Motor drive device

Abstract

To provide a motor drive device capable of improving electric efficiency.SOLUTION: A motor drive device includes: a converter configured to convert an input AC voltage into a first DC voltage; a capacitor connected in parallel to the converter and configured to smooth the first DC voltage; a three-phase inverter circuit configured to convert the first DC voltage into an AC voltage by switching operation of a semiconductor switching element connected in parallel to the capacitor and supply a voltage to each phase of a motor; and a single-phase inverter of a second DC voltage connected to each phase output end of the three-phase inverter circuit. The motor drive device superposes the output voltage of the single-phase inverter on the output voltage of the three-phase inverter circuit to supply power to a motor in which each phase voltage of an induced voltage peak value varies with rotation speed. The second DC voltage is set to satisfy a relation 1 / 2×first DC voltage<each phase voltage<(1 / 2×first DC voltage+second DC voltage) relative to each phase voltage of the induced voltage peak value at a predetermined rotation speed of the motor.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a motor drive device. [Background technology]

[0002] Conventionally, power conversion devices have been proposed that relax the limitations imposed by DC power supply voltages to supply a large AC output voltage to a load. For example, a power conversion device described in Patent Document 1 includes a three-level inverter connected to a common DC power supply and having switching units for three phases that can output three levels of output voltage per phase, and single-phase inverters connected to individual DC power supplies and provided corresponding to each phase of the three-level inverter. An output terminal of each phase switching unit of the three-level inverter is connected to one AC output terminal of the corresponding single-phase inverter, and the other AC output terminal of each of these single-phase inverters is connected to an AC motor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-166251 Summary of the Invention [Problem to be solved by the invention]

[0004] In permanent magnet synchronous motors, which have been widely used in recent years as motors connected to the output of inverters, an induced voltage occurs as the rotor rotates. If the induced voltage of the motor is controlled so that it does not exceed the DC voltage, losses in the inverter and motor increase, and electrical efficiency may decrease. An object of the present invention is to provide a motor drive device that can improve electrical efficiency. [Means for solving the problem]

[0005] The present invention, which was completed with the above object in mind, is a motor drive device comprising: a converter that converts an input AC voltage into a first DC voltage; a capacitor connected in parallel to the converter and smoothing the first DC voltage; a three-phase inverter circuit that converts the first DC voltage into an AC voltage by the switching operation of semiconductor switching elements connected in parallel to the capacitor and supplies the AC voltage to each phase of a motor; and single-phase inverters of a second DC voltage connected to each phase output terminal of the three-phase inverter circuit, wherein the output voltage of the single-phase inverter is superimposed on the output voltage of the three-phase inverter circuit to supply power to the motor in which each phase voltage of an induced voltage peak value varies with rotational speed, and the second DC voltage is set so that, with respect to each phase voltage of an induced voltage peak value at a predetermined rotational speed of the motor, 1 / 2 × first DC voltage < each phase voltage < (1 / 2 × first DC voltage + second DC voltage). Here, the second DC voltage is a fixed value that satisfies ½ × first DC voltage ≦ second DC voltage ≦ first DC voltage in a rotation speed region where each phase voltage of the induced voltage peak value of the motor is equal to or less than the first DC voltage, and may vary according to the rotation speed in a rotation speed region where the rotation speed exceeds the first DC voltage. The second DC voltage may be equal to or higher than the first DC voltage. The second DC voltage may be set to be equal to or higher than the first DC voltage by boost charging using a winding inductance of the motor during initial charging. Furthermore, the three-phase inverter circuit may perform the switching operation so that the voltages of both poles produce one pulse per half cycle of the output AC cycle resulting from the rotation of the motor when the rotation speed of the motor is such that each phase voltage of the induced voltage peak value is equal to or greater than 1 / 2 × the first DC voltage. Furthermore, the three-phase inverter circuit may perform the switching operation so that the voltages of both poles produce multiple pulses per half cycle of the output AC cycle resulting from the rotation of the motor when the rotation speed of the motor is such that each phase voltage of the induced voltage peak value is less than 1 / 2 × the first DC voltage. The single-phase inverter may also have four semiconductor switching elements, which may be divided into high-frequency switched switching elements and low-frequency switched switching elements, and the high-frequency switched switching elements and the low-frequency switched switching elements may be switched between. Also, an auxiliary converter may be provided between the first DC voltage and the second DC voltage. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide a motor drive device that can improve electrical efficiency. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a diagram illustrating an example of a schematic configuration of a motor drive device according to an embodiment of the present invention. [Figure 2] 1A is a diagram showing an example of a voltage waveform of each half-bridge inverter, FIG. 1B is a diagram showing an example of a voltage waveform of a single-phase inverter, and FIG. 1C is a diagram showing an example of a voltage waveform of an inverter unit. [Figure 3] 1 is a diagram illustrating an example of a charging section and a discharging section of a capacitor of a single-phase inverter. [Figure 4] 3A and 3B are diagrams illustrating examples of operational waveforms of switching elements that configure a half-bridge inverter and a single-phase inverter. [Figure 5] FIG. 1 is a diagram illustrating an example of a schematic configuration of a motor drive device according to a comparative example. [Figure 6] FIG. 10 is a diagram illustrating an example of the relationship between the rotation speed of a motor and the phase voltage of an induced voltage peak value. [Figure 7] 10 is a diagram illustrating an example of the relationship between each phase voltage of the induced voltage peak value of the motor and the switching operation of the three-phase inverter circuit. FIG. [Figure 8] FIG. 2 is a diagram illustrating an example of the relationship between the rotation speed of a motor and the DC voltage of a single-phase inverter. [Figure 9]10A and 10B are diagrams illustrating an example of a modified example of a charging section and a discharging section of a capacitor of a single-phase inverter. [Figure 10] FIG. 10 is a diagram illustrating an example of a schematic configuration of a motor drive device according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First Embodiment FIG. 1 is a diagram showing an example of a schematic configuration of a motor drive device 1 according to the first embodiment. The motor drive device 1 includes a motor 5 for a compressor 1000 used in, for example, a refrigerator (not shown) or an air conditioner (not shown), and a control device 10 that controls the driving of the motor 5. The motor drive device 1 also includes an AC power supply 20, a converter 30, a smoothing capacitor 40, and an inverter unit 50 that converts a DC voltage into an AC voltage and supplies the AC voltage to the motor 5.

[0009] The compressor 1000 can be exemplified as a device that performs a suction stroke to fill a cylinder with refrigerant, a compression stroke to compress the refrigerant in the cylinder, and a discharge stroke to release the compressed refrigerant to the outside of the compressor 1000. The compression mechanism of the compressor 1000 is not particularly limited, and can be exemplified as a rotary type, a reciprocating type, or a scroll type, for example. The motor 5 can be, for example, a permanent magnet synchronous motor (PMSM), but the type of the motor 5 is not particularly limited.

[0010] The converter 30 is connected to the AC power supply 20 and converts the AC voltage from the AC power supply 20 into a DC voltage. The smoothing capacitor 40 is connected to the DC output terminal of the converter 30, and smoothes the DC voltage Vdc that is the output of the converter 30. The smoothing capacitor 40 has two capacitors connected in series between the output terminals of the converter 30.

[0011] The inverter unit 50 includes a three-phase inverter circuit 60 configured with three sets of half-bridge inverters 61, 62, 63 each consisting of two series-connected switching elements, and single-phase inverters 71, 72, 73 connected in series to output lines of the three sets of half-bridge inverters 61, 62, 63. Hereinafter, when there is no need to distinguish between the single-phase inverters 71, 72, 73, they may be simply referred to as the "single-phase inverter 70." The three-phase inverter circuit 60 is a two-level inverter capable of outputting two levels. For example, the half-bridge inverter 61 has an upper switching element 61a and a lower switching element 61b.

[0012] 1, the single-phase inverter 70 has a full-bridge inverter consisting of four switching elements and a capacitor 75. For example, the single-phase inverter 71 has an upper left switching element 71a, a lower left switching element 71b, an upper right switching element 71c, and a lower right switching element 71d. The output voltages of the single-phase inverters 71, 72, 73 for each phase are superimposed on the output voltages of the half-bridge inverters 61, 62, 63 for each phase, and the sum of the output voltages of the half-bridge inverters 61, 62, 63 and the output voltages of the single-phase inverters 71, 72, 73 is output from the inverter unit 50.

[0013] (Control device 10) The control device 10 includes a setting unit 110 that sets a control value Vt, and an output unit 120 that outputs a signal to drive the motor 5 using the control value Vt set by the setting unit 110 and the rotational speed ωa of the motor 5. The control device 10 also includes a speed detection unit 130 that detects the rotational speed ωa of the motor 5, and a current detection unit 140 that detects the current Ia supplied to the motor 5.

[0014] The setting unit 110 can, for example, calculate the control value Vt based on a speed command value ωr output to the control device 10 from a higher-level control device, for example, a control device that comprehensively controls the operation of a refrigerator (not shown), and the actual rotation speed ωa of the motor 5 detected by the speed detection unit 130. The setting unit 110 outputs the calculated control value Vt to the output unit 120. In this way, the control device 10 is configured as a speed feedback control system so that the rotation speed ωa of the motor 5 coincides with the speed command value ωr.

[0015] The output unit 120 outputs a signal for driving the motor 5 based on the rotor position, and performs PWM chopping using a PWM waveform. PWM chopping means finely turning on and off the signal for driving the motor 5. By finely turning on and off the signal, it is possible to control the current supplied to each phase of the motor 5 according to the duty ratio, and therefore the output torque.

[0016] The speed detection unit 130 can detect the rotational speed ωa (rpm) of the motor 5 by grasping the change in the rotor position per unit time using the relationship between the waveforms of the induced voltages from the multiple phase windings of the motor 5 and the reference voltage.

[0017] The current detection unit 140 includes a shunt resistor (not shown) connected to the emitter terminal side of the lower switching element of the half-bridge inverters 61, 62, 63, and a current detection circuit (not shown) that detects a current (hereinafter, may be referred to as "current Ia") flowing through the shunt resistor. The current detection circuit can be, for example, configured with an amplifier circuit that detects the peak current flowing through the shunt resistor and a peak hold circuit.

[0018] In the motor driving device 1 configured as above, the control device 10 controls the inverter unit 50, thereby controlling the driving of the motor 5. Fig. 2(a) is a diagram showing an example of the voltage waveforms of the half-bridge inverters 61, 62, and 63. Fig. 2(b) is a diagram showing an example of the voltage waveform of the single-phase inverter 70. Fig. 2(c) is a diagram showing an example of the voltage waveform of the inverter unit 50. FIG. 3 is a diagram showing an example of a charging section and a discharging section of the capacitor 75 of the single-phase inverter 70. In FIG. FIG. 4 is a diagram showing an example of operating waveforms of the switching elements that configure the half-bridge inverter 61 and the single-phase inverter 71. In FIG.

[0019] The control device 10 controls the driving of the three-phase inverter circuit 60 so that each half-bridge inverter 61, 62, 63 of the three-phase inverter circuit 60 outputs a voltage of each polarity as one pulse per half cycle, with the midpoint of the smoothing capacitor 40 set to ½×Vdc. Hereinafter, the voltage pulse of each polarity, with this pulse width as half a cycle, may be referred to as a basic voltage pulse. More specifically, as shown in FIG. 2(a), the control device 10 controls the driving of the three-phase inverter circuit 60 so that each half-bridge inverter 61, 62, 63 of the three-phase inverter circuit 60 outputs a basic voltage pulse of ±½×Vdc per half cycle, with the midpoint of the smoothing capacitor 40 set to ½×Vdc, relative to the control value Vt.

[0020] The control device 10 also performs high-frequency PWM control on the single-phase inverter 70 so that the output voltage of the single-phase inverter 70 is an output voltage that compensates for the difference between the output voltages of the half-bridge inverters 61, 62, and 63 and the control value Vt. As shown in FIG. 2(b), the control device 10 controls the driving of the single-phase inverter 70 so that the output voltage of the single-phase inverter 70 has the opposite polarity to the output voltage of the inverter unit 50 in regions where the phase is close to 0 and 180 degrees within a half cycle, and the same polarity as the output voltage of the inverter unit 50 in regions where the phase is close to 90 degrees. The control device 10 controls the driving of the single-phase inverter 70 so that the DC voltage of the single-phase inverter 70 is set to DC voltage Vs, and the output voltage of the single-phase inverter 70 is set to -Vs when the polarity is opposite to that of the output voltage of the inverter unit 50, or to Vs when the polarity is the same as that of the output voltage of the inverter unit 50, and has a frequency higher than the frequency of the fundamental voltage pulse. The DC voltage Vs can be exemplified as 1 / 2×Vdc, and FIGS. 2(b) and 2(c) show an example in which the DC voltage Vs is 1 / 2×Vdc.

[0021] In other words, the control device 10 controls the driving of the three-phase inverter circuit 60 and the single-phase inverter 70 so that the voltage waveform of the inverter unit 50 becomes the waveform shown in FIG. 2(c). As a result, as shown in FIG. 3, in the region where the phase is close to 0 degrees and 180 degrees, the single-phase inverter 70 operates to charge the capacitor 75, and conversely, in the region where the phase is close to 90 degrees, the single-phase inverter 70 operates to discharge the capacitor 75.

[0022] Furthermore, the control device 10 switches, every half cycle, between the switching elements on the left side and the switching elements on the right side of the single-phase inverter 70. More specifically, as shown in Fig. 4, the control device 10 first PWM chops the upper right switching element 71c and the lower right switching element 71d, and after half a cycle, switches to PWM chopping the upper left switching element 71a and the lower left switching element 71b.

[0023] Then, in the motor drive device 1 described above, when Vs = 1 / 2 × Vdc, by the control device 10 controlling the driving of the inverter unit 50 as described above, the total voltage of the output voltages of the half-bridge inverters 61, 62, 63 of each phase and the output voltages of the single-phase inverters 71, 72, 73 of each phase becomes as shown in Fig. 2(c).

[0024] In the motor drive device 1 configured as described above, the Vs and the number of turns of the motor 5 are set so that the phase voltage Vm of the induced voltage peak value at a predetermined rotational speed (rpm) of the motor 5 satisfies 1 / 2 × Vdc < Vm < 1 / 2 × Vdc + Vs. This is due to the following reasons. Fig. 5 is a diagram showing an example of the schematic configuration of a motor drive device 600 according to a comparative example. Fig. 6 is a diagram showing an example of the relationship between the rotational speed of the motor 5 in the motor drive device 1 and the phase voltage Vr of the induced voltage peak value. Fig. 6 also shows an example of the relationship between the rotational speed of the motor 605 in the motor drive device 600 according to the comparative example and the phase voltage Vrc of the induced voltage peak value.

[0025] As a motor drive device 600 according to the comparative example, consider a device different from the motor drive device 1 in which the inverter unit 650 corresponding to the inverter unit 50 and the motor 605 corresponding to the motor 5 are different. And the inverter unit 650 is different from the inverter unit 50 in that it does not include the single-phase inverters 71, 72, 73.

[0026] The maximum output voltage of the inverter section 650 of the motor drive device 600 is Vdc, while the maximum output voltage of the inverter section 50 of the motor drive device 1 is 1 / 2×Vdc + Vs. The motor 5 can be driven within the range where the phase voltage Vr of each peak value of the induced voltage does not exceed the output voltage of the inverter section 50. Similarly, the motor 605 can be driven within the range where the phase voltage Vrc of each peak value of the induced voltage of the motor 605 does not exceed the output voltage of the inverter section 650. Therefore, it is possible to increase the phase voltage Vr of each peak value of the induced voltage of the motor 5 higher than the phase voltage Vrc of each peak value of the induced voltage of the motor 605. As a result, the number of turns of the motor 5 can be made more than the number of turns of the motor 605. Even in the rotational speed region (the shaded portion in FIG. 6) where the phase voltage Vrc of each peak value of the induced voltage of the motor 605 becomes 1 / 2 or more of the maximum output voltage Vdc of the inverter section 650 and the motor 605 cannot be driven, the motor 5 can be driven because the maximum output voltage of the inverter section 50 is 1 / 2×Vdc + Vs. In other words, it is possible to set the number of turns of the motor 5 so that the phase voltage Vm of each peak value of the induced voltage at a predetermined rotational speed of the motor 5 (for example, the rotational speed in the rotational speed region of the shaded portion in FIG. 6) satisfies 1 / 2×Vdc < Vm < 1 / 2×Vdc + Vs. Thus, in the motor drive device 1, the number of turns of the motor 5 can be made more than the number of turns of the motor 605, and the efficiency of the motor 5 can be made higher than the efficiency of the motor 605.

[0027] Also, in the motor drive device 600 according to the comparative example, it is possible to increase the rotational speed by reducing the torque of the motor 605 by performing field-weakening control, but the loss increases due to the field-weakening current. On the other hand, in the motor drive device 1 according to the present embodiment, since it is not necessary to perform field-weakening control even in the rotational speed region of the shaded portion in FIG. 6, the loss due to the field-weakening current can be prevented. However, in the motor drive device 1 according to the present embodiment, field-weakening control may be performed in the rotational speed region where the phase voltage Vr of each peak value of the induced voltage of the motor 5 becomes 1 / 2×Vdc + Vs or more, which is the maximum output voltage of the inverter section 50.

[0028] As described above, the motor drive device 1 includes a converter 30 that converts an input AC voltage into a DC voltage Vdc (an example of the first DC voltage), and a smoothing capacitor 40 (an example of a capacitor) that is connected in parallel with the converter 30 and smooths the DC voltage Vdc. Further, the motor drive device 1 converts the DC voltage Vdc into an AC voltage by the switching operation of a semiconductor switching element connected in parallel with the smoothing capacitor 40, and supplies voltage to each phase of the motor 5. The motor drive device 1 also includes a three-phase inverter circuit 60 and a single-phase inverter 70 connected to each phase output terminal of the three-phase inverter circuit 60, where the single-phase inverter 70 outputs a DC voltage Vs (an example of the second DC voltage). Then, the motor drive device 1 superimposes the output voltage of the single-phase inverter 70 on the output voltage of the three-phase inverter circuit 60, and supplies power to the motor 5 in which each phase voltage Vr varies with the rotation speed and the induced voltage peak value. The DC voltage Vs is set such that 1 / 2×Vdc<Vm<(1 / 2×Vdc+Vs) with respect to each phase voltage Vm of the induced voltage peak value at a predetermined rotation speed of the motor 5.

[0029] In the motor drive device 1 configured as described above, the number of turns of the motor 5 can be made larger than, for example, the number of turns of the motor 605 in the motor drive device 600 according to the comparative example. Therefore, the efficiency of the motor 5 can be increased compared to the efficiency of the motor 605. Further, for example, in the rotation speed range (e.g., the shaded portion in FIG. 6) where field weakening control needs to be performed in the motor drive device 600 according to the comparative example, field weakening control is not required. Therefore, losses due to the field weakening current can be prevented.

[0030] In other words, in the motor drive device 1, the phase voltage Vm of the induced voltage peak value at a predetermined rotation speed of the motor 5 is preferably set to satisfy the relationship 1.35×Vin≦Vm≦2.23×Vin with respect to the input AC voltage effective value Vin. The minimum value of the DC voltage Vdc converted by the converter 30 from the input AC voltage effective value Vin is 3×√2×Vin / π≈1.35×Vin. Furthermore, when a boost PFC circuit is connected to the input, the minimum boost ratio is approximately 1.1, and assuming that the DC voltage Vdc = Vc, the maximum value of the DC voltage Vdc is 1.35×Vin×1.1×1.5=2.23×Vin.

[0031] Furthermore, in the motor drive device 1, the control device 10 controls the driving of the three-phase inverter circuit 60 so that the three-phase inverter circuit 60 outputs one pulse of voltage at each half cycle, with the midpoint of the smoothing capacitor 40 set to ½×Vdc. In other words, the three-phase inverter circuit 60 performs switching operation so that the voltage at each half cycle of the output AC voltage resulting from the rotation of the motor 5 produces one pulse. This allows the drive frequency of the three-phase inverter circuit 60 to be reduced, thereby reducing switching loss.

[0032] FIG. 7 is a diagram showing an example of the relationship between the phase voltage Vr of the induced voltage peak value of the motor 5 and the number of switching operations of the three-phase inverter circuit 60. In FIG. The control device 10 may control the drive of the three-phase inverter circuit 60 to output one pulse of voltage at each polarity per half cycle only when the rotation speed of the motor 5 is such that the phase voltage Vr of the induced voltage peak value is equal to or greater than ½ × Vdc. In other words, the control device 10 may perform switching operations to output multiple pulses of voltage at each polarity per half cycle of the output AC cycle resulting from the rotation of the motor 5 when the rotation speed of the motor 5 is such that the phase voltage Vr of the induced voltage peak value is less than ½ × Vdc. This allows the Vs of the single-phase inverter 70 to be controlled to a constant value, thereby stabilizing the control of the single-phase inverter 70. For example, as shown in FIG. 7, when the phase voltage Vr of the induced voltage peak value of the motor 5 is less than ½ × Vdc, the number of switching operations may be increased as the phase voltage Vr decreases.

[0033] Furthermore, in the motor drive device 1, the single-phase inverter 70 (for example, single-phase inverter 71) has four semiconductor switching elements (for example, switching elements 71a, 71b, 71c, and 71d). The four semiconductor switching elements are divided into switching elements (for example, switching elements 71c and 71d) that are switched at high frequency (in other words, PWM chopping) and switching elements (for example, switching elements 71a and 71b) that are switched at low frequency, and the switching elements that are switched at high frequency and the switching elements that are switched at low frequency are switched. This makes it possible to equalize the losses of the four switching elements and suppress localized heat generation.

[0034] The control device 10 may perform high-frequency switching on all four switching elements without dividing the four switching elements into switching elements that are switched at high frequencies and switching elements that are switched at low frequencies.

[0035] Furthermore, in the motor drive device 1 described above, the value of the DC voltage Vs is not particularly limited. For example, the DC voltage Vs may be equal to or greater than Vdc. This allows the inverter unit 50 to output a higher voltage, eliminating the need for field-weakening control. Furthermore, since the voltage applied to the motor 5 can be increased, the current can be reduced, thereby reducing conduction loss in the inverter unit 50. If the DC voltage Vs is equal to or higher than Vdc, it is preferable that the initial charging be performed by boost charging using the winding inductance of the motor 5. This eliminates the need for a separate charging means for boosting the voltage, allowing the motor drive device 1 to have a simple configuration.

[0036] Furthermore, the single-phase inverter 70 may be configured as a multilevel circuit. When the single-phase inverter 70 is configured as a multilevel circuit, the DC voltage Vs should be set to the maximum voltage that the single-phase inverter 70 can output. This allows the voltage applied to the motor 5 to be increased, thereby reducing the current and the conduction loss of the inverter unit 50.

[0037] <Modification> FIG. 8 is a diagram showing an example of the relationship between the rotation speed of the motor 5 and the DC voltage Vs of the single-phase inverter 70. In the motor drive device 1 described above, the DC voltage Vs is a fixed value. However, the DC voltage Vs may be set to a fixed value satisfying 1 / 2×Vdc≦Vs≦Vdc up to a rotational speed at which the phase voltage Vr of the induced voltage peak value of the motor 5 exceeds 1 / 2×Vdc+Vs, and Vs may be varied so as to satisfy Vr<1 / 2×Vdc+Vs when the rotational speed at which the phase voltage Vr of the induced voltage peak value of the motor 5 exceeds Vdc. This allows the three-phase inverter circuit 60 to output approximately one pulse per period of the electrical angular frequency while lowering the DC voltage Vs, thereby reducing switching loss in the three-phase inverter circuit 60 and the single-phase inverter 70. Furthermore, the DC voltage Vs can be kept at the minimum necessary voltage, thereby reducing loss in the single-phase inverter 70.

[0038] An example of a means for varying the DC voltage Vs is to adjust the output phase of the three-phase inverter circuit 60 to control the charging and discharging sections of the capacitor 75. FIG. 9 is a diagram showing an example of a modified example of the charging section and the discharging section of the capacitor 75 of the single-phase inverter 70. In FIG. As shown in Fig. 9, the control device 10 causes the phase of the output pulses of each half-bridge inverter 61, 62, 63 relative to the control value Vt to be different from the phase shown in Fig. 2(a). In the phase shown in Fig. 2(a), the control device 10 controls the driving of the single-phase inverter 70 so that the period during which the single-phase inverter 70 performs the operation of charging the capacitor 75 and the period during which the single-phase inverter 70 performs the operation of discharging the capacitor 75 are the same. In contrast, as shown in Fig. 9, the control device 10 controls the driving of the single-phase inverter 70 so that the period during which the single-phase inverter 70 performs the operation of charging the capacitor 75 is longer than the period during which the single-phase inverter 70 performs the operation of discharging the capacitor 75.

[0039] That is, in the motor drive device 1 according to the modified example, the DC voltage Vs is a fixed value that satisfies 1 / 2×Vdc≦Vs≦Vdc in the rotational speed range where the phase voltage Vr of the induced voltage peak value of the motor 5 is equal to or less than the DC voltage Vdc, and varies according to the rotational speed in the rotational speed range where the voltage Vr exceeds Vdc. This allows the DC voltage Vs to be set to a value that corresponds to the operating state, thereby suppressing switching loss in the single-phase inverter 70. Furthermore, by satisfying 1 / 2×Vdc≦Vs≦Vdc, a separate DC power supply for the single-phase inverter 70 is not required, which means that the motor drive device 1 can be configured without an external power supply, thereby simplifying the structure.

[0040] Second Embodiment FIG. 10 is a diagram showing an example of a schematic configuration of a motor drive device 2 according to the second embodiment. The motor drive device 2 according to the second embodiment differs from the motor drive device 1 according to the first embodiment in that an auxiliary converter 200 is provided between the DC voltage Vdc and the DC voltage Vs of the single-phase inverter 70. The differences from the first embodiment will be described below. The same components in the first and second embodiments are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0041] The auxiliary converter 200 can be, for example, a so-called isolated DC-DC converter that is provided between the smoothing capacitor 40 and the capacitor 75 of the single-phase inverter 70 and exchanges power between the DC voltage Vdc and the DC voltage Vs of the single-phase inverter 70. By including the auxiliary converter 200 in the motor drive device 2, it becomes possible to control the DC voltage Vs of the single-phase inverter 70 to be constant. [Explanation of symbols]

[0042] 1, 2... motor drive device, 5... motor, 10... control device, 30... converter, 40... smoothing capacitor, 50... inverter section, 60... three-phase inverter circuit, 61, 62, 63... half-bridge inverter, 70, 71, 72, 73... single-phase inverter, 200... auxiliary converter

Claims

1. a converter for converting an input AC voltage into a first DC voltage; a capacitor connected in parallel with the converter to smooth the first DC voltage; a three-phase inverter circuit that converts the first DC voltage into an AC voltage by switching operations of semiconductor switching elements connected in parallel to the capacitor, and supplies the AC voltage to each phase of a motor; a single-phase inverter of a second DC voltage connected to each phase output terminal of the three-phase inverter circuit; Equipped with A motor drive device that supplies power to the motor in which each phase voltage of an induced voltage peak value varies with rotation speed by superimposing an output voltage of the single-phase inverter on an output voltage of the three-phase inverter circuit, The second DC voltage is, with respect to each phase voltage of an induced voltage peak value at a predetermined rotation speed of the motor, The voltages are set so as to satisfy the relationship: 1 / 2 × first DC voltage < each phase voltage < (1 / 2 × first DC voltage + second DC voltage). Motor drive device.

2. the second DC voltage is a fixed value that satisfies ½×first DC voltage≦second DC voltage≦first DC voltage in a rotation speed region where each phase voltage of an induced voltage peak value of the motor is equal to or less than the first DC voltage, and that varies according to the rotation speed in a rotation speed region where the second DC voltage exceeds the first DC voltage. The motor drive device according to claim 1 .

3. the second DC voltage is equal to or greater than the first DC voltage; The motor drive device according to claim 1 .

4. the second DC voltage is set to be equal to or higher than the first DC voltage by boost charging using a winding inductance of the motor during initial charging; The motor drive device according to claim 3 .

5. the three-phase inverter circuit performs the switching operation so that voltages of both poles become one pulse per half cycle of an output AC cycle resulting from rotation of the motor when the rotation speed of the motor is such that each phase voltage of an induced voltage peak value of the motor is equal to or greater than 1 / 2 × the first DC voltage; The motor drive device according to claim 1 .

6. the three-phase inverter circuit performs the switching operation so that voltages of both poles become multiple pulses per half cycle of an output AC cycle resulting from rotation of the motor when the rotation speed of the motor is such that each phase voltage of an induced voltage peak value of the motor is less than 1 / 2 × the first DC voltage; The motor drive device according to claim 1 .

7. The single-phase inverter has four semiconductor switching elements, The four semiconductor switching elements are divided into switching elements that are switched at high frequencies and switching elements that are switched at low frequencies, and the switching elements that are switched at high frequencies and the switching elements that are switched at low frequencies are switched. The motor drive device according to claim 1 .

8. an auxiliary converter is provided between the first DC voltage and the second DC voltage; The motor drive device according to claim 1 .

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