1️⃣ Improve efficiency
2️⃣ Improve power factor
3️⃣ Reduce machine cost
4️⃣ None of the above
✅ Answer (Detailed Solution Below)
✔️ Option 2: Improve power factor
📖 Detailed Solution
🔹 The air gap contributes maximum reluctance in the magnetic circuit.
🔹 To establish air gap flux, magnetizing current is required.
🔹 Higher flux density requires higher magnetizing current.
🔹 Magnetizing current is reactive in nature.
🔹 High reactive current increases phase angle.
🔹 Large phase angle reduces power factor.
🔹 When air gap flux density is kept low:
Required magnetizing current decreases.
Reactive current reduces.
Phase angle decreases.
Power factor improves.
👉 Hence, low air gap flux density improves power factor.
⚙️ Effect of Low Air Gap Flux Density
✔️ Magnetizing current ↓
✔️ Reactive current ↓
✔️ Phase angle ↓
✔️ Power factor ↑
✔️ Stable operation
📈 If Air Gap is Increased
🔹 Permeability decreases.
🔹 Magnetizing inductance decreases.
🔹 Magnetizing current increases.
🔹 Power factor becomes poor.
🔹 Leakage flux increases.
🔹 Maximum torque reduces.
❌ Why Other Options are Incorrect
(1) Improve Efficiency ❌
🔹 Efficiency may improve indirectly.
🔹 But main objective is power factor improvement.
(3) Reduce Machine Cost ❌
🔹 Lower flux density may increase size.
🔹 Does not mainly reduce cost.
(4) None of the Above ❌
🔹 Power factor improvement is correct.
📌 Additional Information
🔹 Magnetizing current dominates at light load.
🔹 Poor power factor causes:
Higher line current
More losses
Poor system performance
🔹 Designers aim to minimize Im.
📌 Key Exam Notes
⭐ Low flux density → Low Im
⭐ Low Im → High PF
⭐ High PF → Better system efficiency
⭐ Air gap design → Critical
📝 Final Summary
📍 Air gap flux density is kept low to reduce magnetizing current and improve power factor.
👉 Hence, Option 2 is Correct.