Vehicle and control method therefor

1. Field of the Invention

The invention relates to a vehicle that includes a three-phase alternating-current motor, an electrical storage unit that is able to exchange electric power with the three-phase alternating-current motor via an inverter, a smoothing capacitor that smoothes voltage between the inverter and the electrical storage unit and a collision detecting unit that detects a collision of the vehicle, and also to a control method for the vehicle.

2. Description of Related Art

There is suggested a vehicle of this type, which includes a motor generator, an inverter circuit that drives the motor generator, a secondary battery unit that supplies electric power to the inverter circuit via a system main relay, a high-voltage capacitor that is provided between the system main relay and the inverter circuit and a radar sensor that measures the relative speed between an obstacle and the vehicle and the distance between the obstacle and the vehicle (for example, see Japanese Patent Application Publication No. 2005-20952 (JP-A-2005-20952)). In this vehicle, when a collision of the vehicle is predicted on the basis of information from the radar sensor, the system main relay is turned off, the inverter circuit is controlled so as to discharge the electric charge remaining in the high-voltage capacitor using coils of the respective phases of the motor generator while torque is not generated from the motor generator. In addition, there is suggested a vehicle of this type, which includes a motor generator, an inverter that has pairs of serially connected upper arm transistor and lower arm transistor and that drives the motor generator and a capacitor that is provided between a direct-current power supply and the inverter, and in which, when there occurs a short-circuit failure in any one of the upper arm and lower arm transistors of the inverter, the vehicle is caused to stop running, and the upper arm or lower arm transistor serially connected to the upper arm or lower arm transistor that has a short-circuit failure is turned on to fix the upper and lower arms of the short-circuit phase in a conductive state to thereby consume the electric charge stored in the capacitor as a short-circuit current (for example, see Japanese Patent Application Publication No. 2008-54420 (JP-A-2008-54420)). In this vehicle, when there occurs a short-circuit failure in the inverter as described above, the upper and lower arms of the short-circuit phase are fixed in a conductive state to form a path that passes current, in addition to a closed circuit between the motor generator and the inverter, to thereby prevent an overcurrent state of the inverter or conductive line even when large counter electromotive force is continuously generated in the motor generator, for example, while the vehicle is being towed.

Incidentally, in a vehicle of this type, when a collision actually occurs, the electric charge stored in the capacitor between the direct-current power supply and the inverter should be promptly discharged in terms of ensuring safety. However, when the upper and lower arms of the inverter are fixed in a conductive state after occurrence of a collision of the vehicle by applying the invention described in JP-A-2008-54420, large current flows through the upper and lower arms fixed in a conductive state, so there is a concern that the transistors may be adversely influenced. Furthermore, when counter electromotive force is generated in the motor generator, current discharged from the capacitor and current resulting from the counter electromotive force flow at the same time, so there is a concern that the transistors may be adversely influenced. In addition, when current resulting from the counter electromotive force is returned to the motor generator via the inverter and half-wave current flows through the phases of the motor generator, a permanent magnet of the motor generator may possibly be demagnetized.

The invention provides a vehicle that, in the event of a collision, promptly discharges the electric charge stored in a smoothing capacitor while further appropriately protecting an inverter and a three-phase alternating-current motor.

A first aspect of the invention relates to a vehicle. The vehicle includes: a three-phase alternating-current motor; an inverter that includes a plurality of upper arm transistors, each of which tends to reduce its resistance as an input voltage increases and turns on when an input voltage higher than a predetermined value is applied, a plurality of lower arm transistors, each of which is serially connected to a corresponding one of the upper arm transistors, and a plurality of diodes, each of which is connected in antiparallel with a corresponding one of the upper arm transistors and the lower arm transistors; an electrical storage unit that is able to exchange electric power with the three-phase alternating-current motor via the inverter; a smoothing capacitor that smoothes a Voltage between the inverter and the electrical storage unit; a control unit that controls the inverter; and a collision detecting unit that detects a collision, wherein the control unit turns off all the upper arm transistors of the inverter and turns on all the lower arm transistors of the inverter after the collision detecting unit has detected a collision, and then, when no counter electromotive force is generated in the three-phase alternating-current motor, the control unit applies an input voltage lower than the predetermined value to at least any one of the upper arm transistors of the inverter, and turns on at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied.

According to the above aspect, after the collision detecting unit has detected a collision, all the upper arm transistors of the inverter are turned off, and all the lower arm transistors of the inverter are turned on. By so doing, when counter electromotive force is generated in the three-phase alternating-current motor after the collision detecting unit has detected a collision, current resulting from the counter electromotive force flows through the lower arm transistors and diodes of the inverter while being substantially equally distributed. Thus, current is prevented from concentratively flowing through a single lower arm transistor to thereby make it possible to protect the lower arm transistors, and half-wave current is not caused to flow through the phases of the three-phase alternating-current motor to thereby make it possible to prevent demagnetization of the permanent magnet of the three-phase alternating-current motor. In addition, when all the upper arm transistors are turned off and all the lower arm transistors are turned on, it is possible to prevent the upper arm transistors and lower arm transistors of the inverter from overheating because of current discharged from the smoothing capacitor and current resulting from the counter electromotive force, flowing at the same time. Note that, when counter electromotive force is generated in the three-phase alternating-current motor, torque in the direction to stop the rotation is generated in the three-phase alternating-current motor, and then no counter electromotive force is generated at the stage at which the rotation of the three-phase alternating-current motor has decreased to a certain degree because of the generated torque. In addition, as the collision detecting unit has detected a collision, all the upper arm transistors of the inverter are turned off, and all the lower arm transistors of the inverter are turned on. Then, when no counter electromotive force is generated in the three-phase alternating-current motor, an input voltage lower than the predetermined value is applied to at least any one of the upper arm transistors of the inverter, and at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied, is turned on. In this way, the resistance of at least any one of the upper arm transistors of the inverter, to which the input voltage lower than the predetermined value is applied, is larger than the resistance of the at least any one of the upper arm transistors, which is completely turned on, and a current that flows through the at least any one of the upper arm transistors reduces. Thus, when the at least any one of the lower arm transistors, corresponding to the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied, is turned on, it is possible to promptly discharge the electric charge stored in the smoothing capacitor while preventing large current from flowing through these upper arm transistors and lower arm transistors. As a result, in the vehicle according to the aspect of the invention, it is possible to promptly discharge the electric charge stored in the smoothing capacitor while further appropriately protecting the inverter and the three-phase alternating-current motor in the event of a collision.

In addition, in the above aspect, the vehicle may include a plurality of the three-phase alternating-current motors and a plurality of the inverters, and the control unit may turn off all the upper arm transistors of the plurality of inverters and may turn on all the lower arm transistors of the plurality of inverters after the collision detecting unit has detected a collision, and then, when no counter electromotive force is generated in any one of the plurality of three-phase alternating-current motors, the control unit may apply an input voltage lower than the predetermined value to at least any one of the upper arm transistors included in at least any one of the plurality of inverters and may turn on at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied.

In the vehicle according to the above aspect, after the collision detecting unit has detected a collision, all the upper arm transistors of the plurality of inverters are turned off, and all the lower arm transistors of the plurality of inverters are turned on. By so doing, when counter electromotive force is generated in any one of the three-phase alternating-current motors, only current resulting from the counter electromotive force generated in the any one of the three-phase alternating-current motors flows through the lower arm transistors and diodes of the inverter corresponding to the any one of the three-phase alternating-current motors while being substantially equally distributed. Thus, it is possible to prevent the upper arm transistors and lower arm transistors of the plurality of inverters from overheating because of current discharged from the smoothing capacitor and current resulting from the counter electromotive force, flowing at the same time. In addition, current resulting from counter electromotive force may be caused not to flow toward the smoothing capacitor, so it is possible to cause the smoothing capacitor not to store new electric charge. Furthermore, because the flow path of current resulting from the counter electromotive force is formed, the current flows through the phases of the three-phase alternating-current motor in which counter electromotive force is generated and the torque in the direction to stop the rotation is applied to the three-phase alternating-current motor, so it is possible to early decrease the rotation of the three-phase alternating-current motor to thereby stop generation of counter electromotive force. Then, in the vehicle according to this aspect, as the collision detecting unit has detected a collision, all the upper arm transistors of the plurality of inverters are turned off and all the lower arm transistors of the plurality of inverters are turned on, and then, when no counter electromotive force is generated in any one of the plurality of motors, an input voltage lower than the predetermined value is applied to at least any one of the upper arm transistors included in at least any one of the plurality of inverters, and at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied is turned on. By so doing, it is possible to promptly discharge the electric charge stored in the smoothing capacitor while preventing large current from flowing through the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied and the at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors. As a result, in the vehicle according to this aspect, it is possible to promptly discharge the electric charge stored in the smoothing capacitor while further appropriately protecting the plurality of inverters and the plurality of three-phase alternating-current motors in the event of a collision.

A second aspect of the invention relates to a control method for a vehicle. The vehicle includes a three-phase alternating-current motor, an inverter that includes a plurality of upper arm transistors, each of which tends to reduce its resistance as an input voltage increases and turns on when an input voltage higher than a predetermined value is applied, a plurality of lower arm transistors, each of which is serially connected to a corresponding one of the upper arm transistors, and a plurality of diodes, each of which is connected in antiparallel with a corresponding one of the upper arm transistors and the lower arm transistors, an electrical storage unit that is able to exchange electric power with the three-phase alternating-current motor via the inverter, a smoothing capacitor that smoothes a voltage between the inverter and the electrical storage unit, and a collision detecting unit that detects a collision. The control method includes: turning off all the upper arm transistors of the inverter and turning on all the lower arm transistors of the inverter after the collision detecting unit has detected a collision; and when no counter electromotive force is generated in the three-phase alternating-current motor, applying an input voltage lower than the predetermined value to at least any one of the upper arm transistors of the inverter, and turning on at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the input voltage lower than the predetermined value is applied.

According to the above aspects, it is possible to promptly discharge the electric charge stored in the smoothing capacitor while further appropriately protecting the inverter and the three-phase alternating-current motor in the event of a collision.

Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic configuration diagram of a hybrid vehicle according to an embodiment of the invention;

FIG. 2 is a schematic configuration diagram of an electrical drive system equipped for the hybrid vehicle according to the embodiment;

FIG. 3 is a view that illustrates an example of a path of current flowing through an inverter and a motor MG 1 when counter electromotive force is generated in the motor MG 1 according to the embodiment;

FIG. 4 is a view that illustrates an example of a path of current flowing from a smoothing capacitor to the inverter according to the embodiment;

FIG. 5 is a schematic configuration diagram of a hybrid vehicle according to an alternative embodiment to the embodiment; and

FIG. 6 is a flowchart that shows an example of collision discharge control routine executed by a motor ECU after a collision of a vehicle is detected in the hybrid vehicle according to the alternative embodiment to the embodiment.

FIG. 1 is a schematic configuration diagram of a hybrid vehicle 20 that serves as a vehicle according to an embodiment of the invention. The hybrid vehicle 20 according to the embodiment includes an engine 22 , an engine electronic control unit (hereinafter, referred to as “engine ECU”) 24 , a planetary gear 30 , a motor MG 1 , a reduction gear 35 , a motor MG 2 , drive wheels 39 a and 39 b , a battery 50 , an inverter 41 , an inverter 42 , a motor electronic control unit (hereinafter, referred to as “motor ECU”) 40 , a battery electronic control unit (hereinafter, referred to as “battery ECU”) 52 and a hybrid electronic control unit (hereinafter, referred to as “hybrid ECU”) 70 . The engine 22 uses gasoline, light oil, or the like, as fuel. The engine ECU 24 executes drive control over the engine 22 . A carrier 34 of the planetary gear 30 is connected to a crankshaft 26 via a damper 28 . The crankshaft 26 serves as an output shaft of the engine 22 . The motor MG 1 is connected to a sun gear 31 of the planetary gear 30 and is able to generate electric power. The reduction gear 35 is coupled to a ring gear shaft 32 a that is a drive shaft connected to a ring gear 32 of the planetary gear 30 . The motor MG 2 is connected to the reduction gear 35 . The drive wheels 39 a and 39 b are connected to the ring gear shaft 32 a via a gear mechanism 37 and a differential gear 38 . The battery 50 is connected to an electric power line 54 via a system main relay SMR, and is, for example, a lithium ion secondary battery or a nickel metal hydride secondary, battery. The inverter 41 is interposed between the motor MG 1 and the electric power line 54 . The inverter 42 is interposed between the motor MG 2 and the electric power line 54 . The motor ECU 40 executes drive control over the motors MG 1 and MG 2 via the inverters 41 and 42 . The battery ECU 52 manages the battery 50 . The hybrid ECU 70 controls the vehicle overall while communicating with the engine ECU 24 , the motor ECU 40 , the battery ECU 52 , and the like.

The motor MG 1 and the motor MG 2 each are configured as a known synchronous generator motor that includes a rotor in which a permanent magnet is embedded and a stator in which three phase coils are wound. As shown in FIG. 2 , the inverter 41 is formed of six transistors T 11 to T 16 and six diodes D 11 to D 16 that are respectively connected in antiparallel with the transistors T 11 to T 16 , and the inverter 42 is formed of six transistors T 21 to T 26 and six diodes D 21 to D 26 that are respectively connected in antiparallel with the transistors T 21 to T 26 . Each of the transistors T 11 to T 16 and T 21 to T 26 tends to reduce its resistance as the gate voltage (input voltage) increases, and turns on when the gate voltage higher than or equal to a predetermined value V 1 is applied. In addition, the transistors T 11 to T 16 and T 21 to T 26 are arranged in pairs so as to be a source side and a sink side with respect to a positive electrode bus 54 a and a negative electrode bus 54 b that are shared by the inverters 41 and 42 as the electric power line 54 , three phase coils (U phase, V phase and W phase) of the motors MG 1 and MG 2 each are connected to a connecting point between a corresponding one of the pairs of transistors. Thus, the percentage of an ON time of each of the transistors T 11 to T 16 is controlled in a state where voltage is applied between the positive electrode bus 54 a and the negative electrode bus 54 b to thereby make it possible to form a revolving magnetic field in the three phase coils. Thus, it is possible to drive the motor MG 1 for rotation. Similarly, the percentage of an ON time of each of the transistors T 21 to T 26 is controlled in a state where voltage is applied between the positive electrode bus 54 a and the negative electrode bus 54 b to thereby make it possible to form a revolving magnetic field in the three phase coils. Thus, it is possible to drive the motor MG 2 for rotation. In addition, a smoothing capacitor 57 is connected between the positive electrode bus 54 a and the negative electrode bus 54 b at a position adjacent to the inverters 41 and 42 with respect to the system main relay SMR. The smoothing capacitor 57 smoothes voltage between the inverters 41 and 42 and the battery 50 . Hereinafter, the three transistors T 11 to T 13 of the inverter 41 and the three transistors T 21 to T 23 of the inverter 42 are termed “upper arm transistors”, and the three transistors T 14 to T 16 of the inverter 41 , each of which is serially connected to a corresponding one of the upper arm transistors T 11 to T 13 , and the three transistors T 24 to T 26 of the inverter 42 , each of which is serially connected to a corresponding one of the upper arm transistors T 21 to T 23 , are termed “lower arm transistors”.

The inverter 41 is controlled by the motor ECU 40 to execute drive control over the motor MG 1 , and the inverter 42 is controlled by the motor ECU 40 to execute drive control over the motor MG 2 . A signal required to execute drive control over the motor MG 1 is input to the motor ECU 40 , and a signal required to execute drive control over the motor MG 2 is input to the motor ECU 40 . The signals, for example, include a signal from a rotational position detecting sensor 43 that detects the rotational position of a rotor of the motor MG 1 , a signal from a rotational position detecting sensor 44 that detects the rotational position of a rotor of the motor MG 2 , phase currents Iu 1 and Iv 1 from current sensors 45 U and 45 V that respectively detect phase currents flowing through the U phase and V phase of the three phase coils of the motor MG 1 , phase currents Iu 2 and Iv 2 from current sensors 46 U and 46 V that respectively detect phase currents flowing through the U phase and V phase of the three phase coils of the motor MG 2 , a voltage VH from a voltage sensor 57 a that detects the voltage of the smoothing capacitor 57 , and the like. In addition, switching control signals are output from the motor ECU 40 to the transistors T 11 to T 16 of the inverter 41 and the transistors T 21 to T 26 of the inverter 42 . Then, the motor ECU 40 communicates with the hybrid ECU 70 , executes drive control over the motors MG 1 and MG 2 by control signals from the hybrid ECU 70 , and, where necessary, outputs data about the operating states of the motors MG 1 and MG 2 to the hybrid ECU 70 . Note that the motor ECU 40 also computes the rotational speed Nm 1 of the motor MG 1 on the basis of the signal from the rotational position detecting sensor 43 and computes the rotational speed Nm 2 of the motor MG 2 on the basis of the signal from the rotational position detecting sensor 44 .

When the thus configured hybrid vehicle 20 according to the embodiment is ready to run, the hybrid ECU 70 sets a required torque Tr on the basis of an accelerator operation amount Acc from an accelerator pedal position sensor 84 that detects the depression amount of an accelerator pedal 83 and a vehicle speed V from a vehicle speed sensor 88 , generates command signals, such as a target rotational speed Ne and target torque Te of the engine 22 , a torque command Tm 1 of the motor MG 1 and a torque command Tm 2 of the motor MG 2 , on the basis of the set required torque Tr, and transmits the generated command signals to the engine ECU 24 and the motor ECU 40 . In addition, the acceleration of the vehicle from an acceleration sensor 58 is input to the hybrid ECU 70 . The acceleration sensor 58 serves as a collision detecting unit that is attached to a vehicle body (for example, the center portion, side portions, or the like, of the front of the vehicle). Then, the hybrid ECU 70 , for example, determines that there occurs a collision of the vehicle when the input acceleration exceeds a collision determination threshold or a threshold for activating a safety device, such as an airbag, turns off the system main relay SMR to isolate the battery 50 from the electrical system of the vehicle in order to increase the safety of the vehicle in the event of a collision, and transmits a predetermined collision occurrence signal to the ECUs 24 , 40 , 52 , and the like.

In the above hybrid vehicle 20 , in the event of a collision, the electric charge stored in the smoothing capacitor 57 should be promptly discharged in order to early reduce the gate voltages of the inverters 41 and 42 . Then, when the hybrid ECU 70 determines that there occurs a collision, the motor ECU 40 according to the embodiment controls at least any one of the inverters 41 and 42 in accordance with the following procedure to thereby discharge the electric charge stored in the smoothing capacitor 57 .

The motor ECU 40 according to the embodiment receives the collision occurrence signal from the hybrid ECU 70 as the hybrid ECU 70 determines that there occurs a collision of the vehicle. When the motor ECU 40 receives the signal, the motor ECU 40 turns off all the upper arm transistors T 11 to T 13 of the inverter 41 and the upper arm transistors T 21 to T 23 of the inverter 42 and turns on all the lower arm transistors T 14 to T 16 of the inverter 41 and the lower arm transistors T 24 to T 26 of the inverter 42 , and then determines whether counter electromotive force is generated in any one of the motors MG 1 and MG 2 on the basis of the phase currents Iu 1 and Iv 1 of the motor MG 1 , detected by the current sensors 45 U and 45 V, and the phase currents Iu 2 and Iv 2 of the motor MG 2 , detected by the current sensors 46 U and 46 V. That is, when the right or left drive wheel 39 b or 39 a rotates at an idle in a state where the right or left drive wheel 39 b or 39 a is separated from a road surface after a collision of the hybrid vehicle 20 or when the hybrid vehicle 20 is not completely stopped because of an oblique collision, the motor MG 1 or MG 2 may continue rotating even after occurrence of a collision. In addition, even when rotation of the drive wheels 39 a and 39 b is stopped after occurrence of a collision, the motor MG 1 may be forced by the engine 22 to continue rotating if the engine 22 is operating. Then, in such a case, when counter electromotive force is generated in any one of the motors MG 1 and MG 2 and then all the upper arm transistors T 11 to T 13 of the inverter 41 and all the upper arm transistors T 21 to T 23 of the inverter 42 are turned off and all the lower arm transistors T 14 to T 16 of the inverter 41 and all the lower arm transistors T 24 to T 26 of the inverter 42 are turned on, current resulting from the counter electromotive force flows through the phases of the motors MG 1 and MG 2 and the phases of the inverters 41 and 42 as indicated by the broken arrows in FIG. 3 . Thus, by monitoring the phase currents Iu 1 , Iv 1 , Iu 2 and Iv 2 , it is possible to determine whether counter electromotive force is generated in any one of the motors MG 1 and MG 2 . Note that, instead of the phase currents Iu 1 , Iv 1 , Iu 2 and Iv 2 , it may be determined on the basis of the rotational speeds Nm 1 and Nm 2 of the motors MG 1 and MG 2 whether counter electromotive force is generated in any one of the motors MG 1 and MG 2 . In any cases, the motor ECU 40 according to the embodiment functions as a counter electromotive force generation determining unit that determines whether counter electromotive force is generated in any one of the motors MG 1 and MG 2 .

When the motor ECU 40 determines that counter electromotive force is generated in at least any one of the motors MG 1 and MG 2 after occurrence of a collision, the motor ECU 40 continues to turn off all the upper arm transistors T 11 to T 13 of the inverter 41 and/or upper arm transistors T 21 to T 23 of the inverter 42 , connected to the motors MG 1 and/or MG 2 in which counter electromotive force is generated, and to turn on all the lower arm transistors T 14 to T 16 of the inverter 41 and/or lower arm transistors T 24 to T 26 of the inverter 42 , connected to the motors MG 1 and/or MG 2 in which counter electromotive force is generated. By so doing, for example, when counter electromotive force is generated in the motor MG 1 , current resulting from the counter electromotive force circulates between the inverter 41 and the motor MG 1 while being substantially equally distributed among the lower arm transistors T 14 to T 16 and the diodes D 14 to D 16 of the inverter 41 as indicated by the broken arrows in FIG. 3 . Here, in the case where all the upper arm transistors T 11 to T 13 of the inverter 41 are turned off and only the transistor T 14 among the lower arm transistors is turned on when counter electromotive force is generated in the motor MG 1 , current resulting from the counter electromotive force of the motor MG 1 all passes through the transistor T 14 and then flows again to the motor MG 1 via the diodes T 15 and T 16 as indicated by the alternate long and two short dashes line in FIG. 3 , so there is a concern that current resulting from the counter electromotive force of the motor MG 1 concentratively flows through the transistor T 14 to adversely influence the transistor T 14 and, in some cases, the transistor T 14 breaks. In addition, in this case, half-wave current flows (only current in any one of positive and negative directions constantly flows) through the U, V and W phases of the motor MG 1 , so there is a concern that the permanent magnet of the motor MG 1 is demagnetized. In contrast to this, as in the case of the hybrid vehicle 20 according to the present embodiment, for example, when it is determined that counter electromotive force is generated in the motor MG 1 after occurrence of a collision, all the upper arm transistors T 11 to T 13 of the inverter 41 are turned off, and all the lower arm transistors T 14 to T 16 of the inverter 41 are turned on. This prevents current from concentratively flowing through any one of the lower arm transistors T 14 to T 16 to make it possible to protect the lower arm transistors T 14 to T 16 , and stops half-wave current from flowing through the phases of the motor MG 1 to make it possible to prevent demagnetization of the permanent magnet of the motor MG 1 . In addition, when all the upper arm transistors T 11 to T 13 of the inverter 41 are turned off and all the lower arm transistors T 14 to T 16 of the inverter 41 are turned on, current discharged from the smoothing capacitor 57 and current resulting from the counter electromotive force do not flow at the same time to thereby make it possible to prevent the transistors T 11 to T 16 of the inverter 41 from overheating. Note that when counter electromotive force is generated in any one of the motors MG 1 and MG 2 , torque in the direction to stop the rotation is generated in the any one of the motors MG 1 and MG 2 , so the rotation of the any one of the Motors MG 1 and MG 2 early decreases, and no counter electromotive force is generated at the stage at which the rotation has decreased to a certain degree.

Then, when the motor ECU 40 determines that no counter electromotive force is generated in any one of the motors MG 1 and MG 2 immediately after occurrence of a collision, or when counter electromotive force is generated in at least any one of the motors MG 1 and MG 2 immediately after occurrence of a collision but the rotation of the motors MG 1 and/or MG 2 decreases and then no counter electromotive force is generated, the motor ECU 40 applies a gate voltage V 2 lower than the predetermined value V 1 to at least any one of the upper arm transistors T 11 to T 13 of the inverter 41 and the upper arm transistors T 21 to T 23 of the inverter 42 , and applies a gate voltage higher than or equal to the predetermined value V 1 to at least any one of the lower arm transistors T 14 to T 16 of the inverter 41 and the lower arm transistors T 24 to T 26 of the inverter 42 , which is serially connected to the at least any one of the upper arm transistors to which the gate voltage V 2 is applied, to thereby completely turn on the at least any one of the lower arm transistors T 14 to T 16 . For example, as shown in FIG. 4 , when the gate voltage V 2 is applied to the upper arm transistor T 11 of the inverter 41 and the lower arm transistor T 14 serially connected to the upper arm transistor T 11 is completely turned on, the upper arm transistor T 11 enters a so-called half-on state, so the resistance of the upper arm transistor T 11 is larger than the resistance of the upper arm transistor T 11 , which is completely turned on, and a current that flows through the upper arm transistor T 11 is smaller than a current that flows through the upper arm transistor T 11 completely turned on. Then, when the upper arm transistor T 11 is placed in a half-on state and the lower arm transistor T 14 is completely turned on, current from the smoothing capacitor 57 flows through the upper arm transistor T 11 and the lower arm transistor T 14 as indicated by the broken arrow in FIG. 4 . Thus, it is possible to cause, the smoothing capacitor 57 to promptly discharge while preventing large current from flowing through the upper arm transistor T 11 and the lower arm transistor T 14 owing to an increase in the resistance of the half-on upper arm transistor T 11 .

Note that, in the hybrid vehicle 20 according to the embodiment, the number of the half-on upper arm transistors of the inverters 41 and 42 is determined on the basis of the voltage VH of the smoothing capacitor 57 at the time when it is determined that no counter electromotive force is generated in any one of the motors MG 1 and MG 2 after occurrence of a collision. That is, when it is determined that no counter electromotive force is generated in any one of the motors MG 1 and MG 2 after occurrence of a collision, for example, any one of the upper arm transistors T 11 to T 13 of the inverter 41 and any one of the upper arm transistors T 21 to T 23 of the inverter 42 may be placed in a half-on state, all the upper arm transistors T 11 to T 13 of the inverter 41 or all the upper arm transistors T 21 to T 23 of the inverter 42 may be placed in a half-on state or all the upper arm transistors T 11 to T 13 of the inverter 41 and all the upper arm transistors T 21 to T 23 of the inverter 42 may be placed in a half-on state on the basis of the voltage VH of the smoothing capacitor 57 . In addition, when it is determined that no counter electromotive force is generated in any one of the motors MG 1 and MG 2 after occurrence of a collision, the lower arm transistor(s) corresponding to the half-on upper arm transistor(s) just needs to be turned on; instead, all the lower arm transistors of the inverters 41 and/or 42 that include the half-on upper arm transistor(s) may be turned on.

As described above, in the hybrid vehicle 20 according to the embodiment, after the hybrid ECU 70 determines that there occurs a collision on the basis of the acceleration of the vehicle from the acceleration sensor 58 , all the upper arm transistors T 11 to T 13 and/or T 21 to T 23 of the inverters 41 and/or 42 connected to the motors MG 1 and/or MG 2 in which counter electromotive force is generated are turned off and all the lower arm transistors T 14 to T 16 and/or T 24 to T 26 of the inverters 41 and/or 42 connected to the motors MG 1 and/or MG 2 in which counter electromotive force is generated are turned on. By so doing, current resulting from the counter electromotive force flows through the lower arm transistors T 14 to T 16 and/or T 24 to T 26 and the diodes D 14 to D 16 and/or D 24 to D 26 while being substantially equally distributed. Thus, current is prevented from concentratively flowing through any one of the lower arm transistors T 14 to T 16 and T 24 to T 26 to thereby make it possible to protect the lower arm transistors T 14 to T 16 and T 24 to T 26 , and half-wave current is not caused to flow through the phases of the motor MG 1 or MG 2 to thereby make it possible to prevent demagnetization of the permanent magnets of the motors MG 1 and MG 2 . In addition, when all the upper arm transistors T 11 to T 13 and/or T 21 to T 23 are turned off and all the lower arm transistors T 14 to T 16 and/or T 24 to T 26 are turned on, it is possible to prevent the upper arm transistors T 11 to T 13 and T 21 to T 23 and the lower arm transistors T 14 to T 16 and T 24 to T 26 of the inverters 41 and 42 from overheating because of current discharged from the smoothing capacitor 57 and current resulting from the counter electromotive force, flowing at the same time. Note that, when counter electromotive force is generated in any one of the motors MG 1 and MG 2 , torque in the direction to stop the rotation is generated in the any one of the motors MG 1 and MG 2 , so no counter electromotive force is generated at the stage at which the rotation of the any one of the motors. MG 1 and MG 2 has decreased to a certain degree because of the generated torque. In addition, when it is determined that no counter electromotive force is generated in any one of the motors MG 1 and MG 2 after it is determined that there occurs a collision and then all the upper arm transistors T 11 to T 13 and/or T 21 to T 23 of the inverters 41 and/or 42 connected to the motors MG 1 and/or MG 2 in which counter electromotive force is generated are turned off and all the lower arm transistors T 14 to T 16 and/or T 24 to T 26 of the inverters 41 and/or 42 connected to the motors MG 1 and/or MG 2 in which counter electromotive force is generated, the gate voltage V 2 lower than the predetermined value V 1 is applied to at least any one of the upper arm transistors T 11 to T 13 and T 21 to T 23 of the inverters 41 and 42 , and at least any one of the lower arm transistors T 14 to T 16 and the T 24 to T 26 , which is serially connected to the at least any one of the upper arm transistors to which the gate voltage V 2 is applied, is turned on: In this way, the resistance of at least any one of the upper arm transistors T 11 to T 13 and T 21 to T 23 of the inverters 41 and 42 , to which the gate voltage V 2 lower than the predetermined value V 1 is applied, is larger than the resistance of the at least any one of the upper arm transistors T 11 to T 13 and T 21 to T 23 , which is completely turned on, and a current that flows through the at least any one of the upper arm transistors reduces. Thus, when at least any one of the lower arm transistors, corresponding to the at least any one of the upper arm transistors to which the gate voltage V 2 lower than the predetermined value V 1 is applied, is turned on, it is possible to promptly discharge the electric charge stored in the smoothing capacitor 57 while preventing large current from flowing through these upper arm transistor(s) and lower arm transistor(s). As a result, in the vehicle according to the present embodiment, it is possible to promptly discharge the electric charge stored in the smoothing capacitor 57 while further appropriately protecting the inverters 41 and 42 and the motors MG 1 and MG 2 in the event of a collision.

FIG. 5 is a schematic configuration diagram that shows a hybrid vehicle 20 B according to an alternative embodiment to the present embodiment. Note that, in order to avoid overlap description, like reference numerals denote similar components of the hybrid vehicle 20 B to those of the hybrid vehicle 20 according to the above described embodiment, and the illustration and detailed description are omitted. In addition, for the sake of easy description, the reduction gear 35 , the gear mechanism 37 , and the like, are not shown in FIG. 5 . The hybrid vehicle 20 B shown in the drawing includes, in addition to the components of the above described hybrid vehicle 20 , for example, a motor MG 3 and an inverter 47 . The motor MG 3 is configured as a synchronous generator motor and inputs or outputs power to or from a rear wheel-side drive shaft 32 b that is coupled to rear wheels 39 c and 39 d . The inverter 47 is connected to the battery 50 via the smoothing capacitor 57 in parallel with the inverters 41 and 42 , and drives the motor MG 3 . The inverter 47 has the same configuration as the configurations of the above described inverters 41 and 42 . In the hybrid vehicle 20 B that includes the plurality of motors MG 1 to MG 3 and the inverters 41 , 42 and 47 in this way as well, when there occurs a collision of the vehicle, the electric charge stored in the smoothing capacitor 57 should be promptly discharged in order to early decrease the gate voltages of the inverters 41 , 42 and 47 . Therefore, in the hybrid vehicle 20 B according to the alternative embodiment, when the motor ECU 40 receives a collision occurrence signal from the hybrid ECU 70 as the hybrid ECU 70 determines that there occurs a collision of the vehicle, the motor ECU 40 executes collision discharge control routine illustrated in FIG. 6 .

When the collision discharge control routine is started, the motor ECU 40 initially turns off all the upper arm transistors of all the inverters 41 , 42 and 47 and turns on all the lower arm transistors of all the inverters 41 , 42 and 47 (S 100 in FIG. 6 ). Subsequently, on the basis of the phase currents that flow through the phases of the motors MG 1 , MG 2 and MG 3 , detected by current sensors (not shown), it is determined whether counter electromotive force is generated in any one of the motors MG 1 , MG 2 and MG 3 (S 110 in FIG. 6 ). Then, when the motor ECU 40 determines that counter electromotive force is generated in any one of the motors MG 1 , MG 2 and MG 3 after a collision is detected, the motor ECU 40 at least continues to turn off all the upper arm transistors of the inverter connected to the any one of the motors MG 1 , MG 2 and MG 3 in which counter electromotive force is generated and continues to turn on all the lower arm transistors of that inverter.

On the other hand, when the motor ECU 40 determines that no counter electromotive force is generated in any one of the motors MG 1 , MG 2 and MG 3 after a collision is detected, or when counter electromotive force is generated in any one of the motors MG 1 , MG 2 and MG 3 after a collision is detected but the rotation of that motor has decreased and then no counter electromotive force is generated, the motor ECU 40 executes half-on discharge control in which the gate voltage V 2 lower than the predetermined value V 1 is applied to at least any one of the upper arm transistors included in at least any one of the inverters 41 , 42 and 47 and a gate voltage higher than the predetermined value V 1 is applied to at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the gate voltage V 2 is applied, to completely turn on the at least any one of the lower arm transistors (S 120 in FIG. 6 ). By so doing, the at least any one of the upper arm transistors to which the gate voltage V 2 is applied is placed in a so-called half-on state, so the resistance of the at least any one of the upper arm transistors is larger than the resistance of the at least any one of the upper arm transistors, which is completely turned on, and a current that flows through the at least any one of the upper arm transistors is smaller than a current that flows through the at least any one of the upper arm transistors, completely turned on. Then, the target upper arm transistor(s) is placed in a half-on state and the target lower arm transistor(s) is completely turned on. By so doing, current from the smoothing capacitor 57 flows through those upper arm transistor(s) and lower arm transistor(s). Thus, it is possible to cause the smoothing capacitor 57 to promptly discharge while preventing large current from flowing through the upper arm transistor(s) and the lower arm transistor(s) owing to an increase in the resistance of the half-on upper arm transistor(s).

After the motor ECU 40 starts half-on discharge control, the motor ECU 40 determines whether the electric charge stored in the smoothing capacitor 57 has been completely discharged on the basis of the voltage VH, which is the voltage between the terminals of the smoothing capacitor 57 , detected by a voltage sensor (not shown) (S 130 in FIG. 6 ), and, when it is determined that the electric charge stored in the smoothing capacitor 57 has not been completed, the motor ECU 40 continues the above half-on discharge control. Then, when it is determined that the electric charge stored in the smoothing capacitor 57 has been completely discharged, all the inverters are shut down (S 140 in FIG. 6 ), and the routine ends. Note that, in S 120 of FIG. 6 , the number of inverters that include a half-on upper arm transistor or the number of half-on upper arm transistors in that inverter may be selectively determined.

As described above, in the hybrid vehicle 20 B according to the alternative embodiment, after the hybrid ECU 70 determines that there occurs a collision on the basis of the acceleration of the vehicle from the acceleration sensor 58 , all the upper arm transistors of all the inverters 41 , 42 and 47 are turned off, and all the lower arm transistors of all the inverters 41 , 42 and 47 are turned on. By so doing, when counter electromotive force is generated in any one of the motors MG 1 , MG 2 and MG 3 , only current resulting from the counter electromotive force generated in the any one of the motors flows through the lower arm transistors and diodes of the inverter corresponding to that motor while being substantially equally distributed. Thus, it is possible to prevent the upper arm transistors and lower arm transistors of the plurality of inverters 41 , 42 and 47 from overheating because of current discharged from the smoothing capacitor 57 and current resulting from the counter electromotive force, flowing at the same time. In addition, current resulting from the counter electromotive force may be caused not to flow toward the smoothing capacitor 57 , so it is possible to cause the smoothing capacitor 57 not to store new electric charge. Furthermore, because the flow path of current resulting from the counter electromotive force is formed, the current flows through the phases of the motor in which counter electromotive force is generated and the torque in the direction to stop the rotation is applied to the motor, so it is possible to early decrease the rotation of the motor to stop generation of counter electromotive force. Then, in the hybrid vehicle 20 B, all the upper arm transistors of all the inverters 41 , 42 and 47 are turned off as the hybrid ECU 70 detects a collision. Then, all the lower arm transistors of all the inverters 41 , 42 and 47 are turned on. After that, when no counter electromotive force is generated in any one of the motors MG 1 , MG 2 and MG 3 , the gate voltage V 2 lower than the predetermined value V 1 is applied to at least any one of the upper arm transistors included in at least any one of the inverters 41 , 42 and 47 . Then, at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors to which the gate voltage V 2 is applied, is turned on. By so doing, it is possible to promptly discharge the electric charge stored in the smoothing capacitor 57 while preventing large current from flowing through the at least any one of the upper arm transistors to which the gate voltage V 2 lower than the predetermined value V 1 is applied and the at least any one of the lower arm transistors, which is serially connected to the at least any one of the upper arm transistors. As a result, in the hybrid vehicle 20 B according to the alternative embodiment, it is possible to promptly discharge the electric charge stored in the smoothing capacitor 57 while appropriately protecting the inverters 41 , 42 and 47 and the plurality of motors MG 1 , MG 2 and MG 3 in the event of a collision.

Note that the aspect of the invention is described by taking the hybrid vehicle 20 or 20 B that includes the plurality of motors MG 1 , MG 2 , and the like; however, of course, the aspect of the invention may be applied to a hybrid vehicle that includes a single electric motor that is able to input or output power or an electric vehicle that includes a single or plurality of electric motors that are able to input or output power.

A mode for carrying out the aspect of the invention is described using the embodiments; however, the aspect of the invention is not limited to such embodiments. The aspect of the invention may be implemented in various forms without departing from the scope of the invention.