In star delta starting, the phase voltage is 58 % of the line voltage when motor is first connected in star. The reduced voltage leads to reduced motor current. The reduced current improves the motor starting characteristics, and the motor starts without any electrical and mechanical stress. That is why star delta starter preferred.
The induction motor takes 6 to 7 times of its rated current at starting time. At starting, the rotor impedance is highly inductive( Xl=2*pi*sfs*L) as the frequency of the rotor current is equal to the stator supply frequency.
In this condition, the motor draws high current and delivers very less torque at starting.
To limit the starting current of an induction motor,the supply voltage needs to be reduced to reduce the voltage induced in the rotor circuit. The motor current will reduce with the decrease of the supply voltage. The small rating motors can be started on Direct on Line, however, as the rating of the motor increase, the starting of the motor must be done by star delta starter.
n star delta starter, the winding of the stator is first connected in the star to reduce the stator phase voltage by 42 % of the supply voltage. The voltage across the winding will be 58 % of the supply voltage and the starting current will reduce accordingly. If the line voltage is 440 volts, the phase voltage across the stator winding is 0.58*440=255 Volts.
After running the motor in star for a certain time, the timer switches on the delta contactor, and now the motor winding gets connected in delta. The full supply voltage is impressed on the stator winding when connected in delta.