Arc voltage fluctuations in D.C. plasma spray torches result from the movement of the arc attachment inside the anode nozzle. These fluctuations express the variation of the arc length and the power dissipation and strongly influence torch performance and coating quality.
In this study, two torches with swirl or straight injection of the plasma gas have been investigated. To characterize the arc instability, voltage fluctuations and acoustic emissions from the plasma jet have been recorded and analyzed on line using a workstation operating with LabView environment. The length and the stability of the jet have been quantified in terms of jet centreline deviation from the torch axis (JetRMS) using a video system. Different
arc currents, nozzle diameters, Ar/H2 plasma gas flow rates have been examined.
A good correlation has been found between the frequency of the voltage and acoustic signals. This study has allowed to compare the dominant values of the frequencies according to the type of plasma gas injection and the different operating parameters. Thus an arc current and plasma gas flow rate increase and a nozzle diameter decrease lead to an higher restrike mode frequencies.