📝 Consider the following statements:
1️⃣ It prevents cogging
2️⃣ It produces more uniform torque
3️⃣ It increases starting torque
4️⃣ It reduces motor ‘hum’ during its operation
Select the correct answer using the codes given below.
📋 Options:
(a) 2, 3 and 4
(b) 1, 2 and 3
(c) 1, 2 and 4
(d) 1, 3 and 4
✅ Answer (Detailed Solution Below)
✔️ Option (c): 1, 2 and 4
📖 Detailed Solution
🔹 In squirrel-cage induction motors, rotor bars are skewed to improve electromagnetic performance.
🔹 Skewing means rotor slots are inclined to the shaft axis.
🔹 This prevents direct alignment of stator and rotor teeth.
🔹 Hence, cogging is eliminated.
🔹 Harmonic torque is reduced.
🔹 Torque ripple decreases.
🔹 Output torque becomes smoother.
🔹 Air-gap flux becomes more uniform.
🔹 Magnetic noise reduces.
🔹 Motor runs quietly.
👉 Therefore, statements 1, 2, and 4 are correct.
⚙️ Effects of Rotor Bar Skewing
✔️ Prevents magnetic locking (cogging)
✔️ Reduces slot harmonics
✔️ Produces uniform torque
✔️ Reduces vibration
✔️ Reduces humming noise
✔️ Improves smooth running
📈 Effect of Skewing on Starting Torque
🔹 Skewing increases effective rotor bar length.
🔹 Rotor resistance increases slightly.
🔹 Leakage reactance increases.
🔹 Increased reactance reduces current.
🔹 Starting torque decreases slightly.
👉 Hence, skewing does not increase starting torque.
❌ Why Statement (3) is Incorrect
“It increases starting torque” ❌
🔹 Skewing increases leakage reactance.
🔹 This reduces starting current.
🔹 Lower current produces lower torque.
🔹 Therefore, starting torque reduces slightly.
🧲 Relation with Slot Harmonics
🔹 Slot harmonics produce parasitic torques.
🔹 These cause crawling and vibration.
🔹 Skewing eliminates these effects.
🔹 Crawling is prevented.
📌 Additional Information
Why Skewing Gives Uniform Torque
🔹 Flux meets uniform reluctance.
🔹 Harmonic peaks are averaged.
🔹 Torque ripple is minimized.
📌 Key Exam Notes
⭐ Skewing → Prevents cogging
⭐ Skewing → Reduces noise
⭐ Skewing → Smooth torque
⭐ Skewing → Slightly reduces starting torque
📝 Final Summary
📍 Skewing of rotor bars prevents cogging, produces uniform torque, and reduces motor hum, but it does not increase starting torque.
👉 Hence, Option (c) is Correct.