Improving performance in gearless mill drives avoiding failures caused by stress concentration from electrical origin

Abstract

The configuration of gearless mill drives (GMD) fed by high-power cycloconverters employed in mineral grinding is a mature technology, with high performance for torque and speed control that has been applied successfully in modern concentrators for wet grinding with mill diameters up to 42-feet and 28 MW. Lower ore grades push the search for economy of scale with the trend to larger sizes, powers and productivity of GMDs. Global trend to automation and enhanced control of process quality require high integration and performance of equipment in Concentrators. Risk management has to deal with the high impact on production of larger GMD’s and availability, due to planned maintenance and un-planned downtimes. In order to avoid events causing unexpected downtimes like failures, great efforts have been done for improving performance and reliability of GMD’s. Because of recent unexpected failures in the drive system of some large GMD’s in stator and rotor subsystems, great efforts have been needed for facing the challenge to designers and users for identifying the causes of such failures. The phenomena involved in such events have shown a complex nature with manifestations of mechanical and electrical damages to diverse components of ring motors. This paper presents a discussion on the main electrical and electromagnetic phenomena that can generate stress conditions in gearless drives with the aim of avoiding failures and outages. Current efforts for further research and contributions for better design with improved reliability of GMD’s are proposed.