REPORT ON DIRECT-ON-LINE (DOL) STARTER FOR 3-PHASE MOTOR
DECLARATION
I hereby declare that the report titled "Control Diagram and Working of a 3-Phase Motor
Using Direct-On-Line (DOL) Starter" is the result of my own work. All information presented
in this report has been derived from authentic references and personal understanding of
the electrical control systems. I confirm that this work has not been submitted, in whole or
in part, for any other purpose.
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APPROVAL
This is to certify that the report titled "Control Diagram and Working of a 3-Phase Motor
Using DOL Starter" has been examined and approved as meeting the requirements for
submission.
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ACKNOWLEDGMENT
I would like to express my sincere gratitude to my instructor/supervisor for their
continuous support and guidance during the preparation of this report. Special thanks to
my peers, laboratory staff, and everyone who contributed to my understanding of motor
control systems. Their invaluable insights have helped me complete this report successfully.
OVERVIEW
A Direct-On-Line (DOL) Starter is one of the simplest methods to start a 3-phase induction
motor by applying the full line voltage directly to the motor terminals. This report presents
a comprehensive control diagram of the DOL starter, explaining its components, working
principle, wiring procedure, and operational characteristics. The DOL starter is widely used
due to its simplicity, ease of use, and effectiveness in starting motors under moderate load
conditions.
OBJECTIVES
- To study and understand the control diagram for starting and stopping a 3-phase
induction motor using a DOL starter.
- To explain the function of each component in the DOL starter control and power circuits.
- To demonstrate the step-by-step wiring procedure for connecting the DOL starter.
- To highlight the operational sequence for starting, running, and stopping the motor.
- To ensure awareness of safety precautions and best practices when handling electrical
control systems.
- To provide practical knowledge of industrial motor control systems for real-world
applications.
COMPONENTS USED
- MCB (Miniature Circuit Breaker)
- O/L (Overload Relay)
- Contactor (K1) with Main and Auxiliary Contacts
- Start (ON) Push Button (NO)
- Stop (OFF) Push Button (NC)
- Indicator Lamp
- 3-Phase Induction Motor
DETAILED EXPLANATION
**Power Connections**
1. Connect the 3-phase power supply lines (L1, L2, L3) to the input terminals of the MCB.
2. From the MCB output, connect to the input of the O/L relay.
3. Connect the output terminals of the O/L relay to the motor terminals (U, V, W).
**Control Circuit Connections**
1. Connect a control voltage supply (typically 230V AC or 24V DC) between live (L) and
neutral (N).
2. Connect the live line through: MCB → O/L (95-96) → OFF button (NC) → ON button (NO)
→ Contactor coil A1.
3. Connect A2 of the coil to neutral (N).
4. Connect the auxiliary holding contact (K1) in parallel with the ON button to maintain coil
energization.
5. Connect the indicator lamp in parallel with the contactor coil to indicate motor status.
**Operation**
- **Starting**: Press the ON button to energize the contactor coil, closing the main and
auxiliary contacts, starting the motor, and illuminating the indicator.
- **Stopping**: Press the OFF button to de-energize the contactor coil, stopping the motor,
and turning off the indicator.
- **Overload Protection**: The O/L relay will trip in case of excessive current, breaking the
control circuit and stopping the motor.
SAFETY CONSIDERATIONS
- Always isolate power supply before wiring.
- Properly earth the motor and starter components.
- Use rated MCBs and O/L relays matching motor capacity.
- Check phase sequence for correct motor rotation.
- Test control circuit with the motor disconnected initially.
APPLICATIONS
Common in pumps, compressors, fans, and conveyors.
ADVANTAGES
Simple and cost-effective.
Easy installation and maintenance.
LIMITATIONS
High starting current (6-8 times FLC).
Mechanical and electrical stress on motor and load.
CONCLUSION
This report outlines the practical implementation of a Direct-On-Line Starter, covering its
working principle, wiring procedure, and operational sequence. The DOL starter serves as a
fundamental building block in electrical engineering and industrial motor control, offering
simplicity, efficiency, and reliability.
