Thyristor-supplied tandem cold mill

A modern tandem mill has a high performance, which requires a complex, sophisticated control system. Antiparallel-connected thyristor convertors combine fast response with high efficiency for supplying the continuously controllable power to vary the speed of the d.c. motors. The changeover time from one antiparallel convertor to the other when a reversal in power is required (e.g. at the commencement of mill slowdown) is made very short to avoid tension disturbances in the strip. Although a thyristor supply presents problems of voltage dips to the a.c. supply, this can be alleviated by careful design. In order to accelerate quickly, the reel control incorporates a new control system, which utilises the full available power of the motor during speed changes. This involves a spillover scheme to weaken the motor field when the voltage approaches the rated value, and this is also used on the stand-speed control. Other features on the mill drive include digital position control for the thyristor-supplied screw motors, automatic gauge control, which incorporates direct control of interstand tension by means of stand-speed variation, comprehensive data logging and automatic slowdown provided by a digital process-control computer.