Direct online starter (DOL starter) working principle, control wiring, advantages and disadvantages

DIRECT ON-LINE (DOL) 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. 

It 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.HP motor required special unit called a motor starter. 

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.

The advantages of a DOL starter:

1. Simple and most economical starter.
2. More comfortable to design, operate and control.
3. Provides nearly full starting torque at starting.
4. Easy to understand and troubleshoot.
5. DOL starter connects the supply to the delta winding of the motor.

The disadvantages of a DOL starter:

1. High starting current (5-8 times of full load current).
2. DOL Starter causes a significant dip in voltage, hence suitable only for small motors.
3. DOL Starter reduces the lifespan of the machine.
4. Mechanically tough.
5. Unnecessary high starting torque

DOL Starter Applications

The applications of DOL starters are primarily motors where a high inrush current does not cause excessive voltage drop in the supply circuit (or where this high voltage drop is acceptable).

Direct on line starters are commonly used to start small water pumps, conveyor belts, fans, and compressors.