Cold rolling reversing mill: analysis and improvement of the dynamic behavior of the control system

The authors present the method used for the analysis of a 6000-kW main drive thyristor supply system of a cold rolling Z-mill at Acesita, Brazil. The main drive is powered by three DC motors connected in parallel, and fed by a common thyristor armature supply. Since commissioning, strong interactions were observed among the field and armature supplies which resulted in operational problems and instabilities. A careful analysis of the systems dynamics, assisted by the digital simulation of the complete system, indicated the need for redesigning the control loops. After this had been done, the problems were eliminated, allowing the operation of the mill up to top design speed. The analysis, simulation, and results obtained are presented