Electric Technology

Direct Online Starter (DOL): Working principals and Applications

Dol starter: Basic working and Applications

A Direct Online (DOL) Starter may be a method of starting a 3 phase induction motor.

Despite this direct connection, no harm is completed to the motor. A DOL motor starter contains protection devices, and sometimes condition monitoring. A direct on line starter can only be used in the high inrush current of the motor doesn’t cause an excessive drop within the supply circuit. If a high drop must be avoided, a star-delta starter should be used instead. Direct on line starters are commonly wont to start small motors, especially 3 phase cage induction motors.

As we know, the equation for armature current in the motor. DOL Starter Formula. The value of back emf (E) depends upon speed (N), i.e. E is directly proportional to N.

At starting, the value of E is zero. So starting current is very high. In a small rating motor, the rotor has a more considerable axial length and little diameter. So it gets accelerated fastly. Hence, speed increases, and thus the value of armature current decreases rapidly. Therefore, small rating motors smoothly run when it’s connected on to a 3-phase supply. If we connect an outsized motor directly across a 3-phase line, it might not run smoothly and can be damaged, because it doesn’t get accelerated as fast as a smaller motor since it’s short axial length and bigger diameter more massive rotor. However, for giant rated motors, we will use an oil-immersed DOL starter.

Dol starter: Basic working and Applications

Working of DOL starter

The control circuit is connected to any two phases and energized from them only. When we press the start button, the current flows through the contactor coil (magnetizing coil) and control circuit also. The current energizes the contactor coil and leads to close the contacts, and hence 3-phase supply becomes available to the motor.

If we press the stop button, the current through the contact becomes discontinued, hence supply will not be available, and a similar thing will happen when the overload relay operates. Since the supply of motor breaks, the machine will come to rest.

A direct online starter consists of two buttons, a GREEN button for starting a RED for stopping. The purpose of the motor and a GREEN button for starting. The DOL starter comprises of an MCCB or circuit breaker, contactor, and an overload relay for cover. These two buttons, Green, and Red.

To start the motor, we close the contact by pushing the Green Button, and full line voltage appears on the motor. A contractor can be of 3 poles or 4-poles. Below given contactor is of 4-pole type.

If there is any fault, the auxiliary coil gets de-energized, and hence the starter disconnects the motor from the supply main.

Dol starter: Basic working and Applications

Features, Advantages/Disadvantages & Applications of DOL Starter


It is very simple to design, operate, and maintain.

It is the cheapest and economical starter.

It has a compact design and occupies less space.

It provides 100% of the starting torque.

The control circuit (green and red button) is simple and a layperson can operate it.

Understanding and troubleshooting the system is easier.


The starting current is very high.

The starting high current may damage the motor thus only low rating motors should be used.

The high inrush current causes a voltage dip in the power lines that can be dangerous for other appliances connected in parallel.

The high starting torque can be unnecessary in some cases.

High starting torque causes mechanical stress reducing the life span of the motor itself.

There is no control over the starting current and torque.


It provides high starting current.

It provides high starting torque.

It causes voltage dip in the power mains.

It has the simplest controlling mechanism.

It is suitable for low power rated motor.


The DOL starters are used for a motor having low power ratings.

Where the starting current does not damage the windings of the motor.

It is used in applications where the starting current does not cause huge variations in the line voltage.

Direct online starters are used for small water pumps, conveyor belts, fans, and compressors.

Parts of DOL Starter:

A DOL starter is made of following parts:

Circuit breaker or Fuse:

The circuit breaker or fuse is directly connected to the power mains and it is used for protection against short circuits. It trips the power supply in case of a short circuit to protect the system from any potential hazards.

Magnetic Contactors:

A magnetic contactor is an electromagnetic switch that operates electromagnetically to switch the power supplied to the motor.

The magnetic field generated by the coil is used for switching the terminals. The passing current through the coil magnetizes the iron core that is surrounded by the coil. The magnetic force pulls on the armature to close or open the contacts.

The magnetic contactors have three NO (Normally open) main contacts used for power supply to the motor and auxiliary contacts (NO and NC) with a lesser rating used for the control circuit. The coil is connected to the voltage source through auxiliary contacts. Also, keep in mind that the coil used for a single-phase and three-phase supply vary as the supply voltages are different.

Overload Relay:

OLR or also known as overload relay is the last part used in the DOL starter and it is used for protection against overloading of the motor. It breaks the current flow when it exceeds a certain limit but it also tolerates the high starting current. So the OLR is carefully selected in such a way that its tripping current limit does not fall below the starting current range.