1️⃣ Introduce additional leakage reactance
2️⃣ Eliminate slot harmonics
Which of the statements given above is/are correct?
📋 Options:
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
✅ Answer (Detailed Solution Below)
✔️ Option (c): Both 1 and 2
📖 Detailed Solution
🔹 In squirrel-cage induction motors, rotor slots are skewed.
🔹 Skewing means rotor slots are inclined to the shaft axis.
🔹 This prevents direct alignment with stator slots.
🔹 It improves air-gap flux distribution.
Effect 1: Introduction of Leakage Reactance
🔹 Skewing increases effective rotor bar length.
🔹 Magnetic coupling between stator and rotor reduces.
🔹 Leakage flux increases.
🔹 Leakage reactance increases.
🔹 Higher leakage reactance reduces starting and maximum torque.
Effect 2: Elimination of Slot Harmonics
🔹 Slot harmonics are caused by slotting of stator and rotor.
🔹 These harmonics produce parasitic torques.
🔹 They cause:
- Cogging
- Crawling
- Noise
- Torque pulsations
🔹 Skewing averages harmonic effects along rotor length.
🔹 Air-gap reluctance becomes uniform.
🔹 Slot harmonics are reduced or eliminated.
🔹 Motor runs smoothly and quietly.
👉 Therefore, both statements are correct.
⭐ Major Effects of Rotor Skewing
✔️ Increases leakage reactance
✔️ Reduces slot harmonics
✔️ Prevents cogging
✔️ Reduces crawling
✔️ Reduces noise
✔️ Improves torque smoothness
❌ Why Other Options are Incorrect
(a) 1 Only ❌
🔹 Skewing also reduces slot harmonics.
(b) 2 Only ❌
🔹 Skewing also increases leakage reactance.
(d) Neither 1 nor 2 ❌
🔹 Both effects occur in practice.
📈 Impact on Motor Performance
Parameter Effect of Skewing Leakage Reactance Increases Slot Harmonics Reduces Noise Reduces Starting Torque Decreases Maximum Torque Decreases Smoothness Improves
| Parameter | Effect of Skewing |
|---|---|
| Leakage Reactance | Increases |
| Slot Harmonics | Reduces |
| Noise | Reduces |
| Starting Torque | Decreases |
| Maximum Torque | Decreases |
| Smoothness | Improves |
📌 Additional Information
🔹 Uniform air-gap reluctance → Uniform torque
🔹 Reduced tooth harmonics → Less vibration
🔹 Reduced hum → Quiet operation
📌 Key Exam Notes
⭐ Skewing → Leakage reactance ↑
⭐ Skewing → Slot harmonics ↓
⭐ Skewing → Cogging ↓
⭐ Skewing → Crawling ↓
📝 Final Summary
📍 Skewing of rotor slots increases leakage reactance and reduces slot harmonics, thereby improving smoothness and reducing noise in induction motors.
👉 Hence, Option (c) is Correct.