Why motor is started on star and then run on delta?

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings, During starting the motor windings are connected in star configuration and this reduces the voltage across each winding, This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally. Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.

Most induction motors are started DOL(Directly On Line), but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges(Max. Instantaneous input current drawn by an electrical device when turned on).To limit the starting current surge, large induction motors are started at reduced voltage(Star Configuration) and then have full supply voltage reconnected(Delta Configuration) when they run up to near rotated speed.