1️⃣ Rotating at synchronous speed around stator
2️⃣ Constant in magnitude
3️⃣ Magnitude depending upon the load on the motor
4️⃣ None of the above
✅ Answer (Detailed Solution Below)
✔️ Option 3: Magnitude depending upon the load on the motor
📖 Detailed Solution
🔹 In a 3-phase induction motor, the stator produces a rotating magnetic field (RMF) at synchronous speed .
🔹 This rotating flux cuts the rotor conductors.
🔹 Due to this cutting action, EMF is induced in the rotor bars.
⚙️ Relation Between Rotor EMF and Slip
🔹 The induced rotor EMF is proportional to slip.
Where:
Er = Rotor induced EMF
s = Slip
Ns = Synchronous speed
N = Rotor speed
⚙️ Effect of Load on Rotor EMF
🔹 When Load Increases
✔️ Motor speed decreases
✔️ Slip increases
✔️ Relative speed increases
✔️ Rotor EMF increases
🔹 When Load Decreases
✔️ Motor speed increases
✔️ Slip decreases
✔️ Relative speed decreases
✔️ Rotor EMF decreases
👉 Therefore, rotor EMF depends directly on load.
📈 Operating Conditions and Rotor EMF
| Condition | Slip (s) | Rotor EMF |
|---|---|---|
| Starting | s = 1 | Maximum |
| Full Load | Small | Medium |
| No Load | Very Small | Minimum |
❌ Why Other Options are Incorrect
(1) Rotating at Synchronous Speed ❌
🔹 This describes stator flux speed.
🔹 It does not describe EMF magnitude.
(2) Constant in Magnitude ❌
🔹 Rotor EMF changes with slip.
🔹 Slip varies with load.
🔹 Hence, EMF is not constant.
(4) None of the Above ❌
🔹 Option (3) is correct.
📘 Important Concept
🔹 Rotor behaves like secondary of transformer.
🔹 Rotor EMF ∝ Relative speed.
🔹 Relative speed depends on load.
👉 Hence, rotor EMF depends on load.
⭐ Important Exam Points
✔️ Rotor EMF ∝ Slip
✔️ Slip ∝ Load
✔️ Load ↑ → EMF ↑
✔️ Load ↓ → EMF ↓
📌 Key One-Line Exam Statement ⭐
The magnitude of EMF induced in rotor bars depends on slip and hence varies with motor load.
📝 Final Conclusion
📍 The EMF induced in rotor bars is proportional to slip, and since slip varies with load, the rotor EMF magnitude depends on the load on the motor.
👉 Hence, Option 3 is Correct.