DOL Starter(Direct On Line Starter)

 

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

What is DOL Starter?

A DOL starter (also known as a direct on line starter or across the line starter) is a method of starting a 3 phase induction motor. In a DOL Starter, an induction motor is connected directly across its 3-phase supply, and the DOL starter applies the full line voltage to the motor terminals. Despite this direct connection, no harm is done to the motor. A DOL motor starter contains protection devices, and in some cases, condition monitoring. 

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 apply full line voltage to 3 phase motor. This is the main disadvantage of dol starter.

Small HP motor can be started simply plugging into the power supply. But large HP motor required 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 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 asynchronous motor, draws 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 then 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 dol motor starter and how dol starter work. Explanation of direct online motor starter with control and power circuit diagram is given below.

Table of Contents

Direct online starter (DOL starter) control 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 direct On-Line motor starter.

DOL Starter power circuit diagram

Direct online starter circuit Wiring diagram: 

Direct online starter DOL diagram

DOL starter of three phase induction motors control and power circuit consist 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 head toward the full speed, current begins to drop but it will not drop significantly till 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 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 start push button on the direct online motor starter is pressed, contactor coil gets energized and it latches contactor pole due to electromagnetic action. 3 phase supply get applied to the induction motor terminal and motor start running

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

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

Component of direct online starter(DOL Starter):

1)Contactor :

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

Overload protection is provided along with the contactors 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 used. A contactor is an electromechanical switch which 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 condition 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 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 open type push button, to start motor, this push button need to press and supply get applied to motor through power contactor.

Stop push button:

This is NC type push button we need to press to stop motor from running condition. after pressing stop button contactor coil get de-energized with the opening of contacts. Due to this switching operation motor get disconnected from supply and motor stops rotating.

 

                                        

                                        

Advantages of DOL Starter(direct online starter) 

• Direct On-Line motor starter is 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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