A solid state direct on line starter for medium voltage induction motors with minimized current and torque pulsations

This paper describes a thyristorized static switch which is designed and im plemented specifically for frequent direct-on-line starting of medium volta ge large induction motors. The circuit configuration chosen utilizes back-t o-back connected series thyristor strings on two of the supply lines. Both the starting current and the torque pulsations of the induction motor are m inimized by a controlled, non-simultaneous switching of motor phases to the supply. This reduces the electrical stresses on the power system and the m echanical stresses on the shaft as compared to conventional switching eleme nts. The optimum switching strategy Is found by the use of a hybrid mathema tical model in ABC/dq0 form. The validity of the results obtained for mediu m voltage, squirrel-cage induction machine are verified by the use of a phy sical simulator consisting of a low voltage, small induction machine and a shaft torque measuring system. The system has been in operation successfull y in an Iron and Steel Plant since 1991.