Power conversion device and air conditioner

Abstract

A power conversion device (200) comprises: a non-boost rectifier circuit (210) that converts, into a first DC voltage, a first AC voltage outputted from an AC power source (100); a reactor (220) connected to the rectifier circuit (210); a capacitor (230) that smooths the first DC voltage to a second DC voltage; a voltage detection unit (240) that detects the second DC voltage; an inverter (260) that is connected in parallel to the capacitor (230), and that converts the second DC voltage into a second AC voltage having a desired voltage and a desired frequency and outputs the second AC voltage to a motor (310); and a control unit (270) that controls the operation of the inverter (260). The capacitance of the capacitor (230) is set on the basis of a carrier frequency, the resistance value of the motor (310), and a regeneration determination count which is for determining, by the control unit (270), that a regenerative voltage is generated in the motor (310) when the number of times the second DC voltage satisfies a regeneration determination condition has reached a prescribed regeneration determination count.

Description

Power conversion device and air conditioner

[0001] The present disclosure relates to a power conversion device that performs power conversion and an air conditioner.

[0002] Conventionally, power conversion devices have been commonly used, including a converter section that generates a ripple DC voltage from an AC voltage using a diode bridge rectifier circuit or the like, a smoothing section that smooths the ripple DC voltage using a reactor and a capacitor, and an inverter section that has multiple switching elements and converts the smoothed DC voltage into an AC voltage having a desired voltage and frequency, and outputs the AC voltage having the desired voltage and frequency to a motor to rotate it. In recent years, power conversion devices having the above configuration have been required to be smaller, more efficient, and less expensive. In particular, the reactors and capacitors used in the smoothing section tend to be larger and more expensive than other elements. Therefore, there is an increasing need for power conversion devices with smaller reactor and capacitor capacities.

[0003] If the capacitor's capacity is reduced, the amount of energy stored in the capacitor will decrease. Therefore, if a regenerative voltage is generated in the motor, the voltage across the capacitor increases as the regenerative voltage from the motor is absorbed, which may result in the voltage across the capacitor exceeding its withstand voltage. While a regenerative resistor is used to protect the capacitor's withstand voltage, this method constantly generates loss due to the regenerative resistor during operation of the power conversion device, thereby reducing the efficiency of the power conversion device. Furthermore, measures to dissipate heat generated by the regenerative resistor are required, which creates a cost issue. To address these issues, Patent Document 1 discloses a technology in which a motor drive device absorbs regenerative energy using an adjustment circuit consisting of a rectifier circuit, a capacitor, and a load.

[0004] JP 2011-200020 A

[0005] However, according to the above-mentioned conventional technology, the adjustment circuit is provided as a separate circuit between the wiring between the smoothing capacitor and the inverter and the wiring connecting the inverter and the motor. Furthermore, the rectifier circuit of the adjustment circuit is provided with a diode for each wiring connecting the inverter and the motor. This increases the circuit size, which leads to a problem that the board size, and therefore the device size, increases even if the capacitance of the smoothing capacitor is reduced.

[0006] The present disclosure has been made in view of the above, and has an object to provide a power conversion device that can suppress element damage due to regenerative voltage while suppressing an increase in circuit size.

[0007] In order to solve the above-mentioned problems and achieve the object, a power conversion device according to the present disclosure includes a non-boost rectifier circuit that converts a first AC voltage output from an AC power source into a first DC voltage, a reactor connected to the rectifier circuit, a capacitor that smoothes the first DC voltage into a second DC voltage, a voltage detection unit that detects the second DC voltage, an inverter connected in parallel to the capacitor that converts the second DC voltage into a second AC voltage of a desired voltage and frequency and outputs the second AC voltage to a motor, and a control unit that controls operation of the inverter. The capacitance of the capacitor is set based on a carrier frequency used by the control unit to generate an inverter drive signal for controlling operation of the inverter, a resistance value of the motor, and a regeneration determination number for determining that a regenerative voltage is being generated in the motor when the number of times that the second DC voltage satisfies a specified regeneration determination condition reaches a specified regeneration determination number.

[0008] The power conversion device according to the present disclosure has an advantage that it is possible to suppress element damage caused by regenerative voltage while suppressing an increase in circuit size.

[0009] FIG. 1 shows an example of the configuration of a power conversion device according to embodiment 1. FIG. 2 shows an example of the configuration of a power conversion device according to embodiment 1. FIG. 3 shows an example of the configuration of a power conversion device according to embodiment 1. FIG. 4 shows an example of the configuration of a power conversion device according to embodiment 1. FIG. 5 shows an example of the state in which the power supply voltage is lost or momentarily reduced in the power conversion device according to embodiment 1. FIG. 6 shows an example of the state in which the capacitor is charged from the motor in the power conversion device according to embodiment 1. FIG. 7 shows an operation of a control unit of the power conversion device according to embodiment 1 to generate inverter drive signals UP and UN with a voltage command in sine wave mode. FIG. 8 shows an operation of a control unit of the power conversion device according to embodiment 1 to generate inverter drive signals UP and UN with a voltage command in trapezoidal wave mode. FIG. 9 shows an example of the configuration of a control unit provided in the power conversion device according to embodiment 1. FIG. 10 shows the induced voltage characteristics of the motor of a compressor connected to the power conversion device according to embodiment 1. FIG. 11 shows a flowchart showing the operation of the control unit of the power conversion device according to embodiment 1. FIG. 12 shows an example of the configuration of an air conditioner according to embodiment 2.

[0010] Hereinafter, a power conversion device and an air conditioner according to an embodiment of the present disclosure will be described in detail with reference to the drawings.

[0011] First Embodiment Fig. 1 is a first diagram showing a configuration example of a power conversion device 200 according to a first embodiment. The power conversion device 200 is connected to an AC power supply 100 and a compressor 300, and converts an AC voltage output from the AC power supply 100 into a DC voltage, which is then converted into an AC voltage of a desired voltage and frequency and output to the compressor 300. In the following description, the AC voltage output from the AC power supply 100 will be referred to as a first AC voltage, and the AC voltage output from the power conversion device 200 will be referred to as a second AC voltage. The power conversion device 200 includes a rectifier circuit 210, a reactor 220, a capacitor 230, a voltage detection unit 240, a current detection unit 250, an inverter 260, and a control unit 270.

[0012] The rectifier circuit 210 includes a plurality of rectifying diodes 211, six diodes 211 in the example of FIG. 1 , and converts a first AC voltage output from the AC power supply 100 into a first DC voltage and outputs the first DC voltage. In the first embodiment, the rectifier circuit 210 is a non-boost type circuit. Furthermore, the rectifier circuit 210 is not configured as a power factor correction circuit.

[0013] 1, one end of the reactor 220 is connected to the output end of the rectifier circuit 210, and the other end is connected to the capacitor 230. Note that the installation position of the reactor 220 within the power conversion device 200 is not limited to the example in FIG.

[0014] In the example of FIG. 1 , one end of the capacitor 230 is connected to the output terminal of the rectifier circuit 210 via the reactor 220, and the other end is connected to the output terminal of the rectifier circuit 210. The capacitor 230 is a smoothing capacitor that smoothes the first DC voltage converted by the rectifier circuit 210 into a second DC voltage. It can also be said that the capacitor 230 smoothes the first DC voltage to generate the second DC voltage. The pulsating component contained in the second DC voltage is smaller than the pulsating component contained in the first DC voltage. In the first embodiment, as described below, a small-sized, i.e., small-capacity, smoothing capacitor 230 is used. The capacitor 230 may be, for example, an electrolytic capacitor or a film capacitor, but the type of the capacitor 230 is not limited thereto.

[0015] Voltage detection unit 240 is connected to capacitor 230 and detects DC bus voltage Vdc, which is a second DC voltage output from capacitor 230. Voltage detection unit 240 has an RC filter with a time constant that serves as a cutoff frequency for frequency components higher than a specified multiple of the resonant frequency of the LC circuit of reactor 220 and capacitor 230, for example, frequency components five to ten times higher, and detects DC bus voltage Vdc using this RC filter. Voltage detection unit 240 outputs a detected value, which is the voltage value of DC bus voltage Vdc, to control unit 270.

[0016] The inverter 260 is connected in parallel to the capacitor 230 and, under the control of the control unit 270, converts the DC bus voltage Vdc, which is the smoothed second DC voltage, into a second AC voltage of a desired voltage and frequency. Specifically, the inverter 260 includes a switching element 261 and six diodes 262 connected in parallel to the switching element 261. The inverter 260 has a plurality of legs, three legs in the example of FIG. 1 , in which the upper arm switching element 261 and the lower arm switching element 261 are connected in series. The inverter 260 turns on and off each switching element 261 based on an inverter drive signal, which is a PWM (Pulse Width Modulation) signal output from the control unit 270, to generate the second AC voltage, i.e., converts the DC bus voltage Vdc, which is the smoothed second DC voltage, into the second AC voltage. The inverter 260 outputs the second AC voltage to the motor 310 included in the compressor 300 .

[0017] Current detection unit 250 is connected between inverter 260 and motor 310, and detects current Iuvw output from inverter 260. Current Iuvw is a current flowing through each of the U-phase, V-phase, and W-phase layers of motor 310. Current detection unit 250 outputs a detected value, which is the current value of current Iuvw output from inverter 260, to control unit 270. Note that current detection unit 250 may detect currents for two of the U-phase, V-phase, and W-phase if control unit 270 can calculate the current for one phase from the currents for two phases.

[0018] The control unit 270 controls the operation of the inverter 260 so as to output a second AC voltage to the motor 310 of the compressor 300 connected to the inverter 260. Specifically, the control unit 270 generates an inverter drive signal for controlling the operation of the inverter 260 based on the detection values ​​acquired from the voltage detection unit 240 and the current detection unit 250, and outputs the signal to the inverter 260. Note that, although not shown in FIG. 1 , the control unit 270 may further control the operation of the inverter 260 using a detection value that is a current value detected by a current detection unit that detects the current flowing from the capacitor 230 to the inverter 260. As will be described later, the control unit 270 controls the operation of the inverter 260 using a dq rotating coordinate system that rotates in synchronization with the rotor position of the motor 310.

[0019] The compressor 300 connected to the power conversion device 200 includes a motor 310. In the first embodiment, the motor 310 is a three-phase motor including a winding 311 for each phase, i.e., the U-phase, the V-phase, and the W-phase.

[0020] FIG. 1 illustrates an example in which the AC power supply 100 connected to the power conversion device 200 is a three-phase AC power supply, but this is merely an example and is not limiting. FIG. 2 is a second diagram illustrating an example of the configuration of the power conversion device 200 according to the first embodiment. As shown in FIG. 2, the AC power supply 100 connected to the power conversion device 200 may be a single-phase AC power supply. In this case, in the power conversion device 200, the rectifier circuit 210 includes four diodes 211 and converts a first AC voltage output from the AC power supply 100 into a first DC voltage and outputs the first DC voltage. In addition, in the example of FIG. 2, one end of the reactor 220 is connected to the AC power supply 100 and the other end is connected to the input end of the rectifier circuit 210. Note that, when the AC power supply 100 is a single-phase AC power supply, the installation position of the reactor 220 in the power conversion device 200 is not limited to the example of FIG. 2.

[0021] In the configuration of the power conversion device 200 described above, the reactor 220 and the capacitor 230 are often larger in size than the other electrical components. When a large reactor 220 and a large capacitor 230 are used, the power factor of the power conversion device 200 tends to deteriorate. Therefore, by reducing the capacitance of the reactor 220 and the capacitor 230, the power factor of the power conversion device 200 can be improved. Furthermore, by reducing the capacitance of the reactor 220 and the capacitor 230, the power conversion device 200 can have a smaller circuit, which also enables cost reduction. The small smoothing capacitor 230 can be an electrolytic capacitor or a film capacitor, and any type of capacitor is acceptable. The power conversion device 200 uses the circuit configuration described above, i.e., the rectifier circuit 210, the reactor 220, and the capacitor 230, to convert a first AC voltage into a first DC voltage, and further converts the first DC voltage into a second DC voltage, and uses the second DC voltage as the input voltage of the inverter 260.

[0022] Here, if the capacitance of capacitor 230 is reduced, the energy that can be charged in capacitor 230, i.e., the charging energy of capacitor 230, will be reduced. For example, assume that the AC voltage input to power conversion device 200 from AC power supply 100, i.e., the power supply voltage, is lost or momentarily drops while power conversion device 200 is driving motor 310. Fig. 3 is a diagram showing an example of a state in which the power supply voltage is lost or momentarily drops in power conversion device 200 according to embodiment 1. When the power supply voltage is lost or momentarily drops in power conversion device 200, capacitor 230 will pass a current to inverter 260, and charge will be discharged from capacitor 230. As can be seen from the relationship Q=CV, in power conversion device 200, when the capacity of capacitor 230 is small, the charge stored in capacitor 230 is smaller than when the capacity of capacitor 230 is large, and the speed at which charge is released from capacitor 230 is faster, so the voltage input from capacitor 230 to inverter 260, i.e., the DC bus voltage Vdc which is the voltage across capacitor 230, drops sharply.

[0023] Like a typical power conversion device, the power conversion device 200 generates a drive power supply for the switching element 261 of the inverter 260 using the DC bus voltage Vdc, which is the voltage across the capacitor 230. Therefore, when the voltage across the capacitor 230 drops, the inverter 260 stops the switching operation of the switching element 261, and becomes configured like a diode rectifier circuit. In this case, in the power conversion device 200, the induced voltage φf*ωe generated in the motor 310 appears as a virtual power supply due to the voltage difference between the induced voltage φf*ωe of the motor 310 and the DC bus voltage Vdc, which is the voltage across the capacitor 230, and the induced voltage φf*ωe of the motor 310 charges the capacitor 230. FIG. 4 is a diagram showing an example of a state in which the capacitor 230 is charged from the motor 310 in the power conversion device 200 according to the first embodiment. In the power conversion device 200, when the capacity of the capacitor 230 is small, the rate at which electric charge is charged to the capacitor 230 is faster, i.e., the rate at which the regenerative voltage rises is faster, compared to when the capacity of the capacitor 230 is large, and therefore the time required for the voltage across the capacitor 230 to reach or exceed the withstand voltage of the capacitor 230 is shorter. Therefore, in the power conversion device 200, when the capacity of the capacitor 230 is small, protection against the regenerative voltage is necessary.

[0024] As the DC bus voltage Vdc, which is the voltage across capacitor 230, decreases, an RLC series circuit is formed by the resistance R of motor 310, the inductance L of motor 310, and the capacitance C of capacitor 230, with the differential voltage between DC bus voltage Vdc and induced voltage φf*ωe of motor 310 as a power source. In this case, an electric charge is charged in capacitor 230 according to the time constant of the RLC circuit. The time constant of the RLC circuit is determined by the capacitance C of capacitor 230; the larger the capacitance C of capacitor 230, the larger the time constant, and it takes time for the voltage across capacitor 230 to rise to the withstand voltage.

[0025] Therefore, in the present embodiment, in power conversion device 200, control unit 270 determines that a regenerative voltage is being generated based on the detection results of the voltage across capacitor 230 a specified number of times or more of which have been detected as regeneration determinations after the power supply voltage is lost or momentarily dropped, and the capacitance and carrier frequency of capacitor 230 are set so that control unit 270 can suppress the regenerative voltage by operating switching of switching element 261 of inverter 260. The carrier frequency is the frequency of a carrier used when control unit 270 controls the switching of switching element 261 of inverter 260.

[0026] 5 is a diagram showing an operation in which the control unit 270 of the power conversion device 200 according to the first embodiment generates the inverter drive signals UP and UN based on a voltage command in the sine wave mode. ＊ is a diagram showing a case where the voltage command Vu is in a sine wave mode. ＊ The maximum amplitude of the voltage command Vu for the U phase is Vdc / 2. ＊ The control unit 270 compares the voltage command Vu ＊ When the carrier is greater than the voltage command Vu, the control unit 270 generates each inverter drive signal so that the inverter drive signal UP for the switching element 261 of the upper arm of the U phase is high and the inverter drive signal UN for the switching element 261 of the lower arm of the U phase is low. ＊ is larger, the inverter drive signals are generated so that the inverter drive signal UP for the upper arm switching element 261 of the U phase is low and the inverter drive signal UN for the lower arm switching element 261 of the U phase is high. ＊ Based on the result of comparing the carrier with the inverter drive signal VP for the upper arm switching element 261 of the V phase and the inverter drive signal VN for the lower arm switching element 261 of the V phase, the control unit 270 generates a voltage command Vw for the W phase. ＊Based on the result of comparing the carrier with the inverter drive signal WP for the upper arm switching element 261 of the W phase, and the inverter drive signal WN for the lower arm switching element 261 of the W phase are generated.

[0027] The voltage command is not limited to a sine wave shape. Fig. 6 is a diagram showing the operation of the control unit 270 of the power conversion device 200 according to the first embodiment to generate the inverter drive signals UP and UN with a voltage command in a trapezoidal wave mode. ＊ is a trapezoidal wave mode, and the voltage command Vu ＊ The maximum amplitude of is Vdc / 2. As in the case where the voltage command is in the sine wave mode, the control unit 270 generates inverter drive signals for the upper arm and lower arm switching elements 261 of each phase based on the result of comparing the voltage command for each phase with the carrier.

[0028] The following describes a specific configuration and operation of the control unit 270. Fig. 7 is a diagram showing an example configuration of the control unit 270 included in the power conversion device 200 according to the first embodiment. As shown in Fig. 7, the control unit 270 includes a regenerative voltage determination unit 271, a voltage command control unit 272, a modulation factor calculation unit 273, and a PWM signal generation unit 274.

[0029] The regenerative voltage determination unit 271 acquires from the voltage detection unit 240 the DC bus voltage Vdc, which is the second DC voltage detected by the voltage detection unit 240. The regenerative voltage determination unit 271 acquires the DC bus voltage Vdc from the voltage detection unit 240, for example, at the timing of the peaks, or the valleys, or the timing of the peaks and valleys of the carrier wave shown in FIGS. 5 and 6 . The regenerative voltage determination unit 271 determines whether a regenerative voltage is being generated in the motor 310 based on the DC bus voltage Vdc acquired from the voltage detection unit 240. The regenerative voltage determination unit 271 determines that a regenerative voltage is being generated in the motor 310 when the number of times that the DC bus voltage Vdc satisfies a specified regeneration determination condition reaches a specified regeneration determination number. The specified number of regeneration determinations is the number of times that the DC bus voltage Vdc satisfies the specified regeneration determination conditions, which is necessary for the regeneration voltage determination unit 271 of the control unit 270 to determine that regeneration voltage is being generated in the motor 310.

[0030] For example, when the DC bus voltage Vdc detected by the voltage detection unit 240 becomes equal to or lower than a specified voltage for a specified period, the regenerative voltage determination unit 271 determines that the DC bus voltage Vdc has suddenly dropped and that the power supply voltage has been lost or momentarily dropped in the power conversion device 200, and starts counting the number of regeneration determinations described above. The condition under which the DC bus voltage Vdc detected by the voltage detection unit 240 becomes equal to or lower than a specified voltage for a specified period, which is used by the regenerative voltage determination unit 271 to determine that the power supply voltage has been lost or momentarily dropped in the power conversion device 200, is assumed to indicate a fluctuation different from the fluctuation in the voltage across the capacitor 230 when the power conversion device 200 is operating normally.

[0031] For example, the regenerative voltage determination unit 271 determines that a regenerative voltage is being generated in the motor 310 when the difference between the DC bus voltage Vdc previously detected by the voltage detection unit 240 and the DC bus voltage Vdc currently detected by the voltage detection unit 240 is equal to or greater than a specified value as a regenerative determination condition, and determines that a regenerative voltage is being generated in the motor 310 when the number of times the regenerative determination condition is satisfied is equal to or greater than the regenerative determination count described above. The regenerative voltage determination unit 271 may determine that the regenerative determination condition is being generated when the difference becomes constant, when the DC bus voltage Vdc detected by the voltage detection unit 240 simply increases, when the DC bus voltage Vdc detected by the voltage detection unit 240 exceeds a specified voltage that takes into account the withstand voltage of the capacitor 230, or when the DC bus voltage Vdc detected by the voltage detection unit 240 becomes constant at a voltage that is equal to or greater than a specified voltage that takes into account the withstand voltage of the capacitor 230. The regeneration determination condition indicates a change in the DC bus voltage Vdc that is different from a change in the DC bus voltage Vdc due to the first DC voltage supplied from the AC power supply 100 via the rectifier circuit 210, that is, the power supply voltage.

[0032] When the regenerative voltage determination unit 271 determines that a regenerative voltage is being generated in the motor 310, it instructs the PWM signal generation unit 274 to generate and output an inverter drive signal to the inverter 260 to turn on the switching element 261 of the lower arm of each leg. The specified number of regenerative determinations may be one, but is preferably two or more to prevent erroneous detection due to sudden noise or the like. The number of regenerative determinations may be counted by the number of consecutive times that the DC bus voltage Vdc confirmed by the regenerative voltage determination unit 271 satisfies the regenerative determination condition described above, or by the total number of times that the DC bus voltage Vdc confirmed by the regenerative voltage determination unit 271 satisfies the regenerative determination condition described above.

[0033] The voltage command control unit 272 acquires the current Iuvw output from the inverter 260 from the current detection unit 250. The voltage command control unit 272 estimates the position of a rotor (not shown) of the motor 310 based on the current Iuvw. This is because, when the power conversion device 200 is used in an air conditioner or the like, position sensorless control is often performed, which does not use a position sensor to detect the position of the rotor of the motor 310 for the compressor 300. The voltage command control unit 272 estimates an estimated phase angle θ, which is the direction of the rotor magnetic poles on the dq axes, and an estimated speed ω, which is the rotor speed, for the rotor. The voltage command control unit 272 estimates a speed command ω ＊ and the estimated speed ω, the voltage command V ＊ Specifically, the voltage command control unit 272 generates a speed command ω ＊ The voltage command V ＊ The voltage command V ＊ Regarding the voltage command V, using the notation of the dq axes of the rotor magnetic poles mentioned above, ＊２ = d-axis voltage command Vd ＊２ +q-axis voltage command Vq ＊２ It can also be expressed as follows: Speed ​​command ω ＊When the power conversion device 200 is used as a refrigeration cycle-applied device such as an air conditioner, the estimated phase angle θ is based on, for example, the temperature detected by a temperature sensor (not shown), information indicating the set temperature instructed from a remote controller (not shown), which is an operation unit, information on the selection of the operation mode, and information on instructions to start and stop operation. The operation mode is, for example, heating, cooling, dehumidification, etc. The voltage command control unit 272 outputs the estimated phase angle θ to the PWM signal generation unit 274, and generates the voltage command V ＊ is output to the modulation factor calculation unit 273.

[0034] The modulation factor calculation unit 273 obtains the DC bus voltage Vdc from the voltage detection unit 240 and calculates the voltage command V from the voltage command control unit 272. ＊ The modulation factor calculation unit 273 calculates the modulation factor Kh in the power conversion device 200 by the following equation (1).

[0035] Kh = √(Vd ＊２ +Vq ＊２ ) / (Vdc / √(2)) …(1)

[0036] In formula (1), √(Vd ＊２ +Vq ＊２ ) is Vd ＊２ +Vq ＊２ √(2) represents the square root of 2. The modulation factor calculation unit 273 calculates the modulation factor Kh obtained by calculation and the voltage command V obtained from the voltage command control unit 272. ＊ to the PWM signal generating unit 274.

[0037] The PWM signal generation unit 274 obtains the estimated phase angle θ from the voltage command control unit 272, and calculates the modulation factor Kh and the voltage command V from the modulation factor calculation unit 273. ＊ The PWM signal generator 274 obtains the voltage command V ＊ From the dq coordinates, the AC voltage command Vuvw ＊ The PWM signal generating unit 274 converts the modulation factor Kh and the coordinate-converted voltage command Vuvw ＊The control unit 270 applies a voltage to the motor 310 by outputting the PWM signal generated by the PWM signal generation unit 274 to the switching element 261 of the inverter 260 as the inverter drive signal described above. When the PWM signal generation unit 274 is instructed by the regenerative voltage determination unit 271 to generate and output an inverter drive signal for turning on the switching element 261 of the lower arm of each leg of the inverter 260, as described above, the PWM signal generation unit 274 generates and outputs the inverter drive signal for turning on the switching element 261 of the lower arm of each leg of the inverter 260.

[0038] When the switching element 261 of the lower arm of each leg of the inverter 260 is turned on, the path of the current due to the regenerative voltage generated in the motor 310 becomes a closed path between the motor 310 and the inverter 260. Therefore, the current flowing between the motor 310 and the inverter 260 is attenuated by the resistance of the motor 310, etc. This allows the power conversion device 200 to stop the motor 310 in a situation where a regenerative voltage is generated in the motor 310.

[0039] Here, we will explain the capacitance of capacitor 230 used in power conversion device 200. If the number of regeneration determinations specified for determining that regenerative voltage is being generated by regenerative voltage determination unit 271 described above is set to N, the number of regeneration determinations by regenerative voltage determination unit 271 must be N or more times during the charging time of capacitor 230, and therefore the capacitance of capacitor 230 is set as shown in equation (2).

[0040] C>1 / fc / N / R…(2)

[0041] In equation (2), C is the capacitance of capacitor 230, fc is the carrier frequency of the carrier used by PWM signal generation unit 274 of control unit 270, N is the number of regeneration determinations specified by regeneration voltage determination unit 271 of control unit 270 to determine that a regenerative voltage is being generated, and R is the resistance value of motor 310. N is an integer equal to or greater than 1. In this way, the capacitance C of capacitor 230, which can prevent the voltage across it from exceeding the withstand voltage due to charge accumulation caused by the regenerative voltage, is set by the carrier frequency fc, the number of regeneration determinations N specified by regeneration voltage determination unit 271 of control unit 270, and the resistance value R of motor 310.

[0042] Furthermore, when a small-capacity capacitor 230 is used in the power conversion device 200, the carrier frequency fc is set as shown in equation (3) based on the relationship between the capacitance C of the capacitor 230 and the carrier frequency fc.

[0043] C*fc>1 / N / R…(3)

[0044] In other words, the product of the carrier frequency fc and the capacitance C of the capacitor 230 is determined by the resistance value R of the motor 310 and the reciprocal of the regeneration determination count N specified by the regenerative voltage determination unit 271 of the control unit 270. It is also possible to express Equation (3) in such a way that C is moved to the right-hand side. By setting the capacitance C and carrier frequency fc of the capacitor 230 to satisfy Equations (2) and (3), the power conversion device 200 can reduce the capacitance C of the capacitor 230 while suppressing failures due to regenerative voltage. Thus, the capacitance C of the capacitor 230 is set by the carrier frequency fc used by the control unit 270 when generating an inverter drive signal for controlling the operation of the inverter 260, the resistance value R of the motor 310, and the regeneration determination count N, which is used by the control unit 270 to determine that a regenerative voltage is being generated in the motor 310 when the number of times the second DC voltage satisfies the specified regeneration determination condition reaches the specified regeneration determination count N. The carrier frequency fc used by the control unit 270 is set based on the capacitance C of the capacitor 230, the resistance value R of the motor 310, and the number of regeneration determinations N described above.

[0045] The regenerative voltage is the induced voltage φf*ωe generated in the motor 310 and can be calculated from the rotation speed and induced voltage constant of the motor 310. Figure 8 shows the induced voltage characteristics of the motor 310 of the compressor 300 connected to the power conversion device 200 according to the first embodiment. The induced voltage φf*ωe of the motor 310 is generated by the magnetic flux generated by the magnet provided in the motor 310. However, air conditioners equipped with the power conversion device 200 often employ flux-weakening control to suppress the magnetic flux of the magnet, particularly at high rotation speeds. In this case, even if the voltage across the capacitor 230 does not drop suddenly, the induced voltage φf*ωe of the motor 310 becomes higher than the voltage across the capacitor 230. If the voltage generated in the motor 310 is, for example, approximately 1.5 times the voltage across the capacitor 230, the difference between the induced voltage φf*ωe of the motor 310 and the voltage across the capacitor 230 causes the motor 310 to charge the capacitor 230. In such an operating state of the power conversion device 200, the capacitance C of the capacitor 230 and the carrier frequency fc are set as described above, and the control unit 270 performs the control as described above, so that the power conversion device 200 can suppress element destruction due to regenerative voltage.

[0046] Note that the power conversion device 200 may include a regenerative protection capacitor in addition to the capacitor 230, which is the main circuit capacitor for driving the motor. FIG. 9 is a third diagram showing a configuration example of the power conversion device 200 according to the first embodiment. The power conversion device 200 shown in FIG. 9 is configured by adding a regenerative protection circuit 281 to the power conversion device 200 shown in FIG. 1 . The regenerative protection circuit 281 includes a regenerative protection capacitor 282 and a diode 283. The regenerative protection circuit 281 has the regenerative protection capacitor 282 and the diode 283 connected in series and connected in parallel to the capacitor 230. While the capacitor 230 repeatedly charges and discharges while the motor 310 is running, the regenerative protection capacitor 282 of the regenerative protection circuit 281 is configured to only be charged by the diode 283 connected in series, and is intended to absorb regenerative voltage. The regenerative protection circuit 281 shown in Figure 9 has the simplest configuration and can suppress an increase in circuit size, but may also include other components such as a discharge resistor and a switching element. The type of regenerative protection capacitor 282 is not limited to an electrolytic capacitor or a film capacitor. Furthermore, by setting the combined capacitance of the capacitor in the upstream stage of inverter 260 of power conversion device 200, i.e., capacitor 230 and regenerative protection capacitor 282, to the aforementioned capacitance C, the capacitance of regenerative protection capacitor 282 can be set to a range including zero, for example.

[0047] 10 is a flowchart showing the operation of the control unit 270 of the power conversion device 200 according to the first embodiment. In the control unit 270, the regenerative voltage determination unit 271 acquires the voltage across the capacitor 230, i.e., the DC bus voltage Vdc, from the voltage detection unit 240 (step S1). The regenerative voltage determination unit 271 determines whether the power supply voltage in the power conversion device 200 has been lost or momentarily dropped (step S2). As described above, the regenerative voltage determination unit 271 determines whether the power supply voltage in the power conversion device 200 has been lost or momentarily dropped based on whether the DC bus voltage Vdc has become equal to or lower than a specified voltage for a specified period of time. If the power supply voltage in the power conversion device 200 has not been lost or momentarily dropped (step S2: No), the regenerative voltage determination unit 271 returns to step S1. When the power supply voltage is lost or momentarily drops in the power conversion device 200 (step S2: Yes), the regenerative voltage determination unit 271 starts counting the number of regenerative determinations N of the DC bus voltage Vdc to determine that regenerative voltage is being generated in the motor 310 (step S3).

[0048] The regenerative voltage determination unit 271 acquires the voltage across the capacitor 230, i.e., the DC bus voltage Vdc, from the voltage detection unit 240 (step S4). If the number of times that the regeneration determination condition is satisfied for the DC bus voltage Vdc detected by the voltage detection unit 240 does not reach the regeneration determination count N (step S5: No), the regenerative voltage determination unit 271 returns to step S4. If the number of times that the regeneration determination condition is satisfied for the DC bus voltage Vdc detected by the voltage detection unit 240 reaches the regeneration determination count N (step S5: Yes), the regenerative voltage determination unit 271 determines that a regenerative voltage is being generated in the motor 310 and instructs the PWM signal generation unit 274 to generate and output an inverter drive signal to the inverter 260 to turn on the switching elements 261 of the lower arms of each leg. The PWM signal generation unit 274 generates and outputs an inverter drive signal to the inverter 260 to turn on the switching elements 261 of the lower arms of each leg (step S6).

[0049] Next, a description will be given of the hardware configuration of the control unit 270 included in the power conversion device 200. Fig. 11 is a diagram showing an example of a hardware configuration realizing the control unit 270 included in the power conversion device 200 according to embodiment 1. The control unit 270 is realized by a processor 910 and a memory 920.

[0050] The processor 910 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 920 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 920 is not limited to these, and may also be a magnetic disk, optical disk, compact disk, minidisc, or DVD (Digital Versatile Disc).

[0051] As described above, according to the present embodiment, the capacitance C of capacitor 230 used in power conversion device 200 is set based on the carrier frequency fc used by control unit 270 when generating an inverter drive signal for controlling the operation of inverter 260, the resistance value R of motor 310, and the regeneration determination count N described above, which is used by control unit 270 to determine that a regenerative voltage is being generated in motor 310 when the number of times the second DC voltage satisfies a specified regeneration determination condition reaches the specified regeneration determination count N. Furthermore, the carrier frequency fc used by control unit 270 is set based on the capacitance C of capacitor 230, the resistance value R of motor 310, and the regeneration determination count N described above. Furthermore, when control unit 270 determines that a regenerative voltage is being generated in motor 310, it generates and outputs to inverter 260 an inverter drive signal that turns on switching elements 261 of the lower arms of each leg. As a result, even if the capacity of capacitor 230 is reduced to achieve smaller size, higher efficiency, lower costs, etc., power conversion device 200 can avoid a situation in which the voltage across capacitor 230 due to the regenerative voltage exceeds the withstand voltage of capacitor 230, even if the power supply voltage is lost or drops instantaneously and a regenerative voltage is generated in motor 310. Power conversion device 200 can prevent element breakdown due to the regenerative voltage while suppressing an increase in circuit size.

[0052] Embodiment 2. Fig. 12 is a diagram illustrating a configuration example of an air conditioner 500 according to Embodiment 2. The air conditioner 500 includes an AC power source 100, a power conversion device 200, a compressor 300 having a motor 310, a four-way valve 400, an outdoor heat exchanger 410, an expansion valve 420, an indoor heat exchanger 430, and refrigerant piping 440. In Fig. 12, the compressor 300, the four-way valve 400, the outdoor heat exchanger 410, the expansion valve 420, and the indoor heat exchanger 430 are connected via the refrigerant piping 440. A motor 310 is provided inside the compressor 300 to operate a compressor mechanism (not shown). The motor 310 has a stator and a rotor. The motor 310 is a compressor motor that is driven by input of a second AC voltage having a desired voltage and frequency generated by the power conversion device 200. The air conditioner 500 can perform air conditioning control by compressing the refrigerant inside the compressor 300 as a result of the rotation of the motor 310 inside the compressor 300, and circulating the refrigerant between the outdoor heat exchanger 410 and the indoor heat exchanger 430 via the refrigerant piping 440. The power conversion device 200 and the motor 310 are electrically connected, and the power conversion device 200 is connected to the AC power supply 100. The power conversion device 200 generates an AC voltage to be supplied to the motor 310 using a first AC voltage supplied from the AC power supply 100.

[0053] In this way, the power conversion device 200 is mounted on the air conditioner 500, but this is just an example and the present invention is not limited to this. In addition to the air conditioner 500, the power conversion device 200 can be applied to products equipped with a refrigeration cycle, such as refrigerators, freezers, and heat pump water heaters.

[0054] 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.

[0055] 100 AC power supply, 200 power conversion device, 210 rectifier circuit, 211, 262, 283 diode, 220 reactor, 230 capacitor, 240 voltage detection unit, 250 current detection unit, 260 inverter, 261 switching element, 270 control unit, 271 regenerative voltage determination unit, 272 voltage command control unit, 273 modulation factor calculation unit, 274 PWM signal generation unit, 281 regenerative protection circuit, 282 regenerative protection capacitor, 300 compressor, 310 motor, 311 winding, 400 four-way valve, 410 outdoor heat exchanger, 420 expansion valve, 430 indoor heat exchanger, 440 refrigerant piping, 500 air conditioner, 910 processor, 920 memory.

Claims

1. A power conversion device comprising: a non-boost type rectifier circuit that converts a first AC voltage output from an AC power source into a first DC voltage; a reactor connected to the rectifier circuit; a capacitor that smoothes the first DC voltage into a second DC voltage; a voltage detection unit that detects the second DC voltage; an inverter connected in parallel to the capacitor that converts the second DC voltage into a second AC voltage of a desired voltage and frequency and outputs the second DC voltage to a motor; and a control unit that controls the operation of the inverter, wherein the capacitance of the capacitor is set based on a carrier frequency used by the control unit when generating an inverter drive signal for controlling the operation of the inverter, a resistance value of the motor, and the regeneration judgment number for the control unit to determine that a regenerative voltage is being generated in the motor when the number of times that the second DC voltage satisfies a specified regeneration judgment condition has reached a specified regeneration judgment number.

2. The power conversion device according to claim 1, wherein the carrier frequency used by the control unit is set based on a capacitance of the capacitor, a resistance value of the motor, and the number of regeneration determinations.

3. The power conversion device according to claim 1 or 2, wherein the inverter has a configuration having a plurality of legs in which upper arm switching elements and lower arm switching elements are connected in series, and when the control unit determines that a regenerative voltage is being generated in the motor, it generates and outputs to the inverter an inverter drive signal for turning on the lower arm switching elements of each leg.

4. A power conversion device as described in any one of claims 1 to 3, wherein the control unit starts counting the number of regeneration determinations when the second DC voltage detected by the voltage detection unit becomes equal to or lower than a specified voltage for a specified period of time.

5. The power conversion device according to claim 1, wherein the number of regeneration determinations is two or more.

6. A power conversion device as claimed in any one of claims 1 to 5, further comprising: a regenerative protection circuit consisting of a regenerative protection capacitor and a diode in parallel with the capacitor, wherein the combined capacitance of the capacitor and the regenerative protection capacitor is set to the capacitance of the capacitor, thereby setting the capacitance of the regenerative protection capacitor to a range including 0.

7. An air conditioner comprising a power conversion device according to any one of claims 1 to 6.

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