Influence of configuration and operating conditions on the electric arc instabilities of a plasma spray torch: role of acoustic resonance

A theoretical approach is proposed to explain the particularities of the power spectrum of a plasma spray torch voltage. This is founded on an acoustic resonance of the rear part of the plasma torch which is occupied by the cold gas before it reaches the arc region. The spectrum is characterized by the presence of a sharp peak in the range 3-8 kHz, with an amplitude that represents up to +/- 30% of the voltage mean value. The peak frequency presents an evolution that is governed by the torch operating conditions, the thermal properties of the plasma forming gas and the geometrical configuration of the electrode assembly. The theory is compared with experimental values of the peak frequency recorded for different operating conditions and torch configurations. The injection ring through which the gas is injected also plays an important role. The torch behaviour is carefully characterized in order to collect all the experimental data required for the analysis on the involved phenomena.