Direct Online Starter (Simplified)

As the name suggests if an electrical machine (often a motor) is connected to supply directly with no provisions to limit the starting current drawn by the motor , the starting method is called the direct online starter,
DOL starter is used for induction motors of power ratings up-to 5hp.


the simplified form of most commonly used  DOL starter circuit has the following components :

1. a normally open (NO) push button type switch.
   
   
2. a normally closed (NC) push button type switch.
   
3. A Contactor (follow the link for contactor info) with auxiliary NO switch.
   
   

The electrical drawing used worldwide is called the single line diagram , but before making our DOL circuit in single line diagram we will
draw the whole schematic like an artist rather than the engineer ,
i.e.
we will draw the whole step by step image of what the circuit would look like
before converting it to proper electrical drawing .

1.  Supply the contactor coil with rated volts to make it operate , the circuit would look like

Now notice that the coil is supplied supply directly without any switch
so the contactor is activated , with

• the power contacts in closed state.
• auxiliary NC open and Auxiliary NO closed.

 2. Next, we add one NO push button switch in series with the coil supply circuit.

 

Notice the contactor operates when push button is pressed and returns to normal state when push button is not in pressed state.

but we need the contactor to remain in the activated state even when push button is released.
for that to happen we need a second NO contact which operates only when the contactor operates ,
that NO is placed in shunt with the NO push button so that

1. when we press the NO push button the contactor gets activated and so does the second NO contact, so now there are two paths for  current to get to the coil and so on.
2.  when the NO push button is released the second NO is still activated so the contactor is activated too.

 Notice that this purpose can be  served by the auxiliary NO contact.

the modified circuit would look like

STEP 1


STEP 2


STEP 3



This circuit is still missing one key component
i.e. a provision to stop the machine manually .
for that , we need to cut the supply to the coil
that is achieved by adding one NC push button in series before the NO push button .


With power connections the circuit would look like:



This is the basics of DOL starter used in industries .
But in actual applications
other components for indications
and protections like overload, over and under voltage , over temperature are installed .

the detailed circuitry and single line diagram of that can be viewed following the link bellow.

Direct online starter with complete protections installed.(link offline for now, will update soon!)