What is the motor rating that need to use wye-delta starting?
The use of a wye-delta starter is determined more by the size of the motor relative to the strength of the supply. If starting the motor across-the-line (or direct-on-line) produces voltage drop at the motor in excess of 15%, or the switchgear bus voltage drops to the point that other connected loads would be adversely affected, a wye-delta starter can be used. In this starting method, the motor is initially energized with its windings connected in wye, and draws much less inrush current. Shortly after the motor is energized and has started accelerating the load, it is quickly switched to a delta winding connection, where it finishes accelerating the load and runs continuously. There are other starting methods available, such as reduced voltage autotransformers, part-winding starting, and even variable frequency drives.
