Understanding the Basics of DOL Starter Control Diagram and Power Connection

A Direct-On-Line (DOL) starter is a type of motor starter that is commonly used in industrial and commercial settings. It is a simple and cost-effective way to start and control a motor, making it a popular choice for many applications. In this article, we will discuss the basics of a DOL starter control diagram and how it works.

What is a DOL Starter?

A DOL starter is a type of motor starter that is used to start and control the speed of an AC induction motor. It is called a “direct-on-line” starter because it directly connects the motor to the power supply without any intermediate devices. This means that the motor receives full voltage from the power supply, allowing it to start and run at full speed.

DOL Starter Control Diagram

Images shows control circuit diagram wiring that connects all the parts like overload relay, contactor, MCB, indication lamps of the DOL starter.
DOL Starter Control Wiring connection showing Push buttons,thermal overload relay

A DOL starter has two circuits, a control circuit that is responsible for controlling the starter’s operation and the other is a power circuit that connects the motor to a three-phase power supply.

The control circuit is the wiring that connects all the components of the DOL starter. It includes the power supply, the contactor, the overload relay, and the start and stop buttons. The control circuit is responsible for controlling the operation of the motor and ensuring its safe and efficient operation.

DOL starter directly applies full line voltage to the motor because of this, the motor draws a large starting current which is 6 to 7 times the full load current.

Start Push Button: this push button is normally NO type and is particularly used to start the induction motor.

Stop Push Button: The stop push button is Normally NC type element that De-energizes the contactor coil when it is pressed, disconnecting the motor from the line.

Power contactors NO connected across the start push button that acts as a holding contact to keep the contactor latched.

As shown in the dol starter control diagram overload relays NC contact connected in series to isolate the three-phase induction motor from the supply in an overload condition.

How Does a DOL Starter Control Diagram Work?

  • When the Start button is pressed, the control circuit energizes the contactor coil.
  • The contactor closes its main contacts, directly connecting the motor to the line voltage.
  • The motor starts running, drawing current from the supply.
  • The OLR continuously monitors the motor current.
  • If the current exceeds the safe limit for an extended period, the OLR trips, de-energizing the contactor coil and disconnecting the motor.
  • The Stop button can be pressed at any time to de-energize the contactor and stop the motor.

DOL Starter Power Circuit Diagram

Dol starter power circuit Diagram with MCB and contactor connection

Miniature Circuit Breaker (MCB): Protects the entire circuit from overcurrents and short circuits.

Contactor: The Contactor is the main component of the DOL starter. It is an electrically controlled switch, it is responsible for the direct electrical flow. When the Contactor’s coil is electrically charged, the magnetic field generated pulls in the contactor’s switch, subsequently making a connection between the incoming power supply and the motor.

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The power circuit consists of a contactor which gets latched as we press the start button and connect the motor to the power supply.

Thermal Overload Relay (OLR): The thermal overload relay is connected to a power contactor for protection. An overload relay has a bimetallic Strip that gets bent when excessive fault current flows through it.

The power circuit Diagram consists of an MCB or fuse unit which is connected in series with the supply. Power contactor, thermal overload relay for protection.

Overload Relay, depicted as a series element in the control circuit. When a motor draws extreme current, possibly due to overload or fault, the Overload Relay trips—breaking the control circuit and disengaging the Contactor. This defensive automation prevents damage to the motor winding.

Troubleshooting DOL Starter Common Issues

The motor doesn’t start when the start button is pushed.

The initial step in troubleshooting is confirming if the circuit power is on. If the power supply is functional, the next step is to check the control circuit. Verify the healthiness of the start and stop Push buttons. Sometimes, a faulty button might break the power flow. If both buttons are working fine, check the condition of the Contactor. If there are visible damages or if it doesn’t engage when the start button is pushed, a replacement may be necessary.

The motor does not stop when the stop button is pressed.

In such an issue, the immediate suspect is the stop button. However, the Contactor may also cause this problem. An issue with the Contactor can cause it to keep the circuit closed, hence keeping the motor running even when the stop button is actuated.

The Circuit Breaker trips frequently.

If the circuit breaker trips frequently, it points out the excessive current going through the circuit. The primary suspect here should be the motor. Motors tend to draw more current when they are near the end of their life cycle or when they are operating under high load. Hence, inspect the motor for any abnormalities.

Frequent tripping of the Overload Relay.

The main cause of frequent tripping of an overload relay is an overload of the motor, which is a common problem in motor starters. To troubleshoot this problem, check the setting of the overload relay, ensuring it’s not set too low.

Investigate whether the motor is overloaded or not. If everything seems normal, consider a mechanical problem or a short circuit within the motor windings.


A DOL starter control diagram is a simple and effective way to start and control an AC induction motor. It consists of several components that work together to ensure the safe and efficient operation of the motor. By understanding the basics of a DOL starter control diagram, you can better understand how this type of motor starter works and how it can be used in various applications.

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