Why motor first run in star connection and then in delta?

All 3 Phase Motors are connected through Self starters in which Automatically switching is done from Star to Delta Connections of the winding.

As  all of you know any Electrical equipment is to be connected to +ve and -Ve  supply to complete the circuit but in 3 phase its not possible to connect neutral to the 3 phase motor. so the motor supply first is connected to Star connection where the winding connecting acts as Neutral and motor starts to pick up the Speed or Torque, it needs to draw high current for starting from idle load to movementum, Once the Speed is reached as per the requirement of the roatar pump of motor then the load is Automatically switched to Delta connection through timmer fixed inside the starter and connetced to 3 Phase contactors where neutral is not required for smooth running of the motor.

so the sequence of Star and delats is used to run the motors.

Star Delta Motors are preferred as a method of starting in Low starting Torque Applications. The starting current of Induction motors are six to seven times the full load current. Starters are employed to reduce the starting current.

A star Delta Starter reduces the starting current by connecting the motor winding in Star at the time of starting. This reduces the voltage across the winding.The voltage across the winding will be( 1/Sqrt(3))*Line Voltage.

After the starting time set in the timer the winding will be connected in Delta. The voltage across the winding will be equal to the line voltage.

The switching of winding configuration is done by devices called contactors. Typical application of Star Delta motors are compressors

Note: Since the Torque is Proportional to Square of voltage. A high starting torque is not possible.