Why is star delta starter preferred with an induction motor?

Star delta starter is used to reduce the starting current drawn by the motor.
Start the motor in STAR and then, when the motor has gained sufficient speed, change its connections to DELTA to allow the motor to run at its full speed and torque from then on.
The motor starts as star connected, ie, voltage reduced by root(3), current reduced by root(3) and power reduced by 3.
After the motor starts, it is switched to delta connection and runs normally.

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

Descriptions: A Dual starterconnects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. This type of starting is suitable for small motors below5 hp (3.75 kW).

Reduced-voltage starters are employed with motors above5 hp. Although Dual motor startersare available for motors less than150kW on400 V and for motors less than1 MW on6.6 kV.

 

Supply reliability and reserve power generation dictates the use of reduced voltage or not to reduce the starting current of an induction motor the voltage across the motor need to be reduced. This can be done by1. Autotransformer starter,2. Star-delta starteror3. Resistor starter. Now-a-days VVVF drive used extensively for speed control serves this purpose also.

Because the first few current having access to high-speed Delta is connected for higher torque

• In addition, starting torque is increased and it prevents the damage of motor winding.

Induction motors are the mostly used ones ,they have high starting current hence  cannot be started at  D.O.L  unless designed  for it .Therefore Star Delta starter  is used .In star  two coils are in series in between the phases therefore reduced current  and voltage  is delivered to motor which is desired for starting  after few seconds the connection is in delta  i.
e  a single coil between the phases hence more voltage and current is drawn  by motor for normal running .

An induction motor draws large current at start. The current drawn by motor is more because the rotor has more inductive reactance and the reactance gets reduced as motor accelerates and become minimum when motor attains its synchronous speed.

 

The voltage is reduced to limit the starting current of an induction motor.The star delta starter is one of the starting method for squirrel cage induction motor. In star the voltage gets reduced to % of the line voltage and correspondinlgly the starting current gets reduced.

Read More:Star Delta starter for Squirrel Cage Induction Motor

Elaborating in furhter to previous answer.
In Delta connection VL = VPh VL - Line voltage Vph - Phase voltage & IPh = IL /1.732 IL - Line current IPh - Phase current Hence Phase current is less compared to star connection where line current = phase current. 
 

slam... can you tell me plz.. if we have a star connections motor can we convert into delta .I mean we can run on delta supply?

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

Due to its simplicity and low cost