Soft starting of large induction motors at constant current with minimised starting 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 been carried out in dynamic state using a hybrid induction machine model which takes account of disconnected, two-phase and three-phase operational modes of the machine. Some simple control strategies have been proposed to keep the current constant at any preset value during starting, and to eliminate supply frequency starting torque pulsations. These are shown to be very effective in the elimination of reclosing transient torque and current components at any speed at the reconnection instant to the supply after an interruption, 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.