Describe and start a compound motor using a four-point starter with a diagram. (50 Marks)

Introduction

A compound motor is a type of DC motor that has both series and shunt field windings. It provides high starting torque and good speed regulation. To safely start a compound motor, a four-point starter is used to limit the starting current and protect the motor.

Definition

A compound motor is a DC motor that combines both series and shunt field windings to achieve advantages of both types of motors.

Types of Compound Motor

• Cumulative Compound Motor
• Differential Compound Motor

Four-Point Starter

A four-point starter is used to start DC motors safely by limiting the initial current. It has four terminals: Line (L), Armature (A), Field (F), and an additional terminal (N) for holding coil.

Main Parts of Four-Point Starter

• Starting Resistance
• Handle
• Overload Release (OLR)
• No-Voltage Coil (NVC)

Working Principle

When the starter handle is moved from OFF to ON position, resistance is gradually removed from the armature circuit. This limits the starting current. The no-voltage coil holds the handle in position and releases it in case of supply failure. The overload relay protects the motor from excessive current.

Procedure to Start Compound Motor

1. Check all connections properly.
2. Ensure supply voltage is correct.
3. Keep starter handle at OFF position.
4. Switch ON the main supply.
5. Slowly move starter handle to ON position.
6. Observe motor starting smoothly.
7. Ensure motor runs at rated speed.

Diagram (Text Representation)

        L ---- Starter ---- A (Armature)
         |                |
         |                |-- Series Field
         |                |
         |                |-- Shunt Field
         |                |
         N ---------------
    

Advantages

• High starting torque
• Good speed regulation
• Protection against overload

Applications

• Lifts
• Cranes
• Rolling mills

Precautions

• Do not start motor without starter
• Check field connections
• Do not overload motor

Conclusion

The compound motor was successfully started using a four-point starter. The starter provides safe operation and protection to the motor.