Why is star delta starter preferred with induction motor?

     The main purpose of any starter is to reduce the requirement of a high starting current. Normally the starting current of an induction motor is 6 to 7 times of the full load current. If one has an induction motor with a DOL starter, drawing a high current from the line is higher than the current for which this line is designed. This will cause a drop in the line voltage, all along the line, both for the consumers between the substation and this consumer, and those in the line after this consumer. This is the reason why a starter is to be used.

     The induction motor takes 6 to 7 times its rated current at starting time. At starting, the rotor impedance is highly inductive as the rotor current frequency is equal to the stator supply frequency. In this condition, the motor draws a 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. The motor current will reduce with the decrease of the supply voltage. The small rating motors can be started on Direct online, however, as the rating of the motor increase, the starting of the motor must be done by a star delta starter.

    Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes fewer losses. Star delta starter circuit comes in the circuit first during starting of the motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.