1️⃣ Noise, parasitic torque, starting torque and pull-out torque
2️⃣ Noise and parasitic torque, but increases starting torque and pull-out torque
3️⃣ Noise and pull-out torque, but increases parasitic torque and starting torque
4️⃣ Noise, parasitic torque and starting torque, but increases pull-out torque
✅ Answer (Detailed Solution Below)
✔️ Option 1: Noise, parasitic torque, starting torque and pull-out torque
📖 Detailed Solution
🔹 In squirrel-cage induction motors, rotor slots are skewed.
🔹 Skewing means rotor bars are inclined to the shaft.
🔹 This prevents alignment of stator and rotor teeth.
🔹 Slot harmonics are suppressed.
🔹 Parasitic torques such as cogging and crawling are reduced.
🔹 Air-gap flux becomes more uniform.
🔹 Magnetic hum decreases.
🔹 Motor runs more quietly.
🔹 However, skewing increases:
- Rotor bar length
- Rotor resistance
- Leakage reactance
🔹 Increased leakage reactance reduces current.
🔹 Reduced current lowers torque.
👉 Therefore:
✔️ Noise reduces
✔️ Parasitic torque reduces
✔️ Starting torque reduces
✔️ Pull-out (maximum) torque reduces
⚙️ Effects of Skewing on Motor Performance
| Parameter | Effect |
|---|---|
| Noise | Decreases |
| Parasitic Torque | Decreases |
| Starting Torque | Decreases |
| Pull-out Torque | Decreases |
| Smoothness | Improves |
⭐ Why Skewing Reduces Torque
🔹 Skewing increases leakage reactance.
🔹 Leakage flux increases.
🔹 Effective air-gap flux decreases.
🔹 Mutual coupling weakens.
🔹 Torque production reduces.
❌ Why Other Options are Incorrect
(2) Increases Starting and Pull-out Torque ❌
🔹 Skewing never increases torque.
(3) Increases Parasitic Torque ❌
🔹 Skewing reduces parasitic torque.
(4) Increases Pull-out Torque ❌
🔹 Pull-out torque decreases due to reduced flux.
📈 Relation with Slot Harmonics
🔹 Slot harmonics cause:
- Torque ripple
- Noise
- Cogging
- Crawling
🔹 Skewing eliminates these harmonics.
🔹 Improves torque smoothness.
📌 Additional Information
Advantages of Skewing
✔️ Quiet operation
✔️ Smooth torque
✔️ Less vibration
✔️ Reduced crawling
✔️ Improved reliability
Disadvantages of Skewing
❌ Slight loss of torque
❌ Increased leakage reactance
📌 Key Exam Notes
⭐ Skewing → Reduces noise
⭐ Skewing → Reduces parasitic torque
⭐ Skewing → Lowers starting torque
⭐ Skewing → Lowers pull-out torque
📝 Final Summary
📍 Rotor skewing reduces noise and parasitic torque but also slightly reduces starting and maximum torque due to increased leakage reactance.
👉 Hence, Option 1 is Correct.