Soft Starter

DOL circuit output is given to soft starter.which inturn speed up the motor smoothly. DOL just acts as a switch for the soft starter.

A soft starter just like a VSD ramps a motor up to feed using power factor and frequency till full load RPM is reached

Is the Wikipedia article on soft starters not available in that part of the world?

I think for the most part a Soft Starter is used INSTEAD of a DOL starter. If you put a DOL starter in from of a Soft Starter, it becomes simply an Isloation Contactor, so it is no longer a "DOL" starter. If you put it in parallel to the Soft Starter, it becomes a Bypass Starter.

Softstarters can be used in different fashions such as (1) In-Line and (2) Internally bypassed and (3) Externally bypassed.

(1) In-Line: The softstarter always remains in line and motor receives power from the thyristors all the times. This allows the motor to stop with the softstarter but there is a power loss keeps occurring in the thyristors and additional cooling system is required for large size softstarters.

(2) Internally bypassed: In this case, the softstarter itself contains the in-built bypass contactor which is also a part of the softstarter. After the motor start is over, the thyristors of the soft starter are bypassed with the contactor. This minimizes the power loss within the thyristors during the motor running condition.

(3) Externally bypassed: Softstarter is connected in parallel to the DOL starters. Once the motor starting is complete, the softstarter is bypassed with the motor starters and it is completely isolated from the system. The starter takes care of the motor protection system during the motor running condition. A single softstarter may be used to start multiple motors in this configuration. It doesn't allow the motor for soft stopping.

It seems to me the OP's question is related to the configuration 3.

This link may help:

https://www05.abb.com/global/scot/scot209.nsf/veritydisplay/2985284834bcff7fc1256f3a00274038/$File/1SFC132002M0201.pdf (page 64)

- MS