High voltage induction motors for use in hazardous areas: evolution of legal aspects and technical investigations related to safety problems

A new European Community Directive, dealing with Ex equipment, will shortly be published, giving the opportunity to review the standard safety concepts (EExd, e, p, etc.). According to the European Council statement, the safety degree is based on the number of independent defensive measures (barriers) adopted to reduce the likelihood of an undesired event occurrence. Moreover, the safety designation will require an increasing assumption of responsibility by the motor manufacturer. Consequently, a better understanding of the causes related to an explosion and the actions that must be taken to avoid it occurring are both necessary. Explosions can be caused by incendive sparking originated in the air gap during starting, or due to current transfer at undesired joints or corona discharges on contaminated end-windings in high voltage motors. The paper covers the performance of corona suppressing systems (CSS) in aggressive environments, where a heavy degradation may lead to undesired effects as the inception of high magnitude surface discharges. The methodology used to evaluate the actual condition of CSS during ageing progression is based on detection of partial discharge (PD) patterns. The results of an accelerated multistress ageing on wound “motorettes” are reported: the partial discharge amplitude distribution (PDAD) technique is found quite effective to detect problems. Taking into account the energy related to the PD, the hazard of explosion is also considered