Studies on High Froquency Arc (1st Report)

The dynamic characteristics of H.F. W-W arc

Abstract

The high frequency (3kHz) arc characteristics using W-W electrodes were studied experimentally and theoretically comparing with those of 60Hz and D.C. arcs.
As the result of experimental research, it is recognized that the polarity exchange which is an undesirable property of A.C. welding arc is very easy, i.e. restriking voltage seems not to be necessary for high frequency arc in W-W electrodes and electrical dynamic characteristics of high frequency arc is resistive over all the current range of this experiment. (Fig. 5, 6 & 7)
The resistive characteristics of 3kHz arc suggests that the temperature of arc column is held constant, since the resistance of arc column directly depends on the instantaneous temperature distribution in it.
The radiation from 3Hz arc column is intensive even in the period of current zero and the phase of the small perturbation of radiation lags by about 1/4 cycle from that of the electric power input, while at 60Hz it is in the same phase as that of the instantaneous input. (Fig. 8, 9 & 10)
Such optical data mean that dynamic A.C. arc characteristics is affected remarkably by the thermal inertia of arc column.
In the second part of this paper, the thermal energy balance at A.C. arc column is discussed.
The experimental data are found quantitatively close to the results of theoretical analysis at 3kHz, qualitatively even at 60Hz.
From such a theoretical treatment, it is concluded that temperature change of A.C. arc column depends on current frequency ω, effective electric power and mean thermal energy of arc column as follows:
If ω»ω₀(ω₀=3e₀i0/8M₀C_p0T₀), the dynamic characteristics of A.C. arc column becomes resistive, i.e. at a high frequency arc, the arc column can not respond to the instantaneous current, but is sustained nearly constant.
Accordingly, in arc with W-W electrodes, the polarity exchange becomes very easy at high frequency arc due to the residual plasma at current zero.
Thus, a high frequency arc has excellent characteristics for arc stability.