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 an apparatus 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 by using a dual-inverter type drive apparatus that uses one motor and two inverters. In the dual-inverter type drive apparatus, 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 the dual-inverter type drive apparatus, when the other ends of a plurality of stator coils are mutually connected at the neutral point, it can be used as a normal closed type motor drive apparatus. In the apparatus of Patent Document 1, the other ends of a plurality of stator coils are mutually connected 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

Summary of the Invention

Problems to be Solved by the Invention

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

Means for Solving the Problems

[0006] The charging system disclosed in this specification includes a motor, a first / second inverter, a neutral point switching element, a power receiving terminal, a circuit selector, and a controller. One end of a plurality of stator coils of the motor 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 connected to each other at the neutral point. The power receiving terminal includes a connection positive terminal and a connection negative terminal, and an external power source having an output voltage lower than that of the battery can be connected. The circuit selector switches the connection relationship among the first / second inverters, the power receiving terminal, and the neutral point.

[0007] The first inverter includes a plurality of first series-connected bodies connected in parallel between the battery positive terminal and the first ground terminal. Each first series-connected body includes 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 (connection point between the first upper switching element and the first lower switching element) of each first series-connected body. The second inverter includes a plurality of second series-connected bodies connected in parallel between the battery positive terminal and the second ground terminal. Each second series-connected body includes 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 (connection point between the second upper switching element and the second lower switching element) of each second series-connected body.

[0008] The circuit selector can select any one of a motor drive mode, a first charging mode, a second charging mode, and a neutral point charging mode. In the motor drive mode, the first ground terminal and the second ground terminal are connected to the battery negative electrode. The motor drive mode is selected when driving the motor. In the first charging mode, the first ground terminal is connected to the battery negative electrode, and the second ground terminal is disconnected from the battery negative electrode and connected to the connection positive terminal. In the second charging mode, the second ground terminal is connected to the battery negative electrode, and the first ground terminal is disconnected from the battery negative electrode and connected to the connection positive terminal. In the neutral point charging mode, the neutral point is connected to the connection positive terminal, and the first ground terminal is connected to the battery negative electrode.

[0009] When the controller drives the motor with the first inverter and the second inverter, it sets the selector to the motor drive mode and keeps the neutral point switching element off. When the controller appropriately controls the first inverter and the second inverter, the motor is driven. When the controller drives the motor with only the first inverter, it sets the selector to the motor drive mode and keeps the neutral point switching element on. When the controller stops the second inverter and controls the first inverter, the motor is driven by the AC power sent from the first inverter.

[0010] When an external power source is connected to the power receiving terminal, the controller executes the following process. The controller compares the cumulative stress of the first lower switching element (the first cumulative stress), the cumulative stress of the second lower switching element (the second cumulative stress), and the cumulative stress of the neutral point switching element (the third cumulative stress). When 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 appropriately. The voltage of the external power source is boosted by the operation of the first lower switching element, and the battery is charged.

[0011] When the first cumulative stress is the smallest, the controller holds the neutral point switching element off and sets the circuit selector to the second charging mode, and turns on and off the second lower switching element 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 the smallest, the controller holds the neutral point switching element on and sets the circuit selector to the neutral point charging mode, and turns on and off the first lower switching element 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 the neutral point charging mode, the current of the external power supply flows through the neutral point switching element to the stator coil. However, in the first charging mode, the current of the external power supply bypasses the neutral point switching element and flows through the second lower switching element to the stator coil. In the second charging mode, the current of the external power supply bypasses the neutral point switching element and flows through the first lower switching element to the stator coil.

[0013] In the charging system disclosed in this specification, the current of the external power supply flows through 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 alleviated.

[0014] Note that the cumulative stress is calculated based on the cumulative value of the rising temperature per unit time of the switching element. The details and further improvements of the technology disclosed in this specification will be described in the following "Mode for Carrying Out the Invention".

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0016] The charging system 2 of the embodiment will be described with reference to the drawings. FIG. 1 shows a circuit diagram of the charging system 2. The charging system 2 includes 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, a battery 60 and an external power source 70 are depicted in FIG. 1.

[0017] The charging system 2 can charge the battery 60 with the external power source 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 the external power source 70 whose output voltage is lower than that of the battery 60.

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

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

[0020] Each of the three sets of first series-connected 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-connected 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-connected bodies 21a, 21b, and 21c. The three sets of second series-connected 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-connected 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-connected 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 disposed 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 includes a plurality of temperature sensors 36, and each temperature sensor 36 measures the temperature of each of the plurality of neutral point switching elements 35.

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

[0026] The circuit selector 40 will be described. The circuit selector 40 includes five switches 40a - 40e. The 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. The 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] The switch 40c connects the first ground terminal 10g to the connection positive terminal 45p or disconnects the first ground terminal 10g from the connection positive terminal 45p. The switch 40d connects the neutral point 32 to the connection positive terminal 45p or disconnects the neutral point 32 from the connection positive terminal 45p. The switch 40e connects the second ground terminal 20g to the connection positive terminal 45p or disconnects the second ground terminal 20g from the connection positive terminal 45p.

[0028] The circuit selector 40 (switches 40a - 40e) is controlled by the controller 50. By appropriately controlling the switches 40a - 40e of the circuit selector 40 by the controller 50, the connection relationship among the first inverter 10, the second inverter 20, the neutral point 32, and the power receiving terminal 45 is switched. The state of the circuit selector 40 is hereinafter referred to as "mode". The modes selectable by 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. By closing switch 40a, the first ground terminal 10g is connected to the battery negative terminal 60n. By opening switch 40b and closing switch 40e, the second ground terminal 20g is disconnected from the battery negative terminal 60n and connected to the connection positive terminal 45p. By opening switch 40d, the neutral point 32 is disconnected from the connection positive terminal 45p.

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

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

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

[0034] When driving the motor 30 with both the first inverter 10 and the second inverter 20, the controller 50 holds the neutral point switching element 35 off and sets the circuit selector 40 to the motor drive mode. Holding the switching element off means opening the switching element. At this time, one end of the plurality of 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 on and off each switching element of the first inverter 10 and the second inverter 20, the motor 30 is driven. Since the motor 30 is driven by two inverters, it can output high torque.

[0035] In FIG. 1, the state where the switches 40a and 40b are closed and the remaining switches 40c - 40e are open is depicted. That is, in FIG. 1, the circuit selector 40 is set to the motor drive mode. Also, in FIG. 1, the line connecting the external power supply 70 and the power receiving terminal 45 is drawn as a virtual line. That is, in FIG. 1, the external power supply 70 is not connected to the charging system 2.

[0036] When driving the motor 30 with the first inverter 10 only, the controller 50 holds the neutral point switching element 35 on and sets the circuit selector 40 to the motor drive mode. By holding the neutral point switching element 35 on, the other ends of the plurality of stator coils 31 are connected to each other at the neutral point 32. At this time, the motor 30 becomes a closed type motor with the other ends Y - connected. When the controller 50 stops the second inverter 20 and appropriately turns on and off each switching element of the first inverter 10, 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 can be used as a boost converter. Therefore, the output voltage of the external power supply 70 may be lower than that of the battery 60. Note that the positive terminal 70p of the external power supply 70 is connected to the connection positive terminal 45p of the power receiving terminal 45, and the negative terminal 70n of the external power supply 70 is connected to the connection negative terminal 45n.

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

[0039] Temperature sensors 14, 24, and 36 are attached to each of the switching elements 13, 23, and 35, and the temperature of each switching element is measured. 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 the temperature of each switching element. The cumulative stress is a value obtained by accumulating 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 deterioration of the switching element. The temperature drop per unit time does not significantly contribute to the deterioration of the switching element. Therefore, the temperature drop per unit time is not taken into account in the cumulative stress.

[0040] When the withstand voltages of the first lower switching element 13, the second lower switching element 23, and the neutral point switching element 35 are different, each cumulative stress is multiplied by a coefficient corresponding to the withstand voltage. Hereinafter, the cumulative stress of the first lower switching element 13 is referred to as the first cumulative stress, and the cumulative stress of the second lower switching element 23 is referred to as the second cumulative stress. Also, the cumulative stress of the neutral point switching element 35 is referred to as the third cumulative stress.

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

[0042] The flow of current when each switching element is used will be described. When the second cumulative stress is the smallest among the three cumulative stresses, the controller 50 holds the neutral point switching element 35 off and sets the circuit selector 40 to the first charging mode (Fig. 2). The thick arrow line in Fig. 2 indicates the flow of current in the first charging mode. Note that in the following figures from Fig. 2 onwards, the illustration of the temperature sensor is omitted for clarity.

[0043] The output current of 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 appropriately turns on and off the first lower switching element 13, the voltage is boosted in the stator coil 31, and current flows from the stator coil 31 to the battery 60 through the freewheeling diode of the first upper switching element 12. That is, the battery 60 is charged by the external power supply 70.

[0044] When the first cumulative stress is the smallest among the three cumulative stresses, the controller 50 holds the neutral point switching element 35 off and sets the circuit selector 40 to the second charging mode (Fig. 3). The thick arrow line in Fig. 3 indicates the flow of current in the second charging mode.

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

[0046] When the third cumulative stress among the three cumulative stresses is the smallest, the controller 50 holds the neutral point switching element 35 in the on state and sets the circuit selector 40 to the neutral point charging mode (Fig. 4). The thick arrow line in Fig. 4 indicates the current flow in the neutral point charging mode.

[0047] The output current of the external power supply 70 flows through the power receiving terminal 45, the switch 40d of the circuit selector 40, and the neutral point switching element 35 to the stator coil 31. 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 appropriately turns on and off the first lower switching element 13, the voltage is boosted in the stator coil 31, and current flows from the stator coil 31 to the battery 60 through the freewheeling diode of the first upper switching element 12. That is, 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 the smallest, the controller 50 controls the circuit selector 40 so that the current of the external power supply 70 flows through the first lower switching element 13 to the stator coil 31. When the second cumulative stress (i.e., the cumulative stress of the second lower switching element 23) is the smallest, the controller 50 controls the circuit selector 40 so that the current of the external power supply 70 flows through the second lower switching element 23 to the stator coil 31. When the third cumulative stress (i.e., the cumulative stress of the neutral point switching element 35) is the smallest, the controller 50 controls the circuit selector 40 so that the current of the external power supply 70 flows through the neutral point switching element 35 to the stator coil 31.

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

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

[0051] For example, when the cumulative stress of the first lower switching element 13 of the first series connection body 11a is smaller than the cumulative stress of the first lower switching elements 13 of the first series connection bodies 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 body 11a becomes larger than the current flowing through the first lower switching elements 13 of the first series connection bodies 11b and 11c. By such processing, the cumulative stress in the plurality of first lower switching elements 13 is also equalized. The same applies to the plurality of second lower switching elements 23 and the plurality of neutral point switching elements.

[0052] Points to note regarding the technology described in the embodiments will be described. The first cumulative stress may be the sum of the respective cumulative stresses of the plurality of first lower switching elements 13, or may be the average of the cumulative stresses of the plurality of first lower switching elements 13. The same applies to the second cumulative stress and the third cumulative stress. The same calculation formula is used for the first / second / third cumulative stresses. The first / second / third cumulative stresses may be based on the cumulative value of the temperature rise per unit time of each switching element. Typically, the first / second / third cumulative stresses may be a value obtained by multiplying the cumulative value of the temperature rise per unit time of each switching element by a coefficient.

[0053] In charging that employs the first charging mode, the second inverter 20 is not used. Therefore, in the first charging mode, the switch 40b may be either closed or open. In charging that employs the second charging mode, the first inverter 10 is not used. Therefore, in the second charging mode, the switch 40a may be either closed or open. In charging that employs the neutral point charging mode, the second inverter 20 is not used. Therefore, in the neutral point charging mode, the switch 40b may 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 be rephrased as a "driving device".

[0055] The expression "holding the switching element on" is equivalent to "closing the switching element", which means electrically connecting the devices connected to each end of the switching element. The expression "holding the switching element off" is equivalent to "opening 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 positive terminal 60p and the battery negative terminal 60n, but its illustration is omitted. Also, a smoothing capacitor is connected between the connection positive terminal 45p and the connection negative terminal 45n, but its illustration is omitted.

[0057] As described above, specific examples of the present invention have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Also, the technology illustrated in this specification or the drawings can achieve multiple purposes simultaneously, and achieving one of those purposes itself has technical utility.

Explanation of Reference Numerals

[0058] 2: Charging system 10, 20: Inverter 10g, 20g: Ground terminal 11a - 11c, 21a - 21c: Series connection body 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 source

Claims

1. A charging system for charging a battery using an external power source having an output voltage lower than that of the battery, a motor having a plurality of stator coils, a first inverter connected to one end of the plurality of stator coils, a second inverter connected to the other end of the plurality of stator coils, a neutral point switching element for connecting the other ends of the plurality of stator coils to a neutral point, a power receiving terminal having a connection positive terminal and a connection negative terminal to which the external power source can be connected, and the connection negative terminal is connected to the battery negative terminal of the battery, a circuit selector for switching the connection relationship between the first inverter, the second inverter, the neutral point, and the power receiving terminal, a controller, and comprising, the first inverter has a first ground terminal, a plurality of first series-connected bodies in which a first upper switching element and a first lower switching element are connected in series, and a plurality of first series-connected bodies connected in parallel between the battery positive terminal of the battery and the first ground terminal, and comprising, one end of each of the stator coils is connected to the midpoint of each of the first series-connected bodies, the second inverter has a second ground terminal, a plurality of second series-connected bodies in which a second upper switching element and a second lower switching element are connected in series, and a plurality of second series-connected bodies connected in parallel between the battery positive terminal and the second ground terminal, and comprising, the other end of each of the stator coils is connected to the midpoint of each of the second series-connected bodies, the circuit selector has a motor drive mode for connecting the first ground terminal and the second ground terminal to the battery negative electrode, a first charging mode for connecting the first ground terminal to the battery negative electrode and disconnecting the second ground terminal from the battery negative electrode and connecting it to the connection positive terminal, a second charging mode for connecting the second ground terminal to the battery negative electrode and disconnecting the first ground terminal from the battery negative electrode and connecting it to the connection positive terminal, a neutral point charging mode for connecting the neutral point to the connection positive terminal and connecting the first ground terminal to the battery negative electrode, and any one of them can be selected, the controller When driving the motor with the first inverter and the second inverter, set the circuit selector to the motor drive mode and keep the neutral point switching element off. When the external power supply is connected to the power receiving terminal, Compare 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. When the second cumulative stress is the smallest, keep the neutral point switching element off and set the circuit selector to the first charging mode. Turn the first lower switching element on and off to charge the battery. When the first cumulative stress is the smallest, keep the neutral point switching element off and set the circuit selector to the second charging mode. Turn the second lower switching element on and off to charge the battery. When the third cumulative stress is the smallest, keep the neutral point switching element on and set the circuit selector to the neutral point charging mode. Turn the first lower switching element on and off to charge the battery. Charging system.

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

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

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