1️⃣ Faraday’s laws of electromagnetic induction
2️⃣ Lenz’s law
3️⃣ Fleming’s right-hand rule
4️⃣ Lorentz force law
✅ Answer (Detailed Solution Below)
✔️ Option 2: Lenz’s Law
📖 Detailed Solution
🔹 When a 3-phase AC supply is applied to the stator:
✔️ A rotating magnetic field (RMF) is produced.
✔️ RMF rotates at synchronous speed (Ns).
✔️ This rotating field cuts the rotor conductors.
⚙️ Step 1: Induction of EMF (Faraday’s Law)
🔹 Due to relative motion between RMF and rotor:
✔️ EMF is induced in rotor conductors.
✔️ Rotor current flows (rotor is short-circuited).
👉 This step is explained by Faraday’s Law.
⚙️ Step 2: Direction of Rotor Rotation (Lenz’s Law)
🔹 According to Lenz’s Law:
👉 The induced current flows in such a direction that it opposes the cause producing it.
🔹 Cause of induced current = Relative speed between RMF and rotor.
🔹 To oppose this cause:
✔️ Rotor tries to reduce relative speed.
✔️ Rotor starts rotating in the same direction as RMF.
👉 Hence, rotor follows the stator field.
⚙️ Why Rotor Never Reaches Synchronous Speed
🔹 If rotor speed = RMF speed:
✔️ Relative speed = 0
✔️ No EMF induced
✔️ No rotor current
✔️ No torque
🔹 Rotor slows down again.
👉 Therefore, rotor always runs slightly below Ns.
📐 Mathematical Relation (Slip)
Where:
Ns = Synchronous speed
Nr = Rotor speed
For torque production:
✔️ Slip must be greater than zero.
❌ Why Other Options are Incorrect
(1) Faraday’s Law ❌
🔹 Explains induction of EMF.
🔹 Does not explain direction of motion.
(3) Fleming’s Right-Hand Rule ❌
🔹 Used for generators.
🔹 Not used for induction motor direction.
(4) Lorentz Force Law ❌
🔹 Applies to charged particles.
🔹 Not used for motor rotation explanation.
⭐ Important Exam Points
✔️ Faraday’s Law → EMF induction
✔️ Lenz’s Law → Direction of rotation
✔️ Rotor follows RMF
✔️ Rotor speed < Synchronous speed
📌 Key One-Line Exam Statement ⭐
The rotor rotates in the same direction as the stator field due to Lenz’s law, which makes the rotor oppose relative motion.
📝 Final Conclusion
📍 The induced rotor current always opposes the relative motion between the rotor and the rotating magnetic field. To reduce this relative speed, the rotor rotates in the same direction as the stator field. This behavior is explained by Lenz’s Law.
👉 Hence, Option 2 is Correct.