Direct Online (DOL) Starter

Direct Online (DOL) Starter – Working, Circuit Diagram, Advantages & Disadvantages

🔹 What is a Direct Online (DOL) Starter?

A Direct Online Starter (DOL Starter) is the simplest and most commonly used method to start induction motors. In this method, the motor is directly connected to the full line voltage. Because of this, it draws 6–8 times the rated current at startup.

👉 DOL starters are generally used for motors up to 7.5 kW (10 HP), where the inrush current can be tolerated by the electrical system.

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🔹 Main Parts of a DOL Starter

A basic DOL starter consists of the following components:

1. Electromagnetic Contactor – Connects/disconnects supply to motor.

2. Overload Relay (O/L Relay) – Protects the motor from overcurrent and overheating.

3. Main MCB – Provides protection against short-circuits and faults.

4. ON/OFF Push Buttons – Used to manually start and stop the motor.

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🔹 Circuit Diagrams of DOL Starter

1. Power Circuit Diagram

The Power Circuit shows how the 3-phase supply is connected to the motor through the contactor and overload relay.

• The overload relay is placed nearest to the motor to ensure quick tripping during abnormal conditions.
  

  

  DOL Power Circuit
  
  

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2. Control Circuit Diagram

The Control Circuit governs the operation of the contactor using push buttons and overload relay.

• The Stop Push Button (NC) and Overload Relay NC are in series.

• The Start Push Button (NO) energizes the contactor coil.

• The contactor’s auxiliary contacts (13-14) provide the hold-on feature.

(Diagram: DOL Control Circuit)

Direct Online Starter Control Circuit

👉 Depending on the contactor coil rating, the control circuit can be 220V or 440V.

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🔹 Working Principle of DOL Starter

1. When the Start Button is pressed, current flows through O/L relay NC → Stop PB NC → Start PB → Contactor coil.

2. The contactor closes, supplying full line voltage to the motor.

3. The auxiliary contact (13-14) holds the circuit, so the motor keeps running even after the Start button is released.

4. Pressing the Stop Button or tripping of the Overload Relay de-energizes the contactor, stopping the motor.

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🔹 Advantages of DOL Starter

• ✅ Simple design, easy to operate.

• ✅ Low initial and maintenance cost.

• ✅ Compact and reliable for small motors.

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🔹 Disadvantages of DOL Starter

• ❌ High inrush current (6–8 times rated current).

• ❌ Causes voltage dips in supply.

• ❌ Leads to thermal and mechanical stress on motor windings.

• ❌ Suitable only for motors ≤ 7.5 kW.

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🔹 Applications of DOL Starter

• Pumps

• Fans & blowers

• Compressors

• Small conveyors

• Workshop machinery

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✅ Pro Tip: DOL starters can be enhanced with features like indicators, remote start/stop, limit switches, reverse-forward control, and level switches for advanced applications.

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🔹 Comparison with Other Starters

• Star-Delta Starter → Used for motors above 7.5 kW, reduces starting current.

• Auto-Transformer Starter → Provides reduced voltage starting for large motors.

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📌 Conclusion

The DOL Starter remains the most economical, simple, and widely used starter for low-rating induction motors. However, for large motors, advanced starters like Star-Delta or Soft Starters are preferred to avoid voltage dips and mechanical stress.