Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2002-08-08
2003-10-28
Hirshfeld, Andrew H. (Department: 2854)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S540000, C324S754090
Reexamination Certificate
active
06639410
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for measuring an insulation resistance characteristic of a capacitive electronic part such as a capacitor.
2. Description of the Related Art
A measuring apparatus shown in
FIG. 1
has been used to measure an insulation resistance characteristic of a capacitive electronic part such as a capacitor in the past. Specifically, a direct current (dc) measurement power supply
1
has one terminal thereof grounded and the other terminal thereof connected to one terminal of a measured capacitor
3
via a current-limiting resistor
2
. One terminal of a voltmeter
4
is connected between the current-limiting resistor
2
and measured capacitor
3
. The other terminal of the measured capacitor
3
is connected to an ammeter
5
, and a leakage current flowing through the measured capacitor
3
is measured using the ammeter
5
.
A measurement voltage E applied to the measured capacitor
3
is measured using the voltmeter
4
, and a current is flowing through the measured capacitor
3
is measured using the ammeter
5
. An insulation resistor R characteristic of the measured capacitor
3
can be calculated as follows:
R=E/I
In the foregoing measuring apparatus, a measured value of the insulation resistance R may contain an error because of a noise caused by the measurement power supply
1
, a hum caused by a power supply or the like, and a noise caused by the measured capacitor
3
itself. This poses a problem.
Reasons why the error occurs will be described with reference to FIG.
2
.
Assume that the capacitance of the measured capacitor
3
is C, an insulation resistance is R, a dc voltage generated by the measurement power supply
1
is E, and a voltage (alternating voltage component) derived from a noise caused by the measurement power supply
1
or a hum caused by a mains power supply is e.
Except for the noise component e, I=Ir (leakage current) would be established and the insulation resistance R characteristic of the measured capacitor
3
could be calculated according to R=E/I. However, in reality, the current I does not equal the leakage current Ir determined with the insulation resistance but contains a noise component Ic flowing through the capacitor. Namely, I=Ir+Ic is established. This results in an error of a measured value.
For example, assume that an insulation resistance, characteristic of a capacitor, exhibiting an insulation resistance of 50 M&OHgr; and offering a capacitance of 10 &mgr;F, is measured using a voltage of 50 V, and an output of a power supply contains a noise of 10 mVrms. In this case, the leakage current Ir and noise component Ic are calculated as follows:
Ir=
50 V/50 M&OHgr;=1 &mgr;A
Ic=
10 mVrms/(½&pgr;×60×10 &mgr;F)=approx. 38 &mgr;Arms
The current of 1 &mgr;A that should be measured is buried under the noise current 38 &mgr;A that is 30 or more times larger than the current of 1 &mgr;A. This does not allow precise measurement. If the values of the current I detected for a long period of time are integrated, an average of the values of the noise component Ic of the current I approaches 0. Measurement now becomes possible. However, it takes too much time to complete the measurement. This poses a problem.
When a resistor Rs is, as shown in
FIG. 3
, inserted in the middle of a path leading from the measured capacitor
3
to the ammeter
5
, the noise component Ic can be reduced. For example, assuming that Rs equals 50 k&OHgr;, Ir and Ic are calculated as follows:
Ir=
50 V/(50 M&OHgr;+50 k&OHgr;)=approx. 1 &mgr;A
Ic=
10 mVrms/(50 k&OHgr;+½&pgr;×60×10 &mgr;F)=approx. 0.2 &mgr;Arms
The noise component is as small as 0.2 &mgr;A relative to the current to be measured, 1 &mgr;A. This enables precise measurement. However, since the resistor offering a resistance that is as large as 50 k&OHgr; is used, a charging current used to charge the capacitor with the capacitance C flows for a time that is several times longer than a time, constant RC=50 k&OHgr;×10 &mgr;F=500 ms. Measurement cannot be started until the charging current ceases. It therefore takes too much time to complete the measurement. This poses a problem.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an insulation resistance measuring apparatus capable of highly precisely measuring an insulation resistance characteristic of a capacitive electronic part in a short period of time while being unaffected by various kinds of noises.
For accomplishing the above object, according to the first embodiment of the present invention, there is provided an insulation resistance measuring apparatus that applies a predetermined measurement voltage to a capacitive electronic part, and measures a current flowing through the electronic part so as to calculate an insulation resistance characteristic of the electronic part. Herein, a noise clipper circuit is connected on a path leading from a measurement power supply through the capacitive electronic part to a current detector. The noise clipper circuit is realized with a circuit having a resistor Ra and a switching means RE
1
connected in parallel with each other. The switching means RE
1
is controlled to remain closed in an early stage of charging the capacitive electronic part. When charging the capacitive electronic part has progressed sufficiently, the switching means RE
1
is controlled to open.
For achieving precise measurement in a short period of time, while a large current is flowing through the noise clipper circuit, or in other words, while a charging current used to charge a capacitor is flowing, a resistance Ra should be reduced. When the current flowing through the noise clipper circuit gets smaller, or in other words, when the charging current used to charge the capacitor ceases and only a current determined with the insulation resistance R flows, the resistance Ra should be increased. According to the first embodiment of the present invention, a noise clipper circuit consists of a resistor Ra and a switching means RE
1
which are connected in parallel with each other. In an early stage of charging a capacitive electronic part, the switching means RE
1
is closed. When charging the capacitive electronic part has progressed sufficiently, the switching means RE
1
is opened.
In the early stage of charging the capacitive electronic part, since the switching means RE
1
is closed, the noise clipper circuit offers almost no resistance. In other words the resistance offered by the noise clipper circuit is so small that the capacitive electronic part can be charged smoothly. Thereafter, when charging the capacitive electronic part has been nearly completed, the switching means RE
1
is opened. This causes the resistance offered by the noise clipper circuit to increase. A noise component of a charging current is cut off. This results in precise measurement.
For measuring a final insulation resistance characteristic of a capacitive electronic part, the noise clipper circuit composed of the resistor Ra and switching means RE
1
alone will suffice. For detecting a temporal variation of the charging current for charging the capacitive electronic part, when the charging current gets smaller with the switching means RE
1
closed, the variation of the charging current may not be measured correctly because of the adverse effect of a noise current. However, when charging the capacitive electronic part is performed insufficiently, if the switching means RE
1
is opened, charging the capacitive electronic part is retarded because of the resistor Ra.
According to the second embodiment of the present invention, preferably, a noise, clipper circuit is realized with a parallel circuit having a first resistor Ra, a first switching means RE
1
, and a series circuit connected in parallel with one another. The series circuit consists of a second resistor Rb and a second switching means RE
2
. In this
Burns Doane , Swecker, Mathis LLP
Hamdan Wasseem H.
Hirshfeld Andrew H.
Murata Manufacturing Co. Ltd.
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