A fraction KV1, of the supply voltage V1 is applied to the stator at the time of beginning by means of auto-transformer.
Figure: Auto-transformer Starting (k < 1)
This limits the initial current to k Is. As the motor attains its full speed, the full voltage is set through the auto-transformer.
The starting current and starting torque are decreased to k2 times their corresponding values with direct-on-line starting.
This method is utilized for delta operated motors. The stator phases are primary connected in star by a special switch (triple pole double throw switch). As the steady state speed is attained, the stator is connected in delta by changing the position of the same switch. The star-delta starter decreases the starting torque to one-third of that generated by direct switching in delta. Star-Delta starter is cheap as compared to auto-transformer starter and is therefore utilized extensively.