Starter and Its Necessity

 

A. Starter and Its Necessity

A starter is a device used to safely start and control an electric motor. When an induction motor is switched directly to supply, it draws a very high current (5–7 times the full load current). This can:

  • Damage motor windings due to overheating.

  • Cause a sudden voltage drop in the supply system.

  • Stress mechanical parts due to abrupt torque.

Therefore, starters are necessary to:

  • Limit the starting current.

  • Provide smooth acceleration of the motor.

  • Protect the motor against overloads and faults.

  • Ensure longer motor life and reliable operation.

 B. Types of Starters

1. Direct-On-Line (DOL) Starter

  • Working: Connects the motor directly to the supply.

  • Features: Simple, inexpensive, compact.

  • Use: Suitable for small motors (up to ~5 HP).

  • Limitation: High starting current, not suitable for large motors.

2. Star-Delta Starter

  • Working: Motor starts in star connection (voltage per phase reduced to 1/√3), then switches to delta connection for normal running.

  • Features: Reduces starting current to one-third of DOL.

  • Use: Common for medium to large motors.

  • Limitation: Only applicable to motors designed for delta running.

3. Autotransformer Starter

  • Working: Uses an autotransformer to reduce voltage during starting, then gradually increases to full supply.

  • Features: Smooth voltage control, reduced starting current.

  • Use: Large motors requiring controlled acceleration.

  • Limitation: More expensive and bulky compared to star-delta.

4. Rotor Resistance Starter (for Slip-Ring Induction Motors)

  • Working: External resistances are inserted into the rotor circuit during starting. Resistance is gradually reduced as the motor accelerates.

  • Features: Provides high starting torque with limited current.

  • Use: Slip-ring induction motors in heavy-duty applications (cranes, hoists).

  • Limitation: Requires slip-ring motors, not applicable to squirrel-cage motors.

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