Soft starting of large induction motors at constant current with minimizedstarting torque pulsations

The performance of a voltage-controlled large induction motor soft starter has been improved, resulting in nearly perfect current and torque profiles. The performance analysis of the soft-starter motor-load combination has be en carried out in the dynamic state using a hybrid induction machine model which takes account of disconnected two-phase and three-phase operational m odes of the machine. Some simple control strategies have been proposed to k eep the current constant at any preset value during starting, and to elimin ate supply-frequency starting torque pulsations. These are shown to be very effective in the elimination of reclosing transient torque and current com ponents at any speed at the reconnection instant to the supply after an int erruption, or at the instant of bus transfer. The proposed strategies have been verified experimentally on a laboratory machine using a special torque measurement system in the dynamic state.