Mainly Induction Motor takes 5-6 times of FLC while starting
in DOL mode.So soft starter is used to start the motor for
reducing starting current and smooth starting.Softstarter
which gives the voltage from 30% to 100% percentage of
voltage by certain time(Its in seconds) to the motor.By
means of thyristor softstarter is controlling the
voltage.After 100% voltage reached,the inbuilt DOL bypass
contactor is come into picture.
Advantages of softstarter:Motor life be good,Mechanical
equip life be good.
Disadvantage:It is mainly not applicable for heavy starting
for starting a rated motor with a DOL starter which draws 5-
6 times of load current initially which causes voltage
drops in line while a soft starter takes comparatively less
current and gradually decrease to the rated current.
but the main draw back of soft starter is its torque
remains low at the starting.