SU64854A1 - Device for measuring resistance - Google Patents

Description

Devices for measuring grounding resistance are known, which contain a transformer or an inductor for supplying an alternating current to a circuit composed of a series-connected measured ground, auxiliary grounding and measuring resistance, a zero DC indicator connected via a rectifier to the secondary winding of an auxiliary transformer located in the circuit between the test earthing and the probe, a measuring transformer that supplies a current to the circuit from the measuring resistance , compensating voltage drop on the ground between the tested ground and the probe.

The proposed device differs from the known ones in that between the middle point of the secondary winding of the auxiliary transformer connected via the indicator rectifiers to the ends of the potentiometer and the potentiometer slider is an additional secondary winding of the supply transformer or auxiliary transformer connected to the inductor. This compensates for the initial deviation of the indicator.

FIG. 1 is a schematic diagram of the device according to the invention; FIG. 2 - voltage curves in the potentiometer circuit.

The measuring current I and.zm is supplied from the AC mains through the supply transformer I to the circuit: measuring resistance 2, test ground 3, auxiliary ground 4. The drop in voltage UN from the measuring resistance is fed through a measure TejlbHbm transformer 5 to compensate for voltage drop U between the tested ground 5 and probe 6. As an indicator of the potential difference V between the probe and the secondary winding of the measuring transformer, a zero current indicator was used 7. When measuring the resistance ground test 3 arrows .NO 64854. 2 -

The indicator is set to zero by moving the slider of measuring resistance 2 — in this position, there is no current in the primary (1 winding of the auxiliary transformer 8 and the resistance of the measuring resistance is equal to the resistance of the test ground 3.

The power supply of the zero indicator circuit is produced from the secondary winding of the auxiliary transformer connected via rectifiers 9 to the ends of potentiometer 10, in parallel to which the indicator is connected. Between the middle point of the secondary winding of the auxiliary transformer and the potentiometer slider, an additional secondary winding P of the supply transformer is supplied, supplying an alternating voltage of 2-3 V from the same source that supplies the measuring current, "", to the test earthing. The indicated voltage must be in phase with the measuring current. The purpose of the additional winding is to compensate for the influence of stray currents. In the absence of a nanogram on the terminals of the primary winding of an auxiliary transformer, the rectified currents of the additional secondary windings of the supply transformer, when the potentiometer's motor is in the middle position, create equal drops of voltage across the arms, directed oppositely, between the ends of the potentiometer of the potential difference will not, and the deviation indicator will not give. Wandering currents call the same deviation of arrows 11 of the indicator.

On the curves, the solid line shows the additional voltage, the dotted line indicates the voltage supplied by the auxiliary transformer, and the bold line indicates the total voltage. Upward from the horizontal axis, the voltage acting in the chain of the shoulder / 2 potentiometer, winz in the chain of the shoulder, the shaded part of the straightened voltage.

If there is a coincidence in the frequency and phase of the additional voltage of the auxiliary transformer (curve a), the DC voltage on the measuring system will be the highest.

At the same time, the frequency of the auxiliary transformer and the phase shift between them in 90 (curve 6) mean values of rectified voltage in the centers of both arms of the potentiometer are equal, therefore, the constant voltage on the measuring voltage is paiuio to zero.

With a phase shift other than 90, the notes at the ends of the potentiometer can be equalized by changing the shoulder resistance ratio by moving the slider to the note of the meter.

When the frequency of the voltage supplied by the auxiliary transformer is of even multiplicity in relation to the additional voltage (even harmonic), the average values of the rectified voltages in the chains of both arms and the potentiometer are equal (curve c), which means the constant voltage on the measuring system equal to the bullet.

With the frequency of the voltage supplied by the auxiliary transformer of odd multiplicity with respect to the additional voltage (odd harmonics), the average values of the rectified voltage at the prices of both lengths will not be equal (curve d. In this case, equalize the potentials. At the ends of the potentiometer You can move the engine.

When the frequency of the voltage supplied by the auxiliary transformer at an equal frequency of additional voltage and not being harmonic with respect to it, the average values of the rectified voltage in the prices of both shoulders of the notocimeter

an integer number of periods of additional voltage will be p (curve /).

So, from a review of the operation of the indicator circuit, it can be seen that the indicator does not give a deviation if the voltage of the stray current has a frequency that is not equal to the frequency of the additional voltage, and is not its odd harmonic.

When the frequency of the stray current is equal to the frequency of the additional voltage or is its odd harmonic, the indicator does not give a deviation only if there is a phase shift between the voltage of the stray current and the additional voltage of 90 °. With all the other phase shift, the indicator will give a deviation, it can be returned to zero by changing the ratio of the potentiometer arms.

If the equilibrium indicator applies the Itz potential difference between the probe and the secondary winding of the measuring transformer 1, due to the measuring current 7 ,, equal to the frequency of the additional current, the device will give a deviation.

Thus, it can be seen that in the presence of stray-alternating currents, this indicator allows to measure the ground resistance. To do this, before passing the measuring current through the tested earthing, put the indicator arrow to zero by moving the potentiometer slider. Then, it is necessary to turn on the measuring current and compensate for the voltage drop at the tested ground. Wandering of direct current affects measurement, since the indicator is plugged into the probe circuit through the auxiliary transformer.

This circuit, eliminating the influence of stray currents, can also be applied when supplying a ground circuit from a local source - an inductor and a simulated current.

To perform the functions of an additional secondary winding, an additional transformer will need to be applied, the primary winding of which is incident to the id. and the secondary supplies an additional voltage to the indicator circuit.

Subject ir r and d and

A device for measuring grounding resistance, containing a transformer or inductor for supplying alternating current to a circuit composed of series-connected measurements of earth ground, auxiliary ground 1P1 and measuring resistance, is a zero constant current indicator, connected via rectifiers to the secondary winding of an auxiliary transformer located in the circuit between isyitu and the earth ground and the probe, the measuring transformer, which supplies this circuit with the measuring voltage of 15, the computer Sensitive to voltage on the ground between the tested ground and the probe, characterized in that, to compensate for the initial deflection of the indicator, n the chain between the middle point of the secondary winding of the auxiliary transformer connected through the rectifier of the indicator with the ends of the potentiometer, and the slider of the latter included an additional secondary the winding of the supply transformer or auxiliary transformer connected to the inductor.

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