DOL Starter: Working Principle, Parts and Advantage

DOL Starter is the simplest and cheapest method used for starting a three-phase induction motor. Direct online starter is used for up to 5 HP motors and is more popular due to the low price. Other starting methods are used to start 3-phase induction motors like star delta starter, autotransformer starter, variable frequency drive, soft starter.

It directly applies full line voltage to the induction motor and the asynchronous motor draws a very high starting current up until it achieves full speed.

The starting current is up to 6 to 7 times of full load current. This is the main disadvantage of a DOL starter.

A small HP motor can be started simply by plugging into the power supply. But a large HP motor required a special unit called a motor starter.

DOL Starter for 3-phase induction motors contains parts like miniature circuit breakers (MCBs) for motor protection purposes, overload relays, power contactors, fuse units, start buttons, and stop buttons.

Small size D O L starter is operated manually but for larger HP motors, it is operated using an electromechanical contactor to switch the motor ON and OFF.

DOL Motor Starter can be used if a high starting current cannot cause an excessive voltage drop in the motor circuit.

It finds applications in various industries where the motor load is relatively small and does not require precise control.

In this article, we will discuss the DOL Starter Working Principle, its advantages disadvantages, and parts for a 3-phase starter.

DOL Starter Working Principle

The DOL starter working principle is relatively straightforward. When the starter is switched on, it creates a direct electrical connection between the motor and the power supply.

This means that the induction motor receives the full voltage from the power supply, allowing it to start running immediately.

To start the induction motor, the start push button needs to be pressed. After that, the main power contactor coil is energized due to electromechanical action and this latch contactor pole.

Full line voltage gets applied to the motor terminals. and the motor starts running. The motor will draw a very high inrush current for a short time.

After that start push button gets released and the supply to the contactor coil continues through the holding contact provided to the main contactor of the motor starter.

As the motor accelerates and heads towards full speed, the current begins to drop but it will not drop significantly till the motor achieves full speed, about 85% of synchronous speed.

The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but it will not affect the magnitude of the starting current.

See also  What is Soft Starter,Working Principle,diagram,advantages

At the time of stopping the motor, we need to press the stop push button on the DOL Motor starter panel.

This disconnects the supply to the coil, and the contactor coil gets de-energized and releases the contactor latched pole, and the motor supply gets disconnected.

Parts of DOL Starter

DOL Starter is a motor control device that includes a contactor, overload relay, push buttons, indication lamps, and fuses.


A contactor is a heavy-duty relay with a high current rating, used to connect an electrical motor to the power supply.

The rating of contactors ranges from 10 amps to several hundred amperes. Large contactors are made from an alloy that contains silver.

Arcing during the switching operation of the contactor causes contact to oxidize. However, silver oxide is still a good conductor.

Power Contactor

Overload protection is provided along with the contactor to protect the motor from overloading. A contactor is not used to interrupt a short circuit current, Unlike the circuit breaker.

The contactor creates noise while switching operations. If when requires silent operation solid state relay is used. A contactor is an electromechanical switch that operates on energizing and de-energizing the contactor coil.

The contactor has three main power contacts depicted as L1 L2 L3 in the starter wiring diagram.

The contacts that connect a motor to the power supply and also have additional contacts are referred to as Auxiliary contacts, which act as hold-on contacts when starting push-button release.

Thermal Overload Relay:

Most motor winding failure occurs due to overload. Motor overload tends to the heating of winding that results in the weakening of its insulation.

A minor overload does not cause a motor failure immediately but it will eventually shorten the expected lifetime.

Thermal Overload Relay dol motor starter

To prevent this situation thermal overload relay or OLR is provided.

Thermal overload relay has a bimetallic strip that bends because of overheating caused by overcurrent.

Resulting in disconnecting the motor from the supply and preventing further motor winding damage also insulation failure.

Miniature circuit breaker:

A miniature circuit breaker (MCB) is a Switch that automatically disconnects the motor from the supply when an abnormal condition happens.

To protect the motor from short circuit conditions and avoid damage to motor winding MCB is used. generally, MCB is used in circuits in place of conventional fuses

Push buttons:

There are two push buttons used in the dol motor starter circuit, one is a green color for the start button and the other is red for the stop push button.

Start push button: this is normally an open-type push button, to start the motor, this push button needs to be pressed, and the supply gets applied to the motor through a power contactor.

Stop push button: This is the NC type of push button we need to press to stop the motor from running condition.

See also  Star Delta Starter-Working Principle, Theory, Control Wiring Diagram and Connection

After pressing the stop button contactor coil gets de-energized with the opening of contacts.

Due to this switching operation motor gets disconnected from the supply and the motor stops rotating.

Indication lamps:

Indication lamps are used in the DOL starter circuit to indicate the status of the motor.

Generally, three types of indication lamps are used in direct online starter green for motor off indication, red indication lamp to indicate the motor is running and third assigned to trip.

Advantages of DOL Starter

This starter offers several advantages, making it a preferred choice in various applications:

  • Quick start: starter provides instant motor start-up
  • Cost-effective: The Direct online motor starter is the Cheapest compared to other starters like star-delta and soft starters.
  • Easy implementation: DOL is the simplest motor starter to design and maintain.
  • Simple troubleshooting: due to the simple wiring connection troubleshooting is simple
  • High starting torque: 100% torque during motor start-up.
  • D O L starter used up to 5HP motor depending upon insulation class of motor.

Disadvantages DOL Starter

Despite its advantages, it has some limitations that should be considered:

  • High starting current: typically 7 to 8 time rated current applied at starting
  • Potential motor damage: The high starting torque may subject the motor winding to excessive stress.
  • Size limitations: The DOL starter is used for motors with ratings up to 5 HP, unsuitable for high-rated motors.

Applications of DOL Starter

  • It finds application in various scenarios where the high inrush current of the motor does not pose any detrimental effects. Some common applications include:
  • D O L starters are mainly used to start smaller rating motors up to 5HP
  • It finds applications, where the starting current does not cause a huge voltage dip in the system
  • used for applications that required high initial torque
  • They are commonly used in pumps, fans, small motors, compressors, small water pumps, and industrial conveyor belts. Fans, air curtains, exhaust fan, blower etc.
  • These applications benefit from simplicity and cost-effectiveness, as they do not require complex control systems or precise speed regulation.

Comparison of DOL Starter and Star-Delta starter

Sr NoDOL StarterStar Delta Starter
1This starter is used up to 5HP motorIt can be used up to 20HP motor
2Starting currently is so highThe starting current is reduced by 1/3
3Construction is simplewiring is complex compared to the DOL starter
4Can be used with any motor i.e start connected or delta-connected motoronly used where all six winding terminal leads are accessible
5Starting Torque is highThe starting Torque is low

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