Soft Start of an Induction Motor with Reduction in Torque Pulsations during Starting

This project deals with the thyristorised soft starting of 3 phase Induction Motor. Three phase induction motors of ratings beyond 50KW when started directly from a 3 phase supply draws 3 to 5 times the rated current and starting torque can be 2 times larger than in the normal operation. These high starting current will cause overheating of the windings and a large voltage drop and influence the normal operation of the motor when connected to the main power supply. Large starting torque pulsations may cause severe mechanical stress on the shaft of the motor and the load. So it is necessary to control motor starting current value. Solid state starters or Soft Starters are designed to control such starting current of the motor. Soft starter is done by connecting SCR s in antiparallel in each phase. The voltage to the stator terminals can be controlled by varying the conduction period of thyristors. The high starting current can be considerably reduced by soft start, thereby the electrical and mechanical stress on the motor can be minimized. Energy saving during starting can also be achieved and hence losses can also be minimized. The soft starter is developed for 3 Phase, 440V, 50Hz 1HP induction motor and is tested in laboratory. The starting current of the induction motor with and without soft starter are observed and tabulated. It is observed that the starting high inrush current is reduced to 30 to 50% of its rated value thus minimizing the starting torque pulsations and increasing the life of the motor.