1️⃣ Time harmonics
2️⃣ Space harmonics
3️⃣ Slot harmonics
4️⃣ Reverse rotating fields
✅ Answer (Detailed Solution Below)
✔️ Option 3: Slot harmonics
📖 Detailed Solution
🔹 In induction motors, stator and rotor slots produce harmonic components.
🔹 These harmonics are called slot harmonics.
🔹 Slot harmonics interact with the main magnetic field.
🔹 This interaction produces:
- Torque pulsations
- Noise
- Vibration
- Cogging
🔹 Due to skewing, bars do not face the same stator slot continuously.
🔹 Harmonic effects are averaged out.
🔹 Parasitic torque reduces.
🔹 Torque becomes smoother.
👉 Hence, skewing is used to reduce torque due to slot harmonics.
⚙️ What are Slot Harmonics?
🔹 Caused by slotting of stator and rotor.
🔹 Produce high-frequency flux variations.
🔹 Create pulsating and parasitic torques.
🔹 Lead to cogging and crawling.
⭐ Effects of Skewing
✔️ Suppresses slot harmonics
✔️ Reduces cogging
✔️ Reduces humming noise
✔️ Improves smooth running
✔️ Reduces torque ripple
✔️ Improves starting performance
✔️ Increases rotor resistance slightly
🧲 Cogging (Magnetic Locking)
🔹 Occurs when stator and rotor teeth align.
🔹 Produces strong magnetic attraction.
🔹 Rotor may refuse to start.
🔹 Also caused when:
Harmonic frequency = Slot frequency
Torque modulation occurs.
Prevention of Cogging
✔️ Different stator and rotor slots
✔️ Skewing of rotor slots
✔️ Proper design of slots
📈 Why Skewing Reduces Slot Harmonics
🔹 Bars lie under different stator teeth.
🔹 Harmonic torque is distributed.
🔹 Peaks are reduced.
🔹 Average torque becomes uniform.
❌ Why Other Options are Incorrect
(1) Time Harmonics ❌
🔹 Caused by supply waveform distortion.
🔹 Reduced by filters, not skewing.
(2) Space Harmonics ❌
🔹 Due to winding distribution.
🔹 Reduced by chorded windings.
(4) Reverse Rotating Fields ❌
🔹 Caused by negative sequence currents.
🔹 Not controlled by skewing.
📌 Additional Information
Advantages of Skewed Rotor Slots
🔹 Uniform air-gap flux
🔹 Quiet operation
🔹 Smooth torque curve
🔹 Less vibration
🔹 Better reliability
📌 Key Exam Notes
⭐ Skewing → Reduces slot harmonics
⭐ Skewing → Prevents cogging
⭐ Skewing → Reduces noise
⭐ Skewing → Improves torque smoothness
📝 Final Summary
📍 Skewing of rotor slots is mainly used to reduce torque pulsations caused by slot harmonics and to prevent cogging in induction motors.
👉 Hence, Option 3 is Correct.