Star-Delta connection Anaylsis

Why star- delta starter is used for induction motor? 

Three-phase induction motor are widely used in all industries due to their robustness and smooth operation. We should not connect and run any electrical motor directly from the three phase supply. Voltage and current is uncertain in power system. We can get above the rated voltage and current at abnormal conditions. So, protection to the motor is must needed. For this purpose, the device used called as startrer.


What is starter?

• Starter is a device used to limit the starting current from the supply.
• It is a protection device for motor during starting.

What is star-delta starter?

• Three phase winding has been connected in two fashion.

1. Star connection --->>In this connection,

      Vph=Vline/(root(3))

       Iph=Iline

 2. Delta connection--->>In this connection, 

       Vph=Vline

       Iph=Iline/(root(3))

• During starting of Three phase induction motor, rotating magnetic field is created in the stator and due to the electromagnetic induction, current trends to flow in rotobars due to short circuited. 
• At starting, rate of cutting of flux is high. As a result, large amount of current will be produced in rotor. This causes voltage drop in the line and affects other equipment connected to it.

• The motor is first allow to run in star connection because the voltage is reduced by root(3) times the line voltage. So the current is limited.
• After rotor achieving reasonable speed ,the three phase winding is changed to delta connection. So,the motor is operated in rated supply voltage. 
• By this method, we can protect the three phase induction motor from starting current.

Which one performs the changing in star to delta?

• The contactor performs the changing of star to delta connection.
• Contactor is more like a switch,  it is operated using timing circuits for changing star to delta.