DOL Starter in Ladder Programming

What is DOL Starter: Direct On Line Starter is used to start three phase motors below 5 H.P. rating in industries. As the name suggests it connects the motor directly to the line (power supply). Main components of DOL are:

-        

             # Push buttons

-          # MCB

-          # Contactor

Two push buttons are used, one NO for start operation and one NC for stop operation. MCB is used as a protection device so as in case of any short circuit or over current, it trips and breaks the supply. Major used in a DOL starter is Contactor. Contactor works in similar way as relay works. When coil of contactor is energized its power contacts are closed so as to start the connected motor. Control wiring of DOL starter is shown below:

DOL Starter

Now if we are making DOL using a PLC, then we can use any button (NO or NC) for any operation (Start or Stop). Below are the Ladder programs for various combinations of NO and NC.

As we know start button in any program should work as a NO and stop as NC. It doesn't matter whether they are given NO or NC

1. Hardware Condition - Start = NO and Stop = NC

DOL using NO NC

In this circuit I1 is start and I2 is stop. I1 is given NO in hardware so it will be made NO in ladder. I2 is given NC in hardware, so it will be made NO in ladder. If having any confusion please read NO/NC Concept.

2. Hardware Condition - Start = NO and Stop = NO

DOL Using NO NO

3. Hardware Condition - Start = NC and Stop = NO

DOL Using NC NO

4. Hardware Condition - Start = NC and Stop = NC

DOL Using NC NC

To understand it easily, just remember that start buttons are always same in hardware and ladder but stop buttons are always different in hardware and software.