Wye-delta starting is often used with large horsepower motors to reduce in-rush current during the starting period and to reduce starting torque. Wye-delta starting is accomplished by connecting the motor stator windings in wye or star during the starting period and then re- connecting them in delta during the run period. This is sometimes called soft starting. If the stator windings of a motor are connected in delta during the starting period, the starting current will be three times the value if the windings were connected in wye. Assume that a motor is to be connected to a 480 volt three-phase power line. Also assume that the motor windings have an impedance of 0.5 ohms when the motor is first started. If the stator windings are connected in delta (Figure 39 – 1), the voltage across each phase winding will be 480 volts because line voltage and phase volt- age are the same in a delta connection. The amount of
In a wye connected load, the line current and phase current are the same. Therefore, the starting current has been reduced from 1662.72 amperes to 554 amperes by connecting the stator windings in wye instead of delta during the starting period.