Power Distribution and Management

Everything you need to know about Direct On Line (DOL) Motor Starter

Different starting methods are required just to start an induction motor as they draw more starting current. In order to prevent any kind of damage to the winding due to the high current flow, various kinds of starters are employed.

One of the simplest forms of motor starter that is used for induction motors is the Direct On Line starter. It consists of an MCCB or Circuit Breakers, Contactor and an overload relay for protection. It has a very specific contactor which is electromagnetic, and it can be opened by the thermal overload relay, in case of a mishap.

Usually, the contactor is controlled by a different start and stop buttons, and then an auxiliary contact on the contactor is used which is placed across the start button, as to hold in contact.

Now Let Us Talk About The Principle of Direct On Line Starter (DOL)

So to begin with, the contactor is closed, applying full line voltage towards the motor windings. So first the motor will extract a very high inrush current for a brief period of time, the magnetic field in the iron and the current will be limited to the locked rotor current that is present in the motor.

In the next step, the motor will develop locked rotor torque and commence to accelerate towards full speed. And, as they do that the current will begin to drop, but it won’t drop significantly until the motor is at high speed, typically around 85% of synchronous speed. One of the functions of a motor design is the starting current curve and the terminal voltage, and it is also completely independent of the motor load.

Different Parts of DOL Starters

DOL part – Contactor

The magnetic contactors that are present act as electromagnetically operated switches that provide a safe and convenient means for connecting branch circuits. They also use electromagnetic energy to close switches. The electromagnet consists of a coil of wire that is placed on the iron core.

So what happens here is, when the current flows through the coil, the iron of the magnet becomes magnetized and attracts the iron bar that is known as the armature. When the interruption of the current flows through the coil of wire, it causes the armature to drop out because of the presence of an air gap in the magnetic circuit.

Line-voltage magnetic motors are electromechanical appliances which provide a safe, economic, and convenient means for starting and stopping motors and also have the advantage of being controlled remotely.

The main objective of the contactor is to control the machinery that uses electric motors. It consists of a coil that is connected to the voltage sounds and quite often for the single-phase motors, 230 V coils are used and for three-phase, 415V coils.

Overload Relay (Overload protection)

When talking about overload protection, it is there for the electric motor and helps prevent burnout and to ensure maximum operating life. Due to overload, a motor draws excessive current that leads to overheating.

As you might be aware that as the motor winding insulation deteriorates because of overheating, there are established limits on the motor operating temperature that stops a situation like this.

You can further divide the overload relays into three parts:

  1. Thermal Relay: As the name suggests, thermal overload relays depend on the rising temperature, which is caused by the overload current to trip, and it can be further bifurcated into melting alloy and bimetallic.
  2. Magnetic Relay: They only react to current excesses and are not affected by the temperature.
  3. Electronic Relay: This kind of electric relay comes as a combination of the high-speed trip, adjustability, and ease of installation. They are ideal in many precise applications.


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