What is the condition to use star _ delta method to starting induction motor ?

1. It depends upon the kW rating of the motor. In my experience, I have used star-delta starting for motors in kW range of tokW.

2. In some process requirements, you may require to start the motor in DOL although its kW rating might be high.

 So, you have to choose this starting method based on process requirement and size of the motor.