Direct Online Starter | DOL Starter, working,principle,Control,wiring,power,diagram

How Direct online starter (DOL) work, pdf

Different starting methods are used to start 3 phase induction motor like star delta starter, autotransformer starter, variable frequency drive,soft starter,dol starter(direct online starter) because the Induction motor draws a large current at starting.

In this article, we will discuss DOL starter (Direct Online Starter) - Working Principle and Construction, DOL Starter Diagram, control diagram, power circuit, and wiring, Single & 3 Phase starter

Direct On-Line is the simplest and cheapest method used for starting three-phase motor. Dol is used for up to 5 HP motor. dol starter is more popular due to the low price. 

DOL starter connects the motor directly to supply without a reduction in supply voltage and applies full line voltage to 3 phase motor. this is the main disadvantage of dol starter.

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

Direct online starter for 3-phase induction motor contain parts like miniature circuit breaker (MCB) for motor protection purpose, overload relay, power contactor, fuse unit, start button, stop button.

Small size dol starter is operated manually but for larger HP motor, dol motor starter are operated using electromechanical contactor (relay) to switch 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.

D O L starter use for starting the small motor, compressor, small water pumps, industrial conveyor belts. Fans, air curtains, exhaust fan, blower, etc.

A reversing starter can connect the motor for rotation in either direction. Such a starter contains two DOL circuits—one for clockwise operation and the other for counter-clockwise operation, with mechanical and electrical interlocks to prevent simultaneous closure.

For three phase motors, this is achieved by changing the wires connecting any two phases. Single phase AC motors and direct-current motors require additional devices for reversing rotation

In the case of three phases, the asynchronous motor draws a very high starting current up till achieving full speed. This starting current up to 6 to 7 time of full load current.

To reduce this starting current larger motor has starting methods like star delta starter or variable frequency drive.to reduced voltage dips on the power supply.

The demerits of dol starter is that gives the highest possible current to motor. Which is 6 to 7 time of rated current of the motor.

Besides the DOL starter draws the starting current there also exists a current peak that can rise up to 14 times the rated current since the motor is not energized from the first moment when starting.

So, here the question arises what is a dol motor starter and how dol starter work. Explanation of direct online motor starter with control and power circuit diagram is given below.

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Direct online starter (DOL starter) control diagram : 

dol control circuit diagram
Fig. direct online starter dol control circuit diagram

DOL starter control circuit diagram consist components like Main power contactor, start button, stop push button and overload relay is connected in series this circuit is called control circuit of dol motor starter. All controlling of starter are controlled by this circuit.

DOL starter power circuit diagram :

Direct Online starter power diagram has fuse unit (F1), Main power contactor (K1), overload relay (F2). Also, we can connect MCB for protection purpose in a direct On-Line motor starter.
Dol starter power circuit diagram
DOL Starter power circuit diagram

Direct online starter circuit Wiring diagram: 

Direct online starter DOL diagram
Direct online starter DOL diagram

DOL starter of three phase induction motors control and power circuit consists MCB or fuse unit which connected in series with the supply. Power contactor, thermal overload relay for protection and stop and start push button.

Working Principle of Direct online starter (DOL Starter ) :

1) To start the motor, start push button is pressed. After that, the main power contactor coil energized due to electromechanical action and this latch contactor pole.it applies full line voltage to the motor terminals. and the motor starts running. The motor will draw a very high inrush current for a short time.
2) As the motor accelerates and heads toward full speed, the current begins to drop but it will not drop significantly till the motor achieved full speed, about 85% of synchronous speed.
3) The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of high starting current, but it will not affect the magnitude of starting current.
4)the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque developed by the motor is less than the torque of load at any speed during the start period. the motor stops accelerating.
5) If motor torque developed by DOL Motor starter is less for load, then the motor must be replaced by the motor which can develop high starting torque.

DOL starter working :

1)When the start push button on the direct online motor starter is pressed, the contactor coil gets energized and it latches contactor pole due to electromagnetic action. 3 phase supply get applied to the induction motor terminal and the motor start running

2) after that start push button gets released and the supply to contactor coil continued through latching contact provided to the main contractor of D O L starter. 

3) At the time of stopping the motor we need to press the stop push button on dol starter circuit, this disconnect supply to the coil and contactor coil gets de-energized, and it releases the contactor latched pole, and the motor supply gets disconnected.

Component of direct online starter(DOL Starter):

1)Contactor :

A contactor is a heavy duty relay with a high current rating, used for powering up an electrical motor. The current rating for contactors varies from 10 amps to several hundred amps. The high current contactor is made from an alloy containing silver. Arcing during the switching operation of contactor causes contact to oxidize. However, silver oxide is still a good conductor.

Overload protection is provided along with the contractors to start the motor. Contactor is not used to interrupt a short circuit current, Unlike the circuit breaker used. Contactor size varies from small to large for high current appliances.

Contactor creates noise while switching operation. If when requires silent operation solid state relay is used. A contactor is an electromechanical switch that operates on energization and de-energized of the coil. 
The contactor has 3 NO contact which connects a motor to the power supply and one additional contact refer as Auxiliary contact, which is act as hold on contact when start push-button release. To maintain energization of contactor coil. 
If some time power fails abruptly due to this contactor coil get de-energized,it open contactor terminal and motor get disconnected from the supply.

2) Over load relay (overload protection ):

Most of motor winding failure occurs due to overload. Motor overload tends to the heating of motor winding that results in the weakening of winding insulation. A minor overload does not cause a motor failure immediately but it will eventually shorten the expected lifetime. 
To prevent this situation thermal overload relay is provided into a direct online starter circuit. 

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

This disconnect motor from supply and prevent further motor winding damage also insulation failure. 

3) Miniature circuit breaker (MCB) :

To protect the motor from short circuit conditions and avoid damage to motor winding MCB is used in DOL circuit.

4 ) Push buttons :

There are two push buttons are used in dol motor starter, one is a green color for the start button and the other is red for the stop push button. 
Switching of power supply carried out by using electromechanical contactor which has 3 to 4 pole

Start push button: 

this is normally an open type push button, to start the motor, this push button needs to press, and the supply gets applied to the motor through a power contactor.

Stop push button:

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

Advantages of DOL Starter(direct online starter) 

    • Direct On-Line motor starter is the Cheapest Motor Starter.
    • DOL is the simplest motor starter to design and maintain. 
    • 100% starting torque.
    • D O L starter used up to 5HP motor depending upon insulation class of motor. 

      Disadvantages of DOL Starter 

        • High starting current 
        • typically 7 to 8 time rated current applied at starting
        • due to high current stress applied on motor winding. 
        • not suitable for high rating motor. 
        • create a voltage dip on the power system
        • High starting torque  

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