What is a DOL Starter ?

What is a  DOL Starter ?

DOL Defines an initiator

DOL Starter (Direct On Line Starter) is a method of starting a three-phase induction motor. In the DOL starter, the induction motor is directly connected to its 3-phase power supply, and the DOL starter applies the full line voltage to the motor terminals. The motor is protected even when connected directly to the power supply. DOL motor starters include protection and, in some models, condition monitoring. The following is the wiring diagram of the DOL starter:

Starting mechanism

The wiring diagram for DOL stater is shown below. The direct-in-line starter consists of two buttons, a green button for starting and a red button for stopping the motor. DOL starters include MCCB or circuit breakers, contactors, and overload relays for protection. These two buttons, namely green and red or start and stop buttons control the contacts.

To start the motor, press the green button to close the contact, thereby applying a full line voltage to the motor. Contactors can have 3 or 4 poles; The figure below shows a 4-pole contactor.

It contains three NO (normally open) contacts to connect the motor to the power cord, and a fourth contact is the "hold contact" (auxiliary contact) to energize the contactor coil after releasing the start button.

In the event of any failure, the auxiliary coil will lose power, so the starter will disconnect the motor from the power supply.

Working principle

The working principle of the DOL starter begins with the connection of the three-phase main power supply to the motor. The control circuit is connected to any two phases and is powered only from them.

When we press the start button, the current also flows through the contactor coil (magnetizing coil) and the control circuit.

The current energizes the contactor coil and causes the contacts to close, so the motor can use a three-phase power supply. The control circuit of the DOL Starter is as follows.

Advantages of DOL Starter

• Simple and most economical starter.

• More comfortable design, operation and control.

• Provides almost full starting torque at starting.

• Easy to understand and troubleshoot.

• The DOL starter connects the power supply to the triangular winding of the motor

Disadvantages of DOL Starter

• High starting current (5-8 times full load current).

• DOL Starter causes a significant drop in voltage and is therefore only suitable for small motors.

• DOL Starter will shorten the service life of the machine.

• High mechanical strength.

• Unnecessarily high starting torque

DOL starter application

Applications for DOL starters are primarily motors where high inrush current does not cause an excessive voltage drop in the power supply circuit (or where such a high voltage drop is acceptable).

Direct in-line starters are commonly used to start small pumps, conveyor belts, fans, and compressors. In the case of asynchronous motors (such as three-phase squirrel-cage motors), the motor will consume a high starting current until it is running at full speed.