Abstract:
An object is to provide an inverter device for an automobile capable of detecting a ground fault with simple circuitry, and in summary, the inverter device has an inverter (inverter module) which generates an AC output from a battery loaded on a vehicle body to drive a motor, and the inverter device comprises a ground fault detection circuit connected between a negative line of the battery and the vehicle body, and a controller, wherein the ground fault detection circuit includes a serial circuit of a resistor element and a condenser element or a serial circuit of a plurality of resistor elements, and a potential at a connection point of the elements is input to the controller to detect a ground fault.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an inverter device for an automobile which generates an AC output from a battery loaded on a vehicle body to drive a motor. 
   2. Description of the Related Art 
   Recently, an air conditioner equipped with an electrically driven compressor driven by a battery power source has been developed as an air conditioner for an electric automobile. This air conditioner comprises a battery (DC power source), an inverter device and an electrically driven compressor. 
   The above-mentioned inverter device comprises a switch as a switching device connected in serial to the battery; a charging device connected in parallel to this switch and including a resistor and a switch; an inverter module configured by molding, into a mold package, a switching element group including a switching element and an unshown diode to absorb a switching surge; a condenser; etc. The switching element group of the inverter module converts a DC voltage from the battery into a three-phase AC voltage, and applies it to the electrically driven compressor to drive a motor of the electrically driven compressor. 
   The above-mentioned condenser stably supplies a voltage to the switching element group. The resistor of the charging device restricts an incoming current through the condenser and an incoming current generated in the condenser when the DC voltage of the battery is applied. That is, the presence of the resistor makes it possible to, when the battery is connected, open the switch, close the switch of the charging device, and pass a current via the resistor, thereby restricting the incoming current produced when the voltage of the battery is applied. This makes it possible to prevent such a disadvantage that a high current or a high voltage is applied to the inverter module to damage the switching element group and the like in the inverter module. 
   In such an inverter device, a ground fault might occur, for example, in a positive line and a negative line of the battery, or inside the motor of the electrically driven compressor. Especially when the ground fault has occurred in the negative line, a leak current does not run despite the ground fault because a high voltage side is usually insulated, which has not allowed the ground fault to be detected. Thus, if the ground fault is left as it is without being noticed and if a further ground fault occurs at another place, a leak current might run to result in an electric shock, or a fire or damage of equipment due to the current. 
   Furthermore, means to detect such a ground fault has also been developed which is provided with an oscillating circuit to form a predetermined pulse waveform, and a ground fault detection circuit including a detection circuit to detect the pulse waveform from the oscillating circuit, thereby detecting the occurrence of the ground fault from a change in the pulse waveform of the oscillating circuit (e.g., refer to Japanese Patent Publication Laid-open No. 2933490). 
   However, when the ground fault detection circuit as mentioned above is provided, the circuit to detect the ground fault is complicated, leading to a problem of an increased production cost. 
   SUMMARY OF THE INVENTION 
   The present invention has been attained to solve the foregoing problem of prior art, and it is an object of the present invention to provide an inverter device for an automobile capable of detecting a ground fault with simple circuitry. 
   That is, an inverter device for an automobile of the present invention has an inverter which generates an AC output from a battery loaded on a vehicle body to drive a motor, and the inverter device comprises a ground fault detection circuit connected between a negative line of the battery and the vehicle body, and a controller, wherein the ground fault detection circuit includes a serial circuit of a resistor element and a condenser element or a serial circuit of a plurality of resistor elements, and a potential at a connection point of the elements is input to the controller to detect a ground fault. 
   Furthermore, in the inverter device for the automobile of the present invention, the controller, in the invention described above, judges that that a ground fault has occurred in the negative line of the battery when the input potential has decreased. 
   Still further, in the inverter device for the automobile of the present invention, the controller, in the invention described above, judges that a ground fault has occurred in a positive line of the battery or in an output of the inverter when the input potential has increased. 
   According to an inverter device for an automobile of the present invention, the inverter device has an inverter which generates an AC output from a battery loaded on a vehicle body to drive a motor, and the inverter device comprises a ground fault detection circuit connected between a negative line of the battery and the vehicle body, and a controller, wherein the ground fault detection circuit includes a serial circuit of a resistor element and a condenser element or a serial circuit of a plurality of resistor elements, and a potential at a connection point of the elements is input to the controller to detect a ground fault, whereby, for example, when a ground fault has occurred in the negative line of the battery, a value of a current running to the ground fault detection circuit decreases, and the potential at the connection point of the elements decreases. Thus, when the input potential of the controller has decreased, it is judged that a ground fault has occurred in the negative line of the battery, so that it is possible to accurately detect the ground fault in the negative line of the battery. 
   Furthermore, for example, when a ground fault has occurred in the positive line of the battery or in an output of the inverter, the value of the current running to the ground fault detection circuit increases, so that the potential at the connection point of the elements decreases. Thus, when the input potential of the controller has increased, it is judged that a ground fault has occurred in the positive line of the battery or in the output of the inverter, so that it is possible to detect the ground fault in the positive line of the battery and the ground fault in the output of the inverter. 
   In this way, such simple circuitry makes it possible to detect the occurrence of the ground fault in each section of the inverter device, thereby also allowing production cost to be reduced as low as possible. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an electric circuit diagram of one embodiment of an air conditioner for an electric automobile Comprising an inverter device of the present invention; 
       FIG. 2  is a diagram showing the flow of a current when a ground fault has occurred in a negative line of the inverter device of  FIG. 1 ; 
       FIG. 3  is a diagram showing the flow of the current when a ground fault has occurred in a positive line of the inverter device of  FIG. 1 ; 
       FIG. 4  is a diagram showing the flow of a current when a ground fault has occurred in an UVW harness of the inverter device of  FIG. 1 ; and 
       FIG. 5  is an electric circuit diagram of another embodiment of the air conditioner for the electric automobile comprising the inverter device of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Embodiments of the present invention will be described below in detail with reference to the drawings. 
   Embodiment 1 
     FIG. 1  is a power source circuit diagram of one embodiment of an air conditioner for an electric automobile comprising an inverter device  8  of the present invention. In  FIG. 1 ,  1  is a main battery as a DC power source of the electric automobile, and this provides power to an electrically driven compressor  10  of the air conditioner via the inverter device  8  of the present invention. 
   The above-mentioned inverter device  8  comprises a switch  2  as a switching device, a charging circuit  7  as a charging device, a condenser  30 , a discharging resistor  31 , the inverter module  35  (inverter), and the like. 
   The above-mentioned inverter module  35  comprises a switching element group  12  provided in a mold package  40  to convert the voltage into the three-phase AC voltage by switching. This switching element group  12  comprises a switching element  14  and an unshown diode to absorb a switching surge, and is connected between a positive line  4  (e.g., about DC +350 V) and a negative line  6  of the battery  1 . 
   Furthermore, a DC voltage is output from the battery  1 , but a voltage converted into a three-phase AC is supplied to a stator coil  10 C (three phase) of a motor  10 M of the electrically driven compressor  10  by the inverter module  35  of the inverter device  8  described later. That is, the voltage is switched by the switching element group  12  of the inverter module  35 , so that the three-phase AC is supplied to a U phase, V phase, W phase of the stator coil  10 C via an UVW harness  80  (an output of the inverter). 
   Furthermore, the switch  2  is connected to the positive line  4  between the battery  1  and the switching element group  12 . The condenser  30  stably supplies a voltage to the switching element group  12 , and is connected between the positive line  4  that is between the switch  2  and the switching element group  12 , and the negative line  6 . The condenser  30  is charged from the battery  1  via the charging circuit  7  described later. Further, the discharging resistor  31  discharges the charged condenser  30 , and is connected between the positive line  4  that is between the condenser  30  and the switching element group  12 , and the negative line  6 . 
   The above-mentioned charging circuit  7  comprises a serial circuit of a switch  3  and a positive characteristic thermister  18 , and is connected in parallel to the switch  2 . This charging circuit  7  restricts an incoming current through the condenser  30  when the voltage of the battery  1  is applied. 
   Then, a controller  60  of the inverter device  8 , in accordance with an operation command from an unshown controller of the air conditioner, first closes the switch  3  while the switch  2  is open (open-up), and passes a current from the battery  1  to the condenser  30  via the positive characteristic thermister  18 , thus charging the condenser  30 . The positive characteristic thermister  18  heats by itself to increase a resistance value, and thus serves to restrict an increase in a value of a flowing current. This restricts the incoming current and protects the condenser  30  and the switching element group  12 . 
   Next, the controller  60  closes the switch  2  at the moment when the charging of the condenser  30  is completed, and then opens the switch  3  of the charging circuit  7 , and after this, the voltage of the battery  1  is applied to the switching element group  12  via the switch  2 . The controller  60  controls the turning on/off of the switching element  14  of the switching element group  12  to generate a three-phase AC voltage at a predetermined frequency, and, as described above, applies it to the stator coil  10 C of the motor  10 M of the electrically driven compressor  10  to drive the same. 
   Then, in accordance with an operation specifying instruction from the controller of the air conditioner, the controller  60  opens (opens up) the switch  2 , and stops the operation of the electrically driven compressor  10 . 
   In addition, an airtight container  10 A of the electrically driven compressor  10  is grounded to a vehicle body B, and the stator coil  10 C and the like inside are electrically insulated from the airtight container  10 A. 
   Here, a ground fault detection circuit  75  of the present invention will be described. The ground fault detection circuit  75  is connected between the negative line  6  of the battery  1  and the vehicle body B, and comprises a serial circuit of a resistor element  77  on the negative line  6  side and a condenser element  78  on the vehicle body B side. Further, a potential at a connection point of the resistor element  77  and the condenser element  78  is input to the controller  60 . 
   It is to be noted that a plurality of unshown pins is attached to the mold package  40  to connect the switching element group  12  in the mold package  40  with the battery  1 , the switch  2 , the electrically driven compressor  10  and the like outside the mold package  40 . These pins allow the devices inside and outside the mold package  40  to be connected without trouble. 
   With the configuration described above, the operation of the ground fault detection circuit  75  will be described. Here, even in a normal state where no ground fault has occurred, a leak current is running from the stator coil  10 C of the motor  10 M to the vehicle body B via the airtight container  10 A through a refrigerant (indicated by a sign of a condenser or a resistor in the airtight container  10 A in  FIG. 1 ) or the like within the airtight container  10 A. That is, a leak current at a frequency switched by the switching element group  12  runs to the airtight container  10 A, the vehicle body B, the condenser element  78  and the resistor element  77  of the ground fault detection circuit  75 , and the negative line  6  of the battery  1  as indicated by a broken-line arrow in  FIG. 1 , so that a certain potential appears at the connection point of the condenser element  78  and the resistor element  77 . That is, a potential V0 at the connection point of the condenser element  78  and the resistor element  77  is input to the controller  60  even in the normal state. 
   Here, when a ground fault has occurred between the negative line  6  and the vehicle body B as shown in  FIG. 2 , the leak current from the motor  10 M of the electrically driven compressor  10  described above runs from the part where the ground fault has occurred to the negative line  6 . Thus, there is little current running to the ground fault detection circuit  75 , and the potential of the connection point input to the controller  60  will decrease or will be zero. When, for example, the potential input to the controller  60  has decreased below predetermined upper and lower ranges of the V0, the controller  60  judges that a ground fault has occurred in the negative line  6 , and issues a predetermined warning. 
   On one hand, if a ground fault has occurred between the positive line  4  and the vehicle body B, a ground-fault current is added to the leak current from the motor  10 M of the electrically driven compressor  10 . That is, as shown in  FIG. 3 , in addition to the leak current from the motor  10 M of the electrically driven compressor  10 , a current also leaks from the part where the ground fault has occurred and runs to the ground fault detection circuit  75 . Consequently, the current running to the ground fault detection circuit  75  increases, and the potential of the connection point input to the controller  60  will increase. 
   On the other hand, when a ground fault has occurred in the UVW harness  80  which is the output of the inverter module (inverter), the ground-fault current is also added to the leak current from the motor  10 M of the electrically driven compressor  10 . That is, as shown in  FIG. 4 , in addition to the leak current from the motor  10 M of the electrically driven compressor  10 , a current also leaks from the part of the UVW harness  80  where the ground fault has occurred and runs to the ground fault detection circuit  75 . Consequently, as in the case described above where the ground fault has occurred between the positive line  4  and the vehicle body B, the current running to the ground fault detection circuit  75  increases, and the potential of the connection point input to the controller  60  will increase. Therefore, when the potential input to the controller  60  has increased beyond the predetermined upper and lower ranges of the V0, the controller  60  judges that a ground fault has occurred in the positive line  4  or in the UVW harness  80 , and issues a predetermined warning. 
   In this way, when the potential has increased beyond the potential within the predetermined ranges resulting from the leak current running in the normal state, the controller  60  judges that a ground fault has occurred in the positive line  4  or in the UVW harness  80 , and when the input potential has decreased below the predetermined ranges, the controller  60  judges that a ground fault has occurred in the negative line  6 , thereby making it possible to detect the occurrence of the ground fault in the inverter device  8  and in the UVW harness  80  which is the output of the inverter. 
   In particular, the ground fault detection circuit of the present invention also allows the detection of the ground fault in the negative line  6  of the battery  1  occurrence of which has heretofore been difficult to detect, so that the safety and reliability of the inverter device  8  can be improved. 
   Furthermore, since the ground fault detection circuit  75  of the present invention comprises a simple structure of the serial circuit of the resistor element  77  and the condenser element  78 , the increase of costs due to the provision of the ground fault detection circuit  75  can be reduced as low as possible. 
   Embodiment 2 
   In addition, the ground fault detection circuit  75  comprises the serial circuit of the resistor element  77  and the condenser element  78  in the embodiment described above, but, for example, as shown in  FIG. 5 , it may also comprise a serial circuit of a plurality of resistor elements (in  FIG. 5 , two resistor elements  77 ,  79 ). In this case, the resistor element  79  on the vehicle body B side used has a sufficiently large resistance value.