All About Working Principles and Applications of DOL Starters


Direct On-Line starter (DOL) or across the line starter is a technique of starting a 3-phase induction motor in an electrical system. In an induction motor, a DOL Starter directly connects it to its 3-phase supply, where the DOL starter extends the total line voltage to the terminals of the motor on the electrical system. 

In spite of this direct connection, the motor stays unharmed or damaged. A DOL starter includes protection devices and sometimes a condition monitoring device as well.

Working Principle Of DOL Starter

The working principle of a DOL starter begins with a direct connection to the motor’s main 3-Phase. The control circuit of the DOL starter is connected to two specific phases and only gets energized from them. 

When you press the start button, the current starts to flow through the contactor coil, also known as the magnetizing coil, and passes through the control circuit. This current then energizes the contactor coils and results in closing the contacts, thus making the 3-phase supply open to the motor. 

Similarly, when you press the stop button, the current that was flowing through the contact gets cut-off, which stops the current supply to the motor and is similar to how the overload relay operates. 

When the current supply to the motor breaks, the machine shuts down usually. The contactor coil continues to receive supply even when you release the start button as it still gets current supply from the primary contacts of the DOL starter.

Motor Starter with Overload Protection

When a motor starts to draw too much current to match the load requirements, where this load requirement extends beyond the graded limit, then it is known as overload, and it is a type of over current. 

Overload protection enables circuits to open up at relatively low currents, which are slightly higher than the graded rating of the motor. If overload currents continue to occur for extended durations, damage is likely to happen. But with a DOL starter, the equipment’s circuit won’t trip when a high amount of current flows for a short period, such as when you start a motor.

Features Of DOL Starters

For enabling protection to the equipment, overload relays are used as it limits the amount of current that gets drawn into the circuit. However, it doesn’t necessarily mean that overload relays can protect the device against a short circuit. This is where a fuse or an enclosed DOL starter is employed to protect the system against over current. 

If you want to protect your induction motors against potential electrical damages, and you need DOL starters, then lookout for the following features:

  • DOL starters up to 45 kW that is non-reversing or reversing (with or without isolation)
  • Star-delta starters up to 132 kW (with or without isolation)
  • Several variations can be requested based on individual requirements
  • Level of protection as per IEC 529: IP55 or 65 (Depends on the model)

Applications Of DOL Starters

Mainly, DOL starters are used for induction and electrical motors, in which a high inflow of current does not cause an unwarranted or sudden voltage drop in the supply circuit of the motor. 

DOL starters are generally used to start water stations, conveyor belts grids, and compressors. If it is an asynchronous motor (like a 3-phase motor), then the motor draws a high level of starting current until it is operating on full speed.

Some applications where DOL Starters are used as an integral component of the commercial electrical system are: 

  • Industrial Usage
  • Infrastructure networks
  • Buildings (Including Smart Buildings)
  • Automotive sector
  • DOL starting and stopping of a motor

You will find a wide-ranging variety of enclosed soft-start and DOL starters available in the market. Still, it is suggested that you consult with a licensed professional and find out what suits your requirements the best. You will find simple to use DOL starters that are easy to install, are compact, and designed in such a way that the DOL starters can be mounted on walls or placed remotely on a machine. Find the best protection for your motors and equipment today!