Voltage fluctuations are investigated for a wide range of operating conditions using the Praxair SG-100 plasma torch operated with Ar/H2 as plasma gas. The voltage signals are characterized using statistically averaged values and power spectra. The process of anode erosion and its effect on the characteristics of the signal are described. It has been found that erosion has only little effect on the average voltage, but two characteristics of the voltage fluctuations obtained from image analysis of the voltage trace are found to be excellent indicators of anode erosion. An estimate of arc root speeds based on the voltage traces yields values of 50-100 m/s. It is estimated that the range of arc root movement and arc root speed is significantly reduced to less than one half of the original value due to anode erosion. Investigation of the effect of operating conditions and anode erosion on power spectra has been investigated and it has been observed that high frequency peaks are predominantly a characteristic of the geometry of the anode.

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