A) stator magnetic field and rotor rotate in opposite directions
B) rotor rotates at a speed less than synchronous speed and in the same direction
C) rotor rotates at a speed more than synchronous speed and in the same direction
D) none of the above
✅ Answer:
➡️ C) rotor rotates at a speed more than synchronous speed and in the same direction
🧠Detailed Theory + Solution
◆ Step 1: Slip Formula
Where:
→ = synchronous speed
→ = rotor speed
◆ Step 2: Normal Motor Operation
→ Rotor speed
s > 0
➡️ This is motoring region
◆ Step 3: Condition for Negative Slip 🚨
➡️ If rotor is driven by external prime mover
→ Rotor speed becomes greater than synchronous speed
N_r > N_s
So,
s = \frac{N_s - N_r}{N_s} < 0
➡️ Slip becomes negative
◆ Step 4: Physical Meaning
→ Rotor is moving faster than stator field
→ Relative motion reverses
➡️ Machine starts delivering power
👉 Works as induction generator
◆ Step 5: Operating Regions Summary
◆ → Motoring region
◆ → Synchronous speed
◆ → Generating region
◆ → Braking region
❌ Why Other Options Are Incorrect
◆ A) Opposite direction → slip > 1 (not negative)
◆ B) → positive slip
◆ D) Incorrect since option C is correct
🔑 Key Points to Remember
◆ Negative slip occurs when rotor speed exceeds synchronous speed
◆ Requires external mechanical input (prime mover)
◆ Machine acts as induction generator
◆ Power flows from rotor → supply
🎯 Final Conclusion
➡️ Slip becomes negative only when:
✔️ Rotor rotates faster than synchronous speed in same direction
👉 Correct answer: C) ✔️