Charging system

Abstract

To provide a technology for mitigating accumulation stress of a switching element in a system for charging a battery using a dual inverter type device.SOLUTION: A charging system comprises a motor, first / second inverters, a neutral point switching element, a power-receiving terminal, a circuit selector, and a controller. The motor has a plurality of stator coils, each of which has one end connected to the first inverter and the other end connected to the second inverter. The neutral point switching element connects the other ends of the stator coils with each other at a neutral point. To the power-receiving terminal, an external power source having output voltage lower than that of a battery is connected. The circuit selector changes over a relation of connection among the first / second inverters, the power-receiving terminal, and the neutral point. The controller controls the circuit selector so that current flows from the external power source to the stator coils through a switching element having switching elements with low accumulation stress of the switching element of the first / second inverters and the neutral point switching element.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The technology disclosed in this specification relates to a charging system that charges a battery by using the stator coil of a motor and the switching element of an inverter as a boost converter.

Background Art

[0002] It is known that the circuit of the stator coil of a motor and the switching element of an inverter can be used as a boost converter. Patent Document 1 discloses a device that charges a battery with an external power source whose output voltage is lower than that of the battery by using the stator coil of a motor and the switching element of an inverter as a boost converter. In the technology of Patent Document 1, a battery is charged using a dual-inverter type drive device that uses one motor and two inverters. In the dual-inverter type drive device, an open-winding type motor is used. One end of the stator coil of the motor is connected to the first inverter, and the other end is connected to the second inverter.

[0003] In a dual-inverter type drive device, when the other ends of a plurality of stator coils are connected to each other at the neutral point, it can be used as a normal closed-type motor drive device. In the device of Patent Document 1, the other ends of a plurality of stator coils are connected to each other at the neutral point, and an external power source is connected to the neutral point. The current of the external power source flows through the neutral point to the stator coil. By turning on and off the lower switching element of the first inverter, the voltage of the external power source is boosted by the stator coil. The power of the external power source flows to the battery through the neutral point / stator coil / upper switching element of the first inverter. Hereinafter, for convenience of explanation, the charging method of sending power to the battery through the neutral point is referred to as neutral point charging.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] Repeated neutral-point charging increases the cumulative stress (cumulative load) on the neutral-point switching element that connects the other end of the stator coil to the neutral point. This specification provides a technique for mitigating the cumulative stress on the neutral-point switching element in a charging system using a dual-inverter type drive device. [Means for solving the problem]

[0006] The charging system disclosed herein comprises a motor, first / second inverters, a neutral point switching element, a power receiving terminal, a circuit selector, and a controller. One end of the motor's multiple stator coils is connected to the first inverter, and the other end is connected to the second inverter. When the neutral point switching element is turned on, the other ends of the stator coils are interconnected at the neutral point. The power receiving terminal has a positive terminal and a negative terminal, and can be connected to an external power source with an output voltage lower than that of the battery. The circuit selector switches the connection relationship between the first / second inverters, the power receiving terminal, and the neutral point.

[0007] The first inverter comprises a plurality of first series connections connected in parallel between the battery positive terminal and the first ground terminal. Each first series connection comprises a first upper switching element and a first lower switching element connected in series. One end of each stator coil is connected to the midpoint of each first series connection (the connection point between the first upper switching element and the first lower switching element). The second inverter comprises a plurality of second series connections connected in parallel between the battery positive terminal and the second ground terminal. Each second series connection comprises a second upper switching element and a second lower switching element connected in series. The other end of each stator coil is connected to the midpoint of each second series connection (the connection point between the second upper switching element and the second lower switching element).

[0008] The circuit selector can choose between motor drive mode, first charging mode, second charging mode, and neutral point charging mode. In motor drive mode, the first and second ground terminals are connected to the battery negative terminal. The motor drive mode is selected when driving the motor. In first charging mode, the first ground terminal is connected to the battery negative terminal, and the second ground terminal is disconnected from the battery negative terminal and connected to the connected positive terminal. In second charging mode, the second ground terminal is connected to the battery negative terminal, and the first ground terminal is disconnected from the battery negative terminal and connected to the connected positive terminal. In neutral point charging mode, the neutral point is connected to the connected positive terminal, and the first ground terminal is connected to the battery negative terminal.

[0009] When the motor is driven by both the first and second inverters, the controller sets the selector to motor drive mode and keeps the neutral point switching element in the off position. The motor is driven when the controller appropriately controls the first and second inverters. When the motor is driven by only the first inverter, the controller sets the selector to motor drive mode and keeps the neutral point switching element in the on position. When the controller stops the second inverter and controls the first inverter, the motor is driven by AC power supplied from the first inverter.

[0010] When an external power supply is connected to the power receiving terminal, the controller performs the following actions: The controller compares the cumulative stress of the first lower switching element (first cumulative stress), the cumulative stress of the second lower switching element (second cumulative stress), and the cumulative stress of the neutral point switching element (third cumulative stress). If the second cumulative stress is the smallest, the controller keeps the neutral point switching element off, sets the circuit selector to the first charging mode, and turns the first lower switching element on and off as appropriate. The voltage of the external power supply is boosted by the operation of the first lower switching element, and the battery is charged.

[0011] When the first cumulative stress is at its lowest, the controller keeps the neutral point switching element off, sets the circuit selector to the second charging mode, and switches the second lower switching element on and off as appropriate. The voltage of the external power supply is boosted by the operation of the second lower switching element, and the battery is charged. When the third cumulative stress is at its lowest, the controller keeps the neutral point switching element on, sets the circuit selector to the neutral point charging mode, and switches the first lower switching element on and off as appropriate. The voltage of the external power supply is boosted by the operation of the first lower switching element, and the battery is charged.

[0012] In neutral point charging mode, the current from the external power supply flows to the stator coil through the neutral point switching element. However, in the first charging mode, the current from the external power supply bypasses the neutral point switching element and flows to the stator coil through the second lower switching element. In the second charging mode, the current from the external power supply bypasses the neutral point switching element and flows to the stator coil through the first lower switching element.

[0013] In the charging system disclosed herein, current from an external power supply is passed to the stator coil through the element with the smallest cumulative stress among the neutral point switching element, the first lower switching element, and the second lower switching element. Therefore, the load on the neutral point switching element is relieved.

[0014] The cumulative stress is calculated based on the cumulative value of the temperature rise of the switching element per unit time. Further details and improvements of the technology disclosed herein are described in the following "Modes for Carrying Out the Invention". [Brief explanation of the drawing]

[0015] [Figure 1] This is a circuit diagram of the charging system in the embodiment. [Figure 2] This is a circuit diagram showing the current flow in the first charging mode. [Figure 3] This is a circuit diagram showing the current flow in the second charging mode. [Figure 4] This is a circuit diagram showing the current flow in neutral point charging mode. [Modes for carrying out the invention]

[0016] The charging system 2 of the embodiment will be described with reference to the drawings. Figure 1 shows the circuit diagram of the charging system 2. The charging system 2 comprises a first inverter 10, a second inverter 20, a motor 30, a neutral point switching element 35, a circuit selector 40, a power receiving terminal 45, and a controller 50. In addition to the charging system 2, Figure 1 also shows a battery 60 and an external power supply 70.

[0017] The charging system 2 can charge the battery 60 with an external power supply 70 connected to the power receiving terminal 45. When charging the battery 60, the lower switching element of the first inverter 10 or the second inverter 20 and the stator coil 31 of the motor 30 function as a boost converter. Therefore, the battery 60 can be charged with an external power supply 70 that has a lower output voltage than the battery 60.

[0018] The charging system 2 and battery 60 are installed in the electric vehicle. The motor 30 is connected to the axle (not shown) of the electric vehicle, and the charging system 2 also functions as a drive system that drives the motor 30 with power from the battery 60 to run the electric vehicle. The motor 30 is a three-phase AC motor and has multiple stator coils 31.

[0019] The configuration of the first inverter 10 will now be described. The first inverter 10 has a first ground terminal 10g and three sets of first series connectors 11a, 11b, and 11c. The three sets of first series connectors 11a, 11b, and 11c are connected in parallel between the positive terminal (battery positive terminal 60p) of the battery 60 and the first ground terminal 10g.

[0020] Each of the three sets of first series connection bodies 11a, 11b, and 11c includes a first upper switching element 12 and a first lower switching element 13 connected in series. The first upper switching element 12 is connected to the battery positive terminal 60p, and the first lower switching element 13 is connected to the first ground terminal 10g. One end of each of the three stator coils 31 is connected to the midpoint (connection point of the first upper switching element 12 and the first lower switching element 13) of each of the three sets of first series connection bodies 11a, 11b, and 11c. The first inverter 10 includes a plurality of temperature sensors 14, and each temperature sensor 14 measures the temperature of each of the plurality of first lower switching elements 13.

[0021] The configuration of the second inverter 20 will be described. The second inverter 20 has a second ground terminal 20g and three sets of second series connection bodies 21a, 21b, and 21c. The three sets of second series connection bodies 21a, 21b, and 21c are connected in parallel between the battery positive terminal 60p and the second ground terminal 20g.

[0022] Each of the three sets of second series connection bodies 21a, 21b, and 21c includes a second upper switching element 22 and a second lower switching element 23 connected in series. The second upper switching element 22 is connected to the battery positive terminal 60p, and the second lower switching element 23 is connected to the second ground terminal 20g. The other end of each of the three stator coils 31 is connected to the midpoint (connection point of the second upper switching element 22 and the second lower switching element 23) of each of the three sets of second series connection bodies 21a, 21b, and 21c. The second inverter 20 includes a plurality of temperature sensors 24, and each temperature sensor 24 measures the temperature of each of the plurality of second lower switching elements 23.

[0023] Freewheeling diodes are connected in anti-parallel to the switching elements of the first inverter 10 and the second inverter 20. The freewheeling diode may be a separate element from the switching element or may be a diode included in the switching element.

[0024] The charging system 2 includes a plurality of neutral point switching elements 35. Each of the plurality of neutral point switching elements 35 is positioned between the other end of each of the plurality of stator coils 31 and the neutral point 32. When the plurality of neutral point switching elements 35 are held ON, the plurality of stator coils 31 are interconnected at the neutral point 32. The charging system 2 also includes a plurality of temperature sensors 36, each of which measures the temperature of each of the plurality of neutral point switching elements 35.

[0025] The power receiving terminal 45 has a positive terminal 45p and a negative terminal 45n. An external power supply 70 is connected to the power receiving terminal 45. The positive terminal 70p of the external power supply 70 is connected to the positive terminal 45p, and the negative terminal 70n of the external power supply 70 is connected to the negative terminal 45n. The negative terminal 45n is connected to the negative terminal 60n of the battery. The positive terminal 45p is connected by the circuit selector 40 to either the first ground terminal 10g, the second ground terminal 20g, or the neutral point 32.

[0026] The circuit selector 40 will now be described. The circuit selector 40 includes five switches 40a-40e. Switch 40a connects the first ground terminal 10g to the battery negative terminal 60n or disconnects the first ground terminal 10g from the battery negative terminal 60n. Switch 40b connects the second ground terminal 20g to the battery negative terminal 60n or disconnects the second ground terminal 20g from the battery negative terminal 60n.

[0027] Switch 40c connects the first ground terminal 10g to the positive terminal 45p, or disconnects the first ground terminal 10g from the positive terminal 45p. Switch 40d connects the neutral point 32 to the positive terminal 45p, or disconnects the neutral point 32 from the positive terminal 45p. Switch 40e connects the second ground terminal 20g to the positive terminal 45p, or disconnects the second ground terminal 20g from the positive terminal 45p.

[0028] The circuit selector 40 (switches 40a-40e) is controlled by the controller 50. The controller 50 appropriately controls the switches 40a-40e of the circuit selector 40 to switch the connection relationship between the first inverter 10, the second inverter 20, the neutral point 32, and the power receiving terminal 45. The state of the circuit selector 40 will be referred to as "mode" below. The modes that can be selected with the circuit selector 40 are as follows:

[0029] Motor drive mode: Close switches 40a and 40b, and open switches 40c-40e. By closing switches 40a and 40b, the first ground terminal 10g and the second ground terminal 20g are connected to the battery negative terminal 60n.

[0030] First charging mode: Close switches 40a and 40e, and open the remaining switches 40b, 40c, and 40d. Closing switch 40a connects the first ground terminal 10g to the battery negative terminal 60n. Opening switch 40b and closing switch 40e disconnects the second ground terminal 20g from the battery negative terminal 60n and connects to the connected positive terminal 45p. Opening switch 40d disconnects the neutral point 32 from the connected positive terminal 45p.

[0031] Second charging mode: Close switches 40b and 40c, and open the remaining switches 40a, 40d, and 40e. Closing switch 40b connects the second ground terminal 20g to the battery negative terminal 60n. Opening switch 40a and closing switch 40c disconnects the first ground terminal 10g from the battery negative terminal 60n and connects to the connected positive terminal 45p. Opening switch 40d disconnects the neutral point from the connected positive terminal 45p.

[0032] Neutral point charging mode: Close switches 40a and 40d, and open the remaining switches 40b, 40c, and 40e. Closing switch 40a connects the first ground terminal 10g to the battery negative terminal 60n. Closing switch 40d connects the neutral point 32 to the connected positive terminal 45p.

[0033] The controller 50 controls the circuit selector 40 and selects one of the four modes described above. Next, the operation of the controller 50 will be explained.

[0034] When the motor 30 is driven by both the first inverter 10 and the second inverter 20, the controller 50 keeps the neutral point switching element 35 in the off position and sets the circuit selector 40 to motor drive mode. Keeping the switching element in the off position means opening the switching element. At this time, one end of the multiple stator coils 31 is connected to the AC terminal of the first inverter 10, and the other end is connected to the AC terminal of the second inverter 20. The motor 30 becomes an open-type motor. When the controller 50 appropriately turns the switching elements of the first inverter 10 and the second inverter 20 on and off, the motor 30 is driven. Since the motor 30 is driven by two inverters, it can output high torque.

[0035] Figure 1 shows switches 40a and 40b closed, and the remaining switches 40c-40e open. In other words, in Figure 1, the circuit selector 40 is set to motor drive mode. Also, in Figure 1, the line connecting the external power supply 70 and the power receiving terminal 45 is drawn as a dotted line. In other words, in Figure 1, the external power supply 70 is not connected to the charging system 2.

[0036] When the motor 30 is driven by the first inverter 10 alone, the controller 50 keeps the neutral point switching element 35 ON and sets the circuit selector 40 to motor drive mode. By keeping the neutral point switching element 35 ON, multiple stator coils The other end of 31 is interconnected at the neutral point 32. At this time, the motor 30 becomes a closed-type motor with the other end Y-connected. When the controller 50 stops the second inverter 20 and appropriately switches each switching element of the first inverter 10 on and off, the motor 30 is driven by the first inverter 10.

[0037] The charging system 2 can charge the battery 60 by connecting an external power supply 70 to the power receiving terminal 45. The charging system 2 can use the first inverter 10 or the second inverter 20 and the stator coil 31 as a boost converter. Therefore, the output voltage of the external power supply 70 may be lower than that of the battery 60. The positive terminal 70p of the external power supply 70 is connected to the positive terminal 45p of the power receiving terminal 45, and the negative terminal 70n of the external power supply 70 is connected to the negative terminal 45n of the power receiving terminal 45.

[0038] Furthermore, the charging system 2 can connect the positive terminal 70p of the external power supply 70 to the stator coil 31 using any of the first lower switching element 13, the second lower switching element 23, or the neutral point switching element 35.

[0039] Each of the switching elements 13, 23, and 35 is equipped with a temperature sensor 14, 24, and 36, respectively, which measures the temperature of each switching element. The temperature of each switching element is transmitted to the controller 50. The controller 50 evaluates the cumulative stress of each switching element based on its temperature. Cumulative stress is the cumulative value of the temperature rise per unit time of each switching element. The cumulative value of the temperature rise per unit time serves as an indicator of the degradation of the switching element. The temperature decrease per unit time does not contribute significantly to the degradation of the switching element. Therefore, the temperature decrease per unit time is not taken into account in the cumulative stress.

[0040] Furthermore, if the yield strengths of the first lower switching element 13, the second lower switching element 23, and the neutral point switching element 35 are different, a coefficient corresponding to the yield strength is multiplied by each cumulative stress. Hereafter, the cumulative stress of the first lower switching element 13 will be referred to as the first cumulative stress, the cumulative stress of the second lower switching element 23 will be referred to as the second cumulative stress, and the cumulative stress of the neutral point switching element 35 will be referred to as the third cumulative stress.

[0041] When the controller 50 charges the battery 60 with an external power supply 70 connected to the power receiving terminal 45, it compares the first, second, and third cumulative stresses and uses the switching element with the smallest cumulative stress to guide the current from the external power supply 70 to the stator coil 31. Therefore, the cumulative stress on the neutral point switching element 35 is reduced.

[0042] The current flow when using each switching element is explained below. When the second cumulative stress is the smallest of the three cumulative stresses, the controller 50 keeps the neutral point switching element 35 in the OFF position and sets the circuit selector 40 to the first charging mode (Figure 2). The thick arrows in Figure 2 show the current flow in the first charging mode. Note that the temperature sensor is omitted from Figure 2 onward for clarity.

[0043] The output current from the external power supply 70 flows to the stator coil 31 via the power receiving terminal 45, the switch 40e of the circuit selector 40, and the second lower switching element 23. The stator coil 31 and the first lower switching element 13 have a circuit configuration equivalent to a boost converter. When the controller 50 turns the first lower switching element 13 on and off as appropriate, the voltage in the stator coil 31 is boosted, and current flows from the stator coil 31 to the battery 60 through the freewheeling diode of the first upper switching element 12. In other words, the battery 60 is charged by the external power supply 70.

[0044] If the first cumulative stress is the smallest of the three cumulative stresses, the controller 50 keeps the neutral point switching element 35 in the OFF position and sets the circuit selector 40 to the second charging mode (Figure 3). The thick arrows in Figure 3 show the current flow in the second charging mode.

[0045] The output current from the external power supply 70 flows to the stator coil 31 via the power receiving terminal 45, the switch 40c of the circuit selector 40, and the first lower switching element 13. The stator coil 31 and the second lower switching element 23 have a circuit configuration equivalent to a boost converter. When the controller 50 turns the second lower switching element 23 on and off as appropriate, the voltage in the stator coil 31 is boosted, and current flows from the stator coil 31 to the battery 60 through the freewheeling diode of the second upper switching element 22. In other words, the battery 60 is charged by the external power supply 70.

[0046] If the third cumulative stress is the smallest of the three cumulative stresses, the controller 50 keeps the neutral point switching element 35 ON and sets the circuit selector 40 to neutral point charging mode (Figure 4). The thick arrows in Figure 4 show the current flow in neutral point charging mode.

[0047] The output current from the external power supply 70 flows to the stator coil 31 via the power receiving terminal 45, the switch 40d of the circuit selector 40, and the neutral point switching element 35. Similar to the first charging mode, the stator coil 31 and the first lower switching element 13 have a circuit configuration equivalent to a boost converter. When the controller 50 turns the first lower switching element 13 on and off as appropriate, the voltage in the stator coil 31 is boosted, and current flows from the stator coil 31 to the battery 60 through the freewheeling diode of the first upper switching element 12. In other words, the battery 60 is charged by the external power supply 70.

[0048] When the first cumulative stress (i.e., the cumulative stress of the first lower switching element 13) is smallest, the controller 50 controls the circuit selector 40 so that the current from the external power supply 70 flows to the stator coil 31 through the first lower switching element 13. When the second cumulative stress (i.e., the cumulative stress of the second lower switching element 23) is smallest, the controller 50 controls the circuit selector 40 so that the current from the external power supply 70 flows to the stator coil 31 through the second lower switching element 23. When the third cumulative stress (i.e., the cumulative stress of the neutral point switching element 35) is smallest, the controller 50 controls the circuit selector 40 so that the current from the external power supply 70 flows to the stator coil 31 through the neutral point switching element 35.

[0049] As described above, when charging the battery 60, the controller 50 controls the circuit selector 40 so that the current from the external power supply 70 flows to the stator coil 31 through the switching element with the least cumulative stress. Therefore, the stress on the neutral point switching element 35 is reduced.

[0050] Furthermore, the controller 50 may adjust the on / off timing of each first lower switching element 13 so that, when the first cumulative stress is smallest, the current flowing through the first lower switching element 13 with the smallest cumulative stress among the multiple first lower switching elements 13 is greater than the current flowing through the first lower switching element 13 with the largest cumulative stress.

[0051] For example, if the cumulative stress of the first lower switching element 13 of the first series connection 11a is less than the cumulative stress of the first lower switching elements 13 of the first series connections 11b and 11c, the controller 50 adjusts the on / off timing of each first lower switching element 13 so that the current flowing through the first lower switching element 13 of the first series connection 11a is greater than the current flowing through the first lower switching elements 13 of the first series connections 11b and 11c. This process also equalizes the cumulative stress in multiple first lower switching elements 13. The same applies to multiple second lower switching elements 23 and multiple neutral point switching elements.

[0052] The following points should be noted regarding the technology described in the examples. The first cumulative stress may be the sum of the cumulative stresses of each of the multiple first lower switching elements 13, or it may be the average of the cumulative stresses of the multiple first lower switching elements 13. The same applies to the second and third cumulative stresses. The same calculation formula is used for the first, second, and third cumulative stresses. The first, second, and third cumulative stresses should be based on the cumulative value of the temperature rise per unit time of each switching element. Typically, the first, second, and third cumulative stresses may be the cumulative value of the temperature rise per unit time of each switching element multiplied by a coefficient.

[0053] In charging using the first charging mode, the second inverter 20 is not used. Therefore, in the first charging mode, switch 40b can be either closed or open. In charging using the second charging mode, the first inverter 10 is not used. Therefore, in the second charging mode, switch 40a can be either closed or open. In charging using the neutral point charging mode, the second inverter 20 is not used. Therefore, in the neutral point charging mode, switch 40b can be either open or closed.

[0054] The charging system 2 can drive the motor 30 with the first inverter 10 and the second inverter 20. Therefore, the charging system 2 may also be referred to as a "drive device".

[0055] The expression "to keep the switching element ON" is equivalent to "to close the switching element," which means electrically connecting the devices connected to each end of the switching element. The expression "to keep the switching element OFF" is equivalent to "to open the switching element," which means electrically disconnecting the devices connected to each end of the switching element.

[0056] A filter capacitor is connected between the battery's positive terminal 60pF and negative terminal 60nF, but its diagram is omitted. Similarly, a smoothing capacitor is connected between the connected positive terminal 45pF and negative terminal 45nF, but its diagram is also omitted.

[0057] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. The technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of symbols]

[0058] 2: Charging system 10, 20: Inverter 10g, 20g: Ground terminal 11a-11c, 21a-21c: Series connection 12, 22: Upper switching element 13, 23: Lower switching element 14, 24, 36: Temperature sensor 30: Motor 31: Stator coil 32: Neutral point 35: Neutral point switching element 40: Circuit selector 40a-40e: Switch 45: Power receiving terminal 50: Controller 60: Battery 70: External power supply

Claims

1. This is a charging system that charges the battery using an external power supply with a lower output voltage than the battery itself. A motor equipped with multiple stator coils, A first inverter connected to one end of a plurality of stator coils, A second inverter connected to the other end of the multiple stator coils, A neutral point switching element that connects the other ends of multiple stator coils to the neutral point, A power receiving terminal having a positive terminal and a negative terminal to which the external power supply can be connected, wherein the negative terminal is connected to the negative terminal of the battery, A circuit selector that switches the connection relationship between the first inverter, the second inverter, the neutral point, and the power receiving terminal, Controller and It is equipped with, The first inverter is, The first ground edge and A plurality of first series connectors comprising a first upper switching element and a first lower switching element connected in series, wherein a plurality of first series connectors are connected in parallel between the positive terminal of the battery and the first ground terminal of the battery, It is equipped with, One end of each stator coil is connected to the midpoint of each of the first series connections. The second inverter is, At the end of the second field, A plurality of second series connectors comprising a second upper switching element and a second lower switching element connected in series, wherein a plurality of second series connectors are connected in parallel between the positive terminal of the battery and the second ground terminal, It is equipped with, The other end of each stator coil is connected to the midpoint of each of the second series connections. The aforementioned circuit selector A motor drive mode in which the first ground terminal and the second ground terminal are connected to the battery negative terminal, A first charging mode in which the first ground terminal is connected to the battery negative terminal and the second ground terminal is disconnected from the battery negative terminal and connected to the connected positive terminal, A second charging mode in which the second ground terminal is connected to the battery negative terminal, and the first ground terminal is disconnected from the battery negative terminal and connected to the connected positive terminal, A neutral point charging mode is provided in which the neutral point is connected to the positive terminal of the connection, and the first ground terminal is connected to the negative terminal of the battery. You can choose either of the following: The aforementioned controller, When the motor is driven by the first inverter and the second inverter, the circuit selector is set to the motor drive mode, and the neutral point switching element is kept in the OFF position. When the external power supply is connected to the power receiving terminal, The first cumulative stress of the first lower switching element, the second cumulative stress of the second lower switching element, and the third cumulative stress of the neutral point switching element are compared. When the second cumulative stress is smallest, the neutral point switching element is kept off and the circuit selector is set to the first charging mode, and the first lower switching element is turned on and off to charge the battery. When the first cumulative stress is smallest, the neutral point switching element is kept off and the circuit selector is set to the second charging mode, and the second lower switching element is turned on and off to charge the battery. When the third cumulative stress is smallest, the neutral point switching element is kept ON, the circuit selector is set to neutral point charging mode, and the first lower switching element is turned ON and OFF to charge the battery. Charging system.

2. The charging system according to claim 1, wherein the controller adjusts the on / off timing of each first lower switching element such that, when the first cumulative stress is smallest, the current flowing through the first lower switching element with the smallest cumulative stress among the plurality of first lower switching elements is greater than the current flowing through the first lower switching element with the largest cumulative stress.

3. The first cumulative stress is based on the cumulative value of the temperature rise per unit time of the first lower switching element. The second cumulative stress is based on the cumulative value of the temperature rise per unit time of the second lower switching element. The charging system according to claim 1 or 2, wherein the third cumulative stress is based on the cumulative value of the temperature rise per unit time of the neutral point switching element.

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