RU2328749C1 - Method of measurement of magnetising current of transformer that operates under load - Google Patents

Abstract

FIELD: electromechanics.

SUBSTANCE: suggested method consists in the following: measurements of instantaneous value of magnetising current is carried out from two shunts, one of which is included into primary winding of transformer, and the second one is included in the secondary winding, at that measuring terminals of shunts are connected serially back-to-back, and shunts are calibrated so that ratio of voltage drops in shunts during flowing of same current is equal to transformer transformation ratio.

EFFECT: development of method for measurement of magnetising current of transformer that operates under load.

4 dwg

Description

The method relates to electrical engineering, can be used for demonstration or research purposes.

Closest to the proposed one is the method [1] - Patent G01R 31/34 No. 1017083 "Method for measuring the magnetizing current of an induction motor" Ushakov AV, Mladentsev VI, which involves the measurement of current parameters only in the primary winding of the transformer, in the absence of load, that is, in idle mode. (The prototype is an asynchronous motor. An induction motor can generally be regarded as a transformer, then the primary winding is the stator winding, and rotation of the asynchronous motor with synchronous speed is the idle operation of the transformer).

The disadvantage of this method [1] is the impossibility of measuring the magnetizing current when operating under load. When the transformer is operating under load, the magnetizing current is part of the primary current and there is no section of the electric circuit where the magnetizing current would flow without a load component and could be measured. However, it is known that the magnetizing current with the appearance of the load current does not remain constant, the same as in idle mode, but decreases. The reason for the decrease in the magnetizing current is the voltage drop on the scattering inductance of the magnetic flux of the primary winding and the active resistance of the primary winding.

The solution to the problem of measuring a magnetizing transformer operating under load is achieved by measuring the instantaneous value of the magnetizing current from two shunts, one of which is connected to the primary and the second to the secondary winding of the transformer, and the measuring terminals of the shunts are connected in series, and the shunts are calibrated so so that the ratio of the voltage drops on the shunts when the same current flows is equal to the transformation ratio of the transformer.

Measurement by the proposed method is as follows.

Shunts are installed in the primary and secondary windings. The ratio between the rated currents of the shunts is chosen as close as possible to the transformation ratio of the transformer. The terminals of the shunts are connected so that the voltage drop from the current of the secondary winding i ' ₂ (t) is subtracted from the voltage drop from the current i ₁ (t). The oscilloscope in such a circuit should show the current i ^μ (t), since i ^μ (t) = i ₁ (t) -i ' ₂ (t). But this is only true with the exact selection of shunts, when the same current flowing through the shunts, will create voltage drops on the measuring terminals of the shunts, the ratio of which is equal to the transformation coefficient of the transformer.

It is not possible to select the ratio between the rated currents of the shunts exactly equal to the transformation coefficient of the transformer (in this case, the same current flowing through the shunts will create voltage drops on the measuring terminals of the shunts whose ratio is equal to the transformation coefficient of the transformer) using standard shunts. Therefore, before measuring, it is necessary to scale.

During scaling, a reduced voltage is applied to the transformer, while saturation is excluded and i ^μ (t) is negligible. The load turns on. The magnetizing current is measured by an oscilloscope according to the described scheme.

Undervoltage is necessary in order to exclude magnetizing current during setup. In this case, with correctly selected shunts, the voltage drop across the shunts is equal and the oscilloscope connected to the series-connected shunts should show 0 regardless of the load.

If these conditions are not fulfilled accurately, a sinusoid will be observed on the oscilloscope screen. This is the difference in voltage drops on shunts. To select the resistance of the shunt at one of the shunts, disconnect the conductor suitable for the measuring terminal and connect directly to the working surface of the shunt. Moving the conductor along the surface of the shunt, they achieve a straight line on the screen of the oscilloscope (no signal). The conductor is fixed. It is this position of the conductor that will correspond to the scaled circuit. The circuit is ready for measurements.

In order to verify that the measurement results are correct, the following experiments can be done.

- Measurements are carried out at reduced voltage and various loads. In all cases, the oscilloscope readings should be zero.

- Measurements to idle. Gradually increase the supply voltage. In this case, the magnetizing current should increase, and then become a non-sinusoidal characteristic shape.

The inventive method meets the requirement of "novelty" because it has new features: the instantaneous value of the magnetizing current is measured from two shunts, one of which is connected to the primary and the second to the secondary winding of the transformer, and the measuring terminals of the shunts are connected in series, and the shunts are calibrated in this way that the ratio of the voltage drops on the shunts when equal current flows corresponds to the transformation ratio of the transformer.

It is this combination of features that provides the method with new features that are not obvious:

- the ability to measure the magnetizing current of a transformer operating under load;

- measurements should be made using the instantaneous values of the magnetizing current, which makes it possible to measure a non-sinusoidal magnetizing current in the regime of strong saturation of the magnetic circuit;

- measure the magnetizing current when the transformer is powered with a non-sinusoidal current;

- the ability to clearly demonstrate the operation of the transformer under load (how with increasing load the magnetizing current decreases);

- new possibilities for the study of transformers (by changing the magnetizing current from changing the location of the windings, changing the shape of the magnetic circuit, one can judge the inductance of the magnetic flux of individual windings)

This allows us to conclude that the criterion of "inventive step".

Figure 1 presents a diagram for the allocation of the magnetizing current: Tr.1 - transformer; Sh1 - a shunt in the primary circuit; Sh2 - a shunt in the secondary circuit; R _b - load resistance (ballast rheostat).

Figure 2 presents the calibrated shunt Ш2.

In figure 3. oscillograms of the magnetizing current i ^μ (t) are presented for various calibrations of the shunt Ш2 (a, b, c) and in the saturation mode of the transformer primary winding (d).

In figure 4. real oscillograms of the magnetizing current are presented for a weak (a) and stronger saturation of the magnetic circuit (b).

Measurement by the proposed method, presented in figures 1-4, is as follows.

In the primary and secondary windings of the transformer Tr. 1, shunts Ш1 and Ш2 are installed (Fig. 1). The ratio between the rated currents of the shunts is chosen as close as possible to the transformation ratio of the transformer. The terminals of the shunts Ш1 and Ш2 are connected so that the voltage drop from the current of the secondary winding i ' ₂ (t) on the shunt Ш2 is subtracted from the voltage drop at the shunt Ш1 from the current i ₁ (t). The oscilloscope in such a circuit should show the current i ^μ (t), since i ^μ (t) = i ₁ (t) - i ' ₂ (t).

During scaling, one of the shunts disconnects the conductor, suitable for the measuring terminal, and connects directly to the working surface of the shunt (figure 2). On the oscilloscope screen, depending on the connection point, one of the pictures shown in Fig. 3 will be observed.

Moving the conductor along the surface of the shunt, achieve a straight line on the screen of the oscilloscope (figv - no signal). The conductor is fixed. It is this position of the conductor that will correspond to the scaled circuit. The circuit is ready for measurements.

In figure 4. real oscillograms of the magnetizing current are given for weak (a) and stronger saturation of the magnetic circuit (b)

The method was tested using a commercially available welding transformer TDM-259 with a wedge magnetic shunt. Supply voltage 220 V; secondary winding voltage at idle - 65 V.

Shunts used:

in the primary winding 100 A; 75 mV;

in the secondary winding 200 A; 75 mV;

Load R _b - ballast rheostat RB-300.

Power was supplied from a laboratory potential regulator (adjustable source of alternating sinusoidal voltage)

The test results allow us to conclude that the claimed invention meets the criterion of "industrial applicability".

Information sources

1. Patent G01R 31/34 “Method for measuring the magnetizing current of an induction motor” Ushakov AV, Mladentsev VI Publication No. 1017083, publication date 2005.08.20, Registration Application No. 3360117/07.

Claims (1)

A method of measuring the magnetizing current of a transformer operating under load, characterized in that the instantaneous value of the magnetizing current is measured from two shunts, one of which is connected to the primary and the second to the secondary winding of the transformer, and the measuring terminals of the shunts are connected in series, and the shunts are calibrated in such a way so that the ratio of the voltage drops on the shunts when the same current flows is equal to the transformation ratio of the transformer.

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