Measurement of Electrical Parameters of AC Arcs

Measurements of electric arc parameters are normally made with canventional electromechanical instruments with the tacit assumption that the measurements are mean values. This may be a close approximation in the case of dc arcs supplied from a battery or from a rectified and smoothed dc source. It is, however, not true for dc arcs in plasma torches, arcs supplied with unsmoothed dc, ac arcs used for welding, or in arc furnaces where effects due to nonlinear variation of arc voltage with arc current, sudden random fluctuations in arc voltage, and high frequency components of the arc voltage occur. Electricity supply authorities use electrodynamic instruments to measure power and kVA in arc loads which, as arc fumaces, are among the largest single loads in the country, again assuming that mean values are obtained in this way. Possible sources of error in these assumptions which may have important effects on process control and operating costs, are examined. Some methods of measurement are shown to be subject to errors not normally appreciated, and are of little value, except for comparative purposes. Methods of minimizing errors are also considered.