Understand how the DOL (Direct On Line) starter circuit works!

One type of 3-phase electric motor starter circuit that is most often used is the DOL (Direct On Line) circuit. This circuit directly connects a 3-phase electric motor to a power source through a component called a contactor (You should learn what a contactor is and how it works before continuing to read this article). In this article, we will learn and understand how the DOL circuit works for 3-phase electric motor starters!

I have made the simplest DOL starter wiring diagram which we can understand easily. The picture is like this.

Explanation of the components in the Wiring Diagram

In the scheme above, first, we have a voltage source with a sinusoidal symbol which is an AC (Alternating current) voltage source.

Then the SWITCH contact acts as a connector and voltage breaker for the entire DOL circuit. Usually, I use 1 phase MCB for this case.

After going through the SWITCH (MCB), we have a protection component called the Thermal Overload relay which is in the form of a normally closed contact. This component will protect the electric motor when an overload occurs.

After contacting the Overload Relay, we go to the Stop Push Button (NC). This button is usually red and functions to turn off the voltage source to the next component.

Then we encounter the next 2 components which are connected in parallel, namely the Start Push Button (NO) and Aux Contact NO (C). The first component will function to start the 3-phase electric motor and the second component is the locking circuit.

Finally, we come to the last component, namely the Main Contactor coil which I symbolize with C.

When the voltage source successfully enters Coil C, all the contacts on this component will change position. Contacts that were initially NO will change to Close, and those that were originally NC will change to Open.

Explanation of how the DOL circuit works

We already understand the function of each component in the wiring diagram above. Now it’s time to fully understand how it works.

When the SWITCH contact is closed, electric current will flow to the Overload Relay Contact and then flow to the Stop Push Button (NC). Here, the flow of electric current stops, until you press the Start Push Button (NO).

When you press the Start Push Button (NO), the flow of electric current will continue to the Main Contactor Coil (C) and turn it on.

When you release the Start Push Button (NO), the current flow to C should stop. This is where the role of Aux Contact (NO) works. This contact will hold the flow of electricity to Coil C of the Main Contactor, and keep it ON.

The main contactor will remain ON until you decide to turn it off by pressing the Stop Push Button (NC).

Applying Wiring diagrams to real components

Above, we have seen the wiring diagram for the DOL starter circuit, understood the components, and understood how it works.

Now it’s time for me to show you what this wiring diagram looks like when applied to real electrical components.

In this case, I will use components that you can easily find on the market. The following is a list of components.

The following is a DOL Starter wiring diagram if equipped with terminal numbers for the components in the table above.

It will look like this when wiring the components. You can also see a 3-phase voltage source connected to a 3-phase electric motor.

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