JPS632349B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ground resistance meter using the AC potentiometric method.

First, an outline of a ground resistance meter using the AC potentiometric method will be explained with reference to FIG. This earth resistance meter consists of an oscillation circuit 1, an output transformer 2, a current transformer 3, a variable resistor 4 connected to the secondary side of the current transformer 3, and a synchronous detector that operates in synchronization with the oscillation circuit 1. 5, galvanometer 6 and three measurement terminals E, P,
In this case, one terminal of the secondary winding of the output transformer 2 is connected to the measurement terminal C,
The other terminal is connected to the measurement terminal E via the primary winding of the current transformer 3. The synchronous detector 5 is
It is connected between the variable resistor 4 and the measurement terminal P. When measuring the ground resistance, the measurement terminals E, P, and C are connected to the ground object 7 to be measured and the auxiliary ground electrodes 8 and 9, respectively. The oscillation output of the oscillation circuit 1 appears as a measurement current in the secondary winding of the output transformer 2, and this measurement current is passed through the measurement terminal C, the auxiliary grounding electrode 9, the ground 10, the grounding object to be measured 7,
It flows through a measuring loop which returns to the output transformer 2 via the measuring terminal E and the primary winding of the current transformer 3. As a result, a voltage drop occurs between the measurement terminals E and P due to the grounding resistance of the grounding object 7 to be measured and the measurement current, and on the other hand,
Since this measurement current induces a current on the secondary side of the current transformer 3, a voltage drop also occurs between the measurement terminal E and the variable resistor 4. These two voltages are detected by a synchronous detector 5 using a switching signal synchronized with the oscillation waveform of the oscillation circuit 1. Therefore, by operating the variable resistor 4 to balance the galvanometer 6 and reading the dial scale of the variable resistor 4 at that time, the grounding resistance of the grounding object 7 to be measured can be determined. In this case, in order to avoid the negative influence of external noise such as ground voltage on the measurement accuracy, the two voltages mentioned above are synchronously detected by the synchronous detector 5, but this method is not very effective when actually measuring in the field. No, and the cause was unknown.

As a result of intensive research into the above-mentioned drawbacks, the inventors of the present invention found that the oscillation circuit 1 was the cause. That is, a self-excited oscillation circuit called a Royer circuit (see FIG. 2) has been consistently used as the oscillation circuit 1 of the earth resistance meter. Since the Royer circuit itself is well known, we will not explain it in detail here, but
To put this briefly, a pair of transistors 1
2 and 13, and an oscillation output transformer 14. In this case, the core of the transformer 14 is different from a normal iron core, and has a hysteresis loop that is close to a rectangular shape. This Royer circuit utilizes the magnetic saturation characteristics of the core of the oscillation output transformer 14, and the saturation magnetic flux φs of the core, the voltage V of the power supply 15,
Primary side collector winding Nc of the oscillation output transformer 14
A rectangular wave having an oscillation period with a duty ratio of 1:1 determined by is oscillated. Note that the oscillation half cycle period T=4Ncφs/V. by the way,
As shown in FIG. 1, an electrical device 11 is connected to the grounding object 7 to be measured.
Earth voltage at commercial frequency may be generated due to leakage current from 1. In this case, since a commercial frequency current flows in the measurement loop due to this earth voltage, the secondary winding 1 of the oscillation output transformer 14
A magnetomotive force is applied to the core via 6. As a result, the magnetic flux saturation point of the core is biased, and the oscillation waveform is phase modulated by the earth voltage. Therefore, as shown in FIG. 3, the voltage waveform between the measurement terminals E and P is a superposition of the square wave test current that has undergone phase modulation by the earth voltage and the earth voltage. The synchronous detector 5 also detects earth voltage components. As a result, the balance point of the galvanometer 6 shifts, causing a measurement error.

Therefore, an object of the present invention is to provide a ground resistance meter that can accurately measure ground resistance without being affected by external noise such as ground voltage.

Embodiments of the present invention will be described below with reference to FIGS. 4 and 5.

The earth resistance meter of the present invention uses a two-transformer inverter circuit 17 shown in FIG. 4 as an oscillation circuit.
This inverter circuit 17 supplies a measurement current to the measurement loop shown in FIG. 2-transformer inverter circuit 17
is a publication published by Nikkan Kogyo Shimbun, “Inverter
As introduced in ``Converter'' (written by Mogi), the converter includes a pair of transistors 18 and 19, an oscillation transformer 20, and an output transformer 21. The oscillation transformer 20 has a magnetically saturated core with a nearly rectangular hysteresis loop, and a feedback resistor 22 is connected between its primary winding and the collector of one of the transistors 18. The secondary winding serves as the base winding of each of transistors 18 and 19. The output transformer 21 is a normal non-saturation type transformer, and its primary winding is connected between the collectors of the transistors 18 and 19, and one terminal of the secondary winding is connected to a measurement terminal. C, and the other terminal is connected to measurement terminal E via the primary winding of current transformer 3.

This inverter circuit 17 operates as follows. First, assume that transistor 19 is off and transistor 18 begins to conduct. When the transistor 18 starts energizing and starts supplying a load current through the output transformer 21, its collector voltage becomes higher than the collector voltage of the transistor 19, so the current flows through the feedback resistor 22 to the oscillation transformer 20. flows to the primary winding of the As a result, a voltage is induced in the secondary winding of the oscillation transformer 20 in a direction that further pushes the transistor 18 in the energizing direction and, conversely, pushes the transistor 19 into the cut-off direction. This feedback effect causes transistor 18 to quickly saturate and provide full load current. The current flowing through the feedback resistor 22 increases linearly due to the primary side inductance of the oscillation transformer 20, and eventually the oscillation transformer 20 saturates. Then, the current on the primary side increases rapidly, so the voltage drop across the feedback resistor 22 increases, and therefore the voltage across the primary winding decreases. That is, the feedback voltage decreases. As a result, transistor 18 is released from saturation and transistor 19 begins to conduct current. Then, a feedback effect occurs in the opposite direction to that described above, so that transistor 18 is cut off and transistor 19 begins to conduct current. In this case, the primary current of the oscillation transformer 20 is reversed, and the transistor 19 is rapidly energized and saturated in the same way as when the transistor 18 was energized earlier, supplying the full load current. When the core of the oscillation transformer 20 is saturated, the voltage of the primary winding decreases as described above, so the phenomenon progresses in the opposite direction. In this way, the transistors 18 and 19 are energized alternately, so that a pulse oscillation output appears in the secondary winding of the output transformer 21, and this oscillation output is fed into the measurement loop in FIG. 1 as a measurement current. Supplied.

The oscillation period of this two-transformer inverter circuit 17 is determined by the saturation characteristics of the oscillation transformer 20. Therefore, even if a commercial frequency noise current caused by earth voltage or the like flows into the measurement loop including the secondary winding of the output transformer 21,
The oscillation waveform of this inverter circuit 17 is not phase-modulated by the noise current.

FIG. 5 relates to another embodiment of the oscillation circuit applied to the ground resistance meter of the present invention. 1
and a pair of transistors 27, 28 connected to the output sides of each of the second inverters 23, 24 and turned on and off alternately according to their outputs;
The oscillation frequency of this oscillation circuit is determined by the CR time constant of the capacitor 25 and resistor 26 of the non-stable multivibrator. Therefore, the oscillation waveform of this oscillation circuit is not subjected to phase modulation due to noise current caused by earth voltage or the like, as in the previous embodiment. As is clear from the description of the above embodiments, the present invention uses an oscillation circuit consisting of an oscillator and a non-saturating output transformer, so the configuration of the oscillator is limited to these embodiments. isn't it.

According to the present invention, the rectangular wave measurement current flowing in the measurement loop is not subjected to phase modulation due to earth voltage or the like, so the synchronous detector 5 can detect the voltage between the measurement terminals E and P and the measurement current. When synchronously detecting the voltage between the terminal E and the variable resistor 4, the earth voltage component is not detected together with these voltages. Therefore, it becomes possible to measure the grounding resistance of the grounding object 7 to be measured with high precision.

[Brief explanation of the drawing]

Figure 1 is a schematic circuit diagram of a grounding resistance meter using the AC voltage difference method, Figure 2 is a circuit diagram of an oscillation circuit used in a conventional earthing resistance meter, and Figure 3 is a measurement using phase modulation using the earth voltage. A current waveform diagram, FIG. 4 is a circuit diagram of an oscillation circuit applied to the earth resistance meter of the present invention,
FIG. 5 is a circuit diagram showing another oscillation circuit similarly applied to the present invention. In the figure, 1 is an oscillation circuit, 2 is an output transformer, 3 is a current transformer, 4 is a variable resistor, 5 is a synchronous detector, 6 is a galvanometer, 7 is a grounding object to be measured, 8 and 9 are auxiliary grounding poles, 11 is electrical equipment, 17 is a two-transformer inverter circuit, 18, 19, 27, 28 are transistors, 20 is a saturated transformer for oscillation, 21, 29 is a non-saturated transformer for output, 23, 24 is CMOS
An inverter, 25 a capacitor, and 26 a resistor.

Claims (1)

[Claims] 1 An oscillation circuit having a first measurement terminal connected to a grounding object to be measured, second and third measurement terminals connected to each of two auxiliary grounding electrodes, and an output transformer, the transformer one terminal of the secondary winding of the output transformer is connected to the second measurement terminal, and the other terminal of the secondary winding of the output transformer and the first measurement terminal a current transformer having a connected primary winding; and a variable resistor connected in parallel to the secondary winding of the current transformer, one end of which is connected to the first measurement terminal. a synchronous detector connected between the variable resistor and the third measurement terminal and operated by a switching signal synchronized with an oscillation waveform of the oscillation circuit; a secondary winding of the output transformer;
an alternating current potential difference in which a measurement current is supplied from the oscillation circuit via the output transformer to a measurement loop including the second measurement terminal, ground, the first measurement terminal, and the primary winding of the current transformer; 1. A grounding resistance meter, wherein the oscillation circuit includes an oscillator and a non-saturated transformer that supplies a measurement current to the measurement loop.