor drive device 1 according to embodiment 2 changes the on-duty of switching elements Q1, Q2, Q3, Q4, Q5, and Q6 in accordance with the magnitude of fluctuation in voltage of smoothing capacitor 12.

[0061] As described above, the smaller the capacitance of smoothing capacitor 12, the greater the change in average voltage relative to a change in capacitance, and the greater the fluctuation in power that can be output to motor 14. Therefore, when a film capacitor is used as smoothing capacitor 12, if the capacitance of smoothing capacitor 12 decreases due to deterioration over time and the effects of environmental temperature, the power that can be output to motor 14 will decrease. The motor drive device 1 according to embodiment 2 takes this into consideration and suppresses the decrease in power that can be output to motor 14 caused by changes in the capacitance of the film capacitor.

[0062] When the capacitance of the smoothing capacitor 12 decreases, the voltage of the smoothing capacitor 12 becomes closer to the full-wave rectified waveform output by the rectifier circuit 10, and the voltage fluctuation becomes larger. The calculation unit 15b calculates and holds the magnitude of the voltage fluctuation from the voltage detected by the voltage detection unit 15c. The amount of change in the current I2 input to the inverter 13, which is changed according to the magnitude of the current output from the rectifier circuit 10 as described in the first embodiment, is hereinafter referred to as the "correction amount." The calculation unit 15b changes the correction amount according to the magnitude of the voltage fluctuation of the smoothing capacitor 12. Specifically, the calculation unit 15b increases the correction amount as the voltage fluctuation increases.

[0063] When the capacitance of smoothing capacitor 12 decreases, the voltage fluctuation of smoothing capacitor 12 increases and the average voltage decreases. Therefore, by increasing the correction amount in accordance with the decrease in the capacitance of smoothing capacitor 12, the current I3 that charges smoothing capacitor 12 from inverter 13 can be increased, thereby suppressing the voltage fluctuation and the decrease in the average voltage of smoothing capacitor 12. This makes it possible to suppress a decrease in the output power caused by a change in the capacitance of the film capacitor used in smoothing capacitor 12.

[0064] The magnitude of the change in the correction amount, which is changed according to the magnitude of the power fluctuation of the smoothing capacitor 12, can be determined by storing in advance in the calculation unit 15b a table of change amounts according to the magnitude of the voltage fluctuation. Also, a target value can be set for the magnitude of the voltage fluctuation, and the magnitude of the change amount can be determined by feedback so that the deviation between the correction amount after the change and the target value becomes small.

[0065] 9 is a flowchart showing the flow of the operation of the motor drive device according to the second embodiment. When the inverter 13 starts operating, in step S1, the current detection unit 15d detects the magnitude of the output current output from the rectifier circuit 10. In step S2, the calculation unit 15b determines the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 based on the magnitude of the output current detected by the current detection unit 15d and the correction amount. After determining the on-duty in step S2, in step S3, the drive unit 15a turns on and off the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 at a predetermined cycle. The on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 here is the on-duty determined in step S2.

[0066] Furthermore, when the inverter 13 starts operating, in step S4, apart from the processes of steps S1 to S3, the voltage detection unit 15c detects the voltage across the smoothing capacitor 12. In step S5, the calculation unit 15b calculates the magnitude of the voltage fluctuation based on the voltage across the smoothing capacitor 12 detected in step S4, and holds this. Then, in step S6, the calculation unit 15b calculates the amount of change in the correction amount according to the magnitude of the voltage fluctuation. In step S7, the calculation unit 15b changes the correction amount by the amount of change calculated in step S6. The correction amount changed in step S7 is used in the process of determining the on-duty in step S2.

[0067] In the above-mentioned first and second embodiments, the motor drive device 1 is configured to include the DC conversion circuit 11, but the present invention is not limited to this. In cases such as when a three-phase AC power supply is used as the AC power supply 8, the DC conversion circuit 11 may not be provided, and the smoothing capacitor 12 may be connected to the output of the rectifier circuit 10. Even in a configuration without the DC conversion circuit 11, by using a film capacitor with a small capacitance as the smoothing capacitor 12, it is possible to suppress fluctuations in the voltage across the smoothing capacitor 12 and fluctuations in the current flowing through the inverter 13.

[0068] Next, a description will be given of the hardware configuration of the calculation unit 15b of the control unit 15 included in the motor drive device 1 according to the above-mentioned embodiments 1 and 2. Fig. 10 is a diagram showing an example of the hardware configuration of the calculation unit of the motor drive device according to the embodiments 1 and 2. Fig. 10 shows a hardware configuration in the case where the function of the calculation unit 15b is realized by using hardware that executes a program.

[0069] The calculation unit 15b has a processor 91 that executes various processes, a memory 92 that is a main memory, and a storage device 93 that stores information. The processor 91 may be a calculation means such as a calculation device, a microprocessor, a microcomputer, a CPU (Central Processing Unit), or a DSP (Digital Signal Processor). The memory 92 may be a non-volatile or volatile semiconductor memory such as a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory), or an EEPROM (Electrically Erasable Programmable Read Only Memory). The storage device 93 stores a program for executing a process of changing the on-duty of the switching elements Q1, Q2, Q3, Q4, Q5, and Q6 according to the magnitude of the voltage fluctuation of the smoothing capacitor 12 and a process of changing the correction amount according to the magnitude of the voltage fluctuation of the smoothing capacitor 12. The processor 91 reads the program stored in the storage device 93 into the memory 92 and executes it. The processor 91 reads out a program stored in the storage device 93 into the memory 92 and executes it, thereby implementing the functions of the calculation unit 15b.

[0070] The calculation unit 15b is configured to have a function of converting the voltage output from the current detection unit 15d from analog to digital, and can be digitally controlled using a microcontroller. When the calculation unit 15b is configured with a microcontroller, a reference voltage circuit is not required, and it is also possible to integrate analog circuits and miniaturize the control unit 15. When the calculation unit 15b is configured with a microcontroller, it is also possible to integrate peripheral circuits such as a memory that holds the calculation results and table data, and miniaturize the control unit 15.

[0071] The configurations shown in the above embodiments are merely examples of the contents, and may be combined with other known technologies. Parts of the configurations may be omitted or modified without departing from the spirit of the invention. [Explanation of symbols]

[0072] 1 motor drive device, 2 compressor, 3 blower, 4 heat exchanger, 5 outdoor unit, 6 piping, 7 indoor unit, 8 AC power supply, 9 noise filter, 10 rectifier circuit, 11 DC conversion circuit, 12 smoothing capacitor, 13 inverter, 14 motor, 15 control unit, 15a drive unit, 15b calculation unit, 15c voltage detection unit, 15d current detection unit, 17 current sensor, 91 processor, 92 memory, 93 storage device, 100 air conditioner, Cin capacitor, Lin coil, Lu, Lv, Lw windings, Q1, Q2, Q3, Q4, Q5, Q6 switching elements.

Claims

1. A rectifier circuit that converts an AC voltage input from an AC power source into a DC voltage; a smoothing capacitor that smoothes the output voltage output from the rectifier circuit; an inverter having a plurality of switching elements for converting an input DC voltage into an AC voltage and outputting the AC voltage to a winding of a motor; a control unit having a current detection unit that detects a magnitude of an output current output from the rectifier circuit and a drive unit that outputs a signal to the plurality of switching elements to cause them to perform a switching operation, the smoothing capacitor is a film capacitor, During a constrained current supply in which current is supplied to the windings while the rotation of the motor is stopped, the current input to the inverter is increased as the output current increases, The motor drive device, during the constrained current supply, wherein the drive unit outputs to the switching element the signal that increases an on-duty in the switching operation as the output current increases.

2. The control unit includes a voltage detection unit that detects a voltage across the smoothing capacitor, The motor drive device according to claim 1 , wherein the control unit increases a change amount of the current output from the inverter to the motor as the fluctuation in the voltage between both ends increases.

3. An air conditioner comprising the motor drive device according to claim 1 or 2.

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