Direct On Line DOL Motor Starter

Direct On Line DOL Motor Starter

Generally we know induction motor starting current is 6 to 7 times of full load current for this purpose we are using starters. there are different types of starter available in the market to prevent this high current to  protect the motor from damage.

The direct online motor starter is the cheapest and simple construction to start the induction motor. DOL motor starter is not limiting the starting current and its directly start the motor.The Direct On Line Motor Starter (DOL) consist of start and stop control switches and overload current protection relays, magnetic contractors with auxiliary relay NO & NC contacts.

 dol starter electricalmastar

Normally the magnetic contactor is operated by start/stop push buttons. and NO auxiliary contact of magnetic contactor is connected between start and stop push buttons.


Working of Direct On Line Starter DOL

During start, the magnetic contactor is activated, and its giving full line voltage to the motor windings. The motor will consider high starting current during short time, and this high current connected to rotor and the motor rotate with locked rotor current of the motor. The motor will  generate locked rotor torque and starts to accelerate at full speed.

when ever the motor will start the starting current of motor is very high after the motor obtained 80% of synchronous speed the motor current will be reduce.  during the starting time of motor there will be chance for  load affect because of high starting current.

During starting time  the torque developed by the motor is  greater the load torque at starting speeds , then motor will reach at  full speed  the motor torque is lesser then load torque . if your using DOL starter then motor should be produce high starting torque otherwise we need to change the motor with high starting torque.

The  difference between torque developed by the motor and  the load torque is called acceleration torque, and if the motor accelerate torque only  change due to the motor speed torque & load speed torque curve. The starting time of motor is fully dependent on motor  acceleration torque and the load torque.

The working principle of DOL motor starter is very simple. the main part of direct online starter is magnetic contactor and relay coil. the main 3 phase supply is connected to magnetic contactor and its connected to over load relay and its finally connected to motor terminal. the operation motor is controlled by start and stop push buttons. one of the phase supply is taken from the main supply and its connected to over load relay terminal NC contact and its connected to stop push button contact 95 and its connected to start push button contact 96 and its connected to auxiliary contact of magnetic contactor A1 and common supply is directly connected from A2 auxiliary contact. when ever the start button is pressed the magnetic contactor auxiliary contact get activated and supply is connected to motor terminal. and motor will start to run.

Connecting of DOL Starter

1. Main Contact in magnetic contactor


  • Magnetic Contactor is connecting between Supply Voltage, auxiliary relay coil and thermal overload relay.
  • R1 of magnetic Contactor Connect (NO) to R Phase through MCCB
  • Y1 of magnetic Contactor Connect (NO) to Y Phase through MCCB
  • B1 of magnetic Contactor Connect (NO) to B Phase through MCCB.

NO Auxiliary Contact

  • Relay contact terminal 13-14 & 53-54 are NO contact in the magnetic contactor. at normal condition it is in open position. whenever the relay get energized its act like NC contacts.
  • Contactor terminal 53 is connecting to Start push Button Point (94)
  • Contactor terminal  54  is connected to neutral or common point of Start/Stop Button.

NC Auxiliary Contact

  • overload relay terminal 95-96 is a normally closed NC contact. when ever motor get over load then this thermal relay get activate and its operated into NO contact.

2. Relay wiring  Connection

  • relay coil generally having A1 and A2 contact terminal. A1 of Relay Coil is always  connecting to phase supply and A2 is always connecting to neutral through thermal over load relay’s NC contact (95).

3. Thermal Overload Relay Connection:

  • generally thermal overload relay having 3 main contact terminal T1,T2,T3 . Main power supply R,Y,B  are  connect to Thermal Overload Relay terminal T1,T2,T3
  • Normally Overload Relay is Connecting between Main Contactor and Motor terminal
  • NC contact  of thermal overload relay is connecting to stop push button and common connection of start/stop button.


Wiring Diagram of DOL Starter

The below diagram show the general connection of direct online starter .

Image result for dol starter



Advantages of DOL Starter

  1. simple operation
  2. low cost
  3. maintenance less
  4. simple construction and easy installation.

Disadvantages of DOL Starter:

  1. its not reduce starting current of the motor
  2. its can not work with more then 5 HP motor
  3. it produce high starting current 6 to 8 times of full load current
  4. its producing mechanical harassment of the motor

Application of DOL starter:

  • low kw 3 phase induction motor
  • water pump motor
  • cooling tower pumps
  • blowers


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