**Abstract**

During the present scientific research, instantaneous values of the electric arc voltage were obtained for the alternative current three-phase shaft electric arc furnace SF-125. The calculation of the electric arc voltage signal were provided for early melting stage on the base of designed mathematical model, measured signals of phase voltage and electric arc current and predetermined parameters of electric circuit. The obtained results made it possible to accomplish the comparative analysis of electric arc current and electric arc voltage harmonics fluctuations including analysis of the relative RMS values for odd, even and total harmonics. Consequently, it is clear that harmonic composition of the electric arc voltage is more stable against non-symmetrical modes and random disturbances of electric arc length in comparison with harmonic composition of the electric arc voltage. Therefore, the method of heating stage diagnostic based on the data about harmonic composition of the electric arc voltage may be efficiently applied in electric modes control systems, providing the rise of energy efficiency for steelmaking complex. Nevertheless, at the late stages of melting heating stage diagnostic methods based on the data about harmonic composition of electric arc current and electric arc voltage have equal efficiency. Due to this fact, resource-intensive calculation of the electric arc voltage instantaneous values becomes unreasonable.

**Keywords**

Electric arc furnace, electric arc, Fourier transformation, harmonic data analysis, heating stage diagnostic.

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