DOL starter, also called Direct online Starter, is one of the simplest types of motor starters used to start an induction motor.
I can see DOL starters everywhere in my work (just for low-power motors).
In this article, I will answer 6 important questions for beginners about this starting method. Let’s dive directly into the questions.
Why DOL Starter is Required?
DOL starter is required for motors with low inrush current, To control and protect it with low cost and simple circuit.
When we are talking about induction motors in industrial applications, we should know that it needs a control circuit.
DOL is one of the most economical and suitable methods for starting, protecting, and controlling small-size induction motors with no high inrush current.
Besides starting motors, the DOL starter offers over-current protection and overload protection during the operation as I mentioned above.
It consists of a fuse to protect against overcurrent due to any short circuit. And similarly, it consists of relays to offer protection against overload conditions.
What Is The Power Range of Motors I Can Start With DOL?
One of the questions I found during my search on the net about DOL is, Can I Start a 10 HP induction motor using the DOL starter? Yes, you can start a 10 HP directly online.
DOL Starter is normally used for starting small motors. It is one of the simplest types of starters and is used for starting motors below 10 HP.
However, in some cases, we can start motors ranging from 5 to 25HP using a DOL starter.
The motor will cause a voltage drop in the power system, so if the power system is not good enough to handle this drop it’s not recommended to start the motor with DOL.
But it is suggested to use specialized starting equipment like VFDs, solid-state devices, Auto transformers, Star Delta Starter or soft starters, etc.
So it is clear that we can start a 10HP induction motor using a DOL starter. All we need is to ensure that the power supply will withstand the voltage drop at the start.
Why Do We Use A Contactor in DOL Starter?
You may ask a good question, why do we use a contactor in DOL? Why not switch the motor on and off using a circuit breaker? In DOL starter, The contactor is a main part of the circuit and is used in order to:
- Make it easy to provide over-current protection for the motor with the help of an over-current relay. When the motor draws over-current, the overload relay activates the contactor which in turn switches off the motor.
- Control the motor locally or remotely (manual or automatic control).
- Provide a safe method to connect the motor to the branch circuit.
DOL starter consists of a circuit breaker, Contactor, and over-current relay. While the contactor is an electromagnetic switch type. Which activates under the condition of a thermal overload relay.
The principle of operation contactor in DOL starter is simple. When the contactor is closed. Full line voltage is applied at the windings of the motor. The contactor acts as a switch based on electromagnetic induction.
When the switches are closed, they provide a connection between circuits. Besides this, we can control it remotely, manually, or automatically. We can easily control electrical systems and machinery by using contactors.
When DOL Starter Is A Good Choice?
DOL starter is the best choice for motors in case there is no risk of a large amount of inrush current, or when your power system is strong enough to handle this inrush current.
As I mentioned, Some power systems can handle DOL for a 25 hp motor without any effects on other loads and motors.
Besides this A DOL starter is suitable for use in small-range induction motors applications like Compressors, Conveyor built, water pumps, Fans, and three-phase squirrel cage induction motors.
Does the DOL Starter Reduce The Starting Current?
DOL starter does not reduce the starting currents of the motor.
As if we look at the construction and working principle of DOL starter. It consists of push buttons, contactors, and protective relays.
In this case, it just protects the motor from overcurrent and offers overload protection. As on higher temperature relay get disconnected.
DOL starter is just a starting, control, and protection for the low-rate motors. And provides full voltage to the motor.
For more information about motor starting methods, read my detailed article here.
How Do You Keep A DOL Starter Working Properly?
It is important to test the DOL starters regularly to avoid any mishaps. As induction motors require a large amount of starting current, this current affects the contactor contacts day after day.
- The first thing to test in the DOL starter is the contactor. The contactor acts as a switch in DOL Starter. It is directly connected with input voltage and offers continuity of voltage. Make sure that all contacts are working properly. This is done by the AVO meter.
One of the most common contactors faults is phase failure; this means one contact phase is always closed, even if the contactor coil is de-energized. Read my article about contactor failure here.
- Functionality test. In the Functionality test, when we energize the whole circuit, Press the Start button, and check the contactor operation, you can hear a click sound of the contactor.
It means that the contactor is now connected and in a close position. If you use a phase rotation tester or AVO meter it will also indicate that the circuit is live now.
- Check all parts of the circuit, i.e circuit breaker, fuses, and overload relay. Activate the relay, by pressing the test button to see if it will de-energize the contactor.
- Now, again switch off the breaker and check whether you hear any click or not. During this situation, the contactor disconnects. Now put the breaker again in the close position. This time you will not hear any clicking sound. Which indicates that the contactors did not close automatically. So the Starter is in safe mode and you can use it. And there is no chance of restarting. Make sure you follow the same procedure before using a DOL starter.
