🔌 What is a Three-Point Starter?

A three-point starter is a device used to safely start and stop a DC shunt motor. It also protects the motor from overloads and loss of voltage.

The name “three-point” comes from the three terminals:

• L → Line (incoming supply)
• F → Field winding
• A → Armature winding

🛠️ Main Components and Their Roles

1. Starting Arm (SR):
   • This is the handle you move to start the motor.
   • It’s spring-loaded, so if you let go, it will return to the OFF position unless something holds it in place.
2. Starting Resistance (Rs):
   • Limits the initial current to the armature. DC motors draw high current at startup, so this protects the motor.
3. Studs (Contact Points):
   • As the arm moves across these points, it slowly cuts out the resistance in steps.
   • By the time it reaches the last stud, all resistance is removed and the motor runs normally.
4. Hold-On Coil (NVR – No Voltage Release):
   • Acts like a magnet to hold the starter arm in the ON position.
   • If the power fails or the field circuit breaks, the coil de-energizes, the magnetic pull is lost, and the arm returns to OFF. This prevents damage to the motor.
5. Overload Release (OLR):
   • A protective coil connected in the line.
   • If the current exceeds safe levels (i.e., overload), it trips, disconnecting the hold-on coil and sending the starter arm back to OFF.

🔄 Step-by-Step Operation

1. Power On:
   Switch ON the main supply.
2. Move the Starter Arm (SR):
   Begin pushing the arm to the right. As it contacts stud 1:
   • The field winding gets full supply.
   • The armature gets limited current through the full starting resistance.
3. Keep Moving the Arm:
   • With each step (stud), the resistance in series with the armature reduces.
   • This allows the motor to speed up safely.
4. Final Stud (ON Position):
   • All resistance is removed.
   • The Hold-On Coil (NVR) now keeps the arm in place using the current from the field circuit.
5. Safety During Faults:
   • Power failure or field disconnection → Coil loses power → Arm springs back to OFF.
   • Overload → OLR trips → Coil deactivates → Arm goes to OFF.

⚠️ Important Design Note

As the arm moves, part of the resistance is bypassed. If the field current flows through this bypassed resistance, it may drop. So, a brass arc is often used at the first contact to ensure the field circuit gets a steady current directly, avoiding this problem.

Interactive version of this concept

Here are the advantages and disadvantages of a Three-Point Starter used in DC shunt motors:

✅ Advantages of Three-Point Starter

1. Protects the Motor During Startup:
   • Limits high inrush current to the armature by inserting resistance in series during starting.
2. No-Voltage Protection:
   • If the supply fails or drops, the Hold-On Coil de-energizes and the arm returns to OFF, preventing accidental restarting.
3. Overload Protection:
   • The Overload Release trips if the motor draws excess current, preventing overheating or damage.
4. Simple and Cost-Effective:
   • Mechanically straightforward design that is easy to install and maintain.

❌ Disadvantages of Three-Point Starter

1. Field Weakening Risk:
   • Since the Hold-On Coil is in series with the field circuit, reducing field current (e.g., for speed control) may unintentionally release the starter arm.
2. Manual Operation:
   • Requires manual engagement and monitoring; not suitable for automated systems.
3. No Reverse Protection:
   • It doesn’t prevent motor reversal or handle directional control.
4. Limited to DC Shunt Motors:
   • Not suitable for series motors or modern AC motor systems.

Would you like a comparison with a Four-Point Starter too?