1️⃣ Parasitic torque and noise but increases pull-out torque
2️⃣ Parasitic torque and noise but increases starting torque
3️⃣ Noise but increases pull-out torque and parasitic torque
4️⃣ Parasitic torque, noise, pull-out torque and starting torque
✅ Answer (Detailed Solution Below)
✔️ Option 4: Parasitic torque, noise, pull-out torque and starting torque
📖 Detailed Solution
🔹 In squirrel-cage induction motors, rotor slots are inclined to the shaft.
🔹 This inclination is called skewing.
🔹 Skewing prevents alignment of stator and rotor teeth.
🔹 Slot harmonics are suppressed.
🔹 Harmonic torques (parasitic torques) reduce.
🔹 Air-gap flux becomes more uniform.
🔹 Magnetic hum decreases.
🔹 Motor runs quietly and smoothly.
🔹 However, skewing increases:
- Rotor bar length
- Rotor resistance
- Leakage reactance
🔹 Increased leakage reactance reduces rotor current.
🔹 Reduced current lowers torque.
👉 Therefore, skewing reduces parasitic torque, noise, starting torque, and pull-out torque.
⚙️ Effects of Skewing on Motor Performance
| Parameter | Effect |
|---|---|
| Noise | Decreases |
| Parasitic Torque | Decreases |
| Starting Torque | Decreases |
| Pull-out Torque | Decreases |
| Smoothness | Improves |
🧲 Cogging (Magnetic Locking)
🔹 Occurs when stator and rotor slots align.
🔹 Magnetic reluctance becomes minimum.
🔹 Rotor gets locked.
🔹 Motor refuses to start.
Causes of Cogging
✔️ Same number of stator and rotor slots
✔️ Slot harmonics
✔️ Harmonic synchronous torque
Prevention of Cogging
✔️ Skewing rotor bars
✔️ Proper slot selection
✔️ Different stator and rotor slots
🌊 Crawling
🔹 Motor runs at ≈ 1/7 synchronous speed.
🔹 Caused by 7th harmonic field.
🔹 Produces parasitic torque.
Prevention of Crawling
✔️ Proper stator winding design
✔️ Reducing harmonics
✔️ Proper rotor slot selection
✔️ Skewing
⭐ Why Torque Reduces Due to Skewing
🔹 Skewing increases leakage reactance.
🔹 Leakage flux increases.
🔹 Main flux decreases.
🔹 Mutual coupling weakens.
🔹 Torque reduces.
❌ Why Other Options are Incorrect
(1) and (2) ❌
🔹 Skewing never increases torque.
(3) ❌
🔹 Parasitic torque always reduces.
📌 Additional Advantages of Skewing
✔️ Quiet operation
✔️ Uniform torque
✔️ Less vibration
✔️ Improved reliability
📌 Key Exam Notes
⭐ Skewing → Reduces cogging
⭐ Skewing → Reduces crawling
⭐ Skewing → Lowers torque
⭐ Skewing → Smooth operation
📝 Final Summary
📍 Skewing of rotor slots reduces parasitic torque and noise but also reduces starting and maximum torque due to increased leakage reactance.
👉 Hence, Option 4 is Correct.