Insulation measuring apparatus

1. An insulation measuring apparatus, comprising:

a measuring circuit including a first capacitor as a flying capacitor;

a control unit that is adapted to read a voltage set on the first capacitor to decide an insulation state of a power supply, and control a path configuration of the measuring circuit configured when one selected from a power supply voltage, a positive-electrode side ground fault resistor voltage, and a negative-electrode side ground fault resistor voltage is to be set on the first capacitor;

a switching section provided in a path located between the measuring circuit and the control unit, for shutting off the path between the measuring circuit and the control unit; and

a second capacitor provided between a ground and a path located between the switching section and the control unit, wherein

the control unit is adapted to control the path configuration of the measuring circuit when the control unit is to read the voltage set on the first capacitor, by turning ON the switching section to set a voltage corresponding to the voltage that is set on the first capacitor on the second capacitor, and then by turning OFF the switching section to read the voltage set on the second capacitor and to discharge an electric charge corresponding to the voltage being set on the first capacitor, and

in the measuring circuit, a resistance value of a path that is controlled to discharge the electric charge of the first capacitor is being set to a value which the electric charge corresponding to the voltage set on the first capacitor is dischargeable in a reading period of the voltage by the control unit.

2. The insulation measuring apparatus according to claim 1 , wherein the control unit turns ON the switching section to discharge the electric charge corresponding to the voltage set on the second capacitor after reading the voltage set on the second capacitor.

3. The insulation measuring apparatus according to claim 1 , wherein, upon setting the voltage on the first capacitor, the control unit charges the first capacitor for a first predetermined time in which the first capacitor is not fully charged, and then discharges the first capacitor for a second predetermined time, and

the control unit calculates the voltage which is to be set when the first capacitor is fully charged by reflecting a charging rate in the first predetermined time and a charging rate in the second predetermined time in a read value of the voltage being set on the first capacitor.

4. The insulation measuring apparatus according to claim 3 , wherein the second predetermined time is set as a time elapsed from a start of discharge to a first coincidence point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a plus variation product whose specification error is at a plus maximum is employed as the first capacitor.

5. The insulation measuring apparatus according to claim 3 , wherein the second predetermined time is set as a time elapsed from a start of discharge to a second coincidence point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a plus variation product whose specification error is at a minus maximum is employed as the first capacitor.

6. The insulation measuring apparatus according to claim 3 , wherein,

where a point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a plus variation product whose specification error is at a plus maximum is employed as the first capacitor is set as a first coincidence point, and

where a point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a minus variation product whose specification error is at a minus maximum is employed as the first capacitor is set as a second coincidence point,

the second predetermined time is set from the start of discharge to a point located between the first coincidence point and the second coincidence point.

7. The insulation measuring apparatus according claim 3 , wherein the first capacitor is formed of a ceramics capacitor or a film capacitor.

8. An insulation measuring apparatus, comprising:

a measuring circuit including a first capacitor as a flying capacitor;

a control unit that is adapted to read a voltage set on the first capacitor to decide an insulation state of a power supply, and control a path configuration of the measuring circuit configured when one selected from a power supply voltage, a positive-electrode side ground fault resistor voltage, and a negative-electrode side ground fault resistor voltage is to be set on the first capacitor; and

a second capacitor which is provided between a ground and a path located between the measuring circuit and the control unit, and on which a voltage is set when the voltage is set on the first capacitor, wherein

the control unit is adapted to configure a path through which electric charges accumulated on the first capacitor and the second capacitor are discharged when starting an operation of reading the voltage set on the second capacitor, and to compensate a reduction of voltage, which is caused by the discharge in a measuring period of the voltage set on the second capacitor, when measuring the insulation state, and

in the measuring circuit, a resistance value of a path that is controlled to discharge the electric charge of the first capacitor is being set to a value which the electric charge corresponding to the voltage set on the first capacitor is quickly discharged in a reading period of the voltage by the control unit.

9. The insulation measuring apparatus according to claim 8 , wherein, upon setting the voltage on the first capacitor, the control unit charges the first capacitor for a first predetermined time in which the first capacitor is not fully charged, and then discharges the first capacitor for a second predetermined time, and

the control unit calculates the voltage which is to be set when the first capacitor is fully charged by reflecting a charging rate in the first predetermined time and a charging rate in the second predetermined time in a read value of the voltage being set on the first capacitor.

10. The insulation measuring apparatus according to claim 9 , wherein the second predetermined time is set as a time elapsed from a start of discharge to a first coincidence point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a plus variation product whose specification error is at a plus maximum is employed as the first capacitor.

11. The insulation measuring apparatus according to claim 9 , wherein the second predetermined time is set as a time elapsed from a start of discharge to a second coincidence point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a plus variation product whose specification error is at a minus maximum is employed as the first capacitor.

12. The insulation measuring apparatus according to claim 9 , wherein,

where a point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a plus variation product whose specification error is at a plus maximum is employed as the first capacitor is set as a first coincidence point, and

where a point at which a voltage transition occurred from a start of charge on an assumption that a standard product whose specification error is zero is employed as the first capacitor coincides with a voltage transition occurred from a start of charge on an assumption that a minus variation product whose specification error is at a minus maximum is employed as the first capacitor is set as a second coincidence point,

the second predetermined time is set from the start of discharge to a point located between the first coincidence point and the second coincidence point.

13. The insulation measuring apparatus according claim 9 , wherein the first capacitor is formed of a ceramics capacitor or a film capacitor.