Patent Publication Number: US-11387766-B2

Title: Control circuit for electric power converter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority from Japanese Patent Application No. 2018-89494 filed on May 7, 2018, the contents of which are hereby incorporated by reference in their entirety into this application. 
     BACKGROUND 
     1 Technical Field 
     The present disclosure relates to control circuits for electric power converters. 
     2 Description of Related Art 
     There are known control circuits for electric power converters that have, for each of phase windings of a multi-phase rotating electric machine, upper-arm and lower-arm switches electrically connected to the phase winding. These control circuits are configured to forcibly turn off all of the upper-arm and lower-arm switches of the electric power converter upon determination that an abnormality has occurred in at least one of the electric power converter and the rotating electric machine. 
     SUMMARY 
     According to the present disclosure, there is provided a control circuit for an electric power converter. The electric power converter includes a plurality of switch pairs each consisting of an upper-arm switch and a lower-arm switch that are electrically connected in series with each other. Each of the switch pairs has a corresponding one of a plurality of phase windings of a multi-phase rotating electric machine electrically connected to a node between the upper-arm and lower-arm switches of the switch pair. Each of the upper-arm and lower-arm switches has a diode electrically connected in antiparallel thereto. The switch pairs have a DC power source electrically connected in parallel thereto. All of the upper-arm switches of the switch pairs together constitute an upper-arm switch group and all of the lower-arm switches of the switch pairs together constitute a lower-arm switch group. The control circuit includes an abnormality determiner, a voltage determiner and an abnormality-handling controller. The abnormality determiner is configured to determine whether an abnormality has occurred in at least one of the electric power converter and the rotating electric machine. The voltage determiner is configured to determine whether a line-to-line voltage between the phase windings of the rotating electric machine when a counterelectromotive force is generated in the phase windings is higher than a voltage of the DC power source. The abnormality-handling controller is configured to perform, when it is determined by the abnormality determiner that an abnormality has occurred in at least one of the electric power converter and the rotating electric machine and it is determined by the voltage determiner that the line-to-line voltage is higher than the voltage of the DC power source, an all-phase short circuit control of turning on all of the switches of one of the upper-arm switch group and the lower-arm switch group and turning off all of the switches of the other of the upper-arm switch group and the lower-arm switch group. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a schematic diagram illustrating the overall configuration of a rotating electric machine control system which includes a control circuit according to a first embodiment; 
         FIG. 2  is a schematic diagram illustrating the configuration of the control circuit according to the first embodiment; 
         FIG. 3  is a schematic diagram illustrating the configuration of a composite IC and a first isolation transmitter of the control circuit according to the first embodiment; 
         FIG. 4  is a flow chart illustrating a safe-state determination process performed by a safe-state determiner of the control circuit according to the first embodiment; 
         FIG. 5  is a flow chart illustrating an overall-state determination process performed by an overall-state determiner of the control circuit according to the first embodiment; 
         FIG. 6  is a graph illustrating the characteristics of a voltage sensor included in the rotating electric machine control system; 
         FIG. 7  is a time chart illustrating the changes with time of (a) a power source voltage, (b) the presence or absence of an ASC disabling command and (c) the content of a compulsory switching command for bringing the rotating electric machine control system into a safe state according to the first embodiment; 
         FIG. 8  is a schematic diagram illustrating the configuration of a control circuit according to a second embodiment; 
         FIG. 9  is a schematic diagram illustrating the configuration of a composite IC and a first isolation transmitter of the control circuit according to the second embodiment; 
         FIG. 10  is a schematic diagram illustrating the configuration of a control circuit according to a third embodiment; 
         FIG. 11  is a flow chart illustrating a safe-state determination process performed by a safe-state determiner of the control circuit according to the third embodiment; 
         FIG. 12A  is a graph illustrating the relationship between the rotational speed of a rotor and d-axis and q-axis currents in a rotating electric machine when an ASC control is performed by the control circuit according to the third embodiment; 
         FIG. 12B  is a graph illustrating the relationship between the rotational speed of the rotor and the phase-current amplitude in the rotating electric machine when the ASC control is performed by the control circuit according to the third embodiment; 
         FIG. 12C  is a graph illustrating the relationship between the d-axis current and the q-axis current in the rotating electric machine when the ASC control is performed by the control circuit according to the third embodiment; 
         FIG. 12D  is a graph illustrating the relationship between the rotational speed of the rotor and torque generated by the rotating electric machine when the ASC control is performed by the control circuit according to the third embodiment; 
         FIG. 13  is a schematic diagram illustrating the overall configuration of a rotating electric machine control system which includes a control circuit according to a fourth embodiment; 
         FIG. 14  is a schematic diagram illustrating the configuration of the control circuit according to the fourth embodiment; 
         FIG. 15  is a flow chart illustrating a safe-state determination process performed by a safe-state determiner of the control circuit according to the fourth embodiment; 
         FIG. 16  is a schematic diagram illustrating the configuration of a control circuit according to a fifth embodiment; 
         FIG. 17  is a flow chart illustrating a monitoring process performed by a monitoring microcomputer of the control circuit according to the fifth embodiment; 
         FIG. 18  is a schematic diagram illustrating the configuration of a control circuit according to a sixth embodiment; 
         FIG. 19  is a flow chart illustrating a safe-state determination process performed by a safe-state determiner of the control circuit according to the sixth embodiment; 
         FIG. 20  is a time chart illustrating the changes with time of (a) a fourth output voltage, (b) a power source voltage, (c) the presence or absence of an ASC disabling command and (d) the content of a compulsory switching command for bringing the rotating electric machine control system into a safe state according to the sixth embodiment; 
         FIG. 21  is a schematic diagram illustrating the configuration of a control circuit according to one modification; 
         FIG. 22  is a schematic diagram illustrating the configuration of a control circuit according to another modification; and 
         FIG. 23  is a schematic diagram illustrating the configuration of a control circuit according to yet another modification. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     There is known, for example from Japanese Patent Application Publication No. JP2016013040A, a control circuit for an electric power converter which performs, upon determination that an abnormality has occurred in a current sensor, a shutdown control of forcibly turning off all of the upper-arm and lower-arm switches of the electric power converter; the current sensor is configured to detect electric current flowing in each of the phase windings of the rotating electric machine. 
     However, the inventor of the present application has found the following problems with the known control circuit. 
     The rotational speed of a rotor of the rotating electric machine may be high when the shutdown control is performed by the known control circuit; thus, when a counterelectromotive force is generated in the phase windings of the rotating electric machine, the line-to-line voltage between the phase windings may become higher than the voltage of a DC power source. The DC power source is electrically connected in parallel to switch pairs each consisting of one upper-arm switch and one lower-arm switch that are electrically connected in series with each other. In this case, though all of the upper-arm and lower-arm switches are turned off, so-called regeneration is performed such that electric current induced in the phase windings of the rotating electric machine flows through a closed circuit that includes the phase windings, the DC power source and diodes each being connected in antiparallel to one of the upper-arm and lower-arm switches. Consequently, the DC voltage on the DC power source side of the electric power converter rises considerably; thus reliability degradation or a failure may occur in at least one of the DC power source, components of the electric power converter (e.g., the switches and a smoothing capacitor) and other electrical machines and devices connected with the DC power source. 
     In contrast, the above-described control circuit according to the present disclosure has the following advantages. 
     When the all-phase short circuit control is performed, no closed circuit is formed which includes the diodes, the phase windings of the rotating electric machine and the DC power source. Instead, a closed circuit is formed which includes the turned-on switches of the electric power converter and the phase windings of the rotating electric machine. Consequently, it becomes possible to cause electric current induced in the phase windings of the rotating electric machine to flow within the formed closed circuit, thereby preventing regeneration from being performed. As a result, it becomes possible to prevent the DC voltage on the DC power source side of the electric power converter from being considerably increased and thus the torque generated by the rotating electric machine from being rapidly increased. 
     Exemplary embodiments and their modifications will be described hereinafter with reference to  FIGS. 1-23 . It should be noted that for the sake of clarity and understanding, identical components having identical functions throughout the whole description have been marked, where possible, with the same reference numerals in each of the figures and that for the sake of avoiding redundancy, descriptions of identical components will not be repeated. 
     First Embodiment 
       FIG. 1  shows the overall configuration of a rotating electric machine control system which includes a three-phase rotating electric machine  10 , a three-phase inverter (i.e., an electric power converter)  20  and a control circuit  30  according to the first embodiment. 
     In the present embodiment, the rotating electric machine control system is used in a vehicle such as an electric vehicle or a hybrid vehicle. 
     The rotating electric machine  10  is employed as a main rotating electric machine in the vehicle. The rotating electric machine  10  has a rotor mechanically connected with driving wheels (not shown) of the vehicle; thus mechanical power (or torque) can be transmitted between the rotor and the driving wheels. In the present embodiment, the rotating electric machine  10  is configured as a synchronous machine, more particularly as an interior permanent magnet synchronous machine. 
     The rotating electric machine  10  has three phase windings  11  electrically connected to a storage battery (i.e., a DC power source)  21  via the inverter  20 . The output voltage of the storage battery  21  is, for example, higher than or equal to 100V. In addition, on the input side of the inverter  20 , there is provided a smoothing capacitor  22  to smooth the input voltage of the inverter  20 . 
     The inverter  20  includes three switch pairs each corresponding to one of three phases of the rotating electric machine  10  and consisting of an upper-arm switch SWH and a lower-arm switch SWL that are electrically connected in series with each other. 
     For each of the three phases, to a node (or junction point) between the upper-arm and lower-arm switches SWH and SWL of the switch pair corresponding to the phase, there is connected a first end of that one of the phase windings  11  of the rotating electric machine  10  which corresponds to the phase. On the other hand, second ends of the phase windings  11  are connected together at a neutral point. Moreover, the phase windings  11  are arranged in the rotating electric machine  10  so as to be offset from each other by 120° in electrical angle. 
     In the present embodiment, each of the upper-arm and lower-arm switches SWH and SWL is configured with a voltage-controlled semiconductor switching element, more particularly with an IGBT (Insulated Gate Bipolar Transistor). Moreover, each of the upper-arm switches SWH has an upper-arm diode DH, which is a freewheeling diode, connected in antiparallel thereto. Similarly, each of the lower-arm switches SWL has a lower-arm diode DL, which is a freewheeling diode, connected in antiparallel thereto. 
     Each of the upper-arm switches SWH has its collector (i.e., high potential-side terminal) electrically connected with a positive terminal of the storage battery  21 . On the other hand, each of the lower-arm switches SWL has its emitter (i.e., low potential-side terminal) electrically connected with a negative terminal of the storage battery  21 . 
     As shown in  FIGS. 1 and 2 , the rotating electric machine control system includes a voltage sensor  23   a , a phase-current sensor  23   b  and an angle sensor  23   c.    
     The voltage sensor  23   a  (see  FIG. 2 ) is configured to output a voltage signal Vs indicative of a terminal voltage of the smoothing capacitor  22 . 
     The phase-current sensor  23   b  is configured to output a current signal indicative of at least two of three phase currents respectively flowing in the three phase windings  11  of the rotating electric machine  10 . 
     The angle sensor  23   c  is configured to output an angle signal indicative of an electrical angle of the rotating electric machine  10 . The angle sensor  23   c  is implemented by, for example, a resolver, an encoder or an MR (Magneto-Resistive) sensor that includes a magnetoresistance-effect element. 
     In addition, in the present embodiment, the voltage sensor  23   a  is built in the control circuit  30 , whereas both the phase-current sensor  23   b  and the angle sensor  23   c  are arranged outside the control circuit  30 . 
     As shown in  FIG. 2 , the control circuit  30  includes a microcomputer  50  that generates switching (abbreviated to SW in  FIG. 2 ) commands for controlling switching of the switches SWH and SWL of the inverter  20  and thereby controlling a controlled value of the rotating electric machine  10  to its desired value. The controlled variable is, for example, torque. 
     Specifically, the microcomputer  50 , which corresponds to a “command generator”, generates the switching commands on the basis of the output signals of the sensors  23   a - 23   c . More specifically, the microcomputer  50  generates the switching commands so as to have the upper-arm switch SWH and the lower-arm switch SWL alternately turned on in each of the three switch pairs of the inverter  20 . 
     Moreover, the microcomputer  50  calculates, based on the voltage signal Vs outputted from the voltage sensor  23   a , the terminal voltage of the smoothing capacitor  22  as a power source voltage VDC. The microcomputer  50  also calculates, based on the current signal outputted from the phase-current sensor  23   b , at least two of the three phase currents respectively flowing in the three phase windings  11  of the rotating electric machine  10 . The microcomputer  50  also calculates, based on the angle signal outputted from the angle sensor  23   c , the electrical angle θe of the rotating electric machine  10 . Then, based on the calculated electrical angle θe, the microcomputer  50  further calculates the electrical angular speed we of the rotating electric machine  10 . 
     Next, the configuration of the control circuit  30  according to the present embodiment will be described in detail with reference to  FIG. 2 . 
     The control circuit  30  further includes a low-voltage power supply circuit  40 , which corresponds to a “power supply unit”, and a composite IC (Integrated Circuit)  60  in addition to the microcomputer  50 . 
     The composite IC  60  includes a power supply controller  61 , a power supply monitor  62  and a voltage converter  63 . 
     The power supply controller  61  is configured to control the low-voltage power supply circuit  40 . 
     The power supply monitor  62  is configured to output, upon determination that predetermined conditions are satisfied, a rest signal to the microcomputer  50 . Upon the reset signal being inputted to the microcomputer  50 , operation of the microcomputer  50  is stopped; thus the generation and output of the switching commands are stopped. 
     The voltage converter  63  is configured to covert the voltage inputted from a low-voltage DC power source  24  into a voltage of such a level as to be used in the composite IC  60 . Then, the voltage converter  63  outputs the resultant voltage. 
     The low-voltage power supply circuit  40  has a function of stepping down an output voltage VB of the low-voltage DC power source  24 . The output voltage VB of the low-voltage DC power source  24  is lower than the output voltage of the storage battery  21 . 
     In addition, at least part of various functions of the control circuit  30  may be realized by software stored in a tangible memory device and a computer (e.g., the microcomputer  50 ) that executes the software, by hardware, or by a combination of the aforementioned means. 
     The low-voltage power supply circuit  40  generates a first output voltage V 1   r , a second output voltage V 2   r  and a third output voltage V 3   r  by stepping down the output voltage VB of the low-voltage DC power source  24 . The first output voltage V 1   r  is set to be lower than the output voltage VB of the low-voltage DC power source  24 . The second output voltage V 2   r  (e.g., equal to 5V) is set to be lower than the first output voltage V 1   r . The third output voltage V 3   r  (e.g., equal to 1.2V) is set to be lower than the second output voltage V 2   r.    
     Divided-voltage values of the first, second and third output voltages Vr 1 , Vr 2  and Vr 3  are inputted to the power supply monitor  62 . Then, based on the inputted divided-voltage values, the power supply monitor  62  detects the first, second and third output voltages Vr 1 , Vr 2  and Vr 3 . 
     The second output voltage V 2   r  is supplied to the voltage sensor  23   a , the phase-current sensor  23   b  and the angle sensor  23   c , thereby enabling these sensors  23   a - 23   c  to respectively output the voltage signal, the current signal and the angle signal. 
     The microcomputer  50  includes a CPU (Central Processing Unit) and peripheral circuits which include, for example, an A/D converter and an input/output unit for exchanging signals with external devices or circuits. The third output voltage V 3   r  of the low-voltage power supply circuit  40  is supplied to the CPU of the microcomputer  50 , whereas the second output voltage V 2   r  of the low-voltage power supply circuit  40  is supplied to the peripheral circuits of the microcomputer  50 . 
     The control circuit  30  further includes a first isolation transmitter  80 , a drive circuit unit  90 , an isolation power supply  91 , an abnormality detector  92  and a second isolation transmitter  93 . 
     In the present embodiment, each of the first isolation transmitter  80  and the second isolation transmitter  93  is constituted of a photocoupler that is an optical isolation element. 
     The isolation power supply  91 , whose electric power source is the low-voltage DC power source  24 , is configured to supply electric power to drive circuits that together constitute the drive circuit unit  90 . 
     The drive circuit unit  90  includes the drive circuits that respectively correspond to the six switches SWH and SWL of the inverter  20 . That is, the number of the drive circuits is equal to six in the present embodiment. In addition, the drive circuits respectively drive the switches SWH and SWL of the inverter  20  upon supply of electric power from the isolation power supply  91  to the drive circuit unit  90 . 
     The composite IC  60  further includes a switch (abbreviated to SW in  FIG. 2 ) controller  70 , an overall-state determiner  64  and an overvoltage detector  65 . 
     The switch controller  70  is configured to transmit the switching commands, which are outputted from the microcomputer  50 , to the drive circuit unit  90  via the first isolation transmitter  80 . 
     The overvoltage detector  65  detects the power source voltage VDC on the basis of the voltage signal Vs outputted from the voltage sensor  23   a . Moreover, when the detected power source voltage VDC is higher than a power source voltage threshold, the overvoltage detector  65  determines that an overvoltage abnormality has occurred and notifies the switch controller  70  of the occurrence of the overvoltage abnormality. 
     In addition, instead of determining the power source voltage VDC by itself, the overvoltage detector  65  may acquire the power source voltage VDC detected by the microcomputer  50 . 
       FIG. 3  shows both the configuration of the switch controller  70  and the configuration of a photodiode-side part of the first isolation transmitter  80 . 
     As shown in  FIG. 3 , the switch controller  70  includes three upper-arm controller  71 H for respectively controlling the upper-arm switches SWH of the inverter  20 , three high switches  72 H respectively corresponding to the upper-arm switches SWH, three lower-arm controller  71 L for respectively controlling the lower-arm switches SWL of the inverter  20 , and three low switches  72 L respectively corresponding to the lower-arm switches SWL. 
     In the present embodiment, each of the high and low switches  72 H and  72 L is configured with an N-channel MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). 
     On the other hand, the first isolation transmitter  80  includes three upper-arm photodiodes  82 H respectively corresponding to the upper-arm switches SWH of the inverter  20 , three upper-arm resistors  81 H respectively corresponding to the upper-arm switches SWH, three lower-arm photodiodes  82 L respectively corresponding to the lower-arm switches SWL of the inverter  20 , and three lower-arm resistors  81 L respectively corresponding to the lower-arm switches SWL. 
     Each of the upper-arm resistors  81 H has a first end to which the first output voltage V 1   r  of the low-voltage power supply circuit  40  is applied, and a second end to which the drain of the corresponding high switch  72 H is connected via the corresponding upper-arm photodiode  82 H and a corresponding one of six terminals Tr of the composite IC  60 . In addition, the source of the corresponding high switch  72 H is grounded. 
     Similarly, each of the lower-arm resistors  81 L has a first end to which the first output voltage V 1   r  of the low-voltage power supply circuit  40  is applied, and a second end to which the drain of the corresponding low switch  72 L is connected via the corresponding lower-arm photodiode  82 L and a corresponding one of the six terminals Tr of the composite IC  60 . In addition, the source of the corresponding low switch  72 L is grounded. 
     To each of the upper-arm controllers  71 H, there is inputted, from the microcomputer  50 , the switching command for controlling switching of the upper-arm switch SWH of the inverter  20  corresponding to the upper-arm controller  71 H. Specifically, in the present embodiment, each of the upper-arm controllers  71 H transmits, by turning on the corresponding high switch  72 H, an ON command as the switching command to the corresponding drive circuit of the drive circuit unit  90  via the corresponding upper-arm photodiode  82 H and the corresponding upper-arm phototransistor (not shown) of the first isolation transmitter  80 . Moreover, each of the upper-arm controllers  71 H transmits, by turning off the corresponding high switch  72 H, an OFF command as the switching command to the corresponding drive circuit of the drive circuit unit  90  via the corresponding upper-arm photodiode  82 H and the corresponding upper-arm phototransistor (not shown) of the first isolation transmitter  80 . Upon receipt of the switching command, the corresponding drive circuit determines whether the switching command is the ON command or the OFF command. Then, the corresponding drive circuit turns on the corresponding upper-arm switch SWH when the switching command is determined to be the ON command and turns off the corresponding upper-arm switch SWH when the switching command is determined to be the OFF command. 
     To each of the upper-arm controllers  71 H, there is also inputted a compulsory switching command that is outputted from the overall-state determiner  64 . The compulsory switching command is either a shutdown command or an ASC (Active Short Circuit) command. 
     Each of the upper-arm controllers  71 H determines whether the compulsory switching command has been inputted thereto from the overall-state determiner  64 . Moreover, when it is determined that no compulsory switching command has been inputted thereto, each of the upper-arm controllers  71 H turns on or off the corresponding high switch  72 H according to the switching command from the microcomputer  50 . Consequently, the ON command or the OFF command is transmitted to the corresponding drive circuit of the drive circuit unit  90  via the first isolation transmitter  80 . As a result, the corresponding upper-arm switch SWH of the inverter  20  is turned on or off by the corresponding drive circuit depending on whether the switching command is the ON command or the OFF command. On the other hand, when it is determined that the compulsory switching command has been inputted thereto, each of the upper-arm controllers  71 H keeps the corresponding high switch  72 H off regardless of whether the switching command from the microcomputer  50  has been inputted thereto. Consequently, the OFF command is transmitted to the corresponding drive circuit of the drive circuit unit  90  via the first isolation transmitter  80 . As a result, the corresponding upper-arm switch SWH of the inverter  20  is turned or kept off. 
     To each of the lower-arm controllers  71 L, there is inputted, from the microcomputer  50 , the switching command for controlling switching of the lower-arm switch SWL of the inverter  20  corresponding to the lower-arm controller  71 L. Specifically, in the present embodiment, each of the lower-arm controllers  71 L transmits, by turning on the corresponding low switch  72 L, an ON command as the switching command to the corresponding drive circuit of the drive circuit unit  90  via the corresponding lower-arm photodiode  82 L and the corresponding lower-arm phototransistor (not shown) of the first isolation transmitter  80 . Moreover, each of the lower-arm controllers  71 L transmits, by turning off the corresponding low switch  72 L, an OFF command as the switching command to the corresponding drive circuit of the drive circuit unit  90  via the corresponding lower-arm photodiode  82 L and the corresponding lower-arm phototransistor (not shown) of the first isolation transmitter  80 . Upon receipt of the switching command, the corresponding drive circuit determines whether the switching command is the ON command or the OFF command. 
     Then, the corresponding drive circuit turns on the corresponding lower-arm switch SWL when the switching command is determined to be the ON command and turns off the corresponding lower-arm switch SWL when the switching command is determined to be the OFF command. 
     To each of the lower-arm controllers  71 L, there is also inputted a compulsory switching command that is outputted from the overall-state determiner  64 . The compulsory switching command is either a shutdown command or an ASC command. 
     Each of the lower-arm controllers  71 L determines whether the compulsory switching command has been inputted thereto from the overall-state determiner  64 . Moreover, when it is determined that no compulsory switching command has been inputted thereto, each of the lower-arm controllers  71 L turns on or off the corresponding low switch  72 L according to the switching command from the microcomputer  50 . Consequently, the ON command or the OFF command is transmitted to the corresponding drive circuit of the drive circuit unit  90  via the first isolation transmitter  80 . As a result, the corresponding lower-arm switch SWL of the inverter  20  is turned on or off by the corresponding drive circuit depending on whether the switching command is the ON command or the OFF command. On the other hand, when it is determined that the shutdown command has been inputted thereto as the compulsory switching command, each of the lower-arm controllers  71 L keeps the corresponding low switch  72 L off regardless of whether the switching command from the microcomputer  50  has been inputted thereto. Consequently, the OFF command is transmitted to the corresponding drive circuit of the drive circuit unit  90  via the first isolation transmitter  80 . As a result, the corresponding lower-arm switch SWL of the inverter  20  is turned or kept off. In contrast, when it is determined that the ASC command has been inputted thereto as the compulsory switching command, each of the lower-arm controllers  71 L keeps the corresponding low switch  72 L on regardless of whether the switching command from the microcomputer  50  has been inputted thereto. Consequently, the ON command is transmitted to the corresponding drive circuit of the drive circuit unit  90  via the first isolation transmitter  80 . As a result, the corresponding lower-arm switch SWL of the inverter  20  is turned or kept on. 
     In addition, the logical value representing the ON command and the logical value representing the OFF command may be opposite to the logical values transmitted with the configuration shown in  FIG. 3 . 
     Referring back to  FIG. 2 , the abnormality detector  92  detects abnormalities occurring in the upper-arm and lower-arm switches SWH and SWL of the inverter  20  and the drive circuit unit  90 . Moreover, upon determination that an abnormality (to be referred as switch-side abnormality hereinafter) has occurred in at least one of the upper-arm and lower-arm switches SWH and SWL and the drive circuit unit  90 , the abnormality detector  92  transmits an abnormality signal to the overall-state determiner  64  via the second isolation transmitter  93 . 
     In addition, as the abnormalities occurring in the upper-arm and lower-arm switches SWH and SWL, the abnormality detector  92  detects at least one of an overcurrent abnormality in which the electric current flowing through any one of the switches exceeds an overcurrent threshold, an overheating abnormality in which the temperature of any one of the switches exceeds a temperature threshold, and a voltage deviation abnormality in which the voltage Vce between the collector and the emitter in any one of the switches falls outside a predetermined range. On the other hand, as the abnormalities occurring in the drive circuit unit  90 , the abnormality detector  92  detects at least one of an abnormality in which no electric power is outputted from the isolation power supply  91  to the drive circuit unit  90 , an abnormality in which the voltage supplied by the isolation power supply  91  to the drive circuit unit  90  drops below a low-voltage threshold, an abnormality in which the voltage supplied by the isolation power supply  91  to the drive circuit unit  90  exceeds an overvoltage threshold, and an abnormality in which variation in the voltage supplied by the isolation power supply  91  to the drive circuit unit  90  exceeds a predetermined value. 
     Upon determination that an abnormality has occurred in at least one of the sensors  23   a - 23   c  and the microcomputer  50 , the microcomputer  50  stops the generation and output of the switching commands and outputs a switching stop command to the overall-state determiner  64 . 
     In addition, as the abnormalities occurring in the sensors  23   a - 23   c , the microcomputer  50  detects at least one of an abnormality in which any one of the sensors has no signal outputted therefrom, an abnormality in which the output signal of any one of the sensors drops below a first predetermined threshold, an abnormality in which the output signal of any one of the sensors exceeds a second predetermined threshold, and an abnormality in which the output characteristics of any one of the sensors are not normal. On the other hand, the determination as to whether an abnormality has occurred in the microcomputer  50  is made based on the results of a diagnosis performed by the microcomputer  50  itself or by a diagnosing unit provided outside the microcomputer  50 . 
     The composite IC  60  further includes a safe-state determiner  66 . The safe-state determiner  66  calculates the power source voltage VDC on the basis of the voltage signal Vs outputted from the voltage sensor  23   a . Then, the safe-state determiner  66  determines, based on the calculated power source voltage VDC, the content of the compulsory switching command to be outputted from the overall-state determiner  64 . 
       FIG. 4  shows a safe-state determination process performed by the safe-state determiner  66 . In addition, this process is repeatedly performed in a predetermined cycle by the safe-state determiner  66 . 
     In step S 10 , the safe-state determiner  66  calculates the power source voltage VDC on the basis of the voltage signal Vs outputted from the voltage sensor  23   a , and determines whether the calculated power source voltage VDC is higher than a predetermined voltage Vgmth. 
     In addition, this determination is made for determining whether regeneration is being performed. The safe-state determiner  66  corresponds to both a “power source voltage detector” and a “voltage determiner”. In the present embodiment, the predetermined voltage Vgmth is set to a minimum value in a normal range of the terminal voltage of the storage battery  21  or to a value higher than the minimum value and lower than a maximum value in the normal range. Here, the normal range denotes the range within which the terminal voltage of the storage battery  21  varies during normal operation of the storage battery  21 . 
     If the determination in step S 10  results in a “NO” answer, i.e., if the calculated power source voltage VDC is not higher than the predetermined voltage Vgmth, the process proceeds to step S 11 . 
     In step S 11 , the safe-state determiner  66  determines that the line-to-line voltage between the phase windings  11  of the rotating electric machine  10  is not higher than the terminal voltage of the storage battery  21  and thus no regeneration is being performed. Then, the safe-state determiner  66  determines that when it is determined in step S 20  of  FIG. 5  (to be described later) that at least one of the abnormalities has occurred, the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the shutdown (abbreviated to SD in  FIGS. 4 and 5 ) command. Thereafter, the process terminates. 
     On the other hand, if the determination in step S 10  results in a “YES” answer, i.e., if the calculated power source voltage VDC is higher than the predetermined voltage Vgmth, the process proceeds to step S 12 . 
     In step S 12 , the safe-state determiner  66  determines that the line-to-line voltage between the phase windings  11  of the rotating electric machine  10  is higher than the terminal voltage of the storage battery  21  and thus regeneration is being performed. Then, the safe-state determiner  66  determines that when it is determined in step S 20  of  FIG. 5  that at least one of the abnormalities has occurred, the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the ASC command. Thereafter, the process terminates. 
     In addition, the results of the determinations made by the safe-state determiner  66  in steps S 11  and S 12  of the above process are inputted to the overall-state determiner  64 . 
       FIG. 5  shows an overall-state determination process performed by the overall-state determiner  64 . In addition, this process is repeatedly performed in a predetermined cycle by the overall-state determiner  64 . 
     In step S 20 , the overall-state determiner  64  determines whether at least one of the following events has occurred: (a) the notification of a switch-side abnormality from the abnormality detector  92 ; (b) the input of the switching stop command from the microcomputer  50 ; and (c) the notification of the overvoltage abnormality from the overvoltage detector  65 . 
     In addition, the determination as to whether a switch-side abnormality has occurred can be made by determining whether the abnormality signal has been transmitted from the abnormality detector  92  via the second isolation transmitter  93 . In the present embodiment, the overall-state determiner  64  corresponds to an “abnormality determiner” that performs step S 20 . 
     If the determination in step S 20  results in a “NO” answer, i.e., if none of the events (a)-(c) has occurred, the process proceeds to step S 21 . 
     In step S 21 , the overall-state determiner  64  refrains from outputting the compulsory switching command to the switch controller  70 . That is, the overall-state determiner  64  allows continuation of the switching control (or drive) of the upper-arm and lower-arm switches SWH and SWL of the inverter  20  according to the switching commands outputted from the microcomputer  50 . 
     On the other hand, if the determination in step S 20  results in a “YES” answer, i.e., if at least one of the events (a)-(c) has occurred, the process proceeds to step S 22 . 
     In step S 22 , the overall-state determiner  64  further determines whether both the following two conditions are satisfied: (1) a determination result has been inputted from the safe-state determiner  66  and the determination result indicates that the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command to the ASC command; and (2) an ASC disabling command, which will be described in detail later, has not been inputted from the microcomputer  50 . 
     If the determination in step S 22  results in a “NO” answer, i.e., if neither of the above two conditions is satisfied, the process proceeds to step S 23 . In other words, the process proceeds to step S 23  if at least one of the following two conditions is satisfied: (3) a determination result has been inputted from the safe-state determiner  66  and the determination result indicates that the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command to the shutdown command; and (4) the ASC disabling command has been inputted from the microcomputer  50 . 
     In step S 23 , the overall-state determiner  64  outputs the shutdown command as the compulsory switching command to the upper-arm and lower-arm controllers  71 H and  71 L of the switch controller  70 . Consequently, a shutdown control is performed by the switch controller  70  to turn off all of the upper-arm and lower-arm switches SWH and SWL of the inverter  20 . 
     In contrast, if the determination in step S 22  results in a “YES” answer, i.e., if both the above two conditions are satisfied, the process proceeds to step S 24 . 
     In step S 24 , the overall-state determiner  64  outputs the ASC command as the compulsory switching command to the upper-arm and lower-arm controllers  71 H and  71 L of the switch controller  70 . Consequently, an All-phase Short Circuit control (to be referred to as ASC control hereinafter) is performed by the switch controller  70  to turn off all of the upper-arm switches SWH of the inverter  20  while turning on all of the lower-arm switches SWL of the inverter  20 . 
     In addition, the overall-state determiner  64  also corresponds to an “abnormality-handling controller” that performs steps S 22 -S 24 . 
     Next, the ASC disabling command will be described in detail. 
     In the present embodiment, as shown in  FIG. 6 , the voltage sensor  23   a  outputs the voltage signal Vs that decreases with increase in the actual terminal voltage of the storage battery  21 . On the other hand, the second output voltage V 2   r  of the low-voltage power supply circuit  40  may become low during the activation of the low-voltage power supply circuit  40  or in the case of a failure of the low-voltage power supply circuit  40 . Moreover, when the second output voltage V 2   r  becomes low, the voltage signal Vs of the voltage sensor  23   a  may also become low though the actual terminal voltage of the storage battery  21  is low. In this case, the detected power source voltage VDC becomes high; thus though the terminal voltage of the storage battery  21  is actually low, it is erroneously determined to be high. Consequently, though it is actually necessary to perform the shutdown control, the ASC control may be accidently performed instead. 
     In view of the above, in the present embodiment, the microcomputer  50  determines whether the second output voltage V 2   r  of the low-voltage power supply circuit  40  is lower than a predetermined voltage. Moreover, when the second output voltage V 2   r  of the low-voltage power supply circuit  40  is determined to be lower than the predetermined voltage, the microcomputer  50  outputs the ASC disabling command to the overall-state determiner  64 . In contrast, when the second output voltage V 2   r  of the low-voltage power supply circuit  40  is determined to be higher than or equal to the predetermined voltage, the microcomputer  50  refrains from outputting the ASC disabling command to the overall-state determiner  64 . Accordingly, when it is determined in step S 22  of  FIG. 5  that the ASC disabling command has been inputted from the microcomputer  50 , the overall-state determiner  64  determines the second output voltage V 2   r  of the low-voltage power supply circuit  40  to be lower than the predetermined voltage; thus in subsequent step S 23 , the overall-state determiner  64  outputs the shutdown command as the compulsory switching command to the upper-arm and lower-arm controllers  71 H and  71 L of the switch controller  70 . Consequently, the ASC control is not performed even when the line-to-line voltage between the phase windings  11  of the rotating electric machine  10  is determined in step S 12  of  FIG. 4  to be higher than the terminal voltage of the storage battery  21 . As a result, it becomes possible to prevent the ASC control from being accidentally performed. 
     Moreover, the microcomputer  50  may output a continuous pulse signal as the ASC disabling command to the overall-state determiner  64 . In this case, the overall-state determiner  64  determines that the ASC disabling command has been inputted thereto when it is determined that the pulse signal has been inputted thereto, and determines that the ASC disabling command has not been inputted thereto when it is determined that the pulse signal has not been inputted thereto. Consequently, it becomes possible to prevent the ASC disabling command from being accidentally transmitted to the overall-state determiner  64  due to signal sticking. In addition, it should be noted that the microcomputer  50  may also output, instead of a pulse signal, a digital signal of logical H or L as the ASC disabling command. 
       FIG. 7  illustrates the changes with time of (a) the power source voltage VDC, (b) the presence or absence of the ASC disabling command outputted from the microcomputer  50  and (c) the content of the compulsory switching command for bringing the rotating electric machine control system into a safe state. 
     At a time instant t 1 , it is determined by the safe-state determiner  66  that: the power source voltage VDC becomes higher than the predetermined voltage Vgmth; and the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command to the ASC command. 
     Then, at a time instant t 2 , it is determined by the safe-state determiner  66  that: the power source voltage VDC becomes lower than the predetermined voltage Vgmth; and the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command to the shutdown (abbreviated to SD in  FIG. 7 ) command. 
     Thereafter, at a time instant t 3 , the ASC disabling command is inputted from the microcomputer  50  to the overall-state determiner  64 . 
     Consequently, at a time instant t 4 , though the power source voltage VDC again becomes higher than the predetermined voltage Vgmth, it is still determined by the safe-state determiner  66  that the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command to the shutdown command. 
     Thereafter, at a time instant t 5 , the output of the ASC disabling command from the microcomputer  50  is stopped. 
     According to the present embodiment, it is possible to achieve the following advantageous effects. 
     In the present embodiment, the ASC control is performed instead of the shutdown control upon: (1) determination by the overall-state determiner  64  that at least one of a switch-side abnormality, the input of the switching stop command and the overvoltage abnormality has occurred; and (2) determination by the safe-state determiner  66  that the line-to-line voltage between the phase windings  11  of the rotating electric machine  10  when a counterelectromotive force is generated in the phase windings  11  is higher than the terminal voltage of the storage battery  21 . When the ASC control is performed, no closed circuit is formed which includes the upper-arm and lower-arm diodes DH and DL of the inverter  20 , the phase windings  11  of the rotating electric machine  10  and the storage battery  21 . Instead, a closed circuit is formed which includes the turned-on lower-arm switches SWL of the inverter  20  and the phase windings  11  of the rotating electric machine  10 . Consequently, it becomes possible to cause electric current induced in the phase windings  11  of the rotating electric machine  10  to flow within the formed closed circuit, thereby preventing regeneration from being performed. As a result, it becomes possible to prevent the terminal voltage of the smoothing capacitor  22  from being considerably increased and the torque generated by the rotating electric machine  10  from being rapidly increased. 
     Moreover, in the present embodiment, the overall-state determiner  64 , the safe-state determiner  66  and the switch controller  70  are integrated into the single composite IC  60 . Consequently, it becomes possible to shorten the time from when the determination in step S 20  of  FIG. 5  results in a “YES” answer until the ASC control or the shutdown control is performed. 
     Modification of First Embodiment 
     The safe-state determination process shown in  FIG. 4 , which is performed by the safe-state determiner  66 , may be suitably modified. 
     For example, hysteresis may be provided in switching between the determination in step S 11  and the determination in step S 12 . Specifically, in this case, when the voltage signal Vs of the voltage sensor  23   a  is determined to be lower than a first predetermined threshold ASCL, the safe-state determiner  66  determines that the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the ASC command. On the other hand, when the voltage signal Vs of the voltage sensor  23   a  is determined to be higher than a second predetermined threshold ASCH that is higher than the first predetermined threshold ASCL, the safe-state determiner  66  determines that the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the shutdown command. With the above configuration, it is possible to prevent chattering. 
     Second Embodiment 
     A control circuit  30  according to the second embodiment has a similar configuration to the control circuit  30  according to the first embodiment. Therefore, the differences therebetween will be mainly described hereinafter. 
       FIG. 8  shows the configuration of the control circuit  30  according to the second embodiment.  FIG. 9  shows both the configuration of the switch controller  70  and the configuration of the photodiode-side part of the first isolation transmitter  80  according to the second embodiment. 
     As shown in  FIG. 9 , in the present embodiment, the switch controller  70  includes an arm controller  73 , an upper-arm SD unit  74 , a lower-arm SD unit  75  and a lower-arm ASC unit  76 , all of which are driven by the second output voltage V 2   r  of the low-voltage power supply circuit  40  supplied thereto. 
     Moreover, in the present embodiment, the high switches  72 H and the low switches  72 L are provided in the first isolation transmitter  80 , not in the switch controller  70  as in the first embodiment. 
     Furthermore, in the present embodiment, each of the low switches  72 L consists of a first low switch  72 La and a second low switch  72 Lb that are electrically connected in parallel to each other. In addition, each of the first and second low switches  72 La and  72 Lb is configured with an N-channel MOSFET. 
     Moreover, in the present embodiment, the first isolation transmitter  80  further includes a first switch  83 H, an upper-arm driver  84 H, a second switch  83 L and a lower-arm driver  84 L. 
     In the present embodiment, the arm controller  73  of the switch controller  70  determines whether the compulsory switching command has been inputted thereto from the overall-state determiner  64 . Moreover, when it is determined that no compulsory switching command has been inputted thereto, the arm controller  73  outputs an ON command to each of the upper-arm SD unit  74  and the lower-arm SD unit  75  and an OFF command to the lower-arm ASC unit  76 . 
     Then, the upper-arm SD unit  74  outputs the inputted ON command to the upper-arm driver  84 H of the first isolation transmitter  80  via a first one of three terminals Tr of the composite IC  60 . The lower-arm SD unit  75  outputs the inputted ON command to the lower-arm driver  84 L of the first isolation transmitter  80  via a second one of the three terminals Tr of the composite IC  60 . The lower-arm ASC unit  76  outputs the inputted OFF command to each of the gates of the second low switches  72 Lb of the first isolation transmitter  80  via a third one of the three terminals Tr of the composite IC  60 . In addition, the switching commands from the microcomputer  50  are respectively inputted to the gates of the first low switches  72 La and the high switches  72 H. 
     Upon receipt of the ON command, the upper-arm driver  84 H turns on the first switch  83 H, thereby energizing the upper-arm photodiodes  82 H. Consequently, the switching commands from the microcomputer  50  for controlling switching of the upper-arm switches SWH of the inverter  20  are transmitted to the drive circuit unit  90  via the first isolation transmitter  80 . Moreover, upon receipt of the ON command, the lower-arm driver  84 L turns on the second switch  83 L, thereby energizing the lower-arm photodiodes  82 L. Consequently, the switching commands from the microcomputer  50  for controlling switching of the lower-arm switches SWL of the inverter  20  are transmitted to the drive circuit unit  90  via the first isolation transmitter  80 . 
     On the other hand, when it is determined that the shutdown command has been inputted thereto as the compulsory switching command, the arm controller  73  outputs an OFF command to each of the upper-arm SD unit  74 , the lower-arm SD unit  75  and the lower-arm ASC unit  76 . 
     Upon receipt of the OFF command from the upper-arm SD unit  74  via the first terminal Tr of the composite IC  60 , the upper-arm driver  84 H turns off the first switch  83 H, thereby terminating the energization of the upper-arm photodiodes  82 H. Consequently, all of the upper-arm switches SWH of the inverter  20  are turned or kept off. Moreover, upon receipt of the OFF command from the lower-arm SD unit  75  via the second terminal Tr of the composite IC  60 , the lower-arm driver  84 L turns off the second switch  83 L, thereby terminating the energization of the lower-arm photodiodes  82 L. Consequently, all of the lower-arm switches SWL of the inverter  20  are turned or kept off. As a result, the shutdown control is performed. 
     In contrast, when it is determined that the ASC command has been inputted thereto as the compulsory switching command, the arm controller  73  outputs an OFF command to the upper-arm SD unit  74  and an ON command to each of the lower-arm SD unit  75  and the lower-arm ASC unit  76 . 
     Upon receipt of the OFF command from the upper-arm SD unit  74  via the first terminal Tr of the composite IC  60 , the upper-arm driver  84 H turns off the first switch  83 H, thereby terminating the energization of the upper-arm photodiodes  82 H. Consequently, all of the upper-arm switches SWH of the inverter  20  are turned or kept off. Moreover, upon receipt of the ON command from the lower-arm SD unit  75  via the second terminal Tr of the composite IC  60 , the lower-arm driver  84 L keeps the second switch  83 L on, thereby allowing the lower-arm photodiodes  82 L to be continuously energized. Furthermore, upon input of the ON command from the lower-arm ASC unit  76  to the gates of the second low switches  72 Lb via the third terminal Tr of the composite IC  60 , the second low switches  72 Lb are turned on. Consequently, all of the lower-arm switches SWL of the inverter  20  are turned or kept on. As a result, the ASC control is performed. 
     According to the present embodiment, it is also possible to achieve the same advantageous effects as described in the first embodiment. 
     Moreover, according to the present embodiment, it becomes possible to reduce the number of the terminals Tr of the composite IC  60  from six to three. 
     Third Embodiment 
     A control circuit  30  according to the third embodiment has a similar configuration to the control circuit  30  according to the first embodiment. Therefore, the differences therebetween will be mainly described hereinafter.  FIG. 10  shows the configuration of the control circuit  30  according to the third embodiment. 
     As shown in  FIG. 10 , in the present embodiment, the composite IC  60  further includes a line-to-line voltage estimator  67  to which is inputted the angle signal outputted from the angle sensor  23   c.    
     The line-to-line voltage estimator  67  calculates, based on the inputted angle signal, the rotational speed Nm of the rotor of the rotating electric machine  10 . Then, based on the calculated rotational speed Nm and using the equation “Vc=K×Nm”, the line-to-line voltage estimator  67  estimates the line-to-line voltage Vc between the phase windings  11  of the rotating electric machine  10  when a counterelectromotive force is generated in the phase windings  11 . Here, K is a coefficient that depends on the amount of magnetic flux ϕ generated by magnetic poles of the rotor. 
     Alternatively, the line-to-line voltage estimator  67  may estimate the line-to-line voltage Vc on the basis of the electrical angular speed we instead of the rotational speed Nm of the rotor. 
     The line-to-line voltage Vc estimated by the line-to-line voltage estimator  67  is inputted to the safe-state determiner  66 . Then, based on the line-to-line voltage Vc and the power source voltage VDC, the safe-state determiner  66  determines the content of the compulsory switching command to be outputted from the overall-state determiner  64 . 
       FIG. 11  shows a safe-state determination process performed by the safe-state determiner  66  according to the present embodiment. In addition, this process is repeatedly performed in a predetermined cycle by the safe-state determiner  66 . 
     In step S 13 , the safe-state determiner  66  determines whether the line-to-line voltage Vc estimated by the line-to-line voltage estimator  67  is higher than the power source voltage VDC calculated based on the voltage signal Vs outputted from the voltage sensor  23   a . In addition, this determination is made for determining whether regeneration is being performed. 
     If the determination in step S 13  results in a “NO” answer, i.e., if the line-to-line voltage Vc is not higher than the power source voltage VDC, the process proceeds to step S 11 . 
     In step S 11 , the safe-state determiner  66  determines that no regeneration is being performed. Then, the safe-state determiner  66  determines that when it is determined in step S 20  of  FIG. 5  that at least one of the abnormalities has occurred, the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the shutdown (abbreviated to SD in  FIG. 11 ) command. Thereafter, the process terminates. 
     On the other hand, if the determination in step S 13  results in a “YES” answer, i.e., if the line-to-line voltage Vc is higher than the power source voltage VDC, the process proceeds to step S 12 . 
     In step S 12 , the safe-state determiner  66  determines that regeneration is being performed. Then, the safe-state determiner  66  determines that when it is determined in step S 20  of  FIG. 5  that at least one of the abnormalities has occurred, the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the ASC command. Thereafter, the process terminates. 
     In addition, the results of the determinations made by the safe-state determiner  66  in steps S 11  and S 12  of the above process are inputted to the overall-state determiner  64 . 
     As above, in the present embodiment, the line-to-line voltage Vc estimated by the line-to-line voltage estimator  67  is compared with the power source voltage VDC that is calculated based on the voltage signal Vs outputted from the voltage sensor  23   a . Consequently, it becomes possible to accurately determine whether the line-to-line voltage Vc between the phase windings  11  of the rotating electric machine  10  when a counterelectromotive force is generated in the phase windings  11  is higher than the terminal voltage of the storage battery  21 . As a result, it becomes possible to suppress electric current from flowing in the phase windings  11 , thereby suppressing increase in the temperature of the rotating electric machine  10 . 
     In the present embodiment, when the line-to-line voltage Vc estimated based on the rotational speed Nm of the rotor is determined to be higher than the power source voltage VDC, it is determined that the line-to-line voltage Vc when a counterelectromotive force is generated in the phase windings  11  of the rotating electric machine  10  is higher than the terminal voltage of the storage battery  21 . Moreover, when the line-to-line voltage Vc is determined to be higher than the terminal voltage of the storage battery  21 , it is determined that the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the ASC command. In contrast, when the line-to-line voltage Vc is determined to be not higher than the terminal voltage of the storage battery  21 , it is determined that the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command to the shutdown command. 
     It should be noted that in the present embodiment, the estimation of the line-to-line voltage Vc is made based on the rotational speed Nm of the rotor for the purpose of preventing the torque (or braking torque) generated by the rotating electric machine  10  from becoming high during occurrence of an abnormality. 
     When the ASC control is performed, electric current flows within the phase windings  11  of the rotating electric machine  10  and the inverter  20 , thereby preventing regeneration, in which electric current flows to the storage battery  21  side, from being performed. In this case, since electric current flows through the phase windings  11  of the rotating electric machine  10 , torque is generated by the rotating electric machine  10 . The generated torque increases with the absolute value of the q-axis current Iq. Moreover, as can be seen from the following equations for determining the d-axis current Id and the q-axis current Iq, the absolute value of the q-axis current Iq increases with decrease in the electrical angular speed we (or the rotational speed Nm of the rotor). This is because the resistance component increases with decrease in the rotational speed Nm of the rotor. The following equations can be derived by substituting Vd=0 and Vq=0 into well-known voltage equations, where Vd is the d-axis voltage and Vq is the q-axis voltage. In addition, both Vd and Vq are set to zero because all of the lower-arm switches SWL of the inverter  20  are turned on in the ASC control. 
     
       
         
           
             
               
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       FIGS. 12A-12D  show the relationships between various parameters when the ASC control is performed. Specifically,  FIG. 12A  shows the relationship between the rotational speed Nm of the rotor and the d-axis and q-axis currents Id and Iq.  FIG. 12B  shows the relationship between the rotational speed Nm of the rotor and the phase-current amplitude.  FIG. 12C  shows the relationship between the d-axis current Id and the q-axis current Iq.  FIG. 12D  shows the relationship between the rotational speed Nm of the rotor and the torque generated by the rotating electric machine  10 . 
     As shown in  FIG. 12A , when the rotational speed Nm of the rotor is low, the absolute value of the q-axis current Iq is small. However, when the rotational speed Nm of the rotor drops below a given rotational speed, the absolute value of the q-axis current Iq turns to increase. Consequently, as shown in  FIG. 12D , in a low-rotational speed region where the rotational speed Nm of the rotor is lower than the given rotational speed, the braking torque generated by the rotating electric machine  10  increases with decrease in the rotational speed Nm. The increase in the braking torque may cause sudden deceleration of the vehicle. Therefore, in the low-rotational speed region, it is desirable to perform the shutdown control, not the ASC control. 
     In the case of determining whether regeneration is being performed on the basis of comparison between the power source voltage VDC and the predetermined voltage Vgmth (see step S 10  of  FIG. 4 ), it may be erroneously determined in the low-rotational speed region that regeneration is being performed. For example, the rotational speed Nm of the rotor may drop into the low-rotational speed region with the terminal voltage of the smoothing capacitor  22  remaining high. In this case, though the rotational speed Nm of the rotor has dropped, the change in the amount of electric charge stored in the smoothing capacitor  22  is small. Consequently, though the rotational speed Nm of the rotor falls within the low-rotational speed region, the power source voltage VDC would be determined by the safe-state determiner  66  to be higher than the predetermined voltage Vgmth; thus it would be determined by the safe-state determiner  66  that the rotating electric machine control system can be brought into a safe state by setting the compulsory switching command to the ASC command. Hence, upon the determination in step S 22  of  FIG. 5  resulting in a “YES” answer, the ASC control would be performed in the low-rotational speed region. As a result, the braking torque generated by the rotating electric machine  10  would be increased, which may cause sudden deceleration of the vehicle. 
     In contrast, in the present embodiment, the line-to-line voltage Vc is estimated based on the rotational speed Nm of the rotor; thus the amplitude of the estimated line-to-line voltage Vc increases with the rotational speed Nm of the rotor. Therefore, when the estimated line-to-line voltage Vc is determined to be higher than the power source voltage VDC, the rotational speed Nm of the rotor would be definitely higher than the given rotational speed and thus definitely fall outside the low-rotational speed region. That is, in the present embodiment, the line-to-line voltage Vc estimated in the low-rotational speed region would be definitely lower than the minimum value in the range within which the power source voltage VDC varies during normal operation of the storage battery  21 . Hence, according to the present embodiment, it is possible to reliably prevent the ASC control from being performed in the low-rotational speed region, thereby reliably preventing sudden deceleration of the vehicle. 
     In addition, in the present embodiment, though the shutdown control is performed in the low-rotational speed region, it would not cause any problems because regeneration is not performed when the line-to-line voltage Vc is not higher than the terminal voltage of the storage battery  21 . 
     Modifications of Third Embodiment 
     In step S 13  of  FIG. 11 , the predetermined voltage Vgmth, which is used in step S 10  of  FIG. 4 , may be used instead of the power source voltage VDC. In this case, it is still possible to prevent the ASC control from being performed in the low-rotational speed region. 
     The line-to-line voltage Vc may be estimated based on both the rotational speed Nm of the rotor and the temperature of the rotor detected by a temperature sensor or estimated by a temperature estimator. 
     The control circuit  30  may further include an angle information converter that converts the angle signal outputted from the angle sensor  23   c  into angle information. In this case, the angle information converter may be built in either the microcomputer  50  or the composite IC  60 , or provided as a dedicated IC (Integrated Circuit). 
     Instead of the angle sensor  23   c , a rotational speed sensor may be employed which detects the rotational speed (i.e., the mechanical angular speed) of the rotor of the rotating electric machine  10 . 
     The rotating electric machine control system may further include a voltage sensor that detects the line-to-line voltage Vc between the phase windings  11  of the rotating electric machine  10 . In this case, in step S 13  of  FIG. 11 , the line-to-line voltage Vc detected by the voltage sensor is used instead of the estimated line-to-line voltage Vc. 
     Fourth Embodiment 
     A control circuit  30  according to the fourth embodiment has a similar configuration to the control circuit  30  according to the first embodiment. Therefore, the differences therebetween will be mainly described hereinafter. 
     In the present embodiment, as shown in  FIG. 13 , the rotating electric machine control system further includes an input current sensor  23   d  that is configured to detect electric current flowing from the inverter  20  to the storage battery  21  side and output a current signal indicative of the detected electric current. 
       FIG. 14  shows the configuration of the control circuit  30  according to the present embodiment. 
     As shown in  FIG. 14 , in the present embodiment, to the input current sensor  23   d , there is supplied the second output voltage V 2   r  of the low-voltage power supply circuit  40 . The current signal outputted from the input current sensor  23   d  is inputted to the microcomputer  50 . Then, based on the inputted current signal, the microcomputer  50  calculates the electric current flowing from the inverter  20  to the storage battery  21  side. 
     Moreover, in the present embodiment, instead of the voltage signal Vs outputted from the voltage sensor  23   a , the current signal outputted from the input current sensor  23   d  is inputted to the safe-state determiner  66 . In addition, to the safe-state determiner  66 , there is also inputted from the overall-state determiner  64  information on the switching state of each of the upper-arm and lower-arm switches SWH and SWL of the inverter  20 . 
     In the present embodiment, the safe-state determiner  66  determines a safe state of the rotating electric machine control system on the basis of the current signal outputted from the input current sensor  23   d.    
       FIG. 15  shows a safe-state determination process performed by the safe-state determiner  66  according to the present embodiment. In addition, this process is repeatedly performed in a predetermined cycle by the safe-state determiner  66 . 
     In step S 14 , the safe-state determiner  66  determines whether the ASC control is being performed. 
     If the determination in step S 14  results in a “YES” answer, the process immediately terminates without performing the subsequent steps. 
     In contrast, if the determination in step S 14  results in a “NO” answer, the process proceeds to step S 15 . 
     In step S 15 , the safe-state determiner  66  determines that the shutdown control is being performed. Then, the safe-state determiner  66  further determines, based on the current signal outputted from the input current sensor  23   d , whether electric current is flowing from the inverter  20  to the storage battery  21  side. 
     In addition, when the counterelectromotive force generated in the phase windings  11  of the rotating electric machine  10  exceeds the terminal voltage of the storage battery  21  during the execution of the shutdown control, regenerative current flows from the inverter  20  to the storage battery  21  side. 
     If the determination in step S 15  results in a “NO” answer, the process proceeds to step S 11 . 
     In step S 11 , the safe-state determiner  66  determines that no regeneration is being performed. Then, the safe-state determiner  66  determines that the shutdown state is currently a safe state of the rotating electric machine control system. Thereafter, the process terminates. 
     On the other hand, if the determination in step S 15  results in a “YES” answer, the process proceeds to step S 12 . 
     In step S 12 , the safe-state determiner  66  determines that regeneration is being performed. Then, the safe-state determiner  66  determines that the rotating electric machine control system can be brought into a safe state (i.e., all-phase short circuit state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the ASC command. Thereafter, the process terminates. 
     In addition, the results of the determinations made by the safe-state determiner  66  in steps S 11  and S 12  of the above process are inputted to the overall-state determiner  64 . 
     According to the present embodiment, it is also possible to achieve the same advantageous effects as described in the first embodiment. 
     Moreover, according to the present embodiment, it becomes possible to accurately determine whether regeneration is being performed. Consequently, it becomes possible to reduce chances of the ASC control being performed during occurrence of an abnormality. As a result, it becomes possible to suppress braking torque generated by the rotating electric machine  10  from being increased in the low-rotational speed region. 
     Fifth Embodiment 
     A control circuit  30  according to the fifth embodiment has a similar configuration to the control circuit  30  according to the second embodiment. Therefore, the differences therebetween will be mainly described hereinafter. 
       FIG. 16  shows the configuration of the control circuit  30  according to the fifth embodiment. 
     As shown in  FIG. 16 , in the present embodiment, the control circuit  30  further includes a monitoring microcomputer  100  which corresponds to a “monitor”. 
     Hereinafter, for the sake of convenience of explanation, the microcomputer  50  will be referred to as “control microcomputer  50 ”. Moreover, the switching stop command outputted from the control microcomputer  50  will be referred to as “first switching stop command”. 
     In the present embodiment, the monitoring microcomputer  100  is provided to monitor whether any abnormality has occurred in the control microcomputer  50 . In addition, the monitoring method may be a well-known method such as a method using a watchdog timer. 
     To the monitoring microcomputer  100 , there is supplied the second output voltage V 2   r  of the low-voltage power supply circuit  40 . Moreover, to the monitoring microcomputer  100 , there are inputted the angle signal outputted from the angle sensor  23   c  and the voltage signal Vs outputted from the voltage sensor  23   a.    
       FIG. 17  shows a monitoring process performed by the monitoring microcomputer  100 . In addition, this process is repeatedly performed in a predetermined cycle by the monitoring microcomputer  100 . 
     In step S 30 , the monitoring microcomputer  100  determines whether any abnormality has occurred in the control microcomputer  50 . 
     If the determination in step S 30  results in a “NO” answer, the process immediately terminates without performing the subsequent steps. 
     In contrast, if the determination in step S 30  results in a “YES” answer, the process proceeds to step S 31 . 
     In step S 31 , the monitoring microcomputer  100  further determines whether it is necessary to perform the ASC control. 
     Specifically, in this step, the monitoring microcomputer  100  determines whether the line-to-line voltage Vc between the phase windings  11  of the rotating electric machine  10  when a counterelectromotive force is generated in the phase windings  11  is higher than the terminal voltage of the storage battery  21 . In addition, this determination may be made using the method of step S 10  of  FIG. 4  based on the voltage signal Vs outputted from the voltage sensor  23   a . Alternatively, this determination may be made using the method of step S 13  of  FIG. 11  based on both the angle signal outputted from the angle sensor  23   c  and the voltage signal Vs outputted from the voltage sensor  23   a . Moreover, in the case of the rotating electric machine control system including the input current sensor  23   d , this determination may alternatively be made using the method of step S 15  of  FIG. 15 . Furthermore, this determination may alternatively be made using a method that is a combination of at least two of the aforementioned methods. 
     If the determination in step S 31  results in a “NO” answer, i.e., if it is unnecessary to perform the ASC control, the process proceeds to step S 33 . 
     In step S 33 , the monitoring microcomputer  100  outputs a second switching (abbreviated to SW in  FIG. 17 ) stop command to the overall-state determiner  64 . Thereafter, the process terminates. 
     In addition, the second switching stop command is outputted for the same purpose as the first switching stop command. 
     On the other hand, if the determination in step S 31  results in a “YES” answer, i.e., if it is necessary to perform the ASC control, the process proceeds to step S 32 . 
     In step S 32 , the monitoring microcomputer  100  outputs an ASC command to the overall-state determiner  64  via the safe-state determiner  66 . Thereafter, the process terminates. 
     In addition, the monitoring microcomputer  100  may output a continuous pulse signal as the ASC command. In this case, it is possible to prevent the ASC command from being accidentally transmitted to the overall-state determiner  64  due to signal sticking. 
     According to the present embodiment, it is also possible to achieve the same advantageous effects as described in the second embodiment. 
     Moreover, according to the present embodiment, when an abnormality has occurred in the control microcomputer  50 , the ASC command can be outputted from the monitoring microcomputer  100 , thereby causing the ASC control to be performed. 
     Modifications of Fifth Embodiment 
     Instead of the monitoring microcomputer  100 , a monitoring IC may be provided in the control circuit  30  to monitor the control microcomputer. 
     The safe-state determiner  66  may be omitted from the composite IC  60 . In this case, in step S 32  of  FIG. 17 , the monitoring microcomputer  100  may output the ASC command directly to the overall-state determiner  64 . 
     Step S 30  may be omitted from the process shown in  FIG. 17 . In this case, the monitoring microcomputer  100  performs step S 31  regardless of any abnormality occurring in the control microcomputer  50 . 
     Sixth Embodiment 
     A control circuit  30  according to the sixth embodiment has a similar configuration to the control circuit  30  according to the second embodiment. Therefore, the differences therebetween will be mainly described hereinafter. 
       FIG. 18  shows the configuration of the control circuit  30  according to the sixth embodiment. 
     As shown in  FIG. 18 , in the present embodiment, to the safe-state determiner  66 , there are inputted the monitoring results of the power supply monitor  62 . 
     In the present embodiment, the low-voltage power supply circuit  40  further generates, by stepping down the output voltage VB of the low-voltage DC power source  24 , a fourth output voltage V 4   r  in addition to the first, second and third output voltages V 1   r , V 2   r  and V 3   r.    
     More particularly, in the present embodiment, the fourth output voltage V 4   r  is equal to the second output voltage V 2   r . The divided-voltage value of the fourth output voltage V 4   r  is inputted to the power supply monitor  62 . Then, based on the inputted divided-voltage value, the power supply monitor  62  detects the fourth output voltage V 4   r.    
     Moreover, in the present embodiment, to the voltage sensor  23   a , the phase-current sensor  23   b  and the angle sensor  23   c , there is supplied the fourth output voltage V 4   r  instead of the second output voltage V 2   r . On the other hand, to the microcomputer  50 , there are supplied both the second and fourth output voltages V 2   r  and V 4   r.    
     The power supply monitor  62  has a function of monitoring both the power supply controller  61  and the low-voltage power supply circuit  40 . Upon determination that an abnormality has occurred in at least one of the power supply controller  61  and the low-voltage power supply circuit  40 , the power supply monitor  62  notifies the safe-state determiner  66  of the occurrence of the abnormality. In addition, the power supply monitor  62  monitors the low-voltage power supply circuit  40  on the basis of the detected output voltages V 1   r -V 4   r.    
     In the present embodiment, the safe-state determiner  66  determines a safe state of the rotating electric machine control system on condition that no abnormality related to the output signals of the sensors  23   a - 23   c  has occurred. 
     In addition, “abnormalities related to the output signals of the sensors  23   a - 23   c ” denote detection accuracy degradation abnormalities such that deviations of the detected values of the physical quantities, which are detected based on the output signals of the sensors  23   a - 23   c , from the actual values of the physical quantities are greater than predetermined thresholds. Specifically, taking the voltage signal Vs of the voltage sensor  23   a  as an example, the abnormality related to the voltage signal Vs denotes the detection accuracy degradation abnormality of the voltage sensor  23   a  such that the deviation of the power source voltage VDC detected based on the voltage signal Vs of the voltage sensor  23   a  from the actual terminal voltage of the smoothing capacitor  22  is greater than a predetermined threshold. 
     In the present embodiment, the abnormalities related to the output signals of the sensors  23   a - 23   c  include: abnormalities occurring in the low-voltage power supply circuit  40 ; abnormalities occurring in the sensors  23   a - 23   c ; abnormalities occurring in interfaces between the sensors  23   a - 23   c  and the microcomputer  50 ; abnormalities occurring in the microcomputer  50 ; abnormalities occurring in electrical paths connecting the low-voltage power supply circuit  40  and the sensors  23   a - 23   c ; abnormalities occurring in electrical paths connecting the sensors  23   a - 23   c  and the aforementioned interfaces; and abnormalities occurring in electrical paths connecting the aforementioned interfaces and the microcomputer  50 . 
     Moreover, the abnormalities related to the output signals of the sensors  23   a - 23   c  also include those abnormalities which occur during the activation and shutdown of the low-voltage power supply circuit  40 . This is because during the activation and shutdown of the low-voltage power supply circuit  40 , the output voltages V 1   r -V 4   r  of the low-voltage power supply circuit  40  become unstable, thereby lowering the accuracy of detection of the physical quantities based on the output signals of the sensors  23   a - 23   c.    
     In the present embodiment, the abnormities occurring in the low-voltage power supply circuit  40  are determined as follows. When the fourth output voltage V 4   r  of the low-voltage power supply circuit  40  drops below a first threshold VtLS, the power supply monitor  62  determines that an undervoltage abnormality has occurred in the low-voltage power supply circuit  40 . Thereafter, when the fourth output voltage V 4   r  of the low-voltage power supply circuit  40  exceeds a second threshold VtHS that is higher than the first threshold VtLS, the power supply monitor  62  determines that there is no undervoltage abnormality occurring in the low-voltage power supply circuit  40  (see  FIG. 20( a ) ). 
       FIG. 19  shows a safe-state determination process performed by the safe-state determiner  66  according to the present embodiment. In addition, this process is repeatedly performed in a predetermined cycle by the safe-state determiner  66 . 
     It should be noted that steps S 10 -S 12  of the safe-state determination process according to the present embodiment are identical to steps S 10 -S 12  of the safe-state determination process according to the first embodiment (see  FIG. 4 ). Therefore, descriptions of steps S 10 -S 12  will not be repeated hereinafter. 
     If the determination in step S 10  results in a “YES” answer, the process proceeds to step S 16 . 
     In step S 16 , the safe-state determiner  66  further determines whether any abnormality related to the output signals of the sensors  23   a - 23   c  has occurred. 
     If the determination in step S 16  results in a “NO” answer, the process proceeds to step S 12 . 
     In contrast, if the determination in step S 16  results in a “YES” answer, the process proceeds to step S 11 . Consequently, though it has been determined in step S 10  that regeneration is being performed, the safe-state determiner  66  determines in step S 11  that the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command, which is to be outputted from the overall-state determiner  64 , to the shutdown (abbreviated to SD in  FIG. 19 ) command. 
     In addition, when any abnormality related to the output signals of the sensors  23   a - 23   c  has occurred so that the inverter  20  becomes unable to continue its normal operation, the switch controller  70  may stop switching of the upper-arm and lower-arm switches SWH and SWL of the inverter  20 . 
       FIG. 20  illustrates the changes with time of (a) the fourth output voltage V 4   r  of the low-voltage power supply circuit  40 , (b) the power source voltage VDC, (c) the presence or absence of the ASC disabling command outputted from the microcomputer  50  and (d) the content of the compulsory switching command for bringing the rotating electric machine control system into a safe state. 
     It should be noted that: (b)-(d) of  FIG. 20  respectively correspond to (a)-(c) of  FIG. 7  described in the first embodiment; and time instants t 1 -t 5  shown in  FIG. 20  respectively correspond to the time instants t 1 -t 5  shown in  FIG. 7 . 
     As shown in  FIG. 20 , at a time instant tb, it is determined by the power supply monitor  62  that the fourth output voltage V 4   r  drops below the first threshold VtLS. That is, it is determined by the power supply monitor  62  that the undervoltage abnormality has occurred in the low-voltage power supply circuit  40 . Consequently, from the time instant tb, the compulsory switching command is set to the shutdown command and thus the ASC control is not performed. 
     In contrast, at a time instant ta, it is determined by the power supply monitor  62  that the fourth output voltage V 4   r  exceeds the second threshold VtHS. That is, it is determined by the power supply monitor  62  that there is no undervoltage abnormality occurring in the low-voltage power supply circuit  40 . Consequently, during the time period from the time instant ta to the time instant tb, the compulsory switching command is set to either the shutdown command or the ASC command depending on both the result of the determination in step S 10  of  FIG. 19  and the presence or absence of the ASC disabling command outputted from the microcomputer  50 . As a result, during the time period of from ta to tb, either the shutdown control or the ASC control is performed depending on both the result of the determination in step S 10  of  FIG. 19  and the presence or absence of the ASC disabling command outputted from the microcomputer  50 . 
     In addition, prior to the time instant ta, the compulsory switching command is set to the shutdown command and thus the ASC control is not performed. 
     As described above, in the present embodiment, when any abnormality related to the output signals of the sensors  23   a - 23   c  has occurred, the safe-state determiner  66  determines that the rotating electric machine control system can be brought into a safe state (i.e., shutdown state) by setting the compulsory switching command to the shutdown command even if the line-to-line voltage Vc is determined to be higher than the terminal voltage of the storage battery  21 . Consequently, the ASC control is not performed even when the determination in step S 20  of  FIG. 5  results in a “YES” answer. As a result, it becomes possible to prevent the ASC control from being accidentally performed in situations where the ASC control should not be performed. 
     Moreover, in the present embodiment, the system of supplying electric power from the low-voltage power supply circuit  40  to the sensors  23   a - 23   c  is separated from the system of supplying electric power from the low-voltage power supply circuit  40  to the microcomputer  50 . Consequently, the microcomputer  50  can continue its normal operation when the fourth output voltage V 4   r  of the low-voltage power supply circuit  40  becomes abnormal. 
     Modifications of Sixth Embodiment 
     The system of supplying electric power from the low-voltage power supply circuit  40  to the sensors  23   a - 23   c  and the system of supplying electric power from the low-voltage power supply circuit  40  to the microcomputer  50  may be integrated into a single system as shown in  FIG. 2 . 
     Specifically, the control circuit shown in  FIG. 2  may be modified so that: the monitoring results of the power supply monitor  62  are inputted to the safe-state determiner  66 ; and the ASC control is not performed when any abnormality related to the output signals of the sensors  23   a - 23   c  has occurred. Moreover, the control circuit shown in  FIG. 8  may also be modified so that: the monitoring results of the power supply monitor  62  are inputted to the safe-state determiner  66 ; and the ASC control is not performed when any abnormality related to the output signals of the sensors  23   a - 23   c  has occurred. 
     While the above particular embodiments and modifications have been shown and described, it will be understood by those skilled in the art that various further modifications, changes, and improvements may be made without departing from the spirit of the present disclosure. 
     For example,  FIG. 21  shows the configuration of a control circuit that is a combination of the control circuit according to the third embodiment (see  FIG. 10 ) and the control circuit according to the sixth embodiment (see  FIG. 18 ). In the control circuit shown in  FIG. 21 , the monitoring results of the power supply monitor  62  are inputted to the safe-state determiner  66 ; and the output voltages of the low-voltage power supply circuit  40  are divided into two systems. 
       FIG. 22  shows the configuration of a control circuit that is a combination of the control circuit according to the fourth embodiment (see  FIG. 14 ) and the control circuit according to the sixth embodiment (see  FIG. 18 ). In the control circuit shown in  FIG. 22 , the monitoring results of the power supply monitor  62  are inputted to the safe-state determiner  66 ; and the output voltages of the low-voltage power supply circuit  40  are divided into two systems. 
       FIG. 23  shows the configuration of a control circuit that is a combination of the control circuit according to the fifth embodiment (see  FIG. 16 ) and the control circuit according to the sixth embodiment (see  FIG. 18 ). In the control circuit shown in  FIG. 23 , the monitoring results of the power supply monitor  62  are inputted to the safe-state determiner  66 ; and the output voltages of the low-voltage power supply circuit  40  are divided into two systems. 
     The ASC control may be modified to turn on all of the upper-arm switches SWH of the inverter  20  and turn off all of the lower-arm switches SWL of the inverter  20 . 
     When at least one of the upper-arm switches SWH of the inverter  20  is stuck ON, the ASC control may be performed to turn on all of the upper-arm switches SWH and turn off all of the lower-arm switches SWL of the inverter  20 . In contrast, when at least one of the lower-arm switches SWL of the inverter  20  is stuck ON, the ASC control may be performed to turn on all of the lower-arm switches SWL and turn off all of the upper-arm switches SWH of the inverter  20 . 
     When at least one of the upper-arm switches SWH of the inverter  20  is stuck OFF, the ASC control may be performed to turn off all of the upper-arm switches SWH and turn on all of the lower-arm switches SWL of the inverter  20 . 
     In contrast, when at least one of the lower-arm switches SWL of the inverter  20  is stuck OFF, the ASC control may be performed to turn off all of the lower-arm switches SWL and turn on all of the upper-arm switches SWH of the inverter  20 . 
     The voltage sensor  23   a  may alternatively be configured so that the voltage signal Vs outputted from the voltage sensor  23   a  increases with the terminal voltage of the storage battery  21 . 
     The ASC disabling may not be performed. For example, in step S 22  of  FIG. 5 , the condition (2) regarding the ASC disabling command may be removed. 
     Each of the upper-arm and lower-arm switches SWH and SWL of the inverter  20  may be configured with, instead of an IGBT, other semiconductor switching elements such as an N-channel MOSFET having a body diode built therein. 
     Each of the upper-arm and lower-arm switches SWH and SWL of the inverter  20  may be configured with two or more switching elements that are connected in parallel with each other. For example, each of the upper-arm and lower-arm switches SWH and SWL may be configured with a SiC switching element and a Si switching element that are connected in parallel with other, or with an IGBT and a MOSFET that are connected in parallel with other. 
     The command generator, which generates the switching commands, is not limited to the microcomputer  50 . For example, the command generator may be implemented by a control IC or a dedicated IC with a motor control function. 
     The IC on which the power supply controller  61 , the power supply monitor  62 , the overall-state determiner  64  and the switch controller  70  are mounted is not limited to the composite IC. 
     The voltage sensor  23   a  may alternatively be provided outside the control circuit  30 . 
     The phase-current sensor  23   b , the angle sensor  23   c  and the input current sensor  23   d  may alternatively be built in the control circuit  30 . 
     Each of the first isolation transmitter  80  and the second isolation transmitter  93  may alternatively be constituted of a magnetic coupler that is a magnetic isolation element. In this case, in performing the ASC control, the composite IC  60  may output to the magnetic couplers a command to stop the output of the switching commands generated by the microcomputer  50 . At the same time, the composite IC may output to the drive circuit unit  90  both an ON command for turning on all of the lower-arm switches SWL and an OFF command for turning off all of the upper-arm switches SWH (alternatively, both an ON command for turning on all of the upper-arm switches SWH and an OFF command for turning off all of the lower-arm switches SWL). 
     The controlled variable of the rotating electric machine is not limited to torque. For example, the controlled variable may alternatively be the rotational speed Nm of the rotor of the rotating electric machine  10 . 
     The rotating electric machine  10  is not limited to an interior permanent magnet synchronous machine. For example, the rotating electric machine  10  may alternatively be configured as a field winding synchronous machine. 
     Moreover, the rotating electric machine  10  is not limited to a synchronous machine. For example, the rotating electric machine  10  may alternatively be configured as an induction machine. 
     Furthermore, the rotating electric machine  10  is not limited to an in-vehicle main rotating electric machine. For example, the rotating electric machine  10  may alternatively be an electric motor employed in, for example, an electric power steering system or an air conditioning electric compressor. 
     The number of phases of the multi-phase rotating electric machine  10  is not limited to three. For example, the number of phases of the rotating electric machine  10  may alternatively be six or nine. In addition, in the case of the number of phases of the rotating electric machine  10  being equal to six, the inverter  20  may include, for example, 12 switches. 
     The rotating electric machine control system shown in  FIG. 1  may further have a boost (or stepping-up) converter provided between the storage battery  21  and the inverter  20 . 
     The low-voltage power supply circuit  40  is not limited to a voltage stepping-down circuit. For example, the low-voltage power supply circuit  40  may alternatively be a voltage boosting (or stepping-up) circuit or a voltage boosting/stepping-down circuit. 
     The rotating electric machine control system may alternatively include two or more pairs of rotating electric machines and inverters. For example, in the case of the rotating electric machine control system including two pairs of rotating electric machines and inverters, the total number of the switches of the inverters may be equal to 12.