Patent Publication Number: US-2022224257-A1

Title: Rotating body drive system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the U.S. bypass application of International Application No. PCT/JP2020/036485 filed on Sep. 25, 2020 which designated the U.S. and claims priority to Japanese Patent Application No. 2019-181703 filed on Oct. 1, 2019, the contents of both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to rotating body drive systems that drive rotating bodies. 
     BACKGROUND 
     Vehicle drive systems are often provided with a plurality of drive systems (power systems) each including a motor that drives a wheel and an inverter that drives the motor. Such a technique is shown in JP 2009-35243 A. 
     SUMMARY 
     A rotating body drive system according to the present disclosure includes a first motor driving a predetermined first rotating body, a second motor driving a second rotating body different from the first rotating body, a first inverter driving the first motor, and a second inverter driving the second motor. 
     The rotating body drive system further includes a first switch, a gang switch, a determination unit, and a control unit. The first switch, when turned ON, connects between the first inverter and the first motor so that current can be passed therethrough and, when turned OFF, disconnects the connection. The gang switch, when turned ON, connects between the first inverter and the second motor so that current can be passed therethrough and, when turned OFF, disconnects the connection. The determination unit determines whether the first rotating body is in an abnormal state. The control unit controls the first switch and the gang switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features of the present disclosure will be made clearer by the following detailed description, given referring to the appended drawings. In the accompanying drawings: 
         FIG. 1  is a schematic diagram illustrating a rotating body drive system according to a first embodiment; 
         FIG. 2  is a circuit diagram illustrating a rotating body drive system; 
         FIG. 3  is a circuit diagram illustrating a rotating body drive system in a normal state; 
         FIG. 4  is a circuit diagram illustrating a rotating body drive system in a first countermeasure state; 
         FIG. 5  is a circuit diagram illustrating a rotating body drive system in a second countermeasure state; 
         FIG. 6  is a flowchart illustrating connection control; and 
         FIGS. 7A and 7B  are simplified circuit diagrams of the circuit diagrams shown in  FIGS. 4 and 5 , respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the vehicle drive systems in JP 2009-35243 A, malfunctions that could occur in any of the drive systems can be compensated by other drive systems. Therefore, vehicles are prevented from being brought into a state in which they cannot travel. 
     However, even with such vehicle drive systems, if any of the drive wheels has been caught in mud, snow, or the like on the road surface or run on a frozen road which deteriorates traction of the wheel, only the torque of the wheels other than the wheel in question can be transferred to the ground, as in normal vehicle drive systems. Therefore, it will be difficult, for the vehicle to escape from the mud, snow, or the like on the road surface or the frozen road. 
     To ensure traveling performance which is sufficient for the vehicle to move out of the mud, snow, or the like on the road surface or the frozen road, using only the torque of the drive wheels other than the drive wheel deteriorated in traction, it will be necessary to use high power inverters. 
     Similar issues could be raised in rotating body drive systems other than the vehicle drive systems. Specifically, in drone drive systems, for example, if one or more propellers of the drone have broken down due to damage or the like, the drone has to perform a soft landing while maintaining flight, using only the power of the remaining propellers. In this case, to ensure flight performance sufficient for the drone to maintain flight using only the remaining propellers, it will be necessary to use high power inverters. 
     The present disclosure has been made in light of the circumstances described above and mainly aims that, in a predetermined abnormal state of a rotating body, sufficiently high power is ensured in other rotating bodies without the need of providing a high-power inverter. 
     A rotating body drive system according to the present disclosure includes a first motor driving a predetermined first rotating body, a second motor driving a second rotating body different from the first rotating body, a first inverter driving the first motor, and a second inverter driving the second motor. 
     The rotating body drive system further includes a first switch, a gang switch, a determination unit, and a control unit. The first switch, when turned ON, connects between the first inverter and the first motor so that current can be passed therethrough and, when turned OFF, disconnects the connection. The gang switch, when turned ON, connects between the first inverter and the second motor so that current can be passed therethrough and, when turned OFF, disconnects the connection. The determination unit determines whether the first rotating body is in an abnormal state. The control unit controls the first switch and the gang switch. 
     In the rotating body drive system, the control unit establishes a normal state in which the first switch is ON and the gang switch is OFF in the case where the determination unit determines that there is no abnormality in the state of the first rotating body. In this normal state, the first motor is driven by the first inverter, while the second motor is driven by the second inverter. 
     On the other hand, in the rotating body drive system, the control unit establishes a first countermeasure state in which the first switch is OFF and the gang switch is ON in the case where the determination unit determines that there is abnormality in the state of the first rotating body. In this first countermeasure state, the second motor is driven by both the first inverter and the second inverter. 
     According to the present disclosure, in the case where the determination unit determines that there is abnormality in the state of the first rotating body, the control unit establishes a first countermeasure state so that the second motor can be driven by both the first inverter and the second inverter. Accordingly, compared to the case where the second motor is driven using only the second inverter, torque of the second rotating body can be enhanced. Thus, sufficiently high power can be ensured in the second rotating body when the first rotating body is in an abnormal state, without the need of providing a high-power second inverter. 
     First Embodiment 
     Referring to the drawings, some embodiments of the present disclosure will be described. It should be noted, however, that the present disclosure should not be construed as being limited to these embodiments but may be appropriately modified and implemented in the scope not departing from the spirit of the present disclosure. 
       FIG. 1  is a schematic diagram illustrating a vehicle drive system  300  according to the present embodiment. The vehicle drive system is installed in a vehicle  400 , and includes a host ECU  10 , a battery  20 , a first drive system  100 , a second drive system  200 , and a gang switch  50 . 
     The first drive system  100  includes a first wheel  190 , a first motor  160 , a first inverter  120 , a first control unit  110 , and a first switch  150 . In the present embodiment, the first wheel  190  corresponds to a front-left wheel and is rotated together with a first rotary shaft  180 . 
     When the first rotary shaft  180  is rotatably driven via reduction gears or the (not shown), the first motor  160  rotatably drives the first wheel  190 . The first inverter  120  converts power of direct current supplied from the battery  20  into alternating current and supplies the converted alternating current to the first motor  160  to drive the first motor  160 . The first control unit  110  controls the first inverter  120  to control the first motor  160  driving the first wheel  190 . 
     Specifically, the first drive system  100  inputs first motor information i 1  obtained based on the first motor  160  being driven, into the first control unit  110 . The first motor information i 1  may include, for example, information related to rotation angle of the rotor of the first motor  160  relative to the stator thereof, information related to current passed through a U-phase coil  164 , a V-phase coil  165 , and a W-phase coil  166  of the stator of the first motor  160 , or other information. The first control unit  110  controls the first inverter  120  using the first motor information i 1  to control the first wheel  190  being driven. 
     The second drive system  200  includes a second wheel  290 , a second motor  260 , a second inverter  220 , a second control unit  210 , and a second switch  250 . The second wheel  290  corresponds to a front-right wheel and is rotated together with a second rotary shaft  280 . 
     A more specific description of the second drive system  200  is similar to the description of the first drive system  100  above, reading as follows. In other words, the term first can be read as second, the term left can be read as right, the term U phase can be read as X phase, the term V phase can be read as Y phase, the term W phase can be read as Z phase, and the reference signs can be read as corresponding reference signs. 
       FIG. 2  is a circuit diagram illustrating the vehicle drive system  300 . First, the first drive system  100  will be described. The first motor  160  includes a U-phase coil  164 , a V-phase coil  165 , and a W-phase coil  166 , with first ends of these coils being connected to each other via a neutral point. 
     The first inverter  120  includes a first upper wiring  122 , three upper arms ( 124  to  126 ), three connecting wires ( 134  to  136 ), three lower arms ( 144  to  146 ), and a first lower wiring  148 . 
     The first upper wiring  122  has a first end connected to the positive terminal of the battery  20 . The first lower wiring  148  has a first end connected to the negative terminal of the battery  20 . 
     The three connecting wires ( 134  to  136 ) include a U-phase wire  134 , a V-phase wire  135 , and a W-phase wire  136 . The U-phase wire  134  has a first end connected to an end of the U-phase coil  164  opposite to the neutral point. The V-phase wire  135  has a first end connected to an end of the V-phase coil  165  opposite to the neutral point. The W-phase wire  136  has a first end connected to an end of the W-phase coil  166  opposite to the neutral point. 
     The three upper arms ( 124  to  126 ) include a U-phase upper arm  124 , a V-phase upper arm  125 , and a W-phase upper arm  126 . The U-phase upper arm  124  has a first end connected to the first upper wiring  122 , and a second end connected to the U-phase wire  134 . Also, the U-phase upper arm  124  is provided with a U-phase upper switch Ua at an intermediate portion thereof in the length direction. The V-phase upper arm  125  has a first end connected to the first upper wiring  122 , and a second end connected to the V-phase wire  135 . Also, the V-phase upper arm  125  is provided with a V-phase upper switch Va at an intermediate portion thereof in the length direction. The W-phase upper arm  126  has a first end connected to the first upper wiring  122 , and a second end connected to the W-phase wire  136 . Also, the W-phase upper arm  126  is provided with a W-phase upper switch Wa at an intermediate portion thereof in the length direction. 
     The three lower arms ( 144  to  146 ) include a U-phase lower arm  144 , a V-phase lower arm  145 , and a W-phase lower arm  146 . The U-phase lower arm  144  has a first end connected to the first lower wiring  148 , and a second end connected to the U-phase wire  134 . Also, the U-phase lower arm  144  is provided with a U-phase lower switch Ub at an intermediate portion thereof in the length direction. The V-phase lower arras  145  has a first end connected to the first lower wiring  148 , and a second end connected to the V-phase wire  135 . Also, the V-phase lower arm  145  is provided with a V-phase lower switch Vb at an intermediate portion thereof in the length direction. The W-phase lower arm  146  has a first end connected to the first lower wiring  148 , and a second end connected to the W-phase wire  136 . Also, the W-phase lower a  146  is provided with a W-phase lower switch Wb at an intermediate portion thereof in the length direction. 
     The three connecting wires ( 134  to  136 ) are provided with the first switch  150 . Specifically, the first switch  150  includes a U-phase switch  154 , a V-phase switch  155 , and a W-phase switch  156 . The U-phase switch  154  is provided between the upper and lower arms ( 124 ,  144 ) of the U-phase wire  134  and the U-phase coil  164 . The V-phase switch  155  is provided between the upper and lower arms ( 125 ,  145 ) of the V-phase wire  135  and the V-phase coil  165 . The W-phase switch  156  is provided between the upper and lower arms ( 126 ,  146 ) of the W-phase wire  136  and the W-phase coil  166 . 
     In the following description, the expression that the first switch  150  is ON refers to that the three switches ( 154  to  156 ) configuring the first switch  150  are all ON, and the expression that the first switch  150  is OFF refers to that the three switches ( 154  to  156 ) are all OFF. The first switch  150 , when turned ON, connects between the first inverter  120  and the first motor  160  so that current can be passed therethrough and, when turned OFF, disconnects the connection. 
     The first control unit  110  controls ON and OFF states of the three upper switches (Ua, Va, Wa) and the three lower switches (Ub, Vb, Wb) to control the first inverter  120 . 
     Next, the second drive system  200  will be described. The second motor  260  includes an X-phase coil  264 , a Y-phase coil  265 , and a Z-phase coil  266 . The second inverter  220  includes a second upper wiring  222 , three upper arms ( 224  to  226 ), three connecting wires ( 234  to  236 ), three lower arms ( 244  to  246 ), and a second lower wiring  248 . 
     The three upper arms ( 224  to  226 ) include an X-phase upper arm  224 , a Y-phase upper arm  225 , and a Z-phase upper arm  226 . The X-phase upper arm  224  is provided with an X-phase upper switch Xa, the Y-phase upper arm  225  is provided with a Y-phase upper switch Ya, and the Z-phase upper arm  226  is provided with a Z-phase upper switch Za. 
     The three lower arms ( 244  to  246 ) include an X-phase lower arm  244 , a Y-phase lower arm  245 , and a Z-phase lower arm  246 . The X-phase lower arm  244  is provided with an X-phase lower switch Xb, the Y-phase lower arm  245  is provided with a Y-phase lower switch Yb, and the Z-phase lower arm  246  is provided with a Z-phase lower switch Zb. 
     The three phase wirings ( 234  to  236 ) include an X-phase wire  234 , a Y-phase wire  235 , and a Z-phase wire  236 . The second switch  250  includes an X-phase switch  254 , a Y-phase switch  255 , and a Z-phase switch  256 . 
     A more specific description of the second drive system  200  is similar to the description of the first drive system  100  above, reading as follows. In other words, the term first can be read as second, the term U phase can be read as X phase, the term V phase can be read as Y phase, the term W phase can be read as Z phase, and the reference signs can be read as corresponding reference signs. 
     Next, the gang switch  50  will be described. The gang switch  50  includes a UX gang switch  54 , a VY gang switch  55 , and a WZ gang switch  56 . Specifically, a battery  20 -side portion of the U-phase wire  134  with reference to the U-phase switch  154  (the side opposite to the first motor  160 ) is connected to a battery  20 -side portion of the X-phase wire  234  with reference to the X-phase switch  254 , via a UX coupling wire  34 . The UX coupling wire  34  is provided with the UX gang switch  54 . 
     Also, a battery  20 -side portion of the V-phase wire  135  with reference to the V-phase switch  155  is connected to a battery  20 -side portion of the Y-phase wire  235  with reference to the Y-phase switch  255 , via a VY coupling wire  35 . The VY coupling wire  35  is provided with the VY gang switch  55 . Furthermore, a battery  20 -side portion of the W-phase wire  136  with reference to the W-phase switch  156  is connected to a battery  20 -side portion of the Z-phase wire  236  with reference to the Z-phase switch  256 , via a WZ coupling wire  36 . The WZ coupling wire  36  is provided with the WZ gang switch  56 . 
     In the following description, the expression that the gang switch  50  is ON refers to that the three switches ( 54  to  56 ) configuring the gang switch  50  are all ON, and the expression that the gang switch  50  is OFF refers to that the three switches ( 54  to  56 ) are all OFF. The gang switch  50 , when turned ON, connects between the first inverter  120  and the second motor  260  so that current can be passed therethrough in the state in which the second switch  250  is ON, and connects between the second inverter  220  and the first motor  160  so that current can be passed therethrough in the state in which the first switch  150  is ON. When the gang switch  50  is turned OFF, the first inverter  120  and the second motor  260  are disconnected from each other so that current cannot be passed therethrough, while the second inverter  220  and the first motor  160  are disconnected from each other so that current cannot be passed therethrough. 
     Referring back to  FIG. 1 , a further description will be provided. The host ECU  10  includes a determination unit  14  and a control unit  15 . The determination unit  14  determines whether traction of the first wheel  190  and the second wheel  290  is abnormal. Specifically, for example, if the rotating speed of the first wheel  190  or the second wheel  290  is higher than that of other wheels, traction can be determined to be abnormal. 
     Furthermore, for example, the vehicle  400  may include a first tire pressure monitoring system (TPMS) that detects tire pressure of the first wheel  190 , and a second tire pressure monitoring system (TPMS) that detects tire pressure of the second wheel  290 . If the tire pressure detected by these systems is low, the determination unit  14  can determine the traction as being abnormal. This is because, if the tire pressure is low, there is a high probability that traction is reduced due to the tire floating, or the like. 
     In the following description, if traction of neither of the first wheel  190  and and the second wheel  290  is determined to be abnormal by the determination section  14 , this state is referred to as a normal state. Also, of the first and second wheels  190  and  290 , if traction of only the first wheel  190  is determined to be abnormal by the determination unit  14 , this state is referred to as a first abnormal state, and if traction of only the second wheel  290  is determined to be abnormal, this state is referred to as a second abnormal state. In addition, if traction of both the first second wheels  190  and  290  is determined to be abnormal by the determination unit  14 , this state is referred to as a dual abnormal state. 
     In the first abnormal state, the second drive system  200  inputs the second motor information i 2  into not only the second control unit  210  but also the first control unit  110 , Also, in the second abnormal state, the first drive system  100  inputs the first motor information i 1  into not only the first control unit  110  but also the second control unit  210 . 
     In the normal state, the control unit  15  establishes a normal state s 0  in which the first and second switches  150  and  250  are ON and the gang switch  50  is OFF. In the first abnormal state, a first countermeasure state s 1  is established in which the first switch  150  is OFF, the second switch  250  is ON, and the gang switch  50  is ON. In the second abnormal state, a second countermeasure state s 2  is established in which the first switch  150  is ON, the second switch  250  is OFF, and the gang switch  50  is ON. 
       FIG. 3  is a circuit diagram illustrating the vehicle drive system  300  in the normal state s 0 . It should be noted that, although the figure shows the U-phase lower switch Ub, the V-phase upper switch Va, and the W-phase lower switch Wb as being ON, and the X-phase lower switch Xb, the Y-phase upper switch Ya, and the Z-phase lower switch Zb as being ON, this is a state at a predetermined moment. The upper switches (Ua, Va, Wa, Xa, Ya, Za) and the lower switches (Ub, Vb, Wb, Xb, Yb, Zb) repeat ON and OFF states at individual predetermined timings to switch directions of the current passed through the motors ( 160 ,  260 ). The same applies to  FIGS. 4 and 5 . 
     In the normal state s 0  shown in  FIG. 3 , the first inverter  120  supplies current to the first motor  160  to drive the first motor  160 , while the second inverter  220  supplies current to the second motor  260  to drive the second motor  260 . 
     In the normal state s 0 , the first motor  160  is driven with power that is equal to or lower than predetermined first upper limit power which is determined by the performance (critical power) of the first inverter  120 . Furthermore, the second motor  260  is driven with power that is equal to or lower than predetermined second upper limit power which is determined by the performance (critical power) of the second inverter  220 . It should be noted that, in the present embodiment, the first upper limit power is equal to the second upper limit power. 
       FIG. 4  is a circuit diagram illustrating the vehicle drive system  300  in the first countermeasure state s 1 . In the first countermeasure state s 1 , the first switch  150  is OFF and therefore the first inverter  120  does not supply current to the first motor  160 . On the other hand, since the second switch  250  and the gang switch  50  are ON, the second inverter  220  and the first inverter  120  supply current to the second motor  260  to drive the second motor  260  with both inverters  120  and  220 . 
     Specifically, in the first countermeasure state s 1 , the second drive system  200  inputs the second motor information i 2  into not only the second control unit  210  but also the first control unit  110  as mentioned above. The first control unit  110  controls the switches (Ua, Va, Wa, Ub, Vb, Wb) of the first inverter  120  using the second motor information i 2  to control the first inverter  120  driving the second motor  260 . 
     In the first countermeasure state s 1 , cooperation between the inverters ( 120 ,  220 ) produces power higher than the second upper limit power to drive the second motor  260  with the produced power. However, the period of driving the second motor  260  with the power higher than the second upper limit power should be within a predetermined second time limit. The second time limit is time which is determined based on the time taken for the second motor  260  to cause predetermined deterioration in performance by being driven with the power higher than the second upper limit power. 
     Specifically, the predetermined deterioration in performance may be, for example, demagnetization of the rotor magnets or short circuiting of a predetermined circuit, due to overheat of the second motor  260 . The second time limit may be a constant or a variable which is determined based on the magnitude of current passing through the second motor  260  or the temperature of the second motor  260 . The second time limit, if it is a variable, may be obtained using a map or may be obtained using a function. 
     In the first countermeasure state s 1 , if the period of driving the second wheel  290  with power higher than the second upper limit power exceeds the second time limit, the state may be returned to the normal state s 0 , or the first countermeasure state s 1  may be kept unchanged with the power of the inverters  120  and  220  reduced. 
       FIG. 5  is a circuit diagram illustrating the vehicle drive system  300  in the second countermeasure state s 2 . In the second countermeasure state s 2 , the second switch  250  is OFF and therefore the second inverter  220  does not supply current to the second motor  260 . On the other hand, since the first switch  150  and the gang switch  50  are ON, the first inverter  120  and the second inverter  220  supply current to the first motor  160  to drive the first motor  160  with both inverters  120  and  220 . 
     The second countermeasure state s 2  can be more specifically described by reading the above description for the first countermeasure state s 1  as follows. Specifically, the terms first and second can be read in reverse, and the reference signs can be read as corresponding reference signs. 
       FIG. 6  is a flowchart illustrating connection control performed by the host ECU  10 . The initial state is the normal state s 0 . From this state, first, the determination unit  14  detects information related to traction of the first wheel  190  and the second wheel  290  (S 611 ). Next, it is determined whether traction of the first wheel  190  is abnormal (S 612 ). 
     If traction of the first wheel  190  is determined to be abnormal at S 612  (YES at S 612 ), it is determined whether traction of the second wheel  290  is abnormal (S 613 ). If traction of the second wheel  290  is determined to be abnormal (YES at S 613 ), this means that the dual abnormal state is established in which traction of both wheels is abnormal, and therefore, the connection control is terminated, with the normal state s 0  unchanged. 
     On the other hand, if traction of the second wheel  290  is determined not to be abnormal at S 613  (NO at S 613 ), this means that the first abnormal state is established in which only traction of the first wheel  190  is abnormal, and therefore, the first motor  160  is stopped being driven by the first inverter  120  (S 614 ). Then, the first switch  150  is turned OFF and at the same time the gang switch  50  is turned ON (S 615 ) to change state to the first countermeasure state s 1 . Then, the first inverter  120  starts driving the second motor  260  (S 616 ) so that the second motor  260  is driven by both inverters  120  and  220 . In this state, the connection control is terminated. 
     On the other hand, if traction of the first wheel  190  is determined not to be abnormal at S 612  (NO at S 612 ), it is determined whether traction of the second wheel  290  is abnormal (S 623 ). If traction of the second wheel  290  is determined not to be abnormal (NO at S 623 ), this means that the normal state is established in which traction of neither of the wheels is abnormal, and therefore the connection control is terminated, with the normal state s 0  unchanged. 
     On the other hand, if traction of the second wheel  290  is determined to be abnormal at S 623  (YES at S 623 ), this means that the second abnormal state is established in which only traction of the second wheel  290  is abnormal, and therefore the second motor  260  is stopped being driven by the second inverter  220  (S 624 ). Then, the second switch  250  is turned OFF and at the same time the gang switch  50  is turned ON (S 625 ) to change state to the second countermeasure state s 2 . Then, the second inverter  220  starts driving the first motor  160  (S 626 ) so that the first motor  160  is driven by both inverters  120  and  220 . In this state, the connection control is terminated. 
     It should be noted that, after terminating the connection control, the following control is performed. Specifically, if the state has been changed to the first countermeasure state s 1  under the connection control, state is returned to the normal state s 0  under conditions that traction of the first wheel  190  has become normal. Furthermore, if the state has been changed to the second countermeasure state s 2  under the connection control, state is returned to the normal state s 0  under conditions that traction of the second wheel  290  has become normal. 
     Furthermore, in the first countermeasure state s 1 , if the period of driving the second wheel  290  with power higher than the second upper limit power has exceeded the second time limit, state is returned to the normal state s 0 , or the first countermeasure state s 1  is kept unchanged with the power of the inverters  120  and  220  reduced. Also, in the second countermeasure state s 2 , if the period of driving the first wheel  190  with power higher than the first upper limit power has exceeded the first time limit, state is returned to the normal state s 0 , or the second countermeasure state s 2  is kept unchanged with the power of the inverters  120  and  220  reduced. 
     According to the present embodiment, the following advantageous effects can be achieved. In the first abnormal state in which traction of the first wheel  190  is abnormal, the second motor  260  is driven by both inverters ( 120 ,  220 ) so that torque of the second wheel  290  can be enhanced. Details of this will be described referring to  FIG. 7 . In the following description, the maximum value of current that can be passed through the upper switches (Ua, Va, Wa, Xa, Ya, Za) and the lower switches (Ub, Vb, Wb, Xb, Yb, Zb) is referred to as Imax. 
     If the circuit at a predetermined moment in the normal state s 0  shown in  FIG. 3  is simplified by omitting circuit through which no current is passed, a circuit diagram as shown in  FIG. 7A  can be obtained. Herein, the positive terminal of the battery  20  and the second motor  260  are connected to each other via one upper switch (Ya). On the other hand, the negative terminal of the battery  20  and the second motor  260  are connected to each other via two lower switches (Xb, Zb) which are parallel to each other. Therefore, the maximum value of current that can be passed through the second motor  260  corresponds to the maximum value (Imax) of current that can be passed through the single upper switch (Ya). 
     If the circuit at a predetermined moment in the first countermeasure state s 1  shown in  FIG. 4  is simplified by omitting circuits through which no current is passed, a circuit as shown in  FIG. 7B  can be obtained. Herein, the positive terminal of the battery  20  and the second motor  260  are connected to each other via two upper switches (Va, Ya) which are parallel to each other. On the other hand, the negative terminal of the battery  20  and the second motor  260  are connected to each other via four lower switches (Ub, Wb, Xb, Zb) which are parallel to each other. Therefore, the maximum value of current that can be passed through the second motor  260  corresponds to the maximum value (2×Imax) of total current that can be passed through the two upper switches (Va, Ya). 
     As described above, while an Imax current at a maximum can be passed through the second motor  260  in the normal state s 0 , a 2×Imax current at a maximum can be passed through the second motor  260  in the first countermeasure state s 1 . Therefore, in the first countermeasure state s 1 , the second motor  260  can output about twice the power of the maximum power (second upper limit power) of the normal state s 0 . 
     Therefore, in the first abnormal state in which the first wheel  190  has been caught in mud, snow, or the like on the road surface or run on a frozen road, torque of the second wheel  290  can be sufficiently enhanced by changing state into the first countermeasure state s 1 . Thus, the vehicle  400  can be brought into a state of easily moving out of the mud, snow, or the like on the road surface or the frozen road, without the need of providing a second inverter  220  having high power. 
     Similarly, in the second abnormal state in which the second wheel  290  has been caught in mud, snow, or the like on the road surface or run on a frozen road, torque of the first wheel  190  can be sufficiently enhanced by changing state into the second countermeasure state s 2 . Thus, the vehicle  400  can be brought into a state of easily moving out of the mud or the like on the road surface, without the need of providing a first inverter  120  having high power. 
     Specifically, in the present embodiment, the first wheel  190  is a left wheel, and the second wheel  290  is a right wheel. Accordingly, in the first abnormal state in which the first wheel  190  as a left wheel has been caught in mud or the like on the road surface and traction has been deteriorated, the vehicle  400  can be brought into a state of easily moving out of the mud or the like by changing state into the first countermeasure state s 1  and enhancing torque of the second wheel  290  as a right wheel. On the contrary, in the second abnormal state in which the second wheel  290  as a right wheel has been caught in mud or the like on the road surface and traction has been deteriorated, the vehicle  400  can be brought into a state of easily moving out of the mud or the like by changing state into the second countermeasure state s 2  and enhancing torque of the first wheel  190  as a left wheel. 
     Furthermore, in the first countermeasure state s 1 , the second drive system  200  inputs the second motor information i 2  into the first control unit  110 , and accordingly, the first control unit  110  can use the second motor information i 2  to control the first inverter  120  driving the second motor  260 , without any problem. Similarly, in the second countermeasure state s 2 , the first drive system  100  inputs the first motor information i 1  into the second control unit  210 , and accordingly, the second control unit  210  can use the first motor information i 1  to control the second inverter  220  driving the first motor  160 , without any problem. 
     When changing state from the normal state s 0  to the first countermeasure state s 1 , the first inverter  120  is caused to stop driving the first motor  160 , and then the first switch  150  is turned OFF and at the same time the gang switch  50  is turned ON for change of state into the first countermeasure state s 1 . Thus, the first switch  150  is turned OFF after current is stopped passing therethrough. Therefore, compared to the case where the first switch  150  is turned OFF while current is passed therethrough, insulation breakdown is unlikely to occur between the terminals of the first switch  150 . Thus, withstand voltage required of the first switch  150  can be minimized. 
     Similarly, when changing state from the normal state s 0  to the second countermeasure state s 2 , the second inverter  220  is caused to stop driving the second motor  260 , and then the second switch  250  is turned OFF and at the same time the gang switch  50  is turned ON for change of state into the second countermeasure state s 2 . Thus, withstand voltage required of the second switch  250  can be minimized as in the case of the first switch  150 . 
     Furthermore, in the first countermeasure state s 1 , the second inverter  220  drives the second motor  260  with power higher than the second upper limit power within a predetermined second time limit. The second time limit is determined based on the time taken for the second motor  260  to cause predetermined deterioration in performance by being driven with power higher than the second upper limit power. Thus, the second motor  260  can be prevented from causing the deterioration in performance. 
     Similarly, in the second countermeasure state s 2 , the first inverter  120  drives the first motor  160  with power higher than the first upper limit power within a predetermined first time limit. The first time limit is determined based on the time taken for the first motor  160  to cause predetermined deterioration in performance by being driven with power higher than the first upper limit power. Thus, the first motor  160  can be prevented from causing the deterioration in performance. 
     Other Embodiments 
     The embodiment described above can be modified and implemented as follows. For example, the first drive system  100  may drive both the right and left front wheels instead of driving only the left front wheel, and the second drive system  200  may drive both the right and left rear wheels instead of driving only the right rear wheel. In other words, the first wheel  190  may be both of the right and left front wheels, and the second wheel  290  may be both of the right and left rear wheels. In this case, for example, if the front wheels as the first wheel  190  are caught in mud or the like on the road surface, torque can be concentrated on the rear wheels as the second wheel  290  to bring the vehicle into a state of easily moving out of the mud or the like. 
     Also, for example, the second switch  250  may be omitted so that state is changed from the normal state s 0  to the first countermeasure state s 1  only in the first abnormal state, and that, in the second abnormal state, the normal state s 0  remains unchanged without changing state to the second countermeasure state s 2 . In this case, for example, the wheels, which are preferred to be set as the first wheel  190 , are the shoulder side wheels among the right and left wheels, the rear side wheels among the front and rear wheels, or the like, which are more likely to be caught in mud, show, or the like on the road surface or run on a frozen road. 
     Furthermore, for example, the circuit diagrams ( FIGS. 2 to 5 and 7 ) indicate the upper switches (Ua, Va, Wa, Xa, Ya, Za) and the lower switches (Ub, Vb, Wb, Xb, Yb, Zb) with normal symbols of transistors; however, these switches may be MOSFETs, IGBTs, or the like. 
     Also, for example, the vehicle drive system  300  may be used as a drone drive system that drives the propellers of drones. Specifically, for example, in the case where a drone has four propellers, i.e., front-left, front-right, rear-left, and rear-right propellers, the front-left and rear-right propellers as first diagonal propellers may be driven using the first motor  160 , and the front-right and rear-left propellers as second diagonal propellers may be driven using the second motor  260 . In this case, for example, in the occurrence of malfunctions, such as damage, in the first diagonal propellers, power can be concentrated on the second diagonal propellers to perform soft landing or the like using power of the second diagonal propellers. 
     The present disclosure has been described based on embodiments; however, the present disclosure should not be construed as being limited to these embodiments or structures. The scope of the present disclosure should encompass various modifications or equivalents. In addition, various combinations, or modes, and further, other combinations or modes including one or more additional elements or fewer elements of the various combinations or modes should also be included within the category or idea of the present disclosure.