Power conversion device

Abstract

An inverter (3) having a driving motor (5), a first capacitor (2c), and a second capacitor having a plurality of capacitors (2a, 2b) connected in series with each other are connected in parallel to a DC power source (1), a switching circuit (4) having a plurality of switching elements (4a-4f) is connected between the inverter (3) and the second capacitor (2a, 2b), and a control circuit (6) for controlling the inverter (3) and the switching circuit (4) is provided. The control circuit (6) performs control for switching the switching circuit (4) according to a torque command and a rotational speed command for the motor (5), supplying current to the inverter (3) from the second capacitor (4a, 4b) in a 3-level operation, and supplying current to the inverter (3) from both the first capacitor (2c) and the second capacitor (2a, 2b) in a 2-level operation.

Description

TECHNICAL FIELD

[0001] The present application relates to a power conversion device. BACKGROUND

[0002] As a conventional power conversion device, for example, there is an example of a circuit structure disclosed in Patent Literature 1 described below. In this conventional technology, in order to reduce the loss of an inverter, the operation of the inverter is set to be able to select and switch between 2-level operation and 3-level operation.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT LITERATURE

[0005] Patent Literature 1: Japanese Patent No. 5386640 SUMMARY

[0006] Thus, in the conventional inverter device, in order to reduce only the inverter, there is an example in which the inverter is operated in a manner of selecting and switching between 2-level operation and 3-level operation, but reduction of the total loss of a load (for example, a motor) connected to the inverter is not sufficiently considered. Therefore, there is a problem in that even if the loss of the inverter can be reduced, the high-frequency iron loss of the motor increases, and the total loss of the device as a whole increases.

[0007] In addition, in the case of an inverter device for an electric vehicle, in order to extend the driving distance, in a driving pattern defined by a fuel efficiency standard called WLTC (Worldwide harmonized Light duty driving Test Cycle), it is necessary to reduce the total loss of the inverter and the motor.

[0008] When the driving pattern of the WLTC is mapped on the characteristics (hereinafter referred to as NT characteristics) between the rotational speed and the torque of the motor, in the driving pattern of the WLTC, the mode in which the inverter and the motor operate in a low current region is high, so it is preferable to minimize the total loss by performing 3-level operation to reduce both the switching loss of the inverter and the high-order harmonic iron loss of the motor in this region, and to achieve miniaturization of the inverter.

[0009] Further, in the conventional power conversion device, even in the case of performing 2-level operation, only a capacitor connected in series with 3-level operation operates, so there is a problem in that the size of the capacitor becomes large, hindering miniaturization.

[0010] The present application discloses a technology for solving the above problems, and aims to provide a power conversion device capable of reducing the total loss of the inverter and the motor, and achieving miniaturization of the inverter.

[0011] The power conversion device disclosed in the present application has an inverter that drives a motor, a first capacitor, and a second capacitor having a plurality of capacitors connected in series with each other, which are connected in parallel to a direct-current power supply, a switching circuit having a plurality of switching elements is connected between the inverter and the second capacitor, and the power conversion device has a control circuit that controls the inverter and the switching circuit, the control circuit performs control that switches the switching circuit according to a torque command and a rotational speed command for the motor, supplies current to the inverter from the second capacitor at the time of 3-level operation, and supplies current to the inverter from both the first capacitor and the second capacitor at the time of 2-level operation.

[0012] According to the power conversion device disclosed in the present application, the inverter and the motor are controlled according to a predetermined operation map of the inverter, so that the total loss of the inverter and the motor is minimized according to the voltage of the direct-current power supply of the inverter and the torque and the rotational speed of the motor, and thus the total loss of the inverter and the motor can be reduced.

[0013] In addition, by limiting the region of 3-level operation to a low current region equivalent to the WLTC operation mode at the time of affecting the running distance of an electric vehicle, the size of the capacitor can be suppressed from increasing, and the inverter can be miniaturized. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a configuration diagram of the power conversion device according to Embodiment 1 of the present application.

[0015] Figure 2 is a diagram that explains a current path at the time of 3-level operation according to Embodiment 1 of the present application.

[0016] Figure 3 is a diagram that explains another current path at the time of 3-level operation according to Embodiment 1 of the present application.

[0017] Figure 4 is a diagram that explains further another current path at the time of 3-level operation according to Embodiment 1 of the present application.

[0018] Figure 5 is a waveform diagram that shows a relationship between a change in switching voltage of the inverter and a change in phase current of the motor in conjunction with a change in current path at the time of 3-level operation according to Embodiment 1 of the present application.

[0019] Figure 6 is a circuit diagram that shows a configuration of a necessary capacitor in a case where the inverter performs only 2-level operation.

[0020] Figure 7is a circuit diagram showing the structure of a capacitor necessary in the case where the inverter is made to act only at 3 levels.

[0021] Figure 8 is a circuit diagram showing the structure of a capacitor necessary in the case where the inverter is switched to act at 2 levels and 3 levels (corresponding to the present application).

[0022] Figure 9 is a waveform chart showing the temporal change of the phase current of the motor in the case where the inverter is made to act at 2 levels in Embodiment 1 of the present application.

[0023] Figure 10 is a waveform chart showing the temporal change of the phase current of the motor in the case where the inverter is made to act at 3 levels in Embodiment 1 of the present application.

[0024] Figure 11 is a chart explaining the difference in the motor loss at the time of 2-level actuation and at the time of 3-level actuation in Embodiment 1 of the present application.

[0025] Figure 12 is a chart explaining the difference in the inverter loss at the time of 2-level actuation and at the time of 3-level actuation in Embodiment 1 of the present application.

[0026] Figure 13 is a flowchart showing the processing procedure for making an actuation map for minimizing the total loss of the device as a whole in which the motor loss and the inverter loss are combined in advance in Embodiment 1 of the present application.

[0027] Figure 14 is an explanatory diagram of one example of an actuation map made in accordance with the flowchart of Figure 13 in Embodiment 1 of the present application.

[0028] Figure 15 is a characteristic chart schematically showing the relationship between the total loss of the inverter and the motor (left vertical axis) and the size of the inverter (right vertical axis) with respect to the inverter current (horizontal axis) in Embodiment 1 of the present application.

[0029] Figure 16 is a block diagram of the power conversion device according to Embodiment 2 of the present application.

[0030] (Symbol Explanation)

[0031] 1: DC power supply; 2a, 2b: 2nd capacitor; 2c: 1st capacitor; 3: inverter; 3a to 3f: switching element; 4: switching circuit; 4a to 4f: switching element; 5: motor; 6: control circuit. DETAILED DESCRIPTION

[0032] Embodiment 1

[0033] Figure 1 is a configuration diagram of a power conversion device according to Embodiment 1 of the present application.

[0034] The power conversion device according to Embodiment 1 has an inverter 3 for driving a motor 5, and a first capacitor 2c and a pair of capacitors 2a, 2b (hereinafter, these capacitors 2a, 2b are referred to as second capacitors) connected in series with each other are connected in parallel with respect to a direct current power source 1. Further, the inverter 3 is connected to the direct current power source 1.

[0035] The inverter 3 includes, for example, six switching elements 3a to 3f composed of IGBTs (Insulated Gate Bipolar Transistors) and diodes connected in anti-parallel.

[0036] Further, a switching circuit 4 including six switching elements 4a to 4f composed of a pair of opposing reverse blocking IGBTs is connected between the connection points of the three alternating current outputs of the inverter 3 and the second capacitors 2a, 2b.

[0037] Specifically, a pair of switching elements 4a, 4b is connected to the midpoint of the second capacitors 2a, 2b and the midpoint of a pair of switching elements 3a, 3b constituting a branch of the inverter 3, and the other pair of switching elements 4c, 4d is connected to the midpoint of the second capacitors 2a, 2b and the midpoint of a pair of switching elements 3c, 3d constituting a branch of the inverter 3, and further, the other pair of switching elements 4e, 4f is connected to the midpoint of the second capacitors 2a, 2b and the midpoint of a pair of switching elements 3e, 3f constituting a branch of the inverter 3. Further, the alternating current outputs of the inverter 3 are connected to the motor 5, respectively.

[0038] The control circuit 6 calculates the total loss of each loss of the inverter 3 and the motor 5 in total, with respect to each of the 2-level operation and the 3-level operation, and takes in operation information such as the voltage of the direct current power source 1, each voltage of the second capacitors 2a, 2b and the current flowing therethrough, and the temperature of each of the inverter 3 and the switching circuit 4 from a sensor provided outside, not shown. Further, the information of the phase current and the rotational position of the motor 5 is taken in, and further, the information of the torque command and the rotational speed command (information of the NT characteristic) provided from a controller of a higher level, not shown, is taken in.

[0039] Then, the control circuit 6 selects and switches the 2-level operation and the 3-level operation in such a manner that the total loss of the inverter 3 and the motor 5 becomes minimum, based on the various information taken in as described above, and beforehand creates an operation map in which information of the carrier frequency of the inverter 3 for reducing the loss depending on the operation state of the inverter 3 and the motor 5 and a region definition for selecting whether to perform the 2-level operation or the 3-level operation are defined on the NT characteristic of the motor 5 (refer to the operation map described later Figure 14 ), and registers the information of the operation map to a storage device not shown.

[0040] When driving control is performed on the motor 5, the control circuit 6 refers to the operation map, decides the carrier frequency of the inverter 3 and whether to perform the 2-level operation or the 3-level operation, based on the information of the command values of the torque and the rotational speed of the motor 5, the temperature information, and the information of the current flowing in the 2nd capacitors 2a, 2b, and controls the operation of the inverter 3 and the switching circuit 4.

[0041] Figures 2-4 The current path of the 1-phase quantity flowing in the 1st and 2nd capacitors 2a to 2c when the inverter 3 performs the 3-level operation is shown (indicated by a thick solid line in the figure). Also, Figure 5 The relationship of the change of the switching voltage of the inverter 3 and the phase current of the 1-phase quantity of the motor 5 in this case is shown.

[0042] When the current phase is low and the phase current of the motor 5 is small, the switching circuit 4 becomes on and the current flows in the 2nd capacitors 2a, 2b (indicated by the states of the symbols (A), (B) of Figure 2 , Figure 3 , and Figure 5 ). On the other hand, when the current phase is high and the phase current of the motor 5 is large, the switching circuit 4 becomes off and the current flows in both the 1st capacitor 2c and the 2nd capacitors 2a, 2b (indicated by the state of the symbol (C) of Figure 4 , Figure 5 ).

[0043] When the command value of the motor phase current of the inverter 3 becomes large in accordance with the torque command and the rotational speed command to the motor 5, the inverter 3 is switched from the 3-level operation to the 2-level operation by the control of the control circuit 6. In this case, the switching circuit 4 becomes off and the current flows in both the 1st capacitor 2c and the 2nd capacitors 2a, 2b. That is, the current path shown in Figure 4 is obtained. When switched to this 2-level operation, the current flowing in each of the capacitors 2a to 2c is ideally divided in the capacity ratio of the capacity of the 1st capacitor 2c and the series capacity of the 2nd capacitors 2a, 2b.

[0044] Generally, the size of each capacitor 2a to 2c is proportional to its capacitance value, which affects the torque ripple of the motor 5 or the withstand voltage of the inverter 3, so it needs to be set to a capacitance value for suppressing voltage ripple. In addition, for the self-heating of the capacitor due to current ripple, in order to reduce the internal resistance to be below the allowable temperature, the capacitance value needs to be defined.

[0045] Figure 6 is a circuit diagram showing the structure of the capacitors necessary when the inverter 3 performs only 2-level operation, Figure 7 is a circuit diagram showing the structure of the capacitors necessary when performing only 3-level operation, Figure 8 is a circuit diagram showing the structure of the capacitors necessary when performing 2-level operation and 3-level operation alternately as in the present application.

[0046] Here, for simplicity of explanation, the capacitor capacity necessary for flowing the maximum current in the inverter 3 is set to 4C.

[0047] When performing only 2-level operation, as shown in Figure 6 , the first capacitor 2c is connected in parallel to the direct current power supply 1, so the capacitance of the first capacitor 2c is 4C. In addition, when performing only 3-level operation, as shown in Figure 7 , the second capacitors 2a, 2b are connected in parallel to the direct current power supply 1, so the capacitance of each capacitor 2a, 2b needs to be 8C and the total becomes 16C. In addition, when performing 2-level operation and 3-level operation alternately as in the present application, as shown in Figure 8 , the first capacitor 2c and the second capacitors 2a, 2b are connected in parallel to the direct current power supply 1, so the capacitance of the first capacitor 2c needs to be 3.5C, the capacitance of each of the second capacitors 2a, 2b needs to be 1C, and the total becomes 5.5C.

[0048] Therefore, when the capacitor capacity necessary for flowing the maximum current is set to 4C, and assuming that the volume is proportional to the capacitance, in the 3-level structure Figure 7 , the increase ratio of the size from the 2-level structure Figure 6 is 4.0 times, and in contrast, in the structure Figure 8 of the present application, the increase ratio of the size from the 2-level structure Figure 6 becomes 1.375 times. That is, even if the structure of the present application that can alternately perform 2-level operation and 3-level operation is set, it is possible to suppress the increase in the size of the capacitor.

[0049] Figure 9 is a waveform diagram showing the temporal change of the phase current Im_2lv of the motor 5 when the inverter performs 2-level operation, Figure 10is a waveform chart showing the phase current Im_3lv of the motor 5 in the case where the inverter is made to act with 3 levels. Furthermore, in these charts, the change in the switching voltage of the inverter 3 is also shown.

[0050] In comparing the waveform of the phase current Im_2lv at the time of 2-level action with the waveform of the phase current Im_3lv at the time of 3-level action, the current waveform at the time of 2-level action is distorted compared to the current waveform at the time of 3-level action. The reason for this is that, with respect to the voltage used to generate the current, in the case of 2-level action, the current is controlled with a voltage that is doubled (Vsw = 2 x Vsw / 2) compared to the case of 3-level action. As a result, the higher harmonic iron loss of the motor 5 is greater at the time of 2-level action compared to the time of 3-level action. In addition, the higher the rotational speed of the motor 5, the more pronounced this tendency is. That is, when the rotational speed of the motor 5 is high, the number of switching times within one period of the phase current decreases, so the distortion increases, and the higher harmonic iron loss increases at the time of 2-level action compared to the time of 3-level action.

[0051] Figure 11 shows the motor loss with respect to the time of 2-level action and the time of 3-level action, and Figure 12 shows the inverter loss with respect to the time of 2-level action and the time of 3-level action.

[0052] Here, the motor loss includes the iron loss, copper loss, and mechanical loss that accompany the driving of the motor 5, and in particular, the influence of the high-frequency iron loss that accompanies the change in the phase current is large. In addition, the inverter loss includes the switching loss, conduction loss, and recovery loss of the voltage recovery that accompany the turn-on / turn-off of each switching element 3a to 3f that accompanies the driving of the inverter.

[0053] As is clear from these charts, with respect to the motor loss ( Figure 11 ), as described above, the higher harmonic iron loss of the motor 5 is smaller and the motor loss is reduced at the time of 3-level action compared to the time of 2-level action. The amount of reduction in the loss at the time of switching from the time of 2-level action to the time of 3-level action in this case is set as ΔPm (with the vertical axis set as positive toward the bottom).

[0054] On the other hand, with respect to the inverter loss ( Figure 12 ), it cannot be said that the loss is reduced at the time of 3-level action compared to the time of 2-level action, and depending on the operating state of the inverter 3, by switching from the time of 2-level action to the time of 3-level action, not only is there a case where the inverter loss is reduced ( Figure 12 , which is shown by 3-level action A), but there is also a case where it is instead increased ( Figure 12 , which is shown by 3-level action B). The amount of reduction in the loss at the time of switching from the time of 2-level action to the time of 3-level action in this case is set as ΔPi. Figure 12The increase in the loss at the time of switching from the 2-level operation to the 3-level operation in the case shown in the 3-level operation B of FIG. 3 is set as ΔPinv (the longitudinal axis is set as positive downward).

[0055] Therefore, in order to minimize the total loss of the device in which the motor loss and the inverter loss are combined, it is sufficient to maximize the reduction effect ΔP (= ΔPm + ΔPinv) of the total loss in which they are combined.

[0056] Figure 13 is a flowchart showing a process for preparing an operation map in which the operation of the inverter 3 is registered to the control circuit 6 in advance in order to minimize the total loss of the device in which the motor loss and the inverter loss are combined.

[0057] First, the necessary torque and rotational speed command values are set for the motor 5 (step 1), and the losses generated in the inverter 3 and the motor 5 at the time of the 2-level operation and the 3-level operation, respectively, in the case are calculated (steps 2, 3). The parameters used for the calculation are the voltage of the DC power supply, the phase current, the carrier frequency, the modulation rate, the power factor, and the temperature.

[0058] Specifically, with respect to the inverter loss, each of the parameters of the voltage of the DC power supply 1, the phase current of the motor 5, the modulation rate, the carrier frequency, the power factor at the time of the 2-level operation and the 3-level operation, and the temperature is changed, and the inverter loss at the time of the 2-level operation and the 3-level operation, respectively, is calculated (step 2). In addition, with respect to the motor loss, each of the parameters of the copper loss, the fundamental iron loss, the high-frequency iron loss, the mechanical loss, and the temperature of the motor 5 is changed, and the motor loss at the time of the 2-level operation and the 3-level operation, respectively, is calculated (step 3).

[0059] Next, the inverter loss and the motor loss obtained by changing these parameters are combined, and the total loss of the inverter 3 and the motor 5 is calculated (step 4). Next, the operation condition of the inverter 3 in which the total loss becomes the minimum (in other words, the reduction effect ΔP of the total loss becomes the maximum) is found, the operation condition in which the total loss of the inverter 3 and the motor 5 becomes the minimum is mapped on the NT characteristic of the motor 5, and the region in which the 3-level operation is executed is limited according to the characteristics of the necessary torque and rotational speed at the time of WLTC mode travel (step 5).

[0060] That is, the operation map in which the information of the carrier frequency of the inverter 3 for reducing the total loss according to the voltage of the DC power supply 1 and the operation state of the inverter 3 and the motor 5 and the information of the threshold value Shl for distinguishing the region in which the 2-level operation is performed and the region in which the 3-level operation is performed are defined on the operation map of the NT characteristic of the motor 5 (operation map (3)) is prepared in advance, and the operation map is stored in a storage device or the like not shown which the control circuit 6 has. Figure 14

[0061] ​The control circuit 6, when actually driving the motor 5, refers to the operation map prepared in advance as described above, and determines the carrier frequency of the inverter 3 and whether to perform 2-level operation or 3-level operation, based on information of the command value of the torque and the rotational speed of the motor 5, temperature information, and information of the current flowing in the 2nd capacitors 2a, 2b, and controls the operation of the inverter 3 and the switching circuit 4. In the determination of whether to perform 2-level operation or 3-level operation, for example, in a case where the reduction amount of the high-order harmonic iron loss of the motor 5 when the inverter 3 performs 3-level operation is greater than the increase amount of the loss of the inverter 3 when the inverter 3 is switched from 2-level operation to 3-level operation, the control circuit 6 determines to make the inverter 3 perform 3-level operation.

[0062] Figure 14 is a diagram showing one example of the operation map in which the region for selecting whether to perform 2-level operation or 3-level operation is defined on the NT characteristic representing the relationship between the rotational speed N and the torque T of the motor 5, according to the processing shown in the flowchart of the above-described Figure 13

[0063] Further, the symbol Shl in the figure represents a threshold value for selecting and switching between 3-level operation and 2-level operation. In addition, the symbol Tmax in the figure is a curve showing the maximum torque that can be taken for the rotational speed N of the motor 5.

[0064] Here, in the WLTC mode, the required torque to the motor 5 becomes equal to or lower than the threshold value Shl, so 3-level operation is selected, but this region is lower than the maximum operation point of the inverter 3 and the motor 5 that covers the entire operation region of the vehicle, and is in a low current region.

[0065] Thus, the reason for performing 3-level operation when running in the WLTC mode is that, even if the efficiency is improved in the entire region on the NT characteristic, the improvement effect on fuel efficiency is small due to the frequency, and rather, when 3-level operation is limited to the operation region (utilization current limit) where the fuel efficiency is effective, and the size increase of the additional switching circuit 4, series capacitors 2a, 2b is suppressed, the reduction of the total loss and the miniaturization and low cost of the entire device can be more achieved.

[0066] In addition, it is understood from Figure 14 that, when the torque of the motor 5 increases to exceed the threshold value Shl at the same rotational speed, 3-level operation is switched to 2-level operation. In addition, in the figure, the threshold value Shl is shown as a substantially horizontal line, but in a case where the threshold value Shl is a line inclined like the right oblique lower, when the rotational speed of the motor 5 increases to exceed the threshold value Shl at the same torque, 3-level operation is switched to 2-level operation. In addition, in the figure, the threshold value Shl is shown as a line, and of course, the hysteresis can be provided to the threshold value Shl to set the operation state in a manner to stabilize the operation, and the description is omitted. ​

[0067] In addition, with respect to the carrier frequency, whether it is the carrier frequency fx_3lv at the time of 3-level operation or the carrier frequency fx_2lv at the time of 2-level operation, the carrier frequency increases as the rotational speed N of the motor 5 increases. That is, the carrier frequency at the time of 3-level operation increases as f1_3lv→f2_3lv→f3_3lv. In addition, the carrier frequency at the time of 2-level operation increases as f1_2lv→f2_2lv→f3_2lv.

[0068] Further, in the case of comparing the carrier frequency fx_3lv at the time of 3-level operation and the carrier frequency fx_2lv at the time of 2-level operation, regardless of the rotational speed N of the motor 5, the carrier frequency fx_3lv at the time of 3-level operation is set to be lower than the carrier frequency fx_2lv at the time of 2-level operation, that is, fx_2lv>fx_3lv. The reason for this is that, at the time of 3-level operation, operation is limited to a low current region in which the torque T is small, so the inverter loss is small, and the distortion of the phase current of the motor 5 is smaller than in the case of 2-level operation (fx_2lv>fx_3lv), so it is possible to reduce the high-order harmonic iron loss compared to the case of 2-level operation, reliably reduce the motor loss, and reduce the total loss of the inverter 3 and the motor 5 as a whole. Figure 10

[0069] In addition, with respect to the carrier frequency at the time of 2-level operation, when the switching frequency of the inverter 3 is reduced, the switching loss becomes smaller, but the distortion of the phase current of the motor 5 becomes larger, and the high-frequency iron loss of the motor 5, on the contrary, increases. In this way, the carrier frequency is a parameter that is contrary for the inverter 3 and the motor 5, so it is necessary to set it so as to ensure the controllability of the motor 5 and minimize the total loss of the inverter 3 and the motor 5.

[0070] Figure 15 is a characteristic diagram that schematically shows the relationship of the total loss Lt (left vertical axis) of the inverter 3 and the motor 5 and the size Siz (or cost) of the inverter 3 with respect to the current Iiv (horizontal axis) flowing through the inverter 3.

[0071] The size Siz of the inverter 3 here refers to the size of the additional switching circuit 4 and the 2nd capacitors 2a, 2b and the size of the gate drive circuit that drives the switching circuit 4. Also, when these sizes become larger, the cost also increases in conjunction therewith.

[0072] In order to provide a structure that can switch between 2-level operation and 3-level operation, it is necessary to add components such as the switching circuit 4, the 2nd capacitors 2a, 2b, and the gate drive circuit thereof to the inverter for the structure that performs 2-level operation, and when the current Iiv flowing through the inverter 3 increases, the withstand voltage of these components is required, so the size Siz becomes larger depending on the amount of current.

[0073] ​On the other hand, regarding the loss, when the current Iiv of the inverter 3 is large, regarding the total loss Lt of the inverter 3 and the motor 5, as has been explained in association with Figure 12 by switching from the 2-level operation to the 3-level operation, not only is there a case where the inverter loss decreases and the total loss decreases (indicated by the line of the symbol Lt1 in FIG. 10), but there is also a case where the inverter loss increases and the total loss increases (indicated by the line of the symbol Lt2 in FIG. 10). Figure 15 Figure 6

[0074] Therefore, in a region where the amount of current of the inverter 3 is less than Iiv0 in the map, it is possible to reduce the total loss Lt without unnecessarily increasing the size Siz of the inverter 3.

[0075] As described above, in the power conversion device in Embodiment 1 of the present application, the inverter 3 is provided in a structure that enables selection and switching between 2-level operation and 3-level operation, and by taking advantage of the fact that, in the case where the inverter 3 is caused to perform 3-level operation, it is possible to reduce both the switching loss of the inverter 3 and the high-order harmonic iron loss of the motor 5, an operation map is prepared in advance on the NT characteristics of the motor 5 in such a manner that the information of the carrier frequency and the information of the threshold value Shl that distinguishes between the region where 2-level operation is performed and the region where 3-level operation is performed are defined, and the operation map is stored to the control circuit 6, and based on the operation map, the inverter selects 2-level operation and 3-level operation, so that it is possible to cause the inverter 3 to operate at the carrier frequency at which the total loss of the inverter 3 and the motor 5 is the smallest.

[0076] In addition, as components that are required in order to minimize the total loss of the inverter 3 and the motor 5 during WLTC mode travel, which is a mode of travel that is an index of the fuel efficiency of a vehicle, and in order to make switching between 2-level operation and 3-level operation possible, the 1st capacitor 2c and the 2nd capacitors 2a and 2b that are connected in series with each other are connected in parallel with the direct-current power supply 1, so that it is possible to suppress an increase in size and to achieve a reduction in the size of the inverter 3.

[0077] Embodiment 2.

[0078] In the power conversion device of the above-described Embodiment 1, in order to provide the inverter 3 in a structure that enables selection and switching between 2-level operation and 3-level operation, the switching circuit 4 that is attached to the inverter 3 is provided in a structure that uses IGBTs of the reverse blocking type.

[0079] In contrast to this, in this Embodiment 2, as Figure 16 indicated, the inverter 3 and the switching circuit 4 are both provided in a structure that uses MOS (Metal-Oxide-Semiconductor), and even in the case where this structure is adopted, it is possible to achieve the same effects as in Embodiment 1.​​

[0080] Further, the present application describes various exemplary embodiments, but the various features, modes, and functions described in the embodiments are not limited to the application of the specific embodiments, and can be applied to the embodiments alone or in various combinations.

[0081] Therefore, an infinite number of modifications not exemplified can be conceived within the scope of the technology disclosed in the present application. For example, a case where at least one constituent element is modified, a case where something is added or omitted, and further a case where at least one constituent element is extracted and combined with the constituent element of another embodiment.

Claims

1. A power conversion device, wherein, An inverter equipped with a drive motor is provided. A first capacitor and a second capacitor, which consists of multiple capacitors connected in series, are connected in parallel to a DC power supply. A switching circuit with multiple switching elements is connected between the inverter and the second capacitor. Furthermore, the power conversion device includes a control circuit that controls the inverter and the switching circuit. The control circuit performs the following control: switching the switching circuit according to torque and speed commands for the motor; supplying current to the inverter from the second capacitor when operating at level 3; and supplying current to the inverter from both the first and second capacitors when operating at level 2. The control circuit performs the following control: if the reduction in the higher harmonic iron loss of the motor when the inverter operates at 3 levels is greater than the increase in inverter losses when the inverter switches from 2-level operation to 3-level operation, the inverter is operated at 3 levels.

2. The power conversion device according to claim 1, wherein, The control circuit performs control that increases the current supplied to the motor during 2-level operation compared to 3-level operation.

3. The power conversion device according to claim 1 or 2, wherein, The control circuit performs the following control: when the motor is at the same speed, it switches to level 2 operation when high torque is required, and switches to level 3 operation when low torque is required.

4. The power conversion device according to claim 1 or 2, wherein, The control circuit performs the following control: when the motors have the same torque, it switches between 2-level and 3-level operation according to the motor speed.

5. The power conversion device according to claim 1 or 2, wherein, The control circuit performs the following control: when the motor is at the same speed, the carrier frequency during 3-level operation is made to be lower than the carrier frequency during 2-level operation.

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