1️⃣ Reduce the relative speed between the rotating stator field and the stationary rotor conductors
2️⃣ Oppose the rotor current
3️⃣ Increase the relative speed between the rotating stator field and stationary rotor conductors
4️⃣ None of the above
✅ Answer (Detailed Solution Below)
✔️ Option 1: Reduce the relative speed between the rotating stator field and the stationary rotor conductors
📖 Detailed Solution
🔹 The operation of a 3-phase induction motor is based on electromagnetic induction and Lenz’s Law.
⚙️ Step-by-Step Working Principle
🔹 Step 1: Production of Rotating Magnetic Field
✔️ When a 3-phase AC supply is applied to stator windings:
✔️ A rotating magnetic field (RMF) is produced.
✔️ RMF rotates at synchronous speed (Ns).
🔹 Step 2: Induction of Rotor EMF
✔️ At starting, rotor is stationary.
✔️ RMF cuts rotor conductors.
✔️ Due to relative speed, EMF is induced.
✔️ Rotor current flows (rotor is short-circuited).
👉 Explained by Faraday’s Law.
🔹 Step 3: Production of Torque
✔️ Rotor conductors carrying current are in magnetic field.
✔️ Mechanical force acts on conductors.
✔️ Net force produces torque.
👉 Explained by Lorentz force principle.
🔹 Step 4: Direction of Rotation (Lenz’s Law)
🔹 According to Lenz’s Law:
👉 Induced current always opposes the cause producing it.
🔹 Cause = Relative speed between RMF and rotor.
🔹 To oppose this cause:
✔️ Rotor moves in same direction as RMF.
✔️ Relative speed reduces.
👉 Hence, rotor follows stator field.
📐 Why Rotor Runs Below Synchronous Speed
🔹 If rotor speed = Ns:
✔️ Relative speed = 0
✔️ No EMF
✔️ No current
✔️ No torque
🔹 Motor cannot sustain speed.
👉 Therefore:
Rotor always runs slightly below synchronous speed.
⚙️ Concept of Relative Speed
| Quantity | Value |
|---|---|
| Stator field speed | Ns |
| Rotor speed | Nr (< Ns) |
| Relative speed | Ns − Nr |
✔️ Rotor tries to minimize (Ns − Nr).
❌ Why Other Options are Incorrect
(2) Oppose the Rotor Current ❌
🔹 Rotor current is necessary for torque.
🔹 Motor does not oppose its own current.
(3) Increase Relative Speed ❌
🔹 Motor always tries to reduce relative speed.
🔹 Increasing it would stop operation.
(4) None of the Above ❌
🔹 Option (1) is correct.
📘 Comparison with Other Machines
Induction Motor
✔️ Rotor speed < Ns
✔️ Relative speed ≠ 0
Synchronous Motor
✔️ Rotor speed = Ns
✔️ Relative speed = 0
DC Motor
✔️ Field and armature rotate together
✔️ Relative speed = 0
⭐ Important Exam Points
✔️ RMF induces rotor current
✔️ Lenz’s Law decides direction
✔️ Rotor tries to catch RMF
✔️ Slip must exist
📌 Key One-Line Exam Statement ⭐
The rotor rotates in the same direction as the stator field to reduce the relative speed between them as per Lenz’s Law.
📝 Final Conclusion
📍 The induced rotor current opposes the relative motion between the stator’s rotating magnetic field and the rotor. To reduce this relative speed, the rotor rotates in the same direction as the stator field.
👉 Hence, Option 1 is Correct.