Power conversion device and heat pump device

Abstract

A power conversion device (100) comprises: a rectifier (3) that is a converter for rectifying AC power supplied from an AC power supply (1); a smoothing unit (4) that is connected to the output end of the converter and smooths power output by the converter; an inverter (5) that is connected to both ends of the smoothing unit and generates AC power to be output to a load (6); a current detector (7) that detects the current flowing between the smoothing unit and the inverter; and a control unit (9) that controls the inverter. The control unit divides, on the basis of a voltage vector indicating the state of each switching element constituting the inverter, a unit time that is a period having a predetermined length into a plurality of current detection intervals, calculates, for each of the plurality of current detection intervals, the products of the time widths of the current detection intervals and the current values detected by the current detector in the current detection intervals, and controls the inverter so that the sum value of the calculated products per unit time becomes constant for each unit time.

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device and a heat pump device using the power conversion device. [Background technology]

[0002] In controlling electric motors that drive single rotary compressors or twin rotary compressors that are applied to heat pump devices such as air conditioners and refrigerators, there is a technique for suppressing an increase in power consumption by appropriately compensating for the torque pulsation component according to the state of the electric motor (for example, see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-178814 A Summary of the Invention [Problem to be solved by the invention]

[0004] The motor control device described in Patent Document 1 controls a motor that drives a load with periodically pulsating load torque. However, in the case of a power conversion device equipped with a converter that converts AC power supplied from an AC power source into DC power, if the load torque pulsates at a frequency asynchronous with the power source frequency, the charge / discharge current flowing from the converter to the smoothing capacitor becomes unbalanced between the positive and negative power source voltages. As a result, there is a risk that harmonics in the power source current will increase.

[0005] Furthermore, the motor control device described in Patent Document 1 estimates a three-phase AC current flowing through the motor from a detection result of a current flowing between a converter that rectifies an AC power supply and an inverter, and generates a control signal based on the estimated three-phase AC current. Patent Document 1 also describes that a shunt resistor can be used to detect the current flowing between the converter and the inverter.

[0006] However, when current detection is performed using a shunt resistor, detection is possible only at a specified timing. In general, current detection is performed periodically at a set time interval, so only instantaneous values ​​can be detected, making it difficult to obtain an accurate current value. If an accurate current value cannot be obtained, it becomes difficult to accurately control the inverter. Note that there is a minimum value for the time interval at which current detection is performed, and current detection cannot be performed at an interval shorter than the minimum value.

[0007] The present disclosure has been made in consideration of the above, and aims to provide a power conversion device that is capable of highly accurate load control based on instantaneous values ​​of current obtained at a specified timing. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems and achieve the object, the power conversion device according to the present disclosure includes a converter that rectifies AC power supplied from an AC power source, a smoothing unit connected to an output terminal of the converter and smoothing the power output by the converter, an inverter connected to both ends of the smoothing unit and generating AC power to be output to a load, a current detector that detects a current flowing between the smoothing unit and the inverter, and a control unit that controls the inverter. The control unit controls the current flowing between each switching element of the inverter for a unit time that is a period of a predetermined length. The timing at which the voltage vector, which is a vector whose components are values ​​indicating the on / off state, changes The inverter is controlled so that the total value of the calculated products per unit time is constant for each unit time. Effect of the Invention

[0009] According to the present disclosure, it is possible to realize an electric power conversion device capable of highly accurately controlling a load based on an instantaneous value of a current obtained at a predetermined timing. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing a configuration example of a power conversion device according to a first embodiment; [Diagram 2] FIG. 1 is a diagram for explaining a method in which a control unit generates a control signal for an inverter. [Diagram 3] FIG. 2 is a diagram showing switching patterns of switching elements that configure an inverter. [Figure 4] FIG. 1 is a diagram showing an example of current and voltage waveforms when the power conversion device according to the first embodiment is operating. [Diagram 5] A diagram showing the frequency components contained in the input current Is when symmetry is lost when the frequency of the power supply voltage Vs is 50 Hz. [Figure 6] FIG. 1 is a diagram showing an example of current and voltage waveforms when a power conversion device operates a load at a constant current. [Figure 7] Diagram for explaining a general method of detecting load current [Figure 8] FIG. 1 is a diagram for explaining a method for detecting a DC current Idc flowing from an inverter of a power conversion device to a load. [Figure 9] FIG. 1 is a diagram illustrating an example of a hardware configuration for implementing a control unit included in a power conversion device according to a first embodiment. [Figure 10] FIG. 1 is a diagram showing a configuration example of a heat pump device according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, a power conversion device and a heat pump device according to embodiments of the present disclosure will be described in detail with reference to the drawings.

[0012] Embodiment 1 1 is a diagram illustrating a configuration example of a power conversion device according to a first embodiment. The power conversion device 100 includes a reactor 2, a rectifier 3 which is a converter, a smoothing unit 4, an inverter 5, a current detector 7, a voltage detector 8, and a control unit 9. The power conversion device 100 is connected to an AC power source 1, and converts AC power supplied from the AC power source 1 into three-phase AC power and supplies the power to a load 6.

[0013] The power supply voltage Vs input from the AC power supply 1 is rectified by the rectifier 3 via the reactor 2, and smoothed by being stored in the capacitors 4a and 4b constituting the smoothing unit 4 connected to the output end of the rectifier 3, before being supplied to the inverter 5. If the side of the output terminal of the AC power supply 1 to which the reactor 2 is connected is taken as positive, the capacitor 4a of the smoothing unit 4 is charged when the power supply voltage Vs is positive, and the capacitor 4b is charged when the power supply voltage Vs is negative.

[0014] Here, the AC power source 1 may be a commercial power source of 50 Hz or 60 Hz, or may be an AC voltage generated by a distributed power source such as a stationary storage battery or a solar power generation system.

[0015] The reactor 2 may be an EI or EE type reactor made of laminated electromagnetic steel sheets, or may use an iron core made of ferrite or amorphous material, etc. The material of the windings may be copper, aluminum, or the like.

[0016] The rectifier 3 is realized, for example, by arranging diodes in a bridge shape. Instead of diodes, the rectifier 3 may be configured by power semiconductors such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). The diodes and power semiconductors such as MOSFETs may be made of general silicon materials, or may be wide band gap semiconductors with lower loss.

[0017] The capacitors 4a and 4b are aluminum electrolytic capacitors, small-capacity film capacitors, or the like.

[0018] The configuration of the rectifier 3 is not limited to that shown in Fig. 1. The power conversion device 100 according to the present embodiment may be any device that includes a circuit for rectifying AC power and a capacitor for smoothing the rectified DC power.

[0019] An inverter 5 is connected to both ends of the smoothing unit 4, i.e., both ends of a series circuit consisting of the capacitors 4a and 4b connected in series, and a load 6 is connected to the inverter 5. The load 6 that consumes the AC power generated by the inverter 5 includes an electric motor. The inverter 5 has a plurality of switching elements connected in series arranged in parallel, and operates to apply a polyphase AC voltage to the electric motor included in the load 6. A diode is connected in parallel to each of the switching elements that make up the inverter 5. IGBTs (Insulated Gate Bipolar Transistors), MOSFETs, etc. are widely used as switching elements. In the case of a MOSFET, a parasitic diode is built in, and there are cases where a diode connected in parallel is not separately connected.

[0020] Silicon (Si) is widely used as a material for switching elements, and in recent years, MOSFETs with superjunction structures have also come into widespread use due to demand for higher efficiency. In addition, wide band gap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), and diamond are also becoming more common for achieving even higher efficiency, and it doesn't matter which material is used for the switching elements, as long as it is possible to perform switching operations to apply voltage to the motor.

[0021] There are induction motors and synchronous motors, but any type of motor configuration is acceptable. For example, in the case of a synchronous motor, the stator may be either concentrated winding or distributed winding, and any material such as copper or aluminum wire may be used for the windings as long as it can pass current. As for the rotor, there are surface magnet types and embedded magnet types that use permanent magnet synchronous motors, but any type may be used as long as it has a structure that can generate rotational force.

[0022] When such an electric motor is used to realize, for example, a refrigeration cycle device, the electric motor is used to operate a compression mechanism that compresses a refrigerant and to drive a fan for heat exchange.

[0023] Furthermore, the control unit 9 outputs a pulse width modulation (PWM) signal, which is a signal for controlling a switching element to control the voltage supplied from the inverter 5 to the load 6, based on the DC current Idc detected by the current detector 7 and the DC voltage Vdc detected by the voltage detector 8. Note that the current detector 7 may be any device capable of detecting a current, such as a shunt resistor or a DCCT (Direct Current Transformer).

[0024] A method will be described below in which the control unit 9 generates a PWM signal as a control signal for the inverter 5. FIG.

[0025] As shown in the upper part of FIG. 2, the control unit 9 compares a carrier signal whose amplitude is 1 / 2 of the DC voltage Vdc with Vu*, Vv*, and Vw*, which are command values ​​of the voltages of the U-phase, V-phase, and W-phase to be applied to the motor included in the load 6, to generate a PWM signal for operating each switching element of the inverter 5. V* is the amplitude of the voltage command value (Vu*, Vv*, and Vw*) of each phase. The PWM signal generated by the control unit 9 is shown in the lower part of FIG. 2. In the lower part of FIG. 2, UP is a control signal for the upper left switching element of the inverter 5 shown in FIG. 1, UN is a control signal for the lower left switching element, VP is a control signal for the upper center switching element, VN is a control signal for the lower center switching element, WP is a control signal for the upper right switching element, and WN is a control signal for the lower right switching element.

[0026] Fig. 3 is a diagram showing switching patterns of the switching elements that constitute the inverter 5. In detail, Fig. 3 shows the relationship between the direction of the voltage applied to the motor (voltage direction) in each switching pattern of the switching elements that constitute the inverter 5, the DC current Idc (detected current) that flows between the rectifier 3 and the inverter 5 when a voltage vector (V0 to V7) is output, and the PWM signals (UP, VP, WP, UN, VN, WN). The voltage vector represents the state of each switching element that constitutes the inverter 5. The numerical values ​​(0 to 7) combined with "V" correspond to the states of the UP, VP, and WP switching elements.

[0027] The direction of the voltage applied to the motor, indicated as "voltage direction" in Fig. 3, is determined from the detection result of DC voltage Vdc by voltage detector 8. DC current Idc, indicated as "detected current", is detected by current detector 7. In Fig. 3, the current flowing in the direction from rectifier 3 to inverter 5 is considered positive. Generally, in controlling a motor connected to an inverter, the current at a specific moment when a PWM signal corresponding to a voltage vector is output to the inverter is detected, and the detected instantaneous value is used to control the motor. Such a method of controlling a motor is well known, so a detailed explanation will be omitted.

[0028] Next, consider control for suppressing speed pulsation by controlling the torque of the electric motor in accordance with the pulsation of the load torque as in the technique described in the above-mentioned Patent Document 1. Fig. 4 is a diagram showing an example of current and voltage waveforms when the power conversion device 100 according to the first embodiment operates. Fig. 4 shows waveforms in the case where speed pulsation is suppressed by controlling the torque of the electric motor in accordance with the pulsation of the load torque.

[0029] When the power conversion device 100 controls the torque of the motor in accordance with the pulsation of the load torque, it increases or decreases the effective power output to the motor in accordance with the pulsation of the load torque. Therefore, as shown in FIG. 4, the DC current Idc pulsates, and the capacitors 4a and 4b constituting the smoothing unit 4 consume electric charge in accordance with the pulsation of the load torque. Therefore, the timing of charging the capacitors 4a and 4b by the power supply voltage Vs becomes asynchronous with the power supply frequency, and the symmetry of the positive and negative sides of the input current Is is lost. Note that the waveforms shown in FIG. 4 indicate, from the top, the power supply voltage Vs, the input current Is, the DC voltage Vdc, the motor currents Iu, Iv, and Iw, the DC current Idc, and the voltage vector. When the symmetry between the positive side waveform and the negative side waveform of the input current Is is lost, the harmonics contained in the input current Is increase.

[0030] When the input current Is and the power supply voltage Vs are synchronized, the input current Is is dominated by odd-order harmonic components such as the third, fifth, seventh, etc., with the fundamental wave being the same frequency component as the power supply voltage Vs, and the generation of even-order harmonics is minor. However, when the input current Is flows asynchronously and irregularly with the power supply voltage Vs, the symmetry of the waveform of the input current Is is lost as described above. As a result, current components other than odd-order harmonics, specifically even-order harmonics and inter-order harmonics, are generated. Inter-order harmonics are harmonics that do not fall into either the odd-order harmonics or even-order harmonics of the input current Is. For example, if the fundamental wave frequency is 50Hz, the second-order harmonic is 100Hz, and is a frequency component between 50Hz and 100Hz.

[0031] Here, the harmonic current of the power supply current (corresponding to the input current Is in the power conversion device 100) has a limit value set in 61000-3-2, a standard established by JIS (Japanese Industrial Standards) and IEC (International Electrotechnical Commission). In this standard, the limit value of odd-order harmonics is set higher than that of even-order harmonics. Therefore, in a case where components other than odd-order harmonics are generated, the even-order harmonics increase and tend to exceed the limit value. In addition, the method of handling inter-order harmonics is determined in 61000-4-7, a standard established by JIS and IEC. In this standard 61000-4-7, it is specified that inter-order harmonics are grouped for each frequency in a specific range and are added to adjacent odd-order harmonics or even-order harmonics. Therefore, as the inter-order harmonics increase, the harmonics treated as odd-order harmonics and the harmonics treated as even-order harmonics increase. Therefore, the occurrence of interharmonics cannot be ignored, and it is important to suppress their occurrence.

[0032] In the case of Figure 4, there are places where the charging current per charge is large (for example, the positive charging current in the first cycle from the left) and places where it is small (for example, the positive charging current in the fourth cycle from the left) due to an imbalance in the input current Is, and in particular in the region where the charging current is large, there are concerns about the effects of a decrease in inductance due to the large input current Is flowing through reactor 2, and increased losses due to excessive current flowing through rectifier 3. For this reason, it is necessary to control the flow of such currents, in other words, to prevent the charging current per charge from becoming too large.

[0033] When the DC current Idc is small, the drop in the DC voltage Vdc is small, and the amount of charge by the power supply voltage Vs is small, so the input current Is drops. When the power supply voltage Vs is low (close to zero) and the DC current Idc is large, the DC voltage Vdc drops significantly. When a positive or negative peak occurs in the power supply voltage Vs in this state, the charging current to the capacitors 4a and 4b increases, and the input current Is increases. In such a case where the symmetry of the input current Is is lost and the input current Is becomes unbalanced, the input current Is contains the following components. As an example, the current components contained in the input current Is when the power supply frequency is 50 Hz are shown in FIG. 5. FIG. 5 is a diagram showing the frequency components contained in the input current Is when the symmetry is lost when the frequency of the power supply voltage Vs is 50 Hz. It should be noted that the frequency components contained in the input current Is when the symmetry is lost when the power supply frequency is 60 Hz can also be calculated in the same manner as described below.

[0034] The frequencies (0, 5, 10, ..., 100) listed in the leftmost column of Fig. 5 are the frequencies of the periodically changing waveform of the DC current Idc (see Fig. 4), that is, the fluctuation frequencies of the DC current Idc flowing through the load 6. For ease of explanation, hereinafter, this frequency may be referred to as the load current fluctuation frequency.

[0035] The ranges of values ​​(75-125, 125-175, ..., 275-325) shown in the first row of Fig. 5 indicate frequency ranges and are the units for grouping the interharmonics described above. That is, in grouping according to the provisions of the above-mentioned standard 61000-4-7, the interharmonics included in each of the ranges shown in the first row of Fig. 5 are grouped together. For example, interharmonics included in the range of 75-125 Hz belong to the same group. The values ​​written in each column other than the leftmost column from the third row onwards in Fig. 5 indicate the frequencies of the interharmonics included in the input current Is.

[0036] The entries (2nd, 3rd, ..., 6th) other than the leftmost column of the second row in Fig. 5 indicate the order of harmonics of the power supply frequency and indicate where the grouped interharmonics are added. For example, the interharmonics in the column marked "2nd" (75Hz, 80Hz, ...), i.e., the interharmonics in the range of 75 to 125Hz, are added to the 2nd harmonic. Half of the interharmonics at the boundaries of the grouping ranges (125Hz, 175Hz, 225Hz, ...) are added to one of the odd harmonics and even harmonics corresponding to each of the two ranges that contain this interharmonic, and the remaining half is added to the other.

[0037] A description will be given of specific current components that are generated when the DC current Idc flowing through the load 6 is periodically changed. In the following description, the frequency (50 Hz) of the fundamental wave component of the power supply voltage Vs will be referred to as power supply frequency 1f, and the frequencies (100 Hz, 150 Hz, 200 Hz, ...) of the harmonic components of the power supply voltage Vs will be referred to as power supply frequency 2f, power supply frequency 3f, power supply frequency 4f, .... In order to avoid complicating the description, the DC current Idc flowing through the load 6 may be referred to as a load current.

[0038] Current components that occur regardless of fluctuations in the load current of the AC power supply 1 include current components with frequencies shown in (1) below.

[0039] (1) Power frequency 1f, power frequency 3f, power frequency 5f,…, power frequency (2n-1)f, (n=1,2,3,…)

[0040] "Power supply frequency 1f" represents the frequency of the fundamental wave of the power supply current, and "power supply frequency (2n-1)" represents the frequency of the 2n-1-th harmonic of the power supply current. In other words, the fundamental wave component and odd-order harmonic components are generated regardless of fluctuations in the load current of the AC power supply 1.

[0041] The current components occurring in the input current Is due to the influence of the fundamental wave component of the load current include the current components with the frequencies shown in (2) to (7) below. Note that "power supply frequency 3f" represents the frequency of the third harmonic of the power supply current, and "power supply frequency 5f" represents the frequency of the fifth harmonic of the power supply current.

[0042] (2) Power supply frequency 1f-{Power supply frequency 1f-|Power supply frequency 1f-load current fluctuation frequency|} (3) Power supply frequency 1f + {Power supply frequency 1f - | Power supply frequency 1f - load current fluctuation frequency |} (4) Power supply frequency 3f-{Power supply frequency 1f-|Power supply frequency 1f-load current fluctuation frequency|} (5) Power supply frequency 3f + {power supply frequency 1f - | power supply frequency 1f - load current fluctuation frequency |} (6) Power supply frequency 5f-{Power supply frequency 1f-|Power supply frequency 1f-load current fluctuation frequency|} (7) Power supply frequency 5f + {power supply frequency 1f - | power supply frequency 1f - load current fluctuation frequency |}

[0043] In addition, the current components occurring in the input current Is due to the influence of the second harmonic component of the load current include the current components with frequencies shown in (8) to (13) below. These are shown in parentheses in Fig. 5 (for example, "(80, 120)").

[0044] (8) Power supply frequency 1f-{Power supply frequency 1f-|Power supply frequency 1f-load current fluctuation frequency×2|} (9) Power supply frequency 1f + {Power supply frequency 1f - | Power supply frequency 1f - load current fluctuation frequency × 2 |} (10) Power supply frequency 3f-{Power supply frequency 1f-|Power supply frequency 1f-load current fluctuation frequency x 2|} (11) Power supply frequency 3f + {power supply frequency 1f - | power supply frequency 1f - load current fluctuation frequency × 2 |} (12) Power supply frequency 5f-{Power supply frequency 1f-|Power supply frequency 1f-load current fluctuation frequency x 2|} (13) Power supply frequency 5f + {power supply frequency 1f - | power supply frequency 1f - load current fluctuation frequency × 2 |}

[0045] In this way, current components other than those of the frequencies shown in (1) are generated depending on the power supply frequency and the load current fluctuation frequency, and the amount of generation increases as the fluctuation range of the load current becomes larger. As shown in Fig. 5, in most cases, the interharmonics between the odd and even harmonics, which are multiples of 50Hz (power supply frequency), are generated. When the grouping process defined in the above-mentioned standard 61000-4-7 is performed, for example, when the load current fluctuation frequency is 30Hz, current components with frequencies of 80Hz, 90Hz, 110Hz, and 120Hz are generated, but these are grouped into the even harmonic of 100Hz, resulting in an increase in even harmonics. Since the limit value of even harmonics is set low in the above-mentioned standard 61000-3-2, if such interharmonics grouped into even harmonics occur, the even harmonics are more likely to exceed the limit value.

[0046] Interharmonics of the frequencies shown in (2) to (13) above do not occur if the load current fluctuation frequency is zero, that is, if the load current is a direct current. Therefore, if the control unit 9 controls the load 6 so that the load current is a direct current, it is clear that the imbalance of the input current Is does not occur as shown in Fig. 6, and the occurrence of interharmonics can be suppressed. Fig. 6 is a diagram showing an example of current and voltage waveforms when the power conversion device 100 operates the load 6 with a constant current.

[0047] In this way, by operating the load 6 with a constant current so that a constant charge is always consumed from the capacitors 4a and 4b constituting the smoothing unit 4, the variation in charging and discharging of the capacitors 4a and 4b is eliminated. As a result, the power supply voltage Vs and the input current Is have a waveform that is synchronized, and the input current Is has a waveform that contains many odd-order harmonics. In general, the power supply current (corresponding to the input current Is in this embodiment) is synchronized with the power supply voltage Vs and contains many odd-order harmonics. Therefore, as described above, in the standard 61000-3-2, the limit value of odd-order harmonics is set high, and the limit value of even-order harmonics is set low. Therefore, it is necessary to suppress the generation of inter-order harmonics grouped into even orders.

[0048] From the above, in order to prevent the generation of even harmonics, it is necessary to suppress the AC component occurring in the DC current Idc flowing from the inverter 5 to the load 6, and it is therefore necessary to accurately detect the DC current Idc.

[0049] Conventionally, as shown in Fig. 7, the current is detected at the timing when the voltage vector shown in Fig. 3 is output within one cycle of the carrier signal, and the detection result is used to control the inverter. Fig. 7 is a diagram for explaining a general method of detecting a load current. The upper part of Fig. 7 shows the relationship between the carrier signal and the voltage commands (Vu*, Vv*, Vw*) of each phase applied to the motor. The lower part of Fig. 7 shows the current flowing through the motor and the timing at which the current is detected (corresponding to the current measurement points represented by ●). In the load current detection method shown in Fig. 7, the current is detected once in the period from when the voltage vector changes to when it changes again.

[0050] However, in reality, as can be seen from Figure 7, current flows continuously through the motor, and the current changes even in the same section of the voltage vector. Therefore, the detected value changes depending on the detection timing, and there is a possibility that an accurate value cannot be obtained. In addition, there are sections where no current flows, so detection is not easy. There are methods to finely time the current detection timing, but while the period of the carrier signal is several tens to several hundreds of microseconds, the analog-to-digital conversion speed of a typical microcontroller is about several microseconds, and there is a risk that the time resolution will be insufficient. In addition, frequent analog-to-digital conversion is not realistic because it may affect the processing time of the entire microcontroller.

[0051] Therefore, in the power conversion device 100 according to the present embodiment, current detection is performed using a method as shown in Fig. 8. Fig. 8 is a diagram for explaining a method for detecting a direct current Idc flowing from the inverter 5 to the load 6 of the power conversion device 100. In the method shown in Fig. 8, one carrier period is defined as a unit time that is a period of a predetermined length, and an average value per unit time of the direct current Idc is calculated based on a current value (instantaneous value) detected at a certain timing within one carrier period and a PWM signal output to the inverter 5.

[0052] In the power conversion device 100, the time when the real vector is output from the control unit 9 to the inverter 5 can be estimated based on the relationship between the carrier signal used when generating a PWM signal for the inverter 5 and the voltage commands Vu*, Vv*, and Vw*. The real vector here is a voltage vector other than the V0 vector and the V7 vector shown in FIG. 3, that is, a voltage vector output when a current flows from the inverter 5 to the load 6. The control unit 9 can estimate the time when each of the V1 vector to the V6 vector is output. For example, in the case of time t1 shown in FIG. 8, the time when the real vector is output is the difference between the point where the voltage command Vw* and the carrier signal intersect and the point where the voltage command Vv* and the carrier signal intersect. In this regard, the timing of the rise of the carrier signal is v=a×t+b, and the voltage commands Vv* and Vw* are respectively substituted for v to find t and take the difference. Note that v is the voltage command value on the vertical axis in FIG. 8, t is the time on the horizontal axis in FIG. 8, and a and b are constants. The area can be calculated by multiplying the t1 calculated in this way and the current I1 detected in that section. In the same way, I2×t2, I3×t3, and I4×t4 shown in Figure 8 are calculated and added together. By dividing this by the time of the carrier cycle, the average load current, which is the average value of the current flowing through the load 6 in one carrier cycle, can be calculated. By using this method, even when it is difficult to precisely time the current detection, such as when using an inexpensive shunt resistor, it is possible to suppress detection errors in the average load current and control the DC current Idc flowing through the load 6 to a constant value.

[0053] In addition, the timing of current detection in the current detection intervals, which are time intervals (t1 to t4) corresponding to each voltage vector, is preferably near the center of each current detection interval. By performing detection near the center of the current detection interval, it is possible to suppress current detection errors and calculate the area with high accuracy. However, if detection is performed near the center when the width of the current detection interval is narrow, ringing occurs when the current detection interval switches, and this ringing continues until the center, which may affect the detection accuracy. Therefore, when the width of the current detection interval is narrow, detection may be performed after waiting until the ringing effect is eliminated.

[0054] Furthermore, the peak current of the DC current Idc pulsates at a frequency six times that of the current flowing through the motor. Therefore, the average value may be calculated by adding the areas of the currents as described above at a period of six times the frequency. Furthermore, in order to eliminate the influence of other pulsations, the average value may be calculated at a period other than one carrier period. For example, a period that is an integer multiple of one carrier period may be used as the unit time, and the average value of the DC current Idc per unit time may be calculated.

[0055] Also, while FIG. 8 shows an example in which current measurement is performed once in each current detection section, current measurement may be performed multiple times in one current detection section, and the measured values ​​may be averaged and used in the above area calculation.

[0056] By controlling the torque generated by the motor so that the current value (DC current Idc) determined in this way is constant, the amount of charge consumed from capacitors 4a and 4b in accordance with the cycle of the load torque pulsation becomes uniform for each cycle of the load torque pulsation, making it possible to reduce harmonics in the input current Is. This makes it possible to effectively reduce harmonic currents even when the capacity of reactor 2 and capacitors 4a and 4b is small, leading to smaller, lighter, and less costly equipment.

[0057] A method of controlling the torque generated by the motor so that the DC current Idc flowing through the load 6 is constant can be easily realized by using known technology. For example, the control unit 9 compensates for a q-axis current command value, which is the torque current of the motor, and operates the motor so that the DC current Idc flowing through the load 6 is constant, thereby making it possible to easily manipulate the active power and control the DC current Idc flowing through the load 6 to be constant.

[0058] In addition, the charging and discharging of the capacitors 4a and 4b is also affected by the electromotive force and voltage drop caused by the reactor 2, and by the capacitance of the capacitors 4a and 4b. If the reactor 2 and the capacitors 4a and 4b have a sufficiently large capacity (reactor 2: several mH, capacitors 4a and 4b: several hundred μF), the fluctuation of the DC voltage Vdc is small, so that components other than odd harmonics are unlikely to occur in the input current Is even if pulsation is superimposed on the DC current Idc. Therefore, a control that pulsates the load torque like the conventional control may be used while taking into account the amount of current generated other than odd harmonics. In this way, it is possible to achieve both a reduction in vibration of the compressor driven by the controlled motor and a reduction in harmonic current.

[0059] In a load connected to an electric motor, such as a general single rotary compressor or twin rotary compressor, in order to suppress an increase in vibration accompanying speed fluctuations due to pulsation of the load torque, the torque output by the electric motor is made to approximately match the pulsation of the load torque to prevent speed fluctuations. In addition, the output torque of the electric motor is suppressed so that power consumption does not become too large.

[0060] In this type of operation, the DC current Idc flowing through the load 6 fluctuates according to the load torque, and as mentioned above, the harmonic current of the input current Is deteriorates. In the case of an inverter that drives an electric motor, the load power is determined by the product of the mechanical angular frequency ω and the torque τ of the electric motor. Therefore, it is possible to control the DC current Idc so that it approaches a constant value by varying the mechanical angular frequency ω or the torque τ.

[0061] However, there is a concern that vibrations may increase if the pulsation of the DC current Idc flowing through the load 6 is suppressed to prevent a further increase in the power supply harmonic current. Generally, in the case of an electric motor, power fluctuations are proportional to the load torque and inversely proportional to the moment of inertia. If fluctuations in the DC current Idc flowing through the load 6 due to the load torque cannot be tolerated, this can be addressed by designing the moment of inertia to be larger. By taking such measures, it is possible to achieve both vibration suppression and power supply harmonic current suppression.

[0062] In addition, in the load of an electric motor such as a conventional general single rotary compressor or twin rotary compressor, in order to suppress an increase in vibration associated with speed fluctuations due to load torque pulsation, the torque output by the electric motor is made to approximately match the load torque pulsation to prevent speed fluctuations, and in addition, the output torque of the electric motor is suppressed to prevent excessive power consumption.

[0063] In this type of operation, the load current fluctuates according to the load torque, and as mentioned above, the harmonic current of the input current deteriorates. In the case of an inverter that drives an electric motor, the load power is determined by the product of the mechanical angular frequency ω and the torque τ. Therefore, it is possible to control the load current, or DC current Idc, so that it approaches a constant value by varying the mechanical angular frequency ω or the torque τ.

[0064] However, if the load current pulsation is suppressed so as not to further increase the power supply harmonic current, there is a concern that the vibration of the power supply current will increase. In the case of a general electric motor, the power fluctuation is proportional to the load torque and inversely proportional to the moment of inertia. When the amount of fluctuation in the load current due to the fluctuation in the load torque is unacceptable, it is possible to deal with this by designing the moment of inertia to be large. By taking such measures, it is possible to achieve both the suppression of vibration and the suppression of the power supply harmonic current. That is, the control unit 9 of the power conversion device 100 controls the load 6 via the inverter 5 so that the amount of fluctuation in the direct current Idc detected by the current detector 7 is within an allowable range, in other words, so that the amount of fluctuation in the direct current Idc is equal to or less than a specified value. The allowable range is a range in which the even-order harmonics affected by the generation of inter-order harmonics in the power supply current can be maintained at or below the limit value of the standard, and is determined in advance taking into consideration the inductance of the reactor 2, the capacitance of the capacitors 4a and 4b, etc.

[0065] The switching frequency of the inverter that drives the air conditioner compressor is higher than the 40th harmonic, which is the power supply harmonic regulation value of 2 kHz (when the power supply mains number is 50 Hz) or 2.4 kHz (when the power supply frequency is 60 Hz). The load current in the inverter is pulsating according to the switching frequency, but this pulsating current is unlikely to affect the power supply harmonics. Therefore, the pulsating component of the load current should be suppressed below the frequency of the 40th harmonic, which is the power supply harmonic regulation value.

[0066] Next, a hardware configuration of the control unit 9 included in the power conversion device 100 will be described. Fig. 9 is a diagram illustrating an example of a hardware configuration realizing the control unit 9 included in the power conversion device 100 according to the first embodiment. The control unit 9 of the power conversion device 100 is realized by, for example, a processor 91 and a memory 92 illustrated in Fig. 9.

[0067] The processor 91 is a CPU (Central Processing Unit, also referred to as a central processing unit, processing unit, arithmetic unit, microprocessor, microcomputer, processor, or DSP (Digital Signal Processor)). The memory 92 is a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory), or the like. The memory 92 stores a program for operating as the control unit 9 of the power conversion device 100, and the control unit 9 is realized by the processor 91 reading and executing the program. The above-mentioned program stored in the memory 92 may be provided to a user or the like in a state in which it is written in a storage medium such as a CD (Compact Disc)-ROM or a DVD (Digital Versatile Disc)-ROM, or may be provided via a network.

[0068] The control unit 9 can also be realized by a dedicated processing circuit, for example, a single circuit, a composite circuit, an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), or a circuit that combines these.

[0069] As described above, in the power conversion device 100 according to this embodiment, the control unit 9 controlling the inverter 5 defines a unit time as an integer multiple of one period of the carrier signal, divides the unit time into a plurality of current detection intervals based on the voltage vector, calculates the product of the time width of the current detection interval and the DC current Idc detected by the current detector 7 in the current detection interval for each of the plurality of current detection intervals, and controls the inverter 5 so that the total value of the calculated products per unit time is constant for each unit time. As a result, even when a shunt resistor or the like for which it is difficult to improve the resolution of current detection is used as the current detector 7, the DC current flowing between the smoothing unit 4 and the inverter 5 can be detected with high accuracy, and the control of the load 6 based on the instantaneous value of the current obtained at a predetermined timing can be performed with high accuracy.

[0070] Embodiment 2 In this embodiment, a description will be given of an apparatus that can be realized by applying the power conversion apparatus 100 described in embodiment 1. As an example, a heat pump apparatus that uses the power conversion apparatus 100 described in embodiment 1 will be described.

[0071] 10 is a diagram showing a configuration example of a heat pump device 200 according to the second embodiment. The heat pump device 200 according to the second embodiment includes the power conversion device 100 described in the first embodiment.

[0072] The heat pump device 200 also includes a refrigeration cycle having a configuration in which a four-way valve 902, a compressor 903 constituting the load 6 shown in FIG. 1, a heat exchanger 906, an expansion valve 908, and a heat exchanger 910 are attached via refrigerant piping 912.

[0073] Compressor 903 is provided with a compression mechanism 904 that compresses the refrigerant circulating inside refrigerant pipes 912, and an electric motor 905 that operates compression mechanism 904. Electric motor 905 constitutes load 6 shown in FIG. 1, and is driven by receiving power from power conversion device 100.

[0074] The heat pump device 200 having such a configuration can be used in, for example, an air conditioner, a heat pump water heater, a refrigerator, a freezer, and the like.

[0075] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or the embodiments may be combined with each other. Also, parts of the configurations may be omitted or modified without departing from the spirit of the invention. [Explanation of symbols]

[0076] 1 AC power source, 2 reactor, 3 rectifier, 4 smoothing section, 4a, 4b capacitors, 5 inverter, 6 load, 7 current detector, 8 voltage detector, 9 control section, 100 power conversion device, 200 heat pump device, 902 four-way valve, 903 compressor, 904 compression mechanism, 905 electric motor, 906, 910 heat exchanger, 908 expansion valve, 912 refrigerant piping.

Claims

1. A converter that rectifies AC power supplied from an AC power source; a smoothing unit connected to an output end of the converter and smoothing the power output by the converter; an inverter connected to both ends of the smoothing unit to generate AC power to be output to a load; a current detector that detects a current flowing between the smoothing unit and the inverter; A control unit that controls the inverter; Equipped with the control unit divides a unit time, which is a period of a predetermined length, into a plurality of current detection intervals based on a timing at which a voltage vector, which is a vector whose components are values ​​indicating the on / off states of each switching element constituting the inverter, changes, calculates, for each of the plurality of current detection intervals, a product of a time width of the current detection interval and a current value detected by the current detector in the current detection interval, and controls the inverter so that a total value of the calculated products per unit time is constant for each unit time. Power conversion equipment.

2. The unit time is an integer multiple of one period of a carrier signal used to generate a control signal to be output to the inverter. The power conversion device according to claim 1 .

3. The current detector is configured with a shunt resistor. The power conversion device according to claim 1 or 2.

4. A heat pump device comprising the power conversion device according to any one of claims 1 to 3.

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