Types and comparison of induction motor starters

As the name indicates, starters are used to start or stop a motor. It consists of an electromechanical switch and protection devices (overload and/or short-circuit protection).

Why should we need a starter?

An induction motor draws 5-10 times its rated current during start. In order to reduce the starting current, we need a starter. If we don’t use a starter, inrush currents may heat up the motor winding and the life of motor may reduce.

Comparison of three phase induction motor starters

Method	Starting current	Starting Torque	Pros	Cons
Direct-on-Line (DOL)	5-10 times the full load current	5-10 times the full load torque	Economical and Higher starting torque.	High in-rush current
Star-Delta Starter	2-3 times the full load current	2-3 times the full load torque	Inrush current can be reduced to one-third when compared to DOL.	Starting torque will reduce
Auto-transformer	2-3 times the full load current	2-3 times the full load torque	Inrush current can be reduced to one-third when compared to DOL.	Starting torque will reduce
Soft starter	3-5 times the full load current	1.5-2.5 times the full load torque	Inrush current can be reduced. Starting torque can be controlled.	Starting torque will be less
Variable Speed drive	1.1-1.5 times full load current	1.5-2.5 times the full load torque	Speed and torque control, communication and Energy saving.	Higher cost