Rung 1 Main contactor : The main contactor depends upon the normally open input start push button (I1), normally closed stop button (I2) and normally closed overload relay. It means that Main contactor will only be energized if start button is pressed, while stop is not pressed and overload relay is not activated. A normally open input named (Q1) is added in parallel to the start button I1. By doing so, a push button is created which means that once motor is started, it will be kept started even if start button is released
Rung 2 Star contactor: Star contactor depends upon main contactor, normally close contacts of timer (T1), and normally close contacts of output delta contactor (Q3). So star contactor will only be energized if main contactor is ON, time output is not activated and delta contactor is not energized.
Timer T1: Timer T1 measures the time after which the winding connection of star delta starter is to be changed. It will start counting time after main contactor is energized.
Rung 3 Delta contactor: Delta contactor will be energized when main contactor (Q1) is energized, timer T1 is activated and star contactor (Q3) is de-energized.