What is Direct Online Starter (DOL)? Working Principle, Wiring Diagram, Applications

There are different types of methods used to start induction motor as these motors utilize more power for starting.  When the flow of current through windings is high then there will be a chance to damage the motor. To overcome this problem, different kinds of starter techniques are used.  The simple type of starter method is DOL (Direct on Line Starter).  The Direct Online starter includes a circuit breaker or MCCB, overload relay and contactor for the protection of motor. The unlocking of an electromagnetic contactor can be done through the thermal overload relay beneath fault conditions. Usually, the controlling of contactor can be done by using separate buttons like start and stop. An auxiliary contact is used on the contactor across the start button because the contactor is electrically latched while the working of an induction motor.

What is Direct Online Starter or DOL?

The most fundamental, economical, as well as an easy method for running a three-phase induction motor is named as the DOL starter. This starter joints three phase motors straight across the supply of three phases. The attractiveness of this kind of starter is, it can be connected straightly to the motor, and it doesn’t affect the induction motor.  The Direct Online Starter consists of a protective device as well as main contacts. Let us discuss an overview of DOL.

DOL Starter
DOL Starter

Construction of DOL Starter

The DOL starter consists of two switches namely Green and Red where the green switch is used to start and red switch is used to stop the motor. The DOL starter comprises a circuit breaker (or) MCCB, overload relay & contactor for protecting the motor. The two switches of the motor control the contacts. The motor can be started when we shut the contact by pressing the green switch, and the full-line voltage comes out to the induction motor.

Generally, the contactors are 3-poles contactors or 4-pole contactors. For instance, a 4-pole type contactor consists of three normally open contacts and one is auxiliary or holds on contact. The three NO contacts are used to connect the induction motor to supply lines whereas the auxiliary contact is used to boost the contactor coil when the start button is opened.

Construction of DOL Starter
Construction of DOL Starter

If any error takes place, then holds on contactor gets deactivated. Therefore, the DOL starter separates the induction motor from the mains supply.

DOL Starter Wiring

The wiring or connections of DOL starter includes mainly four parts such as main contact, normally open contacts, normally closed contacts, and connections of relay coil as well as thermal overload relay.

The connection of Main Contact

The connection of the main contact can be done by using the following steps.

  • The Contactor can be connected between relay coil, thermal overload relay, and voltage supply
  • The L1 contactor is connected to R-phase using MCCB
  • The L1 contactor is connected to Y-phase using MCCB
  • The L3 contactor is connected to B-phase using MCCB

The connection of NO Contact

The connection of NO contact can be done by using the following steps.

  • The normally open contact is from 13 to 14 or 53 to 54 contactor points
  • The 53-Contactor Point connected to 94-Start Button & Point of Contactor-54 is connected to general Start wire or stop switch.

Normally Closed Contact

The normally closed contacts are 95 to 96

The Connection of Relay Coil

The Relay Coil (A1) is connecting to any supply segment & A2 relay coil is connected to the NC Connection (95) of the thermal overload relay.

The connection of Thermal Overload Relay

The connection of thermal overload relay can be done by using the following steps.

  • The T1, T2, and T3 are connected to the relay
  • This relay is connected among motor as well as main contactor
  • The normally closed connection of this relay is connected to stop switch as well as the mutual connection of start or stop switch.

DOL Starter Working

The 3 phase DOL starter wiring diagram is shown below. The DOL starter joints the 3 phase main wiring with the induction motor namely L1, l2 and L3 when the start switch is pressed.  Generally, the Direct Online starter working can be done in two different stages namely DOL starter control circuit and DOL starter power circuit. The control circuit is attached to any two of the phases and activated from the two phases only. Whenever we push the start switch, then the current will flows through the control circuit as well as contactor wind also. The contactor coil can be boosted by the current to make the contacts close, and thus three phase supply becomes obtainable for the induction motor.

When we push the stop button, then the flow of current through the contact will be stopped, therefore power supply to the induction motor will not be accessible, as well as the same thing will occur while overload relay works.

As the motor supply breaks, then the machine will move toward to relax. The contactor coil gets power supply even though we open start switch because it will obtain supply from the main contacts as demonstrated in the above Direct Online starter circuit diagram.

Advantages & Disadvantages

The advantages of this starter include the following.

  • The Direct Online starter is an inexpensive starter.
  • It gives almost complete starting torque at the beginning.
  • The designing, operating and controlling this starter is very easy.
  • Understanding and troubleshooting are very easy.

The disadvantages of this starter include the following.

  • Starting current is very high
  • This starter causes an important dip in voltage, therefore appropriate only for small motors.
  • The lifespan of the machine can be reduced.
  • It is hard mechanically.
  • Needless high opening torque

Applications of DOL Starter

The applications of this starter include the following.

These starters are very useful in motors when the 3-phase induction motor is at low rating as well as the input is very low (5V) then that will function at low speeds & lower rating.

These starters are used where maximum currents do not cause any damage such as to run small compressors, conveyor belts, water pumps, fans, etc.

Thus, this is all about Direct Online Starter (DOL), and the main features of this DOL include three phase connection, High starting torques, voltage dips, mechanical load, current peaks, and very easy switching devices. These are mainly applicable to where maximum currents do not cause any damage such as to run small compressors, conveyor belts, water pumps, fans, etc. What is single phase DOL starter wiring diagram?


  1. This article is really helpful for me

    1. Tarun Agarwal says:

      Hi Rahul
      Thank you so much for your feedback

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