Soft Starter and Star Delta Starter

This should have been asked in Electrical Engineering forum. Anyway here is the answer:

Start-Delta and Softstarter are both the starting method of induction motors for reducing the motor starting current. However, they are quite different in their functionalities and constructions.

Start-Delta starter: It is a starting method that reduces the starting current and starting torque. The device normally consists of three contactors, an overload relay and a timer for setting the time in the star-position (starting position). For the starting period (time set by the timer), the motors windings are connected in start (Y) and then the connection is changed to delta during normal run. Because of the start connection, the starting current is 1/3^rd of the DOL (Direct-On-Line) starting; however the starting torque is also 1/3^rd of the DOL starting torque. For this type of starting, the motor windings must have both ends (2 terminals for each phase, total 6 ends) terminated in the terminal.

Softstarter: A softstarter is an electronic device which controls the voltage during the starting period with having thyristors in the main circuit. As the motor voltage is low during start with soft starter starting, the starting current and starting torque is also low. It contains the motor protection capability and hence separate motor protection relay is not required. It is also used to control the motor start/stop. Soft starters can be used for both LV and MV voltage motors.

For more details, see these links:

http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/2985284834bcff7fc1256f3a00274038/$File/1SFC132002M0201.pdf

http://www.fairford.co.uk/userimages/_files/soft_start_docs/sd0018802__basic_soft_starter_principles.pdf

- MS