1️⃣ Increase the strength of rotor bars
2️⃣ Reduce the magnetic hum and locking tendency of rotor
3️⃣ Improve cooling of rotor
4️⃣ Increase motor speed
✅ Answer (Detailed Solution Below)
✔️ Option 2: Reduce the magnetic hum and locking tendency of rotor
📖 Detailed Solution
🔹 Rotor skewing means rotor slots are inclined to the shaft axis.
🔹 Due to skewing, rotor bars do not align directly with stator slots.
🔹 This prevents magnetic locking (cogging).
🔹 Air-gap flux becomes more uniform.
🔹 Harmonic torque reduces.
🔹 Magnetic noise (hum) decreases.
🔹 Motor runs smoothly and quietly.
👉 Therefore, skewing is done to reduce hum and locking tendency.
⚙️ What is Rotor Skewing?
🔹 Rotor slots are inclined.
🔹 Not parallel to shaft.
🔹 Bars lie under different stator teeth.
🔹 Improves electromagnetic performance.
⭐ Functions of Skewed Rotor Slots
✔️ Prevents cogging
✔️ Reduces magnetic hum
✔️ Improves smooth running
✔️ Reduces harmonic torque
✔️ Makes air-gap flux uniform
✔️ Improves starting performance
📈 Additional Advantages
1️⃣ Increase in Rotor Resistance
🔹 Bar length increases.
🔹 Resistance increases slightly.
🔹 Starting torque improves.
🔹 Starting current reduces.
2️⃣ Harmonic Elimination
Where:
α = Skew angle (mechanical)
n = Harmonic order
P = Number of poles
❌ Why Other Options are Incorrect
(1) Increase Strength of Rotor Bars ❌
🔹 Skewing does not improve mechanical strength.
(3) Improve Cooling ❌
🔹 Cooling is done by fan and ventilation system.
(4) Increase Motor Speed ❌
🔹 Speed depends on supply frequency and poles.
🔹 Skewing does not change speed.
📌 Key Exam Notes
⭐ Skewing → Prevents cogging
⭐ Skewing → Reduces noise
⭐ Skewing → Improves torque smoothness
⭐ Skewing → Better starting
📝 Final Summary
📍 Rotor slots are skewed in squirrel-cage induction motors to reduce magnetic hum and prevent locking by improving flux distribution.
👉 Hence, Option 2 is Correct.