The main purpose of any starter is to reduce the requirement of 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, which 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, who are in the line after this consumer. This is the reason for which a starter is to be used.
It is T.P.D.T switch used to first start the motor with the winding connected in star and then switch for delta connection in running position. TPDT stands for Triple Pole, Double Throw.
Why Star Delta Starter Preferred in Induction Motor?
If the winding of Induction Motor is connected in star, the voltage per phase supplied to each winding is reduced by 0.577.In general the voltage per phase in delta connection is Vs, the phase current in each stator winding is (Vs/Zs), where Zs represents the impedance per phase of the motor at standstill or start.
The line current or the input current to the motor is Ist (starting current) = (1.732*Vs)/Zs which is the current when it has to be started by DOL starter.
Now, if the stator winding is connected as star, the phase or line current drawn from supply at start (standstill) = (Vs/Zs)/(1.732)
which is 0.333= (0.577*0.577) of the starting current, if DOL starter is used.
The voltage per phase in each stator winding is now Vs/√3. So the starting current is reduced by 33.3%. Because of the reduction in starting current, starting torque reduces.
Therefore we can conclude that by using Star Delta starter, the starting current is reduced to approximately two-thirds. Since starting current is reduced, the voltage drops during the starting of motor in systems are reduced.