A) rotates at the speed of rotor in the air gap
B) is at standstill with respect to stator mmf
C) rotates at slip speed with respect to stator mmf
D) rotates at synchronous speed with respect to rotor
✅ Answer:
➡️ B) is at standstill with respect to stator mmf
🧠Detailed Theory + Solution
◆ Step 1: Stator MMF
➡️ When 3-phase supply is given → rotating magnetic field (RMF) is produced
➡️ This RMF rotates at synchronous speed with respect to stator
◆ Step 2: Rotor Current & MMF
➡️ Rotor conductors cut stator field → EMF induced
➡️ Rotor current frequency:
f_r = s f
➡️ Rotor MMF rotates at slip speed (sNs) relative to rotor
◆ Step 3: Rotor MMF Speed w.r.t Stator
Rotor has two motions:
→ Mechanical rotation =
→ MMF rotation w.r.t rotor =
So total speed:
\text{Rotor MMF speed w.r.t stator} = N + sN_s
But we know:
N = (1 - s) N_s
So,
N + sN_s = (1 - s)N_s + sN_s = N_s
◆ Step 4: Key Result 🚨
➡️ Rotor MMF rotates at synchronous speed w.r.t stator
➡️ Stator MMF also rotates at
◆ Step 5: Final Interpretation
➡️ Both MMFs rotate at same speed and direction
👉 Therefore:
✔️ Relative speed = 0
✔️ They appear stationary to each other
❌ Why Other Options Are Incorrect
◆ A) Rotor speed → Wrong, MMF ≠ rotor speed
◆ C) Slip speed → Only relative to rotor, not stator
◆ D) Synchronous w.r.t rotor → Incorrect reference
🔑 Key Points to Remember
◆ Rotor MMF speed w.r.t rotor = slip speed (sNs)
◆ Rotor MMF speed w.r.t stator = synchronous speed (Ns)
◆ Stator & rotor fields lock together magnetically
◆ This is essential for torque production
📊 Summary Table
| Component | Speed w.r.t Stator | Speed w.r.t Rotor |
|---|---|---|
| Stator MMF | ||
| Rotor MMF | ||
| Rotor (Physical) | 0 |
🎯 Final Conclusion
➡️ Rotor MMF and stator MMF both rotate at same synchronous speed
✔️ Hence, they are at standstill relative to each other