DOL Starter – Standalone with overload protection

Description

Direct on line starters, aka across the line starters are a way of starting up 3 phase induction motors. With Direct on Line (DOL) starters, the induction motor is connected directly across all of its 3 phase supply, and the starter applied the entire line voltage to the terminals on the motor

Even though it is directly coupled, there isn’t harm to the motor, as all of our direct on line starters contain protection equipment.

Since the starter connect the supply line directly to the motor, there is a very high inrush current compared to the full load of the motor (up to 8 times higher) This increase load rapidly decreases as the motor reaches the rated RPM

A DOL starter should be used only in instances when the inrush current does not cause significant voltage drop in the supply. If voltage drop is a problem – it is best to consider star delta, variable frequency drives or soft starters.
DOL’s are mostly used to start up smaller sized motor – specifically 3 phase squirrel cage induction motors (usually limited to around 11KW motor size maximum)

If larger motors are connected direct on line, they would not start up smoothly, as the motor would not get accelerated as quickly as a smaller motor, since the larger motor has a larger axial length and diameter and rotor.

A DOL starter contains 3 x Normally Open contacts that connect the motor to supply, and the fourth contact is the auxiliary contact, which, when energized, energizes the contactor coil after the start button is released, to keep the three supply contacts in place.

If there is a problem or a fault, the, the auxiliary coil will get de-energized, and the starter will disconnect from the supply, stopping the motor

Overload Protection
When a motor draws excessive current to meet the load requirements such that this load requirement goes beyond the rated limit, this is known as overload. Thermal overload is installed as standard on all our DOL starters, which is a kind of failsafe that occurs when the motor draws excessive load or current, which cause overheating of the motor and the switchgear.

Overload protection is important, as overloads will likely damager the equipment if they persist over and extended period of time.

For most motors, the maximum rating of the overload protection device should be set at 125% of the full load ampere rating of the motor – which should be indicated on the motors specification plate.

Of the stop button is pressed, the current through the supply contact is interrupted, and the similar thing will happen when the overload relay operates. Since the supply of motor breaks, the motor will stop.

Advantages of DOL Starter

1. Simple & cost effective
2. Easy to design, operate and control
3. Nearly full torque availability at startup
4. It connects the supply to the delta winding of the motor

Disadvantages of DOL Starter

1. High starting current (up 8 times of full load).
2. They cause  significant dips in voltage – so only useable on small motors
3. Reduced motor lifespan
4. High starting torque is usually unnecessary

DOL Starter Applications
DOL starters are usually used for motors when high inrush current doesn’t cause significant voltage drop in the supply
DOL’s are primarily used for starting small water pumps, conveyor belts, fans, and compressors, hammer mills and stone mills.