Fast dynamics and accurate control around zero speed are normally required from process technologies. It is from this perspective that the Ward Leonard field control system is described. Several industrial plants utilizing the Ward Leonard control scheme are in use in South Africa. Multiprocessor digital control systems have proved themselves as the most reliable control system available today. Problems arising from feeding a system with large time constant have been overcome by the use of a PID armature current controller and adequate forcing. It has been shown that iron losses and line losses have an effect on the sizing of the thyristor set and that the voltage forcing has to be limited to the dynamic optimum of the system. The speed-time curves for a roughing mill controlled by the old magnetic amplifier have been compared with those of a multiprocessor control operating on the same mill.
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