A 3-phase induction motor is an electric motor that uses three alternating currents (AC) to generate a rotating magnetic field. This rotating magnetic field induces an electromotive force (EMF) in the stator, which causes the rotor to rotate and produce mechanical power. Induction motors are commonly used in industrial and commercial settings, such as pumps, fans, conveyors, and machine tools. They are also used in residential and commercial applications, such as air conditioners, refrigerators, and washing machines. Because of their simple and robust design, they are widely used in many industrial and commercial applications that require a reliable and efficient operation.

How Does a Direct Online Starter Work?


A direct online starter (DOL) controls the electrical power supply to a 3-phase induction motor. It comprises several components, including a contactor, an overload relay, and a control circuit.

  1. Contactor: The contactor is an electrically operated switch that is used to turn the power on and off to the motor. When the contactor is closed, it completes the electrical circuit and power flows to the motor, causing it to start. When the contactor is open, the circuit is broken and power is cut off to the motor, causing it to stop.
  2. Overload relay: The overload relay is a safety device that monitors the current flowing through the motor. If the current exceeds a certain level, indicating that the motor is overloaded, the overload relay will open the contactor and cut power to the motor, protecting it from damage.
  3. Control Circuit: The control circuit controls the contactor and the overload relay. It receives input from various sensors and devices, such as push buttons, timers, and programmable controllers, and uses this input to control the contactor and the overload relay.

When the control circuit receives a signal to start the motor, it closes the contactor, completing the electrical circuit and allowing power to flow to the motor. The motor starts and the control circuit monitors the current flowing through the motor to ensure it is not overloaded. If the current exceeds a certain level, indicating that the motor is overloaded, the control circuit opens the contactor and cuts power to the motor, protecting it from damage.


Construction of 3-Phase Induction Motor

It consists of two main parts:

  1. Stator 
  2. Rotor

The stator is the stationary part of the motor and it is made up of a series of stacked laminations of silicon steel. Inside the stator, there are three windings, one for each phase, that is evenly spaced and connected in a Y or delta configuration. These windings are connected to a 3-phase AC power source, which creates a rotating magnetic field within the stator.

The rotor is the rotating part of the motor and is typically made of cast iron or aluminium. Inside the rotor are two main types: squirrel cage rotor and wound rotor. A Squirrel cage rotor has a set of conductors that are shorted together at the ends, similar to a squirrel cage. The wound rotor has windings similar to the stator winding and is connected to external resistance through slip rings.

The rotor is positioned inside the stator and rotates due to the electromagnetic interaction between the stator’s magnetic field and the rotor’s conductors. As the rotor rotates, it generates mechanical power that can be used to drive loads such as pumps, fans, and conveyors.

In short, the 3-phase induction motor consists of a stationary stator and a rotating rotor, which are essential in creating the electromagnetic interaction that produces mechanical power.


Importance of a Direct Online Starter for a 3-Phase Induction Motor

A direct online starter (DOL) is a motor starter used to start and stop a 3-phase induction motor. The importance of a DOL starter for a 3-phase induction motor can be summarized in the following points:

  1. Protection: DOL starter provides protection to the motor from overloading and short-circuiting. It has built-in overcurrent protection and thermal protection that helps to prevent damage to the motor.
  2. Control: DOL starter allows for precise control over the starting and stopping of the motor. It can start and stop the motor at specific times or in response to certain conditions, improving efficiency and reducing wear and tear on the motor.
  3. Starting Torque: DOL starter provides high starting torque, which is important for applications that require high starting torque such as heavy-duty machinery.
  4. Inrush Current: DOL starter reduces the initial inrush current that occurs during the starting of the motor, this helps in reducing stress on the motor and the power supply circuit.
  5. Cost-effective: DOL starter is a cost-effective solution for starting and stopping a three-phase induction motor. It is simple and easy to install, and it does not require any additional equipment.



In conclusion, the DOL starter is a crucial device for 3-phase induction motors as it provides protection, control, high starting torque, reduced inrush current and cost-effectiveness. It helps to protect the motor from damage and prolongs its lifespan while also improving efficiency and performance. It is a simple and cost-effective solution for starting and stopping a motor and is widely used in industrial and commercial applications.DOL starter works by controlling the electrical power supply to a three-phase induction motor using contactor, overload relay and control circuit to start and stop the motor and protect it from overloading and short-circuiting.

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