What is DOL Starter? Working Principle and Connection

The DOL Starter is a crucial component in the electrical power distribution system for industrial and commercial applications. It is responsible for starting large electric motors in a controlled manner ensuring that the power supply to the motor is regulated and protected from overloading and short-circuiting. The working principle, connection and basics of the DOL Starter are essential for anyone involved in the design, maintenance and operation of electrical power systems. In this article, we will explore the key elements of the DOL Starter, including its function, construction and how it is connected to the power supply and motor.

What is DOL starter?

The DOL starter (Direct Online Starter) is a type of motor starter that connects the electric motor directly to the line voltage. It is the simplest and cheapest method to start an induction motor. In the DOL start, the terminals of the motor are connected to the power supply through a contactor and overload protection device.

It is also known as a direct online starter or across-the-line starter.

Although the DOL starter is the most common starting method available on the market, it is important to note that it has some restrictions. The starting current of an induction motor is 6 to 8 times greater than the rated current when directly connected to the power supply voltage. The control of current and torque is not possible. For this reason, large power motors cannot be able to start with this method.

Direct online (DOL) is the direct connection of the terminal voltage to the motor stator with no additional components. The DOL starter is most commonly used for small motors. Because the startup of a small motor has a low impact on the system and causes less voltage drop.

It must be taken into account that for a certain motor the torque and current values are fixed, irrespective of the load, for a constant voltage. In cases where the motor starting current is excessively high, harmful consequences may occur. If DOL starting is not possible due to these problems, an indirect connection system can be used to reduce starting current such as star delta starter, soft starter and variable frequency drive starter.

Direct online starters are used on a large scale due to their list of beneficial features. The following are some features of DOLs:

• Start and stop the motor.
• Provide the necessary overload and short circuit protection for the motor.
• Facilitate remote on/off control feature.
• Allow you to switch the current rapidly.
• Isolate the motor from the main supply.

DOL Starter Working Principle

The controller of the DOL starter is typically a simple push-button (but could be a selector switch, limit switch, float switch, etc.). After pushing the start button, the coil of the contactor is energized and the contactor closes. This provides the supply current to the motor. To turn the motor off, a stop button is provided.

To protect the motor from overcurrent, the control circuit is wired through a normally closed auxiliary contact of the overload relay. When the overload relay trips, the normally closed auxiliary contact opens and de-energizes the contactor coil and the contactor main contacts open.

DOL starter connection

Below you can see the power and control circuit diagram of the DOL starter.

Features of DOL starter

• It has the shortest starting time.
• It is a cheap solution compared to star-delta and soft starter.
• It is very easy to implement.
• Troubleshooting is very easy.
• Provides 100% torque at starting.
• It is compact in size.
• High starting (inrush) current causes a voltage drop in the power supply which can cause interference in other equipment connected to the same installation.
• It has a high starting torque. It could cause heavy wear & tear on couplings, gearboxes, etc.
• The only possible way to stop the motor is to make a direct stop. The direct stop may not be preferred in pump and conveyor systems.
• Magnetization peaks can occur. (up to 20 times the rated current)
• It could oversize cables and contactors compared to other starting methods.
• Limitation in the number of starts/hours.
• Only suitable for motors of rating less than 5…10 kW

DOL starter components

In a DOL starter circuit, you need the below components:

Control device

• Contactors

Overcurrent protection devices

• Overload relays
• Motor protection circuit breakers
• Molded case or air circuit breakers
• Switch disconnectors

Monitoring and auxiliary devices

• Pilot devices (push buttons, pilot lamps, selector switches, etc..)
• Control transformers
• Measurement instruments (ammeters, voltmeters, etc..)

Applications of DOL starter

DOL starter is very useful in motors that have low power ratings. It is used where maximum currents do not cause any damage such as to run small compressors, conveyors, pumps, fans, etc.