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SarkariResultOutBMS · 2024

Previous Year Question

A small air-gap in a three-phase induction motor helps in
  1. reducing the chances of crawling
  2. increasing the starting torque
  3. reducing the chances of cogging
  4. reducing the magnetizing current

Correct answer: D

Explanation

A smaller air gap reduces magnetic reluctance, so less magnetizing current is required to establish the working flux. This improves power factor and reduces magnetizing reactive demand, although the gap cannot be made arbitrarily small because of mechanical clearance and harmonic/unbalanced-pull considerations. The listed benefit corresponds to option D.

Formula

Reluctance Rm=l/(μA); smaller air gap → lower magnetizing mmf/current

Step-by-step solution

Effect of Small Air Gap explanatory diagram

Correct Answer

D — reducing the magnetizing current

Concept and Reasoning

A smaller air gap reduces magnetic reluctance, so less magnetizing current is required to establish the working flux. This improves power factor and reduces magnetizing reactive demand, although the gap cannot be made arbitrarily small because of mechanical clearance and harmonic/unbalanced-pull considerations. The listed benefit corresponds to option D.

Calculation / Key Relation

Reluctance Rm=l/(μA)

smaller air gap → lower magnetizing mmf/current

Exam Takeaway

Small gap mainly reduces magnetizing current.

Common Mistake

Do not ignore required mechanical clearance between stator and rotor.

Common mistake

Do not ignore required mechanical clearance between stator and rotor.

Exam tip

Small gap mainly reduces magnetizing current.

Quick method

Small gap mainly reduces magnetizing current.