The determination of the characteristics of a welding transformer with DAQ system.

Suciu, Lenuta ; Marta, Constantin ; Tatucu, Iancu 等

1. INTRODUCTION

In the process of monitoring, control and adjustment of industrial welding transformers it is necessary to acquire a large volume of information of difference origin, as well as to stock transmit and process it, in order to make certain decision or to intervene.

The application part of the paper presents the acquisition of several signals, during the idle running of a welding transformer, with the help of the programming environment LabVIEW.

2. IDLE RUNNING OF THE WELDING TRANSFORMER

The idle running regime is characterised by the fact that the transformer is connected only to the primary network and the secondary circuit of the transformer is open, Z=[infinity] or [I.sub.z]=0.[3] The equations of the idle running transformer are:

[U.sub.1] = [I.sub.10] x [Z.sub.1] - [U.sub.e1] (1)

[U,.sub.20] = [U.sub.e1] = -[Z.sub.1m] x [I.sub.10] (2)

[I.sub.10] = [I.sub.1] (3)

where [U,.sub.20] is the secondary voltage reduced to the primary, corresponding to the idle running regime, [I.sub.10] is the complex expression of current [i.sub.10] passing through the primary winding in this case, called idle current.

[FIGURE 1 OMITTED]

This current has a small value compared to the nominal current: [I.sub.10]=(0,02/0,10) [I.sub.n]. [2] For the high-power transformers: [I.sub.10]=(0,015/0,30) [I.sub.n].

The idle current of the transformer varies depending on the nominal power of the transformer, ranging between approx. 10% and approx. 2.5% of [I.sub.n] in the case of the mono-phased transformers' powers varying between 0.6 and 40 000kVA. The purpose of the paper is the reduction of the failure risk of the most expensive pieces of welding equipment. Consequently it is necessary to monitor their operation, by collecting and processing detailed information about the condition of the transformers, in order to avoid their possible breakdown, with the help of the data acquisition systems.

The monitoring by means of data acquisition systems must provide information for a very reliable estimation of the transformer's remaining life duration. For this purpose we should remind two aspects.

The risk of a transformer's failure increases after 10 years of operation, and consequently a warning system is absolutely necessary.

Taking into account the high price for the purchase of a new transformer and even for its repair (depending on the defect), the necessity of introducing a monitoring and diagnosis system, combined with a rapid and efficient protection, is fully justified.

3. DATA ACQUISITION, ANALYSIS AND DISPLAY

In order to acquire the voltage and current from the montage, we used the SCXI 1125 module, which is a module of signal conditioning, with isolated programmable channels, with the help of which we took the signals acquired and digitised by the SCXI 1600 module. At the signal filtering the accepted frequency is of 4 Hz or 10 Hz, the average filtering value is of 5,000 points at 10000 samples per second, the effective value of the working voltage is of 300 V, scanning is flexible, the sampling rate is of 333kS/s, the input impedance 1 Gff, the output impedance 4,5 Mff, for parallel module 330 ff, parallel or multiplex functioning module.

After the data acquisition and the supply of necessary data, the results are graphically shown.

4. PLOTTING OF CHARACTERISTICS

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In order to perform the plotting of the characteristics of the electric transformer in idle running regime in figure 2 we represented the block diagram of the virtual instrument for the module of data continuous acquisition during the transformer's' idle running, and the front panel in figure 3. In the first stage we verified the precision of the transducer for smaller values of the primary voltage, primary current, secondary voltage, and in the second case we verified the running of the transducer at higher values of the primary voltage, primary current, secondary voltage. From the two running tests we found that at high values of the primary voltage the current is deformed and at small values the current has a perfectly sinusoidal shape, the primary voltage is dephased by 180[degrees] before the primary current. In the block diagram in Figure 2 we used a "while"--type time loop which is executed until the condition becomes real, i.e. in the present case the acquisition of the effective value of the primary voltage is executed up to the value of 200V that we preset or until we press the stop button.

The characteristics were obtained by modifying the voltage at the terminals of the transformer's primary. We also introduced a time delay--Time Delay for the data acquisition to be performed simultaneously with the transducer working time, with the purpose of avoiding the occurrence of parasite signals. As a result of the course of the programme from two running tests we remarked that at high values of the current depending on frequency the current is deformed, and when applying the Fourier transformed only the fundamental, 1st order harmonic appears.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Unlike the case when at small currents more harmonics occur, harmonic of the 1st, 3rd, 5th order, as the electric transformer has an uneven number of harmonics, Figure 3 and 4. After the course of the programme we remark that the secondary voltage is dephased by 180[degrees] before the primary current, and the secondary one by 180[degrees] behind the current, considering as phase origin the current depending on the primary voltage.

5. CONCLUSIONS

By processing from the primary data we can extract the useful information, we obtain statistic data, we model the phenomena, we elaborate optimum actions and strategies, we perform transformations in order to obtain characterisations in the time and frequency ranges.

With this paper we aimed at determining the characteristics of the mono-phase electric transformer with the help of one of the newest and most rapid programming environments, i.e. LabVIEW.

In view of performing a numerical processing, we need to transform the analogic signals in numerical signals with a data acquisition system.

The use of the data acquisition systems allows the recording of events taking place within the electrical equipment of the machine-building companies, in view of certain analyses focused on the quality of welded joinings.

Moreover, the analysis of certain post-breakdown unwanted events allows the identification of the causes of certain events or anomalies in the interior of the electric transformers, as well as their localisation.

The economic effects of the use of the equipment proposed to be realised will be, among others, the reduction of the own technologies losses from the welding installations, the observance of the electric power quality standards, the minimisation of the failure rate, as well as the reduction of the electric power consumption.

6. REFERENCES

Cotet, F. & Ciobanu O. (1998). Fundamentals of programming in Lab VIEW, MatrixRom, ISBN 9739390-56-0

Munteanu, R. et al. (1999). Computer Aided Analysis of Electromagnetic Measurements Systems. In : International Computer Science Conference "MicroCAD'99". Proceedings section F, pp 87-92, Miskolc, Hungary

Oancea, C-D. & Oancea, C. (2002). Computer aided measurements, Printech Publishing House, Bucharest, ISBN 973-652-645-3

Oprea, C. et al. (2005). Parameter estimation of a transformer in an electrical machines virtual laboratory using LabVIEW, Oradea University Annals, ISSN: 1223-2106

Tatucu, I. & Suciu, L. (2006). Numerical model for the study of electromagnetic processes in non stationary regime. Eftimie Murgu University Resita Annals, ISSN 1453-7394