DOL Starter (Direct On Line Starter) Diagram & Working Principle

Updated: Feb 11, 2021

What is DOL Starter?

A direct on line starter or DOL starter is used to start the 3 phase induction motor through connecting the three phase supply to motor stator winding. The full line voltage is applied to stator through DOL starter. The major parts on a DOL starter are overload relay as a protective device, contactor, no voltage release relay. A wiring diagram of DOL starter is as given below.

power diagram of dol starter

The initial current drawn by the induction motor depends on the magnitude of the back emf. The back emf depends on the speed of the motor. The back emf is zero at motor start because speed of the motor is zero. The applied voltage in DOL is maximum and the back emf is zero, that is why motor draws a very large current in comparison to motor full load current. The motor draws about 6 to 8 times current of its FLC when started on DOL.The DOL motor starter is best suited for the small rating induction motor. The high current drawn by the motor cause voltage drop in the cable and thus the current drawn by the motor is further increase on account of the line voltage drop.

The current drawn by the motor can be expressed with the following mathematical expression.

Induced EMF in Induction Motor

From above equation it is clear that in the absence of back emf at start, the current depends on the winding resistance. The winding resistance of the motor is very low, and that is why motor draws large current at start.

The motor output KW depends on the square of the diameter of the rotor, length of the rotor and synchronous speed of the motor.

Motor KW Equation

The rotor of the small rating motor has more axial length and small diameter, therefore it accelerates faster than large rating motor and the higher starting current gets decreased fast.Therefore, the DOL starter is most suitable for small rating motors.

The large rating motor takes more time to accelerate to its full speed and thus the motor keep on drawing more current for longer period of time, and this may damage the motor insulation.

DOL Starter Wiring Diagram

The wiring diagram of the DOL starter is very simple. The starter has switch fuse unit or MCCB, thermal overload relay, magnetic contactor and the push buttons. The push buttons are used for starting and stopping the motor. The red button is used for motor stopping and the green button is used for motor starting.

DOL Wiring Diagram

When we press green button, the contactor gets supply and it gets energized and the three phase supply reached to motor terminal and motor starts rotating. The contactor is latched through the auxilary contact of the contactor, which is wired in parallel with the start push button. Thus, after release of green button the motor keep non running. When the stop switch-RED Button- is pressed the supply to contactor coil is interrupted and it deenergize the contactor and motor To start the motor, we close the contact by pushing the Green Button, and the full line voltage appears to the motor. A contactor can be of 3 poles or 4-poles. Below given contactor is of 4-pole type. In case of the fault the overload relay contact wired in the series with the sop button, stops the motor.

3 Phase Motor Starter with Overload Protection

The motor protection relay is the integral part of the DOL motor starter. In the case of any fault in the motor or abnormality in the supply side the motor must be stopped to prevent the motor from damaging. If the single phasing occurs the motor draws a large current and if the current keeps on flowing for a longer period the motor insulation may get failed due to temperature rise.

The other case of motor drawing large current is the excessive load on the motor or bearing failures of the motor itself. The overload relay senses the excessive current in the form of heat energy in all the cases whether it is single phasing or excessive load of driven equipment. If the current exceed above the set current of the overload relay, it trips the motor.

Nowadays, microprocessor based overload relay are used to protect the motor from the damage. The relay senses the over current, instantaneous over current, locked rotor and single phasing. Fault finding is very easy if such type of relay is used for motor protection.. Also, in place of switch fuse unit and motor protection relay, MPCB( Motor protection Circuit breaker) can be used, which gives the functions of both SFU and overload relay.

DOL Starter Working Principle

The DOL starter provides three phase supply to motor when the start button of thy starter is pressed. When the magnetic contactor is energized by giving the supply to magnetic contactor the motor starts rotating. The running of the motor is ensured by latching the contactor through its auxilary contactor.

The control circuit for a DOL Starter is shown below.

When the stop button is pressed, the supply to magnetic contactor coil is stopped , and the three phase supply going to motor is interrupted and motor gets stopped.

Advantages of DOL Starter The advantages of a DOL starter include:

  1. Very simple and cheaper

  2. Easy design and operation

  3. Provides approximately full starting torque at starting for low rating motors.

  4. Easy to troubleshoot

Disadvantages of DOL Starter

The disadvantages of a DOL starter include:

  1. Motor draw high starting current (6-8 times of its full load current).

  2. DOL Starter causes a significant dip in line voltage, hence preferred for small motors.

  3. DOL Starter reduces the lifespan of the machine because motor goes under severe stress at every start

DOL Starter Applications

DOL starter is used to start the motor of small rating or large rating motor used for the application to start the equipment like compressor at no load.

Read More: Star Delta Starter Working Principle,Control and Power Diagram
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