1️⃣ Reduce the chance of crawling
2️⃣ Increase the starting torque
3️⃣ Reduce the chance of cogging
4️⃣ Reduce the magnetizing current
✅ Answer (Detailed Solution Below)
✔️ Option 4: Reduce the magnetizing current
📖 Detailed Solution
🔹 The air gap increases the reluctance of the magnetic circuit.
🔹 Higher reluctance requires more magnetomotive force (MMF).
🔹 More MMF means higher magnetizing current.
🔹 When air gap is reduced:
Reluctance decreases.
Permeability increases.
Required MMF reduces.
Magnetizing current decreases.
🔹 Lower magnetizing current improves power factor.
🔹 Efficiency also improves.
👉 Hence, a smaller air gap reduces magnetizing current.
⚙️ Effect of Small Air Gap
✔️ Lower reluctance
✔️ Lower magnetizing current
✔️ Better power factor
✔️ Higher efficiency
✔️ Improved magnetic coupling
📈 If Air Gap is Increased
🔹 Permeability decreases.
🔹 Magnetizing inductance decreases.
🔹 Leakage reactance increases.
🔹 Leakage flux increases.
🔹 Magnetizing current increases.
🔹 Power factor becomes poor.
❌ Why Other Options are Incorrect
(1) Reduce Chance of Crawling ❌
🔹 Crawling is caused by harmonics.
🔹 Reduced mainly by rotor slot skewing.
(2) Increase Starting Torque ❌
🔹 Slight effect only.
🔹 Starting torque mainly depends on rotor resistance.
(3) Reduce Chance of Cogging ❌
🔹 Smaller air gap increases flux density.
🔹 May increase cogging tendency.
🔹 Prevented by skewing slots.
📌 Additional Information
Crawling
🔹 Occurs near 1/7th synchronous speed.
🔹 Due to harmonic torque.
Cogging
🔹 Magnetic locking of rotor.
🔹 Due to slot harmonics.
👉 Both are reduced by slot skewing, not air gap reduction.
📌 Key Exam Notes
⭐ Small air gap → Low Im
⭐ Large air gap → High Im
⭐ Power factor → Improves with small air gap
⭐ Efficiency → Improves
📝 Final Summary
📍 A smaller air gap reduces magnetic reluctance, lowers magnetizing current, and improves power factor and efficiency.
👉 Hence, Option 4 is Correct.