The Star Delta starting method of squirrel cage induction motor lies on the concept of reduced voltage control. The motor is started in star connected coil so that the starting voltage is somewhat low value, after some time when the motor is stable enough then the coil of motor is shifted to delta connected.
As the squirrel cage rotor has the rotor circuit short circuited there is no means to add extra resistance to its rotor circuit to control the starting current inrush. Reducing the line voltage while starting would be a good option.
How Star Delta starter improves the high inrush starting current:
It is the starting phase: the motor windings are star connected and the voltage on them is VL/√3, The current in the motor winding and on the line results to be I= VL/(√3*Zw), Zw is the winding impedance. Thus the starting current is reduced by a factor of 1/√3.
[alvaa]By analyzing the current and voltage formulas previously given, it can be seen how Y starting phase occurs at a voltage value equal to 0.577 times the line voltage VL and requires a current absorption 0.33 times lower than in case of direct-on-line starting and delta connected motor.
In Star Delta starting concept – the stator coil is involved. That is the the stator coil connection is made to STAR at starting and then in normal running condition stator coil connection is made to DELTA.