Induction motor used barring gear of STG is not getting rotate through soft starter even full voltage is applied by soft starter in 5 to 7 sec. But if I am providing full voltage through DOL, then motor is running normal.

If soft starter is providing full voltage after 7 to 8 sec then motor should rotate. In DOL it is working fine.

 

The soft starter has some losses; it won't be applying full Voltage but will be close.
How easily does it pull away DOL?

 

Are you absolutely sure the soft starter is applying full voltage after 8 seconds? Or are you assuming, because that's what you set the ramp time for? Some soft starters, especially cheaper ones where they use marginally rated devices, will go into current limit if the motor current attempts to exceed what they can handle. In a few of them, they TELL you that it has gone DOL, but if you connect a meter, you will see it is not true.